Abstract
Polyanion compounds are attractive for use as cathode materials because they generally have higher potentials for a given Mn+/(n+1)+ redox couple in comparison to the oxide analogue.1 In addition, the strong bonding within the polyanion group provides similar safety characteristics to oxides even though they exhibit higher working potentials.2 The phospho-olivine LiFePO4 has been thoroughly investigated as a cathode material for use in lithium-ion batteries since its introduction in 1997.3 It has been determined that NaMPO4 (M = Fe and Mn) compounds prefer the electrochemically inactive maricite structure and not the olivine structure when using conventional synthesis methods,4 which has left the research community seeking unorthodox methods to access the olivine phase.5 , 6 Herein we report on a NaCoPO4 (NCP) polymorph (Red-phase) that has previously only been observed as an impurity while attempting to synthesize other polymorphs.7 The Red-phase was synthesized by a microwave-assisted solvothermal process using tetraethylene glycol as the solvent. Ex-situ XRD and XANES measurements indicate that during cycling the structure undergoes reversible changes and the Co2+/3+ redox couple is partially active near 4.4 V vs. Na. The silicate family of cathode materials is rich in polymorphism8 and has also been thoroughly investigated for use in lithium-ion batteries. Among the polymorphs investigated is Pn-Li2MnSiO4 9 that was accessed by ion-exchange of the parent Pn-Na2MnSiO4 compound. Yet, minimal work has been reported on the sodium analogue.9 , 10 Considering the air sensitivity reported of the lithium version,11 herein we report preliminary results on the effect of air exposure on the electrochemistry of Pn-Na2MnSiO4. § cjohnson@anl.gov Acknowledgements Work supported by U. S. DOE, Office of Science under Contract No. DE-AC02-06CH11357. We also thank the Argonne LDRD program office for additional support.
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