JP2018152161A - Negative electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel lithium-doped negative electrode material.SOLUTION: A negative electrode material contains lithium and a Si-containing negative electrode active material that contains silicon and oxygen, and also contains a LiSiOcrystal and a LiSiOcrystal, or a lithium silicide crystal.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a negative electrode material.

  As a negative electrode active material of a lithium ion secondary battery, a negative electrode active material containing Si 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.

  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, and describes a lithium ion secondary battery including the silicon material as a negative electrode active material. Has been.

  Among these negative electrode active materials, a Si-containing negative electrode active material containing silicon and oxygen has a large theoretical capacity and is useful as a negative electrode active material for a lithium ion secondary battery. However, these Si-containing negative electrode active materials also have a large irreversible capacity. The inventor of the present invention studied to introduce lithium into the Si-containing negative electrode active material in order to cancel the irreversible capacity. Hereinafter, in the present specification, introducing lithium into the Si-containing negative electrode active material is referred to as “lithium doping” or simply “lithium doping”.

Various methods have been proposed as a method for doping lithium into the Si-containing negative electrode active material. For example, in Patent Document 6, a mixture of metallic lithium and / or organolithium compound and silicon oxide is mechanically mixed with a mixing device such as a ball mill under heating conditions in the range of 60 ° C. to 100 ° C., and silicon oxide and A method is disclosed in which lithium is reacted to lithium-doped a Si-containing negative electrode active material. The document describes that the Li-doped Si-containing negative electrode active material contains Li ₄ SiO ₄ crystals. Patent Document 7 discloses a method in which a mixture of lithium hydride and silicon oxide is heated to react silicon oxide and lithium, and then a specific component is removed by acid treatment to dope lithium into the Si-containing negative electrode active material. It is disclosed. The document describes that a Li-doped Si-containing negative electrode active material contains Li ₂ Si ₂ O ₅ crystals.

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 No. 4985949 JP2015-153520A

The inventor of the present invention aimed to develop a lithium-doped negative electrode material that is completely different from that obtained by the above-described conventional method.
The present invention has been made in view of such circumstances, and an object thereof is to provide a new lithium-doped negative electrode material.

The negative electrode material of the present invention is
Si-containing negative electrode active material containing silicon and oxygen, and lithium,
It is a negative electrode material containing Li ₄ SiO ₄ crystal and Li ₂ SiO ₃ crystal or lithium silicide crystal.

  The method for producing a negative electrode material of the present invention is a novel lithium-doped negative electrode material.

It is a phase diagram of silicon material. 3 is an explanatory diagram schematically showing a lithium doping step in the method for producing a negative electrode material of Example 1. FIG. It is an XRD analysis chart of the negative electrode material of Example 1- Example 9, the comparative example 1, and the comparative example 2. FIG. 10 is an XRD analysis chart of negative electrode materials of Example 10 and Example 11.

  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 material of the present invention contains a Si-containing negative electrode active material containing silicon and oxygen, and lithium, and contains a Li ₄ SiO ₄ crystal and a Li ₂ SiO ₃ crystal, or a lithium silicide crystal. It is.

The negative electrode material of the present invention is a lithium-doped negative electrode material, and the irreversible capacity of the Si-containing negative electrode active material can be canceled by performing lithium doping on the Si-containing negative electrode active material, thereby improving the initial efficiency of the lithium ion secondary battery. There are advantages that can be improved.
The negative electrode material of the present invention is greatly different from the conventional negative electrode materials disclosed in the above patent documents in terms of composition. This means that the negative electrode material of the present invention has a larger lithium doping amount than the conventional negative electrode material. Such a negative electrode material of the present invention enables charge / discharge at a lower potential (vs. Li reference) in addition to being able to preferably cancel the irreversible capacity. The negative electrode material of the present invention can be produced by doping a large amount of lithium into a Si-containing negative electrode active material.

  As a method of manufacturing the negative electrode material of the present invention in which a large amount of lithium is doped in the Si-containing negative electrode active material, for example, the same method as the conventional negative electrode material manufacturing method disclosed in Patent Document 6 and Patent Document 7 is used. It may be adopted. That is, by adopting a lithium doping method in which a solid lithium source is brought into direct contact with the Si-containing negative electrode active material to react with each other, and the amount of the lithium source with respect to the Si-containing negative electrode active material is increased, A large amount of lithium may be doped.

  However, according to the conventional method, a large amount of unreacted lithium can be contained in the negative electrode material that is a reaction product. For this reason, there is a problem that it is difficult to dope a desired amount of lithium to the Si-containing negative electrode active material. Furthermore, according to the method, a lithium compound that is derived from a lithium source and does not contribute to the battery reaction, such as lithium oxide, can be included in the negative electrode material.

  The inventor of the present invention has obtained the idea that gaseous lithium, that is, zero-valent lithium gas is brought into contact with the Si-containing negative electrode active material in place of such a conventional method, so that the negative electrode material of the present invention can be easily obtained. A method of manufacturing was found. Hereinafter, if necessary, a method for producing a negative electrode material of the present invention by bringing a zero-valent lithium gas into contact with a Si-containing negative electrode active material is simply referred to as a production method of the present invention. It is called gas.

  The manufacturing method of this invention has a lithium dope process which makes zero valent lithium gas contact a Si containing negative electrode active material. As will be described later, according to the production method of the present invention having the lithium doping step, it is possible to dope an optimum amount of lithium to the Si-containing negative electrode active material, and unreacted lithium or lithium oxide. It is also possible to suppress mixing into the negative electrode material. That is, it can be said that the negative electrode material of the present invention obtained by the production method is greatly reduced in the amount of unreacted lithium, lithium oxide and the like and contains a large amount of lithium. Therefore, it can be said that the negative electrode material of the present invention is a novel negative electrode material suitable as a negative electrode active material for a lithium ion secondary battery. Hereinafter, in order to help understanding of the present invention, first, the production method of the present invention will be described.

Lithium gas is generated by heating solid or liquid metallic lithium (hereinafter simply referred to as metallic lithium as necessary) as a lithium gas generation source. In addition, the reaction product obtained by the reaction of lithium in a zero-valent gas state with the Si-containing negative electrode active material, that is, the negative electrode material of the present invention obtained by the production method of the present invention is cooled and finally obtained. It is thought that it becomes solid. The negative electrode material manufacturing method of the present invention may be performed under conditions that can cause such a reaction.
For example, in the method for producing a negative electrode material of the present invention, the heating temperature of metallic lithium, the amount of metallic lithium and a Si-containing negative electrode active material, and the atmospheric temperature of the reaction chamber in which the reaction between the lithium gas and the Si-containing negative electrode active material is performed Depending on the size of the reaction chamber, the atmospheric pressure, and the like, it may be set as appropriate so that lithium gas is generated from metallic lithium and the lithium gas reaches the Si-containing negative electrode active material in the gaseous state.

  In the method for producing a negative electrode material of the present invention, lithium gas may be brought into contact with the Si-containing negative electrode active material, and the positional relationship between metallic lithium and the Si-containing negative electrode active material is not particularly limited. For example, metallic lithium may be disposed in the reaction chamber together with the Si-containing negative electrode active material, or may be disposed outside the reaction chamber and only lithium gas may be introduced into the reaction chamber. However, in order to suppress unreacted lithium derived from metallic lithium from adhering to the negative electrode active material or the negative electrode material, it is preferable to avoid contact between metallic lithium and the Si-containing negative electrode active material and to separate them from each other. . That is, it is preferable that the metallic lithium is disposed so as not to contact the Si-containing negative electrode active material in the lithium doping step.

  In order to avoid contact between metallic lithium and the Si-containing negative electrode active material and to increase the contact frequency between the lithium gas and Si-containing negative electrode active material, the Si-containing negative electrode active material is more gas in the reaction chamber than metallic lithium. It is preferable to arrange on the downstream side of the distribution path.

The reaction between the lithium gas and the Si-containing negative electrode active material suppresses the oxidation of the lithium-doped negative electrode active material or metallic lithium and promotes the gasification of metallic lithium, so that an inert gas such as argon gas exists. It is preferable to carry out under an inert atmosphere and / or under a reduced pressure atmosphere.
In order to make the reaction chamber have a reduced pressure atmosphere, the gas in the reaction chamber may be discharged to the outside using a pressure reducing device such as a suction pump. In addition, in order to make the reaction chamber an inert atmosphere, an inert gas may be supplied from the outside to the reaction chamber. By the movement of these gases, the gas flows in the reaction chamber, and the above-described gas flow path is formed.

  By the way, lithium oxide obtained by oxidizing metallic lithium does not generate lithium gas. Therefore, in order to produce a lithium-doped negative electrode active material, that is, the negative electrode material of the present invention with a high yield while reducing lithium loss, it is preferable to suppress oxidation of metallic lithium in the lithium doping step.

  In order to suppress the oxidation of metallic lithium, it is preferable not only to reduce the pressure in the reaction chamber and / or the inert atmosphere as described above, but also to remove oxygen from the vicinity of metallic lithium. In order to remove oxygen from the vicinity of metallic lithium, the lithium doping process may be performed in the presence of an oxygen scavenger. The oxygen scavenger refers to a material that reacts with oxygen in the atmosphere in preference to metallic lithium. Furthermore, the oxygen scavenger is preferably present as a solid at a temperature at which metallic lithium generates lithium gas, in consideration of handleability.

  Titanium and zirconium are preferably used as oxygen scavengers in the production method of the present invention because they have properties suitable for the oxygen scavenger described above. As the oxygen scavenger, titanium or zirconium can be used, or oxides of these metals can be used. Note that the oxide here refers to a low-order oxide having a small oxidation number. These oxygen scavengers may be used alone or in combination of two or more.

In order to more reliably suppress the oxidation of metallic lithium using the oxygen-trapping material, it is considered that the oxygen-trapping material is preferably arranged in a space where metallic lithium is present in the lithium doping process. In addition, in order to suppress oxidation of metallic lithium, lithium gas generated from the metallic lithium, and lithium-doped negative electrode active material, it is preferable to uniformly distribute an oxygen scavenger in the entire reaction chamber.
However, in order to avoid the reaction between the oxygen scavenger and the Si-containing negative electrode active material, it is preferable to separate the oxygen scavenger and the Si-containing negative electrode active material.
The metallic lithium and the oxygen scavenger may be in non-contact or in contact with each other. When the metal lithium and the oxygen scavenger are brought into contact with each other, the oxygen scavenger can also function as a reducing agent that can obtain oxygen from the metal lithium.

In order to efficiently capture oxygen in the reaction chamber, it is preferable that the oxygen-trapping material has a large specific surface area. Specifically, the oxygen scavenger material is preferably a porous material. For example, a porous material of titanium called sponge titanium is particularly preferably used as an oxygen scavenger in the production method of the present invention in two aspects of material and shape.
The porosity of the oxygen-trapping material that is a porous material is preferably about 10% to 90% by volume. The porosity can be measured with a known measuring apparatus typified by a mercury intrusion porosimeter.

Examples of the Si-containing negative electrode active material include Si alone and SiO _x (0.3 ≦ x ≦ 1.6) that disproportionates to SiO ₂ , a composite that combines SiO _x and a carbon-based material, and Patent Document 5 described above. Can be mentioned.

Hereinafter, the silicon material disclosed in Patent Document 5 (in this specification, simply referred to as “silicon material”) will be described in detail. For example, the silicon material may include a step of reacting CaSi ₂ with an acid to synthesize a layered silicon compound containing polysilane as a main component, and a step of heating the layered silicon compound at 300 ° C. or higher to release hydrogen. It is manufactured after that.

The method for producing a silicon material described in Patent Document 5 is represented as follows by using an ideal reaction formula when hydrogen chloride is used as the acid.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, in the step of synthesizing the layered silicon compound including the upper reaction, the layered silicon compound is hardly manufactured as containing only Si ₆ H ₆ , and the layered The silicon compound is represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an acid anion, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a product. In the above chemical formula, inevitable impurities such as Ca that can remain are not taken into consideration. A silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. For efficient insertion and desorption reaction of charge carriers such as lithium ions, the plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. . The length in the longitudinal direction of the plate-like silicon body is preferably within a range of 0.1 μm to 50 μm. The plate-like silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. This laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, the above plate-like silicon body is preferably in a state where amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite 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 preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scherrer equation using X-ray diffraction measurement on the silicon material and using the half-value width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart. .

  The abundance and size of the plate-like silicon body, amorphous silicon and silicon crystallite contained in the silicon material mainly depend on the heating temperature and the heating time. The heating temperature is preferably in the range of 350 ° C to 950 ° C, more preferably in the range of 400 ° C to 900 ° C.

  The average particle diameter of the silicon material or the negative electrode material of the present invention when measured with a general laser diffraction particle size distribution measuring device is preferably in the range of 0.6 to 30 μm, more preferably in the range of 1 to 20 μm. The range of 2 to 10 μm is more preferable, and the range of 3 to 8 μm is particularly preferable. In the present specification, the average particle diameter means a 50% cumulative diameter (D50).

Lithium doped in the Si-containing negative electrode active material forms a compound with silicon or silicon and oxygen. Taking the case where the Si-containing negative electrode active material has the composition formula Si _0.7 O _0.3 as an example, the compound will be described based on the phase diagram of the negative electrode material shown in FIG.
The phase diagram shown in FIG. 1 was obtained using phase diagram calculation software (Factsage Corp., Computational Mechanics Research Center). The composition formula Si _0.7 O _0.3 is a typical composition formula of silicon materials.

  In the phase diagram of the negative electrode material shown in FIG. 1, the vertical axis represents temperature, and the horizontal axis represents the molar ratio of lithium in the lithium-doped negative electrode active material (Li / (Li + O + Si)). Hereinafter, as necessary, the “lithium-doped negative electrode active material” is referred to as a negative electrode material, and the “molar ratio of lithium in the lithium-doped negative electrode active material” is referred to as a lithium doping amount.

The aspect of the negative electrode material obtained by doping lithium into the negative electrode active material having the composition formula Si _0.7 O _0.3 varies depending on the silicon doping amount and the temperature, as shown in FIG. For example, in a region where the amount of lithium doping is very small, lithium silicate is generated and a large amount of silicon remains, but in a region where the amount of lithium doping is very large, lithium silicide is generated in addition to lithium silicate.

Specifically, in the state diagram shown in FIG. 1, the negative electrode material is Li ₂ Si ₂ O ₅ + SiO ₂ + Si or Li ₂ Si ₂ O ₅ + Li _{2 in} a region at room temperature and a lithium doping amount of less than 0.18. It takes the form of SiO ₃ + Si. On the other hand, in the region where the lithium doping amount is 0.18 or more, the negative electrode material is Li ₄ SiO ₄ + Li ₂ SiO ₃ + Si, Li ₄ SiO ₄ + Li ₁₂ Si ₇ + Si, Li ₄ SiO ₄ + Li ₁₂ Si ₇ + Li. ₇ Si ₃ , Li ₄ SiO ₄ + Li ₁₃ Si ₄ + Li ₇ Si ₃ , Li ₄ SiO ₄ + Li ₁₃ Si ₄ + Li ₂₂ Si ₅ , or Li ₄ SiO ₄ + Li + Li ₂₂ Si ₅ .

  The aspect of the lithium silicate and lithium silicide in these negative electrode materials corresponds to the lithium doping amount in the negative electrode material.

As described above, the negative electrode material of the present invention has a larger lithium doping amount than the conventional negative electrode material. Specifically, the negative electrode material of the present invention contains lithium silicate and lithium silicide, or, when not containing lithium silicide, contains Li ₄ SiO ₄ and Li ₂ SiO ₃ as lithium silicate.
If the amount of lithium doped in the negative electrode material is too small, sufficient lithium may not be brought into the lithium ion secondary battery. Since the negative electrode material of the present invention is sufficiently lithium-doped, it is suitable as a negative electrode material for a lithium ion secondary battery.

  By the way, if the amount of lithium doping in the negative electrode material is excessive, there may be cases where lithium exceeding the required amount is brought into the lithium ion secondary battery due to the negative electrode material. Accordingly, there is a preferred range for the lithium doping amount of the negative electrode material of the present invention.

The negative electrode material of the present invention contains lithium silicate and lithium silicide, or, when not containing lithium silicide, contains Li ₄ SiO ₄ and Li ₂ SiO ₃ as lithium silicate. More preferably, Li ₄ SiO ₄ is contained as the lithium silicate, and Li ₁₂ Si ₇ is contained as the lithium silicide.
As a corresponding range of the lithium doping amount, it can be said that the lithium doping amount of the negative electrode material of the present invention is 0.18 or more, and preferably 0.18 or more and 0.63 or less. The lithium doping amount of the negative electrode material of the present invention is more preferably 0.23 or more and 0.58 or less.

The preferable range of the lithium doping amount described above is a preferable range when a Si-containing negative electrode active material having a composition formula of Si _0.7 O _0.3 is used. The range varies slightly depending on the type of the Si-containing negative electrode active material. In either case, the negative electrode material of the present invention contains lithium silicate and lithium silicide, or lithium silicate when lithium silicide is not contained. As a lithium doping amount containing Li ₄ SiO ₄ and Li ₂ SiO ₃ as a lithium doping amount in the negative electrode material of the present invention.

The amount of lithium doping in the negative electrode material of the present invention can be adjusted by appropriately setting the relative abundance of the Si-containing negative electrode active material and metallic lithium lithium in the reaction chamber. Specifically, if the relative amount of metallic lithium with respect to the Si-containing negative electrode active material is increased, the lithium content in the negative electrode material increases, and a negative electrode material containing lithium silicide is obtained. On the other hand, if the relative amount of metallic lithium with respect to the Si-containing negative electrode active material is reduced, the lithium content in the negative electrode material is reduced, and a negative electrode material containing no lithium silicide is obtained. In this case, if the relative amount of metallic lithium with respect to the Si-containing negative electrode active material is a certain level or more, the negative electrode material of the present invention containing Li ₄ SiO ₄ and Li ₂ SiO ₃ as lithium silicate is obtained, and the Si-containing negative electrode active material if very fewer relative amount of metal lithium to, or contains only _Li 2 _Si 2 _{O 5} as the lithium silicate or _the negative electrode material containing only Li 2 _Si 2 _{O 5} and _Li 2 SiO _3, i.e., the conventional negative electrode A material is obtained. Even when the amount of metallic lithium relative to the Si-containing negative electrode active material is large, it is possible to reduce the lithium content in the negative electrode material by shortening the reaction time.

  In addition, the relative amount of Si-containing negative electrode active material and lithium gas in the reaction chamber can be set by appropriately adjusting the amount of metallic lithium relative to the Si-containing negative electrode active material, the temperature of the reaction chamber, the time required for the lithium doping process, etc. It is.

  Further, after the lithium doping step, before lowering the temperature of the reaction chamber, it is preferable to introduce an inert gas into the reaction chamber and replace the argon gas in the reaction chamber with the inert gas. This is because if the temperature of the reaction chamber is lowered as it is after the lithium doping step, lithium particles in which the lithium gas is cooled may be mixed into the negative electrode material when lithium gas remains in the vicinity of the negative electrode material. . Note that the reaction chamber is preferably heated by a heater or the like in order to suppress the temperature drop of the lithium gas and to suppress liquefaction or solidification of unreacted lithium.

  In the lithium doping step, lithium gas that has reacted with the Si-containing negative electrode active material becomes a part of the negative electrode material. On the other hand, the lithium gas that has not reacted with the Si-containing negative electrode active material may partially remain on the negative electrode material and remain as solid lithium particles, but does not substantially constitute the negative electrode material. . In addition, lithium metal may be included in metallic lithium, which is a source of lithium gas. However, lithium oxide is in a gaseous state because its boiling point is much higher than that of lithium and its vapor pressure is much lower. hard. Therefore, the lithium oxide contained in the metallic lithium does not constitute lithium gas, and the lithium oxide also does not substantially constitute the negative electrode material.

  According to such a lithium doping step, the negative electrode material of the present invention in which the amount of unreacted lithium, lithium oxide, etc. is greatly reduced can be obtained.

The lithium silicate and lithium silicide contained in the negative electrode material of the present invention may be amorphous or in a crystalline state, but at least a part of these is presumed to be in a crystalline state.
For example, the negative electrode material in the region where the lithium doping amount is 0.23 or more includes only a liquid phase and lithium silicate and optionally Si at a high temperature exceeding 650 ° C., for example, and does not have lithium silicide. However, similarly, the negative electrode material in the region where the lithium doping amount is 0.23 or more includes both lithium silicate and lithium silicide at an intermediate temperature of 650 ° C. or less.
According to the production method of the present invention, the reaction between gaseous lithium and the Si-containing negative electrode active material occurs at such a high temperature that lithium can exist in gaseous form. Therefore, when the lithium doping step is performed under the condition that the lithium doping amount is 0.23 or more, lithium silicide generated by the reaction between the lithium gas and the Si-containing negative electrode active material is generated as a crystalline phase during the reaction. Lithium silicate is also produced as a crystalline phase.

The silicon material has a special structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. For this reason, when a silicon material is selected as the Si-containing negative electrode active material, in order to maintain the structure, in the lithium doping process, basically, the silicon material is preferably solid, The temperature is preferably lower than the temperature at which Si melts. For reference, the melting point of Si is 1414 ° C., and the melting point of SiO ₂ is 1650 ± 75 ° C.

The Si-containing negative electrode active material has a large capacity, but is inferior in conductivity compared to a general negative electrode active material such as graphite. The negative electrode material of the present invention may further have a carbon coat layer. In this case, the conductivity of the negative electrode material of the present invention can be improved, and by integrating a carbon coat layer as a conductive material into the negative electrode material, compared with a case where the negative electrode material and the conductive material are simply mixed. Thus, the negative electrode material and the conductive material can be brought into close contact with each other, and excellent conductivity can be imparted to the negative electrode material of the present invention.
The carbon coat layer may be formed on the Si-containing negative electrode active material after the lithium doping step, or may be formed on the Si-containing negative electrode active material before the lithium doping step. Specifically, if the Si-containing negative electrode active material is carbon-coated by a known method such as chemical vapor deposition, a carbon coat layer can be easily formed on the surface of the Si-containing negative electrode active material.

  The carbon coating on the Si-containing negative electrode active material is preferably performed after the lithium doping step. The volume of the Si-containing negative electrode active material changes with charging / discharging, that is, as lithium enters and exits. By performing carbon coating on the Si-containing negative electrode active material doped with lithium and increasing its volume, Even if the volume of the negative electrode active material changes, a carbon coat layer that is not easily broken can be obtained.

  As the carbon coating method, in addition to the above-described chemical vapor deposition (CVD), a physical vapor deposition method such as a vacuum vapor deposition method, a sputtering method, an ion plating method, or the like can be employed. Other methods can also be adopted. The carbon coat layer may be formed only on a part of the surface of the Si-containing negative electrode active material, may be formed on the entire surface of the Si-containing negative electrode active material, or may be partially formed inside the Si-containing negative electrode active material. You can get in.

  The material of the carbon coat layer, that is, the carbon source, preferably contains carbon and does not bring many elements other than carbon into the negative electrode, and the state thereof is not particularly limited such as solid or gaseous. Examples of the carbon source include carbon simple substance, hydrocarbon gas, and organic polymer. When the CVD method is adopted as the carbon coating method, it is preferable to select a hydrocarbon gas such as methane, ethane, propane, or butane from the viewpoint of work efficiency.

  The thickness of the carbon coat layer is not particularly limited, but a thicker one is preferable in consideration of durability and conductivity of the negative electrode material. On the other hand, considering the capacity maintenance and ion conductivity of the negative electrode material, it is preferable that the carbon coat layer is thin. This is because when the amount of the carbon coat layer in the negative electrode material is excessive, the amount of the Si-containing negative electrode active material is relatively reduced, so that the capacity of the negative electrode material is reduced or lithium ions are difficult to pass. Taking these into consideration, the thickness of the carbon coat layer is preferably 1 to 100 nm, more preferably 10 to 50 nm, and even more preferably 5 to 50 nm.

  The carbon coat layer may contain only carbon or may contain other elements. For example, the carbon coat layer may contain at least one metal atom selected from transition metals. Since the metal atom improves the conductivity in the carbon coat layer, the lithium ion conductivity in the negative electrode is improved. As a metal atom selected from transition metals, Cu, Fe, Ni and the like are preferable, and Cu is particularly preferable. The metal atom content in the carbon coat layer is preferably in the range of 0.1 to 10% by mass.

  The negative electrode material of the present invention can be used for a negative electrode for a lithium ion secondary battery. Hereinafter, the negative electrode comprising the negative electrode material of the present invention is referred to as the negative electrode of the present invention as necessary.

  The negative electrode of the present invention has a current collector and a negative electrode active material layer bound to the surface of the current collector.

  The current collector refers to a chemically inert electronic conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. 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.

  The negative electrode active material layer includes the above-described negative electrode material, and the negative electrode material is, as described above, a lithium-doped Si-containing negative electrode active material. The lithium ion secondary battery of the present invention may contain only the negative electrode material of the present invention, that is, a lithium-doped Si-containing negative electrode active material as a negative electrode active material, or other known negative electrode active materials such as graphite. You may use together with the negative electrode material of this invention.

  The negative electrode active material layer in the negative electrode of the present invention contains a conductive additive and / or a binder as necessary, in addition to the negative electrode material of the present invention.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be optionally added when the conductivity of the electrode is insufficient. For example, when the negative electrode material of the present invention has a relatively large number of carbon coat layers, the conductivity of the electrode is sufficiently excellent. If you are, you do not have to add it. 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 negative electrode active material layer alone or in combination of two or more.

  The blending ratio of the conductive assistant in the negative electrode active material layer is preferably a mass ratio of active material: conductive assistant = 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.

  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, and poly (meth) acrylic acid. What is necessary is just to employ | adopt well-known things, such as acrylic resin, alkoxysilyl group containing resin, carboxymethylcellulose, and styrene butadiene rubber.

  A cross-linked polymer obtained by cross-linking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/063882 may be used as a binder.

  Examples of the diamine used in the crosslinked polymer include alkylene diamines such as ethylene diamine, propylene diamine, and hexamethylene diamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophorone diamine, and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine, and naphthalenediamine are exemplified.

  The blending ratio of the binder in the active material layer is preferably a mass ratio of active material: binder = 1: 0.005 to 1: 0.3. 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 blending ratio of the binder in 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.

  In order to form an active material layer on the surface of the current collector, a current collecting 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. An active material may be applied to the surface of the body. Specifically, an active material, a solvent, and, if necessary, a binder and / or a conductive aid are mixed to prepare a slurry. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. 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.

  By the way, when the negative electrode material containing lithium silicide, water, and the water-based binder are used in combination, the slurry containing the negative electrode material and the binder is poor at the time of manufacturing the negative electrode, and the slurry is used as a current collector. It may be difficult to apply the coating. Therefore, it can be said that it is preferable to combine a non-aqueous solvent and a non-aqueous binder with the negative electrode material of the present invention that can contain lithium silicide. Non-aqueous solvents include N-methyl-2-pyrrolidone, methanol, and methyl isobutyl ketone. Examples of the non-aqueous binder include a binder selected from a fluorine-containing resin, a thermoplastic resin, an imide resin, an acrylic resin, an alkoxysilyl group-containing resin, and a crosslinked polymer crosslinked with a polyamine.

  In addition, other lithium ion secondary battery components such as a positive electrode and an electrolyte solution constituting a lithium ion secondary battery in combination with the negative electrode material of the present invention may be appropriately selected based on a conventional method. There is no particular limitation.

  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.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, this invention is not limited by these Examples.

Example 1
(Manufacture of negative electrode materials)
(Silicon material manufacturing process)
CaSi ₂ was added to an aqueous 36% by mass HCl solution in an ice bath and stirred under an argon gas atmosphere. The reaction solution was filtered, the residue was washed with distilled water and acetone, and further dried under reduced pressure to separate the layered silicon compound containing polysilane. The layered silicon compound was heated at 900 ° C. for 1 hour under an argon gas atmosphere to obtain a silicon material. This silicon material was pulverized with a jet mill NJ-30 (Aisin Nano Technologies Co., Ltd.) to obtain a particulate silicon material.

(Lithium doping process)
Using the silicon material obtained in the above silicon material manufacturing process as a Si-containing negative electrode active material, a lithium doping process was performed as follows. An explanatory view schematically showing the lithium doping apparatus used in the manufacturing method of Example 1 is shown in FIG. Hereinafter, based on FIG. 2, the lithium dope process in the manufacturing method of Example 1 is demonstrated.

(apparatus)
As shown in FIG. 2, the lithium doping apparatus 1 is connected to a heating furnace 3 in which a processing chamber 2 is partitioned, a partition wall 5 having a large number of openings 4 and partitioning the processing chamber 2 up and down, and a heating furnace 3. The gas inflow path section 6 and the gas outflow path section 7, the gas cylinder (not shown) connected to the gas inflow path section 6, the decompression pump P connected to the gas outflow path section 7, and the first disposed in the processing chamber. It is composed of one container 8 and a second container 9.

  The processing chamber 2 is partitioned into an upper chamber 20 and a lower chamber 21 by a partition wall 5. The gas flow between the upper chamber 20 and the lower chamber 21 is allowed by the large number of openings 4 provided in the partition wall 5. A tubular gas inflow passage portion 6 is connected to the lower chamber 21, and a tubular gas outflow passage portion 7 is connected to the upper chamber 20. A gas cylinder (not shown) containing argon gas is connected to the gas inflow path section 6, and the argon gas is supplied to the lower chamber 21 through the gas inflow path section 6. On the other hand, a decompression pump P is connected to the gas outflow path section 7. When the decompression pump P is driven by receiving power from a power supply (not shown), the gas in the upper chamber 20 is transferred to the gas outflow path section 7. And flows out to the outside through the decompression pump P. Therefore, a gas flow path from the lower chamber side 21 to the upper chamber side 20 is formed inside the processing chamber 2. A first container 8 for silicon material is disposed in the upper chamber 20, and a second container 9 for metallic lithium is disposed in the lower chamber 21. The upper chamber 20 is a reaction chamber in which the silicon material and lithium gas react.

(Method)
Silicon material S was put in the first container 8 in the upper chamber 20, and metallic lithium L was put in the second container 9 in the lower chamber 21. In the manufacturing method of Example 1, the amounts of metallic lithium and silicon material were such that metallic lithium: silicon material = 1: 3 (mass ratio).

  The decompression pump P was driven to make the upper chamber 20 and the lower chamber 21 have a reduced pressure atmosphere of about 1 to 10 Pa. The entire processing chamber 2 was heated to 550 ° C. by a heater (not shown), and lithium gas was generated from metallic lithium L placed in the second container 9. The lithium gas generated in the lower chamber 21 passed through the opening 4 of the partition wall 5 and moved to the upper chamber 20 to contact the silicon material S. This state was maintained for 6 hours. After 6 hours, argon gas was introduced into the processing chamber 2 and the lithium gas in the processing chamber 2 was replaced with argon. Thereafter, the processing chamber 2 was naturally cooled to room temperature under an argon atmosphere. The negative electrode material of Example 1 was obtained through the above steps.

(Example 2)
The amount of metal lithium and silicon material was the same as that of Example 1 except that metal lithium: silicon material = 1: 4 (mass ratio) and that the heating temperature of the processing chamber was 600 ° C. The negative electrode material of Example 2 was manufactured by the method.

(Example 3)
The amount of metal lithium and silicon material was the same as that of Example 1 except that metal lithium: silicon material = 1: 2 (mass ratio) and that the heating temperature of the processing chamber was 600 ° C. The negative electrode material of Example 3 was manufactured by the method.

(Example 4)
The amount of metallic lithium and silicon material was the same as that of Example 1 except that the amount of metallic lithium: silicon material = 1: 2 (mass ratio) and that the heating temperature of the processing chamber was 650 ° C. The negative electrode material of Example 4 was manufactured by the method.

(Example 5)
The amount of metallic lithium and silicon material was the same as that of Example 1 except that the amount of metallic lithium: silicon material = 1: 1 (mass ratio) and that the heating temperature of the processing chamber was 650 ° C. The negative electrode material of Example 5 was manufactured by the method.

(Example 6)
The amount of metal lithium and silicon material was the same as that of Example 1 except that metal lithium: silicon material = 1: 2 (mass ratio) and that the heating temperature of the processing chamber was 700 ° C. The negative electrode material of Example 6 was manufactured by the method.

(Example 7)
The amount of metallic lithium and silicon material was the same as that of Example 1 except that the amount of metallic lithium: silicon material = 1: 1 (mass ratio) and that the heating temperature of the processing chamber was 700 ° C. The negative electrode material of Example 7 was manufactured by the method.

(Example 8)
The amount of metallic lithium and silicon material was the same as that of Example 1 except that the amount of metallic lithium: silicon material = 1: 2 (mass ratio) and that the heating temperature of the processing chamber was 750 ° C. The negative electrode material of Example 8 was manufactured by the method.

Example 9
The amount of metallic lithium and silicon material was the same as that of Example 1 except that the amount of metallic lithium: silicon material = 1: 1 (mass ratio) and that the heating temperature of the processing chamber was 750 ° C. The negative electrode material of Example 9 was manufactured by the method.

(Comparative Example 1)
The negative electrode material of Comparative Example 1 is a silicon material obtained in the silicon material manufacturing process in the manufacturing method of Example 1.

(Comparative Example 2)
The amount of metal lithium and silicon material was the same as that of Example 1 except that metal lithium: silicon material = 1: 2 (mass ratio) and that the heating temperature of the processing chamber was 500 ° C. The negative electrode material of Comparative Example 2 was produced by the method.

(XRD analysis 1)
The negative electrode materials of Examples 1 to 9, Comparative Example 1 and Comparative Example 2 were subjected to X-ray diffraction measurement (XRD) with a powder X-ray diffractometer using CuKα. FIG. 3 shows an X-ray diffraction chart of each negative electrode material, Li ₄ SiO ₄ , Li ₁₂ Si ₇ , Li ₇ Si ₃ , and Li ₁₃ Si ₄ . In addition, the lithium silicate crystals and lithium silicide crystals contained in each negative electrode material confirmed by FIG. 3 are summarized in Table 1.

As shown in FIG. 3 and Table 1, in the negative electrode materials of Examples 1 to 9, a peak derived from the lithium silicate crystal and a peak derived from the lithium silicide crystal were confirmed. That is, the negative electrode materials of Examples 1 to 9 were all doped with lithium. From this result, it becomes clear that the negative electrode materials of Examples 1 to 9 are the negative electrode materials of the present invention.
Furthermore, from the results shown in FIG. 3 and Table 1, the type of lithium silicate crystal contained in each negative electrode material is set by appropriately setting the amount of lithium and the Si-containing negative electrode active material and the temperature of the lithium doping step. It can be seen that various changes can be made.

  Further, as shown in FIG. 3, in Comparative Example 2 where the reaction temperature was 500 ° C., a peak of silicon crystal around 26 to 30 ° was observed, but it was unique to other lithium silicate crystals and lithium silicide crystals. From the fact that no peak is observed, it is understood that when the lithium doping step is performed under a reduced pressure of about 1 to 10 Pa, lithium doping is not performed on the Si-containing negative electrode active material at a reaction temperature of 500 ° C. Furthermore, in Examples 6 to 9 where the reaction temperature was 700 ° C. or higher, the peak of the silicon crystal was sharp, so that the silicon material partially melted at that temperature, and as a result, It is estimated that large silicon was produced. Therefore, when performing a lithium dope process under reduced pressure of about 1-10 Pa, it can be said that it is preferable that reaction temperature exceeds 500 degreeC and is less than 700 degreeC. The reaction temperature is more preferably 510 ° C. or more and 690 ° C. or less, more preferably 525 ° C. or more and 675 ° C. or less, and particularly preferably 550 ° C. or more and 650 ° C. or less.

(Example 10)
Example 1 except that in the lithium doping process, an oxygen scavenger material made of sponge titanium was placed in the lower chamber together with the second container containing metallic lithium, and the heating temperature of the processing chamber was set to 600 ° C. A negative electrode material of Example 10 was obtained in the same manner.

(Example 11)
A negative electrode material of Example 11 was obtained in the same manner as in Example 10 except that no oxygen scavenger made of titanium sponge was arranged.

(XRD analysis 2)
The negative electrode materials of Example 10 and Example 11 were subjected to X-ray diffraction measurement with a powder X-ray diffractometer using CuKα. FIG. 4 shows an X-ray diffraction chart of each negative electrode material.

  As shown in FIG. 4, in the negative electrode materials of Example 10 and Example 11, since peaks derived from lithium silicide crystals are confirmed, as in the negative electrode materials of Examples 1 to 9, It can be said that the negative electrode materials of Example 10 and Example 11 are also the negative electrode material of the present invention.

Incidentally, in the negative electrode material of Example 10 was carried out with lithium doping process in the presence of oxygen capturing material, as compared with the negative electrode material of Example 11 in the absence of oxygen scavenging material was lithium doping process, Li ₇ Si The ratio of ₃ was large. This result means that the negative electrode material of Example 10 has a larger amount of lithium doping than the negative electrode material of Example 11. That is, it can be said that more lithium gas was generated in the lithium doping process of Example 10 performed in the presence of the oxygen scavenger than in the lithium doping process of Example 11 performed in the absence of the oxygen scavenger. This result supports the usefulness of the oxygen scavenger in producing the negative electrode material of the present invention.

1: Lithium doping apparatus 2: Processing chamber 3: Heating furnace 4: Opening 5: Partition wall 6: Gas inflow path section 7: Gas outflow path section P: Pressure reducing pump 8: First container 9: Second container S: Silicon material L: Metallic lithium

Claims (5)

Si-containing negative electrode active material containing silicon and oxygen, and lithium,
A negative electrode material containing Li ₄ SiO ₄ crystal and Li ₂ SiO ₃ crystal or lithium silicide crystal. The negative electrode material according to claim 1, wherein the lithium silicide crystal is a Li ₁₂ Si ₇ crystal. The negative electrode material according to claim 1 or 2, comprising a Li ₄ SiO ₄ crystal and a Li ₁₂ Si ₇ crystal.   The negative electrode material according to claim 1 or 2, wherein a molar ratio of silicon, oxygen, and lithium represented by Li / (Li + O + Si) is in a range of 0.18 to 0.63.   The negative electrode material according to any one of claims 1 to 4, wherein the Si-containing negative electrode active material is a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in a thickness direction.