Modification Of LiMnPO₄Cathode Material

Olivine-structured lithium manganese phosphate (LiMnPO4) has been intensivelystudied as a promising cathode materials for lithium ion batteries owing to excellentthermal stability, low cost, high safety and environmental benignity. The theoretical energydensity of LiMnPO4is about20%higher than that of commercialized LiFePO4due to itshigher Li+intercalation potential of4.1V (vs. Li+/Li). Moreover, the high operatingvoltage of LiMnPO4is compatible with present non-aqueous organic electrolytes oflithium ion batteries. However, LiMnPO4suffers from poor electronic conductivity andlow lithium diffusivity resulting in its low discharge capacity and poor rate capability. Andthese intrinsic disadvantages hinder LiMnPO4from its practical applications in lithium ionbatteries. In this work, Ti addition, synthesizing LiMnPO4/Li3V2(PO4)3composite material,mixing with LiMn2O4were used to improve the electrochemical performance of LiMnPO4cathode materials.1. Ti additive LiMnPO4cathode materials were successfully synthesized through asolid-state method. The effects of titanium source and the content of titanium on themorphologies, structures and electrochemical properties of the prepared samples werestudied in detail. Sinopharm TiO2, P25TiO2, TiO2nanopowders and Tetra-n-butylTitanate were used as titanium source respectively, the results show TiO2nanopowders isthe best among the several kinds of titanium source. Using TiO2nanopowders as titaniumsource, a series of the (1-x)LiMnPO4·LixTix(PO4)δsamples were prepared by changing thecontent of titanium. The samples with Ti addition exhibit excellent electrochemicalperformance compared to the pure LiMnPO4sample. Meanwhile, the electrochemicalperformance is correlated with the content of Ti. The sample prepared with10%Ti isconsidered to be the optimum.0.9LiMnPO4·Li0.1Ti0.1(PO4)δshows the discharge specific capacityof131mAh·g-1at0.05C, and no capacity fade is observed even after50cycles at0.5C.2. A series of (1-x)LiMnPO4·xLi3V2(PO4)3/C (x=0,0.1,0.2,0.3,0.4,0.5,1)composites cathode materials are successfully synthesized by solid-state method. XRDresults reveal that the composites comprise Li3V2(PO4)3phase and LiMnPO4phase with asmall amount of LiVP2O7impurity. The experimental results show the partial mutualdoping between LiMnPO4and Li3V2(PO4)3phases could happen during the syntheticprocess. And the ratios of LiMnPO4to Li3V2(PO4)3are correlated with the electrochemicalperformances of the composite materials. Among these samples, the x=0.4sample exhibitsthe best electrochemical performance with the discharge specific capacity of154mAh·g-1at0.05C, and more than95mAh·g-1at0.5C after50cycles.3. LiMnPO4and LiMn2O4were mixed in certain proportion to prepare the(1-x)LiMn2O4+xLiMnPO4cathode materials. The content of Mn dissolution andelectrochemical performance of the samples were measured, it turned out that thedischarge capacity and the cycling performance have been enhanced after mixing the twokinds of cathode materials at25℃or55℃. Among them, the x=0.5sample exhibits thelowest content of Mn dissolution and the better electrochemical performance. At25℃,capacity retention of the x=0.5sample is95.8%at1C after50cycles; at55℃, its specificdischarge capacity is131.8mAh·g-1at0.1C, and its capacity retention is96.5%at1C after50cycles.