Transition-Metal Pyrophosphates As Cathodes for Rechargeable Na-Ion Batteries: Effects on the Electrochemical Properties of Composition and Preparation Methods
Authors: Tealdi Cristina , Ricci Monica , Ferrara Chiara , Bruni Giovanna , Quartarone Eliana , Mustarelli Piercarlo 
Published: February 2020

The urgency of decreasing atmosphere pollution is driving industry toward more environmentally-friendly power sources, including solar, wind and hydro. As intermittent renewable energy technologies will become prevalent and integrated into the electrical grid, however, availability of large-scale storage systems will be crucial. At present, Li-ion batteries (LIBs) are one of the most encouraging technologies. On the other hand, grid-scale storage requires low cost, safety, and eco-friendly devices, and LIBs are approaching their technical limits combined with fast lithium sources pauperisation. Therefore, other chemistries must be considered, and Na-ion batteries (SIBs) are under rapidly development. In this context, materials for Na-ion batteries, in particular cathodic ones, are the subject of a growing research interest. [1,2]
Among the cathode materials which are currently proposed for use in Na-ion batteries, an interesting family is that of sodium metal pyrophosphates having general formula Na2MP2O7(M = Co, Cu, Fe, Mn).[3] In fact, chiefly when Fe is employed as the transition metal, all the involved elements are non-toxic, abundant and environmentally-friendly. Pyrophosphate-based cathodes are normally prepared by means of a solid-state reaction process, possibly followed by reduction of the particle size and carbon coating. [4]
In this study we focused our attention both on the Mn-Fe substitution and on the relationships among synthesis procedure and functional properties in determining the electrochemical properties of the cathode material. The solid solution Na2Fe1−x
Mn
x
P2O7 (x = 0, 0.25, 0.5, 0.75, 1) was prepared by conventional solid-state reaction. In addition, the Na2FeP2O7 end-member was also prepared through different synthetic procedures: glucose-assisted, solid-state reaction and wet-chemistry (citrate) methods. The materials were characterized for what concerns their structural, morphological and electrochemical process through X-ray diffraction, scanning electron microscopy, cyclic voltammetry, charge/discharge curves. The results show that the system forms a complete solid solution over the entire composition range. The electrochemical performances are severely affected by the increase of Mn content, at least in the case of conventional high temperature solid-state reaction and standard slurry preparation. [5] The electrochemical results are compared and discussed in terms of morphological considerations. Interestingly, the electrochemical behavior of the Na2FeP2O7 compound is strongly dependent upon the preparation method, especially for high C-rates. This is consistent with the fact that different preparation methods can modulate the morphological properties and the carbon coating of the material.

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