Revisiting the alluaudite NaMnFe2(PO4)3 sodium insertion material: Structural, diffusional and electrochemical insights

Among the gamut of sodium battery insertion materials, NaMnFe2(PO4)(3) was reported as the first alluaudite framework compound albeit with poor electrochemical activity [Chem. Mater. 22 (2010) 5554], We hereby report auto-combustion synthesis of carbon coated NaMnFe2(PO4)(3) alluaudite and its Na+ diffusion, ionic conductivity and electrochemical activity synergizing experiments with bond valence site energy (BVSE) modeling. It registered a 2.8 V redox activity with a reversible capacity of similar to 60 mAh g(-1) with good cycling stability. BVSE calculations revealed an exceptionally low one-dimensional migration barrier of 0.31 eV for Na-ion diffusion. It was in sync with the low activation energy barrier of 0.162 eV derived from ac impedance spectroscopy. NaMnFe2(PO4)(3) alluaudite cathode was found to have conductivity value of 0.5 x 10(-6)S cm(-1) at room temperature. Among the PO4-based sodium battery insertion materials, NaMnFe2(PO4)(3) alluaudite shows excellent ionic conductivity with very low Na+ migration barrier. It can lead to the realization of superior reversible capacity in this alluaudite cathode comparable to LiFePO4. (C) 2018 Elsevier Ltd. All rights reserved.