Long-duration operando study of co-free single- and poly- crystalline LiNiO2 cathodes for high energy dense applications

Li-ion batteries (LIBs) have become the most popular power source for high-energy- density applications such as electric vehicles (EVs) due to the increasing demand for renewable energy storage systems. LiCoO2 (LCO), a commercial LIB cathode material, exhibits good structural stability. However, the practical reversible capacity of LCO is limited to ~150 mAh/g (at 4.2 V vs. graphite), which has led the search for more advanced alternatives. Among these, Co-free lithium nickel oxide (LiNiO23LNO) and its derivatives have identified as alternative promising materials for next-generation LIBs. LNO has a layered structure, and it offers a higher practical capacity of >200 mAh/g at 4.2 V vs. graphite and Li/Li+. However, despite its potential, LNO continues to face several challenges, such as cation mixing during the synthesis, surface oxygen loss and structural instability at high states of delithiation, which contribute to capacity fading over extended cycling. Addressing these degradation pathways is essential to unlocking the full potential of LNO, so it can deliver high capacity and reliable long-term performance for future energy storage systems. This thesis highlights the degradation mechanisms (involved at surface and bulk) of LNO cathode materials in high-mass-loading graphite cells, using advanced in-house and synchrotron characterisation techniques. The cells underwent stressful voltage conditions using constant-current constant-voltage (CCCV) protocols, which exhibited degradation in LNO cathode and increase the overall cell impedance. Through operando X-ray diffraction (XRD) and X-ray absorption (XAS), it was revealed that the bulk structure and electronic environment of LNO remained largely unaffected, even after stressful conditions (2.534.4 V vs. graphite) and prolonged 100 cycles. Ex-situ microscopic analysis further confirmed the absence of particle fractures in aged single-crystalline LNO cathodes. However, surface-sensitive soft XAS data uncovered the reconstruction of a dense, NiO-like rock-salt phase at the cathode surface in the aged cell. This reconstructed phase acts as a kinetic barrier to Li+ diffusion that increase cell impedance, charge transfer resistance, and ultimately leading to capacity fade. To overcome degradation issues, a thin conformal alumina (AlxOy) coating was applied onto LNO powder via particle atomic layer deposition (P-ALD). The electrochemical performance of ALD-coated samples in graphite and Li/half cells demonstrated improved cycling stability over long-term cycling at a C/3 rate compared to uncoated samples. Operando XRD data demonstrated the reversibility of phase transitions after extended cycling in ALD- coated cathode, demonstrating the preservation of the bulk layered structure. Postmortem surface sensitive soft X-ray absorption spectroscopy data revealed that ALD coatings provide an effective approach to inhibit surface rocksalt reconstruction (a barrier to Li+ transport) and enhanced long-term electrochemical performance of the cathode. In addition, the alternative strategies to surface coatings, dual doping with tungsten (W) and magnesium (Mg) were introduced in the LNO cathode using one-step synthesis process. The electrochemical performance of the resulting dual-doped LNO material was carried out in coin cells and 3- electrode El-cells. After 200 cycles, the dual-doped LNO cathodes (vs. graphite) exhibited improved cycling stability, better Li+ kinetics and reversible phase transitions compared to undoped counterparts. These improvements suggest that the dopants effectively stabilised the cathode during long-term cycling and resist against particle cracking issues.