Fundamentals of Ion‐Exchange Synthesis and Its Implications in Layered Oxide Cathodes: Recent Advances and Perspective

Abstract

Layered oxide cathodes such as Ni-rich ternary and Li-rich layered cathode materials have been widely used for lithium-ion batteries owing to their excellent Li + transport properties, high energy density, and relatively low cost. However, such layered cathode materials synthesized by high-temperature sintering face inherent issues such as low structural stability, irreversible migration of transition metal ions, and irreversible redox reactions of oxygen anions. To make a breakthrough from the perspective of material synthesis, a new ion-exchange synthesis has emerged in recent years, which is a promising strategy for synthesis of Li-ion cathodes. Herein, the fundamentals of ion-exchange synthesis and their implications in layered oxide cathodes for lithium-ion batteries is presented. Specifically, ion-exchange synthesis and mechanisms of ion exchange are introduced in detail, followed by a discussion of the reduction of synthetic temperature, the synthesis of novel crystal structures, the inhibited migration of transition metal ions, the increased reversibility of anionic redox, and the optimized surface reconstruction. Finally, a summary and outlook is provided for ion-exchange synthesis of layered oxide cathodes for lithium-ion batteries. It is anticipated that this ion-exchange synthesis will facilitate the commercialization of high-performance cathode materials for next generation Li-ion batteries.

Traditional Electrochemical Zn2+ Intercalation/Extraction Mechanism Revisited: Unveiling Ion‐Exchange Mediated Irreversible Zn2+ Intercalation for the δ‐MnO2 Cathode in Aqueous Zn Ion Batteries

“…Ion exchange involves the process of replacement between an insoluble solid with an exchangeable ion and an ion of the same charge in solution. [ 50 ] Essentially, it is the process of ion replacement reactions between the ions in solution and the exchangeable ions in the ion exchanger. The ion exchange process can be described: $$$$\begin{equation}{{\mathrm{A}}}^ + {{\mathrm{X}}}^ - \left( {{\mathrm{solid}}} \right) + {{\mathrm{B}}}^ + {{\mathrm{Y}}}^ - \left( {{\mathrm{solution}}} \right) \to {{\mathrm{B}}}^ + {{\mathrm{X}}}^ - \left( {{\mathrm{solid}}} \right) + {{\mathrm{A}}}^ + {{\mathrm{Y}}}^ - \left( {{\mathrm{solution}}} \right)\end{equation}$$$$…”

Li/Ni Intermixing: The Real Origin of Lattice Oxygen Stability in Co‐Free Ni‐Rich Cathode Materials

“…The capacity decrease and the kinetics hindrance of NM90 can be attributed to the electrochemical inert and hindrance effect of intermixed Ni 2+ . [ 6,35 ] Then, tested at 55 °C, it is worth noticing that the NM90 delivers a higher discharge capacity of 228.9 mAh g −1 than the NCM90 of 225.9 mAh g −1 in Figure 1d. The increase of their capacity can be attributed to the increase in both electronic and ionic conductivity at an elevated temperature.…”

“…[ 1–3 ] However, the representative high‐capacity Ni‐rich cathodes still suffer from the huge challenges of fast electrochemical failure and thermal instability. [ 4–6 ] These challenges can be attributed to their poor structural stability, such as irreversible phase transitions, mechanical instability and gas generation. [ 7–9 ] It is widely accepted that these degradation are mainly driven by the loss of lattice oxygen.…”

Low‐Electronegativity Cationic High‐Entropy Doping to Trigger Stable Anion Redox Activity for High‐Ni Co‐Free Layered Cathodes in Li‐Ion Batteries

“…Furthermore, when high-Ni cathode is heated in the in the delithiated state, Ni migrates into Li layer even more easily, and the extensive absence of cations will cause the collapse of the TM layer (bulk decomposition) and subsequent O 2 release, leading to poor thermal stability. [5,6] What is regretted is the current doping strategies have not resolved these issues well. [7][8][9][10][11][12][13][14][15][16] Because of the structure instability caused by high Ni content, the further improvement of the specific capacity cannot rely entirely on the higher Ni content (> 80 %) that directly corresponds to the higher capacity (> 185.8 mAh g À 1 at 2.5-4.5 V, 0.5 C).…”