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.