Li-Substituted Layered Spinel Cathode Material for Sodium Ion Batteries

Abstract

The O3-type layered Na(NixFeyMnz)O2 (0 < x, y, z < 1) cathode materials have attracted great interest in sodium-ion batteries due to the abundance and cost of raw materials and their high specific capacities. However, the cycling stability and rate capability at high voltages (> 4.0V) of these materials remains an issue. In this work, we successfully synthesized a Li-substituted layered-tunneled (O3-spinel) intergrowth cathode (LS-NFM) to address these issues. The remarkable structural compatibility and connectivity of the two phases were confirmed by X-ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). LS-NFM electrode reached a discharge capacity of 96 mAh g-1 with a capacity retention of 86% after 100 cycles at a current rate of 100 mA g-1 in a voltage window of 2.0-4.2 V. Moreover, the LS-NFM cathode exhibited an enhanced rate capability in comparison to the un-doped layered cathode (NFM). The enhanced rate capability of LS-NFM can be explained by the significantly increased effective Na + diffusivity measured by galvanostatic intermittent titration technique (GITT) compared to the un-doped control NFM cathode, which can be ascribed to the improved charge transport kinetics through shortened diffusion path by direct connection between the 3D channels in the spinel phase and 2D channels in the layered phase. The results from ex situ hard/soft X-ray adsorption spectroscopy (XAS) suggest that the capacity of LS-NFM cathode is mainly associated with the Ni 2+ /Ni 4+ redox couple, and slightly from the Fe 3+ /Fe 4+ redox couple. The voltage profile of the LS-NFM cathode exhibited a reversible plateau above 4.0 V, indicating great stability at high voltages and structural stabilization by the spinel phase. In addition to the substitution of various transition metals, or the modification of the stoichiometry of each transition metal, this study provides a new strategy to improve electrochemical performance of layered cathode materials for sodium ion batteries.

Probing the charged state of layered positive electrodes in sodium-ion batteries: reaction pathways, stability and opportunities

“…Improved rate capability on the other hand could be ascribed to the improved charge transport kinetics due to direct connection between the 3D channels in the spinel phase and 2D channels in the layered phase modifying the diffusion pathway. 166 A highly reversible transition at high voltages was also reported for the layered-tunnel intergrowth structure with the composition Na 0.6 MnO 2 . 167 The highly reversible P2/tunnel to OP4/tunnel phase transition was detected using in situ XRD upon cycling the composite up to 4.3 V. Taking this further and combining P2, P3 and spinel structures to form the Na 0.5 [-Ni 0.2 Co 0.15 Mn 0.65 ]O 2 nanocomposite, Hou et al were able to synthesise a particularly stable composition for which no phase transitions were detected via ex situ XRD when cycled between 1.5 and 4.0 V vs. Na/Na + .…”

“…An improved performance in terms of capacity and average voltage was found for the O3 layered/spinel composite Na 0.87 -Li 0.25 Ni 0.4 Fe 0.2 Mn 0.4 O 2+d electrode. 166 The voltage prole of the composite material shows a reversible high voltage plateau while this plateau is irreversible in the spinel free material, suggesting that addition of the spinel phase contributed towards the enhanced structural and electrochemical stability, especially at high voltages, the mechanism of which is yet to be explored. Improved rate capability on the other hand could be ascribed to the improved charge transport kinetics due to direct connection between the 3D channels in the spinel phase and 2D channels in the layered phase modifying the diffusion pathway.…”

Constructing Active Sites from Atomic‐Scale Geometrical Engineering in Spinel Oxide Solid Solutions for Efficient and Robust Oxygen Evolution Reaction Electrocatalysts

“…X‐ray absorption near edge structure (XANES) spectra were recorded at the Fe K‐edge of MCFO NS, MFO NS, CFO NP, and other compounds (Figure 3b ). [ 28 , 29 ] The XANES spectra of FeO and Fe 2 O 3 show that white line intensities are located at 7127.8 and 7132.5 eV, corresponding to Fe 2+ and Fe 3+ , respectively. Among the various spinel oxides, CFO NP exhibits a white line intensity at 7132.4 eV, demonstrating that the valence state of Fe in CFO NP is the highest, which is close to Fe 3+ .…”

Layer‐Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries

“…O 3−Fd m Category : Recently, Xiong and co‐workers successfully integrated spinel structure with O3‐layered structure to obtain an intergrowth composite of Na 0.87 Li 0.25 Ni 0.4 Fe 0.2 Mn 0.4 O 2+d (LS–NFM) via Li + substitution and careful optimization of Na + /Li + ratio . It was reported that the O3 phase did not form at high Na + content, while formation of separate spinel phases is more favorable when the Li + content is too high.…”

“…c) Cyclic performances and d) rate capabilities of LS–NFM and single‐phase undoped NFM cathode; log plots during the e) charge and f) discharge process. Reproduced with permission . Copyright 2018, American Chemical Society.…”