Controlled Synthesis And Electrochemical Behavior Of LiMnPO₄Micro-/Nanocrystals

Along with the development and utilization of new energy sources, the rapiddevelopment of portable electronic products, and the increasing pressure on facingenviromental protection, the demand for lithium ion batteries with highperformances is more urgent than ever, which brings new opportunities andchallenges to further development for lithium ion batteries, yet the researches andapplications of lithium ion batteries have achieved great successes. The cathodematerial is an important part of lithium ion battery, which restrict the performancesfor the further improvement and the cost for the further reduction. LiMnPO₄is avery promising cathode material in electrochemical performance and industrial costfor lithium ion battery. However, LiMnPO₄suffers from very low electricalconductivity, which leads to poor electrochemical properties. The main factorsgoverning the electrical conductivity of LiMnPO₄probably include grain size,dispensability, cryallographic orientation and modification, which can bemanipulated for improving the electrical conductivity and enhancing theelectrochemical properties of LiMnPO₄.Several novel morphologies of LiMnPO₄micro-/nanocrystals had beencontrollably synthesized by employing Na2S·9H₂O as a sole additive viahydrothermal/solovthermal routes. The possible formation was proposed. Theeffects of grain size, dispensability, crystallographic orientation, and modificationon electrochemical behaviors of LiMnPO₄cathode material were discussed. Theobtained phases, structures and morphologies of LiMnPO₄samples werecharacterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）,transmission electron microscopy （TEM）. The charge/discharge measurement, cyclicvoltammetry （CV） measurement, and electrochemical impedance spectroscopy （EIS）measurement were carried out to test the electrochemical behavior of LiMnPO₄cathode material. In this work, we have obtained the following conclusions.Several novel LiMnPO₄morphologies had been controllably synthesized byhydrothermal method, including microspheres assembled by plates, wedges, prisms,and disperse morphologies with plates, wedges, prisms, respectively. Thecrystallographic orientations and dispensabilities of LiMnPO₄crystals werecontrollably manipulated. The three dispersed morphologies samples had a similardegree of the dispensabilty as well as size of the grain, and different the shapefeatures and the crystallographic orientations. The three microspheres morphologiessamples possessed a similar size of microsphere, yet the assembly units of themicrospheres had different the shape features and the crystallographic orientations. The dispersed the plates and the microsphere assembled with plates possessed largepercentage of exposed （010） facets as well as small thickness along the [010]direction. The three dispersed morphologies samples exhibited betterelectrochemical properties than the corresponding microspheres samples. Theexcellent electrochemical performance of the dispersed samples can be attributed toits well-dispsered morphology advangtage. The plate-like crystals displayedsuperior electrochemical performance over the wedge-like, and prism-like crystals,which can be attributed to its special crystallographic orientation.The well-dispersed rectangular rods with a length of1-4μm, a width of⁵00nm and a thickness of around200nm were prepared by hydrothermal process. The[010] direction was just along the thinnest part of the rectangular rods. Thewell-dispersed rods with a length of200-600nm, a mean width of ca.500nm andthickness of around200nm, and the well-dispersed rods with a length of200-400nm, a width of ca.200nm and a thickness of around100nm were obtained bysolvothermal route. These rods had a similar shape feature as well as dispensability,and different the grain sizes. The results of the electrochemical measurementsconfirmed that the charge/discharge and the rate capability of the cells wereenhanced, and the cycling stability of the cells was slightly decreased along with thereduction of the grain sizes.The splitting process was proposed to elucidate the growth mechanism of themorphologies synthesized by hydrothermal and solvothermal methods. Themesophases of NH4MnPO₄·H₂O and MnHPO₄·2.25H₂O were formed at the earlyreaction stage, and finally the LiMnPO₄phase was obtained. The crystallographicorientations of the as-prepared crystals could be strongly governed by acetate ionand/or sulphion and/or hydroxyl.The electrochemical properties of doping samples such as LiMn_0.95M_0.05PO₄（M=Al, Ce, Fe, In） and LiMn_1-yFe_yPO₄（y=0.1,0.3,0.5） were worse than that of thewell-dispersed rods, which could be ascribe to the blocking of the diffusion channelfor lithium ion owing to the doping cation in the lithium ion site. The ratecapabilities and cycling stabilities of the LiMnPO₄/C composites obtain by in-situand ex-situ routes with sucrose were better than that of the LiMnPO₄/C compositesprepared by ball milling method with Super P, yet the capacities of the LiMnPO₄/Ccomposites obtained by sucrose slightly decreased at low charging/discharging rate.For example, The LiMnPO₄/C composites prepared by in-situ route with3g sucroseexhibited the discharge capacities of139.4,132.6,129.1,119.9and109.7mA h g^-1at0.05C,0.1C,0.2C,0.5C and1C, respectively, whereas the LiMnPO₄/Ccomposites obtained by ball milling method with Super P delivered the dischargecapacities of142.1,126.1,98.1,81.5and70.5mA h g^-1at0.05C,0.1C,0.2C,0.5C and1C, respectively.93.7%and91.4%of the initial discharge capacities could be retained over50cycles at a charging/discharging rate of0.1C at roomtemperature in cell potential range of2.4-4.5V for the LiMnPO₄/C compositesobtained by3g sucrose and Super P, respectively.