(Battery Division Research Award) Structure-Property Relationships: A Journey From LFP to Lithium Metal

Abstract

Conceptual importance of “Matastable Phase Diagram” and “Liquid Madelung Potential” will be demonstrated by picking up some of my mechanism-solving works on LFP and lithium metal.In Li x FePO4, the ground state phase separation and metastable solid solution are energetically competitive, and the solid solution phase is induced in a non-equilibrium electrochemical reaction, playing critical role for fast kinetics. However, there had been no discussion or experimental report of the crystal structure, optical and transport properties of the metastable solid solution phase due to the extreme technical difficulties; the solid-solution phase Li x FePO4 is metastable during electrochemical process and disappears within a few seconds. We overcame this limitation by quenching Li x FePO4 (x = 2/3) at 350 °C to room temperature. This quenched phase remained stable for a couple of weeks, which enabled sufficient time to measure the several intrinsic properties, including polaron-crystallized superstructure, optical properties with emerging inter-valence charge transfer absorption, and enhanced conductivity over 103 in order.Since the Li/Li+ redox potential (E Li/Li+) positions outside of the thermodynamical potential window of the present electrolyte, suppressing side reactions is essential, which can be realized not only by kinetic suppression by SEI but also by thermodynamic stabilization viz. upshift of E Li/Li+. Remarkable improvement of Coulombic efficiency is exemplified in (locally) salt-concentrated electrolyte, where E Li/Li+ largely upshifts over 0.6 V. We revealed that the unusual upshift of E Li/Li+ is primarily dominated by Coulombic energy penalty of Li+ in the electrolytes rather than the simple Nernst equation with extremely high Li+ activity coefficient.[1] Overall energetics was verified and directly quantified by introducing the “liquid Madelung potential (E LM)” analogous to the conventional concept in solid-state science.[1] Namely, Li+ is electrostatically more stabilized by electron-localized solvents, while relatively less stabilized by electron-delocalized larger sized anion, thus decreasing the overall Coulombic energy gain of Li+ under progressive anion coordination.[1][1] N. Takenaka, S. Ko, A. Kitada, and A. Yamada, submitted (2022) Figure 1