Electrolyte composition for a lithium-ion battery

Abstract

An electrolyte composition for a lithium-ion battery, a lithium-ion battery, and also the use of a fluorine-containing cyclic carbonate component and lithium nitrate for improving the cycle stability and/or for increasing the performance of a lithium-ion battery.

Claims

1. An electrolyte composition for a lithium-ion battery with at least one cathode and at least one anode, the electrolyte composition comprising: at least one aprotic nonaqueous solvent; one fluorine-containing cyclic carbonate component; and at least two lithium salts, wherein one of the lithium salts is lithium nitrate, which is present in a quantity from 0.05% to 20% by weight relative to a total weight of the electrolyte composition, wherein the fluorine-containing cyclic carbonate component is present in a quantity from 5% to 12% by weight relative to the total weight of the electrolyte composition, wherein the anode is a silicon anode or a silicon/carbon composite anode, and wherein the electrolyte composition contains lithium bis(fluorosulfonyl)imide as an additional lithium salt.

2. The electrolyte composition according to claim 1, wherein the fluorine-containing cyclic carbonate component is fluoroethylene carbonate.

3. A lithium-ion battery comprising: a casing; a battery core having at least one cathode and at least one anode, wherein the anode is a silicon anode or a silicon/carbon composite anode; and an electrolyte composition according to claim 1.

4. The electrolyte composition according to claim 1, wherein the fluorine-containing cyclic carbonate component and lithium nitrate improve a cycle stability of a lithium-ion battery.

5. The electrolyte composition according to claim 1, wherein the fluorine-containing cyclic carbonate component and lithium nitrate increase a performance of a lithium-ion battery.

6. The electrolyte composition according to claim 1, wherein the fluorine-containing cyclic carbonate component and lithium nitrate form a surface layer on the anode.

7. The electrolyte composition according to claim 6, further comprising vinylene carbonate.

8. The lithium-ion battery according to claim 3, wherein the battery core further comprises at least one separator element.

9. The lithium-ion battery according to claim 8, wherein the at least one separator element has a microporous structure.

10. The electrolyte composition according to claim 1, wherein the at least one aprotic nonaqueous solvent is propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), tetrahydrofuran (THF), 1,2-dimethoxyethane (DME), 2-methyltetrahydrofuran (2Me-THF), vinylene carbonate (VC), N-methylpyrrolidone (NMP), acetonitrile, or ethyl acetate, or a combination thereof.

11. An electrolyte composition for a lithium-ion battery with at least one cathode and at least one anode, the electrolyte composition comprising: at least one aprotic nonaqueous solvent; one fluorine-containing cyclic carbonate component; and at least two lithium salts, wherein one of the lithium salts is lithium nitrate, wherein the fluorine-containing cyclic carbonate component is present in a quantity from 5% to 12% by weight relative to the total weight of the electrolyte composition, wherein the anode is a silicon anode or a silicon/carbon composite anode, and wherein the electrolyte composition contains lithium bis(fluorosulfonyl)imide as an additional lithium salt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

(2) FIGS. 1 to 4 show capacities and cycle stabilities of various lithium-ion batteries according to exemplary embodiments of the invention.

DETAILED DESCRIPTION

(3) FIG. 1 shows specific capacities SK and cycle stabilities of a) a standard electrolyte LP 71 (1M LiPF.sub.6 in EC/DEC/DMC 1:1:1 wt.), b) LP 71+10% FEC, c) LP 71+0.5% LiNO.sub.3, and d) LP 71+10% FEC+0.5% LiNO.sub.3 in test cells with a silicon/carbon composite working electrode and lithium counter electrode (2 cycles with a C-rate of C/10; 98 cycles with a C-rate of 1C).

(4) FIG. 2 shows the specific capacity SK of a test cell with a silicon/carbon composite working electrode and lithium counter electrode with LP 71+10% FEC+0.5% LiNO.sub.3 in the electrolyte composition (2 cycles with a C-rate of C/10; 98 cycles with a C-rate of 1C).

(5) FIG. 3 shows the capacities and the cycle stabilities of lithium-ion batteries with different electrolyte compositions containing LP 71, 10% FEC and a) 0.1%, b) 0.3%, and c) 0.5% lithium nitrate (2 cycles with a C-rate of C/10; 98 cycles with a C-rate of 1C).

(6) FIG. 4 shows the specific capacity SK (mAh/g) of a lithium-ion battery with a standard electrolyte composition, which is to say with lithium bis(fluorosulfonyl)imide, but without lithium nitrate and fluoroethylene carbonate (curve with circle symbols), and with an electrolyte composition according to an exemplary embodiment of the invention with lithium bis(fluorosulfonyl)imide, lithium nitrate, and fluoroethylene carbonate (curve with square symbols) over the cycles Z (2 cycles with a C-rate of C/10; 98 cycles with a C-rate of 1C). The specific capacity K of a lithium-ion battery with electrolyte composition according to an exemplary embodiment of the invention remains nearly constant from the third cycle after initial use onward over more than 100 cycles. The specific capacity K of a lithium-ion battery with standard electrolyte composition, in contrast, drops from approximately 1000 mAh/g to less than 400 mAh/g after 90 cycles.

(7) The following abbreviations are used in the figures:

(8) SK=Specific capacity

(9) FEC=Fluoroethylene carbonate

(10) Z=Cycles

(11) CE=Coulomb efficiency

(12) Li.sup.+ I=Lithium insertion

(13) Li.sup.| D=Lithium desertion

(14) 1. Test Series with LiPF.sub.6 as the Second lithium salt

(15) To start with, a certain quantity of FEC, which was in the range from 1 to 50 percent by weight, was added to common, commercially available nonaqueous electrolytes, namely LP 71 (1M LiPF.sub.6 in EC/DEC/DMC in a weight ratio of 1:1:1), LP 30 (1M LiPF.sub.6 in EC/DMC in a weight ratio of 1:1), LP 40 (1M LiPF.sub.6 in EC/DEC in a weight ratio of 1:1), LP 50 (1M LiPF.sub.6 in EC/EMC in a weight ratio of 1:1) (Merck company, now BASF), or 1 M LiPF.sub.6 in EC/DMC in a weight ratio of 3:7. After that, LiNO.sub.3 in a quantity of 0.05 to 20 percent by weight was added to this mixture as a second additive, and the electrolyte composition was mixed for several hours with a magnetic stirrer. Then the electrolyte composition was tested in research test cells (half cells versus lithium) with a silicon/carbon working electrode. Composition of the silicon/carbon working electrode is: silicon nanoparticles 20 percent by weight, graphite 60 percent by weight, carbon black 12 percent by weight, and polyacrylic acid binder 8 percent by weight. The cells were assembled in an argon-filled glove box; for this purpose, two Celgard separators were placed between the working electrode and lithium counter electrode and impregnated with different amounts of the electrolyte, namely 100 L to 500 L per separator. After assembly, electrochemical cycling tests were performed at different C-rates. For comparison, all tests were also performed with the standard electrolyte, e.g., LP71, as well as the electrolytes with only one additive apiece, e.g., LP 71+FEC and LP 71+LiNO3, in order to thus investigate and demonstrate the effect of the combination of these two additives (see FIG. 1).

(16) In addition to the investigation of the effect of the novel electrolyte in research test cells, full cells were also constructed that have a silicon/carbon anode and a LiNi.sub.0.33Mn.sub.0.33Co.sub.0.33O.sub.2 (NMC) cathode. The standard 18650 cell design was chosen for this purpose in order to characterize the performance in full cells without a lithium counter electrode.

(17) An advantageous selection of the examples specified above is listed below: 1. Electrolyte composition: LP 30 (1M LiPF.sub.6 in EC/DMC 1:1 wt.)+20 percent by weight FEC+0.1 percent by weight LiNO3 2. Electrolyte composition: LP 40 (1M LiPF.sub.6 in EC/DEC 1:1 wt.)+5 percent by weight FEC+1 percent by weight LiNO.sub.3 3. Electrolyte composition: LP 50 (1M LiPF.sub.6 in EC/EMC 1:1 wt.)+10 percent by weight FEC+0.15 percent by weight LiNO.sub.3+1% VC 4. Electrolyte composition: LP 71 (1M LiPF.sub.6 in EC/DEC/DMC 1:1:1 wt.)+10 percent by weight FEC+0.5 percent by weight LiNO.sub.3 5. Electrolyte composition: LP 71 (1M LiPF.sub.6 in EC/DEC/DMC 1:1:1 wt.)+10 percent by weight FEC+0.3 percent by weight LiNO.sub.3 6. Electrolyte composition: LP 71 (1M LiPF.sub.6 in EC/DEC/DMC 1:1:1 wt.)+10 percent by weight FEC+0.1 percent by weight LiNO.sub.3

(18) 2. Test Series with lithium bis(fluorosulfonyl)imide (LiFSI) as the Second lithium salt

(19) To start with, a certain concentration of LiFSI (0.8 to 1.2 M) was added to common, commercially available nonaqueous solvent combinations such as, e.g., EC/DEC 1:1, EC/DEC 3:7, EC/EMC 1:1, EC/DMC 1:1, EC/EMC 3:7, EC/DMC 3:7, EC/DEC/EMC 1:1:1, EC,DMC,EMC 1:1:1, EC/DEC/DMC 1:1:1, (Sigma Aldrich company), and this conducting salt was dissolved in the solvents. Then a certain quantity of FEC was added, which was in the above-mentioned range from 0.05 to 50 wt. %. After that, LiNO.sub.3 in a quantity of 0.05 to 20 wt. % was added to this new mixture as a second additive, and the electrolyte composition was mixed for several hours with a magnetic stirrer. Then the electrolyte composition was tested in research test cells (half cells versus lithium) with a silicon/carbon working electrode (example compositions: silicon nanoparticles 20 wt. %, graphite 60 wt. %, carbon black 12 wt. % and polyacrylic acid binder 8 wt. %, or silicon nanoparticles 10 wt. %, graphite 77 wt. %, carbon black 5 wt. %, and polyacrylic acid binder 8 wt. %). The cells were assembled in an argon-filled glove box; for this purpose, two Celgard separators were placed between the working electrode and lithium counter electrode and impregnated with different amounts of the electrolyte composition (100 to 500 L per separator). After assembly, electrochemical cycling tests were performed at different C-rates. For comparison, all tests were also performed with the reference electrolyte 1 M LiFSI in EC/DEC/DMC 1:1:1 without additives, in order to thus demonstrate and verify the effect of the novel electrolyte composition (see FIG. 4).

(20) An exemplary selection of the examples specified above is listed below: 1.) Electrolyte composition: (1M LiFSI in EC/DMC/DEC 1:1:1 wt.)+10 wt. % FEC+0.5 wt. % LiNO.sub.3 2.) Electrolyte composition: (0.8M LiFSI in EC/EMC 1:1:1 wt.)+5 wt. % FEC+1 wt. % LiNO.sub.3 3.) Electrolyte composition: (1.2M LiFSI in EC/ /DEC 1:1:1 wt.)+20 wt. % FEC+0.15 wt. % LiNO.sub.3+1% VC 4.) Electrolyte composition: (1M LiFSI in EC/DEC/DMC 1:1:1 wt.)+15 wt. % FEC+0.5 wt. % LiNO.sub.3

(21) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.