JP2017526144A - Anode materials for lithium-ion batteries - Google Patents

Abstract

An anode material for a lithium ion battery, comprising: a carbon shell having an internal hollow space; and silicon particles in the hollow space inside the carbon shell.

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.

  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).

In recent years, silicon (Si) has emerged as one of the most promising electrode materials for next generation high energy LIBs. This is a low voltage suitable for the negative electrode and a maximum of 4,200 mAh / based on the formation of Li _4.4 Si alloy, 10 times higher than the conventional carbon negative electrode (372 mAh / g corresponding to the formation of LiC ₆ ). Provides a high theoretical specific capacity of g. However, since Si forms a Li _4.4 Si alloy, the volume expands up to 400% upon complete lithium insertion (lithiation), and the alloy returns to Si upon lithium extraction (delithiation). It shrinks significantly and forms two important challenges associated with silicon-based negative electrode materials: reduced mechanical integrity of the Si negative electrode material and SEI stability.

  The stress induced by the large capacity change causes cracking and crushing of the Si negative electrode material resulting in loss of electrical connection and eventual capacity reduction. This was believed to be the main reason for the rapid capacity loss in early studies of silicon negative electrode materials. In recent years, designing nanostructured materials, including composites of carbon nanowires, carbon nanotubes, and carbon nanoporous membranes with silicon nanoparticles, has been successful in addressing stability issues in these materials. is there. US Publication No. 2012/0064409 discloses nano graphene reinforced particles in which a graphene sheet and a negative electrode active material such as Si are bonded or aggregated to each other. Due to their small size and open space around the Si nanoparticles, the distortion of the nanostructures can be easily relaxed without mechanical destruction. This nanostructured strategy greatly increased the cycle life of the anode material up to several hundred cycles with 75% capacity retention. However, it is still far from the expected value.

  The stability of SEI at the Si / electrolyte interface is another important factor for obtaining a long cycle life. It is a very difficult problem and has not been addressed with materials that have effectively large volume changes. The decomposition of the electrolyte occurs due to the low potential of the negative electrode and forms a passivation SEI layer on the surface of the negative electrode material during battery charging. The SEI layer is an electrical insulator, but is a lithium ion conductor, which eventually leads to SEI growth stopping at a certain thickness. Despite the fact that silicon nanostructures mainly overcome the problem of mechanical breakdown, the interface with the electrolyte is not static due to repeated volume expansion and contraction. The problem of SEI stability will be described with reference to FIG. FIG. 2 is a schematic diagram of SEI formation on the surface of a conventional negative electrode material made of solid Si. The Si particles 21 expand upon charging by being changed (lithiated) into the Si—Li alloy 23. A thin layer of SEI 22 is formed on the surface of the Si-Li alloy 23 in a lithiated state. During discharge (delithiation), the Si-Li alloy 23 returns to the Si particles 21 and the SEI 22 is broken into individual pieces, exposing the fresh Si surface to the electrolyte. In later cycles, new SEI continues to form on the newly exposed Si surface, which ultimately results in a very thick layer of SEI 22 on the outside of the Si particles 21. This is due to: a) consumption of electrolyte and lithium ions during continuous SEI formation, b) electrical insulation properties of SEI weakening electrical contact between current collector and negative electrode material, c) long lithium through thick SEI It causes battery performance degradation through diffusion distance, and d) decomposition of the negative electrode material caused by mechanical stress. The formation of a stable SEI is important for achieving a long life with the Si negative electrode material, which is generally maintained in the electrode materials of other batteries that undergo large volume changes.

US Publication 2012/0064409

  Here, we have seriously studied the above problem, and the hollow space inside the carbon shell is wide enough for the expansion of the Si particles encapsulated therein by lithiation, and the carbon that traps the Si particles. The shell has discovered a new anode material that allows lithium ions to pass through and forms a stable SEI on its surface.

  According to one aspect of the present invention, there is provided an anode material for a lithium ion battery including a carbon shell having an internal hollow space and silicon particles in the hollow space inside the carbon shell.

  In this design, the liquid electrolyte contacts only the outer surface of the carbon shell and does not substantially penetrate the internal hollow space. During lithiation, lithium ions penetrate the carbon shell and react with the inner Si particles. The carbon shell has mechanical rigidity so that the inner Si particles can expand in the inner hollow space. This internal expansion is possible because silicon softens significantly during critical lithium insertion. During delithiation, it shrinks back to internal Si particles. Overall, the interface with the electrolyte is mechanically constrained and remains stationary during both lithiation and delithiation. Si particles expand and contract only inside the carbon shell and do not come into contact with the SEI-forming substance in the electrolyte. The material so designed provides two attractive points as an anode material: first, the static outer surface allows for stable SEI deployment, and second, the internal hollow space is machine Allows free volume expansion of Si particles without disruption.

1 is a schematic view of a method for producing a composite anode material in an example embodiment. FIG. It is the schematic diagram of SEI formation in the anode material surface which consists of the conventional solid Si. It is a schematic diagram of SEI formation on the surface of the composite anode material of the embodiment. It is a scanning transmission electron microscope (STEM) cross-sectional image of the negative electrode using the composite anode material of the embodiment example as an active substance. 2 is a scanning electron microscope (SEM) image of the composite anode material of Example 1. FIG. It is a STEM cross-sectional image of the negative electrode using the composite anode material of Example 1 as an active material. It is a STEM cross-sectional image of the negative electrode using the composite anode material of Example 2 as an active material.

The invention will now be described with reference to example embodiments.
A configuration of a composite anode material according to an embodiment and a manufacturing method thereof will be described with reference to FIG.

  First, silicon suboxide (SiOx, 0 <X <2) particles 1 having an appropriate particle size are provided as a starting material. The range of x is preferably 0.5 to 1.6, more preferably about 1, i.e. silicon monoxide. The starting material can be prepared by oxidizing silicon or by coating pure silicon with silicon dioxide. Solid silicon monoxide particles are commercially available. Silicon dioxide particles containing silicon particles can be used as starting material. The particle size of the SiOx (starting material) defines the size of the resulting composite anode material.

  Second, the SiOx particles 1 are coated with a carbon layer to form a carbon shell 2. The carbon shell can be formed by thermal decomposition of hydrocarbons such as sugars or using CVD methods. The temperature of thermal decomposition can be 500 degreeC-1800 degreeC. The CVD method can be performed by heating a carbon source. Examples of the carbon source include, but are not limited to, hydrocarbons such as methane, ethane, ethylene, acetylene, and benzene, organic solvents such as methanol, ethanol, toluene, and xylene, and carbon monoxide. The temperature of CVD can be 400-1200 degreeC. The thickness of the carbon layer can be 100 nm or less, preferably 50 nm or less, and most preferably 10 nm or less. The carbon layer can be doped with halogens such as boron (B), nitrogen (N), and fluorine (F). The outer diameter (diameter) of the carbon shell depends on the size and thickness of the starting material (SiOx particles 1), and is 300 nm to 80 μm, preferably 800 nm to 50 μm, and most preferably 1000 nm to 20 μm. In other words, the size of the anode material is the diameter of the carbon shell.

Third, the carbon-coated SiOx particles are heat-treated (heating). The heat treatment can be performed, for example, in a temperature range of 600 to 2000 ° C., preferably 800 to 1200 ° C. under an inert atmosphere such as N ₂ , argon, or a mixture thereof. Heating during carbon coating may be part of this heat treatment. The Si particles 3 are developed by heat treatment, decomposition of silicon monoxide into Si and SiO2, or change of a portion where Si is lost with crystallization of the Si element in the SiOx particles 1 to SiO ₂ 4. The growth of the Si particles 3 can be controlled by heating conditions. The size and number of Si particles are not limited when the total volume of Si particles expanded by lithiation is smaller than the internal space of the carbon shell 2. The size of the Si particles decreases as the temperature increases. The number of Si particles increases as the temperature increases. The size of the Si particles is 1 nm to 20 μm, preferably 5 nm to 10 μm, and most preferably 10 nm to 5 μm. The number of Si particles contained in the hollow space inside the carbon shell is 100 or less, preferably 20 or less, and most preferably 5 or less. If silicon dioxide particles containing silicon particles are used as starting material, this heat treatment can be omitted.

Finally, SiO ₂ 4 is etched (etching) with an etchant such as a hydrogen fluoride (HF) aqueous solution. The etching can be performed at a temperature range of about 0 to 70 ° C. for 0.5 to 48 hours. The concentration of the HF solution can be about 5% to about 50% (concentrated HF). It is preferable that all SiO ₂ 4 is removed. However, when the expansion and contraction of the Si particles 3 are not affected, a part of the SiO ₂ 4 may remain in the carbon shell 2. After etching, an internal hollow space (void) 5 is formed inside the carbon shell 2 to complete the composite anode material 10. The internal hollow space 5 is a space that is substantially closed from the outside of the carbon shell except that lithium ions pass through the carbon shell.

  FIG. 3 is a schematic view of SEI formation on the surface of the composite anode material according to this embodiment. The Si particles 3 expand at the time of charging (lithiation) by changing to the Si—Li alloy 31 in the internal hollow space 5 of the carbon shell 2. A thin layer of SEI 32 having lithium ion conductivity is formed on the surface of the carbon shell 2. During discharge (delithiation), the Si—Li alloy 31 returns to the Si particles 3, but the SEI 32 can maintain the shape as it is. In a later cycle, the SEI 32 can be stably maintained. Thus, the composite anode material of this example embodiment does not consume much of the electrolyte to form SEI.

  FIG. 4 is a scanning transmission electron microscope (STEM) cross-sectional image of a negative electrode using the anode material of the embodiment as an active material. As shown in FIG. 4, the anode material of this embodiment does not break even when compressed during slurry preparation and electrode formation. This is very surprising because it seemed that the carbon shell would be easily destroyed.

  Another exemplary embodiment relates to a lithium ion battery including a negative electrode including an anode material according to the above exemplary embodiment. The anode material preferably has a capacity of at least 600 mAh / g, more preferably a capacity of 800 mAh / g or more. 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 can be produced alone as a secondary battery electrolyte operating 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
First, 10 g of 5 μm diameter SiO particles were coated with amorphous carbon by the CVD method. The subsequent carbon coated SiO was heated at 950 ° C. for 5 hours. 20% HF was used to etch silicon oxide. The final product was washed and dried in a vacuum oven for 24 hours to obtain anode material A. An SEM image of anode material A is shown in FIG.

Example 2
First, 10 g of 5 μm diameter SiO particles were coated with amorphous carbon by the CVD method. The subsequent carbon coated SiO was heated at 1100 ° C. for 5 hours. 20% HF was used to etch silicon oxide. The final product was washed and dried in a vacuum oven for 24 hours to obtain anode material B.

Comparative Example 1
5 μm diameter SiO particles were used as anode material C.

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

Preparation of Test Cell Each of anode materials A to D, carbon black, and polyimide were mixed in N-methylpyrrolidone (NMP) at a weight ratio of 90: 1: 9 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 SETM cross-sectional images of the negative electrode using the anode material A of Example 1 and the anode material B of Example 2 are shown in FIGS. 6 and 7, respectively. 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 (8)

A carbon shell having an internal hollow space;
An anode material for a lithium ion battery comprising silicon particles in a hollow space inside the carbon shell.   2. The anode material according to claim 1, wherein the size and number of the silicon particles satisfy a condition that a total volume of the silicon particles is smaller than a volume of the internal hollow space of the carbon shell in a state expanded by lithiation.   The anode material according to claim 2, wherein the silicon particles have a size of 1 nm to 20 μm.   The anode material according to claim 2, wherein the number of silicon particles is 100 or less.   The anode material according to claim 1, wherein the carbon shell has a thickness of 100 nm or less.   The anode material according to claim 1, wherein the anode material has a diameter of 300 nm to 80 µm as a diameter of the carbon shell.   It is a lithium ion battery containing a positive / negative electrode, Comprising: The said negative electrode is a lithium ion battery containing the anode material of any one of Claims 1-6.   The lithium ion battery of claim 7, wherein the anode material has a capacity of at least 600 mAh / g.