Realizing High-Performance Cathodes with Cationic and Anionic Redox Reactions in High-Sodium-Content P2-Type Oxides for Sodium-Ion Batteries

Abstract

P2-type layered transition-metal oxides with anionic redox reactions are promising cathodes for sodium-ion batteries. In this work, a high-sodium-content P2-type Na7/9Li1/9Mg1/9Cu1/9Mn2/3O2 (NLMC) cathode material is prepared by substituting Li/Mg/Cu for Mn sites in Na2/3MnO2. The Li/Mg ions trigger the anionic redox reaction, while the Cu ions enhance the structure stability during electrochemical cycling. As a result, the oxide has a high reversible capacity of 225 mAh g–1 originating from both cationic and anionic redox activities with a capacity retention of 77% after 100 cycles. The migration energy barrier and Na ion diffusion kinetics are studied using density functional theory (DFT) calculations and the galvanostatic intermittent titration technique. Furthermore, X-ray diffraction, DFT, scanning electron microscopy, and transmission electron microscopy are applied to reveal the structural evolution and charge compensation of NLMC, providing a thorough understanding of the structural and morphology evolution of Na-deficient oxides during cycling. The results are inspiring for the design of a high-Na content P2-type layered oxide cathode for sodium-ion batteries.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

“…Generally, some cathode materials are known to be beneficial for rate performance, but are unfavourable for matching with anodes to build full batteries because of the low initial Na content in their crystal structure. 190,191 Although some materials have a high initial sodium content, which is favourable for matching with the anode to form a full battery, their crystal structure, which is unfavourable for sodium ion transport, results in a poor rate performance, and these materials also experience gradual capacity degradation due to multiple phase transitions over a long period during cycling. [192][193][194] Thus, to develop effective layered oxide cathodes for SIBs, it is a promising method to tailor the cathodes with a radially aligned hierarchical structure, which has a varying concentration gradient of chemical composition from the inside to the outside.…”

Electrochemical Storage Behavior of a High-Capacity Mg-Doped P2-Type Na2/3Fe1−yMnyO2 Cathode Material Synthesized by a Sol–Gel Method

“…Surface-sensitive X-ray photoelectron spectroscopy (XPS) was applied to clarify the valence states and composition of the as-synthesized P2-Na 2/3 Mg 2/9 Fe 2/9 Mn 5/9 O 2 material, and the results are shown in Figure 2. The deconvoluted XPS spectra of Mn 2p (Figure 2a) are fitted into two peaks at 643.67 eV and 655.1 eV, which can be assigned to Mn 4+ (2p 3/2 ) and Mn 4+ (2p 1/2 ) species, respectively [19,20]. The Fe 2p spectrum exhibits two peaks at 724.4 and 711.1 eV, corresponding to Fe 2p 1/2 and Fe 2p 3/2 , respectively (Figure 2b).…”

Predominant P3-Type Solid–Solution Phase Transition Enables High-Stability O3-Type Na-Ion Cathodes

“…On the contrary, low Na content leads to a larger d (O−Na−O) interlayer space, therefore promoting the formation of a P2-type structure. 18,19 O3-type NaNi 0.5 Mn 0.5 O 2 can exhibit high capacity that benefited from high Na content and adequate Ni 2+/4+ redox couple. Nevertheless, it suffers irreversible phase transformation with great volume variation at elevated cutoff voltages, which is detrimental for cycling stability.…”