Electrolyte formulations for wide temperature lithium ion batteries

This application claims priority of U.S. Provisional Patent Application Ser. No. 60/650,001 filed Feb. 3, 2005, which is incorporated herein by reference.

The invention described herein may be manufactured, used, and licensed by or for the United States Government.

The present invention generally relates to a non-aqueous electrolyte rechargeable electrochemical cell, and particularly to the formulation of non-aqueous electrolyte used in Li ion rechargeable cells to achieve a wide operating temperature range.

Rechargeable lithium ion batteries have been commercially available for well over a decade. In spite of the improvements in energy densities and power densities, lithium ion cell technology remains restricted to a narrow temperature range of operation. The electrolyte components ethylene carbonate (EC) and lithium hexafluorophosphate (LiPF 6 ) are responsible for much of the temperature range limitation. As a result, lithium batteries can only deliver the rated capacity and power in a narrow range of −20° C. and +60° C. Below −30° C., lithium cells suffer severe power and energy loss along with the safety risk caused by possible lithium metal deposition; while at temperatures higher than 60° C., the thermal decomposition of LiPF 6 causes permanent degradation of the cell and potential safety hazards. These restrictions limit the usage of lithium ion batteries in a variety of harsh environments experienced by electric or hybrid electric vehicles (EV/HEV), military and space missions.

Efforts to lower the low temperature operational limits of Li ion cells have focused on replacing the majority of the high melting EC with high ratios of low melting solvents such as the linear dialkylcarbonates or esters. Exemplary of this effort is U.S. Pat. No. 6,492,064 (Smart et al.). Unfortunately, the cycle life of such modified Li ion cells is compromised at room temperatures. Moreover, the oxidative decomposition of these volatile co-solvents at the charged surface of the cathode accelerates at elevated temperatures, resulting in gas buildup and shortened cell lifetime.

Efforts to raise the high temperature operational limit of Li ion cells include using a thermally stable lithium salt and have been scarce and rarely successful. An example of the use of a thermally stable salt lithium bis(oxalato)borate (LiBOB) is Xu et al., Electrochemical and Solid - State Letters, 5 (1), A26 (2002). While Li ion cells having an electrolyte based on LiBOB and carbonate mixtures such as EC/dimethylene carbonate (DMC) or EC/propylene carbonate (PC)/DMC can stably cycle at temperatures as high as 70° C., such cells do suffer from lower power and diminished low temperature performance.

Another example for the effort to improve the stability of electrolytes at high temperature is shown by Takami et al., who taught an electrolyte including LiBF 4 dissolved in a gamma-butyrolactone (GBL) reduces the gas production within a lithium ion cell when the cell is stored at high temperatures. However, such cells also suffer from lower power and diminished low temperature performance. The cycling performance of these lithium ion cells deteriorates rapidly at high temperatures, caused by the presence of labile fluorines in the anion BF 4 − . (Takami et al., J. Electrochem. Soc., 149, A9 (2002)). The electrolyte combination of GBL with LiPF 6 also fails to deliver good performances in lithium ion cells even at room temperature, due to the low stability of the electrolyte on anodic graphite. (Chagnes et al., J. Electrochem. Soc., 150, A1255 (2003)).

Currently, no electrolyte composition is available which can simultaneously support the stable operation of Li ion cells at both high and low temperatures. Thus, there exists a need for an electrolyte composition that can simultaneously support the operation of Li ion cells above 60° C. and below −30° C. without serious degradations in cycle life, energy and power.

A non-aqueous rechargeable electrochemical cell includes an electrolyte composition produced through the dissolution of a thermally stable lithium salt in a lactone solvent. The resulting cell has stable performance in a wide temperature range between −40° C. and 80° C. The resulting cell operates across this wide temperature range with a commercially acceptable capacity retention, power loss characteristics, and safety characteristics across this temperature range.

The non-aqueous rechargeable electrochemical cell includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. A non-aqueous electrolyte is in simultaneous contact with the electrodes and the separator and includes a lithium salt dissolved in a non-aqueous electrolyte solvent having the formula:

where R 1 -R 6 are independently in each occurrence a hydrogen radical, an aliphatic moiety containing 1 to 5 carbon atoms, or an aromatic moiety containing 6 to 24 carbon atoms; n is an integer between 0 and 4 inclusive with the proviso that when R 1 -R 6 are all hydrogen radicals, n is not 1.

Alternatively, an electrochemical cell includes as a solvent a lactone having a four to eight membered ring to yield a capacity retention for the cell at −40° C. for more than 50% and at 60° C. after 100 cycles more than 90% relative to the cell operation at 25° C. Regardless of the nature of electrolyte lactone solvent, additional secondary solvents are also optionally provided such as a cyclic carbonate, an acyclic carbonate, an acyclic mono-ester, and combinations thereof.

This invention is further illustrated with reference to the accompanying drawings of exemplary inventive aspects as follows:

FIG. 1 is a plot of the high temperature (60° C.) cycling performances of lithium ion button cells based on different electrolytes including a thermally stable salt LiBOB and the lactone GBL (electrolytes D and E). The baseline lithium ion cell uses an electrolyte composition (1.0 molar LiPF 6 /EC:DMC weight ratio 1:1, electrolyte A) that is common in commercial state-of-the-art lithium ion cells. A LiBOB electrolyte without GBL (electrolyte C) is also included for comparison.

FIG. 2 is a plot of the high temperature (60° C.) cycling performances of lithium ion button cells based on electrolytes G, H, I, J, K, M and N.

FIG. 3 is a plot of the high temperature (75° C.) cycling performances of the electrolytes C and D of FIG. 1 in state-of-the-art industrial 8 ampere hour (Ah) lithium ion cells, which were hermetically sealed in steel cans and which use LiNiO 2 -based cathodes. The baseline lithium ion cells used 1.0 molar LiPF 6 /EC:PC:DMC in a weight ratio 1:1:3 (electrolyte B).

FIG. 4 is a plot of the cell voltage-dependence of the overall impedance of lithium ion cells for electrolytes A and C and 1 molar LiBOB in GBL (electrolyte F).

The present invention has utility as an electrolyte and an operative battery. An inventive non-aqueous electrolyte rechargeable battery includes an electrode group of a positive electrode and a negative electrode; a separator interposed between the positive and the negative electrodes; and a non-aqueous electrolyte permeating the electrode group and impregnating the separator. The inventive non-aqueous electrolyte includes at least one lithium salt dissolved in one or more non-aqueous solvents.

A positive electrode (synonymously described herein as a cathode) typically is a lithiated metal oxide or mixed metal oxides. Preferably, the lithiated metal oxide or mixed metal oxides has layer or tunnel structures to facilitate lithium ion diffusion within these structures. More preferably, lithiated metal oxide powder is coated as a thin film, with resort to a polymeric binder and other conventional optional conductive additives, onto a metal substrate. Lithiated metal oxide cathode materials operative herein illustratively include LiCoO 2 , LiNiO 2 doped with various amounts of metals, (LiNi x M′ y M″ (1-x-y) O 2 , where M′ and M″ are each independently one of Co, Al and Mn), spinel (LiMn 2 O 4 ), olivine phosphates (LiMPO 4 ) where M is at least one of Co, Ni, Fe, and Mn, or partially substituted forms thereof; or the mixed metal oxides of the formula Li 1-x (M 1 y M 2 z M 3 1-y-z )O 2 (0≦x<1, 0≦y,z<1) where M 1 , M 2 , and M 3 are each independently one of Co, Ni, Fe, and Mn, or the mixture several of these metals.

A negative electrode (synonymously described herein as an anode) typically is lithium metal or materials that can be lithiated at potentials within +2.0 V of the lithium metal potential. Such anodic materials illustratively include amorphous and ordered carbonaceous materials, Li—Al alloys, Li—Sn alloys, Li—Sn—O alloys, Sn, SnO and SnO 2 , lithiated TiO 2 such as Li 4 Ti 5 O 12 with spinel structure. More preferably, anode powder is coated as a thin film, with the help of a polymeric binder and other conductive additives, onto a metal substrate.

A conventional optional separator is either a porous polymeric film that is impregnated with the non-aqueous electrolyte solutions, or a polymer material able to form a gel with the electrolyte solutions. Separators operative herein illustratively include non-woven polyolefin films, fluorine-containing polymers such as poly(vinylidene fluoride) (PVdF), (hexafluoro propylene) copolymer (PVdF-HFP), polyacrylates, methacrylates, acrylonitriles, and copolymers of styrenes and butadienes, copolymers and polymer blends of the aforementioned polymeric species.

An inventive non-aqueous electrolyte solution includes one or more thermally stable salts, such as stable lithium salts dissolved in a non-aqueous solvent or a mixture of non-aqueous solvents. An electrolyte lithium salt illustratively includes lithium bis(trifluoromethane sulfonyl)imide (LiIm), lithium trifluoromethane sulfonate (LiTf), lithium tetrafluoroborate (LiBF 4 ), lithium bis(oxalato)borate (LiBOB), lithium tris(oxalato)phosphate (LiTOP), and lithium difluoro(oxalato)borate (LiBOF) or combinations thereof. Preferably the thermally stable lithium salt is LiBOB.

The non-aqueous solvent or solvent mixture includes at least one cyclic lactone as defined by the following general structure:

where R 1 -R 6 are independently in each occurrence a hydrogen radical, an aliphatic moiety containing 1 to 5 carbon atoms, or an aromatic moiety containing 6 to 24 carbon atoms, and n is an integer between 0 and 4, inclusive. Cyclic lactones operative herein illustratively include beta-propiolactone (BPL), beta-butyrolactone (BBL), alpha-methyl-gamma-butyrolactone (AMGBL), gamma-butyrolactone (GBL), gamma-valerolactone (GVL), delta-valerolactone (DVL), gamma-caprolactone (GCL), epsilon-caprolactone (ECL), gamma-octanolactone (GOL), gamma-nanolactone (GNL), gamma-decanolactone (GDL), delta-decanolactone (DDL), gamma-undecanolactone (GUL), delta-undecanolactone (DUL), and delta-dodecanolactone (DDDL).

An electrolyte solution including the combination of at least a thermally stable lithium salt and at least a cyclic lactone yields a lithium cell able to deliver stable performances at both low and high temperatures.

The present invention is further illustrated with reference to the following examples, which are for illustrative purpose, but the spirit of the present invention is by no way limited by these examples. To help the illustration of the examples, a number of electrolytes as designated by letters A, B, C, etc. with different salt-solvent combinations are used as shown in Table 1.