Lithium ion cells with high performance electrolyte and silicon oxide active materials achieving long cycle life, fast charge and high thermal stability
Improved electrolytes for lithium-based cells can include a dual salt combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide or lithium bis(trifluoro-methanesulfonyl)imide, and a solvent that includes dimethyl carbonate, ethylmethyl carbonate and 5 to 25 volume percent of fluoroethylene carbonate. The improved electrolytes can include additives triethyl phosphate, ethoxy(pentafluoro)cyclotriphosphazene, 1,3-propane sultone, or mixtures thereof, and have small limited amounts of additional cosolvents and/or lithium-free organic additives. The improved electrolytes can be used to prepare lithium-based cells with silicon-based active materials as negative electrodes and nickel rich lithium metal oxides as positive electrodes. The lithium-based cells can achieve high energy, high power, fast charge and long cycle life along with good thermal stability.
1 . A lithium ion cell comprising:
a negative electrode comprising from about 75 wt % to about 96 wt % an active material from about 0.1 wt % to about 7 wt % nanoscale conductive carbon and from about 4 wt % to about 20 wt % polymer binder, wherein the active material comprises from about 45 wt % to about 100% silicon-based active material, and from 0 to about 55 wt % graphitic carbon;
a positive electrode comprising a lithium metal oxide, conductive carbon, and a polymer binder;
a separator between the negative electrode and the positive electrode;
electrolyte comprising from about 1.1M to about 2.2M lithium salt and non-aqueous solvent, wherein the lithium salt consists essentially of LiPF 6 , from about 60 mole percent to about 100 mole percent LiTFSI, and no more than about 2 mole percent optional other lithium salts, wherein the non-aqueous solvent comprises from about 5 vol % to about 25 vol % fluoroethylene carbonate, 50 vol % to 90 vol % dimethyl carbonate, ethylmethyl carbonate or a mixture thereof, and no more than about 35 vol % optional cosolvents selected from the group consisting of propylene carbonate, ethyl acetate, methyl acetate, propyl acetate, and mixtures thereof; and
a container enclosing the negative electrode, the positive electrode, the separator and the electrolyte;
wherein the cell has a room temperature discharge specific capacity at a rate of 4 C of at least about 120 mAh/g between 2.5V and a selected charge voltage based on the weight of the cathode active material.
2 . The lithium ion cell of claim 1 wherein the silicon-based active material comprises a silicon-silicon oxide carbon composite material.
3 . The lithium ion cell of claim 1 wherein the graphitic carbon has a BET surface area from about 1 m 2 /g to about 20 m 2 /g.
4 . The lithium ion cell of claim 1 wherein the polymer binder of the negative electrode comprises a blend of polyimide and a water based polymer binder.
5 . The lithium ion cell of claim 1 wherein the polymer binder of the negative electrode comprises a blend of at least about 50 wt % polyimide and a second polymer binder selected from the group consisting of polyvinylidene fluoride, cellulose, styrene-butadiene rubber, lithiated polyacrylic acid, copolymers thereof and mixtures thereof, wherein the polyimide has an elongation of at least about 40% and the second polymer binder has an elasticity greater than that of the polyimide.
6 . The lithium ion cell of claim 1 wherein the polymer binder of the negative electrode comprises a water soluble binder.
7 . The lithium ion cell of claim 6 wherein the water soluble binder comprises poly (acrylamide-co-acrylate salt) having at least about 5 mole percent of the acrylate salt moiety and at least about 5 mole percent of the acrylamide moiety.
8 . The lithium ion cell of claim 6 wherein the water soluble binder comprises metal-poly acrylic acid/acrylate (M-PAA) moieties form corresponding monomers, where M is a cation of lithium, sodium, potassium or a mixture thereof.
9 . The lithium ion cell of claim 1 wherein the lithium metal oxide comprises a lithium nickel cobalt manganese oxide approximately represented by the formula LiNi x Mn y Co z O 2 , where 0.45≤x, 0.05≤y, z≤0.35, x+y+z≈1.
10 . The lithium ion cell of claim 9 wherein the lithium metal oxide further comprises a lithium and manganese rich lithium nickel manganese cobalt oxide represented by the formula Li 1+b Ni α Mn β CO γ A δ O 2-z F z , where b+α+β+γ+δ≈1, b ranges from about 0.04 to about 0.3, α ranges from 0 to about 0.4, β range from about 0.2 to about 0.65, γ ranges from 0 to about 0.46, δ ranges from about 0 to about 0.15 and z ranges from 0 to 0.2, with the proviso that both α and γ are not 0, and where A is a metal different from lithium, manganese, nickel and cobalt.
11 . The lithium ion cell of claim 1 further comprising supplemental lithium in an amount from about 80% to about 180% of the negative electrode first cycle irreversible capacity loss, the lithium ion cell having a ratio at the fourth cycle at a discharge rate of C/3 of negative electrode capacity divided by the positive electrode capacity from about 1.10 to about 1.95.
12 . The lithium ion cell of claim 1 wherein the negative electrode active material comprises from about 60 wt % to about 95 wt % silicon-oxide based material and from about 5 wt % to about 40 wt % graphite.
13 . The lithium ion cell of claim 1 wherein the solvent consists essentially of from about 60 vol % to about 85 vol % ethylmethyl carbonate and from about 10 vol % to about 20 vol % fluoroethylene carbonate and no more than 25 vol % optional cosolvent selected from the group consisting of propylene carbonate, ethyl acetate, methyl acetate, propyl acetate, and mixtures thereof.
14 . The lithium ion cell of claim 13 wherein the lithium salt consists essentially of from about 0.3M to about 0.6M LiPF 6 and from about 0.8M to about 1.4M LiTFSI.
15 . The lithium ion cell of claim 14 wherein the electrolyte consists essentially of:
lithium salt,
solvent,
no more than about 10 weight percent (wt %) optional additive selected from the group consisting of triethyl phosphate (TEP), ethoxy (pentafluoro) cyclotriphosphazene (PFPN), 1,3-propane sultone (PS), and mixtures thereof; and
no more than about 5 wt % additional cosolvents and/or lithium-free organic additives.
16 . The lithium ion cell of claim 1 wherein the lithium ion cell has a capacity at the 700th cycle of at least about 80% of the capacity at the 7th cycle when cycled at room temperature from the 10th cycle to the 700th cycle between 2.5V and the selected charge voltage at a rate of 4 C.
17 . A lithium ion cell comprising:
a negative electrode comprising from about 75 wt % to about 96 wt % an active material from about 0.1 wt % to about 7 wt % nanoscale conductive carbon and from about 4 wt % to about 20 wt % polymer binder, wherein the active material comprises from about 45 wt % to about 100% silicon-based active material, and from 0 wt % to about 55 wt % graphitic carbon;
a positive electrode comprising a lithium metal oxide, conductive carbon, and a polymer binder;
a separator between the negative electrode and the positive electrode;
electrolyte comprising from about 1.0M to about 2.5M lithium salt and non-aqueous solvent, wherein the lithium salt consists essentially of LiPF 6 , from about 60 mole percent to about 100 mole percent LiTFSI, and no more than about 2 mole percent optional other lithium salts, and wherein the non-aqueous solvent comprises from about 5 vol % to about 25 vol % fluoroethylene carbonate; and
a container enclosing the negative electrode, the positive electrode, the separator and the electrolyte;
wherein the cell has a room temperature discharge specific capacity at a rate of 4 C of at least about 120 mAh/g between 2.5V and a selected charge voltage based on the weight of the cathode active material and
wherein the lithium ion cell having a capacity at the 700th cycle of at least about 80% of the capacity at the 7th cycle when cycled at room temperature from the 7th cycle to the 700th cycle between 2.5V and the selected charge voltage at a rate of 4 C.