Mitigating Capacity Degradation and Surface Oxygen Loss of Single-Crystalline LiNiO2 Cathodes Via Al2O3 Surface Coating
Authors: Ans Muhammad , Bolloju Satish , Menon Ashok S. , Melgari Paolo , Paez Fajardo Galo , Piper Louis F. 
Published: December 2024

The performance of next-generation Li-ion batteries (LIBs) for electric vehicle applications relies on energy-dense and low-cost lithium-ion chemistries. Single-crystalline LiNiO2 (SC-LNO) cathodes are regarded as promising materials for high-energy-density applications owing to their high practical capacity, resistance to particle cracking and competitive cost [1]. However, under high voltage operation (≥ 4.4 vs. graphite), they experience rapid capacity loss, owing to parasitic electrode-electrolyte reactions expedited by surface oxygen loss [2]. Overcoming this, stabilising the long-duration performance of SC-LNO cathodes to operate at high voltages while maintaining capacity retention is critical for their integration into commercial batteries. Surface engineering by coating could provide an effective approach to tune the electrochemistry of Ni-rich cathodes while inhibiting degradation pathways [3,4]. Aluminium oxide (Al2O3) coating is used as a barrier to eliminate highly reactive electrolyte species (HF) from the cathode surface, thereby enhancing long-term stability and improving high voltage performance [5,6]. It offers a viable strategy to control the surface of SC-LNO from O loss (O-n, n > 2) and protect it from electrolyte attack. In this study, we employ an ultrathin and conformal film of AlOx on SC-LNO powder via Powder Atomic Layer Deposition (P-ALD). To minimise moisture exposure, the slurry and casting processes were performed in a controlled dry room environment. Electrochemical testing was carried out in balanced coin cells (vs. commercial graphite) at 25 oC and cycled between a voltage range of 2.5–4.2 V and 2.5–4.4 V. Following two formation cycles performed at a C/20 rate, the cells were cycled 100 times at C/3 rate. Electrochemical impedance spectroscopy measurements were also performed before and after 100 cycles to assess any impedance changes.

The electrochemical data revealed that AlOx coated SC-LNO cathodes provide significantly higher capacity retention (94.5%) than uncoated ones (66.3%) after 50 cycles at 4.4 V (vs. graphite). Regarding the rate capability test, the coated cathode delivers improved and stable capacities at faster rates compared with uncoated SC-LNO cathodes. Online electrochemical mass spectroscopy (OEMS) analysis of the AlOx coated cathodes indicates that the onset potential for oxygen loss related gas evolution is > 4.4 V vs. Li/Li+. In contrast, for the uncoated SC-LNO sample, this was found to be ~4.2 V. Hence, the AlOx coating serves as a protective layer, for the cathode against electrolyte-induced degradation and transition metal dissolution at high voltage by forming Al-O-F species at the surface [6]. This beneficial interaction enhances the electrochemical performance by effectively inhibiting impedance growth and improving the cycling life and stability of SC-LNO cathodes. This study highlights the crucial role of AlOx coatings in mitigating surface oxygen loss, which is a key factor contributing to the deterioration of cell performance and issues related to Li+ kinetics. Our findings significantly contribute to advancing the development of high-performance SC-LNO cathode materials, offering valuable insights into the application of ALD coatings for enhancing the performance of Li-ion battery cells.

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