LITHIUM BATTERY AND USE OF A GERMANIUM ORGANYL-BASED ELECTROLYTE ADDITIVE AS AN ELECTROLYTE ADDITIVE THEREIN

Abstract

A lithium battery including an anode having an active anode material, a cathode having an active cathode material. The cathode material includes lithium nickel cobalt manganese cobalt oxide (NCM). An electrolyte separates the anode and cathode. The electrolyte includes a solvent or solvent mixture and lithium hexafluorophosphate, and a germanium organyl-based electrolyte additive. Also disclosed are uses of the germanium organyl-based electrolyte additive in the lithium battery for enhancing one characteristic selected from the group consisting of reversible capacity, Coulombic efficiency, cyclic stability and combinations thereof.

Claims

1. A lithium battery comprising: an anode comprising an active anode material a cathode comprising an active cathode material comprising lithium nickel manganese cobalt oxide (LiNi.sub.xCo.sub.yMn.sub.1-x-yO.sub.2) (NCM) with each of x and y not including zero and x+y being smaller than 1 a separator separating anode and cathode, and an electrolyte wherein the electrolyte comprises a solvent or solvent mixture and lithium hexafluorophosphate, wherein the electrolyte further comprises a germanium organyl-based electrolyte additive.

2. The lithium battery according to claim 1, wherein the germanium organyl-based electrolyte additive is a compound of formula 1 ##STR00007## wherein X is Ge; Y1 and Y2 are independently (CH.sub.2).sub.m with m being 0, 1 or 2; and Z.sub.1 and Z.sub.2 are independently selected from the group consisting of nitrile, a substituted or unsubstituted C6- to C14-aryl, and a substituted or unsubstituted C5-C12 heteroaryl with the heteroatom selected from O, N, and S, wherein the optional substituent is selected from the group consisting of C1 to C9 alkyl, and C1 to C9 alkoxyl.

3. The lithium battery according to claim 1, wherein the germanium organyl-based electrolyte additive is selected from the group consisting of the following formulas 2 to 19, with X being Ge, and R being C1 to C9 alkyl, or C1 to C9 alkoxyl: ##STR00008## ##STR00009## ##STR00010## ##STR00011## and a mixture thereof.

4. The lithium battery according to claim 1, wherein the germanium organyl-based electrolyte additive is 3,3′-((diphenylgermanediyl)bis(oxy))dipropanenitrile (DGDP) of formula 2 ##STR00012##

5. The lithium battery according to claim 1, wherein the active cathode material is selected from the group consisting of NCM with 0.3≤x<1.

6. The lithium battery according to claim 1, wherein, in terms of the total amount of electrolyte comprising lithium hexafluorophosphate in a solvent or solvent mixture, 0.01 to 10 wt.-%, or 0.1 to 5 wt.-%, or 0.2 to 1 wt.-%, or 0.25 to 0.75 wt.-% germanium organyl-based electrolyte additive.

7. The lithium battery according to claim 1, wherein the concentration of lithium hexafluorophosphate is in the range of 0.1 M to 2 M, or 0.5 M to 1.5 M, or 0.7 M to 1.2 M.

8. The lithium battery according to claim 1, wherein the solvent or solvent mixture is selected from an organic solvent or solvent mixture, an ionic liquid and/or a polymer matrix.

9. The lithium battery according to claim 1, wherein the organic solvent or solvent mixture is selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3-dioxalane, methylacetate and/or mixtures thereof, preferably selected from the group consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and/or mixtures thereof.

10. The lithium battery according to claim 1, wherein the organic solvent mixture comprises, or consists of, a mixture from ethylene carbonate and at least one further solvent, or ethyl methyl carbonate, in a ratio in terms of weight parts of >1:99 to <99:1, or from >1:9 to <9:1, or from ≥3:7 to ≤1:1.

11. The lithium battery according to claim 1, wherein the electrolyte further comprises an additive selected from the group consisting of chlorethylene carbonate, fluorethylene carbonate, vinylene carbonate, vinylethylene carbonate, ethylene sulfite, ethylene sulfate, propane sulfonate, sulfite, preferably dimethylsulfite and propylene sulfite, sulfate, butyrolactone optionally substituted with F, Cl or Br, phenylethylene carbonate, vinylacetate and trifluoropropylene carbonate.

12. The lithium battery according to claim 1, wherein the active anode material is selected from the group consisting of carbon, graphite, mixtures of silicon and carbon/graphite, silicon, tin, lithium-metal oxide, materials that form alloys with lithium, composites und mixtures thereof, preferably carbon, graphite, mixtures of silicon and carbon/graphite and composites und mixtures thereof.

13. Use of a germanium organyl-based electrolyte additive as additive in a lithium battery as defined in claim 1 for enhancing one characteristic selected from the group consisting of reversible capacity, Coulombic efficiency (C.sub.Eff), cyclic stability, capacity retention and combinations thereof.

Description

[0066] The Figures show:

[0067] FIG. 1: Charge/discharge cycling stability data of the NCM523/graphite cells with 0.05 wt. % 3,3′-((diphenylgermanediyl)bis(oxy))dipropanenitrile (DGBP) in 1M LiPF.sub.6 in EC/EMC 3:7 wt. % (LP57) by using a cell voltage of 4.30 V (cycles 1-3) and 4.55 V (from cycle 4) in comparison to the reference electrolyte (RE). Cut-off voltages: 2.80-4.55 V.

[0068] FIG. 2: Cathode and anode potential profiles for NMC532/graphite cells with RE and RE+0.05 wt. % DGDPin the 1st charge/discharge cycle. Cut-off cell voltages: 3.00-4.55 V.

[0069] The mean values as well as the standard deviation of three cells for the charge/discharge cycling performance of NMC111/graphite cells, with and without 0.05 wt. % DGDP as electrolyte additive, are shown in FIG. 1.

[0070] The additive concentration was set to 0.05 wt. % in terms of the total amount of electrolyte comprising lithium hexafluorophosphate in a solvent or solvent mixture to be consumed during the formation process and to form protective SEI (solid electrolyte interphase)/CEI-layers. Higher concentrations are in this case not necessary and in some cases even have a counter-productive effect on cell performance.

[0071] Already in the first three formation cycle, the cells with the DGDP-containing electrolyte show a higher discharge capacity compared to the RE-containing cell. However, both showed a similar first C.sub.Eff. Thus, the reversible capacity improves and whereas there is no distinguishable improvement with respect to C.sub.Eff within the formation cycles by adding DGDP into the RE.

[0072] In the following cycles, at a charge/discharge rate of 0.5 C (1 C=200 mA g.sup.−1), the DGDP-containing cells outperform the cells with RE, by strongly improving the capacity retention.

[0073] The improvement by DGDP on the long-term cycling performance is maintained after 80 cycles and becomes even more pronounced with higher cycle numbers.

[0074] According to the present invention, germanium organyl-based electrolyte additives, in particular 3,3′-((diphenylgermanediyl)bis(oxy))dipropanenitrile (DGDP), were shown to act as a highly effective cathode electrolyte interphase (CEI)—electrolyte additive for NCM cathodes in LIBs operated at high-voltage. With the use of only 0.05 wt. % DGDP, NCM523/graphite LIB cells showed a superior charge/discharge cycling performance upon cycling at high voltage (4.55 V), compared to the carbonate-based reference electrolyte. The capacity retention could be improved. Furthermore, DGDP is a very effective and therefore cost efficient compound for the application in NCM/graphite cells, as only 0.05 wt. % of DGDP are already effective.

[0075] The present invention provides a variety of different germanium organyl-based electrolyte additives that are accessible by variation of the substituents enabling to customize the electrochemical properties.

EXAMPLES

Example 1: Synthesis of Germanium Organyl-Based Electrolyte Additives

Example 1.1: Synthesis of 43,3′-((diphenylgermanediyl)bis(oxy))dipropanenitrile (DGDP)

[0076] 1.) 5 mmol (0.34 ml) 3-hydroxypropionitriles were dissolved in 50 ml dry THF, cooled to −78° C. and 5 mmol n-BuLi (2.5 M n-BuLi in hexane, 2 ml) were added slowly over a period of 30 minutes. The cooling bath was then removed and stirred for a further 2 hours at room temperature.

[0077] 2.) To 1.) 2.5 mmol (0.53 ml) Diphenylgermanium dichloride were slowly added at room temperature and stirred for a further 48 h at room temperature.

[0078] 3.) The solvent was removed in a vacuum and a white solid remained as a residue. This residue was mixed with 100 ml toluene and then filtered. The filtrate was concentrated to 25 ml and stored at −25° C., forming colourless prisms with a yield of 600 mg. These were dried at 40° C. in a vacuum.

[0079] ESI-MS: theory: 368.06 M

[0080] measured: 391.05 M+Na+

[0081] .sup.1H-NMR (300 MHz, CDCl.sub.3, TMS): δ=7.72 (m, 4H, Ar—H), 7.56 (m, 2H, Ar—H), 7.51 (dd, 4H, Ar—H), 4.02 (t, 4H, CH.sub.2), 2.63 (t, 4H, CH.sub.2).

Example 2: Electrode and Electrolyte Preparation

[0082] The electrolyte preparation and storage as well as the cell manufacturing were carried out in an argon-filled glove box (H.sub.2O and O.sub.2 contents<0.1 ppm). All indicated mixing ratios are based on the mass ratio (% by weight).

[0083] For the preparation of the additive electrolytes according to the invention, 3,3′-((diphenylgermanediyl)bis(oxy))dipropanenitrile (DGDP) was added to this electrolyte mixture. The proportion of the additive (A) indicated in % by weight refers to the electrolyte (E) with additive (A), not to the entire electrolyte mixture including additive, that is, W (A)=m (A)/(m (E)+m (A)).

[0084] The electrodes were prepared in a large scale in cooperation with the battery line of the MEET institute. The cathode contains 93 wt. % LiNi.sub.0.5 Co.sub.0.2Mn.sub.0.3O.sub.2 (NCM-523; CATL), 4 wt. % carbon black (Super C65, Imerys) and 3 wt. % polyvinylidene difluoride (PVdF, Solef 5130, Solvay) as binder. N-methylpyrrolidone (NMP, ALDRICH) was used as dispersant. The LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2 powder was sieved (75 μm) and dried under vacuum for 24 h at 60° C. to prevent agglomerates and remove residual moisture. PVdF and NMP were added into an air-tight container and homogenized over night by a shear mixer at 2500 rpm. Afterwards, carbon black and NCM-523 was homogenized to the solution and mixed for 1.5 h under low vacuum and water cooling. After optimization of viscosity the solid content reached 50%. The electrode paste was cast onto an aluminum foil (Evonik Industries) with an average mass loading of 1.5 mAh cm.sup.−2.

[0085] The anode contains 94.5 wt. % graphite (FSNC-1; Shanshan Technology; D50=15.0±2.0 μm; BET surface area=1.3±0.3 m.sup.2 g.sup.−1), 1.0 wt. % carbon black (Super C65, Imerys), 2.25 wt. % styrene butadiene rubber (SBR, Lipaton SB 5521, Polymer Latex GmbH) and 2.25 wt. % sodium carboxymethyl cellulose (Na-CMC, Walocel CRT 2000 PPA 12, Dow Wolff Cellulosics)), demineralized water was used as a dispersant. Based on the viscosity the solid loading was optimized to 54%. The electrode paste was coated onto copper foil (Evonik Industries) with a mass loading of 2.7 mAh cm.sup.−2. The electrodes were calendered to reach a density of 1.5 g cm.sup.−3. For the LIB cell investigations, the capacity ratio between the NCM523 cathode and anode was set as 1:1.6, to avoid lithium metal plating at the graphite anode.

[0086] A mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 by weight, Solvionic, purity:battery grade) containing 1 M LiPF.sub.6 was used as reference electrolyte (RE; also referred to as “LP57”). DGDPwas added into the reference electrolyte with the desired amount (0.05%) by weight ratio in an argon filled glove box.

Example 3: Cell Setup and Electrochemical Characterization

[0087] All electrochemical investigations were performed using three-electrode Swagelok cells in climatic chambers at 20° C. Lithium metal (Albemarle Corporation; purity: battery grade) was used as the reference electrode (REF; Ø=5 mm). A polypropylene nonwoven (Freudenberg 2190, 3 layers) was used as separator. Each cell contained in total 240 μL (160 μL+80 μL for REF) electrolyte and were assembled in an argon filled glove box.

[0088] Long-term charge/discharge cycling of full cells was evaluated by a battery tester Series 4000 (MACCOR). NMC532/graphite LIB cells were cycled in a voltage range from 2.80 V to 4.55 V with three formation cycles with a charge and discharge rate of 40 mA g.sup.−1, equal to a C-rate of 0.2 C (based on the specific capacity of NCM523 at 4.55 V vs. Li/Li+, 200 mAh g-1, obtained from 3-electrode measurements), followed by subsequent cycles with a charge/discharge rate of 100 mA g-1 (corresponding to 0.5 C). Each charging step included a constant voltage step at 4.55 V until the current dropped below 0.05 C.