Development of Advanced Lithium-ion Rechargeable Cells with Improved Low Temperature Performance

Dueto future plans to explore Mars and the outer planets, NASA has interest in developing lithium-ion rechargeable batteries that are capable of operating at low temperatures. To address these problems, w have initiated research focused upon t h e development of advanced electrolyte systems for lithium-ion cells with improved low temperature performance. Promising electrolyte solutions were selected based on conductivity and stabil i ty considerations and incorporated into C/LiCo02 cells for evaluation. The discharge capacity and rate capability as a function of temperature were evaluated in two types of lithium-ion cells, consisting of graphite-based systems with EC-based electrolytes and coke-based systems with PC-based electrolytes. Some of the experimental lithium-ion cells fabricated with these electrolytes were found to be capable of operating at temperatures as low as -30”C (both charging and discharging at -30”C) and provide more than 557. of the room temperature capacity. Cycle life testing of the these cells at -20”C and at room temperature is in progress. Some of the cells have completed more than 500 cycles to date (1 OOO/. DOD) . INTRODUCTION Lithium-ion cells generally show poor performance at low temperatures. This poor performance is primarily due to the limitations of the electrolyte solutions which contain organic solvents, such as ethylene carbonate (EC) and dimethyl carbonate (DMC), that become highly viscous and freeze at low temperatures resulting in poor conductivity of the medium. Thus, recently there has been interest in developing electrolytes which can result in lithium-ion cells with improved low temperature performance. (Ein-Eli, 1997: Juzkow, 1997) In designing electrolytes that are highly conductive at low temperatures it is necessary to consider a number of important parameters, such as the dielectric constant of the medium, the viscosity, the Lewis acid-base coordination behavior, as well as the appropriate liquid ranges and satt solubilities of the systems. For an electrolyte solution to be a viable candidate for lithium-ion cell applications, it must satisfy a number of requirements in addition to possessing the desired conductivity over the specified temperature range, such as (i) possess good electrochemical stability over a wide voltage window (O to 4.5V), (ii) have the ability to form thin, stable passivating films at the carbonaceous anode electrode, and (iii) display good thermal and chemical stability. All of these factors need to be weighed accordingly depending on the ultimate application intended. An effective way to improve the low temperature conductivity of the electrolyte solution is to extend the liquid range and decrease the viscosity of the solvent system used. This can be accomplished by the addition of solvents which improve the low temperature conductivity of EC and PC-based systems. Possible candidate solvent additives include formates, acetates, cyclic and aliphatic ethers, Iactones, as well as other carbonates, such as diethyl carbonate (DEC). In addition, an optimization of the electrolyte salt concentration can also translate into improved low temperature conductivity. We have previously identified (Smart, 1996) a number of electrolytes which are highly conductive at low temperatures based upon these approaches and successfully demonstrated their use in lithium-ion cells. After evaluating the low temperature conductivity and assessing the relative stability of potential systems, a number of electrolytes were selected for evaluation in lithium-ion experimental cells. The low temperature and cycle life performance of these cells was the basis for selecting six electrolytes for incorporation into prototype cells which were fabricated by Wilson Greatbatch Ltd. according to