Patent Application
20160329562
The present invention relates to a negative electrode active material for nonaqueous electrolyte secondary batteries and a nonaqueous electrolyte secondary battery containing the negative electrode active material.
Silicon (Si) and a silicon oxide represented by SiOx have higher capacity per unit volume as compared to carbon materials such as graphite and therefore have been investigated for applications in negative electrode active materials. In particular, SiOx has a smaller volume expansion coefficient as compared to Si when SiOx stores Li during charge and therefore is expected to be quickly commercialized. For example, Patent Literature 1 discloses SiOx having a carbon coating formed on the surface.
PTL 1: Japanese Published Unexamined Patent Application No. 2004-47404
However, there is a problem in that a nonaqueous electrolyte secondary battery in which SiOx or the like is used as a negative electrode active material has poorer initial charge/discharge efficiency and a larger reduction in capacity in initial cycles as compared to the case where graphite is used as a negative electrode active material.
A major cause of the problem is that the change in volume of SiOx or the like during charge and discharge is larger than that of graphite. The large change in volume of an active material probably causes, for example, the reduction in electrical conductivity of an active material layer, leading to the deterioration of initial charge/discharge efficiency or the like.
In order to solve the problem, a negative electrode active material for nonaqueous electrolyte secondary batteries according to the present invention is a particulate negative electrode active material used in nonaqueous electrolyte secondary batteries. The negative electrode active material includes mother particles made of SiOx (0.5≦X≦1.5), carbon coating layers each covering at least one portion of the surface of a corresponding one of the mother particles, and amorphous carbon particles stuck on the carbon coating layers.
A nonaqueous electrolyte secondary battery according to the present invention includes a negative electrode containing the negative electrode active material, a positive electrode, a separator placed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.
According to the present invention, in a nonaqueous electrolyte secondary battery in which SiOx is used as a negative electrode active material, cycle characteristics and initial charge/discharge efficiency can be improved.
Embodiments of the present invention are described below in detail.
In this specification, the term “approximately **” is intended to include completely the same things and those regarded as substantially the same, as described using the term “approximately the same” as an example. Drawings referred to in the description of the embodiments are those schematically drawn. Dimensional proportions of each component illustrated in the drawings may possibly be different from those of an actual one. Detailed dimensional proportions and the like should be judged in consideration of descriptions below.
A nonaqueous electrolyte secondary battery which is an example of an embodiment of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a nonaqueous electrolyte containing a nonaqueous solvent, and a separator. An example of the nonaqueous electrolyte secondary battery is a structure in which an electrode assembly formed by winding the positive electrode, the negative electrode, and the separator placed therebetween and the nonaqueous electrolyte are housed in an enclosure.
The positive electrode is preferably composed of a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode current collector used is, for example, a conductive thin film, particularly metal or alloy foil which is made of aluminium or the like and which is stable within the potential range of the positive electrode or a film including a metal surface layer made of aluminium or the like. The positive electrode active material layer preferably contains a conductive material and a conductive agent in addition to the positive electrode active material.
The positive electrode active material is not particularly limited and is preferably a lithium transition metal oxide. The lithium transition metal oxide may contain a non-transition metal such as Mg or Al. Examples of the lithium transition metal oxide include lithium cobaltate, olivine-type lithium phosphates typified by lithium iron phosphate, Ni—Co—Mn, Ni—Mn—Al, and Ni—Co—Al. The positive electrode active material may contain one or more of these compounds.
For the conductive material, carbon materials such as carbon black, acetylene black, Ketjenblack, and graphite and mixtures of two or more of the carbon materials can be used.
For the binding agent, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, and mixtures of two or more of these compounds can be used.
As exemplified in
As exemplified in
In the case of using the negative electrode active materials 13a and the negative electrode active materials 13b in combination, when the negative electrode active materials 13b are graphite, the mass ratio of the negative electrode active materials 13a to graphite preferably ranges from 1:99 to 20:80. When the mass ratio thereof is within the above range, both high capacity and enhanced cycle characteristics are likely to be achieved. However, when the percentage of the negative electrode active materials 13a with respect to the mass of the negative electrode active material 13 is less than 1% by mass, the merit of increasing the capacity by adding the negative electrode active materials 13a is small.
In the negative electrode active materials 13a (hereinafter referred to as the negative electrode active material particles 13a), the carbon coating layers 15 are placed on the surfaces of the mother particles 14, which are made of SiOx (0.5≦X≦1.5), and the amorphous carbon particles 16 are stuck to the surfaces of the carbon coating layers 15. SiOx has a structure in which Si is dispersed in an amorphous SiO2 matrix. The presence of dispersed Si can be confirmed by observation using a transmission electron microscope (TEM).
In the nonaqueous electrolyte secondary battery, which contains the negative electrode active material particles 13a, the carbon coating layers 15 on the surfaces of the mother particles 14 can improve a disadvantage of SiOx, which has low electronic conductivity, and the amorphous carbon particles 16, which are stuck to the surfaces of the carbon coating layers 15, improve the binding force between SiOx and a binder by an anchoring effect. When particles stuck to the surfaces of the carbon coating layers 15 are amorphous carbon particles, initial charge/discharge efficiency and cycle characteristics are particularly improved. The reason for this is as described below. In the case where highly crystalline carbon typified by graphite or fine metal particles are stuck to the surface of SiOx, a high-temperature treatment step, an electroless plating step, or the like is necessary. Treating SiOx at high temperature significantly reduces the charge/discharge capacity because of the disproportionation reaction of SiOx. In the case where the SiOx surface is electrolessly plated, irregularities are unlikely to be formed on the surfaces of particles and no sufficient anchoring effect is obtained.
The fact that the amorphous carbon 16 is stuck to the surfaces of the carbon coating layers 15 means that the amorphous carbon 16 is attached to the surfaces of the carbon coating layers 15 even in the case of mixing with a solvent or the like when the negative electrode is prepared. This is different from secondary aggregation.
The mother particles 14 preferably have an average size of 1 μm to 15 μm and more preferably 4 μm to 10 μm. In this specification, the term “average size” refers to the particle size (volume-average particle size: Dv50) at a cumulative volume percentage of 50% in the particle size distribution determined by a laser diffraction/scattering method. Dv50 can be measured using, for example, “LA-750” manufactured by HORIBA. When the average size of the mother particles 14 is too small, the surface area of the particles is too large and the amount of the mother particles 14 reacting with an electrolyte solution is large, hence, the capacity may possibly be reduced. However, when the average size thereof is too large, the influence of the volume expansion of SiOx during charge is large and therefore charge/discharge characteristics may possibly be reduced.
The amorphous carbon particles 16 preferably have an average size of 0.01 μm to 1 μm and more preferably 0.05 μm to 0.8 μm. When the average size of the amorphous carbon particles 16 is too small, surface irregularities of the carbon coating layers 15 on the mother particles 14 are small and therefore no sufficient anchoring effect is likely to be obtained. However, when the average size thereof is too large, the number of the amorphous carbon particles 16 stuck on the carbon coating layers 15 is limited and therefore no sufficient anchoring effect is likely to be obtained.
The amorphous carbon particles 16 are preferably more than 0% to 15% by mass with respect to the mother particles 14 and more preferably 2% to 8% by mass. When the amorphous carbon particles 16 are too few with respect to the mother particles 14, surface irregularities of the carbon coating layers 15 on the mother particles 14 are few and therefore no sufficient anchoring effect is likely to be obtained. However, when the amorphous carbon particles 16 are too many, the fraction of amorphous carbon in the active material is large and the capacity is likely to be reduced.
As a carbon material in the carbon coating layers 15, carbon black, acetylene black, Ketjenblack, graphite, and mixtures of two or more of these materials can be used as is the case with the conductive material in the positive electrode active material layer.
Each of the carbon coating layers 15 preferably covers 50% to 100% of the surface of a corresponding one of the mother particles 14 and more preferably 100%. In the present invention, the fact that the surfaces of the mother particles 14 are covered by the carbon coating layers 15 means that the surfaces of the mother particles 14 are covered by the carbon coating layers 15 that have a thickness of at least 1 nm in the case where a cross section of each particle is observed with a SEM.
The carbon coating layers 15 preferably have an average thickness of 1 nm to 200 nm and more preferably 5 nm to 100 nm in view of the ensuring of electrical conductivity and the diffusivity of Li+ into SiOx forming the mother particles 14 or the like. The coating layers 15 preferably have substantially a uniform thickness over the entire area thereof. The average thickness of the carbon coating layers 15 can be measured in such a manner that cross sections of the negative electrode active material particles 13a are observed using a scanning electron microscope (SEM), a transmission electron microscope (TEM), or the like. When the thickness of the coating layers 15 is too small, the electrical conductivity is reduced and it is difficult to uniformly cover the mother particles 14. However, when the thickness of the coating layers 15 is too large, the diffusivity of Li+ into the mother particles 14 is inhibited and the capacity is likely to be reduced. The percentage of the carbon coating layers with respect to SiOx is preferably 10% by mass or less.
The carbon coating layers 15 can be formed by, for example, a common process such as a CVD process, a sputtering process, or a plating (electroplating or electroless plating) process. For example, in the case where the coating layers 15 are formed on the surfaces of SiOx particles by the CVD process using the carbon material, for example, the SiOx particles and a hydrocarbon gas are heated in a vapor phase and carbon produced by the pyrolysis of the hydrocarbon gas is deposited on the SiOx particles. The hydrocarbon gas used may be a methane gas or an acetylene gas.
The negative electrode active materials 13a preferably have a BET specific surface area of 1 m2/g to 30 m2/g and more preferably 5 m2/g to 30 m2/g. When the BET specific surface area thereof is too small, no sufficient irregularities are formed on the SiOx particles and no sufficient anchoring effect is likely to be obtained. However, when the BET specific surface area thereof is too large, the amount of a binder attached to the surface of SiOx is too large, the dispersibility of the binder is reduced, and the adhesion of the negative electrode is likely to be reduced.
The amorphous carbon particles 16 can be stuck to the carbon coating layers 15 in such a manner that, for example, an aqueous solution containing an organic acid catalyst and the SiOx particles including the carbon coating layers are mixed together and are subjected to hydrolysis and a polymerization reaction at 80° C. to 120° C., water is evaporated, and heat treatment is then performed at 500° C. to 800° C. The aqueous solution containing the organic acid catalyst and the SiOx particles including the carbon coating layers may be mixed with a lithium compound. Examples of the organic acid catalyst include citric acid, malic acid, tartaric acid, lactic acid, and glycolic acid. Examples of the lithium compound include LiOH, Li2CO3, LiF, and LiCl.
SiOx forming the mother particles 14 may contain lithium silicate (such as Li4SiO4, Li2SiO3, Li2Si2O5, or Li8SiO6) in particles.
The following salts can be used as an electrolyte salt in the nonaqueous electrolyte: for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB10Cl10, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, chloroborane lithium, borates, imide salts, and the like. In particular, LiPF6 is preferably used from the viewpoint of ionic conductivity and electrochemical stability. Electrolyte salts may be used alone or in combination. In 1 L of the nonaqueous electrolyte, 0.8 mol to 1.5 mol of the electrolyte salt is preferably contained.
For example, a cyclic carbonate, a linear carbonate, a cyclic carboxylate, or the like is used as a solvent in the nonaqueous electrolyte. Examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC). Examples of the linear carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylate include 7-butyrolactone (GBL) and γ-valerolactone (GVL). Nonaqueous solvents may be used alone or in combination.
The separator used is a porous sheet having ionic permeability and insulating properties. Examples of the porous sheet include microporous thin films, woven fabrics, and nonwoven fabrics. The separator is preferably made of a polyolefin such as polyethylene or polypropylene.
The present invention is further described below with reference to examples. The present invention is not limited to the examples.
SiOx (X=0.93, an average primary particle size of 5.0 μm) surface-coated with carbon was prepared. Coating was performed by a CVD process. The percentage of carbon with respect to SiOx was 10% by mass. The carbon coverage of the surface of SiOx was 100%. The carbon coverage of the SiOx surface was confirmed by a method below. A cross section of each SiOx particle was exposed using an ion milling system (ex. IM4000) manufactured by Hitachi High-Technologies Corporation and was checked using a SEM and a backscattered electron image. The interface between a carbon coating layer and SiOx in the particle cross section was identified from the backscattered electron image. The percentage of carbon coatings, having a thickness of 1 nm or more, present on the surface of each SiOx particle was calculated from the ratio of the sum of the lengths of the interfaces between the carbon coatings having a thickness of 1 nm or more and SiOx to the perimeter of SiOx in the particle cross section. The average of the percentages of the carbon coatings on the surfaces of 30 of the SiOx particles was calculated as a carbon coverage.
To 1,000 g of water, 0.5 moles of Li2CO3 was added, followed by adding 0.2 moles of citric acid, whereby an aqueous solution in which Li2CO3 was completely dissolved was prepared. To the aqueous solution, 1 mole of the above SiOx was added, followed by mixing. The mixed solution was subjected to a dehydrocondensation reaction at 80° C., followed by drying at 120° C., whereby an intermediate was obtained. The intermediate was heat-treated at 600° C. for 5 hours in an Ar atmosphere. The SiOx was washed with pure water. The BET specific surface area of the heat-treated and water-washed SiOx particles was measured using Tristar II 3020 (manufactured by Shimadzu Corporation), resulting in that the BET specific surface area was 20 m2/g.
The fact that amorphous carbon was stuck to the surface of SiOx surface-coated with carbon was confirmed by a method below.
The fact that carbon stuck on the SiOx surface-coated with carbon was amorphous carbon was confirmed by a method below.
Next, for
SiOx and PAN (polyacrylonitrile) serving as a binder were mixed at a mass ratio of 95:5, followed by adding NMP (N-methyl-2-pyrrolidone) serving as a dilution solvent. This was stirred using a mixer (ROBOMIX manufactured by PRIMIX Corporation), whereby negative electrode mix slurry was prepared. The negative electrode mix slurry was applied to a surface of copper foil such that the mass per 1 m2 of a negative electrode mix layer was 25 g/m2. Next, this was dried at 105° C. in air and was rolled, whereby a negative electrode was prepared. The packing density of the negative electrode mix layer was 1.50 g/ml.
To a solvent mixture prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3:7, 1.0 mole per liter of lithium hexafluorophosphate (LiPF6) was added, whereby a nonaqueous electrolyte solution was prepared.
An electrode assembly was prepared in an inert atmosphere using the negative electrode equipped with a Ni tab at the outer periphery thereof, lithium metal foil, and a polyethylene separator placed between the negative electrode and the lithium metal foil. The electrode assembly was put in a battery enclosure composed of an aluminium laminate. Furthermore, the nonaqueous electrolyte solution was poured into the battery enclosure. Thereafter, the battery enclosure was sealed, whereby Battery A1 was prepared.
Battery A2 was prepared in substantially the same manner as that described in Experiment 1 except that the amount of added citric acid was 0.18 moles. The BET specific surface area of heat-treated and water-washed SiOx particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 15 m2/g.
Battery A2 was prepared in substantially the same manner as that described in Experiment 1 except that the amount of added citric acid was 0.25 moles. The BET specific surface area of heat-treated and water-washed SiOx particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 30 m2/g.
Battery Z was prepared in substantially the same manner as that described in Experiment 1 except that untreated SiOx was used as a negative electrode active material (that is, SiOx having no amorphous carbon particles on a carbon coating layer). The BET specific surface area of SiOx particles was measured using Tristar II 3020, resulting in that the BET specific surface area was 5 m2/g.
The above batteries were charged and discharged under conditions below, followed by investigating the initial charge/discharge efficiency given by Equation (1) below and the tenth-cycle capacity retention given by Equation (2) below. The results are shown in Table 1.
After constant-current charge was performed at a current of 0.2 lt (4 mA) until the voltage reached 0 V, constant-current charge was performed at a current of 0.05 lt (1 mA) until the voltage reached 0 V. Next, after a rest was taken for 10 minutes, constant-current discharge was performed at a current of 0.2 lt (4 mA) until the voltage reached 1.0 V.
Initial charge/discharge efficiency (%)=(first-cycle discharge capacity/first-cycle charge capacity)×100 (1)
Tenth-cycle capacity retention (%)=(tenth-cycle discharge capacity/first-cycle discharge capacity)×100 (2)
In Battery Z, in which no amorphous carbon particles are placed on carbon coatings on the surfaces of SiO particles, it is conceivable that no sufficient anchoring effect is obtained between active material particles and a binder and the adhesion between active materials is reduced.
However, in Batteries A1 to A3, since carbon coatings are placed on the surfaces of SiOR particles and amorphous carbon particles are stuck on the carbon coatings, it is conceivable that the surfaces of the particles have irregularities sufficient to obtain an anchoring effect between active material particles and a binder and the adhesion between active materials is improved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-254384 | Dec 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/006198 | 12/12/2014 | WO | 00 |
