Communication–Electrochemical Evaluation of Mn x Fe 1-x PO 4 (0 ≤ x ≤ 1) Cathode Materials for Na-Ion Batteries

Abstract

6 7 We report the electrochemical properties of carbon-coated particles of Mn x Fe 1-x PO 4 (0 ≤ x ≤ 1), a group of promising cathode's ma-terials for sodium-based batteries. The Mn x Fe 1-x PO 4 materials have been obtained by electrochemical delithiation of LiMn x Fe 1-x PO 4 and their electrochemical properties have been evaluated in Na cells. The mechanism of Na insertion/extraction strongly depends on the amount of manganese in the structure: at low amount of Mn (x ≤ 0.2) there is no specific domain associated to the electrochemical activity of Mn; at high amount of Mn (x ≥ 0.5) one can distinguish two voltage regions, one linked to the Mn 2+ /Mn 3+ , and the other one to the Fe 2+ /Fe 3+ redox couples. Energy storage has become a growing global concern over the past 17 decades as a result of a rapid increase of worldwide energy demand. 18 Li-ion batteries have attracted all the attention in energy storage due 19 to their largest energy and power densities. However, Na-ion batteries 20 are considered an interesting alternative to lithium-ion, especially for 21 low cost systems for stationary applications, due to richer sources and 22 lower costs of sodium. 1 23 The development of efficient electrode materials in a large scale, 24 and with satisfactory electrochemical properties is a key factor if Na-25 ion batteries are to be commercialized in the near future. A good strat-26 egy to achieve high energy-density Na-ion batteries is to utilize high 27 voltage cathode. 2–4 Nanostructured polyanion-type materials, such as 28 NaFePO 4 olivine, offer advantages such as high capacity and excel-29 lent stability. Previously, a capacity of 1 Na per formula unit, which 30 corresponds to 154 mA h g −1 at a voltage of about 3 V vs. Na/Na + , has 31 been reported. 5–8 The sodium electrochemical exchange in NaFePO 4 32 involves the formation of Na 2/3 FePO 4 intermediate phase 5–8 which is 33 radically different from what is observed for LiFePO 4 . 9–12 34 In this work, we have investigated the possibility of increas-35 ing the working voltage of Na-ion cells by using carbon-coated 36 Mn x Fe 1-x PO 4 (0 ≤ x ≤ 1) materials as positive electrodes. Car-37 bon coated LiMn x Fe 1-x PO 4 materials have been prepared via solid-38 state synthesis process. The LiMn x Fe 1-x PO 4 /C composites have been 39 delithiated by electrochemical oxidation, and then sodium has been 40 electrochemically reinserted in Na half-cell. The structural character-41 ization of the different materials includes X-ray diffraction (XRD) 42 and transmission electron microscopy (TEM). The electrochemical 43 measurements showed that Mn x Fe 1-x PO 4 /C (0 ≤ x ≤ 1) materials are 44 promising candidates for positive electrode materials for high energy 45 Na-ion batteries. 46 Experimental 47 Preparation of materials.—The LiMn x Fe 1-x PO 4 /C materials were 48 prepared by solid-state reaction according to Yamada et al. 13 Iron (II) 49 oxalate dehydrate; FeC 2 O 4 .2H 2 O (Aldrich, 99%), Ammonium phos-50 phate monobasic; NH 4 H 2 PO 4 (Sigma-Aldrich, 98.5%), Lithium car-51 bonate; Li 2 CO 3 (Sigma-Aldrich, 99%) and manganese (II) carbon-52 ate; MnCO 3 (Alfa-Aesar, 99%), were ball-milled in stoichiometric 53 amounts to match the target molecular formula, with carbon (Super 54 C65, Timcal) in 20 wt% of the mass of LiMn x Fe 1-x PO 4 . After ball– 55 milling the powders were treated at 700 • C, under argon for 12 hours. 56 Materials characterization.—X-ray diffraction was carried-out 57 with a Bruker D8 Advance diffractometer, Transmission electron-58 microscopy (TEM, Tecnai F20ST (FEI)), 200 kV, EDX detector and 59 High Angle Annular Dark Field (HAADF) detector, collection of 60 z E-mail: pkubiak@qf.org.qa chemical EDX maps was done in scanning transmission electron mi-61 croscopy (STEM) mode. 62 Electrochemical measurements.—The composition of the lami-63 nated electrode was: 80% wt active material; 10% wt binder (PVDF 64 P-5130 (Solvay)); 10% carbon black (Super C65 (Timcal)), with a 65 loading of ≈3 mg cm −2 . The electrochemical delithiation was per-66 formed in 2-electrodes Swagelok-type electrochemical cells, in CCCV 67 (constant current -constant voltage mode. Metallic lithium was used 68 as the counter electrode and LiPF 6 1 M in EC/DMC 50–50 wt. as the 69 electrolyte. The electrochemical insertion/extraction of sodium was 70 performed in 2-electrodes Swagelok-type electrochemical cells using 71 metallic sodium as counter electrode and NaPF 6 0.5 M in EC/DMC 72 50–50% wt. as electrolyte.