JP6563361B2 - Method for producing negative electrode for lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for producing a negative electrode for a lithium ion secondary battery using a high frequency plasma method.

In recent years, mobile electronic devices such as mobile phones and notebook personal computers have played an important role in the information society. These electronic devices are required to be driven for a long time, and inevitably, a high energy density of a secondary battery as a driving power source has been desired.

As a power source for transportation devices such as these electronic devices and vehicles, there is a demand for higher performance of lithium ion secondary batteries that are lightweight and can obtain a high energy density. A lithium ion secondary battery has a structure in which an electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent or a lithium solid electrolyte is sandwiched between a negative electrode active material and a positive electrode active material. Charging and discharging are performed by lithium ions moving between materials.

Conventionally, graphite has been used as a negative electrode active material for a lithium ion secondary battery, but the crystallite size is as large as a micron order and is not suitable for high-speed charge / discharge. Theoretically, silicon, an alloy mainly composed of silicon, silicon oxide, and the like that can provide a charge / discharge capacity higher than that of a carbon-based negative electrode material have attracted attention as negative electrode materials. This is because silicon can be used as a negative electrode active material because it forms an alloy with lithium, and more lithium can be taken in compared with graphite, so that a high capacity of the battery can be expected (for example, Non-Patent Document 1). Patent Documents 1 and 2).

Conventionally, SiC, which is a carbide of silicon, has been considered to be electrochemically inactive, do not insert and release lithium ions, and has no capacity (Patent Document 3). However, it has been reported that when the SiC surface is graphitized, lithium can be inserted and used as a negative electrode (Non-patent Documents 2 and 3 and Patent Document 4). Further, a non-aqueous electrolyte secondary battery using a negative electrode material in which a compound of a phase in which C and N are present in a non-equilibrium state in Si is deposited on the surface of a copper foil collector to a thickness of 1 to 30 μm by a CVD method or the like. Related to the invention (Patent Documents 5 and 6) and an invention relating to a negative electrode active material in which an amorphous carbon layer is formed on the surface of an amorphous Si-containing compound represented by SiCx (0.25 ≦ x ≦ 0.95) (Patent Document 7) ) Has been filed for a patent.

In addition, a SiC nanoparticle composite film negative electrode for a lithium ion secondary battery in which silicon nanoparticles and carbon are combined by a plasma film forming method has been reported (Non-Patent Document 4).

JP 2003-077529 A JP 2007-194204 (Patent No. 4671950) Japanese Patent Application Laid-Open No. 11-337996 U.S. Pat. No. 8,734,674 JP 2006-128067 A (Patent No. 4972880) JP 2007-188873 A (Patent No. 4899841) JP 2012-64565 A (Patent No. 5669143)

Uday Kasavajjula, et al., "Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells", Journal of Power Sources, 163, (2007), 1003-1039 T. Sri Devi Kumari, et al., "Nano silicon carbide: a new lithium-insertion anode material on the horizonRSC. Adv., 2013, 3, 15028-15034 M.Shiratani et al., "SiC nanoparticle Composite Anode for Li-ion Batteries", Mater.Res.Soc.Symp.Proc.Vol.1678, DOI: 10.1557 / opl.2014.742 X. Chang et al., "Direct plasma deposition of amorphous Si / C nanocomposites as high performance anodes for lithium ion batteries", J. Mater. Chem. A, 2015, 3, 3522-3528

The development of negative electrode materials that achieve both high capacity and cycle life is important for improving the performance of lithium ion secondary batteries. In order to use silicon as a negative electrode active material, a mechanism for absorbing volume change due to charge / discharge is required. In addition, since Si is not conductive, it needs to be mixed with a conductive additive having conductivity such as carbon in order to secure a conductive path with the current collector.

The negative electrode active material described in Patent Document 5 includes amorphous SiC having a non-equilibrium structure represented by SiCx (0.25 ≦ x ≦ 0.95), and is 5 to 20% by weight based on the total weight. A two-source sputtering apparatus includes a carbon layer made of amorphous carbon on the surface and substantially uses amorphous carbon as an active material. An active material layer having a thickness of 2 μm is formed by sputtering using a Si target and a C target. In order to manufacture the active material having the structure specified here, it is necessary to use means such as a two-source sputtering apparatus, which is difficult to manufacture.

An object of the present invention is to provide a negative electrode using silicon and carbon as active materials by a relatively simple and inexpensive manufacturing method.

The present inventors have found that a Si—C composite film mainly composed of amorphous SiC formed under a specific condition using a high-frequency plasma CVD method acts as an active material.

The present invention provides: (1) feed gas containing by decomposing the raw material gas by a plasma chemical vapor deposition in a method of depositing simultaneously the anode current collector substrate surface carbon and silicon, using a high frequency discharge, as an element of silicon or with a mixed gas of a raw material gas containing carbon as a constitutional element, using a raw material containing silicon element and a carbon element as a raw material gas, the heating temperature of the substrate be less than 850 ° C., more than 15 wt% of the deposits non amorphous Ri Do carbide, a negative electrode the method of manufacturing a lithium ion secondary battery, characterized by depositing a thin film that to act as an active material, a.

The present invention is also characterized in that (2) silicon chloride gas or silicon hydride gas is used as the raw material gas for the silicon component, and alkane gas having 1 to 6 carbon atoms is used as the raw material gas for the carbon component (1 ) For producing a negative electrode for a lithium ion secondary battery.

The present invention also provides (3) a method for producing a negative electrode for a lithium ion secondary battery according to (1) above, wherein silicon tetrachloride gas is used as the raw material gas for the silicon component, and methane is used as the raw material gas for the carbon component, It is.

Moreover, this invention is (4) The manufacturing method of the negative electrode for lithium ion secondary batteries of said (1) characterized by the thickness of the said thin film being 100 nm-10 micrometers.

Moreover, this invention is (5) The manufacturing method of the negative electrode for lithium ion secondary batteries of said (1) characterized by the said collector board | substrate consisting of stainless steel.

Many conventional techniques aim to make Si nanoparticles or carbon amorphous and suppress the formation of SiC as much as possible. However, the present invention can use SiC that is easily formed as it is. I made it.

The method for producing a negative electrode for a secondary battery according to the present invention eliminates the need for a complicated process such as a conventional negative electrode manufacturing process using a conductive additive or a binder, and is relatively high-frequency and low-temperature and high-speed plasma CVD methods. It is only necessary to form a thin uniform SiC film on the current collector substrate surface, which can simplify the process and reduce the cost, and can provide a negative electrode with a high yield. Further, unlike Si, an active material layer with a small volume change can be formed by charging and discharging.

3 is a graph showing measurement results of X-ray photoelectric spectroscopy (XPS) of the negative electrode produced in Example 1. FIG. 4 is a graph showing measurement results of initial charge / discharge characteristics of the half cell produced in Example 1. FIG. 6 is a graph showing measurement results of initial charge / discharge characteristics of a half cell produced in Example 2. It is a graph which shows the cycle characteristic of the half cell produced in Example 1,2. 6 is a graph showing measurement results of initial charge / discharge characteristics of a half cell produced in Example 3. 6 is a graph showing measurement results of initial charge / discharge characteristics of a full cell produced in Example 3.

In the present invention, examples of the negative electrode current collector include copper materials, nickel materials, and stainless steels having high corrosion resistance that are highly conductive and inexpensive. Stainless steel usually has a problem that the contact resistance between the active material layer and the active material layer increases due to the passive film, and it is desirable to use a steel type excellent in both corrosion resistance and surface contact resistance.

In the present invention, an amorphous SiC thin film is deposited on the surface of a current collector substrate by a high frequency plasma chemical vapor deposition (PECVD) method. Usually, plasma used for plasma chemical vapor deposition is a radio frequency (10 kHz to 20 kHz). MHz) or microwaves (0.1-10 GHz) are used, but high frequency plasma is suitable for the deposition of amorphous SiC.

In general, when a thin film is deposited from the gas phase, a large number of nuclei are formed on the substrate, and the nuclei come into contact with each other to form island-like or columnar structures. A crystal film is formed depending on conditions such as the substrate temperature and the raw material gas composition. Conventionally, SiC with a stoichiometric composition (Si: C = 1: 1 silicon carbide) is inactive to Li ions, does not insert and release lithium ions, and has no capacity. Is made to contain SiC as little as possible, or to form SiC having a non-equilibrium composition. However, the amorphous SiC thin film layer formed by the method of the present invention acts as an active material. The mechanism is not clear, but it is speculated that free SiC intervenes in amorphous SiC to form a structure that is not dense but has columnar gaps, and that voids into which lithium ions are inserted are formed. .

A method for forming amorphous SiC on the negative electrode current collector substrate surface will be described below. The amorphous SiC film supplies a source gas serving as a Si component supply source and a C component supply source together with a carrier gas into a chamber of a high-frequency plasma CVD apparatus, and decomposes the source gas to generate Si atoms and C atoms. At the same time , a film is deposited on the surface of the current collector.

In high-frequency plasma CVD (PECVD), when the heating temperature of the current collector substrate is set to less than 850 ° C. to about 100 ° C. and a specific source gas is used, a silicon carbide film having an amorphous structure rather than a polycrystal may be formed. it can. When the heating temperature of the substrate is 900 ° C. or higher, the SiC that grows is a single crystal. The substrate temperature can be adjusted by the energy of the high frequency magnetic field of the high frequency coil 7. The high-frequency plasma CVD is a method using a discharge with a high frequency of 13.56 MHz, and can form an amorphous SiC thin film with a small compressive stress. When a low frequency (380 KHz) is used, a dense film having a large compressive stress is obtained.

As a raw material gas for forming silicon carbide, generally, a silicon hydride gas such as silane, disilane, dichlorosilane, or trichlorosilane and a mixed gas of alkane and hydrogen having 1 to 6 carbon atoms such as methane, ethane, or propane are used. Si-C-H gas using silicon, silicon chloride gas such as silicon tetrachloride and disilicon hexachloride and Si using a mixed gas of alkane and hydrogen having 1 to 6 carbon atoms such as methane, ethane or propane A -C-Cl-H gas is used. However, the raw material is not limited to these gases as long as it is a raw material capable of forming amorphous SiC by a reaction in plasma.

As the Si raw material, a solution containing Si may be vaporized to be a raw material gas. Moreover, Si (C ₂ H ₃₎ containing C in raw material containing Si as _{4, Si (C 4 H 9} ) 4, Si (OC 2 H 5) it a raw material gas if the organosilane _4, etc. Also good. The carrier gas, hydrogen, even with argon, etc. have good.

Preferred conditions for depositing the amorphous SiC film by the high-frequency plasma CVD method are as follows. Since the flow rate of the mixed gas is dependent on the apparatus, it is necessary to select it appropriately according to the apparatus, but the target is usually about 10 sccm to 500 sccm. The current collector substrate temperature is less than 850 ° C. Although an amorphous film can be deposited even at room temperature, the deposition rate is lowered, so that the temperature is preferably 100 ° C. or higher, more preferably 600 ° C. or higher. The process pressure in the chamber is 0.05 to 0.5 torr (6.7 to 66.7 Pa), the applied high frequency output is about 50 to 500 W, the applied high frequency is 13.56 MHz, and the distance from the electrode to the current collector substrate is set. It is good to deposit about 15 minutes-2 hours as about 20-80 mm. The bias on the current collector substrate side in the high frequency plasma CVD method is preferably −100 V to 0 V.

According to the above method, a Si—C based composite film in which amorphous SiC is 15 mass% or more with respect to the total amount of the film is formed. Of the Si component in the film, 66 to 90% is present as SiC. This composite film is usually deposited to a thickness of 100 nm to 10 μm, preferably 300 nm to 1000 nm, particularly preferably 300 nm to 600 nm.

According to the production method of the present invention, excellent characteristics are obtained in which the negative electrode capacity is about 1000 mAh / g in a half cell, and the cycle characteristics are reduced by less than 10% from the initial capacity after 500 cycles.

The negative electrode produced by the method of the present invention can be used as a component for a lithium ion secondary battery. That is, a secondary battery is formed from the negative electrode of the present invention, a positive electrode using a lithium ion compound or the like as an active material, an electrolytic solution disposed between the positive and negative electrodes, and a separator separating the positive and negative electrodes. Can do. There are no particular restrictions on the organic solvent and electrolyte of the electrolytic solution, the positive electrode, the separator, and the structure and size of the outer container constituting the secondary battery, and conventionally known ones can be used.

The positive electrode current collector may be, for example, aluminum, nickel, or stainless steel. The separator may be a porous film made of polyolefin such as polypropylene (PP) or polyethylene (PE), or a porous film made of ceramic.

As the non-aqueous organic solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are suitable. Fluoroether may be used to improve the flame retardancy of the electrolytic solution. The non-aqueous organic solvent may contain an additive such as an organosilicon compound.

As an electrolyte provided in such a secondary battery, various materials having a property of conducting lithium ions can be used without any particular limitation. The technology disclosed herein can be applied to a battery including a liquid electrolyte (electrolytic solution) and a battery including a solid electrolyte. Application to a battery including a solid electrolyte (typically, a solid electrolyte made of an inorganic material) is more preferable. Such a solid electrolyte may use only 1 type, and may use together (for example, mix and use) 2 or more types of solid electrolyte from which a composition differs.

(Preparation of negative electrode) The source gas is decomposed into a stainless steel foil (ferritic stainless steel SUS430, 50 mm x 50 mm, thickness 50 μm) used as a current collector using a plasma CVD apparatus (manufactured by Hiroshima Co., Ltd.), and Si and carbon are simultaneously dissolved. A film was formed to form an amorphous SiC film. Silicon tetrachloride (SiCl ₄ ) gas is used as the Si component source gas, methane (CH ₄ ) gas is used as the C component source gas, and the flow rate (sccm) ratio is SiCl ₄ gas: CH ₄ : H ₂ = 6. : 36: 8. The film was formed at a film formation temperature of 750 ° C., a pressure of 16.7 Pa, an RF output of 200 W, and a film formation time of 90 minutes.

A uniform and dense film of about 500 nm was formed. FIG. 1 shows an XPS spectrum (si2p). The composition of the film was C: O: Si = 73: 4: 23 in mass%. A peak of 99.3 eV corresponding to simple Si could not be confirmed, and it was found that about 90% of Si was present in SiC (carbide). Moreover, although an electron beam diffraction image was acquired in order to confirm crystallinity, a clear diffraction pattern was not confirmed but it was amorphous.

A half cell (2032 coin cell) was prepared using the negative electrode formed by the above method, and the negative electrode characteristics were evaluated. The negative electrode size is φ16 mm, the counter electrode is Li metal, the electrolyte is 1 mol / L LiPF6, EC: DMC (1: 2 v / v%) (no additive), the separator is PP-based, the thickness is 25 μm, and the evaluation temperature is 25 ° C. Constant (constant temperature bath). FIG. 2 shows the initial charge / discharge characteristics. The initial 10 cycles were evaluated for initial performance at a charge / discharge rate of 0.1 C, and then the cycle characteristics were evaluated at a 1 C rate. The capacity (initial capacity) in the first cycle was 1018 mAh / g, and the capacity in the fifth cycle was 1163 mAh / g.

A gas obtained by vaporizing hexamethyldisiloxane is used as the Si component raw material gas, methane (CH4) gas is used as the C component raw material gas, and the vaporized gas at a flow rate (sccm) ratio: CH ₄ : H ₂ = 16: 12 The film was formed under the same conditions as in Example 1 except that the power was 12 and the RF output was 400 W and the film formation time was 45 minutes. A uniform and dense film of about 500 nm was formed. The film was amorphous, and the composition of the film was C: O: Si = 67: 12: 21 by mass%. It was found that about 66% of Si is present in SiC (carbide).

A half cell was produced with the negative electrode formed by the above method, and the negative electrode characteristics were evaluated under the same conditions as in Example 1. The resulting initial charge / discharge characteristics are shown in FIG. The capacity (initial capacity) in the first cycle was 961 mAh / g, and the capacity in the eighth cycle was 956 mAh / g.

The charge / discharge mode of Examples 1 and 2: CC-CV, cut-off voltage: 0.02-2.1 V, cycle characteristics at 1C rate are shown in FIG. After 500 cycles, Example 1 had a high capacity of 982 mAh / g and the capacity reduction rate was 6.7%. Example 2 had a high capacity of 767 mAh / g, and the capacity reduction rate was 1.7%.

Silicon tetrachloride (SiCl ₄ ) gas is used as the Si component source gas, methane (CH ₄ ) gas is used as the C component source gas, and the flow rate (sccm) ratio is SiCl ₄ gas: CH ₄ : H ₂ = 6. : 48:12. The film was formed at a film formation temperature of 750 ° C., a pressure of 20 Pa, an RF output of 150 W, and a film formation time of 60 minutes.

A half cell was produced with the negative electrode formed by the above method, and the negative electrode characteristics were evaluated under the same conditions as in Example 1. The results are shown in FIG. The capacity (initial capacity) in the first cycle was 1127 mAh / g, and the capacity in the fifth cycle was 1190 mAh / g.

A full cell (2032 coin cell) was produced with the negative electrode formed by the above method, and the negative electrode characteristics were evaluated. The negative electrode size is φ16 mm, the counter electrode is lithium cobalt oxide, the electrolyte is 1 mol / L LiPF ₆ , EC: DMC (1: 2 v / v%) (no additive), the separator is PP-based, thickness 25 μm, evaluation temperature The temperature was constant at 25 ° C. (a constant temperature bath). FIG. 6 shows the initial charge / discharge characteristics. In the initial 6 cycles, the initial performance was evaluated at a charge / discharge rate of 0.1 C. The capacity (initial capacity) in the first cycle was 910 mAh / g, and the capacity in the sixth cycle was 1595 mAh / g.

The present invention is expected to progress as a new manufacturing method using silicon and carbon as a negative electrode of a lithium ion battery.

Claims (5)

In a method of decomposing a source gas by plasma chemical vapor deposition and simultaneously depositing carbon and silicon on the negative electrode current collector substrate surface, a source gas containing silicon as a constituent element and a source gas containing carbon as a constituent element using high frequency discharge mixed gas, or using a raw material containing silicon element and a carbon element as a raw material gas, the heating temperature of the substrate be less than 850 ° C., Ri least 15 wt% of the deposits Do amorphous silicon carbide, utilization of the method for producing a negative electrode for a lithium ion secondary battery, characterized by depositing a thin film that to act as agent. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein silicon chloride gas or silicon hydride gas is used as the raw material gas for the silicon component, and alkane gas having 1 to 6 carbon atoms is used as the raw material gas for the carbon component. Manufacturing method. The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein silicon tetrachloride gas is used as the Si component source gas and methane is used as the C component source gas. The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein the thin film has a thickness of 100 nm to 10 μm. The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein the current collector substrate is made of stainless steel.

Images