Constructing a Composite Structure by a Gradient Mg2+ Doping Strategy for High-Performance Sodium-Ion Batteries

Layered P2-Na_0.67Mn_0.67Ni_0.33O₂ has been considered an attractive cathode material for sodium-ion batteries (SIBs). Nevertheless, it is still burdened with hazardous phase transformation of P2-O2 under high voltage and harmful reactions at the interface of the electrode and electrolyte. These result in unfavorable structural degradation and rapid capacity decay. Herein, a gradient Mg²⁺ doping approach is proposed to trigger a structural transformation. During the annealing process, the bulk-diffused Mg²⁺ and surface residual Mg²⁺ induce the formation of the P2/P3@MgO structure. Consequently, this method combines the merits of the composite phases, bulk doping, and surface modification. In consequence, Na⁺ diffusion kinetics and electrochemical performances are remarkably enhanced. The cells using P2/P3@MgO show 69.7% capacity retention at 0.2 C within a voltage range of 1.5-4.5 V for 100 cycles, compared with the 42.6% for P2-Na_0.67Mn_0.67Ni_0.33O₂. This work offers new insights into further developments of advanced layered oxide cathodes for SIBs.

Scalable Nitrate Treatment for Constructing Integrated Surface Structures to Mitigate Capacity Fading and Voltage Decay of Li-Rich Layered Oxides

Capacity fading and voltage decay is one of the biggest obstacles for the practical application of Li-rich layered oxides due to the serious surface-related detrimental reactions. Herein, we develop a versatile and scalable method to construct a robust surface-integrated structure. All the designed samples deliver outstanding capacity and voltage stability, among which the Zn-treated sample possesses the best electrochemical performance. Its capacity retention is larger than 90 % after 400 cycles with a voltage decay ratio as small as 0.73 mV per cycle. What is more, the rules of surface-integrated structure with different cations in terms of capacity and voltage stability is further deciphered by combining with density function theory (DFT) calculations. It is found that to obtain advanced Li-rich layered oxide cathodes, cations in Li-sites should firstly ensure the binding energy of the surface-integrated structure in a lower level and then provide Bader charge for the nearest O atoms as small as possible.