Unveiling the Complementary Manganese and Oxygen Redox Chemistry for Stabilizing the Sodium‐Ion Storage Behaviors of Layered Oxide Cathodes

Abstract

Layered oxide cathodes for sodium‐ion batteries (SIBs) have drawn increasing attention owing to their fascinating additional capacity contributed by oxygen‐redox chemistry. Unfortunately, excessive oxygen redox incurs an irreversible oxygen release, deteriorating the cyclic stability and compromising the advantage of additional capacity. Significant efforts have been made so far to stabilize lattice oxygen, but the potential advantages associated with oxygen loss have been ignored. Herein, a complementary Mn and O redox mechanism is first revealed in a novel P2‐Na0.75Ca0.04[Li0.1Ni0.2Mn0.67]O2 cathode. The partial oxygen release activates more Mn3+/Mn4+ redox capacities upon cycling, which effectively compensate the capacity loss from O2‐/On‐ redox and thus enable an ultra‐stable cycling performance (capacity retentions of 102.1% and 95.2% over 100 and 200 cycles at 5 C, respectively). The fast Na+ diffusion kinetics of the cathode also ensure an exceptional rate capability (133.1 and 68.8 mAh g‐1 at 0.1 and 20 C, respectively). The electrode crystal/electronic structure evolution and charge compensation mechanism upon sodiation/desodiation have been elucidated via systematic operando measurements and theoretical computations. The complementary chemistry is a universal principle that stabilizes the Na‐storage behaviors of Mn‐based oxide cathodes, providing new opportunities for anionic redox to boost the energy density and cycling life of SIBs simultaneously.

Unexpectedly High Cycling Stability Induced by a High Charge Cut‐Off Voltage of Layered Sodium Oxide Cathodes

“…There are two different prismatic sites for Na + , which are sharing faces (Na f ) and edges (Na e ) with the adjacent TMO 6 octahedra, respectively. Rietveld refinement demonstrates that Li + is mainly incorporated into the TM layers because of the similar radius between Li and TM ions, [27] which is further ascertained by the 7 Li magic-angle spinning (MAS) solid-state nuclear magnetic resonance (ss-NMR) spectrum (inset of Figure 1b). ≈ 87% of Li + ions locate in the TM layer (1750 and 1486 ppm) and 13% in the Na layer (720 ppm).…”

“…Such an unusual movement can be attributed to the oxidation of lattice oxygen, resulting in the reduced interlayer repulsion and smaller c-parameters. [24,27] During the discharge process, all the diffraction peaks evolve in totally opposite directions and finally restore to their original positions, demonstrating a highly reversible structural evolution of P2-NLNMO upon sodiation/desodiation. The detailed lattice parameters variation determined from the in situ XRD patterns is depicted in Figure 3b.…”

“…[14,15,[20][21][22][23][24] However, excessive oxygen oxidation will inevitably arouse irreversible lattice oxygen release, structural distortion, and TM migration, resulting in rapid capacity fading. [25][26][27] Many efforts have been devoted to enhancing the anionic redox reversibility, including 4d/5d TM substitution and surface modification. [28][29][30] Notwithstanding, the abovementioned strategies not only complicate the synthesis procedures, but also inevitably sacrifice a portion of capacity.…”

Superstructure Variation and Improved Cycling of Anion Redox Active Sodium Manganese Oxides Due to Doping by Iron

“…The slope region between 2.0 and 3.5 V becomes wider and wider, which has been found in previous reports and has been ascribed to the activation of Mn 4+ /Mn 3+ to compensate for capacity loss from O 2 n− /O 2− . [41][42] After 60 cycles, the discharge capacity decays accompanied with increasing polarization, and only 37.1 mA h g −1 is left after 100 cycles, corresponding to a capacity retention of 33.6%.…”

Structural degradation mechanisms and modulation technologies of layered oxide cathodes for sodium‐ion batteries

“…[8] Several different categories of compounds have been investigated as potential cathode materials. These include layered oxides, [9][10][11][12] polyanionic compounds, [13][14][15][16] Prussian blue analogs, [17,18] and some organics. [19,20] Among them, layered transition-metal (TM) oxides Na x TMO 2 (TM, TM ═ Fe, Mn, Ni, Co, Cr, Ti, V, Cu, and their combinations, 0 < x ≤ 1, the most common structures of Na x TMO 2 are P2, P3, and O3) have been extensively exploited due to their high capacities, good stability, and simple synthesis procedures, and are thus extensively being exploited.…”