ORIGINAL PAPER Synthesis and characterization of LiMn 1-x Fe x PO 4 /carbon nanotubes composites as cathodes for Li-ion batteries T. T. D. Nguyen & L. Dimesso & G. Cherkashinin & J. C. Jaud & S. Lauterbach & R. Hausbrand & W. Jaegermann Received: 20 July 2012 / Revised: 14 December 2012 / Accepted: 8 January 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Carbon nanotubes (CNT) coated with LiMn 1-x Fe x PO 4 (0.2 ≤ x ≤ 0.8), as possible cathode materials, was synthesized by using a sol–gel process (Polyol method), after annealing under flowing nitrogen. X-ray diffraction (XRD) patterns of the composites confirmed the formation of the olivine structured LiMn 1-x Fe x PO 4 phase and no sec- ondary phases were detected. The morphological investiga- tion revealed the formation of agglomerates with particles size ranging between 300 and 700 nm. XRD investigation of composites shows difference of the morphology by doping CNT and carbon black in the composites. Transmission electron microscopy shows the growth of nano-sized par- ticles on CNT (20–70 nm) and the agglomeration of primary particles to form secondary particles. The X-ray photoelec- tron spectroscopy showed that the Fe and Mn ions are in divalent states in the LiMn 1-x Fe x PO 4 composites. The cyclic voltamograms showed the oxidation peaks of iron and man- ganese ions at 3.53–3.63 and 4.05–4.33 V, respectively, while the reduction peaks were found at 3.21–3.42 V (iron reduction) and 3.85–3.93 V (manganese reduction) depend- ing on the iron content in the composition. The LiMn 0.6- Fe 0.4 PO 4 /CNT composite (x =0.4) (with 20 %wt CNT) delivered a specific capacity of 120 mAhg -1 (at a discharge rate of C/20 and RT). Keywords Cathode materials . Lithium iron manganese phosphates . Composites . Sol–gel Introduction Among the new cathode materials which can replace the transition metal oxide in the lithium ion batteries, phosphor- olivine LiMPO 4 (M = Fe, Mn, Co and Ni) have attracted a lot of attention due to the high theoretical capacity and opera- tive voltage vs. Li/Li + (3.4, 4.1 and 4.8 V for M=Fe, Mn and Co, respectively) [1–3]. Further, the low-cost, non- toxicity and high thermal stability meet the requirements for the production of rechargeable lithium batteries in large scale for electric/hybrid vehicles and load leveling systems [1]. The lithium metal phosphates crystallize with an olivine- type structure which contains a distorted hexagonal close- packing of oxygen anions, with three types of cations occu- pying the interstitial sites: (1) corner-sharing MO 6 (M= transition metal) octahedra, which are nearly coplanar to form a distorted two-dimensional square lattice perpendicu- lar to the a-axis; (2) edge-sharing LiO 6 octahedra aligned in parallel chains along the b-axis; and (3) tetrahedral PO 4 groups connecting neighboring planes or arrays. The iron-containing system has been the most extensive- ly studied, although the members of the family show some advantages over the Fe–phosphate. As example, the theo- retical energy density of LiMnPO 4 is 697 Whkg -1 against 586 Whkg -1 for LiFePO 4 whereas the Mn 2+ ↔Mn 3+ redox reaction occurs at 4.1 V vs. Li/Li + against 3.4 V for the Fe 2+ ↔Fe 3+ couple, which is low enough to avoid any decomposition of the non-aqueous electrolyte. Despite the better electrochemical properties, the current density of the Li x MnPO 4 system is several orders of magnitude smaller than that of Li x FePO 4 [4, 5], and the kinetics of the deintercalation/intercalation processes is much slower [1, 6, 7]. In fact, LiFePO 4 is classified as a T. T. D. Nguyen (*) : L. Dimesso : G. Cherkashinin : J. C. Jaud : S. Lauterbach : R. Hausbrand : W. Jaegermann Surface Investigation Division, Materials Science Department, Technische Universität Darmstadt, Petersenstraße 23, 64287 Darmstadt, Germany e-mail: nguyendung@surface.tu-darmstadt.de Ionics DOI 10.1007/s11581-013-0848-7