A high-performance Na-storage cathode enabled by layered P2-type KxMnO2 with enlarged interlayer spacing and fast diffusion channels for sodium-ion batteries

Abstract

Layered manganese-based oxides are considered the most promising candidates for cathode materials for sodium-ion batteries (SIBs) because of their high energy density, facile synthesis process, and abundant elemental resources. However, the relatively large size of Na⁺ diffusing in the narrow layer spacing causes inevitable structural failure and sluggish diffusion kinetics during the insertion/extraction process resulting in poor cycling performance. Here, we designed and synthesized a P2-type K⁺-expanded manganese-based layered metal oxide (K_0.69MnO₂, abbreviated as KMO), which possesses stabilized and enlarged Na⁺ diffusion channels for high energy density SIBs. The distensible layered interlayer space of KMO (6.36 Å) compared to that of pristine P2-Na_0.67MnO₂ (NMO, 5.62 Å) can provide larger ionic diffusion channels and more Na⁺ storage sites, which enables fast Na⁺ diffusion and ensures high rate performance. Consequently, the prepared KMO cathode delivers a high reversible capacity of 167.9 mA h g⁻¹ (at 20 mA g⁻¹), a fast rate capability of 112.5 mA h g⁻¹ at 500 mA g⁻¹, and good cycling stability with a capacity retention of 87.6% after 100 cycles at 50 mA g⁻¹. Furthermore, a full battery assembled with pre-activated KMO as the cathode and commercial hard carbon as the anode could deliver a high energy density of 195.4 W h kg⁻¹ based on the total active mass of the cathode and anode. This investigation provides new insights into designing high-performance cathode materials with fast ion diffusion channels for SIBs and beyond.