CN100466340C - Non-aqueous electrolyte rechargeable battery - Google Patents

Abstract

The present invention discloses a non-aqueous electrolyte secondary battery including: a positive electrode; a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode active material that includes at least Si. The non-aqueous electrolyte includes lithium hexafluorophosphate as a main supporting electrolyte and has an acid content of not less than 50 ppm and not more than 200 ppm. The negative electrode has a potential of not less than 0.6 V and not more than 1.5 V relative to a Li electrode at an end-of-discharge voltage of the battery. The battery is prevented from suffering degradation of storage characteristics caused by the dissolution reaction of Si from the negative electrode during charging and the precipitation reaction of the dissolved Si.

Description

Non-aqueous electrolyte secondary battery

technical field

The present invention relates to a nonaqueous electrolyte secondary battery, and particularly to a nonaqueous electrolyte secondary battery including a preferred negative electrode and a nonaqueous electrolyte.

Background technique

Lithium metal can achieve high voltage and high energy density, and the use of lithium metal as the negative electrode of nonaqueous electrolyte secondary batteries has been extensively researched and developed. This development has led to the commercialization of lithium-ion batteries using graphite materials in the negative electrode, which can reversibly absorb and desorb lithium, and have excellent cycle life and safety.

However, the effective capacity of the battery using the negative electrode based on graphite material is about 350mAh/g, which is very close to the theoretical capacity (372mAh/g) of graphite material. Therefore, as long as graphite material is used in the negative electrode, it is unlikely that the capacity will be greatly improved. At the same time, the functions of portable devices are becoming more and more complex, and the capacity of nonaqueous electrolyte secondary batteries, which are energy sources for such devices, is also required to increase accordingly. Therefore, in order to obtain a higher capacity, it is necessary to have an anode material with a capacity higher than that of graphite.

As materials that can provide a high capacity, alloy materials containing silicon (Si) and alloy materials containing tin have recently attracted attention. These metal elements can electrochemically absorb and desorb lithium ions, and can be charged/discharged at a much higher capacity than graphite. For example, it is known that the theoretical discharge capacity of silicon is 4199 mAh/g, which is 11 times that of graphite.

Therefore, the use of silicon-containing negative electrode active materials is being studied in conjunction with batteries using conventional lithium secondary battery components, such as lithium cobalt oxide positive electrodes, and lithium hexafluorophosphate and ethylene carbonate. A non-aqueous electrolyte composed of a mixed solution of methyl ethyl carbonate. However, if such batteries are stored at high temperatures, especially in a discharged state, they are very susceptible to degradation. Therefore, this type of battery has a problem of failure of the battery function after storage.

To avoid this problem, it is preferable to minimize the negative electrode discharge potential. For example, Japanese Laid-Open Patent Publication No. Hei 11-233155 (Patent Document 1) suggests that by using SiO as the negative electrode active material and controlling the discharge termination potential of the negative electrode with respect to the Li electrode at 0.6 V or lower, the resulting charge/discharge Capacity loss due to cycling is minimized.

However, if the discharge potential is limited as in Patent Document 1, since the average discharge potential of SiO relative to the Li electrode is 0.4V to 0.5V, only about half of the intrinsic capacity of SiO can be utilized, thus sacrificing the inherent high capacity characteristics of SiO .

Contents of the invention

An object of the present invention is to provide a nonaqueous electrolyte secondary battery that can make full use of the high capacity inherent in Si-based negative electrode materials without causing deterioration in storage characteristics and charge/discharge cycle characteristics .

The present invention relates to a nonaqueous electrolyte secondary battery comprising: a positive electrode capable of electrochemically absorbing and desorbing Li, a negative electrode including a negative electrode active material containing at least Si, and a nonaqueous electrolyte.

The nonaqueous electrolyte includes lithium hexafluorophosphate as a lithium salt as a main supporting electrolyte, and the acid content of the nonaqueous electrolyte is not less than 50 ppm and not more than 200 ppm. The potential of the negative electrode relative to the Li electrode is not lower than 0.6V and not higher than 1.5V at the end-of-discharge voltage of the battery.

The present invention was accomplished based on the following studies.

A conventional battery with the above problems was disassembled and the potential of the negative electrode relative to the Li electrode (Li/Li ⁺ ) was measured. As a result, the potential was found to be 1.8 V with respect to Li/Li ⁺ . At the same time, by calculating the Si content dissolved in the electrolyte, it was found that about 1/100 of the Si contained in the negative electrode was dissolved in the non-aqueous electrolyte. In addition, analyzing the coating film formed on the surface of the negative electrode active material due to the side reaction with the nonaqueous electrolyte, a large number of Si-containing compounds were measured, along with Li-containing organic and inorganic compounds.

The reaction was carefully studied, and it was speculated that the following reaction occurred when Si contained in the negative electrode active material was dissolved in the electrolyte:

Si+2HF+nh ⁺ ＝>SiF ₂ +2H ⁺ +(n-1)e ^- (1)

(h ⁺ means hole)

When the battery is in a discharged state (ie, when electrons leave the negative electrode) and HF exists in the non-aqueous electrolyte, Si forms SiF ₂ through the reaction shown in equation (1), and dissolves in the non-aqueous electrolyte. SiF ₂ , as a compound of Si, precipitates on the surface of the negative electrode or other positions of the battery through the following chemical reactions.

SiF ₂ +4HF＝>H ₂ SiF ₆ +H ₂ (2)

These reactions are believed to occur when Si contained in the negative electrode active material is dissolved in the non-aqueous electrolyte and precipitated as a Si compound. In order to suppress the dissolution of Si contained in the negative electrode active material in the non-aqueous electrolyte and the precipitation of Si in the form of Si compounds on the surface of the negative electrode during charging, the present inventors found that the following conditions are effective:

(1) The non-aqueous electrolyte contains lithium hexafluorophosphate as a main lithium salt.

(2) The acid content of the nonaqueous electrolyte is not less than 50 ppm and not more than 200 ppm.

(3) At the end-of-discharge voltage of the battery, that is, when the battery voltage is at the end-of-discharge voltage or when the battery has been discharged to the end-of-discharge voltage, the potential of the negative electrode relative to the Li electrode is not lower than 0.6V and not higher than 1.5V.

The negative electrode active material is preferably Si, Si alloy or Si compound.

The Si compound is preferably an oxide represented by _SiOx , where 0<x<2. The total content of Fe, Ni, Co, Cu and Cr contained in the Si alloy is preferably 1000 ppm or less.

The negative electrode preferably includes a negative electrode current collector and a negative electrode active material thin film deposited on the negative electrode current collector.

The nonaqueous electrolyte preferably contains not more than 5% by weight of bis[1,2-oxalate(2-)-O,O']borate ions.

The present invention can take full advantage of the inherently high capacity of Si-based anode materials. At the same time, the present invention can solve the above-mentioned problems, that is, it can control the dissolution reaction of Si in the discharged state and the film-forming reaction caused by the dissolved Si. Therefore, it is possible to provide a nonaqueous electrolyte secondary battery having excellent storage characteristics and excellent charge/discharge characteristics.

While the novel features of the present invention are set forth with particularity in the appended claims, a better understanding and appreciation of the present invention, both in organization and in content, can be better understood and appreciated from the following detailed description taken in conjunction with the accompanying drawings. Other objects and features related thereto.

Description of drawings

FIG. 1 is a schematic longitudinal sectional view of a cylindrical lithium-ion secondary battery used in an embodiment of the present invention.

Detailed description of the invention

Anode active materials that can electrochemically absorb and desorb Li according to the present invention include Si. More specifically, the negative electrode active material is Si, Si alloy or Si compound.

The Si alloy preferably has a Si content of greater than 50% by weight. Particularly preferred Si alloys have a Si content of 75% by weight or more. Such alloys can provide high capacity.

The impurity element contained in Si alloy, Si or Si compound is preferably Ti, Zr or Sn. Further, P or Sb is also preferably included. In addition, elements that would have an adverse effect if contained therein are Fe, Ni, Co, Cu, or Cr, and the total content of Fe, Ni, Co, Cu, and Cr is desirably 1000 ppm or less.

The reason for this is as follows: Ti, Zr, Sn, P, or Sb, just like Si, has a valence of 4 or higher. Therefore, when Ti, Zr, Sn, P, or Sb is alloyed with Si, holes are less likely to be formed, but free electrons (carriers) are generated, so that these elements have a favorable effect of increasing electrical conductivity. On the other hand, Fe, Ni, Co, Cu, or Cr is trivalent or divalent when combined with Si. Therefore, when alloyed with Si, voids are formed in the alloy, which promotes the dissolution of Si during charging.

The Si compound is preferably SiO _x , where 0<x<2. Especially when 0<x≤1, a negative electrode with high capacity and long life can be obtained. The x value represents the ratio of oxygen to the entire negative electrode active material, and can be measured, for example, by quantifying oxygen according to a combustion method. The compound _SiOx can locally have various compositions, or can have a completely homogeneous composition. An example of a compound in the former case is SiO _0.8 , where the outermost layer is SiO _0.3 , the middle layer is SiO _0.7 , and the bottommost layer is SiO _0.9 .

As described above, the Si-based negative active material is preferably amorphous or low-crystalline. "Low crystalline" as used herein means a crystal grain size of 50 nm or less. The grain size can be calculated from the half-width of the most intense peak in the X-ray diffraction pattern by the Scherrer equation.

Meanwhile, "amorphous" as used herein means that the X-ray diffraction pattern has a broad peak in the range of 2θ=15 to 40°. For a crystalline negative active material, when it expands due to intercalation of Li, the particles or films of the negative active material are broken or damaged. As a result, the reaction area of the negative electrode active material increases, and thus there are more chances of contact with the hydrofluoric acid contained in the non-aqueous electrolyte. Therefore, the dissolution reaction of Si and the precipitation reaction of Si compounds are promoted. On the other hand, for an amorphous or low-crystalline negative active material, it expands when Li is inserted, but since it is divided by tiny grain boundaries (approximately from a few nanometers to 50 nm), the expansion stress is dispersed by the corresponding grain boundaries. and weakened. As a result, cracking or breakage of active material particles or thin films is less likely to occur.

In a preferred mode of the negative electrode including the Si-based negative electrode active material, an electrode mixture layer and a binder are provided on the negative electrode current collector, and the electrode mixture layer contains at least particles of the negative electrode active material. In another preferred mode, the negative electrode active material is provided on the negative electrode current collector in the form of a deposited film or a sintered film formed by a physical or chemical method. In a particularly preferred manner, the deposited film of the negative electrode active material is physically formed on the negative electrode current collector.

The size of the negative electrode active material particles used in the electrode mixture layer is preferably not less than 0.1 μm and not more than 50 μm. The binder may be any material as long as it can bind the negative electrode current collector and the negative electrode active material, and is electrochemically inactive within the battery operating potential range. Suitable exemplary binders include styrene-butylene copolymer rubber, polyacrylic acid, polyethylene, polyurethane, polymethylmethacrylate, polyvinylidene fluoride, polytetrafluoroethylene, carboxymethylcellulose, and methylcellulose Su and so on. These can be used alone or in combination. As for the amount of the binder, the larger the better in terms of maintaining the structure of the electrode mixture layer, but the smaller the better in terms of increasing the battery capacity and improving the discharge characteristics. The electrode mixture layer further preferably contains a conductive agent mainly composed of carbon such as graphite, carbon black or carbon nanotubes. These conductive agents are preferably in contact with the negative electrode active material.

Preferred physical methods for forming deposited films are sputtering, vapor deposition, thermal spray and shot peening, and preferred chemical methods are CVD and electroplating. Among these methods of forming a deposited film, vapor deposition is particularly preferable because vapor deposition can form a film at high speed and is suitable for forming a deposited film of not less than several micrometers and not more than 50 μm. The deposited film does not have to be a flat or smooth film, and the deposited active material can be columnar or island-shaped. The sintered film is desirably provided by forming an electrode mixture layer containing an anode active material, and subjecting it to sintering treatment by heating or plasma.

When the deposited film or sintered film of the negative electrode active material is composed of Si only, the preferred thickness is not less than 1 μm and not more than 20 μm. When the film is thinner than 1 μm, the volume of the current collector relative to the battery is large, thereby making it difficult to produce a battery with a large capacity. On the other hand, if the film is thicker than 20 [mu]m, the stress due to the expansion of the active material will act significantly on the entire current collector or negative electrode, thereby wrinkling the electrode or causing the electrode to eventually be destroyed. Therefore, a thickness greater than 20 μm is not suitable. When the deposited film or sintered film of the negative electrode active material is composed of Si alloy or Si compound, the thickness of the film is preferably not less than 3 μm and not more than 50 μm for the same reason as described above.

Each film thickness mentioned above refers to the thickness before lithium is inserted, and is almost equal to the film thickness in the discharged state (battery state when discharged to the end-of-discharge voltage).

The thickness of the electrode mixture layer containing the negative electrode active material is preferably not less than 10 μm and not more than 100 μm. If the electrode mixture layer is thicker than 100 μm, it is difficult for the non-aqueous electrolyte to penetrate to the vicinity of the current collector, so that the negative active material cannot be fully utilized. Meanwhile, when it is thinner than 10 μm, the volume of the current collector relative to the electrode is too large. As a result, battery capacity deteriorates, which is not desirable.

The nonaqueous electrolyte includes a nonaqueous solvent and lithium hexafluorophosphate as a main supporting electrolyte dissolved in the nonaqueous solvent. Any nonaqueous solvent can be used without particular limitation as long as it is used as the electrolyte of the nonaqueous electrolyte secondary battery. Commonly used examples are solvent mixtures of cyclic carbonates such as ethylene carbonate or propylene carbonate and chain carbonates such as methyl carbonate, ethyl carbonate or methyl carbonate. ethyl ester. In addition, γ-butyrolactone, dimethoxyethane, etc. can also be mixed into a non-aqueous solvent. The concentration of lithium hexafluorophosphate is desirably not lower than 0.5 mol/L and not higher than 2 mol/L. The non-aqueous electrolyte using lithium hexafluorophosphate as the main supporting electrolyte can provide good battery characteristics compared to using other lithium salts. Furthermore, in addition to lithium hexafluorophosphate, small amounts of other lithium salts, for example, lithium tetrafluoroborate or lithium imide salts, can also be added.

The total acid content of the nonaqueous electrolyte is not less than 50 ppm and not more than 200 ppm. As mentioned above, the presence of HF during discharge can cause a dissolution reaction of Si at the negative electrode. In the present invention, by controlling the acid content and the potential of the negative electrode, part of the surface of the negative electrode active material can be adjusted to facilitate the charging/discharging reaction.

The outermost surface of the Si-based negative active material is usually covered with SiO ₂ . Since this _SiO2 is electrochemically inactive for Li and does not conduct electricity, it interferes with the charge/discharge reaction. Therefore, by dissolving the _SiO2 layer and some nearby Si (Si here refers to the Si phase or _SiOx (0<x<2) phase in the Si element, Si alloy) can increase the reaction area of the negative electrode active material, which can Promote electrochemical reactions. If the acid content of the non-aqueous electrolyte is less than 50 ppm, the _SiO2 layer cannot be sufficiently dissolved, and the surface of the active material is not sufficiently damaged, so that it will not be effective. On the other hand, if the acid content is higher than 200 ppm, Si, the negative electrode active material, is excessively dissolved, which is also unsuitable.

The main acid contained in the non-aqueous electrolyte is hydrofluoric acid, but other acids may be contained as long as the total acid content is within the above range. The acid content of the non-aqueous electrolyte can be reduced by reducing the water content initially (at the time of production) contained in the non-aqueous electrolyte. In addition, the acid content can also be effectively reduced by reducing the water content in the positive electrode, negative electrode, separator, and other battery components.

In addition, the nonaqueous electrolyte preferably contains bis[1,2-oxalate(2-)-O,O']borate (hereinafter referred to as BOB) ions not higher than 5% by weight. The BOB ion is an anion having the following structural formula and can neutralize the acid contained in the electrolyte. Therefore, BOB ions can significantly weaken the active acid in the electrolyte, thereby avoiding the excessive dissolution reaction of Si and the precipitation reaction of dissolved Si.

In the present invention, the potential of the negative electrode relative to Li/Li ⁺ is 0.6-1.5V at the end-of-discharge voltage of the battery. If the discharge ends at a potential higher than this range, Si dissolution may occur even if the above conditions are satisfied. The negative electrode potential is particularly preferably 0.6 to 0.9 V with respect to Li/Li ⁺ at the end-of-discharge voltage of the battery. As used herein, "discharge cut-off voltage" refers to the lowest discharge voltage that can be safely discharged. In the present invention, the range of the end-of-discharge voltage is not lower than 1.5V and not higher than 3.0V, and the preferred range is 2.0V-2.5V. Furthermore, even at least in this voltage range, it should be preferred that after the battery is fully discharged to the end of the charge/discharge cycle, both reversible Li (Li electrochemically active) and irreversible Li (Li due to side reactions etc. The reason is Li without potential, such as Li oxide). In this case, the reversible Li can be discharged if over-discharge occurs, thus ensuring the safety of the battery. As for the reversible Li content, a small amount is effective, but ideally not less than 3% and not more than 30% of the discharge capacity (total discharge capacity) in the range of 0 to 3.0 V vs. Li/Li ⁺ . If the content of reversible Li is less than 3%, Li is easily depleted during excessive discharge, so it is difficult to ensure safety. On the other hand, if the content exceeds 30%, the capacity of Li for the battery operating voltage range is small, so it is difficult to obtain a high capacity.

Furthermore, such control of the negative electrode discharge potential can have a significant effect on inhibiting Si dissolution, especially when using oxides of Si, especially in thin film form. The reasons are probably as follows. Firstly, the oxide of Si has in particular a _SiO2 layer. Second, Si oxide in the form of a thin film has a small reaction area and therefore has a significantly higher capacity per unit reaction area than particulate Si oxide, and thus is more likely to cause side reactions.

In order to make the end-of-discharge potential of the negative electrode within the above range and provide a high capacity, it is preferable to add Li to provide an appropriate irreversible capacity of the negative electrode. The method of adding Li includes a method of sticking Li metal foil on the surface of the negative electrode and a method of forming a Li thin film on the surface of the negative electrode by vapor deposition and other methods. The amount of Li added to the surface can be considered as appropriate according to the irreversible capacity of the negative electrode itself.

When the negative electrode of the present invention includes a current collector made of metal foil and a negative electrode mixture layer on each side of the current collector, the current collector is desirably a copper foil or a copper alloy foil. The copper alloy foil preferably has a copper content of 90% by weight or higher. Copper foil or copper alloy foil containing an element such as P, Ag or Cr is effective in terms of improving the strength and flexibility of the current collector.

The thickness of the current collector is preferably not less than 6 μm and not more than 40 μm. It is difficult to handle if the current collector is thinner than 6 μm. In addition, it is impossible for the current collector to have the required strength to crack or wrinkle due to the expansion and contraction of the electrode mixture layer. On the other hand, if the current collector is thicker than 40 μm, the volume ratio of the current collector to the battery is large, thus detrimental to capacity depending on the kind of battery. In addition, if the current collector is thick, it is also difficult to handle, such as difficult to bend.

The nonaqueous electrolyte secondary battery of the present invention includes the above-mentioned negative electrode, a positive electrode capable of electrochemically absorbing and desorbing Li, and a nonaqueous electrolyte.

The positive electrode may be any electrode used as a positive electrode in a nonaqueous electrolyte secondary battery without particular limitation. The positive electrode can be produced by conventional methods. For example, a positive electrode active material, a conductive agent such as carbon black, and a binder such as polyvinylidene fluoride can be mixed in a liquid phase to form a slurry, and this slurry is applied to a positive electrode current collector such as made of Al, and its Drying and winding are performed to obtain a positive electrode.

The positive electrode active material may be any material used as a positive electrode active material in a non-aqueous electrolyte secondary battery without particular limitation. However, lithium-containing transition metal compounds are preferred. Examples of lithium-containing transition metal compounds include, but are not limited to, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and LiMnO ₂ . Such compounds in which transition metal elements are substituted by other elements can also be preferably used. Such examples include LiCo _1-x Mg _x O ₂ , LiNi _1-y Co _y O ₂ , LiNi _1-yz Co _y Mn _z O ₂ , where 0<x<1, 0<y<1, and z is an integer.

In order to prevent internal short circuit between positive and negative electrodes, a separator is provided between the two electrodes. The separator material can be any material, as long as the material can pass the non-aqueous electrolyte reasonably, and can avoid contact between the positive electrode and the negative electrode. Microporous membranes made of polyethylene, polypropylene, etc. are often used in non-aqueous electrolyte secondary batteries, and their thickness is generally not less than 10 μm and not more than 30 μm.

The present invention is applicable to non-aqueous electrolyte secondary batteries of various shapes, such as cylinder, flat, coin and prism, and there is no special limitation on the shape of the battery. The present invention is applicable to batteries in various packaging forms, including batteries in which power generation components such as electrodes and electrolytes are packaged in metal battery boxes or laminated film boxes, and the packaging form of the batteries is not particularly limited.

Hereinafter, the present invention will be described in more detail by way of Examples and Comparative Examples. However, the following examples are only illustrations of preferred modes of the present invention, and should not be construed as any limitation to the present invention.

Detailed ways

Example 1

In this example, negative electrodes and cylindrical batteries were produced in the following manner, and their cycle life and discharge capacity were evaluated.

(1) Production of negative electrodes

Negative pole (i)

A graphite crucible and an electron gun in which Si metal (purity 99.999%, Furuuchi Chemical Corporation, ingot) were placed were placed in a vapor deposition apparatus. An electrolytic copper foil (20 μm thick, available from Furukawa Circuit Foil Co., Ltd.) used as a current collector sheet was fed from a roll to a vapor deposition apparatus at a constant speed (5 cm/min) so that Si deposited on the copper foil surface. The deposition conditions were set at an accelerating voltage of -8 kV and a current of 150 mA. The vacuum in the apparatus was set to 3x10 ^-5 Torr when the following vapor deposition was performed, unless otherwise specified.

After completion of deposition on one side of the current collector sheet, vapor deposition on the other side (non-deposited side) was performed in the same manner to form an active material film on each side. These films were analyzed by X-ray diffraction, and it was found that there was a broad peak at 2θ=15-40° which was considered to be caused by amorphous Si. This result indicates that the active material Si is amorphous. The total thickness of the negative electrode is about 30-36 μm, and the film thickness on each side of the current collector is about 5 μm.

The negative electrode sheet was sent again to the vapor deposition apparatus, and Li was deposited on both sides of the negative electrode from target metal Li (available from Honjo Chemical Corporation) by a resistance heating method. The amount of deposition can be adjusted by changing the conveying speed of the negative electrode sheet from the rollers to the vapor deposition device. In this way, a specified amount of Li can be added to the surface of the negative electrode sheet. For example, in the negative electrode (i) used in battery 1 described later, the conveying speed of the negative electrode sheet was 5 cm/min, and the thickness of deposited Li was about 5 μm. By changing the amount of Li added, the irreversible capacity of the anode can be replenished to control the anode potential at the end of discharge. Likewise, negative electrode (ii), negative electrode (iii), and negative electrode (iv) described below were also supplemented with Li.

After forming the Si thin film, the negative electrode (i) was vacuum-dried at 110°C for 15 hours, and then stored at room temperature in a dry atmosphere having a dew point of -60°C or lower. In addition, after replenishing Li, the negative electrode (i) was also stored at room temperature in a dry atmosphere with a dew point of −60° C. or lower to remove and control the water content in the electrode.

Negative pole (ii)

Ti metal (purity 99.9%, 100-150 μm, available from Japan Pure Chemical Co., Ltd.) and Si metal (purity 99.99%, 100-150 μm, available from Kanto Chemical Co., Inc.) were weighed, and then Blending was performed at a weight ratio of Ti:Si=9.2:90.8.

3.5 kg of this powder mixture was put into a vibration mill (FV-20, available from Chuo Kakohki Industries, Ltd.), and stainless steel metal balls (2 cm in diameter) were placed therein so that the metal balls accounted for 70% of the inner volume of the mill. After the vessel was evacuated, it was filled with Ar (99.999% pure, available from Nippon Sanso Corporation) to provide 1 atmosphere of pressure. The frequency of the mill was set at 720 Hz. Under these conditions, the alloy was produced mechanically for 80 hours.

The Ti-Si alloys obtained from the above operations were collected and their particle size distributions were measured. As a result, it was found to have a wide particle size distribution of 0.5 μm to 80 μm. This Ti—Si alloy was sieved with a sieve (10 μm or less) to obtain an alloy material having a maximum particle size of 8 μm and an average particle size of 5 μm (hereinafter referred to as alloy “a”).

The impurity of this alloy "a" was measured by an ICP spectrometer, and it was found that the content of Fe was 500 ppm, that of Ni was 30 ppm, and that of Cr was 60 ppm. The contents of other transition metal elements are all lower than the lower limit of measurement. The total content of these elements was 590 ppm.

Alloy "a" was analyzed by X-ray diffraction, which showed that the alloy was microcrystalline. Meanwhile, the size of crystal grains (crystallites) was calculated from the half-width of the strongest peak by the Schuler equation, and it was found that the crystal grain size was 18 nm. Further, from the peaks of the X-ray diffraction pattern and observation by a transmission electron microscope, it is estimated that the alloy "a" has a Si single phase (A phase) and a TiSi ₂ phase (B phase). Assuming that the alloy "a" has only these two phases, the ratio of the Si single phase and the TiSi ₂ phase is calculated, and it is found that Si:TiSi ₂ =80:20 (weight ratio).

Alloy "a" was observed with a transmission electron microscope (TEM). Alloy "a" was found to have an amorphous region, a Si single phase comprising grains (crystallites) of about 10 nm in size, and a TiSi ₂ phase comprising grains (crystallites) of about 15-20 nm in size.

Alloy "a" was blended with graphite in a weight ratio of 25:75. Then 100 parts by weight of this mixture was mixed with 2 parts by weight of acetylene black and 8 parts by weight of polyacrylic acid, wherein acetylene black (trade name DENKA BLACK, available from Denki Kagaku Kogyo K.K.) was used as a conductive agent, and polyacrylic acid ( Molecular weight of about 150000, available from Wako Pure Chemical Industries, Ltd.) was used as a binder. The mixture was sufficiently kneaded while adding pure water to obtain a negative electrode mixture slurry. The graphite used was flake graphite (KS-44) with an average particle size of 20 μm, available from Timcal Ltd.

This negative electrode mixture slurry was applied to both sides of a current collector made of a 10 μm thick electrolytic copper foil (available from Furukawa Circuit Foil Co., Ltd.), dried, and wound. The result was a negative electrode sheet comprising a current collector and a negative electrode mixture layer carried on each side thereof. The density of the negative electrode mixture layer was 1.3 g/cm ³ , and the porosity of the negative electrode mixture layer was 40%. The negative electrode sheet was vacuum-dried at 190°C for 12 hours, and then stored in a dry atmosphere with a dew point of -60°C or lower.

Next, a Li metal foil of a specified size (50 μm thick, available from Honjo Chemical Corporation) was pasted on the surface of the negative electrode sheet to supplement the irreversible capacity with an appropriate amount of Li. For example, in the negative electrode (ii) used in the battery 7 described later, the attached Li covers about 1/4 of the negative electrode area.

After Li supplementation, the negative electrode sheet was likewise stored in a dry atmosphere with a dew point of -60°C or lower to remove and control the water content in the electrode.

Negative pole (iii)

SiO (purity 99.9%, average particle size 20 μm, maximum particle size 45 μm, available from Sumitomo Titanium Corporation) was used as the negative electrode active material. SiO was mixed with graphite and acetylene black at a ratio of SiO:graphite:acetylene black=45:52:3 (weight ratio) to compensate for the low conductivity of SiO. The mixture was mixed with 5 parts by weight of polyvinylidene fluoride (available from Kureha Chemical Industry Co., Ltd.), and then sufficiently kneaded with N-methyl-2-pyrrolidone to obtain a negative electrode mixture slurry. The graphite used was graphite (KS4) with an average particle size of 3 μm, available from Timcal Ltd.

In the same method as in the negative electrode (ii), the negative electrode mixture slurry was coated on both sides of the current collector, dried and wound, wherein the current collector was made of 10 μm thick electrolytic copper foil (available from Furukawa Circuit Foil Co., Ltd. obtained). The result was a negative electrode sheet comprising the current collector and the negative electrode mixture layer carried on each side thereof. The density of the negative electrode mixture layer was 1.0 g/cm ³ , and the porosity of the negative electrode mixture layer was 55%.

The negative electrode sheet was vacuum-dried at 80°C for 24 hours, and then stored in a dry atmosphere with a dew point of -60°C or lower.

In the same way as in the negative electrode (ii), a Li metal foil (50 μm thick, available from Honjo Chemical Corporation) of a specified size was pasted on this negative electrode to supplement an appropriate amount of Li. For example, in the negative electrode (iii) used in the battery 13 described later, the attached Li covers about 1/2 of the negative electrode area.

The Li-supplemented negative electrode sheet was stored at room temperature in a dry atmosphere with a dew point of -60°C or lower to remove and control the water content in the electrode.

Negative pole (iv)

In the same method as in negative electrode (i), the current collector sheet comprising electrolytic copper foil is sent into a vapor deposition device equipped with a graphite crucible and an electron gun in which Si metal is placed, and Si is deposited on the surface of the sheet. However, in this example, oxygen gas (available from Nippon Sanso Corporation) with a purity of 99.7% was supplied to the surface of the current collector sheet in the deposition apparatus. Oxygen is supplied from an oxygen cylinder at a flow rate of 80 sccm through a mass flow controller and a gas pipeline, and the gas pipeline is equipped with a nozzle at one end entering the vapor deposition device. The deposition conditions were set as acceleration voltage of -8kV, current of 150mA, and vacuum degree of 1.5x10 ^-3 Torr.

In this way, a thin film of active material is formed on each side of the current collector sheet. The total thickness of the negative electrode, that is, the total thickness of the current collector sheet and the films on both sides is about 42-45 μm, and the thickness of the film on each side is about 10-12 μm. The oxygen content in the film was measured by a combustion method and found to be SiO _0.4 . In addition, the films were subjected to X-ray diffraction analysis. As a result, a crystallization peak believed to be caused by Cu in the current collector sheet was observed, and the film had a broad peak at 2θ = 15-40°. This result indicates that the active material is amorphous.

The negative electrode sheet thus produced was fed again into a vapor deposition apparatus, and Li was deposited on both sides of the negative electrode by heating the target metal Li. For example, when the conveying speed of the negative electrode is set to 3 cm/min, the thickness of Li deposited on each side is about 8 μm.

(2) Preparation of positive electrode

Li ₂ CO ₃ and CoCO ₃ were blended at a predetermined molar ratio, and the mixture was heated at 950° C. to synthesize LiCoO ₂ as a cathode active material. The cathode active material is sieved to 45 μm or less. 100 parts by weight of this positive electrode active material and 5 parts by weight of acetylene black used as a conductive agent, 4 parts by weight of polyvinylidene fluoride used as a binder and an appropriate amount of N-methyl-2- as a dispersion medium The pyrrolidone is mixed, and the mixture is fully kneaded to form a positive electrode mixture slurry.

The positive electrode mixture slurry was applied to both sides of a current collector comprising a 15 μm thick aluminum foil (available from Showa Denko K.K.), dried, and wound. The result was a positive electrode sheet comprising the current collector and the positive electrode mixture layer carried on each side thereof.

The positive electrode sheet was stored at room temperature in a dry atmosphere having a dew point of -60°C or lower, and the electrode was dehydrated by vacuum drying at 80°C immediately before battery assembly as described below.

(3) Assembly of cylindrical battery

The positive electrode sheet and the negative electrode sheet were cut into predetermined shapes to produce a cylindrical lithium ion secondary battery as shown in FIG. 1 .

One end of the aluminum positive electrode lead 14 is connected to the current collector of the positive electrode 11 . One end of the nickel negative electrode lead 15 is connected to the current collector of the negative electrode 12 . A positive electrode 11 , a negative electrode 12 , and a separator 13 insulating these two electrodes are spirally wound to form an electrode body. The separator 13 is a 20 μm thick microporous membrane made of polyethylene resin, which is wider than both the positive and negative electrodes. The outside of the electrode body is completely covered by the end portion of the separator at the end of the winding side. The electrode body was vacuum-dried at 60°C for 10 hours in a dry atmosphere of -60°C (dew point) to evaporate the water content in the electrode body. The separator 13 and other battery components should be completely dried beforehand in order to minimize the possible water contained in the battery.

The upper insulating ring 16 and the lower insulating ring 17 are respectively fixed on the top and bottom of the electrode body, and then the electrode body is placed in the battery case 18 . Subsequently, a non-aqueous electrolyte was injected into the battery case to impregnate the electrode body. The other end of the positive electrode lead 14 is welded to the conductive component 21 electrically connected to the positive terminal 20, and the conductive component 21 is fixed on the sealing plate 19 made of electrical insulating resin. The other end of the negative electrode lead 15 is welded to the inner side of the bottom edge of the battery case 18 . Finally, the opening of the battery case 18 is closed with a sealing plate 19 . This completes a cylindrical lithium ion secondary battery. The battery has a diameter of 18mm, a height of 65mm, and a design capacity of 2000mAh.

Further, in this embodiment, the electrode 22 is installed in the center of the electrode body as a reference electrode for measuring the potential of the negative electrode. The electrode 22 includes a Ni metal wire 23 (the diameter of the wire is 0.5 mm), and the outer surface of the wire is wrapped with a Li metal foil 24 (thickness: 50 μm). The nickel wire 23 passes through the conductive component 21 and the positive electrode terminal 20, and is electrically insulated from the conductive component 21 by an insulating film 25 covering the surface of the nickel wire. The positive electrode terminal 20 is insulated from the nickel wire 23 by a gasket 26 .

Dissolve lithium hexafluorophosphate at a concentration of 1 mol/L in a non-aqueous solvent mixture with a volume ratio of ethylene carbonate and diethyl carbonate of 1:1 to prepare the non-aqueous electrolyte used. The non-aqueous electrolyte was quantitatively and qualitatively analyzed to determine its acid content and acid components. As a result, it was found that the main component was HF, and the total content was 18 ppm.

Using the negative electrode (i), negative electrode (ii), negative electrode (iii) and negative electrode (iv) into which a specified amount of Li was added, Batteries 1 to 6, Batteries 7 to 12, Batteries 13 to 18, and Batteries "a" were fabricated accordingly "~"f". Table 1 presents the Li content added to the negative electrodes of the corresponding batteries.

(4) Battery evaluation

<i>Discharge capacity

Each of these batteries was charged at a constant current of 400mA (0.2C; 1C is the hour-rate current) until the battery voltage reached 4.05V in a constant temperature oven set at 20°C, and then charged at Charge at a constant voltage of 4.05V until the current value drops to 20mA (0.01C). Thereafter, each cell was discharged at a constant current of 0.2C until the cell voltage dropped to 2.0V. Table 1 shows the discharge capacity.

<ii>Discharge storage test

After measuring the discharge capacity, the battery was operated as follows in a constant temperature furnace set at 20°C. First, each battery is charged at a constant current of 0.2C until the battery voltage reaches 4.05V, and then charged at a constant voltage of 4.05V until the current value reaches 0.01C. Next, the battery was discharged at a constant current of 0.2C until the battery voltage dropped to 2.0V (end-of-discharge voltage). After 1 hour, the potential of the negative electrode with respect to the electrode 22 was measured. This potential is indicated as "negative electrode potential during storage" in the following tables. Then, each battery was stored in an oven at 85°C for 3 days. After storage, it was charged and discharged under the same conditions as described above to obtain a discharge capacity. In this way, the capacity recovery rate after storage ((discharge capacity after storage)×100/(capacity before storage)) (%) can be obtained.

<iii>cycle life

After the discharge capacity was measured, the battery was repeatedly charged and discharged in a constant temperature furnace set at 20°C. First, each battery is charged with a constant current of 1C until the battery voltage reaches 4.05V, and then charged at a constant voltage of 4.05V until the current value reaches 0.05C. Next, the battery is discharged at a constant current of 1C until the battery voltage drops to 2.5V. These operations are repeated. The percentage of the discharge capacity at the 100th cycle to the discharge capacity at the 2nd cycle was defined as the capacity retention (%). Table 1 presents the results. The closer the capacity retention rate is to 100%, the better the cycle life is.

[Table 1]

negative electrode Li addition Discharge capacity (mAh) Negative electrode potential during storage (V, relative to Li/Li⁺) Acid content of electrolyte (ppm) Recovery rate (%) Capacity retention after 100 cycles (%) battery 1 Negative pole (i) Deposition thickness 6μm 1320 0.38 64 96 92 battery 2 Negative pole (i) Deposition thickness 4μm 1780 0.59 63 95 91 battery 3 Negative pole (i) Deposition thickness 3μm 1930 0.90 68 92 91 battery 4 Negative pole (i) Deposition thickness 2μm 2010 1.18 71 89 86 battery 5 Negative pole (i) Deposition thickness 1μm 2060 1.48 70 85 82 battery 6 Negative pole (i) no deposition 2100 1.76 71 58 43 battery 7 Negative pole (ii) paste 1/4 area 1290 0.40 85 97 95 battery 8 Negative pole (ii) paste 1/6 area 1760 0.61 87 94 93 battery 9 Negative pole (ii) Paste 1/8 area 1900 0.88 90 93 92 battery 10 Negative pole (ii) paste 1/10 area 1970 1.13 83 90 88 battery 11 Negative pole (ii) Paste 1/15 area 2030 1.50 91 83 78 battery 12 Negative pole (ii) no stickers 2060 1.69 89 63 33 battery 13 Negative pole (iii) paste 1/2 area 1280 0.39 78 97 95 battery 14 Negative pole (iii) paste 1/3 area 1750 0.61 79 94 94 battery 15 Negative pole (iii) paste 1/4 area 1930 0.92 76 92 94 battery 16 Negative pole (iii) paste 1/6 area 1990 1.08 75 89 89 battery 17 Negative pole (iii) Paste 1/8 area 1860 1.49 81 84 85 battery 18 Negative pole (iii) no stickers 1100 1.56 80 65 64 battery 19 graphite did not join 1250 0.31 90 99 95 battery a Negative pole (iv) Deposition thickness 15μm 1450 0.41 65 98 98 battery b Negative pole (iv) Deposition thickness 12μm 1860 0.73 70 99 98 battery c Negative pole (iv) Deposition thickness 10μm 1980 0.98 71 99 98 battery d Negative pole (iv) Deposition thickness 8μm 2050 1.10 73 99 99 battery e Negative pole (iv) Deposition thickness 4μm 1860 1.45 74 98 87 battery f Negative pole (iv) no deposition 1330 1.83 72 64 60

The manufacturing method of Comparative Battery 19 in Table 1 was the same as that of Battery 6 except that a negative electrode prepared as follows was used therein. A mixture of 100 parts by weight of graphite and 6 parts by weight of a binder (polyacrylic acid) and water was mixed to form a slurry, and the slurry was coated on a current collector, dried and wound to make a negative electrode.

As shown in Table 1, all the batteries using the negative electrode of this example showed higher capacity than the comparative battery 19. However, Cell 1, Cell 7, Cell 13, Cell "a" and Cell "f" exhibited only slightly higher capacities than Comparative Cell 19, suggesting that they did not take full advantage of the properties of the inventive anode. Further, battery 18 exhibited a slightly lower capacity than comparative battery 19 because lithium of the positive electrode was consumed due to the irreversible capacity of the negative electrode. The same is true for battery "f". Furthermore, cell 6, cell 12, cell 18, and cell "f" were observed to have particularly low recovery rates for post-discharge storage properties. Careful examination of these batteries revealed that the non-aqueous electrolyte contained a large amount of Si dissolved therein and a Si-containing coating film was formed on the surface of the negative electrode. In addition, examination of other batteries found that the lower the negative electrode potential at the end of discharge (the negative electrode potential during storage in Table 1), the less Si dissolved in the non-aqueous electrolyte and the dissolved amount was below the lower limit of measurement, especially when the negative electrode potential At 0.9 V or less vs. Li/Li ⁺ .

In these batteries, the acid content of the nonaqueous electrolyte is 63 to 91 ppm. In Batteries 1 to 6 using the negative electrode (i), the acid content was the lowest at 63 to 71 ppm. In Batteries 7 to 12 using the negative electrode (ii), the acid content was as high as 83 to 91 ppm. The reasons are probably as follows. In the negative electrode (ii), polyacrylic acid was used as a binder, and polyacrylic acid formed a hydrate on the electrode. Part of the hydrate cannot evaporate during drying at 190 °C, and the water contained in the hydrate reacts with _LiPF6 in the nonaqueous electrolyte to form hydrofluoric acid. In the negative electrode (iii), the acid content is 75 to 81 ppm. Table 1 gives these values.

In addition, Battery 6, Battery 12, and Battery 18 were not as good as the other batteries in terms of capacity retention after 100 cycles. This may be because, with the increase of charge/discharge cycles, repeated discharges lead to the dissolution reaction of Si and the precipitation reaction of dissolved Si, thus hindering the charge/discharge reaction.

Example 2

In this example, the acid content of the non-aqueous electrolyte was investigated. Using the negative electrode of Battery 9, a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1. More specifically, Batteries 20 to 25 were produced by changing the drying conditions of the negative electrode (ii) (vacuum drying at 190° C. for 12 hours) as shown in Table 2. Table 2 presents the measured electrolyte acid content and battery characteristics.

[Table 2]

negative electrode drying conditions Acid content of electrolyte (ppm) Discharge capacity (mAh) Negative electrode potential during storage (V, relative to Li/Li⁺) Recovery rate (%) Capacity retention after 100 cycles (%) battery 20 Negative pole (ii) Store at 20°C for 24 hours in a dry atmosphere with a dew point of -60°C or lower 231 1820 0.85 42 31 battery 21 Negative pole (ii) 60 ℃ hot air drying for 10 hours 189 1880 0.89 70 66 battery 22 Negative pole (ii) 190℃ hot air drying for 10 hours 145 1890 0.88 75 78 battery 23 Negative pole (ii) Vacuum drying at 60°C for 10 hours 102 1910 0.91 82 81 battery 24 Negative pole (ii) Vacuum drying at 190°C for 12 hours 56 1910 0.9 80 80 battery 25 Negative pole (ii) Vacuum drying at 190°C for 48 hours 44 1820 0.88 81 61

As clearly shown in Table 2, even when the termination potential of the negative electrode is 1.5 V or lower relative to Li/Li ⁺ , if the acid content of the non-aqueous electrolyte is greater than 200 ppm, the recovery rate and capacity retention rate after storage in the discharged state are both low. Low, as in the case of battery 20. This may be because a large amount of hydrofluoric acid near the negative electrode tends to make the reaction of reaction formula (1) proceed to the right, thereby promoting the dissolution reaction of Si and the related film formation reaction.

Battery 25 exhibited a lower capacity retention than batteries 22-24 after 100 cycles. When the Si content of the nonaqueous electrolyte of the battery 25 was measured, it was found to be below the lower limit of the measurement. In addition, when observing the surface of the negative electrode active material, it was found that the surface was very flat and smooth. On the other hand, with Batteries 20 to 24, it was found that the surface of the negative electrode active material had minute asperities, and these asperities became larger as the acid content increased. Especially for the cells 20-22, there are deep depressions. This indicates that a large amount of Si dissolution occurs locally.

The surface of the negative electrode active material of the battery 25 has a binding energy attributed to SiO ₂ as measured by X-ray photoelectric spectrometer (XPS). This result indicates that _SiO2 on the outermost surface cannot be dissolved when the acid content is small. Further, since the reaction area of the negative electrode active material is small, the electrochemical reaction cannot proceed smoothly. Probably due to this, the cycle characteristics of the battery 25 (ie, the characteristics obtained after the electrochemical reaction is repeated) deteriorate.

Example 3

In this example, the influence of impurities contained in the negative electrode was studied. The preparation method of the negative electrode active material used in the present embodiment is the same as that in the negative electrode (ii), except that the balls used in the mechanical manufacturing alloy are made of S45C steel (Fe-0.45%C steel) instead of stainless steel, Increase the operating time to 80 hours, 100 hours or 150 hours. The obtained alloy has a wide particle size distribution of 0.5 μm to 80 μm. These were sieved with a sieve (10 μm or less) to obtain an alloy material having a maximum particle size of 8 μm and an average particle size of 5 μm. The alloy powders thus obtained are referred to as alloy "b", alloy "c" and alloy "d".

The impurities of these alloys "b" to "d" were measured by ICP spectrometer, and it was found that transition metal elements other than Fe were lower than the lower limit of measurement. The Fe content was 680 ppm in alloy "b", 980 ppm in alloy "c", and 1320 ppm in alloy "d".

Alloys "b" to "d" were analyzed by X-ray diffraction, and their XRD distributions were similar to those of alloy "a". All these alloys are microcrystalline, and the grain (crystallite) size is calculated from the half-width of the strongest peak by the Schuler equation to be 11-18 nm. The particles of alloys "b" to "d" are presumed to have Si single phase (A phase) and TiSi ₂ phase (B phase). In addition, these alloys "b" to "d" were observed with a transmission electron microscope (TEM), and found that they had an amorphous region, a Si single phase including crystal grains (crystallites) with a size of about 10 nm, and a TiSi ₂ phase of crystal grains (fine crystals) of about 10 to 20 nm.

Using these alloys, a negative electrode mixture layer was formed on each side of the current collector sheet in the same manner as the negative electrode (ii), and Li foil was pasted on the surface of the negative electrode mixture layer to cover 1/8 of the negative electrode area.

Using these negative electrodes, cylindrical lithium ion secondary batteries 26 to 28 were produced in the same manner as battery 9, and evaluated in the same manner as in the above-described examples. Table 3 presents the results and the acid content measured from these cells.

[table 3]

negative active material Impurity content (ppm) Discharge capacity (mAh) Negative electrode potential during storage (V, relative to Li/Li⁺) Acid content of electrolyte (ppm) Recovery rate (%) Capacity retention after 100 cycles (%) battery 26 alloy b 680 1890 0.88 81 91 90 battery 27 alloy c 980 1870 0.85 83 73 83 battery 28 alloy d 1320 1870 0.91 85 32 55

These results indicate that when the impurity content exceeds 1000 ppm, both the recovery rate and the capacity retention rate after storage in the discharged state become low.

Example 4

Batteries 29, 30, and 31 were produced in the same manner as Batteries 3, 9, and 15, respectively, except that the nonaqueous electrolyte used contained 3% by weight of bis[1,2-oxalic acid (2-)-O,O']boronic acid Lithium (hereinafter referred to as LiBOB). Further, Batteries 32, 33 and 34 were also produced in the same manner except that the nonaqueous electrolyte used contained 5% by weight of LiBOB. Further, batteries 35, 36 and 37 were produced in the same manner except that the nonaqueous electrolyte used contained 8% by weight of LiBOB. These batteries were evaluated in the same manner as in the above-mentioned examples. Table 4 presents the results and the acid content measured from these cells.

[Table 4]

negative electrode The amount of LiBO added (weight %) Discharge capacity (mAh) Negative electrode potential during storage (V, relative to Li/Li⁺) Acid content of electrolyte (ppm) Recovery rate (%) Capacity retention after 100 cycles (%) battery 29 Negative pole (i) 3 1950 0.85 75 94 95 battery 30 Negative pole (ii) 3 1930 0.86 77 96 96 battery 31 Negative pole (iii) 3 1940 0.89 79 95 97 battery 32 Negative pole (i) 5 1930 0.91 80 95 91 battery 33 Negative pole (ii) 5 1910 0.94 79 97 92 battery 34 Negative pole (iii) 5 1910 0.92 80 96 94 battery 35 Negative pole (i) 8 1860 0.90 83 94 85 battery 36 Negative pole (ii) 8 1830 0.89 83 95 81 battery 37 Negative pole (iii) 8 1810 0.88 85 95 83

These results indicate that the addition of LiBOB at a concentration of not higher than 5% by weight to the non-aqueous electrolyte can improve the recovery rate after storage in the discharged state and the capacity retention rate after charge/discharge cycles. This may be because LiBOB traps hydrofluoric acid and inhibits its reaction, thereby inhibiting the dissolution of Si and improving battery characteristics. When the added amount of LiBOB exceeds 5% by weight, the capacity retention ratio after charge/discharge cycles tends to decrease. This may be because excess LiBOB increases the viscosity of the non-aqueous electrolyte, thereby hindering the movement of lithium ions. These results indicate that the upper limit of the amount of LiBOB is preferably 5% by weight.

The negative electrode of the nonaqueous electrolyte secondary battery according to the present invention can provide a nonaqueous electrolyte secondary battery having both high capacity and good charge/discharge cycle characteristics. The present invention can be applied to various nonaqueous electrolyte secondary batteries. For example, the present invention is applicable not only to a cylindrical battery as in the embodiment, but also to coin-shaped, prismatic, and flat-shaped batteries, and batteries having an electrode body structure such as rolled or layered. The nonaqueous electrolyte secondary battery according to the present invention can be used as a main power source for mobile communication devices, portable electronic equipment, and the like.

While this invention has been described in terms of presently preferred embodiments, it is to be understood that such disclosure is not to be construed as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the invention pertains after reading the above disclosure. Accordingly, the appended claims should be considered to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims (6)

1. rechargeable nonaqueous electrolytic battery, it comprises: the positive pole of absorption and desorption Li electrochemically, negative pole and nonaqueous electrolyte,

Wherein said negative pole comprises the negative active core-shell material that contains Si at least,

Described nonaqueous electrolyte comprises the lithium hexafluoro phosphate as main supporting electrolyte, and have the acid content that is not less than 50ppm and is not higher than 200ppm and

Described negative pole has the electromotive force that is not less than 0.6V and is not higher than 1.5V with respect to the Li electrode under the final discharging voltage of described battery.

2. according to the rechargeable nonaqueous electrolytic battery of claim 1, wherein said negative active core-shell material is Si, Si alloy or Si compound.

3. according to the rechargeable nonaqueous electrolytic battery of claim 2, wherein said Si compound is the oxide of representing with SiOx, wherein 0＜x＜2.

4. according to the rechargeable nonaqueous electrolytic battery of claim 2, wherein said negative pole comprises anode collector and the negative active core-shell material film that is deposited on the described anode collector.

5. according to the rechargeable nonaqueous electrolytic battery of claim 2, the total content of Fe, Ni, Co, Cu and Cr is 1000ppm or lower in the wherein said Si alloy.

6. according to the rechargeable nonaqueous electrolytic battery of claim 1, wherein said nonaqueous electrolyte contains two [1,2-oxalic acid (2-)-O, the O '] borate ions that are not higher than 5 weight %.

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