(Battery Division Student Research Award sponsored by Mercedes-Benz Research & Development) Solution-processable closo-borate Electrolyte for all-solid-state Sodium–ion Batteries

Abstract

All-solid-state batteries promise to simultaneously yield higher energy and power density, and improved safety as compared to state-of-the-art lithium-ion technologies based on organic liquid electrolytes. A competitive all-solid-state battery requires a solid-state electrolyte with high ionic conductivity near room temperature, combined with high thermal and electro-chemical stability as well as ease of processing into a device. Meeting these requirements simultaneously represents a major challenge. Here we present a sodium-ion conductor in the closo-borate family, namely Na2(B12H12)0.5(B10H10)0.5, that simultaneously offers high sodium-ion conductivity of 0.9 mS/cm at 20 °C, excellent thermal stability up to 450 °C, and importantly a large electro-chemical stability window of 3 V including stability versus metallic sodium enabling the use of a sodium metal anode.[1-2] Using a solution-based impregnation method to create intimate contact between cathode material and solid-state electrolyte, we demonstrate that Na2(B12H12)0.5(B10H10)0.5, can be implemented in a 3 V all-solid-state sodium-ion battery consisting of a sodium metal anode and a NaCrO2 cathode. The cell shows high cycling stability and good rate performance with more than 85% capacity retention after 250 cycles at C/5. [3] We further extend this approach to the use of slurry-based composite electrodes as used in conventional batteries.[4] We show that Na2(B12H12)0.5(B10H10)0.5 can be prepared and infiltrated in its highly conductive phase inside porous sheet-type electrodes using the appropriate solvent. The infiltrated electrodes can be cycled in all-solid-state half cells at room temperature with remarkable stability and rate capability. The method is demonstrated for both NaCrO2 cathode and Sn metal anodes including demonstration of operation in an all-solid-state full cell based on the closo-borate electrolyte.[4,5] [1] L. Duchêne et al., Chem. Commun., 2017, 53, 4195. [2] L. Duchêne et al., Chem. Mater., 2019, 10.1021/acs.chemmater.9b00610 [3] L. Duchêne et al., Energy Environ. Sci., 2017, 10, 2609 [4] D. H. Kim et al., Nano Lett. 2017, 17, 3013. [5] L. Duchêne et al., submitted