Negative electrode for use in lithium-ion battery, lithium-ion battery, and method for producing lithium-ion battery

Abstract

This negative electrode for use in a lithium-ion battery contains at least a silicon-based active material as a negative electrode active material. The ratio of the amount of lithium remaining in the negative electrode without being de-doped to the amount of lithium that can be de-doped from the negative electrode after the first lithiation is 0.07-0.90.

Description

Negative electrode used in a lithium-ion battery, lithium-ion battery, and method for manufacturing a lithium-ion battery

[0001] The following disclosure relates to a negative electrode used in a lithium-ion battery, its manufacturing method, and the like.

[0002] In recent years, lithium-ion batteries have been widely applied in various fields. Patent Document 1 discloses a prelithiation method for increasing the Coulomb efficiency of a lithium-ion battery. Increasing the Coulomb efficiency of a lithium-ion battery corresponds to increasing the energy density of the lithium-ion battery.

[0003] Japanese Patent Application Laid-Open No. 2023-542767

[0004] The negative electrode according to one aspect of the present disclosure is a negative electrode containing at least a silicon-based active material as a negative electrode active material. The capacity of the negative electrode at full charge is defined as the first capacity, the ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity rate, and the capacity corresponding to the lithium ions that have moved to the negative electrode due to lithiation of the negative electrode active material or the lithium ions remaining in the negative electrode due to delithiation after lithiation is defined as the second capacity. The ratio of the second capacity to the first capacity is not less than a value obtained by adding 0.07 to the irreversible capacity rate and not more than a value obtained by adding 0.90 to the irreversible capacity rate.

[0005] The negative electrode according to one aspect of the present disclosure is a negative electrode used in a lithium-ion battery, containing at least a silicon-based active material as a negative electrode active material, and the ratio of the amount of lithium remaining in the negative electrode without being de-doped to the amount of lithium that can be de-doped in the negative electrode after the first lithiation is not less than 0.07 and not more than 0.90.

[0006] The negative electrode according to one aspect of the present disclosure is a negative electrode used in a lithium-ion battery. When discharging with a metal lithium counter electrode at a discharge rate of 0.1 C, the maximum value of the voltage of the negative electrode is not less than 0.20 V and not more than 1.07 V.

[0007] A lithium-ion battery according to one aspect of the present disclosure comprises a positive electrode containing lithium as a positive electrode active material and a negative electrode containing at least a silicon-based active material as a negative electrode active material, and contains 1.07 times or more and 1.90 times or less the maximum amount of lithium that can be contained in the positive electrode.

[0008] A method for manufacturing a lithium-ion battery according to one aspect of the present disclosure includes the steps of doping a negative electrode containing at least a silicon-based active material as a negative electrode active material with an amount of lithium exceeding the amount corresponding to the irreversible capacity of the negative electrode, and dedoping only a portion of the lithium that can be dedoped from the negative electrode.

[0009] A method for manufacturing a lithium-ion battery according to one aspect of the present disclosure includes the steps of: preparing a negative electrode containing at least a silicon-based active material as a negative electrode active material; and a lithiation step of reacting the negative electrode active material with a lithium foil to lithify the negative electrode active material, wherein the capacity of the negative electrode when fully charged is defined as a first capacity, the ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity ratio, the capacity corresponding to the lithium ions that move from the lithium foil to the negative electrode in the lithiation step is defined as a second capacity, and the ratio of the second capacity to the first capacity is greater than or equal to the irreversible capacity ratio plus 0.07 and less than or equal to the irreversible capacity ratio plus 0.90.

[0010] This document schematically shows the structure of a lithium-ion battery in Embodiment 1. It shows an example of the relationship between the negative electrode active material, the remaining Li percentage, the initial energy density, and the initial Coulomb efficiency. It shows an example of the discharge curve during lithiation of the negative electrode. It shows an example of the relationship between the voltage value in the discharge curve and the initial Coulomb efficiency. It shows an example of the relationship between the capacity per unit area and the energy density of a lithium-ion battery. It shows an example of the relationship between the remaining Li percentage, the energy density, the average operating voltage, and the Coulomb efficiency. It schematically shows the relationship between the SOC and the state of the negative electrode during discharge of a lithium-ion battery. It shows an example of the relationship between the remaining Li percentage and the initial Coulomb efficiency. This is a table showing the maximum voltage of the negative electrode when the negative electrode in this disclosure is discharged at 0.1C. The voltage 15 minutes after the discharge has stopped is also shown. This document schematically shows the manufacturing method of a lithium-ion battery in Embodiment 3. It shows various numerical values ​​in the examples.

[0011] [Embodiment 1] There is a need to increase the energy density of lithium-ion batteries compared to conventional batteries.

[0012] Embodiment 1 will be described below. For the sake of clarity, components having the same function as those described in Embodiment 1 will be denoted by the same reference numerals in subsequent embodiments, and their descriptions will not be repeated. For the sake of simplicity, explanations of known technical matters will also be omitted as appropriate.

[0013] The configurations and numerical values ​​described herein are merely examples, unless otherwise consistent with the content. Therefore, unless otherwise consistent with the content, the positional relationships of the components are not limited to the examples in each figure. Also, the components are not necessarily shown to scale. In this specification, the notation "A to B" for two numbers A and B means "greater than or equal to A and less than or equal to B," unless otherwise consistent with the content.

[0014] (Introduction) In a lithium-ion battery, a type of secondary battery, the negative electrode is fully charged during the initial operation, doping it with the maximum amount of Li. At this time, a film of SEI (Solid Electrolyte Interphase), consisting of organic materials such as lithium alkyl carbonate or inorganic materials such as lithium salts, is formed on the surface of the negative electrode. Because lithium ions are consumed due to the formation of this film, the initial discharge capacity is lower than the initial charge amount. The battery capacity lost at this time is called "irreversible capacity".

[0015] Various methods have been proposed to reduce the irreversible capacity of lithium-ion batteries. For example, by using the lithiation method for increasing the Coulomb efficiency of lithium-ion batteries described in Patent Document 1, the irreversible capacity caused by the active material of the lithium-ion battery can be reduced.

[0016] In a broad sense, lithiation refers to the process of lithifying the negative electrode active material contained in the negative electrode by reacting it with lithium. Therefore, during the charging and discharging of a completed lithium-ion battery, lithiation, which dops the negative electrode with Li, and delithiation, which dedops the negative electrode with Li, are performed. In this disclosure, lithiation and delithiation refer to the lithiation and delithiation performed for the first time on the negative electrode when a cell containing the negative electrode and a counter electrode (positive electrode) such as metallic lithium is constructed before the assembly of the lithium-ion battery. Alternatively, it refers to the lithiation and delithiation performed for the first time on the negative electrode during the process from lithium-ion battery assembly to the first charge and discharge. Lithiation in this disclosure can also be called prelithiation. Furthermore, the lithiation and delithiation processes performed for the first time on the negative electrode are collectively referred to as lithiation / delithiation.

[0017] By performing lithiation / delithiation before assembly of lithium-ion batteries, the energy density of the lithium-ion batteries can be increased.

[0018] In recent years, there has been a growing demand for even higher energy density in lithium-ion batteries, driven by the need for lighter and lower profile batteries, as well as the desire to reduce resource consumption.

[0019] From the perspective of increasing the energy density of lithium-ion batteries, using high-capacity or high-potential active materials is one effective method. As an example, silicon-based active materials, which are high-capacity active materials, have been developed for use as anode materials in lithium-ion batteries.

[0020] However, silicon-based active materials, which are high-capacity active materials, have the challenge of significant expansion and contraction during charging and discharging. They also have the challenge of having low Coulomb efficiency (in other words, low initial capacity).

[0021] Furthermore, various chemical species exist within silicon-based active materials. For example, silicon, silicon oxide, lithium silicate, and mixtures thereof all belong to the category of silicon-based active materials. Methods for increasing Coulomb efficiency may differ depending on the chemical species of the silicon-based active material. In addition, materials that combine these silicon-based active materials with carbon materials, etc., to improve issues such as expansion and contraction or conductivity while utilizing the high capacity of silicon-based active materials, are also treated as silicon-based active materials in a broad sense. For example, an active material mixed with graphite is denoted as Si / C or Si-C.

[0022] Furthermore, conventional lithiation methods (e.g., the method described in Patent Document 1) cannot increase the Coulomb efficiency of the silicon-based active material after lithiation to a sufficiently high value that meets recent demands.

[0023] As another example, a lithium metallic anode that does not use an active material has been developed for lithium-ion batteries. Yet another example is the development of anode-free batteries for lithium-ion batteries. These batteries have the advantage of having the lowest irreversible capacity because they do not have an active material in the anode.

[0024] However, in lithium-ion batteries using metallic lithium negative electrodes, or in anode-free batteries, there is no negative electrode active material that has the function of incorporating lithium. Therefore, the location of lithium deposition on the negative electrode following lithium dissolution and leaching cannot be determined.

[0025] Therefore, the edges of the lithium deposition region widen with each repeated charge and discharge cycle of the battery. Depending on the battery's charge and discharge conditions or degradation, lithium may also deposit in a dendrite-like manner. Alternatively, lithium may deposit inside the separator. As a result, metallic lithium eventually reaches the positive electrode of the battery. This can cause a short circuit within the battery.

[0026] Therefore, the inventors of this application (hereinafter referred to as "the inventors") have created a new lithium-ion battery that differs from conventional ones, taking into account the aforementioned problems in conventional lithium-ion batteries. In particular, the inventors have created a new negative electrode for use in lithium-ion batteries that differs from conventional ones.

[0027] (Example of Lithium-ion Battery Configuration) Below, an example of a lithium-ion battery configuration according to one aspect of this disclosure will be described. Figure 1 illustrates a liquid-type lithium-ion battery. However, it should be noted that the basic configuration of the lithium-ion battery according to one aspect of this disclosure is not particularly limited.

[0028] Figure 1 schematically shows the structure of a lithium-ion battery 10 in Embodiment 1. The lithium-ion battery 10 in the example of Figure 1 comprises a positive electrode 1, a negative electrode 2, a separator 3, a non-aqueous electrolyte 4, and a case 5.

[0029] The positive electrode 1 includes a positive electrode active material layer 1A and a positive electrode current collector 1B. For example, the positive electrode 1 may be formed by coating the positive electrode active material layer 1A onto the surface of the positive electrode current collector 1B. In the example shown in Figure 1, the positive electrode 1 is electrically connected to the positive electrode external terminal 8 via the positive electrode lead 6.

[0030] The positive electrode active material layer 1A contains a positive electrode active material. The positive electrode active material in Embodiment 1 may contain lithium (Li). Examples of positive electrode active materials in Embodiment 1 include lithium iron phosphate, lithium manganese phosphate, lithium nickel manganese cobalt oxide, lithium nickel aluminum cobalt oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, and lithium nickel manganese aluminum cobalt oxide. Materials coated with a coating material (e.g., graphite material, oxide, or sulfide) can also be used as the positive electrode active material.

[0031] The negative electrode 2 includes a negative electrode active material layer 2A and a negative electrode current collector 2B. For example, the negative electrode 2 may be formed by coating the negative electrode active material layer 2A onto the negative electrode current collector 2B. In the example shown in Figure 1, the negative electrode 2 is electrically connected to the negative electrode external terminal 9 via the negative electrode lead 7.

[0032] The negative electrode active material layer 2A contains a negative electrode active material. The negative electrode active material in Embodiment 1 only needs to contain at least a silicon-based active material. Examples of silicon-based active materials in Embodiment 1 include silicon oxide (SiO), silicon (Si), and graphite-coated silicon. A composite of graphite material and silicon-based active material can also be used as the negative electrode active material. A mixed active material, which is a mixture of silicon-based active material and graphite active material, can also be used as the negative electrode active material.

[0033] In the example shown in Figure 1, the separator 3 is located between the positive electrode 1 and the negative electrode 2. That is, the lithium-ion battery 10 in the example shown in Figure 1 has a stacked structure in which the positive electrode 1 and the negative electrode 2 are stacked with the separator 3 in between.

[0034] Case 5 houses the positive electrode 1, the negative electrode 2, the separator 3, and the non-aqueous electrolyte 4. To maintain the airtightness of the lithium-ion battery 10, case 5 may have a lid (not shown).

[0035] (First Study) The inventors conducted an experimental study to determine how lithiation of the negative electrode of a lithium-ion battery (e.g., lithium-ion battery 10) according to one aspect of the present disclosure affects the performance of the lithium-ion battery.

[0036] In lithiation, the negative electrode is initially doped with the maximum amount of Li. This doping of the negative electrode is achieved by charging the negative electrode. Charging the negative electrode corresponds to reacting the negative electrode active material with Li. Therefore, by fully charging the negative electrode initially, the negative electrode is doped with the maximum amount of Li. At this time, lithium ions are consumed as a SEI film is formed on the surface of the negative electrode. Furthermore, some of the Li in the lithified negative electrode active material is difficult to remove even when attempting to delithiate it again. These also constitute irreversible capacity. The consumed lithium ions represent an amount of ions equivalent to irreversible capacity.

[0037] Next, in delithiation, the Li doped to the maximum amount in the negative electrode is desorbed to a certain extent. The removal of Li from the negative electrode is done by discharging the negative electrode. Discharging the negative electrode corresponds to releasing lithium ions from the negative electrode (more specifically, the compound of Si and Li in the negative electrode, or the compound of Li with carbon material if carbon material is included). In delithiation, a negative electrode containing a predetermined amount of Li is obtained after discharge.

[0038] In this specification, the ratio of the amount of lithium remaining in the negative electrode without dedoping to the amount of lithium that can be dedoped after the initial lithiation is referred to as the Li remaining rate. The Li remaining rate may also be simply referred to as the "remaining rate". In this specification, the maximum value of the Li remaining rate is expressed as 100%. Furthermore, unless otherwise specified, the Li remaining rate in this specification refers to the Li remaining rate in the negative electrode in the fully discharged state of the lithium-ion battery. The fully discharged state means the state in which the voltage of the lithium-ion battery has reached the discharge termination voltage. Furthermore, the lithium remaining in the negative electrode without dedoping after the initial lithiation is referred to as residual lithium. The amount of residual lithium in the negative electrode is the same as the amount of lithium ions remaining in the negative electrode in the fully discharged state. In other words, the Li remaining rate can also be expressed as the ratio of the lithium ions remaining in the negative electrode in the fully discharged state to the maximum amount of lithium ions that can be extracted from the negative electrode after the initial lithiation.

[0039] In conventional lithium-ion batteries, when fully discharged, the amount of lithium ions doped into the negative electrode becomes virtually zero. In contrast, in the lithium-ion battery of this disclosure, even when fully discharged, the amount of lithium ions doped into the negative electrode does not become zero and remains in the negative electrode. To clarify, in this disclosure, "virtually zero amount of lithium ions" refers to the value excluding the irreversible capacity of the battery cell.

[0040] During the assembly of the lithium-ion battery described herein, a positive electrode with the maximum possible lithium doping is used. Therefore, even if lithium remains in the negative electrode, charging and discharging are performed using only the lithium doped in the positive electrode. Because there is no space on the positive electrode side to accept lithium, the lithium remaining in the negative electrode cannot move to the positive electrode when the battery is operating within the appropriate voltage range. Consequently, the remaining lithium after the initial lithiation / delithiation will continue to remain in the negative electrode even during normal charging and discharging of the lithium-ion battery.

[0041] In cases of discharge over a wider voltage range than the normal operating voltage range, such as under over-discharge conditions, the lithium-ion battery of this disclosure is relatively safe because the excess lithium in the negative electrode has the effect of suppressing a rapid rise in the potential of the negative electrode. The excess lithium in the negative electrode is significant in order to reduce the risk of metal dissolution of the negative electrode current collector or decomposition of the electrolyte due to deep over-discharge, which can lead to the formation of metal deposits on the negative and positive electrodes during recharging, resulting in internal short circuits or reduced safety.

[0042] Thus, the lithium-ion battery of the present disclosure may contain an amount of lithium exceeding the maximum amount that can be doped into the positive electrode, and the amount of lithium exceeding the maximum amount may be included in the negative electrode as residual lithium. For example, the lithium-ion battery of the present disclosure may contain lithium in an amount of 1.07 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. The lithium-ion battery of the present disclosure may contain lithium in an amount of 1.15 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. The lithium-ion battery of the present disclosure may contain lithium in an amount of 1.20 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. Further, the lithium-ion battery of the present disclosure may contain lithium in an amount of 1.30 times or more and less than 1.60 times the maximum amount of lithium that the positive electrode can contain. Also, in the lithium-ion battery of the present disclosure, lithium may not be deposited on the surfaces of the positive electrode and the negative electrode. The lithium-ion battery of the present disclosure can increase the energy density as compared with the conventional one even if lithium is not deposited on the surfaces of the positive electrode and the negative electrode.

[0043] Figure 2 shows an example of the relationship between negative electrode active materials manufactured by different manufacturers, the Li residual rate, the initial energy density, and the initial Coulomb efficiency. In Figure 2, for convenience, the negative electrode active materials are labeled with symbols A to D. In each of the experimental examples described in Embodiment 1, unless otherwise specified, the charge and discharge of the negative electrode during lithiation / delithiation are performed at a discharge rate of 0.1C, and a Li foil is used as the positive electrode. Also, in each of the experimental examples described in Embodiment 1, unless otherwise specified, the ambient temperature is set to 20°C to 30°C. The negative electrode active materials in the example of Figure 2 are all examples of silicon-based active materials. The notation "Si / C" of the negative electrode active material A in the example of Figure 2 represents a mixed active material of silicon and graphite. The mixed active material may be, for example, an aggregate of silicon fine particles coated or solidified with graphite, and includes Si / C active materials that significantly alleviate the expansion and contraction of silicon.

[0044] The progress of lithiation may depend on the ambient pressure. As an example, the ambient pressure during charge and discharge may be set to 10 psi (pound-force per square inch) or more. As another example, the ambient pressure during charge and discharge may be set to 30 psi or more. For example, by performing lithiation at an ambient pressure of 10 psi or more, the possibility of unevenness occurring in the negative electrode after lithiation can be reduced. Thereafter, in a lithium-ion battery formed by laminating a positive electrode, a separator, and a negative electrode that has undergone lithiation / delithiation, a lower ambient pressure than during lithiation / delithiation, such as less than 10 psi, can be set.

[0045] In the example of FIG. 2, the ambient pressure during lithiation and delithiation is 36 psi. Also, the Li residual rate is set to 30%. The Li residual rate of 30% means that when delithiation is performed after doping the maximum amount of lithium into the negative electrode, the ratio of the amount of lithium remaining in the negative electrode without being delithiated is 30 when the amount of lithium that can be delithiated is set to 100.

[0046] As shown in FIG. 2, at this Li residual rate, a high initial energy density exceeding 400 Wh / kg is obtained for any negative electrode active material. The initial energy density means the energy density at the first charge and discharge of a lithium-ion battery using a negative electrode after lithiation / delithiation. In addition, at this Li residual rate, a high initial Coulomb efficiency is obtained for any lithium-ion battery using a negative electrode active material. The initial Coulomb efficiency means the Coulomb efficiency at the first charge and discharge performed after laminating the negative electrode and the positive electrode after lithiation / delithiation. All of the initial Coulomb efficiencies in the example of FIG. 2 exceed the target value of 85% described later, and a value close to the Coulomb efficiency of the positive electrode alone obtained by charging and discharging a half-cell in which a positive electrode, a separator, and lithium are laminated can be obtained. That is, a state in which the Coulomb efficiency of the negative electrode is substantially close to 100% has been achieved even using a silicon-based negative electrode.

[0047] The results in Figure 2 suggest that by appropriately selecting the combination of the negative electrode active material and the remaining Li content of the negative electrode for lithiation / delithiation, it is possible to more effectively increase the energy density of lithium-ion batteries than before. Therefore, the inventors conducted further investigations.

[0048] (Second study) The inventors performed lithiation of the negative electrode by using metallic lithium as the counter electrode and discharging the negative electrode. As can be understood from the above explanation, the remaining Li percentage can be adjusted by adjusting the level of discharge of the negative electrode during lithiation.

[0049] Figure 3 shows an example of a charge-discharge curve during the lithium lithiation of the negative electrode, obtained through experiments conducted by the inventors. The voltage values ​​(values ​​on the vertical axis) shown in Figure 3 represent the voltage of the negative electrode. In the example in Figure 3, the voltage of the negative electrode represents the potential difference between the negative electrode and metallic lithium. In the example in Figure 3, the potential of metallic lithium is set to 0V, which is the reference potential. Although not shown in the figure, the lithium lithiation in this study begins with charging, bringing the voltage of the silicon active material close to that of metallic lithium, and is illustrated from the point where it reaches almost 0V and slowly proceeds with lithium lithiation.

[0050] The negative electrode in the example shown in Figure 3 is manufactured as follows: First, silicon active material and graphite active material are mixed in a weight ratio of 1:1, and the mixture is solidified to prepare mixed silicon and graphite active material particles as the negative electrode active material. Next, binder particles are prepared by mixing carbon nanotubes and acetylene black, which are conductive materials, with the negative electrode active material. Next, a slurry is prepared by mixing methylcellulose, which is a thickening agent, with the binder particles. Next, the slurry is applied to copper foil and dried. Finally, the negative electrode is completed by pressing the copper foil on which the dried slurry is present.

[0051] In Figure 3, the horizontal axis of the charge-discharge curve represents time. However, because the discharge is occurring at a constant rate, the horizontal axis also corresponds to the remaining Li percentage in the discharge region where the voltage begins to rise. In the example in Figure 3, as the discharge progresses (i.e., as Li is detached from the negative electrode), the potential of the negative electrode relative to metallic lithium increases. In other words, in the example in Figure 3, the higher the voltage value in the charge-discharge curve, the lower the remaining Li percentage.

[0052] In the example shown in Figure 3, the maximum voltage value of 1.90V in the charge-discharge curve corresponds to a 0% Li remaining rate, while a voltage value of 0.68V in the charge-discharge curve corresponds to a 20% Li remaining rate. Furthermore, a voltage value of 0.56V in the charge-discharge curve corresponds to a 30% Li remaining rate.

[0053] Next, the inventors experimentally derived the relationship between the voltage value in the charge-discharge curve and the initial Coulomb efficiency. Here, the initial Coulomb efficiency refers to the initial Coulomb efficiency of a lithium-ion battery made by combining a positive electrode active material and a separator using a lithiated / delithiated negative electrode. This relationship can be rephrased as the relationship between the Li remaining rate and the initial Coulomb efficiency. Figure 4 shows an example of the relationship between the voltage value in the charge-discharge curve and the initial Coulomb efficiency obtained by the inventors' experiments. For convenience, the negative electrode active materials from different manufacturers used in the experiment shown in Figure 4 are denoted by the letters E to H. "Si / C" is a mixture of carbon material and silicon active material, including graphite-coated silicon. "SiO" is a general term for active materials consisting of silicon active material solidified with silicon oxide of uncertain valency, or a mixture of silicates, etc. E to H are negative electrode active materials from different manufacturers.

[0054] As shown in Figure 4, it was confirmed that in all cases of negative electrode active materials, lithiation / delithiation effectively improved the initial Coulomb efficiency compared to cases without lithiation / delithiation. Furthermore, it was confirmed that in all cases of negative electrode active materials, the initial Coulomb efficiency at a voltage of 0.68 V (i.e., the initial Coulomb efficiency with a remaining Li of 20%) was higher than the initial Coulomb efficiency at a voltage of 1.90 V (i.e., the initial Coulomb efficiency with a remaining Li of 0%).

[0055] Furthermore, it was confirmed that in all cases of negative electrode active materials, the initial Coulomb efficiency at a voltage of 0.56 V (i.e., the initial Coulomb efficiency at a Li retention rate of 30%) was higher than the initial Coulomb efficiency at a voltage of 0.68 V (i.e., the initial Coulomb efficiency at a Li retention rate of 20%). In Figure 4, 85% is shown as an example target value for the initial Coulomb efficiency. In all cases of negative electrode active materials, the initial Coulomb efficiency at a voltage of 0.56 V exceeds this target value. Thus, in the example in Figure 4, it was confirmed that particularly good initial Coulomb efficiency can be obtained at a Li retention rate of 30%.

[0056] Furthermore, it was confirmed that the initial Coulomb efficiency obtained in the example with a Li remaining rate of 30% shown in Figure 4 was close to the initial Coulomb efficiency of the positive electrode half-cell. A positive electrode half-cell refers to a cell in which the positive electrode and lithium are bonded together via a separator, and then the electrolyte is injected and sealed. The initial Coulomb efficiency of the positive electrode alone can be determined from the positive electrode half-cell. The fact that a Coulomb efficiency almost equivalent to that of the positive electrode half-cell was obtained means that the Coulomb efficiency of the negative electrode of the fabricated lithium-ion battery cell is almost 100%.

[0057] The high initial Coulomb efficiency in the example shown in Figure 4 was achieved at an ambient pressure of 9 psi. This indicates that if a pressure of 36 psi is applied during lithiation and delithiation, the lithium-ion battery can be operated at a pressure lower than 36 psi during subsequent charging and discharging.

[0058] Therefore, in modules and packs incorporating lithium-ion battery cells according to one aspect of this disclosure, the ambient pressure around the cells can be set low. As a result, the structural components of the modules and packs can be simplified. Conventional high-energy-density batteries have problems because the structural components that suppress expansion and contraction are heavy and bulky, but according to this disclosure, the ambient pressure can be reduced, making it possible to realize lithium-ion batteries with high energy density.

[0059] (Third Study) The inventors further conducted experimental studies on the relationship between the remaining Li content of the negative electrode and the performance of the lithium-ion battery. Figure 5 shows an example of the relationship between the capacity per unit area and the energy density of a lithium-ion battery obtained through experiments conducted by the inventors.

[0060] In Figure 5, the legends "Si / C from other companies" and "Si / C manufactured in-house" both represent examples where lithiation / delithiation has not been performed. In the examples in Figure 5, the remaining Li percentages when lithiation / delithiation is performed are shown as "0%", "15%", "30%", "60%", and "90%". In the examples in Figure 5, lithiation / delithiation is performed on Si / C manufactured in-house.

[0061] In the example shown in Figure 5, the energy density per unit area is roughly the same for both cases: when the Li retention rate is 0% and when there is no lithiation / delithiation. On the other hand, when the Li retention rate is higher than 0%, the energy density per unit area is significantly higher than in the cases with 0% Li retention and without lithiation / delithiation.

[0062] In the example shown in Figure 5, when the remaining Li percentage is higher than 0%, high energy densities exceeding 400 Wh / kg are obtained in most of the data. Such high energy densities have not been achieved in conventional lithium-ion batteries using silicon-based active materials.

[0063] Next, the inventors measured the average operating voltage and Coulomb efficiency of lithium-ion batteries for each example with Li remaining percentages of "0%", "15%", "30%", "60%", and "90%". Figure 6 shows an example of the relationship between Li remaining percentage, energy density, average operating voltage, and Coulomb efficiency obtained from the inventors' experiments.

[0064] In the example shown in Figure 6, the average operating voltage and Coulomb efficiency of the lithium-ion battery increase with increasing Li remaining percentage. This increase in average operating voltage and Coulomb efficiency corresponds to an increase in energy density.

[0065] (Regarding the numerical range of the Li remaining rate) The examples above demonstrate that the performance of lithium-ion batteries can be effectively improved by appropriately setting the Li remaining rate in the negative electrode containing silicon-based active material as the negative electrode active material. As an example, as can be seen from Figures 5 and 6, the Li remaining rate may be set to 15% to 90%. This makes it possible to realize lithium-ion batteries with a higher energy density than conventional batteries.

[0066] In the examples shown in Figures 5 and 6, the energy density increases with increasing Li retention. Therefore, as another example, the Li retention rate may be set to 20% to 90%, or to 30% to 90%.

[0067] Incidentally, as described later, in a lithium-ion battery according to one aspect of this disclosure, Li deposition may occur at the negative electrode during charging. If the amount of Li deposition is excessive, it is conceivable that the cycle characteristics of the lithium-ion battery may deteriorate. Therefore, the Li retention rate may be set to a value less than 90%. For example, the Li retention rate may be set to a value of 70% or less. Thus, the Li retention rate may be set to 20% to 70% or to 30% to 70%.

[0068] The inventors have also found that even in batteries where the remaining Li percentage is set high and Li deposition occurs during charging, a lithium-ion battery according to one aspect of this disclosure can achieve superior cycle characteristics compared to lithium metal batteries that use lithium metal as the negative electrode and deposit and dissolve lithium metal. Therefore, for example, when trying to obtain a battery with the highest possible energy density, it is effective to set the remaining Li percentage to a value higher than 70%. In this case, by reducing the thickness of the silicon-based negative electrode, for example, by using a silicon-based negative electrode that is 1 / 5 to 1 / 10 the thickness of one in which Li is not deposited, the energy density can be increased.

[0069] Alternatively, the Li retention rate may be set to a value of 60% or less. Therefore, the Li retention rate may be set to 20% to 60% or to 30% to 60%. As yet another example, the Li retention rate may be set to a value of 45% or less. Therefore, the Li retention rate may be set to 20% to 45% or to 30% to 45%.

[0070] Furthermore, the lower limit of the remaining Li percentage may be 7%. As will be described later, the desired effects of this disclosure can be obtained if the remaining Li percentage is at least 7%.

[0071] (Regarding the numerical range of the negative electrode voltage) As described above, the remaining Li can be adjusted by adjusting the discharge level of the negative electrode during lithiation. For this reason, for example, a numerical range may be set for the maximum value of the negative electrode voltage when metallic lithium is used as the counter electrode and discharged at a discharge rate of 0.1C.

[0072] For example, the maximum value may be set to 0.40V to 0.68V. As shown in Figures 3 and 4 above, a voltage value of 0.68V corresponds to an Li retention rate of 20%. As shown in Figure 9 below, a voltage value of 0.40V corresponds to an Li retention rate of 60%, so the maximum value of the negative electrode voltage may be 0.40V. The Li retention rate may also be set to 7% to 90%. A voltage value of 0.20V corresponds to an Li retention rate of 90%, so the maximum value of the negative electrode voltage may be 0.20V.

[0073] As another example, the maximum value may be set to 0.40V to 0.56V. As shown in Figures 3 and 4, a voltage value of 0.56V corresponds to an Li retention rate of 30%. On the other hand, according to the inventors' research, a voltage value of 0.40V corresponds to an Li retention rate of 60%.

[0074] Furthermore, as shown in Figure 9 below, the voltage value of 1.07V corresponds to a remaining Li rate of 7%. Therefore, for example, the maximum value may be set to 0.20V to 1.07V, or to 0.40V to 1.07V.

[0075] (Fourth consideration) As described above, the negative electrode in a lithium-ion battery according to one aspect of this disclosure is lithiated / delithiated. The positive electrode contains the maximum amount or an amount close to the maximum amount of Li as the positive electrode active material. Therefore, the lithium-ion battery may contain an amount of Li that exceeds the maximum amount that can be doped and dedoped (Li retention rate of 100%) in the negative electrode.

[0076] Therefore, as an example, a lithium-ion battery may contain 1 to 15 times the maximum amount of Li that can be doped and dedoped into the negative electrode.

[0077] For example, if the silicon active material content in the negative electrode is low, the maximum amount of Li that can be doped into the negative electrode decreases. Therefore, the amount of Li deposited in the negative electrode when fully charged is relatively large compared to the maximum amount. Consequently, lithium-ion batteries can contain approximately 15 times the maximum amount of Li that can be doped and dedoped into the negative electrode.

[0078] The inventors investigated the state changes during discharge of a lithium-ion battery designed as described above. As a result, the inventors found that the state of the negative electrode of the lithium-ion battery can change depending on the State of Charge (SOC) of the lithium-ion battery.

[0079] Figure 7 schematically shows the relationship between the state of charge (SOC) and the state of the negative electrode during the discharge of a lithium-ion battery. In the example shown in Figure 7, the Si / C film, which is the negative electrode active material, is located on top of the copper foil, which is the electrode conductor.

[0080] In Figure 7, reference numeral 710 indicates the state of the negative electrode at 100% SOC (i.e., fully charged). The positive electrode, positive electrode current collector, separator, pouch (case), etc., facing the negative electrode active material are not shown in this figure. The example of reference numeral 710 corresponds to the initial state during the discharge process. As shown in the example of reference numeral 710, the inventors have confirmed that in the fully charged negative electrode, Li may be deposited on the surface of the Si / C film 32 formed on the surface of the copper foil 33. The deposited Li layer is indicated by reference numeral 31. Thus, if the amount of Li that can move between the positive and negative electrodes in a lithium-ion battery is greater than the maximum amount of lithium that can be initially doped and dedoped into the negative electrode, Li may be deposited on the negative electrode in the fully charged state.

[0081] The amount of Li deposited at the negative electrode is thought to depend on the Li remaining rate at the negative electrode, assuming that the amount of Li contained in the positive electrode in a fully discharged state, i.e., the amount of Li that can move between the positive and negative electrodes, is constant. For example, if the Li remaining rate at the negative electrode is relatively high, it is conceivable that some of the Li that moves from the positive electrode to the negative electrode during charging may not be able to react with the Si / C film and instead precipitate on the Si / C film. In other words, it is conceivable that some of the Li may not be fully incorporated into the silicon-based active material of the negative electrode and instead precipitate on the Si / C film.

[0082] In Figure 7, reference numeral 720 indicates the state of the negative electrode at SOC 50%. The example of reference numeral 720 corresponds to an intermediate state in the discharge process. As shown in the example of reference numeral 720, the inventors confirmed that at SOC 100%, a portion of the Li deposited on the surface of the Si / C film 32 dissolves. The layer of partially dissolved Li is indicated by reference numeral 41. In other words, at SOC 50%, a portion of the Li deposited on the surface of the Si / C film 32 at SOC 100% remains. The inventors confirmed that a layer of deposits 42 is formed on top of the remaining layer of Li 41.

[0083] Traditionally, it was believed that the lithium (Li) deposited on the surface of the negative electrode would not be utilized. This was because the silicon negative electrode has a high potential, so silicon and graphite are used first for discharge, and the potential of the lithium-ion battery does not drop low enough to utilize the Li deposited on the negative electrode surface.

[0084] However, in the lithium-ion battery of this disclosure, the inventors have confirmed that Li deposited on the surface of the negative electrode also moves to the positive electrode during discharge and can be used during the next charge. Depending on the relationship between the amount of Li that can be doped into the positive electrode and the total amount of Li contained in the lithium-ion battery, in some cases almost all of the Li deposited on the surface of the negative electrode moves to the positive electrode during charging. The inventors also surmise that the phenomenon of Li deposited on the surface of the negative electrode returning to the positive electrode during discharge also affects the improvement of Coulomb efficiency. Furthermore, they have confirmed that the operating voltage also increases in the initial stages of discharge depending on the amount of Li deposited on the negative electrode. This confirms that when lithium is deposited, lithium is used first. It was also found that the discharge energy can be increased by increasing the operating voltage.

[0085] The amount of Li deposited at the negative electrode can be adjusted by adjusting the relationship between the amount of Li that can move between the positive and negative electrodes and the maximum amount of Li that can be doped and dedoped into the negative electrode after lithiation / delithiation (the Li capacity of the negative electrode). If the Li capacity of the negative electrode is smaller than the amount of Li present in the positive electrode in a fully discharged state, Li will be deposited at the negative electrode. If the negative electrode is the same, the amount of Li deposited at the negative electrode can be adjusted by adjusting the amount of Li contained in the positive electrode. The technical scope of lithium-ion batteries of this disclosure also includes lithium-ion batteries in which no Li is deposited at the negative electrode in a fully charged state.

[0086] If lithium is deposited on the negative electrode, a lithium negative electrode cell with high Coulomb efficiency and high energy characteristics can be realized. If the positive electrode capacity does not exceed the capacity of the silicon-based active material in the negative electrode, lithium will not be deposited on the negative electrode even when fully charged, and a silicon-based negative electrode with high cycle characteristics can be realized.

[0087] At low temperatures, the reaction rate decreases, which can prevent the negative electrode from accepting Li, causing Li to precipitate on the negative electrode. This disclosure discusses whether Li precipitates on the negative electrode when a lithium-ion battery is operated under appropriate operating conditions, and does not address whether Li precipitates on the negative electrode under abnormal operating conditions. The appropriate operating temperature range for lithium-ion batteries is generally -20°C to 60°C.

[0088] Figure 8 is a graph showing an example of the relationship between the Li remaining rate and the initial Coulomb efficiency. The horizontal axis of the graph represents the Li remaining rate (the ratio of the amount of lithium remaining in the negative electrode without dedoping to the amount of lithium that can be dedoped in the negative electrode after the initial lithiation). The vertical axis of the graph represents the initial Coulomb efficiency after the battery cell was fabricated. Figure 8 is a graph of the values ​​obtained from two experiments. The experiment that yielded the experimental results shown in Figure 8 was conducted under the same conditions as the experiment that yielded the experimental results shown in Figure 2.

[0089] The positive electrode active material used in Figure 8 is a ternary lithium-ion positive electrode active material using nickel, cobalt, and manganese, and has the performance to achieve an initial Coulomb efficiency of 90-92% when lithium metal is used as the negative electrode. Even with such a positive electrode active material, as shown in Figure 8, the initial Coulomb efficiency can be increased to 88% by reducing the Li retention rate to 7%. An initial Coulomb efficiency of 88% is a technically significant figure that has been difficult to obtain with conventional lithium-ion batteries.

[0090] Furthermore, by setting the remaining Li content to 9%, the initial Coulomb efficiency can be increased to 89%. By setting the remaining Li content to 11.5%, the initial Coulomb efficiency can be increased to approximately 90%. Since the slope of the graph becomes gentler from a remaining Li content of 12% onward, a remaining Li content of 12% can be used as one guideline for the amount of lithium ions to retain.

[0091] While theoretically the desired effect can be obtained if the remaining lithium percentage is less than 100%, it is reasonable to set it at 90% or less considering the usable capacity of the lithium-ion battery.

[0092] Here, the capacity of the negative electrode when fully charged is defined as the first capacity. The capacity corresponding to the lithium ions present in the negative electrode after the delithiation that occurs after the initial lithiation is defined as the second capacity. In this case, the following equation holds:

[0093] ) ((First volume) - (Irreversible volume)) = (Volume corresponding to the amount of lithium that can be dedoped at the anode after the initial lithiation (Dedoped capacity)) ... (Equation 1) (Second volume) = (Irreversible capacity) + (Volume corresponding to the lithium ions that remain at the anode without being dedoped from the dedoped capacity (Undoped capacity)) ... (Equation 2) The ratio of the second volume to the first volume (A) is expressed by the following equation from Equations 1 and 2: A = (Second volume) / (First volume) = [(Irreversible capacity) + (Undoped capacity)] / [(Dedoped capacity) + (Irreversible capacity)] ... (Equation 3) The Li remaining rate, which is the ratio of the undoped capacity to the dedoped capacity, is expressed by the following equation.

[0094] (Li remaining rate) = (undedoped capacity) / (dedoped capacity) ... (Equation 4) Thus, although the Li remaining rate and ratio A do not perfectly coincide mathematically, the inventors have confirmed through experiments that the desirable ranges for these values ​​are roughly the same.

[0095] If we define the irreversible capacity ratio as the ratio of the irreversible capacity to the first capacity, and assume the irreversible capacity is 10%, then the statement "the ratio of the amount of lithium remaining in the anode without being dedoped to the amount of lithium that can be dedoped in the anode after the initial lithiation is 0.07 or more and 0.90 or less" can be rephrased as follows: "The ratio of the second capacity to the first capacity (A) is 0.07 or more and 0.90 or less than or equal to the irreversible capacity ratio plus 0.07." This numerical range can be derived experimentally.

[0096] As described above, the negative electrode according to one aspect of the present disclosure is a negative electrode comprising at least a silicon-based active material as the negative electrode active material, wherein, before the first charging and discharging of the battery cell facing the positive electrode, or during complete discharge within the operating range of the battery cell, the capacity of the negative electrode when fully charged is defined as the first capacity, the ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity ratio, the capacity corresponding to the lithium ions that have moved to the negative electrode by lithiation of the negative electrode active material is defined as the second capacity, and the ratio of the second capacity to the first capacity is greater than or equal to the irreversible capacity ratio plus 0.07 and less than or equal to the irreversible capacity ratio plus 0.90.

[0097] [Embodiment 2] Embodiment 2 describes an example of a method for manufacturing a lithium-ion battery according to one aspect of the present disclosure. The manufacturing method may include a step of doping a negative electrode containing at least a silicon-based active material with an amount of lithium exceeding the amount corresponding to the irreversible capacity, and a step of dedoping only a portion of the Li that can be dedoped from the negative electrode. The doping step and the dedoping step may be carried out under an ambient pressure of 25 psi to 300 psi. The range of the ambient pressure is a range that the inventor has obtained by actual measurement as a preferred range for manufacturing lithium-ion batteries as ambient pressure in the present disclosure. Furthermore, in the lithiation step (the doping step) and the delithiation step (the dedoping step), lithiation and delithiation may be carried out by applying an electric current.

[0098] In the said manufacturing method, if the negative electrode is lithiated / delithiated before assembly of the lithium-ion battery, the processes other than the lithiating / delithiating process may be the same as conventional processes. In the said manufacturing method, in the lithiating / delithiating process, the negative electrode is doped with an amount of lithium exceeding the amount corresponding to the irreversible capacity that is expected to be generated.

[0099] A portion of the doped Li is consumed to form the SEI coating, making dedoping difficult. The amount of Li consumed corresponds to the irreversible capacity. The remaining portion of the doped Li is dedoppable, and in the next step, a portion of this dedoppable Li is left in the negative electrode, while the other portion is dedoped. This process allows for the production of a negative electrode containing residual lithium.

[0100] Using the negative electrode and positive electrode containing residual lithium manufactured in this manner, a lithium-ion battery can be assembled using the same method as before.

[0101] Alternatively, a method may be used in which a larger amount of lithium or lithium compound is placed in the cell, reducing the lithium content of the positive electrode, and completing the lithiation process within the cell. Specifically, a lithium-ion battery is assembled using a negative electrode doped with an amount of lithium or lithium compound exceeding the amount corresponding to the irreversible capacity, and a positive electrode containing less than 100% lithium. The negative electrode may then be delithiated by discharging.

[0102] A method for manufacturing a lithium-ion battery according to one aspect of the present disclosure may include a step of assembling a lithium-ion battery containing a first electrolyte. The second electrolyte used in the doping step and the dedoping step may be a different type of electrolyte from the first electrolyte.

[0103] The first electrolyte can use additives that form a protective film called SEI on the negative electrode, including additives that have the drawback of generating gas. Subsequently, the amount of SEI-forming additives in the second electrolyte can be reduced, allowing for the selection of a composition that matches the rate characteristics or cycle characteristics. In addition to SEI formation, electrolytes for protecting the current collector foil, such as aluminum foil, can be added to the first electrolyte, and the first electrolyte can be operated over a wide voltage range to pre-induce reactions that do not occur in the normal voltage operating range.

[0104] Furthermore, a conventional electrolyte may be used for the first electrolyte, and a gel electrolyte may be used for the second electrolyte. As the first electrolyte, for example, lithium hexafluoride phosphate (LiPF) may be added to a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). ６ A general non-aqueous electrolyte (a solution of ) dissolved at a concentration of 1 mol / L may also be used. In this way, the problems of gel electrolytes, such as difficulty in degassing or uneven operation of the active material due to insufficient penetration into the negative electrode pores, can be resolved by performing the initial charge and discharge using the first electrolyte, and then by using the gel electrolyte as the second electrolyte, operation with excellent cycle characteristics can be achieved.

[0105] As the second electrolyte, the same EC / DEC mixed solvent as the first electrolyte and LiPF ６ A highly reliable electrolyte can be used, which is based on this composition and gelled by adding polymer materials such as polyvinylidene fluoride (PVDF) or polyacrylonitrile (PAN). As a second electrolyte, an electrolyte with low flammability that can be classified as a non-hazardous material without gelling can also be used, such as an electrolyte using a solvent with high viscosity and a high boiling point, or an ionic liquid.

[0106] [Embodiment 3] Embodiment 3 describes another example of a method for manufacturing a lithium-ion battery according to one aspect of the present disclosure. By bringing the lithium foil into direct contact with the negative electrode active material layer of the negative electrode, lithium ions diffuse from the lithium foil to the negative electrode and react. This phenomenon can also be used to perform the initial lithiation (prelithiation) of the negative electrode.

[0107] In other words, a method for manufacturing a lithium-ion battery according to one aspect of the present disclosure may include a step of preparing a negative electrode containing at least a silicon-based active material as a negative electrode active material, and a lithiation step of lithifying the negative electrode active material by reacting the negative electrode active material with lithium foil. In the lithiation step, the electrolyte may be injected without waiting for the reaction between the negative electrode active material and the lithium foil to be completed, and the lithiation step and the step of performing the first charge and discharge may be carried out simultaneously. In other words, in the lithiation step using lithium foil, lithiation may be carried out by a chemical reaction without applying electricity, or lithiation may be carried out by both a chemical reaction and the application of electricity.

[0108] In this disclosure, the capacity of the negative electrode when fully charged is defined as the first capacity. The capacity corresponding to the lithium ions that move from the lithium foil to the negative electrode during the lithiation process is defined as the second capacity. The ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity ratio.

[0109] The initial discharge capacity is lower than the capacity at full charge (first capacity). The battery capacity lost at this time is the irreversible capacity. Generally, the irreversible capacity is about 10% of the capacity at full charge (i.e., the irreversible capacity ratio is about 0.10), but this varies depending on the type of negative electrode active material.

[0110] In the lithiation process, the energy density of the lithium-ion battery can be increased by setting the ratio of the second capacity to the first capacity (second capacity / first capacity) to a value greater than or equal to the irreversible capacity ratio plus 0.07 and less than or equal to the irreversible capacity ratio plus 0.90. The ratio of the second capacity to the first capacity after the lithiation process may be a value greater than or equal to the irreversible capacity ratio plus 0.15 and less than or equal to 0.7. The ratio of the second capacity to the first capacity may be less than or equal to a value greater than or equal to the irreversible capacity ratio plus 0.30 and less than or equal to 0.40. This also applies to Embodiment 2.

[0111] Here, lithium ions present at the negative electrode at the point of interest, other than those corresponding to the irreversible capacity, are referred to as reversible ions. Furthermore, the ratio of the capacity corresponding to the reversible ions at the point of interest to the capacity of the negative electrode when fully charged is referred to as the reversible capacity ratio.

[0112] Assume that the ratio of the capacity corresponding to the lithium ions moved from the lithium foil 21 to the negative electrode 2 by the lithiation process (second capacity) to the capacity of the negative electrode when fully charged (first capacity) is 0.17, and the irreversible capacity ratio is 0.10. In this case, the reversible capacity ratio of the negative electrode after the lithiation process is 0.07.

[0113] In conventional lithium-ionization processes, only an amount of lithium ions equivalent to the irreversible capacity was added to the negative electrode. Adding more lithium ions than the irreversible capacity to the negative electrode was not done because it would lead to a decrease in the capacity of the lithium-ion battery.

[0114] In contrast, in this disclosure, in addition to an amount of lithium ions equivalent to the irreversible capacity, an amount of lithium ions such that the reversible capacity ratio is 0.07 or higher is diffused to the negative electrode during the lithiation process. The inventors have found that this configuration provides the same effects as those described in Embodiment 1.

[0115] Figure 9 is a table showing the maximum voltage (set upper limit voltage) of the negative electrode when metallic lithium is used as the counter electrode and the negative electrode is discharged at 0.1C. The voltage 15 minutes after the discharge has stopped is also shown. In Figure 9, "Value of lithium ions added to irreversible capacity" is the value of the reversible capacity ratio of the negative electrode after the lithiation process, expressed as 100%. As mentioned above, in practice, the reversible capacity ratio is approximately the same as the Li remaining rate, so it is acceptable to interpret the reversible capacity ratio as the Li remaining rate.

[0116] As shown in Figure 9, the upper limit voltage setting of 0.40V corresponds to a reversible capacitance ratio of 60%, so the upper limit voltage setting may be 0.40V. The reversible capacitance ratio may also be set between 7% and 90%. The upper limit voltage setting of 0.20V corresponds to a reversible capacitance ratio of 90%, so the upper limit voltage setting may be 0.20V.

[0117] Furthermore, since the upper limit voltage setting of 1.07V corresponds to a reversible capacitance ratio of 7%, the upper limit voltage setting may be set to 0.20V to 1.07V, or to 0.40V to 1.07V.

[0118] Figure 10 is a schematic diagram showing the manufacturing method of a lithium-ion battery in Embodiment 3. In Figure 10, the lithiation step is performed as a step in assembling a lithium-ion battery by interposing a lithium foil 21 between the negative electrode 2 containing the negative electrode active material and the positive electrode 1. Each step shown in Figure 10 is merely an example, and the lithiation step may be performed as a separate step from the lithium-ion battery assembly step, and before the assembly step.

[0119] In the example shown in Figure 10, the negative electrode 2 and the lithium foil 21 are stacked, and then the separator 3 and the positive electrode 1 are laminated on top of the lithium foil 21. It is sufficient that the lithium foil 21 is positioned between the negative electrode 2 and the positive electrode 1. Alternatively, the lithium foil 21 may be laminated on top of the positive electrode 1, and the positive electrode 1 and the lithium foil 21 may be treated as a laminated film, and the laminated film and the negative electrode 2 may be stacked so that the lithium foil 21 of the laminated film and the negative electrode 2 are in contact via the separator. Alternatively, the lithium foil 21 may be attached to the separator 3, and the laminated film of the separator 3 and lithium foil 21 may be stacked on top of the negative electrode 2 so that the lithium foil 21 and the negative electrode 2 are in contact. Alternatively, the lithium foil 21 attached to the separator 3 may be attached to the side that contacts the positive electrode 1, and then the lithium foil 21 may be stacked on top of the negative electrode 2.

[0120] The assembly process of a lithium-ion battery (in other words, the lithiation process) may be carried out by applying pressure to the laminate of the positive electrode 1, separator 3, lithium foil 21, and negative electrode 2 using a crimping roll. Alternatively, the assembly process may be carried out by winding the strip-shaped laminate with a winding roll. Winding the laminate generates internal pressure, which promotes lithiation.

[0121] The method for manufacturing the lithium-ion battery may include, as a post-assembly step, a step of charging (charging step) and a step of discharging (discharging step) a lithium-ion storage battery comprising a negative electrode 2 containing the lithified negative electrode active material and a positive electrode 1. The charging step and the discharging step may be performed under ambient pressure of 25 psi or more.

[0122] When assembling a lithium-ion battery by winding up the aforementioned laminate, sufficient internal pressure is generated when the laminate is wound up, but charging and discharging may be performed with pressure applied from the outermost periphery. Charging and discharging under an ambient pressure of 25 psi or more only needs to be done the first time.

[0123] The negative electrode is fully charged during the charging process, followed by a discharge process. After the discharge process is completed, the discharge may be performed such that the ratio of the capacity corresponding to the lithium ions remaining in the negative electrode to the first capacity is equal to or greater than the irreversible capacity ratio plus 0.07.

[0124] During the initial charge, not all lithium ions that move from the positive electrode to the negative electrode necessarily return to the positive electrode upon complete discharge; some lithium ions may remain on the negative electrode. The degree of this phenomenon depends on the material of the positive electrode. Therefore, even if the reversible capacity ratio of the negative electrode immediately after the lithiation process is 0.07, if a positive electrode active material is used that contains a large amount of lithium that does not return to the positive electrode, the reversible capacity ratio after the initial discharge may exceed 0.07. Thus, considering the amount of lithium ions that remain on the negative electrode without returning to the positive electrode upon completion of the initial discharge, the amount of lithium ions moved from the lithium foil to the negative electrode during the lithiation process may be set to be smaller.

[0125] For example, let's assume that the target sum of irreversible capacity and reversible capacity after the initial discharge is 0.26, the irreversible capacity is 0.10, and the reversible capacity, which corresponds to the amount of lithium ions that do not return to the positive electrode during the initial discharge, is 0.08. In this case, the amount of lithium ions moved from the lithium foil to the negative electrode in the lithiation process (second capacity) can be set to an amount equivalent to 8% (= 0.26 - 0.10 - 0.08) of the negative electrode's capacity when fully charged (first capacity).

[0126] [Example 1] (Method for separating lithium and cell assembly) As the negative electrode, an active material consisting of a composite of silicon particles and graphite was mixed with an auxiliary agent consisting of graphite, a binder and a thickener were added and dispersed in water to make a slurry, which was then applied to a copper current collector foil and dried to produce a negative electrode plate. After drying, it was pressed.

[0127] A multilayer microporous membrane made of polypropylene and polyethylene was used for the separator. Each component was dried and treated to prevent moisture from being introduced during battery fabrication.

[0128] Next, the negative electrode was constructed by bonding a separator and a sheet of copper foil with a 150 μm lithium foil attached, so that the lithium foil and the negative electrode active material faced each other through the separator. Tabs were ultrasonically welded to the current collector foils of both the negative electrode and the lithium foil, and the materials were wound using a winding device. All subsequent processes after the fabrication of this wound assembly were carried out in a dry atmosphere at the level of battery manufacturing.

[0129] This rolled-up material was inserted into a pouch that was closed on three sides. The pouch did not have to be an aluminum pouch like those used for batteries, but an aluminum pouch was used here for its strength during handling.

[0130] As the first electrolyte, lithium hexafluoride phosphate was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, and vinylene carbonate and fluoroethylene carbonate were further added.

[0131] The first electrolyte was injected into the pouch, and the cell was temporarily assembled. The cell was then held between a mounting jig consisting of a metal plate and urethane resin, and pressure was applied via a spring to maintain a pressure of 36 psi on the negative electrode. When connected to the charge / discharge device, the voltage displayed was positive. After this, lithiation and delithiation were performed at room temperature. Since lithiation and delithiation begin with the process of moving lithium from the lithium foil to the negative electrode, the target voltage was set to 0V and charging was started.

[0132] Generally, when a lithium-ion battery operates, energy is stored during the process where lithium reacts with the negative electrode active material to lithize it, and lithium is lost from the positive electrode active material. This process is called charging, and the reverse is called discharging. Therefore, for the sake of consistency in terminology, the process of lithizing the negative electrode active material will be called charging, and the process of delithizing it will be called discharging.

[0133] Charging was initiated with a constant current and maintained at 0V with constant voltage charging. Subsequently, discharging was performed with a constant current. The maximum voltage indicated in this disclosure is determined by the discharge voltage set to 0V. In this embodiment, the discharge voltage was set to 0.56V. When the voltage reached the upper limit voltage of 0.56V, discharging was stopped, and the device was left for 15 minutes without current flowing or voltage being applied, at which point the voltage dropped to 0.48V. At this point, the initial charge and discharge cycle was completed.

[0134] Next, the electrodes were removed from the pouch. The electrolyte could be washed with dimethyl carbonate or the like, but in this example, the electrode was not washed with the electrolyte before proceeding to the next step.

[0135] Next, the process to finalize the electrode was carried out. For the positive electrode, a ternary oxide containing nickel, cobalt, and manganese was used as the positive electrode active material, which was coated onto an aluminum current collector foil and dried to produce the positive electrode. For the separator, a multilayer microporous film of polypropylene and polyethylene was used. Each component was dried and treated to prevent moisture from being introduced during battery manufacturing.

[0136] Here, the previously lithiated and delithiated negative electrodes were bonded together so that the negative electrode active material and the positive electrode active material faced each other via a separator, wound into a coil, and inserted into an aluminum pouch. Then, the second electrolyte was injected. As the second electrolyte, a general non-aqueous electrolyte was used, which consisted of lithium hexafluoride phosphate dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, with less vinylene carbonate and fluoroethylene carbonate added than in the first electrolyte.

[0137] After injecting the second electrolyte and performing the initial charge and discharge of the battery cell, the cell was degassed and sealed. The Coulomb efficiency at this time was 91.1%, which is equivalent to the initial Coulomb efficiency of 91.0% obtained from the initial charge and discharge of a half-cell with the positive electrode facing lithium, prepared in the same manner.

[0138] By doing so, it was possible to assemble the cell while retaining the desired lithium in the silicon anode, and also to change the composition of the electrolyte used in the prelithiation process from the electrolyte that is actually sealed in the cell and sold as a battery on the market.

[0139] Furthermore, because prelithiation is performed by voltage scanning, the amount of lithium introduced can be determined more accurately, and the variation in the degree of prelithiation in the contained negative electrode active material is small. The status of prelithiation can also be easily grasped from the voltage value or profile, making it an excellent manufacturing method from a traceability standpoint.

[0140] [Example 2] For the positive electrode, a ternary oxide containing nickel, cobalt, and manganese was used, and the positive electrode plate was fabricated by coating it onto an aluminum current collector foil and drying it. For the negative electrode, an active material consisting of a composite material of silicon particles and graphite was mixed with an auxiliary agent consisting of graphite, and a binder and a thickener were added and dispersed in water to prepare a slurry, which was then coated onto a copper current collector foil and dried to prepare a negative electrode plate. After drying, it was pressed. For the separator, a multilayer microporous membrane of polypropylene and polyethylene was used. The above components were dried and then moved to a dry atmosphere process.

[0141] Next, the negative electrode was positioned facing the positive electrode via a separator, and a 6 μm thick lithium foil sheet was placed between the negative electrode and the separator. The 6 μm thickness of the lithium foil sheet corresponds to a capacity equal to 0.2 times the capacity required to fully charge the cell, plus a capacity equivalent to the irreversible capacity. The lithium foil containing the lithium that the positive electrode would contain was pre-bonded to the negative electrode so that only the negative electrode could be handled during lamination. Tabs were ultrasonically welded to the current collector foils of both the negative electrode and the lithium foil, and then wound using a winding device.

[0142] Next, the electrode was placed in an aluminum pouch to form the final product. As the electrolyte, a general non-aqueous electrolyte was used, which consisted of lithium hexafluoride phosphate dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, with vinylene carbonate and fluoroethylene carbonate added.

[0143] The electrolyte was injected, the pouch was clamped between a metal plate and urethane jig, and a pressure of 36 psi was applied via a spring. After the initial charge and discharge of the battery cell in this state, the cell was degassed and sealed.

[0144] In this way, it was possible to assemble the cell while retaining the desired lithium in the silicon anode, and to change the composition of the electrolyte used in prelithiation from the electrolyte that is actually sealed in the cell and sold as a battery on the market. The initial Coulomb efficiency at this time was 91.0%, which is equivalent to the initial Coulomb efficiency of 91% of a half-cell made by facing the same positive electrode with lithium foil.

[0145] During the initial charge and discharge, 36 psi was applied, but when creating a battery pack with multiple cells, the battery pack was designed to operate with only 9 psi of pressure applied, by sandwiching it between plates and securing it with a band.

[0146] By doing as described above, we were able to ensure that the negative electrode contained the desired amount of lithium simply by controlling the thickness of the lithium foil. The process was also almost the same as conventional processes, except for laminating the lithium foil to the negative electrode and the increased drying environment required for handling the lithium foil. As a result, we were able to produce a battery with high Coulomb efficiency and high energy density.

[0147] [Example 3] Another example of the process of manufacturing a negative electrode according to one aspect of the present disclosure is shown. First, the negative electrode was prepared. A silicon-based active material consisting of a composite material of silicon particles and graphite was mixed with an auxiliary agent consisting of graphite, a binder and a thickener were added and dispersed in water to make a slurry, which was then applied to a copper current collector foil and dried to produce a negative electrode plate. After drying, it was pressed.

[0148] There are no particular restrictions based on the amount measured, but here it is 11 mg / cm². ２The electrodes used were as follows: The active material, consisting of a composite of silicon particles and graphite, accounted for 84% of the weight of the electrodes, excluding the copper current collector foil of the negative electrode. These were cut to a test size and vacuum-dried at 110°C for 12 hours in a dry environment for lithium-ion battery testing.

[0149] Next, a separator and a copper foil with lithium foil were prepared. The separator was a multilayer microporous film made of polypropylene and polyethylene. The copper foil with lithium foil had a lithium layer attached to it that was 150 μm thick. The separator and the copper foil with lithium foil were brought into a dry environment for lithium-ion battery testing.

[0150] Tabs for conducting electricity from outside the pouch were ultrasonically welded to the copper foils of the negative electrode and lithium foil, and these were attached facing each other via a separator. The assembly was then inserted into an aluminum pouch with three sides closed, the electrolyte was injected, and the remaining side was sealed using a vacuum sealing machine to remove the air from inside the pouch, thus forming a battery cell.

[0151] A common electrolyte was used, consisting of a solvent combined with linear and cyclic carbonates. Specifically, lithium hexafluoride phosphate was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, and vinylene carbonate and fluoroethylene carbonate were added.

[0152] The cell fabricated in this manner is called a negative electrode cell. With a pressure of 36 psi applied to this negative electrode cell, it was connected to a charge / discharge test machine. Immediately after connection, the charge / discharge test machine showed a cell voltage of 3.4V. This indicates that the potential of the negative electrode relative to the lithium foil is +3.4V. In this state, constant current charging was performed at 0.05C down to 0V (as mentioned earlier, the process of lithiating the negative electrode active material is called the charging process), and then low-voltage charging was performed at 0V until it reached 0.005C. In this process, the silicon-based active material of the negative electrode was lithified. If the negative electrode contains not only silicon-based active material but also graphite-based active material or other materials, those active materials will also be lithified at the same time. The charge capacity obtained here is called the first capacity of the negative electrode when fully charged.

[0153] Next, the voltage limit was set to 1.9V and discharge was started. In this process, delithiation occurs, where lithium moves from the silicon-based active material on the negative electrode to the lithium foil side. The voltage limit of 1.9V was also the voltage value used to calculate the catalog value of the capacity of the silicon-based active material used in this experiment.

[0154] The discharge capacity obtained here is the manufacturer's recommendation and was obtained after a large potential difference of 1.9V relative to Li, and can be considered the maximum capacity that the negative electrode can discharge on the first attempt. The difference between the discharge capacity obtained at this time and the first capacity is called the irreversible capacity, and the ratio of the irreversible capacity to the first capacity is called the irreversible capacity ratio. The first capacity was 109.1mAh, and the discharge capacity when discharged to 1.9V was 95.4mAh, so the irreversible capacity was 13.7mAh. As a result, the irreversible capacity ratio was 0.126.

[0155] Next, a negative electrode cell prepared in the same manner was subjected to constant current charging and low-voltage charging down to 0V. For the next discharge, the discharge upper limit voltage was set to 0.534V and discharge was started. Discharge ended when the voltage of the negative electrode cell reached 0.534V, and after 15 minutes, the voltage had dropped to 0.458V. Since the voltage gradually stabilized, it is thought that the overvoltage during discharge disappeared and the voltage displayed a value close to the potential of the negative electrode. The first capacity of this negative electrode cell was 115.2mAh, and the discharge capacity when discharged to 0.534V was 66.8mAh. The difference between this discharge capacity and the first capacity, 48.4mAh, is the second capacity.

[0156] The irreversible capacitance ratio of the negative electrode cell, determined from the negative electrode cell discharged to 1.9V, was 0.126. The second capacitance of the negative electrode cell, where discharge was stopped at 0.534V, was 48.4mAh, and its ratio to the first capacitance was 0.420. The difference between these two values ​​is 0.294, so the negative electrode that stopped discharge at 0.534V was able to retain lithium in the negative electrode at a ratio equal to the irreversible capacitance ratio plus 0.294.

[0157] Each negative electrode was removed from the negative electrode cell to fabricate a lithium-ion battery cell. For the positive electrode, a ternary oxide containing nickel, cobalt, and manganese was used as the positive electrode active material, which was coated onto an aluminum current collector foil and dried to create the positive electrode. The positive electrode was cut to match the size of the cut negative electrode, with each side being 1 mm smaller than the negative electrode. A multilayer microporous membrane of polypropylene and polyethylene was used as the separator. Each component was dried to prevent moisture from being introduced during battery fabrication. Tab electrodes were ultrasonically welded to the current collectors of the positive and negative electrodes to enable charging and discharging.

[0158] Here, the previously lithiated and delithiated negative electrodes were bonded together so that the negative electrode active material and positive electrode active material faced each other via a separator, inserted into an aluminum pouch, and the electrolyte was injected. As the electrolyte, a general non-aqueous electrolyte was used, which consisted of lithium hexafluoride phosphate dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, with vinylene carbonate and fluoroethylene carbonate added.

[0159] After injecting the electrolyte and performing the initial charge and discharge of the battery cell, the cell was degassed and sealed. The initial Coulomb efficiency at this time was 91.0% for a half-cell with the positive electrode facing lithium, which was prepared in the same way, while the initial Coulomb efficiency of a cell using a negative electrode discharged to 1.9V was 82%, and the initial Coulomb efficiency of a cell using a negative electrode that was stopped discharging at 0.534V was 91.1%.

[0160] From the above, we were able to demonstrate that by using a negative electrode with lithium remaining in a value greater than the irreversible capacity ratio, the initial Coulomb efficiency can be obtained to be almost the same as that of the positive electrode, thereby achieving a high energy density.

[0161] In this embodiment, charging and discharging are performed on a single cell, but connecting multiple cells in series, parallel, or series-parallel configurations is also effective for productivity in efficiently performing lithium lithiation and delithiation. Furthermore, while one pair of negative electrode and lithium foil is placed in a single pouch, multiple pairs may be used, and the negative electrode and lithium foil may be provided on both sides of the copper current collector foil. When controlling the voltage for lithium lithiation and delithiation, connecting each foil in parallel as much as possible allows for lithium lithiation and delithiation of multiple negative electrodes with less variation. In particular, it allows for the retention of lithium at a stable ratio of second capacity despite variations in area and A / C ratio. Additionally, connecting them in series allows for the processing of multiple foils with a small amount of current, enabling the use of inexpensive, thin cables for current transmission, resulting in simpler equipment and lower processing costs.

[0162] [Example 4] Not all lithium ions that moved from the positive electrode to the negative electrode during the initial charge necessarily return to the positive electrode during complete discharge, and some lithium ions may remain at the negative electrode. Conversely, LFP (lithium iron phosphate, LiFePO) ４ There are also excellent active materials, such as those shown above, in which almost all of the lithium in the positive electrode returns to the positive electrode. Considering such cases, it is possible to set a smaller amount of lithium ions to move from the lithium foil to the negative electrode during the lithiation process. The capacitance ratio of the positive and negative electrodes is called the A / C ratio, and this ratio may also be taken into consideration. Even if the capacitance of the positive electrode is the same, the capacitance percentage of the negative electrode that increases due to lithium ions moving from the positive electrode to the negative electrode becomes smaller as the A / C ratio increases. An example of this capacitance ratio is shown in Figure 11.

[0163] Figure 11 is a table comparing the amount of lithium remaining in the negative electrode under various conditions, in a configuration where the negative electrode is lithiated and delithiated by facing it with a lithium negative electrode, the negative electrode is removed, and then a lithium-ion battery cell is fabricated using the positive electrode, separator, and electrolyte. Examples 1 to 6 are shown in Figure 11.

[0164] The A / C ratio is the value obtained by dividing the capacity per unit area of ​​the positive electrode by the capacity per unit area of ​​the negative electrode. This value is calculated from the initial Coulomb efficiency of the battery cell and represents the proportion of lithium that moved from the positive electrode to the negative electrode during the initial charge but did not return to the positive electrode during the initial discharge. It is the remaining value obtained by subtracting the initial Coulomb efficiency of a lithium-ion battery cell made using a negative electrode extracted after lithiation and delithiation of the negative electrode from 1. This value indicates the proportion of lithium that moved from the positive electrode to the negative electrode but did not return to the positive electrode; in other words, it represents the amount of lithium further stored in the negative electrode.

[0165] The reversible capacity rate (considering the A / C ratio) increased at the negative electrode by lithium that did not return to the positive electrode during the initial discharge is calculated from the initial Coulomb efficiency of the battery cell. It is obtained by dividing the proportion of lithium that moved from the positive electrode to the negative electrode during the initial charge but did not return to the positive electrode during the initial discharge by the A / C ratio, and then providing a clear indication of how much the amount of lithium increased for the negative electrode active material.

[0166] The sum of (the percentage of the second capacity) and (the percentage of lithium that moved from the positive electrode to the negative electrode and did not return) represents the percentage of lithium that ultimately remains at the negative electrode after the initial charge and discharge of the lithium-ion battery cell.

[0167] Examples 1 to 4 are examples using the same negative electrode, but with variations in the lithium-ion and delithiation processes, specifically the upper limit voltage during delithiation. For the positive electrode, a ternary oxide containing nickel, cobalt, and manganese was used as the positive electrode active material, which was coated onto an aluminum current collector foil and dried to produce the positive electrode. A multilayer microporous film of polypropylene and polyethylene was used as the separator. Each component was dried and treated to prevent moisture from being introduced during battery fabrication.

[0168] Here, the lithiated and delithiated negative electrodes from Examples 1 to 4 were bonded together with the negative electrode active material and positive electrode active material facing each other via a separator, wound into a coil, and inserted into an aluminum pouch. Next, the electrolyte was injected. As the electrolyte, a general non-aqueous electrolyte was used, which consisted of lithium hexafluoride phosphate dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate, with vinylene carbonate and fluoroethylene carbonate added. Subsequently, the results of the initial charge and discharge of the battery cell while applying a pressure of 9 psi were obtained and are shown in this table. It can be seen that under each condition, the proportion of the second capacity plus the proportion of lithium that moved from the positive electrode to the negative electrode and did not return was greater than the proportion of the second capacity.

[0169] Next, let's discuss Example 5. In Example 5, LFP (lithium iron phosphate) is used as the positive electrode. In this example, all the lithium that moves from the positive electrode to the negative electrode returns to the positive electrode, so although the same upper limit voltage in the delithiation process is set as in Example 1, the (proportion of the second capacitance) + (proportion of lithium that moved from the positive electrode to the negative electrode and did not return) is smaller than in Example 1. In this case, it can be seen that if you want to obtain a more stable and high Coulomb efficiency, it is desirable to further lower the upper limit voltage in the delithiation process.

[0170] Next, Example 6 describes an example where the A / C ratio of the negative electrode is changed. This cell is manufactured under the same conditions as Example 2, but with the negative electrode being thicker. As a result, (the ratio of the second capacitance) + (the ratio of lithium that moved from the positive electrode to the negative electrode and did not return) results in an upper limit voltage in the delithiation process that is similar to that in Example 1. Even in such cases, it is desirable to further lower the upper limit voltage in the delithiation process.

[0171] [Additional Notes] The inventions described in this disclosure have been explained based on the drawings and embodiments. However, the inventions described in this disclosure are not limited to the embodiments described above. That is, the inventions described in this disclosure can be modified in various ways within the scope shown in this disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the inventions described in this disclosure. In other words, it should be noted that it is easy for those skilled in the art to make various modifications or alterations based on this disclosure. Furthermore, it should be noted that these modifications or alterations are included in the scope of this disclosure.

[0172] 1 Positive electrode 1A Positive electrode active material layer (layer containing positive electrode active material) 2 Negative electrode 2A Negative electrode active material layer (layer containing negative electrode active material) 10 Lithium-ion battery

Claims

A negative electrode comprising at least a silicon-based active material as the negative electrode active material, The capacity of the negative electrode when fully charged is defined as the first capacity. The ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity ratio. The second volume is defined as the volume corresponding to the lithium ions that have moved to the negative electrode due to the lithiation of the negative electrode active material, or the lithium ions that remain on the negative electrode due to delithiation after the lithiation. The ratio of the second capacity to the first capacity is greater than or equal to the irreversible capacity ratio plus 0.07 and less than or equal to the irreversible capacity ratio plus 0.90, in the negative electrode. 　 The negative electrode used in lithium-ion batteries, The negative electrode active material includes at least a silicon-based active material, A negative electrode in which the ratio of the amount of lithium remaining in the negative electrode without being dedoped to the amount of lithium that can be dedoped after the initial lithiation is 0.07 or more and 0.90 or less. 　 The negative electrode used in lithium-ion batteries, A negative electrode in which metallic lithium is used as the counter electrode and, when discharged at a discharge rate of 0.1C, the maximum voltage of the negative electrode is 0.20V or more and 1.07V or less. 　 The negative electrode according to claim 3, wherein the maximum voltage of the negative electrode is 0.40V or more and 0.56V or less. 　 A negative electrode according to any one of claims 1 to 4, A lithium-ion battery comprising a positive electrode containing lithium as the positive electrode active material. 　 A positive electrode containing lithium as the positive electrode active material, The negative electrode comprises at least a silicon-based active material as the negative electrode active material, A lithium-ion battery containing 1.07 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. 　 The lithium-ion battery according to claim 6, wherein the lithium contained is 1.15 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. 　 The lithium-ion battery according to claim 6, wherein the lithium contained is 1.20 times or more and less than 1.90 times the maximum amount of lithium that the positive electrode can contain. 　 The lithium-ion battery according to claim 5, wherein the negative electrode is initially doped with lithium and contains lithium greater than 1x and 15x the maximum amount of lithium that can be dedoped. 　 A step of doping a negative electrode containing at least a silicon-based active material as the negative electrode active material with an amount of lithium exceeding the amount corresponding to the irreversible capacity of the negative electrode, A method for manufacturing a lithium-ion battery, comprising the step of dedoping only a portion of the dedopable lithium from the negative electrode. 　 A method for manufacturing a lithium-ion battery according to claim 10, wherein the doping step and the dedoping step are performed under an ambient pressure of 25 psi to 300 psi. 　 The process includes assembling the lithium-ion battery containing a first electrolyte, The method for manufacturing a lithium-ion battery according to claim 10 or 11, wherein the second electrolyte used in the doping step and the dedoping step is an electrolyte of a different type from the first electrolyte. 　 A step of preparing a negative electrode containing at least a silicon-based active material as the negative electrode active material, The process includes a lithiation step in which the negative electrode active material of the negative electrode is reacted with lithium foil to lithify the negative electrode active material, The capacity of the negative electrode when fully charged is defined as the first capacity. The ratio of the irreversible capacity to the first capacity is defined as the irreversible capacity ratio. The second volume is defined as the volume corresponding to the lithium ions that move from the lithium foil to the negative electrode in the lithiation process. A method for manufacturing a lithium-ion battery, wherein the ratio of the second capacity to the first capacity is greater than or equal to the irreversible capacity ratio plus 0.07 and less than or equal to the irreversible capacity ratio plus 0.

90. The method for manufacturing a lithium-ion battery according to claim 13, wherein the lithiation step is performed as a step of assembling the lithium-ion battery by interposing the lithium foil between the negative electrode containing the negative electrode active material and the positive electrode. 　 The method for manufacturing a lithium-ion battery according to claim 13 or 14, wherein the lithiation step is performed under conditions where a pressure of 25 psi or more is applied to the negative electrode. 　 A method for manufacturing a lithium-ion battery according to any one of claims 10 to 12, wherein the doping step is advanced by applying an electric current. 　 A method for manufacturing a lithium-ion battery according to claim 14 or 15, wherein in the lithiation step using the lithium foil, the lithiation is carried out by a chemical reaction without applying an electric current, or the lithiation is carried out by both a chemical reaction and the application of an electric current.

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