A Facile Surfactant-assisted Solid-state Method For The Preparation Of High Power Density LiMnPO₄-based Cathode Materials

Rechargeable lithium ion batteries (LIBs) are one of the most promising energy storagedevices for electric vehicles (EV) and hybrid electric vehicles (HEV) because of their highenergy density, high power density, low cost, superior safety, and stable cycling lifespan.Olivine structured lithium transition metal phosphates (LiMPO4)as the promising cathodematerial due to its appealing features including low cost, non-toxicity, high energy density andthermal stability. However, the inherently low electronic conductivity (<10-10s/cm) and ionicconductivity (<10-16cm2/s) of this material hamper its practical application.To improve the low electronic conductivity and lithium ion diffusion coefficient of theLiMnPO4material, in this paper, the four major research works have been investigated asfollows:1) Optimization of the manganese and iron precursors for the preparation ofLiMnPO4-based cathode materials;2) Study the effect of the calcination temperature and time, the carbon sources, andcarbon contents on the LiMnPO4-based cathode materials to optimize the best synthesisconditions;3) Controll the particle size of the LiMnPO4-based cathode materials usingsurfactant-assisted ball-milling route;4) To improve the lithium ion diffusion of the LiMnPO4-based cathode materials byadjustment of the Fe-doped contents.The experimental results demonstrate:1) The MnxFe1-xC2O4·2H2O (x=1.0,0.85,0.5)complexes act as good precursors for preparation of homogenous LiMnxFe1-xPO4/C solidsolution composites;2) The LiMnxFe1-xPO4/C solid solution composites with goodcrystallinity and uniform distribution of carbon coating layer on the surface of the compositescan be obtained from the calcination the precursors of MnxFe1-xC2O4·2H2O andphenolic resinat600℃for10h;3) The oleic acid as a surfactant, for the scale preparation ofLiMnxFe1-xPO4/C composites, plays a critical role in decreasing particle size of the obtained cathode materials;4) Fe-doped improve the lithium ion diffusion rate can be improved bysuitable Fe-doped content; and5) LiMn0.85Fe0.15PO4/C composite has higher energydensity when it cycled at low current density, and LiMn0.5Fe0.5PO4/C composite hashigher energy density when it cycled at high current density. A compromise composition ofLiMn0.5Fe0.5PO4/C may be the best option for high-power applications such as EV/HEVs thatare keen on rapid charge and durability.Especially, LiMn0.5Fe0.5PO4/C composite with particle size of around40nm and6wt%carbon content, synthesizing bya facile surfactant-assisted solid-state reaction, shows goodelectrochemical performance as a cathode material for LIBs.It delivers discharge capacities of155.0,140.9and121mAhg-1at respective0.1,1and5C (1C=170mAg-1) rates. Meanwhile,it showsstable cycling stability at both25oC (94.8%and90.8%capacity retention after500cycles at1and5C rates, respectively) and55oC (89.2%capacity retention after300cycles at5C rate). The Li-ion diffusion coeffcients of the LiMn0.5Fe0.5PO4/C composite weredetermined to1.7310-11~1.7210-11cm2/s for manganese and1110-11~3.4610-11cm2/s for iron, respectively. The satisfactory electrochemical performance of the obtainedLiMn0.5Fe0.5PO4/C composite could be attributed to the synersgistic effect of the smallparticle size, the presence of a uniform carbon coating layer on the primary particles and thesuitable Fe-doped content.