A High Power–High Energy Na3V2(PO4)2F3 Sodium Cathode: Investigation of Transport Parameters, Rational Design and Realization

Sodium ion batteries are realistic and promising alternatives to lithium due to the abundance of Na and the similar intercalation chemistry of Na when compared to the lithium counterpart. Developing high-power and high-energy sodium batteries is still a significant challenge. Na3V2(PO4)2F3 (NVPF) has been shown to combine excellent charge–discharge kinetics with a competitively high voltage. However, the major issue is, as for the vast majority of electrode materials, the lack of distinct knowledge of fundamental transport parameters, on which an optimized strategy for developing a high-power and high-energy sodium cathode can be based. This work aims at filling this gap. We experimentally investigate the intrinsic ionic and electronic conductivities, as well as the chemical diffusion coefficient of sodium of Na3V2(PO4)2F3 by impedance and dc polarization. On the basis of these results, we develop an optimized design. As the electronic conductivity is found to be much smaller than the ionic one, electronic wiring of the particles (by a graphene network) has higher priority than providing electrolyte contact. This is important since the contact by graphene and electrolyte wetting is partly antagonistic, not so much because of interfacial tensions rather because of the introduced heterogeneities on the nanoscale (cf. Lotus effect). We develop and apply a one-step cost-effective low temperature hydrothermal method without any postheat treatment for the fabrication of a nanoparticulate (∼30–50 nm) NVPF electrode, which provides sufficient porosity and in which every nanoparticle is connected to the graphene network. In terms of rate capability, the performance of this electrode is excellent and at least belongs to the best Na storage performances reported for Na3V2(PO4)2F3 so far.