A dual aliovalent ion doped NASICON ceramic filler embedded in the PEO–NaTFSI polymer matrix for high-performance solid-state sodium-ion batteries

Abstract

All-solid-state sodium-ion batteries (SSSIBs) prevent leakage and fire risks associated with liquid electrolytes by using non-flammable solid electrolytes and electrodes. The primary aim of this work is to develop a NASICON type ceramic electrolyte Na₃Zr₂Si₂PO₁₂ (NZSP) with co-doping of Mg²⁺ and Sc³⁺ to achieve high ionic conductivity. Mg²⁺ doping reduces the grain–grain boundary resistance by forming conducting phase NaMgPO₄ across the grain boundaries. Co-substitution of a trivalent Sc³⁺ at the Zr⁴⁺ site compensates for Na deficiency. As a result, the Na ion mobility through grains and grain boundaries was increased with this co-doping of aliovalent Mg²⁺ and Sc³⁺. The highest grain and grain boundary ionic conductivities (σ_b ∼ 1.78 × 10⁻³ S cm⁻¹ and σ_gb ∼ 0.4 × 10⁻³ S cm⁻¹) and Na ion transference number (t_Na⁺ ∼ 0.998) have been achieved for the optimized composition of Na_3.3Zr_1.8Mg_0.10Sc_0.10Si₂PO₁₂ (NZSP-0.1MS). The NZSP-0.1MS ceramic electrolyte is further employed in quasi-solid cells; a decent discharge capacity of 91 mA h g⁻¹ at 0.1C rate was obtained for the “Zn doped (NVZP) (cathode)‖NZSP-0.1MS‖NVZP(anode)” ceramic full cell. The NZSP-0.1MS ceramic electrolyte filler is further incorporated into the polymer–salt matrix to develop the PEO–NaTFSI/NZSP-0.1MS composite electrolyte. The all-solid-state cell comprised of composite electrode (NVZP/NZSP-0.1MS/PVDF-NaTFSI/C) and composite electrolyte PEO–NaTFSI/NZSP-0.1MS delivered a maximum discharge capacity of 105 mA h g⁻¹ at 0.1C, at 60 °C with good rate performance and cyclic stability retaining 85% capacity over 80 cycles. The specific capacity of the catholyte‖PEO–NaTFSI/NZSP-0.1MS‖anolyte full cell has further decreased to 78 mA h g⁻¹ with a subsequent increase in the C-rate from 0.1 to 1C. Interestingly, the capacity was restored to 97 mA h g⁻¹ after returning to 0.1C, which suggests a better electrochemical stability of the cell components. This study presents an all solid-state sodium-ion cell exhibiting superior electrochemical performances comparable to those of NVP-based liquid electrolyte full cells, providing an avenue for developing a high-performance and safe sodium-ion battery technology.