Discrete Metal-Oxide Clusters with Organofunctionalization as High-Performance Anode Materials

Grafting of organic groups on the surface of metal-oxide clusters greatly enhances the Li+ ion capacity. Triply organo-substituted polyoxometalates (RN-POMs) display an excellent Li+ ion capacity (∼1900 mA h g-1 at 0.134 C; ∼1300 mA h g-1 at 1.07 C) and diffusivity (1.36 × 10-11 cm2 s-1) as an anode material after a high-rate lithiation activation process. Employment of the RN-POMs also mitigates chemical and mechanical degradations, which impair the cycling stability of the electrode materials. Molecular structures of the RN-POMs exhibit 2 terminal alkylimido and 1 bridging aminopyridyl groups on the exterior of the oxometallic core, which create a functional void for the Li+ transportation and storage. The organofunctionalization on the RN-POMs exerts structural and electronic influences compared to the bare POM: larger effective surface areas and pore volume, lower charge-transfer resistance, higher specific capacity, and better cycling performance. The specific capacity and conductivity of the anode materials fabricated with the RN-POMs are further greatly enhanced when subjected to the high-rate lithiation activation process. The capacity performance is increased by more than 300%, the Li+ ion diffusivity rate is improved by 2 orders, and the charge-transfer resistance is decreased by more than 60% to 25.9 ω. For comparison, unstable cycling performance and low capacity are observed for the bare POM. The results suggest that the employment of the RN-POMs offers several advantages over conventional two-dimensional anode materials. Material instability resulting from the volume expansion is effectively mitigated. Higher conductivity, accommodation of more Li, and faster Li+ diffusion kinetics are achieved. This work demonstrates that organofunctionalization on discrete oxometallic clusters opens a new avenue to the design of electrode materials for high-performance lithium-ion batteries.