Improved Safety, High Performance Electrolytes for Li-Ion Batteries

Rechargeable lithium-ion batteries with improved safety and high performance are needed for numerous applications including electric vehicles, consumer electronics, and military platforms.  Current lithium-ion batteries typically utilize an electrolyte composed of a lithium salt (e.g. LiPF₆) and flammable organic solvents (e.g. ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, etc.).  Using conventional flammable electrolytes, current lithium-ion batteries can combust under failure mechanisms such as overcharging, internal shorting, defects, physical damage, overheating, etc. 

Non-flammable electrolytes are currently being developed to provide Li-ion batteries that do not catch fire under failure or abuse conditions. Various non-flammable electrolytes based on ionic liquids, phosphates, phosphonates, and other fire retardant additives have been developed to provide non-flammable lithium-ion conducting electrolytes.  However, most non-flammable electrolytes developed to date result in decreased battery performance, particularly under high rate (≥3 C) and low temperature conditions (≤ -30 ºC) which are required for numerous applications. Improved electrolytes are needed to meet the performance and safety requirements of next generation Li-ion batteries.  

For the electrolyte, needed properties include (i) adequate ionic conductivity over the desired temperature range, (ii) the ability to form stable and conductive  interfaces with the anode and cathode, (iii) high temperature stability, and (iv) flame retardant properties. The electrolyte composition is a compromise between these factors, and different electrolyte compositions are needed depending on the application’s temperature range, electrode materials, rate, etc.

We have explored the development of Li-ion conducting electrolytes that are both non-flammable and provide high performance through developing mixtures composed of salts, solvents and additives. Electrolytes were prepared in an inert atmosphere (Argon) glovebox. Electrolyte properties measured included ionic conductivity versus temperature and flammability.  Flammability was measured using multiple methods: wick test, flash point, oxygen consumption calorimetry, and simulated venting.  The performance of the electrolyte was evaluated in lab-scale cells (coin and pouch) over various charge/discharge rates and temperatures. Electrochemical impedance spectroscopy was used to probe the interfacial resistance.

From our analysis, we determined that the solvents, co-solvents, and additives used within typical electrolytes have different flammabilities.  In addition, the different flammability tests showed differing results indicating that the nature of the test plays a critical role and differing experimental test conditions can lead to widely varied flammabilities. Using the information determined from flammability testing, we developed electrolyte mixtures designed to provide wide temperature operation, high rates, and be flame retardant.  Electrochemical testing of the electrolyte in half and full cells showed that the electrolyte composition affected the battery’s capacity, cycle life, low temperature performance and high temperature performance.  Electrochemical impedance spectroscopy results showed that specific electrolyte compositions resulted in similar charge transfer resistances as conventional flammable electrolytes. Full cells using specifically designed electrolytes exhibited similar capacities at low temperatures (-40 ºC) and at high rates (5C) as cells using baseline electrolytes. From current testing performed, electrolytes containing specific flame retardant compounds, interfacial formers, and wide temperature components have the capability to provide batteries with significantly lower flammability and similar capacities, rates, cycle lives, and temperature ranges as batteries with conventional flammable electrolytes.