Electrolyte effect on performance and lifetime of sodium – ion batteries

Abstract

Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries (LIBs) due to their sodium abundance, lower cost, non-toxicity, and environmental benefits. Intercalation-type anode materials obtained from biomass are being actively researched. However, SIBs face challenges such as lower energy density and initial Coulomb efficiency (ICE) compared to LIBs. Numerous studies are aimed at improving anode and cathode materials, as well as electrolytes. Issues such as degradation and modeling at the electrode-electrolyte interface and the formation of stable thin SEI/CEI layers with high ionic conductivity remain unresolved.

This study estimates the effects of non-aqueous ester-based liquid electrolytes on SIB performance, as well as the effects of FEC and DMC additives. Testing was carried out on coin-type half cells using NaPF6 EC/PC electrolytes with and without FEC, NaPF6 EC/PC/DMC and 2M NaTFSI EC/DMC. Hard carbon from biomass with an active substance content of 92.13% and an expected specific capacity of 350 mAh/g was used as the working electrode, and metal sodium was used as the counter electrode. Galvanostatic cycling, impedance measurements before and after cycling, and SEM images of the electrodes after cycling were performed.

All cells showed a low ICE of 38–50% in the first cycle, likely due to the high polarization of the metal counter electrode. Cells with commercial NaPF6 EC/PC electrolyte had the highest ICE of 50% and stable long-term performance with CE around 97-98% and capacity retention of 80-75 mAh/g over 140 cycles. Adding 3% FEC slightly increased the capacity after the second cycle from 110 to 125 mAh/g, but caused the capacity to drop from 80 to 55 mAh/g after 140 cycles and increased cell impedance.

Electrolytes NaPF6 EC/PC/DMC and 2M NaTFSI EC/DMC showed higher irreversible capacity in the first cycle and ICE - 43% and 38%, respectively. Long-term cycling of C/2 revealed a significant drop in capacity from 45 to 30 mAh/g for the latter electrolyte, indicating instability due to moderate salt concentration and high polarization. SEM images of the spent electrodes were taken to visualize the degradation mechanisms.

To better study SIB electrolytes, it is recommended to use full cells to reduce polarization due to Na reactivity during charging, and scan SEIs with SEM and EDS to analyze their thickness and morphology, which helps model improved electrolytes.