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. EC_0.45:PC_0.45:DMC_0.1 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.

Materials prepared by chemical Li deintercalation with NO₂BF₄ from Li_1.20Mn_0.54Co_0.13Ni_0.13O₂ and chemical Li reinsertion with LiI show very similar chemical composition, oxidation state of each transition metal ion, structural properties and electrochemical performance to those of the material recovered after the 1^st electrochemical cycle. Investigations combining redox titration, magnetic measurement, neutron diffraction and chemical analyzes reveal that uncommon redox processes are involved during the first charge at high voltage and explain the charge overcapacity and large reversible discharge capacity obtained for this material. This further assesses our proposal that oxygen, in addition to nickel and cobalt, participates to the redox processes in charge: within the bulk oxygen is oxidized without oxygen loss, whereas at the surface oxygen is oxidized to O₂ and irreversibly lost from the structure. During the subsequent discharge, in addition to nickel, cobalt and oxygen, manganese is also slightly involved in the redox processes (reduction) to compensate for the initial surface oxygen loss.