Polyanion compounds are attractive for use as cathode materials because they generally have higher potentials for a given M^n+/(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 Co^2+/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-Li₂MnSiO₄⁹ 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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