Scalable Glass-Fiber-Polymer Composite Solid Electrolytes for Solid-State Sodium-Metal Batteries

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10-4 S cm-1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm-2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g-1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

Polycarbonate-Based Solid-Polymer Electrolytes for Solid-State Sodium Batteries

Solid-polymer electrolytes comprised of polypropylene carbonate (PPC) and varied sodium bis(fluorosulfonyl)imide (NaFSI) salt concentrations are investigated for implementation as a conductive solid polymer electrolyte into solid-state cathode composites utilizing a sodium-layered oxide active material. The ionic conductivity generally increases with NaFSI salt content, reaching ≈1 mS cm-1 at 80 °C at the highest salt concentration (PPC:NaFSI = 0.5:1). Through an all-in-one slurry casting method, Na2/3Ni1/3Mn2/3O2 cathode composites are fabricated in which the dispersed PPC electrolyte acts as the primary binder. Enabled by a bilayer polymer electrolyte system, cycling performance with the PPC cathode electrolyte is optimized with respect to salt concentration and anode material. The best cyclability is achieved with a moderate salt concentration electrolyte (PPC:NaFSI = 5:1), showcasing an initial capacity of 83 mA h g-1 with a remarkable 80% capacity retention after 150 cycles at C/5 rate and 60 °C. The superior performance of the lower salt concentration electrolyte is attributed to better electrochemical stability, as confirmed by linear sweep voltammetry and online electrochemical mass spectrometry measurements. These results underscore the potential of carbonate-based polymer electrolytes and the importance of balancing electrolyte conductivity and stability in cell design.