Advanced P2-Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ Cathode Material with Suppressed P2–O2 Phase Transition toward High-Performance Sodium-Ion Battery

Abstract

As a promising cathode material of sodium-ion battery, P2-type NaNiMnO (NNMO) possesses a theoretically high capacity and working voltage to realize high energy storage density. However, it still suffers from poor cycling stability mainly incurred by the undesirable P2-O2 phase transition. Herein, the electrochemically active Fe ions are introduced into the lattice of NNMO, forming NaNiMnFe O ( x = 0, 1/24, 1/12, 1/8, 1/6) to effectively stabilize the P2-type crystalline structure. In such Fe-substituted materials, both Ni/Ni and Fe/Fe couples take part in the redox reactions, and the P2-O2 phase transition is well restrained during cycling, as verified by ex situ X-ray diffraction. As a result, the optimized NaNiMnFeO (1/12-NNMF) has a long-term cycling stability with the fading rate of 0.05% per cycle over 300 cycles at 5 C. Furthermore, the 1/12-NNMF delivers excellent rate capabilities (65 mA h g at 25 C) and superior low-temperature performance (the capacity retention of 94% at -25 °C after 80 cycles) owing to the enhanced Na diffusion upon Fe doping, which is deduced by the studies of electrode kinetics. More significantly, the 1/12-NNMF also displays remarkable sodium-ion full-cell properties when merged with an LS-Sb@G anode, thus implying the possibility of their practical application.

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.…”

“…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.…”

“…[5,6] The cathode is a very important factor determining the electrochemical performance and accounting for the highest cost proportion (≈32.4%) of SIBs; the development of appropriate cathode materials that can accommodate fast and stable sodium-ion insertion/extraction is of great significance. [28] Generally, the layered sodium transition metal oxides could be classified into two main groups, namely, P2 and O3, based on the occupation sites of Na ions (P: prismatic and O: octahedral), where the numbers "2" and "3" represent the number of transition metal layers in each repeated unit. [28] Generally, the layered sodium transition metal oxides could be classified into two main groups, namely, P2 and O3, based on the occupation sites of Na ions (P: prismatic and O: octahedral), where the numbers "2" and "3" represent the number of transition metal layers in each repeated unit.…”

“…[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 .…”