Realizing Two-Electron Transfer in Ni(OH)₂ Nanosheets for Energy Storage

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⁺ (t_2g⁶e_g¹)-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. <i>In situ</i> 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.