Sodium Reactivity in Na-M-O Systems (with M = Mn, V), Cathodes for Na-Ion Batteries
Authors: Adamczyk Evan , Pralong Valerie 
Published: February 2020

Among the cathodes of the sodium ion batteries, the manganese and vanadium based oxide materials present many advantages due to their high energy density, low-cost and low-toxicity. In particular, numerous layered materials have been reported in the system Na-Mn-O1,2. These materials are interesting because they show weak interlayer interactions with free space allowing sodium diffusion. In search of new phases, we present two lamellar compounds of this system with interesting electrochemical properties as well as the first sodium extraction/insertion from Na2V3O7.
The Kagome network of Na4Mn2O5 is made of layers of corner-sharing square-pyramids of MnO5
3. This material was synthesized by a conventional solid-state method starting from Mn2O3 and Na2O and it shows a first charge capacity of 380 mAh/g and a reversible capacity of 130 mAh/g. By mechanical alloying and from the same precursors, a nanostructured material shows a different structure, close to NaMnO2. The reversible capacity is then improved to 220 mAh/g.
Here, we also report for the first time the electrochemical activity of another lamellar phase of this system: Na2Mn3O7. This material, obtained via a simple synthesis from cheap sodium and manganese salts, consists of Mn-vacancy-[Mn3O7]2- layers built up with edge-sharing MnO6 octahedra, separated by NaO6 and NaO5 polyhedra. Starting from this phase, a reversible capacity of 2 Na/f.u. (160 mAh/g) is obtained through a plateau at 2.1 V with a low polarization of 100 mV4. Thus, the electrochemical process allows a reduced phase Na4Mn3O7 to be obtained, which cans intercalate/de-intercalate two sodium per f.u., reversibly5. Interestingly, an additional reversible redox process, corresponding to the extraction of 1.5 Na+, is observed on oxidation at 4.1 V due to the oxygen redox activity, consistent with DFT calculations6. Based on the theoretical explanation given by Ceder et al.7, this oxygen redox activity is explained by the presence of ☐-O-Na axes due to the Mn vacancies in the [Mn3O7]2- layers8. Therefore, by cycling this material in the potential range 4.7-1.5 V, the reversible specific capacity reaches 200 mAh/g.
In comparison, no oxygen activity has been observed for another Na2M3O7 phase: Na2V3O7. We report the sodium extraction from Na2V3O7 which is a tunnel type structure built of [V3O7]2-
∞ nanotubes hold by sodium ions9. In this case a reversible charge capacity of 80 mAh/g at 2.8 V vs Na+/Na is due to the V4+/V5+ redox activity. Moreover, no oxygen-redox reaction was observed in the vanadium (5+) oxide Na4V2O7.
The mechanism of extraction as well as the structures of the as-prepared and oxidized phases will be discussed in this presentation.
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