(571f) Engineered Electrolytes for Storing Energy at Ultra-Low Temperatures

Rechargeable batteries operation at ultralow temperatures (below -40 ℃) is becoming significant for cold-climate applications, especially for carrying out defense and exploration missions under extremely frigid conditions. Nevertheless, even state-of-the-art lithium-ion batteries (LIBs) suffer from severe performance degradation with temperature decreasing and stop their operation at ultralow temperatures. They require thermal insulation and electrically generated heat at the expense of wasted energy and additional mass to maintain >0 ℃ internal temperatures for the battery operation. Electrolyte related issues such as electrolyte freezing and inefficient Li⁺ transport have been identified as a primary factor for the poor performance at ultralow temperatures. The currently used high melting point carbonate-based solvents freeze around -10 ℃ and show high Li⁺ desolvation energy, dramatically decreasing ionic conductivity at low temperature (LT). Therefore, designing and engineering novel electrolytes with tailored chemical, electrochemical and physical properties are critical for addressing the challenges surrounding LT-LIBs. Here, we present desirable ether-based electrolytes aiming for Li⁺ transference enhancement, minimal desolvation energy, and inorganic-rich electrolyte-electrode interphases to facilitate LT-LIB operation by engineering electrolyte solvation and interphase structures.

We introduce several low melting point ether solvents (dipropyl ether (DPE, -122 ℃), tetrahydrofuran (THF, -109 ℃), and cyclopentyl methyl ether (CPME, -140 ℃)) with weakly solvating and high concentration electrolyte strategies for the LT-LIB operation. ^1-4 The developed electrolytes show extensive anion participation into Li⁺ solvation, which enables ethers to apply LIB electrolytes and facilitate Li⁺ transport in wide temperature ranges. Moreover, the distinct Li⁺ solvation produces various inorganic species in the electrolyte-electrodes interphases. Low insertion activation energy and high interfacial energy against Li⁺ at the interfacial regions minimize polarization and mitigate Li plating/dendrite growth.^{1, 2} Density functional theory calculations and molecular dynamics simulations corroborate the benefits of the distinct Li⁺ solvation. The resulting LT electrochemical performance with high Ni cathodes shows ~65 % capacity retention at -40 ℃ compared to capacities at room temperature. Furthermore, a developed electrolyte based on CPME allows the batteries to charge and discharge repeatedly even at -100 ℃.³

This work precedes further investigation into extreme low temperature applications such as defense and outer space, with exploration into alternative electrodes, which can circumvent kinetic hindrances in conventional intercalation electrodes. Apart from that, further investigations for fundamental underpinning principles for Li⁺ transport at LT can magnify the research community’s understanding of the LT-LIB challenges.