Correlation between Surface Reactions and Electrochemical Performance of Al₂O₃‐ and CeO₂‐Coated NCM Thin Film Cathodes

Abstract

Depositing ultrathin oxide coatings has been proven a successful approach to stabilize the surface of LiNixCoyMnzO2 active cathode material in lithium‐ion batteries (LIB). The beneficial effect of Al2O3 coatings arises at least partly from spontaneous reactions between coating and liquid electrolyte. However, it remains unclear if comparable surface reactions occur for other oxide coatings. One difficulty is the characterization of reaction products at the cathode–electrolyte interface due to the multi‐phase properties of composite cathodes. Here, thin films are utilized as model systems to correlate surface reactions with the performance of Al2O3‐ and CeO2‐coated nickel cobalt manganese oxides (NCM). Electrochemical characterization confirms that an Al2O3 coating improves long‐term cycling stability, while CeO2‐coated thin films perform even worse than uncoated counterparts. The analysis of the surface reaction products using X‐ray photoelectron spectroscopy shows that both coatings are fluorinated upon contact with liquid electrolyte in agreement with thermodynamic considerations. The fluorinated Al2O3 coating is stable during cycling, resulting in the improved cell performance. In contrast, the fluorinated CeO2 coating changes chemical composition, facilitating corrosion of the NCM surface. The results demonstrate the importance of a detailed analysis of surface reactions to evaluate the suitability of ultrathin oxide layers as protective coatings for LIBs.

Operando Dissolution of RF Magnetron-Sputtered Thin-Film LiCoO2 Cathodes during Galvanostatic Cycling: In Situ Evaluation of Dissolution and Performance Data

“…This is likely due to polarization issues across the electrode surface, as the mainly two-dimensional thin-film surface provides heterogeneous Li + diffusion pathways in comparison to the three-dimensional composite LCO electrode. 45 The overall dissolution mechanism changes only slightly for the thin-film cathodes so that each step of the cycling protocol exhibits a similar dissolution across several cycles. Composite cathodes, on the other hand, experience a significant change in their dissolution behavior.…”

“…Additionally, the tf-LCO sample tolerates the asymmetric electrode configuration of the flow cell (the geometric area of the cathode is much larger than the tip area of the counter electrode), which is impossible to accomplish with composite electrodes. This is likely due to polarization issues across the electrode surface, as the mainly two-dimensional thin-film surface provides heterogeneous Li + diffusion pathways in comparison to the three-dimensional composite LCO electrode . The overall dissolution mechanism changes only slightly for the thin-film cathodes so that each step of the cycling protocol exhibits a similar dissolution across several cycles.…”

“…As the relative dissolved amounts are comparable to literature values for c-LCO, we suggest that the compact, 2-dimensional morphology of the thin-film electrode enables facilitated dissolution of transition metals. For instance, improved Li-ion diffusion has been reported for electrode thin-films, 45 due to a decreased path length (a consequence of the film thickness and the absence of porous particles and grain boundaries). A similar effect might well be possible for the transition metal dissolution mechanism in that the thin-film morphology does not interfere with the release of TM ions.…”

Rational Design of Multifunctional Surface Modification for Ni-Rich Layered Cathodes of Lithium-Ion Batteries

“…Many strategies have been attempted in the literature to alleviate the hurdles of Ni-rich LNCM cathode materials, such as (1) incorporating surface coating layers; , (2) use of electrolyte additives; , (3) use of dopant-functionalized cathode materials; , and (4) development of single-crystalline LNCM cathode materials. , It is worth mentioning that the surface coating approach is based on the formation of cathode–electrolyte interphase (CEI) layers at the LNCM interface. Since CEI layers have low electronic conductivity but high ionic conductivity, they can suppress decomposition of the electrolytes by preventing contact between the electrolyte and LNCM cathode. , Precursors with low electrical conductivity, such as Al 2 O 3 , SiO 2 , and Cr 2 O 3 , have been reported to be effective at forming the CEI layers.…”

“…24,25 Moreover, fluoride species (a byproduct of lithium salt in the electrolyte) triggers metal dissolution through a chemical reaction in the cell, leading to irreversible degradation of Ni-rich LNCM cathode materials. 26,27 Since these undesired reactions immediately arise from the unstable interfacial property between the Ni-rich LNCM cathode and electrolyte, stabilizing the interfacial property of Ni-rich LNCM cathode materials has been recognized as the optimal approach to increase the lifespan of the cell.…”