Bulk and Nanostructured Closo-Hydroborate Salts of Lithium and Sodium As Superionic Electrolytes for Solid-State Batteries

Abstract

Liquid organic electrolytes continue to dominate metal-ion battery technologies despite lingering concerns over safety, capacity loss due to parasitic side reactions, limited operating temperature range, and costly packaging needed to prevent electrolyte leakage. Although several solid-state electrolytes with ionic conductivity approaching that of liquid organic electrolytes have been recently demonstrated, including perovskites, garnets, and sulfides, challenges exist in integrating these into durable battery systems. The main issues of the existing solid electrolytes are their low ionic conductivity, low electrochemical stability and large interfacial resistance between the electrodes and the solid electrolyte. We propose a new class of superionic solid electrolytes based on closo-borate and carborate anions, such as B12H12 2-, CB11H12 -, B10H10 2-, CB9H10 -. The facile cation conduction pathways in these materials are enabled by the spacious, vacancy-rich, interstitial network afforded by a sub-lattice of large, orientationally mobile anions. Many of these materials display superionic conductivity in bulk, for instance a conductivity as high as 0.03 S cm-1 was achieved in NaCB9H10 at room temperature. This talk will describe three strategies used to further improve the conductivities of bulk closo-hydroborate and carborate anions by 1) ball-milling to create vacancies and stabilize disordered phases, 2) anion mixing to create preferred pathways for facile cation diffusion, and 3) nanoconfinement of salts inside mesoporous silica or alumina with functionalized surfaces to enhance ion-transport through nano-channels. We will show examples where nanostructuring was efficient in increasing the Li and Na-ion conductivity, in some cases by two orders of magnitude compared to bulk. We will present results from neutron scattering and nuclear magnetic resonance techniques which provide invaluable insights into the dynamical nature of the reorienting anions. Finally, we will describe the remaining challenges and barriers which remain to be solved to enable the use of nanostructured electrolytes in all-solid-state batteries at device-relevant temperatures.