Enhancing long-term stability and energy/power density of sodium ion battery through fluorination-induced electron density engineering in covalent organic nanosheets

The structure-related performance of covalent organic nanosheets (CONs) as anode materials in sodium-ion batteries is investigated by the introduction of an electron-withdrawing atom such as fluorine in the unit structure of CON. Then, strong permanent dipoles induced by fluorine are well preserved through the CON framework; these dipoles can provide a large number of accessible sites for Na⁺ on the self-assembled frameworks. The effect of the dipole on the Na⁺ storage capacity of fluorinated CON-37 is demonstrated by subsequently comparing that of non-fluorinated CON-35 with respect to various electrochemical behaviors. As a result, the interaction between Na⁺ and the π framework of CON-37 is reduced with respect to CON-35 as fluorine could withdraw the electron density of the framework. Therefore, the charging/discharging capacity of CON-37 can be improved as a result of better ion and charge carrier conductivity with high stability for up to 2500 subsequent cycles. Furthermore, its chemical structure is extremely well maintained for up to 2500 cycles. The CON-37 electrode exhibits stable cycling performance and a rate capability, maintaining a highly improved reversible discharge capacity of ∼ 350 mA h g⁻¹ for up to 2500 cycles at a current density of 100 mA g^−1.