A Na-Free Surface Enables "Three-In-One" Enhancement of Structural, Interfacial and Air Stability for Sodium-Ion Battery Cathodes.

O3-type layered oxides are regarded as one of the most promising cathode materials for sodium-ion batteries. However, the multistep phase transitions, severe electrode/electrolyte parasitic reactions, and moisture sensitivity are challenging for their practical application because of the highly active Na+. Here, a Na-free layer is built on the surface of NaNi1/3Mn1/3Fe1/3O2 (NMF111) via a leaching treatment and the subsequent surface reconstruction. Accordingly, both the structural degradation from bulk to surface and the overgrowth of the solid electrolyte interface (SEI) are greatly ameliorated, which results in the improved capacity retention of modified NMF111 from 58.3% to 89.6% after 400 cycles at 1 C. Besides, the Na-free surface with rock-salt structure prevents the H+/Na+ exchange and then enables good reversibility and low polarization of the optimal NMF111 when exposed to wet air (50% RH) for 4 days. This work opens a new avenue for the comprehensive cyclability improvement of layered oxides via surface reconstruction.

An Active Strategy to Reduce Residual Alkali for High-Performance Layered Oxide Cathode Materials of Sodium-Ion Batteries.

Residual alkali is one of the biggest challenges for the commercialization of sodium-based layered transition metal oxide cathode materials since it can even inevitably appear during the production process. Herein, taking O3-type Na0.9Ni0.25Mn0.4Fe0.2Mg0.1Ti0.05O2 as an example, an active strategy is proposed to reduce residual alkali by slowing the cooling rate, which can be achieved in one-step preparation method. It is suggested that slow cooling can significantly enhance the internal uniformity of the material, facilitating the reintegration of Na+ into the bulk material during the calcination cooling phase, therefore substantially reducing residual alkali. The strategy can remarkably suppress the slurry gelation and gas evolution and enhance the structural stability. Compared to naturally cooled cathode materials, the capacity retention of the slowly cooled electrode material increases from 76.2% to 85.7% after 300 cycles at 1 C. This work offers a versatile approach to the development of advanced cathode materials toward practical applications.

Tuning the Solvation Structure in Water-Based Solution Enables Surface Reconstruction of Layered Oxide Cathodes toward Long Lifespan Sodium-Ion Batteries.

Layered oxides of sodium-ion batteries suffer from severe side reactions on the electrode/electrolyte interface, leading to fast capacity degradation. Although surface reconstruction strategies are widely used to solve the above issues, the utilization of the low-cost wet chemical method is extremely challenging for moisture-sensitive Na-based oxide materials. Here, the solvation tuning strategy is proposed to overcome the deterioration of NaNi1/3Mn1/3Fe1/3O2 in water-based solution and conduct the surface reconstruction. When capturing the water molecules by the solvation structure of cations, here is Li+, the structural collapse and degradation of layered oxides in water-based solvents are greatly mitigated. Furthermore, Li(H2O)3EA+ promotes the profitable Li+/Na+ exchange to build a robust surface, which hampers the decomposition of electrolytes and the structural evolution upon cycling. Accordingly, the lifespan of Li-reinforced materials is prolonged to three times that of the pristine one. This work represents a step forward in understanding the surface reconstruction operated in a water-based solution for high-performance sodium layered oxide cathodes.