The Synthesis,Modification And Electrochemical Properties Research Of LiMnPO₄ For Li-ion Batteries

Lithium-ion batteries(LIBs)are expected to maximize the development of electric vehicles and hybrid vehicles as a performance outstanding electrochemical energy storage device.Cathode materials in LIBs play a key role in determining energy density,power density,safety and environmental friendliness.LiMnPO4 has a higher operating voltage(4.1 V vs Li/Li+)compared to the widely applied LiFePO4,which increases the theoretical energy density by about 20%.However,its shortcomings limit the application like low electric conductivity,low ion diffusion rate,and "Jahn-Teller effect" of Mn3+bringing about the poor cycling performance.In this work,a series of composite materials were prepared by using carbon material,LiCoPO4 surface modification and cation doping strategies,respectively,which improved the electronic conductivity,used to prevent the disproportionation reaction of Mn3+,and Fe2+ and Co2+ reduced the Mn3+ "Jahn-Teller effect" and improved the cycle stability and rate performance.Eventually,a lithium ion battery cathode material with excellent performance was expected.The contents of this work are listed as follows:(1)LiMnPO4 nanoparticles were prepared by solvothermal method,and phenolic resin was formed on the surface of LiMnP04 by aldol condensation reaction between resorcinol and formaldehyde.After calcination,a carbon layer formed a conductive network to enhance the electrons conductivity of LiMnPO4.Consequently,the electrochemical results showed that the sample with 5.41 wt.%carbon had a higher specific discharge capacity(155 mAh g-1)at 0.1 C,capacity retention reached 85.88%after 60 cycles at 0.1 C/1C.(2)LiCoPO4 and LiMnP04 possess similar structures and lattice parameters,so they can be combined together in a better way,which reduced the side reaction of LiMnPO4 and electrolyte.First,2%LiCoPO4 surface-modified LiMnPO4 was prepared via two-step solvothermal method,and then combined with the exfoliated grapheme,which was used to improve the rate performance.The electrochemical results showed that the cycling and rate performance of LiMnPO4 were improved remarkably after surface modification.The discharge specific capacity was 78 mAh g-1 at 5 C.The capacity retention rate reached 80.6%after 700 cycles at 0.2 C/5 C.At the same time,the full cell assembled with graphite also shows good electrochemical performance.(3)LiMn0.8Fe0.15Co0.05SPO4 was synthesized by solvothermal method as cathode material for LIBs.Fe2+ and Co2+ with less ionic radius could effectively reduce the volume change during charge and discharge to increase the structural stability.The carbon coating could solve the low electronic conductivity and improve the rate performance.Two samples with different Fe and Co contents were prepared by solvothermal method.XRD and SEM showed that LiMn0.8Fe0.15Co0.05PO4 has more lithium ion reaction sites,and IR proved that LiMn0.8Fe0.15Co0.055PO4 has less Mn2+-Li+reverse defect.From the electrochemical results,LiMn0.8Fe0.15Co0.05PO4 delivered a higher discharge specific capacity(68 mAh g-1)at 10 C and the discharge specific capacity has no observable attenuation after 50 cycles at 0.2 C/1C...