“…Voltage hysteresis can be clearly visualized by plotting charging and discharging voltage V against cumulative capacity q , which is defined here as the net amount of electrons transferred from the cathode to the external circuit, for incremental electric charge dq > 0 on charging ( Chg ) and dq < 0 on discharging ( Dchg ):
At a definite voltage V , incremental capacity dq is measurable by an externally connected potentiostat, but internally dq is ultimately redistributed and compensated by a set of redox‐active species, denoted with i , such as TM and O in the DRX material:
Among various redox‐mechanism studies for alkali transition‐metal oxide cathodes, recent spectroscopic and ab initio research has provided evidence correlating voltage hysteresis with oxygen redox, in which the lattice oxygen is oxidized to form various species (e.g., trapped O 2 [
13,16 ] or non‐dimerized O − [
17,18 ] ) accompanied with varying local coordination (e.g., TM migration, [
12,19,20 ] defect formation, [ 21 ] or ligand‐to‐metal charge transfer [
22 ] ). However, few studies can experimentally decouple and quantify the incremental capacity contributions from both TM and O redox ( dq i in Equation 2), particularly if these redox processes simultaneously occur at a given voltage.…”