Sodium Reactivity in Na-M-O Systems (with M = Mn, V), Cathodes for Na-Ion Batteries

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-O 1,2 . 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 Na 2 V 3 O 7 . The Kagome network of Na 4 Mn 2 O 5 is made of layers of corner-sharing square-pyramids of MnO 5 3 . This material was synthesized by a conventional solid-state method starting from Mn 2 O 3 and Na 2 O 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 NaMnO 2 . 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: Na 2 Mn 3 O 7 . This material, obtained via a simple synthesis from cheap sodium and manganese salts, consists of Mn-vacancy-[Mn 3 O 7 ] 2- layers built up with edge-sharing MnO 6 octahedra, separated by NaO 6 and NaO 5 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 mV 4 . Thus, the electrochemical process allows a reduced phase Na 4 Mn 3 O 7 to be obtained, which cans intercalate/de-intercalate two sodium per f.u., reversibly 5 . 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 calculations 6 . 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 [Mn 3 O 7 ] 2- layers 8 . 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 Na 2 M 3 O 7 phase: Na 2 V 3 O 7 . We report the sodium extraction from Na 2 V 3 O 7 which is a tunnel type structure built of [V 3 O 7 ] 2- ∞ nanotubes hold by sodium ions 9 . In this case a reversible charge capacity of 80 mAh/g at 2.8 V vs Na + /Na is due to the V 4+ /V 5+ redox activity. Moreover, no oxygen-redox reaction was observed in the vanadium (5+) oxide Na 4 V 2 O 7 . The mechanism of extraction as well as the structures of the as-prepared and oxidized phases will be discussed in this presentation. 1 X. Ma, H. Chen and G. Ceder, J. Electrochem. Soc. , 2011, 158 , A1307–A1312. 2 J. Billaud, R. J. Clément, A. R. Armstrong, J. Canales-Vázquez, P. Rozier, C. P. Grey and P. G. Bruce, J. Am. Chem. Soc. , 2014, 136 , 17243–17248. 3 G. Brachtel and R. Hoppe, Z. Für Anorg. Allg. Chem. , 1980, 468 , 130–136. 4 E. Adamczyk and V. Pralong, Chem. Mater. , 2017, 29 , 4645–4648. 5 E. Adamczyk, E. Anger, M. Freire and V. Pralong, Dalton Trans . , 2018, 47 , 3112-3118. 6 Z. Zhang, D. Wu, X. Zhang, X. Zhao, H. Zhang, F. Ding, Z. Xie and Z. Zhou, J. Mater. Chem. A , 2017, 5 , 12752. 7 D.-H. Seo, J. Lee, A. Urban, R. Malik, S. Kang and G. Ceder, Nat. Chem. , 2016, 8 , 692–697. 8 B. M. de Boisse, S. Nishimura, E. Watanabe, L. Lander, A. Tsuchimoto, J. Kikkawa, E. Kobayashi, D. Asakura, M. Okubo and A. Yamada, Adv. Energy Mater. , 2018, 8 , 1800409. 9 E. Adamczyk, M. Gnanavel and V. Pralong, Materials , 2018, 11 , 1021.