Patent Application
20160056463
The present invention relates to technology of a nonaqueous electrolyte secondary battery including a positive electrode that contains lithium-nickel composite oxides.
Currently, a nonaqueous electrolyte secondary battery represented by a lithium ion secondary battery is widely used for consumer applications such as small portable devices because of its high energy density. In a general lithium ion secondary battery, a transition metal oxide such as LiCoO2 has been used as a positive electrode active material, a carbon material such as graphite has been used as a negative electrode active material, and a nonaqueous electrolyte obtained by dissolving an electrolyte salt such as LiPF6 in a nonaqueous solvent such as a carbonic acid ester has been used as an electrolyte solution.
Moreover, a nonaqueous electrolyte secondary battery using, as a negative electrode active material, lithium titanate that allows insertion/detachment reaction of the lithium ion to occur at an electric potential relative to that of lithium of about 1.5 V, the electric potential being nobler when lithium titanate is compared with carbon materials, has been proposed in recent years (see, for example, Patent Literatures 1 and 2).
Furthermore, a nonaqueous electrolyte secondary battery using, as a positive electrode active material, a lithium-nickel composite oxide represented by the general formula LiNixM1-xO2 (where 0.7≦x<1, and M represents one or more metals) has been proposed (see, for example, Patent Literature 2).
Patent Literature 1
Japanese Patent Laid-Open Publication No. 2010-153258
Patent Literature 2
Japanese Patent Laid-Open Publication No. 2007-80738
Now, various proposals have been made on a high-capacity nonaqueous electrolyte secondary battery, however a nonaqueous electrolyte secondary battery having a further higher capacity has been demanded in applying the nonaqueous electrolyte secondary battery to a power source for electric power storage facilities and to a power source for vehicles such as an HEV.
Moreover, regarding the lithium-nickel composite an irreversible change in the crystal structure is liable to occur during charge and discharge, and therefore there is a problem that cyclability is greatly lowered.
Thus, it is an object of the present invention to provide a nonaqueous electrolyte secondary battery that may achieve a high capacity and may suppress lowering of the cyclability.
The nonaqueous electrolyte secondary battery of an embodiment of the present invention includes a negative electrode having a negative electrode active material layer; a positive electrode having a positive electrode active material layer; a separator interposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte, the battery being a negative electrode restricted nonaqueous electrolyte secondary battery that stops charging as a result of an electric potential drop of the negative electrode, wherein the size of the negative electrode active material layer is larger than the size of the positive electrode active material layer, and the positive electrode active material layer contains: a lithium-nickel composite oxide A represented by the general formula LiNixM1-xO2 (where 0.7≦x<1, and M represents one or more metals); and a lithium-nickel composite oxide B represented by the general formula LiNixCoyM1-x-yO2 (where 0<x≦0.5, 0<y<1, and M represents one or more metals excluding Co).
According to the present invention, a nonaqueous electrolyte secondary battery that achieves a high capacity and suppresses lowering of the cyclability may be provided.
Hereinafter, an embodiment of the present invention will be explained. The present embodiment is an example of practicing the present invention, and the present invention is not limited to the present embodiment.
The negative electrode 1 includes is negative electrode collector and a negative electrode active material layer provided on the negative electrode collector. The negative electrode active material layer is preferably arranged on both faces of the negative electrode collector, but may be provided on one face of the negative electrode collector. The negative electrode active material layer contains a negative electrode active material, and may also contain a negative electrode additive or the like added therein in addition to the negative electrode active material.
The negative electrode active material includes publicly known negative electrode active materials used for nonaqueous electrolyte secondary batteries such as a lithium ion battery, and examples thereof include carbon-based active materials, silicon-based active materials containing silicon and lithium titanate. Examples of the carbon-based compound include artificial graphite, natural graphite, hardly graphitizable carbon and easily graphitizable carbon. Examples of the silicon-based active material include silicon, silicon compounds, and partially substituted compounds or solid solutions thereof. The silicon compound is preferably, for example, silicon oxides represented by SiOa (where 0.05<a<1.95).
The negative electrode active material here is particularly preferably lithium titanate from the viewpoint of having a small volume expansion during charge and discharge, exhibiting a favorable cyclability, and so on, more preferably lithium titanate represented by the chemical formula Li4+xTi5O12 (0≦x≦3), and an example thereof includes Li4Ti5O12. In addition, lithium titanate in which a part of Ti is substituted with another element such as, for example, Al or Mg may be used.
The negative electrode additive is, for example, a binder and a conductive agent. Examples of the conductive agent include acetylene black, carbon black and graphite. Moreover, examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) and fluorine-based rubber.
The negative electrode collector is constituted by, for example, a publicly known conductive material used for nonaqueous electrolyte secondary batteries such as a lithium ion battery, and examples of the negative electrode collector include nonporous conductive substrates. The thickness of the negative electrode collector is preferably in the range of, for example, about 1 μm or more and about 500 μm or less.
The positive electrode 2 includes a positive electrode collector and a positive electrode active material layer. The positive electrode active material layer is preferably arranged on both faces of the positive electrode collector, but may be arranged only on one face side of the positive electrode collector. The positive electrode active material layer contains a positive electrode active material, and may also contain a positive electrode additive added therein in addition to the positive electrode active material.
The positive electrode active material contains a lithium-nickel composite oxide A represented by the general formula LiNixM1-xO2 (where 0.7≦x<1, and M represents one or metals) and a lithium-nickel composite oxide B represented by the general formula LiNixCoyM1-x-yO2 (where 0<x≦0.5, 0<y<1, and M represents one or more metals excluding Co). The lithium-nickel composite oxide B is a positive electrode active material in which it is harder for an irreversible change in the crystal structure during Charge and discharge to occur than in the lithium-nickel composite oxide A. It is considered that the reason for this is because the composition ratio of Ni in the composite oxide is smaller in the lithium-nickel composite oxide B.
The positive electrode additive is, for example, a binder or a conductive agent. As the binder and the conductive agent, the same substances used for the negative electrode 1 may be used.
The positive electrode collector is constituted by, for example, a publicly known conductive material used for nonaqueous electrolyte secondary batteries such as a lithium ion secondary battery, and examples thereof include nonporous conductive substrates.
Hereinafter, explanation will be given on how to achieve a high capacity and suppress lowering of the cyclability for the nonaqueous electrolyte secondary battery in the present embodiment.
In the present embodiment, the size of the negative electrode active material layer 12 is designed to be larger than the size of the positive electrode active material layer 14. That is to say, as shown in
In addition, in the case of carrying out positive electrode restriction that stops charging of the nonaqueous electrolyte secondary battery in the flat region of the electric potential of the negative electrode, namely by an electric potential change of the positive electrode, the electric potential at which the reaction occurs between the unopposed region of the negative electrode active material layer and the positive electrode active material layer at the end portion of the outer circumference may not be detected from the electric potential of the positive electrode, and therefore it is difficult to raise the utilization coefficient of the unopposed region of the negative electrode active material layer to thereby achieve a high capacity.
As shown in
In this way, by making the size of the negative electrode active material layer larger than the size of the positive electrode active material layer, and carrying out the negative electrode restriction that stops charging of the nonaqueous electrolyte secondary battery as a result of an electric potential drop of the negative electrode, the utilization coefficient of the unopposed region of the negative electrode active material layer rises high capacity may be achieved. However, when the utilization coefficient of the unopposed region of the negative electrode active material layer rises, excessive lithium is inserted and detached from the positive electrode active material. Therefore, in the positive electrode active material consisting of the lithium-nickel composite oxide A represented by LiNixM1-xO2 (where 0.7≦x<1, and M represents one or more metals), the irreversible change in the crystal structure is brought about during charge and discharge to lower the cyclability of the nonaqueous electrolyte secondary battery.
However, the positive electrode active material of the present embodiment contains the lithium-nickel composite oxide B represented by the general formula LiNixCoyM1-x-yO2 (where 0<x≦0.5, 0<y<1 and M represents one or more metals excluding Co) in addition to the lithium-nickel composite oxide A represented by the general formula LiNixM1-xO2 (where 0.7≦x<1 and M represents one or more metals) and therefore may suppress lowering of the cyclability of the nonaqueous electrolyte secondary battery by suppressing the irreversible change in the crystal structure of the lithium-nickel composite oxide A or the like.
Hereinafter, preferable conditions and other constituent elements of the nonaqueous electrolyte secondary battery of the present embodiment will be explained.
The metal M in the lithium-nickel composite oxide A represented by the general formula LiNixM1-xO2 is preferably at least one metal selected from Co, Al, Mn, and Ti from the viewpoint of achieving a high capacity, more preferably Co or Al, and examples include LiNi0.82Co0.15Al0.03.
The metal M in the lithium-nickel composite oxide B represented by the general formula LiNixCoyM1-x-yO2 (where 0<x≦0.5, 0<y<1 and M represents one or more metals excluding Co) is preferably at least one metal selected from Mn, Al, and Ti from the viewpoint Of suppressing lowering of the cyclability and improving the polarization performance, and examples include LiNi0.5Co0.3Mn0.2.
The mass ratio of the lithium-nickel composite oxide B to the lithium-nickel composite oxide. A is preferably in the range of 0.1 or more and less than 0.5. In the case where the mass ratio of the lithium-nickel composite oxide B to the lithium-nickel composite oxide A is less than 0.1, it sometimes occurs that lowering of the cyclability may not be suppressed, and in the case where the mass ratio is 0.5 or more, the ratio of the lithium-nickel composite oxide A is lowered, and therefore it sometimes occurs that the battery capacity is lowered.
The method for producing the lithium-nickel composite oxides A and B is not particularly limited, and the lithium-nickel composite oxides A and B are obtained by mixing a lithium oxide as a Li source and oxides of Ni and other metals, and firing the resultant mixture under an air atmosphere. The composition ratio of each metal in the lithium-nickel composite oxides A and B is adjusted by the molar ratio of each oxide in the mixture, or the like.
The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt. As the nonaqueous solvent, for example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) may be used. These are preferably used in combination of plural kinds.
The nonaqueous solvent of present embodiment is not limited to contain other solvents as specifically described above and may contain, for example, cyclic ethers such as tetrahydrofuran (THF) and 2-methyl tetrahydrofuran (2MeTHF); chain ethers such as dimethoxyethane (DME); γ-butyrolactone (GBL), acetonitrile (AN), sulfolane (SL), and various ionic liquids, or various normal-temperature molten salts.
The electrolyte salt used in the present embodiment is not particularly limited, and the electrolyte salts such as, for example, LiClO4, LiBF4, LiAsF6, LiPF6, LIPF(CF3)5, LiPF2(CF3)4, LiPF3(CF3)3, LiPF4(CF3)2, LiPF5(CF3), LiPF3(C2F5)3, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LIN(C2F5CO)2, LiI, LiAlCl4, and LiBC4O8 may be used alone or in combination of two or more.
Among others, LiPF6 is preferably used because the ion conductivity is favorable. The concentration of these electrolyte salts is preferably set to 0.5 to 2.0 mol/dm3. Furthermore, the concentration of the electrolyte salts is more preferably set to 1.5 to 2.0 mol/dm3. Moreover, the electrolyte salt may also be used when at least one selected from the group consisting of: carbonates such as vinylene carbonate and butylene carbonate; benzenes such as biphenyl and cyclohexylbenzene; sulfur compounds such as propanesultone; and ethylene sulfide, hydrogen fluoride, triazole-based cyclic compounds, fluorine-containing esters, a hydrogen fluoride complex of tetraethylammonium fluoride and derivatives thereof, phosphazene and derivatives thereof, amide group-containing compounds, imino group-containing compounds, and nitrogen-containing compounds, is contained in the electrolyte salt. Moreover, the electrolyte salt may also be used when at least one selected from CO2, NO2, CO, SO2, and the like is contained therein.
As the separator 3, for example, a sheet or the like made of a resin having a predetermined ion permeability, mechanical strength, insulation properties, and so on may be used. The thickness of the separator 3 is preferably in the range of, for example, about 10 μm or more and about 300 μm or less. Moreover, the porosity of the separator 3 is preferably in the range of about 30% or more and about 70% or less. In addition, the porosity is expressed as percentages of the total volume of pores that are contained in the separator 3 relative to the volume of the separator 3.
In addition, the nonaqueous electrolyte secondary battery 30 in
Hereinafter, the present invention will be further explained by an example, but the present invention is not limited to the Example.
Ni0.80Co0.15Al0.05O2, was used as a lithium-nickel composite oxide A, LiNi0.35Co0.35Mn0.30O2 was used as a lithium-nickel composite oxide B, and a positive electrode active material consisting of the lithium-nickel composite oxides A and B in a mass ratio of A to B of 8:2, a carbon powder as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the positive electrode active material, the conductive agent, and the binder was 100:5:2.55, then the resultant mixture was kneaded, and thereafter N-methyl-2-pyrrolidone as a dispersion medium was added therein to prepare a positive electrode slurry. The positive electrode slurry was applied on both faces of an aluminum foil (thickness 15 μm) as a positive electrode collector and dried to manufacture positive: electrode active material layers on the aluminum foil, and thereafter the positive electrode active material layers on the aluminum foil were rolled with a rolling roller to manufacture a positive electrode. Moreover, a positive electrode lead was attached to the obtained positive electrode.
Li4Ti5O12 as a negative electrode active material, a Carbon powder as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the negative electrode active material, the conductive agent, and the binder was 100:7:3, then the resultant mixture was kneaded, and thereafter N-methyl-2-pyrrolidone as a dispersion medium was added thereto to prepare a negative electrode slurry. The negative electrode slurry was applied on both faces of an aluminum foil (thickness 15 μm) as a negative electrode collector and dried to manufacture negative electrode active material layers on the aluminum foil, and thereafter the negative electrode active material layers on the aluminum foil were rolled with a rolling roller to manufacture a negative electrode. Moreover, a negative electrode lead was attached to the obtained negative electrode.
The ratio of the area of the negative electrode active material layer to the area of the positive electrode active material layer (area of negative electrode active material layer/area of positive electrode active material layer) was 1.27.
Lithium hexaflucrophosphate (LiPF6) was dissolved in a mixed solvent obtained by mixing propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a volume ratio of 25:70:5, so that the concentration was 1.2 mol/L to prepare a nonaqueous electrolyte (electrolyte solution), and then LiPO2F2 was dissolved therein as an additive in a concentration of 0.9 wt % relative to the total weight of the electrolyte solution.
The positive electrode and negative electrode manufactured as described above were laminated with a separator interposed therebetween, and the obtained laminated product was wound to manufacture an electrode group. The electrode group was housed in an aluminum laminate film as an exterior body, the aforementioned nonaqueous electrolyte was injected into the aluminum laminate film, and thereafter the aluminum laminate film was tightly sealed to manufacture a test cell 1.
The test cell 1 was housed in a thermostatic chamber at 20° C., charged by a constant current/constant voltage system as described below, and discharged by a constant current system. However, the end of charging was determined by negative electrode restriction. The test cell 1 was charged at a constant current of 1C rate (1C is defined as a value of current at which the whole battery capacity can be consumed in 1 hour) until the voltage at the negative elect rode became 1.4 V or lower and the battery voltage became 2.8 V. After the battery voltage reached 2.8 V, the test cell was charged at a constant voltage of 2.8 V until the current value reached 0.05C. Next, the charge was suspended for 20 minutes, and thereafter the test cell after charging was discharged at a constant current of 1C rate until the battery voltage became 1.5 V. Such charge and discharge were repeated for 1000 cycles. A ratio (a value determined as percentages) of the discharging capacity of each cycle after the first cycle to the discharging capacity at the first cycle was calculated and determined as a discharging capacity retention ratio. It may be said that the cyclability is lowered more as the discharging capacity retention ratio is lowered.
A test cell 2 was manufactured in the same Manner as in Example except that only LiNi0.80Co0.15Al0.05O2 was used as a positive electrode active material. Moreover, the cyclability of the test cell 2 was also evaluated under the same conditions as in the evaluation of the test cell 1.
As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-065934 | Mar 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/001344 | 3/10/2014 | WO | 00 |
