阅读说明：本技术 非水电解质二次电池 (Nonaqueous electrolyte secondary battery ) 是由 奥野幸穗 福冈隆弘 石黑祐 曽我正宽 于 2020-03-02 设计创作，主要内容包括：非水电解质二次电池的负极合剂具备：包含含Si材料及碳材料的负极活性物质、和碳纳米管。含Si材料包含锂硅酸盐相和/或碳相内分散有Si颗粒的第1复合材料、和SiO-(2)相内分散有Si颗粒的第2复合材料中的至少第1复合材料。第1复合材料相对于第1复合材料与第2复合材料的总计的质量比X、和第1复合材料与第2复合材料的总计相对于第1复合材料、第2复合材料与碳材料的总计的质量比Y满足关系式(1)：100Y-32.2X～(5)+65.479X～(4)-55.832X～(3)+18.116X～(2)-6.9275X-3.5356＜0、X≤1、且0.06≤Y。非水电解质包含LiPF-(6)和LiN(SO-(2)F)-(2)。(A negative electrode mixture for a nonaqueous electrolyte secondary battery is provided with: a negative electrode active material containing a Si-containing material and a carbon material, and carbon nanotubes. The Si-containing material comprises a No. 1 composite material in which Si particles are dispersed in a lithium silicate phase and/or a carbon phase, and SiO 2 At least 1 st composite material of 2 nd composite material having Si particles dispersed therein. The mass ratio X of the 1 st composite material to the total of the 1 st composite material and the 2 nd composite material, and the mass ratio Y of the total of the 1 st composite material and the 2 nd composite material to the total of the 1 st composite material, the 2 nd composite material and the carbon material satisfy the relational expression (1): 100Y-32.2X 5 +65.479X 4 ‑55.832X 3 +18.116X 2 -6.9275X-3.5356 < 0, X < 1 and 0.06 < Y. The non-aqueous electrolyte contains LiPF 6 And LiN (SO) 2 F) 2 。)

1. A nonaqueous electrolyte secondary battery includes: a positive electrode, a negative electrode and a non-aqueous electrolyte,

the negative electrode includes a negative electrode mixture containing: a negative electrode active material comprising a silicon-containing material and a carbon material, and a carbon nanotube,

the silicon-containing material comprises at least the 1 st composite material of a 1 st composite material and a 2 nd composite material,

the 1 st composite material is provided with a lithium ion conductive phase and silicon particles dispersed in the lithium ion conductive phase, wherein the lithium ion conductive phase contains a silicate phase and/or a carbon phase, the silicate phase contains at least one selected from the group consisting of alkali metal elements and group 2 elements,

The 2 nd composite material is provided with: SiO 2₂Phase, and the SiO₂The silicon particles are dispersed in the phase and,

a mass ratio X of the 1 st composite material to a total of the 1 st composite material and the 2 nd composite material, and a mass ratio Y of a total of the 1 st composite material and the 2 nd composite material to a total of the 1 st composite material, the 2 nd composite material, and the carbon material satisfy relational expression (1):

100Y-32.2X⁵+65.479X⁴-55.832X³+18.116X²-6.9275X-3.5356＜0、

x is less than or equal to 1 and Y is less than or equal to 0.06,

the nonaqueous electrolyte includes lithium hexafluorophosphate and lithium bis (fluorosulfonyl) imide: LFSI.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the mass ratio X and the mass ratio Y satisfy a relational expression (2):

100Y-2.1551×exp(1.3289X)＜0、

x is less than or equal to 1 and Y is less than or equal to 0.06.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the carbon material contains graphite.

4. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein a content of the carbon nanotubes in the negative electrode mixture is 0.1 mass% or more and 0.5 mass% or less with respect to the entire negative electrode mixture.

5. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein a concentration of the LFSI in the nonaqueous electrolyte is 0.2mol/L or more.

6. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein a concentration of the LFSI in the nonaqueous electrolyte is 0.2mol/L or more and 0.4mol/L or less.

Technical Field

The present invention relates to a nonaqueous electrolyte secondary battery using a silicon-containing material for a negative electrode active material.

Background

A nonaqueous electrolyte secondary battery represented by a lithium ion secondary battery includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes: and a negative electrode mixture containing a negative electrode active material capable of electrochemically occluding and releasing lithium ions. A method of using a high-capacity silicon-containing material in the negative electrode active material is being studied.

Patent document 1 proposes to provide Li_2uSiO_2+u(0 < u < 2) and a silicon-containing material in which silicon particles are dispersed in the lithium silicate phase.

Further, studies have been made on a conductive agent, and patent document 2 proposes a method of using a Carbon Nanotube (CNT) having a coating layer containing metallic lithium formed on the surface thereof as a conductive agent for a negative electrode.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2016/035290 pamphlet

Patent document 2: japanese patent laid-open publication No. 2015-138633

Disclosure of Invention

Problems to be solved by the invention

A method of including a negative electrode mixture containing a silicon-containing material containing silicon particles and CNTs can be considered. The isolation of the silicon particles is likely to occur because cracks are generated in the silicon particles along with expansion and contraction of the silicon particles during charge and discharge, or gaps are formed around the silicon particles along with contraction of the silicon particles. At the beginning of the cycle, even if the silicon particles are isolated, the conductive path is ensured by the CNTs and the capacity is maintained.

However, the active surface of the silicon particles is easily exposed as the particles are isolated, and the active surface may come into contact with the nonaqueous electrolyte to cause a side reaction. When CNTs are included, side reactions are likely to occur, and the composite material is likely to undergo deterioration due to etching accompanying the side reactions in the middle and later of the cycle, resulting in a decrease in capacity.

Means for solving the problems

In view of the above, one aspect of the present invention relates to a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the negative electrode includes a negative electrode mixture containing: a negative electrode active material containing a silicon-containing material and a carbon material, and carbon nanotubes, wherein the silicon-containing material contains at least the 1 st composite material out of a 1 st composite material and a 2 nd composite material, the 1 st composite material contains a lithium ion conductive phase and silicon particles dispersed in the lithium ion conductive phase, the lithium ion conductive phase contains a silicate phase and/or a carbon phase, the silicate phase contains at least 1 selected from the group consisting of alkali metal elements and group 2 elements, and the 2 nd composite material contains SiO₂Phase, and the aforementioned SiO₂Silicon particles dispersed in the phase, wherein a mass ratio X of the 1 st composite material to the total of the 1 st composite material and the 2 nd composite material, and a mass ratio Y of the total of the 1 st composite material and the 2 nd composite material to the total of the 1 st composite material, the 2 nd composite material and the carbon material satisfy the following relational expression (1):

100Y-32.2X⁵+65.479X⁴-55.832X³+18.116X²-6.9275X-3.5356 < 0, X < 1 and 0.06 < Y,

The nonaqueous electrolyte contains lithium hexafluorophosphate and lithium bis (fluorosulfonyl) imide: LFSI.

ADVANTAGEOUS EFFECTS OF INVENTION

The present invention can improve the cycle characteristics of a nonaqueous electrolyte secondary battery having a negative electrode containing a silicon-containing material.

While the novel features of the present invention are set forth in the appended claims, the invention, in accordance with other objects and features thereof, will be understood more fully from the following detailed description taken in conjunction with the accompanying drawings.

Drawings

Fig. 1 is a partially cut schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

Detailed Description

A nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes: a negative electrode mixture containing a negative electrode active material capable of electrochemically occluding and releasing lithium ions and carbon nanotubes (hereinafter referred to as CNTs). The negative electrode active material includes a silicon-containing material and a carbon material.

The silicon-containing material includes at least the 1 st composite material of the 1 st composite material and the 2 nd composite material. High capacity can be obtained with the 1 st composite. The 1 st composite material comprises: a lithium ion conducting phase comprising a silicate phase and/or a carbon phase, and silicon particles dispersed within the lithium ion conducting phase. The silicate phase contains at least 1 selected from the group consisting of alkali metal elements and group 2 elements.

The 2 nd composite material is provided with: SiO 2₂Phase, and SiO₂Silicon particles dispersed in the phase. The silicon particles of the 1 st composite material have a larger average particle diameter than the silicon particles of the 2 nd composite material, and are easily isolated by expansion and contraction during charge and discharge.

The mass ratio X of the 1 st composite material to the total of the 1 st composite material and the 2 nd composite material, and the mass ratio Y of the total of the 1 st composite material and the 2 nd composite material to the total of the 1 st composite material, the 2 nd composite material and the carbon material satisfy the following relational expression (1).

100Y-32.2X⁵+65.479X⁴-55.832X³+18.116X²-6.9275X-3.5356 < 0, X.ltoreq.1, and 0.06. ltoreq.Y (1)

The non-aqueous electrolyte comprises lithium hexafluorophosphate (LiPF)₆) And lithium bis (fluorosulfonyl) imide (LiN (SO)₂F)₂) (hereinafter referred to as LFSI. ). By using LiPF₆The nonaqueous electrolyte with wide potential window and high conductivity can be obtained. In addition, a passive film is easily formed on the surface of a battery component such as a positive electrode current collector, and corrosion of the positive electrode current collector and the like is suppressed.

When the negative electrode mixture containing the 1 st composite material contains CNTs, the conductive path of the isolated silicon particles is secured, and on the contrary, the etching degradation of the 1 st composite material accompanied by the side reaction of the silicon particles (active surface) with the nonaqueous electrolyte becomes easy to progress. Through a non-aqueous electrolyte Comprising a LiPF₆Hydrogen fluoride generated by the reaction with a small amount of water contained in the battery participates in the above-mentioned side reaction, and the CNT promotes LiPF₆Reaction with water.

In contrast, in the present invention, the nonaqueous electrolyte contains LiPF as a lithium salt₆And LFSI. LFSI is less likely to generate hydrogen fluoride even when it is in contact with water, and can form a good-quality SEI (Solid Electrolyte Interface) on the particle surface of the No. 1 composite material. By using LFSI, LiPF can be reduced₆The concentration of (c). Even if LiPF in the non-aqueous electrolyte₆A part of the electrolyte is replaced by LFSI, and a nonaqueous electrolyte with wide potential window and high conductivity can be maintained. By using LFSI, when a negative electrode mixture containing the 1 st composite material and CNT is used, the 1 st composite material is inhibited from being deteriorated by etching due to the above-mentioned side reaction, and a high capacity can be maintained in the middle and after cycles.

The silicon-containing material may also comprise a 2 nd composite material. However, from the viewpoint of increasing the capacity and improving the cycle characteristics, the mass ratio X needs to satisfy the relational expression (1). The 2 nd composite material has a small capacity as compared with the 1 st composite material, but is advantageous in that the expansion at the time of charging is small.

By using a silicon-containing material and a carbon material in combination in the negative electrode active material, stable cycle characteristics can be obtained. However, from the viewpoint of improving cycle characteristics, the mass ratio Y needs to satisfy the relational expression (1). When Y is 0.06 or more, the effect of increasing the capacity of the silicon-containing material can be sufficiently obtained. Y is preferably 0.06 or more and 0.14 or less. In this case, it is easy to achieve both high capacity and improvement in cycle characteristics.

From the viewpoint of further improving the cycle characteristics in the middle and later periods, the mass ratio X and the mass ratio Y preferably satisfy the following relational expression (2).

100Y-2.1551 Xexp (1.3289X) < 0, X ≦ 1, and 0.06 ≦ Y (2)

(CNT)

When CNT is used as the conductive agent, the effect of securing a conductive path of the isolated silicon particles can be remarkably obtained. Since CNTs are fibrous, it is easier to ensure contact between an isolated silicon particle and a negative electrode active material around the silicon particle, and to form a conductive path between the isolated silicon particle and the negative electrode active material around the silicon particle, as compared to spherical conductive particles such as acetylene black.

From the viewpoint of ensuring a conductive path of the isolated silicon particles, the average length of the CNTs is preferably 1 μm or more and 100 μm or less, and more preferably 5 μm or more and 20 μm or less. Similarly, the average diameter of the CNTs is preferably 1.5nm or more and 50nm or less, and more preferably 1.5nm or more and 20nm or less.

The average length and average diameter of CNTs were determined by image analysis using a Scanning Electron Microscope (SEM). Specifically, a plurality of CNTs (for example, about 100 to 1000 CNTs) are arbitrarily selected, the length and diameter are measured, and the average value is obtained. The length of the CNT is a length when the CNT is linear.

From the viewpoint of ensuring a conductive path of the isolated silicon particles and suppressing the etching degradation of the 1 st composite material, the content of the CNT in the negative electrode mixture may be 0.1 mass% or more and 0.5 mass% or less, or may be 0.1 mass% or more and 0.4 mass% or less with respect to the entire negative electrode mixture. When the content of CNT in the negative electrode mixture is 0.1 mass% or more with respect to the entire negative electrode mixture, the cycle characteristics are easily improved. When the content of CNT in the negative electrode mixture is 0.5 mass% or less with respect to the entire negative electrode mixture, the 1 st composite material is easily inhibited from being deteriorated by etching. Examples of the analysis method of CNTs include raman spectroscopy, thermogravimetric analysis, and the like.

(non-aqueous electrolyte)

In the nonaqueous electrolyte, as the lithium salt dissolved in the nonaqueous solvent, LiPF is included₆And LFSI. From the viewpoint of improving the cycle characteristics in and after the middle period, the concentration of LFSI in the nonaqueous electrolyte is preferably 0.2mol/L or more, more preferably 0.2mol/L or more and 1.1mol/L or less, and still more preferably 0.2mol/L or more and 0.4mol/L or less. From fully obtaining the LiPF-based₆From the viewpoint of the effect of (3), LiPF in the nonaqueous electrolyte₆The concentration of (B) is preferably 0.3mol/L or more. LiPF in the non-aqueous electrolyte from the viewpoint of suppressing the etching degradation of the 1 st composite material ₆The concentration of (B) is preferably 1.3mol/L or less. Based on the full use of LFSI and LiPF₆From the viewpoint of the effects of (a), LFSI and LiPF in the nonaqueous electrolyte₆The total concentration of (A) is preferably 1mol/L to 2 mol/L.

Well balanced LFSI-based effects and LiPF-based effects₆In view of the effect of (A), LFSI and LiPF are examples of lithium salts₆The ratio of LFSI in the total of (a) is preferably 5 mol% or more and 90 mol% or less, and more preferably 10 mol% or more and 30 mol% or less. Removing LFSI and LiPF from lithium salt₆In addition, other lithium salts, LFSI and LiPF in lithium salt can be contained₆The proportion of the total amount of (B) is preferably 80 mol% or more, more preferably 90 mol% or more. By mixing LFSI and LiPF in lithium salt₆The ratio of the total amount of (a) to (b) is controlled to be in the above range, and a battery having excellent cycle characteristics can be easily obtained. As lithium salts (LFSI and LiPF) in nonaqueous electrolytes₆) The analytical method (2) may be, for example, Nuclear Magnetic Resonance (NMR), Ion Chromatography (IC), Gas Chromatography (GC), or the like.

(negative electrode active Material)

The negative electrode active material contains a silicon-containing material capable of electrochemically occluding and releasing lithium ions. The silicon-containing material is advantageous for high capacity of the battery. The silicon-containing material comprises at least a 1 st composite material.

(1 st composite Material)

The 1 st composite material comprises: a lithium ion conducting phase comprising a silicate phase and/or a carbon phase, and silicon particles dispersed within the lithium ion conducting phase. The silicate phase contains at least 1 selected from the group consisting of alkali metal elements and group 2 elements. Namely, the 1 st composite material contains: at least one of a composite material including a silicate phase and silicon particles dispersed in the silicate phase (hereinafter also referred to as an LSX material), and a composite material including a carbon phase and silicon particles dispersed in the carbon phase (hereinafter also referred to as an Si — C material). By controlling the amount of silicon particles dispersed in the lithium ion conductive phase, high capacity can be achieved. Stress generated by expansion and contraction of the silicon particles during charge and discharge is relaxed by the lithium ion conductive phase. Therefore, the 1 st composite material is advantageous for the high capacity and the improvement of cycle characteristics of the battery. However, the silicate phase is more excellent as a lithium ion conductive phase than the carbon phase in terms of a small number of sites that can react with lithium and high initial charge/discharge efficiency.

From the viewpoint of high capacity, the average particle diameter of the silicon particles before the initial charging is usually 50nm or more, preferably 100nm or more. LSX materials can be made, for example, by the following method: the mixture of the silicate and the raw material silicon is pulverized and micronized using a pulverizing device such as a ball mill, and then heat-treated in an inert atmosphere. Alternatively, the LSX material may be produced by synthesizing fine particles of silicate and fine particles of raw material silicon without using a pulverizing device, and heat-treating the mixture in an inert atmosphere. In the above, by adjusting the mixing ratio of the silicate and the raw material silicon and the particle size of the raw material silicon, the amount and size of the silicon particles dispersed in the silicate phase can be controlled, and the high capacity can be easily achieved.

From the viewpoint of suppressing cracking of the silicon particles themselves, the average particle diameter of the silicon particles before the initial charging is preferably 500nm or less, and more preferably 200nm or less. After the primary charging, the average particle diameter of the silicon particles is preferably 400nm or less. By making the silicon particles finer, the volume change during charge and discharge is reduced, and the structural stability of the 1 st composite material is further improved.

The average particle diameter of the silicon particles was measured using an image of a cross section of the 1 st composite material obtained by a Scanning Electron Microscope (SEM). Specifically, the average particle diameter of the silicon particles is determined by taking the average of the maximum diameters of arbitrary 100 silicon particles.

The silicon particles dispersed in the lithium ion conductive phase have a phase of silicon (Si) alone in a granular form and are composed of single or multiple crystallites. The crystallite size of the silicon particles is preferably 30nm or less. When the crystallite size of the silicon particles is 30nm or less, the volume change amount due to expansion and contraction of the silicon particles accompanying charge and discharge can be reduced, and the cycle characteristics can be further improved. For example, when the silicon particles shrink, voids are formed around the silicon particles, and contact points between the particles and the surroundings are reduced, whereby the particles are prevented from being isolated, and a decrease in charge and discharge efficiency due to the isolation of the particles is prevented. The lower limit of the crystallite size of the silicon particles is not particularly limited, and is, for example, 5nm or more.

The crystallite size of the silicon particles is more preferably 10nm or more and 30nm or less, and still more preferably 15nm or more and 25nm or less. When the crystallite size of the silicon particles is 10nm or more, the surface area of the silicon particles can be kept small, and therefore deterioration of the silicon particles with generation of irreversible capacity is unlikely to occur.

The crystallite size of the silicon particles was calculated from the half-value width of the diffraction peak of the Si (111) plane attributed to the X-ray diffraction (XRD) pattern of the silicon particles by the scherrer equation.

From the viewpoint of high capacity, the content of the silicon particles in the 1 st composite material is preferably 30% by mass or more, more preferably 35% by mass or more, and still more preferably 55% by mass or more. In this case, the lithium ion diffusibility is good, and excellent load characteristics are easily obtained. On the other hand, the content of the silicon particles in the 1 st composite material is preferably 95% by mass or less, more preferably 75% by mass or less, and still more preferably 70% by mass or less, from the viewpoint of improving cycle characteristics. In this case, the surface of the silicon particles exposed without being covered with the lithium ion conductive phase is reduced, and the reaction between the electrolytic solution and the silicon particles is easily suppressed.

The content of silicon particles can be determined by Si-NMR. Preferable measurement conditions for Si-NMR are shown below.

A measuring device: solid nuclear magnetic resonance spectrum measuring device manufactured by Varian, Inc. (INOVA-400)

And (3) probe: varian7mm CPMAS-2

MAS：4.2kHz

MAS speed: 4kHz

Pulse: DD (45 degree pulse + signal capture time 1H decoupling)

Repetition time: 1200 seconds

Observation width: 100kHz

Observation center: near-100 ppm

Signal capture time: 0.05 second

Cumulative number of times: 560

Sample amount: 207.6mg

The silicate phase contains at least one of an alkali metal element (a group 1 element other than hydrogen in the long periodic table) and a group 2 element in the long periodic table. The alkali metal element includes lithium (Li), potassium (K), sodium (Na), and the like. The group 2 element includes magnesium (Mg), calcium (Ca), barium (Ba), and the like. Among them, a silicate phase containing lithium (hereinafter, also referred to as a lithium silicate phase) is preferable from the viewpoint of small irreversible capacity and high initial charge-discharge efficiency. That is, the LSX material is preferably a composite material comprising a lithium silicate phase and silicon particles dispersed within the lithium silicate phase.

The silicate phase is, for example, a lithium silicate phase (oxide phase) containing lithium (Li), silicon (Si), and oxygen (O). Atomic ratio of O to Si in lithium silicate phase: O/Si is, for example, greater than 2 and less than 4. When O/Si is more than 2 and less than 4 (z in the formula is 0 < z < 2), it is advantageous from the viewpoint of stability and lithium ion conductivity. Preferably, O/Si is more than 2 and less than 3 (z in the formula mentioned later is 0 < z < 1). Atomic ratio of Li to Si in lithium silicate phase: Li/Si is, for example, more than 0 and less than 4. The lithium silicate phase may contain a trace amount of other elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al) in addition to Li, Si, and O.

The lithium silicate phase may have the formula: li_2zSiO_2+z(0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, and the like, z preferably satisfies the relationship 0 < z < 1, and more preferably, z is 1/2.

Lithium silicate phase of LSX and SiO_xSiO of (2)₂In contrast, there are few sites that can react with lithium. Thus, LSX and SiO_xIn contrast, irreversible capacity accompanying charge and discharge is less likely to occur. When the silicon particles are dispersed in the lithium silicate phase, excellent charge and discharge efficiency can be obtained at the initial stage of charge and discharge. In addition, since the content of silicon particles can be arbitrarily changed, a high-capacity negative electrode can be designed.

The composition of the silicate phase of the 1 st composite material can be analyzed, for example, by the following method.

The battery is decomposed, the negative electrode is taken out, washed with a non-aqueous solvent such as ethylene carbonate, dried, and then cross-section processing of the negative electrode mixture layer is performed by a cross-section polisher (CP), to obtain a sample. A reflected electron image of the cross section of the sample was obtained using a field emission scanning electron microscope (FE-SEM), and the cross section of the 1 st composite material was observed. Qualitative and quantitative analysis of the element (acceleration voltage 10kV, beam current 10nA) was performed on the silicate phase of the observed 1 st composite material using an Auger Electron Spectroscopy (AES) analyzer. For example, the composition of the lithium silicate phase is determined based on the contents of lithium (Li), silicon (Si), oxygen (O), and other elements obtained.

The 1 st composite material and the 2 nd composite material can be distinguished from each other in the cross section of the sample. In general, the average particle size of the silicon particles in the 1 st composite material is larger than that of the silicon particles in the 2 nd composite material, and by observing the particle sizes, it is possible to easily distinguish between the two.

In the cross-sectional observation and analysis of the sample, a carbon sample stage may be used for fixing the sample in order to prevent Li diffusion. In order to prevent the cross section of the sample from being altered, a transfer container that holds and conveys the sample without exposing the sample to the atmosphere may be used.

The carbon phase may be composed of amorphous carbon having low crystallinity (i.e., amorphous carbon), for example. The amorphous carbon may be, for example, hard carbon or soft carbon, or may be other than these. Amorphous carbon can be obtained by, for example, sintering a carbon source in an inert atmosphere and pulverizing the obtained sintered body. The Si — C material can be obtained by, for example, mixing a carbon source and silicon as a raw material, stirring the mixture while crushing the mixture with a stirrer such as a ball mill, and then firing the mixture in an inert atmosphere. Examples of the carbon source include sugars such as carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose, and water-soluble resins. When the carbon source and the raw material silicon are mixed, for example, the carbon source and the raw material silicon may be dispersed in a dispersion medium such as alcohol. In the above, by adjusting the mixing ratio of the carbon source and the raw material silicon and the particle size of the raw material silicon, the amount and size of silicon particles dispersed in the carbon phase can be controlled, and high capacity can be easily achieved.

The 1 st composite material is preferably formed into a particulate material (hereinafter also referred to as 1 st particle) having an average particle diameter of 1 to 25 μm, more preferably 4 to 15 μm. In the case of the particle size range, stress due to volume change of the 1 st composite material accompanying charge and discharge is easily relaxed, and good cycle characteristics are easily obtained. The surface area of the 1 st particle is also moderate, and the capacity decrease due to the side reaction with the electrolyte is also suppressed.

The average particle diameter of the 1 st particle means a particle diameter (volume average particle diameter) having a volume cumulative value of 50% in a particle size distribution measured by a laser diffraction scattering method. For example, "LA-750" manufactured by HORIBA, Ltd can be used as the measuring apparatus.

The 1 st particle may also be provided with a conductive material covering at least a part of the surface thereof. The silicate phase lacks electron conductivity, and thus the conductivity of the 1 st particle also tends to decrease. By covering the surface of the 1 st particle with a conductive material, the conductivity can be dramatically improved. The conductive layer is preferably thin to such an extent that the average particle diameter of the 1 st particles is not substantially affected.

(2 nd composite Material)

The silicon-containing material may further comprise: has SiO₂Phase and SiO₂A 2 nd composite of silicon particles dispersed in a phase. SiO for No. 2 composite Material _xX is, for example, about 0.5 to 1.5. The 2 nd composite material is prepared by heat treating SiO and disproportionating SiO₂Phase and SiO₂And a fine Si phase (silicon particles) dispersed in the phase. In the 2 nd composite material, the silicon particles are smaller than in the 1 st composite material, and the average particle diameter of the silicon particles in the 2 nd composite material is, for example, about 5 nm. In the 2 nd composite material, the silicon particles are small, and therefore the improvement width of the cycle characteristics based on the use of LFSI is small as compared with the case of the 1 st composite material. From the viewpoint of increasing the capacity and improving the cycle characteristics, the mass ratio of the 2 nd composite material to the total of the 1 st composite material and the 2 nd composite material satisfies (1-X).

(carbon Material)

The negative electrode active material may further include a carbon material that electrochemically stores and releases lithium ions. The carbon material has a smaller degree of expansion and contraction during charge and discharge than the silicon-containing material. By using the silicon-containing material and the carbon material in combination, the contact state between the negative electrode active material particles and between the negative electrode mixture layer and the negative electrode current collector can be maintained more favorably during repeated charge and discharge. That is, a high capacity of the silicon-containing material can be given to the anode and the cycle characteristics can be improved. From the viewpoint of increasing the capacity and improving the cycle characteristics, the mass ratio of the carbon material to the total of the 1 st composite material, the 2 nd composite material and the carbon material satisfies (1-Y). When the 1 st composite material contains a carbon phase as a lithium ion conductive phase, the carbon phase as the lithium ion conductive phase is not included in the mass of the carbon material.

Examples of the carbon material used for the negative electrode active material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among them, graphite having excellent charge/discharge stability and a small irreversible capacity is preferable. Graphite means a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. The carbon material may be used alone or in combination of two or more.

The nonaqueous electrolyte secondary battery will be described in detail below.

[ negative electrode ]

The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on a surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium to the surface of a negative electrode current collector and drying the negative electrode slurry. The dried coating film may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

The negative electrode mixture contains a negative electrode active material and CNT as essential components. The negative electrode mixture may contain a binder, a conductive agent other than CNTs, a thickener, and the like as optional components.

As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, a net, or a punched sheet) can be used. Examples of the material of the negative electrode current collector include stainless steel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balance between the strength of the negative electrode and weight reduction.

Examples of the binder include resin materials such as fluorine resins such as polytetrafluoroethylene (ptfe) and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resins; polyimide resins such as polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymers; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyether sulfone; rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One kind of the binder may be used alone, or two or more kinds may be used in combination.

Examples of the conductive agent other than the CNT include carbon-based materials such as acetylene black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as benzene derivatives. The conductive agent may be used alone or in combination of two or more.

Examples of the thickener include cellulose derivatives (such as cellulose ether) such as carboxymethyl cellulose (CMC) and modified products thereof (including salts such as Na salt), methyl cellulose, and the like; saponified products of polymers having vinyl acetate units such as polyvinyl alcohol; polyethers (e.g., polyalkylene oxides such as polyethylene oxide) and the like. The thickener may be used alone or in combination of two or more.

The dispersion medium is not particularly limited, and examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[ Positive electrode ]

The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium to the surface of a positive electrode current collector and drying the positive electrode slurry. The dried coating film may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components. NMP or the like can be used as a dispersion medium of the positive electrode slurry.

As the positive electrode active material, for example, a lithium-containing composite oxide can be used. Examples thereof include Li_aCoO₂、Li_aNiO₂、Li_aMnO₂、Li_aCo_bNi_1-bO₂、Li_aCo_bM_1-bO_c、Li_aNi_1-bM_bO_c、Li_aMn₂O₄、Li_aMn_2-bM_bO₄、LiMPO₄、Li₂MPO₄F (M is at least 1 selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B). Here, a is 0 to 1.2, b is 0 to 0.9, and c is 2.0 to 2.3. The value a indicating the molar ratio of lithium increases and decreases by charging and discharging.

Among them, Li is preferable_aNi_bM_1-bO₂(M is at least 1 selected from the group consisting of Mn, Co and Al, 0 & lta & lt 1.2, 0.3 & ltb & lt 1.) and a lithium nickel composite oxide. From the viewpoint of high capacity, it is more preferable that 0.85. ltoreq. b.ltoreq.1 is satisfied. From the viewpoint of stability of the crystal structure, Li containing Co and Al as M is more preferable_aNi_bCo_cAl_dO₂(0＜a≤1.2、0.85≤b＜1、0＜c＜0.15、0＜d≤0.1、b+c+d＝1)。

As the binder and the conductive agent, the same ones as exemplified in the negative electrode can be used. As the binder, an acrylic resin may also be used. As the conductive agent, graphite such as natural graphite and artificial graphite can be used.

The shape and thickness of the positive electrode current collector may be selected according to the shape and range of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum alloy, and titanium.

[ non-aqueous electrolyte ]

The nonaqueous electrolyte contains a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent. The lithium salt includes at least LiPF₆And LFSI. The concentration of the lithium salt in the nonaqueous electrolyte is preferably 0.5mol/L to 2mol/L, for example. By setting the lithium salt concentration in the above range, a nonaqueous electrolyte having excellent ion conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

The nonaqueous electrolyte may also contain other than LiPF₆And lithium salts other than LFSI. As a means for removing LiPF₆And lithium salts other than LFSI, for example, LiClO₄、LiBF₄、LiAlCl₄、LiSbF₆、LiSCN、LiCF₃SO₃、LiCF₃CO₂、LiAsF₆、LiB₁₀Cl₁₀Lower aliphatic carboxylic acid lithium, LiCl, LiBr, LiI, borate salts, imide salts, and the like. Examples of the borate include lithium bis (1, 2-benzenediolate (2-) -O, O ') borate, lithium bis (2, 3-naphthalenediolate (2-) -O, O ') borate, lithium bis (2,2 ' -biphenyldiolate (2-) -O, O ') borate, lithium bis (5-fluoro-2-diolate-1-benzenesulfonic acid-O, O ') borate, and the like. As the imide salt, lithium bistrifluoromethanesulfonate (LiN (CF) may be mentioned₃SO₂)₂) Lithium nonafluorobutanesulfonate trifluoromethanesulfonate (LiN (CF)₃SO₂)(C₄F₉SO₂) Lithium bis (pentafluoroethanesulfonate) (LiN (C))₂F₅SO₂)₂) And the like.

Examples of the nonaqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylates, and chain carboxylates. Examples of the cyclic carbonate include Propylene Carbonate (PC) and Ethylene Carbonate (EC). Examples of the chain carbonate include diethyl carbonate (DEC), Ethyl Methyl Carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylic acid ester include γ -butyrolactone (GBL) and γ -valerolactone (GVL). Examples of the chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The nonaqueous solvent may be used alone or in combination of two or more.

[ separator ]

In general, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous film, woven fabric, nonwoven fabric, or the like can be used. As the material of the separator, polyolefin such as polypropylene or polyethylene is preferable.

An example of the structure of the nonaqueous electrolyte secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and a nonaqueous electrolyte are housed in a case. Alternatively, a wound electrode assembly may be replaced with another electrode assembly such as a laminated electrode assembly in which positive and negative electrodes are laminated with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be of any type such as cylindrical, rectangular, coin, button, and laminate.

Hereinafter, the structure of a rectangular nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery of the present invention will be described with reference to fig. 1. Fig. 1 is a partially cut schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

The battery includes a bottomed rectangular battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed in the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, the positive electrode, and the separator around a flat plate-shaped winding core, and then pulling out the winding core.

One end of the negative electrode lead 3 is attached to a negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via an insulating plate (not shown) made of resin. The negative electrode terminal 6 is insulated from the sealing plate 5 by a gasket 7 made of resin. One end of the positive electrode lead 2 is attached to a positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4 also serving as a positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and separates the negative electrode lead 3 from the battery case 4. The peripheral edge of the sealing plate 5 is fitted to the opening end of the battery case 4, and the fitted portion is laser welded. In this way, the opening of the battery case 4 is sealed by the sealing plate 5. The injection hole of the nonaqueous electrolyte provided in the sealing plate 5 is closed by a plug 8.

Examples

Hereinafter, examples of the present invention will be specifically described, but the present invention is not limited to the following examples.

EXAMPLE 1

[ preparation of the composite Material (LSX Material) ]

In atomic ratio: silica and lithium carbonate were mixed so that Si/Li became 1.05, and the mixture was fired in air at 950 ℃ for 10 hours to obtain Li ₂Si₂O₅(z-1/2). The obtained lithium silicate was pulverized so that the average particle diameter became 10 μm.

And (3) mixing the raw materials in a ratio of 45: 55 mass ratio of lithium silicate (Li) having an average particle diameter of 10 μm₂Si₂O₅) And raw material silicon (3N, average particle diameter 10 μm). The mixture was charged into a jar (SUS, volume: 500mL) of a planetary ball mill (manufactured by Fritsch Co., Ltd., P-5), 24 SUS balls (diameter 20mm) were placed in the jar, a lid was closed, and the mixture was pulverized at 200rpm for 50 hours in an inert atmosphere.

Next, the powdery mixture was taken out in an inert atmosphere, and fired at 800 ℃ for 4 hours in an inert atmosphere under pressure applied by a hot press to obtain a sintered body (LSX material) of the mixture.

Then, the LSX material was pulverized, passed through a 40 μm mesh screen, and the obtained LSX particles were mixed with coal pitch (MCP 250, manufactured by JFE chemical corporation), and the mixture was fired at 800 ℃ in an inert atmosphere, thereby forming a conductive layer containing conductive carbon on the surface of the LSX particles. The coverage of the conductive layer was 5 mass% with respect to the total mass of the LSX particles and the conductive layer. Thereafter, using a sieve, LSX particles having an average particle diameter of 5 μm of the conductive layer were obtained.

The average particle diameter of the silicon particles determined by the above-described method was 100 nm. The crystallite size of the silicon particles calculated from the diffraction peaks assigned to the Si (111) plane and using the scherrer equation was 15nm by XRD analysis of the LSX particles.

AES analysis of the lithium silicate phase resulted in the composition of the lithium silicate phase being Li₂Si₂O₅. The content of silicon particles in the LSX particles as determined by Si-NMR was 55 mass% (Li)₂Si₂O₅Content of (b) 45 mass%).

[ production of negative electrode ]

After water was added to the negative electrode mixture, the mixture was stirred by a mixer (manufactured by PRIMIX Corporation, t.k.hivis MIX) to prepare a negative electrode slurry. The negative electrode mixture used was a mixture of a negative electrode active material, CNT (average diameter 9nm, average length 12 μm), lithium salt of polyacrylic acid (PAA-Li), sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR). In the negative electrode mixture, the mass ratio of the negative electrode active material, CNT, CMC-Na, and SBR is set to 100: 0.3: 0.9: 1.

the negative electrode active material uses a mixture of a silicon-containing material and graphite. The silicon-containing material is at least the 1 st composite material out of the 1 st composite material and the 2 nd composite material. The composite of item 1 used LSX particles obtained as described above. SiO particles having an average particle size of 5 μm (x is 1, and the average particle size of silicon particles is about 5 nm) were used for the 2 nd composite material.

In the negative electrode mixture, the mass ratio X of the 1 st composite material to the total of the 1 st composite material and the 2 nd composite material was set to the value shown in table 1. In the negative electrode mixture, the mass ratio Y of the total of the 1 st composite material and the 2 nd composite material to the total of the 1 st composite material and the 2 nd composite material and graphite was set to the value shown in table 1.

Then, the surface of the copper foil is coated with a coating solution of 1m²The negative electrode slurry was applied so that the mass of the negative electrode mixture (2) was 140g, the coating film was dried and then rolled to form a copper foil having a density of 1.6g/cm on both sides³The negative electrode mixture layer to obtain a negative electrode.

[ production of Positive electrode ]

And (3) mixing the following raw materials in a ratio of 95: 2.5: 2.5 Mass ratio of Mixed lithium Nickel composite oxide (LiNi)_0.8Co_0.18Al_0.02O₂) N-methyl-2-pyrrolidone (NMP) was added to acetylene black and polyvinylidene fluoride, and then stirred by a mixer (manufactured by PRIMIX Corporation, t.k.hivis MIX) to prepare a positive electrode slurry. Next, a positive electrode slurry was applied to the surface of the aluminum foil, the coating film was dried and then rolled to form a density of 3.6g/cm on both sides of the aluminum foil³And (3) obtaining the positive electrode.

[ preparation of non-aqueous electrolyte ]

The lithium salt is dissolved in a nonaqueous solvent to prepare a nonaqueous electrolyte. The nonaqueous solvent used was a mixed solvent of Ethylene Carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 3: 7). LiPF is used as lithium salt₆And LFSI. LiPF in non-aqueous electrolyte₆The concentration of (2) was set to 0.95 mol/L. The LFSI concentration in the nonaqueous electrolyte was set to 0.4 mol/L.

[ production of nonaqueous electrolyte Secondary Battery ]

Each electrode was provided with a tab, and the positive electrode and the negative electrode were wound in a spiral shape with a separator interposed therebetween so that the tabs were located at the outermost peripheral portions, thereby producing an electrode group. The electrode group was inserted into an outer case made of an aluminum laminate film, vacuum-dried at 105 ℃ for 2 hours, and then a nonaqueous electrolyte was injected to seal the opening of the outer case, thereby producing batteries a1 to a 90.

Batteries C1 to C90 were produced in the same manner as batteries a1 to a90, respectively, except that the nonaqueous electrolyte did not contain LFSI.

[ evaluation 1]

The following charge-discharge cycle test was performed on battery a 1.

Constant current charging was performed at a current of 0.3It to a voltage of 4.2V, and thereafter, constant voltage charging was performed at a voltage of 4.2V to a current of 0.015 It. After that, constant current discharge was performed at a current of 0.3It to a voltage of 2.75V. The pause time between charging and discharging was set to 10 minutes. The charging and discharging are carried out in an environment of 25 ℃.

Note that (1/X) It represents a current, (1/X) It (a) rated capacity (Ah)/X (h), and X represents a time for charging or discharging electricity in the rated capacity portion. For example, 0.5It means X is 2, and the current value is rated capacity (Ah)/2 (h).

The charge and discharge were repeated under the above conditions. The capacity retention rate R was determined as the ratio (percentage) of the 300 th cycle discharge capacity to the 1 st cycle discharge capacity_A1And (4) obtaining.

The capacity retention rate R was determined for battery C1, which had the same configuration as battery a1 except that the nonaqueous electrolyte contained no LFSI, by the same method as described above_C1. Using the determined R_A1And R_C1The rate of change in the capacity retention rate of battery a1 with respect to battery C1 (hereinafter referred to simply as the rate of change in the capacity retention rate of battery a 1) was determined by the following equation. In this manner, changes in the capacity retention rate due to the addition of LFSI were examined.

The change rate (%) of the capacity retention rate of battery a1 was (R)_A1-R_C1)/R_C1×100

Similarly, the change rates of the capacity retention rates of the batteries a2 to a90 were obtained using the batteries a2 to a90 and the batteries C2 to C90, respectively.

The evaluation results are shown in table 1. The numerical values (percentages) in the cells of table 1 show the rate of change in the capacity retention rate, and the battery numbers are shown in parentheses. For example, the rate of change in the capacity retention rate of battery a1 is shown in the cell of battery a 1.

[ Table 1]

When the LFSI concentration in the nonaqueous electrolyte is 0.4mol/L, the batteries a1 to a9, a11 to a16, a21 to a24, a31 to a33, a41 to a42, and a51 satisfying the relational expression (1) have a rate of change in capacity retention of 0.5% or more, and the cycle characteristics are greatly improved. Among them, batteries a1 to A3, a11 to a12, and a21 that satisfy relational expression (2) had a rate of change in capacity retention of 1% or more, and the cycle characteristics were further improved.

EXAMPLE 2

LFSI concentration in the nonaqueous electrolyte was set to 0.2mol/L and LiPF in the nonaqueous electrolyte₆Is set to 1.15mol/L, except thatOtherwise, batteries B1 to B90 were produced in the same manner as batteries a1 to a90, respectively.

[ evaluation 2]

The capacity retention rate R of battery B1 was determined in the same manner as in evaluation 1 above_B1. The capacity maintenance rate R of battery B1 thus obtained was used _B1And capacity retention rate R of battery C1, which is the same in construction as battery B1 except that the nonaqueous electrolyte does not contain LFSI_C1The rate of change in the capacity retention rate of battery B1 was obtained by the following equation.

The change rate (%) of the capacity retention rate of battery B1 was (R)_B1-R_C1)/R_C1×100

Similarly, the change rates of the capacity retention rates of batteries B2 to B90 were obtained using batteries B2 to B90 and batteries C2 to C90, respectively.

The evaluation results are shown in table 2. The numerical values (percentages) in the squares of table 2 show the rate of change in the capacity retention rate, and the battery numbers are shown in parentheses. For example, the grid of battery B1 shows the rate of change in the capacity retention rate of battery B1.

[ Table 2]

When the LFSI concentration in the nonaqueous electrolyte is 0.2mol/L, the rate of change in the capacity retention rate is 0.25% or more in batteries B1 to B9, B11 to B16, B21 to B24, B31 to B33, B41 to B42, and B51 satisfying relational expression (1), and the cycle characteristics are greatly improved. Among them, batteries B1 to B3, B11 to B12, and B21 satisfying relational expression (2) have a change rate of the capacity retention rate of 0.5% or more, and the cycle characteristics are further improved.

Industrial applicability

The nonaqueous electrolyte secondary battery of the present invention is useful for a main power supply of a mobile communication device, a portable electronic device, or the like.

The present invention has been described in its preferred form, and it is not intended that the disclosure be interpreted in a limiting sense. Various modifications and alterations will become apparent to those skilled in the art upon reading the foregoing disclosure. Accordingly, it is intended that the appended claims be interpreted as including all such alterations and modifications as fall within the true spirit and scope of the invention.

Description of the reference numerals

1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug