Abstract
Ni-rich layered transition-metal oxides with high specific capacity and energy density are regarded as one of the most promising cathode materials for next generation lithium-ion batteries. However, the notorious surface impurities and high air sensitivity of Ni-rich layered oxides remain great challenges for its large-scale application. In this respect, surface impurities are mainly derived from excessive Li addition to reduce the Li/Ni mixing degree and to compensate for the Li volatilization during sintering. Owing to the high sensitivity to moisture and CO2 in ambient air, the Ni-rich layered oxides are prone to form residual lithium compounds (e.g. LiOH and Li2CO3) on the surface, subsequently engendering the detrimental subsurface phase transformation. Consequently, Ni-rich layered oxides often have inferior storage and processing performance. More seriously, the residual lithium compounds increase the cell polarization, as well as aggravate battery swelling during long-term cycling. This review focuses on the origin and evolution of residual lithium compounds. Moreover, the negative effects of residual lithium compounds on storage performance, processing performance and electrochemical performance are discussed in detail. Finally, the feasible solutions and future prospects on how to reduce or even eliminate residual lithium compounds are proposed.
Highlights
LiCoO2 is one of the earliest successfully commercialized cathode materials, with a low energy density, high cost and toxicity, it is not suitable to be applied as a power battery material (Lu et al, 2019; Xian et al, 2020; Cheng et al 2020)
LiPF6 is adopted as a slurry additive to wipe off alkaline residual lithium compounds in NCM811 slurry, forming into LiF and Li3PO4 (Zhang et al, 2019d)
The origins and negative effects of residual lithium compounds on air storage performance, processing performance, and electrochemical performance of Ni-rich cathode materials were analyzed
Summary
LiCoO2 is one of the earliest successfully commercialized cathode materials, with a low energy density, high cost and toxicity, it is not suitable to be applied as a power battery material (Lu et al, 2019; Xian et al, 2020; Cheng et al 2020). The external Ni3+ ions with high chemical activity have a tendency to be reduced into Ni2+, together with the lattice oxygen release, the growth of residual lithium compounds, and the surface phase transition during air exposure (Jo et al, 2014b; Tian et al, 2018; Yang et al, 2019a). During the electrode slurry preparation, the high alkaline Ni-rich cathode materials (pH ≈ 11) will give rise to polyvinylidene fluoride (PVDF) degradation and slurry gelation, worsening the processability of the electrode slurry (Ross et al, 2000) Another issue is that due to the intrinsic insulation of surface impurities, the Li+ diffusion is seriously restricted, leading to the increased cell polarization and inferior cycling performance (Chen et al, 2019; Wang et al, 2019). Understanding the origin of residual lithium compounds and the possible solutions of how to remove them appear to be necessary
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