Electrochemical Reactions of LiCoO2-Li3BO3 Composite Electrode Constructed on Li6.6La3Zr1.6Ta0.4O12 solid Electrolyte Sheet Studied By in Situ Hard X-Ray Photoelectron Spectroscopy (HAXPES)
Authors: Iwama Tsukasa , Ohnishi Tsuyoshi , Masuda Takuya 
Published: December 2024

LiCoO2 is one of the commercially used cathode materials in lithium-ion batteries due to its relatively high cycle performance and theoretical capacity density of 274 mAh g-1. However, the practical capacity density is limited to ~140 mAh g-1 under the operation below 4.2 V vs. Li+/Li because the high-voltage charging often results in rapid capacity fading.[
1
] The degradation mechanism under high-voltage charging has been studied from the structural and electronic aspects using various techniques, such as x-ray diffraction (XRD)[
2
] and x-ray photoelectron spectroscopy (XPS).[
3
] During the high-voltage charging up to 4.7 V, LiCoO2 transforms from the H3 phase to the H1-3 and then to the O1 phase.[
1
] While only the oxidation of Co3+ to Co4+ occurs as a result of the electron extraction from Co 3d (t2g) states during charging up to 4.2 V, the oxidation of O2- to O- also occurs as a result of the electron extraction from O 2p states due to the hybridization of t2g and O 2p orbitals.[
3
] These structural changes and oxygen redox are considered as an origin of rapid capacity fading under the high-voltage operation. Although understanding of the high-voltage degradation mechanism may lead to the development of high potential positive electrode materials, change of chemical and electronic structure of LiCoO2 under high-voltage operation yet to be fully clarified.

In the present study, we developed a laboratory-based operando hard x-ray photoelectron spectroscopy (HAXPES) system equipped with a Cr-Kα source (5414.9 eV) and bias application apparatus, that enables us HAXPES measurements of all-solid-state cells during charge/discharge cycles, and applied it to investigate the electrochemical reactions of a LiCoO2-Li3BO3 composite electrode layer constructed on a Li6.6La3Zr1.6Ta0.4O12 solid electrolyte sheet.

The galvanostatic charge/discharge potential profiles exhibited plateaus corresponding to the delithiation from the H1 and H3-LiCoO2 at around 3.9 and 4.6 V during the charging, respectively. In the subsequent discharging, the cell voltage rapidly dropped from 4.7 V to around 4.2 V without the plateau region at around 4.6 V. In addition to the absence of 4.6 V plateau, the capacity density of 3.9 V plateau in the discharging process was substantially smaller than that of the charging process, confirming the irreversibility caused by high-voltage operation. In the Co 2p3/2 region of the pristine cell, a main peak and a broad satellite peak characteristic to LiCoO2 were observed.[
4
] The main peak was deconvoluted into two peaks originated from Co3+ ions of the LiCoO2 and final state effect[
4,5
] at around 780 eV and 780.6 eV, respectively. After the charging up to 4.2 V, the main peak was asymmetrically broadened to a higher binding energy because of appearance of Co4+ peak at around 781 eV. After the subsequent discharging, the full width half maximum (FWHM) of Co4+ peak decreased while of Co3+ ions remained unchanged, confirming the reversible redox reaction of Co3+/4+ ions under the operation below 4.2 V. In the O 1s region of the pristine cell, two peaks corresponding to O2- in LiCoO2 lattice and Li2CO3 adsor After the charging up to 4.7 V, however, a new peak corresponding to peroxo-like oxygen dimer ((O2)n-
, (n<2))[
6
] appeared simultaneously with the disappearance of lattice O2-, as well as the appearance of Co4+ peak as is the case of 4.2 V operation, suggesting the oxidation of lattice O2-. After the subsequent discharging, the FWHM of Co4+ peak and peak intensity of lattice O2- did not fully revert to those of pristine cell, suggesting the irreversible redox reaction to retain Co4+ and (O2)n- species. Further details on the spectral features LiCoO2-Li3BO3 composite electrode under high-voltage operation measured by operando HAXPES system will be discussed.

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