(Invited) Designing Intercalation for High-Valent & Uniform Redox Reactions

Abstract

Many electrochemical materials attain high-valent oxidation state during operation, such as electrocatalysts for oxygen evolution reactions and positive electrodes for lithium-ion batteries. For the latter, the high voltage enabled by high valent redox can further increase the energy density of batteries. In oxides, achieving high oxidation states is typically accompanied by substantial bond contraction as well as participation of the O 2p electronic states, both of which are known to affect the stability and reversibility of the redox reaction. In addition to high valent redox, another desirable characteristic of electrochemical reaction is uniformity. This is especially true for intercalation electrochemistry, since the uniformity directly establishes the mechanical stress within the electrode. In this talk, I will use lithium- and sodium-intercalating layered oxides as a prototypical system for establishing design rules for high-valent and uniform redox. For the first part of the talk, I will discuss the convergence of three phenomena that control the stability of high valent redox: short covalent bonding, mitigation of local strain energy through point defect formation, and coulombically stabilization of oxidized species. In the second part of the talk, I will rationalize unexpected mesoscale heterogeneities such as apparent phase separation in layered oxides, and provide practical design rules on engineering more uniform electrochemical devices.