WO2016031082A1

WO2016031082A1 - Anode material for lithium ion battery

Info

Publication number: WO2016031082A1
Application number: PCT/JP2014/073420
Authority: WO
Inventors: Qian CHENG; Noriyuki Tamura; Kentaro Nakahara
Original assignee: Nec Corporation
Priority date: 2014-08-29
Filing date: 2014-08-29
Publication date: 2016-03-03
Also published as: JP2017526144A

Abstract

The present invention provides an anode material for a lithium-ion battery comprising a carbon shell providing an inner hollow space, and silicon particles included in the inner hollow space of the carbon shell.

Description

DESCRIPTION

TITLE OF THE INVENTION

ANODE MATERIAL FOR LITHIUM ION BATTERY

TECHNICAL FIELD

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

BACKGROUND ART

Among all the rechargeable battery technologies, lithium-ion batteries (LIBs) offer superior performance, and are suitable for a main power source in portable electronics. LIBs are also the most promising power source for electric vehicles and are projected to be enablers of smart grids based on renewable energy technologies. For many of these applications, energy density and cycle life stand out as two important technical parameters which need significant improvements. For example, in 2010 the U.S. Department of Energy has put forth a goal to create a LIB with double the energy density of current batteries and a cycle life of 5000 cycles with 80% capacity retention for electric vehicles, in comparison, the typical high energy batteries used in portable electronics only have a cycle life of 500-1000 cycles: Increasing energy density in LIBs requires developing electrode materials with higher charge capacity or higher voltage. Improving cycle life involves stabilizing two critical components of battery electrodes: active electrode materials and their interface with electrolyte -so-called "solid-electrolyte interphase" (SEI).

Recently, silicon (Si) has emerged as one of the most promising electrode materials for next-generation high energy LIBs. It offers a suitable low voltage for an anode and a high theoretical specific capacity up to 4,200 mAh/g based on the formation of Li₄₄Si alloy, which is 10 times higher than that of conventional carbon anodes (372 mAh/g corresponding to the formation of LiC₆). However, Si expands volumetrically by up to 400% upon full lithium insertion (lithiation) to form the Li₄₄Si alloy, and the alloy can contract significantly to return Si upon lithium extraction (delithiation), creating two critical challenges associated with

silicon-based anode materials: degradation of the mechanical integrity of Si anode materials and SEI stability.

Stress induced by the large volume changes causes cracking and

pulverization of Si anode materials, which leads to loss of electrical contact and eventual capacity fading. This was considered to be main reason for rapid capacity loss in early studies of Si anode materials. Recently, there have been successes in addressing stability issues in these materials by designing nanostructured materials, including composites of carbon nanowires, carbon nanotubes, carbon nanoporous films with Si nanoparticle. US 2012/0064409 A 1 discloses a nano

graphene-enhanced particulate that graphene sheets and anode active materials such as Si are mutually bonded or agglomerated. Due to their small size and open space surrounding Si nanoparticle, the strain in nanostructures can be easily relaxed without mechanical fracture. This nanostructuring strategy has greatly increased the cycle life of the anode material up to a few hundred cycles with 75% capacity retention; however, it is still far from expectations.

The SEI stability at the interface between Si and the liquid electrolyte is another critical factor in obtaining long cycle life; it is a very challenging issue and has not been effectively addressed for materials with large volume changes.

Electrolyte decomposition occurs due to the low potential of the anode and forms a passivation SEI layer on the surface of the anode material during battery charging. The SEI layer is an electronic insulator but a lithium ion conductor; this results in the SEI eventually terminating growth at a certain thickness. Even though Si nanostructures have largely overcome mechanical fracture issues, their interface with the electrolyte is not static due to repetitive volume expansion and contraction. The issue of the SEI stability will be described with reference to Fig. 2. Fig 2 is a schematic diagram of SEI formation on the surface of a conventional anode material composed of solid Si. Si particle 21 expands upon charging (lithiation) by changing into Si-Li alloy 23. A thin layer of SEI 22 forms on the surface of Si-Li alloy 23 in the lithiated and expanded state. During discharging (delithiation), Si-Li alloy 23 returns to Si particle 21, and SEI 22 can break down into separate pieces, exposing fresh Si surface to the liquid electrolyte. In later cycles, new SEI continues to form on newly exposed Si surface, and this finally results in a very thick layer of SEI 22 on the outside of Si particle 21. This results in battery performance degradation through: a) the consumption of electrolyte and lithium ions during continuous SEI formation, b) the electrically insulating nature of the SEI weakening electrical contact between the current collector and the anode material, c) the long lithium diffusion distance through the thick SEI and d) degradation of the anode material caused by mechanical stress from the thick SEI. Formation of a stable SEI is critical for realizing long cycle life in Si anode materials, which also holds in general for other battery electrode materials that undergo large volume changes.

SUMMARY OF THE INVENTION

Here, we have intensively investigated the above issues and found a novel anode material, in which an inner hollow space inside a carbon shell is sufficiently wide for expansion of Si particles encapsulated therein at lithiation and the carbon shell that is confining Si particles allows passing lithium ions therethrough and forming a stable SEI on the surface thereof.

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

In this design, liquid electrolyte only contacts the outer surface of the carbon shell and cannot substantially enter the inner hollow space. During lithiation, lithium ions penetrate through the carbon shell and react with the inner Si particles. The carbon shell is mechanically rigid, so the inner Si particles can expand in the inner hollow space. This inner expansion is possible because silicon is considerably softened upon significant lithium insertion. During delithiation, the inner Si particles shrink back. Overall, the interface with electrolyte is mechanically constrained and remains static during both lithiation and delithiation. The Si particles expand and contract only in the inside of the carbon shell, and they are not in contact with SEI formation substances in the electrolyte. Such a designed material provides two attractive points as an anode material: first, the static outer surface allows for the development of a stable SEI; and second, the inner hollow space allows for free volume expansion of Si particles without mechanical breaking.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 : a schematic diagram of a production process for a composite anode material in one exemplary embodiment;

Fig 2: a schematic diagram of SEI formation on the surface of a

conventional anode material composed of solid Si;

Fig 3: a schematic diagram of SEI formation on the surface of the composite anode material of the exemplary embodiment;

Fig. 4: Scanning Transmission Electron Microscopy (STEM) cross-section image of a negative electrode using the composite anode material of the exemplary embodiment as an active material;

Fig. 5: Scanning Electron Microscopy (SEM) image of the composite anode material of Example 1;

Fig. 6: STEM cross-section image of a negative electrode using the composite anode material of Example 1 as an active material; and

Fig. 7: STEM cross-section image of a negative electrode using the composite anode material of Example 2 as an active material. MODES FOR CARRYING OUT THE INVENTION

The invention will be now described herein with reference to illustrative embodiments.

A configuration and a production process for a composite anode material according to an exemplary embodiment will be described with reference to Fig. 1.

First, silicon sub-oxide (SiOx: 0<x<2) particle 1 having an appropriate particle size is provided as a starting material. The range of x is preferably between 0.5 and 1.6, and 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 including silicon particles can be used as a starting material. The particle size of SiOx (starting material) defines the resultant size of the composite anode material. Second, SiOx particle 1 is coated with carbon layer to prepare carbon shell 2 (C coating). The carbon shell can be formed by pyrolysis of hydrocarbons such as sugar or by using CVD method. Temperatures in pyrolysis can be 500 to 1800°C. The CVD method can be carried out by using carbon source with heating. The carbon source is not restricted, examples of carbon source include hydrocarbons such as methane, ethane, ethylene, acetylene and benzene; organic solvents such as methanol, ethanol, toluene and xylene; and carbon monoxide. Temperatures of CVD can be 400 to 1200°C. The thickness of the carbon layer can be not greater than 100 nm, preferably not greater than 50 nm, most preferably not greater than 10 nm. The carbon layer can be doped with boron (B), nitrogen (N), halogen such as fluorine (F), etc. The outer size (diameter) of the carbon shell depends on the size of the starting material (SiOx particle 1) and thickness, and it can be 300 nm to 80 μπι, preferably 800 nm to 50 μιη, most preferably 1000 nm to 20 μηι. In other words, a size of the anode material is in terms of the diameter of the carbon shell.

Third, the carbon coated SiOx particle is subjected to heat treatment

(Heating). The heat treatment can be carried out at a temperature range of 600 to 2000°C, preferably 800 to 1200°C under inert atmosphere such as N₂, Ar or mixture thereof. Heating at the time of the carbon coating can be also as a part of this heat treatment. According to the heat treatment, Si particles 3 is developed by decomposing silicon monoxide to Si and Si0₂ or by crystalizing Si elements in SiOx particle 1 as well as silicon eliminated part is changed to Si0₂ 4. The growth of Si particles 3 can be controlled by heating conditions. The size and number of Si particles are not limited if the total volume of Si particles in the state of expansion due to lithiation is less than that of the inner space of carbon shell 2. The size of Si particles decreases as the temperature increases. The number of Si particles increases as the temperature increases. The size of Si particles can be 1 nm to 20 μιη, preferably 5 nm to 10 μηι, most preferably 10 nm to 5 μηι. The number of Si particles included in the inner hollow space of carbon shell can be not greater than 100, preferably not greater than 20, most preferably not greater than 5. This heat treatment can be omitted when using the silicon dioxide particle including silicon particles as the starting material.

Finally, Si0₂ 4 is etched with etching solution such as aqueous hydrogen fluoride (HF) solution (Etching). The etching can be carried out at a temperature range of about 0 to 70°C for 0.5 to 48 hours. The concentration of HF solution can be about 5% to about 50% (cone. HF). It is preferable that all Si0₂ 4 is removed, but a part of Si0₂ 4 may be left in the carbon shell 2 if it does not affect the expansion and contract of Si particles 3. After the etching, inner hollow space

(void) 5 is created in the inside of carbon shell 2 so that composite anode material 10 is completed. Inner hollow space 5 is substantially a closed space from an outside of the carbon shell except that lithium ions allow passing the carbon shell.

Fig 3 is a schematic diagram of SEI formation on the surface of the composite anode material according to this exemplary embodiment. Si particle 3 expands upon charging (lithiation) by converting to Si-Li alloy 31 in inner hollow space 5 of carbon shell 2. A thin layer of SEI 32 which is lithium ion conductive forms on the surface of carbon shell 2. During discharging (delithiation), Si-Li alloy 31 returns to Si particle 3, but SEI 32 can remain the shape as it is. In later cycles, SEI 32 can stably remain. In this way, the composite anode material of the exemplary embodiment does not consume lots of electrolyte for forming SEI.

Fig. 4 is Scanning Transmission Electron Microscopy (STEM) cross-section image of a negative electrode using the anode material of the exemplary embodiment as an active material. As shown in Fig. 4, the anode material of the exemplary embodiment does not break during preparing slurry and after pressing at the electrode formation. This is very surprised because it appeared that the carbon shell would be readily broken.

Another exemplary embodiment relates to a lithium-ion battery including a negative electrode comprising the anode material according to the above exemplary embodiment. The anode material preferably has a capacity of at least 600 mAh/g, more preferably 800 mAh/g or more. The battery also comprises a positive electrode comprising an active material, an electrolyte comprising a lithium salt dissolved in at least one non-aqueous solvent and a separator configured to allow electrolyte and lithium ions to flow between a first side of the separator and an opposite second side of the separator.

As for the active material of the positive electrode, but there is also no particular restriction on the type or nature thereof, known cathode materials can be used for practicing the present exemplary embodiment. The cathode materials may be at least one material selected from the group consisting of 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 phosphates, metal sulfides, and combinations thereof. The cathode material may also be at least one compound selected from chalcogenide compounds, such as titanium disulfate or molybdenum disulfate. More preferred are lithium cobalt oxide (e.g., Li_xCo0₂ where 0.8<x<l), lithium nickel oxide (e.g., LiNi0₂) and lithium manganese oxide (e.g., LiMn₂0₄ and LiMn0₂) because these oxides provide a high cell voltage. Lithium iron phosphate is also preferred due to its safety feature and low cost. All these cathode materials can be prepared in the form of a fine powder, nano-wire, nano-rod, nano-fiber, or nano-tube. They can be readily mixed with an additional conductor such as acetylene black, carbon black, and ultra-fine graphite particles.

For the preparation of the positive and negative electrodes, a binder can be used. Examples of the binder include polytetrafluoroethylene (PTFE),

polyvinylidene fluoride (PVDF), ethylene propylenediene copolymer (EPDM), or styrene-butadiene rubber (SBR). The positive and negative electrodes can be formed on a current collector such as copper foil for the negative electrode and aluminum or nickel foil for the positive electrode. However, there is no particularly significant restriction on the type of the current collector, provided that the collector can smoothly path current and have relatively high corrosion resistance. The positive and negative electrodes can be stacked with interposing a separator therebetween. The separator can be selected from a synthetic resin nonwoven fabric, porous polyethylene film, porous polypropylene film, or porous PTFE film.

A wide range of electrolytes can be used for manufacturing the battery. Most preferred are non-aqueous and polymer gel electrolytes although other types of electrolytes can be used. The non-aqueous electrolyte to be employed herein may be produced by dissolving an electrolyte salt (Li salt) in a non-aqueous solvent. Any known non-aqueous solvent which has been employed as a solvent for a lithium secondary battery can be employed. A mixed solvent comprising ethylene carbonate (EC) and at least one kind of non-aqueous solvent whose melting point is lower than that of ethylene carbonate and whose donor number is 18 or less

(hereinafter referred to as a second solvent) may be preferably employed as the non-aqueous solvent. The second solvent to be used in the mixed solvent with EC functions to make the viscosity of the mixed solvent lowering than that of which EC is used alone, thereby improving an ion conductivity of the mixed solvent.

Furthermore, when the second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is employed, the aforementioned ethylene carbonate can be easily and selectively solvated with lithium ion, so that the reduction reaction of the second solvent with the carbonaceous material well developed in graphitization is assumed to be suppressed. Further, when the donor number of the second solvent is controlled to not more than 18, the oxidative decomposition potential to the lithium electrode can be easily increased to 4 V or more, so that it is possible to manufacture a lithium secondary battery of high voltage. Preferable second solvents are dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), γ- butyrolactone (y-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA).

These second solvents may be employed singly or in a combination of two or more. More desirably, this second solvent should be selected from those having a donor number of 16.5 or less. The viscosity of this second solvent should preferably be 28 cps or less at 25°C. The mixing ratio of the aforementioned ethylene carbonate in the mixed solvent should preferably be 10 to 80% by volume. If the mixing ratio of the ethylene carbonate falls outside this range, the conductivity of the solvent may be lowered or the solvent tends to be more easily decomposed, thereby deteriorating the charge/discharge efficiency. More preferable mixing ratio of the ethylene carbonate is 20 to 75% by volume. When the mixing ratio of ethylene carbonate in a non-aqueous solvent is increased to 20% by volume or more, the solvating effect of ethylene carbonate to lithium ions will be facilitated and the solvent

decomposition-inhibiting effect thereof can be improved.

Further, in the electrolyte, in order to maintain a stable quality SEI layer on the negative electrode surface, additives may be added. The SEI layer has a role to suppress reactivity with the electrolyte solution (decomposition), and subjected to desolvation reactions due to delithiation of the lithium ion battery, and to suppress the structural physical degradation of the anode material. Examples of the additives include vinylene carbonate (VC), propane sultone (PS), and cyclic disulfonic acid ester.

Examples of the Li salt according to this exemplary embodiment include

LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, L1CIO₄, LiAlCL,, LiN(C«F₂«₊₁S0₂)(CmF₂/w₊₁S0₂) (n and m are natural numbers), and LiCF₃S0₃. 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.

A casing for the battery in the exemplary embodiment may be, for example, a laminate film in which a substrate, a metal foil and a sealant are sequentially laminated. Examples of a substrate which can be used include a resin film with a thickness of 10 to 25 μπι made of polyester (PET) or Nylon. A metal foil may be an aluminum film with a thickness of 20 to 40 μιη. A sealant may be a rein film with a thickness of 30 to 70 μπι made of polyethylene (PE), polypropylene (PP), modified polypropylene (PP) or an ionomer.

EXAMPLE

Example 1

1 Og SiO particles with diameter of 5 μηι were first coated with amorphous carbon by CVD method. Then the carbon coated SiO was heated at 950°C for 5h. 20% HF was used to etch the silicon oxide. The final product was washed and dried in vacuum oven for 24h to obtain anode material A. SEM images of anode material A are shown in Fig. 5.

Example 2

lOg SiO particles with diameter of 5 μπι were first coated with amorphous carbon by CVD method. Then the carbon coated SiO was heated at 1100°C for 5h. 20% HF was used to etch the silicon oxide. The final product was washed and dried in vacuum oven for 24h to obtain anode material B. Comparative Example 1

SiO particles with diameter of 5 μπι were used as anode material C.

Comparative Example 2

Si particles with diameter of 5 μιη were used as anode material D.

Fabrication of a test cell

Slurry was prepared by mixing each of anode materials A to D, carbon black, and polyimide in a ratio of 90: 1 :9 in N-methylpyrrolidone (NMP). The slurry was coated on a Cu foil and dried at 120°C for 15 min to form a thin substrate. Then, the thin substrate was pressed to 45 μπι thick with the loading density of 50 g/m and then heat treated at 200°C for 2h in N₂ atmosphere to prepare a negative electrode. SETM cross-section images of a 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, while a metal lithium foil was used as a counter electrode. A separator made of porous polypropylene film was interposed between the working electrode and counter electrode. The electrolyte prepared by dissolving LiPF₆ in a mixed solvent of ethyl carbonate (DEC) and ethylene carbonate (EC) in a ratio of 7:3 in a concentration of 1 M, then a laminate half-cell was fabricated.

The test cell was evaluated in initial charge capacity, coulombic efficiency, rate capabilities of 1C charge/0.1C discharge and 6C charge/0.1C discharge and capacity retention 1C after 100 cycles. Results are shown in Table 1.

Table 1

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.

Claims

1. An anode material for a lithium-ion battery, comprising:

a carbon shell providing an inner hollow space, and

silicon particles included in the inner hollow space of the carbon shell.

2. The anode material according to claim 1, wherein size and number of the silicon particles satisfy that a total volume of the silicon particles in an expanded state by lithiation is less than a volume of the inner hollow space of the carbon shell.

3. The anode material according to claim 2, wherein the size of silicon particles is from 1 ran to 20 μιη.

4. The anode material according to claim 2, wherein the number of the silicon particles is not greater than 100.

5. The anode material according to claim 1, wherein a thickness of the carbon shell is not greater than 100 nm.

6. The anode material according to claim 1 , wherein a size of the anode material is from 300 nm to 80 μηι in terms of a diameter of the carbon shell.

7. A lithium ion battery comprising positive and negative electrodes, the negative electrode comprises the anode material according to any one of claims 1 to 6.

8. The lithium ion battery according to claim 7, wherein the anode material has a capacity of at least 600 mAh/g.