Use of zeolite-type additives in solid-state Na battery prototypes with enhanced low-temperature performance

Rechargeable solid-state sodium batteries are considered important high-energy next-generation rechargeable batteries. Sodium, being abundant and affordable, can be combined with safe and stable solid electrolytes, addressing safety issues encountered with high-energy batteries based on active metal anodes like Li and Na, which employ liquid aprotic electrolyte solutions. Solid electrolytes based on polymeric matrices provide flexibility and good adhesion to the electrodes without requiring external pressure during battery manufacturing and cycling. However, one drawback is the low conductivity at room temperature and high impedance at the interfaces. Here, we report on the development of Na||NVP solid batteries based on composite polymer electrolytes containing PEO:NaFSI matrices in which particles containing Na-X type zeolite, which has a stoichiometry of Na₂[(SiO₂)_2.5(AlO₂)], and approximately 20 % Na content by weigh are embedded. The presence of the zeolite additive significantly improves the electrochemical parameters of the Na||NVP cells, including Na ionic conductivity, salt diffusion coefficient, electrochemical window, and interfacial properties. The optimal concentration of the zeolite-X embedded in PEO:NaFSI was 3 % by weight. Batteries containing this composite solid electrolyte exhibited 83 mAh/g (of the cathode) with ∼ 99 % stability and high coulombic efficiency after over 500 charge/discharge cycles at 40 °C (1C).