JP6384596B2 - Anode materials for lithium-ion batteries - Google Patents

Description

  The present invention relates to a negative electrode (anode) material for a lithium ion battery. In particular, the present invention relates to a composite negative electrode material of carbon and silicon.

Conventional technology

  Among all rechargeable battery technologies, lithium ion batteries (LIBs) provide superior performance and are suitable for the main power source of portable electronic devices. LIB is also the most promising power source for electric vehicles and is predicted to be a smart grid enabler based on renewable energy technology. For many of these applications, energy density and cycle life stand out as two important technical parameters that require significant improvement. For example, in 2010, the US Department of Energy has twice the energy density of current batteries for electric vehicles, and typical high energy batteries used in portable electronics are only 500-1000 The goal is to create a LIB with a cycle life of 5000 cycles with 80% capacity retention compared to having only a cycle life of a cycle. Increasing the energy density in LIB requires the development of electrode materials with higher charge capacity or higher voltage. Improving cycle life involves stabilizing two important components of the battery electrode of the active electrode material and its interface with the electrolyte, the so-called SEI (Solid Electrolyte Interface).

  Compared to the negative electrode, lithium titanate is an alternative to graphite with excellent cycling properties, but with a lower energy density. Other alternatives to graphite, such as tin oxide and silicon, have the potential to provide increased energy density. However, these other alternatives for negative electrode materials, particularly silicon, are commercially unsuitable due to poor charge / discharge cycles associated with structural changes and unusually large volume expansions, with lithium insertion / alloying. I know that. Structural changes and large volume changes destroy the structural integrity of the electrodes and reduce cycle efficiency.

  In recent years, composite anode materials of carbon / Li storable substances have been proposed (for example, JP-A Nos. 2004-119176, 2004-349253, and 2005-71938). These disclose that Li-storable materials such as Si, Sn and their oxides are embedded in the carbon matrix.

JP 2004-119176 A JP 2004-349253 A JP 2005-71938 A

A conventional composite anode material is manufactured by previously manufacturing an active material that is a Li-storable material and then coating the active material with carbon. Therefore, the content of the active material varies from particle to particle.
An object of the present invention is to provide an anode material for a lithium ion battery including an active material-embedded hard carbon having a smaller content change in each particle, and a lithium ion battery including the anode material.

  That is, according to one aspect of the present invention, there is provided an anode material for a lithium ion battery including active material-embedded hard carbon, wherein the active material includes at least one oxide selected from silicon and tin, and the oxidation material An anode material is provided wherein the article is produced by solvothermal synthesis in a medium comprising a precursor of hard carbon from its precursor.

  According to one aspect of the present invention, since the active material is produced in situ with the carbon precursor decomposition, the anode material for a lithium ion battery containing active material-embedded hard carbon having a smaller content change in each particle Can provide.

2 is an SEM image of anode material A produced in Example 1. FIG. XRD of anode material A produced in Example 1. 4 is an SEM image of anode material B manufactured in Example 2. FIG.

  The invention will now be described with reference to embodiments.

Anode Material One embodiment of the present invention relates to an anode material for a lithium ion battery containing active material embedded hard carbon.
The anode material of this embodiment is obtained by solvent thermal synthesis, particularly hydrothermal synthesis using water as a solvent. First, a carbon precursor solution is provided by dissolving the carbon precursor in a solvent such as water. Next, a precursor for the active material is added to the carbon precursor solution, and then the mixture is heated under a high pressure atmosphere. As the reaction apparatus, a pressure vessel such as an autoclave is usually used. Solvent thermal (hydrothermal) synthesis converts the precursor for the active material into an active material having a crystalline form, particularly a nanocrystalline form. In the present embodiment example, the active material includes at least one oxide selected from silicon and tin. The active material is typically silicon dioxide or tin dioxide, but they may contain non-oxidized metal moieties. During solvent thermal (hydrothermal) synthesis, the carbon precursor is decomposed and adsorbed onto the active material or aggregates of the active material. The decomposed carbon precursor is then carbonized at high temperature in an inert atmosphere and converted to hard carbon.

  Examples of the carbon precursor include polymers such as polyimide, furan resin, phenol resin, polyvinyl alcohol, cellulose resin, epoxy resin and polystyrene resin, and sugars such as sucrose. In the case of hydrothermal synthesis, the carbon precursor is preferably soluble in water, and saccharides are suitable.

  The precursor for the active material can be an inorganic or organic compound of silicon or tin. Examples of precursors for active materials include inorganic or organic salts such as silicon or tin chloride, sulfate, carbonate, organic silicon such as 3-aminopropylmethyldiethoxysilane, butyl (trichloro) stannane, or An organic tin compound is mentioned.

  The solvent used for the solvent thermal synthesis is a solvent that can dissolve the carbon precursor. Water is preferably used, and a water-soluble solvent such as alcohol can be used together with water.

  The concentration of the carbon precursor solution is preferably 0.2 to 6 mol / L. Solvent thermal synthesis is carried out at a temperature below the supercritical temperature of the solvent. For example, the hydrothermal synthesis is performed at a temperature lower than 374 ° C., which is the supercritical temperature of water, preferably 160 to 300 ° C. for 1 to 24 hours.

  The size of the anode material can be between 20 nm and 80 μm, more preferably between 100 nm and 50 μm, and most preferably between 500 nm and 20 μm. The size of the active material in the hard carbon can be less than 100 nm, preferably less than 50 nm, and most preferably less than 10 nm. Hard carbon can be doped with boron, nitrogen or the like. The ratio of hard carbon to active material is preferably 50: 1 to 1: 1.

Here, a novel active material composite material embedded in hard carbon was designed as a lithium ion battery anode material. Its structural advantage is
1) The active material embedded in the hard carbon does not come into direct contact with the electrolytic solution (solvent) during charging and discharging. Therefore, SEI is formed only on the surface of hard carbon during cycling, and has high Coulomb efficiency and long cycle life.
2) The active material is coated with hard carbon having a lower resistance than a pure active material.
3) Since the active material is produced in situ by decomposing the hard carbon precursor, the active material is dispersed in the hard carbon at an atomic level uniform dispersion. Therefore, the content deviation of the active material in each particle can be reduced.

Lithium Ion Battery Another embodiment relates to a lithium ion battery including a negative electrode including an anode material according to the above embodiment. The anode material has a capacity of at least graphite, ie a capacity of 372 mAh / g. The battery is also configured such that the positive electrode containing the active material, the electrolyte containing a lithium salt dissolved in at least one non-aqueous solvent, and the electrolyte and lithium ions flow from the first side to the opposing second side. A separator is provided.

The type and properties of the positive electrode active material are not particularly limited, but known cathode materials can be used for carrying out the present invention. Cathode materials are lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate And at least one material selected from the group consisting of salts, metal sulfides, and combinations thereof. The positive electrode active material may also be at least one compound selected from chalcogen compounds such as titanium disulfuric acid or molybdenum disulfuric acid. More preferably, lithium cobalt oxide (eg, Li _x CoO ₂ , 0.8 ≦ X ≦ 1), lithium nickel oxide (eg, LiNiO ₂ ), lithium manganese oxide (eg, LiMn ₂ O ₄ and LiMnO _2). ). Lithium iron phosphate is preferred because of its safety and low cost. All these cathode materials can be prepared in the form of fine powders, nanowires, nanorods, nanofibers or nanotubes. They can be easily mixed with additional conductive agents such as acetylene black, carbon black, and ultrafine graphite particles.

  A binder can be used for the production of the electrode. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene propylene diene copolymer (EPDM), and styrene-butadiene rubber (SBR). The positive electrode and the negative electrode can be formed on a current collector such as a copper foil for the negative electrode and an aluminum or nickel foil for the positive electrode. However, as long as the current can flow smoothly and have a relatively high corrosion resistance, there is no particular limitation on the type of current collector. The positive electrode and the negative electrode can be stacked with a separator in between. The separator can be selected from a synthetic resin nonwoven fabric, a porous polyethylene film, a porous polypropylene film, or a porous PTFE film.

  A wide range of electrolytes can be used to practice the present invention. Most preferred are non-aqueous and polymer gel electrolytes, although other types can be used. The non-aqueous electrolyte employed here is manufactured by dissolving an electrolyte salt in a non-aqueous solvent. Any known non-aqueous solvent that has been employed as a solvent for lithium secondary batteries can be used. A non-solvent mainly composed of a mixed solvent composed of ethylene carbonate (EC) and at least one non-aqueous solvent (hereinafter referred to as second solvent) having a melting point lower than that of the aforementioned ethylene carbonate and a donor number of 18 or less. An aqueous solvent is preferably employed. This non-aqueous solvent is (a) stable to a negative electrode containing a carbonaceous material with a well-developed graphite structure, (b) effective in suppressing reduction or oxidative decomposition of the electrolyte, and (c) It is advantageous in that it has high conductivity. A non-aqueous electrolyte composed only of ethylene carbonate (EC) is advantageous in that it is stable against decomposition upon reduction with a graphitized carbonaceous material. However, EC has a relatively high melting point of 39 ° C. to 40 ° C., a relatively high viscosity, and thus its own conductivity is low, so that EC alone can be produced as a secondary battery electrolyte that operates at or below room temperature. Is not suitable. The second solvent used by mixing with EC functions to lower the viscosity of the mixed solvent than that of EC alone, thereby improving the ionic conductivity of the mixed solvent. Furthermore, when a second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is used, the above-mentioned ethylene carbonate can easily and selectively solvate lithium ions and graphitize. It is considered that the reduction reaction of the second solvent with the well-developed carbonaceous material is suppressed. Furthermore, when the donor number of the second solvent is controlled to 18 or less, the oxidative decomposition voltage with respect to the lithium electrode can be easily increased to 4 V or more, and a high voltage lithium secondary battery can be manufactured. Preferred second solvents are dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γBL), acetonitrile (AN), Ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA). These 2nd solvents can be used individually or in combination of 2 or more types. More preferably, the second solvent should be selected from those having a donor number of 16.5 or less. The viscosity of the second solvent is preferably 28 cps or less at 25 ° C. The mixing ratio of the ethylene carbonate in the mixed solvent is preferably 10 to 80% by volume. When the mixing ratio of ethylene carbonate is out of this range, the conductivity of the solvent tends to decrease or the solvent tends to decompose more easily, thereby deteriorating the charge / discharge performance. A more preferable mixing ratio of ethylene carbonate is 20 to 75% by volume. When the mixing ratio of ethylene carbonate in the non-aqueous solvent is increased to 20% by volume or more, the solvation effect of ethylene carbonate with respect to lithium ions is promoted, and the solvent decomposition suppressing effect is improved.

  In addition, an additive may be added to the electrolytic solution in order to maintain a stable quality SEI layer on the surface of the negative electrode. The SEI layer suppresses reaction (decomposition) with the electrolytic solution, and is exposed to a desolvation reaction due to delithiation of the lithium ion battery, and has a role of suppressing physical deterioration in the structure of the anode material. Examples of the additive include vinylene carbonate (VC), propane sultone (PS), and cyclic sulfonic acid ester.

Examples of Li salt according to this embodiment _{is, LiPF 6, LiBF 4, LiAsF} 6, LiSbF 6, L 1 ClO 4, LiAlCl 4, LiN (C n F 2n + 1 SO 2) (C m F 2m + 1 SO 2) (N and m are natural numbers), and LiCF ₃ SO ₃ . However, the Li salt is not limited to these. One of these Li salts may be used, or two or more of these Li salts may be used in combination.

  The battery case in the present embodiment may be, for example, a laminate film in which a base material, a metal foil, and a sealant are sequentially laminated. Examples of the substrate that can be used include a resin film made of polyester (PET) or nylon and having a thickness of 10 to 25 μm. The metal foil may be an aluminum film having a thickness of 20 to 40 μm. The sealant may be a resin film having a thickness of 30 to 70 μm made of polyethylene (PE), polypropylene (PP), modified polypropylene (PP), or ionomer.

Example 1
6.84 g (20 mmol) of sucrose was added to 100 ml of deionized water, and the mixture was magnetically stirred at 60 ° C. for 30 minutes to obtain a carbon precursor solution. 3.78 g (20 mmol) of SnCl ₂ was then slowly added to the carbon precursor solution. The mixture was charged into a 300 ml Teflon-coated stainless steel autoclave. Subsequently, the autoclave was placed in an oven and heated at 180 ° C. for 4 hours. The autoclave was naturally cooled to room temperature. The dark precipitate was collected, washed several times with deionized water and dried in an oven at 100 ° C. for 24 hours. The obtained powder was carbonized in an oven under an N ₂ atmosphere to obtain an anode material A. Carbonization was performed at a final temperature of 1000 ° C. with a N ₂ flow rate of 100 ml / min, a heating rate of 5 ° C./min. An SEM image of anode material A is shown in FIG. As shown in FIG. 1, the anode material A is a spherical particle having a smooth surface morphology. FIG. 2 is an X-ray diffraction of anode material A. The (110) plane of SnO ₂ in anode material A shifted to a higher incident angle (2θ) compared to SnO ₂ synthesized by other methods, while the other planes remained at the same incident angle. . Furthermore, the strength ratio of SnO ₂ to (101) of (110) face in anode material A is higher than 1.1, and the strength ratio of SnO ₂ synthesized by other methods is always less than 1.

Example 2
6.84 g (20 mmol) of sucrose was added to 100 ml of deionized water, and the mixture was magnetically stirred at 60 ° C. for 30 minutes to obtain a carbon precursor solution. Then 10 ml (48 mmol) 3-aminopropylmethyldiethoxysilane was slowly added to the carbon precursor solution. The mixture was charged into a 300 ml Teflon-coated stainless steel autoclave. Subsequently, the autoclave was placed in an oven and heated at 180 ° C. for 4 hours. The autoclave was naturally cooled to room temperature. The dark precipitate was collected, washed several times with deionized water and dried in an oven at 100 ° C. for 24 hours. The obtained powder was carbonized in an oven under an N ₂ atmosphere to obtain an anode material B. Carbonization was performed at a final temperature of 1000 ° C. with a N ₂ flow rate of 100 ml / min, a heating rate of 5 ° C./min. An SEM image of the anode material B is shown in FIG. During hydrothermal synthesis, silicon oxide grows into rod-like crystals.

Comparative Example 1
An average diameter of 10 μm SnO ₂ particles was used as anode material C.

(Comparative Example 2)
Average diameter 5 μm SiO particles were used as anode material D.

Preparation of Test Cell Each of the anode materials A to D, carbon black, and PVDF were mixed at a weight ratio of 91: 1: 8 into N-methylpyrrolidone (NMP) to prepare a slurry. The slurry was applied on a copper foil and dried at 120 ° C. for 15 minutes to form a thin substrate. Next, the substrate was pressed to a thickness of 45 μm with a load of 50 g / m ² , and heat-treated at 200 ° C. for 2 hours in an N ₂ atmosphere to produce a negative electrode.
The negative electrode was used as a working electrode, and a metal lithium foil was used as a counter electrode. A separator made of a porous polypropylene film was inserted between the working electrode and the counter electrode. 1M LiPF ₆ was dissolved in a mixed solvent of diethyl carbonate (DEC) and ethylene carbonate (EC) at a volume ratio of 7: 3 to prepare an electrolyte solution, and a laminated half cell was prepared.

  The test cell was evaluated with initial charge capacity, coulomb efficiency, rate characteristics of 1C charge / 0.1C discharge and 6C charge / 0.1C discharge, and a capacity maintenance ratio of 1C after 100 cycles. The results are shown in Table 1.

  While the present invention has been described with reference to the exemplary embodiments, the present invention is not limited to the above exemplary embodiments. It will be understood that various changes may be made to the structure and details of the invention which will be apparent to those skilled in the art within the spirit and scope of the invention as defined in the claims.

Claims (14)

A method for producing an anode material for a lithium ion battery in which a silicon oxide active material is embedded in hard carbon ,
Conversion of the silicon oxide precursor into an active material having a nanocrystal form and embedding in the hard carbon by solvent thermal synthesis in a medium containing the silicon oxide precursor and the hard carbon precursor A method for producing an anode material , characterized in that The method for producing an anode material according to claim 1, wherein the solvent thermal synthesis uses water as a solvent. The method for producing an anode material according to claim 2, wherein the hard carbon precursor is a saccharide. The method for producing an anode material according to any one of claims 1 to 3, wherein the anode material has a size of 20 nm to 80 µm. The method for producing an anode material according to any one of claims 1 to 4, wherein the size of the active material is less than 100 nm. The method for producing an anode material according to any one of claims 1 to 5 , wherein the hard carbon is doped with boron or nitrogen. Method for producing the anode material according to any one of claims 1 to 6 the precursor of silicon oxide is 3-aminopropyl methyl diethoxy silane. A lithium ion battery manufacturing method including positive and negative electrodes, wherein the anode material included in the negative electrode is manufactured by the anode material manufacturing method according to any one of claims 1 to 7. Battery manufacturing method . The method of manufacturing a lithium ion battery according to claim 8, wherein the anode material has a capacity of at least 372 mAh / g. An anode material for a lithium ion battery, wherein an active material is embedded in hard carbon, the active material is an oxide of silicon, and the particle size of the active material is less than 10 nm. Anode material to be used. The anode material according to claim 10, wherein the anode material has a size of 20 nm to 80 μm. The anode material according to claim 10, wherein the hard carbon is doped with boron or nitrogen. It is a lithium ion battery containing a positive / negative electrode, Comprising: The lithium ion battery whose anode material contained in the said negative electrode is an anode material of any one of Claims 10-12. The lithium ion battery of claim 13, wherein the anode material has a capacity of at least 372 mAh / g.