Electrolyte for Lithium Based Energy Accumulators

Abstract

An electrolyte for a lithium-ion battery which includes lithium hexafluorophosphate and lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. A lithium-ion battery which includes an electrolyte containing lithium hexafluorophosphate and lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. A motor vehicle which uses a lithium-ion battery having electrolytes containing lithium hexafluorophosphate and lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. The electrolyte can increase the service life of the lithium-ion battery.

Claims

1. An electrolyte for a lithium-ion battery, the electrolyte comprising: lithium hexafluorophosphate; and lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate.

2. The electrolyte according to claim 1, wherein the 2-pentafluoroethoxy-1,1,2,2 tetrafluoroethanesulfonate is present in the amount of 0.01 to 10 mol %, based on the amount of lithium hexafluorophosphate present.

3. The electrolyte according to claim 1, wherein the 2-pentafluoroethoxy-1,1,2,2 tetrafluoroethanesulfonate is present in the amount of 0.5 to 5 mol %, based on the amount of lithium hexafluorophosphate present.

4. The electrolyte according to claim 1, wherein the concentration of lithium hexafluorophosphate ranges from 0.1 M to 2 M.

5. The electrolyte according to claim 1, wherein the concentration of lithium hexafluorophosphate ranges from 0.5 M to 1.5 M.

6. The electrolyte according to claim 1, wherein the concentration of lithium hexafluorophosphate ranges from 0.7 M to 1.2 M.

7. The electrolyte according to claim 1, wherein the electrolyte further comprises an organic solvent, an ionic liquid and/or a polymer matrix.

8. The electrolyte according to claim 7, wherein the organic solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, glutaronitrile, adiponitrile, pimelonitrile, γ-butyrolactone, γ-valerolactone, dimethoxyethane, 1,3 dioxalane, methyl acetate, and mixtures thereof.

9. The electrolyte according to claim 7, wherein the organic solvent is selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and mixtures thereof.

10. The electrolyte according to claim 7, wherein the organic solvent comprises a mixture of ethylene carbonate and at least one further organic solvent in a ratio of 1:99 to 99:1.

11. The electrolyte according to claim 7, wherein the organic solvent comprises a mixture of ethylene carbonate and at least one further organic solvent in a ratio of 1:9 to 9:1.

12. The electrolyte according to claim 7, wherein the organic solvent comprises a mixture of ethylene carbonate and at least one further organic solvent in a ratio of 3:7 to 1:1.

13. The electrolyte according to claim 10, wherein the at least one further organic solvent is ethyl methyl carbonate.

14. The electrolyte according to claim 1, wherein the electrolyte further comprises a compound selected from chloroethylene carbonate, fluoroethylene carbonate, vinylene carbonate, vinylethylene carbonate, ethylene sulfite, ethylene sulfate, propanesulfonates, sulfites, sulfates, butyrolactones optionally substituted by F, Cl or Br, phenylethylene carbonate, vinyl acetate and/or trifluoropropylene carbonate.

15. A lithium-ion battery comprising: an anode; a cathode; a separator; and an electrolyte comprising lithium hexafluorophosphate and lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate.

16. The lithium-ion battery according to claim 15, wherein the cathode comprises an active cathode material selected from the group consisting of lithium metal oxide, layered oxides, spinels, olivine compounds, silicate compounds, HE-NCM, and mixtures thereof.

17. The lithium-ion battery according to claim 15, wherein the anode comprises an active anode material selected from the group consisting of carbon, graphite, mixtures of silicon and carbon/graphite, silicon, lithium metal oxide, materials which can be alloyed with lithium, and mixtures thereof.

18. A motor vehicle comprising a lithium-ion battery as claimed in claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1 shows the effect of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LIFEES) as an additive in LiPF.sub.6-containing electrolytes in lithium half-cells with NCM (4.6 V versus Li/Li.sup.+) on the discharge capacity.

[0060] FIG. 2 shows the effect of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LIFEES) as an additive in LiPF.sub.6-containing electrolytes on the internal resistance of the cell.

[0061] FIG. 3 shows the effect of lithium 2-pentafluoro ethoxy-1,1,2,2-tetrafluoroethanesulfonate (LIFEES) as an additive in LiPF.sub.6-containing electrolytes on the self-discharge in LMNO half-cells (4.95 V versus Li/Li.sup.+).

DETAILED DESCRIPTION OF THE DRAWINGS

Example 1

Preparation of the Electrolyte Solutions

[0062] The electrolyte mixtures were prepared in a glovebox with H.sub.2O and O.sub.2 content of less than 0.5 ppm. All mixing ratios stated are based on the mass ratio (wt %).

[0063] An electrolyte containing 1 M LiPF.sub.6 in EC:EMC (1:1) was prepared by initially introducing 50 wt % of ethylene carbonate (EC) and 50 wt % of ethyl methyl carbonate (EMC) and dissolving the required amount of LiPF.sub.6 in this solvent mixture, to get a concentration of 1 M LiPF.sub.6. This electrolyte was used as a comparative electrolyte.

[0064] For preparing the additized electrolytes of the invention, lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate was added to this electrolyte mixture. The fraction in wt % of the additive (A) is based on the electrolyte (E) without additives, not on the overall electrolyte mixture including additives; i.e., w(A)=(A)/m(E). The water content of the electrolytes according to Karl-Fischer was less than 20 ppm.

Electrochemical Investigations

[0065] The experiments were conducted in a three-electrode arrangement in modified Swagelok® T-pieces (tube connectors, stainless steel body) with a nickel manganese cobalt oxide (NMC) electrode (12 mm diameter) or lithium nickel manganese oxide (LNMO) (12 mm diameter) as working electrode and with lithium foil (12 mm or 5 mm diameter, respectively) as counter electrode and reference electrode. The internal cell constituents were insulated from the stainless steel housing by a polyester film. The electrodes were separated by a glass fiber filter which was impregnated with the corresponding electrolyte (200 μL). On account of the sensitivity of the cell components to hydrolysis and to air, the cells were constructed in a glovebox.

Measurements at Constant Current

[0066] The measurements at constant current were carried out on a Series 4000 battery tester (from Maccor) at 20° C.±2° C. The NMC half-cells were cycled in the potential range from 3.0 V to 4.6 V versus Li/Li.sup.+. For the LMNO half-cells, a potential range of 3.0 V to 4.95 V versus Li/Li.sup.+ was selected.

The Test Plan Employed Was as Follows

[0067] After three forming cycles with a charge and discharge rate (C and D rate) of C/5 (here 150 mAh g.sup.−1 correspond to a C rate of 1 C) the cycling behavior was verified over 50 cycles with a charge and discharge rate of 1 C. This was followed by a D rate test. The cells were discharged at different D rates from D/5 to 5D and charged in each case at C/2. The D rates employed here were D/5, D/3, D/2, 1D, 2D, 3D, and 5D. After the D rate test came five cycles with charge and discharge rates of C/5, in order to verify whether the cathode material has suffered damage as a result of the loading test. The last phase included the test of the long-term stability, where the cycling behavior with a charge and discharge rate of 1 C is monitored over 100 cycles.

Example 2

Determination of the Cycling Behavior of Lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in an NMC Half-Cell

[0068] The preparation of an electrolyte containing 1 M lithium hexafluorophosphate (LiPF.sub.6) in a solvent mixture of ethylene carbonate and ethyl methyl carbonate (EC:EMC 1:1) with the addition of 1 wt % or 3 wt % of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate took place as described under example 1. The cycling behavior was determined using an NMC half-cell at constant current as described above.

[0069] FIG. 1 plots the discharge capacity against the number of cycles of 1 M LiPF.sub.6 in a solvent mixture of ethylene carbonate and ethyl methyl carbonate (EC:EMC 1:1) with and without the addition of 1 wt % or 3 wt % of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in an NMC half-cell. As FIG. 1 shows, the addition of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate reduced the drop in the discharge capacity with increasing number of cycles (capacity fading). With increasing fraction of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, there was a slowdown in capacity fading, but the discharge capacity achieved in the initial cycles was also lower.

Example 3

Investigation of the Internal Resistance of Lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in an NMC Half-Cell

[0070] Preparation of the electrolytes and construction of the cells took place as in example 1. After the forming of the cells at a rate of C/5 in the potential range from 3.0 V to 4.6 V versus Li/Li.sup.+, the cells were equilibrated for five hours while the open circuit voltage was measured. The AC impedance of both cells was measured in the frequency range from 1 MHz to 1 mHz (amplitude 5 mV) at 20° C.

[0071] FIG. 2 shows the Nyquist plot of 1 M LiPF.sub.6 in a solvent mixture of ethylene carbonate and ethyl methyl carbonate (EC:EMC 1:1) with and without the addition of 1 wt % of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in an NMC half-cell. As FIG. 2 shows, the addition of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate produced a higher outer-layer resistance, which comes about likely as a result of the oxidative decomposition of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate and an associated formation of a passivation layer on the NMC cathode.

Example 4

Measurement of the Self-Discharge of Lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in an LMNO Half-Cell

[0072] FIG. 3 shows the self-discharge of a lithium-ion half-cell with 1 M lithium hexafluorophosphate (LiPF.sub.6) in a solvent mixture of ethylene carbonate and ethyl methyl carbonate (EC:EMC 1:1) with and without the addition of 3 wt % of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. The working electrode used was the cathode material lithium nickel manganese oxide (LMNO). The electrolyte was prepared and the cell constructed as in Example 1. After three forming cycles at C/5 in the potential range from 3.0 V to 4.95 V versus Li/Li.sup.+, charging again took place up to an end-of-charge potential of 4.95 V versus Li/Li.sup.+, after which the open circuit voltage between the LMNO electrode and the lithium reference electrode was monitored over time. As FIG. 3 shows, the addition of 3 wt % of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate reduced the self-discharge. The profile of the self-discharge follows the course of the discharge curve of the cathode material.

[0073] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.