Understanding the Performance of Symmetric Sodium-Ion Batteries with Liquid and Solid Electrolytes

While solid-state electrolytes are often regarded as promising alternatives to liquid electrolytes due to enhanced safety, their practical implementation remains limited by the charge transfer process along with interfacial challenges. In this work, a symmetric Na 3 V 2 (PO 4 ) 3 /C full cell is employed as a model system to systematically compare charge storage performances using liquid and solid electrolyte configurations. By utilizing identical active materials in both electrodes, the study isolates the electrolyte and other contributions such as internal resistance due to Na chemical diffusion, charge transfer process, and interphase evolutions to the observed storage performances. Total heat generation from these cells is reported at 50% SoC at 0.1 C and 1 C from the estimation of reversible and irreversible heat losses. Observed voltage polarization is correlated with the irreversible heat loss due to internal resistance derived from the deconvolution of impedance data using the distribution of relaxation time/distribution of phasance method. The solid-state cell exhibits higher voltage polarization reflecting more irreversible heat generation compared to its liquid counterpart especially at high current. The anolyte in the symmetric solid-state cell is identified as a dominant performance bottleneck, where restricted solid–solid contact and interphase heterogeneity are believed to hinder Na-ion diffusion and charge transfer processes. Interphase analysis further shows that the investigated liquid electrolyte promotes the formation of comparatively conductive inorganic species, including NaF-rich surface layers, whereas the composite solid electrolyte undergoes more pronounced electrolyte decomposition, resulting in thicker and compositionally complex interphases that impede ionic transport. These findings provide mechanistic insights for the observed voltage polarization in liquid and solid electrolyte systems that help in the rational design of improved solid electrolyte interfaces and composite electrode architectures for solid-state sodium-ion batteries.