Polymorphism in Li₄Zn(PO₄)₂ and Stabilization of its Structural Disorder to Improve Ionic Conductivity

Realization of the vulnerability of current rechargeable battery systems drives the research of solid electrolytes. In the search for a new Li ion conductor, we explored the rich crystal chemistry of Li4Zn­(PO4)2, which presents a low-temperature monoclinic (α-) and a high-temperature orthorhombic (β-) polymorph. We solved the crystal structure of the β phase and found that it has a disordered Li/Zn-sublattice while showing the largest conductivity; however, it could not be stabilized at room temperature by quenching. We discovered that the partial substitution of Zn2+ with Ga3+ in Li4–xZn1–xGax(PO4)2 first leads to an intermediate β′ phase. Increasing the Ga content to 0.5 mol pfu enables us to stabilize the pure β phase at room temperature, which exhibits a conductivity by several orders of magnitude higher than the pristine sample. The crystal structures of the new β′/β-phases have been solved to elucidate the conduction mechanism, which confirms the high sensitivity of ionic conductivity on disorder.