Recent progress in the development of sodium insertion materials for Na-ion batteries show that they can be considered as interesting alternative to Li-ion batteries owing to more abundant and widespread sodium resources and lower cost as compared to lithium [1,2].

Olivine transition metal phosphates isostructural to the well-known LiFePO₄ are good positives electrode candidates for Na-ion batteries. Indeed, it has been shown that 1 Na per formula unit can be reversibly deintercalated from olivine NaFePO₄ at a potential of 2.8 V vs. Na/Na⁺ delivering a theoretical capacity of 154 mAh/g [3,4]. However, materials working at higher voltage and with higher cycling potential are highly desirable to increase the energy density. Following this approach, mixed Na[Fe,Mn]PO₄compositions with olivine structure are very promising[5].

In this work, we describe the synthesis and characterisation of the solid solution NaMn_xFe_1-xPO₄ (0≤x≤1) prepared from their lithium counterparts (LiMn_xFe_1-xPO₄). We have analysed the structural evolution of these materials with varying Mn content and the electrochemical properties of carbon coated NaMn_xFe_1-xPO₄(0≤x≤1) phosphate materials have been characterized in Na half-cells by galvanostatic cycling under various conditions. The results obtained will be here presented and discussed in terms of energy density and efficiency.

[1]B. L. Ellis, W. R. M. Makahnouk, Y. Makimura, K. Toghill and L. F. Nazar, Nat. Mater. (2007), 6, 749.

[2]V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energy Environ. Sci., (2012), 5, 5884.

[3] P. Moreau, D. Guyomard, J. Gaubicher, F. Boucher. Chem Mater. (2010), 22, 4126-4128.

[4] M. Casas-Cabanas, V. Roddatis, D. Saurel, P. Kubiak, J. Carretero-Gonzalez, V. Palomares, P. Serras and T. Rojo, J. Mater. Chem. (2012), 22, 17421.

[5] K. T. Lee, T. N. Ramesh, F. Nan, G. Botton, L. F. Nazar. Chem. Mater. (2011), 23, 3593-3600.