Materials Discovery and Process Development of Bulk Ionic Glass (BIG) Solid Electrolytes Through Geometric Frustration for All Solid State Batteries

Bulk ionic glasses that are ductile and thin are promising as solid electrolytes for all solid-state batteries. The ductility or brittleness of these materials is dependent on composition and processing parameters. The most successful example of this is amorphous inverted or ionic glass Lithium Phosphorous Oxynitride (LIPON) which has enabled the use of Lithium metal as an anode. LIPON’s high resistance or toughness to fracture and high ionic conductivity is due to the high alkali content (3:1 ratio of alkali modifier to P network former that makes Li⁺ the dominant species rather than a network modifier) but currently requires expensive vacuum deposition to fabricate. This work investigates the concept of geometric frustration to stabilize entropy in bulk ionic glasses thus promoting melt quenching with low quench rates as a new method to fabricate similar inverted glasses with high alkali content. The compositional series comprising of alkali elements Li-Mg-Ca with different phosphates in increasing R values (ratio of charge or moles of modifying alkali oxide to charge or moles of Phosphorus pentoxide) is explored. Glasses were fabricated by melting and pouring them at 1350°C. The series ranged in composition from an alkali to phosphate ratio of 1:1 to 2.5:1. Differential scanning calorimetry (DSC) data showed that the glasses in this range had glass transition temperature (T_g) in the range of 416°C – 452°C. Crystallization temperatures (T_c) ranged from 507°C - 550°C . Raman data confirmed that the glasses range from an alkali:phosphate ratio of 1:1 to nearly 2.5:1. These results demonstrate that a melt quench pouring based approach is promising to process bulk inverted ionic glasses paving the way for the research and development of thin film electrolyte based Li metal batteries.