Na⁺-Conductive Na₂Ti₃O₇-Modified P2-type Na_2/3Ni_1/3Mn_2/3O₂ via a Smart in Situ Coating Approach: Suppressing Na⁺/Vacancy Ordering and P2–O2 Phase Transition

Abstract

Sodium-ion batteries (SIBs) have shown great superiority for grid-scale storage applications because of their low cost and the abundance of sodium. P2-type Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode materials have attracted much attention for their high capacities and operating voltages as well as their simple synthesis processes. However, Na + /vacancy ordering and the P2−O2 phase transition are unavoidable during Na + insertion/extraction, leading to undesired voltage plateaus and deficient battery performances. We show that this defect can be effectually eliminated by coating a moderate Na + conductor Na 2 Ti 3 O 7 with a smart in situ coating approach and a concomitant doping of Ti 4+ into the bulk structure. Based on the combined analysis of ex situ X-ray diffraction, scanning electron microscopy, electrochemical performance tests, and electrochemical kinetic analyses, Na 2 Ti 3 O 7 coating and Ti 4+ doping effectively refrain Na + /vacancy ordering and P2−O2 phase transition during cycling. Additionally, the Na 2 Ti 3 O 7 coating layer suppresses particle exfoliation and accelerates Na + diffusion at the cathode and electrolyte interface. Hence, Na 2 Ti 3 O 7 -coated Na 2/3 Ni 1/3 Mn 2/3 O 2 exhibits excellent cycling stability (almost no capacity decay after 200 cycles at 5 C) and outstanding rate capability (31.1% of the initial capacity at a high rate of 5 C compared to only 10.4% for the pristine electrode). This coating strategy can provide a new guide for the design of prominent cathode materials for SIBs that are suitable for practical applications. KEYWORDS: sodium-ion batteries, cathode material, Na + conductor-Na 2 Ti 3 O 7 , in situ coating approach, phase transition, Na + /vacancy ordering

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

“…The electrochemical impedance spectra (EIS) of the above three samples are compared in Figure S9 (Supporting Information). The CV curves and charge/discharge profiles featuring smoother peaks or plateaus than those of the bulk materials, [28,36,41,43,44,46] along with the well-maintained voltage plateau at ≈4.2 V during cycling, suggest that the hierarchical nanofibers could effectively alleviate the Na + /vacancy ordering and P2-O2 phase transition occurring for the pristine materials. In addition, the electrochemical performance of the P3-type Na 2/3 Ni 1/3 Mn 2/3 O 2 (annealed at 700 °C for 6 h) has also been evaluated ( Figure S10 in the Supporting Information), which shows a much lower specific capacity with inferior cycling stability relative to the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofibers.…”

“…This is because the P2 phase material possesses facile Na-ion diffusion in the presence of multi-Na vacancies compared with the P3 phase. [28,35,36,38,39,41,42,[44][45][46]48,51,53] This originates from the delicately tailored fibrous nanostructure composed of nanograins with high reactivity and high porosity that would accelerate the electronic/ionic transportation. The CV curves and charge/discharge profiles featuring smoother peaks or plateaus than those of the bulk materials, [28,36,41,43,44,46] along with the well-maintained voltage plateau at ≈4.2 V during cycling, suggest that the hierarchical nanofibers could effectively alleviate the Na + /vacancy ordering and P2-O2 phase transition occurring for the pristine materials.…”

“…More importantly, the capacity can swiftly restore to ≈155 mA h g −1 when the rate is turned back to 0.1 C, demonstrating a great tolerance for the rapid Na + insertion/extraction. Figure 3c presents the high-resolution Ni 2p spectra of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofibers at different charged/discharged states: the pristine electrode featuring Ni 2p 1/2 and Ni 2p 3/2 peaks at 872.1 and 854.6 eV, respectively (accompanied by two satellite peaks), demonstrates the bivalent state of Ni; [46] when the electrode is charged to 4.0 V and then to 4.5 V, the Ni 2+ is successively oxidized to Ni 3+ and Ni 4+ ; when discharged to 2.0 or 1.5 V, the Ni 2+ can almost recover. [28,35,36,38,39,41,42,[44][45][46]48,51,53] This originates from the delicately tailored fibrous nanostructure composed of nanograins with high reactivity and high porosity that would accelerate the electronic/ionic transportation.…”

“…Figure 4a depicts the typical charge-discharge GITT profiles in the second cycle; the cell was charged and discharged at a constant current flux (20 mA g −1 ) for 30 min and then relaxed for 120 min to make the voltage reach equilibrium (as schematically described in Figure S14, Supporting Information). [28,36,46] Figure 4c presents the CV curves collected at various sweep rates ranging from 0.1 to 1.0 mV s −1 ; with the scan rate increasing, the redox currents increase drastically, while the potential intervals between the oxidation and reduction peaks enlarge slightly, reflecting the minor polarization of the nanofiber cathode. [28,36,46] Figure 4c presents the CV curves collected at various sweep rates ranging from 0.1 to 1.0 mV s −1 ; with the scan rate increasing, the redox currents increase drastically, while the potential intervals between the oxidation and reduction peaks enlarge slightly, reflecting the minor polarization of the nanofiber cathode.…”

“…[36,37] Dahn's group first investigated the Na-intercalation/deintercalation of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 , [38] revealing that the rapid capacity decay caused by the undesirable P2-O2 phase transition (with a large volume change of ≈23%) when charged to above 4.2 V was the main challenge hindering the wide application of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 . doping to stabilize the crystal structure, [28,[41][42][43] surface coating to suppress the unfavorable side reactions, [44][45][46] and lowering the charge cut-off voltage to eliminate the phase transformation, [36,47,48] have been adopted for modifying P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 . doping to stabilize the crystal structure, [28,[41][42][43] surface coating to suppress the unfavorable side reactions, [44][45][46] and lowering the charge cut-off voltage to eliminate the phase transformation, [36,47,48] have been adopted for modifying P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 .…”