Towards high energy density sodium ion batteries through electrolyte optimization

A comprehensive study is reported entailing optimization of sodium ion electrolyte formulation and compatibility studies with positive and negative electrode materials. EC:PC:DMC and EC:PC:DME were found to exhibit optimum ionic conductivities and lower viscosities. Yet, hard carbon negative electrode materials tested in such electrolytes exhibit significant differences in performance, rooted in the different resistivity of the SEI, which results in too large polarization and concomitant loss of capacity at low potentials when DME is used as a co-solvent. EC0.45:PC0.45:DMC0.1 was found to be the optimum composition resulting in good rate capability and high capacity upon sustained cycling for hard carbon electrodes. Its compatibility with positive Na3V2(PO4)2F3 (NVPF) electrodes was also confirmed, which led to the assembly of full Na-ion cells displaying an operation voltage of 3.65 V, very low polarisation and excellent capacity retention upon cycling with ca. 97 mA h g−1 of NVPF after more than 120 cycles together with satisfactory coulombic efficiency (>98.5%) and very good power performance. Such values lead to energy densities comparable to those of the current state-of-the-art lithium-ion technology.