The Role of Surface Coating in P'2-Na0.67Mn0.67Ni0.33O2: Enhancing Capacity and Stability of Layered Cathodes for Sodium-ion Batteries

P'2-Na0.67Mn0.67Ni0.33O2 (NMNO) is one of the most promising cathodic materials for new generation sodium-ion batteries (SIBs). One drawback that is preventing its commercialization is however the high instability in the potential window 2.5-4.5 V vs. Na+/Na, caused by degradation reactions affecting the material's structure and composition. Herein we propose a strategy to overcome this weak spot introducing the surface coating of the material particles with inert magnesium oxide, that has been chosen as coating agent because of its low molecular weight. This protective layer is able to reduce the structural instability of the material without modifying the mixed conduction properties and thus globally stabilizing the electrochemical performances. In fact, the electrode of NMNO coated with 7 % of MgO, after an initial stabilization, was able to deliver over 90 mAh g(-1) of reversible specific capacity at a current value of 50 mA g(-1), with a capacity retention of 70 % after 250 cycles, to be compared with the 30 mAh g-1 of the uncoated material. The effect of the coating on the particles surface is here investigated with differential electrochemical mass spectrometry, to analyze the influence of the treatment on the electrode/electrolyte interphase.