소듐이차전지용 저가 고용량 복합인산염계 양극 소재 연구

Abstract

Recent global warming and the exhaustion of fossil fuel concerns have accelerated advancements to develop renewable energy sources. To cope with ever growing energy demand and supply sustainable energy in an efficient way, large-scale energy storage systems (ESSs) have become an important area of research in recent years. Among various candidates, Na-ion batteries (NIBs) are considered to be the optimal choice for these applications because of the availability and low cost of sodium, as well as the similar electrochemistry to the established Li-ion battery technology. In this respect, the search for new electrode materials for NIBs based on low cost redox couples such as Fe or Mn has been intensively performed in recent years. For better electrode materials for NIBs, my prime interest is in designing new mixed-phosphate compounds based on low cost transition metal redox couples such as Fe2+/Fe3+ and Mn2+/Mn3+. In Chapter 2, a new iron-based mixed polyanion compound, Na4Fe3(PO4)2(P2O7), for NIBs is introduced. Structural characterization of a newly synthesized mixed-polyanion compound with three-dimensional Na pathways was performed using combined X-ray and neutron diffraction studies. The electrode exhibited average potential ~ 3.2 V (vs. Na+/Na) and energy density of 320 Wh kg-1. Also, the reversible electrode operation was found from ion-exchanged sample of Li3NaFe3(PO4)2(P2O7) in Li-ion cells. The electrode delivers about 92 % of theoretical capacity (~140 mAh g-1) with an average voltage of 3.4 V (vs. Li+/Li). This research firstly suggested that a significant opportunity exists to explore new open-framework electrodes for NIBs with high electrochemical performances by combination of (PO4)3- and (P2O7)4- polyanions. In Chapter 3, I investigated the electrochemical mechanism of NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4) in Na-ion cells using first principles calculations and experiments. I discovered that the de/sodiation of the NaxFe3(PO4)2(P2O7) electrode occurs via one-phase reaction with a reversible Fe2+/Fe3+ redox reaction. The electrode accompanies an exceptionally small volumetric change of less than 4% during electrochemical cycling, which is attributed to the open framework of polyanion compounds with flexible P2O7 dimer in the structure. Although the structural distortion in NaFe3(PO4)2(P2O7) reduces Na de/intercalation kinetics at the last step of charge resulting in incomplete utilization of Na (~ 82 % of theoretical capacity), high rate capability was confirmed with the negligible capacity reduction from C/20 to C/5. Also, stable cycle retention up to 20 cycles were confirmed. In situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) revealed that the partially charged electrodes, NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4), are thermally stable up to 530 °C. The understanding of electrochemical mechanism of NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4) shown here will give a direction to the optimization of the new Na4Fe3(PO4)2(P2O7) electrode for Na rechargeable batteries. Chapter 4 introduces a new 3.8 V-class manganese-based mixed-polyanion compound, Na4Mn3(PO4)2(P2O7), for Na rechargeable batteries. The electrode shows a highly reversible electrochemical activities with a high Mn2+/Mn3+ redox potential of 3.84 V (vs. Na+/Na) and the largest energy density of 416 Wh kg-1 yet reported for a manganese-based polyanion cathode. Ex situ XRD and X-ray absorption spectroscopy (XAS) studies revealed that the reversible multi-phase reaction occurs during electrochemical cycling. Although Jahn-Teller distortion of Mn3+ causes large volume change ~7 % upon charge reaction, the Na ion mobility is not decreased, rather it is increased. It is due to the unique Jahn-Teller distortion in the crystal structure, which opens up Na diffusion channels. This feature stabilize the electrode, providing high power capability and cycle stability in Na-ion cells. I believe that this compound could be a new strong competitor for large scale NIBs based on its high voltage, large energy density and cycle stability. Although the iron- and manganese-based electrode materials display promising electrochemical activities in Na-ion cells as shown from Chapter 2-4, the low energy density of Na4Fe3(PO4)2(P2O7) electrode and large volume change with multiple structural transitions in Na4Mn3(PO4)2(P2O7) electrode upon electrochemical cycling are still big challenges for its application to large-scale ESSs. In Chapter 5, I will introduce Fe- and Mn-based binary metal mixed-phosphate cathodes, Na4MnxFe3-x(PO4)2(P2O7) (x = 1, 2), which is successfully synthesized for the first time here. The electrochemical analysis of these binary mixed-phosphate materials shows that the substitution of Mn in the structure increases the energy density by utilizing Mn2+/Mn3+ redox couples and upshifts the Fe 2+/Fe3+ redox potential in Na-ion cells. Ex situ structural investigation reveals that they operate via one-phase reaction upon charge and discharge processes with a remarkably low volume change of 2.07 % for Na4MnFe2(PO4)¬(P2O7), which is one of the lowest value among Na battery cathodes reported thus far. With a merit of open framework structure and low volume change, more than a half of the theoretical capacity is obtained at 20 C from both electrodes, and in the case of the Na4MnFe2(PO4)2(P2O7) electrode, it exhibits a stable cycle performances up to 3,000 cycles at 1C and room temperature. I believe that these materials can be a strong competitor for large-scale Na-ion battery cathodes based on their low costs, long-term cycle stability and high energy density.