Fluorinated and safety-reinforced copolymer solid electrolytes for high-voltage all-solid-state sodium metal batteries

Abstract

Poly (ethylene oxide)-based all-solid-state electrolytes are extensively regarded as highly promising contenders for the upcoming era of sodium metal batteries. Nevertheless, the intrinsic elevated highest occupied molecular orbital (HOMO) and the strong sodium-ion adsorption energy lead to a narrow electrochemical stability window (ESW) and a sluggish ion transport kinetics, thus retarding its widespread application in high-voltage batteries. About these shortcomings, we propose a strategy of fluorine substitution through a ready UV-initiated free-radical copolymerization to regulate the orbital level sodium-ion adsorption energy of poly (ethylene oxide) electrolytes. The characterization confirmed the success in the introduction of fluorine into the polymer (referred to as P-co-F). DFT calculations demonstrated a lower HOMO (-7.061 eV) in fluorine-substituted polymer electrolyte, which enabled to expand the ESW up to 5.55 V. Besides, the sodium-ion adsorption energy on P-co-F was decreased from −2.78 eV to −3.43 eV, facilitating the rapid transport of sodium ion. In addition, Polyethylene terephthalate (PET) was adopted as a support frame, which would reinforce the mechanical properties of polymer electrolytes. The electrochemical tests suggested P-co-F exhibited a profound interfacial stability in the Na//Na symmetrical system throughout an operational duration of 4690 h at 0.1 mA cm−2. The assembled all-solid-state battery manifested remarkable stability during continuous cycling at room temperature for 200 cycles, retaining an impressive 96.59 % of its initial capacity at 0.1C. The intriguing fluorine-induced effect, coupled with a cost-effective safety substrate, will provide valuable insights for the rational molecular design of innovative electrolytes and easy production scalability for all-solid-state Na metal batteries.