Electrochemical properties of iron substituted lithium manganese phosphate prepared by sol-gel method

Abstract

Olivine LiMn0.4Fe0.6PO4 (LMFP) materials were synthesized by the modified sol–gel method with the addition of sucrose as an additional carbon source. Electrochemical properties of LMFP were advanced. The materials doped possess high reversible capacity of 107.4 mAh·g at 0.2 C-rate and excellent cycling stability (the capacity retention of 85.8% via 50 cycles), exhibiting the improved electrochemical properties. Introduction Currently, energy and environment are the most important topics. One of the greatest challenges is how to make use of Green Energy according to the strategy of sustainable development and the replacement of fossil fuels [1]. During the past decades, lithium ion batteries (LIBs) have dominated the portable electronic market and been applied to electric vehicles (EVs), hybrid EVs (HEVs) and smart grids, due to their high energy density and safety [2, 3]. All the time, lithium transition metal oxides are used as the main cathode active materials. However, these materials have some limitations for the intrinsic drawbacks, such as a poor thermal stability [4]. For this reason, the olivine-LiMPO4 (M =Mn, Fe, Co, Ni) compounds are drawing more and more interests owing to their low cost, environmentally benign nature, high capacity and thermal stability. As a promising candidate, lithium manganese phosphate LiMnPO4 is expected for its high energy density (700 Wh/kg) and aboundant resource [5]. Unfortunately, LiMnPO4 is being affected by the actual defect, such as slow lithium diffusion kinetics, low electronic and polaronic conductivity. In addition, the high structural strain at the phase boundary between charged and discharged phase and the instable structure due to Jahn-Teller distortion of Mn ion limit its application [6, 7, 8]. Hence, many efforts have been conducted to improve its properties. Recently, partial substitution by transition metal in LiMnPO4 is employed to achieve high energy density and stability. Indeed, some researches demonstrate an increase in kinetics when some of the Mn ions are replaced with Fe to form the solid solution LiMnxFe1-xPO4 [9]. Therefore, the solid solutions with an olivine structure is regarded as a promising material. What is more, the partial substitution of Mn ions by Fe ions can effectively improve the specific and rate capabilities, which may be attributed to the excellent contact area between active materials and electrolyte [10, 11]. In this work, we synthesize the LMFP utilizing Fe substitution by sol-gel method and investigate improved performance of LMFP, principally. LiMnPO4 sample is prepared as a comparison, as well. The results indicate that iron substituted lithium manganese phosphate exhibits distinctly improved electrochemical properties, compared to the simple lithium manganese phosphate. Experimental LiMn0.4Fe0.6PO4 was prepared by the modified sol-gel method, reported in a previous study [12]. All reagents were analytical grade. LiH2PO4, FeC2O4·2H2O, Mn(COOCH3)2·4H2O were dissolved in deionized water in the correct stoichiometric ratios to get a mixed solution, and a critic 5th International Conference on Environment, Materials, Chemistry and Power Electronics (EMCPE 2016) © 2016. The authors Published by Atlantis Press 342 acid solution was added to the preceding solution. Sucrose was added as an additional carbon source. After placed in oil bath at 80 °C for 10 h, the mixture was dried at 100 °C for 12 h in a vacuum oven. Finally, the gel was calcined at 700 °C for 10 h to obtain the composite. The structure was characterized by powder X-ray diffraction (XRD, Rint-2000, Rigaku, Japan) using Cu-Kα radiation over the 2θ range of 10°-80°. Electrochemical tests were performed by the coin cells (2032 type) that were assembled in an argon glove box. The cathode materials were formed from the active materials, a poly (tetrafluoroethylene) binder and acetylene black in 80:10:10 weight ratio. 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 vol. %) was used as the electrolyte between the lithium metal anode and the cathode. The capacities and cycle performances of the cells were carried out on a LAND CT2001A tester (Wuhan, China) in the voltage range of 2.5-4.5 V. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 10 kHz to 10 mHz, with a 5 mV a.c. input signal by CHI660C (Shanghai, China). Results and discussion 10 20 30 40 50 60 70 80 2 theta (degree) PDF#77-0178 In te ns ity (a .u .) x=1 x=0.4 LiMnxFe1-xPO4 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 2.0 2.5 3.0 3.5 4.0 4.5 5.0