Experimental & Computational Evidence for Reduced Surface Layer Trapping Li-ions in Ni-rich Li-ion Battery Cathodes
Authors: Shah Nickil Ajit , Menon Ashok S. , Paez Fajardo Galo , Ans Muhammad , Loveridge Melanie J , Piper Louis F. 
Published: December 2024

Lithium-ion batteries have proved their dominance in the mobile vehicle market; however, their energy densities need to be increased to facilitate a swifter transition to a greener future.[1] Of the promising materials, nickel-rich layered oxide cathodes emerge as a key contender, where their operation at high voltages allows access to higher capacities, providing an overall increase to the W⋅h/kg.[2] However, at high voltages, degradation in the form of oxygen-loss from the cathode arises for layered oxides.[3] For NMC811, an emerging cathode material, this effect is especially prominent as a mode of suspected degradation when operated under wider voltage windows (2.5–4.4 V). The nature of these species evolved at the surface and their contribution to electrode degradation, are not yet fully understood, and more details need to be uncovered to reliably access higher energy densities in current and next generation cathode materials.
Here, we report the use of O K-edge soft X-ray absorption (sXAS) measurements to probe into species variation at the cathode’s surface for single crystal NMC811 full cells cycled at voltages below and above their oxygen loss threshold. The results confirm that operation at high voltage (above oxygen loss) results in the evolution of a layer forming at the surface. Our calculations using Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD) have allowed us to confirm the nature of this surface being comprised of a reduced nickel oxygen species as an Ni-O ‘rock-salt’ type layer (RSL). We coupled these results with surface measurements using hard X-ray photoelectron spectroscopy (HAXPES) and observed trapped lithium for cells in their charged state, suggesting there is a kinetic barrier hindering their mobility.
We then proceeded to understand the relationship of this trapped lithium to the bulk structure using operando XRD measurements. Here, we show that structural fatigue is generated in NMC811 as a consequence of high voltage cycling. This feature of fatigue was evident from the evolution of the 003 reflection from the NMC phase and was seen to develop with prolonged cycling (Fig. 1b & c). This ‘fatigue’ feature in literature has been attributed to trapped lithium in the bulk and has only been observed after long-term cycling.[4] This feature is known to be associated to a reconstructed part of the bulk structure with poor lithium kinetics[4] and here, we observe this feature as early as the first cycle which we attribute to the formation of the RSL. Mitigating the formation of this RSL layer will therefore reduce the kinetic capacity loss, thus this discovery proves promising towards engineering future cathodes with higher utilisable capacities.
[1] Drive to electrify. Nature Climate Change, 14, (2024), 299.
[2] W. Li, E. M. Erickson, A. Manthiram, High-nickel layered oxide cathodes for lithium-based automotive batteries, Nature Energy, 5, (2020), 26–34.
[3] H. Zhang, H. Liu, L. F. J. Piper. M. S. Whittingham, G. Zhou, Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation, Chemical Review, 122, (2022), 5641–5681.
[4] C. Xu, K. Märker, J. Lee, A. Mahadevegowda, P. J. Reeves, S. J. Day, M. F. Groh, S. P. Emge,C. Ducati, B. L. Mehdi, C. C. Tang, C. P. Grey, Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries, Nature Materials, 1, (2021), 84–92.
[5] A. S. Menon, N. Shah, J.A. Gott, E. Fiamegkou, M. J. W. Ogley, G. J. P. Fajardo, N. Vaenas, I. Ellis, N. Ravichandran, P. Cloetens, D. Karpov, J.M. Warnett, P. Malliband, D. Walker, G. West, M. Loveridge, L. F. J. Piper, Quantifying Electrochemical Degradation in Single-Crystalline LiNi0.8Mn0.1Co0.1O2–Graphite Pouch Cells through Operando X-Ray and Postmortem Investigations, PRX Energy 3, (2024) 013004.

Figure 1