Design of (thio) phosphates for high performance lithium ion batteries

Abstract

Empirical bond length - bond valence relations provide insight into the link between structure and ion transport in solid electrolytes and mixed conductors. Building on our earlier systematic adjustment of bond valence (BV) parameters to the bond softness, here we link the squared BV mismatch to the absolute energy scale and use it as a Morse-type interaction potential for analyzing low-energy ion migration paths in ion or mixed-conducting solids by either an energy landscape approach or molecular dynamics (MD) simulations and compare the results to experimental characterizations. For a wide range of lithium oxide and lithium sulfide compounds we could thus model ion migration pathways and mechanisms revealing significant differences to an earlier geometric approach. This novel BV-based force-field has then been applied to investigate a range of mixed conductors, focusing on cathode materials for lithium ion battery (LIB) applications to promote a systematic design of LIB cathodes that combine high energy density with high power density. To demonstrate the versatility of the new BV-based force field it is applied in exploring various strategies to enhance the power performance of phosphate-based safe low-cost cathode materials including LiFePO4, LiVPO 4F and thiophosphate solid electrolytes for rechargeable all-solid-state lithium Li-ion batteries (AS-LIBs). The argyrodite-type Li 6PS5X (X = Cl, Br, I) thiophosphates were prepared by mechanical milling and subsequent annealing. Samples are characterized structurally by neutron and X-ray powder diffraction as well as electrochemically by impedance spectroscopy. A room temperature conductivity of 10-3.1 S/cm renders the anion-disordered Li6PS 5X solid electrolytes (with X=Cl, Br) suitable for AS-LIBs.