Ni-rich layered oxide materials have been considered as promising cathode materials for high energy density Li-ion batteries because of their high reversible capacity. One of their catastrophic failure modes is the formation of residual lithium compounds on the oxide surface when it is exposed to air. In this paper, it is demonstrated that water is essential for the formation of residual lithium at room temperature. Furthermore, hydrophobic LiNi 0.8 Co 0.1 Mn 0.1 O 2 is introduced to suppress the formation of residual lithium because the hydrophobic surface inhibits contact between water and LiNi 0.8 Co 0.1 Mn 0.1 O 2 . Hydrophobic LiNi 0.8 Co 0.1 Mn 0.1 O 2 is obtained through surface engineering using hydrophobic organic molecules, such as polydimethylsiloxane. Hydrophobic polydimethylsiloxane-grafted LiNi 0.8 Co 0.1 Mn 0.1 O 2 suppresses the formation of residual lithium even in humid air, leading to the negligible surface degradation of LiNi 0.8 Co 0.1 Mn 0.1 O 2 . As a result, hydrophobic LiNi 0.8 Co 0.1 Mn 0.1 O 2 shows excellent electrochemical performance even after storage in humid air for 2 weeks.

LiNiO 2 is a promising cathode material for lithium ion batteries because of its high specific capacity (approximately 220 mA h g −1 ). However, there are several challenging issues in the development of LiNiO 2 , including its poor cycle and rate performance because of its structural deterioration due to thermodynamically unstable Ni 3+ . This paper demonstrates the role of Na + in the electrochemical performance and structural stability of [Li 1-x Na x ]NiO 2 (x = 0, 0.005, 0.01, 0.025, and 0.05). Charge disproportionation Ni 3+ → Ni 2+ and Ni 4+ in LiNiO 2 increases the cation mixing of Li + and Ni 2+ during cycling, resulting in the poor cycle performance of LiNiO 2 . However, Na + in [Li 1-x Na x ]NiO 2 mitigates the charge disproportionation because of the larger size of Na + than Li + , leading to the improved structural stability of [Li 1-x Na x ]NiO 2 . Consequently, Na + -doped LiNiO 2 alleviates the increase in the cation mixing of Li + and Ni 2+ during cycling compared to bare LiNiO 2 . This results in the improved cycle performance of [Li 1-x Na x ]NiO 2 (x = 0.05), such as approximately 76% of capacity retention after 100 cycles. Moreover, the substitution of Li + with Na + in LiNiO 2 improves the storage characteristics of [Li 1-x Na x ]NiO 2 , leading to a negligible capacity loss even after long-term storage.

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Crack formation is considered one of the significant failure modes of layered oxide cathode materials for Na-ion batteries because particle cracks accelerate electrolyte decomposition, transition metal dissolution, and electrical contact...