Revealing the Origin of Transition‐Metal Migration in Layered Sodium‐Ion Battery Cathodes: Random Na Extraction and Na‐Free Layer Formation

Abstract

Cation migration often occurs in layered oxide cathodes of lithium-ion batteries due to the similar ion radius of Li and transition metals (TMs). Although Na and TM show a big difference of ion radius, TMs in layered cathodes of sodium-ion batteries (SIBs) can still migrate to Na layer, leading to serious electrochemical degeneration. To elucidate the origin of TM migration in layered SIB cathodes, we choose NaCrO 2 , a typical layered cathode suffering from serious TM migration, as a model material and find that the TM migration is derived from the random desodiation and subsequent formation of Na-free layer at high charge potential. A Ru/Ti co-doping strategy is developed to address the issue, where the doped active Ru is first oxidized to create a selective desodiation and the doped inactive Ti can function as a pillar to avoid complete desodiation in Ru-contained TM layers, leading to the suppression of the Na-free layer formation and subsequent enhanced electrochemical performance.

Recent Progress in the Emerging Modification Strategies for Layered Oxide Cathodes toward Practicable Sodium Ion Batteries

“…The co-doped Na 0.67 Mn 0.8 Ni 0.1 Mg 0.1 O 2 cathode exhibits best rate performance, delivering high reversible capacities of 110, 66, and 37 mA h g −1 at the high current densities of 480 (2 C), 1200 (5 C) and 1920 mA g −1 (8 C), respectively. Chu et al [91] prepared a Ru/Ti co-doped Na 0.85 Cr 0.85 Ru 0.10 Ti 0.05 O 2 (NCRT) cathode via a simple solidstate reaction. The theoretical calculation based on the DFT was first introduced to predict the impact of Ru/Ti co-doping on the crystal structure and electron structure.…”

“…Chu et al. [ 91 ] prepared a Ru/Ti co‐doped Na 0.85 Cr 0.85 Ru 0.10 Ti 0.05 O 2 (NCRT) cathode via a simple solid‐state reaction. The theoretical calculation based on the DFT was first introduced to predict the impact of Ru/Ti co‐doping on the crystal structure and electron structure.…”

“…d) The spatial electron distribution of NCTO at the region of −1.0 ≤ E-E f ≤ 0 (Left) and −2.0 ≤ E-E f ≤ −1.0 (Right).The crystal structures of e) NC and f) NCRT at x = 0.67, respectively. Reproduced with permission [91]. Copyright 2023, Wiley-VCH.…”

O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.15O2 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries

“…Among various sodium cathode candidates, layered transition metal oxides Na x TMO 2 (where TM refer to transition metal ion) have attracted much attention due to its advantages such as high specific capacity, simple preparation, and environmental friendliness. − Compared with P2- and P3-type layered oxides, the sufficient sodium content enables O3-type layered oxides to be advantageous over counterparts in full-cell applications. − However, their cycle life is still subject to undesired structural degradation caused by transition metal slab sliding upon charging to high voltages (>4 V), leading to fast capacity decay and poor cycling stability in NIBs. − In addition, most O3-type materials suffer from notable performance deterioration when storing in a humid environment, which undoubtedly increases their cost of transportation and preservation. ,,− For O3-NaNi 0.5 Mn 0.5 O 2 , when the charging cutoff voltage is greater than 4.1 V, its specific capacity is up to 180 mAh g –1 . , However, a series of complex phase transitions (O3–O′3–P3–P′3–P3′–O1) cause significant internal stress, leading to the collapse of layered structure, resulting in capacity decay and poor rate performance. ,, In addition, when O3-NaNi 0.5 Mn 0.5 O 2 was exposed to air for 2 h, the structure changed from O3 phase to O′3- and P3-Na 1– y Ni 0.5 Mn 0.5 O 2 , which also leads to the degradation in electrochemical performance. , Undoubtedly, further practical application of O3-type layered oxides for NIBs requires addressing both phase transition reversibility during deep desodiation and humid sensitivity when exposed to ambient air.…”

Crystal Facet Design in Layered Oxide Cathode Enables Low-Temperature Sodium-Ion Batteries

“…The GITT measurement was first performed to study the Na + diffusion coefficients in both electrodes. According to previous reports, if the voltage is a linear function of the square root of time during the pulse time, the rate of Na + diffusion can be determined by following the simplified formula: D normalN normala + = 4 π τ ( m B V M M B A ) 2 ( Δ E s Δ E τ ) 2 , τ ≪ L 2 D in which D Na + (cm 2 s –1 ) is the Na + diffusion rate; τ (s) represents the testing time in each pulse; m B (g), M B (g mol –1 ), and V M (cm 3 mol –1 ) are the weight, molar weight, and molar volume of the active materials, respectively; Δ E τ and Δ E s are the variation of voltage during each pulse time and quasi-equilibrium potential during one step (Figure S8a, Supporting Information), respectively; and A represents the surface area of the electrode. , The GITT curves for NCO-AC and NCO-O are displayed in Figure a,b, respectively. The Na + diffusion coefficients were determined for both samples according to eq due to the linear relationship of V vs τ 1/2 (Figure S8b, Supporting Information).…”