1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Materials Science inc. Nanomaterials & Polymers The Doping of FeNb 11 O 29 as a Way to Improve Its Electrochemical Performances Marcella Bini,* Irene Quinzeni, and Daniele Spada [a] FeNb 11 O 29 , with the possibility to exchange up to 23 electrons per formula unit and a theoretical capacity value of 400 mAh/g, is nowadays a very promising anode material for Li-ion batteries. However, the improvement of its electrochemical performances is still an open challenge, and the doping is a well-recognized method to obtain this goal. In this paper, orthorhombic Fe 0.8 Mn 0.2 Nb 11 O 29 and Fe 0.8 V 0.2 Nb 11 O 29 samples were prepared by solid state synthesis. The redox processes, as determined by cyclic voltammetry, involve only Fe and Nb ions as in the undoped compound. FeNb 11 O 29 is a well-known mixed oxide with pseudocapacitive behavior, which is increased with the Mn doping. The doped samples show better performances during long term cycling at 2 C, with capacities of 240 and 220 mAh/g and capacity retention of 100.9 and 98.7% for Mn and V doped respectively. 1. Introduction The search of more performing electrode materials for lithium ion batteries is an intriguing although arduous challenge. [1–3] Very recently, as advanced anode material, emerged an intercalation compound, FeNb 11 O 29 possessing an interesting theoretical capacity of 400 mAh/g .[4–7] Thanks to three redox couples Fe 2 + /Fe 3 + , Nb 4 + /Nb 5 + and Nb 3 + /Nb 4 + up to 23 electrons can be transferred per formula unit so reaching a theoretical capacity higher than that of graphite. The high working potential of about 1.6 V guarantees cell safety and its open ReO 3 type structure, constituted only by octahedra, can promote the Li + diffusion in open and large channels. It is known that FeNb 11 O 29 can stabilize in two stable polymorphs [8,9] presenting subtle structural differences. At syn- thesis temperatures below 1250 °C the most stable phase is the monoclinic one, which undergoes an irreversible phase trans- formation to the orthorhombic symmetry at 1250 °C, as for its analogous parent compound Nb 12 O 29 . [10,11] This last shear compound was thoroughly characterized and, together with its structural features, magnetic, spectroscopic and electrochem- ical properties are reported. [12–14] For what concerns instead FeNb 11 O 29 , at present, only few studies are devoted to deepening the structural and physico- chemical features of both polymorphs. [8,9] It was proved that the pure orthorhombic polymorph guarantees higher capacity values and high stability for long term cycling also at C rates as high as 10 C, [7] but at slower C-rates a progressive capacity loss (up to 20% of the initial values) was verified.. The introduction of dopant ions could be an interesting and an easy way to improve the cycling stability, thanks to the formation of stronger covalent bonds that can reduce the crystal structure’s stress during repeated Li intercalation/de-intercalation. In the electrochemical literature, the doping can be considered as a way to increase the cell life and performances, as an alternative to the carbon coating of active material, the reduction of particle sizes or the addition of stabilizers to the electrolyte. [15–19] In the literature, at our knowledge, there are very few papers regarding the doping of FeNb 11 O 29 . Recently, Cr [6] and Mn and V [9] doping were proposed, with the aim to improve the structural stability and the electrical conduction of the material. In all the cases, better electrochemical performances with respect to the undoped compound, but also interesting spectroscopic and magnetic features were observed. The increase in structural stability and of capacity retention for the doped samples, suggests that this could be an interesting way to obtain more performing materials in order to satisfy the high requests of energy market, in particular of electric vehicles. In this paper, the doping effect of Mn and V ions on the electrochemical performances of orthorhombic FeNb 11 O 29 was studied. Fe 0.8 Mn 0.2 Nb 11 O 29 and Fe 0.8 V 0.2 Nb 11 O 29 were prepared by using conventional solid state syntheses at high temperature. Structural and morphological features, such as purity level and particle sizes and shapes were verified by X-ray powder diffraction (XRD) and Scanning Electron Microscopy (SEM). Cyclic voltammetry, charge-discharge cycles at different C rates, long term cycling at 2 C and 10 C were carried out to determine the electrochemical performances of the doped samples to be compared with those of the undoped FeNb 11 O 29 . 2. Results and discussion The formation of the desired orthorhombic doped compounds was verified by XRD. The comparison with the undoped sample demonstrates that no traces of reagents nor impurity phases [a] Prof. M. Bini, I. Quinzeni, D. Spada Dept. of Chemistry, University of Pavia, viale Taramelli 16, 27100 Pavia Italy E-mail: bini@unipv.it Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201901182 Full Papers DOI: 10.1002/slct.201901182 5656 ChemistrySelect 2019, 4,5656–5661 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim