JP5725362B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lithium secondary battery (typically a lithium ion battery) and a method for manufacturing the same. Specifically, the present invention relates to a lithium secondary battery excellent in cycle characteristics used as a high-output power source for driving a vehicle and a method for manufacturing the same.

  Lithium ion batteries and other lithium secondary batteries are widely used for consumer use (such as power supplies for personal computers and portable terminals) because they are smaller, lighter, and have higher energy density than existing batteries. Moreover, since the output density is high, it is preferably used as a high-output power source for driving a vehicle such as a hybrid vehicle (HV).

  This type of lithium secondary battery (typically a lithium ion battery) has a configuration in which an electrode body composed of a positive electrode and a negative electrode and an electrolyte (typically an electrolyte solution) are contained in a battery case. The electrodes (positive electrode and negative electrode) are electrode mixture layers (mainly composed of active materials capable of reversibly occluding and releasing charge carriers (typically lithium ions) on corresponding positive and negative current collectors) Specifically, a positive electrode mixture layer and a negative electrode mixture layer) are respectively formed. For example, in the case of a positive electrode of a lithium secondary battery, a positive electrode active material such as a lithium transition metal composite oxide, a highly conductive material powder (conductive material), a binder (binder), etc. are mixed in an appropriate solvent. A porous positive electrode mixture layer is formed by applying the prepared slurry-like (including paste and ink-like) composition to the positive electrode current collector.

  In the battery, charge and discharge are performed by moving charge carriers (typically lithium ions) between the positive and negative electrodes. However, since the electrode active material repeatedly expands and contracts with such charge / discharge (that is, insertion and extraction of lithium ions in the electrode mixture layer), the electrode mixture layer deteriorates (collapses) every time the charge / discharge cycle is repeated. However, it is known that the charge / discharge capacity decreases. Patent document 1 is mentioned as a prior art regarding this kind of problem. Patent Document 1 discloses a technique in which polyvinylidene fluoride (PVdF) having a melting point of 165 ° C. or less is used as a binder for the positive electrode mixture layer.

JP 2003-157830 A

  By the way, there is a plug-in hybrid vehicle (PHV) equipped with a lithium secondary battery as a power source for driving a motor as a vehicle that has been developed and put into practical use in recent years. Hybrid vehicles (HV) already on the market use both an engine (internal combustion engine) and a battery for driving a motor as a power source. When the charge capacity of the motor driving battery decreases, the engine is driven to automatically operate the battery. Recharge. On the other hand, in addition to the HV mode (hybrid vehicle mode), the plug-in hybrid vehicle (PHV) has an EV mode (electric vehicle mode) that uses only the battery that is connected and charged in advance to an external power source such as a household power source. It is designed to run in either mode.

  As described above, since the PHV battery is used differently from the conventional HV battery, the required performance is also significantly different from the conventional battery. For example, in the case of a battery for PHV, use in the EV mode (that is, traveling with only pre-charged electricity) must be considered. Therefore, the battery is required to have high energy density (rated capacity) in addition to high output density (output characteristics) and durability (good cycle characteristics). In addition, the PHV battery is not automatically charged even if the charge capacity decreases. For this reason, rapid discharge (that is, acceleration of an automobile) is frequently performed even in a region where the SOC (State of Charge) is low (for example, SOC is 30% or less). Generally, it is known that charging / discharging in such a region has a high internal resistance of the battery, and output characteristics and cycle characteristics are deteriorated more than charging / discharging in a region where SOC is high. For this reason, a battery for PHV needs to be able to cope with rapid discharge in a wide SOC range (especially in a region where the SOC is 30% or less). As described above, PHV batteries are required to have high power density and improved durability under unprecedented severe conditions, and it is difficult for the technology described in Patent Document 1 to deal with such problems.

  The present invention has been made in view of the above points, and an object of the present invention is a lithium secondary battery having a high energy density, which has improved battery performance (for example, cycle characteristics and output density) as compared with the related art. It is to provide a lithium secondary battery, particularly the above-described vehicle driving power source. Another object of the present invention is to provide a suitable method for producing such a lithium secondary battery.

In order to achieve the above object, according to the present invention, there is provided a lithium secondary battery comprising an electrode body having a positive electrode and a negative electrode, and an electrolyte, wherein the positive electrode is formed on the positive electrode current collector and the positive electrode current collector. A positive electrode mixture layer, and the positive electrode mixture layer includes a positive electrode active material and polyvinylidene fluoride (PVdF) as a binder. Here, a mass ratio (α / β) of α crystal and β crystal based on a nuclear magnetic resonance spectrum (NMR) of the polyvinylidene fluoride is 0.42 or more and 0.56 or less. To do.
The lithium secondary battery having such a configuration is characterized by having PVdF in which α / β satisfies the above value. That is, PVdF satisfying the above α / β can suppress swelling or dissolution in the electrolyte, and can maintain the shape of the positive electrode mixture layer for a long period of time. Therefore, in the lithium secondary battery provided with such a positive electrode mixture layer, battery performance (for example, cycle characteristics and output density) can be improved.

As a preferable embodiment of the lithium secondary battery disclosed herein, the melting start temperature based on the differential scanning calorimetry (DSC) of the polyvinylidene fluoride (PVdF) is 144 ° C. or higher and 153 ° C. or lower. Can be mentioned.
PVdF satisfying such a melting start temperature can more effectively suppress swelling or dissolution in the electrolyte, and thus can maintain the shape of the positive electrode mixture layer over a long period of time. For this reason, in a lithium secondary battery provided with such a positive electrode mixture layer, even better battery performance (for example, cycle characteristics and output density) can be realized.

As a preferred embodiment of the lithium secondary battery disclosed herein, a value (spring constant ratio) obtained by dividing the spring constant after the positive electrode is immersed in the electrolyte for 30 days by the spring constant before immersion is 0.8. It is mentioned that it is above 1.0.
A positive electrode that satisfies the above-described spring constant ratio (that is, the spring constant is well maintained) is excellent in stretchability and can suitably follow the expansion and contraction of the positive electrode mixture layer accompanying charge / discharge. In addition, since the shape of the positive electrode mixture layer can be maintained over a long period of time, battery performance (for example, cycle characteristics and output density) can be improved in a lithium secondary battery including such a positive electrode.

As a preferable embodiment of the lithium secondary battery disclosed herein, the porosity of the positive electrode mixture layer is 25% or more and 40% or less.
A lithium secondary battery that satisfies the above porosity can achieve both high output density and high energy density. Moreover, since it is excellent in elasticity and can follow the expansion and contraction of the composite material layer accompanying charge / discharge, the lithium secondary battery provided with such a positive electrode has high energy density and battery performance (for example, Cycle characteristics and output density) can be improved.

As a preferred embodiment of the lithium secondary battery disclosed herein, the positive electrode current collector is formed in a long shape, and the positive electrode mixture layer is formed on the long positive electrode current collector. The negative electrode is configured by forming a negative electrode mixture layer on a long negative electrode current collector corresponding to the long positive electrode current collector, and the long positive electrode and the long positive electrode current collector. A negative electrode is laminated and wound to form a wound electrode body.
For example, an assembled battery in which a plurality of lithium secondary batteries including the wound electrode body are arranged and restrained in the arrangement direction is liable to cause performance deterioration due to fluctuations in surface pressure (constraint pressure), and thus disclosed herein. The lithium secondary battery (single cell) to be used can be particularly preferably used. Therefore, this invention can also provide an assembled battery having a configuration in which a plurality of lithium secondary batteries disclosed herein are connected and arranged.

  The lithium secondary battery disclosed herein has a high energy density, and can improve battery performance (for example, cycle characteristics and output density) as compared with the related art. In particular, vehicles (typically electric motors such as plug-in hybrid vehicles (PHV), hybrid vehicles (HV), electric vehicles (EV), fuel cell vehicles (FCV)) are capable of exhibiting such high performance. Can be suitably used as a power source (drive power source) for a motor mounted on the motor.

  Here, a method for producing a lithium secondary battery including an electrode body having a positive electrode and a negative electrode and an electrolyte is provided. The manufacturing method includes a step of producing a positive electrode having a positive electrode mixture layer and a positive electrode current collector that holds the positive electrode mixture layer, and a step of constructing a lithium secondary battery using the positive electrode. . Here, the step of producing the positive electrode includes applying a slurry-like composition containing a positive electrode active material and polyvinylidene fluoride as a binder onto the positive electrode current collector to form the positive electrode mixture layer. It includes pressing the positive electrode mixture layer to have a predetermined thickness and heat-treating the pressed positive electrode mixture layer at a temperature of 145 ° C. or higher and 158 ° C. or lower.

  As another preferable embodiment of the production method disclosed herein, the step of producing the positive electrode includes α crystal and β based on a nuclear magnetic resonance spectrum of polyvinylidene fluoride contained in the positive electrode mixture layer by the heat treatment. And the mass ratio (α / β) to the crystal is 0.42 or more and 0.56 or less.

  As another preferred embodiment of the production method disclosed herein, the step of producing the positive electrode has a melting start temperature based on a differential scanning calorimetry of polyvinylidene fluoride contained in the positive electrode mixture layer by the heat treatment. For example, the temperature may be 144 ° C or higher and 153 ° C or lower.

  As another preferable embodiment of the production method disclosed herein, the pressing in the step of producing the positive electrode may be performed so that the porosity of the positive electrode mixture layer is 25% or more and 40% or less. .

  As another preferred embodiment of the production method disclosed herein, in the step of producing the electrode body, a positive electrode current collector formed in an elongated shape as the positive electrode current collector is used to form a positive electrode. The laminated positive electrode and the corresponding long negative electrode are laminated to produce a wound electrode body.

It is a perspective view which shows typically the external shape of the assembled battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium secondary battery which concerns on one Embodiment of this invention. It is a side view which shows typically the vehicle (automobile) provided with the lithium secondary battery which concerns on one Embodiment of this invention. It is a graph which shows transition of cycle characteristics about Examples 1 and 2 and Comparative Examples 1 and 2. It is a graph which shows the capacity | capacitance maintenance factor of cycling characteristics about Example 1, 2 and Comparative Examples 1,2. It is a graph which shows the capacity | capacitance maintenance factor of an output characteristic about Examples 1, 2 and Comparative Examples 1-3. It is a graph showing transition of the spring constant ratio after electrolyte solution immersion about Example 1, 2 and Comparative Examples 1-3. It is a graph which shows the relationship between cycling characteristics and a spring constant ratio about Example 1, 2 and Comparative Examples 1-3. It is a graph which shows the relationship between an output characteristic and a spring constant ratio about Example 1, 2 and Comparative Examples 1-3. It is a graph which shows the relationship between (alpha) / (beta) ratio of PVdF in NMR, and a spring constant ratio. It is a graph which shows the relationship between the melting start temperature of PVdF in DSC, and a spring constant ratio. It is a figure for demonstrating the measuring method of a spring constant.

  In this specification, “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term that includes a so-called storage battery such as a lithium secondary battery and a power storage element such as an electric double layer capacitor. In addition, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged / discharged by movement of charges accompanying the lithium ions between the positive and negative electrodes. A secondary battery generally referred to as a lithium ion battery (or a lithium ion secondary battery), a lithium polymer battery, or the like is a typical example included in the lithium secondary battery in this specification. The “active material” refers to a substance (compound) that participates in power storage on the positive electrode side or the negative electrode side. That is, it refers to a substance that is involved in the insertion and extraction of electrons during battery charge / discharge.

  Hereinafter, preferred embodiments of the lithium secondary battery disclosed herein will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in this field. The lithium secondary battery having such a structure can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field.

As a positive electrode of the lithium secondary battery disclosed herein, a granular positive electrode active material is attached on a positive electrode current collector as a positive electrode mixture together with a conductive material, PVdF as a binder, or the like (positive electrode active material layer) (Also referred to as “.”) Is used.
Here, as a material of the positive electrode current collector, a member mainly composed of a metal having good conductivity such as aluminum, nickel, titanium, stainless steel, or the like can be used. The shape of the current collector is not particularly limited because it may vary depending on the shape of the battery constructed using the obtained electrode, and a rod-like body, a plate-like body, a foil-like body, a net-like body, or the like may be used. it can. In a battery provided with a wound electrode body to be described later, a foil-like body made of an alloy mainly composed of aluminum (aluminum alloy) is used. The thickness of the foil-shaped current collector is not particularly limited, but about 5 μm to 200 μm (more preferably 10 μm to 30 μm) can be preferably used in consideration of the capacity density of the battery and the strength of the current collector.

As the positive electrode active material used here, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, an oxide containing lithium and a transition metal element as constituent metal elements such as lithium nickel oxide (eg LiNiO ₂ ), lithium cobalt oxide (eg LiCoO ₂ ), lithium manganese oxide (eg LiMn ₂ O ₄ ) Examples thereof include lithium transition metal oxides), phosphates including lithium and transition metal elements such as lithium manganese phosphate (LiMnPO ₄ ) and lithium iron phosphate (LiFePO ₄ ) as constituent metal elements. Among them, a positive electrode active material (typically substantially lithium nickel cobalt, which is mainly composed of lithium nickel cobalt manganese composite oxide (also referred to as NCM, for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ). A positive electrode active material made of a manganese composite oxide can be preferably used because of its excellent thermal stability and high energy density.

  Here, the lithium nickel cobalt manganese composite oxide is an oxide having Li, Ni, Co, and Mn as constituent metal elements, and at least one other metal element (Li, Ni, Co, and Mn) in addition to Li, Ni, Co, and Mn. The meaning also includes oxides containing transition metal elements and / or typical metal elements other than Ni, Co, and Mn. Such metal elements are, for example, one or more elements of Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. It can be. The same applies to lithium nickel oxide, lithium cobalt oxide, and lithium manganese oxide. As such a lithium transition metal oxide (typically in particulate form), for example, a lithium transition metal oxide powder prepared by a conventionally known method can be used as it is. For example, lithium transition metal oxide powder substantially composed of secondary particles having an average particle diameter in the range of about 1 μm to 25 μm can be preferably used as the positive electrode active material.

  As the conductive material used here, one kind or two or more kinds of substances conventionally used in lithium secondary batteries can be used without particular limitation. For example, carbon materials such as carbon powder and carbon fiber are preferably used. Alternatively, conductive metal powder such as nickel powder may be used. As the carbon powder, various carbon blacks (for example, acetylene black (AB), furnace black, ketjen black (KB)), graphite powder, and the like can be used. Among these, a preferable carbon powder is acetylene black (AB). The smaller the particle size of the primary particles of the conductive material, the wider the contact area. Therefore, the conductive path in the positive electrode mixture layer (that is, the contact area between the positive electrode active material and the conductive material) is advantageous. For example, when carbon powder is used, it is preferable that the average particle size of primary particles constituting the powder is in the range of about 10 nm to 200 nm (typically about 15 nm to 100 nm). Moreover, although it does not specifically limit, the usage-amount of the electrically conductive material with respect to 100 mass% of positive electrode active materials can be 1 mass%-20 mass% (preferably 5 mass%-15 mass%), for example. Moreover, you may contain various additives (For example, the inorganic compound (for example, phosphate and carbonate) which generate | occur | produces gas at the time of an overcharge) etc.

  The positive electrode mixture layer of the lithium secondary battery disclosed herein contains at least polyvinylidene fluoride (PVdF) as a binder in addition to the positive electrode active material. The concept of PVdF here may include a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and other monomers, a modified product thereof, a modified product, and the like. Monomers that can be copolymerized with vinylidene fluoride in the copolymer include, for example, fluorine-based monomers such as vinyl fluoride, ethylene trifluoride, ethylene trifluoride, tetrafluoroethylene, and hexafluoropropylene; ethylene, It may be one or more selected from non-fluorinated monomers such as methyl methacrylate and acrylic acid; The copolymerization ratio of vinylidene fluoride in the copolymer is preferably 50% by mass or more (typically 70% by mass or more, for example, 85% by mass or more). Examples of the modified product include a vinylidene fluoride homopolymer or copolymer carboxylic acid modified product.

  The positive electrode mixture layer of the lithium secondary battery disclosed herein has a composition that includes a solid component (optional component) other than the conductive material and the PVdF as necessary, as long as the effects of the present invention are not significantly impaired. possible. For example, a polymer other than PVdF (typically, a polymer other than PVdF among polymers described later as a binder for the negative electrode mixture layer) can be contained as an optional component. However, the content of the polymer which is an optional component is preferably 1/2 or less, more preferably 1/5 or less of the mass of PVdF. If the content of such an arbitrary polymer is too large, the spring constant of the positive electrode, which will be described later, decreases due to repeated charge and discharge (that is, the electrode mixture layer deteriorates (collapses)), and the cycle characteristics and the like may decrease. In a preferred embodiment, the positive electrode mixture layer substantially does not contain a polymer (binder) other than PVdF, or the content of the polymer (binder) is 5% by mass or less of the whole positive electrode mixture layer (further 3% by mass). % Or less).

Then, the powdered material containing the positive electrode active material and the conductive material is dispersed in a suitable dispersion medium together with a binder containing PVdF and kneaded to form a slurry (including pastes and inks). (Hereinafter, referred to as “positive electrode mixture slurry”). A method in which an appropriate amount of this positive electrode mixture slurry is applied to one or both sides of the positive electrode current collector to form a positive electrode mixture layer and then dried can be preferably employed. This drying can be performed under heating as necessary. In a preferred embodiment, the drying temperature is about 120 ° C. or lower (typically 80 ° C. or higher and lower than 120 ° C.), and the drying time is about 30 seconds to 2 minutes.
After drying the positive electrode mixture slurry, press treatment and heat treatment are performed. Thus, the thickness and density of the positive electrode mixture layer can be adjusted.

  As the dispersion medium used here, one or two or more organic solvents conventionally used in the production of lithium secondary batteries can be used without any particular limitation. For example, amide, alcohol, ketone, ester, amine, ether, nitrile, cyclic ether, aromatic hydrocarbon and the like can be mentioned. More specifically, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide, 2-propanol, ethanol, methanol, acetone, methyl ethyl ketone, methyl propenoate, Cyclohexanone, methyl acetate, ethyl acetate, methyl acrylate, diethyl triamine, N, N-dimethylaminopropylamine, acetonitrile, ethylene oxide, tetrahydrofuran (THF), dioxane, benzene, toluene, ethylbenzene, xylene dimethyl sulfoxide (DMSO), dichloromethane, Examples include trichloromethane and dichloroethane. Of these, N-methyl-2-pyrrolidone (NMP) is particularly preferably used. For example, the dispersion state of the positive electrode mixture paste and the drying rate of the positive electrode mixture paste are changed by mixing other dispersion media (for example, xylene) having different boiling points into N-methyl-2-pyrrolidone (NMP). Can be made.

  The said press process can be used without specifically limiting 1 type, or 2 or more types among the techniques conventionally used at the time of preparation of a lithium secondary battery. For example, a roll press method, a flat plate press method, etc. can be used suitably. In addition, when the positive electrode mixture layer is too bulky, the curvature after pressing is large and it is difficult to use as a wound electrode described later.

  The heat treatment temperature (H) can be, for example, H ≧ 120 ° C., usually H ≧ 130 ° C. is appropriate, for example, H ≧ 140 ° C., preferably H ≧ 145 ° C. If the heat treatment temperature H is too low, the treatment time until the desired α / β is realized becomes long, and the productivity of the battery is lowered, or the degree of treatment (α / β value) becomes uneven. It can happen. The heat treatment temperature H can be, for example, H <160 ° C., and preferably H ≦ 158 ° C. If the heat treatment temperature is too high, the PVdF may be completely melted after the positive electrode mixture layer is formed, and the crystal form of the PVdF may change. In such a case, the performance (for example, cycle characteristics and output density) of the lithium secondary battery is deteriorated such that the porosity of the positive electrode mixture layer changes or the conductive path in the positive electrode mixture layer cannot be maintained. It is possible that In addition, although the example which performs the drying of the positive electrode mixture slurry and the heat treatment of the formed positive electrode mixture layer as separate processes has been described above, these (drying and heat treatment) may be performed as a series of operations.

The mass (weight per unit area) of the positive electrode mixture layer per area of the positive electrode current collector can be, for example, 15.0 mg / cm ^{2 to} 50.0 mg / cm ² (solid content). Usually, a suitable result can be realized by setting it to 25.0 mg / cm ^{2 to} 40.0 mg / cm ² (solid content). If this amount is too small, the energy density per unit battery is small, and if it is too large, the internal resistance of the battery tends to increase. For the same reason, the density of the positive electrode composite material layer may be a ^{2.2g / cm 3 ~3.0g / cm 3} , typically a 2.5g ^/ cm ³ ~2.93g ^/ cm 3 Is appropriate.

The porosity of the positive electrode mixture layer formed on the positive electrode current collector after the heat treatment step can be set to, for example, 20% to 50%. Usually, a preferable result can be realized by setting the content to 23 to 40%. When the density of the positive electrode mixture layer is low, the energy density of the battery decreases. On the other hand, if the density of the positive electrode mixture layer is too high, as shown in Comparative Example 3 described later, the output density particularly in a region where the SOC is low is lowered.
In addition, the porosity in this specification is the ratio of the space | gap in a positive mix layer. For example, the porosity of the positive electrode mixture layer is expressed by the following formula (Va (cm ³ )) from the apparent volume Va (cm ³ ) of the positive electrode mixture layer and the volume Vb (cm ³ ) of the void formed inside the positive electrode mixture layer ( Calculated according to 1).
(Vb / Va) × 100 (1)
The apparent volume Va of the positive electrode mixture layer is calculated by the following formula (2) from the area S in plan view of the sample of the positive electrode sheet and the thickness T of the positive electrode mixture layer. The area Sa in a plan view of the positive electrode sheet is obtained, for example, by cutting a sample of the positive electrode sheet into a square or a rectangle with a punching machine or a cutter. Further, the thickness T of the positive electrode mixture layer is obtained by using, for example, a caliper or a thickness measuring machine.
Va = S × T (2)
The volume Vb of the void formed inside the positive electrode mixture layer is measured, for example, by using a mercury porosimeter that measures the pore distribution of the porous body by a mercury intrusion method. In this measurement, “void” means a void opened to the outside (open hole), and a closed space (closed hole) in the positive electrode mixture layer is not included in the “void”. In the mercury intrusion method, first, the positive electrode sheet is immersed in mercury in a vacuumed state. In this state, when the pressure applied to mercury increases, mercury gradually enters into a smaller space. For this reason, the size of the voids in the positive electrode mixture layer and the volume distribution thereof can be determined based on the relationship between the amount of mercury that has entered the positive electrode mixture layer and the pressure applied to the mercury. For example, when Autopore III9410 manufactured by Shimadzu Corporation is used, the volume distribution of voids corresponding to a pore range of 50 μm to 0.003 μm can be grasped by measuring at 4 psi to 60000 psi.

The porosity of the positive electrode mixture layer is, for example, the mass W (g) of the positive electrode mixture layer, the apparent volume V (cm ³ ) of the positive electrode active material layer 34, and the true density ρ ( It is also possible to calculate from the following equation (3) from the value obtained by dividing the mass W by the actual volume that does not include voids. The true density of the positive electrode mixture layer is measured by a density measuring device such as a constant volume expansion method (gas displacement pycnometer method), for example.
(1-W / ρV) × 100 (3)

Further, the porosity of the positive electrode mixture layer is, for example, the area Sb occupied by the voids per unit cross-sectional area of the positive electrode mixture layer in the cross-sectional sample of the positive electrode mixture layer and the apparent cross-sectional area Sa of the positive electrode mixture layer. From the above, it can also be calculated by the following equation (4).
(Sb / Sa) × 100 (4)
Sa and Sb can be measured by, for example, observing a cross section of the positive electrode mixture layer with a scanning electron microscope (SEM). According to the cross-sectional image, the voids formed in the cross-section of the constituent material of the positive electrode mixture layer can be determined based on the difference in color tone and shade. The sample for observation can obtain an arbitrary cross section of the positive electrode sheet by CP processing (Cross Section Polisher processing), and the porosity can be accurately approximated as the number of the samples increases.

In a preferred embodiment of the technology disclosed herein, the polyvinylidene fluoride (PVdF) constituting the positive electrode mixture layer is formed by the crystal of the PVdF being an α crystal and a β crystal based on a nuclear magnetic resonance spectrum (NMR). It is characterized by a mass ratio (α / β) of 0.42 or more and 0.56 or less. This mass ratio α / β can be determined by solid-state NMR ( ¹⁹ F-NMR) measurement using ¹⁹ F as an observation nucleus. More specifically, α / β can be calculated by waveform separation of α / 2 near −78 ppm and α / 2 + β around −95 ppm in the NMR measurement. In the present specification, as a numerical value indicating a preferable range of α / β, the third decimal place may be rounded off. In this case, for example, α / β = 0.562 satisfies the condition that “α / β is 0.42 or more and 0.56 or less”, but α / β = 0.568 is not satisfied.

  When the value of α / β is too small, the positive electrode composite material layer has poor stretchability, and it may be difficult to maintain the shape of the positive electrode composite material layer for a long period of time. If the value of α / β is too large, PVdF swells or dissolves in the electrolyte, and the conductive path in the positive electrode mixture layer (that is, the contact area between the positive electrode active material and the conductive material) decreases. Therefore, battery performance (for example, cycle characteristics and output characteristics) may be deteriorated. When α / β is in the preferred range described above, for example, in a cycle characteristic test performed by the method described in the examples described later, the capacity retention after 2000 cycles can be improved by 2 times or more. Therefore, according to this aspect, the performance (for example, cycle characteristics) of the lithium secondary battery can be improved.

  Usually, a positive electrode mixture slurry in which a positive electrode active material, a commercially available PVdF as a binder, and a conductive material are dispersed in an appropriate dispersion medium (for example, an organic solvent such as N-methyl-2-pyrrolidone) is used as a current collector. In the positive electrode mixture layer formed by applying and drying under general conditions (for example, 80 ° C. to 120 ° C.), α / β of PVdF is about 0.59, which is not the preferred range disclosed herein. (Comparative Example 1 described later). As a method of obtaining a positive electrode mixture layer in which α / β of PVdF is in the preferable numerical range disclosed herein, there is a method of employing the positive electrode manufacturing method disclosed here (that is, the heat treatment step described above).

  In a preferred embodiment of the technology disclosed herein, the polyvinylidene fluoride (PVdF) constituting the positive electrode mixture layer has a melting start temperature of 140 ° C. or higher based on differential scanning calorimetry (DSC) of the PVdF crystal. (Preferably 144 ° C. or higher) 155 ° C. or lower (preferably 153 ° C. or lower). If the melting start temperature is lower than the region where the melting start temperature is applied (that is, the PVdF crystal size is small), the PVdF may swell or dissolve in the electrolyte due to a slight temperature change in the battery usage environment. Moreover, when the value of the melting start temperature is too high, there is a possibility that it cannot follow the expansion and contraction of the electrode mixture layer accompanying charge / discharge. When the melting start temperature is in the preferred range described above, for example, in a cycle characteristic test performed by the method described in the examples described later, the capacity retention after 2000 cycles can be improved by 2 times or more. Therefore, according to this aspect, the performance (for example, cycle characteristics) of the lithium secondary battery can be improved.

  In addition, the said melting start temperature can be grasped | ascertained by the DSC measurement which heats up at a rate of 10 degree-C / min from at least 50 degreeC to 200 degreeC (for example, 25 degreeC to 250 degreeC). More specifically, for example, it can be determined by DSC measurement performed by the method described in the examples described later. The melting start temperature can be adjusted by the positive electrode manufacturing method disclosed here (that is, the heat treatment step described above). In a typical aspect of the technology disclosed herein, the heat treatment temperature of the positive electrode mixture layer and the melting start temperature of PVdF in the positive electrode mixture layer are approximately the same.

In the lithium secondary battery disclosed herein, a value (spring constant ratio) obtained by dividing the spring constant after the positive electrode is immersed in an electrolyte described later for 30 days by the spring constant before immersion is 0.8 or more and 1.0. It is preferable that it is less than. A positive electrode that satisfies the spring constant ratio (that is, the spring constant is well maintained) is excellent in stretchability and can suitably follow the expansion and contraction of the positive electrode mixture layer that accompanies charge / discharge. In addition, since the shape of the positive electrode mixture layer can be maintained over a long period of time, performance (for example, cycle characteristics and output density) can be improved in a lithium secondary battery including such a positive electrode.
Here, the spring constant is a proportionality constant obtained by dividing the load P when a load is applied to the electrode by the displacement X. More specifically, the displacement X when the load P (kgf: 1 kgf = about 9.8 N) is applied to the flat surface and compressed is read and calculated from the following equation (5).
Spring constant k (kgf / mm) = P (kgf) / X (mm) (5)
The spring constant can be measured using, for example, a compression tester shown in FIG. First, an arbitrary number of electrodes (positive electrode sheets) 10 provided with a positive electrode mixture layer are prepared. A pair of flat plates 90 are brought into contact with both sides of the flat surface 10A of the electrode (that is, so as to be horizontal with the positive electrode mixture layer formed on the positive electrode sheet). The electrode (positive electrode sheet) 10 is sandwiched between the pair of flat plates 90 from both sides, and a load is applied to the flat surface 10A of the electrode (positive electrode sheet) 10 to compress (typically elastic deformation).
By the above method, the spring constant kb before the positive electrode sheet is immersed in the electrolyte and the spring constant ka after the immersion are calculated. Using this spring constant, the ratio of the spring constant is obtained from the following equation (6).
kb / ka (6)
More specifically, it can be determined by, for example, the method described in Examples described later.

In the negative electrode of the lithium secondary battery disclosed herein, a negative electrode active material layer (also referred to as a negative electrode active material layer) is formed by adhering a granular negative electrode active material together with a binder or the like as a negative electrode mixture onto a negative electrode current collector. .) Is used.
Here, examples of the material for the negative electrode current collector include copper, nickel, titanium, and stainless steel. The form is not particularly limited, and a rod-like body, a plate-like body, a foil-like body, a net-like body, or the like can be used. In a battery including a wound electrode body to be described later, a foil-like body made of copper or an alloy (copper alloy) containing copper as a main component is used. The thickness of the foil-shaped current collector is not particularly limited, but about 5 μm to 200 μm (more preferably 10 μm to 50 μm) can be preferably used in consideration of the capacity density of the battery and the strength of the current collector.

  As the negative electrode active material used here, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, amorphous carbon (carbon powder), oxides such as lithium titanate (LTO), tin (Sn), silicon (Si) and lithium alloys, and the like can be given. As amorphous carbon, graphite (graphite), non-graphitizable carbonaceous (hard carbon), graphitizable carbonaceous (soft carbon), or a combination thereof including at least a part of graphite structure (layered structure) Can be used.

  As a binder used here, 1 type, or 2 or more types of various polymer materials normally used when manufacturing a lithium secondary battery can be used, without specifically limiting. For example, when the negative electrode mixture layer is formed using an aqueous liquid slurry, a polymer material that is dissolved or dispersed in water can be preferably employed. Examples of such polymer materials include cellulose polymers, fluorine resins, vinyl acetate copolymers, rubbers, and the like. More specifically, carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), styrene butadiene rubber (SBR), acrylic acid-modified SBR resin (SBR latex), and the like. Alternatively, when a negative electrode mixture layer is formed using a solvent-based liquid slurry (a solvent-based composition in which the main component of the dispersion medium is an organic solvent), PVdF, polyvinylidene chloride (PVdC), polyethylene oxide (PEO) A polymer material that is dispersed or dissolved in an organic solvent such as) can be preferably used. In addition, various polymer materials (for example, carboxymethyl cellulose (CMC)) that can function as a thickener, conductive materials, and the like may be added. Moreover, although it does not specifically limit, the usage-amount of the binder with respect to 100 mass% of negative electrode active materials can be 1-10 mass% (preferably 2-5 mass%), for example.

A powdery material containing the granular negative electrode active material and, if necessary, a conductive material or the like together with a suitable binder (for example, an organic solvent such as N-methylpyrrolidone (NMP) or an aqueous solvent such as water) ) And kneading to prepare a slurry-like composition (including paste-like and ink-like) (hereinafter referred to as “negative electrode mixture slurry”). A method of applying an appropriate amount of this negative electrode mixture slurry to one or both surfaces of the negative electrode current collector and drying it can be preferably employed. This drying can be performed under heating as necessary.
After drying the negative electrode mixture slurry, the thickness and density of the negative electrode mixture layer are appropriately subjected to press treatment (for example, various conventionally known press methods such as a roll press method and a flat plate press method can be employed). Can be adjusted.

An electrode body in which the positive electrode and the negative electrode are laminated is prepared and accommodated in an appropriate battery case together with the electrolytic solution to construct a lithium secondary battery. In the typical configuration of the lithium secondary battery disclosed herein, a separator is interposed between the positive electrode and the negative electrode.
As a battery case, the material and shape which are used for the conventional lithium secondary battery can be used. Examples of the material include relatively light metal materials such as aluminum and steel, and resin materials such as PPS and polyimide resin. The shape (outer shape of the container) is not particularly limited, and may be, for example, a cylindrical shape, a rectangular shape, a rectangular parallelepiped shape, a coin shape, a bag shape, or the like. The case may be provided with a safety mechanism such as a current interruption mechanism (a mechanism capable of interrupting current in response to an increase in internal pressure when the battery is overcharged).

As the electrolytic solution used here, one kind or two or more kinds similar to the nonaqueous electrolytic solution used in the conventional lithium secondary battery can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which an electrolyte (lithium salt) is contained in a suitable nonaqueous solvent.
As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, Examples include 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. Of these, nonaqueous solvents mainly composed of carbonates are preferably used. For example, the nonaqueous solvent contains one or more carbonates, and the total volume of these carbonates is 60% by volume or more (more preferably 75% by volume or more, and further preferably 90% by volume) of the total volume of the nonaqueous solvent. The non-aqueous electrolyte solution occupying the above and substantially 100% by volume) is preferably used. Further, it may be a solid (gel) electrolytic solution in which a polymer is added to the liquid electrolytic solution. Various additives (for example, an overcharge inhibitor (a compound that decomposes in an overcharged state and generates a large amount of gas)) may be appropriately added.
Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiClO _{4 and the} like are exemplified. Of these, LiPF ₆ is preferably used. The concentration of the electrolyte is not particularly limited, but if the concentration of the electrolyte is too low, the amount of lithium ions contained in the electrolytic solution tends to be insufficient, and the ionic conductivity tends to decrease. On the other hand, if the concentration of the supporting electrolyte is too high, the viscosity of the non-aqueous electrolyte solution becomes too high, and the ionic conductivity tends to decrease. For this reason, a nonaqueous electrolytic solution containing an electrolyte at a concentration of about 0.1 mol / L to 5 mol / L (preferably about 0.8 mol / L to 1.5 mol / L) is preferably used.

  As the separator used here, various porous sheets similar to those conventionally used for lithium secondary batteries can be used. For example, a porous resin sheet (film, nonwoven fabric, etc.) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like can be mentioned. Such a porous resin sheet may have a single layer structure, or may have a two or more layers structure (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). Although it is not particularly limited, preferred porous sheets (typically porous resin sheets) used as the separator base material have an average pore diameter of about 0.001 μm to 30 μm and a thickness of 5 μm to 100 μm. Examples of the porous resin sheet are (more preferably 10 μm to 30 μm). The porosity (porosity) of the porous sheet may be, for example, about 20% to 90% by volume (preferably 30% to 80% by volume). Note that in a lithium secondary battery (lithium polymer battery) using a solid electrolyte, the electrolyte may also serve as a separator.

  Although not intended to be particularly limited, as a schematic configuration of the lithium secondary battery according to one embodiment of the present invention, a flatly wound electrode body (winding electrode body), a non-aqueous electrolyte solution, As an example, a lithium secondary battery (unit cell) in a form in which the battery is housed in a flat box-shaped (cuboid) container is shown in FIGS. An example of an assembled battery having a configuration in which unit cells according to an embodiment of the present invention are connected and arranged is shown in FIG. 1 as a schematic configuration diagram. In the following drawings, members / parts having the same action are denoted by the same reference numerals, and redundant description may be omitted or simplified. The dimensional relationship (length, width, thickness, etc.) in each figure does not reflect the actual dimensional relationship.

  FIG. 1 shows an assembled battery 200. A lithium secondary battery (unit cell) 100 constituting the assembled battery 200 is shown in FIG. The lithium secondary battery 100 includes a wound electrode body 80 and a battery case 50. FIG. 3 is a view showing a wound electrode body 80.

  An example of an assembled battery (typically, an assembled battery in which a plurality of single cells are connected in series) including the lithium secondary battery 100 according to an embodiment of the present invention as a single battery and a plurality of the single batteries. Is shown in FIG. The assembled battery 200 includes a plurality of (typically 10 or more, preferably about 10 to 30, for example, 20) lithium secondary batteries (single cells) 100 each having a positive electrode terminal 70 and a negative electrode terminal 72. Are arranged one by one so as to be alternately arranged, and the wide surfaces of the battery case 50 are arranged in the facing direction (stacking direction). A cooling plate 110 having a predetermined shape is sandwiched between the arranged cells 100. The cooling plate 110 functions as a heat dissipating member for efficiently dissipating heat generated in each unit cell 100 during use, and preferably a cooling fluid (typically between the unit cells 100). Air) (for example, a shape in which a plurality of parallel grooves extending vertically from one side of the rectangular cooling plate to the opposite side are provided on the surface). A cooling plate made of metal having good thermal conductivity or lightweight and hard polypropylene or other synthetic resin is suitable.

A pair of end plates (restraint plates) 120 are arranged at both ends of the unit cells 100 and the cooling plate 110 arranged as described above. One or a plurality of sheet-like spacer members 150 as length adjusting means may be sandwiched between the cooling plate 110 and the end plate 120. The unit cell 100, the cooling plate 110, and the spacer member 150 arranged in the above manner are applied with a predetermined restraining pressure in the stacking direction by a fastening restraint band 130 attached so as to bridge between both end plates. It is restrained. More specifically, by tightening and fixing the end portion of the restraining band 130 to the end plate 120 with screws 155, the unit cells and the like are restrained so that a predetermined restraining pressure is applied in the arrangement direction. Thereby, restraint pressure is also applied to the wound electrode body 80 housed in the battery case 50 of each unit cell 100.
Then, between the adjacent unit cells 100, one positive terminal 70 and the other negative terminal 72 are electrically connected by a connecting member (bus bar) 140. Thus, the assembled battery 200 of the desired voltage is constructed | assembled by connecting each cell 100 in series.

As shown in FIG. 2, a lithium secondary battery 100 according to an embodiment of the present invention includes a long positive electrode sheet 10 and a long negative electrode sheet 20 that are flattened via long separators 40A and 40B. The electrode body (winding electrode body) 80 wound in the shape of a battery is housed in a flat box-shaped (cuboid shape) battery case 50 together with a non-aqueous electrolyte (not shown).
The battery case 50 includes a flat cuboid case main body 52 having an open upper end, and a lid 54 that closes the opening. On the upper surface (that is, the lid 54) of the battery case 50, there are a positive electrode terminal 70 that is electrically connected to the positive electrode 10 of the wound electrode body 80 and a negative electrode terminal 72 that is electrically connected to the negative electrode 20 of the electrode body 80. Is provided.

FIG. 3 is a view showing a long sheet structure (electrode sheet) in a stage before assembling the wound electrode body 80. Of the positive electrode sheet 10 in which the positive electrode mixture layer 14 is formed along the longitudinal direction on one or both surfaces (typically both surfaces) of the long positive electrode current collector 12, and the long negative electrode current collector 22. The negative electrode sheet 20 on which the negative electrode mixture layer 24 is formed along the longitudinal direction on one side or both sides (typically both sides) is superposed together with the two long separators 40A and 40B in the longitudinal direction. Turn to produce a wound electrode body. A flat wound electrode body 80 is obtained by squashing and winding the wound electrode body from the side surface direction.
The positive electrode sheet 10 is formed such that the positive electrode mixture layer 14 is not provided (or removed) at one end along the longitudinal direction, and the positive electrode current collector 12 is exposed. Similarly, the wound negative electrode sheet 20 is not provided with (or removed from) the negative electrode active material layer 24 at one end along the longitudinal direction, so that the negative electrode current collector 22 is exposed. Is formed. A positive current collector 74 (FIG. 2) is attached to the exposed end of the positive current collector 12, and a negative current collector 76 (FIG. 2) is attached to the exposed end of the negative current collector 22, respectively. Are electrically connected to the positive terminal 70 and the negative terminal 72, respectively.

  The lithium secondary battery disclosed herein can be used for various applications, but has a high output density and a high energy density, and has a good cycle characteristic. Therefore, for example, as shown in FIG. 4, the lithium secondary battery 100 disclosed herein can be suitably used as a power source (drive power source) for a motor mounted on a vehicle 1 such as an automobile. Although the kind of vehicle 1 is not specifically limited, Typically, a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), and a fuel cell vehicle (FCV) are mentioned. Further, the lithium secondary battery 100 may be used alone (that is, a single battery), or may be used in the form of an assembled battery that is connected in series and / or in parallel.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

[Construction of lithium secondary battery]
<Example 1>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM) powder as the positive electrode active material powder, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder, The mixture is mixed so that the mass ratio of the material becomes 93: 4: 3, and diluted with N-methylpyrrolidone (NMP) so that NV is about 60 mass%, and the slurry-like composition for forming the positive electrode mixture layer ( A positive electrode mixture slurry) was prepared. As PVdF, trade name “KF polymer # 7305” manufactured by Kureha Co., Ltd. was used. This positive electrode mixture slurry has a basis weight (applied amount in terms of solid content, that is, dry mass of the positive electrode mixture layer) on both sides of a long aluminum foil (positive electrode current collector) having a thickness of approximately 15 μm. It apply | coated so that it might be set to 0 g / cm < ² >, The obtained positive electrode was dried with the hot air of 80 degreeC for about 5 minutes, and the positive mix layer was formed. Subsequently, this positive electrode sheet was pressed so that the density of the positive electrode mixture layer was 2.55 g / cm ³ and wound into a roll with a tension of 0.4 N / mm. And it put into the oven of 145 degreeC, and performed the heat processing in nitrogen atmosphere for 3 hours, and produced the positive electrode sheet (Example 1).

Next, natural graphite (powder), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) as a negative electrode active material have a mass ratio of 98: 1: 1 and NV of 45% by mass. It mixed with ion-exchange water so that it might become, and the slurry slurry for water-system negative mix layer formation (slurry for negative mix layers) was prepared. The slurry for the negative electrode mixture layer has a weight per unit area (coating amount in terms of solid content, that is, dry mass of the positive electrode mixture layer) on both sides of a long copper foil (negative electrode current collector) having a thickness of about 10 μm. It apply | coated so that it might become 1.7 g / cm < ² >, and the negative mix layer was formed. The obtained negative electrode was dried with hot air at 80 ° C. for about 5 minutes, and the negative electrode sheet was pressed so that the density of the composite layer was 1.4 g / cm ³ to prepare a negative electrode sheet.

Then, the positive electrode sheet and the negative electrode sheet prepared above were overlapped and wound via two separators (here, a porous polyethylene sheet (PE) was used) to prepare an electrode body. Such an electrode body is mixed with a non-aqueous electrolyte (here, ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7) and LiPF ₆ as an electrolyte at a concentration of about 1 mol / L. A dissolved electrolyte solution was used.) And was housed in a cylindrical battery case. A lid was attached to the opening of the battery case, welded and joined to form a 18650 type (diameter 18 mm, height 65 mm) lithium secondary battery.

<Example 2>
Except for using a mixture of NMP and xylene at a ratio of 95% by mass: 5% by mass as a dispersion medium at the time of kneading the positive electrode mixture slurry, and similar to Example 1 except that the drying temperature was 115 ° C. A positive electrode sheet (Example 2) was produced. Using this positive electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.

<Comparative Example 1>
A positive electrode sheet (Comparative Example 1) was produced in the same manner as in Example 1 except that the winding and heat treatment were not performed after the positive electrode mixture layer was dried. Using this positive electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.

<Comparative Example 2>
A positive electrode sheet (Comparative Example 2) was prepared in the same manner as in Example 1 except that the drying temperature was 115 ° C. and that the heat treatment was performed in a 160 ° C. oven for 10 hours in a nitrogen atmosphere. Using this positive electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.

<Comparative Example 3>
A positive electrode sheet (Comparative Example 3) was produced in the same manner as Comparative Example 1 except that the positive electrode sheet was pressed so that the density of the positive electrode mixture layer after pressing was 2.70 g / cm ³ . Using this positive electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.
Table 1 below shows an outline of the manufacturing process of each positive electrode sheet.

[Cycle characteristics evaluation]
Each battery thus constructed was subjected to a cycle characteristic test (durability test) of 2000 cycles at a temperature of 60 ° C. In this example, charging is performed at a constant current constant voltage up to 4.1 V at a charging rate of 4 C (CC-CV charging), and discharging is performed at a constant current constant voltage up to 3.0 V at a discharging rate of 4 C (CC-CV discharging). ) Evaluation was performed by a charge / discharge treatment in which the operation was repeated 2000 times. The capacity retention rate (%) of the cycle characteristics was calculated as the ratio of the discharge capacity at the 2000th cycle to the discharge capacity at the 1st cycle ((capacity after 2000 cycles / initial capacity) × 100 (%)).

[Output characteristics evaluation]
Each battery constructed as described above was evaluated for output characteristics at a temperature of 25 ° C. First, each battery in a fully charged state (SOC 100%) was discharged to 3.0 V at a discharge rate of 1C. Thereafter, the battery was charged at a constant current of 1 C until the SOC of the battery reached 30%, and was charged at a constant voltage for 2 hours to adjust the SOC. And about the battery which adjusted the said SOC to 30%, IV resistance was calculated | required from the voltage drop 10 second after discharging with the discharge rate of 20C, and it converted into the output.

The evaluation results of the cycle characteristics of each battery are shown in FIGS. 5 and 6, and the evaluation results of the output characteristics are shown in FIG. FIG. 7 shows the relative value (%) of the output characteristics of each battery when the output characteristics of Example 1 are used as the reference (100).
From FIG. 6, in Comparative Example 1 in which heat treatment was not performed and in Comparative Example 2 in which the heat treatment temperature was high (and / or the heat treatment time was too long), a significant decrease in capacity was observed after 2000 cycles of charge and discharge. The decrease in the capacity retention rate observed in this example is that the positive electrode active material repeatedly expands and contracts with charge and discharge, so that the conductive path in the positive electrode mixture layer (that is, the contact area between the positive electrode active material and the conductive material). ) Decreased. In the past, it was possible to cope with the decrease in the conductive path by increasing the density of the positive electrode mixture layer.
However, as shown in FIG. 7, in Comparative Example 3 in which the density of the positive electrode mixture layer was increased, the output characteristics at SOC 30% were greatly deteriorated. This is presumably because the rate limiting diffusion of lithium ions occurred in the positive electrode mixture layer because the porosity in the positive electrode mixture layer was small. Therefore, for lithium secondary batteries that require output characteristics in a low SOC region, it is difficult to improve cycle characteristics by conventional means (that is, densification of the positive electrode mixture layer). Therefore, the present inventors tried to improve cycle characteristics and output characteristics in a low SOC region by increasing a spring constant ratio described later without changing the porosity of the positive electrode mixture layer.

[Measurement of spring constant]
In order to more closely reproduce the actual battery usage state, the produced positive electrode sheet is laminated via a separator and accommodated in a laminate cell together with an electrolytic solution, and each of six spring constant measuring cells provided only with the positive electrode sheet. Separately constructed. The constructed cell was stored in an environment of 60 ° C., then disassembled and taken out of the positive electrode sheet. The preservation period was 0 days to 60 days, and 6-point sampling was performed by changing the preservation days. This positive electrode sheet was dried at 60 ° C. to obtain a sample for measuring the spring constant. The spring constant was measured using the method shown in FIG. 13 described above under the following conditions.
Sample: 10 positive electrode sheets obtained as described above were superposed Measurement device: “AG-10kN” (manufactured by Shimadzu Corporation)
Maximum load: Up to 2000kgf
Load speed: 0.1 mm / sec
The used load (kgf) and the displacement (mm) of the sample accompanying it were plotted, and the spring constant was determined from the slope of the straight line when the load was 1000 kgf to 2000 kgf. The spring constant ratio in FIGS. 8 to 10 is the ratio of the spring constant after being immersed in the electrolytic solution to the spring constant before being immersed in the electrolytic solution (the spring constant after being immersed in the electrolytic solution / before being immersed in the electrolytic solution). Spring constant).

FIG. 8 shows changes in the spring constant ratio of the positive electrode sheet accompanying immersion in the electrolytic solution. As the number of soaking days increased, the spring constant gradually decreased. This is probably because a part of PVdF in the positive electrode mixture layer swells and / or dissolves in the electrolytic solution. In particular, in Comparative Example 2 where the heat treatment temperature was high (and / or the heat treatment time was too long), such a tendency was intense.
Further, FIG. 9 and FIG. 10 show the relationship between the spring constant ratio after being immersed in the electrolytic solution for 30 days, the cycle characteristic evaluation, and the output characteristic evaluation. From FIG. 9, it was found that the capacity retention after 2000 cycles of the lithium secondary battery can be 70% or more by keeping the spring constant ratio after immersion for 30 days at 0.8 or more and less than 1.0. Further, from FIG. 10, by setting the spring constant ratio to 0.8 or more and 0.9 or less, in addition to the above-mentioned good cycle characteristics, it has good output characteristics in a low SOC region (SOC is 30% in this example). It was possible to obtain a lithium secondary battery having the same.

  The spring constant ratio varies depending on the amount and / or quality of the conductive material and / or PVdF. However, since there is a large contradiction to performance when the amount and quality of the conductive material are changed, the quality of the PVdF (specifically, the crystal By changing the size and crystal form of the PVdF, the swelling and dissolution of the PVdF was suppressed, and the spring constant ratio was examined to be a high value. As shown below, the crystal form was evaluated by the mass ratio (α / β) of α crystal and β crystal in NMR measurement, and the crystal size was evaluated at the melting start temperature in DSC measurement.

[NMR measurement]
A positive electrode mixture layer (measurement sample) was collected from the prepared positive electrode sheet (Example 2, Comparative Examples 1 and 2), filled in a sample tube, and subjected to NMR measurement under the following conditions. Further, in order to ascertain a suitable range for the invention disclosed herein, an additional sample for NMR measurement was prepared. The additional sample was obtained by appropriately changing only the heat treatment temperature in the production process of the positive electrode sheet (Example 2) in the range of 0 ° C. to 145 ° C., and measured together with the sample.
Measuring device: “Advanced 400” (manufactured by BRUKER)
Measurement method: Single pulse method Observation nucleus: ¹⁹ F-NMR
Sample tube: Solid NMR sample tube made of zirconia From the obtained NMR spectrum, the mass ratio (α / β) of α crystal and β crystal was determined by the method described above.

[DSC measurement]
A positive electrode mixture layer (measurement sample) was collected from each of the prepared positive electrode sheets (Example 1, Comparative Examples 1 and 2), sealed in a measurement container, and subjected to DSC measurement under the following conditions. In addition, in order to determine a suitable range for the invention disclosed herein, an additional sample for DSC measurement was prepared. The additional sample was obtained by appropriately changing only the heat treatment temperature in the production process of the positive electrode sheet (Example 1) in the range of 0 ° C. to 160 ° C., and measured together with the sample.
Sample amount: about 12mg
Measuring device: “DSC8230” (manufactured by Rigaku Corporation)
Measurement temperature range: 25 ° C to 250 ° C
Temperature rising rate: 10 ° C./min Measuring container: aluminum pan The melting start temperature was read from the obtained DSC chart.

FIG. 11 shows the relationship between the mass ratio (α / β) between the α crystal and the β crystal in the NMR measurement and the spring constant ratio. In addition, the spring constant ratio in FIG. 11 shows the relative ratio of each spring constant when the spring constant before immersion of the electrolyte solution of Example 2 is used as a reference (100). In Comparative Example 1 in which no heat treatment was performed, the value of α / β was as large as 0.59. On the other hand, Example 2 which showed the above-mentioned favorable battery performance showed a value of α / β of 0.42, and when the value of α / β was 0.42 to 0.56, the spring constant ratio was 0.8. The above high value was shown. The reason is considered that the crystal form of PVdF was changed by performing the heat treatment at 120 ° C. to 155 ° C. (that is, the α crystal decreased and / or the β crystal increased). In Comparative Example 2 where the heat treatment temperature was high (and / or the heat treatment time was too long), the value of α / β was as low as 0.36. The above results support the technical significance of setting the preferable range of α / β to 0.42 or more and 0.56 or less.
The relationship between the melting start temperature and the spring constant ratio in DSC measurement is shown in FIG. In addition, the spring constant ratio in FIG. 12 shows the relative ratio of each spring constant when the spring constant before immersion of the electrolyte solution of Example 1 is used as a reference (100). In Comparative Example 1 in which heat treatment was not performed, the melting start temperature was a low value of 137 ° C. On the other hand, Example 1 which showed the said favorable battery performance showed melting start temperature 148 degreeC, and when the melting start temperature was 144 to 153 degreeC, the spring constant ratio showed the high value of 0.8 or more. The reason for this is considered to be the same as above. In Comparative Example 2 where the heat treatment temperature was high (and / or the heat treatment time was too long), the melting start temperature was as high as 158 ° C. The above-mentioned results support the technical significance of setting the preferable range of the melting start temperature to 144 ° C to 153 ° C.
As described above, the spring constant can be increased and low by optimizing the crystal form of PVdF (ie, α / β crystal in NMR measurement) and the size of the crystal (ie, melting start temperature in DSC measurement). It has been shown that a lithium secondary battery having excellent cycle characteristics can be provided while ensuring output characteristics in the SOC region.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

1 Automobile (vehicle)
10 Positive electrode sheet (positive electrode)
12 Positive electrode current collector 14 Positive electrode mixture layer 20 Negative electrode sheet (negative electrode)
22 Negative electrode current collector 24 Negative electrode mixture layer 40A, 40B Separator sheet 50 Battery case 52 Case body 54 Cover body 70 Positive electrode terminal 72 Negative electrode terminal 74 Positive electrode current collector plate 76 Negative electrode current collector plate 80 Winding electrode body 90 Flat plate 100 Lithium 2 Secondary battery 110 Cooling plate 120 End plate 130 Restraint band 140 Connection member 150 Spacer member 155 Screw 200 Battery assembly

Claims (10)

A lithium secondary battery comprising an electrode body having a positive electrode and a negative electrode, and a liquid electrolyte,
The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector,
The positive electrode mixture layer includes a positive electrode active material and polyvinylidene fluoride as a binder,
Here, the mass ratio of the alpha crystals and beta crystal based on nuclear magnetic resonance spectrum of the polyvinylidene fluoride (alpha / beta) is Ri der 0.42 to 0.56 or less,
Melting start temperature based on differential scanning calorimetry of the polyvinylidene fluoride Ru der 144 ° C. or higher 153 ° C. or less, a lithium secondary battery. 2. The lithium secondary battery according to claim 1, wherein a value obtained by dividing a spring constant after the positive electrode is immersed in the electrolyte for 30 days by a spring constant before the immersion is 0.8 or more and less than 1.0. The lithium secondary battery according to claim 1 or 2 , wherein a porosity of the positive electrode mixture layer is 25% or more and 40% or less. The positive electrode current collector is formed in an elongated shape, and the positive electrode mixture layer is formed on the elongated positive electrode current collector,
The negative electrode is configured by forming a negative electrode mixture layer on a long negative electrode current collector corresponding to the long positive electrode current collector,
The lithium secondary battery according to any one of claims 1 to 3 , wherein the elongated positive electrode and the elongated negative electrode are laminated and wound to form a wound electrode body. The lithium secondary battery according to any one of claims 1 to 4 , which is used as a driving power source for a vehicle. A method for producing a lithium secondary battery comprising an electrode body having a positive electrode and a negative electrode, and a liquid electrolyte, comprising:
Producing a positive electrode having a positive electrode mixture layer and a positive electrode current collector that holds the positive electrode mixture layer; and
Building a lithium secondary battery using the positive electrode;
Including
Here, in the step of producing the positive electrode, the polyvinylidene fluoride contained in the positive electrode mixture layer has the following conditions: (1) Mass ratio of α crystal and β crystal based on nuclear magnetic resonance spectrum (α / β ) Is 0.42 or more and 0.56 or less; (2) The melting start temperature based on differential scanning calorimetry is 144 ° C. or more and 153 ° C. or less;
Applying a slurry-like composition containing a positive electrode active material and polyvinylidene fluoride as a binder onto the positive electrode current collector to form the positive electrode mixture layer;
Pressing the positive electrode mixture layer to a predetermined thickness; and heat-treating the pressed positive electrode mixture layer at a temperature of 145 ° C. or higher and 158 ° C. or lower;
A method for manufacturing a lithium secondary battery. The step of producing the positive electrode is such that the ratio of the spring constant before and after the positive electrode is immersed in the electrolyte for 30 days (spring constant after immersion / spring constant before immersion) is 0.8 or more and less than 1.0. The manufacturing method of the lithium secondary battery of Claim 6 performed. The method for producing a lithium secondary battery according to claim 6 or 7 , wherein two or more kinds of organic solvents having different boiling points are used as the dispersion medium in the slurry composition . The production of the lithium secondary battery according to any one of claims 6 to 8 , wherein the pressing in the step of producing the positive electrode is performed such that the porosity of the positive electrode mixture layer is 25% or more and 40% or less. Method. In the step of producing the electrode body, a positive electrode current collector is formed using a positive electrode current collector formed in a long shape as the positive electrode current collector,
The method for producing a lithium secondary battery according to any one of claims 6 to 9 , wherein the formed positive electrode and a corresponding long negative electrode are laminated to produce a wound electrode body.

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