Flexible trilayer cellulosic paper separators engineered with the BaTiO3 ferroelectric fillers for high energy density sodium-ion batteries

Abstract

Cellulose-based paper separators are employed in sodium-ion batteries (SIBs) as a viable and economical substitute of conventional separators, owing to their sustainability, scalability, safety and cost-effectiveness. We design a full cell configuration having Na₃V₂(PO₄)₃ as the cathode and pre-sodiated hard carbon as the anode with different separators and compare the electrochemical performance of these ceramic-impregnated polymer-coated cellulose paper separators with that of commercial glass fiber separators. Notably, the paper-based multilayer separators provide desirable characteristics such as excellent electrolyte wettability, thermal stability up to 200 °C, and ionic conductivity, which are essential for the efficient operation of SIBs. The cellulose separator is coated with a layer of polyvinylidene fluoride polymer, followed by a second layer of styrene butadiene rubber (SBR) polymer in which ferroelectric filler BaTiO₃ is integrated, which interacts with the polymer hosts through Lewis acid–base interactions and improves the conduction mechanism for the Na⁺ ions. The final lamination is performed by varying the SBR concentrations (0.5, 0.75, and 1.0 w/v%). The incorporated polymer matrices improve the flexibility, adhesion and dispersion of the nanoparticles and affinity of the electrolyte to the electrode. The morphology of the paper separators shows uniform interconnected fibers with a porous structure. Interestingly, we find that the paper separator with 0.75 w/v% content of SBR exhibits decreased interfacial resistance and improved electrochemical performance, having retention of 62% and nearly 100% coulombic efficiency up to 240 cycles, as compared to other concentrations. Moreover, we observe the energy density to be around 376 W h kg⁻¹ (considering the cathode weight), which was found to be comparable to that of the commercially available glass fiber separator. Our results demonstrate the potential of these multilayer paper separators towards achieving sustainability and safety in energy storage systems.