Study on Novel Cathode Materials Involving Anionic Redox Reactions with High Capacity for Rechargeable Ion Batteries


Student thesis: Doctoral Thesis

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Award date11 Jan 2021


Na-ion batteries (NIBs) are being actively explored for largescale energy storage applications as a promising alternative to Li-ion batteries (LIBs), predominantly due to their low cost and the earth abundance of sodium. However, the current energy and power densities of NIBs are still far from practical requirements. The anionic redox reactions (ARRs) have been intensively studied due to the benefit of extra capacity from oxygen activity in Li-rich layered cathode materials for LIBs. Similar ARRs have also been reported in both Na-rich and Na-deficient oxides. This work, aiming to explore more cathode materials with a high capacity from ARRs, examines several novel sodium-based metal oxides that were originally not studied for NIBs as positive electrode materials. Their redox mechanism and structural evolution have also been investigated. Moreover, one Na-ion cathode, LiVO3, reflects the analog of LIBs, which also delivers a remarkable specific capacity and energy density.

First, Na2SeO3, as a Na-rich nonmetal oxide, is studied for the first time as a cathode material for NIBs between 1.5-4.7 V. This material can deliver a discharge capacity of 232 mAh g−1 after electrochemical activation, which is one of the highest capacities from sodium-based cathode materials. X-ray photoelectron spectroscopy (XPS) indicates that the oxidation state of selenium remains unchanged during the charging process. The theoretical simulation shows that after removal of Na, spin is situated around oxygen atoms near the Na vacancy, and the projected density of state (PDOS) of oxygen electrons is close to the Fermi level. These suggest the involvement of oxygen during charge/discharge.

Second, a sodium-rich vanadium compound, Na4V2O7, is investigated as a cathode material for NIBs. After ball milling with carbon, Na4V2O7 can deliver a high reversible capacity of 194 mAh g−1 between 1.2 and 4.7 V. XPS and X-ray absorption spectroscopy (XAS) indicate the charge compensation is activated by oxygen redox reactions. In the subsequent discharge, electron transfer mainly originates from the vanadium redox reaction, with a partial oxygen redox reaction. The material exhibits a negligible volume variation of 1.04% during Na+ (de-)intercalation, as verified by in situ synchrotron X-ray diffraction (SXRD), demonstrating that Na4V2O7 is structurally stable, which is considered to be the benefit of robust VO4 tetrahedrons inside the structure.

Third, NaVO3 is re-investigated due to its unique layered structure made of the VO4 framework. Although our group haspreviously shown that monoclinic NaVO3 can deliver a reversible capacity of about 100 mAh g−1 with ARR through electrochemical characterization, the structural evolution and electrochemical mechanism in different voltage ranges remain elusive. By combining both cationic (V4+/V5+) and anionic (O2−/O ) redox couples between 1.2 V and 4.7 V, NaVO3 delivers a remarkable specific capacity of 245 mAh g−1 (1.2–4.7 V) at 10 mA g−1 . In situ SXRD studies show that the material structure is virtually invariant during Na+ (de-)intercalation, with the a and b lattice parameters changing by only 0.13% and 0.19%, respectively. The stable VO4 tetrahedral framework, as further disclosed by density functional theory (DFT) calculation, also enables the material to give superior rate and cycle capabilities, with a capacity of 164 mAh g−1 (67% utilization) at a current of 1000 mA g−1(about 5C) and capacity retention of 90% after 50 cycles.

Fourth, to expand the NaVO3 case into the analog for LIBs, LiVO3 has been investigated as a cathode material by charging to 4.8 V to activate ARRs. It shows an initial charge capacity of 140 mAh g−1, corresponding to 0.56 Li extraction per formula unit. When cycling between 4.8 V and 2.4 V, a reversible capacity of 130 mAh g−1 is delivered, with excellent cycle retention (93% capacity retention after 100 cycles). With the potential window being enlarged to 2.0–4.8 V, a higher reversible capacity of 220 mAh g−1 with good cycle performance (90% capacity retention after 100 cycles) is achieved by allowing more charge transfer from the vanadium redox reaction. The available capacity can be extended to 350 mAh g−1 by deep discharging the electrode to 1.5 V with a remarkable energy density of 912 Wh kg−1 . The redox reaction mechanisms of cationic (V4+/V5+) and anionic (O2−/O) redox couples have been determined through XPS and XAS measurements and the structural evolution between different voltage ranges has been disclosed by ex situ XRD.

Finally, novel Na-rich 4d/5d transition metal oxides—Na2MoO4 and Na2WO4—with the same spinel structure have been revealed as cathode materials for NIBs for the first time. There is a roughly 1.22 Na and 1.12 Na extraction for W-based and Mo-based cathodes, respectively, in the initial charge process, although both transition metals (TMs) in the pristine materials are initially in the highest oxidation state of 6+. The charge transfer is probably from oxygen redox reactions that need to be characterized in the future. With TMO4 tetrahedrons in the structure, ex situ XRD measurement also reveals an invariant structure evolution upon sodium extraction/insertion.