Interfacial Chemical Reactions and Stability in 5V Lithium Ion Cells

The development of high-voltage batteries poses important challenges in today’s material science research. Upon operation at voltages higher than 4.8 V, the main issues are the electrochemical stability window of electrolytes, the limited stability of redox active cathode, and the degradation of the cathode-electrolyte interface (CEI). In order to overcome these limitations, the electrodes are often coated with an additional buffer layer protecting them from degradation. Unfortunately, the choiceof such coating is never easy. An improved understanding of the interfacial interactions at the atomic level provides a deeper insight into the chemical stability of protective layers at high voltage, and therefore, allows us to develop routes for material synthesis with optimal chemical composition and/or layer thickness. 
In this context, we have explored the chemical stability of a MoO3coating in a LiCoPO4-LiCo2P3O10(LCP-LCPO) cathode upon operation of the battery cell up to 5.1 V. The electronic properties of LCP-LCPO and their interfacial chemical composition were investigated at the BACH beamline of Elettra by PhotoElectron Spectroscopy (PES) and X-ray Absorption Near Edge Structure (XANES). The LCP-LCPO thin film cathodes were assembled in battery cells and subsequently charged/discharged at selected potentials in a glove box under argon atmosphere in the Darmstadt Laboratories.  The treated samples were then transferred to the beamline end-station for the measurements using a special UHV suitcase, avoiding any exposure to atmosphere.
PES and XANES measurements with synchrotron radiation, together with first principle calculations, enabled access to the difference in the ionization potentials of cathode and anode, which defines the working voltage of lithium ion batteries (LIB). The evolution of unoccupied and occupied electronic states fully agrees with ab-initio calculations on the electronic structure of LCP-LCPO cathode, which exhibits a high electronic conductivity with a Co2+/Co3+redox potential of ~4.8 V.
PES measurements clearly show agradual shift of Co 3p and Li 1s core levels, and valence band (VB) upon charging/discharging of the cathode (Figure 1). Notably, the electronic configuration of LCP-LCPO, namely the oxidation state of Co, the work function, the ionization potential, the occupation of the Co3d-O2p hybridized empty states and the change of their hybridization and of polarization in the Li-O-P bond, are fully reversible upon de-/lithiation of the cathode during the first electrochemical cycle (Figure 1). In contrast, the MoO3layer on the top of LCP−LCPO undergoes chemical reactions with the electrolyte on charging the cathode to 5.1 V. This leads to surface reconstruction of the coating, the reduction of Mo(VI), and gradual leaching of the oxide layer during the electrochemical cycle. 

 figure 1

Figure 1.  Evolution of the occupied and unoccupied electronic states during electrochemical de-/lithiation of the LCP−LCPO cathode material(a)The valence band (VB) structure and Li 1s- and Co 3p- core-levels are shifted to the Fermi level (EFLCP=0 eV) and from Eupon the charging (red) and the discharging cathode (blue), respectively. The shape and the intensity of the photoelectron spectra and their energy position are mostly reversible. (b)The electronic configuration at the oxygen- and phosphorous- sites are fully reversible, as evidenced by the O K- and P L- XANES. Adapted from ACS Appl. Mater. Interfaces 2022, 14, 1, 543–556. Copyright 2022 American Chemical Society.


The high stability of the LCP-LCPO cathode against chemical reactions with the electrolyte can be better understood looking at the experimental energy level diagrams (Figure 2a), showing that Fermi level (EF) of LCP-LCPO is higher than the oxidation potential of the electrolyte. This makes the electronic charge transfer between the electrolyte and LCP−LCPO unfavorable. The higher stability and the limited degradation of the CEI at high operation voltages was achieved thanks to the MoO3coating. In fact, the decomposition of MoOcontribute to the formation of a stable CEI through an electron transfer mechanism from the electrolyte oxidation level to the valence state of MoO(Figure 2b). 
This study provides a deeper insight into the development of high-voltage cathode materials with improved electronic conductivity: tailoring the olivine structure of LCP with the orthophosphate structure of LCPO and coupling the system to a MoOcoating provides an enhancement of the electronic conductivity of the system, and a complete restoration of the electronic properties upon limited time electrochemical cycles.

Figure 2

Figure 2.  Electronic level energy diagrams of (a) the LCP-LCPO cathode/electrolyte interface and (b)the MoO3 coating/electrolyte interface, as deduced from the PES. Adapted from ACS Appl. Mater. Interfaces 2022, 14, 1, 543–556. Copyright 2022 American Chemical Society.


This research was conducted by the following research team:

Gennady Cherkashinin1, Robert Eilhardt1,Silvia Nappini2,Matteo Cococcioni3,Igor Píš2,4,Simone dal Zilio2,Federica Bondino2Nicola Marzari5Elena Magnano2,6and Lambert Alff1


Institute of Materials Science, Technische Universität Darmstadt, Darmstadt, Germany
IOM CNR Laboratorio TASC, Trieste, Italy
Physics Department, University of Pavia, Pavia, Italy 
Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy 
5  Theory and Simulation of Materials (THEOS), and National Centre for Computational Design and Discovery of Novel Materials (MARVEL), Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland 
6  Department of Physics, University of Johannesburg, Johannesburg, South Africa

Contact persons:

GennadyCherkashinin, e-mail:  


G. Cherkashinin, R. Eilhardt, S. Nappini, M. Cococcioni, I. Píš, S. dal Zilio, F. Bondino, N. Marzari, E. Magnano, and L. Alff, “Energy Level Alignment at the Cobalt Phosphate/Electrolyte Interface: Intrinsic Stability vs Interfacial Chemical Reactions in 5 V Lithium Ion Batteries”, ACS Appl. Mater. Interfaces 14, 543 (2022), DOI: 10.1021/acsami.1c16296

Last Updated on Monday, 21 February 2022 17:56