Electrolytes having improved low temperature performance

Abstract

The present invention provides a novel electrolyte solvent which is usable with a variety of carbonaceous and metal oxide electrode active materials, providing improved performance over a broad temperature range, and which is stabilized to maintain cell capacity over a number of cycles.

What is claimed is:  
     
         1 . An electrochemical cell which comprises a first electrode, a counter electrode which forms an electrochemical couple with said first electrode, and an electrolyte; said first electrode comprising particles of carbonaceous intercalation active material; and said electrolyte comprising a solvent mixture and a solute; said solvent mixture comprising ethylene carbonate (EC), propylene carbonate (PC), and a compound represented by R′COOR″, where R′ and R″ are each independently selected from the group consisting of ethyl and propyl.  
     
     
         2 . The electrochemical cell according to  claim 1  wherein said solvent mixture consists essentially of said propylene carbonate (PC), said ethylene carbonate (EC), said compound represented by R′COOR″, and optionally one or more other organic solvents; with said compound represented by R′COOR″ being present in an amount by weight which is not greater than the combined amount of said EC, said PC, and said one or more other solvents.  
     
     
         3 . The electrochemical cell according to  claim 2  wherein each said one or more other solvents is selected from the group consisting of carbonates; lactones; propionates; five member heterocyclic ring compounds; and organic solvent compounds having a low alkyl (1-4 carbon group connected through an oxygen to a carbon, and comprising C—O—C bonds.  
     
     
         4 . The electrochemical cell according to  claim 2  wherein each said one or more other organic solvents is selected from the group consisting of methyl ethyl carbonate (MEC), diethyl carbonate (DEC), dipropyl carbonate (DPC), dimethyl carbonate (DMC), butylene carbonate (BC), dibutyl carbonate (DBC), vinylene carbonate (VC), diethoxy ethane (DEE), and mixtures thereof.  
     
     
         5 . The electrochemical cell according  claim 1  wherein said solvent mixture consists essentially of said ethylene carbonate (EC), said propylene carbonate (PC), said compound represented by R′COOR″, and dimethyl carbonate (DMC).  
     
     
         6 . The electrochemical cell according to  claim 5  having said EC/DMC/R′COOR″ in a weight ratio of about 25:40:35 and said PC present in an amount less than said compound represented by R′COOR″.  
     
     
         7 . A lithium ion electrochemical cell which comprises a first electrode, a counter electrode which forms an electrochemical couple with said first electrode, and an electrolyte; each of said electrodes having an intercalation active material, with at least one said active material being a carbonaceous active material; and said electrolyte comprising a solvent mixture and a solute; said solvent mixture consisting essentially of ethylene carbonate (EC), a compound represented by R′COOR″, where R′ and R″ are each independently selected from the group consisting of ethyl and propyl, and dimethyl carbonate (DMC).  
     
     
         8 . The electrochemical cell according to  claim 7  wherein said active material of said first electrode is carbonaceous and consists of graphite particles.  
     
     
         9 . The electrochemical cell according to  claim 7  wherein said active material of said counter electrode is lithium transition metal oxide compound.  
     
     
         10 . The electrochemical cell according to  claim 9  wherein said lithium transition metal oxide compound is selected from the group consisting of LiMn 2 O 4 , LiNiO 2 , LiCoO 2 , and mixtures thereof.  
     
     
         11 . The electrochemical cell according to  claim 9  wherein said lithium transition metal oxide is lithium manganese oxide.  
     
     
         12 . The electrochemical cell according to  claim 11  wherein said lithium manganese oxide is represented by the nominal general formula Li 1+x Mn 2−x O 4 (−0.2≦x≦0.2).  
     
     
         13 . The electrochemical cell according to  claim 7  wherein said electrolyte further comprises a separator in the form of a solid matrix forming a network with voids interpenetrated by said solvent mixture and said solute.  
     
     
         14 . The electrochemical cell according to  claim 13  wherein said matrix is selected from the group consisting of polymeric acrylate, porous polypropylene, porous polyethylene, and glass fiber material.  
     
     
         15 . The electrochemical cell according to  claim 7  wherein said compound represented by R′COOR″ is ethyl propionate (EP) and said solvent mixture consists of said EC\DMC\EP in a weight ratio of 25:40:35.