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.
References:

W. Kong, J. Zhang, D. Wong, W. Yang, J. Yang, C. Schulz and X. Liu, Angew Chem Int Ed Engl, 60, 27102 (2021)

T. Ohnishi, K. Mitsuishi and K. Takada, ACS Applied Energy Materials, 4, 14372 (2021)

D. Ensling, G. Cherkashinin, S. Schmid, S. Bhuvaneswari, A. Thissen and W. Jaegermann, Chemistry of Materials, 26, 3948 (2014)

T. Iwama, T. Ohnishi and T. Masuda, Electrochemistry, 91, 117005 (2023)

L. Dahéron, R. Dedryvère, H. Martinez, M. Ménétrier, C. Denage, C. Delmas and D. Gonbeau, Chemistry of Materials, 20, 583 (2008)

C. Guhl, J. Rohrer, P. Kehne, T. Ferber, L. Alff, K. Albe, W. Jaegermann, P. Komissinskiy and R. Hausbrand, Energy Storage Mater, 37, 190 (2021)