All-solid-state batteries (ASBs) have attracted extensive attention in recent years because of enhanced safety, improved power/energy density and stability compared to conventional Li-ion battery technologies based on organic liquid electrolytes. However, ionic transport in solid electrolyte is often a bottleneck to high-performance ASBs. Based on the preliminary works that we have done, I propose to do systematical studies with using novel synchrotron and lab X-ray based in situ characterizations to investigate the ionic transport mechanism in selected Li and Na solid electrolytes, and to apply the findings to material design and battery-level optimization.
For the preliminary studies, LiTaSiO5, Na3SbSe4, and their doped variants were investigated to reveal the relationship between their structures and ionic conduction behaviors. For LiTaSiO5, first-principles computation predicted that Zr-doping will deliver decent Li diffusion through concerted migration mechanism. As a new family of fast lithium ion conductors, Li(1+x)Ta(1-x)ZrxSiO5, were then synthesized, characterized, and tested in experiments. The improved conductivities and decreased activation energies are consistent with computational predictions. For Na3SbSe4, we synthesized and investigated this compound based on the structure of Na ionic conductor Na3SbS4. A series of intermediate variants of Na3SbSe(4-x)Sx system were also explored. As a novel ionic conductor, cubic Na3SbSe4 showed an excellent ionic conductivity of 0.85 mS cm–1 at room temperature and good stability in a wide voltage range. Further analysis of crystal structure and conduction behavior of the Na3SbSe(4-x)Sx system indicated a non-monotonic relationship between the size of unit cell and ionic conductivity, providing new insights on the ionic conduction mechanism for sulfide electrolytes.
Based on the preliminary results, the following works are proposed: 1. Further exploring the sulfide electrolyte for all-solid-state sodium ion batteries through aliovalent doping to see their impacts on crystal structure and ionic conductions. 2. Using Na3SbS4 as a model system to further investigate how the structural features of sulfide electrolyte, such as the density of vacancies/interstitial sites, the unit cell size, and order/disorder status, affect ionic conduction behavior.