Commercially available Li-ion batteries have limited thermal stabilities, with safe operation range limited to about 80 ^oC. Nevertheless, several industries, as aerospace, oil & gas, military and biomedical, require operations to be performed at extreme environments, with peak temperatures reaching 120-150 ^oC and above. The market for specialty batteries is currently dominated by primary cells, raising safety concerns, requiring extensive maintenance and limiting the power output of the energy storage units. The development of a Li-ion battery that fulfills the safety and cycle life requirements for these applications is a challenging scientific problem and a great market opportunity.

The ubiquity of organic solvents in the vast majority of Li-ion cells greatly limits the temperature range, due to the volatility of the electrolyte. The use of ionic liquids has been proposed as an alternative to extend the thermal stability of batteries, but there are no thin film separators capable of offering the required mechanical stability at high temperatures. Similar problems are faced by polymer electrolytes, where volume change and thermal aging decrease cycle life of devices upon long exposure to high temperatures. While solid state ceramic electrolytes possess the desired stability at elevated temperatures, their poor interfacial and wetting properties tend to limit their use to thin film configurations. None of the electrolyte systems currently used meets the requirements for proper operation of large scale devices up to very high temperatures.

To tackle this issue we developed a gel-like composite electrolyte containing hexagonal boron nitride (BN) and a solution of LiTFSI in the ionic liquid 1-methyl-1-propylpiperidinium bis(trifluormethane)sulfonimide (PP13). The BN acts as a binder, providing mechanical sustentation even at elevated temperatures, while the ionic liquid offer a medium for ion transport. The ionic conductivities ranged from 0.2 mS/cm at room temperature to 4 mS/cm at 150 ^oC, with an average Li-ion transference number of 0.10. The electrolyte held remarkable electrochemical stability even at 120 ^oC, presenting an anodic stability of 5.5 V and a reversible lithium plating/stripping behavior.

Tests on a half-cell configuration using Lithium Titanate (LTO) showed negligible capacity fade for testing periods over a month at 120 ^oC, with high coulombic efficiencies attained. The accelerated lithiation kinetics at high temperatures allowed operation even at a high 3C rate, with great capacity retention even for 600 cycles. The half-cells were able to provide stable performance even at 150 ^oC, showing the superb electrochemical and thermal stability of the electrolyte system. The cells were still functional at room temperature, providing 60% of the full capacity, showing that our electrolyte system presents a record upper temperature limit for a Li-ion cell that can also operate at 25 ^oC.

Preliminary tests on a full-cell configuration at 120 ^oC using LTO and Li_1+xMn₂O₄ yielded good cyclic stability, with a capacity of 70 mAh/g and a voltage output of 2.2 V.

The BN-PP13-based composite showed exciting and so far unmatched performance even at highly extreme conditions. Nevertheless, the development of a proper electrolyte system to allow high temperature operation of Li-ion batteries is just the first step of many. The level of performance required for commercial applications will only be achieved with optimization of all device components, including electrode binders and cell packaging.