JP2010027409A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of exhibiting a stable output characteristic, and capable of detecting easily a state (charge state, abnormality or the like) thereof. <P>SOLUTION: Positive active materials contained in the lithium ion secondary battery 100 are positive active materials comprising compounds expressed by the same compositional expression (for example, LiFePO<SB>4</SB>), and for carrying out two phase coexistence type charge and discharge, and are the positive active materials (the first positive active material 153b and the second positive active material 153c) of two kinds or more different in lithium ion diffusion coefficients. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery.

Lithium ion secondary batteries are attracting attention as power sources for portable devices and as power sources for electric vehicles and hybrid vehicles. Currently, as a lithium ion secondary battery, a positive electrode active material made of LiMO ₂ (M is Co, Ni, Mn, V, Al, Mg, etc.), a negative electrode active material made of graphite, a Li salt, and a non-aqueous solvent are used. The thing which has the nonaqueous electrolyte solution which consists of has become the mainstream (for example, refer patent documents 1-3). This lithium ion secondary battery has the advantage of exhibiting a high discharge voltage and high output.

JP 2005-336000 A JP 2003-100300 A JP 2003-059489 A JP 2003-36889 A JP 2006-12613 A

  By the way, the lithium ion secondary batteries disclosed in Patent Documents 1 to 3 have characteristics that the battery voltage gradually increases as the battery is charged during charging, and conversely the battery voltage gradually decreases as the battery discharges during discharging. have. For this reason, since the change of the battery voltage at the time of charging / discharging becomes large, the output fluctuation is large, and there is a problem that the output characteristics change depending on the state of charge (charged amount) of the battery and are difficult to use.

On the other hand, Patent Documents 4 and 5 propose secondary batteries using, as a positive electrode active material, a lithium transition metal composite oxide having an olivine structure represented by a composition formula LiFePO ₄ or the like. The lithium transition metal composite oxide having an olivine structure represented by LiFePO ₄ or the like has a substantially constant charge / discharge potential even during charge / discharge, and hardly changes even when lithium ions are desorbed and occluded. The reason is, for example, lithium transition metal composite oxide having an olivine structure represented by LiFePO _4, when absorption and desorption of Li, believed to be because the two-phase coexistence state between LiFePO ₄ and FePO ₄ .

Therefore, by using a positive electrode active material that performs charge and discharge in a two-phase coexistence type, such as LiFePO ₄ , a lithium secondary battery that has little change in input density and output density due to change in charge state and stable output characteristics is configured. It becomes possible to do. However, such a lithium ion secondary battery has a small change in battery voltage during charge and discharge. For this reason, it is difficult to detect (determine) the state (charging state, abnormality, etc.) based on the battery voltage.

  The present invention has been made in view of the present situation, and provides a lithium ion secondary battery that can exhibit stable output characteristics and can easily detect the state (charged state, abnormality, etc.). For the purpose.

  The solution is a lithium ion secondary battery including a positive electrode active material, wherein the positive electrode active material is a positive electrode active material that is composed of a compound represented by the same composition formula and performs charge and discharge in a two-phase coexistence type. And it is a lithium ion secondary battery which is 2 or more types of positive electrode active materials from which a lithium ion diffusion coefficient differs.

  The lithium ion secondary battery of the present invention uses, as a positive electrode active material, a positive electrode active material made of a compound represented by the same composition formula and performing charge and discharge in a two-phase coexistence type. For this reason, the change of the input density and the output density accompanying the change of the state of charge is small, and the lithium secondary battery with stable output characteristics is obtained.

However, conventionally, since such a lithium ion secondary battery has a small change in battery voltage during charging and discharging, it has been difficult to detect the state (charged state, abnormality, etc.).
On the other hand, in the lithium ion secondary battery of the present invention, two or more kinds of positive electrode active materials having different lithium ion diffusion coefficients are mixed although they are composed of compounds represented by the same composition formula. Thereby, since the behavior of the battery voltage at the time of charging / discharging can be made as follows, it becomes easy to detect the state of the secondary battery while ensuring stable output characteristics.

  Specifically, for example, when a first positive electrode active material and a second positive electrode active material having a lithium ion diffusion coefficient larger than that of the first positive electrode active material are mixed, charging is started from a state where the charged amount is 0%. Then, diffusion of lithium ions proceeds in the first positive electrode active material having a small lithium ion diffusion coefficient prior to the second positive electrode active material. Therefore, the battery voltage at the initial stage of charging is a battery voltage based only on the reaction with the first positive electrode active material (desorption of Li ions), and the battery voltage value is substantially constant over a predetermined charging range (this range is The first flat range). Thereafter, when the reaction (desorption of Li ions) in the first positive electrode active material is completed, the battery voltage is now based on the reaction (desorption of Li ions) in the second positive electrode active material, and a predetermined charging range. Over this, the battery voltage value becomes substantially constant (this range is defined as a second flat range). When discharging, the reverse of charging.

  By the way, since the lithium ion diffusion resistance increases as the lithium ion diffusion coefficient increases, the battery voltage based on the reaction in the second positive electrode active material is larger than the battery voltage based on the reaction in the first positive electrode active material. Become. For example, when the lithium ion diffusion coefficient of the second positive electrode active material is six times the lithium ion diffusion coefficient of the first positive electrode active material, the battery voltage based on the reaction in the second positive electrode active material is the first positive electrode active material. It becomes about 50 mV larger than the battery voltage based on the reaction at.

  For this reason, when this lithium ion secondary battery is charged, a voltage difference ΔV due to the lithium ion diffusion resistance difference is generated between the first flat range and the second flat range. In other words, a step S occurs at a position between the first flat range and the second flat range in the battery voltage curve indicating the behavior of the battery voltage accompanying the change in the charged amount. Since it is easy to detect this voltage difference ΔV (step S on the curve), the state of the secondary battery can be easily detected by detecting this voltage difference ΔV (step S).

  For example, the controller for estimating the storage amount of the lithium ion secondary battery of the present invention stores the value of the storage amount Q at the position of the step S on the battery voltage curve (referred to as the reference value QK) in advance. When the controller detects a voltage difference ΔV (step S on the curve) during charging / discharging, it can be determined that the amount of charge stored in the lithium ion secondary battery has reached the reference value QK.

  An example of a method for detecting the voltage difference ΔV (step S) by the controller is a pattern matching method. Specifically, the controller draws the battery voltage curve in real time based on the battery voltage value detected every predetermined time, and the battery voltage curve and a reference battery voltage curve stored in the controller in advance. Contrast (pattern matching). Thereby, in the lithium ion secondary battery which is charging / discharging, it can be detected whether voltage difference (DELTA) V (step S) generate | occur | produced.

  Furthermore, it is preferable to detect the occurrence of the voltage difference ΔV (step S) using the value of dV / dQ, which is the ratio of the change amount dV of the battery voltage V to the change amount dQ of the storage amount Q of the secondary battery. . Since the amount of change in the value of dV / dQ tends to be larger than the amount of change in the battery voltage V, it is possible to accurately detect the occurrence of the voltage difference ΔV. Thereby, it is possible to accurately determine when the charged amount of the lithium ion secondary battery has reached the reference value QK.

  It is also possible to detect battery abnormality using the pattern matching method described above. Specifically, for example, in the controller, the position of the step S on the battery voltage curve drawn based on the battery voltage value detected every predetermined time and the reference battery voltage curve K stored in advance in the controller When the position of the step S is greatly deviated, it can be determined that the battery is abnormal. Further, when the step S does not occur on the battery voltage curve drawn based on the battery voltage value detected every predetermined time in the controller, it can be determined that the battery is abnormal.

As the positive electrode active material, for example, LiFe _(1-X) M _x PO ₄ (M is Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, B, Nb). And a compound represented by 0 ≦ X ≦ 0.1). Two or more kinds of compounds represented by LiFe _(1-X) M _X PO ₄ and having different lithium ion diffusion coefficients may be mixed. Note that the lithium ion diffusion coefficient can be varied by varying the firing conditions (such as firing temperature and firing time) when the positive electrode active material is manufactured.

Also, as the positive electrode active _{material, LiMn (1-X) M} X PO 4 (M is, Cr, Co, Cu, Ni , V, Mo, Ti, Zn, Al, Ga, B, is at least one of Nb , 0 ≦ X ≦ 0.1) or LiCo _(1-X) M _x PO ₄ (M is at least one of Cr, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, B, and Nb. And a compound represented by 0 ≦ X ≦ 0.1) may be used. Two or more LiMn _(1-X) M _X PO ₄ or LiCo _(1-X) M _X PO ₄ having different lithium ion diffusion coefficients may be mixed.

Further, in the above lithium ion secondary battery, the positive electrode active materials are all LiFe _(1-X) M _x PO ₄ (M is Mn, Cr, Co, Cu, Ni, V, Mo, A lithium ion secondary battery that is at least one of Ti, Zn, Al, Ga, B, and Nb and is a compound represented by 0 ≦ X ≦ 0.1) is preferable.

In a lithium ion secondary battery using LiFe _(1-X) M _X PO ₄ as a positive electrode active material, an amount of electricity corresponding to about 80% of the theoretical battery capacity is charged with a substantially constant battery voltage of about 3.4 V. Can be discharged. Therefore, a stable output with little fluctuation can be obtained over a wide range of about 80% of the theoretical battery capacity.

  However, conventionally, in such a secondary battery, since the battery voltage fluctuation is extremely small over a wide range of about 80% of the theoretical battery capacity, the state of the battery over a wide range of about 80% of the theoretical battery capacity. There is a possibility that (charge state, etc.) cannot be detected.

On the other hand, in the lithium ion secondary battery of the present invention, two or more positive electrode active materials having different lithium ion diffusion coefficients are mixed. That is, two or more positive electrode active materials having the same composition formula as LiFe _(1-X) M _X PO ₄ but different lithium ion diffusion coefficients are mixed.

  Thereby, over a wide range of about 80% of the theoretical battery capacity, while the battery voltage fluctuation is extremely small, the battery voltage fluctuation is extremely small (hereinafter also referred to as a small voltage fluctuation range M). A voltage difference ΔV (step S on the curve) based on the difference in lithium ion diffusion coefficient can be developed. For this reason, even within the small voltage fluctuation range M, by detecting this voltage difference ΔV (step S on the curve), it is possible to appropriately detect the state of the secondary battery (such as the charged state). Therefore, the lithium ion secondary battery of the present invention is a secondary battery that is easy to detect the state while ensuring extremely stable output characteristics.

Furthermore, in the above lithium ion secondary battery, the positive electrode active material is preferably a lithium ion secondary battery that is a compound represented by LiFePO ₄ .

In particular, a lithium ion secondary battery using LiFePO ₄ as a positive electrode active material exhibits a very stable output characteristic over a wide range of about 80% of the theoretical capacity of the battery due to small fluctuations in battery voltage. However, conventionally, in such a secondary battery, battery voltage fluctuations are extremely small, and there is a possibility that the state of the battery (such as a charged state) cannot be detected.

In contrast, the lithium ion secondary battery of the present invention uses two or more positive electrode active materials having different lithium ion diffusion coefficients. That is, mix different LiFePO ₄ with 2 or more kinds of lithium ion diffusion coefficients. Thereby, the voltage difference ΔV (step S on the curve) based on the difference in the lithium ion diffusion coefficient can be developed in the middle of the small voltage fluctuation range M. For this reason, even within the small voltage fluctuation range M, by detecting this voltage difference ΔV (step S on the curve), it is possible to appropriately detect the state of the secondary battery (such as the charged state).

  Furthermore, in any one of the above lithium ion secondary batteries, the positive electrode active material includes a first positive electrode active material and a second positive electrode active material having a lithium ion diffusion coefficient larger than that of the first positive electrode active material. A lithium ion secondary battery is preferably included.

  In the lithium ion secondary battery of the present invention, since a voltage difference ΔV (step S) based on the difference in lithium ion diffusion coefficient between the first positive electrode active material and the second positive electrode active material appears, the state of the secondary battery is detected. It becomes easy.

  Furthermore, when the lithium ion secondary battery is a lithium ion secondary battery in which the lithium ion diffusion coefficient of the second positive electrode active material is five times or more than the lithium ion diffusion coefficient of the first positive electrode active material. good.

If the difference in lithium ion diffusion coefficient between the first positive electrode active material and the second positive electrode active material is too small, the voltage difference ΔV (step S) may not appear clearly.
On the other hand, in the lithium ion secondary battery of the present invention, the lithium ion diffusion coefficient of the second positive electrode active material is 5 times or more than the lithium ion diffusion coefficient of the first positive electrode active material. Thereby, since the voltage difference ΔV (step S) clearly appears based on the difference in the lithium ion diffusion coefficient, the state of the secondary battery can be detected appropriately.

Furthermore, in any one of the above lithium ion secondary batteries, the positive electrode active material includes a third positive electrode active material and a fourth positive electrode active material in addition to the first positive electrode active material and the second positive electrode active material. wherein, the first 1 D ₁ the lithium ion diffusion coefficient of the positive electrode active material, the second cathode active D ₂ lithium ion diffusion coefficient of a substance, the third cathode active lithium-ion diffusion coefficient of a substance D _3, the fourth Lithium ion secondary battery satisfying the relationship of 5D ₁ ≦ D ₂ ≦ 20D ₁ , 5D ₂ ≦ D ₃ ≦ 20D ₂ , 5D ₃ ≦ D ₄ ≦ 20D ₃ , where D ₄ is the lithium ion diffusion coefficient of the positive electrode active material Is preferable.

  By mixing the first to fourth positive electrode active materials that satisfy such a relationship, the voltage difference ΔV (step S) clearly appears between the three flat ranges (three locations) of the first to fourth flat ranges. A small voltage fluctuation range M can be secured. Thereby, in the small voltage fluctuation range M, it becomes possible to more appropriately detect the state (charged state, etc.) of the secondary battery by using (detecting) the three voltage differences ΔV (steps S). .

Example 1
Next, a lithium ion secondary battery 100 according to Example 1 of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the lithium ion secondary battery 100 is a rectangular sealed lithium ion secondary battery including a rectangular parallelepiped battery case 110, a positive electrode terminal 120, and a negative electrode terminal 130. Among these, the battery case 110 is made of metal, and includes a rectangular housing portion 111 that forms a rectangular parallelepiped housing space, and a metal lid portion 112. An electrode body 150, a positive current collecting member 122, a negative current collecting member 132, and the like are accommodated in the battery case 110 (rectangular accommodation portion 111).

  The electrode body 150 is an oblong cross section, and is a flat wound body formed by winding a sheet-like positive electrode plate 155, a negative electrode plate 156, and a separator 157 (see FIGS. 2 and 3). The electrode body 150 is positioned at one end portion (right end portion in FIG. 1) in the axial direction (left and right direction in FIG. 1), and a positive electrode winding portion 155b in which only a part of the positive electrode plate 155 overlaps in a spiral shape, It is located at the other end (left end in FIG. 1) and has a negative electrode winding part 156b in which only a part of the negative electrode plate 156 overlaps spirally.

  The positive electrode plate 155 is coated with a positive electrode mixture 152 containing a positive electrode active material (a first positive electrode active material 153b and a second positive electrode active material 153c) at a portion other than the positive electrode winding portion 155b (see FIG. 3). ). The negative electrode plate 156 is coated with a negative electrode mixture 159 including a negative electrode active material 154 at a portion other than the negative electrode winding portion 156b (see FIG. 3). The positive electrode winding part 155 b is electrically connected to the positive electrode terminal 120 through the positive electrode current collecting member 122. The negative electrode winding part 156 b is electrically connected to the negative electrode terminal 130 through the negative electrode current collecting member 132.

In particular, in Example 1, a mixture of the first positive electrode active material 153b and the second positive electrode active material 153c is used as the positive electrode active material. The first positive electrode active material 153b and the second positive electrode active material 153c are both compounds represented by the composition formula LiFePO ₄ . A compound represented by LiFePO ₄ (positive electrode active material) is an active material that performs charge / discharge of a two-phase coexistence type, and a charge / discharge reaction is performed in a state where two crystals having different crystal structures coexist. .

Incidentally, the first positive electrode active material 153b and the second positive electrode active material 153c are both compounds represented by the composition formula LiFePO ₄ , but have different lithium ion diffusion coefficients. Specifically, the lithium ion diffusion coefficient D ₁ of the first positive electrode active material 153b is 2.0 × 10 ⁻¹⁴ (cm ² / S), and the lithium ion diffusion coefficient D ₂ of the second positive electrode active material 153c is 12 × 10 ⁻¹⁴ (cm ² / S). Accordingly, the lithium ion diffusion coefficient D ₂ of the second positive electrode active material 153c is 5 times or more (specifically, 6 times) the lithium ion diffusion coefficient D ₁ of the first positive electrode active material 153b.
In the lithium ion secondary battery 100 of Example 1, the first positive electrode active material 153b and the second positive electrode active material 153c are mixed at 20:80 (weight ratio).

  In Example 1, a natural graphite-based carbon material is used as the negative electrode active material 154. Specifically, a natural graphite material having an average particle diameter of 20 μm, a lattice constant C0 of 0.67 nm, a crystallite size Lc of 27 nm, and a graphitization degree of 0.9 or more is used.

  Next, FIG. 4 shows a charge characteristic diagram and FIG. 5 shows a discharge characteristic diagram of the lithium ion secondary battery 100. FIG. 4 shows the behavior of the battery voltage V (the inter-terminal voltage between the positive terminal 120 and the negative terminal 130 in the first embodiment) when the lithium ion secondary battery 100 is charged with a current of 3C. Is shown. FIG. 5 shows the behavior of the battery voltage V when the lithium ion secondary battery 100 is discharged with a current of 3C. In FIGS. 4 and 5, as a comparative example, a battery voltage curve of a lithium ion secondary battery manufactured using only the first positive electrode active material 153 b as the positive electrode active material is indicated by a two-dot chain line.

The theoretical electric capacity (the lithium ion secondary battery 100 is capable of theoretically maximally storing the first positive electrode active material 153b and the second positive electrode active material 153c (LiFePO ₄ ) included in the lithium ion secondary battery 100 of the first embodiment. The current value that can be charged in 1 hour is 1C because it is a positive electrode regulation, which is the battery theoretical capacity). The storage rate (%) is the ratio of the stored amount (the amount of electricity charged in the lithium ion secondary battery 100) to the theoretical battery capacity.

  As can be seen from FIGS. 4 and 5, the lithium ion secondary battery 100 has a battery voltage around 3.4 V, a battery voltage fluctuation range as small as 0.2 V, and a battery theoretical capacity (the charge rate is 0 in FIG. 4). The amount of electricity corresponding to about 80% of the range of ˜100% can be charged and discharged. Therefore, a stable output with little fluctuation can be obtained over a wide range of about 80% of the theoretical battery capacity. Note that a range corresponding to about 80% of the theoretical battery capacity is extremely small, and a small voltage variation range M (see FIGS. 4 and 5).

By the way, as shown with a dashed-two dotted line in FIG.4 and FIG.5, the lithium ion secondary battery of a comparative example (as a positive electrode active material, lithium ion diffusion coefficient is 2.0 * 10 < ^-14 > cm < ² > / S 1st. In the case of using only the positive electrode active material 153b), the battery voltage fluctuation is extremely small over a wide range of about 80% of the theoretical battery capacity. For this reason, the battery state (charged state, etc.) may not be properly detected over a wide range of about 80% of the theoretical battery capacity.

On the other hand, in the lithium ion secondary battery 100 of Example 1, two types of positive electrode active materials (first positive electrode active material 153b and second positive electrode active material 153c) having different lithium ion diffusion coefficients are mixed. That is, two types of positive electrode active materials (first positive electrode active material 153b and second positive electrode active material 153c) having the same composition formula as LiFePO ₄ but different lithium ion diffusion coefficients are mixed.

  As a result, as shown by a solid line in FIGS. 4 and 5, the battery voltage fluctuation is extremely small (having a small voltage fluctuation range M) over a wide range of about 80% of the theoretical battery capacity, but the voltage is small. In the middle of the fluctuation range M, a voltage difference ΔV (step S on the curve) based on the difference in the lithium ion diffusion coefficient can be expressed.

  Specifically, due to the lithium ion diffusion resistance difference between the first positive electrode active material 153b and the second positive electrode active material 153c between the first flat range F1 and the second flat range F2 where the battery voltage is substantially constant. A voltage difference ΔV is generated. In other words, a step S occurs at a position between the first flat range F1 and the second flat range F2 in the battery voltage curve indicating the behavior of the battery voltage (see FIGS. 4 and 5). Since the change rate of the battery voltage at the step S is considerably larger than the change rate of the battery voltage in the first flat range F1 and the second flat range F2, the occurrence of the step S (voltage difference ΔV) is easily detected. be able to. Therefore, it becomes easy to detect the state of the lithium ion secondary battery 100 by detecting the voltage difference ΔV (step S).

  In the lithium ion secondary battery 100 of Example 1, as shown in FIGS. 4 and 5, the voltage difference ΔV (step S) appears in the vicinity of a storage rate of 20% (discharge depth of 80%). This is because the mixing ratio of the first positive electrode active material 153b and the second positive electrode active material 153c is 20:80 (weight ratio). That is, since the ratio of the first positive electrode active material 153b having a small lithium ion diffusion coefficient is set to 20 wt%, a voltage difference ΔV (step S) appears at a position in the vicinity of the storage rate of 20% correspondingly. . Therefore, when the occurrence of the voltage difference ΔV (step S) is detected during charge / discharge, it can be determined that the storage rate of the lithium ion secondary battery 100 has reached 20%.

  Here, a method for detecting the voltage difference ΔV (step S) will be specifically described. For example, when the lithium ion secondary battery 100 is charged and discharged, the battery voltage of the lithium ion secondary battery 100 is detected every predetermined time. Then, based on the detected battery voltage, the battery voltage curve (see FIGS. 4 and 5) is drawn in real time, and the battery voltage curve and the controller that controls the lithium ion secondary battery 100 are stored in advance. A reference battery voltage curve is compared (pattern matching). From this comparison, when it is determined that the step S has occurred in the charged / discharged lithium ion secondary battery 100, it can be determined that the storage rate of the lithium ion secondary battery 100 has reached 20%.

  Further, the voltage difference ΔV (step S) is accurately detected by using the value of dV / dQ, which is the ratio of the change amount dV of the battery voltage V to the change amount dQ of the storage amount Q of the lithium ion secondary battery 100. can do. Here, a Q-dV / dQ curve (during charging) representing the relationship between the charged amount Q of the lithium ion secondary battery 100 and dV / dQ is shown in FIG. As shown in FIG. 6, since the amount of change in dV / dQ value is larger than the amount of change in battery voltage V, it is possible to accurately detect the occurrence of voltage difference ΔV (step S). Specifically, when the voltage difference ΔV (step S) occurs, a clear peak P appears on the Q-dV / dQ curve.

  Therefore, when the lithium ion secondary battery 100 is charged / discharged, the battery voltage and charge (discharge) electricity amount of the lithium ion secondary battery 100 are detected, and dV / based on the detected battery voltage and charge (discharge) electricity amount. Calculate the value of dQ. Then, based on the calculated value of dV / dQ, while drawing a Q-dV / dQ curve (see FIG. 6) in real time, this Q-dV / dQ curve and the reference Q stored in advance in the controller -DV / dQ curve is compared (pattern matching). Based on this comparison, it is determined whether or not the lithium ion secondary battery 100 has reached a state corresponding to the peak P on the Q-dV / dQ curve. When it is determined that the state corresponding to the peak P has been reached, it can be determined that the voltage difference ΔV (step S) has occurred, and thus it can be determined that the storage rate of the lithium ion secondary battery 100 has reached 20%. it can.

  Moreover, it is also possible to detect abnormality of the lithium ion secondary battery 100 using a pattern matching method. Specifically, in the controller that controls the lithium ion secondary battery 100, while drawing a battery voltage curve (see FIGS. 4 and 5) in real time based on the battery voltage value detected every predetermined time, the battery voltage The curve is compared (pattern matching) with a reference battery voltage curve stored in advance in the controller. When the position of the step S on the battery voltage curve drawn based on the battery voltage value detected every predetermined time and the position of the step S on the reference battery voltage curve stored in advance in the controller are greatly shifted Can be determined as an abnormality of the lithium ion secondary battery 100. Further, even when the step S does not occur on the battery voltage curve drawn on the basis of the battery voltage value detected every predetermined time in the controller, it can be determined that the lithium ion secondary battery 100 is abnormal.

  Further, in the assembled battery in which a plurality of lithium ion secondary batteries 100 are electrically connected, the battery voltage of each lithium ion secondary battery 100 is detected every predetermined time (and the value of dV / dQ is calculated). Thus, the occurrence of the voltage difference ΔV (step S on the curve) of each lithium ion secondary battery 100 is detected. At this time, when there is a lithium ion secondary battery 100 in which the occurrence of the voltage difference ΔV (step S on the curve) is significantly different from that of other lithium ion secondary batteries 100, the lithium ion secondary battery 100 is It can be determined to be abnormal.

  As described above, by detecting the voltage difference ΔV (step S on the curve) even within the small voltage fluctuation range M, the state of the lithium ion secondary battery 100 (charged state such as storage rate and SOC, abnormality, etc.) Can be detected appropriately. Therefore, it can be said that the lithium ion secondary battery 100 of Example 1 is a lithium ion secondary battery that is easy to detect the state while ensuring extremely stable output characteristics.

Here, the manufacturing method of the lithium ion secondary battery 100 of the first embodiment will be described.
First, LiFePO ₄ LiFePO ₄ lithium ion diffusion coefficient of 2.0 × 10 ^-14 cm ² / S (the first positive electrode active material 153b), the lithium ion diffusion coefficient is 12 × 10 ^-14 cm ² / S (Second positive electrode active material 153c) was prepared.

  Next, the first positive electrode active material 153b and the second positive electrode active material 153c were mixed at a ratio of 20:80 (weight ratio) to obtain a mixed positive electrode active material. Thereafter, the mixed positive electrode active material, (first positive electrode active material 153b and second positive electrode active material 153c), acetylene black (conductive aid), and polyvinylidene fluoride (binder resin) are mixed at 85: 5: 10 (weight ratio). ) And mixed with N-methylpyrrolidone (dispersion solvent) to prepare a positive electrode slurry. Next, this positive electrode slurry was applied to the surface of the aluminum foil 151, dried, and then pressed. Thereby, the positive electrode plate 155 in which the positive electrode mixture 152 was coated on the surface of the aluminum foil 151 was obtained (see FIG. 3).

Incidentally, different lithium ion diffusion coefficients LiFePO ₄ (the first positive electrode active material 153b and the second positive electrode active material 153c), a known solid phase synthesis method (e.g., JP reference 2002-15735) was used to 600-1000 By adjusting the firing temperature in the range of ° C., it can be obtained by controlling the growth state of the particles.
Specifically, lithium carbonate is used as the lithium source, ferrous oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) is used as the iron source, ammonium hydrogen phosphate is used as the phosphorus source, and the firing precursor Generate a body. Thereafter, by firing the obtained precursor at different temperatures, it is possible to obtain the first positive electrode active material 153b and the second positive active material 153c represented by LiFePO _4.

  Further, 95: 2.5: 2.5 (weight ratio) of a natural graphite-based carbon material (negative electrode active material 154), a styrene-butadiene copolymer (binder resin), and carboxymethylcellulose (thickener). ) In water to prepare a negative electrode slurry. Next, this negative electrode slurry was applied to the surface of the copper foil 158, dried, and then pressed. Thereby, the negative electrode plate 156 in which the negative electrode mixture 159 was applied to the surface of the copper foil 158 was obtained (see FIG. 3). In Example 1, a natural graphite-based carbon material having an average particle size of 20 μm, a lattice constant C0 of 0.67 nm, a crystallite size Lc of 27 nm, and a graphitization degree of 0.9 or more is used. Yes. In Example 1, the coating amounts of the positive electrode slurry and the negative electrode slurry are adjusted so that the ratio between the theoretical capacity of the positive electrode and the theoretical capacity of the negative electrode is 1: 1.5.

  Next, a positive electrode plate 155, a negative electrode plate 156, and a separator 157 were laminated and wound to form an electrode body 150 having an oval cross section (see FIGS. 2 and 3). However, when the positive electrode plate 155, the negative electrode plate 156, and the separator 157 are stacked, an uncoated portion of the positive electrode plate 155 that is not coated with the positive electrode mixture 152 protrudes from one end portion of the electrode body 150. Thus, the positive electrode plate 155 is arranged. Furthermore, the negative electrode plate 156 is arranged so that an uncoated portion of the negative electrode plate 156 not coated with the negative electrode mixture 159 protrudes from the opposite side of the positive electrode plate 155 from the uncoated portion. . Thereby, the electrode body 150 (refer FIG. 1) which has the positive electrode winding part 155b and the negative electrode winding part 156b is formed. In Example 1, a polypropylene / polyethylene / polypropylene three-layer structure composite porous membrane is used as the separator 157.

Next, the positive electrode winding part 155 b of the electrode body 150 and the positive electrode terminal 120 are connected through the positive electrode current collecting member 122. Further, the negative electrode winding portion 156 b of the electrode body 150 and the negative electrode terminal 130 are connected through the negative electrode current collecting member 132. Then, this was accommodated in the square accommodating part 111, the square accommodating part 111 and the cover body 112 were welded, and the battery case 110 was sealed. Next, after injecting an electrolyte through an injection port (not shown) provided in the lid 112, the injection port is sealed, whereby the lithium ion secondary battery 100 of Example 1 is completed. . In Example 1, lithium hexafluorophosphate (LiPF ₆ ) was used as an electrolytic solution in a solution in which EC (ethylene carbonate) and DEC (diethyl carbonate) were mixed at a volume ratio of 4: 6. 1 mol dissolved one is used.

Here, a method for calculating the lithium ion diffusion coefficient of the first positive electrode active material 153b and the second positive electrode active material 153c will be described. In Example 1, a secondary battery B was produced using only the first positive electrode active material 153b as the positive electrode active material. For the secondary battery B, reference literature (document name: Solid State Ionics vol.148 P45, work) author: based on the techniques described in PPprosini etc.) to calculate the lithium ion diffusion coefficient D _1. Similarly, the secondary battery C was produced by using only the second cathode active material 153c as the positive electrode active material, based on the technique described in the above references was calculated lithium ion diffusion coefficient D _2. This will be described in detail below.

(Production of secondary battery)
First, a positive electrode plate was produced using only the first positive electrode active material 153b as the positive electrode active material. Specifically, the first positive electrode active material 153b, acetylene black (conducting aid), and polyvinylidene fluoride (binder resin) are mixed at a ratio of 85: 5: 10 (weight ratio), and this is mixed with N-methyl. A positive electrode slurry was prepared by mixing pyrrolidone (dispersing solvent). Next, this positive electrode slurry was applied to the surface of the aluminum foil 151, dried, and then pressed to obtain a positive electrode plate. Further, a lithium metal plate was prepared as a counter electrode plate of the positive electrode plate. Then, the positive electrode plate and the lithium metal plate were laminated with a separator interposed therebetween to produce an electrode body, and a secondary battery B was produced using this electrode body.
Moreover, the secondary battery C was produced as described above using only the second positive electrode active material 153c as the positive electrode active material.

(Impedance measurement)
Next, impedance measurement was performed on the fabricated secondary batteries B and C using a Solartron 1255 WB type electrochemical measurement system (frequency analyzer + potential galvanostat). Specifically, a potential of 5 mV is obtained for a secondary battery in which the storage amount is adjusted to an amount of electricity (storage rate 60%) corresponding to 60% of the battery capacity predicted from the positive electrode theoretical capacity (battery theoretical capacity (Ah)). While giving an amplitude, the measurement frequency was varied in the range of 0.01 Hz to 100 kHz, and the impedance was measured from the synchronized current value. Thereby, the secondary battery B, and C, respectively, to draw a (2πf / sec ^{-1) -1/2} and Z components, were calculated &Dgr; E / .delta.x from the slope of the graph drawn.

(Calculation of lithium ion diffusion coefficient)
Next, using the obtained impedance measurement results, the lithium of the first positive electrode active material 153b is calculated by a calculation method described in a reference (document name: Solid State Ionics vol.148 P45, author: PPprosini, etc.). the ion diffusion coefficient D ₁ to calculate the lithium ion diffusion coefficient D ₂ of the second positive electrode active material 153c. Specifically, each lithium ion diffusion coefficient was calculated based on the following formula (1).

D = 1/2 [{V _M / (S × F × A)} × (δE / δx)] ² (1)
Here, D is the lithium ion diffusion coefficients, the V _M volume per positive electrode active material 1 mole, S is the contact area between the positive electrode active material and the electrolyte, F is the Faraday constant, A and &Dgr; E / .delta.x the above This is a value obtained from the impedance measurement result. The contact area S between the positive electrode active material and the electrolytic solution can be calculated based on, for example, the porosity of the positive electrode plate. Or it can also measure using a well-known pore distribution measuring apparatus.

From the formula (1), the lithium ion diffusion coefficient D ₁ of the first positive electrode active material 153b was calculated to be 2.0 × 10 ⁻¹⁴ (cm ² / S). In addition, the lithium ion diffusion coefficient D ₂ of the second positive electrode active material 153c was calculated to be 12 × 10 ⁻¹⁴ (cm ² / S).

(Example 2)
In Example 2, compared to Example 1, only the mixing ratio of the first positive electrode active material 153b and the second positive electrode active material 153c was changed to manufacture the lithium ion secondary battery 200.
Specifically, the first positive electrode active material 153b and the second positive electrode active material 153c were mixed at a ratio of 50:50 (weight ratio) to obtain a mixed positive electrode active material. Thereafter, using this mixed positive electrode active material, a positive electrode plate 255 in which the positive electrode mixture 252 was coated on the surface of the aluminum foil 151 was produced in the same manner as in Example 1 (see FIG. 3). Next, a positive electrode plate 255, a negative electrode plate 156, and a separator 157 were stacked and wound to form an electrode body 250 having an oval cross section (see FIGS. 2 and 3). Thereafter, in the same manner as in Example 1, a lithium ion secondary battery 200 (see FIG. 1) was produced.

  Here, a charge characteristic diagram of the lithium ion secondary battery 200 is shown in FIG. 7, and a discharge characteristic diagram is shown in FIG. FIG. 7 shows the behavior of the battery voltage V (inter-terminal voltage between the positive terminal 120 and the negative terminal 130) when the lithium ion secondary battery 200 is charged with a current of 3C. FIG. 8 shows the behavior of the battery voltage V when the lithium ion secondary battery 200 is discharged with a current of 3C. In FIGS. 7 and 8, as a comparative example, a battery voltage curve of a lithium ion secondary battery manufactured using only the first positive electrode active material 153b as the positive electrode active material is shown by a two-dotted line.

The theoretical electric capacity that the first positive electrode active material 153b and the second positive electrode active material 153c (LiFePO ₄ ) included in the lithium ion secondary battery 200 of the second embodiment can theoretically accumulate to the maximum extent (the lithium ion secondary battery 200 is The current value that can be charged in 1 hour is 1C because it is a positive electrode regulation, which is the battery theoretical capacity). The storage rate (%) is the ratio of the stored amount (the amount of electricity charged in the lithium ion secondary battery 200) to the theoretical battery capacity.

  As shown in FIGS. 7 and 8, the lithium ion secondary battery 200 of Example 2 is similar to the lithium ion secondary battery 100 of Example 1 and has a battery voltage fluctuation range of about 3.4 V. Can be reduced to 0.2 V, and an amount of electricity corresponding to about 80% of the theoretical battery capacity (in the range of 0 to 100% in FIG. 7) can be charged and discharged. Therefore, a stable output with little fluctuation can be obtained over a wide range of about 80% of the theoretical battery capacity.

Also in the lithium ion secondary battery 200 of the second embodiment, two types of LiFePO ₄ (first positive electrode active material 153b and second positive electrode active material 153c) having different lithium ion diffusion coefficients are mixed. As a result, as shown by the solid lines in FIGS. 7 and 8, the battery voltage fluctuation is extremely small (having a small voltage fluctuation range M) over a wide range of about 80% of the theoretical battery capacity, but the voltage is small. In the middle of the fluctuation range M, a voltage difference ΔV (step S on the curve) based on the difference in the lithium ion diffusion coefficient can be expressed.

  Specifically, due to the lithium ion diffusion resistance difference between the first positive electrode active material 153b and the second positive electrode active material 153c between the first flat range F1 and the second flat range F2 where the battery voltage is substantially constant. Voltage difference ΔV occurs. In other words, a step S occurs at a position between the first flat range F1 and the second flat range F2 in the battery voltage curve indicating the behavior of the battery voltage (see FIGS. 7 and 8). This voltage difference ΔV (step S on the curve) can be easily detected as described in the first embodiment. Therefore, it is easy to detect the state of the lithium ion secondary battery 200 by detecting the voltage difference ΔV (step S).

  In the lithium ion secondary battery 200 of the second embodiment, as shown in FIGS. 7 and 8, the voltage difference ΔV (step S) appears in the vicinity of a storage rate of 50% (discharge depth of 50%). This is because the mixing ratio of the first positive electrode active material 153b and the second positive electrode active material 153c is 50:50 (weight ratio). That is, since the ratio of the first positive electrode active material 153b having a small lithium ion diffusion coefficient is set to 50 wt%, a voltage difference ΔV (step S) appears at a position in the vicinity of the power storage rate corresponding to 50%. . Therefore, when the voltage difference ΔV (step S) is detected, it can be determined that the storage rate of the lithium ion secondary battery 200 has reached 50%.

  Thus, from the results of Examples 1 and 2, by adjusting the mixing ratio of the first positive electrode active material 153b and the second positive electrode active material 153c, the storage rate (depth of discharge) at which the voltage difference ΔV (step S) appears. ) Value can be adjusted. That is, by adjusting the mixing ratio of the first positive electrode active material 153b and the second positive electrode active material 153c, the voltage difference ΔV (step S) can appear at a desired position on the battery voltage curve.

  In the above, the present invention has been described with reference to the first and second embodiments. However, the present invention is not limited to the above-described embodiments, and it can be applied as appropriate without departing from the scope of the present invention. Nor.

For example, in the lithium ion secondary batteries 100 and 200 of Examples 1 and 2, two types of LiFePO ₄ having different lithium ion diffusion coefficients were used as the positive electrode active material, but 3 to 5 types of lithium ion diffusion coefficients were different. LiFePO ₄ may be used. As a result, a plurality of voltage differences ΔV (steps S) are generated during charging / discharging of the lithium ion secondary battery. By detecting the plurality of voltage differences ΔV (steps S), a plurality of states (such as power storage rate and SOC) are detected. ) Can be detected.

In the lithium ion secondary batteries 100 and 200 of Examples 1 and 2, LiFePO ₄ was used as the positive electrode active material, but LiMnPO ₄ or LiCoPO ₄ may be used. By using two or more types of LiMnPO ₄ having different lithium ion diffusion coefficients or two or more types of LiCoPO ₄ having different lithium ion diffusion coefficients as the positive electrode active material, a clear voltage difference ΔV (step S on the curve) can be obtained. Can be expressed. By detecting this voltage difference ΔV (step S), the state of the lithium ion secondary battery can be easily detected.

1 is a cross-sectional view of lithium ion secondary batteries 100 and 200 according to Examples 1 and 2. FIG. It is sectional drawing of the electrode bodies 150 and 250 concerning Example 1,2. FIG. 3 is a partial enlarged cross-sectional view of electrode bodies 150 and 250 according to Examples 1 and 2 and corresponds to an enlarged view of a portion B in FIG. FIG. 3 is a charging characteristic diagram of the lithium ion secondary battery 100. 3 is a discharge characteristic diagram of the lithium ion secondary battery 100. FIG. 3 is a diagram showing a Q-dV / dQ curve (at the time of charging) of the lithium ion secondary battery 100. FIG. FIG. 6 is a charging characteristic diagram of the lithium ion secondary battery 200. 3 is a discharge characteristic diagram of a lithium ion secondary battery 200. FIG.

Explanation of symbols

100, 200 Lithium ion secondary battery 150, 250 Electrode body 153b First positive electrode active material 153c Second positive electrode active material 154 Negative electrode active material 155, 255 Positive electrode plate 156 Negative electrode plate 157 Separator

Claims (5)

A lithium ion secondary battery including a positive electrode active material,
The positive electrode active material is
A lithium ion secondary battery comprising a compound represented by the same composition formula and a two-phase coexisting type charge / discharge positive electrode active material having different lithium ion diffusion coefficients. The lithium ion secondary battery according to claim 1,
All of the positive electrode active materials are LiFe _(1-X) M _x PO ₄ (M is at least one of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, B, and Nb. A lithium ion secondary battery which is any one of the compounds represented by 0 ≦ X ≦ 0.1). The lithium ion secondary battery according to claim 2,
All of the positive electrode active materials are lithium ion secondary batteries which are compounds represented by LiFePO ₄ . It is a lithium ion secondary battery as described in any one of Claims 1-3,
The positive electrode active material is
A first positive electrode active material;
And a second positive electrode active material having a lithium ion diffusion coefficient larger than that of the first positive electrode active material. The lithium ion secondary battery according to claim 4,
A lithium ion secondary battery in which a lithium ion diffusion coefficient of the second positive electrode active material is five times or more than a lithium ion diffusion coefficient of the first positive electrode active material.

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