JP2019204684A - Negative electrode active material and method for manufacturing negative electrode - Google Patents

Abstract

To further improve a method for manufacturing a silicon-containing negative electrode active material by doping a lithium-doped negative electrode active material with lithium.SOLUTION: A method for manufacturing a lithium-doped negative electrode active material comprises a doping step of bringing a silicon-containing negative electrode active material with a lithium complex solution prepared by dissolving a lithium metal and a polycyclic aromatic compound in an organic solvent, thereby doping the negative electrode active material with lithium. In the doping step, a negative electrode active material slurry containing the negative electrode active material and the organic solvent is kept below 50°C and then the lithium complex solution is dropped.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a negative electrode active material and a method for producing a negative electrode.

  As a negative electrode active material for a lithium ion secondary battery, a negative electrode active material containing silicon having a high lithium ion storage capacity is known. For example, Patent Literature 1 and Patent Literature 2 describe lithium ion secondary batteries in which the negative electrode active material is silicon. Patent Literature 3 and Patent Literature 4 describe lithium ion secondary batteries in which the negative electrode active material is SiO.

  For the purpose of further improving the battery performance of the lithium ion secondary battery, a search for a new negative electrode active material containing silicon is also in progress. As a new negative electrode active material containing silicon, Patent Document 5 discloses a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. Patent Document 5 further describes that a carbon-silicon composite was manufactured by coating the silicon material with carbon, and that the composite can be used as a negative electrode active material of a lithium ion secondary battery. Has been.

By the way, the irreversible capacity | capacitance of the negative electrode active material containing a silicon is large. For this reason, the technique which cancels the irreversible capacity | capacitance of the negative electrode active material containing the said silicon | silicone by doping lithium to these negative electrode active materials previously is proposed.
Hereinafter, if necessary, the negative electrode active material containing silicon is referred to as a silicon-containing negative electrode active material, the negative electrode having a silicon-containing negative electrode active material is referred to as a silicon-containing negative electrode, and the silicon-containing negative electrode active material doped with lithium is referred to as lithium. This is referred to as a doped negative electrode active material, and a silicon-containing negative electrode doped with lithium is referred to as a lithium-doped negative electrode.

Non-Patent Document 1, Non-Patent Document 2, and Patent Documents 6 to 8 introduce various methods for doping lithium into a silicon-containing negative electrode active material.
In Patent Document 6, as a method for doping lithium into a silicon-containing negative electrode active material, a solution containing metallic lithium dissolved in liquid ammonia, or a solution obtained by dissolving n-butyl lithium in an organic solvent, a silicon-containing negative electrode active material, Or the method of immersing the negative electrode using the said silicon-containing negative electrode active material is introduced.
Patent Document 7 introduces a method of kneading and mixing metallic lithium and a silicon-containing negative electrode active material in the presence of a solvent.
Non-Patent Document 1 introduces a method of obtaining a lithium-doped negative electrode by immersing a silicon-containing negative electrode in a dope solution in which metallic lithium is dissolved in a solution in which naphthalene is dissolved in tetrahydrofuran.
Non-Patent Document 2 introduces a method of obtaining a lithium-doped negative electrode by immersing a silicon-containing negative electrode in a solution obtained by dissolving naphthalene and metallic lithium in butyl methyl ether.
In Patent Document 8, a silicon-containing negative electrode is immersed in an alkali metal complex obtained by mixing an alkali metal, an organic solvent solvated with the alkali metal, and a ligand having electrophilic substitution reactivity. For example, a method for obtaining a lithium-doped negative electrode has been introduced. In the examples of Patent Document 8, polycyclic aromatic compounds such as naphthalene, fluorene, azulene, acenaphthylene, biphenylene, pyrene, tetracene, and benzanthracene are used as the ligand, and tetrahydrofuran, propylene are used as the organic solvent. Carbonate, ethylene carbonate, dimethoxyethane, and dimethyl sulfoxide are used.

The inventor of the present invention actually manufactured a lithium-doped negative electrode active material by these methods. Then, it turned out that the lithium dope negative electrode active material suitable for a lithium ion secondary battery is not necessarily obtained by all these methods.
For example, the inventor of the present invention doped the above silicon material with lithium by the method introduced in Patent Documents 6 and 7. As a result, the obtained lithium-doped negative electrode active material had a very small amount of lithium doped, although the oxygen content increased.
That is, in order to obtain a lithium-doped negative electrode active material suitable for a lithium ion secondary battery, the conventionally known lithium doping method is not sufficient, and a method for doping lithium into a silicon-containing negative electrode active material is not sufficient. There is a need for improvement.

JP 2014-203595 A Japanese Patent Laying-Open No. 2015-57767 JP 2015-185509 A JP-A-2015-179625 International Publication No. 2015/114692 Japanese Patent Laid-Open No. 10-294104 Japanese Patent Laid-Open No. 2006-189331 JP-A-2015-43310

Masuo, Y. and five others, “Examination of pre-doping method to flaky silicon negative electrode using Li-Naphtharene complex solution”, Abstracts of Battery Conference, 19 November 2014, Volume 55th, Page 88 Toru Tabuchi, two others, "Li-doping process for LixSiO-negative active materialized by chemical method for the 50th month, 50"

  The present invention has been made in view of the above circumstances, and an object of the present invention is to further improve a method for producing a lithium-doped negative electrode active material by doping lithium into a silicon-containing negative electrode active material.

The method for producing a negative electrode active material of the present invention that solves the above problems is as follows.
A negative electrode active material containing silicon, contacting a lithium complex solution obtained by dissolving metallic lithium and a polycyclic aromatic compound in an organic solvent, and doping the negative electrode active material with lithium;
The dope step is a method for producing a lithium-doped negative electrode active material, in which a negative electrode active material suspension containing the negative electrode active material and an organic solvent is made 50 ° C. or lower and the lithium complex solution is dropped.

  According to the method for producing a negative electrode active material of the present invention, a lithium-doped negative electrode active material that can further improve the battery characteristics of a lithium ion secondary battery can be produced by doping lithium into the silicon-containing negative electrode active material.

2 is a SEM image of a lithium-doped negative electrode active material of Example 1. 4 is a SEM image of a lithium-doped negative electrode active material of Comparative Example 3. 4 is a SEM image of a lithium-doped negative electrode active material of Comparative Example 2. 2 is a surface curve of the lithium-doped negative electrode active material of Example 1. 10 is a surface curve of a lithium-doped negative electrode active material of Comparative Example 3. 6 is a graph showing powder resistance of each negative electrode active material of Example 1, Example 4, and Comparative Examples 1 to 4.

  The best mode for carrying out the present invention will be described below. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from these numerical ranges can be used as new upper and lower numerical values.

  The negative electrode active material obtained by the method for producing a negative electrode active material of the present invention is a lithium-doped silicon-containing negative electrode active material. Hereinafter, as necessary, the negative electrode active material is referred to as a negative electrode active material of the present invention, and a negative electrode including the negative electrode active material of the present invention in a negative electrode active material layer is referred to as a negative electrode of the present invention. Furthermore, the negative electrode active material production method of the present invention is referred to as the production method of the present invention, and the negative electrode production method including the production method of the present invention is referred to as the negative electrode production method of the present invention as necessary.

The manufacturing method of this invention has a dope process.
The doping step is a step in which a lithium complex solution in which metallic lithium and a polycyclic aromatic compound are dissolved in an organic solvent is brought into contact with a silicon-containing negative electrode active material, and the negative electrode active material is doped with lithium. In the step, the negative electrode active material suspension containing the negative electrode active material and the organic solvent is brought to 50 ° C. or lower, and the lithium complex solution is dropped.

  In the production method of the present invention, the silicon-containing negative electrode active material is doped with lithium by the doping step to obtain a lithium-doped negative electrode active material. A doping method using a lithium complex solution is introduced in Non-Patent Documents 1 and 2 or Patent Document 8. Hereinafter, a doping method using a lithium complex solution is referred to as a wet doping method as necessary.

  The inventor of the present invention actually manufactured a lithium-doped negative electrode active material by the wet doping method introduced in Patent Document 8. And the negative mix slurry for manufacturing a negative electrode was actually adjusted using the lithium dope negative electrode active material obtained by the said method. The negative electrode mixture slurry contains a lithium-doped negative electrode active material, a solvent, and a binder. The negative electrode mixture slurry is applied on the negative electrode current collector to form a negative electrode mixture layer, and the negative electrode current collector on which the negative electrode mixture layer is formed is dried and heated, whereby the negative electrode active material layer is formed. A negative electrode formed on the negative electrode current collector is obtained.

  However, when the inventor of the present invention prepared a negative electrode mixture slurry using the lithium-doped negative electrode active material obtained by the above wet doping method, the negative electrode mixture slurry was gelled, and the negative electrode current collector Could not be applied. That is, the conventional wet doping method cannot obtain a lithium-doped negative electrode active material suitable for coating and forming a negative electrode active material layer.

The inventor of the present invention can produce a negative electrode mixture slurry that can be applied to a negative electrode current collector by suppressing gelation by treating the lithium-doped negative electrode active material obtained by the wet doping method with CO _2. I found out. The inventor manufactured a lithium ion secondary battery using the negative electrode manufactured using the negative electrode mixture slurry. However, the battery performance of this lithium ion secondary battery did not exceed expectations. Specifically, it was considered that the lithium ion secondary battery could be further improved, particularly in terms of initial capacity and capacity maintenance rate.

  The inventor of the present invention has noticed that lithium doping may be uneven in the lithium-doped negative electrode active material obtained by the wet doping method. When lithium doping is uneven, for example, when a thick layer or large particles of lithium adhere to the surface of the silicon-containing negative electrode active material, the insertion and extraction of lithium by the negative electrode active material There is a possibility that the initial capacity and capacity retention rate of the lithium ion secondary battery may be affected by the thick layer or large grains.

  The inventor of the present invention has obtained an idea that a doping process by a wet doping method is performed under mild conditions as a result of further intensive studies to uniformly dope lithium into a silicon-containing negative electrode active material. Then, a lithium-doped negative electrode active material was actually manufactured by a wet doping method under mild conditions, and a lithium-ion secondary battery was manufactured using the lithium-doped negative electrode active material. The battery characteristics of the secondary battery were improved.

That is, in the production method of the present invention, the wet dope process is performed by dropping the lithium complex solution to a negative electrode active material suspension containing a silicon-containing negative electrode active material and an organic solvent at 50 ° C. or lower. By carrying out like this, the lithium dope negative electrode active material which can suppress the non-uniform dope of lithium and can improve the battery characteristic of a lithium ion secondary battery can be manufactured.
Further, as will be described later, the lithium-doped negative electrode active material obtained by such a new doping method has a negative electrode mixture slurry that can be applied to a negative electrode current collector, with gelation suppressed even without CO ₂ treatment. It has also been found that it can be formed and has a very advantageous characteristic in manufacturing a lithium ion secondary battery.
Hereafter, the manufacturing method of this invention is demonstrated for every process.

  In the doping step in the production method of the present invention, a lithium complex solution in which metallic lithium and a polycyclic aromatic compound are dissolved in an organic solvent is brought into contact with the silicon-containing negative electrode active material. In that case, a lithium complex solution is dripped at the negative electrode active material suspension containing a silicon-containing negative electrode active material and an organic solvent. Further, the temperature of the negative electrode active material suspension at this time is set to 50 ° C. or lower.

[Doping process]
Specifically, in the wet doping method, lithium is doped into the silicon-containing negative electrode active material by bringing the lithium complex solution into contact with the silicon-containing negative electrode active material. In the lithium complex solution, lithium ions, organic solvents and polycyclic aromatic compounds are considered to form complexes. Accordingly, as the organic solvent, an aprotic organic solvent having a hetero element such as O, N, or S capable of coordinating with lithium may be selected. Further, as the polycyclic aromatic compound, a compound having a stable chemical structure capable of maintaining a radical anion may be selected. In the manufacturing method of this invention, it can be said that the stable complex solution suitable for industrial production of a lithium dope negative electrode active material can be obtained by selecting a polycyclic aromatic compound with metallic lithium and an organic solvent.

  The organic solvent should just satisfy | fill said conditions, and what is necessary is just to select the 1 type or multiple types of what is described in the nonpatent literature 1, patent document 2, patent document 8, etc.

  More specifically, examples of the organic solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, and dimethyl sulfoxide, γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-hexanolactone, and δ-octanolactone. Cyclic esters such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, butyl acetate, propyl acetate, methyl propionate, methyl formate, ethyl formate, vinyl acetate, methyl acrylate, methyl methacrylate, etc. Esters, oxetane, tetrahydrofuran, tetrahydropyran, 2-methyltetrahydrofuran, 2-methyltetrahydropyran, furan, furfural, 1,2-dioxane, 1,3-dioxane, 1,4-dioxane Sun, 2,2-dimethyl-1,3-dioxolane, cyclic ethers such as crown ether, chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, acetonitrile, Nitriles such as pionitrile, acrylonitrile, malononitrile, glutaronitrile, methoxyacetonitrile, 3-methoxypropionitrile, formamide, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone, hexamethylsulfoltri Amides such as amides may be mentioned. In addition, pyridine or the like may be used.

  As a polycyclic aromatic compound, what is necessary is just to select the 1 type or multiple types of what is described in the nonpatent literature 1, the patent document 2, and the patent document 8.

  More specifically, examples of the polycyclic aromatic compound include azulene, naphthalene, biphenylene, 1-methylnaphthalene, 2-methylnaphthalene, sapotallin and other aromatic aromatic hydrocarbons, acenaphthene, acenaphthylene, anthracene, fluorene, phenalene. , Tricyclic aromatic hydrocarbons such as phenanthrene, benzanthracene (benz [a] anthracene), benzo [a] fluorene, benzo [c] phenanthrene, chrysene (benzo [a] phenanthrene), fluoranthene, pyrene, tetracene (benz) [B] anthracene), tetracyclic aromatic hydrocarbons such as triphenylene, benzopyrene, benzo [a] pyrene, benzo [e] pyrene, benzo [a] fluoranthene, benzo [b] fluoranthene, benzo [j] fluoranthene, benzo [K] Fluo Nthene, dibenz [a, h] anthracene, dibenz [a, j] anthracene, pentacene, perylene, picene, tetraphenylene, and other 5-ring aromatic hydrocarbons, anthanthrene, 1,12-benzoperylene, circulene, corannulene , Coronene, dicolonylene, diindenoperylene, helicene, heptacene, hexacene, ketrene, ovalene, zetrene and the like aromatic hydrocarbons.

  In some cases, hydrogen of these various polycyclic aromatic hydrocarbons may be substituted with a substituent such as a methyl group. However, in order to efficiently form a complex with lithium, it can be said that it is preferable that the number of the substituents in the polycyclic aromatic compound after substitution is small. The number of the substituents in the polycyclic aromatic compound is preferably in the range of 0 to 2, more preferably in the range of 0 to 1, and particularly preferably 0.

  Furthermore, considering the efficiency of the doping process, it is considered preferable that the lithium complex solution contains a large amount of lithium. In this case, it is considered preferable to select a polycyclic aromatic compound that forms a complex with lithium, which is not bulky. Therefore, it is preferable to select a polycyclic aromatic compound having a small number of rings. For example, it is preferable to use a 2- to 4-ring aromatic compound, and it is preferable to use a 2- to 3-ring aromatic compound. It is more preferable to use a bicyclic aromatic compound.

  In the lithium complex solution, the molar ratio of metallic lithium to polycyclic aromatic compound is preferably in the range of 1: 5 to 1: 1, and in the range of 1: 3 to 1: 1. Is more preferable, and it is particularly preferable to be within the range of 1: 1.5 to 1: 1.

  The concentration of metallic lithium in the lithium complex solution is preferably in the range of 0.1 to 5 mol / L, more preferably in the range of 0.2 to 3 mol / L, and 0.5 to 2 mol / L. More preferably, it is in the range.

  As the organic solvent used for the negative electrode active material suspension, the same one as used for the lithium complex solution may be used, or a different one may be used. As the organic solvent, it is preferable to use the same organic solvent as the lithium complex solution.

The concentration of the lithium complex solution and the dropping speed are not particularly limited, but the dropping is preferably performed while stirring the negative electrode active material suspension, and it is more preferable to maintain the stirring state after the dropping for a predetermined time.
The dropwise addition of the lithium complex solution is performed while keeping the negative electrode active material suspension at 50 ° C. or lower. The temperature is preferably 40 ° C. or less, more preferably less than 25 ° C., and still more preferably 10 ° C. or less. There is no particular lower limit to the temperature, but it is reasonable to set the temperature to −10 ° C. or higher. In addition to the negative electrode active material suspension, the lithium complex solution is more preferably set to 50 ° C. or lower.

  The amount of the lithium complex solution with respect to the silicon-containing negative electrode active material is not particularly limited, but if the amount of lithium with respect to the silicon-containing negative electrode active material is too small, it is difficult to dope a sufficient amount of lithium into the silicon-containing negative electrode active material. In addition, if the amount of lithium is excessive with respect to the silicon-containing negative electrode active material, the silicon-containing negative electrode active material is thickly covered with lithium, which may make it difficult to sufficiently function as the negative electrode active material. Therefore, it is considered that there is a preferable range for the amount of the lithium complex solution with respect to the silicon-containing negative electrode active material.

  Specifically, the amount (total amount) of lithium supplied to 1 g of the silicon-containing negative electrode active material is preferably in the range of 0.01 to 0.20 g, and 0.015 to 0.15 g. It is more preferable that it is within the range, and it is considered that it is more preferable that it is within the range of 0.02 to 0.12 g. The amount of lithium supplied per minute per 1 g of the silicon-containing negative electrode active material is preferably in the range of 0.00004 to 0.007 g, and in the range of 0.00006 g to 0.005 g. It is more preferable that it is in the range of 0.00008 g to 0.004 g.

  By controlling the lithium concentration of the lithium complex solution and the dropping rate of the lithium complex solution so that lithium and the silicon-containing negative electrode active material have such a relationship, the amount of lithium doped into the silicon-containing negative electrode active material can be reduced. It can be optimized and the silicon-containing negative electrode active material can be uniformly doped with lithium.

  Through the above doping process, a lithium-doped negative electrode active material is formed in the complexed mixed liquid of the lithium complex solution and the silicon-containing negative electrode active material. The lithium-doped negative electrode active material can be separated from the complexed mixed solution by a known method such as filtration. You may wash | clean the lithium dope negative electrode active material after isolation | separation as needed. As the solvent used for cleaning, it is preferable to use the same organic solvent as used in the dope process, but various other organic solvents described above may be used.

  In the manufacturing method of the present invention, the dope process may be composed of a pretreatment process and a main dope process. The main doping process in this case is the same as the above doping process except that the silicon-containing negative electrode active material after the pretreatment process is used. Therefore, the description of this dope process is omitted in this specification.

The pretreatment step is a step of treating the silicon-containing negative electrode active material with a fluorine anion-containing solution. By treating the silicon-containing negative electrode active material with a fluorine anion-containing solution, SiO ₂ present on the surface of the silicon-containing negative electrode active material is removed. Thereby, it is estimated that the specific surface area increases on the surface of the silicon-containing negative electrode active material, and the concentration of Si contributing to the battery reaction also increases. That is, the silicon-containing negative electrode active material that has undergone the pretreatment step can impart excellent battery characteristics to the lithium ion secondary battery.

Examples of the fluorine anion-containing solution include a solution containing an acid selected from HF, HBF ₄ or HPF _6, and a solution containing an ammonium fluoride salt such as ammonium fluoride, tetramethylammonium fluoride, and tetrabutylammonium fluoride. Is exemplified. As content of the said acid or ammonium fluoride salt in a fluorine anion containing solution, 2-20 mass% is preferable, 5-15 mass% is more preferable, 7-13 mass% is further more preferable. Considering the treatment efficiency of the pretreatment step, it is particularly preferable to use hydrofluoric acid as the fluorine anion-containing solution.

  Further, the fluorine anion-containing solution may be a mixed acid solution containing an acid that supplies the fluorine anion and an acid other than the acid. An acid other than the acid supplying the fluorine anion may act as a catalyst, and the reaction rate of the pretreatment process may be increased. As an acid other than the acid supplying the fluorine anion, a strong acid is preferable. Specific examples of the strong acid include hydrogen chloride, hydrogen bromide, hydrogen iodide, nitric acid, and sulfuric acid.

  The mass ratio of the acid supplying the fluorine anion to the acid other than the acid in the mixed acid solution is preferably in the range of 1: 0.1 to 1:10, and in the range of 1: 0.5 to 1: 5. The inside is more preferable, and the range of 1: 1 to 1: 3 is more preferable.

  Examples of the solvent for the fluorine anion-containing solution include water, methanol, ethanol, propanol, 2-propanol, and tetrahydrofuran.

  From the viewpoint of suitably controlling the oxygen removal reaction by the fluorine anion, the pretreatment step is preferably performed by a method of adding a silicon-containing negative electrode active material to the fluorine anion-containing solution under stirring, and further, −10 to 10 ° C. It is preferable to be performed within the range.

  In order to isolate the silicon-containing negative electrode active material treated in the pretreatment step, a filtration step, a washing step, and a drying step may be appropriately performed. The pretreatment step is preferably performed in an inert gas atmosphere.

  The manufacturing method of the negative electrode active material of this invention can also be regarded as one process of the manufacturing method of a negative electrode. According to the method for producing a negative electrode, a negative electrode capable of imparting excellent battery characteristics to a lithium ion secondary battery can be produced by using the negative electrode active material of the present invention having at least a surface modified by a pretreatment step.

  Hereinafter, the silicon-containing negative electrode active material used in the production method of the present invention and other battery components will be described.

The silicon-containing negative electrode active material used in the production method of the present invention is literally a negative electrode active material containing silicon. This type of negative electrode active material is considered to occlude and release lithium when silicon is alloyed with lithium.
Examples of such a silicon-containing negative electrode active material include silicon simple substance (metal silicon), silicon-based materials such as SiO _x (0.3 ≦ x ≦ 1.6) disproportionate to silicon simple substance and silicon dioxide, silicon simple substance or Examples include composites in which a silicon-based material and a carbon-based material described later are combined. Furthermore, it is also preferable to use the silicon material disclosed in Patent Document 5 as the silicon-based material. Patent Document 5 discloses a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. In Patent Document 5, a layered silicon compound mainly composed of polysilane obtained by reacting CaSi ₂ and an acid to remove Ca is synthesized, and the layered silicon compound is heated at 300 ° C. or more to release hydrogen. In addition, a lithium ion secondary battery including the silicon material manufactured as an active material is described.

  As the silicon-containing negative electrode active material used in the method for producing a negative electrode active material of the present invention, the silicon material disclosed in Patent Document 5 can be preferably used. In the present specification, unless otherwise specified, the “silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction” disclosed in Patent Document 5 is simply referred to as a silicon material.

As a specific synthesis process of the silicon material, a process of obtaining a layered silicon compound mainly composed of layered polysilane obtained by reacting CaSi ₂ with an acid to remove Ca, and heating the layered silicon compound at 300 ° C. or higher. A step of synthesizing the silicon material through both steps of the heating step can be exemplified.

The raw material CaSi ₂ generally has a structure in which a Ca layer and a Si layer are laminated. CaSi ₂ may be synthesized by a known production method, or a commercially available one may be adopted. CaSi ₂ is preferably pulverized and powdered in advance.

In the step of obtaining a layered silicon compound, Si forms Si—H bonds while Ca is replaced with acid H in layered CaSi ₂ . The layered silicon compound has a layered shape because the basic skeleton of the Si layer by the raw material CaSi ₂ is maintained.

  Acids include hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, sulfuric acid, nitric acid, phosphoric acid, formic acid, acetic acid, methanesulfonic acid, tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroarsenic acid And fluoroantimonic acid, hexafluorosilicic acid, hexafluorogermanic acid, hexafluorotin (IV) acid, trifluoroacetic acid, hexafluorotitanic acid, hexafluorozirconic acid, trifluoromethanesulfonic acid, and fluorosulfonic acid. These acids may be used alone or in combination.

  In particular, it may be preferable to employ an acid capable of generating a fluorine anion as the acid. By employing the acid, Si—O bonds that can occur in the layered silicon compound and bonds between Si and anions of other acids (for example, Si—Cl bonds in the case of hydrochloric acid) can be reduced. Note that when a Si—O bond or a Si—Cl bond exists in the layered silicon compound, a Si—O bond or a Si—Cl bond may exist in the silicon material even after the next heating step.

In the step of obtaining the layered silicon compound, the acid may be used in an amount capable of supplying 2 equivalents or more of protons to CaSi ₂ . Although this step may be performed without a solvent, it is preferable to employ water as a solvent from the viewpoint of separation of a target product and removal of by-products such as CaCl ₂ . The reaction conditions in this step are preferably reduced pressure conditions such as vacuum or an inert gas atmosphere, and are preferably temperature conditions of room temperature or lower such as an ice bath. Examples of the temperature range in this step include −40 to 20 ° C., −20 to 15 ° C., and −10 to 10 ° C. What is necessary is just to set the reaction time of the same process suitably.

The chemical reaction in the step of obtaining the layered silicon compound when hydrochloric acid is used as the acid is represented by the following ideal reaction formula.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂

In the above reaction formula, Si ₆ H ₆ corresponds to an ideal layered silicon compound.

The step of obtaining the layered silicon compound is preferably performed in the presence of water. Since Si ₆ H ₆ can react with water, the layered silicon compound is rarely obtained only with a compound of Si ₆ H ₆ , and usually contains an element derived from oxygen or an acid.

  After the step of obtaining the layered silicon compound, it is preferable to appropriately perform a filtration step for filtering out the layered silicon compound, a washing step for washing the layered silicon compound, and a drying step for drying the layered silicon compound as necessary.

Next, a heating process for heating the layered silicon compound at 300 ° C. or higher will be described. This step is a step of obtaining a silicon material by removing hydrogen, water, etc. from the layered silicon compound. The chemical reaction of this step is represented by an ideal reaction formula as follows.
Si ₆ H ₆ → 6Si + 3H ₂ ↑

  However, since the layered silicon compound actually used in the heating process contains oxygen and acid-derived elements and also contains unavoidable impurities, the actually obtained silicon material also contains oxygen and acid-derived elements and is unavoidable. It also contains impurities.

  The heating step is preferably performed in a non-oxidizing atmosphere having a lower oxygen content than in normal air. Examples of the non-oxidizing atmosphere include a reduced-pressure atmosphere including a vacuum and an inert gas atmosphere. The heating temperature is preferably within the range of 300 ° C to 1000 ° C, more preferably within the range of 500 ° C to 900 ° C, and even more preferably within the range of 600 ° C to 900 ° C. If the heating temperature is too low, hydrogen may not be sufficiently released. On the other hand, if the heating temperature is too high, crystallization of silicon in the silicon material proceeds excessively, and therefore, when the silicon material is used as the negative electrode active material, there is a risk of performance degradation. What is necessary is just to set a heating time suitably according to heating temperature, and it is also preferable to determine a heating time, measuring the quantity of hydrogen etc. which escapes out of a reaction system. By appropriately selecting the heating temperature and the heating time, the ratio of amorphous silicon and silicon crystallites contained in the silicon material to be manufactured, and the size of the silicon crystallites can be adjusted. The shape and size of a nano-level layer including amorphous silicon and silicon crystallites included in the silicon material can also be adjusted.

  The size of the silicon crystallite is preferably nano-sized. Specifically, the silicon crystallite size is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still more preferably in the range of 1 nm to 50 nm, and particularly in the range of 1 nm to 10 nm. preferable. Note that the silicon crystallite size is calculated from the Scherrer equation using X-ray diffraction measurement (XRD measurement) on the silicon material and using the half-value width of the diffraction peak of the Si (111) plane of the obtained XRD chart. The

By the heating step, a silicon material having a structure in which a plurality of plate-like silicon bodies are stacked in the thickness direction can be obtained. This structure can be confirmed by observation with a scanning electron microscope or the like. Considering the case where a silicon material is used as a negative electrode active material of a lithium ion secondary battery, the plate-like silicon body has a thickness in the range of 10 nm to 100 nm for efficient insertion and desorption reaction of lithium ions. Are preferable, and those within the range of 20 nm to 50 nm are more preferable. The length of the plate-like silicon body in the major axis direction is preferably in the range of 0.1 μm to 50 μm. The plate-like silicon body preferably has a (length in the major axis direction) / (thickness) range of 2 to 1000. The laminated structure of the plate-like silicon body is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

The particle size of the silicon material is not particularly limited. However, the smaller the average particle size of the silicon material, the greater the effect of the pretreatment process. Further, if the average particle size of the silicon material is too small, the effect of the pretreatment process becomes excessive, and it may be difficult to maintain a sufficient function as the negative electrode active material.
In order to effectively perform the above pretreatment step, the average particle size of the silicon material is preferably in the range of 1 to 50 μm, more preferably in the range of 1.5 to 25 μm, More preferably, it is in the range of 10 μm.

  By the way, the silicon-containing negative electrode active material typified by the above silicon material is considered to be inferior in conductivity as compared with other negative electrode active materials such as graphite. Therefore, the silicon-containing negative electrode active material is generally carbon coated and used for the negative electrode. However, as will be described later, one embodiment of the negative electrode active material obtained by the production method of the present invention can improve battery characteristics typified by the initial discharge capacity and capacity retention rate without performing carbon coating.

  By the way, in the case of using a finely divided silicon-containing negative electrode active material, if carbon coating is performed by a thermal CVD method, aggregation of the silicon-containing negative electrode active material occurs during the carbon coating, and the battery of the lithium ion secondary battery The characteristics may deteriorate. Accordingly, when a finely divided silicon-containing negative electrode active material is used in the production method of the present invention, it is particularly preferable not to perform carbon coating by a thermal CVD method. In this specification, the thermal CVD method means a method in which a silicon-containing negative electrode active material and a carbon source gas are heated to a temperature higher than the carbonization temperature of the carbon source gas to form a carbon coat layer. In addition, the fine particle here means specifically that the average particle diameter of the silicon-containing negative electrode active material is 2.8 μm or less, 2.6 μm or less, 2.4 μm or less, 2.2 μm or less, 2.0 μm or less. Means within each range.

  On the other hand, when a silicon-containing negative electrode active material having a relatively large average particle size is used in the production method of the present invention, a lithium ion secondary battery excellent in battery characteristics can be obtained even if carbon coating is performed. Accordingly, when carbon coating is performed in the production method of the present invention, it is preferable to use a silicon-containing negative electrode active material having a relatively large average particle size. Specifically, when carbon coating is performed, a preferable range of the average particle diameter of the silicon-containing negative electrode active material is 3.2 μm or more, 3.4 μm or more, 3.6 μm or more, 3.8 μm or more, 4.0 μm or more. Each range is mentioned.

  The average particle size of the negative electrode active material of the present invention is not particularly limited, but is preferably in the range of 0.5 μm to 10.0 μm, more preferably in the range of 1.0 μm to 8.0 μm. It is particularly preferable that it is in the range of 0.5 μm to 6.0 μm.

  The negative electrode active material of the present invention obtained by the production method of the present invention contains silicon and lithium. The lithium content in the negative electrode active material of the present invention is not particularly limited, but is preferably in the range of 1.0% to 20.0% by mass percentage, and in the range of 1.5% to 12.0%. It is more preferable that it is in the range of 2.0% to 8.0%.

  As described above, according to the manufacturing method of the present invention, lithium is wet-doped under mild conditions. For this reason, the negative electrode active material of the present invention obtained by the production method of the present invention is uniformly doped with lithium and has a smooth surface. The surface roughness of the negative electrode active material of the present invention is preferably less than 0.01 μm, preferably in the range of 0.001 μm to 0.008 μm, and in the range of 0.003 μm to 0.006 μm. Is particularly preferred.

The negative electrode active material of the present invention can be used for a negative electrode for a lithium ion secondary battery. The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer contains the negative electrode active material of the present invention.
In the negative electrode active material layer, the negative electrode active material of the present invention in which the silicon-containing negative electrode active material is lithium-doped and other negative electrode active materials not containing silicon may be used in combination. Other negative electrode active materials include general negative electrode active materials, that is, group 14 elements such as carbon, germanium, and tin, group 13 elements such as aluminum and indium, group 12 elements such as zinc and cadmium, antimony, bismuth, and the like. A group 15 element such as magnesium, alkaline earth metals such as magnesium and calcium, and at least one simple substance, compound or alloy of group 11 elements such as silver and gold may be selected. Specific examples of the alloy or compound include tin-based materials such as Ag—Sn alloy, Cu—Sn alloy, and Co—Sn alloy, and carbon-based materials such as various graphites. Further, as the negative electrode active _{material, Nb 2 O 5, TiO 2} , Li 4 Ti 5 O 12, WO 2, MoO 2, Fe 2 O 3 oxide such, _{or, Li 3-x M x N} (M = Co Nitride represented by Ni, Cu) may be employed. In the negative electrode active material layer, the negative electrode active material of the present invention is essential, and one or more of these other negative electrode active materials can be used. A plurality of silicon-containing negative electrode active materials may be used in the negative electrode active material of the present invention. In addition, when using together multiple types of silicon-containing negative electrode active material, the quantity of the silicon-containing negative electrode active material used as a reference | standard by a dope process becomes the sum of the said multiple types of silicon-containing negative electrode active material. Moreover, the quantity of the negative electrode active material used as a reference | standard by a dope process is a silicon-containing negative electrode active material to the last, and excludes about the negative electrode active material which does not contain silicon.

  In addition to the negative electrode active material and the binder of the present invention, the negative electrode active material layer can appropriately contain an additive such as a conductive additive in an appropriate amount as necessary. Similarly, since the positive electrode active material layer in the lithium ion secondary battery can contain an appropriate amount of additives in addition to the positive electrode active material described later, in the following section, the negative electrode active material layer and the positive electrode active material layer Is comprehensively described. Hereinafter, if necessary, the negative electrode and the positive electrode are collectively referred to as an electrode, the negative electrode active material and the positive electrode active material are collectively referred to as an active material, and the negative electrode active material layer and the positive electrode active material layer are collectively referred to as an active material layer. That's it.

  The binder plays a role of securing an active material layer composition such as an active material to the surface of the current collector and maintaining a conductive network in the electrode. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, poly ( Examples thereof include acrylic resins such as (meth) acrylic acid, styrene-butadiene rubber (SBR), and carboxymethyl cellulose. These binders may be used singly or in plural.

  In addition, as the binder, a polymer having a hydrophilic group such as a carboxyl group may be employed. By blending a polymer having a hydrophilic group as a binder, more excellent battery performance can be imparted to the negative electrode of the present invention having the negative electrode active material of the present invention. Examples of the polymer having a hydrophilic group include carboxyl group-containing polymers such as polyacrylic acid and polymethacrylic acid. As the binder, a cross-linked polymer obtained by cross-linking these carboxyl group-containing polymers with diamine, and a mixture or a reaction product of a carboxyl group-containing polymer and a polyamideimide are preferable. This type of binder can be produced and used by the method disclosed in WO2016 / 063882.

  The blending ratio of the binder to the active material layer is preferably a mass ratio of active material: binder = 1: 0.001 to 1: 0.3, and 1: 0.005 to 1: 0. .2 is more preferable, and 1: 0.01 to 1: 0.15 is still more preferable. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be added arbitrarily when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent. The conductive auxiliary agent may be any chemically inert electronic high conductor, and examples thereof include carbon black, graphite, Vapor Grown Carbon Fiber, and various metal particles. The Examples of carbon black include acetylene black, ketjen black (registered trademark), furnace black, and channel black. These conductive assistants can be added to the active material layer alone or in combination of two or more.

  The blending ratio of the conductive additive to the active material layer is preferably a mass ratio of active material: conductive additive = 1: 0.005 to 1: 0.5, and 1: 0.01 to 1: 0. .2 is more preferable, and 1: 0.03 to 1: 0.1 is even more preferable. This is because if the amount of the conductive auxiliary is too small, an efficient conductive path cannot be formed, and if the amount of the conductive auxiliary is too large, the moldability of the active material layer is deteriorated and the energy density of the electrode is lowered.

  A current collector refers to a chemically inert electronic conductor that continues to pass current through an electrode during battery discharge or charge. As the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel, etc. Metal materials can be exemplified. The current collector may be covered with a known protective layer. What collected the surface of the electrical power collector by the well-known method may be used as an electrical power collector.

  The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

  As a method of forming the active material layer on the surface of the current collector, a conventionally known method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method can be used. The electrode mixture slurry may be applied to the surface of the electric body. The slurry is applied to the surface of the current collector and then dried. In order to increase the electrode density, the dried product may be compressed.

  The negative electrode of the present invention can constitute a battery such as a lithium ion secondary battery together with the positive electrode, the electrolytic solution, and, if necessary, a separator. As the positive electrode, the electrolytic solution, the separator, etc., those manufactured by a conventional method may be used.

It does not specifically limit about the positive electrode active material used for a positive electrode, The general positive electrode active material for lithium ion secondary batteries can be used.
As a general positive electrode active material for a lithium ion secondary battery, a general formula of a layered rock salt structure: Li _a Ni _b Co _c Mn _{d De} O _f (0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, At least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, 1.7 ≦ f ≦ 3) And lithium composite metal oxide, Li ₂ MnO ₃ . Further, as a positive electrode active material, a spinel structure metal oxide such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a spinel structure metal oxide and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe). Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element contained in the basic composition may be substituted with another metal element.
The positive electrode can be produced in the same manner as the negative electrode of the present invention described above, using the positive electrode active material described above.

  The negative electrode of the present invention may be placed in a battery container together with a positive electrode and a known separator as necessary, and a lithium ion secondary battery may be injected by injecting an electrolytic solution.

The electrolytic solution includes an organic solvent and a lithium salt dissolved in the organic solvent.
As the organic solvent, cyclic esters, chain esters, ethers and the like can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. In the electrolytic solution, these organic solvents may be used alone or in combination. The organic solvent used for the electrolytic solution may be the same as the organic solvent used for the dope process, or may be different. In consideration of bringing the organic solvent used in the dope process into the lithium ion secondary battery, the organic solvent used in the electrolytic solution and the organic solvent used in the dope process are preferably the same.

Examples of the lithium salt include LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) _{2 and the} like.

As an electrolytic solution, an organic solvent such as ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, and dimethyl carbonate, and a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ from 0.5 mol / l. A solution dissolved at a concentration of about 1.7 mol / l can be exemplified.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art. Moreover, each component shown in this specification including the embodiment and the following examples can be arbitrarily extracted and combined.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. In addition, this invention is not limited by these Examples.

Example 1
(Method for producing negative electrode active material)
(Silicon material manufacturing process)
50 g of CaSi ₂ (Ca content: 38.9% by mass) was added to 507 g of a 35% by mass HCl aqueous solution in an ice bath and stirred under an argon gas atmosphere. The molar ratio of CaSi ₂ to HCl was 1:10. After confirming that foaming disappeared from the reaction solution, the mixture was further stirred for 3 hours under the same conditions. Thereafter, the reaction solution was heated to room temperature and filtered. The filtration residue was washed with 300 ml of distilled water three times, then with 300 ml of ethanol, and dried under reduced pressure to separate the layered silicon compound containing polysilane. The layered silicon compound was heated at 900 ° C. for 1 hour in an argon gas atmosphere with an O ₂ content of 1% by volume or less to obtain a silicon material.

This silicon material was pulverized with a jet mill NJ-30 (Aisin Nano Technologies). The average particle diameter D50 of the silicon material particles after pulverization by the jet mill was 4.335 μm. Note that D10 of the silicon material was 1.404 μm, and D90 was 9.508 μm.
In this specification, D50, D10, and D90 mean D50, D10, and D90, respectively, when measured with a general laser diffraction particle size distribution measuring apparatus. Moreover, an average particle diameter means D50.

(Dope process)
The above silicon material was used for the doping process as follows.
In a glove box, 6.46 g of naphthalene was dissolved in 44.8 g of dehydrated tetrahydrofuran, 0.35 g of lithium metal was added, and the mixture was stirred at room temperature (25 ° C.) for 3 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.05 mol.

  The above lithium complex solution was placed in a dropping funnel. Separately, 7.0 g of silicon material and 44.8 g of dehydrated tetrahydrofuran were placed in a flask to form a negative electrode active material suspension, and the flask was attached to a dropping funnel. The lithium complex solution was dropped into the flask over 3 hours while stirring the negative electrode active material suspension in the flask at 0 ° C. in an argon gas atmosphere. After completion of dropping, the mixture in the flask was further stirred for 15 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material. The negative electrode active material of Example 1 was obtained through the above steps.

  In addition, in the dope process of Example 1, the density | concentration and dripping speed | rate of the lithium complex solution were controlled so that 0.050028g of lithium might be supplied per 1g with respect to 1g of silicon materials.

  The average particle diameter of the negative electrode active material of Example 1 was 4.354 μm, and aggregation and granulation due to lithium doping were not observed.

(Example 2)
A negative electrode active material of Example 2 was obtained in the same manner as in Example 1 except that the lithium concentration of the lithium complex solution was different and that the dropping in the doping process was performed at room temperature.
Details of the dope process in Example 2 will be described below.

(Dope process)
The above silicon material was used for the doping process as follows.
In a glove box, 9.2 g of naphthalene was dissolved in 32.0 g of dehydrated tetrahydrofuran, 0.5 g of metallic lithium was added, and the mixture was stirred at room temperature for 3 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.07 mol.

  The above lithium complex solution was placed in a dropping funnel. Separately, 10.0 g of silicon material and 64.1 g of dehydrated tetrahydrofuran were placed in a flask to form a negative electrode active material suspension, and the flask was attached to a dropping funnel. The lithium complex solution was dropped into the flask over 3 hours while stirring the negative electrode active material suspension in the flask at room temperature (25 ° C.) under an argon gas atmosphere. After completion of dropping, the mixture in the flask was further stirred for 15 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material. Through the above steps, the negative electrode active material of Example 2 was obtained.

  In addition, in the dope process of Example 2, the density | concentration and dripping speed | rate of the lithium complex solution were controlled so that 0.050028g of lithium might be supplied per 1g with respect to 1g of silicon materials.

Example 3
Except that SiO was used as the silicon-containing negative electrode active material, and the lithium complex solution was dropped while stirring the negative electrode active material suspension at 0 ° C., the same doping step as in Example 2 was performed. The negative electrode active material of Example 3 was obtained.
Therefore, in the doping step of Example 3, the concentration and dropping rate of the lithium complex solution are controlled so that 0.05 g of lithium is supplied at a rate of 0.00028 g per minute per 1 g of the silicon-containing negative electrode active material. It was.

(Comparative Example 1)
A negative electrode active material of Comparative Example 1 was produced in the same manner as in Example 1 except that the dope process was not performed. The negative electrode active material of Comparative Example 1 is the same as the negative electrode active material before the doping step of Example 1, that is, the silicon material used in Example 1.

(Comparative Example 2)
In Comparative Example 2, the dropping in the doping process was performed at 60 ° C., and the CO ₂ treatment process was performed after the doping process. Details of the dope process and the CO ₂ treatment process in Comparative Example 2 will be described below.
(Dope process)
In a glove box, 3.7 g of naphthalene was dissolved in 12.8 g of dehydrated tetrahydrofuran, 0.2 g of metallic lithium was added, and the mixture was stirred at room temperature for 2 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.03 mol.

  The above lithium complex solution was placed in a dropping funnel. Separately, 2.0 g of the same silicon material as used in Example 1 and 12.8 g of dehydrated tetrahydrofuran were placed in a flask to form a negative electrode active material suspension, and the flask was attached to a dropping funnel. The lithium complex solution was dropped into the flask over 1.5 hours while stirring the negative electrode active material suspension in the flask at 60 ° C. in an argon gas atmosphere. After completion of dropping, the mixture in the flask was further stirred for 19 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material.

  In the doping process of Comparative Example 2, the concentration and dropping rate of the lithium complex solution were controlled such that 0.1 g of lithium was supplied at a rate of 0.00056 g per minute per 1 g of silicon material.

(CO ₂ treatment process)
The lithium-doped negative electrode active material obtained in the doping step was left in CO ₂ gas for 2 hours. The negative electrode active material of Comparative Example 2 was obtained through the above steps.

(Comparative Example 3)
In Comparative Example 3, in the dope process, the lithium complex solution was not dropped into the negative electrode active material suspension, but the silicon material was added to the lithium complex solution all at once, and the temperature at this time was room temperature. Also, it was CO ₂ treatment step after Comparative Example 3, doping step.
Details of the doping process and the CO ₂ treatment process in Comparative Example 3 will be described below.

(Dope process)
In a glove box, 0.74 g of naphthalene was dissolved in 10.2 g of dehydrated tetrahydrofuran, 0.04 g of metallic lithium was added, and the mixture was stirred at room temperature for 4 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.006 mol.

  To the above lithium complex solution, 0.82 g of the same silicon material as in Example 1 was added and stirred at room temperature for 19 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material.

(CO ₂ treatment process)
The lithium-doped negative electrode active material obtained in the doping step was left in CO ₂ gas for 1 hour. The negative electrode active material of Comparative Example 3 was obtained through the above steps.

(Comparative Example 4)
In Comparative Example 4, as in Comparative Example 3, a silicon material was added to the lithium complex solution at one time at room temperature in the doping step, and a CO ₂ treatment step was performed after the doping step.
Details of the dope process and the CO ₂ treatment process in Comparative Example 4 will be described below.
(Dope process)
In a glove box, 5.3 g of naphthalene was dissolved in 17.8 g of dehydrated tetrahydrofuran, 0.29 g of metallic lithium was added, and the mixture was stirred at room temperature for 2 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.04 mol.
To this lithium complex solution, 0.82 g of the same silicon material as used in Example 1 was added to obtain a complexed mixed solution. The complexed mixture was stirred at room temperature for 80 hours, then filtered, and the filter residue was washed with tetrahydrofuran. The filtered residue after washing was dried under reduced pressure to obtain a lithium-doped negative electrode active material.
(CO ₂ treatment process)
The lithium-doped negative electrode active material obtained in the doping step was left in CO ₂ gas for 2 hours. The negative electrode active material of Comparative Example 4 was obtained through the above steps.

(Comparative Example 5)
SiO before the doping step was used as the negative electrode active material of Comparative Example 5.

(Analysis of negative electrode active material 1)
(1. Component analysis)
Using a multi-type ICP emission spectroscopic analyzer ICPE-9820 (manufactured by Shimadzu Corporation), the lithium amounts of the lithium-doped negative electrode active materials of Examples 1 to 3 and Comparative Examples 2 to 4 were measured. In Comparative Example 3 and Comparative Example 4 was a sample of that before CO ₂ process. The analysis results are shown in Table 1.

  Each of the negative electrode active materials of Examples 1 to 3 and Comparative Examples 2 to 4 in which the doping process was performed was doped with lithium.

(2. Surface observation)
The lithium-doped negative electrode active materials of Example 1, Comparative Example 2, and Comparative Example 3 were observed with a SEM (scanning electron microscope). The SEM image of the lithium doped negative electrode active material of Example 1 is shown in FIG. 1, the SEM image of the lithium doped negative electrode active material of Comparative Example 3 is shown in FIG. 2, and the SEM image of the lithium doped negative electrode active material of Comparative Example 2 is shown. 3 shows.

As shown in FIGS. 1 to 3, the surface of the lithium-doped negative electrode active material of Comparative Example 2 and Comparative Example 3 has a white portion and a dark portion that are biased, and the contrast between them is conspicuous. In contrast, the surface of the lithium-doped negative electrode active material of Example 1 appears uniform gray.
It is thought that what looks white on the surface of each lithium dope negative electrode active material shown by FIGS. 1-3 is lithium. It is considered that a region with a large amount of lithium and a region with a very small amount of lithium are present on the surfaces of the lithium-doped negative electrode active materials of Comparative Examples 2 and 3. That is, it is considered that the lithium distribution on the surfaces of the lithium-doped negative electrode active materials of Comparative Example 2 and Comparative Example 3 is uneven. On the other hand, in Example 1 that appears to be uniform gray, it is considered that lithium is uniformly dispersed over the entire surface of the lithium-doped negative electrode active material.

  The main difference between Example 1 and Comparative Example 3 is whether or not the coating process was performed by dripping, and the main difference between Example 1 and Comparative Example 2 was whether or not the coating process was performed at 50 ° C. or less. It is. From this result, it can be said that a lithium-doped negative electrode active material in which lithium is uniformly doped can be obtained by performing the coating process by dropping and performing the coating process at 50 ° C. or lower.

(3. Surface roughness)
3D-SEM images of the lithium-doped negative electrode active material of Example 1 and the lithium-doped negative electrode active material of Comparative Example 3 were taken, and surface curves thereof were obtained. The surface curve of the lithium-doped negative electrode active material of Example 1 is shown in FIG. 4, and the surface curve of the lithium-doped negative electrode active material of Comparative Example 3 is shown in FIG.

  4 and 5, the surface curve of the lithium-doped negative electrode active material of Example 1 was relatively gentle, whereas the surface curve of the lithium-doped negative electrode active material of Comparative Example 3 was relatively uneven. It was a big one. The arithmetic average roughness Ra of the surface of the lithium-doped negative electrode active material of Example 1 is calculated based on FIG. 4, and the arithmetic average roughness Ra of the surface of the lithium-doped negative electrode active material of Comparative Example 3 is calculated based on FIG. did. As a result, the average roughness Ra of the surface of the lithium-doped negative electrode active material of Example 1 was 0.005 μm, and the average roughness Ra of the surface of the lithium-doped negative electrode active material of Comparative Example 3 was 0.01 μm.

  From the above results, when the average roughness Ra of the surface of the lithium-doped negative electrode active material is less than 0.01 μm, it can be considered that the bias of lithium dope in the lithium-doped negative electrode active material is small. Moreover, it can be considered that the lithium-doped negative electrode active material is more uniformly doped with lithium as the average roughness Ra is smaller. Specifically, the average roughness Ra of the surface of the lithium-doped negative electrode active material is preferably in the range of 0.001 μm to 0.008 μm, and more preferably in the range of 0.001 μm to 0.007 μm. Preferably, it can be said that it is still more preferable that it exists in the range of 0.003 micrometer-0.006 micrometer.

(Negative electrode manufacturing process)
A negative electrode mixture slurry was produced by mixing the negative electrode active material of Example 1, a solvent, and a binder.
Specifically, 72.5 parts by mass of the negative electrode active material of Example 1, 13.5 parts by mass of acetylene black as a conductive additive, 14 parts by mass of polyamideimide as a binder, and an appropriate amount of N-methyl-as a solvent 2-Pyrrolidone was mixed to obtain a negative electrode mixture slurry.
An electrolytic copper foil having a thickness of 20 μm was prepared as a negative electrode current collector. The negative electrode mixture slurry was applied to the surface of the copper foil using a doctor blade and dried to form a negative electrode mixture layer on the negative electrode current collector. Thereafter, the current collector and the negative electrode composite material layer were firmly and closely joined by a roll press. This was heated and dried at 240 ° C. for 2 hours under reduced pressure to produce the negative electrode of Example 1 in which the negative electrode active material layer was formed on the negative electrode current collector.

(Method for producing lithium ion secondary battery)
A lithium ion secondary battery (half cell) was manufactured using the negative electrode obtained in the negative electrode manufacturing step as an evaluation electrode. A metal lithium foil having a thickness of 500 μm was used as the counter electrode.
The counter electrode was cut to φ14 mm and the evaluation electrode was cut to φ11 mm, and a glass filter manufactured by Hoechst Celanese and Celgard 2400 manufactured by Celgard were interposed as separators between the electrodes. This electrode body was accommodated in a CR2032-type coin battery member manufactured by Hosen Co., Ltd., which is a battery case. Then, a nonaqueous electrolytic solution in which LiPF ₆ was dissolved at a concentration of 1M was injected into a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1, and the battery case was sealed, and the battery case was sealed. The lithium ion secondary battery of Example 1 was obtained.

  Using each negative electrode active material of Example 2, Example 3 and Comparative Examples 1 to 5, each negative electrode and lithium of Example 2, Example 3 and Comparative Examples 1 to 5 were the same as Example 1. An ion secondary battery was manufactured.

(Evaluation of lithium ion secondary battery 1)
About each lithium ion secondary battery of Example 1- Example 3 and Comparative Example 1- Comparative Example 5, it charges until the voltage with respect to the counter electrode of an evaluation electrode will be 0.01V at 25 degreeC and electric current 0.2mA, and then Discharge was performed at 25 ° C. and a current of 0.2 mA until the voltage with respect to the counter electrode of the evaluation electrode became 1V. The discharge capacity at this time was defined as the initial discharge capacity. Further, (initial discharge capacity / initial charge capacity) × 100 was calculated as the initial efficiency. In addition, letting the evaluation electrode occlude Li is referred to as charging, and discharging Li from the evaluation electrode is referred to as discharging.
Next, for each of the lithium ion secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 5, 1V-0.01V is charged to 25V at a current of 0.5 mA up to 0.01V and discharged to 1V. 50 charge / discharge cycles were performed. For each lithium ion secondary battery after 50 cycles, the discharge capacity was measured under the same conditions as the measurement of the initial discharge capacity. The discharge capacity at this time was calculated as the post-cycle discharge capacity, and (post-cycle discharge capacity / initial discharge capacity) × 100 was calculated as the capacity retention rate (%). The results are shown in Table 2.

  As shown in Table 2, in the lithium ion secondary batteries of Examples 1 to 3, high initial discharge capacity, initial efficiency, and high capacity retention ratio were compatible. This is because in the lithium ion secondary battery of each example, when producing the negative electrode active material, lithium doping was performed by dropping in the doping process, and the doping process was performed at a temperature lower than 60 ° C. it is conceivable that. Moreover, when Example 1 and Example 2 are compared, it can be said that the temperature of the dope process is preferably lower in order to improve the initial discharge capacity and the capacity retention rate.

  Further, in the doping process of Example 1 and Example 2, 0.05 g of lithium was supplied at a rate of 0.00028 g per minute with respect to 1 g of the silicon-containing negative electrode active material. On the other hand, in the doping process of Comparative Example 2, 0.1 g of lithium was supplied to 1 g of the silicon-containing negative electrode active material at a rate of 0.00056 g per minute. The lithium ion secondary battery of Comparative Example 2 is greatly inferior in terms of the initial discharge capacity, the initial efficiency, and the capacity maintenance rate as compared with the lithium ion secondary battery of Example 1 and the lithium ion secondary battery of Example 2. Therefore, it is considered that the supply amount and supply speed of lithium in the dope process also affect the initial discharge capacity, initial efficiency, and capacity retention rate of the lithium ion secondary battery. That is, in the doping step, less than 0.1 g of lithium is supplied to 1 g of the silicon-containing negative electrode active material, and the amount of lithium supplied per minute to 1 g of the silicon-containing negative electrode active material is 0.00. It is thought that it is good to carry out at the speed | velocity | rate which becomes less than 00005g. Preferably, the amount of lithium supplied to 1 g of silicon-containing negative electrode active material in the doping step is 0.07 g or less, and the amount of lithium supplied per minute to 1 g of silicon-containing negative electrode active material. Is considered to be supplied at a speed of 0.00039 g or less.

  In addition, the above-mentioned 2. In view of the results of surface observation, the silicon-containing negative electrode active material is uniformly doped with lithium by performing a dripping step at a low temperature. As a result, the initial discharge capacity and capacity retention rate of the lithium ion secondary battery are improved. Is considered to be realized.

  Next, the influence of the silicon material particle size and the silicon coating of the silicon material on the lithium doping was examined by the following examples and comparative examples.

Example 4
(Method for producing negative electrode active material)
(Silicon material manufacturing process)
The silicon material produced in the same manner as in Example 1 was pulverized with a jet mill NJ-30 (Aisin Nano Technologies, Inc.). The average particle diameter D50 of the silicon material particles of the cyclone recovered product after pulverization by the jet mill is 1.912 μm, and it can be said that the silicon material is a finely divided silicon material. In addition, D10 of the silicon material was 0.795 μm, and D90 was 3.875 μm.

(Dope process)
The above silicon material was used for the doping process as follows.
In a glove box, 11.54 g of naphthalene was dissolved in 40.0 g of dehydrated tetrahydrofuran, 0.63 g of metallic lithium was added, and the mixture was stirred at room temperature for 3 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.09 mol.

  The above lithium complex solution was placed in a dropping funnel. Separately, 10.0 g of silicon material and 80.1 g of dehydrated tetrahydrofuran were placed in a flask to form a negative electrode active material suspension, and the flask was attached to a dropping funnel. The lithium complex solution was dropped into the flask over 2 hours 30 minutes while stirring the negative electrode active material suspension in the flask at 0 ° C. in an argon gas atmosphere. After completion of dropping, the mixture in the flask was further stirred for 15 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material. Through the above steps, the negative electrode active material of Example 4 was obtained.

  In addition, in the dope process of Example 4, the density | concentration and dripping speed | rate of the lithium complex solution were controlled so that 0.063g lithium might be supplied to 1g silicon material at a rate of 0.00042g per minute.

  The average particle diameter of the negative electrode active material of Example 4 was 1.934 μm.

(Comparative Example 6)
A negative electrode active material of Comparative Example 6 was produced in the same manner as in Example 4 except that the dope process was not performed. The negative electrode active material of Comparative Example 6 is the same as the negative electrode active material before the doping step of Example 4, that is, the silicon material used in Example 4.

(Comparative Example 7)
The same fine particulate silicon material as used in Example 4 was used, and carbon coating was performed by a thermal CVD method to obtain a negative electrode active material of Comparative Example 7. The detail of the manufacturing method of the negative electrode active material of the comparative example 7 is demonstrated below.
(Carbon coating process)
Using a silicon material having an average particle diameter of 1.912 μm obtained in the same silicon material production process as in Example 4, carbon coating was performed by thermal CVD as follows.
5.07 g of silicon material was placed in a carbon container and placed in a rotary kiln type heating furnace. The inside of the furnace was replaced with argon gas until the oxygen concentration became 0.95% or less. After replacement with argon gas, the temperature in the furnace was raised to 900 ° C. over 1 hour while flowing argon gas at a flow rate of 1.6 L / min, and then cooled to 880 ° C. over 15 minutes. While maintaining the temperature in the furnace at 880 ° C., a mixed gas of propane gas having a flow rate of 0.4 L / min and argon gas having a flow rate of 1.6 L / min was passed through the furnace for 30 minutes.
The negative electrode active material of Comparative Example 7 was obtained through the above steps. The negative electrode active material of Comparative Example 7 is a carbon-coated finely divided silicon material.

  The average particle diameter of the negative electrode active material of Comparative Example 7 was 2.459 μm.

(Analysis of negative electrode active material 2)
Similarly to Analysis 1 of the negative electrode active material described above, the amount of lithium in the negative electrode active material of Example 4 was measured. As a result, the negative electrode active material of Example 4 was doped with lithium.

(Evaluation of lithium ion secondary battery 2)
Using the negative electrode active materials of Example 4, Comparative Example 6 and Comparative Example 7, the negative electrodes and lithium ion secondary batteries of Example 4, Comparative Example 6 and Comparative Example 7 were produced in the same manner as Example 1.
For the lithium ion secondary batteries of Example 4, Comparative Example 6 and Comparative Example 7, the initial discharge capacity was measured in the same manner as in Evaluation 1 of the lithium ion secondary battery. Moreover, about the lithium ion secondary battery of Example 4 and Comparative Example 7, the capacity | capacitance maintenance factor was measured similarly to evaluation 1 of said lithium ion secondary battery. The results of evaluation 2 of the lithium ion secondary battery are shown in Tables 3 and 4 together with the results of Analysis 2 of the negative electrode active material. Tables 3 and 4 also show the results of Example 1.

The lithium ion secondary battery of Example 1 was obtained by supplying 0.05 g of lithium to 1 g of the silicon-containing negative electrode active material at a rate of 0.00028 g per minute in the doping step. It is. On the other hand, the lithium ion secondary battery of Example 4 supplies 0.063 g of lithium to 1 g of silicon-containing negative electrode active material at a rate of 0.00042 g per minute in the doping step. It was obtained. That is, the negative electrode active material of Example 4 has a larger lithium doping amount and doping rate than the negative electrode active material of Example 1.
However, as shown in Table 3, the lithium ion secondary battery of Example 4 using the negative electrode active material of Example 4 has the initial discharge capacity, the initial efficiency, and the capacity retention rate. Compared to the ion secondary battery, it showed the same performance. Rather, the capacity retention rate of the lithium ion secondary battery of Example 4 was superior. Therefore, it can be said that the battery characteristics of the lithium ion secondary battery can be further improved when a finely divided silicon-containing negative electrode active material is used in the production method of the present invention.

Incidentally, as described above, since the resistance of the silicon-containing negative electrode active material is relatively large, it is common to use the silicon-containing negative electrode active material as a negative electrode after carbon coating. However, as shown in Table 4, in Comparative Example 7 in which carbon coating was performed on a fine particle silicon material having an average particle size of 1.9 μm by a thermal CVD method, the average particle size after carbon coating was 2.5 μm. It was increasing. The initial discharge capacity of the lithium ion secondary battery of Comparative Example 7 is significantly lower than the initial discharge capacity of the lithium ion secondary battery of Comparative Example 6 using the silicon material before carbon coating as the negative electrode active material. It was.
The result is that the silicon-containing negative electrode active material is agglomerated or granulated by carbon-coating a fine particulate silicon-containing negative electrode active material by a thermal CVD method. As a result, the battery of the lithium ion secondary battery Suggests a negative impact on performance.

  On the other hand, the lithium of Example 1 in which such a finely divided silicon material or a silicon material having a relatively large average particle diameter was used as a negative electrode active material by carbon doping without lithium coating by the doping process of the present invention. The ion secondary battery and the lithium ion secondary battery of Example 4 exhibited excellent initial discharge capacity and capacity retention rate. From this result, when lithium doping is performed by the production method of the present invention, a lithium ion secondary battery having excellent battery characteristics can be obtained without performing carbon coating regardless of the particle diameter of the silicon-containing negative electrode active material. I can say that.

As will be described later, when lithium doping is performed by the production method of the present invention using a silicon-containing negative electrode active material having a relatively large average particle size, a negative electrode that can exhibit excellent battery characteristics even when carbon-coated. An active material is obtained. On the other hand, as described above, when a finely divided silicon-containing negative electrode active material is used, if the silicon-containing negative electrode active material is carbon coated, the battery characteristics of the lithium ion secondary battery may be deteriorated.
Therefore, when a finely divided silicon-containing negative electrode active material is used in the production method of the present invention, it is preferable not to perform carbon coating of the silicon-containing negative electrode active material. The fine particle here means specifically that the average particle diameter of the silicon-containing negative electrode active material is 2.8 μm or less. Preferably, the average particle diameter of the silicon-containing negative electrode active material can be within a range of 2.6 μm or less, 2.4 μm or less, 2.2 μm or less, or 2.0 μm or less.

(Example 5)
(Method for producing negative electrode active material)
(Carbon coating process)
Using a silicon material having an average particle diameter of 4.335 μm obtained in the same silicon material manufacturing process as in Example 1, carbon coating was performed by a thermal CVD method as follows.
15 g of silicon material was put in a carbon container and placed in a rotary kiln type heating furnace. The inside of the furnace was replaced with argon gas until the oxygen concentration became 1% or less. After replacement with argon gas, the temperature in the furnace was raised to 900 ° C. over 1 hour while flowing argon gas at a flow rate of 1.6 L / min, and then cooled to 880 ° C. over 15 minutes. While maintaining the temperature in the furnace at 880 ° C., a mixed gas of propane gas having a flow rate of 0.4 L / min and argon gas having a flow rate of 1.6 L / min was passed through the furnace for 30 minutes.

(Dope process)
The dope process was performed as follows using the silicon material after the carbon coating.
In a glove box, 6.46 g of naphthalene was dissolved in 44.8 g of dehydrated tetrahydrofuran, 0.35 g of lithium metal was added, and the mixture was stirred at room temperature for 3 hours to prepare a lithium complex solution. The amount of lithium and the amount of naphthalene contained in this lithium complex solution were each 0.05 mol.

  The above lithium complex solution was placed in a dropping funnel. Separately, 7.0 g of the carbon-coated silicon material and 44.8 g of dehydrated tetrahydrofuran were placed in a flask to form a negative electrode active material suspension, and the flask was attached to a dropping funnel. The lithium complex solution was dropped into the flask over 3 hours while stirring the negative electrode active material suspension in the flask at 0 ° C. in an argon gas atmosphere. After completion of dropping, the mixture in the flask was further stirred for 15 hours. After completion of the stirring, filtration was performed, the filtration residue was washed with tetrahydrofuran, and the washed filtration residue was dried under reduced pressure to obtain a lithium-doped negative electrode active material. Through the above steps, the negative electrode active material of Example 5 was obtained.

  In the doping process of Example 5, the concentration and dropping rate of the lithium complex solution were controlled so that 0.05 g of lithium was supplied at a rate of 0.0002778 g per minute per 1 g of the carbon-coated silicon material. It was done. Since the mass increase due to the carbon coating is slight, the carbon-coated silicon material 1g described above can be read as the silicon material 1g.

(Comparative Example 8)
A negative electrode active material of Comparative Example 8 was produced in the same manner as Example 5 except that the dope process was not performed. The negative electrode active material of Comparative Example 8 is the same as the negative electrode active material before the doping step of Example 5, that is, the silicon material used in Example 5.

(Analysis of negative electrode active material 3)
Similarly to Analysis 1 of the negative electrode active material described above, the amount of lithium in the negative electrode active material of Example 5 was measured. As a result, the negative electrode active material of Example 5 was doped with lithium.

(Evaluation 3 of lithium ion secondary battery)
Using the negative electrode active material of Example 5 and Comparative Example 8, the negative electrode and lithium ion secondary battery of Example 5 and Comparative Example 8 were produced in the same manner as Example 1.
For the lithium ion secondary batteries of Example 5 and Comparative Example 8, the initial discharge capacity was measured in the same manner as in Evaluation 1 of the lithium ion secondary battery. Moreover, about the lithium ion secondary battery of Example 5, the capacity | capacitance maintenance factor was also measured similarly to evaluation 1 of said lithium ion secondary battery. The results of Evaluation 3 of the lithium ion secondary battery are shown in Table 5 together with the results of Analysis 3 of the negative electrode active material.

  As shown in Table 5, in the lithium ion secondary battery of Example 5 in which a silicon material having an average particle diameter of 4.335 μm was carbon coated by a thermal CVD method and then doped by dropping, As shown in Comparative Example 7 shown in FIG. 4, the initial discharge capacity was not deteriorated, and excellent initial discharge capacity, initial efficiency, and capacity retention ratio were exhibited.

  From this result, it is understood that when a silicon coating is applied to the silicon-containing negative electrode active material by a thermal CVD method and a doping process is further performed, a material having a relatively large average particle diameter may be used. Specifically, if the silicon-containing negative electrode active material having an average particle diameter exceeding 1.912 μm of the average particle diameter of the silicon-containing negative electrode active material used in Comparative Example 7, both the carbon coating and the doping step are performed. It can be said that it is suitable. More specifically, the silicon-containing negative electrode active material in this case preferably has an average particle diameter of 3.2 μm or more, more preferably 3.4 μm or more, and still more preferably 3.6 μm or more. It is more preferable that the thickness is 8 μm or more, and 4.0 μm or more is particularly preferable.

(Analysis of negative electrode active material 4)
(4. NMR measurement signal intensity ratio)
The negative electrode active materials of Example 1, Comparative Example 3 and Comparative Example 4 were subjected to solid ⁷ Li-NMR measurement. A CMX300 type nuclear magnetic resonance apparatus manufactured by Chemagnetics was used as the apparatus, and the measurement was performed at room temperature with a 5 mm probe, a repetition time of 10 seconds, and an integration count of 2500 times. An aqueous lithium chloride solution (0 ppm) was used as an external standard.
Example 1 In the NMR spectra of the negative electrode active materials of Comparative Example 3 and Comparative Example 4, a main signal near 0 ppm was observed. On the other hand, no signal around 260 ppm derived from metallic lithium was observed in each NMR spectrum. Further, in the NMR spectrum of the negative electrode active material of Comparative Example 4, a signal not observed in the negative electrode active material of Example 1 was observed around 18 ppm. Waveform separation was performed for each NMR spectrum to obtain a signal intensity ratio at -400 ppm to 600 ppm. From these analysis results, a lithium compound expected to be contained in each negative electrode active material of Example 1, Comparative Example 3 and Comparative Example 4 and a signal intensity ratio for each lithium compound were calculated. The results are shown in Table 6.

From the result of Table 6, it is thought that the ratio of lithium inorganic salts, such as lithium carbonate, in the surface of a negative electrode active material reduces by performing the dope process by dripping like Example 1. FIG. For example, it is known that lithium carbonate does not participate in charging / discharging of a lithium ion secondary battery at a general voltage. For this reason, it is thought that the manufacturing method of this invention which performs the dope process by dripping can contribute to the capacity | capacitance increase of a lithium ion secondary battery by reducing the lithium carbonate of the surface of a negative electrode active material.
Moreover, in the lithium dope negative electrode active material obtained with the manufacturing method of Example 1 by dripping, the ratio of lithium and silicon alloys such as Li ₁₂ Si ₁₄ having a chemical shift of 4 to 6 ppm is high. Thus, it is thought that lithium doped as an alloy with silicon is likely to contribute to the charge / discharge reaction of the lithium ion secondary battery. That is, it is considered that the manufacturing method of the present invention in which the doping process is performed by dripping can contribute to the capacity increase of the lithium ion secondary battery by being able to dope lithium into the silicon-containing negative electrode active material in the state of an alloy with silicon.

Specifically, in the negative electrode active material of the present invention, in the NMR spectrum obtained by solid ⁷ Li-NMR, the signal intensity ratio with a chemical shift of 4 to 6 ppm is considered to be 7 or more, more preferably 10 or more. .

(5. NMR measurement relaxation time)
The negative electrode active materials of Example 1, Comparative Example 3 and Comparative Example 4 were measured for solid ⁷ Li-NMR relaxation time. The relaxation period T1 was measured at a sample rotation speed of 10 KHz using an AVANCE 600 type nuclear magnetic resonance apparatus (600 Hz) manufactured by Bruker BioSpin Corporation. The results are shown in Table 7.

If the relaxation time T1 is short, it means that there are many nuclei capable of transferring the energy in the vicinity of the nucleus to which energy is given at the time of measurement. In the case of solid ⁷ Li-NMR measurement, This suggests that there is a high possibility that other Li exists in the vicinity.
When Example 1 and Comparative Example 3 are compared, the negative electrode active material of Example 1 is smaller in the measured amount of lithium than the negative electrode active material of Comparative Example 3, but the negative electrode active material of Comparative Example 3 The relaxation time T1 is shorter than This result is considered to mean that Li was uniformly dispersed in the negative electrode active material of Example 1 as compared with the negative electrode active material of Comparative Example 3. This is supported by the fact that the lithium ion secondary battery of Example 1 is superior to the lithium ion secondary battery of Comparative Example 3 in terms of initial discharge capacity and capacity retention rate.

  Further, when Example 1 and Comparative Example 4 are compared, the negative electrode active material of Comparative Example 4 has a relaxation time T1 shorter than that of the negative electrode active material of Example 1. Moreover, the lithium ion secondary battery of Comparative Example 4 is inferior to the lithium ion secondary battery of Example 1 in terms of initial discharge capacity and capacity retention rate. This is probably because the negative electrode active material of Comparative Example 4 has a much larger measured value of Li amount than the negative electrode active material of Example 1. From this, it can be said that the battery performance of the lithium ion secondary battery is lowered in this case, although the relaxation time T1 is shortened even when the measured value of the Li amount in the negative electrode active material is excessive.

Considering the above results, in order to provide good battery performance with the lithium-doped negative electrode active material, both the measured value of Li amount, that is, the lithium content is low and the relaxation time T1 is somewhat short. Is good.
Specifically, it can be said that the lithium content of the negative electrode active material is preferably in the range of 2.0 to 8.0% by mass and the relaxation time T1 is preferably 1.8 seconds or less.

  In addition, as a more preferable lower limit value of the lithium content of the negative electrode active material, values of 2.25%, 2.5%, 2.75%, and 3% can be given. As a preferable upper limit value of the lithium content of the negative electrode active material, respective values of 7.5%, 6.5%, 5.5%, and 4.5% may be mentioned. More preferable upper limit values of the relaxation time T1 may include values of 1.78 seconds, 1.75 seconds, 1.73 seconds, and 1.7 seconds. Examples of preferable lower limit values of the relaxation time T1 include values of 1.55 seconds, 1.58 seconds, 1.6 seconds, and 1.63 seconds.

(6. Powder resistance)
For each of the negative electrode active materials of Example 1, Example 4, and Comparative Examples 1 to 4, the powder resistance was measured. As a measuring device, a powder resistance measuring device manufactured by Mitsubishi Chemical Analytech Co., Ltd. was used, and the powder resistance of each negative electrode active material at each load was measured while applying a load to each negative electrode active material in the range of 4 kN to 20 kN. . The characteristics of each negative electrode active material used in the test are shown in Table 8, and the results are shown in FIG. In addition, the vertical axis | shaft of FIG. 6 is powder resistance (ohm * cm), and a horizontal axis is a load (kN).
In addition, the absolute value of the slope of the straight line between two points of loads 4 kN and 8 kN in the graph shown in FIG. 6 was calculated. The results are shown in Table 9.

As shown in FIG. 6 and Table 9, in the graph in which the vertical axis represents the powder resistance (Ω · cm) and the horizontal axis represents the load (kN), the slope of the straight line between the two points of loads 4 kN and 8 kN is shown. The size was the largest in Example 4, followed by Example 1. Compared with these, in the negative electrode active materials of Comparative Examples 1 to 4, the magnitude of the inclination was small. From this result, it is considered that whether or not it is the negative electrode active material of the present invention manufactured by the manufacturing method of the present invention can be determined by the magnitude of the inclination.
Specifically, in the negative electrode active material of the present invention, the absolute value of the slope is preferably 2 × 10 ⁷ or more, more preferably 3 × 10 ⁷ or more, still more preferably 4 × 10 ⁷ or more, and particularly preferably 5 It is considered to be 10 ⁷ or more.

Claims (15)

A negative electrode active material containing silicon, contacting a lithium complex solution obtained by dissolving metallic lithium and a polycyclic aromatic compound in an organic solvent, and doping the negative electrode active material with lithium;
The said dope process is a manufacturing method of the lithium dope negative electrode active material which makes the negative electrode active material suspension containing the said negative electrode active material and the organic solvent 50 degrees C or less, and dripping the said lithium complex solution.   2. The method for producing a negative electrode active material according to claim 1, wherein in the doping step, the lithium complex solution is dropped by setting the negative electrode active material suspension to less than 25 ° C. 3.   3. The method for producing a negative electrode active material according to claim 1, wherein in the doping step, 0.07 g or less of lithium is supplied at a rate of 0.00039 g / min or less to 1 g of the negative electrode active material. .   The negative electrode according to any one of claims 1 to 3, wherein a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in a thickness direction is used as the negative electrode active material containing silicon. A method for producing an active material. The dope process includes
A pretreatment step of treating the negative electrode active material with a fluorine anion-containing solution;
A main doping step in which a lithium complex solution obtained by dissolving metallic lithium and a polycyclic aromatic compound in an organic solvent is brought into contact with the negative electrode active material after the pretreatment step, and the negative electrode active material is doped with lithium. The manufacturing method of the negative electrode active material as described in any one of Claims 1-4. As the negative electrode active material containing silicon, a material having an average particle diameter of 3.2 μm or more is used.
The manufacturing method of the negative electrode active material as described in any one of Claims 1-5 including the carbon coating process of carbon-coating the negative electrode active material containing the silicon before the said dope process. As the negative electrode active material containing silicon, a material having an average particle diameter of 2.8 μm or less is used.
The manufacturing method of the negative electrode active material as described in any one of Claims 1-5 which does not include the process of carbon-coating by thermal CVD after the said dope process.   The manufacturing method of a negative electrode which has a negative electrode manufacturing process which manufactures a negative electrode using the negative electrode active material obtained by the manufacturing method of Claim 6 or Claim 7.   The manufacturing method of a lithium ion secondary battery which has a negative electrode manufacturing process of Claim 8.   A negative electrode active material containing lithium-doped silicon having a surface roughness Ra of 0.0001 μm to 0.008 μm.   The negative electrode active material according to claim 10, wherein the silicon-containing negative electrode active material is a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction.   The average particle diameter of the said negative electrode active material is 2.8 micrometers or less, The negative electrode active material of Claim 10 or Claim 11 which is not carbon-coated. Solid ⁷ A negative electrode active material containing lithium-doped silicon, wherein an NMR spectrum obtained by Li-NMR measurement has a chemical shift of 4 to 6 ppm and a signal intensity ratio of 7 or more. A negative electrode active material containing lithium-doped silicon, wherein the lithium content is 2.0% to 8.0% by mass and the relaxation time T1 by solid ⁷ Li-NMR measurement is 1.8 seconds or less. material. In the graph in which the vertical axis represents powder resistance (Ω · cm) and the horizontal axis represents load (kN), the absolute value of the slope of the straight line between the two points of load 4 kN and load 8 kN is 2 × 10 ⁷ or more. A negative electrode active material containing lithium-doped silicon.