Yufeng Xue

The theoretical capacity of a given electrode material is ultimately determined by the number of electrons transferred in each redox center. The design of multi-electron transfer processes could break through the limitation of one-electron transfer and multiply the total capacity but is difficult to achieve because multiple electron transfer processes are generally thermodynamically and kinetically more complex. Here, we report the discovery of two-electron transfer in monolayer Ni(OH)2 nanosheets, which contrasts with the traditional one-electron transfer found in multilayer materials. First-principles calculations predict that the first oxidation process Ni2+ → Ni3+ occurs easily, whereas the second electron transfer in Ni3+ → Ni4+ is strongly hindered in multilayer materials by both the interlayer hydrogen bonds and the domain H structure induced by the Jahn–Teller distortion of the Ni3+ (t2g6eg1)-centered octahedra. In contrast, the second electron transfer can easily occur in monolayers because all H atoms are fully exposed. Experimentally, the as-prepared monolayer is found to deliver an exceptional redox capacity of ∼576 mA h/g, nearly 2 times the theoretical capacity of one-electron processes. In situ experiments demonstrate that monolayer Ni(OH)2 can transfer two electrons and most Ni ions transform into Ni4+ during the charging process, whereas bulk Ni(OH)2 can only be transformed partially. Our work reveals a new redox reaction mechanism in atomically thin Ni(OH)2 nanosheets and suggests a promising path toward tuning the electron transfer numbers to multiply the capacity of the relevant energy storage materials.

The substantial capacity gap between available anode and cathode materials for commercial Li-ion batteries (LiBs) remains, as of today, an unsolved problem. Oxygen vacancies (OVs) can promote Li-ion diffusion, reduce the charge transfer resistance, and improve the capacity and rate performance of LiBs. However, OVs can also lead to accelerated degradation of the cathode material structure, and from there, of the battery performance. Understanding the role of OVs for the performance of layered lithium transition metal oxides holds great promise and potential for the development of next generation cathode materials. This review summarises some of the most recent and exciting progress made on the understanding and control of OVs in cathode materials for Li-ion battery, focusing primarily on Li-rich layered oxides. Recent successes and residual unsolved challenges are presented and discussed to stimulate further interest and research in harnessing OVs towards next generation oxide-based cathode materials.

Ni oxides and oxyhydroxides (NiOx) have been studied for a long time as cathode materials for alkaline batteries and electrocatalysts for the oxygen evolution reaction (OER). Yet, understanding of the connection between their atomic and electronic structures and electrochemical performance or stability is still incomplete. In this work, we use first-principles density functional theory (DFT) calculations to revisit the structure, electronic properties, and OER activity of β-NiOOH, the catalytically active phase of NiOx. Following extensive DFT-based screening, we identify a hitherto overlooked structure characterized by a uniform distribution of H atoms on the NiO2 layers. All the Ni3+ cations in this structure exhibit an identical tg6eg1 electronic configuration with an occupied 3dz2 orbital. Comparison of the calculated bond lengths with extended X-ray absorption fine structure (EXAFS) data unequivocally supports this structure relative to all other low-energy configurations. Based on this structure, we uncover and detail defect-dominated OER mechanisms on the basal β-NiOOH (001) surface, with overpotentials as low as 0.39 V. The present results should provide a valuable contribution to ongoing efforts for understanding and developing enhanced transition-metal hydroxide catalysts for the OER.