Exploring the Ni redox activity in polyanionic compounds as conceivable high potential cathodes for Na rechargeable batteries

Although nickel-based polyanionic compounds are expected to exhibit a high operating voltage for batteries based on the Ni2+/3+ redox couple activity, some rare experimental studies on the electrochemical performance of these materials are reported, resulting from the poor kinetics of the bulk materials in both Li and Na nonaqueous systems. Herein, the electrochemical activity of the Ni2+/3+ redox couple in the mixed-polyanionic framework Na4Ni3(PO4)2(P2O7) is reported for the first time. This novel material exhibits a remarkably high operating voltage when cycled in sodium cells in both carbonate- and ionic liquid-based electrolytes. The application of a carbon coating and the use of an ionic liquid-based electrolyte enable the reversible sodium ion (de-)insertion in the host structure accompanied by the redox activity of Ni2+/3+ at operating voltages as high as 4.8 V vs Na/Na+. These results present the realization of Ni-based mixed polyanionic compounds with improved electrochemical activity and pave the way for the discovery of new Na-based high potential cathode materials

Microstructure of LiCoO2 with and without "AIPO(4)" nanoparticle coating: Combined STEM and XPS studies

"AlPO 4"-coated LiCoO 2 was shown to exhibit markedly improved capacity retention relative to bare LiCO 2 upon cycling to 4.7 V, Scanning and transmission electron microscopy imaging showed that the coating thickness of "AlPOV 4"-coated LiCoO 2 varied from ??? 10 to ??? 100 nm. Energy-dispersive X-ray mapping revealed that the coating was not single-phase "AlPO 4", rather consisting of P-rich thick regions (???100 nm) and Al-rich thin regions (???10 nm). Detailed X-ray photoelectron spectroscopy (XPS) studies of the "AlPO 4"-coated LiCoO 2 in comparison to bare LiCoO 2 and various reference compounds such as Li 2CO 3, Li 3PO 4, and AlPO4 indicate that (1) AlPO 4 is absent on the surface; (2) the surface consisted of Li 3PO 4 and heavily Al substituted LiAl yCo 1-yO 2, which may result from AlPO 4 nanoparticles reacting with bare LiCoO 2 during the coating heat treatment at 700??C; and (3) the amount of surface Li 2CO 3 is markedly reduced in the coated sample relative to the bare LiCoO 2. The existence of Li 3PO 4 in "AlPO 4"-coated LiCoO 2 was confirmed with X-ray powder diffraction. The coating microstructure of "AlPO 4"- coated LICoO 2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics by particle surface microstructure are discussed in detail.close524