NSRRC Activity Report 2022

058 NSRRC ACTIVITY REPORT 2022 Fig. 2 : Operando Ni K-edge X-ray absorption spectra of (a) HLC and (b) LLC cathodes during cycling. Ni K-edge absorption energies of (c,d) HLC and (e,f) LLC cathodes and their corresponding charge–discharge curves. The K-edge energies are evaluated at a normalized intensity of 0.5. [Reproduced from Ref. 1] Fig. 3 : (a) Reaction mechanism of HLC and LLC cathodes and the corresponding CV results. (b) Cation migration mechanism. Schematic of the cation migration in HLC and LLC cathodes. Colors for different elements: O is red, Li is green, and TM is purple. [Reproduced from Ref. 1] 2(f) ). The oxidation state of Ni is maintained from 4.6 V to 4.4 V, followed by substantial changes at potentials of less than 4.4 V, likely resulting from anion reduction and TM reduction processes, respectively. The 2D XAS mapping at the Ni K-edge on the HLC and LLC particles is recorded at 4.6 V and 4.4 V during discharge. The full charged state of Ni (Ni 4+ ) is distributed uniformly on the HLC particle at 4.6 V, which is consistent with in operando XAS results. Figure 3(a) shows the proposed reaction mechanism of HLC and LLC cathodes, which will be discussed further, and the corresponding CV results. During the charge process, TM oxidation is performed (region I), followed by anion oxidation (region II) in both the HLC and LLC cathodes. Severe voltage decay is observed for the HLC cathode, and anion reduction occurs at a lower potential region along with TM reduction process. The LLC cathode exhibits sequential anion and TM reduction reactions during the discharge process. The TM layer is known to resemble a honeycomb-like structure, yielding a high amount of the Li 2 MnO 3 phase (C2/m structure) in the HLC material. The lattice oxygen tends to form an oxygen dimer with short covalent bonds at a high-potential region during the delithiation process, which causes the metal-oxygen de- coordination and cation migration. The voltage decay could originate from the cation migration during the