Patent Application
20190198915
The present invention relates to nonaqueous electrolyte secondary batteries and methods for manufacturing such batteries.
Recently, nonaqueous electrolyte secondary batteries with high energy densities have been used for applications such as power supplies for driving hybrid electric vehicles (PHEVs and HEVs) and electric vehicles (EVs). There is a growing need for a nonaqueous electrolyte secondary battery with a higher performance for such applications.
Japanese Published Unexamined Patent Application No. 2011-187440 (Patent document 1) proposes a technique involving the use of a nonaqueous electrolyte containing lithium fluorosulfonate.
An object of the present invention is to provide a nonaqueous electrolyte secondary battery with good low-temperature output characteristics and high-temperature storage characteristics.
A first aspect of the present invention provides a nonaqueous electrolyte secondary battery. This nonaqueous electrolyte secondary battery includes an electrode assembly and a nonaqueous electrolyte. The electrode assembly includes a positive electrode and a negative electrode. The positive electrode contains a positive electrode active material, lithium carbonate, and lithium phosphate. The negative electrode contains a negative electrode active material. The nonaqueous electrolyte contains lithium fluorosulfonate.
The thus-configured nonaqueous electrolyte secondary battery has good low-temperature output characteristics and high-temperature storage characteristics.
The nonaqueous electrolyte secondary battery according to the first aspect of the present invention preferably further includes a housing having an opening and containing the electrode assembly and the nonaqueous electrolyte, a sealing member sealing the opening, a terminal attached to the sealing member and electrically connected to the positive electrode or the negative electrode, and a current-interrupting mechanism disposed on a conduction path between the positive electrode or the negative electrode and the terminal. The current-interrupting mechanism is activated to break the conduction path and thereby interrupt a current therethrough when the internal pressure of the battery reaches a predetermined level or higher. This nonaqueous electrolyte secondary battery is reliable against overcharge.
The nonaqueous electrolyte secondary battery according to the first aspect of the present invention preferably further includes a housing having an opening and containing the electrode assembly and the nonaqueous electrolyte, a sealing member sealing the opening, a terminal attached to the sealing member and electrically connected to the positive electrode or the negative electrode, and a short-circuiting mechanism that is activated to forcedly short-circuit the positive electrode and the negative electrode when the internal pressure of the battery reaches a predetermined level or higher. This nonaqueous electrolyte secondary battery is reliable against overcharge. The nonaqueous electrolyte secondary battery preferably further includes a fusible portion disposed on a conduction path between the positive electrode or the negative electrode and the terminal. The fusible portion is blown by a short-circuit current upon activation of the short-circuiting mechanism.
The positive electrode active material is preferably a lithium transition metal oxide, and the negative electrode active material is preferably a carbonaceous material. The lithium transition metal oxide preferably contains at least one transition metal selected from nickel, cobalt, and manganese. Preferred carbonaceous materials include graphite and amorphous carbon.
A second aspect of the present invention provides a method for manufacturing a nonaqueous electrolyte secondary battery. This nonaqueous electrolyte secondary battery includes an electrode assembly, a nonaqueous electrolyte, a housing having an opening and containing the electrode assembly and the nonaqueous electrolyte, and a sealing member sealing the opening. The electrode assembly includes a positive electrode and a negative electrode. The positive electrode contains a positive electrode active material, lithium carbonate, and lithium phosphate. The negative electrode contains a negative electrode active material. The nonaqueous electrolyte contains lithium fluorosulfonate. This method includes placing the nonaqueous electrolyte in the housing.
Embodiments of the present invention will now be described in detail. It should be noted that the following embodiments are provided for illustration purposes only and are not intended to limit the invention.
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The flat-wound electrode assembly 1 is constructed by winding together the positive electrode 21 and the negative electrode 22 with the separator therebetween and forming them into a flat shape. This is done such that the exposed region 4 of the positive electrode substrate 21a and the exposed region 5 of the negative electrode substrate 22a are wound at the opposite ends of the flat-wound electrode assembly 1.
The exposed region 4 of the positive electrode substrate 21a is electrically connected to a positive electrode terminal 7 via a positive electrode current collector 6. The exposed region 5 of the negative electrode substrate 22a is electrically connected to a negative electrode terminal 9 via a negative electrode current collector 8. The positive electrode current collector 6 and the positive electrode terminal 7 are preferably made of aluminum or aluminum alloy. The negative electrode current collector 8 and the negative electrode terminal 9 are preferably made of copper or copper alloy. The positive electrode terminal 7 includes a flange 7a and an insertion portion 7b formed on one side of the flange 7a. The insertion portion 7b is inserted into a terminal mounting hole formed in a sealing member 3. The negative electrode terminal 9 includes a flange 9a and an insertion portion 9b formed on one side of the flange 9a. The insertion portion 9b is inserted into a terminal mounting hole formed in the sealing member 3.
A current-interrupting mechanism 50 is disposed on the conduction path between the positive electrode 21 and the positive electrode terminal 7. When the internal pressure of the battery 20 reaches a predetermined level or higher, the current-interrupting mechanism 50 is activated to break the conduction path between the positive electrode 21 and the positive electrode terminal 7 and thereby interrupt a current therethrough.
The positive electrode terminal 7 is attached to the sealing member 3 with an outer insulator 11 therebetween. The negative electrode terminal 9 is attached to the sealing member 3 with an outer insulator 13 therebetween.
The flat-wound electrode assembly 1 is covered with a resin insulating sheet 14 and is contained in a metal prismatic housing 2. The sealing member 3 is fitted and laser-welded to the opening edge of the prismatic housing 2.
The sealing member 3 has a liquid electrolyte inlet 15 through which a nonaqueous liquid electrolyte is injected. The liquid electrolyte inlet 15 is sealed by a sealing plug 16 such as a blind rivet. The sealing member 3 also has a gas release valve 17 that ruptures and releases gas from the battery 20 when the internal pressure of the battery 20 exceeds the activation pressure of the current-interrupting mechanism 50.
The construction of the current-interrupting mechanism 50 disposed in the nonaqueous electrolyte secondary battery 20 will now be described with reference to
The positive electrode terminal 7 includes the flange 7a and the insertion portion 7b and has a through-hole 7x formed inside. The insertion portion 7b of the positive electrode terminal 7 is inserted into through-holes formed in the outer insulator 11, the sealing member 3, a first insulator 10, and a conductor 51. The end of the insertion portion 7b is crimped to hold together the positive electrode terminal 7, the outer insulator 11, the sealing member 3, the first insulator 10, and the conductor 51.
The periphery of a deformable plate 52 is welded to the lower edge of a cylindrical portion of the conductor 51. A thin portion 6d of a base 6a of the positive electrode current collector 6 is laser-welded to the center of the deformable plate 52 to form a joint 54. In this way, the exposed region 4 of the positive electrode substrate 21a is electrically connected to the positive electrode terminal 7 via the positive electrode current collector 6, the deformable plate 52, and the conductor 51. The thin portion 6d has an annular groove 6e formed around the joint 54.
When the internal pressure of the prismatic housing 2 exceeds a predetermined level, the deformable plate 52 deforms such that the center thereof is inverted toward the sealing member 3. The deformation of the deformable plate 52 causes the thin portion 6d of the positive electrode current collector 6 to rupture at the annular groove 6e, thus breaking the conduction path.
The through-hole 7x formed in the positive electrode terminal 7 is sealed by a terminal plug 18. The terminal plug 18 includes an elastic member 18a and a metal plate 18b.
Although the current-interrupting mechanism illustrated in this embodiment is disposed on the conduction path of the positive electrode, it may instead be disposed on the conduction path of the negative electrode.
Methods for manufacturing the positive electrode 21, the negative electrode 22, the flat-wound electrode assembly 1, and the nonaqueous liquid electrolyte used as the nonaqueous electrolyte for the nonaqueous electrolyte secondary battery 20 will now be described.
A lithium transition metal oxide represented by the formula Li(Ni0.35Co0.35Mn0.30)0.995Zr0.005O2 is used as a positive electrode active material. A positive electrode mixture slurry is prepared by weighing the lithium transition metal oxide, carbon powder, serving as a conductor, polyvinylidene fluoride (PVdF), serving as a binder, lithium carbonate, and lithium phosphate in a mass ratio of 87.2:7:2:2:1.8 and mixing them with N-methyl-2-pyrrolidone (NMP), serving as a dispersion medium.
A positive electrode protective layer slurry is prepared by mixing alumina powder, PVdF, carbon powder, and NMP, serving as a dispersion medium, in a mass ratio of 21:4:1:74.
The positive electrode mixture slurry and positive electrode protective layer slurry prepared as described above are applied to each side of an aluminum foil, serving as the positive electrode substrate 21a, using a die coater. The electrode is dried to remove NMP, serving as a dispersion medium, is pressed to a predetermined thickness using a roller press, and is cut to a predetermined size to form the positive electrode 21. The positive electrode 21 has the exposed region 4 of the positive electrode substrate 21a, where no positive electrode active material mixture layer 21b is formed, on each side of the positive electrode 21. The exposed region 4 of the positive electrode substrate 21a extends longitudinally along one lateral edge of the positive electrode 21.
A negative electrode mixture slurry is prepared by dispersing graphite powder, serving as a negative electrode active material, carboxymethylcellulose (CMC), serving as a thickener, and styrene-butadiene rubber (SBR), serving as a binder, in water in a mass ratio of 98:1:1.
A negative electrode protective layer slurry is prepared by mixing alumina powder, a binder (acrylic resin), and NMP, serving as a dispersion medium, in a mass ratio of 30:0.9:69.1 and dispersing the mixture in a bead mill.
The negative electrode mixture slurry prepared as described above is applied to each side of a copper foil, serving as the negative electrode substrate 22a, using a die coater. The electrode is dried to remove water, serving as a dispersion medium, and is pressed to a predetermined thickness using a roller press. The negative electrode protective layer slurry prepared as described above is then applied to the negative electrode active material mixture layer 22b. The electrode is dried to remove NMP, serving as a dispersion medium, thus forming the negative electrode protective layer 22c. The electrode is cut to a predetermined size to form the negative electrode 22. The negative electrode 22 has the exposed region 5 of the negative electrode substrate 22a, where no negative electrode active material mixture layer 22b is formed, on each side of the negative electrode 22. The exposed region 5 of the negative electrode substrate 22a extends longitudinally along each lateral edge of the negative electrode 22.
The positive electrode 21 and negative electrode 22 fabricated as described above are wound together with a 20 μm thick polypropylene separator therebetween and are pressed into a flat shape to form the flat-wound electrode assembly 1. This is done such that the exposed region 4 of the positive electrode substrate 21a and the exposed region 5 of the negative electrode substrate 22a are wound at the opposite ends of the flat-wound electrode assembly 1 in the axial direction. The outermost layer of the flat-wound electrode assembly 1 is formed by the separator.
A solvent mixture is prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio (at 25° C. and 1 atm) of 3:3:4. LiPF6 is added as a solute to the solvent mixture in an amount of 1 mol/L. Lithium fluorosulfonate is then added to the solvent mixture in an amount of 1% by mass of the nonaqueous liquid electrolyte.
The process of forming the conduction path, including the current-interrupting mechanism 50, of the positive electrode will now be described. A resin outer insulator 11 is placed on the top surface of an aluminum sealing member 3. A resin first insulator 10 and a conductor 51 made of aluminum are placed on the bottom surface of the sealing member 3. The insertion portion 7b of a positive electrode terminal 7 made of aluminum is inserted into through-holes formed in the outer insulator 11, the sealing member 3, the first insulator 10, and the conductor 51. The end of the insertion portion 7b is crimped to hold together the positive electrode terminal 7, the outer insulator 11, the sealing member 3, the first insulator 10, and the conductor 51. The joint between the end of the insertion portion 7b and the conductor 51 is laser-welded.
The periphery of the deformable plate 52 is then laser-welded to the lower edge of the cylindrical portion of the cup-shaped conductor 51 to seal the lower end of the cylindrical portion of the conductor 51. The deformable plate 52 is a thin aluminum plate protruding downward.
A resin second insulator 53 is placed on the bottom surface of the deformable plate 52 and is attached to the first insulator 10 in a latching manner. A protrusion on the bottom surface of the second insulator 53 is inserted into a through-hole formed in the base 6a of an aluminum positive electrode current collector 6. The end of the protrusion is heated and widened to hold together the base 6a of the positive electrode current collector 6 and the second insulator 53. The through-hole 7x is filled with N2 gas to a predetermined pressure from the top of the positive electrode terminal 7 to press the deformable plate 52 against the base 6a of the positive electrode current collector 6. The base 6a is laser-welded to the deformable plate 52. The terminal plug 18 is inserted into the through-hole 7x of the positive electrode terminal 7.
The conduction path of the negative electrode is formed as follows. A resin outer insulator 13 is placed on the top surface of the sealing member 3. A resin inner insulator 12 and the negative electrode current collector 8 are placed on the bottom surface of the sealing member 3. The insertion portion 9b of the negative electrode terminal 9 is inserted into through-holes formed in the outer insulator 13, the sealing member 3, the inner insulator 12, and the negative electrode current collector 8. The end of the insertion portion 9b is crimped to hold together the negative electrode terminal 9, the outer insulator 13, the sealing member 3, the inner insulator 12, and the negative electrode current collector 8. The joint between the end of the insertion portion 9b and the negative electrode current collector 8 is laser-welded.
The positive electrode current collector 6 attached to the sealing member 3 as described above is joined to the exposed region 4 of the positive electrode substrate 21a of the wound electrode assembly 1 by resistance welding. The negative electrode current collector 8 attached to the sealing member 3 as described above is joined to the exposed region 5 of the negative electrode substrate 22a of the wound electrode assembly 1 by resistance welding.
The wound electrode assembly 1 is covered with a box-shaped insulating sheet 14 and is inserted into a prismatic housing 2 made of aluminum. The sealing member 3 is fitted and laser-welded to the prismatic housing 2. The nonaqueous liquid electrolyte prepared as described above is injected through the liquid electrolyte inlet 15 of the sealing member 3. The liquid electrolyte inlet 15 is sealed with the sealing plug 16 to obtain the nonaqueous electrolyte secondary battery 20.
A nonaqueous electrolyte secondary battery of Example 1 was fabricated as described above.
A nonaqueous electrolyte secondary battery of Comparative Example 1 was fabricated as in Example 1 except that the positive electrode active material mixture layer 21b contained no lithium phosphate and the nonaqueous electrolyte contained no lithium fluorosulfonate.
A nonaqueous electrolyte secondary battery of Comparative Example 2 was fabricated as in Example 1 except that the positive electrode active material mixture layer 21b contained no lithium phosphate.
The low-temperature output characteristics, high-temperature storage characteristics, and overcharge characteristics of the nonaqueous electrolyte secondary batteries of Example 1 and Comparative Examples 1 and 2 were measured by the following procedures.
Each nonaqueous electrolyte secondary battery was charged at a charge current of 2 A to a charge depth of 27% (SOC 27%) at 25° C. and was then allowed to stand at −30° C. for 5 hours. Thereafter, the nonaqueous electrolyte secondary battery was discharged at 10, 20, 30, 40, 50, 60, 70, or 80 A for 10 seconds. The battery voltage was measured after discharge at each current and was plotted against the current. The low-temperature output characteristics were determined as the output calculated from the I-V characteristics for discharge. The charge depth after discharge was returned to the initial charge depth by charging the battery at a constant current of 2 A.
Each nonaqueous electrolyte secondary battery was charged at a constant current of 4 A to a voltage of 4.1 V at 25° C. and was then charged at a constant voltage of 4.1 V for 2 hours. Thereafter, the nonaqueous electrolyte secondary battery was discharged at a constant current of 4 A to a voltage of 3 V and was then discharged at a constant voltage of 3 V for 3 hours. The discharge capacity during this process was measured to determine the initial battery capacity.
Next, the nonaqueous electrolyte secondary battery was charged at a constant current of 2 A to a charge depth of 80% (SOC 80%) at 25° C. and was then allowed to stand at 60° C. for 100 days. Thereafter, the nonaqueous electrolyte secondary battery was charged at a constant current of 4 A to a voltage of 4.1 V at 25° C. and was then charged at a constant voltage of 4.1 V for 2 hours. Thereafter, the nonaqueous electrolyte secondary battery was discharged at a constant current of 4 A to a voltage of 3 V and was then discharged at a constant voltage of 3 V for 3 hours. The discharge capacity during this process was measured to determine the battery capacity after storage at high temperature. The capacity retention was calculated as the percentage of the battery capacity after storage at high temperature to the initial battery capacity.
Each nonaqueous electrolyte secondary battery was charged at a charge current of 4 A to a charge depth of 100% (SOC 100%) at 25° C. and was then allowed to stand at 60° C. for 2 hours. Thereafter, the nonaqueous electrolyte secondary battery was charged at a charge current of 125 A until the current was interrupted due to the activation of the current-interrupting mechanism or other reasons.
Table 1 summarizes the results of the low-temperature output characteristics, high-temperature storage characteristics, and overcharge characteristics measurements for the nonaqueous electrolyte secondary batteries of Example 1 and Comparative Examples 1 and 2. In Table 1, the result of the low-temperature output characteristics measurement for Example 1 is expressed as 100%, and the results of the low-temperature output characteristics measurements for Comparative Examples 1 and 2 are expressed as percentages relative to that for Example 1.
As shown in Table 1, the nonaqueous electrolyte secondary battery of Example 1, in which the positive electrode active material mixture layer contained lithium carbonate and lithium phosphate and the nonaqueous electrolyte contained lithium fluorosulfonate, had good low-temperature output characteristics and high-temperature storage characteristics and was reliable with no abnormal behavior upon overcharge. These results can be explained as follows.
The addition of lithium fluorosulfonate improves the low-temperature output characteristics and the capacity retention after storage at high temperature probably because it adsorbs onto the positive and negative electrode active materials to reduce the reaction resistance at low temperature and also inhibits irreversible lithium formation through the reaction with the liquid electrolyte. A comparison between Example 1 and Comparative Example 2 shows that the addition of lithium phosphate to the positive electrode active material mixture layer produces a synergistic effect with lithium fluorosulfonate to further improve the low-temperature output characteristics of the nonaqueous electrolyte secondary battery. The addition of lithium carbonate and lithium phosphate to the positive electrode active material mixture layer allows the current-interrupting mechanism to activate immediately and also inhibits heat generation and abnormal reactions in the battery. This is probably because lithium carbonate emits the amount of gas required to activate the current-interrupting mechanism within a short period of time, and lithium phosphate inhibits oxidative degradation of the liquid electrolyte upon overcharge and thus inhibits heat generation due to oxidative degradation of the liquid electrolyte.
As discussed above, the present invention provides a nonaqueous electrolyte secondary battery with good low-temperature output characteristics and high-temperature storage characteristics.
Lithium carbonate is preferably present in the positive electrode active material mixture layer in an amount of 0.1% to 5% by mass, more preferably 0.5% to 4% by mass, even more preferably 1% to 3% by mass, of the positive electrode active material present in the positive electrode active material mixture layer.
Lithium phosphate is preferably present in the positive electrode active material mixture layer in an amount of 0.1% to 5%, more preferably 0.5% to 4%, even more preferably 1% to 3%, of the mass of the positive electrode active material present in the positive electrode active material mixture layer.
Lithium fluorosulfonate is preferably present in the nonaqueous electrolyte in an amount of 0.25% to 2.5%, more preferably 0.5% to 2%, even more preferably 0.75% to 1.5%, of the total mass of the nonaqueous electrolyte. These are the amounts of lithium fluorosulfonate present in the nonaqueous electrolyte during the fabrication of the battery.
The current-interrupting mechanism 50 may be replaced with a short-circuiting mechanism serving as a pressure-sensitive safety mechanism. The pressure-sensitive short-circuiting mechanism is preferably disposed near the sealing member.
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The positive electrode active material used in the present invention is preferably a lithium transition metal oxide. Examples of lithium transition metal oxides include lithium cobaltate (Li1+aCoO2 (where 0≤a≤0.2)), lithium manganate (Li1+aMn2O4 (where 0≤a≤0.2)), lithium nickelate (Li1+aNiO2 (where 0≤a≤0.2)), lithium nickel manganese oxide (Li1+aNi1−xMnxO2 (where 0≤a≤0.2 and 0<x<1)), lithium nickel cobalt oxide (Li1+aNi1−xCoxO2 (where 0≤a≤0.2 and 0<x<1)), and lithium nickel cobalt manganese oxide (Li1+aNixCoyMnzO2 (where 0≤a≤0.2, 0<x<1, 0<y<1, 0<z<1, and x+y+z=1)). These lithium transition metal oxides may contain other elements such as aluminum, titanium, zirconium, niobium, boron, tungsten, magnesium, and molybdenum. One such example is a lithium transition metal oxide represented by the formula Li1+aNixCoyMnzMbO2 (where M is at least one element selected from aluminum, titanium, zirconium, niobium, boron, magnesium, and molybdenum, 0≤a≤0.2, 0.2≤x≤0.5, 0.2≤y≤0.5, 0.2≤z≤0.4, 0≤b≤0.02, and a+b+x+y+z=1)). A preferred positive electrode active material is a lithium nickel cobalt manganese oxide containing zirconium and tungsten.
The negative electrode active material may be a carbonaceous material capable of absorbing and desorbing lithium ions. Examples of carbonaceous materials capable of absorbing and desorbing lithium ions include graphite, nongraphitizable carbon, graphitizable carbon, fibrous carbon, coke, and carbon black. Among these, graphite is preferred. Examples of noncarbonaceous materials include silicon, tin, and alloys and oxides thereof.
The nonaqueous electrolyte may contain nonaqueous solvents (organic solvents) such as carbonates, lactones, ethers, ketones, esters, and mixtures thereof. Examples of such nonaqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate and linear carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. In particular, mixtures of cyclic carbonates and linear carbonates are preferred. The nonaqueous electrolyte may also contain unsaturated cyclic carbonates such as vinylene carbonate (VC).
The nonaqueous electrolyte may contain electrolyte salts commonly used in conventional lithium-ion secondary batteries. Examples of such electrolyte salts include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, LiAsF6, LiClO4, Li2B10Cl10, Li2B12Cl12, LiB(C2O4)2, LiB(C2O4)F2, LiP(C2O4)3, LiP(C2O4)2F2, LiP(C2O4)F4, and mixtures thereof. Among these, LiPF6 is preferred. The electrolyte salt is preferably dissolved in the nonaqueous solvent in an amount of 0.5 to 2.0 mol/L. In particular, the nonaqueous electrolyte preferably contains LiPF6 and LiB(C2O4)2.
The separator is preferably a porous separator made of a polyolefin such as polypropylene (PP) or polyethylene (PE). Particularly preferred are separators having a three-layer structure of PP and PE (PP/PE/PP or PE/PP/PE). Alternatively, the separator may be a polymer electrolyte.
While detailed embodiments have been used to illustrate the present invention, to those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and is not intended to limit the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-126143 | Jun 2015 | JP | national |
This application is a divisional of U.S. application Ser. No. 15/189,495 filed on Dec. 22, 2016, which claims priority to Japanese Patent Application No. 2015-126143 filed in the Japan Patent Office on Jun. 24, 2015, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15189495 | Jun 2016 | US |
| Child | 16293736 | US |
