JP7710425B2 - Method for controlling charging and discharging of lithium secondary battery - Google Patents

Description

The present invention relates to a method for controlling charging and discharging a lithium secondary battery.

In recent years, secondary batteries containing Si-containing materials in the negative electrode have been known for the purpose of increasing capacity, etc. Although the Si-containing materials have a high theoretical capacity, they undergo large expansion and contraction (volume change) with charge-discharge cycles, and the decrease in capacity retention rate with charge-discharge cycles is an issue (see Patent Documents 1 to 4). In this regard, Patent Document 1 describes that a lithium secondary battery containing a negative electrode active material represented by Li _x Si in the negative electrode can be charged and discharged within a range where the value of X is 0≦X≦2.33, thereby suppressing the decrease in capacity retention rate with charge-discharge cycles.

JP 2000-173669 A JP 2004-349016 A JP 2005-332805 A JP 2009-259695 A

In recent years, there has been a demand for lithium secondary batteries with even higher energy density and improved cycle characteristics. The present invention was made in consideration of the above circumstances, and its purpose is to provide a method for controlling the charge and discharge of a lithium secondary battery that can suppress volumetric changes associated with charge and discharge cycles and maintain a high energy density for a long period of time.

Through intensive research by the inventors, it was newly discovered that factors that cause large volume changes in lithium secondary batteries containing Si-containing materials in the negative electrode include the large expansion and contraction that occurs when Si itself reacts with Li, as well as the large polarization of the charge/discharge voltage, which slows down the diffusion of Li within the solid, making the reaction prone to unevenness. Therefore, the inventors came up with the idea of creating a battery configuration that can obtain a high energy density, and charging and discharging the secondary battery within the range of Si usage in which both expansion and contraction and polarization are kept small. After extensive research, the inventors have completed the present invention.

The present invention provides a method for controlling the charge and discharge of a lithium secondary battery, comprising an electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte, the positive electrode including a lithium transition metal composite oxide, the lithium transition metal composite oxide including Ni, and a molar ratio of Ni to all transition metal elements being 70 mol% or more, the negative electrode including graphite and a Si-containing material, the ratio of Si included in the Si-containing material being 20 mass% or more and 70 mass% or less when the total mass of the graphite and the Si-containing material is taken as 100 mass%, and when Li is absorbed into the Si during charging to form a Li-Si alloy (LixSi), charging is controlled so that the maximum value of the molar ratio X of Li to Si is 1.3 or more and 1.9 or less, and within 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.

According to the present invention, by increasing the content of Si in the negative electrode to 20 mass% or more, setting the molar ratio of Si to 1.3 to 1.9 (equivalent to Li ₁₂ Si ₇ ), and designing the positive electrode according to the capacity of the negative electrode, it is possible to realize a high energy density equal to or higher than that when only graphite is used. In addition, the volume change caused by the charge/discharge cycle is suppressed compared to, for example, Patent Document 1, and a high energy density can be exhibited for a long period of time.

FIG. 1 is a perspective view illustrating a secondary battery according to an embodiment. FIG. 2 is a schematic vertical cross-sectional view taken along line II-II in FIG. 1 is a graph showing a change in c-axis length of a Ni-containing lithium transition metal composite oxide during charging and discharging. 1 is a graph showing the relationship between the Si content and the energy density. 1 is a graph showing the relationship between the Li insertion capacity and the expansion rate of LixSi. 1 is a graph showing the relationship between differential capacity (dQ/dV) and voltage. 1 is a graph showing the change in cell thickness in Example 1 and Comparative Example 6. 13 is a graph showing the change in cell thickness in Comparative Example 3. 1 is a graph showing the changes in capacity and capacity retention rate in Example 1 and Comparative Example 6.

Below, preferred embodiments of the technology disclosed herein will be described with reference to the drawings. Note that matters other than those specifically mentioned in this specification that are necessary for implementing the present invention (for example, the general configuration and manufacturing process of a battery that do not characterize the present invention) can be understood as design matters for a person skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and common technical knowledge in the field.

In this specification, the term "secondary battery" refers to any power storage device that can be repeatedly charged and discharged, and is a concept that encompasses so-called storage batteries (chemical batteries) such as lithium secondary batteries, and capacitors (physical batteries) such as electric double-layer capacitors. In addition, in this specification, "lithium secondary battery" refers to any secondary battery that uses lithium ions (Li ions) as electrolyte ions and realizes charging and discharging by the transfer of charge associated with Li ions between the positive and negative electrodes. In addition, the notation "A to B" indicating a range in this specification includes the meaning of A or more and B or less, as well as "greater than A" and "smaller than B."

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

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.

The exterior body 10 is a housing that contains the electrode body 20 and a non-aqueous electrolyte (not shown). As shown in FIG. 1, the exterior body 10 is formed in a flat, bottomed rectangular parallelepiped shape (square). The battery 100 is preferably a square secondary battery equipped with a square exterior body 10. However, the shape of the exterior body 10 is not limited to a square shape, 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 that used conventionally, and is not particularly limited. The exterior body 10 is made of a lightweight metal material with good thermal conductivity, 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.

2, the exterior body 10 here includes a case body 12 having an opening 12h, and a lid (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 that extend from the bottom wall 12a and face each other, and a pair of short side walls 12c that extend from the bottom wall 12a and face each other. The lid 14 is attached to the case body 12 so as to close the opening 12h of the case body 12. The lid 14 faces the bottom wall 12a of the case body 12. The exterior body 10 is integrated by joining (e.g., welding) the lid 14 to the periphery of the opening 12h of the case body 12. The exterior body 10 is hermetically sealed (sealed).

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 square-shaped (typically rectangular) positive electrode and a square-shaped (typically rectangular) negative electrode are stacked in the stacking direction with a separator between them, or a wound electrode body in which a strip-shaped positive electrode and a strip-shaped negative electrode are stacked with a strip-shaped separator between them and wound around a winding axis.

As shown in FIG. 2, the positive electrode has a positive electrode collector 21 and a positive electrode active material layer (not shown) fixed on at least one surface of the positive electrode collector 21. The positive electrode collector 21 is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. The density of the positive electrode (average packing density of the positive electrode active material layer) is preferably 3.2 g/cm ³ or more. 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 in accordance with a 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.

The positive electrode active material layer contains a positive electrode active material capable of reversibly absorbing and releasing charge carriers. The positive electrode active material is a high Ni-containing lithium transition metal composite oxide, that is, the lithium transition metal composite oxide contains Ni as an essential component and contains a lithium transition metal composite oxide having a molar ratio of Ni to all transition metal elements of 70 mol% or more. The high Ni-containing lithium transition metal composite oxide is preferably a layered compound, and more preferably a compound represented by Li _a MO ₂ (wherein a is 0<a<2, and M is one or more transition metals including Ni). 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, even more preferably 85 mol% or more, and particularly preferably 90 mol% or more. The reason for this will be explained with reference to FIG. 3.

Figure 3 is a graph showing the change in c-axis length of Ni-containing lithium transition metal composite oxide during charging and discharging. As shown in Figure 3, according to the inventor's study, in a high Ni-containing lithium transition metal composite oxide, that is, when the Ni molar ratio is 70 mol% or more, or even 80 mol% or more (Ni80 in Figure 3), a large contraction occurs in the c-axis when the lithium metal reference potential becomes 4.1 V (vs. Li/Li+) or more. This allows the expansion of the negative electrode, which will be described later, to be absorbed favorably, and the volume change of the electrode body 20 can be mitigated. This tendency does not occur when the Ni molar ratio is 57 mol% (Ni57 in Figure 3). Moreover, when the Ni molar ratio is 90 mol% or more (Ni92 in Figure 3), it becomes even more remarkable. This is because in a composition with a high Ni ratio, the H3 structure appears at the above potential, and large contraction occurs at that time.

When the total mass of the positive electrode active material is taken as 100 mass%, the proportion of the high Ni-containing lithium transition metal composite oxide is preferably 60 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more.

The positive electrode active material layer may contain a positive electrode active material other than the high Ni-containing lithium transition metal composite oxide, or any other component than the positive electrode active material, such as a conductive material, a positive electrode binder, or various additive components. As the conductive material, for example, a carbon material such as acetylene black (AB) may be used. It is preferable that the positive electrode active material layer contains a positive electrode binder. As the positive electrode binder, for example, a fluororesin such as polyvinylidene fluoride (PVdF) may be used, and it is particularly preferable that the positive electrode binder contains PVdF.

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 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 is preferably 1.4 g/ ^cm3 or more. This makes it easier to design the capacity of a high-capacity negative electrode.

The negative electrode active material layer contains a negative electrode active material capable of reversibly absorbing 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 conventionally used for this type of application, such as SiC and SiOx, can be used without any particular limitations. Among them, SiC is preferable. The graphite may be in a composite state with the Si-containing material, such as SiC, or may be separate from the Si-containing material. In this embodiment, when the total mass of the graphite and the Si-containing material is 100 mass%, the proportion of Si contained in the Si-containing material is 20 to 70 mass%. The reason for this will be explained with reference to Table 1 and FIG. 4.

Table 1 is a table summarizing the Si content, capacity, and energy density. Figure 4 is a graph showing the relationship between the Si content and energy density. As shown in Table 1 and Figure 4, according to the inventor's study, by increasing the Si content to 20 mass% or more, a high energy density equivalent to or higher than that when only graphite is used (capacity: 372 mAh/g, energy density: 1.38 Wh/g-negative electrode active material) can be achieved, even with charge/discharge control that limits the range of Si use, as described below. The Si content is 70 mass% from the viewpoint of suppressing expansion of the negative electrode, and may be approximately 50 mass% or less, for example 30 mass% or less.

The negative electrode active material layer may contain a negative electrode active material other than graphite and a Si-containing material, or any other component than the negative electrode active material, such as a conductive material, a negative electrode binder, or various additive components. The negative electrode active material layer preferably contains a conductive material. As the conductive material, for example, carbon black such as acetylene black (AB) or Ketjen black, carbon nanotubes (CNT), carbon fibers such as vapor grown carbon fiber (VGCF), activated carbon, graphite, or other carbon materials can be suitably used, and among them, it is preferable to include at least one of carbon black, carbon nanotubes, and carbon fibers. This can suitably increase the conductivity of the Si-containing material, reduce internal resistance, and mitigate volume change.

The negative electrode active material layer preferably contains a negative electrode binder. For example, a cellulose-based binder, a polyacrylic-based binder, a rubber-based binder, etc. can be used as the negative electrode binder. Any negative electrode binder that has been used in lithium secondary batteries can be used as the negative electrode binder without any particular restrictions. 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. In addition, when the total solid content of the negative electrode is taken as 100 mass%, the proportions of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are preferably 0.5 to 3 mass%, respectively. This makes it easier to follow the expansion and contraction of the Si-containing material accompanying charge and discharge, and improves the cycle characteristics.

Cellulosic binders include all linear polymers containing at least β-glucose as a repeating unit and their derivatives. Typically, they are compounds in which some or all of the hydroxyl groups in the β-glucose structure, which is the repeating unit, are replaced with alkoxy groups, and their derivatives. Examples of cellulose binders that can be used include carboxymethyl cellulose (CMC), carboxyethyl cellulose, and hydroxyethyl cellulose, with CMC being the most preferred.

The polyacrylic binder includes all polymeric compounds formed by 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, a polymer (homopolymer or copolymer) containing units of acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, acrylic acid salts, or methacrylic acid salts (hereinafter referred to as "acrylic units") can be used, and polyacrylic acid (PAA) is particularly preferred. As a 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 with charging and discharging.

Rubber-based binders include all rubber-like polymeric materials that contain carbon-carbon double bonds in the main chain. Examples of rubber-based binders that can be used include styrene butadiene rubber (SBR), butyl rubber (IIR), butadiene rubber (BR), and acrylonitrile butadiene rubber (NBR). By including a rubber-based binder in the negative electrode binder, the tensile strength and elastic modulus can be improved, and by using it in combination with a polyacrylic binder in particular, cycle deterioration can be effectively suppressed. In addition, an increase in internal resistance can be suppressed, and energy density and cycle characteristics can be improved.

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. A suitable separator is, for example, a resin porous sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator preferably has a total thickness of 20 μm or less. The separator is preferably a heat-resistant separator having a substrate portion made of a resin porous sheet such as PE or PP, and a heat-resistant layer provided on at least one surface of the substrate portion.

As shown in FIG. 2, a laminated portion is formed in the center (shaded portion) of the long side direction X of the electrode body 20, in which the positive electrode active material layer and the negative electrode active material layer are laminated in an insulated state. Meanwhile, at the left end of the long side direction X of the electrode body 20, a part of the positive electrode collector 21 (positive electrode collector exposed portion) protrudes from the laminated portion. A positive electrode lead member 23 is attached to the positive electrode collector exposed portion. Also, at the right end of the long side direction X of the electrode body 20, a part of the negative electrode collector 22 (negative electrode collector exposed portion) protrudes from the laminated portion. A negative electrode lead member 24 is attached to the negative electrode collector exposed portion.

As shown in FIG. 1, the positive electrode terminal 30 and the negative electrode terminal 40 are disposed at both ends of the long side direction X of the lid 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 each protrude from the same surface of the exterior body 10 (specifically, the lid 14). However, the positive electrode terminal 30 and the negative electrode terminal 40 may each protrude from different surfaces of the exterior body 10. As shown in FIG. 2, the positive electrode terminal 30 is electrically connected to the positive electrode of the electrode body 20 through 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 through the negative electrode lead member 24 inside the exterior body 10.

The non-aqueous electrolyte may be the same as that used in the past, 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 (LiPF ₆ ). However, the non-aqueous electrolyte may be in a solid state (solid electrolyte) and integrated with the electrode body 20.

When the battery 100 described above is charged, Li is absorbed 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 rate (volume change rate) of the Li-Si alloy. As shown in FIG. 5, the larger the molar ratio X of Li, the higher the Li insertion capacity of Si, but on the other hand, the expansion rate also increases. Therefore, in this embodiment, the range of use of Si during charging is controlled so that the maximum value of the molar ratio X of Li to Si is 1.3 to 1.9 (up to Li ₁₂ Si ₇ ). This suppresses volume change associated with charge and discharge cycles, and high energy density can be exhibited for a long period of time. From the viewpoint of exhibiting the above effect 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 energy density, the maximum value of the molar ratio X may be 1.6 or more, or 1.7 or more.

On the other hand, when the battery 100 is discharged, Li is released from the Li-Si alloy (LixSi) at the negative electrode. The range of Si used during discharge may be controlled so that the minimum value of the molar ratio X of Li to Si is 0 (up to the equivalent of Si).

Voltage control is desirable for charging and discharging. Figure 6 is a graph showing the relationship between differential capacity (dQ/dV) and voltage for a secondary battery containing a Si-containing material in the negative electrode. "Differential capacity (dQ/dV)" indicates the amount of capacity change per unit voltage. The peak where dQ/dV is rising indicates that charging and discharging was more active than other voltage regions for some reason. Based on this knowledge, in voltage control, the charging and discharging voltage may be set, for example, within the voltage range indicated by the arrow in Figure 6 so as not to exceed the peak where dQ/dV is rising (the reaction peak in Figure 6). The charging and discharging rate (C rate) is preferably 0.2C or more, and more preferably 0.3C or more. The charging and discharging rate is preferably 2C or less, and even more preferably 1C or less.

In addition, the negative electrode capacity when the molar ratio X of Li to Si is 1.3 to 1.9 can be simply obtained from the theoretical capacity relative to the Si content (see Table 1). More specifically, when a half cell is prepared using a Si-containing material with Li metal as the counter electrode and the initial charge and discharge is performed, the dQ/dV curve (see FIG. 6) or the dV/dQ curve shows that the peak of Si→Li ₂ Si and the peak of Li ₂ Si→Li _3.25 Si are separated into two peaks. Based on this, the capacity (the range of the arrow in FIG. 6) at x=2.0 (Li ₂ Si) can be calculated, and the negative electrode capacity (initial negative electrode discharge capacity) when the molar ratio X is 1.3 to 1.9 can also be obtained from the capacity ratio.

In this embodiment, 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 (positive/negative electrode capacity ratio) is 1.0 to 1.3. If the weight of the negative electrode is increased to increase the positive/negative electrode capacity ratio, the energy density decreases. Therefore, the positive/negative electrode capacity ratio is set to 1.3 or more. Furthermore, if the positive/negative electrode capacity ratio is less than 1.0, Li deposition is likely to occur, and cycle characteristics decrease. Therefore, the positive/negative electrode capacity ratio is set to 1.0 or more. From the viewpoint of exerting the above effect at a high level, the positive/negative electrode capacity ratio is more preferably 1.0 to 1.2. Considering, for example, changes in the capacity ratio during high current charging/discharging, the positive/negative electrode capacity ratio is preferably 1.05 or more, and more preferably 1.1 or more.

The positive/negative electrode capacity ratio can be calculated by calculating the capacity of the positive electrode and the capacity of the negative electrode separately and using the following formula: negative electrode capacity ÷ positive electrode capacity. The capacity of the positive electrode is calculated by preparing a half cell with Li metal as the counter electrode and performing the initial charge in a voltage range corresponding to the full cell (for example, 2.5 to 4.3 V) (initial positive electrode charge capacity). The capacity of the negative electrode is calculated by adding up the above-mentioned Si usage capacity (initial negative electrode discharge capacity when the molar ratio X of Li to Si is 1.3 to 1.9) and the graphite capacity in the Si usage voltage range (control range of the above molar ratio X). When the irreversible capacity of the charge/discharge of the negative electrode is greater than that of the positive electrode, it is preferable to calculate the capacity after subtracting the irreversible capacity.

The battery 100 can be used for various purposes, but can be suitably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. The type of vehicle is not particularly limited, but examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

The following describes some examples of the present invention, but is not intended to limit the present invention to these examples.

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

Next, a bag-shaped aluminum laminate film was prepared as a battery case. The electrode body and an amount of nonaqueous electrolyte that would immerse the electrode body were placed in the laminate film and sealed. This produced a laminate cell.

<Evaluation of initial characteristics> Next, a metal plate (SUS plate) with an area covering the laminate cell was placed on top of the prepared laminate cell, and a thickness gauge was placed on top of it. Then, the initial charge and discharge was performed at a charge and discharge rate of 0.2C so that the molar ratio X of Li to Si was in the range shown in Table 2, and the energy density and the expansion rate (rate of change in thickness) of the laminate cell at this time were measured. Note that 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. Note that in Table 2, the energy density is expressed per unit mass of the negative electrode active material. Also, the expansion rate is during charging, and is expressed as a percentage of the thickness before charging (100%).

As shown in Table 2, Comparative Example 1, in which the proportion of Si in the negative electrode active material is low at 10 mass%, Comparative Example 2, in which the range of Si use is narrowed to 0≦x≦1.2, Comparative Example 4, in which the positive/negative electrode capacity ratio is high at 1.4, and Comparative Example 5, in which the molar ratio of Ni is low at 57 mol%, had lower energy densities than Comparative Example 6, in which only graphite was used as the negative electrode active material. In addition, Comparative Example 3, in which the range of Si use is widened to 0≦x≦2.0, and Comparative Example 6, in which only graphite was used as the negative electrode active material, had high expansion coefficients. The reason for the high expansion coefficient in Comparative Example 6 (100% graphite) is that graphite has a stage structure like Si, and expansion and contraction occurred except for the intermediate Stage 2, although not as much as Si.

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

As shown in Table 2 and FIG. 7, Comparative Example 6, which used only graphite as the negative electrode active material, had a larger gradient of the graph than Example 1, which contained 30 mass% Si, i.e., the cycle expansion rate was relatively high. Also, as shown in Table 2, FIG. 7 and FIG. 8, Comparative Example 3, which performed a charge-discharge cycle with the Si usage range of 0≦x≦2.0 (up to Li ₁₃ Si ₄ equivalent), had a larger gradient of the graph than Example 1, which performed a charge-discharge cycle with the Si usage range of 0≦x≦1.7 (up to Li ₁₂ Si ₇ equivalent), i.e., the cycle expansion rate was relatively high. Also, as shown in Table 2 and FIG. 9, Comparative Example 6 (100% graphite) had a relatively low capacity retention rate compared to Example 1.

In contrast to these Comparative Examples 1 to 6, Example 1, i.e., a battery cell in which the molar ratio of Ni is 70 mol% or more (here 85 mol%), the ratio of Si is 20 mass% or more (here 30 mass%), the range of Si use is 0≦x≦1.7 (up to Li ₁₂ Si ₇ equivalent), charge/discharge cycles are performed, and the positive/negative electrode capacity ratio is 1.0 or more (here 1.1), had a smaller cycle expansion rate and a superior capacity retention rate, despite having a larger absolute capacity than Comparative Example 6 (100% graphite). Such results demonstrate the significance of the technology disclosed herein.

As described above, specific aspects of the technology disclosed herein include those described in the following sections.
Item 1: A method for controlling charge and discharge of a lithium secondary battery comprising: an electrode assembly including a positive electrode and a negative electrode; and a non-aqueous electrolyte; wherein the positive electrode comprises a lithium transition metal composite oxide, the lithium transition metal composite oxide comprises Ni, and a molar ratio of Ni to all transition metal elements is 70 mol% or more; the negative electrode comprises graphite and a Si-containing material, and when the total mass of the graphite and the Si-containing material is taken as 100 mass%, a ratio of Si contained in the Si-containing material is 20 mass% or more and 70 mass% or less; and when Li is absorbed into the Si during charging to form a Li-Si alloy (LixSi), charging is controlled so that a maximum value of a molar ratio X of Li to Si is 1.3 or more and 1.9 or less; and within the control range of the molar ratio X, a 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 includes at least one of a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder as a negative electrode binder;
Item 2. The charge/discharge control method according to item 1.
Item 3: The negative electrode binder includes 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 mass%, the proportions of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are each 0.5 mass% to 3 mass%.
Item 3. The charge/discharge control method according to item 1 or 2.
Item 4: The negative electrode contains at least one of carbon black, carbon nanotubes, and carbon fibers as a conductive material;
Item 4. 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/ ^cm3 or more.
Item 5. 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/ ^cm3 or more.
Item 6. The charge/discharge control method according to any one of items 1 to 5.

The above describes an embodiment of the present invention, but the above embodiment is merely an example. The present invention can be embodied in various other forms. The present invention can be embodied based on the contents disclosed in this specification and the technical common sense in the relevant field. The technology described in the claims includes various modifications and variations of the above-exemplified embodiment.

10: Exterior body 20: Electrode body 100: Battery (lithium secondary battery)

Claims (6)

An electrode assembly 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 contains Ni, and a molar ratio of Ni to all transition metal elements is 70 mol % or more;
The negative electrode includes graphite and a Si-containing material,
When the total mass of the graphite and the Si-containing material is taken as 100 mass%, a ratio of Si contained in the Si-containing material is 20 mass% or more and 70 mass% or less,
When Li is absorbed into the Si during charging to form a Li-Si alloy (LixSi), charging is controlled so 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 charging and discharging of a lithium secondary battery. The negative electrode includes 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 claim 1 . the negative electrode binder includes a cellulose-based binder, a polyacrylic-based binder, and a rubber-based binder;
When the total solid content of the negative electrode is taken as 100 mass%, the proportions of the cellulose-based binder, the polyacrylic-based binder, and the rubber-based binder are each 0.5 mass% or more and 3 mass% or less.
The charge/discharge control method according to claim 2 . 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 claim 1 . The density of the positive electrode is 3.2 g/ ^cm3 or more;
The charge/discharge control method according to claim 1 . The density of the negative electrode is 1.4 g/ ^cm3 or more.
The charge/discharge control method according to claim 1 .