Understanding the Effect of Atomic-Scale Surface Migration of Bridging Ions in Binding Li3PO4 to the Surface of Spinel Cathode Materials

Spinel cathode materials (e.g., LiMn2O4 and LiNi0.5Mn1.5O4) with strongly bonded surface coatings are desirable for delivering improved electrochemical performance in long-term cycling. Here, we report that the introduction of bridging ions such as Fe and Co, which can diffuse into both the spinel cathode materials and Li3PO4, the latter is found to cover the spinel surface in the form of dense and uniform particles (similar to 2-3 nm). Detailed structural analysis of the surface reveals that the bridging ions diffuse into the 16c site of the spinel structure to form ion-doped spinel cathode materials, which contribute to the formation of strong bonds between the surface and Li3PO4, possibly via spinel-(surface bridging ions)-Li3PO4 bonds. The critical role of the surface bridging ions is further investigated by heating the as-formed Li3PO4-coated spinel cathode materials (with bridging ions) to high temperatures, resulting in further diffusion of bringing ions from the surface to the interior of the spinel materials and consequently depletion of the surface spinel-(surface bridging ions)-Li3PO4 bonds. This leads to the gradual growth of surface Li3PO4 particles (similar to 20 nm) and the exposure of the spinel surface.