Fluorine-Stabilized β-Nickel Hydroxides: Composition, Structural Features, and Electrochemical Properties

Most commercial Ni-based alkaline batteries use β-Ni(OH)₂ as the cathode material. These batteries are particularly attractive due to the use of low-cost materials and nonflammable aqueous electrolytes, which enhance safety and recyclability. However, these electrodes suffer from structural instability during charge-discharge cycles caused by phase transitions between β-Ni(OH)₂/β-NiOOH and α-Ni(OH)₂/γ-NiOOH redox couples, leading to capacity fading over time. Therefore, stabilizing the β polymorph remains a significant challenge. In this study, nickel hydroxyfluorides (Ni(OH)_2-xF_x) were synthesized using microwave-assisted hydrothermal routes. It was possible to increase the fluorine content up to 0.48 while maintaining the β-Ni(OH)₂ structural type. Powder X-ray diffraction and vibrational spectroscopy (FTIR, Raman) revealed substantial structural variations with fluorine incorporation. The highly electronegative fluorine plays a crucial role in enhancing both intraslab interactions, by increasing lattice rigidity, and interslab interactions, by affecting the level of the O-H bonds and H-H electrostatic repulsion. UV-vis-NIR spectroscopy confirmed the increased bond ionicity of the fluorinated samples, evidenced by an optical bandgap opening, a decrease of crystal field stabilization energy, and a slight increase of the Racah parameter. Thermogravimetric analysis showed a gradual increase in the decomposition temperature with higher fluorine content. Interestingly, the thermal decomposition of nickel hydroxyfluorides resulted in a novel synthesis method for nickel oxyfluorides (Ni_1-(y/2) □_y/2O_1-yF_y), stabilizing nickel vacancies. Galvanostatic cycling tests indicated a significant capacity decrease in the fluorine-containing samples. To further understand the mechanisms behind the reduced electrochemical performance, chemical oxidation tests were conducted. These tests demonstrated the inability of the fluorinated samples to be oxidized, suggesting that the high electronegativity of fluorine prevents nickel oxidation.