UC San Diego

Solid-state batteries are an emerging next-generation energy storage technology with the potential to offer improved safety, higher energy density, and longer cycle life. The solid electrolyte is the key component that determines the electrochemical performance of a solid-state battery, especially at room temperature. Chloride-based solid electrolytes have emerged as promising catholyte materials due to their moderately high ionic conductivity, good mechanical pliability, and superior oxidation stability. However, Na-ion conducting ternary chloride phases, like Na3YCl6 and Na2ZrCl6, exhibit very low ionic conductivities, significantly lagging their Li-ion counterparts, which results in poor room temperature battery performance. Additionally, when fabricating solid-state battery cathodes, the solid electrolyte must be combined with active materials in high weight fractions in order to achieve sufficient ionic percolation within the cathode composite. This requirement drastically hinders the practicality of solid-state batteries as the solid electrolyte is conventionally designed to be electrochemically inactive and is effectively electrochemical ‘deadweight’, significantly lowering the energy density of the cell.In this dissertation, the first challenge was addressed by developing novel chloride compositions with enhanced ionic conductivity. It was found that NaCl-deficient compositions led to the formation of nanocrystalline and amorphous Na2.25-xY0.25Zr0.75Cl6-x (1.375  x  2.000) solid electrolytes with enhanced ionic conductivity due to reduced activation energy barriers and fast chemical exchange between prismatic Na+ local environments. Moreover, it was shown that reducing the ratio of NaCl to ZrCl4 has direct implications on polyhedral connectivity, leading to the formation of new structures with composition NaZr2Cl9 that are built from face-sharing [Zr2Cl9]- polyanionic species, rather than isolated [ZrCl6]2- species. The second challenge was addressed by modifying the well-known Na2ZrCl6 solid electrolyte via aliovalent substitution of inactive Zr4+ cations with redox-active M5+ (M = Nb or Ta) cations to create a series of Na2–xMxZr1–xCl6 solid-solutions that possess both high ionic conductivities and active sites for Na+ storage. It was discovered that both the niobium- and tantalum-containing chlorides exhibit rather high electrochemical potentials (2.2–2.8 V vs. Na9Sn4), making them ideal catholytes to pair with commonly used oxide cathode materials like NaCrO2. This synergistic pairing leads to a cathode composite with an 83–102% increase in energy density and 39–81% improvement in areal discharge capacity compared to a redox-innocent solid electrolyte. These studies present innovative design strategies to improve ionic conductivity and cathode energy density through composition control and aliovalent substitution, offering new opportunities for the development of high-performance solid-state batteries.