Method for controlling a secondary battery

Abstract

To provide a secondary battery in which deformation of a rectangular battery case is suppressed while metal deposition is also suppressed.SOLUTION: In a control method of a secondary battery including a flat electrode body including a positive electrode and a negative electrode, and a prismatic battery case that houses the electrode body, the electrode body has the charging rate of a predetermined value of 0.2 C or more, and has a peak in the range of 80% to 90% in a graph in which the horizontal axis is plotted as "depth of charge (SOC)" and the vertical axis is plotted as "change in thickness of the electrode body", and when the depth of charge (SOC) is in the range of less than 60%, a gap exists between the electrode body and the prismatic battery case in the thickness direction of the electrode body, and when the depth of charge (SOC) is at the apex of the peak, the gap between the electrode body and the prismatic battery case disappears.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for controlling a secondary battery. [Background technology]

[0002] A secondary battery typically includes an electrode assembly in which a positive electrode and a negative electrode are stacked in a predetermined thickness direction while being insulated from each other, and a battery case that houses the electrode assembly. When such a secondary battery is charged, the electrode assembly expands in the thickness direction, which may cause deformation of the battery case or damage to the welds of the battery case (see Patent Documents 1 and 2). In relation to this, Patent Document 1 describes that deformation and damage to the prismatic battery case can be suppressed by providing an excess space in the prismatic battery case that corresponds to the expansion volume of the electrode assembly caused by charging. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-104902 A [Patent Document 2] JP 2015-115203 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the above technology is applied to a secondary battery that is charged at a relatively high charge rate of, for example, 0.2 C or more, there is room for improvement. That is, the inventors' investigations revealed that when a gap is newly provided between the electrode body and the prismatic battery case, metal deposition (e.g., Li deposition) is likely to occur when the state of charge (SOC) is high, particularly when the SOC is 80 to 90%. This tendency is particularly noticeable in a mode in which charging is repeated at a charge rate of 0.2 C or more, such as in an in-vehicle battery.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide a control method for a secondary battery that suppresses deformation of a rectangular battery case and also suppresses metal deposition.

Means for Solving the Problems

[0006] A control method for a secondary battery including a flat electrode body including a positive electrode and a negative electrode, and a rectangular battery case that houses the electrode body, wherein the electrode body has a charging rate of 0.2C or more. In a graph in which "State of Charge (SOC)" is plotted on the horizontal axis and "Change in thickness of the electrode body" is plotted on the vertical axis, the State of Charge (SOC) has a peak in the range of 80% or more and 90% or less, and when the State of Charge (SOC) is in the range of less than 60%, there is a gap between the electrode body and the rectangular battery case in the thickness direction of the electrode body, and at the State of Charge (SOC) that is the peak apex, a control method for a secondary battery is provided in which the gap between the electrode body and the rectangular battery case is eliminated.

[0007] When the charging rate is 0.2C or more, the electrode body expands in the State of Charge (SOC) range where metal deposition (for example, Li deposition) is likely to occur, that is, in the range of 80 to 90% of SOC, and the thickness changes greatly. Therefore, in the present invention, a gap is secured between the electrode body and the rectangular battery case in a state where the SOC is low, while in a state where the SOC is high, the change in the thickness of the electrode body is utilized to eliminate the gap between the electrode body and the rectangular battery case. As a result, it is possible to effectively suppress metal deposition (for example, Li deposition) while suppressing deformation of the rectangular battery case.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0009] Hereinafter, with reference to the drawings, preferred embodiments of the technology disclosed herein will be described. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present invention (for example, the general configuration and manufacturing process of a battery that does not characterize the present invention) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the relevant field.

[0010] In this specification, the term "secondary battery" refers to all rechargeable power storage devices, and includes so-called storage batteries (chemical batteries) such as lithium-ion secondary batteries and nickel-metal hydride batteries, and capacitors (physical batteries) such as electric double layer capacitors. The "state of charge (SOC)" refers to the state of charge when the upper limit voltage is obtained as 100% and the lower limit voltage is obtained as 0% in the range of the operable voltage that can be reversibly charged and discharged. In addition, in this specification, the notation "A~B" indicating a range includes the meaning of "greater than A" and "less than B" in addition to the meaning of "A or more and B or less".

[0011] <Secondary battery 100> FIG. 1 is a perspective view of the secondary battery 100. FIG. 2 is a schematic longitudinal sectional view taken along line II-II of FIG. 1. FIG. 3 is a schematic longitudinal sectional view taken along line III-III of FIG. 1, where (A) represents the first SOC state where the SOC is less than 60%, and (B) represents the second SOC state where the SOC is a predetermined SOC within the range of 80 to 90%. In the following description, the reference signs L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, up, and down, respectively, and the reference signs X, Y, and Z in the drawings represent the width direction, the thickness direction orthogonal to the width direction, and the up-down direction of the secondary battery 100, respectively. However, these are merely directions for convenience of explanation and do not limit the installation form of the secondary battery 100 in any way.

[0012] As shown in FIG. 2, the secondary battery 100 includes a rectangular battery case 10, an electrode body 20, a positive electrode terminal 30, and a negative electrode terminal 40. Although not shown in the figure, the secondary battery 100 further includes an electrolyte here. The secondary battery 100 is configured such that the electrode body 20 and an electrolyte (not shown) are housed inside the rectangular battery case 10. Here, the secondary battery 100 is a non-aqueous electrolyte secondary battery, specifically a lithium-ion secondary battery. In this specification, the "lithium-ion secondary battery" generally refers to a secondary battery that uses lithium ions (Li ions) as electrolyte ions and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0013] The rectangular battery case 10 is a housing that houses the electrode body 20 and an electrolyte (not shown). As shown in FIG. 1, the rectangular battery case 10 is formed in a flat and bottomed rectangular parallelepiped shape (rectangular). The material of the rectangular battery case 10 may be the same as those conventionally used, and there is no particular limitation. The rectangular battery case 10 is composed of, for example, a lightweight and thermally conductive metal material such as aluminum, aluminum alloy, or stainless steel.

[0014] As shown in FIG. 2, the rectangular battery case 10 here includes a case body 12 having an opening 12h and a lid body (sealing plate) 14 that closes the opening 12h. As shown in FIG. 1, the case body 12 includes a flat bottom wall 12a, a pair of long side walls 12b extending from the bottom wall 12a and facing each other, and a pair of short side walls 12c extending from the bottom wall 12a and facing each other. The bottom wall 12a is substantially rectangular. The bottom wall 12a faces the opening 12h. The long side walls 12b and the short side walls 12c each have a flat surface. In a plan view, the area of the long side wall 12b is larger than the area of the short side wall 12c. As shown in FIG. 3(A), the long side wall 12b faces a flat portion 20f of the electrode body 20, which will be described later.

[0015] The lid body 14 is attached to the case body 12 so as to close the opening 12h of the case body 12. The lid body 14 faces the bottom wall 12a of the case body 12. Here, the lid body 14 is substantially rectangular. The rectangular battery case 10 is integrated by joining (for example, welding) the lid body 14 to the periphery of the opening 12h of the case body 12. The rectangular battery case 10 is hermetically sealed.

[0016] The electrode body 20 has a flat outer shape. The electrode body 20 is housed inside the rectangular battery case 10 such that the thickness direction thereof coincides with the direction connecting the pair of long side walls 12b. As shown in FIG. 3(A), the electrode body 20 has a pair of flat portions 20f at both ends in the thickness direction Y. The pair of flat portions 20f faces the long side walls 12b of the rectangular battery case 10. Also, the end faces of the electrode body 20 orthogonal to the thickness direction Y face the bottom wall 12a, the lid body 14, and the pair of short side walls 12c, respectively. Although the rectangular battery case 10 and the electrode body 20 directly face each other in FIG. 3(A), an insulating sheet (for example, a resin sheet made of a resin such as polypropylene (PP); also referred to as an electrode body holder) or the like may be interposed between the rectangular battery case 10 and the electrode body 20.

[0017] The electrode body 20 has a positive electrode and a negative electrode (not shown). Here, the electrode body 20 is a laminated electrode body in which a rectangular (typically rectangular) positive electrode and a rectangular (typically rectangular) negative electrode are laminated in the thickness direction (lamination direction) via a separator. The thickness direction of the electrode body 20 is the lamination direction of the positive electrode and the negative electrode, and coincides with the thickness direction Y of the secondary battery 100. Note that the electrode body 20 may be, for example, a flat wound electrode body in which a strip-shaped positive electrode and a strip-shaped negative electrode are laminated via a strip-shaped separator and wound around a winding axis.

[0018] As shown in FIG. 2, the positive electrode has a positive electrode current collector 21 and a positive electrode active material layer (not shown) fixed on at least one surface of the positive electrode current collector 21. The positive electrode current collector 21 is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. The filling density (average filling density) of the positive electrode active material layer is preferably 3.4 g / cm 3 or more. When the filling density of the positive electrode active material layer is a predetermined value or more, metal precipitation (for example, Li precipitation) is likely to occur particularly at a charging rate of 0.2C or more. Therefore, it is particularly effective to apply the technology disclosed herein.

[0019] The positive electrode active material layer contains a positive electrode active material capable of reversibly occluding and releasing a charge carrier. Examples of the positive electrode active material include lithium transition metal composite oxides such as lithium nickel cobalt manganese-containing composite oxides. The positive electrode active material layer may contain optional components other than the positive electrode active material, such as a conductive material, a binder, and various additive components. As the conductive material, a carbon material such as acetylene black (AB) can be used. As the binder, polyvinylidene fluoride (PVdF) or the like can be used.

[0020] As shown in FIG. 2, the negative electrode has a negative electrode current collector 22 and a negative electrode active material layer (not shown) fixed on at least one surface of the negative electrode current collector 22. The negative electrode current collector is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The filling density (average filling density) of the negative electrode active material layer is typically smaller than that of the positive electrode active material layer, and is 1.4 g / cm3 The above is preferable, for example, 1.4 to 1.6 g / cm 3 can be. When the packing density of the negative electrode active material layer is equal to or greater than a predetermined value, the resistance of the negative electrode increases, and especially at a charging rate of 0.2C or more, the capacity on the negative electrode side significantly decreases. As a result, metal precipitation (for example, Li precipitation) is likely to occur at this charging rate. Therefore, it is particularly effective to apply the technology disclosed herein. The theoretical capacity ratio of the negative electrode to the positive electrode is preferably 1 to 1.2 when the charging rate is a predetermined value of 0.2C or more. Thereby, the effect of the technology disclosed herein can be more stably exhibited.

[0021] The negative electrode active material layer contains a negative electrode active material capable of reversibly occluding and releasing charge carriers. Examples of the negative electrode active material include carbon materials such as graphite, and materials containing silicon such as SiOx and Si-C composites (Si-containing materials). The negative electrode active material preferably contains a Si-containing material. According to the study of the present inventor, thereby, in the range of 80 to 90% of the state of charge (SOC) where metal precipitation is likely to occur, the electrode body 20 (especially the negative electrode) is likely to expand, and the effect of the technology disclosed herein can be more stably exhibited. The negative electrode active material (for example, Si-containing material) preferably has a BET specific surface area of 3 m 2 / g or less from the viewpoint of better exhibiting the effect of the technology disclosed herein.

[0022] The negative electrode active material more preferably contains graphite and a Si-containing material. The proportion of the Si-containing material is typically 2% by mass or more, preferably 3% by mass or more in terms of Si when the total amount of the negative electrode active material is 100% by mass. Thereby, in the range of 80 to 90% of the SOC, the electrode body 20 (especially the negative electrode) is likely to expand, and the effect of the technology disclosed herein can be more stably exhibited. The proportion of the Si-containing material in the total amount of the negative electrode active material may be 20% by mass or less, 10% by mass or less in terms of Si. Also, when the total of the mass of graphite and the mass of the Si-containing material is 100% by mass, the proportion of the Si-containing material is preferably 3% by mass or more in terms of Si.

[0023] The negative electrode active material layer may contain optional components other than the negative electrode active material, such as a binder, various additive components, etc. As the binder, for example, rubber-based binders such as styrene-butadiene rubber (SBR), cellulose-based binders such as carboxymethyl cellulose (CMC), acrylic-based binders such as polyacrylic acid (PAA), etc. can be used. The negative electrode active material layer preferably contains a rubber-based binder. Thereby, the electrode body 20 can be smoothly expanded and contracted with charge and discharge. From the above viewpoints, the proportion of the binder in the entire negative electrode active material layer is preferably 0.5% by mass or more.

[0024] The separator is a member that is interposed between the positive electrode active material layer of the positive electrode and the negative electrode active material layer of the negative electrode in the thickness direction Y and insulates them. As the separator, for example, a porous sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is suitable. The separator preferably has a total thickness of 20 μm or less. Thereby, in the range where the SOC is 80 - 90%, the reaction force absorption effect of the separator is suppressed to be small, and the electrode body 20 can be expanded more greatly. The separator is preferably a heat-resistant separator having a base material portion made of a porous sheet made of a resin such as PE and a heat-resistant layer provided on at least one surface of the base material portion.

[0025] The separator is preferably an adhesive separator having an adhesive layer provided on at least one surface of the above base material portion and / or the heat-resistant layer. The adhesive layer is a layer containing a resin material having a melting point of 100 °C or less and adhesiveness (or adhesivity). Examples of the resin material that can be contained in the adhesive layer include fluorine resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE). The separator is preferably adhered and integrated with the positive electrode and / or the negative electrode via the adhesive layer. Thereby, the electrode body 20 can be made to stand on its own in a direction where the thickness direction Y coincides with the direction connecting the pair of long side walls 12b, and the position in the rectangular battery case 10 can be stabilized.

[0026] In this embodiment, when the charging rate of the electrode body 20 is set to a predetermined value of 0.2C or more, in a graph with "state of charge (SOC)" on the horizontal axis and "change in the thickness of the electrode body 20 (amount of expansion)" on the vertical axis, the SOC has a peak in the range of 80 to 90%. This range of SOC is an SOC region where metal precipitation (e.g., Li precipitation) is likely to occur. The above charging rate is more preferably 0.3C or more. From the perspective of suppressing the areal density of the negative electrode active material layer from becoming too large and realizing a high energy density, the above charging rate is preferably 3C or less, and more preferably 1C or less.

[0027] The peak that appears in the range of 80 to 90% of the SOC indicates that in this SOC region, the electrode body 20 expands rapidly, and the thickness of the electrode body 20 (the length in the thickness direction Y) changes significantly. The expansion of the electrode body 20 in this SOC region is preferably mainly due to the expansion of the negative electrode (e.g., the Si-containing material in the negative electrode active material layer). Note that "having a peak" means that there is a maximum value of the peak within the range of SOC 80 to 90%. The specific method of creating the above graph and the like will be described in detail in the examples described later.

[0028] As shown in FIG. 2, in the central portion (hatched portion) in the width direction X of the electrode body 20, a laminated portion is formed in which the positive electrode active material layer and the negative electrode active material layer are laminated in an insulated state. On the other hand, a part of the positive electrode current collector 21 (positive electrode current collector exposed portion) protrudes from the laminated portion at the left end in the width direction X of the electrode body 20. A positive electrode lead member 23 is attached to the positive electrode current collector exposed portion. Also, a part of the negative electrode current collector 22 (negative electrode current collector exposed portion) protrudes from the laminated portion at the right end in the width direction X of the electrode body 20. A negative electrode lead member 24 is attached to the negative electrode current collector exposed portion.

[0029] As shown in FIG. 1, the positive terminal 30 and the negative terminal 40 are arranged at both ends in the width direction X of the lid body 14. The positive terminal 30 and the negative terminal 40 project outside the rectangular battery case 10. Here, the positive terminal 30 and the negative terminal 40 project from the same surface (specifically, the lid body 14) of the rectangular battery case 10, respectively. However, the positive terminal 30 and the negative terminal 40 may project from different surfaces of the rectangular battery case 10, respectively. As shown in FIG. 2, the positive terminal 30 is electrically connected to the positive electrode of the electrode body 20 through the positive electrode lead member 23 inside the rectangular battery case 10. The negative terminal 40 is electrically connected to the negative electrode of the electrode body 20 through the negative electrode lead member 24 inside the rectangular battery case 10.

[0030] The electrolyte may be the same as the conventional one and is not particularly limited. The electrolyte is, for example, a non-aqueous liquid electrolyte (non-aqueous electrolyte solution) containing a non-aqueous solvent and a supporting salt. The non-aqueous solvent includes, for example, carbonates such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The supporting salt is, for example, a fluorine-containing lithium salt such as lithium hexafluorophosphate (LiPF6). However, the electrolyte may be in a solid state (solid electrolyte) and integrated with the electrode body 20.

[0031] As shown in FIG. 3(A), in the present embodiment, when the SOC is in the first SOC state of less than 60% (for example, when the SOC is in the state of 20 to 30%), in other words, when the electrode body 20 is in the state before significant expansion, there is a gap S between the flat portion 20f of the electrode body 20 and the long side wall 12b of the rectangular battery case 10 in the thickness direction Y. Thereby, even if the electrode body 20 expands with charging, the load on the rectangular battery case 10 can be reduced, and deformation or damage of the rectangular battery case 10 can be suppressed.

[0032] When the SOC is 0% (fully discharged state), the width of the gap S is preferably 5% or more, more preferably 6% or more of the total thickness of the negative electrode in the thickness direction Y. The width of the gap S can be 15% or less, 10% or less, or 8% or less of the total thickness of the negative electrode. Also, when the SOC is 0% (fully discharged state), the width of the gap S is preferably 2.5% or more, more preferably 3% or more of the thickness of the rectangular battery case 10 in the thickness direction Y. By satisfying at least one of the above, the load on the rectangular battery case 10 can be better reduced, and deformation or damage to the rectangular battery case 10 can be suppressed at a high level.

[0033] In this embodiment, the voltage defined as SOC 0% (fully discharged state) is 3V. Also, the SOC is more preferably the SOC at the first charge and discharge. In this case, the discharge (cut-off) voltage at the first discharge is 3V, and the discharge rate is preferably about 1 / 10C to 1 / 2C. Also, the "width of the gap S" refers to the total width when the gap S is provided on the front side and the rear side in the thickness direction Y as shown in Fig. 3(A). Also, the "total thickness of the negative electrode" can be obtained by multiplying the thickness per sheet of the negative electrode in the thickness direction Y (the total thickness of the negative electrode current collector 22 and the negative electrode active material layer) by the number of stacked sheets in the thickness direction Y.

[0034] When the secondary battery 100 is charged from the first SOC state, the electrode body 20 expands. Then, as shown in Fig. 3(B), when the SOC is in the range of 80 to 90% and reaches the second SOC state which is the peak apex of the above graph, in other words, when the electrode body is in the most expanded state, the gap between the flat portion 20f of the electrode body 20 and the long side wall 12b of the rectangular battery case 10 is eliminated in the thickness direction Y. The range where the SOC is 80% or more, especially the range where the SOC is 80 to 90%, is an SOC region where metal precipitation (e.g., Li precipitation) is likely to occur. By making the gap S between the electrode body 20 and the rectangular battery case 10 disappear at the timing when the electrode body 20 expands the most, metal precipitation (e.g., Li precipitation) can be effectively suppressed while suppressing deformation or damage to the rectangular battery case 10.

[0035] Incidentally, the charging rate from the first SOC state to the second SOC state is preferably 0.2C or more, more preferably 0.3C or more. The charging rate from the first SOC state to the second SOC state may be constant. Also, the charging rate in the range where the SOC is 80 - 90% is 0.2C or more, more preferably 0.3C or more. The charging rate is preferably 3C or less, and more preferably 1C or less.

[0036] In the range where the SOC is 80 - 90% (for example, the second SOC state), the electrode body 20 may press the rectangular battery case 10 with a weak force. The force (pressing force) with which the electrode body 20 presses the rectangular battery case 10 is preferably 1 MPa or less, and more preferably 0.4 MPa or less, for example, during the first charge. Thereby, deformation or damage of the rectangular battery case 10 can be suppressed at a high level, and the durability of the secondary battery 100 can be improved.

[0037] When the secondary battery 100 is further charged from the second SOC state, the electrode body 20 may contract. When the SOC is 100%, it is preferable that the electrode body 20 and the rectangular battery case 10 are not in contact. However, at SOC 100%, the electrode body 20 and the rectangular battery case 10 may be in contact.

[0038] <Control method (design method, manufacturing method) of the secondary battery 100> The secondary battery 100 can be realized by using, for example, an electrode body having the above-described configuration and controlling (designing or manufacturing) so that the gap S as described above is ensured when the SOC is 0% (the state of being discharged to 3V). For other configurations than the above, appropriate adjustment can be made as design matters for those skilled in the art. In an example of the control method (design method, manufacturing method), a secondary battery as described above may be preliminarily manufactured and tried to be controlled.

[0039] As a result, when the expansion of the electrode body 20 is insufficient (for example, the pressing force is insufficient) or when the timing of contact between the electrode body 20 and the rectangular battery case 10 is too late, for example, the electrode body 20 may be configured to expand more easily, or the gap S when the SOC is 0% (the state of discharging to 3V) may be reduced. More specifically, for example, the content of the Si-containing material contained in the negative electrode may be increased, the packing density of the negative electrode active material layer may be increased, or the size of the rectangular battery case 10 may be reduced.

[0040] On the other hand, when the electrode body 20 expands too much (for example, the pressing force is too strong) or when the timing of contact between the electrode body 20 and the rectangular battery case 10 is too early, for example, the electrode body 20 may be configured to expand less easily, or the gap S when the SOC is 0% (the state of discharging to 3V) may be increased. More specifically, for example, the content of the Si-containing material contained in the negative electrode may be reduced, the packing density of the negative electrode active material layer may be reduced, or the size of the rectangular battery case 10 may be increased.

[0041] <Usage of the secondary battery 100> The secondary battery 100 can be used for various applications. For example, it can be suitably used as a power source (driving power source) for a motor mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited. For example, plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), battery electric vehicles (BEV), etc. can be mentioned.

[0042] Hereinafter, several examples related to the present invention will be described, but the present invention is not intended to be limited to such examples.

[0043] 〔Test I: Measurement of change in thickness of electrode body〕 First, a positive electrode active material layer (packing density: 3.5 g / cm) containing lithium nickel cobalt manganese composite oxide (LiNiCoMnO2) as a positive electrode active material3 ) was used to fabricate a positive electrode. Also, a negative electrode active material layer containing graphite as the negative electrode active material (90% by mass of the total negative electrode active material) and Li-doped SiOx (10% by mass of the total negative electrode active material, equivalent to 3.2% in terms of Si), and SBR as a binder (packing density: 1.6 g / cm 3 ) was used to fabricate a negative electrode. Next, these positive and negative electrodes were cut into rectangular shapes and alternately laminated via a separator to fabricate an electrode body. As the separator, one with an adhesive layer containing PVdF applied on both sides (total thickness: 20 μm) was used. Then, the above electrode body was pressed to adhesively bond and integrate the positive electrode, separator, and negative electrode with each other.

[0044] Next, a bag-shaped aluminum laminate film was prepared as the battery case. Then, the electrode body and a non-aqueous electrolyte in an amount sufficient for the electrode body to be immersed were placed in the laminate film and sealed. Thereby, a preliminary laminate cell for testing was fabricated.

[0045] Next, a metal plate (SUS plate) with an area covering the fabricated laminate cell was placed on the laminate cell, and a thickness gauge was placed thereon. Then, charging and discharging were performed at a charging rate of 1 / 3C or 1 / 20C within a voltage range where the state of charge (SOC) was 20 - 95% or 0 - 100%, and the swelling amount (change in thickness) of the laminate cell at this time was measured. Since the thickness of the aluminum laminate film is on the order of several tens of microns and is extremely thin compared to the thickness in the stacking direction of the electrode body (several tens of mm), here, the swelling amount of the cell ≒ the change in the thickness of the electrode body is regarded. The results are shown in FIGS. 4 and 5. In FIGS. 4 and 5, the change in the thickness of the electrode body is represented as a ratio (thickness change rate, %) when the total thickness of the negative electrode is taken as 100%.

[0046] Figure 4 is a graph showing the relationship between the SOC and the thickness change rate of the electrode body when the charging rate is 1 / 3C (about 0.33C, that is, a predetermined value of 0.2C or more). As shown in Figure 4, when charging the region of SOC 80 to 90% at a charging rate of 0.2C or more (preferably 0.3C or more), significant expansion occurred in the electrode body around SOC 80%, and a peak having a maximum value was confirmed around SOC 85%. Further, since the expansion of the electrode body decreased when exceeding SOC 85%, it can be seen that it was temporary.

[0047] Although not particularly limitedly construed, according to the study of the present inventors, this temporary expansion of the electrode body is considered to be caused by the negative electrode, particularly silicon (Si) contained in the negative electrode. Specifically, it is considered to be caused by the resistance difference between Si and graphite in the negative electrode active material layer, as well as the reaction unevenness of Si particles. That is, in the negative electrode, expansion temporarily occurred due to the progress of the reaction in the portion that easily reacts around SOC 80%, and then it is considered that the temporary expansion subsided due to the movement and homogenization of Li ions by the potential difference in the negative electrode. In particular, since Si has slow diffusion in the solid and is likely to cause reaction unevenness, it is considered that such a peak appeared prominently.

[0048] Figure 5 is a graph showing the relationship between the SOC and the thickness change rate of the electrode body when the charging rate is 1 / 20C (0.05C, that is, less than 0.2C). As shown in Figure 5, when charging the region of SOC 80 to 90% at a charging rate of less than 0.2C, no peak was observed in the region of SOC 80 to 90%. Figure 6 is a graph summarizing the relationship between the charging rate and the thickness change rate of the electrode body. As shown in Figure 6, it was found that the above-mentioned temporary expansion of the electrode body specifically appears when the charging rate is set to a predetermined value of 0.2C or more, for example, 0.2 to 1C.

[0049] FIG. 7 is a graph further investigating the relationship between the amount of Si (in terms of Si conversion) in the negative electrode active material and the SOC at which the temporary expansion of the electrode body begins. In the graph, the "positive to negative electrode ratio" represents the theoretical capacity ratio of the negative electrode to the positive electrode. From FIG. 7, when the theoretical capacity ratio is 1.0 to 1.2, for example, when adding Si to the negative electrode to suppress Li deposition in the range of 80 to 90% of SOC due to the expansion of the negative electrode, it is found that the amount of Si should be 2% or more, and more preferably 3% or more.

[0050] As described above, in this test example, it was found that by mixing an appropriate amount of the Si material that causes temporary expansion and temporarily rapidly expanding in the range of SOC 80 to 90% where Li deposition is likely to occur, the gap between the electrode body and the rectangular battery case can be eliminated. As a result, Li deposition can be suppressed while ensuring a gap when SOC is less than 60%.

[0051] 〔Test II: Specific Examples〕 Based on the preliminary results of Test I described above, several secondary batteries were controlled by using a rectangular battery case and adjusting the content of the Si-containing material in terms of Si conversion, the packing density of the negative electrode active material layer, and the width of the gap. The results are shown in Table 1 below. Regarding matters other than those shown in Table 1, the configuration is the same as that described in Test I.

[0052]

Table 1

[0053] As shown in Table 1, in Comparative Example 1, there was no gap between the electrode body and the rectangular battery case in any of the SOC states including the first SOC state and the second SOC state. Therefore, in Comparative Example 1, the reaction force of the cell (the reaction force at the end of life) tended to increase, and deformation of the battery case, which is one of the problems of the present application, was likely to occur. In Comparative Example 2, since the gap in the first SOC state was too large, in the second SOC state, for example, the charge-discharge reaction became non-uniform, Li deposition occurred, and the deposited Li accumulated in the gap. As a result, when cycle charge-discharge was repeated, the reaction force of the cell finally reached a high value. In Comparative Example 3, the gap in the first SOC state was the same as in Examples 1 to 5, but since the packing density of the negative electrode active material layer was low, the expansion in the second SOC state was small. As a result, as in Comparative Example 2, Li deposition occurred in the second SOC state.

[0054] On the other hand, in Examples 1 to 5, a gap was secured between the electrode body and the rectangular battery case in the first SOC state, and there was no such gap in the second SOC state, and Li deposition was also suppressed.

[0055] As described above, specific aspects of the technology disclosed herein include those described in the following respective items. Item 1: A method for controlling a secondary battery, comprising a flat electrode body including a positive electrode and a negative electrode, and a rectangular battery case for housing the electrode body, wherein the electrode body has a peak in a range where the state of charge (SOC) is 80% or more and 90% or less on a graph in which the horizontal axis is the state of charge (SOC) and the vertical axis is the change in the thickness of the electrode body with a charging rate set to a predetermined value of 0.2C or more, and when the state of charge (SOC) is in a range of less than 60%, there is a gap between the electrode body and the rectangular battery case in the thickness direction of the electrode body, and the gap between the electrode body and the rectangular battery case disappears at the state of charge (SOC) at the peak apex. Item 2: The control method of the secondary battery according to Item 1, wherein the negative electrode contains graphite and an Si-containing material as negative electrode active materials, and when the total amount of the negative electrode active materials is 100% by mass, the Si-containing material is 3% by mass or more in terms of Si. Item 3: The control method of the secondary battery according to Item 1 or Item 2, wherein when the state of charge (SOC) is 0%, in the thickness direction, the width of the gap between the electrode body and the rectangular battery case is 5% or more of the total thickness of the negative electrode. Item 4: The control method of the secondary battery according to any one of Items 1 to 3, wherein when the state of charge (SOC) is 0%, in the thickness direction, the width of the gap between the electrode body and the rectangular battery case is 2.5% or more of the thickness of the rectangular battery case.

[0056] As described above, the embodiments of the present invention have been described, but the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the content disclosed in this specification and common general knowledge in the art. The technology described in the claims includes various modifications and changes of the above-exemplified embodiments.

Explanation of Reference Numerals

[0057] 10 Rectangular battery case 20 Electrode body 100 Secondary battery

Claims

1. A method for controlling a secondary battery, comprising: a flat electrode body including a positive electrode and a negative electrode; and a rectangular battery case for housing the electrode body, wherein when the charging rate is set to a predetermined value of 0.2C or more, in a graph plotting "state of charge (SOC)" on the horizontal axis and "change in thickness of the electrode body" on the vertical axis, the electrode body has a peak with a maximum value in the range where the state of charge (SOC) is 80% or more and 90% or less. Here, the "change in thickness of the electrode body" is the change in the amount of expansion from before charging until the state of charge (SOC) on the horizontal axis is reached, when the state of charge (SOC) is in the range of less than 60%, there is a gap between the electrode body and the rectangular battery case in the thickness direction of the electrode body, at the state of charge (SOC) at the apex of the peak, the gap between the electrode body and the rectangular battery case is eliminated, A method for controlling a secondary battery.

2. The negative electrode contains graphite and an Si-containing material as negative electrode active materials, when the total amount of the negative electrode active material is 100% by mass, the Si-containing material is 3% by mass or more in terms of Si, The method for controlling a secondary battery according to Claim 1.

3. when the state of charge (SOC) is 0%, the width of the gap between the electrode body and the rectangular battery case in the thickness direction is 5% or more of the total thickness of the negative electrode, The method for controlling a secondary battery according to Claim 1 or 2.

4. when the state of charge (SOC) is 0%, the width of the gap between the electrode body and the rectangular battery case in the thickness direction is 2.5% or more of the thickness of the rectangular battery case, The method for controlling a secondary battery according to Claim 1 or 2.

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