JP2009187753A - Power storage element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power storage element of a type in which lithium ions are stored beforehand in a negative electrode by a horizontal doping method, in which pre-doping of the lithium ions is completed in a short time. <P>SOLUTION: This is a power storage element 200 in which a power generation element in which a positive electrode 1p capable of carrying lithium ions or anions reversibly and a negative electrode 1n consisting of a material capable of storing and releasing lithium ions are arranged in opposition through a separator 30 is used as one unit and an electrode laminate 100 laminating at least one unit or more of the power generation elements is sealed tightly together with an electrolytic solution containing lithium salt, and all or a part of metal lithium 20 is arranged so as to be in electrical contact with a current collector on the surface of the current collector 11n on the negative electrode side, and the lithium ions originating from the metal lithium are stored beforehand in the negative electrode. The total value of a pore volume per unit area respectively of the positive electrode, the negative electrode, and the separator of the power generation element is 1.7 μL or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention includes a positive electrode capable of reversibly carrying lithium ions or anions, a negative electrode which is made of a material capable of occluding and releasing lithium ions, and is disposed opposite to the positive electrode via a separator, and a lithium salt. The present invention relates to a power storage element that includes an electrolyte and is formed by stacking a positive electrode, a separator, and a negative electrode. Specifically, the present invention relates to a power storage element in which lithium ions are occluded in advance in a negative electrode.

  The above-mentioned type of storage element (hereinafter referred to as pre-doped storage element) is capable of rapid charging and has lithium ions previously stored in the negative electrode, so that the potential of the negative electrode is lowered and a large voltage can be obtained, resulting in high energy. Capacity can be obtained. Therefore, it is expected to be used for wind power generation load leveling devices, instantaneous power failure countermeasure devices, regenerative power storage for automobiles, and the like.

  In a conventional pre-doped type storage element, a lithium ion occlusion (pre-doping) method, for example, as described in Japanese Patent No. 3485935, a negative electrode or a positive electrode is formed on a mesh-like current collector, and these are alternately arranged. There is a method of pre-doping lithium ions into the negative electrode through this mesh by disposing a current collector with metallic lithium pasted on the outside of the electrode laminate laminated with a separator sandwiched between them to have electrical contact with the negative electrode. In this method, metallic lithium and the negative electrode face each other, and lithium ions are predoped from a direction perpendicular to the negative electrode.

  Hereinafter, when such a pre-doping method is referred to as a “vertical doping method”, the vertical doping method has a problem that the cost increases because a current collector having a hole such as a mesh is used. .

  Therefore, the present inventors formed a negative electrode on the surface of the sheet-like current collector as a pre-doping method that is easy to manufacture and reduce the price, and adhered lithium metal to the same surface as the current collector surface, The “horizontal dope method” is adopted in which the negative electrode is doped along the surface of the current collector. In addition, the horizontal dope method has the advantage that the amount of stored lithium ions can be accurately set and the subsequent battery reaction can be made uniform as compared with the vertical dope method. Yes. In such a horizontal doping method, the negative electrode can be easily and inexpensively formed on the surface of the current collector by a coating technique such as die coating, gravure coating, and reverse coating.

  However, in the horizontally doped pre-doped storage element, the assembly was completed because lithium ions were diffused to the negative electrode located far from the fixed position on the negative electrode current collector where the metal lithium was arranged. It was necessary to leave the electricity storage element for an extremely long time until lithium ions were uniformly occluded in the negative electrode active material. Naturally, if the electrode area is increased in order to meet the demand for higher output for the power storage element, the aging period for occluding lithium ions becomes longer.

  Aging over a long period of time reduces productivity and increases manufacturing costs. In this case, even if the reliability can be ensured, the possibility of cost reduction due to the ease of manufacturing the horizontally doped storage element is hindered. Accordingly, an object of the present invention is to provide a highly reliable and high-power storage element at low cost by completing pre-doping of lithium ions in a short time.

  In order to achieve the above object, the present inventors first focused on the fact that the mass transfer path is formed in the horizontal direction on the electrode surface in the horizontal doping method. In the horizontal dope method, the lithium metal dissolved in the electrolytic solution serving as a lithium ion transfer medium is occluded in the negative electrode active material while diffusing the electrode surface in parallel (horizontal), so the lithium ion transfer path It was thought that the more the amount of the electrolyte solution present therein, the smoother the movement of lithium ions.

  The electrolytic solution includes pores (pores in the negative electrode, pores in the positive electrode, pores in the separator) in each element constituting the power storage element, and interfaces (positive electrode and separator, negative electrode and separator in each component). As a result, in the cell composed of a pair of positive electrode and negative electrode arranged opposite to each other via the separator, the unit in the positive electrode, the negative electrode, and the separator It has been found that there is an optimum condition for achieving the above object in the value of the pore volume per area.

The present invention has been made on the basis of the above knowledge, and is formed by arranging a positive electrode capable of reversibly carrying lithium ions or anions and a negative electrode made of a material capable of occluding and releasing lithium ions through a separator. In a power storage element formed by sealing and sealing an electrode laminate formed by laminating at least one unit of power generation elements as a unit with a power generation element together with an electrolyte containing a lithium salt,
The surface of the current collector on the negative electrode side is arranged so that all or part of the metal lithium is in electrical contact with the current collector, and lithium ions originating from the metal lithium are previously stored in the negative electrode. Being
The power storage element has a total pore volume per unit area of the positive electrode, the negative electrode, and the separator of the power generation element of 1.7 μL or more.

Further, in the electric storage device, the pore volume per unit area of the negative electrode 0.3 [mu] L / cm ² or more, the pore volume per unit area of the positive electrode 0.7μL / cm ² or more, per unit area of the separator It is more preferable to use a power storage element that satisfies any one of the conditions in which the pore volume is 0.7 μL / cm ² or more, or a power storage element that satisfies a combination of these conditions.

  The present inventors focused on the mode diameter of the negative electrode material as a condition for diffusing lithium ions more uniformly into the negative electrode, and attempted to optimize the mode diameter. The present invention also extends to a power storage element having an optimized mode diameter. In any one of the above power storage elements, the negative electrode has a mode diameter of 0.1 μm or more and 6.0 μm by pore distribution measurement. The power storage element is as follows.

  Furthermore, the present inventors examined the relationship between the amount of electrolytic solution serving as a lithium ion transfer medium and the pore volume in the horizontal pre-doping method based on the following considerations (1) to (3).

(1) When the amount of the electrolyte is small with respect to the capacity of the pore volume of the electrode laminate (A / B <1), it has a pore portion where the electrolyte does not exist, and the migration path of lithium ions is sufficient Cannot be secured.
(2) Even if the electrolytic solution is included only in the pores of the positive electrode, the negative electrode, and the separator, an electrolytic solution having a total pore volume or more is necessary.
(3) The electrolytic solution does not exist only in the pores of the positive electrode, the negative electrode, and the separator, and there is a portion that is retained in the gap inside the electric storage element.

  As a result of the examination based on these considerations, it has been found that there is an optimum condition in the relationship between the amount of electrolyte and the pore volume, and the storage element that satisfies the optimum condition is also within the scope of the present invention. In any one of the power storage elements, the volume A of the electrolyte applied to the power storage element, and the total value B of the respective pore volumes of the positive electrode, the negative electrode, and the separator of the electrode laminate included in the power storage element; The ratio A / B is 1.4 or more.

  According to the present invention, pre-doping of lithium ions can be completed in a short time, whereby a highly reliable and high-power storage element can be provided at low cost.

=== Structure of power storage element ===
FIG. 1 shows a schematic structure (A) on the negative electrode side and a schematic structure (B) on the positive electrode side of the electricity storage device in the first example of the present invention. A plan view and a side view of a negative electrode body (negative electrode body) 1n and a positive electrode body (positive electrode body) 1p are shown in (A) and (B), respectively. In the negative electrode body 1n, a copper foil having a terminal connection portion 12n projecting on one side of a substantially rectangular shape is used as a negative electrode current collector 11n, and the front and back surfaces of the substantially rectangular region other than the terminal connection portion 12n of the negative electrode current collector 11n. The negative electrode active material is applied, a part of the substantially rectangular region is peeled off in a predetermined shape to provide an electrode uncoated portion 13n where the negative electrode active material is not coated, and metal lithium 20 is applied to the uncoated portion 13n. It is based on a pasted structure.

  As a specific method for producing the negative electrode body 1n, first, N-methyl-2-pyrrolidinone (NMP) as a solvent was mixed with a non-graphitizable carbon material and a polyvinylidene fluoride resin mixed at a weight ratio of 95: 5. ) And kneaded into a paste form is applied in a substantially rectangular shape on both the front and back surfaces of a 14 μm thick copper foil to be the negative electrode current collector 11n and dried. The negative electrode material on one of the front and back surfaces of the negative electrode current collector 11n is applied so that the pore volume per unit area of the negative electrode material becomes a predetermined value after drying. The pore volume can be controlled by adjusting the coating weight, thickness, and porosity. And after cutting copper foil so that it may become an electrode body shape provided with the terminal connection part 12n, the negative electrode active material of the part used as the electrode non-application part 13n is peeled. In the present embodiment, the left and right center of the coating region is peeled off in a strip shape extending vertically to form the electrode uncoated portion 13n to form the negative electrode portion 10n, and the strip-shaped metal along the shape of the uncoated portion 13n. Lithium 20 is attached. The carbon material may be easily graphitizable carbon, non-graphitizable carbon, graphite or the like, but is not particularly limited as long as it is a material capable of occluding and releasing lithium ions.

  On the other hand, in the positive electrode body 1p, an aluminum foil having a terminal connection portion 12p protruding on one side of a substantially rectangular shape is used as a positive electrode current collector 11p, and the front and back sides of a substantially rectangular region other than the terminal connection portion 12p of the positive electrode current collector 11p. It is based on a structure in which a positive electrode active material is applied on both sides. As a specific method for producing the positive electrode body 1p, 90 parts by weight of activated carbon powder, 10 parts by weight of acetylene black, and 10 parts by weight of polyvinylidene fluoride powder are mixed, and NNP is added to the mixture and kneaded to form a paste. The kneaded material is applied in a substantially rectangular shape on both the front and back surfaces of an aluminum foil having a thickness of 20 μm and dried. Similarly to the negative electrode, the positive electrode material on one surface of the current collector is adjusted so that the pore volume per unit area of the negative electrode material becomes a predetermined value after drying.

  After the positive electrode material is applied to both surfaces of the aluminum foil current collector and dried, the aluminum foil is cut so as to have an electrode body shape having the positive electrode terminal connection portion 12p. Then, when facing the negative electrode body 1n, the electrode uncoated portion is peeled off in a strip shape extending vertically from the left and right center of the coated region so that the position matches the uncoated region 13n to which the metallic lithium is stuck. 13p is formed.

  The bipolar electrode bodies (1p, 1n) thus created are laminated so as to face each other via a separator so as to match the shapes of the two uncoated electrodes (13p, 13n). Assembled into. In this example, the separator uses a polyolefin microporous film, but as a material, a porous film using at least one kind of polyethylene, polypropylene, aramid, PET, cellulose, cellophane, etc. Any suitable material can be adopted as long as it has transparency and can electrically separate the positive and negative electrodes. Further, the porosity of the separator is not particularly limited as long as the positive and negative electrodes are not short-circuited. Usually, the porosity is 30 to 70%.

  2A and 2B are a cross-sectional view and a plan view of the electrode stack 100. A configuration comprising one separator 30 and a pair of positive and negative electrodes facing each other through the separator 30 is defined as one positive and negative electrode facing portion 50, and the electrode stack 100 is a laminate of a plurality of positive and negative electrode facing portions 50. Yes (A). In this embodiment, 10 sets of positive and negative electrode facing portions 50 are stacked. Then, the respective current collectors are stacked so that the terminal connection portions (12p, 12n) of the positive electrode and the negative electrode are staggered so as to protrude in the same direction and do not overlap vertically in the stacking direction ( B).

Further, after assembling the electrode laminate 100 in this way, as shown in FIG. 3, the positive terminal connection portions 12p and the negative terminal connection portions 12n of the respective positive and negative electrode facing portions 50 are stacked. The electrode laminate 100 is arranged in a state where the lead terminals 60 are led out to the outside in an aluminum laminate film outer package 70 which is welded to the lead terminal plate 60 in a lump and fused in three sides to form a bag shape. Then, an electrolytic solution (propylene carbonate adjusted so that the concentration of LiPF ₆ is 1 mol / L) is injected into the outer package 70, and the opening of the bag-shaped outer package 70 is vacuum-sealed. Is completed. In the above structure, the method for attaching metallic lithium to the negative electrode is not limited to the position and arrangement shown in FIG. 1, and for example, various positions and arrangements are available as shown in FIGS. Conceivable.

  The structure of the above-described power storage element 200 is the same as that of a conventionally known power storage element. However, the present invention pays attention to the pore structure in each component of the positive electrode, the negative electrode, and the separator, and achieves shortening of the pre-doping period by optimizing the pore volume per unit area in each component. .

=== Evaluation Method for Pre-Dope ===
In the electricity storage device having the above structure, a wide variety of electricity storage devices having different pore volume in each of the negative electrode, the positive electrode, and the separator, and the total pore volume obtained by summing these volumes, and the mode diameter in the negative electrode material are used as samples. Produced. And the pre-dope speed | rate in these samples, the uniformity of pre-dope, etc. were evaluated.

  The pre-doping speed is determined by measuring the cell voltage Va1 after 1 hour from the start of the storage and the cell voltage Va2 after 100 hours after each sample was stored in a high temperature bath at 45 ° C. for 100 hours. The voltage value difference (cell voltage change) ΔVa (= Va1−Va2) was evaluated. That is, the larger the ΔVa, the faster the pre-doping speed.

  Further, whether or not lithium ions are pre-doped into the negative electrode sufficiently and uniformly can be evaluated by measuring the cell resistance value R after completing the pre-doping of the electric storage element. The cell resistance value R was determined by allowing the sample to stand for 10 days, charging it at 1 A to 25 ° C. to 3.8 V, resting for 1 minute, then discharging at a current value of 100 A, and discharging the voltage value Vb1 immediately before the start of discharge. The voltage value Vb2 at the start of 100 msec was obtained from the voltage value difference ΔVb (= Vb1−Vb2) and the current value I (= 100 A) by the equation R = ΔVb / I.

For the pore volume, the pore diameter D was calculated from the pressure P measured using a pore distribution measuring device (Porosimeter PoreMaster 60 manufactured by Quantachrome) based on the mercury intrusion method, and finally the pore volume was calculated. . Specifically, the pressure P is measured in the range of 20 to 60,000 PSI (0.138 to 414 MPa), and first, the pore (straight) diameter D is determined according to the Washburn equation D = 4γcos θ / P.
Calculated based on Here, γ is the surface tension of mercury (480 dyn / cm), and θ is the contact angle (140 °) between mercury and the pore wall surface. And, for example, the pore volume of the separator is
Apparent volume of separator-(Separator weight / Separator material density)
Calculated by The apparent volume can be obtained from the outer diameter size of the separator.

=== Evaluation of pre-doping rate by pore volume ===
Table 1 shows the pore volume of each of the negative electrode material, the positive electrode material, and the separator for each sample, the total pore volume, and the cell voltage change ΔVa that serves as an index of the pre-doping rate.

In Table 1, the samples of Examples 1 to 16 correspond to the power storage elements according to the present invention, and the samples of Comparative Examples 1 to 4 correspond to the conventional power storage elements. Here, focusing on the total pore volume, each sample of the example of Example 5 that is 1.70 μL / cm ² or more had a cell voltage change ΔVa of about 0.6 V at the minimum. On the other hand, in each sample of the comparative example, it is 0.5 V or less, indicating that the pre-doping rate is reduced. Therefore, the total value of the pore volume per unit area of each of the negative electrode, the positive electrode, and the separator is desirably 1.7 μL / cm ² .

As described above, in the horizontal dope method, metallic lithium dissolved in the electrolytic solution serving as a lithium ion transfer medium is occluded in the negative electrode active material while diffusing the electrode surface in parallel (horizontal), so the lithium ion It is considered that the movement of lithium ions is performed more smoothly as the amount of the electrolyte present in the movement path is larger. That is, in the horizontal dope method, the electrolytic solution contains pores in each component (pores in the negative electrode, pores in the positive electrode, pores in the separator), and interfaces (positive electrode and separator, As the total volume of the pore volume of each component including the electrolyte solution is larger, the amount of the electrolyte solution that forms the diffusion path increases. Table 1 shows that the lower limit of the total volume is 1.7 μm / cm ² .

Incidentally, as can be seen from the results in Table 1, the total value of the pore volume and the value of the cell voltage change ΔVa are not proportional. This suggests that while there is a total pore volume value above a certain level as a necessary condition for increasing the pre-doping rate, there is an optimum condition for the pore volume in each component of the negative electrode, positive electrode, and separator. ing. Here, about Examples 1-16 and Comparative Examples 1-4, the pore volume of each component was compared and the result of Table 1 was summarized. FIG. 5 shows the classification of each of Examples 1 to 16 (s1 to s16) according to the pore volume of each component as a summary. From this figure, the necessary conditions for the respective pore volumes of the negative electrode, the positive electrode, and the separator can be found. It can be said that the negative electrode is 0.3 μm / cm ² or more, and the positive electrode and the separator are 0.7 μm / cm ² or more, respectively. And about Examples 1-10 which satisfy | fill all of these conditions, cell voltage change (DELTA) Va is 0.79V-1.06V, and is much larger than 0.58V-0.65V of Examples 11-16. It can be seen that the pre-doping rate is extremely high.

=== Evaluation of pre-dope uniformity by negative electrode mode diameter ===
Next, the negative electrode pre-doped with lithium ions was evaluated for the relationship between the mode diameter serving as an index of the pore size in the pore structure and the uniformity of the pre-doping. In this evaluation, a sample having the same pore volume of the positive electrode and the separator was used. Here, the pore volume of each of the positive electrode and the separator is set to 3.44 μL / cm ² and 1,38 μL / cm ^2, and Examples 7 to 10 in Table 1 and a new sample as a comparative example (Comparative Example 5) are used. )It was used. Then, the uniformity of pre-doping, that is, whether or not lithium ions are uniformly occluded in the negative electrode was evaluated based on the cell resistance value R of each sample.

Table 2 summarizes the relationship between the mode diameter of the negative electrode and the cell resistance value.

  In Table 2, the cell resistance value R was determined by the above-described equation R = ΔVb / I. The resistance value R in the table is shown as a relative value when the resistance value R in Example 10 is set as a reference (100%). From Table 2, it can be seen that the cell resistance value of Comparative Example 5 is increased by nearly 70% from the reference. And it turned out that it is desirable for the mode diameter of a negative electrode to be 0.1-6.0 micrometers.

=== About electrolyte volume ===
If the electrolyte solution having the same volume as each of the positive electrode, the negative electrode, and the separator is not supplied to each positive and negative electrode facing portion 50 of the electrode laminate 100, there is a portion where the electrolyte solution is lacking. In addition, since the electrolytic solution can be distributed at the interface between the constituent elements, an excess amount of the electrolytic solution with respect to the pore volume is given to the electrode laminate 100 in order to realize a good pre-dope. There is a need. Therefore, the volume of the electrolytic solution sealed in the exterior body 70 of the power storage element 200, that is, the volume of the electrolytic solution applied to the power storage element 200 (referred to as A), and the pore volume in the positive and negative electrode facing portion 50 of the electrode stack 100. Of the positive electrode part 10p, the negative electrode positive electrode part 10n, and the total value of the pore volume in the separator 30 (referred to as B), the conditions under which good lithium pre-doping is possible were studied. Here, using the sample of Example 1, the conditions were examined by the cell voltage change ΔVa when the ratio A / B was changed.

Table 3 shows the results of the study.

  From the examination results, it was found that when the ratio A / B is less than 1.4, a region where the electrolytic solution is insufficient is generated, and lithium ions are difficult to diffuse uniformly along the electrode surface during pre-doping. As described above, in the present invention, the ratio A / B ≧ 1.4 is defined for the volume of the electrolyte contained in the positive and negative electrode facing portion 50.

It is a figure which shows the electrode structure of the electrical storage element in the Example of this invention. It is a schematic structural drawing of the electrode laminated body which comprises the electrical storage element in the said Example. It is the schematic of the electrical storage element in the said Example. It is a figure which shows the other sticking position and arrangement | positioning of metallic lithium in the said electrical storage element. It is a figure which shows the conditions of the pore structure in the said Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1p Positive electrode body 1n Negative electrode body 10p Positive electrode part 10n Negative electrode part 11p Positive electrode collector 11n Negative electrode collector 12p, 12n Terminal connection part 20 Metal lithium 30 Separator 50 Positive / negative electrode opposing part 100 Electrode laminated body 200 Electric storage element

Claims (6)

At least 1 unit or more of a power generation element in which a positive electrode capable of reversibly carrying lithium ions or anions and a negative electrode made of a material capable of occluding and releasing lithium ions are arranged to face each other through a separator. In an electricity storage element formed by sealing and sealing an electrode laminate formed by laminating power generation elements together with an electrolyte containing a lithium salt,
The surface of the current collector on the negative electrode side is arranged so that all or part of the metal lithium is in electrical contact with the current collector, and lithium ions originating from the metal lithium are previously stored in the negative electrode. Being
The total value of the pore volume per unit area of each of the positive electrode, the negative electrode, and the separator of the power generation element is 1.7 μL or more. ^2. The electricity storage device according to claim 1, wherein a pore volume per unit area of the negative electrode is 0.3 μL / cm ² or more. The electrical storage element according to claim 1 or 2, wherein a pore volume per unit area of the positive electrode is 0.7 µL / cm ² or more. The electrical storage element according to claim 1, wherein a pore volume per unit area of the separator is 0.7 μL / cm ² or more.   5. The electric storage element according to claim 1, wherein the negative electrode has a mode diameter of 0.1 μm or more and 6.0 μm or less by pore distribution measurement.   The ratio A / B between the volume A of the electrolyte applied to the electric storage element and the total value B of the respective pore volumes of the positive electrode, the negative electrode, and the separator of the electrode laminate included in the electric storage element is 1.4. It is the above, The electrical storage element in any one of Claims 1-5 characterized by the above-mentioned.

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