JP2011090806A - Electrode for lithium secondary battery, and lithium secondary battery equipped with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium secondary battery and a lithium secondary battery equipped with the same with cycle characteristics further enhanced in one containing silicon. <P>SOLUTION: A coin-type battery 20 is provided with a positive electrode 22 containing an amorphous silane compound obtained by doping lithium in stratified polysilane as an active material, a negative electrode 23 containing an active material capable of occluding/releasing lithium, and an ion conductive medium for conducting lithium ion, intervening between the positive electrode 22 and the negative electrode 23. The amorphous silane compound can be obtained by doping lithium up to the vicinity of 0V in terms of a lithium reference potential. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same.

  As an electrode of a lithium secondary battery, silicon is known to have a high energy density of about 3600 mAh / g at room temperature, and has recently attracted attention as an electrode material for a lithium secondary battery. However, although silicon has a high theoretical capacity, the volume expansion coefficient during the charge / discharge process may be very large. Due to this volume change, for example, cracks in the active material particles, poor contact between the active material and the current collector, and the like occur, resulting in a short charge / discharge cycle life. In addition, due to the same cause, the irreversible capacity is remarkably increased, and the battery capacity may be reduced. For these reasons, several reports have been made on improvements of silicon that uses silicon as an active material.

  For example, Patent Document 1 has a layer containing silicon particles on a current collector and a surface coating layer formed on the silicon layer, and a fine space extending in the thickness direction is formed in the surface coating layer. An improved lithium secondary battery has been proposed. In this battery, the silicon layer can be pressed by the surface coating layer, and the stress resulting from the volume change can be relieved by the fine space. Patent Document 2 proposes a lithium secondary battery in which a silicon thin film is formed on a current collector by melting and evaporating a silicon-containing raw material in an inert gas atmosphere. In this battery, by controlling the synthesis conditions, it is possible to generate a break in the thickness direction of the thin film and relieve the stress resulting from the volume change.

In Non-Patent Document 1, when crystalline silicon is formed on a current collector and the electrode is reduced to near 0 V and doped with lithium, it passes through amorphous Li _x Si (x is an arbitrary number). A lithium secondary battery produced by crystalline Li ₁₅ Si ₄ is described. In this secondary battery, the crystalline Li ₁₅ Si ₄ and the amorphous Li _x Si are repeatedly altered by charging and discharging, and the active material is peeled off from the current collector because of a large change in volume. Or the electrical connection is cut off, and the battery capacity may be reduced. On the other hand, it has been proposed that when lithium is doped in a range exceeding 0.05 V, it can be maintained in an amorphous Li _x Si state.

JP 2005-44672 A JP 2007-123096 A

Electrochemical Solid-State Letters 7, A93-A96, (2004)

  However, in the lithium secondary batteries of Patent Documents 1 and 2 described above, although the volume change of silicon is further suppressed, the volume change of silicon itself still exists, and further improvement in cycle characteristics is desired. It was. Further, the above-described lithium secondary battery of Non-Patent Document 1 has a problem in that the battery capacity cannot be sufficiently obtained because the potential range of charge / discharge is limited, and further improvement in cycle characteristics has been desired.

  This invention is made | formed in view of such a subject, and provides the electrode for lithium secondary batteries which can improve cycling characteristics more in the thing containing silicon, and a lithium secondary battery provided with the same. Main purpose.

  As a result of diligent research to achieve the above-described object, the present inventors have found that when layered polysilane is used as an electrode active material and the electrode is reduced to near 0 V and doped with lithium, volume change and crystal structure change are unlikely to occur. It has been found that a crystalline silane compound is produced and the cycle characteristics can be improved, and the present invention has been completed.

  That is, the electrode for a lithium secondary battery of the present invention contains an amorphous silane compound obtained by doping lithium into a layered polysilane as an active material.

  The lithium secondary battery of the present invention includes the above-described lithium secondary battery electrode and an ion conductive medium that contacts the lithium secondary battery electrode and conducts lithium ions.

The present invention can further improve cycle characteristics in the case of containing silicon as an active material. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, layered polysilane is a stacked body of layers formed of silicon atoms, and there is a space between each layer. In this layered polysilane, lithium ions easily enter the interlayer through the electrolytic solution, so that more silicon atoms can be used in the charge / discharge reaction, and it is assumed that the layered polysilane exhibits a high capacity. In addition, when silicon particles are used as the active material, crystalline Li _x Si _y (x and y are arbitrary numbers) and the like are generated by doping with lithium up to 0 V, which is filled with a large volume change. It is considered that the discharge characteristics are adversely affected. On the other hand, it is considered that the layered polysilane does not generate crystalline Li _x Si _y or the like even when doped with lithium up to 0 V, but generates an amorphous silane compound. This amorphous silane compound is presumed to be capable of suppressing the large volume change and improving the cycle characteristics because the amorphous silane compound retains its state even after repeated charge and discharge and occludes and releases lithium.

It is explanatory drawing of the structure of layered polysilane. 2 is a cross-sectional view illustrating a schematic configuration of a coin-type battery 20. FIG. It is a measurement result result of XRD of layered polysilane. It is an IR spectrum measurement result of layered polysilane. It is a Raman spectrum measurement result of layered polysilane. It is a NMR spectrum measurement result of layered polysilane. It is a measurement result of the charging / discharging cycle of the evaluation cell of Example 1. It is explanatory drawing of the initial stage charge / discharge curve of the evaluation cell of Example 2, and the electric potential of the produced electrode. It is an X-ray-diffraction spectrum measurement result of the electrode of Example 2 in initial stage charge / discharge.

  The electrode for a lithium secondary battery of the present invention contains an amorphous silane compound obtained by doping lithium into a layered polysilane as an active material. In this way, cycle characteristics can be further improved. This amorphous silane compound may be obtained by doping lithium to a predetermined low potential range including 0 V at the lithium reference potential. In this way, it is easy to produce an amorphous silane compound from the layered polysilane. The predetermined low potential range may be, for example, near 0 V at the lithium reference potential, preferably in the range of 0 V to 0.050 V, more preferably in the range of 0 V to 0.020 V, and 0 V More preferably, the range is 0.010 V or less. Further, this amorphous silane compound may exhibit a broad peak having an apex in the range where 2θ in X-ray diffraction is 20 ° or more and 28 ° or less. In addition, this amorphous silane compound is obtained by doping lithium into a layered polysilane having an apex of a peak derived from the 001 plane in the range where 2θ in X-ray diffraction is 13.5 ° or more and 16.5 ° or less. Also good. In addition, when an amorphous silane compound is used for a positive electrode, it will be in the state which discharge | released lithium at the time of charge, and will be in the state which occluded lithium at the time of discharge. On the other hand, when an amorphous silane compound is used for a negative electrode, it is in a state where lithium is occluded during charging and is in a state where lithium is released during discharging. The amorphous silane compound is amorphous and the specific structure is unknown, but it is presumed that the amorphous silane compound has a structure and characteristics similar to those of layered polysilane.

Layered polysilane used for the lithium secondary battery electrode of the present invention, in Raman spectroscopic analysis, may show a plurality of peak in the range of 360 cm ^-1 or more 2200 cm ^-1 or less. Peaks in the Raman spectroscopic analysis, 370 cm ^-1 vicinity, 490 cm ^-1 vicinity, 630 cm ^-1 vicinity, 730 cm ^-1 vicinity may be those present in the vicinity of 840 cm ^-1 and near 2100 cm ^-1. Further, this layered polysilane may exhibit a peak having a peak in a range of −90 ppm to −110 ppm in ²⁹ Si-NMR measurement. The peak of the ²⁹ Si-NMR spectrum may have a vertex in the vicinity of −100 ppm. An explanatory view of the structure of the layered polysilane is shown in FIG.

  The lithium secondary battery electrode of the present invention is, for example, a mixture of an active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like electrode material that is applied to the surface of the current collector and dried. However, it may be compressed to increase the electrode density as necessary. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, What mixed 1 type (s) or 2 or more types, such as ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles, for example, a polytetrafluoroethylene (PTFE), a polyvinylidene fluoride (PVDF), a fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Examples of the solvent for dispersing the active material, conductive material, and binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine. Organic solvents such as ethylene oxide and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors for electrodes include copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

  The lithium secondary battery of the present invention includes the above-described lithium secondary battery electrode and an ion conductive medium that contacts the lithium secondary battery electrode and conducts lithium ions. Specifically, the lithium secondary battery of the present invention includes a positive electrode including a positive electrode active material capable of inserting and extracting lithium, a negative electrode that is an electrode for the lithium secondary battery, and a positive electrode and a negative electrode. And an ion conductive medium that conducts lithium ions. Further, the lithium secondary battery of the present invention includes a positive electrode that is an electrode for the above-described lithium secondary battery, a negative electrode including a negative electrode active material capable of occluding and releasing lithium, and a lithium ion interposed between the positive electrode and the negative electrode. It is good also as what has an ion conduction medium which conducts. That is, the electrode for a lithium secondary battery of the present invention may be used as a positive electrode or a negative electrode.

When the electrode for the lithium secondary battery of the present invention is used as a negative electrode, the positive electrode of the lithium secondary battery of the present invention is, for example, a paste prepared by mixing a positive electrode active material, a conductive material and a binder, and adding an appropriate solvent. What was made into the shape of a positive electrode material may be applied and dried on the surface of the current collector, and may be compressed to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), Li _(1-x) Mn Lithium manganese composite oxide such as ₂ O ₄ , lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , lithium nickel composite oxide such as Li _(1-x) NiO ₂ , lithium such as LiV ₂ O ₃ Vanadium composite oxides, transition metal oxides such as V ₂ O _{5, and the} like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. Moreover, what was illustrated by the electrode mentioned above can each be used for the electrically conductive material, binder, solvent, etc. which are used for a positive electrode. Current collectors of the positive electrode include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, etc., as well as aluminum and titanium for the purpose of improving adhesion, conductivity and oxidation resistance. What processed the surface of copper etc. with carbon, nickel, titanium, silver, etc. can be used. For these, the surface can be oxidized. The shape of the current collector can be any of those exemplified for the electrodes described above.

  In the case where the electrode for a lithium secondary battery of the present invention is used as a positive electrode, the negative electrode of the lithium secondary battery of the present invention may have a negative electrode active material formed on a current collector, for example. Examples of the negative electrode active material include lithium, a lithium alloy, and a tin compound. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as the electrode described above.

As the ion conduction medium of the lithium secondary battery of the present invention, a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, or the like can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane, nitriles such as acetonitrile and benzonitrile,
Examples include furans such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can.

The supporting salt contained in the lithium secondary battery of the present invention is, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Examples include LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3. It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. If the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous electrolyte.

  Further, instead of the liquid ion conducting medium, a solid ion conducting polymer may be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, and polyvinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, an inorganic solid powder bound by an organic binder, or the like can be used as the ion conductive medium.

  The lithium secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. An example of this lithium secondary battery is shown in FIG. FIG. 2 is a cross-sectional view schematically illustrating the configuration of the coin-type battery 20. The coin-type battery 20 includes a cup-shaped battery case 21, a positive electrode 22 having a positive electrode active material provided at a lower portion of the battery case 21, and a negative electrode active material having a positive electrode 22 via a separator 24. A negative electrode 23 provided at a position facing each other, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed in an opening of the battery case 21 and sealing the battery case 21 via the gasket 25. ing. In the coin-type battery 20, the space between the positive electrode 22 and the negative electrode 23 is filled with a non-aqueous electrolyte containing a lithium salt. Here, the positive electrode 22 contains, as an active material, an amorphous silane compound obtained by doping lithium into layered polysilane by reducing the electrode to near 0V. The negative electrode 23 uses lithium metal.

According to the lithium secondary battery of the present embodiment described in detail above, a layered polysilane having a space between silicon layers is used as an electrode, and lithium is easily occluded / released, so that it has a high battery capacity. . In addition, the amorphous silane compound produced by doping lithium into the layered polysilane is stable and does not change to crystalline Li _x Si _y or the like, and the volume change is also suppressed. It will be longer. Therefore, cycle characteristics can be further improved.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the electrode for lithium secondary batteries of this invention concretely is demonstrated as an Example.

[Example 1]
(1) Synthesis of layered polysilane The layered polysilane (Si ₆ H ₆ ) was synthesized under the following conditions. In this synthesis, 3 g of calcium silicide (CaSi ₂ ) was added to 100 ml of concentrated hydrochloric acid cooled to −30 ° C., and left in a dark room at −30 ° C. for 1 week. This treatment turned the black calcium silicide to yellow. The yellow solid was filtered under pressure in an Ar atmosphere, washed with degassed hydrochloric acid (−30 ° C.), washed with degassed HF (hydrogen fluoride) aqueous solution (−30 ° C.), and further degassed acetone (−30 The layered polysilane was synthesized by washing at 110 ° C. overnight under reduced pressure. The synthesized layered polysilane was taken as Example 1. The chemical reaction formula for the synthesis of this layered polysilane is shown in Formula (1).

3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂ (1)

(2) XRD measurement The obtained layered polysilane was subjected to XRD measurement. The measuring device used was RINT-TTR manufactured by Rigaku Corporation. CuKα rays were used as the radiation source. FIG. 3 shows the result of XRD measurement of layered polysilane. As shown in FIG. 3, in Example 1, it was found that the 001 plane of the layered polysilane was 14.8 °. From this, it was found that the layered polysilane layer had a period of 0.59 nm. Moreover, when it calculated using ChemDraw (trademark) of chemical structural formula drawing software, the thickness of the polysilane layer was calculated to be 0.43 nm. From these results, the interlayer space of the layered polysilane was calculated to be 0.16 nm. Since lithium ions have a diameter of 0.146 nm, it was expected that the volume change due to insertion / extraction of lithium ions between layers of the layered polysilane was small.

(3) IR measurement The IR spectrum was measured about the obtained layered polysilane. The measuring apparatus used was a Magna 760 type Fourier infrared spectrometer and a Nic-Plan infrared microscope manufactured by Nicorey. FIG. 4 shows the IR spectrum measurement result of the layered polysilane. As shown in FIG. 4, in Example 1, a peak in the vicinity of 2100 cm ⁻¹ derived from Si—H stretching and a peak of 1100 cm ⁻¹ derived from Si—OH stretching were observed. From this, it was speculated that the silicon atom forming the six-membered ring has a Si—H bond. In addition, although it is the reason which the peak derived from Si-OH expansion-contraction was detected by layered polysilane, when it measured in air | atmosphere, it was estimated that it generate | occur | produced by the influence of the water | moisture content.

(4) Raman spectrum measurement The Raman spectrum was measured about the obtained layered polysilane. The measuring device used was an NRS-3300 laser Raman spectrometer manufactured by JASCO. The excitation wavelength was 532 nm, the objective lens was 20 times, the exposure time was 30 seconds, the laser power was 2.2 mW, and measurement was performed in a quartz cell (inert gas atmosphere). FIG. 5 shows the Raman spectrum measurement result of the layered polysilane. The peak derived from Si—H of the Raman spectrum was identified by performing simulation (model structure is Si ₂₄ H ₃₆ ) using Hartley-Fock / 6-31G (d) on the Raman spectrum of the layered polysilane. As a result, the vibration peak Si backbone 370,490,630cm ^-1, 730cm ^-1 is swinging vibration of Si-H adjacent peaks of 840 cm ^-1 is H-Si-H bending, 2100 cm ^- It was inferred that ¹ was Si—H stretching.

(5) NMR spectrum measurement About the obtained layered polysilane, the NMR spectrum was measured. ²⁹ Si-NMR was measured using a JEOL superconducting Fourier transform nuclear magnetic resonance apparatus (JNM-LA500). FIG. 6 shows the NMR spectrum measurement result of the layered polysilane. As shown in FIG. 6, in the ²⁹ Si-NMR spectrum of the layered polysilane, a peak having a peak in the range of −90 ppm to −110 ppm was obtained. This peak was presumed to be HSi (Si) ₃ of layered polysilane by quantum chemical calculation using RHF / 6-31G (d).

(6) Performance evaluation of electrode The obtained layered polysilane was used as an active material of a lithium secondary battery (evaluation cell), and its characteristics were examined. Specifically, the above-described layered polysilane as an active material, carbon black as a conductive material (TB5500 manufactured by Tokai Carbon), and Teflon powder (F104, manufactured by Daikin Industries, “Teflon” as a binder) are registered trademarks (hereinafter referred to as “Teflon”). 10) was mixed at a weight ratio of 40: 55: 5 and weighed on a stainless steel mesh having a diameter of 15 mm to obtain an electrode for a lithium secondary battery of Example 1. Using a metallic lithium as a counter electrode, a polyethylene microporous membrane as a separator, and a 1M LiPF ₆ solution as an electrolyte, an evaluation cell of a bipolar cell made in Japan Tomcell of Example 1 was produced using this Example 1 as a working electrode. . As a solvent for the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1: 1 (manufactured by Toyama Pharmaceutical) was used. The battery performance was evaluated by setting the lower limit potential to 0 V, the upper limit potential to 1.0 V, and repeating the constant current measurement at 0.7 mA 10 times at room temperature. The measurement was performed using the same two evaluation cells (number of measurement points n = 2). FIG. 7 shows the measurement results of the charge / discharge cycle of the evaluation cell of Example 1. In FIG. 7, the charge capacity per electrode weight is shown in the upper part, and the charge capacity divided by the discharge capacity (referred to as charge / discharge efficiency) is shown in the lower part. As shown in FIG. 7, the electrode using layered polysilane as the active material had an initial capacity of about 600 mAh / g, and 400 mAh / g after 10 cycles. The charge / discharge efficiency was about 1.0 except for the first time, and it was found that the charge / discharge efficiency had good cycle characteristics. From the above, it was found that the layered polysilane is useful as an active material for a lithium secondary battery.

Next, the crystal change of layered polysilane was examined. In an inert gas atmosphere with a dew point of −90 ° C., 10 mg of a mixture of the above-described layered polysilane, carbon black (TB5500, manufactured by Tokai Carbon Co., Ltd.) and Teflon powder in a weight ratio of 20: 75: 5 was weighed. A lithium secondary battery electrode of Example 2 was obtained by pressure bonding to a 15 mm stainless steel mesh. Using a metallic lithium as a counter electrode, a polyethylene microporous membrane as a separator, and a 1M LiPF ₆ solution as an electrolyte, an evaluation cell of a bipolar cell made in Japan Tom Cell of Example 2 was produced using this Example 2 as a working electrode. . As a solvent for the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1: 1 (manufactured by Toyama Pharmaceutical) was used. Using the produced evaluation cell, discharging and charging were performed at a constant current of 0.7 mA with a lower limit potential of 0 V and an upper limit potential of 3.0 V. Specifically, in the first discharge, before charging / discharging (Fresh; No. 1), 0.4 V (No. 2), 0.33 V (No. 3), and 0.0 V (No. 4). The electrode which completed charging / discharging each and the electrode (No. 5) charged to 3V after discharge of 0V were produced. FIG. 8 is an explanatory diagram of the initial charge / discharge curve of the evaluation cell of Example 2 and the potential of the fabricated electrode. Each of the produced electrodes was washed and dried with diethyl carbonate in an inert gas atmosphere having a dew point of −90 ° C., and X-ray diffraction measurement was performed in this inert gas atmosphere. X-ray diffraction measurement was performed under the conditions of 50 kV-300 mA, divergence slit ½ °, divergence length limiting slit 10 mm, scattering slit ½ °, and light receiving slit 0.15 mm in an inert atmosphere. FIG. 9 shows the results of X-ray diffraction spectrum measurement of the electrode of Example 2 in the initial charge / discharge. As shown in FIG. 9, the 2θ = 15 ° peak derived from the period of the layered polysilane was detected by lithium doping up to 0.33 V, but disappeared at 0 V. In addition, a broad peak having an apex in the range of 2θ between 20 ° and 28 ° appeared. This broad peak was presumed to support the formation of amorphous Li _x Si (also referred to as an amorphous silane compound).

Here, when silicon particles are used as the active material, when lithium is electrochemically added (discharged) to crystalline silicon, it changes to amorphous Li _x Si, and further lithium is added electrochemically. It has been reported that it changes to crystalline Li _x Si _y (see Non-Patent Document 1; Electrochemical Solid-State Letters 7, A93-A96, (2004)). Although not shown, when an evaluation cell using crystalline silicon particles as an active material was produced and the performance of the electrode was evaluated, crystalline Li _x Si _y was confirmed. This crystalline Li _x Si _y is considered to have an adverse effect on the charge / discharge characteristics, such as once generated, it does not disappear even in the charge / discharge cycle, and the lithium release property is low. However, the electrode of Example 2 of the present invention did not produce crystalline Li _x Si _y or the like as in the case of using silicon particles, but produced an amorphous silane compound. And since it was maintaining a broad peak even if charging / discharging was repeated, it turned out that the state of an amorphous silane compound is maintained even if charging / discharging is repeated. This is because, in layered polysilane, the distance between adjacent silicon layers (for example, 0.46 nm) is wider than the Si-Si bond distance (for example, 0.224 nm). This was presumed to be because crystallization of silicon and silicon could be prevented. As described above, since the amorphous silane compound is structurally stable, the volume change hardly occurs, and a high discharge capacity is exhibited in the charge / discharge cycle, and it is presumed that the charge / discharge cycle life becomes longer. Moreover, since it is estimated that it has the stable structure according to a layered structure, it is thought that it can respond also to rapid charging / discharging. At present, the structure of the amorphous silane compound is not clear, but since the layered polysilane has a layered structure having a six-membered silicon ring, the layered polysilane is not completely amorphous. It is inferred that the structure (or feature) close to is left.

  The present invention is applicable to the battery industry.

  20 coin-type battery, 21 battery case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 sealing plate.

Claims (4)

  An electrode for a lithium secondary battery, comprising an amorphous silane compound obtained by doping lithium into a layered polysilane as an active material.   The electrode for a lithium secondary battery according to claim 1, wherein the amorphous silane compound is obtained by doping lithium to a predetermined low potential range including 0 V at a lithium reference potential.   The amorphous silane compound shows a broad peak having an apex in the range of 2θ in X-ray diffraction of 20 ° to 28 °, and 2θ in X-ray diffraction of 13.5 ° to 16.5 °. The electrode for a lithium secondary battery according to claim 1, which is obtained by doping lithium into a layered polysilane showing a peak of a peak derived from the 001 plane. The electrode for a lithium secondary battery according to any one of claims 1 to 3,
An ion conductive medium that is in contact with the lithium secondary battery electrode and conducts lithium ions;
Rechargeable lithium battery.