Amorphous, Highly Disordered Carbon Fluorides as a Novel Cathode for Sodium Secondary Batteries

A strategy for enabling a novel carbon fluoride (CF_x) to be a high-performance cathode material for sodium batteries is proposed and realized. An amorphous and highly disordered CF_x, denoted as d-CF_x, is prepared through a facile synthesis route. Herein we report on a Na/d-CF_x cell reversibly discharging/charging at such a low overpotential and capacity-decay rate. The polarization of the Na/d-CF_x cell is about 780 mV, the lowest value reported in the state-of-the-art Na/CF_x system. The initial discharge capacity is 582.5 mA h/g, with a capacity of 412.5 mA h/g after 12 cycles. The designed d-CF_x improves the Na⁺ diffusion rate and facilitates the reaction with larger sodium ions during the discharging process. Amorphous discharge products NaF are easily decomposed, enhancing the charge reaction. The small grain size of NaF is believed to be another key factor that enables the minimal charge gap of Na/d-CF_x cells and a slower capacity-decay rate, which can facilitate the decomposition of NaF and the reformation of C–F bonds during the charging process. Generally speaking, this study encourages the further exploration of secondary sodium/CF_x batteries, and the cyclability of the sodium anode is included.