Towards Novel High-Voltage Cathode Polyanionic Materials: Exploration of the Na2Ο–V2O3–P2O5 System
Authors: Kovrugin Vadim M. , Chen Fan , Chotard Jean-Noël , David Rénald , Masquelier Christian 
Published: February 2020

Polyanionic-type materials have a wider variety of crystal architectures compared to structurally simple transition metal oxides, thus leading to a huge diversity of possible structural features beneficial to some specific electrochemical properties of the batteries [1, 2]. In particular, the 4-, 5-, and 6-fold coordinations and various oxidation states (from +2 to +5) of vanadium in polyanionic compositions may result in the formation of various structural building units, each with its own structural role in the crystal structures. Electrochemically, vanadium also provides the opportunity of exchanging more than one electron per transition metal upon charge/discharge processes. At the same time, natural abundance of sodium and its low cost compared to lithium used in widespread electrode materials have recently attracted a significant attention of the research community to such Na-containing polyanion-based materials [3,4].
However, there are still only 5 reported compositions in the Na2O–V2O3–P2O5 system so far (Fig. 1). Among them, the only NASICON-type Na3V3+
2(PO4)3 phase with a theoretical capacity of about 118 mAh/g at 3.4 V vs. Na/Na+ has been deeply investigated owing to its stable long-term cyclability and high mobility of sodium ions [5]. It was recently also demonstrated that the metal substitution in NASICON-type phases may lead to increasing of the theoretical gravimetric capacity and the average operating voltage due to the access to the V4+/V5+redox couple [6].
In order to continue further exploration of the Na2O–V2O3–P2O5 system, herein we present new compounds, which were structurally and electrochemically characterized. We also report the results of our investigation of the influence of a substitution of a part of vanadium by aluminum in the Na7V3+
4(P2O7)4(PO4) and Na3V3+
2(PO4)3 compositions on the electrochemical behavior of these materials.
This work was carried out under the framework of the NAIADES project titled “Na-ion battery demonstration for electric storage” supported by the Regional Council of Picardie and the University of Picardie Jules Verne.

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Figure 1