LiMn1−xFexPO4 Nanorods Grown on Graphene Sheets for Ultrahigh‐Rate‐Performance Lithium Ion Batteries

Olivine-type lithium transition-metal phosphates LiMPO4 (M=Fe, Mn, Co, or Ni) have been intensively investigated as promising cathode materials for rechargeable lithium ion batteries (LIBs) owing to their high capacity, excellent cycle life, thermal stability, environmental benignity, and low cost. However, the inherently low ionic and electrical conductivities of LiMPO4 seriously limit Li + insertion and extraction and charge transport rates in these materials. In recent years, these obstacles have been overcome for LiFePO4 by reducing the size of LiFePO4 particles to the nanoscale and applying conductive surface coatings such as carbon, which leads to commercially viable LiFePO4 cathode materials. Compared to LiFePO4, LiMnPO4 is an attractive cathode material owing to its higher Li intercalation potential of 4.1 V versus Li/Li (3.4 V for LiFePO4), providing about 20% higher energy density than LiFePO4 for LIBs. [14–19] Importantly, the 4.1 V intercalation potential of LiMnPO4 is compatible with most of the currently used liquid electrolytes. However, the electrical conductivity of LiMnPO4 is lower than the already insulating LiFePO4 by five orders of magnitude, making it challenging to achieve high capacity at high rates for LiMnPO4 using methods developed for LiFePO4. [14–19] Doping LiMnPO4 with Fe has been pursued to enhance conductivity and stability of the material in its charged form. Recently, Martha et al. have obtained improved capacity and rate performance for carbon-coated LiMn0.8Fe0.2PO4 nanoparticles synthesized by a high-temperature solid-state reaction. Graphene is an ideal substrate for growing and anchoring insulating materials for energy storage applications because of its high conductivity, light weight, high mechanical strength, and structural flexibility. The electrochemical performance of various electrode materials can be significantly boosted by rendering them conducting with graphene sheets. Recent work has shown improved specific capacities and rate capabilities of simple oxide nanomaterials (Mn3O4, Co3O4, and Fe3O4) grown on graphene as LIB anode materials. However, it remains a challenge to grow nanocrystals on graphene sheets in solution for materials with more sophisticated compositions and structures, such as LiMn1 xFexPO4, which is a promising but extremely insulating cathode material for LIBs. Herein we present a two-step approach for synthesis of LiMn1 xFexPO4 nanorods on reduced graphene oxide sheets stably suspended in solution. Fe-doped Mn3O4 nanoparticles were first selectively grown onto graphene oxide by controlled hydrolysis. The oxide nanoparticle precursors then reacted solvothermally with Li and phosphate ions and were transformed into LiMn1 xFexPO4 on the surface of reduced graphene oxide sheets. With a total content of 26 wt% conductive carbon, the resulting hybrid of nanorods and graphene showed high specific capacity and unprecedentedly high power rate for LiMn1 xFexPO4 type of cathode materials. Stable capacities of 132 mAhg 1 and 107 mAhg 1 were obtained at high discharge rates of 20C and 50C, which is 85% and 70% of the capacity at C/2 (155 mAhg ), respectively. This affords LIBs with both high energy and high power densities. This is also the first synthesis of LiMn0.75Fe0.25PO4 nanorods that have an ideal crystal shape and morphology for fast Li diffusion along the radial [010] direction of the nanorods. Figure 1 shows our two-step solution-phase reaction for the synthesis LiMn0.75Fe0.25PO4 nanorods on reduced graphene oxide (for experimental details, see the Supporting Information). The first step was to selectively grow oxide nanoparticles at 80 8C on mildly oxidized graphene oxide (mGO) stably suspended in a solution. Controlling the hydrolysis rate of Mn(OAc)2 and Fe(NO3)3 by adjusting the H2O/N,N-dimethylformamide (DMF) solvent ratio and the reaction temperature afforded selective and uniform coating of circa 10 nm nanoparticles of Fe-doped Mn3O4 (Supporting Information, Figure S1a; X-ray diffraction data in Figure S1b) on the mGO sheets without free growth of nanoparticles in solution. Importantly, our mGO was made by a modified Hummers method (Supporting Information), with which a sixfold lower concentration of KMnO4 oxidizer was used to afford milder oxidation of graphite. The resulting mGO sheets contained a lower oxygen content than Hummers GO (ca. 15% vs. ca. 30% measured by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy) and showed higher electrical conductivity when chemically reduced than [*] H. Wang, Y. Liang, H. Sanchez Casalongue, Y. Li, G. Hong, Prof. H. Dai Department of Chemistry Stanford University, Stanford, CA 94305 (USA) E-mail: hdai@stanford.edu Y. Yang, L. Cui, Prof. Y. Cui Department of Materials Science and Engineering Stanford University, Stanford, CA 94305 (USA) E-mail: yicui@stanford.edu [] These authors contributed equally to this work.