Jun John Xu

Synthesis and Characterization of Carbon-Coated Lithium Transition Metal Phosphates LiMPO4 (M = Fe , Mn, Co, Ni) Prepared via a Nonaqueous Sol-Gel Route

An organic solvent-based sol-gel method has been utilized for the synthesis of lithium transition metal phosphates. With this simple and versatile method, particles with sub-μm size and uniform size distribution are obtained for all the LiMPO 4 (M = Fe, Mn, Co. Ni) materials investigated. Homogeneous in situ carbon coating of a few weight percent is achieved with all of them, except LiCoPO 4 , where the in situ carbon coating is only 0.6 wt %. The best-performing as-prepared LiFePO 4 , with in situ surface carbon coating of 1.8 wt %, achieves an electronic conductivity on the order of 10 -2 S/cm and exhibits highly promising electrochemical performance, with only slight dependence on the carbon content of the composite electrode in certain range. Carbon-coated LiFePO 4 samples prepared at lower temperatures exhibit lower electronic conductivity, evidently due to lower specific conductivity of the surface carbon. The dependence of the morphology and electrochemical performance of the synthesized LiFePO 4 on the precursor concentration and the aging time of the gel is investigated. The results also yield information for understanding of the mechanisms of the sol-gel synthesis process, which are discussed. Electrochemical performance of the carbon-coated LiMnPO 4 and LiCoPO 4 was tested and discussed.

A Nanocrystalline Ferric Oxide Cathode for Rechargeable Lithium Batteries

Lithium intercalation hosts as the positive electrode (cathode) are central to the chemistry and key to the energy density of rechargeable lithium batteries. The high cost and toxicity of the commercially used LiCoO 2 cathode have prompted extensive searches for alternatives. While manganese oxides have been investigated most extensively, iron oxides are even more attractive from cost and environmental standpoints. However, search for iron oxides of conventional crystalline structures and micrometer particle sizes as lithium intercalation cathodes has been greeted with disappointing results. Here we report a nanocrystalline ferric oxide that simultaneously exhibits very high lithium intercalation capacity, excellent rate capability and capacity retention upon cycling. These properties reveal thermodynamics and kinetics of the nanocrystalline material inherently different from those of its microcrystalline counterpart, and suggest promise of nanostructured intercalation compounds as electrodes for rechargeable lithium batteries.

Amorphous manganese oxides as lithium intercalation hosts prepared by oxidation of Mn(II) precursors

Amorphous manganese oxides were prepared by oxidation of Mn(II) precursors in aqueous solutions at room temperature. The oxidation procedure offers a number of advantages including a good control of chemical composition and morphology. X-ray and electron diffraction of as-synthesized materials reveal a complete lack of long-range order. Synthesized amorphous manganese dioxide with a low sodium content (0.09 Na/Mn) exhibits a specific capacity of 350, 290, and 250 mAh/g at discharge/charge rates of C/100, C/50, and C/5, respectively. The performance indicates not only extremely high lithium intercalation capacities, but also excellent rate capabilities. Electrochemical behavior during discharge/charge cycling suggests occurrence of local structural relaxations in the amorphous material; however, no trend toward a spinel-like behavior is observed. It is found that the presence of a large amount of sodium in the amorphous material (0.27 Na/Mn) lowers the lithium intercalation capacity but greatly improves capacity retention upon cycling. The sodium ions, which reside stably in the amorphous host as an electrochemically inactive species, may help stabilize the local structure thus leading to excellent cycling performance.

Nanosized Amorphous Iron Oxyhydroxide for Reversible Lithium Intercalation

An iron oxyhydroxide was synthesized via a low-temperature aqueous solution route and investigated as a lithium intercalation host. The chemical composition of the material was determined to be Na 0.01 Fe(OOH) 0.99 . The material possessed a nanosized morphology and a nearly amorphous structure resembling α-FeOOH or the mineral goethite. It exhibited an open-circuit voltage (OCV) of 3.1 V vs. Li + /Li. At a slow discharge rate of C/100 or 62 μA/cm 2 and between the OCV and 1.5 V, close to 0.9 Li per Fe was intercalated into the material, corresponding to a specific capacity of 260 mAh/g. This high intercalation capacity was almost completely reversible. At a high discharge rate of C/10 or 0.53 mA/cm 2 , a specific capacity of 215 mAh/g was attained, indicating excellent rate performance. At the same time, the material exhibited nearly perfect capacity retention upon cycling over 30 cycles. These superior electrochemical properties are attributable to the nearly amorphous structure and nanosize morphology of the material, and point to the promise of intercalation hosts of unconventional, noncrystalline structures, At the same time, the results indicate that the hydroxide ions present in the material did not interfere with lithium intercalation, permitting nearly perfect reversibility of the intercalation process and excellent capacity retention upon cycling.

Effect of Copper Doping on Intercalation Properties of Amorphous Manganese Oxides Prepared by Oxidation of Mn(II) Precursors

Copper-doped amorphous manganese oxides were synthesized via a room-temperature aqueous solution route involving oxidation of Mn(II) precursors. Doping of copper ions did not significantly affect the low rate specific capacity or the intrinsic lithium intercalation capacity of the amorphous materials. However, even a small amount of copper doping dramatically increased the specific capacity at high rates. Larger amounts of copper doping also significantly enhanced capacity retention upon cycling. For an amorphous manganese dioxide doped with 0.18 Cu per Mn, a specific capacity of 275 mAh/g was attained between 4.0 and 1.5 V vs. Li + /Li at a high discharge/charge rate of C/5 (0.78 mA/cm 2 ), with capacity fading of less than 0.5% per cycle upon cycling. It is believed that copper doping has the dual effect of both improving the lithium intercalation kinetics and stabilizing the local structure of the amorphous materials during lithium intercalation and cycling.