Patent Application
20190123337
The present invention relates to a negative electrode for a lithium secondary battery, including a mixture of a silicon oxide and a silicon alloy as an active material. The present invention also relates to a lithium secondary battery including the negative electrode.
Recently, for expanding the use of electric vehicles (xEV), it is necessary to increase the driving distance per charge. Lithium secondary batteries used as the power sources for xEV are strongly desired to have a high energy density in view of weight saving.
For increasing the energy density, increasing the capacity of a battery is one of the solutions. Using a solid-solution positive electrode material constituted of Li2MnO3 as a matrix structure in a positive electrode and a negative electrode material constituted of an alloy mainly based on silicon and silicon oxide in a negative electrode is mentioned as a method (Patent Literature 1).
Silicon has a theoretical capacity of 4200 mAh/g, which is extremely higher than the theoretical capacity (372 mAh/g) of a carbon material (graphite) presently primarily used in practice; however the volume thereof significantly changes by charge/discharge. Because of this volume change, a decrease of battery capacity is a matter of concern (Patent Literature 2).
In contrast, silicon oxide, SiOχ, provides a relatively high capacity and has satisfactory life characteristics. However, since an initial charge/discharge efficiency thereof is low, an effect of increasing the energy density of a battery is not sufficient (Patent Literature 3).
Recently, use of an alloy of silicon and another metal (hereinafter referred to as a Si alloy) has been investigated. Patent Literature 4 proposes use of a silicon solid-solution having one or more semimetal elements (except silicon) belonging to Group 3 to Group 5 incorporated in silicon, as a negative electrode active material, in which the element incorporated in silicon is abundantly present on the crystal grain boundaries of the silicon solid solution than the inside the crystal grains.
Further, Patent Literature 5 proposes use of particles of a transition metal-silicon alloy, which contains the same transition metal as used in a lithium transition metal oxide serving as a positive electrode active material and Si, as a negative electrode active material.
Patent Literature 1: International Publication No. WO 2012/120782
Patent Literature 4: International Publication No. WO 2013/002163
A negative electrode including a silicon oxide (hereinafter referred to as SiOχ) has a high capacity; however the initial charge/discharge efficiency is low. In addition, the true density of SiOχ is low so that it is difficult to increase the density of the electrode. A negative electrode including a Si alloy has higher initial charge/discharge efficiency than a negative electrode including SiOχ. The true density of the Si alloy is high so that the electrode density can be increased. However, there is a problem that the cycle life is short.
An object of the present invention is to provide a negative electrode for a lithium secondary battery providing a high electrode density, i.e., a high volumetric energy density and improved in life characteristics, and a lithium secondary battery using the negative electrode.
According to one aspect of the present invention, there is provided a negative electrode for a lithium secondary battery having a negative electrode active material layer formed on a collector, in which the negative electrode active material layer includes at least first particles; second particles and a binder, and the first particles are formed of SiOχ (0<χ<2.0); the second particles are formed of a Si alloy; the Si alloy includes Si and at least one element selected from metal elements except Li, Mn, Fe, Co and Ni, and semimetal elements; and the central particle size D50 of the first particles is larger than the central particle size D50 of the second particles.
According to another aspect of the present invention, there is provided a lithium secondary battery including the above-mentioned negative electrode for a lithium secondary battery.
According to one aspect of the present invention, it is possible to provide a negative electrode for a lithium secondary battery providing a high volumetric energy density and improved in life characteristics and a lithium secondary battery using the negative electrode.
Now, example embodiments will be described with reference to the drawings; however, the present invention is not limited to the example embodiments alone.
(1) Structure of Negative Electrode for Lithium Secondary Battery
(Negative Electrode Active Material)
In a negative electrode active material according to the example embodiment, the first particles, which are formed of SiOχ (0<χ<2.0), can have a cluster structure or an amorphous structure, and the surface of the particles can be coated with a conductive material. The conductive material includes a carbon material such as graphite, amorphous carbon, diamond-like carbon, fullerene, carbon nanotube and carbon nanohorn, a metal material, an alloy material or an oxide material.
The second particles are formed of a Si alloy, and the Si alloy includes Si and at least one element selected from metal elements except Li, Mn, Fe, Co and Ni, and semimetal elements. Note that pure Si is not regarded as an alloy.
The central particle size D50 of the first particles 4 formed of SiOχ included in the negative electrode active material layer 2 is not particularly limited; however, for example, D50 is preferably 1 μm or more and 35 μm or less, more preferably 2 μm or more and 10 μm or less, and further preferably 3 μm or more and 6 μm or less. Usually, powdery SiOχ to be used in the negative electrode active material of a lithium ion secondary battery is produced by grinding a silicon oxide raw material having a certain size.
The silicon oxide powder herein has a SiO2 film formed on the surface. The SiO2 film herein serves as an insulator when the silicon oxide is used as the negative electrode active material of a lithium ion secondary battery, with the result that resistance is generated and an electrolyte is decomposed. For these reasons, the SiO2 film formed on the surface of silicon oxide fine powder becomes a causative factor of decreasing initial efficiency and cycle characteristics of a lithium ion secondary battery.
Powdery silicon oxide obtained by grinding contains a large amount of fine powder having a diameter of less than 1 μm, which is generated in the grinding. If silicon oxide has a large amount of fine powder, the surface area per unit mass increases, in other words, the area of the SiO2 film formed on the surface increases. Accordingly, when silicon oxide is used as the negative electrode active material of a lithium ion secondary battery, the silicon oxide having a central particle size D50 of 1 μm or more is preferably used in order to prevent a decrease of the initial efficiency and deterioration of cycle characteristics.
If the central particle size D50 exceeds 35 μm, a number of huge silicon oxide particles come to be contained. In this case, if the silicon oxide, a conductive aid and a binder are mixed and used as a negative electrode material for a lithium ion secondary battery, lithium ions cannot get into the interior portion of a huge silicon oxide particle. As a result, the performance of SiOχ cannot be sufficiently provided, with the result that the initial efficiency decreases. Accordingly, the central particle size D50 is preferably 35 μm or less.
Second particles 5 formed of Si alloy have a smaller central particle size D50 than the first particles. For example, the central particle size D50 thereof is preferably 0.1 μm or more and 5 μm or less, more preferably 0.1 μm or more and 3 μm or less and further preferably, 0.1 μm or more and 2 μm or less. If the central particle size D50 thereof is 5 μm or less, it is possible to suppress reduction in particle size due to volume change and degradation of battery characteristics caused by formation of lithium dendrite in charging time. In contrast, if D50 is 0.1 μm or more, an increase of contact resistance can be suppressed.
If the central particle size D50 of the second particles is larger than D50 of the first particles, expansion of volume is large, with the result that initial charge/discharge efficiency significantly decreases and cycle characteristics significantly degrade. For this reason, the central particle size D50 of the first particles must be larger than D50 of the second particles. Note that, the central particle size D50 of the active material can be measured by a laser diffraction/scattering type particle size distribution measuring device.
In order to increase conductivity, the surface of the first particles 4, SiOx, is preferably covered with carbon. The mass ratio of SiOχ and the surface-covered carbon can fall within the range of 99.9/0.1 to 80/20. If the mass ratio falls within this range, the contact resistance between particles is reduced; reduction of SiOχ ratio and negative electrode capacity can be avoided. The mass ratio more preferably falls within the range of 99.5/0.5 to 85/15, and further preferably within the range of 99/1 to 90/10.
The second particles 5, Si alloy, preferably has an initial charging capacity of 4000 mAh/g or less and 1000 mAh/g or more when Li is used as a counter electrode. The theoretical capacity of Si is 4200 mAh/g; however, if the initial charging capacity is 4000 mAh/g or less, a large volume change by charge/discharge is suppressed, with the result that deterioration of the battery can be prevented. If the initial charging capacity is 1000 mAh/g or more, an advantage: high energy density of the battery, can be obtained. The initial charging capacity is more preferably 2000 mAh/g or more and 3800 mAh/g or less and further preferably 2500 mAh/g or more and 3500 mAh/g or less.
Note that, the initial charging capacity can be obtained by charging the battery within the range of 0.02 V to 1 V at 25° C.
As the Si alloy, for example, an alloy of silicon (Si) and a metal element is used in order to increase true density and obtain a high volumetric energy density. Examples of the metal element include beryllium (Be), magnesium (Mg), aluminum (Al), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), copper (Cu), zinc (Zn), gallium (Ga), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), palladium (Pd), ruthenium (Ru), cadmium (Cd), indium (In), tin (Sn), tantalum (Ta), tungsten (W), platinum (Pt), gold (Au), lead (Pb) and bismuth (Bi). Also, an alloy of silicon and a semimetal can be used. Examples of the semimetal include metals except silicon, such as boron (B), germanium (Ge), arsenic (As), antimony (Sb) and tellurium (Te). However, as a metal of Si alloy, lithium (Li), manganese (Mn), cobalt (Co), nickel (Ni) and iron (Fe) are excluded, because these elements are frequently used in a positive electrode material (e.g., LiMn2O4, Li2MnO3, LiNiO2, LiFePO4) of a battery. If Li, Mn, Ni and Fe, which easily elute and precipitate, are used in a Si alloy, ions of these metals are preferentially deposit on the Si alloy particles, with the result negative electrode resistance tends to increase and battery characteristics can degrade.
Provided that the Si alloy is represented by Si1-ψMψ where M represents a metal or semimetal constituting the Si alloy together with silicon, the range of ψ is preferably 0.01 or more and 0.5 or less. If ψ is 0.5 or less, a reduction of the initial charging capacity of the silicon alloy is suppressed, with the result that a high capacity of 1000 mAh/g or more can be attained. In addition, a decrease of the energy density of a battery can be suppressed. If ψ is 0.01 or more, single crystallization of silicon can be suppressed and a volume change associated with charge/discharge and causing deterioration of a battery decreases compared to pure silicon. The range of ψ is more preferably 0.02 or more and 0.4 or less and further preferably 0.03 or more and 0.3 or less.
Provided that the mass ratio of the second particles 5 relative to the total mass of the first particles 4 and the second particles 5 is represented by ω, ω is preferably larger than 0% and 50% or less, more preferably 1% or more and 40% or less and further preferably 5% or more and 20% or less. If the ratio of the second particles increases, volumetric energy density increases; however, the amount of Si alloy, which is likely to cause cycle deterioration associated with charge/discharge, increases. As a result, the cycle life of the battery becomes short. If the ratio of the second particles is low, the effect of increasing energy density becomes low.
(Binder)
As the binder 6, for example, polyimide, polyamide, polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose and modified acrylonitrile rubber particles can be used. The amount of binder for a negative electrode to be used herein is preferably 7 to 20 parts by mass relative to the negative electrode active material of 100 parts by mass in consideration of trade off relationship between “sufficient bonding force” and “imparting high energy”.
(Other Additives)
To the negative electrode active material layer, a conductive aid can be added other than the first particles 4 and the second particles 5 serving as a negative electrode active material, and the binder 6. As the conductive aid, e.g., carbon black, carbon fiber and graphite can be used alone or in combination of two types or more.
(Negative Electrode Current Collector)
As the negative electrode current collector 3, copper, stainless steel, nickel, cobalt, titanium, gadolinium or an alloy thereof can be used, and particularly, stainless steel is preferably used. As the stainless steel, martensite type, ferrite type or austenite/ferrite two-phase type can be used. For example, number JIS 400s in martensite type such as SUS 420J2 having a chromium content of 13%, number JIS 400s in ferrite type such as SUS 430 having a chromium content of 17% and number JIS 300s in austenite/ferrite two-phase type such as SUS 329 J4L having a chromium content of 25%, a nickel content of 6% and a molybdenum content of 3%, can be used. Alternatively, a composite alloy of these can be used.
(Method for Manufacturing Negative Electrode)
The negative electrode 1 for a lithium secondary battery according to an example embodiment of the present invention can be manufactured as follows. A negative-electrode mix is prepared by homogeneously mixing the first particles 4, second particles 5 and binder 6. The mix is dispersed in an appropriate dispersion medium such as N-methyl-2-pyrrolidone (NMP) to prepare a negative-electrode mix slurry. The negative-electrode mix slurry obtained is applied to one or both surfaces of a negative electrode current collector and dried to form a negative electrode active material layer. At this time, pressure can be applied for molding. As the application method, which is not particularly limited, a method known in the art can be used. For example, a doctor blade method and a die coater method can be mentioned. Alternatively, a negative electrode active material layer is formed in advance, and thereafter, a thin-metal film serving as a negative electrode current collector can be formed by a deposition method or a sputtering method to form a negative electrode current collector.
In the negative electrode for a lithium secondary battery according to the present invention, an active material is prepared by homogeneously mixing the second particles, Si alloy, having a higher initial charge/discharge efficiency than SiOχ and a high true density, with the first particles, SiOχ having a low initial charge/discharge efficiency and a low true density. Owing to this, the electrode density is increased and the charge/discharge efficiency is improved. In addition, if the median diameters of the first particles and the second particles are controlled as mentioned above, volumetric expansion of the metal and alloy phase can be sufficiently effectively reduced, with the result that a secondary battery having an excellent balance among the energy density, cycle life and charge/discharge efficiency can be obtained.
In the above manner, a negative electrode for a lithium secondary battery providing a high volumetric energy density and improved in life characteristics can be obtained and a lithium secondary battery using the negative electrode can be provided.
The negative electrode for a lithium secondary battery of the present invention is used as an electrode of a lithium secondary battery. As an example, the structure of a film-packaged stacked lithium secondary battery 7 will be described. The film-packaged stacked lithium secondary battery 7 according to the example embodiment is constituted of an electrode stack 12 sandwiched by film exteriors 13a and 13b, as shown in
The film-packaged stacked lithium secondary battery 7 is produced from the electrode stack 12 and the film exteriors 13a, 13b, for example, as follows. The electrode stack 12 is sandwiched by the film exteriors 13a, 13b. An inlet is provided on the side of the film exteriors 13a, 13b except the side where the positive electrode terminal 15 and the negative electrode terminal 16 are present. The three sides except the side having the inlet are heat-sealed. Subsequently, the side at which positive and negative electrode terminals are present is allowed to face the bottom or a different side having no terminals is turned up, and then, an electrolytic solution (not shown) is introduced. Finally, the side having the inlet is heat-sealed to complete the production of a battery. As the film exteriors 13a, 13b each having a resin layer, for example, an aluminum laminate film having high corrosion resistance is used. Note that both ends of the side having the inlet can be heat-welded to narrow the inlet. In
The positive electrode 10 and the negative electrode 1 are prepared. The positive electrode 10 and the negative electrode 1 are stacked with the separator 11 interposed therebetween to form the electrode stack 12, as shown in
[Film Package]
The film package 13 can use a material prepared by providing a resin layer on the front and back surfaces of a substrate, i.e., a metal layer. As the metal layer, a metal layer having a barrier property, such as a property of preventing electrolytic solution leakage and a property of preventing moisture invasion from outside, can be selected, and e.g., aluminum and stainless steel can be used. On at least one of the surfaces of the metal layer, a heat-sealable resin layer such as a modified polyolefin layer is provided. Further, a heat-sealable resin layer is provided onto the surfaces of the electrode stack 12 each facing to the film exteriors 13a and 13b so that the heat-sealable resin layers are arranged to face each other and the periphery of a portion in which the electrode stack 12 is to be housed is heat-sealed to form an outer container. On the surfaces of the film exteriors opposite to the surface having the heat-sealable resin layer formed thereon, a resin layer such as a nylon film or a polyester film can be provided.
[Non-Aqueous Electrolytic Solution]
In the example embodiment, a non-aqueous electrolytic solution is used as the electrolytic solution. The non-aqueous electrolytic solution is prepared by dissolving an electrolytic salt in a non-aqueous solvent. As the non-aqueous solvent, for example, the following organic solvents can be used. Examples of the organic solvents include cyclic carbonates, linear carbonates, aliphatic carboxylic acid esters, γ-lactones such as γ-butyrolactone, linear ethers, cyclic ethers, phosphoric acid esters and fluorides of these organic solvents. These can be used alone or as a mixture of two or more thereof. To these organic solvents, a lithium salt which is a kind of the electrolytic salt, and a functional additive(s) can be dissolved.
Examples of the cyclic carbonates can include, but are not particularly limited to, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and vinylene carbonate (VC). As the fluorinated cyclic carbonates, e.g., compounds prepared by substituting part or whole hydrogen atoms of the cyclic carbonates with fluorine atoms, can be mentioned. More specifically, for example, 4-fluoro-1,3-dioxolan-2-one (also referred to as monofluoroethylene carbonate), (cis or trans) 4,5-difluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolane-2-one and 4-fluoro-5-methyl-1,3-dioxolan-2-one can be used. Of the cyclic carbonates listed above, e.g., ethylene carbonate, propylene carbonate and 4-fluoro-1,3-dioxolan-2-one are preferable as the cyclic carbonates, in view of withstand voltage and conductivity. The cyclic carbonates can be used alone or in combination of two or more thereof.
Examples of the linear carbonates include, but are not particularly limited to, dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC) and dipropyl carbonate (DPC). As the linear carbonate, a fluorinated linear carbonate is included. As the fluorinated linear carbonate, for example, compounds prepared by substituting part or whole hydrogen atoms of the linear carbonates with fluorine atoms can be mentioned. Specific examples of the fluorinated linear carbonate include bis(fluoroethyl) carbonate, 3-fluoropropylmethyl carbonate and 3,3,3-trifluoropropylmethyl carbonate. Of these, dimethyl carbonate is preferably in view of withstand voltage and conductivity. The linear carbonate can be used alone or in combination of two or more thereof.
Examples of the aliphatic carboxylic acid esters include, but are not particularly limited to, ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate and methyl formate. In the carboxylic acid ester, a fluorinated carboxylic acid ester is included. As the fluorinated carboxylic acid ester, e.g., compounds prepared by substituting part or whole hydrogen atoms of ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate or methyl formate, with fluorine atoms, can be mentioned. Examples thereof that can be used include ethyl pentafluoropropionate, ethyl 3,3,3-trifluoropropionate, methyl 2,2,3,3-tetrafluoropropionate, 2,2-difluoroethyl acetate, methyl heptafluoroisobutyrate, methyl 2,3,3,3-tetrafluoropropionate, methyl pentafluoropropionate, methyl 2-(trifluoromethyl)-3,3,3-trifluoropropionate, ethyl heptafluorobutyrate, methyl 3,3,3-trifluoropropionate, 2,2,2-trifluoroethyl acetate, isopropyl trifluoroacetate, tert-butyl trifluoroacetate, ethyl 4,4,4-trifluorobutyrate, methyl 4,4,4-trifluorobutyrate, butyl 2,2-difluoroacetate, ethyl difluoroacetate, n-butyl trifluoroacetate, 2,2,3,3-tetrafluoropropyl acetate, ethyl 3-(trifluoromethyl)butyrate, methyl tetrafluoro-2-(methoxy)propionate, 3,3,3-trifluoropropyl-3,3,3-trifluoropropionate, methyl difluoroacetate, 2,2,3,3-tetrafluoropropyl trifluoroacetate, 1H,1H-heptafluorobutyl acetate, methyl heptafluorobutyrate and ethyl trifluoroacetate.
Examples of the linear ethers include, but are not particularly limited to, dipropyl ether, ethyl tert-butyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H, 1H,2′H,3H-decafluorodipropyl ether, 1,1,2,3,3,3-hexafluoropropyl-2,2-difluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, propyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1H, 1H,5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1H-perfluorobutyl-1H-perfluoroethyl ether, methyl perfluoropentyl ether, methyl perfluorohexyl ether, methyl 1,1,3,3,3-pentafluoro-2-(trifluoromethyl)propyl ether, 1,1,2,3,3,3-hexafluoropropyl 2,2,2-trifluoroethyl ether, ethyl nonafluorobutyl ether, ethyl 1,1,2,3,3,3-hexafluoropropyl ether, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, 1H,1H,2′H-perfluorodipropyl ether, heptafluoropropyl 1,2,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, ethyl nonafluorobutyl ether, methyl nonafluorobutyl ether, 1,1-difluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,3,3-tetrafluoropropyl) ether, 1,1-difluoroethyl-2,2,3,3,3-pentafluoropropyl ether, 1,1-difluoroethyl-1H, 1H-heptafluorobutyl ether, 2,2,3,4,4,4-hexafluorobutyl-difluoromethyl ether, bis(2,2,3,3,3-pentafluoropropyl) ether, nonafluorobutyl methyl ether, bis(1H,1H-heptafluorobutyl) ether, 1,1,2,3,3,3-hexafluoropropyl-1H,1H-heptafluorobutyl ether, 1H, 1H-heptafluorobutyl-trifluoromethyl ether, 2,2-difluoroethyl-1,1,2,2-tetrafluoroethyl ether, bis(trifluoroethyl) ether, bis(2,2-difluoroethyl) ether, bis(1,1,2-trifluoroethyl) ether, 1,1,2-trifluoroethyl-2,2,2-trifluoroethyl ether and bis(2,2,3,3-tetrafluoropropyl) ether.
As the cyclic ethers, although they are not particularly limited to, e.g., tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane and 2-methyl-1,3-dioxolane are preferable. Cyclic ethers partly fluorinated such as 2,2-bis(trifluoromethyl)-1,3-dioxolane and 2-(trifluoroethyl) dioxolane can be used.
Examples of the phosphoric acid ester compounds include, but are not particularly limited to, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, 2,2,2-trifluoroethyldimethyl phosphate, bis(trifluoroethyl)methyl phosphate, bistrifluoroethylethyl phosphate, tris(trifluoromethyl) phosphate, pentafluoropropyldimethyl phosphate, heptafluorobutyldimethyl phosphate, trifluoroethylmethylethyl phosphate, pentafluoropropylmethylethyl phosphate, heptafluorobutylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, pentafluoropropylmethylpropyl phosphate, heptafluorobutylmethylpropyl phosphate, trifluoroethylmethylbutyl phosphate, pentafluoropropylmethylbutyl phosphate, heptafluorobutylmethylbutyl phosphate, trifluoroethyldiethyl phosphate, pentafluoropropyldiethyl phosphate, heptafluorobutyldiethyl phosphate, trifluoroethylethylpropyl phosphate, pentafluoropropylethylpropyl phosphate, heptafluorobutylethylpropyl phosphate, trifluoroethylethylbutyl phosphate, pentafluoropropylethylbutyl phosphate, heptafluorobutylethylbutyl phosphate, trifluoroethyldipropyl phosphate, pentafluoropropyldipropyl phosphate, heptafluorobutyldipropyl phosphate, trifluoroethylpropylbutyl phosphate, pentafluoropropylpropylbutyl phosphate, heptafluorobutylpropylbutyl phosphate, trifluoroethyldibutyl phosphate, pentafluoropropyldibutyl phosphate, heptafluorobutyldibutyl phosphate, tris(2,2,3,3-tetrafluoropropyl) phosphate, tris(2,2,3,3,3-pentafluoropropyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate, tris(1H,1H-heptafluorobutyl) phosphate and tris(1H,1H,5H-octafluoropentyl) phosphate.
Examples of the supporting electrolyte for the electrolyte include lithium salts such as LiPF6, LiAsF6, LiAlCl4, LiClO4, LiBF4, LiSbF6, LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2 and LiB10Cl10. Other examples of the supporting electrolyte include a lithium salt of a lower aliphatic carboxylic acid, chloroborane lithium, lithium tetraphenylborate, LiBr, LiI, LiSCN and LiCl. The supporting electrolytes can be used alone or in combination of two types or more. The concentration of the supporting electrolyte preferably falls within the range of 0.3 mol/l or more and 5 mol/l in the electrolytic solution.
[Positive Electrode]
The positive electrode is formed, for example, by bonding a positive electrode active material to a positive electrode current collector with a positive electrode binder. Examples of the positive electrode material (positive electrode active material) include, but are not particularly limited to, a laminar material, a spinel material and an olivine material. The laminar material is represented by general formula: LiMO2 (M represents a metal element) and more specifically, includes lithium metal composite oxides having a layered structure and represented by
LiCo1-xMxO2 (0≤x<0.3, M represents a metal except Co);
LiyNi1-xMxO2 (A)
(In the formula (A), 0≤x<0.8, 0<y≤1.0 and M represents at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti and B). In particular,
LiNi1-xMxO2 (0.05<x<0.3, M represents a metal element including at least one element selected from Co, Mn and Al);
Li(LixM1-x-zMnz)O2 (B)
(In the formula (B), 0.1≤x<0.3, 0.33≤z≤0.8, and M is at least one of Co and Ni); and
Li(M1-zMnz)O2 (C)
(In the formula (C), 0.33≤z≤0.7, M is at least one of Li, Co and Ni).
In the above formula (A), the content of Ni is preferably high, in other words, x is preferably less than 0.5 and further preferably 0.4 or less. Examples of such a compound include LiαNiβCoγMnδO2 (1≤α≤1.2, β+γ+δ=1, β≥0.6, γ≤0.2) and LiαNiβCoγAlδO2 (1≤α≤1.2, β+γ+δ=1, β≥0.6, γ≤0.2). Particularly, LiNiβCoγMnδO2 (0.75≤β≤0.85, 0.05≤γ≤0.15, 0.10≤δ≤0.20) is mentioned. More specifically, for example, LiNi0.8Co0.05Mn0.15O2, LiNi0.8Co0.1Mn0.12, LiNi0.8Co0.15Al0.05O2, LiNi0.8Co0.1Al0.1O2 and LiNi0.6Co0.2Mn0.2O2 can be preferably used.
In view of thermal stability, it is preferable that the content of Ni does not exceed 0.5; in other words, in the formula (A), x is 0.5 or more. It is also preferable that the content of a predetermined transition metal does not exceed the half. As such a compound, LiαNiβCoγMnδO2 (1≤α≤1.2, β+γ+δ=1, 0.2≤β≤0.5, 0.1≤γ≤0.4, 0.1≤δ≤0.4) is mentioned. More specifically, e.g., LiNi0.4Co0.3Mn0.3O2 (abbreviated as NCM433), LiNi1/3Co1/3Mn1/3O2, LiNi0.5Co0.2Mn0.3O2 (abbreviated as NCM523), LiNi0.5Co0.3Mn0.2O2 (abbreviated as NCM532) and LiNi0.4Mn0.4Co0.2O2 can be mentioned (however, in these compounds, the contents of individual transition metals can vary within about 10%).
In the above formula (B), Li(Li0.2Ni0.2Mn0.6)O2, Li(Li0.15Ni0.3Mn0.55)O2, Li(Li0.15Ni0.2Co0.1Mn0.55)O2, Li(Li0.15Ni0.15Co0.15Mn0.55)O2 and Li(Li0.15Ni0.1Co0.2Mn0.55)O2 are preferable.
Examples of the spinel material that can be used include:
LiMn2O4;
a material enhanced in lifespan by partially substituting Mn of LiMn2O4 and operated at about 4 V with respect to lithium, for example,
LiMn2-xMxO4 (in the formula, 0<x<0.3, M represents a metal element including at least one metal selected from Li, Al, B, Mg, Si and a transition metal);
a material represented operated at a high voltage of about 5 V such as LiNi0.5Mn1.5O4; and
a material, which has components similar to LiNi0.5Mn1.5O4, and is obtained by substituting a part of the material of LiMn2O4 with a transition metal, charged/discharged at a high potential and further adding another element, for example, represented by
Lia(MxMn2-x-yYy)(O4-wZw) (D)
(in the formula (D), 0.4≤x≤1.2, 0≤y, x+γ<2, 0≤a≤1.2, 0≤w≤1; M represents a transition metal element and contains at least one element selected from the group consisting of Co, Ni, Fe, Cr and Cu; Y represents a metal element and contains at least one element selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca; and Z represents at least one element selected from the group consisting of F and Cl).
In the formula (D), M preferably contains a transition metal element selected from the group consisting of Co, Ni, Fe, Cr and Cu, in a proportion of 80% or more of the composition ratio x, more preferably 90% or more and acceptably 100%; Y contains a metal element selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca preferably in a proportion of 80% or more of the composition ratio y, more preferably 90% or more and acceptably 100%.
The olivine material is represented by general formula:
LiMPO4 (E)
(in the formula (E), M represents at least one element of Co, Fe, Mn and Ni).
More specifically, e.g., LiFePO4, LiMnPO4, LiCoPO4 and LiNiPO4 are mentioned. Materials in which these constituent elements are partly substituted with another element, for example, parts of oxygen atoms are substituted with fluorine atoms, can be used.
Other than these, as a positive electrode active material, e.g., a NASICON-structured material and a lithium transition metal silicon composite oxide can be used. The positive electrode active materials can be used alone and as a mixture of two or more thereof.
Of these positive electrode active materials, positive electrode active materials represented by general formulas (A), (B), (C) and (D) are particularly preferable, because an effect of increasing the energy density of the battery can be expected.
The specific surface areas of these positive electrode active materials are, for example, 0.01 to 20 m2/g, preferably 0.05 to 15 m2/g, more preferably 0.1 to 10 m2/g and further preferably 0.15 to 8 m2/g. If the specific surface area falls within the above range, the contact area with the electrolytic solution can be controlled to fall within an appropriate range. More specifically, if the specific surface area is 0.01 m2/g or more, lithium ions tend to smoothly enter and leave, with the result that resistance can be further reduced. In contrast, if the specific surface area is 8 m2/g or less, promotion of decomposing the electrolytic solution and elution of constituent elements of the active material can be further suppressed.
The central particle size of the lithium composite oxide particles is preferably 0.01 to 50 m and more preferably 0.02 to 40 μm. If the particle size is 0.01 μm or more, elution of constituent elements of the positive electrode material can be further suppressed and deterioration of the positive electrode material in contact with the electrolytic solution can be further suppressed. If the particle size is 50 μm or less, lithium ions tend to smoothly enter and leave, with the result the resistance can be further reduced. The particle size can be measured by a laser diffraction/scattering particle size distribution measuring device.
To the positive electrode active material layers 8a, 8b, a conductive aid and a binder are added. As the conductive aid, e.g., carbon black, carbon fiber and graphite can be used alone or in combination of two or more thereof. Examples of the binder that can be used include polyimide, polyamide, polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose and modified acrylonitrile rubber particles.
[Current Collector]
As the positive electrode current collector 9, aluminum, stainless steel, nickel, cobalt, titanium, gadolinium or alloys of them can be used.
[Separator]
The material of the separator 11 is not particularly limited as long as it is a material such as a nonwoven fabric and a microporous membrane generally used in nonaqueous electrolytic solution secondary batteries. As an example of the material, a polyolefin resin such as polypropylene and polyethylene, a polyester resin, an acrylic resin, a styrene resin, or a nylon resin can be used. Particularly, a polyolefin microporous membrane is preferable since it is excellent in ion permeability and physically isolation of the positive electrode and the negative electrode. If necessary, a layer containing inorganic particles can be formed on the separator 11. Examples of the inorganic particles include insulating oxide, nitride, sulfide and carbide. Of them, the inorganic particles preferably include SiO2, TiO2 and Al2O3. Furthermore, a flame retardant resin having a high melting point such as aramid and polyimide, can be used. In order to increase the impregnating ability of the electrolytic solution, a material having a small contact angle of the electrolytic solution with the separator 11 is preferably selected. In order to keep satisfactory ion permeability and appropriate thrust strength, the film thickness is 5 to 25 μm and further preferably 7 to 16 μm.
Now, a method for producing the film-packed stacked lithium secondary battery 7 according to an example embodiment of the present invention will be described below.
First, for the electrodes for a secondary battery, the positive electrode 10 having the positive electrode active material layers 8a, 8b formed on both surfaces of the positive electrode current collector 9 and the negative electrode 1 having the negative electrode active material layers 2a, 2b formed on both surfaces of the negative electrode current collector 3, are prepared, as shown in
Next, as shown in
While the electrode stack 12 is housed in the film package 13, which is sealed except the inlet, the electrolytic solution (not shown) is introduced into the film package 13 through the inlet. In order to seal the inlet of the film package 13 housing the electrode stack 12 and the electrolytic solution, unsealed outer peripheral portions of the film exteriors 13a, 13b are mutually joined by, e.g., welding. In this manner, the entire periphery of the film package 13 is sealed.
In the aforementioned example embodiments, an electrolytic solution is used; however, e.g., a solid electrolyte containing an electrolytic salt, a polymer electrolyte, a solid state or gel-state electrolyte prepared by mixing or dissolving an electrolytic salt to e.g., a polymer compound can be also used. These can serve also as a separator.
In the aforementioned example embodiments, a battery having a laminate of electrodes is described; the present invention can employ a roll design of electrodes and can be applied to a cylindrical and prismatic batteries.
In the aforementioned example embodiments, a lithium ion secondary battery is described; however, the present invention is effective if it is applied to a secondary battery other than lithium ion secondary batteries.
Now, the effect of an example embodiment will be specifically described by way of Examples and Comparative Examples.
[Production of Positive Electrode]
93% by mass of overlithiated lithium manganate (Li1.2Ni0.2Mn0.6O2), 3% by mass of powdery polyvinylidene fluoride and 4% by mass of powdery graphite were homogeneously mixed to prepare a positive-electrode mix. The positive-electrode mix prepared was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive-electrode mix slurry. The positive-electrode mix slurry was uniformly applied to one of the surfaces of aluminum (Al) foil serving as a positive electrode current collector, dried at about 120° C., molded and pressurized by using a punching die and a press machine to form a rectangle positive electrode. Note that, the unit weight of the positive electrode was set to be 20 g/cm2 and the density of the positive electrode was set to be 2.9 g/cm3.
[Production of Negative Electrode]
A negative electrode active material (85 mass %), which was prepared by mixing carbon-coated silicon oxide (abbreviated as SiOC) particles having a D50 of 5 μm and boron-added Si alloy (Si0.98B0.02) particles having a D50 of 0.4 μm in the ratio of 95 (mass %):5 (mass %); a polyimide binder (13 mass %) and fibrous graphite (2 mass %) were homogeneously mixed to prepare a negative-electrode mix. The negative-electrode mix was dispersed in NMP to prepare a negative-electrode mix slurry. Subsequently, the negative-electrode mix slurry was applied one of the surfaces of stainless steel (SUS) foil, dried at about 90° C., further dried at 350° C. in a nitrogen atmosphere, and molded into a rectangle negative electrode by a punching die. Note that, the outer size of sides of the negative electrode is set to be larger by 1 mm than the outer size of the positive electrode. The unit weight of the negative electrode was set to be 2.6 g/cm2 and the density of the negative electrode was set to be 1.31 g/cm3. Note that, a nonaqueous polyimide binder was used herein; however, an aqueous binder such as SBR (styrene butadiene copolymer), CMC (sodium carboxymethyl cellulose), a mixture of SBR and CMC, PAA (polyacrylic acid) and an aqueous polyimide binder can be used with water as a dispersion medium in preparing slurry.
[Production of Electrolytic Solution]
Ethylene carbonate (EC), tris(2,2,2-trifluoroethyl) phosphate (TTFEP) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (FE1) were mixed in volume ratio of EC/TTFEP/FE1=2/3/5 and dissolved in 0.8 mol/l LiPF6 to prepare an electrolytic solution.
[Production of Stacked Non-Aqueous Electrolyte Secondary Battery]
A positive electrode having a positive electrode terminal connected thereto and a negative electrode having a negative electrode terminal connected thereto were stacked such that the active material layers of them face each other with a porous aramid separator (15 μm) interposed therebetween to produce an electrode stack. At the stacking, the positive electrode and the negative electrode were staked so that the clearance between the edge of the positive electrode and the edge of the negative electrode in each side became 1 mm. The electrode stack was sandwiched by film exteriors made of aluminum laminate film. The outer periphery except the inlet was heat-sealed and the electrolytic solution prepared above was introduced through the inlet. Thereafter, the inlet was sealed by heat-sealed to produce a stacked lithium ion secondary battery. Note that, with respect to the electrode area, provided that the ratio of the initial charging capacity per unit area of the negative electrode and the initial charging capacity per unit area of the positive electrode is represented by A (negative electrode)/C (positive electrode), a ratio of A/C was set to be 1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and Si0.98B0.02 (D50: 0.4 μm) in a ratio of 85 (mass %):15 (mass %). Note that the unit weight of the negative electrode was set to be 2.4 g/cm2 and the density of the negative electrode was set to be 1.36 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and tin-added Si alloy (Si0.93Sn0.07) (D50: 0.4 μm) in a ratio of 95 (mass %):5 (mass %). Note that the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.32 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and Si0.93Sn0.07 (D50: 0.4 μm) in a ratio of 85 (mass %):15 (mass %). Note that the unit weight of the negative electrode was set to be 2.6 g/cm2 and the density of the negative electrode was set to be 1.36 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and titanium-added Si alloy (Si0.95Ti0.05) (D50: 0.5 μm) in a ratio of 95 (mass %):5 (mass %). Note that the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.32 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and aluminum-added Si alloy (Si0.95Al0.05) (D50: 0.6 μm) in a ratio of 95 (mass %):5 (mass %). Note that the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.32 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and chromium-added Si alloy (Si0.95Cr0.05) (D50: 0.6 μm) in a ratio of 95 (mass %):5 (mass %). Note that the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.31 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and copper-added Si alloy (Si0.95Cu0.05) (D50: 0.5 μm) in a ratio of 95 (mass %):5 (mass %). Note that, the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.31 g/cm3. A laminated lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A negative-electrode mix was prepared by homogeneously mixing SiOC (85 mass %), a polyimide binder (13 mass %) and fibrous graphite (2 mass %) and dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a negative-electrode mix slurry. Subsequently, a rectangle negative electrode was formed in the same manner as in Example 1 by using the negative-electrode mix slurry. Note that, the unit weight of the negative electrode was set to be 2.6 g/cm2 and the density of the negative electrode was set to be 1.23 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and boron-added Si alloy (Si0.9B0.1) (D50: 10 μm) in a ratio of 95 (mass %):5 (mass %). Note that, the unit weight of the negative electrode was set to be 2.7 g/cm2 and the density of the negative electrode was set to be 1.36 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
A rectangle negative electrode was formed in the same manner as in Example 1 by using a negative electrode active material prepared by mixing SiOC and manganese-added Si alloy (Si0.95Cu0.05) (D50: 0.5 μm) in a ratio of 85 (mass %):15 (mass %). Note that, the unit weight of the negative electrode was set to be 2.6 g/cm2 and the density of the negative electrode was set to be 1.35 g/cm3. A stacked lithium ion secondary battery was produced by using the positive electrode, separator and electrolytic solution of Example 1 so as to have A/C=1.1.
The levels of the negative electrodes using in Examples 1 to 8, Comparative Examples 1 to 3 are shown in Table 1.
Stacked lithium secondary batteries produced in Examples and Comparative Examples were subjected to repeating cycles four times in an environment of 45° C. In each cycle, the batteries were constantly charged at a current value of 0.1 C up to 4.5 V and constantly discharged at a current value of 0.1 C up to 1.5 V. The charge/discharge efficiency obtained in the first cycle, and the volumetric energy density obtained in the fourth cycle in each level are shown together with the electrode density in Table 2. The volumetric energy densities mentioned in Table 2 were obtained by calculating the discharge energy based on the discharge capacity at the fourth discharge time and average discharge voltage and dividing the discharge energy by the cell volume. Note that, the cell volume was obtained by multiplying the laminate area of an outer package and the thickness of the cell. Note that, unit C represents a relative current amount, and 0.1 C means a current value at which the discharge is completed in just 10 hours by discharging a constant current with a battery having a capacity of a nominal capacity value.
As is apparent from Table 2, the electrode density, volumetric energy density and initial charge/discharge efficiency in each of Examples 1 to 8 are higher than in Comparative Example 1 employing no Si alloy. In addition, in Comparative Example 2 where the central particle size D50 of Si alloy is larger than the central particle size D50 of SiOχ, since the initial charge/discharge efficiency is low, the volumetric energy density is also low.
Subsequently, cycle characteristic was evaluated by repeating a cycle consisting of constant current charge at a current value of 0.3 C up to 4.5 V and a constant current discharge at a current value of 0.3 C up to 1.5 V, 35 times. At this time, a change of the discharge capacity retention rate based on the discharge capacity at the first cycle as 100% is shown in
As shown in Table 3, in Examples 2 and 4 where Si alloy addition amount was large, the discharge capacity retention rate after 35 cycles was lower than in Comparative Example 1; while each of the volumetric energy density at the first cycle was high than in Comparative Example 1. The volumetric energy density at the 35th cycle in Example 2 was higher than in Comparative Example 1 and the volumetric energy density at the 35th cycle in Example 4 was same as in Comparative Example 1. It is found that, in Examples 1, 3, 5 to 8 where Si alloy addition amount is low, the discharge capacity retention rate and both volumetric energy densities are larger than in Comparative Examples. Furthermore, it is found that, in Comparative Example 2 where the central particle size D50 of Si alloy is larger than the central particle size D50 of SiOχ, the discharge capacity retention rate rapidly faded and shows an extremely low value at the 35th cycle. It was found that, in Comparative Example 3 employing Si alloy including Mn, the content of which in the positive electrode Li1.2Ni0.2Mn0.6O2 is the largest, the discharge capacity retention rate at the 35th cycle is lower than in Examples employing Si alloy including other elements. This is presumably because the elution amount of Mn from the positive electrode is large.
From the results mentioned above, the electrode density, and the initial charge/discharge efficiency were improved by mixing second particles composed of a Si alloy having a central particle size D50 smaller than that of first particles composed of SiOχ, to the first particles, with the result that high volumetric energy density was obtained. This application is based upon and claims the benefit of priority from Japanese patent application No. 2016-082179, filed on Apr. 15, 2016, the disclosure of which is incorporated herein in its entirety by reference.
The present invention can be applied to power supplies for mobile devices such as mobile phones and notebook computer; power supplies for electric vehicles such as electric cars, hybrid cars, electric motorcycles and electric assisted bicycles; power supplies for moving/transport mediums such as electric trains, satellites and submarines; and electricity storage systems for storing electricity.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-082179 | Apr 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/012968 | 3/29/2017 | WO | 00 |
