Electrochemical and structural characterization of polyanion electrode materials for sodium-ion batteries

Abstract

As battery applications have extended from portable electronics to large-scale storage such as electric buses or stationary storage connected to renewable energy product, sodium-ion batteries (SIBs) have been great attentions for large-scale applications because of its abundance and cost-effectiveness. Nevertheless the advantages of SIBs for next generation batteries, the difference between Na and Li such as ionic radius, atomic weight, standard reduction potential, and coordination preference provide inferior electrochemical performancein SIBs than lithium-ion batteries (LIBs), requiring substantial improvement of SIBs to be practically used in the diverse real application. Therefore, not only finding suitable electrode materials with high performance, but also the reaction mechanism through various analysis techniques, particularly the structural features along with the electrochemical characteristics of the materials, are the foremost necessary. Here the electrochemical properties and its reaction mechanism of Na-based compounds for SIBs are investigated in structural aspects of electrode materials. Firstly, vanadium ortho-diphosophate, $Na_7 V_4 (P_2 O_7)_4 PO_4$ , was firstly discovered as a cathode material for SIBs and analyzed its crystal structure via complementary Rietveld refinement using X-ray and neutron diffraction, which has not been even documented in the materials database. By combination of computational and experimental study, we reveal that well-defined high voltage profile and unprecedented cycle life of $Na_7 V_4 (P_2 O_7)_4 PO_4$ originate from the presence of the intermediate phase and its unique structural rearrangement during electrochemical cycles. Secondly, we revisit vanadium ortho-diphosophate,$Na_7 V_4 (P_2 O_7)_4 PO_4$ and demonstrate that the amphoteric nature allows $Na_7 V_4 (P_2 O_7)_4 PO_4$ to engage two vanadium redox couples, $V^{3+} /V ^{4+}$ and $V^{2+}/V^{3+}$, for cathode and anode operations, respectively, within the stable voltage window of organic electrolytes. In-situ electrochemical synchrotron X-ray diffraction indicates that Na $Na_7 V_4 (P_2 O_7)_4 PO_4$ reacts with Na ions based on a single-phase reaction in the low voltage range. Upon paring into both electrodes, symmetric full-cell shows an output voltage of 2.81 V. Lastly, amorphous iron phosphate with lattice water, namely $FePO_4 \cdot xH_2 O$ was proposed as a sodium-ion battery cathode. The combination of lattice water and amorphous structure significantly improves the electrochemical performance of $FePO_4 \cdot xH_2 O$. The presence of lattice later facilitated Na ion diffusion in the framework and stabilizes the overall structure for prolonged cycling, leading to superior cycling and rate performance.