Redox Chemistry of LiCoO$_2$, LiNiO$_2$, and LiNi$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ Cathodes: Deduced via XPS, DFT+DMFT, and Charge Transfer Multiplet Simulations

TL;DR

This work tackles how Li deintercalation modulates the bulk electronic structure of LiCoO2, LiNiO2, and LiNi1/3Mn1/3Co1/3O2 cathodes. By integrating XPS measurements with DFT+DMFT calculations and CTM simulations, the authors connect spectral TM 2p satellites to redox processes and quantify the evolution of TM–O 2p hybridization during delithiation. They demonstrate that oxide redox cannot be treated as a rigid-band hole transfer; delithiation shifts hybridization features toward the Fermi level and reduces satellite intensity, especially for Ni, while Mn in LNMCO acts as a structural stabilizer. The study provides mechanistic insight and a practical Redox indicator based on XPS satellites, offering guidance for designing higher-performance cathodes. Overall, the integrated approach establishes a framework to relate spectroscopic fingerprints to redox chemistry in complex, mixed-transition-metal oxides.

Abstract

Understanding the evolution of the physicochemical bulk properties during the Li deintercalation (charging) process is critical for optimizing battery cathode materials. In this study, we combine X-ray photoelectron spectroscopy (XPS), density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, and charge transfer multiplet (CTM) model simulations to investigate how hybridization between transition metal (TM) 3d and oxygen 2p orbitals evolves with Li deintercalation. Based on the presented approach combining theoretical calculations and experimental studies of pristine and deintercalated cathodes, two important problems of ion batteries can be addressed: i) the detailed electronic structure and involved changes with deintercalation providing information of the charge compensation mechanism, and ii) the precise experimental analysis of XPS data which are dominated by charge transfer coupled to final-state effects affecting the satellite structure. As main result for the investigated Li TM oxides, it can be concluded that the electron transfer coupled to the Li$^{+}$-ion migration does not follow a rigid band model but is modified due to changes in TM 3d and O 2p states hybridization. Furthermore, this integrated approach identifies the 2p XPS satellite peak intensity of TM as an effective indicator of the redox chemistry. With that the redox chemistry of cathodes can be deduced, thus offering a foundation for designing more efficient battery materials.

Figures (8)

• Figure 1: Orbital-resolved density of states (DOS) (left panel) and hybridization function (right panel) of (a) LiCoO$_2$ (LCO), (b) Li$_{1/3}$CoO$_2$ (L$_{1/3}$CO), (c) LiNiO$_2$ (LNO), and (d) Li$_{1/3}$NiO$_2$ (L$_{1/3}$NO). The DOSs close to the Fermi level are zoomed in for more details. The computational cells adopted are shown in the inset of the hybridization function.
• Figure 2: Orbital-resolved density of states (DOS) (left panel) and hybridization function (right panel) of (a) Co in LiNi$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ (LNMCO) and Li$_{1/3}$Ni$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ (L$^{\mathrm{Co}}_{1/3}$NMCO). The hybridization functions of Co in LiCoO$_2$ (LCO) and Co2 in Li$_{1/3}$CoO$_2$ (L$_{1/3}$CO) are shown for comparison, and (b) Ni in LNMCO and L$_{1/3}$NMCO (L$^{\mathrm{Ni}}_{1/3}$NMCO). The hybridization functions of Ni in LiNiO$_2$ (LNO) and Ni2 in Li$_{1/3}$NiO$_2$ (L$_{1/3}$NO) are shown for comparison. The simulation cells corresponding to LNMCO, L$^{\mathrm{Co}}_{1/3}$NMCO, and L$^{\mathrm{Ni}}_{1/3}$NMCO are illustrated on the right side.
• Figure 3: The occupation probabilities as a function of occupation number $n_d$ and expectation value of $S_z$ ($z$-component of the spin angular momentum operator) given by the CTQMC impurity solver in DMFT calculations for (a) Co 3$d$ shell in LNMCO, (b) Ni 3$d$ shell in LNMCO, (c) Co 3$d$ shell in L$^{\mathrm{Co}}_{1/3}$NMCO, and (d) Ni 3$d$ shell in L$^{\mathrm{Ni}}_{1/3}$NMCO. The total probability corresponding to each $n_d$ is given in the box on the right side of each figure. Note that the total probability does not add to 1 because we omit those configurations with probability smaller than 0.0005.
• Figure 4: Schematic representations of (a) charge compensation mechanism in Li-ion battery during charging process; (b) the idea of utilizing DMFT-predicted TM $d$-state occupation probabilities corresponding to different cathode oxidation states to simulate the TM 2$p$ XPS based on charge transfer multiplet (CTM) model; and (c) different definitions of Coulomb interaction and charge transfer energy in the electron removal and addition spectra (adopted from Ref. assat2018fundamentalbisogni2016groundpavarini2016quantum), and in the CTM model.
• Figure 5: Background subtracted and energy calibrated core spectra of (a) Ni 2$p$ XPS of NiO, LNO and LNMCO (b) Co 2$p$ XPS of in-vacuo scratched pristine LCO and LNMCO indicating the non-local screening effects. (c) Ni 2$p$ XPS and (d) Co 2$p$ XPS of in-vacuo scratched pristine LNMCO as compared to the in-vacuo scratched fully deintercalated (charged to 4.8 V) LNMCO.