WO2016031085A1

WO2016031085A1 - Anode material for lithium ion battery

Info

Publication number: WO2016031085A1
Application number: PCT/JP2014/073423
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: JP6384596B2; JP2017526145A

Abstract

The present invention provides an anode material for a lithium-ion battery comprising active materials embedded in hard carbon, wherein the active materials comprises oxide of at least one kind of metals selected from silicon and tin and the oxide is made from its precursor by solvothermal synthesis in a medium comprising a precursor of the hard carbon.

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 hard carbon and active particle.

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).

For negative electrodes, lithium titanate is an alternative to graphite with good cycling properties, but it has a lower energy density. Other alternatives to graphite, such as tin oxide and silicon, have the potential for providing increased energy density. However, these other alternatives for negative electrode materials have been found to be unsuitable commercially due to poor discharge and recharge cycling related to structural changes and anomalously large volume expansions, especially for silicon, that are associated with lithium intercalation/alloying. The structural changes and large volume changes can destroy the structural integrity of the electrode, thereby decreasing the cycling efficiency.

Recently, carbon/Li storable substance composite anode materials are proposed (for example: JP 2004-119176 A, JP 2004-349253 A and JP 2005-71938 A). These disclose that Li storable substance such as Si, Sn and oxide thereof is embedded in carbon matrix.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

The conventional composite anode materials are fabricated by previously manufacturing active materials that is Li storable substance and then coating the active material with carbon. Therefore, the content of the active material varies in each particle.

An object of the present invention is to provide an anode material for a lithium ion battery comprising active materials embedded hard carbon with less content variation of the active materials in each particle and a lithium ion battery including the anode materials.

MEANS FOR SOLVING PROBLEMS

That is, one aspect of the present invention provides an anode material for a lithium-ion battery including active materials embedded in hard carbon, wherein the active materials includes oxide of at least one kind of metals selected from silicon and tin and the oxide is made from its precursor by solvothermal synthesis in a medium comprising a precursor of the hard carbon.

According to the aspect of the present invention, since the active material is prepared in situ with carbon precursor decomposition, it can provide an anode material for a lithium ion battery comprising active materials embedded hard carbon with less content variation of the active materials in each particle.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 : SEM image of anode material A manufactured in Example 1.

Fig. 2: XRD of anode material A manufactured in Example 1. Fig. 3: SEM image of anode material B manufactured in Example 2.

MODES FOR CARRYING OUT THE INVENTION

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

Anode Material

An exemplary embodiment of the present invention relates to an anode material for the lithium ion battery including active materials embedded in hard carbon.

The anode material of the present exemplary embodiment is obtainable by solvothermal synthesis, particularly, hydrothermal synthesis employing water as a solvent. First, a carbon precursor solution is provided by dissolving a carbon precursor in a solvent, such as water. Next, a precursor for an active material is added to the carbon precursor solution and then the mixture is heated under high pressure

atmosphere. As reaction equipment, pressure resistant vessels such as an autoclave are usually employed. By solvothermal (hydrothermal) synthesis, the precursor for the active material is converted into the active material with a crystal form, particularly with a nano-crystal form. In this exemplary embodiment, the active material includes oxide of at least one kind of metals selected from silicon and tin. The active material is usually silicon dioxide or tin dioxide, but they may include non-oxidized metal portions. During solvothermal (hydrothermal) synthesis, the carbon precursor is decomposed and adhered on the active material or an aggregate of the active materials. The

decomposed carbon precursor is then carbonized at high temperature under inert atmosphere to convert into hard carbon.

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

The precursor for the active materials can be inorganic or organic compounds of silicon or tin. Examples of the precursor for the active materials include inorganic or organic salts such as chlorides, sulfates, carbonates and the like of silicon and tin, organosilicon and organotin compounds such as 3-aminopropylmethyldiethoxysilane, butyl(trichloro)stannane.

The solvent used for solvothermal synthesis is a solvent that can dissolve the carbon precursor. Water is preferably used and water-soluble solvents such as alcohols can be used with water.

The concentration of the carbon precursor solution is preferably in a range of

0.2 to 6 moles/L. The solvothermal synthesis is conducted at a temperature less than the super-critical temperature of the solvent. For example, the hydrothermal synthesis is conducted at a temperature less than 374°C that is super-critical temperature of water, preferably in a range of 160 to 300°C for 1 to 24 hours.

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

Here we have designed a novel active materials embedded in hard carbon composite materials for lithium ion battery anode materials. The benefits for the structure are:

1) The active materials which embedded in the hard carbon do not directly contact with the electrolyte (solvent) during charging and discharging. So the SEI is only formed on the surface of hard carbon while cycling, it may have higher coulombic efficiency and long cycle life.

2) The active materials are covered with hard carbon which has lower resistance than the pure active materials.

3) Since the active materials are prepared in situ with carbon precursor decomposition, the active materials are distributed in the hard carbon with atomic level uniform distribution. Therefore, it can reduce the content variation of the active materials in each particle.

Lithium-ion Battery

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 that of graphite, i.e., 372 niAh/g. 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 (γ-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, LiAlCl₄, LiN(C«F2«₊₁S0₂)(CwF₂/w₊₁S0₂) (« 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 μτη made of polyester (PET) or Nylon. A metal foil may be an aluminum film with a thickness of 20 to 40 μπι. 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

6.84 g (20 mmole) of sucrose was added to 100 ml of deionized water and the mixture was magnetically stirred for 30 min at 60 °C to obtain a carbon precursor solution. Then, 3.78 g (20 mmole) of SnCl₂ was gradually added to the carbon precursor solution. The mixture was charged in a 300 ml Teflon®-lined stainless autoclave. Subsequently, the autoclave was put into an oven, and then heated at 180 °C for 4 hours. The autoclave was naturally cooled to room temperature. Dark precipitates were collected and washed with deionized water several times and dried in an oven at 100 °C for 24 hours. The obtained powers were carbonized in oven under N₂ atmosphere to obtain anode material A. The carbonization was conducted at 100 ml/min of N₂ flow rate, 5°C/min of temperature raising rate and 1000°C of final temperature. SEM images of anode material A are shown in Fig. 1. As shown in Fig. 1 , anode material A is a spherical particle having a smooth surface morphology. Fig. 2 shows X-ray diffraction of anode material A. The (110) face of Sn0₂ in anode material A shifted to higher incident angle (2 theta) compared with Sn0₂ synthesized by other methods, while other faces stayed on same incident angles. Further, an intensity ratio offace (110) to (101) of Sn0₂ in anode material A is higher than 1.1, where the intensity ratio of Sn0₂ synthesized by other methods is always less than 1. Example 2

6.84 g (20 mmole) of sucrose was added to 100 ml of deionized water and the mixture was magnetically stirred for 30 min at 60 °C to obtain a carbon precursor solution. Then 10 ml (48 mmole) of 3-aminopropylmethyldiethoxysilane was gradually added to the carbon precursor solution. The mixture was charged in a 300 ml Teflon®-lined stainless autoclave. Subsequently, the autoclave was put into an oven, and then heated at 180 °C for 4 hours. The autoclave was naturally cooled to room temperature. Dark precipitates were collected and washed with deionized water several times and dried in an oven at 100 °C for 24 hours. The obtained powers were carbonized in an oven under N₂ atmosphere to obtain anode material B. The carbonization was conducted at 100 ml/min of N₂ flow rate, 5°C/min of temperature raising rate and 1000°C of final temperature. SEM images of anode material B are shown in Fig. 3. During hydrothermal synthesis, silicon oxide is grown to rod-like crystals. Comparative example 1

Sn0₂ particles with average diameter of 10 μιη were used as anode material C.

Comparative example 2

SiO particles with average diameter of 5 μηι 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 PVDF in a weight ratio of 91 : 1 : 8 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 μηι thick with the loading density of 50 g/m² and then heat treated at 200°C for 2h in N₂ atmosphere to prepare a negative electrode.

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 1M, 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 active materials embedded in hard carbon, wherein the active materials comprises oxide of at least one kind of metals selected from silicon and tin and the oxide is made from its precursor by solvothermal synthesis in a medium comprising a precursor of the hard carbon.

2. The anode material according to claim 1, wherein the solvothermal synthesis employs water as a solvent.

3. The anode material according to claim 2, wherein the precursor of the hard carbon is saccharide.

4. The anode material according to claim 1 , wherein the size of the anode material is in a range of 20 nm to 80 μπι.

5. The anode material according to claim 1 , wherein the size of the active materials is less than 100 nm.

6. The anode material according to claim 1, wherein the hard carbon is doped with boron or nitrogen.

7. The anode material according to claim 1 , wherein the active materials comprises Sn0₂ and has an intensity ratio of face (110) to (101) of Sn0₂ in the anode material is higher than 1.1.

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

9. The lithium ion battery according to claim 8, wherein the negative electrode has a capacity of at least 372 mAh/g.