Method for controlling charge and discharge of lithium secondary battery

Abstract

To provide a charging and discharging control method that can achieve high energy density for a long time.SOLUTION: An electrode body includes a positive electrode and a negative electrode. The positive electrode contains a lithium transition metal complex oxide. The lithium transition metal complex oxide contains Ni. The molar ratio of Ni to all the transition metal elements is 70 mol% or more. The negative electrode contains graphite and a Si-containing material. When the total mass of the graphite and the Si-containing material is 100 mass%, the ratio of the Si is 20 mass% or more and 70 mass% or less. When Li is occluded in the Si at charging to become Li-Si alloy (LixSi), charging is controlled so that the maximum value of the molar ratio X of Li to the Si becomes 1.3 or more and 1.9 or less. In the control range for the molar ratio X, the ratio of the capacitance of the negative electrode to the capacitance of the positive electrode is 1.0 or more and 1.3 or less. A charging and discharging control method for such a lithium secondary battery is provided.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a method for controlling charge and discharge of a lithium secondary battery.

Background Art

[0002] In recent years, secondary batteries containing Si-containing materials in the negative electrode have been known for the purpose of increasing the capacity and the like. While Si-containing materials have a high theoretical capacity, they have a large expansion and contraction (volume change) associated with charge and discharge cycles, and a problem is a decrease in the capacity retention rate associated with charge and discharge cycles (see Patent Documents 1 to 4). In relation to this, Patent Document 1 describes that a lithium secondary battery containing a negative electrode active material represented by Li X Si can suppress a decrease in the capacity retention rate associated with charge and discharge cycles by charging and discharging in a range where the value of X is 0 ≦ X ≦ 2.33.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, further increase in energy density and improvement in cycle characteristics have been desired for lithium secondary batteries. The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for controlling charge and discharge of a lithium secondary battery in which volume change associated with charge and discharge cycles is suppressed and high energy density can be exhibited over a long period of time.

Means for Solving the Problems

[0005] As a result of intensive studies by the present inventors, as factors causing large volume changes in a lithium secondary battery containing a Si-containing material in the negative electrode, in addition to the large expansion and contraction when Si itself reacts with Li, the polarization of the charge-discharge voltage is large and the solid-state diffusion of Li is slow, so that it has been newly found that unevenness in the reaction easily occurs. Therefore, it was conceived to charge and discharge a secondary battery in a range of use of Si in which both expansion / contraction and polarization are suppressed to a small level, as a battery configuration capable of obtaining a high energy density. Then, through repeated intensive studies, the present invention was completed.

[0006] According to the present invention, there is provided a charge-discharge control method for a lithium secondary battery, which includes an electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte, the positive electrode includes a lithium transition metal composite oxide, the lithium transition metal composite oxide includes Ni, and the molar ratio of Ni to all transition metal elements is 70 mol% or more, the negative electrode includes graphite and a Si-containing material, and when the total mass of the graphite and the Si-containing material is 100% by mass, the ratio of Si contained in the Si-containing material is 20% by mass or more and 70% by mass or less, and when Li is occluded in the Si to form a Li—Si alloy (LixSi) during charging, charge control is performed such that the maximum value of the molar ratio X of Li to Si is 1.3 or more and 1.9 or less, and in the control range of the molar ratio X, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.0 or more and 1.3 or less.

[0007] According to the present invention, by increasing the content ratio of Si in the negative electrode to 20% by mass or more, setting the range of use of Si to up to a molar ratio of Si of 1.3 to 1.9 (up to equivalent to Li 12 Si7), and designing the positive electrode in accordance with the capacity of the negative electrode, it is possible to realize a high energy density equal to or higher than that in the case of using only graphite, for example. Further, for example, compared with Patent Document 1, volume changes associated with charge-discharge cycles can be suppressed, and a high energy density can be exhibited over a long period of time.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

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Figure 7

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Figure 9

Mode for Carrying Out the Invention

[0009] Hereinafter, preferred embodiments of the technology disclosed herein will be described with reference to the drawings. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present invention (for example, general configurations and manufacturing processes of batteries that do 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, including so-called storage batteries (chemical batteries) such as lithium secondary batteries and capacitors (physical batteries) such as electric double layer capacitors. Also, in this specification, the term "lithium secondary battery" refers to all secondary batteries in which lithium ions (Li ions) are used as electrolyte ions and charging and discharging are achieved by the movement of charges associated with Li ions between the positive and negative electrodes. In addition, in this specification, the notation "A~B" indicating a range shall include the meanings of "greater than A" and "less than B" in addition to the meaning of "A or more and B or less".

[0011] FIG. 1 is a perspective view of a lithium secondary battery 100 (hereinafter sometimes simply referred to as "battery 100"). FIG. 2 is a schematic longitudinal sectional view taken along line II-II of FIG. 1. In the following description, the symbols L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, upper, and lower, respectively, and the symbols X, Y, and Z in the drawings represent the long side direction of the battery 100, the short side direction orthogonal to the long side direction, and the vertical direction, respectively. The short side direction Y is an example of the stacking direction. However, these are merely directions for convenience of explanation and do not limit the installation form of the battery 100 in any way.

[0012] As shown in FIG. 2, the battery 100 includes an exterior body 10, an electrode body 20, a positive electrode terminal 30, a negative electrode terminal 40, and a non-aqueous electrolyte (not shown). The battery 100 is configured such that the electrode body 20 and the non-aqueous electrolyte are housed inside the exterior body 10.

[0013] The exterior body 10 is a housing that accommodates the electrode body 20 and a non-aqueous electrolyte (not shown). As shown in FIG. 1, the exterior body 10 is formed here in a flat and bottomed rectangular parallelepiped shape (rectangular shape). The battery 100 is preferably a rectangular secondary battery including the rectangular exterior body 10. However, the shape of the exterior body 10 is not limited to rectangular, and may be any shape such as a cylinder or a bag shape. The material of the exterior body 10 may be the same as those conventionally used, and there is no particular limitation. The exterior body 10 is composed of, for example, a lightweight and highly thermally conductive metal material such as aluminum, an aluminum alloy, or stainless steel. The exterior body 10 may be a so-called laminate film, for example, an aluminum laminate film having a metal layer containing aluminum and a fusion layer containing resin.

[0014] As shown in FIG. 2, the exterior body 10 includes here 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 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. The exterior body 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 exterior body 10 is hermetically sealed.

[0015] The electrode body 20 has a positive electrode and a negative electrode (not shown). The electrode body 20 may be, for example, a laminated electrode body in which a rectangular (typically rectangular) positive electrode and a rectangular (typically rectangular) negative electrode are stacked in the stacking direction with a separator interposed therebetween, or a wound electrode body in which a strip-shaped positive electrode and a strip-shaped negative electrode are laminated with a strip-shaped separator interposed therebetween and wound around a winding axis.

[0016] 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, stainless steel, etc. The density of the positive electrode (average filling density of the positive electrode active material layer) is 3.2 g / cm 3 or more is preferable. By setting the density of the positive electrode to a predetermined value or more, it becomes easy to design the capacity of the positive electrode to match the high-capacity negative electrode. In the range of use of Si described later, the capacity of the positive electrode is preferably 180 mAh / g or more.

[0017] The positive electrode active material layer contains a positive electrode active material capable of reversibly occluding and releasing charge carriers. The positive electrode active material contains a high-Ni-containing lithium transition metal composite oxide, that is, the lithium transition metal composite oxide contains Ni as an essential component, and the molar ratio of Ni to all transition metal elements is 70 mol% or more. The high-Ni-containing lithium transition metal composite oxide is preferably a layered compound, and Li a MO2 (where a satisfies 0 < a < 2, and M is one or more transition metals including Ni). It is more preferably a compound represented by this formula. In the high-Ni-containing lithium transition metal composite oxide, the molar ratio of Ni to all transition metal elements is more preferably 80 mol% or more, further preferably 85 mol% or more, and particularly preferably 90 mol% or more. The reason for this will be described with reference to Fig. 3.

[0018] Figure 3 is a graph showing the change in the c-axis length of the Ni-containing lithium transition metal composite oxide during charge and discharge. As shown in Figure 3, according to the study by the present inventors, in the case of a high-Ni-containing lithium transition metal composite oxide, that is, when the molar ratio of Ni is 70 mol% or more, further 80 mol% or more (Ni80 in Figure 3), when the potential based on lithium metal becomes 4.1 V (vs. Li / Li+) or more, a large shrinkage occurs in the c-axis. Thereby, the expansion of the negative electrode described later can be preferably absorbed, and the volume change of the electrode body 20 can be alleviated. This tendency does not occur when the molar ratio of Ni is 57 mol% (Ni57 in Figure 3). Further, when the molar ratio of Ni is 90 mol% or more (Ni92 in Figure 3), it becomes more remarkable. This is because in a composition with a high Ni ratio, the H3 structure appears at the above potential, and a large shrinkage occurs at that time.

[0019] When the total mass of the positive electrode active material is 100% by mass, the proportion of the high-Ni-containing lithium transition metal composite oxide is preferably 60% by mass or more, more preferably 80% by mass or more, and still more preferably 85% by mass or more.

[0020] The positive electrode active material layer may contain a positive electrode active material other than the high-Ni-containing lithium transition metal composite oxide, and optional components other than the positive electrode active material, for example, a conductive material, a positive electrode binder, various additive components, etc. As the conductive material, for example, a carbon material such as acetylene black (AB) can be used. The positive electrode active material layer preferably contains a positive electrode binder. As the positive electrode binder, for example, a fluororesin such as polyvinylidene fluoride (PVdF) can be used, and it is preferably included PVdF among them.

[0021] As shown in Figure 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 22 is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The density of the negative electrode (average packing density of the negative electrode active material layer) is typically smaller than that of the positive electrode active material layer, and 1.4 g / cm 3 or more is preferable. Thereby, the capacity design of a high-capacity negative electrode becomes easy.

[0022] The negative electrode active material layer contains a negative electrode active material capable of reversibly occluding and releasing charge carriers. The negative electrode active material contains graphite and a compound containing silicon (Si-containing material). As the Si-containing material, for example, compounds such as SiC and SiOx that have been conventionally used for this type of application can be used without particular limitation. Among them, SiC is preferable. The graphite may be in a state of being composited with the Si-containing material such as SiC, or may be separate from the Si-containing material. In the present embodiment, when the total mass of the graphite and the Si-containing material is 100% by mass, the proportion of Si contained in the Si-containing material is 20 to 70% by mass. The reason for this will be described with reference to Table 1 and FIG. 4.

[0023] Table 1 is a table summarizing the Si content ratio, capacity, and energy density. FIG. 4 is a graph showing the relationship between the Si content ratio and the energy density. As shown in Table 1 and FIG. 4, according to the study by the present inventor, by increasing the proportion of Si to 20% by mass or more, even in the charge-discharge control in which the use range of Si described later is limited, it is possible to achieve an energy density equal to or higher than that when only graphite is used (capacity: 372 mAh / g, energy density: 1.38 Wh / g - negative electrode active material). The proportion of Si is 70% by mass from the viewpoint of suppressing the expansion of the negative electrode, and may be generally 50% by mass or less, for example, 30% by mass or less.

[0024]

Table 1

[0025] The negative electrode active material layer may contain a negative electrode active material other than graphite and the Si-containing material, and optional components other than the negative electrode active material, such as a conductive material, a negative electrode binder, various additive components, etc. The negative electrode active material layer preferably contains a conductive material. As the conductive material, for example, carbon black such as acetylene black (AB) and ketjen black, carbon nanotubes (CNT), carbon fibers such as vapor-grown carbon fibers (VGCF), activated carbon, carbon materials such as graphite can be preferably used, and among them, it is preferable to contain at least one of carbon black, carbon nanotubes, and carbon fibers. Thereby, the conductivity of the Si-containing material can be preferably increased to reduce the internal resistance, and the volume change can be alleviated.

[0026] The negative electrode active material layer preferably contains a negative electrode binder. As the negative electrode binder, for example, a cellulose-based binder, a polyacrylic-based binder, a rubber-based binder, etc. can be used. As these negative electrode binders, those conventionally used in lithium secondary batteries can be used without particular limitation. The negative electrode active material layer preferably contains at least one of a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder, and more preferably contains all of a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder. Further, when the total solid content of the negative electrode is 100% by mass, the proportions of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are each preferably 0.5 to 3% by mass. Thereby, it becomes easier to follow the expansion and contraction of the Si-containing material accompanying charge and discharge, and the cycle characteristics can be improved.

[0027] The cellulose-based binder includes all of linear polymers and their derivatives containing at least β-glucose as a repeating unit. Typically, they are compounds and their derivatives in which part or all of the hydroxy groups in the β-glucose structure, which is the repeating unit, are substituted with alkoxy groups. As the cellulose-based binder, for example, carboxymethyl cellulose (CMC), carboxyethyl cellulose, hydroxyethyl cellulose, etc. can be used, and among them, CMC is preferable.

[0028] The polyacrylic binder encompasses the entirety of high molecular compounds formed by the polymerization of at least one monomer selected from the group consisting of acrylic acid esters, methacrylic acid esters, and acrylonitrile. As the polyacrylic binder, for example, polymers (homopolymers or copolymers) containing units of acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, acrylate salts, or methacrylate salts (hereinafter referred to as "acrylic units") can be used, and among them, polyacrylic acid (PAA) is preferred. As the copolymer, a copolymer containing acrylic units and styrene units, a copolymer containing acrylic units and silicon units, etc. can be used. By including a polyacrylic binder in the negative electrode binder, the adhesiveness can be improved, and the negative electrode can be smoothly expanded and contracted during charge and discharge.

[0029] The rubber binder encompasses all rubber-like polymer materials containing carbon-carbon double bonds in the main chain. As the rubber binder, for example, styrene-butadiene rubber (SBR), butyl rubber (IIR), butadiene rubber (BR), acrylonitrile-butadiene rubber (NBR), etc. can be used. By including a rubber binder in the negative electrode binder, the tensile strength and elastic modulus can be improved, and especially when used in combination with a polyacrylic binder, cycle degradation can be effectively suppressed. Also, the increase in internal resistance can be suppressed, and the energy density and cycle characteristics can be improved.

[0030] 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 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. 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 or PP and a heat-resistant layer provided on at least one surface of the base material portion.

[0031] As shown in FIG. 2, a stacked portion in which a positive electrode active material layer and a negative electrode active material layer are stacked in an insulated state is formed at the central portion (hatched portion) in the long side direction X of the electrode body 20. On the other hand, a part of the positive electrode current collector 21 (positive electrode current collector exposed portion) protrudes from the stacked portion at the left end portion in the long side direction X of the electrode body 20. A positive electrode lead member 23 is attached to the positive electrode current collector exposed portion. Further, a part of the negative electrode current collector 22 (negative electrode current collector exposed portion) protrudes from the stacked portion at the right end portion in the long side direction X of the electrode body 20. A negative electrode lead member 24 is attached to the negative electrode current collector exposed portion.

[0032] As shown in FIG. 1, the positive electrode terminal 30 and the negative electrode terminal 40 are disposed at both end portions in the long side direction X of the lid body 14. The positive electrode terminal 30 and the negative electrode terminal 40 protrude to the outside of the exterior body 10. Here, the positive electrode terminal 30 and the negative electrode terminal 40 protrude from the same surface (specifically, the lid body 14) of the exterior body 10, respectively. However, the positive electrode terminal 30 and the negative electrode terminal 40 may protrude from different surfaces of the exterior body 10, respectively. As shown in FIG. 2, the positive electrode terminal 30 is electrically connected to the positive electrode of the electrode body 20 via the positive electrode lead member 23 inside the exterior body 10. The negative electrode terminal 40 is electrically connected to the negative electrode of the electrode body 20 via the negative electrode lead member 24 inside the exterior body 10.

[0033] The non-aqueous electrolyte may be the same as the conventional one and is not particularly limited. The non-aqueous electrolyte is, for example, a liquid electrolyte (non-aqueous electrolyte solution) containing a non-aqueous solvent and a supporting salt. The non-aqueous solvent contains, 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 non-aqueous electrolyte may be in a solid state (solid electrolyte) and integrated with the electrode body 20.

[0034] When charging the battery 100 as described above, Li is occluded in Si at the negative electrode, and Si becomes a Li-Si alloy (LixSi). FIG. 5 is a graph showing the relationship between the Li insertion capacity and the expansion ratio (volume change ratio) of the Li-Si alloy. As shown in FIG. 5, for Si, the larger the molar ratio X of Li, the higher the Li insertion capacity, but on the other hand, the expansion ratio also becomes larger. Therefore, in the present embodiment, the usage range of Si during charging is controlled such that the maximum value of the molar ratio X of Li to Si is 1.3 to 1.9 (up to equivalent to Si7). Thereby, the volume change associated with the charge-discharge cycle can be suppressed, and a high energy density can be exhibited over a long period. From the viewpoint of exhibiting the above effects at a high level, the maximum value of the molar ratio X is more preferably in the range of 1.5 to 1.8. From the viewpoint of increasing the energy density, the maximum value of the molar ratio X may be 1.6 or more, 1.7 or more. 12 When discharging the battery 100, Li is released from the Li-Si alloy (LixSi) at the negative electrode. The usage range of Si during discharging may be controlled such that the minimum value of the molar ratio X of Li to Si is 0 (up to equivalent to Si).

[0035] Charge and discharge are preferably voltage-controlled. FIG. 6 is a graph showing the relationship between the differential capacity (dQ / dV) and the voltage of a secondary battery including a Si-containing material at the negative electrode. "Differential capacity (dQ / dV)" indicates the amount of capacity change per unit voltage. The peak where dQ / dV is increasing indicates that charge and discharge are occurring more actively in a certain voltage range compared to other voltage regions for some reason. Based on such findings, in voltage control, for example, within the voltage range of the arrow in FIG. 6, the charge-discharge voltage may be set so as not to exceed the peak where dQ / dV increases (reaction peak in FIG. 6). The charge-discharge rate (C rate) is preferably 0.2C or more, more preferably 0.3C or more. The charge-discharge rate is preferably 2C or less, more preferably 1C or less.

[0036]

[0037] In addition, when the molar ratio X of Li to Si is 1.3 to 1.9, the negative electrode capacity can be simply obtained by the theoretical capacity with respect to the Si content (see Table 1). More specifically, a half cell using a Si-containing material with a Li metal counter electrode is fabricated and subjected to the first charge and discharge. When representing the dQ / dV curve (see Fig. 6) or the dV / dQ curve, the peak of Si→Li2Si and the peak of Li2Si→Li 3.25 are separated into two. Based on this, the capacity at x = 2.0 (Li2Si) (the range of the arrow in Fig. 6) is calculated, and the negative electrode capacity (the first negative electrode discharge capacity) when the molar ratio X is 1.3 to 1.9 can also be obtained from the capacity ratio.

[0038] In this embodiment, within the above control range of the molar ratio X, the ratio of the negative electrode capacity to the positive electrode capacity (positive-negative electrode capacity ratio) is 1.0 to 1.3. Increasing the areal density of the negative electrode to increase the positive-negative electrode capacity ratio will result in a decrease in the energy density. Therefore, the positive-negative electrode capacity ratio should be 1.3 or more. Also, when the positive-negative electrode capacity ratio is less than 1.0, Li precipitation is likely to occur and the cycle characteristics deteriorate. Therefore, the positive-negative electrode capacity ratio should be 1.0 or more. From the perspective of achieving the above effects at a high level, the positive-negative electrode capacity ratio is more preferably 1.0 to 1.2. Considering, for example, the change in the capacity ratio during high-current charge and discharge, the positive-negative electrode capacity ratio is preferably 1.05 or more, and more preferably 1.1 or more.

[0039] The positive-negative electrode capacity ratio can be obtained by separately determining the capacity of the positive electrode and the capacity of the negative electrode and using the following formula: negative electrode capacity÷positive electrode capacity. The capacity of the positive electrode is determined by the capacity (the first positive electrode charge capacity) when a half cell with a Li metal counter electrode is fabricated and the first charge is performed within the voltage range corresponding to the full cell (for example, 2.5 to 4.3V). The capacity of the negative electrode is obtained by summing the above-mentioned Si usage capacity (the first negative electrode discharge capacity when the molar ratio X of Li to Si is 1.3 to 1.9) and the capacity of graphite within the Si usage voltage range (the control range of the molar ratio X). When the irreversible capacity of the negative electrode charge and discharge is more than that of the positive electrode, it is preferable to subtract the irreversible capacity before obtaining the value.

[0040] The 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 a vehicle such as a passenger car or a truck. The type of vehicle is not particularly limited, and examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

[0041] Hereinafter, several embodiments of the present invention will be described, but the present invention is not intended to be limited to such embodiments.

[0042] First, a positive electrode including a Ni-containing lithium transition metal composite oxide as a positive electrode active material and PVdF as a positive electrode binder was prepared. The density of the positive electrode active material layer was 3.5 g / cm 3 ). The Ni-containing lithium transition metal composite oxide had the molar ratio (%) of Ni to all transition metal elements as shown in Table 2. Also, a negative electrode including the negative electrode active material shown in Table 2, CMC, SBR, and PAA as negative electrode binders, and carbon black as a conductive material was prepared. The density of the negative electrode active material layer was 1.6 g / cm 3 ). The ratio of Si in the entire negative electrode active material was as shown in Table 2. Next, these positive and negative electrodes were cut into rectangular shapes and alternately laminated via a separator to prepare an electrode body having the positive-to-negative electrode capacity ratio shown in Table 2.

[0043] Next, a bag-shaped aluminum laminate film was prepared as a 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 laminate cell was produced.

[0044] <Evaluation of Initial Characteristics> Next, a metal plate (SUS plate) covering an area of the fabricated laminate cell was placed on the laminate cell, and a thickness gauge was placed thereon. Then, the first charge and discharge were performed at a charge and discharge rate of 0.2C so that the molar ratio X of Li to Si was within the range shown in Table 2. At this time, the energy density and the expansion rate (rate of change in thickness) of the laminate cell were measured. For Comparative Example 6, charge and discharge were performed in the same voltage range as in Example 1. The results are shown in Table 2. In Table 2, the energy density is expressed per unit mass of the negative electrode active material. The expansion rate is that during charging and is expressed as a percentage based on the thickness before charging (100%).

[0045] As shown in Table 2, Comparative Example 1 with a low Si ratio of 10% by mass in the negative electrode active material, Comparative Example 2 with a narrow Si usage range of 0 ≦ x ≦ 1.2, Comparative Example 4 with a high positive / negative electrode capacity ratio of 1.4, and Comparative Example 5 with a low Ni molar ratio of 57 mol% had lower energy densities than Comparative Example 6 using only graphite as the negative electrode active material. Also, Comparative Example 3 with a wide Si usage range of 0 ≦ x ≦ 2.0 and Comparative Example 6 using only graphite as the negative electrode active material had high expansion rates. The reason for the high expansion rate in Comparative Example 6 (graphite 100%) is considered that graphite, like Si, has a Stage structure and undergoes expansion and contraction, although not as much as Si except for the intermediate Stage 2.

[0046] <Evaluation of Cycle Characteristics> Next, a charge and discharge cycle test of 200 cycles was performed at a charge and discharge rate of 0.2C, and the cycle expansion rate and the capacity retention rate were measured. The cycle expansion rate is the slope of the expansion amount (% / Ah) when the horizontal axis is √cycle. The results are shown in Table 2. Also, as an example, FIGS. 7 to 9 show the results up to 15 cycles. FIG. 7 is a graph showing the change in the thickness of the cells of Example 1 and Comparative Example 6 (graphite 100%). FIG. 8 is a graph showing the change in the thickness of the cell of Comparative Example 6 (Si usage range: 0 ≦ x ≦ 2.0). FIG. 9 is a graph showing the change in the capacity and the capacity retention rate of the cells of Example 1 and Comparative Example 6 (graphite 100%). As described above, for Comparative Example 6, charge and discharge were performed in the same voltage range as in Example 1.

[0047] As shown in Table 2 and FIG. 7, Comparative Example 6 using only graphite as the negative electrode active material had a larger slope of the graph than Example 1 containing 30% by mass of Si, that is, a relatively large cycle expansion rate. Also, as shown in Table 2, FIG. 7, and FIG. 8, Comparative Example 3 in which charge-discharge cycles were performed with the Si usage range set to 0 ≦ x ≦ 2.0 (up to the equivalent of Li 13 Si4) had a larger slope of the graph than Example 1 in which charge-discharge cycles were performed with the Si usage range set to 0 ≦ x ≦ 1.7 (up to the equivalent of Li 12 Si7), that is, a relatively large cycle expansion rate. Further, as shown in Table 2 and FIG. 9, Comparative Example 6 (100% graphite) had a relatively low capacity retention rate compared to Example 1.

[0048] For these Comparative Examples 1 to 6, in Example 1, that is, when the molar ratio of Ni was 70 mol% or more (here 85 mol%), the proportion of Si was 20% by mass or more (here 30% by mass), charge-discharge cycles were performed with the Si usage range set to 0 ≦ x ≦ 1.7 (up to the equivalent of Li 12 Si7), and the positive-negative electrode capacity ratio was 1.0 or more (here 1.1), the battery cell had a smaller cycle expansion rate and an excellent capacity retention rate despite having a larger absolute capacity than Comparative Example 6 (100% graphite). Such results indicate the significance of the technology disclosed herein.

[0049]

Table 2

[0050] As described above, specific embodiments of the technology disclosed herein include those described in the following sections. Item 1: A charge-discharge control method for a lithium secondary battery, comprising an electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains a lithium transition metal composite oxide, the lithium transition metal composite oxide contains Ni, and the molar ratio of Ni to all transition metal elements is 70 mol% or more, the negative electrode contains graphite and a Si-containing material, and when the total mass of the graphite and the Si-containing material is 100% by mass, the proportion of Si contained in the Si-containing material is 20% by mass or more and 70% by mass or less, and when Li is occluded in the Si to form a Li-Si alloy (LixSi) during charging, charge control is performed such that the maximum value of the molar ratio X of Li to Si is 1.3 or more and 1.9 or less, and in the control range of the molar ratio X, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.0 or more and 1.3 or less. Item 2: The negative electrode contains at least one of a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder as a negative electrode binder. The charge-discharge control method according to Item 1. Item 3: The negative electrode binder contains a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder, and when the total solid content of the negative electrode is 100% by mass, the proportions of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are each 0.5% by mass or more and 3% by mass or less. The charge-discharge control method according to Item 1 or Item 2. Item 4: The negative electrode contains at least one of carbon black, carbon nanotubes, and carbon fibers as a conductive material. The charge-discharge control method according to any one of Items 1 to 3. Item 5: The density of the positive electrode is 3.2 g / cm 3 or more. The charge-discharge control method according to any one of Items 1 to 4. Item 6: The density of the negative electrode is 1.4 g / cm 3 or more. The charge-discharge control method according to any one of Items 1 to 5.

[0051] The embodiments of the present invention have been described above, 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.

Description of Reference Numerals

[0052] 10 Exterior body 20 Electrode body 100 Battery (lithium secondary battery)

Claims

1. An electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains a lithium transition metal composite oxide, the lithium transition metal composite oxide contains Ni, and the molar ratio of Ni to all transition metal elements is 70 mol% or more, the negative electrode contains graphite and a Si-containing material, when the total mass of the graphite and the Si-containing material is 100% by mass, the ratio of Si contained in the Si-containing material is 20% by mass or more and 70% by mass or less, when Li is occluded in the Si during charging to form a Li-Si alloy (LixSi), charge control is performed such that the maximum value of the molar ratio X of Li to Si is 1.5 or more and 1.8 or less, in the control range of the molar ratio X, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.0 or more and 1.3 or less, A method for controlling charge and discharge of a lithium secondary battery.

2. The negative electrode contains at least one of a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder as a negative electrode binder, The charge and discharge control method according to Claim 1.

3. The negative electrode binder contains a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder, when the total solid content of the negative electrode is 100% by mass, the ratios of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are each 0.5% by mass or more and 3% by mass or less, The charge and discharge control method according to Claim 2.

4. The negative electrode contains at least one of carbon black, carbon nanotubes, and carbon fibers as a conductive material, The charge and discharge control method according to any one of Claims 1 to 3.

5. The density of the positive electrode is 3.2 g / cm ３ or more, The charge and discharge control method according to any one of Claims 1 to 3.

6. The density of the negative electrode is 1.4 g / cm ３ or more. The charge and discharge control method according to any one of Claims 1 to 3.

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