JP2007335086A - Manufacturing method of electrode for lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the lithium must be removed from the negative electrode in advance for normal battery operation since the negative electrode using the silicon as an active material has an expansion rate large enough to easily generate wrinkling in a collector; and a battery built in combination with the negative electrode using the lithium-silicon alloy film capable of reducing the wrinkling as an active material, and the positive electrode containing the lithium such as LiCoO<SB>2</SB>is unable to operate because both of the electrodes contain the lithium. <P>SOLUTION: The manufacturing method of electrodes for a lithium battery includes a primary step for forming the lithium-silicon alloy film on a collector using the vacuum deposition method, and a secondary step for thermal treatment of the films formed by the primary step. The thermal treatment in the secondary step should be performed under the inert gas atmosphere or vacuum condition in the temperature range from 180°C up to 280°C. This method ensures easy manufacturing of the high-capacity negative electrode with the lithium removed from the lithium-silicon alloy film, and allows the high-capacity lithium battery to be manufactured in combination with the positive electrode containing the lithium such as LiCoO<SB>2</SB>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing an electrode for a lithium battery.

  In recent years, the miniaturization and multi-functionalization of portable devices have progressed, and accordingly, the capacity of a battery as a power source for portable devices has been eagerly desired. The theoretical capacity of carbon, which is a negative electrode active material mainly used at present, is 372 mAh / g. Silicon having a theoretical capacity of 4200 mAh / g is considered promising as an active material capable of higher capacity than carbon. Therefore, many materials including silicon and structures of materials including silicon have been studied.

  One of them is an active material film in which silicon is formed as a film on a current collector. There is a report that the silicon film expands to about 400% due to occlusion of lithium, and there is a problem that the current collector is wrinkled by expansion. In addition, there is a problem that the initial discharge capacity becomes smaller than the initial charge capacity at the time of the first charge / discharge after the battery is manufactured (hereinafter, (initial charge capacity) − (initial discharge capacity) is referred to as retention capacity).

  In order to solve these two problems, an active material film made of a lithium silicon alloy film in which lithium and silicon are simultaneously formed has been proposed (for example, see Patent Document 1). Here, the lithium silicon alloy is a mixture of lithium and silicon. The alloy structure includes a solid solution, a eutectic (eutectic mixture), a compound (intermetallic compound), or a material in which they coexist, but is not particularly limited.

  A silicon film expand | swells by occlusion of lithium by charge. Stress caused by the expansion of the silicon film is applied to the current collector, and when the stress exceeds a predetermined value, the current collector exceeds elastic deformation and becomes irreversible, resulting in wrinkles. If the elongation due to the expansion stress is within the elastic deformation range of the current collector, it is desirable because the current collector does not wrinkle.

  By simultaneously forming lithium when forming silicon on the current collector, it is possible to obtain a lithium silicon alloy film considered to have characteristics close to those of a silicon film electrochemically occluded with lithium. The lithium silicon alloy film thus obtained is considered to have no expansion stress applied to the current collector despite containing lithium.

  If a lithium-silicon alloy film in which lithium and silicon are simultaneously formed can be used as an active material film, it can be used as a high-capacity negative electrode that does not cause wrinkles in the current collector.

  In addition, in order to compensate for the retention capacity, it is known that lithium for the retention capacity may be added to the silicon film in advance before the battery is manufactured using the silicon film as an active material film. It has been reported that the retention capacity is about 5% to 8% of the initial charge capacity, and a lithium silicon alloy film containing lithium of this ratio or more may be formed.

As a high-capacity negative electrode other than silicon, there is a lithium-containing composite nitride. A negative electrode in which silicon nitride is added to lithium-containing composite nitride has also been proposed (see, for example, Patent Document 2).
JP 2005-190902 A JP 2003-338282 A

  A value obtained by dividing the amount of lithium atoms in the film of Patent Document 1 by the amount of silicon atoms (hereinafter referred to as “lithium / silicon ratio”) in the range of 0.01 to 0.5 is a condition for supplementing the retention capacity. It can be said that. If the lithium / silicon ratio of the lithium silicon alloy film formed on the current collector is as small as 0.01 or more and 0.5 or less, the film expands greatly when charged as an active material film. That is, it is the amount of lithium exceeding the retention capacity in the lithium silicon alloy film having a large lithium / silicon ratio that helps to relieve the expansion stress.

An active material film made of a lithium silicon alloy having a large lithium / silicon ratio can be obtained by adjusting the film formation amount when lithium and silicon are simultaneously formed. However, when a battery is configured with a positive electrode containing lithium such as lithium cobaltate (LiCoO ₂ ) or lithium nickelate (LiNiO ₂ ) that is widely used at present, the battery cannot be operated.

  In other words, the lithium battery is charged and discharged by the movement of lithium ions between the positive electrode and the negative electrode. However, in this combination, both the positive electrode and the negative electrode contain lithium, so the lithium ions do not move and are charged. Unable to discharge. That is, in order to operate as a battery, there is a problem that lithium must be desorbed from either the positive electrode or the negative electrode in advance.

As a method for desorbing lithium from the active material film before manufacturing the battery, there is an electrode formation method for electrochemical desorption. For example, when considering a negative electrode active material film containing lithium having a retention capacity or more, a model cell is manufactured using a negative electrode active material film and a lithium foil as a counter electrode, and the voltage is about +1.5 V with respect to the lithium foil. Excess lithium can be desorbed from the negative electrode active material film by supplying a constant current of about 0.1 C to the upper limit. However, in this method, after desorbing lithium from the negative electrode active material film, it is necessary to reassemble with a positive electrode such as LiCoO ₂ originally used to produce a battery. Therefore, there is a problem that productivity is extremely poor.

  In Patent Document 2, by mixing a lithium-containing composite nitride and a silicon nitride, lithium contained in the lithium-containing composite nitride reacts with silicon in the silicon nitride to form a lithium-containing composite nitride. This is a technology that electrochemically inactivates lithium originally contained. However, there is a problem that the capacity per volume is reduced by adding silicon nitride.

  SUMMARY OF THE INVENTION The present invention solves the above-described conventional problems, and an object thereof is to provide a production method capable of obtaining a high-capacity lithium battery electrode by a simple method with high productivity.

In order to solve the above-described conventional problems, the lithium battery electrode manufacturing method of the present invention was obtained in the first step of vacuum-depositing a lithium silicon alloy film on a current collector and the first step. A second step of heat-treating the film,
The heat treatment in the second step is a treatment performed in an inert gas atmosphere of 180 ° C. or higher and 280 ° C. or lower or in a vacuum.

  Since this manufacturing method is a heat treatment performed in an inert gas atmosphere or in a vacuum, the method is simple and the productivity is high.

According to the method for manufacturing an electrode for a lithium battery of the present invention, in a negative electrode using a silicon film as an active material film, the negative electrode that reduces expansion of the active material film caused by charging and suppresses wrinkle generation on the current collector, A high-capacity lithium battery can be manufactured in combination with a positive electrode such as LiCoO ₂ .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, the same parts may be denoted by the same numbers, and overlapping descriptions may be omitted.

(Embodiment)
FIG. 1 shows a manufacturing process flow diagram of a lithium battery electrode of the present invention.

  In the first step, a lithium silicon alloy film is formed on the current collector. A lithium silicon alloy film can be obtained by simultaneously forming lithium and silicon.

  The film formation is performed by a vacuum film formation method, in which lithium atoms and silicon atoms flying from a first film formation source mainly made of lithium and a second film formation source mainly made of silicon are collectors. Precipitate on top. Thereby, a lithium silicon alloy film is obtained on the current collector.

  As a vacuum film formation method, a dry process such as a vacuum evaporation method, a sputtering method, a CVD method, or the like can be used. Among them, the vacuum evaporation method is preferable because it has an excellent film formation rate. Below, the case of a vacuum evaporation method is demonstrated to an example.

  FIG. 2 shows a schematic view of a vacuum vapor deposition apparatus for vapor deposition of lithium and silicon simultaneously. In FIG. 2, the vacuum chamber 5 is evacuated to about 0.01 Pa using a vacuum pump such as an oil rotary pump 51, an oil diffusion pump 52, a cryopump, or a turbo molecular pump. Lithium is disposed in the stainless Knudsen cell 1, and high-purity silicon is disposed in the carbon crucible 2. The substrate holder 4 holds a roughened copper foil 3 as a current collector. The shutter 41 controls film formation.

  It is convenient and preferable to use a resistance heating method as a film formation source for vacuum deposition of lithium. As the shape of the evaporation source, the Knudsen cell method can be used as well as the generally used boat heating method. The temperature of the lithium evaporation source is, for example, 500 to 600 ° C. Active lithium heated to a high temperature is likely to be oxidized by reacting with residual gas such as oxygen and moisture in the vacuum chamber. Therefore, it is effective to introduce a small amount of inert gas such as argon in the vicinity of the evaporation source.

  On the other hand, it is desirable to use electron beam heating as a heat source as a film forming source when vacuum depositing silicon. The reason is that during evaporation, silicon is required to be heated to a higher temperature than lithium. A water-cooled copper hearth may be used as a method for holding molten silicon, or a crucible made of carbon, boron nitride, or the like may be used.

  After the vacuum chamber is evacuated to about 0.01 Pa using a vacuum pump such as an oil rotary pump, an oil diffusion pump, a cryopump, or a turbo molecular pump, film formation is performed. At that time, the deposition rate of each deposition source is adjusted so that the lithium / silicon ratio becomes a predetermined value.

  In the second step, heat treatment is performed on the lithium silicon alloy film formed in the first step. In order to prevent atoms in the lithium silicon alloy film formed in the first step, in particular lithium, from reacting with residual gases such as oxygen, nitrogen and moisture in the heat treatment atmosphere and causing deterioration, the heat treatment is performed in an inert gas atmosphere. Or it is desirable to carry out in a vacuum. As the inert gas atmosphere, an inert gas such as argon can be used. The heat treatment must be performed at a temperature of 180 ° C. or higher and 280 ° C. or lower. If the heat treatment temperature is less than 180 ° C., the composition of the active material film hardly changes. By performing heat treatment at 180 ° C. to 280 ° C., lithium in the active material film is greatly reduced. The melting point of lithium is 179 ° C., and it is considered that the lithium in the lithium silicon alloy film is liquefied and discharged from the active material film by setting the heat treatment temperature to 180 ° C. or higher. Further, when the heat treatment temperature exceeds 280 ° C., lithium is considered to be discharged, but the amount of lithium that can be stored and released by silicon in the active material film is greatly reduced. The reason is not clear, but for example, it seems that the current collector metal diffuses and chemically reacts with silicon, so that the silicon does not easily bind to lithium.

  The negative electrode using the active material film obtained through the first step and the second step includes a positive electrode including a positive electrode active material containing lithium that is generally used, such as lithium cobaltate, lithium nickelate, and lithium manganate. A separator made of a microporous film, etc., and a lithium ion conductive electrolyte having a generally known composition in which lithium hexafluorophosphate or the like is dissolved in ethylene carbonate or diethyl carbonate. A lithium secondary battery having a general shape such as a rectangular shape, a rectangular shape, or a coin shape can be manufactured.

  The lithium / silicon ratio of the lithium silicon alloy film formed in the first step needs to satisfy that the current collector is not wrinkled by charge / discharge. In order to eliminate the expansion stress of the active material film caused by charging, a lithium silicon alloy film that is four times as large as 400% corresponding to the thickness of the silicon film to be formed may be formed. It is presumed that it slightly differs depending on the film formation conditions, but when the lithium / silicon ratio was 2.9, a lithium silicon alloy equivalent to five times the silicon film thickness was obtained. Therefore, when proportionally calculated, the lithium / silicon ratio was 2.3. If it is above, the lithium silicon alloy 4 times the silicon film thickness can be obtained.

It is considered that the upper limit of the lithium / silicon ratio should not be segregated in the lithium silicon alloy. A phase of Si ₅ Li ₂₂ has been reported and the lithium / silicon ratio is 4.4.

  Hereinafter, the present invention will be specifically described based on examples.

Example 1
The above-described vacuum deposition apparatus shown in FIG. 2 was used. Lithium was deposited in a stainless Knudsen cell 1 using lithium metal particles made by Honjo Chemical, and vapor deposition was performed by a resistance heating method. Argon gas was introduced at 1 sccm on the side of the stainless Knudsen cell outlet. Silicon 21 is a high-purity chemical ingot made of high-purity silicon, crushed to a size of about 1 cm, placed in a carbon crucible 2 having an inner diameter of 5 cm, and heated with a 270-degree deflection electron beam for vapor deposition. It was. The film-forming substrate also serves as an evaporation mask provided with a roughened copper foil 3 (core material 35 μm thickness, Ra = 2.0 μm) made by Furukawa Circuit Foil, cut into 12 cm square and provided with 10 cm square openings. It was set in the substrate holder 4. The vacuum chamber 5 was evacuated using an oil rotary pump 51 and an oil diffusion pump 52.

  Lithium and silicon deposition rates were measured using two quartz crystal thickness meters (not shown). The quartz crystal thickness meter was installed in a place where the thickness measurement was not affected by the other deposition. A single film was formed in advance using only one material, a ratio was obtained from the obtained film thickness value, and a coefficient was input to the crystal oscillator film thickness meter.

  The silicon deposition rate was controlled to 0.8 nm / sec and lithium to 2.6 nm / sec so that the lithium / silicon ratio was 3. At that time, the current and voltage of the resistance heater were controlled for lithium deposition so that the film formation rate was constant, and the current value was controlled while the electron beam voltage was kept constant for silicon deposition. By opening the shutter 41 for 180 minutes, the film was formed while maintaining this condition.

  When the cross section of the obtained film was observed with an SEM, the thickness was 45 μm, and no pore was seen, and the film was dense.

As a result of ICP emission spectrometry, lithium 1.5 mg / cm ^2, silicon is a composition of that 2.1 mg / cm ^2. The lithium / silicon ratio was 2.9, and the film thickness as lithium was 28 μm, and the film thickness as silicon was 9 μm. The discharge capacity when all deposited lithium contributes to the discharge is 5.8 mAh / cm ² , and all the deposited silicon absorbs and releases 4.4 lithium, which is the theoretical value. The charge / discharge capacity is 8.8 mAh / cm ² .

  The inert atmosphere was replaced by flowing argon of purity 6N at a flow rate of 1 L / min for 30 minutes or more through a quartz glass tube having an inner diameter of 10 cm and a length of 50 cm. A heating wire heater was wound around the quartz glass tube to obtain a heat treatment apparatus. A 1 cm square sample was cut from the film and heat-treated at 180 ° C. for 1 hour in an argon atmosphere. After cooling to room temperature, the sample was removed.

  Using the heat-treated sample as an electrode 6, a coin battery of R2016 was manufactured using a lithium metal foil 8 of 300 μm thickness made by Honjo Chemical as a counter electrode through a separator 7 of a polyethylene microporous film of 20 μm thickness made by Asahi Kasei. . A schematic cross-sectional view of the coin battery is shown in FIG. It consists of a metal disk 9 for collecting current from the electrode 6, a disc spring 10 for pressurizing the electrode, a case 11 for serving as a sealing and external positive / negative terminal, a sealing plate 12 and a gasket 13. As the electrolytic solution, ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1, and a solution prepared by dissolving lithium hexafluorophosphate at 1 mol / L was used. The impregnation with the electrolytic solution was performed by immersing the separator 7 and the electrode 6.

  Charging / discharging was repeated so that the current was constant at 0.2 mA and the final voltage for charging / discharging was 0 V and 1.5 V, and after reaching the final voltage, no current was allowed to flow for 30 minutes. Since lithium metal foil is used for the counter electrode, this time we decided to call the occlusion of lithium in the active material film as charging and the deposition of lithium on the lithium metal foil as discharging.

Table 1 shows the results of charging and discharging. The charge / discharge was started from the discharge because it was assumed that some lithium remained in the electrode even after the heat treatment. As a result, 0.5 mAh / cm ^{2 was} discharged in the first discharge. Thereafter, there was no retention capacity, and charging and discharging at 7.3 mAh / cm ² could be repeated. As a result of disassembling and observing the battery after 5 cycles, the copper foil as the current collector was not deformed such as wrinkles.

To summarize the above results, the initial discharge capacity was 0.5 mAh / cm ^2, which was greatly reduced from 5.8mAh / cm ² a calculated value from the initially deposited lithium amount in the first point. At the second point, there was no retention capacity in charge / discharge in the first cycle after the first discharge. At the third point, wrinkles due to expansion / contraction of the active material film did not occur in the roughened copper foil after charge / discharge.

The first and second points of the results can be considered as evidence that lithium other than the retention capacity +0.5 mAh / cm ² has been liquefied or evaporated in the heat treatment process and disappeared from the film. Here, the vapor pressure of lithium is about 1E-8 Pa at a melting point of 179 ° C., and the vapor pressure is very low. Although sufficient analysis has not been made, it is considered that the lithium-silicon bond in the lithium-silicon alloy film was cut at a temperature equal to or higher than the melting point of lithium, and the lithium was liquefied and discharged from the film by some force.

  Of the lithium incorporated into the film during the co-evaporation process, the lithium corresponding to the retention capacity compensation forms a bond with the electrochemically inactive silicon during the film formation process and the main heat treatment. Estimated. It is presumed that lithium corresponding to the retention capacity compensation has a relatively strong bonding force with silicon, and therefore remains in the film as it is even after the heat treatment step.

  In the third result, an electrode having a lithium-silicon alloy film in which lithium and silicon are vapor-deposited at the same time as an active material film has a rough expansion stress caused by the expansion of the active material film by charging to the same potential with lithium as a counter electrode. It can be considered that the roughened copper foil does not elastically break even when it is applied to the copper halide foil.

  It is said that an electrode using a silicon film on which only silicon is deposited as an active material expands by 400% when charged to the same potential using lithium as a counter electrode. As a result of the experiment, wrinkles occur when the ratio of silicon film thickness / copper foil thickness is 0.2 or more. When a 35 μm thick copper foil is used, a large number of wrinkles are generated when a silicon film of 7 μm or more is formed and charged and discharged.

  In the present invention, silicon having a thickness of 45 μm is formed by simultaneously depositing silicon corresponding to 9 μm and lithium. It is assumed that silicon corresponding to 9 μm is formed with a thickness of 45 μm, which is 5 times, and the expansion stress due to charging is very small or not.

(Example 2)
A battery was prepared and charged / discharged in the same manner as in Example 1 except that the heat treatment temperature in the argon atmosphere was 280 ° C.

Table 1 shows the results of the charge / discharge capacity. In the first discharge, 0.3 mAh / cm ^{2 was} discharged, and thereafter, charging and discharging of 6.9 mAh / cm ² were repeated. As a result of disassembling and observing the battery after 5 cycles, the current collector was not deformed such as wrinkles.

(Comparative Example 1)
A battery was prepared and charged / discharged in the same manner as in Example 1 except that the heat treatment temperature in an argon atmosphere was room temperature (untreated), 150 ° C., and 300 ° C.

Table 1 shows the results of the charge / discharge capacity. When the heat treatment temperature was room temperature and 150 ° C., 5.0 mAh / cm ^{2 was} discharged at the first discharge, and thereafter, 7.5 mAh / cm ² was repeatedly charged and discharged. As a result of disassembling and observing the battery after 5 cycles, the current collector was not deformed such as wrinkles. When the heat treatment temperature was 300 ° C., 0.001 mAh / cm ^{2 was} discharged at the first discharge, and thereafter, 0.08 mAh / cm ² was repeatedly charged and discharged.

  The following can be understood from the above evaluation of the coin battery. In the heat treatment at 150 ° C. or lower, the active material film hardly changes. On the other hand, in the heat treatment at 300 ° C. or higher, the active material film is changed, and silicon atoms that can contribute to occlusion of lithium are almost lost.

  The method for producing an electrode for a lithium battery according to the present invention is useful as a method for producing a lithium secondary battery.

The figure which shows the manufacturing process flow of the electrode for lithium batteries in embodiment of this invention Schematic of a vacuum deposition apparatus for simultaneously depositing lithium and silicon in an embodiment of the present invention Schematic sectional view of a coin battery used in an embodiment of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Stainless steel Knudsen cell 2 Carbon crucible 3 Roughened copper foil 4 Substrate holder 5 Vacuum tank 6 Electrode 7 Separator 8 Lithium metal foil 9 Metal disc 10 Disc spring 11 Case 12 Sealing plate 13 Gasket 21 Silicon 41 Shutter 51 Oil rotation Pump 52 Oil diffusion pump

Claims (5)

A method for producing an electrode for a lithium battery, comprising: a first step of vacuum-depositing a lithium silicon alloy film on a current collector; and a second step of heat-treating the film obtained in the first step,
The method for manufacturing an electrode for a lithium battery, wherein the heat treatment in the second step is a treatment performed in an inert gas atmosphere of 180 ° C. or higher and 280 ° C. or lower or in a vacuum.   2. The method for producing an electrode for a lithium battery according to claim 1, wherein the lithium silicon alloy film has a lithium / silicon ratio of 2.3 or more and 4.4 or less.   The method for producing an electrode for a lithium battery according to claim 1, wherein the lithium silicon alloy film has a lithium / silicon ratio of 2.9 to 4.4.   2. The method for producing an electrode for a lithium battery according to claim 1, wherein the first step is a step of simultaneously performing a step of vacuum-depositing lithium and a step of vacuum-depositing silicon.   5. The method for producing an electrode for a lithium battery according to claim 1, wherein the vacuum film forming step is a vacuum vapor deposition step.