Elucidation of Densification Experienced By LLZO Solid Electrolytes

Abstract

In order to improve the energy density and minimize the safety concerns associated with the liquid electrolytes used in the present day lithium ion batteries, solid electrolytes are being actively considered for use in the next generation energy storage devices.[1] Among various ceramic based solid state electrolytes being investigated, Li7La3Zr2O12 (LLZO) with a garnet crystal structure demonstrates very high ionic conductivity and is extremely stable against lithium.[2] Hence, LLZO is being actively considered as the electrolyte for next generation lithium ion batteries. However, due to poor processing of these ceramics, they contain pores and voids.[3] These open spaces within the bulk not only decrease the effective conductivity of the electrolyte, but also provides open pathways for Li dendrites to propagate. Presence of pores within the ceramic structure can be estimated from the final relative density of the solid electrolyte. High temperature sintering induced densification of the LLZO particles are conducted to improve the relative density of the final solid electrolyte, and this process helps to minimize the void space within the structure.[3, 4] Sometime external pressures are applied to further improve the relative densities.[5] Even though several experimental studies have been conducted for constructing the LLZO solid electrolytes, very little effort has been devoted to actually understand the physical mechanisms that play major role during the densification process.In the present context, a computational methodology has been developed at the mesoscale level to understand the impact of surface and grain-boundary diffusion mechanisms on the overall grain ripening and densification experienced by the LLZO solid electrolytes during the sintering process conducted at elevated temperatures. During densification, the extent of increase in relative density depends on the activation energy associated with the diffusive mass transport processes. Here, the activation energy for LLZO will be estimated from direct comparison of relative densities obtained from the developed computational model and experimental observations.[6] Some strategies, in terms of particle sizes and size distributions, that can lead to improved relative densities within the LLZO solid electrolytes, will also be discussed in this presentation. References Zhang, Z., et al., New horizons for inorganic solid state ion conductors. Energy & Environmental Science, 2018. 11(8): p. 1945-1976. Ramakumar, S., et al., Lithium garnets: synthesis, structure, Li+ conductivity, Li+ dynamics and applications.Progress in Materials Science, 2017. 88: p. 325-411. Rangasamy, E., J. Wolfenstine, and J. Sakamoto, The role of Al and Li concentration on the formation of cubic garnet solid electrolyte of nominal composition Li7La3Zr2O12. Solid State Ionics, 2012. 206: p. 28-32. Sharafi, A., et al., Controlling and correlating the effect of grain size with the mechanical and electrochemical properties of Li7La3Zr2O12 solid-state electrolyte. Journal of Materials Chemistry A, 2017. 5(40): p. 21491-21504. Dzepina, B., D. Balint, and D. Dini, A phase field model of pressure-assisted sintering. Journal of the European Ceramic Society, 2019. 39(2-3): p. 173-182. Shin, R.-H., et al., Effect of Li3BO3 additive on densification and ion conductivity of garnet-type Li7La3Zr2O12 solid electrolytes of all-solid-state lithium-ion batteries. Journal of the Korean Ceramic Society, 2016. 53(6). Figure 1