Low melting point solid electrolytes for scalable manufacturing of all-solid-state li-ion batteries

Abstract

All-solid-state lithium-ion batteries (ASSLBs) without flammable liquid electrolytes offer improved safety for electric vehicles and other applications. However, current inorganic ASSLB manufacturing technology suffers from high cost and low attainable volumetric energy density. Such a fabrication method involves separate fabrications of sintered solid-state electrolyte (SSE) membranes and SSE-comprising ASSLB electrodes, which are then carefully stacked and sintered together at high temperatures and pressures. The energy density is limited by the excessive membrane thickness needed to prevent cracks, and the large fractions of SSE included in electrodes which ensure all active material particles being surrounded by the SSE. In my thesis, I explore a novel manufacturing approach that offers reduced manufacturing costs and improved energy density in ASSLB cells. This approach mimics the fabrication of conventional Li-ion cells with liquid electrolytes, except that SSEs with low melting points are infiltrate into thermally stable electrodes at moderately elevated temperatures (~300 °C or below) in a liquid state and then solidified during cooling. Li anti-perovskites are excellent SSE model materials for the proposed melt-infiltration approach because of their reported high ionic conductivities, low melting points, and low synthesis costs. However, the real performance characteristics of the anti-perovskites have been unclear due to the discrepancies in the reported compositions and conductivity values. In this thesis, high-purity Li hydroxide chloride anti-perovskites were synthesized by a novel approach with a very short reaction time. The structures and compositions of the SSEs were carefully characterized. Ionic conductivities were measured with various approaches and a composition of Li1.9OHCl0.9 was found to have the highest conductivity due to the removal of otherwise unavoidable LiCl impurity. Li1.9OHCl0.9 was then melt-infiltrated into densely packed lithium nickel manganese cobalt oxide (NCM), lithium titanate oxide (LTO), and graphite electrodes with the help of a thin layer Al2O3 coated on the electrodes by atomic layer deposition (ALD) which improved wetting of the molten SSE on the electrodes. The melt-infiltrated electrodes had near zero porosity without additional sintering steps. Inorganic ASSLBs with melt-infiltrated NCM cathodes and both LTO and graphite anodes were fabricated. The cycling data of such cells presented similar voltage profiles and capacity retentions to the cells of the same electrodes with liquid organic electrolytes. The promising performance characteristics of such cells will open new opportunities for the accelerated adoption of ASSLBs for safer electric vehicles.