| Olivine-structured LiFePO4 and LiMnPO4(LiMPO4,M=Fe,Mn)are considered as ideal high-power Li-ion battery cathode materials,however,the relatively low ionic diffusion rate and electrical conductivity would lead to the poor high rate discharge performance and severly hinder their application in high-power Li-ion batteries.Many studies have demonstated that there exists a rapid Li ion channel in olivine LiMPO4.Therefore,reducing the Li ion transport channel to nanoscale is significant for improving high-rate performances,fulfilling fast charging and discharging and finnaly achieving the practical application of high-power LiMPO4 batteries.In this present work,firstly,according to the mechanisms and structures of Li-ion batteries,general situation and development of LiMPO4 cathode materials have been thoroughly discussed.Afterwards,in order to improve the high rate charge and discharge performance of LiMPO4 cathode material,monodispersed LiMPO4 single crystal nanorods have been synthesized via a hydrothermal method.Through the investigation of the effect of Li,Mn,Fe,P precursor reagents which introduce SO42-,K+,NH4+ ions and mineralizer KOH on the synthesis and morphology transformation of LiMPO4,the formation mechanism of LiMPO4[001]nanorods has been figured out.And then,on the basis of the monodispersed LiMPO4 nanorods,LiMPO4@C core-shell nanorods have been prepared.Due to the size reduction along the significant Li+ ion transport pathway and the effective improvement of core-shell carbon coating structures upon electric conductivity and structural stability,the as-prepared LiMPO4@C core-shell nanorods deliver excellent high rate discharge capacities.The main research achievements are summarized as follows:(1)With manganese sulfate,lithium sulfate and ammonium dihydrogen phosphate as preecusor reactant,EG and distilled water in the volume ratio of 1:1 as solvent,flower-like LiMnPO4 nanostructures in space group Pmnb self-assembled with(010)nanosheets have been successfully synthesized under low KOH concentration(0.094 mol/L),and stamen-like LiMnPO4 nanostructures in space group Pmnb self-assembled with[001]nanorods prepared at high KOH concentration(0.125 mol/L).The synergetic effects of the SO42-ions introduced by Li2SO4 and MnSO4 precusor reagents and the K+ ions introduced by mineralizer KOH are responsible for the formation of the nanobelts and nanorods in the LiMnPO4 self-assembly nanostructures.Along(010)facets,there is a Mn-rich face in olivine LiMnPO4.In the hydrothermal system,the preferential combination of SO42-ions on(010)suppress the deposition of LiMnPO4 species along(010)facets,making LiMnPO4 crystals grow to(010)nanosheets at low KOH concentration.At high KOH concentration,a lot of K+ ions are also released into the hydrothermal system due to the ionization of the introduced KOH,the released K+ions also tend to combine with O2-ions situated on(100)planes,which effectively inhibits the deposition of LiMnPO4 species on(100)planes,resulting in the LiMnPO4 olivine crystals grow to[001]nanorods.In order to reduce system energy,the(010)nanosheets formed at low KOH concentration self-assemble to flower-like LiMnPO4 nanostructures and the[001]nanorods formed at high KOH concentration self-assemble to stamen-like LiMnPO4 nanostructures.(2)With manganese sulfate,lithium sulfate as Mn and Li precusors,potassium dihydrogen phosphate as substitute for ammonium dihydrogen phosphate as P precursor,mono-dispersed(010)nanobelts and[001]nanorods have been successfully hydrothermally synthesized at low KOH concentration and high KOH concentration,respectively.The(010)nanobelts have thicknesses of ca.50 nm and widths of 300 nm and lengths of 500 nm,while the[001]nanorods have diameters of 50?80 nm and lengths of 500 nm.The K+ ions introduced by potassium dihydrogen phosphate precursor play essential role in the formation and monodipersion of the nanobelts and nanorods.With more amount of K+ ions were introduced into the hydrothermal system,the combination effect of K+ ions with the O2-ions situated at(100)planes was inhanced,the LiMnPO4 species formed by the dehydration and condensation of the precursors on(100)was suppressed.In consequence,under the synergetic effect of the fast bound K+ions on(100)planes and the fast bound SO42-ions the fonned LiMnPO4 crystallizes to[001]nanorods.Moreover,because more amount of K+ ions are involved in the hydrothermal system,more amount of K+ ions tend to combine with the SO42-ions fast bound on(010)planes,making all the surfaces of the formed LiMnPO4(010)nanobelts and[001]nanorods are covered by the fast bound K+ ions.Due to the electrostatic repulsion,the as-prepared LiMnPO4(010)nanobelts and[001]nanorods are of well monodispersibility.Furthermore,based on this method,with lithium sulfate,manganese sulfate and ferrous sulfate as Li,Mn and Fe precursor and potassium dihydrogen phosphate as P precursor,KOH as mineralizer,monodispersed LiFePO4 and LiMn0.8Fe0.2PO4[001]single-crystal nanorods have been prepared under a same mechanisim.The as-prepared LiFePO4[001]nanorods have diameters less than 100 nm and lengths of 500 nm.The as-prepared LiMn0.8Fe0.2PO4[001]nanorods have diameters of 50?80 um and lengths of 500 nm.(3)Based on the hydrothermally synthesized monodispersed LiMnPO4,LiFePO4 and LiMn0.8Fe0.2PO4[001]nanorods,fully carbon-coated LiMnPO4@C,LiFePO4@C and LiMn0.8Fe0.2PO4@C core-shell nanorods have been successfully synthesized.As for the flower-like LiMnPO4 nanostructures self-assembled with nanosheets and stamen-like LiMnPO4 nanostructures self-assembled with nanorods,due to the sintering bond of the nanosheets and nanorods,the the carbon coatings are impaired in the self-assembled structures,resulting in the flower-like and stamen-like LiMnPO4/C nanostructures.Moreover,due to the the good electronic contact arising from the full carbon coatings and size reducing along the significant Li+ ion transportation pathway,the LiMnPO4@C,LiFePO4@C and LiMn0.8Fe0.2PO4@C core-shell nanorods express excellent diacharge capacities and stable cycle performance.At the rate of 0.1 C,LiMnPO4@C core-shell nanobelts and nanorods diliver discharge capacities of 92.3and 114.2 mAh·g-1,while the flower-like and stamen-like LiMnPO4/C nanostructures only diliver relatively lower discharge capacities of 79.0 and 87.1 mAh·g-1,respectively.As for the LiMnPO4@C core-shell nanorods and nanobelts,due to the reducing of the Li ion transportation pathway of[100],the LiMnPO4@C nanorods derive a high Li-ion diffusion ability compared to the LiMnPO4@C nanobelts.With regard to LiFePO4@C core-shell nanorods,the discharge capacity reaches as high as 173.8 mAh·g-1 at 0.1 C.After 500 cycles,the discharge capacity still achieves 113 mAh·g-1,which corresponds to 96.5%capacity retention of the initial discharge capacity at 10 C,revealing excellent high rate cycle performance.At 0.1 C the discharge capacity of the LiMn0.8Fe0.2PO4@C core-shell nanorods reaches as high as 128.1 mAh·g-1. |
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