JP2014199753A - Secondary battery and negative electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery which has a high capacity and less cycle reduction even with a high current value, and has a small irreversible capacity during initial charge and discharge, and to provide a negative electrode active material.SOLUTION: A negative electrode active material having a lithium silicate phase of continuous phase in a silicon oxide is produced by a step for mixing a raw material containing a lithium material and a silicon oxide material with lithium forming a spinodal phase separation at a compositional ratio of an oxide of silicon, a step for heating at the mixing temperature in the compositional ratio or more, a step for quenching to a temperature lower than the temperature for generating spinodal decomposition in the compositional ratio, and a step for heat treating in a temperature range for forming spinodal phase separation at the compositional ratio.

Description

  The present invention relates to a secondary battery and a negative electrode active material.

  With the rapid market expansion of notebook computers, mobile phones, electric vehicles, stationary storage batteries, etc., secondary batteries with high energy density are required from the viewpoint of miniaturization and weight reduction of devices. As a means for obtaining a secondary battery having a high energy density, there is a method using a negative electrode material having a large capacity per weight.

  As a negative electrode active material for increasing the capacity, metal lithium, aluminum alloyed with lithium, silicon, tin, and the like have been studied (for example, Non-Patent Document 1). Among these, since silicon has a large theoretical capacity, a lithium ion secondary battery using this material as an active material has been proposed (for example, Patent Document 1). However, silicon undergoes a large volume change when reacting with lithium. As a result, current collection performance is reduced by repeated charging and discharging, so that sufficient cycle characteristics are not obtained.

As one of the materials for solving this problem, SiO _x (2>x> 0) has been proposed (for example, Patent Document 2). Although this material has a high capacity and stable cycle characteristics, there is a problem that a so-called irreversible capacity is large, in which all of the Li inserted in the initial charge does not desorb. In contrast, Li ₄ SiO ₄ , Li ₂ SiO ₃ , Li ₂ Si ₂ O ₅ , Li ₄ Si ₃ O ₈ , Li ₆ Si ₄ O _{11 and the} like, Li _y SiO _x (y>) containing Li in advance. 0, 5>x> 0) have been proposed (for example, Patent Document 2). However, these compounds have extremely low electronic conductivity, and it is difficult to desorb and insert lithium at the movement speed of lithium ions required for batteries.

As a material containing Li in advance, a lithium-containing silicon oxide powder represented by the general formula SiLi _x O _{y in} which lithium is fused and a part thereof is crystallized has been proposed (Patent Document 3).

Further, there has been proposed a composite particle having silicon oxide dispersed in a carbonaceous material and a lithium silicate phase mainly contained in Li ₄ SiO ₄ contained in the silicon oxide (Patent Document 4).

JP 2002-83594 A JP-A-6-325765 JP 2003-160328 A JP 2007-59213 A

Solid State Ionics, 113-115, 57, (1998)

  As described above, the compounds proposed in Non-Patent Document 1 and Patent Document 1 are accompanied by a large volume change when reacting with lithium, and current collection performance is reduced by repeated charge and discharge, so that sufficient cycle characteristics are obtained. Absent.

  The compound proposed in Patent Document 2 has extremely low electronic conductivity, and it is difficult to desorb and insert lithium at the lithium ion migration rate required for batteries.

  The compound proposed in Patent Document 3 shows good charge / discharge efficiency and cycle characteristics at a current value as low as 1/1000 or less of the discharge capacity, as described in Examples. However, when the current value is increased to 1/1000 or more of the discharge capacity, the resistance of the active material is high, so that the initial charge / discharge efficiency and cycle characteristics are lowered.

The proposed compounds in Patent Document 4, an electrode area although current value to the discharge capacity for undescribed is unknown, indicates an improvement in current characteristics of 10 mA / cm ² for current density 1 mA / cm ^2. However, the discharge capacity of the negative electrode active material is 704 mAh / g or less, which is only about 1/6 of the theoretical capacity 4200 mAh / g originally possessed by silicon, and further improvement in capacity is desired.

As a result of intensive studies by the inventor, the active materials proposed in Patent Document 3 and Patent Document 4 having comparatively good initial charge / discharge efficiency and cycle characteristics have the following characteristics. It has been found that it is difficult to improve the discharge efficiency, cycle characteristics, and discharge capacity of the active material. Lithium ions pass through a portion where lithium described in Patent Document 3 is fused or a lithium silicate described in Patent Document 4 as an ion conduction path during charging and discharging. However, these are not continuous phases. For this reason, in the active material of Patent Document 3, lithium is consumed by reacting with surrounding SiO ₂ or the like when the current value is increased, so that the initial charge / discharge efficiency is lowered. In addition, although silicon is generated by consumption of lithium, the cycle characteristics are also lowered because silicon particles are easily grown. Moreover, in the active material of patent document 4, since the silicon | silicone which causes charging / discharging reaction with lithium does not exist efficiently around a lithium silicate phase, it is difficult to improve the discharge capacity. Furthermore, since lithium silicate is not a continuous phase, for the same reason as described above, charge / discharge efficiency and cycle characteristics are reduced when the current value is increased.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a secondary battery and a negative electrode active material that have a high capacity even at a high current value, have no cycle reduction, and have a low irreversible capacity during initial charge and discharge. To do.

  One embodiment of the present invention relates to a negative electrode active material having a continuous lithium silicate phase in silicon oxide.

Furthermore, one embodiment of the present invention provides
Mixing a raw material containing a lithium raw material and a silicon oxide raw material in a composition ratio of lithium and silicon oxide forming a spinodal phase separation;
Heating above the mixing temperature in the composition ratio;
Quenching to a temperature lower than the temperature at which spinodal decomposition occurs at the composition ratio;
The present invention relates to a method for producing a negative electrode active material having a continuous lithium silicate phase in silicon oxide, comprising a step of performing a heat treatment in a temperature range in which spinodal phase separation is formed at the composition ratio.

  According to the present embodiment, it is possible to provide a secondary battery and a negative electrode active material excellent in initial charge / discharge efficiency and cycle characteristics.

It is a figure which shows an example of the cross-section of the secondary battery which concerns on embodiment of this invention. “Glass Engineering Handbook”, Asakura Shoten, 1999, p. 192 is a diagram representing the phase separation zone of _{R 2} O-SiO 2 based glass described. “Journal of the American Ceramic Society”, volume 74, Issue 8, p. 1839, FIG. 8 is a phase diagram of the Li ₂ O—SiO ₂ system described in FIG.

[Features of the invention]
The secondary battery according to the present embodiment is a secondary battery including an electrode body including a positive electrode and a negative electrode, and an exterior body that accommodates the electrode body, and the negative electrode active material in the negative electrode is silicon oxide. It has a continuous lithium silicate phase in the product.

  Another embodiment of the negative electrode active material is characterized by having silicon in the vicinity of the lithium silicate continuous phase in the silicon oxide.

  In addition, the method for producing a negative electrode active material according to the present embodiment includes a step of quenching from a mixing temperature and a temperature range in which a spinodal phase separation is formed in a composition ratio of lithium and silicon oxide that forms a spinodal phase separation. It has the process of processing, It is characterized by the above-mentioned.

  In another embodiment of the method for producing a negative electrode active material, in the composition ratio of lithium and silicon oxide forming a spinodal phase separation, a step of quenching from the mixing temperature and a surface of the active material are coated with a carbon material. And a heat treatment in a temperature region for forming a spinodal phase separation.

  In another embodiment of the method for producing a negative electrode active material, in the composition ratio of lithium and silicon oxide forming a spinodal phase separation, a step of quenching from the mixing temperature and a surface of the active material are coated with a carbon material. The method includes a step, a step of performing a heat treatment in a temperature region in which spinodal phase separation is formed, and a heating step of generating silicon from silicon oxide.

[Action]
As the lithium silicate phase becomes a continuous phase, the diffusibility of alkali metal ions such as lithium ions in the active material is improved, so the charge / discharge efficiency and cycle characteristics of the secondary battery are good even when the current value is increased. It becomes. Further, the presence of silicon in the vicinity of the lithium silicate phase increases the capacity per weight of the negative electrode active material. In addition, the presence of the carbon layer on the surface of the negative electrode active material layer improves the electrical conductivity in addition to the ionic conductivity, further improving the charge / discharge efficiency and cycle characteristics of the secondary battery when the current value is increased. To do. In addition, since ionic conductivity and electrical conductivity are improved as compared with conventional silicon oxide negative electrode active materials, an increase in resistance can be suppressed even if the coating film of the negative electrode is thickened. The charge / discharge efficiency and cycle characteristics of the secondary battery when the current value increases are improved.

  Each embodiment of the present invention will be described below. Each figure is a schematic diagram for facilitating the explanation and understanding of the invention, and its shape, dimensions, ratio, etc. are different from the actual device, but these are appropriately determined in consideration of the following explanation and known techniques. The design can be changed.

  A cross-sectional view of the secondary battery according to the present embodiment is shown in FIG. As shown in FIG. 1, the secondary battery according to the present embodiment includes a positive electrode current collector 3 made of a metal such as aluminum foil, and a positive electrode active material layer 1 containing a positive electrode active material provided thereon. It has the negative electrode which consists of the negative electrode current collector 4 which consists of metals, such as a positive electrode and copper foil, and the negative electrode active material layer 2 containing the negative electrode active material provided on it. The positive electrode and the negative electrode are laminated via a separator 5 made of a nonwoven fabric or a polypropylene microporous film so that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. The positive electrode and the negative electrode may be wound in a spiral shape with a separator interposed therebetween. This electrode pair is accommodated in a container formed of exterior bodies 6 and 7 such as an aluminum laminate film. A positive electrode tab 9 is connected to the positive electrode current collector 3, a negative electrode tab 8 is connected to the negative electrode current collector 4, and these tabs are drawn out of the container. An electrolytic solution is injected into the container and sealed. It can also be set as the structure where the electrode group by which the several electrode pair was laminated | stacked was accommodated in the container.

  The positive electrode and the negative electrode according to the embodiment of the present invention include a material capable of inserting and extracting lithium ions. The positive electrode and the negative electrode can also include materials that can occlude and release alkali metal ions other than lithium.

  Hereinafter, the negative electrode, the positive electrode, the electrolyte, the separator, the exterior member, the positive electrode terminal, and the negative electrode terminal will be described.

(Negative electrode)
The negative electrode according to this embodiment includes a negative electrode current collector and a negative electrode active material layer that is supported on one or both surfaces of the negative electrode current collector and includes a negative electrode active material and a binder.

  The negative electrode active material has a continuous phase lithium silicate phase in silicon oxide. When the lithium silicate phase becomes a continuous phase, the diffusibility of alkali metal ions such as lithium ions in the active material is improved. In addition, the alkali metal silicate synthesis reaction by the alkali metal and silicon oxide generated at the first charge can be suppressed, and the first charge / discharge efficiency can be improved.

Examples of lithium silicate include Li ₄ SiO ₄ , Li ₂ SiO ₃ , Li ₂ Si ₂ O ₅ , Li ₈ SiO ₆ , Li ₂ Si ₃ O _{7 and the} like.

As lithium silicate, Li ₄ SiO ₄ is preferably the main component. Li ₄ SiO ₄ is particularly chemically stable and has alkali metal ions, particularly lithium ion conductivity, with respect to Li ₂ SiO _{3 and the} like, thereby improving the diffusion of alkali metal ions in the active material and providing high current characteristics. It is because it can improve.

In the case of using the manufacturing method described later, the main component of the silicate phase becomes _Li 4 SiO _4.

  Further, the lithium silicate phase may contain other compounds in the composition forming the spinodal phase separation. Other compounds include lithium oxide, potassium oxide, sodium oxide, rubidium oxide, cesium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, lead oxide, boron oxide, phosphorus oxide, titanium oxide, aluminum oxide, zirconium oxide, Examples thereof include silicon, phosphorus, lithium hydroxide, and a reaction product of these with silicon oxide. When these compounds are contained, the compound is preferably 1 mol% or more and 30 mol% or less with respect to the total amount of the compound and lithium silicate.

  The continuous lithium silicate phase can be observed with an electron microscope.

The silicon oxide is preferably SiO _x (0 <x ≦ 2.0). From the viewpoint of improving the capacity per weight of the negative electrode active material, SiO _x (0.8 ≦ x ≦ 1.6) is more preferable, and SiO _x (1.0 ≦ x ≦ 1.1) is particularly preferable. is there.

  It is preferable that all or part of the silicon oxide has an amorphous structure. By including the silicon oxide having an amorphous structure, the volume change rate of the active material during charging and discharging can be suppressed, and the cycle characteristics of the secondary battery can be improved. The amorphous structure can be confirmed by X-ray diffraction measurement (general XRD measurement).

The silicon oxide is preferably SiO ₂ whose main component is amorphous. When the main component is amorphous SiO ₂ , volume expansion of the negative electrode during charge / discharge can be suppressed.

  In the present embodiment, the shape of the silicon oxide having a continuous phase of lithium silicate is not particularly limited, but the silicon oxide phase and the lithium silicate phase form a three-dimensional network structure intertwined with each other. It is preferable.

  The ratio of the lithium silicate phase to the silicon oxide phase is not particularly limited, but the lithium silicate phase is preferably 2 mol% or more and 45 mol% or less, more preferably 5 mol% or more and 35 mol% or less with respect to the silicon oxide phase. preferable.

  Further, the silicon oxide contained within 300 nm from the boundary surface between the silicon oxide phase and the lithium silicate phase is preferably 50% or more, preferably 80% or more of the total weight of the silicon oxide.

  Further, it is preferable to have silicon in the vicinity of the lithium silicate continuous phase in the silicon oxide. When silicon exists in the vicinity of the silicate phase, it efficiently reacts with the alkali metal ions diffused in the lithium silicate phase, and the charge / discharge efficiency is improved. Furthermore, the capacity per weight of the negative electrode active material is improved.

  In the present embodiment, the presence of silicon in the vicinity of the lithium silicate continuous phase means that silicon existing within 100 nm from the interface between the silicon oxide phase and the continuous phase of lithium silicate is 5% or more of the total weight of silicon, More preferably, it represents 10% or more.

  In the present embodiment, the silicon content is not particularly limited, but is preferably 20% to 200% (molar ratio) with respect to silicon oxide, more preferably 60% to 150% (molar ratio). More preferably, it is 90% to 110% (molar ratio).

  Silicon expands and contracts when lithium is occluded and released, and is preferably dispersed as finely as possible in order to relieve this stress. Specifically, it is preferably dispersed as a cluster having a size of usually 300 nm or less, preferably 100 nm or less, more preferably 10 nm or less. The silicon of the present invention can be confirmed by transmission electron microscope observation (general TEM observation).

  The negative electrode active material surface is preferably coated with a carbon material. By covering with the carbon material, the electrical conductivity is improved in addition to the ionic conductivity, so that the charge / discharge efficiency and the cycle characteristics of the secondary battery are further improved when the current value is increased.

  As the carbon material coated on the surface of the negative electrode active material, graphite, graphene, amorphous graphite, diamond-like carbon, carbon nanotube, carbon nanohorn, or a composite thereof can be used. Here, graphite with high crystallinity has high electrical conductivity, and is excellent in adhesiveness and voltage flatness with a negative electrode current collector made of a metal such as copper. On the other hand, since amorphous carbon having low crystallinity has a relatively small volume expansion, it has a high effect of relaxing the volume expansion of the entire negative electrode, and deterioration due to non-uniformity such as crystal grain boundaries and defects hardly occurs.

  The coated carbon amount of the negative electrode active material may be 0% by mass with respect to the entire composite active material particles after the coating treatment, but is preferably 1% by mass to 50% by mass, and preferably 2% by mass to 30% by mass. More preferably, it is as follows.

  The negative electrode active material is not particularly limited, but a particulate material can be used. The average particle size is preferably from 0.1 to 30 μm, more preferably from 0.2 to 20 μm. By setting the average particle size of the negative electrode active material to 0.1 μm or more, the specific surface area is increased, and it is possible to suppress a decrease in battery capacity when used as a negative electrode agent for a secondary battery. Moreover, it is prevented that a bulk density becomes small too much and it can also prevent that the charging / discharging capacity per unit volume falls. Furthermore, the negative electrode can be easily formed. Further, by setting the average particle size to 30 μm or less, it is possible to prevent the battery characteristics from being remarkably deteriorated due to foreign matters when applied to the electrode. In addition, electrode formation is facilitated, and the possibility of peeling from the current collector (such as copper foil) can be minimized. In addition, the average particle diameter in this invention is a particle diameter (median diameter) when a cumulative weight will be 50% in the particle size distribution measurement by a laser beam diffraction method.

BET specific surface area of the negative electrode active material is preferably ^{0.5~30m 2 / g, 1~20m 2 /} g is more preferable. If the BET specific surface area is 0.5 m ² / g or more, the surface activity can be increased, the binding force of the binder during electrode preparation is reduced, battery characteristics are deteriorated, and charge and discharge are repeated. The cycle characteristics of the hour can be improved. Moreover, if it is 30 m < ² > / g or less, it can prevent that the absorption amount of the solvent at the time of electrode preparation and the consumption amount of a binder increase. In addition, the BET specific surface area in this invention is a value when it measures by nitrogen gas adsorption amount.

  In the present invention, in addition to silicon oxide having a continuous lithium silicate phase, a material capable of occluding and releasing alkali metal ions may be included as a negative electrode active material.

  As materials that can occlude and release alkali metal ions, known materials can be used, for example, carbon materials such as graphite, graphene, amorphous graphite, diamond-like carbon, carbon nanotubes, carbon nanohorns, Al, Si, etc. , Pb, Sn, Zn, Cd, Sb, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, La, and other metals, lithium titanate, silicon oxide, tin oxide, copper oxide, Examples thereof include oxides such as molybdenum oxide, tungsten oxide, iron oxide, and lead oxide, metal sulfides, and metal nitrides.

  The above negative electrode active materials can be used alone or in admixture of two or more with silicon oxide having a continuous phase lithium silicate phase.

  Next, although the manufacturing method of the silicon oxide which has a continuous lithium silicate phase in this embodiment in this invention is demonstrated, a manufacturing method is not limited to the following method.

  In this embodiment, a method for producing a silicon oxide having a lithium silicate phase as a continuous phase is heated at a mixing temperature or higher in an inert gas atmosphere at a composition ratio of lithium and silicon oxide forming a spinodal phase separation. And a step of performing a heat treatment in a temperature region for forming a spinodal phase separation.

The composition ratio of lithium and silicon oxide is preferably a known ratio of Li ₂ O and SiO ₂ in which spinodal decomposition occurs. The composition ratio is, for example, a diagram showing a phase separation region described in “Glass Engineering Handbook” shown in FIG. 2, Asakura Shoten, 1999, page 192, or “Journal of the American Ceramic Society” shown in FIG. volume 74, Issue 8, p. 1839, FIG. Within the ratio range described in FIG.

The composition ratio for forming the phase separation also changes depending on the quenching conditions and the like and the inclusion of an alkali metal other than Li. For example, when the oxide of lithium and silicon is a Li ₂ O—SiO ₂ two-component system The composition ratio for forming the phase separation of Li ₂ O is approximately 2 mol% to 31 mol%.

  As a raw material of silicon oxide having a continuous phase lithium silicate phase, for example, selected from lithium oxide, lithium carbonate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium silicate and their hydrates, metal lithium, etc. One or more kinds of lithium raw materials and one or more kinds of silicon oxide raw materials selected from silicon oxide, lithium silicate, and the like can be used.

The silicon oxide raw material is a mixture of silicon and SiO ₂ , or those previously fired in a non-oxidizing atmosphere, SiO ₂ fired with a reducing gas such as hydrogen, or SiO ₂ with a predetermined amount of carbon. Or mixed with metal etc. and reduced to a predetermined amount, silicon is baked with oxygen gas or oxide and oxidized to a predetermined amount, silicon compound gas such as silane and oxygen gas are mixed and heated reaction or plasma decomposition What was oxidized to predetermined amount by reaction etc. can be used.

  In addition to the above lithium and silicon oxide materials, sodium oxide, potassium oxide, rubidium oxide, cesium oxide, diphosphorus pentoxide, titanium dioxide, aluminum oxide, zirconium oxide, lead oxide, magnesium oxide, boron oxide, etc. Can be used.

  The rate of temperature rise to the mixing temperature of lithium and silicon oxide is not particularly limited, but is preferably 4 ° C. or more and 2000 ° C. or less, more preferably 10 ° C. or more per minute from the viewpoint of productivity. It is 2000 degrees C or less.

  The mixing temperature of the lithium and silicon oxide is characterized in that the composition ratio of the lithium and silicon oxide is equal to or higher than the temperature at which it becomes a liquid phase. For example, the temperature in the range of LIQUID in FIG. 3 can be used as the temperature at which the liquid phase is obtained. In addition, when mixing temperature changes by including alkali metals other than lithium, the temperature which mixes in the composition is used.

  The holding time at the mixing temperature is not limited as long as lithium and silicon oxide are mixed, but from the viewpoint of productivity, the holding time is preferably 0.1 hours or more and 100 hours or less, more preferably 0.00. 5 hours or more and 70 hours or less.

The cooling rate after mixing is characterized by rapid cooling. Rapid cooling indicates that the cooling rate from the mixed state to the temperature at which spinodal decomposition occurs is 10 ° C./second or more. The faster the cooling rate, the better, preferably 100 ° C./second or more, and more preferably 10 ⁶ ° C./second or more. By increasing the cooling rate, the shape of the spinodal phase separation can be easily controlled, and the charge / discharge efficiency and cycle characteristics are improved. In the composition ratio of lithium and silicon oxide, it is more preferable to maintain the cooling rate to a temperature that is 200 ° C. or more lower than the upper limit temperature for forming the spinodal phase separation. By maintaining the cooling rate to the above temperature, non-uniform nucleation of lithium silicate, silicon oxide, and silicon can be suppressed, and the cycle characteristics of the secondary battery can be improved. Although there is no restriction | limiting in particular in cooling minimum temperature, Room temperature or more is preferable from a viewpoint of productivity.

  As the cooling method, a known method satisfying the above cooling rate condition can be used. For example, a single roller method described in the press method, air cooling method, liquid nitrogen rapid cooling method, “New Glass Handbook”, Maruzen, 1992, page 172 , Double roller method, etc. may be used.

  The reheating step after cooling is characterized in that heat treatment is performed in a temperature region that forms a spinodal phase separation. The temperature for forming the spinodal phase separation is, for example, not more than the range of the dotted line in FIG. 3 in the composition of the oxides of lithium and silicon. In the present embodiment, the heating temperature is preferably 800 ° C. or higher as long as it does not exceed the upper limit of the temperature range for forming the spinodal phase separation. By setting the temperature to 800 ° C. or higher, silicon is easily generated from silicon oxide, and the capacity per weight of the negative electrode active material is improved. In addition, when the temperature which forms a spinodal phase separation changes by including alkali metals other than lithium, the temperature which forms a spinodal phase separation in the composition is made into heating temperature. The heating time is preferably 10 minutes or more, more preferably 15 minutes or more. The heating time is preferably 7 days or less, and more preferably 3 days or less. After the heating, it is preferable to cool to the lower limit temperature of the temperature region forming the spinodal phase separation, and more preferably to room temperature from the viewpoint of productivity.

  The gas atmosphere in the heating and cooling step is not particularly limited, but a known inert gas atmosphere can be used, and examples thereof include nitrogen, argon, helium, krypton, xenon, and mixtures thereof. be able to. However, when silicon is mixed in the raw material or when lithium metal is included, a gas with a limited amount of oxygen is preferable. By limiting the amount of oxygen, a decrease in the amount of silicon after the end of the spinodal phase separation treatment process can be suppressed, and the capacity per weight of the negative electrode active material can be improved. A preferable oxygen amount is 5% or less by volume, particularly preferably an oxygen amount is 1% or less by volume. The remainder is an inert gas such as nitrogen, argon, helium, krypton, xenon or a mixture thereof.

  The step of coating the silicon material having a continuous phase lithium silicate phase in this embodiment with a carbon material can be appropriately selected from the mechanical milling method, the melt carbonization method, the vapor deposition method, and the like.

  The mechanical milling method is a method in which a predetermined amount of a conductive carbon-based material that does not melt during mechanical milling, such as natural graphite, artificial graphite, coke, and carbon fiber, is mixed with silicon oxide having a continuous lithium silicate phase, and then removed. This is a method of mixing in an apparatus in which heat is sufficiently secured. An example of the mixing device is a ball mill.

  The melt carbonization method has a pitch-based material such as petroleum pitch and mesophase pitch, a carbonizable polymer such as polyethylene terephthalate and polyvinyl alcohol, a carbonizable low molecular weight organic material such as citric acid and a continuous lithium silicate phase. In this method, after a predetermined amount of silicon oxide is mixed, heat treatment is performed in an inert gas atmosphere. The heating temperature in the melting method is preferably 400 ° C. or higher, and preferably 700 ° C. or higher for improving conductivity. Especially preferably, it is 800 degreeC or more. The upper limit temperature during heating is an upper limit temperature at which spinodal decomposition of silicon oxide having a lithium silicate phase occurs. Further, when the melt carbonization method is performed at 800 ° C. or higher, the above-described reheating step for forming the spinodal phase separation may be omitted.

  The vapor deposition method is a method for performing thermal CVD by introducing an organic gas or vapor at 600 to 1000 ° C., preferably 800 to 1000 ° C. under normal pressure or reduced pressure. As an organic material used as a raw material for generating an organic gas, a material that can be pyrolyzed at the above heat treatment temperature to generate carbon is selected, particularly in a non-oxidizing atmosphere. For example, methane, ethane, ethylene, acetylene, propane , One or a mixture of two or more selected from hydrocarbons such as butane, butene, pentane, isobutane, hexane, benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, Examples thereof include one or a mixture selected from monocyclic to tricyclic aromatic hydrocarbons such as indene, coumarone, pyridine, anthracene, and phenanthrene. Further, gas light oil, creosote oil, anthracene oil, and naphtha cracked tar oil obtained in the tar distillation step can be used alone or as a mixture of two or more. The upper limit temperature during heating is preferably the upper limit temperature at which spinodal decomposition of silicon oxide having a lithium silicate phase occurs. Further, when the vapor deposition method is performed at 800 ° C. or higher, the reheating step for forming the above-mentioned spinodal phase separation may be omitted.

  In order to make the negative electrode active material of the present invention have a predetermined particle size, it is preferable to use a method of pulverizing or classifying after spinodal phase separation, or pulverizing and classifying. As the pulverization method, a dry pulverization method or a wet pulverization method using a solvent as a medium can be used. Examples of the solvent used in the wet pulverization method include water, toluene, xylene, methanol, ethanol, n-propanol, isopropyl alcohol, isobutyl alcohol, acetone, methyl ethyl ketone, butyl acetate, N, N from the viewpoint of handling and safety. -Dimethylformamide is preferred. The amount of the solvent to be used is preferably 1/10 to 20 times, and particularly preferably 1/5 to 10 times that of the powder material. As the pulverization method, preferably, either or both of a dry pulverization method and a wet pulverization method using water as a medium are used. Examples of pulverizers include mortars, ball mills, circular vibration ball mills, rotational vibration mills, satellite ball mills, planetary ball mills, swirling airflow jet mills, pot mills, centrifugal mills, tower mills, sand mills, attritors, sentry mills, dyno mills, and roller mills. , A pin mill, a tube mill, a rod mill, a jaw crusher, and the like are used, and a pulverizing method using a swirling air flow type jet mill, a ball mill, and a vibrating ball mill is preferable. Furthermore, classification is preferably performed in order to match a predetermined particle size, and an air classifier (for example, a cyclone) or a sieve is preferably used. When classifying with a sieve, a dry method or a wet method using a solvent such as water is preferred. Moreover, although the temperature of grinding | pulverization and classification is based also on the kind of material and solvent to be used, 5-150 degreeC is preferable and 10-90 degreeC is more preferable.

  In another embodiment of the present invention, a silicon oxide having a continuous lithium silicate phase may be produced by chemical vapor deposition or the like. The step of generating silicon, the step of coating the carbon material, and the step of pulverizing and classifying can be combined.

  The negative electrode active material of the present invention may be doped with phosphorus or boron as long as it has a continuous phase composed of a lithium silicate phase. By doping with phosphorus or boron, the electrical conductivity of the negative electrode active material is improved, and in the secondary battery, the charge / discharge efficiency and cycle characteristics of the secondary battery are further improved when the current value is increased.

  You may add a electrically conductive agent to the negative electrode of this invention as needed. By adding a conductive agent, current collecting performance can be improved. A well-known thing can be used as a electrically conductive agent, For example, acetylene black, carbon black, graphite etc. are mentioned.

  The binder for the negative electrode is not particularly limited, but polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer. Examples thereof include polymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, and polyamideimide.

  The content of the negative electrode binder is preferably in the range of 1 to 30% by mass and more preferably 2 to 25% by mass with respect to the total amount of the negative electrode active material and the negative electrode binder. By setting the content to 1% by mass or more, the adhesion between the active materials or between the active material and the current collector is improved, and the cycle characteristics are improved. Moreover, by setting it as 30 mass% or less, an active material ratio can improve and a negative electrode capacity | capacitance can be improved. Moreover, when using the low molecular weight substance which is a precursor of a polyimide or a polyamideimide, it is preferable to perform a hardening process after electrode coating. Although there is no restriction | limiting in the hardening method, The thermosetting method etc. which are heated to the temperature which produces | generates a polymer are mentioned.

  Although it does not restrict | limit especially as a negative electrode collector, Aluminum, nickel, copper, silver, and the alloy containing these 2 or more types are preferable from electrochemical stability. Examples of the shape include foil, flat plate, and mesh.

  The negative electrode can be produced by forming a negative electrode active material layer containing a negative electrode active material and a negative electrode binder on a negative electrode current collector. The negative electrode active material layer can be formed by a general slurry coating method. Specifically, a negative electrode is obtained by preparing a slurry containing a negative electrode active material, a binder and a solvent, applying the slurry onto a negative electrode current collector, drying, compressing and molding as necessary. be able to. Examples of the method for applying the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method. After the negative electrode active material layer is formed in advance, a negative electrode can be obtained by forming a thin film of aluminum, nickel, or an alloy thereof as a current collector by a method such as vapor deposition or sputtering.

(Positive electrode)
The positive electrode according to the present embodiment includes a positive electrode current collector and a positive electrode active material layer that is supported on one or both surfaces of the positive electrode current collector and includes a positive electrode active material, a conductive agent, and a binder.

It does not specifically limit as a positive electrode active material, A normal positive electrode for secondary batteries can be used. For _{example, LiCoO 2, LiNiO 2, LiNi} 0.80 Co 0.15 Al 0.15 O 2, LiMn 2 O 4, LiNi 1/3 Co 1/3 Mn 1/3 O 2, LiNi 0.5 Mn 1. Examples thereof include lithium-containing composite oxides such as ₅ O ₄ , LiFePO ₄ , NASICON type, and lithium transition metal silicon composite oxide. The transition metal portion of these lithium-containing composite oxides may be substituted with other elements, or a mixture of these lithium-containing composite oxides.

As the positive electrode active material, lithium manganate having a layered structure such as LiMnO ₂ , Li _x Mn ₂ O ₄ (0 <x <2) or lithium manganate having a spinel structure; LiCoO ₂ , LiNiO ₂ or a transition metal thereof Lithium transition metal oxides in which a specific transition metal such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ does not exceed half the lithium transition metal oxides; In which Li is made excessive in comparison with the stoichiometric composition. From the viewpoint of increasing the energy density of the secondary battery, Li _α Ni _β Co _γ Al _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0.2), Li _α Ni _β Co _γ Mn _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, γ ≦ 0.2), or a region where the charging curve is 4.5 V or more with respect to lithium metal A material comprising is preferred. As measurement conditions of the charge / discharge curve, the charge / discharge current can be set to 5 mA / g per mass of the positive electrode active material, the charge end voltage can be set to 5.2V, and the discharge end voltage can be set to 3V.

  Examples of the material including a region where the charging curve is 4.5 V or higher with respect to lithium metal include spinel materials, layered materials, and olivine materials.

As a spinel-type material whose charging curve includes a region of 4.5 V or more with respect to lithium metal, the following formula (1):
Li _a (M _x Mn _2−xy Y _y ) (O _4−w Z _w ) (1)
[In the formula (1), x is 0 ≦ x ≦ 1.2, preferably 0.4 <x <1.1, and y is 0 ≦ y, preferably 0 ≦ y <0.5. X + y <2, 0 ≦ a ≦ 1.2, and 0 ≦ w ≦ 1. M includes at least one selected from Co, Ni, Fe, Cr, or Cu, and Y includes at least one selected from Li, B, Na, Al, Mg, Ti, Si, K, or Ca, Z contains at least one of F and Cl. For example, LiNi _0.5 Mn _1.5 O ₄ , LiCoMnO ₄ , LiCrMnO ₄ , LiFeMnO ₄ , LiCu _0.5 Mn ₁ may be used as a material that operates at a high potential of 4.5 V or higher with respect to lithium. _0.5 O ₄ or the like is more preferable. Alternatively, a solid solution of these materials, or a positive electrode active material in which a small amount of Al, Mg, B, Si, Ti, or other metal element is added to these materials may be used.

The olivine-based material has the following formula (2):
LiMPO ₄ (2)
[In Formula (2), M is a transition metal, and preferably includes at least one selected from Fe, Mn, Co, or Ni, and more preferably one of Co or Ni. . For example, LiFePO ₄ , LiMnPO ₄ , LiCoPO ₄ , LiNiPO _{4 and the} like can be mentioned. Moreover, in these materials, you may use what substituted a part of transition metal with another element, or substituted the oxygen part with the fluorine. From the viewpoint of high energy density, in the above formula (2), it is preferable that M contains at least one of Co and Ni because it operates at a high potential of 4.5 V or more with respect to lithium.

The layered material is represented by the following formula (3):
_{Li (Li x M 1-x} -z Mn z) O 2 (3)
[In formula (3), 0 ≦ x <0.3, 0.3 ≦ z ≦ 0.7, and M is at least one selected from Co, Ni, or Fe. The compound represented by this can be mentioned.

  A positive electrode active material can be used individually by 1 type or in combination of 2 or more types.

Specific surface areas of these positive electrode active materials are, for example, 0.01 to ⁵ m ² / g, preferably 0.05 to ⁴ m ² / g, more preferably 0.1 to ³ m ² / g, and 0.2 to ² m. ² / g is more preferable. By setting the specific surface area in such a range, the contact area with the electrolytic solution can be adjusted to an appropriate range. That is, by setting the specific surface area to 0.01 m ² / g or more, lithium ions can be inserted and desorbed smoothly, and the resistance can be further reduced. Moreover, by making a specific surface area 5 m < ² > / g or less, decomposition | disassembly of electrolyte solution can be accelerated | stimulated and it can suppress more that the constituent element of an active material elutes. The specific surface area can be measured by a usual BET specific surface area measurement method.

The center particle diameter of the positive electrode active material is preferably 0.01 to 50 μm, and more preferably 0.02 to 40 μm. By setting the particle size to 0.01 μm or more, elution of constituent elements of the positive electrode active material can be further suppressed, and deterioration due to contact with the electrolytic solution can be further suppressed. In addition, when the particle size is 50 μm or less, lithium ions can be easily inserted and desorbed smoothly, and the resistance can be further reduced. The central particle diameter is 50% cumulative diameter D ₅₀ (median diameter), and can be measured by a laser diffraction / scattering particle size distribution analyzer.

  Examples of the binder for the positive electrode include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, and polytetrafluoroethylene. , Polypropylene, polyethylene, polyimide, and polyamideimide. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The amount of the binder for positive electrode to be used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of binding force and energy density in a trade-off relationship.

  As the positive electrode current collector, for example, aluminum, nickel, silver, stainless steel (SUS), valve metal, or an alloy thereof can be used from the viewpoint of electrochemical stability. Examples of the shape include foil, flat plate, and mesh. In particular, an aluminum foil can be suitably used.

  A conductive auxiliary material may be added to the positive electrode material containing the positive electrode active material for the purpose of reducing impedance. Examples of the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black.

  The positive electrode is prepared by, for example, preparing a slurry containing a positive electrode active material, a binder, and a solvent (and optionally a conductive auxiliary material), applying the slurry onto the positive electrode current collector, and drying the slurry. Can be produced by forming a positive electrode active material layer.

(Separator)
Examples of the separator provided between the positive electrode and the negative electrode include a porous polymer film, a woven fabric, a non-woven fabric, or an ionic conductivity made of a polyolefin such as polyethylene or polypropylene, a fluororesin such as polyimide or polyvinylidene fluoride, and cellulose. Examples include polymer electrolyte membranes. These can be used alone or in combination. In addition, a ceramic material may adhere to or adhere to the separator surface as necessary, for example, to improve safety.

(Electrolyte)
In this embodiment, it is preferable that electrolyte solution contains a cyclic carbonate as a nonaqueous electrolytic solvent. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). Further, some or all of hydrogen in these compounds may be substituted with fluorine. A cyclic carbonate can be used individually by 1 type or in mixture of 2 or more types. EC and PC are preferable because they have a high dielectric constant and excellent electrolyte solubility, and EC is more preferable.

  When operating the positive electrode active material at a high potential of 4.5 V or higher, it is better to use a solvent having higher oxidation resistance than the cyclic carbonate. Examples of the solvent having high oxidation resistance include fluorinated ethers, sulfone compounds, and fluorinated phosphates. These solvents can be used singly or in combination of two or more.

  In the present embodiment, the fluorinated ether compound is represented by the following formula (4).

（式（４）中、Ｒ_１及びＲ_２は、それぞれ独立して、アルキル基又はフッ化アルキル基を示し、Ｒ_１及びＲ_２のうち少なくとも一方はフッ化アルキル基である。）
また、Ｒ_１およびＲ_２の炭素数の合計が１０以下であることが好ましい。

(In Formula (4), R ₁ and R ₂ each independently represent an alkyl group or a fluorinated alkyl group, and at least one of R ₁ and R ₂ is a fluorinated alkyl group.)
In addition, the total number of carbon atoms of R ₁ and R ₂ is preferably 10 or less.

  In formula (4), the fluorine atom content in the fluorinated alkyl group is preferably 50% or more, more preferably 60% or more, based on the total of fluorine atoms and hydrogen atoms. When the content of fluorine atoms is large, the withstand voltage is further improved, and even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, deterioration of battery capacity after cycling is more effectively reduced. Is possible.

Examples of the fluorinated ether compound include CF ₃ OCH ₃ , CF ₃ OC ₂ H ₅ , F (CF ₂ ) ₂ OCH ₃ , F (CF ₂ ) ₂ OC ₂ H ₅ , and CF ₃ (CF ₂ ) CH ₂ O. (CF ₂ ) CF ₃ , F (CF ₂ ) ₃ OCH ₃ , F (CF ₂ ) ₃ OC ₂ H ₅ , F (CF ₂ ) ₄ OCH ₃ , F (CF ₂ ) ₄ OC ₂ H ₅ , F (CF ₂ ) ₅ OCH ₃ , F (CF ₂ ) ₅ OC ₂ H ₅ , F (CF ₂ ) ₈ OCH ₃ , F (CF ₂ ) ₈ OC ₂ H ₅ , F (CF ₂ ) ₉ OCH ₃ , CF ₃ CH ₂ OCH ₃ , CF ₃ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ OCH ₃ , CF ₃ CF ₂ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ O (CF ₂ ) ₂ H, CF ₃ CF ₂ CH ₂ O ( CF _{2) 2} F, HC _{2 CH 2 OCH 3, (CF} 3) (CF 2) CH 2 O (CF 2) 2 H, H (CF 2) 2 OCH 2 CH 3, H (CF 2) 2 OCH 2 CF 3, H (CF 2 ) ₂ CH ₂ OCHF ₂ , H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₂ H, H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₃ H, H (CF ₂ ) ₃ CH ₂ O (CF _{2) 2 H, H (CHF} ) 2 CH 2 O (CF 2) 2 H, (CF 3) 2 CHOCH 3, (CF 3) 2 CHCF 2 OCH 3, CF 3 CHFCF 2 OCH 3, CF 3 CHFCF 2 OCH ₂ CH ₃ , CF ₃ CHFCF ₂ CH ₂ OCHF ₂ , CF ₃ CHFCF ₂ OCH ₂ (CF ₂ ) ₂ F, CF ₃ CHFCF ₂ OCH ₂ CF ₂ CF ₂ H, H (CF ₂ ) ₄ CH ₂ O (C F ₂ ) ₂ H, CH ₃ CH ₂ O (CF ₂ ) ₄ F, F (CF ₂ ) ₄ CH ₂ O (CF ₂ ) ₂ H, H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₂ HO ( CF ₂ ) ₃ H and the like.

  Among the fluorinated ether compounds, a fluorinated ether compound represented by the following formula (4-1) is more preferable.

_{^{X 1 - (CX 2 X 3}} ) n -O- (CX 4 X 5) m -X 6 (4-1)
(In formula (4-1), n and m are each independently 1 to 8. X ^{1 to} X ⁶ are each independently a fluorine atom or a hydrogen atom, provided that at least ^one of X ^{1 to} X ³ One is a fluorine atom, and at least one of X ^{4 to} X ⁶ is a fluorine atom.)

  The fluorinated ether compound is more preferably a compound represented by the following formula (4-2) from the viewpoint of withstand voltage and compatibility with other electrolytes.

_{^{X 1 - (CX 2 X 3}} ) n -CH 2 O-CX 4 X 5 -CX 6 X 7 -X 8 (4-2)
(In the formula (4-2), n is 1 to 8, and X ^{1 to} X ⁸ are each independently a fluorine atom or a hydrogen atom, provided that at least one of X ^{1 to} X ³ is a fluorine atom. And at least one of X ^{4 to} X ⁸ is a fluorine atom.)
In Formula (4-2), when n is 2 or more, a plurality of X ² may be the same or different from each other, and a plurality of X ³ are the same or different from each other. Also good.

  Further, from the viewpoint of voltage resistance and compatibility with other electrolytes, the fluorinated ether compound is more preferably represented by the following formula (4-3).

^{H- (CY 1 Y 2 -CY 3} Y 4) n -CH 2 O-CY 5 Y 6 -CY 7 Y 8 -H (4-3)
(In Formula (4-3), n is 1, 2, 3 or 4. Y ^{1 to} Y ⁸ are each independently a fluorine atom or a hydrogen atom, provided that at least one of Y ^{1 to} Y ⁴ is present. one is a fluorine ^atom, at least one of Y 5 to Y ⁸ is a fluorine atom.)
In the formula ^(4-3), Y 1 ~Y ^4, when n is 2 or more, or different an ^{X 2} is each of plural present, ^{X 3} where there exist a plurality is each Or different.

  The content of the fluorinated ether compound is preferably 15% by volume or more and 90% by volume or less in the nonaqueous electrolytic solvent, more preferably 40% by volume or more and 90% by volume or less, and 50% by volume or more and 90% by volume or less. More preferably, it is 50 volume% or more and 80 volume% or less. When the content of the fluorinated ether compound is too small, the viscosity of the electrolytic solution is increased, so that the conductivity is lowered and the capacity is reduced in the cycle. When the content of the fluorinated ether compound is too large, the dielectric constant of the electrolytic solution is lowered, the supporting salt cannot be dissociated, and the capacity is similarly reduced. In addition, a fluorinated ether compound can be used individually by 1 type or in mixture of 2 or more types.

  In the present embodiment, the sulfone compound is represented by the following formula (5) (hereinafter, the “sulfone compound represented by the formula (5)” may be simply referred to as “sulfone compound”).

（式（５）において、Ｒ_１及びＲ_２は、それぞれ独立に、置換または無置換のアルキル基を示す。Ｒ_１の炭素原子とＲ_２の炭素原子が単結合又は二重結合を介して結合し、環状構造を形成していてもよい）
式（５）で表されるスルホン化合物において、Ｒ_１の炭素数ｎ_１、Ｒ_２の炭素数ｎ_２はそれぞれ１≦ｎ_１≦１２、１≦ｎ_２≦１２であることが好ましく、１≦ｎ_１≦６、１≦ｎ_２≦６であることがより好ましく、１≦ｎ_１≦３、１≦ｎ_２≦３であることが更に好ましい。また、アルキル基は、直鎖状、分岐鎖状、又は環状のものを含む。

(In Formula (5), R ₁ and R ₂ each independently represents a substituted or unsubstituted alkyl group. The carbon atom of R _{1 and} the carbon atom of R ₂ are bonded through a single bond or a double bond. And may form a ring structure)
In sulfone compound represented by formula (5), it is preferable that the carbon number _{n 2} with carbon number _n 1, _{R 2} of _{R 1} is _{1 ≦ n 1 ≦ 12,1 ≦ n} 2 ≦ 12 , respectively, 1 ≦ n ₁ ≦ 6, 1 ≦ n ₂ ≦ 6 are more preferable, and 1 ≦ n ₁ ≦ 3 and 1 ≦ n ₂ ≦ 3 are still more preferable. The alkyl group includes linear, branched, or cyclic groups.

In R ₁ and R ₂ , examples of the substituent include an alkyl group having 1 to 6 carbon atoms (for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, and an isobutyl group), and a C 6-10 carbon atom. An aryl group (for example, a phenyl group, a naphthyl group), a halogen atom (for example, a chlorine atom, a bromine atom, a fluorine atom) etc. are mentioned.

  The sulfone compound is preferably a cyclic sulfone compound represented by the following formula (5-1).

（式（５−１）中、Ｒ_３は、置換または無置換のアルキレン基を示す。）
Ｒ_３において、アルキレン基の炭素数は４〜９であることが好ましく、４〜６であることが更に好ましい。

(In formula (5-1), R ₃ represents a substituted or unsubstituted alkylene group.)
In R ₃ , the alkylene group preferably has 4 to 9 carbon atoms, and more preferably 4 to 6 carbon atoms.

In R ₃ , examples of the substituent include an alkyl group having 1 to 6 carbon atoms (for example, methyl group, ethyl group, propyl group, isopropyl group, butyl group), halogen atom (for example, chlorine atom, bromine atom, fluorine atom). Atom) and the like.

  The cyclic sulfone compound is more preferably a compound represented by the following formula (5-2).

（式（５−２）中、ｍは１〜６の整数である。）
式（５−２）において、ｍは、１〜６の整数であり、１〜３の整数であることが好ましい。

(In formula (5-2), m is an integer of 1-6.)
In Formula (5-2), m is an integer of 1 to 6, and preferably an integer of 1 to 3.

  Preferred examples of the cyclic sulfone compound represented by the formula (5-1) include tetramethylene sulfone, pentamethylene sulfone, hexamethylene sulfone and the like. Preferred examples of the cyclic sulfone compound having a substituent include 3-methylsulfolane and 2,4-dimethylsulfolane. Since these materials are compatible with the fluorinated ether compound and have a relatively high dielectric constant, they have the advantage of being excellent in the dissolution / dissociation action of the lithium salt.

  The sulfone compound may be a chain sulfone compound. Examples of the chain sulfone compound include ethyl methyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, dimethyl sulfone, and diethyl sulfone. Of these, ethyl methyl sulfone, ethyl isopropyl sulfone, and ethyl isobutyl sulfone are preferable. Since these materials are compatible with the fluorinated ether compound and have a relatively high dielectric constant, they have the advantage of being excellent in the dissolution / dissociation action of the lithium salt.

  A sulfone compound can be used individually by 1 type or in mixture of 2 or more types.

  The content of the sulfone compound is preferably 5% by volume or more and 75% by volume or less in the non-aqueous electrolytic solvent, and more preferably 5% by volume or more and 50% by volume or less. If the content of the sulfone compound is too small, the compatibility of the electrolytic solution is reduced. If the content is too high, the viscosity of the electrolytic solution is increased, and the capacity of the cycle characteristics at room temperature is reduced.

  In the present embodiment, the fluorinated phosphate ester is represented by the following formula (6).

（式（６）において、Ｒ^１、Ｒ^２、Ｒ^３は、それぞれ独立に、アルキル基またはフッ化アルキル基を示し、これらのうち少なくとも１つがフッ化アルキル基である。）
フッ化アルキル基とは、少なくとも１つのフッ素原子を有するアルキル基である。式（６）において、Ｒ^１、Ｒ^２およびＲ^３の炭素数は、それぞれ独立に、１〜３であることが好ましい。Ｒ^１、Ｒ^２およびＲ^３の少なくとも１つは、対応する無置換のアルキル基が有する水素原子の５０％以上がフッ素原子に置換されたフッ化アルキル基であることが好ましい。また、Ｒ^１、Ｒ^２およびＲ^３の全てがフッ化アルキル基であり、Ｒ^１、Ｒ^２およびＲ^３が対応する無置換のアルキル基の水素原子の５０％以上がフッ素原子に置換されたフッ化アルキル基であることがより好ましい。フッ素原子の含有率が多いと、耐電圧性がより向上し、リチウムに対して４．５Ｖ以上の電位で動作する正極活物質を用いた場合でも、サイクル後における電池容量の劣化をより低減することできるからである。また、フッ化アルキル基における水素原子を含む置換基中のフッ素原子の比率は５５％以上がより好ましい。

(In Formula (6), R ¹ , R ² and R ³ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of them is a fluorinated alkyl group.)
The fluorinated alkyl group is an alkyl group having at least one fluorine atom. In the formula ^(6), the number of carbon atoms in R 1, ^{R 2} and ^{R 3} each independently is preferably 1-3. At least one of R ¹ , R ² and R ³ is preferably a fluorinated alkyl group in which 50% or more of the hydrogen atoms of the corresponding unsubstituted alkyl group are substituted with fluorine atoms. Also, all of R ¹ , R ² and R ³ are fluorinated alkyl groups, and R ¹ , R ² and R ³ are substituted with fluorine atoms by 50% or more of the hydrogen atoms of the corresponding unsubstituted alkyl groups. More preferably, it is a fluorinated alkyl group. When the content of fluorine atoms is large, the voltage resistance is further improved, and even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, the deterioration of battery capacity after cycling is further reduced. Because it can. Further, the ratio of fluorine atoms in the substituent containing a hydrogen atom in the fluorinated alkyl group is more preferably 55% or more.

  Although it does not specifically limit as fluorinated phosphate ester, For example, phosphoric acid tris (trifluoromethyl), phosphoric acid tris (pentafluoroethyl), phosphoric acid tris (2,2,2-trifluoroethyl) (TTFP) , Tris phosphate (2,2,3,3-tetrafluoropropyl), Tris phosphate (3,3,3-trifluoropropyl), Tris phosphate (2,2,3,3,3-pentafluoropropyl) Among them, tris (2,2,2-trifluoroethyl phosphate) (TTFP) is preferable as the fluorinated phosphate ester compound. One kind can be used alone, or two or more kinds can be used in combination.

  The content of the fluorinated phosphate ester contained in the nonaqueous electrolytic solvent is not particularly limited, but is preferably 0% by volume to 95% by volume in the nonaqueous electrolytic solvent, and is preferably 10% by volume to 95% by volume. Is more preferable, and 20 volume% or more and 70 volume% or less are further more preferable. When the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is 10% by volume or more, the effect of increasing the voltage resistance is further improved. Moreover, the ion conductivity of electrolyte solution improves that the content rate in the nonaqueous electrolytic solvent of fluorine-containing phosphate ester is 95 volume% or less, and the charging / discharging rate of a battery becomes more favorable.

  Since the fluorinated phosphate ester also has high oxidation resistance, oxidative decomposition of the solvent when a 5V class active material is used can be suppressed. As a result, it is possible to improve the capacity maintenance rate of the charge / discharge cycle and reduce gas generation.

  In the present embodiment, the nonaqueous electrolytic solvent may contain an aliphatic carboxylic acid ester, γ-lactone, a cyclic ether, a chain ether other than the above, and the like. Examples of the aliphatic carboxylic acid ester include methyl formate, methyl acetate, ethyl propionate, and derivatives thereof (including fluorinated products). Examples of γ-lactone include γ-butyrolactone and its derivatives (including fluorinated products). Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, and derivatives thereof (including fluorinated products). Examples of the chain ether include 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, and derivatives thereof (including fluorinated products). These can be used individually by 1 type or in mixture of 2 or more types.

  Other nonaqueous electrolytic solvents include, for example, dimethyl sulfoxide, formamide, acetamide, dimethylformamide, dioxolane (eg, 1,3-dioxolane), acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxy Methane, dioxolane derivative, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, anisole, N-methylpyrrolidone, and derivatives thereof (including fluorinated compounds) It can also be used.

  In this embodiment, it is preferable that electrolyte solution further contains the cyclic sulfonate ester represented by following formula (7) as an additive. When the electrolytic solution contains a cyclic sulfonic acid ester, the cyclic sulfonic acid ester forms a film on the surface of the positive electrode, thereby suppressing the reaction of the electrolytic solution and suppressing the volume expansion.

（式（７）中、Ａ及びＢは、それぞれ独立に、アルキレン基又はフッ化アルキレン基を示す。Ｘは、単結合又は−ＯＳＯ_２−基を示す）
式（７）において、アルキレン基の炭素数は、例えば１〜８であり、好ましくは１〜６であり、より好ましくは１〜４である。

(In Formula (7), A and B each independently represent an alkylene group or a fluorinated alkylene group. X represents a single bond or an -OSO ₂ -group).
In Formula (7), carbon number of an alkylene group is 1-8, for example, Preferably it is 1-6, More preferably, it is 1-4.

  The fluorinated alkylene group represents a substituted alkylene group having a structure in which at least one hydrogen atom in the unsubstituted alkylene group is substituted with a fluorine atom. In Formula (7), carbon number of a fluorinated alkylene group is 1-8, for example, Preferably it is 1-6, More preferably, it is 1-4.

Incidentally, -OSO ₂ - groups may be either direction.

  In the formula (7), when X is a single bond, the cyclic sulfonate ester is preferably a cyclic monosulfonate ester, and the cyclic monosulfonate ester is preferably a compound represented by the following formula (7-1).

（式（７−１）中、Ｒ_１０１及びＲ_１０２は、それぞれ独立に、水素原子、フッ素原子、又は炭素数１〜４のアルキル基を示す。ｎは０、１、２、３、又は４である。）

(In Formula (7-1), R ₁₀₁ and R ₁₀₂ each independently represent a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms. N is 0, 1, 2, 3, or 4. .)

In the formula (7), when X is an -OSO ₂ -group, the cyclic sulfonate ester is preferably a cyclic disulfonate ester, and the cyclic disulfonate ester is preferably a compound represented by the following formula (7-2).

（式（７−２）中、Ｒ_２０１〜Ｒ_２０４は、それぞれ独立に、水素原子、フッ素原子、又は炭素数１〜４のアルキル基を示す。ｎは１、２、３、又は４である。また、ｎが２以上の場合、複数個存在するＲ_２０３は互いに同一であっても異なっていてもよく、複数個存在するＲ_２０４は、互いに同一であっても異なっていてもよい。）

(In Formula (7-2), R _{201 to} R ₂₀₄ each independently represent a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms. N is 1, 2, 3, or 4. When n is 2 or more, a plurality of R ₂₀₃ may be the same or different from each other, and a plurality of R ₂₀₄ may be the same or different from each other.

Examples of the cyclic sulfonate ester include 1,3-propane sultone, 1,2-propane sultone, 1,4-butane sultone, 1,2-butane sultone, 1,3-butane sultone, 2,4-butane sultone, 1,3 -Monosulfonic acid ester such as pentane sultone (when X in formula (7) is a single bond), disulfonic acid ester such as methylenemethane disulfonic acid ester, ethylenemethane disulfonic acid ester (X in formula (7) is- OSO ₂ - for group). Among these, 1,3-propane sultone, 1,4-butane sultone, and methylene methane disulfonic acid ester are preferable from the viewpoints of film formation effect, availability, and cost.

  The content of the cyclic sulfonic acid ester in the electrolytic solution is preferably 0.01 to 10% by mass, more preferably 0.1 to 5% by mass, and 0.3 to 3% by mass. Is more preferable. When the content of the cyclic sulfonic acid ester is 0.01% by mass or more, a coating can be more effectively formed on the positive electrode surface to suppress decomposition of the electrolytic solution. When the content of the cyclic sulfonic acid ester is 10% by mass or less, the initial capacity close to the theoretical capacity is ensured by charging at 20 ° C. by adjusting the viscosity and conductivity of the electrolytic solution within a more appropriate range. Can do.

In the present embodiment, the nonaqueous electrolytic solution is obtained by dissolving an electrolyte made of a lithium salt in a nonaqueous electrolytic solvent. Examples of the lithium salt is not particularly limited, for example, lithium imide _{salt, LiPF 6, LiAsF 6, LiAlCl} 4, LiClO 4, LiBF 4, LiSbF 6 and the like. Among these, LiPF ₆ and LiBF ₄ are preferable. Examples of the lithium imide salt include LiN (C _k F _{2k + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (k and m are each a natural number and are preferably 1 or 2 independently). . A lithium salt can be used individually by 1 type, and can also be used in combination of 2 or more type. The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / L. By setting the concentration of the lithium salt within this range, it is easy to adjust the density, viscosity, electrical conductivity, and the like to an appropriate range.

(Battery shape and exterior)
Examples of the shape of the battery include a cylindrical shape, a square shape, a coin shape, a button shape, and a laminate shape.

  In the case of the laminate type, the electrodes and the separator are laminated in a planar shape, and there is no portion with a small R (a region close to the winding core of the wound structure or a region corresponding to a folded portion of the flat wound structure). Therefore, when an active material having a large volume change associated with charging / discharging is used, it is less likely to be adversely affected by the volume change of the electrode associated with charging / discharging than a battery having a wound structure.

  Examples of the battery outer package include stainless steel, iron, aluminum, titanium, alloys thereof, and plated products thereof. As the plating, for example, nickel plating can be used. When the shape of the battery is a laminate type, a laminate film is preferable as the outer package.

  As the laminate film, for example, a film in which a heat welding layer and a metal foil layer are laminated can be used. Examples of the metal foil layer on the resin base layer of the laminate film include aluminum, an aluminum alloy, and a titanium foil. Examples of the material of the heat-welded layer (resin base material layer) of the laminate film include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. Moreover, the resin base material layer and the metal foil layer of the laminate film are not limited to one layer, but may be two or more layers. From the viewpoint of versatility and cost, an aluminum laminate film is preferable.

  When a laminate film is used as the exterior body, battery volume changes and electrode distortions are more likely to occur due to gas generation than when a metal can is used as the exterior body. This is because the laminate film is more easily deformed by the internal pressure of the battery than the metal can. In addition, when sealing a secondary battery using a laminate film as an exterior body, the internal pressure of the battery is usually lower than atmospheric pressure, and there is no extra space inside, so when gas is generated in the battery Immediately leads to battery volume change and electrode deformation. In addition, in the case of a laminate type battery, compared to a battery having a wound structure, when gas is generated between the electrodes, the gap between the electrodes tends to be widened because the gas tends to stay between the electrodes. When used, this tendency becomes more prominent. However, according to the present embodiment, the occurrence of such a problem can be suppressed, and a secondary battery excellent in long-term reliability can be provided even for a laminated battery using a laminate film outer package. it can.

  Examples will be described below, but the present invention is not limited to the examples described below unless the gist of the present invention is exceeded.

Example 1
After charging 4 g of lithium carbonate (manufactured by Wako Pure Chemicals, special grade) and 27 g of silicon monoxide (manufactured by Osaka Titanium Co., Ltd.) into a platinum crucible, the temperature of 1500 ° C. was maintained while circulating argon gas at 10 NL / min in the reactor. And dissolved for 30 minutes, and the melt was cooled to room temperature by the twin roll method. The cooling rate was 10 ⁶ ° C / second.

  After roughly pulverizing the obtained reaction product, 20 g of artificial graphite powder was added to 20 g of the pulverized product, followed by treatment at 120 rpm for 18 hours in an alumina ball mill. The treated powder was placed in an alumina crucible and then heat treated at 900 ° C. for 5 hours under an argon atmosphere.

When the powder after the above treatment was observed with an electron microscope, it had a phase separation structure formed by spinodal decomposition in which two phases were intertwined with each other. As a result of ²⁹ Si-NMR measurement, a peak was found at −66 ppm, and the silicate phase was mainly composed of Li ₄ SiO ₄ . Moreover, the peak derived from silicon was also confirmed from ²⁹ Si-NMR.

(Comparative Example 1)
18 g of silicon monoxide (manufactured by Osaka Titanium) and 2 g of lithium orthosilicate (Li ₄ SiO ₄ ) were treated with an alumina ball mill at 120 rpm for 18 hours, and further 20 g of artificial graphite was added, followed by treatment at 120 rpm for 18 hours. The treated powder was placed in an alumina crucible and then heat treated at 900 ° C. for 5 hours under an argon atmosphere.

  When the powder after the above treatment was observed with an electron microscope, it did not have a phase separation structure formed by spinodal decomposition in which two phases were entangled with each other.

  Charge / discharge characteristics of the active materials obtained in Example 1 and Comparative Example 1 were evaluated.

(Charge / discharge test)
After adding 30 wt% of graphite having an average diameter of 6 μm and 12 wt% of polyvinylidene fluoride to the obtained sample, kneading using N-methylpyrrolidone as a dispersion medium, coating on a copper foil having a thickness of 15 μm, and then adding 12 at 100 ° C. It was vacuum-dried for a time and used as a test electrode. A battery having an EC / DMC (volume ratio 1: 2) solution in which the counter electrode and the reference electrode were dissolved in metal Li and the electrolyte solution was dissolved in LiPF ₆ to 1 M was produced in a low moisture atmosphere with a dew point of -50 ° C.

The charging / discharging test was performed by charging at a current density of 1 mA / cm ² up to a potential difference of 0.01 V between the reference electrode and the test electrode, and further performing a constant voltage charging at 0.01 V for 8 hours, and discharging at 1 mA / cm ² . The current density was up to 1.5V.

  In the charge / discharge test, the charge capacity and the discharge capacity were the amounts of electricity that flowed from the start to the end of charging or discharging. The initial charge / discharge efficiency was determined as a percentage of the discharge capacity at the first cycle to the charge capacity at the first cycle.

Next, similarly, charging is performed at a current density of 1 mA / cm ² until the potential difference between the reference electrode and the test electrode is 0.01 V, and further, constant voltage charging is performed at 0.01 V for 8 hours, and discharging is performed at a current density of 10 mA / cm ^2. It went to 1.5V. Large current characteristics were evaluated by comparing the ratio of capacity at 10 mA / cm ^{2 to} capacity at a current density of 1 mA / cm ² during discharge.

In addition, the 100th cycle of the first cycle is performed by performing 100 cycles of charging at a current density of 1 mA / cm ² to a potential difference of 0.01 V between the reference electrode and the test electrode and discharging to 1.5 V at a current density of 1 mA / cm ^2. The discharge capacity retention rate was measured.

  Table 1 shows the presence / absence of a lithium silicate phase having a continuous phase and the evaluation results for Examples and Comparative Examples.

表１に示すように、リチウムシリケートの連続相を有することで、初回充放電効率、サイクル特性ともに向上することがわかる。

As shown in Table 1, it can be seen that the first charge / discharge efficiency and the cycle characteristics are improved by having a continuous phase of lithium silicate.

  As mentioned above, although embodiment of this invention was described, this invention is not restricted to these, In the category of the summary of the invention as described in a claim, it can change variously. In addition, the present invention can be variously modified without departing from the scope of the invention in the implementation stage. Furthermore, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment.

DESCRIPTION OF SYMBOLS 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode collector 4 Negative electrode collector 5 Separator 6 Laminate exterior 7 Laminate exterior 8 Negative electrode tab 9 Positive electrode tab

Claims (15)

  A negative electrode active material comprising a continuous phase lithium silicate phase in silicon oxide.   2. The negative electrode active material according to claim 1, wherein a content of the lithium silicate is 2 mol% or more and 45 mol% or less with respect to the silicon oxide.   3. The negative electrode active material according to claim 1, wherein the silicon oxide contained within 300 nm from the interface between the silicon oxide phase and the lithium silicate phase is 50% or more of the total weight of the silicon oxide.   4. The negative electrode active material according to claim 1, comprising silicon in the vicinity of a lithium silicate phase in the silicon oxide. 5.   5. The negative electrode active material according to claim 4, wherein 5% or more of the total weight of the silicon is present within 100 nm from the interface between the silicon oxide phase and the lithium silicate phase.   The negative electrode active material according to claim 1, wherein the surface is coated with a carbon material.   A negative electrode for a secondary battery, comprising the negative electrode active material according to claim 1.   It is a secondary battery provided with the electrode body provided with a positive electrode and a negative electrode, a non-aqueous electrolyte, and the exterior body which accommodates the said electrode body, Comprising: The said negative electrode is a negative electrode of Claim 7. A secondary battery. Mixing a raw material containing a lithium raw material and a silicon oxide raw material in a composition ratio of lithium and silicon oxide forming a spinodal phase separation;
Heating above the mixing temperature in the composition ratio;
Quenching to a temperature lower than the temperature at which spinodal decomposition occurs at the composition ratio;
A method for producing a negative electrode active material having a continuous lithium silicate phase in silicon oxide, comprising a step of performing a heat treatment in a temperature range in which spinodal phase separation is formed at the composition ratio.   The negative electrode according to claim 9, further comprising a step of coating a surface with a carbon material before or after the step of performing a heat treatment in a temperature region for forming the spinodal phase separation or simultaneously with the step. A method for producing an active material.   Furthermore, the manufacturing method of the negative electrode active material of Claim 9 or 10 which has a heating process which produces | generates silicon from a silicon oxide.   The method for producing a negative electrode active material according to claim 11, wherein the heating step of generating silicon from the silicon oxide is performed at 800 ° C. or higher.   The step of heating above the mixing temperature, the step of quenching, and the step of performing a heat treatment in a temperature region that forms the spinodal phase separation are performed in an inert gas atmosphere. The manufacturing method of the negative electrode active material as described in any one.   The manufacturing method of the negative electrode for secondary batteries characterized by using the negative electrode active material manufactured by the manufacturing method as described in any one of Claims 9-13.   A method for producing a secondary battery, comprising using a negative electrode for a secondary battery produced by the production method according to claim 14.

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