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.
재료의 풍부함, 가격 경쟁력, 리튬 이차전지와 유사한 작동 원리의 장점으로 나트륨 이차전지의 연구가 최근 십여 년 동안 폭발적으로 이루어 졌으며 그 중 전지의 성능과 가장 밀접한 연관이 있는 전극 소재에 대한 연구가 가장 활발하게 이루어 졌다. 나트륨 이온전지 전극 소재는 리튬 이차전지와의 작동원리 유사성으로 인해 대부분 소재의 골격을 유지한 체 리튬 자리를 나트륨 이온으로 치환한 소재 개발에만 연구가 집중 되었다. 하지만 나트륨은 리튬 보다 큰 원자 반경을 가지기 때문에 충/방전에 따라 구조의 붕괴가 리튬이온 전지에서 보다 극심하며, 사이클 안정성이 떨어지기 때문에 나트륨 이온전지에 적합한 새로운 소재 개발이 필수적이다. 따라서 본 연구에서는 차세대 전지인 나트륨 이차전지에 적합한 소재를 개발하고 이의 반응 메커니즘을 구조적 관점에서 규명하였다.