| LiMnPO4 of the olivine family is expected to be a new-generation cathode material for lithium-ion batteries due to its high voltage platform（4.1 V）and theoretical energy density(697 Wh·kg-1).However,the Jahn-Teller effect of Mn3+will cause the deposition of Mn2+on the negative electrode and make the negative electrode material ineffective,which will seriously affect its discharge capacity and cycle performance.Substituting part of Fe for Mn can effectively alleviate this effect.However,LiMnPO4 and Li Fe PO4 both belong to the olivine structure,their ionic conductance(10-16～1.8×10-14 cm2·s-1)and electronic conductance(10-10～1.8×10-9S·cm-1)are very low.Therefore,in this paper,sucrose（SU）,the quinoline soluble substance from coal pitch（QS）,and graphene oxide（GO）/the quinoline soluble substance from coal pitch（QS）were used as coating carbon sources to study the effect of the coating carbon source addition and carbonization temperature on the structure and properties of LiMn0.8Fe0.2PO4/C.The following main conclusions are reached:（1）Aromatic quinoline soluble substance from coal pitch（QS）were obtained by solvent extraction.QS have C–N and C–S functional groups,which are beneficial for pyrolytic carbon N and S doping.QS pyrolytic carbon has a streamlined structure and high degree of graphitization with a carbon interlayer spacing of about 3.4478（?）.（2）QS was coated on the surface of LiMn0.8Fe0.2PO4 by mechanical stirring method,and LiMn0.8Fe0.2PO4/C cathode material was obtained by cracking.LiMn0.8Fe0.2PO4/C exhibited the best electrochemical performance when the amount of QS is 6.4 wt.%and the carbonization temperature is 670°C:The specific capacities at 0.1,1 and 5 C current densities are 159.2,143.6 and 117.6 m Ah·g-1,respectively.At the current density of 1 C,the high specific capacity of 133.5 m Ah·g-1 can be maintained with 92.8%of capacity retention after 150 cycles.（3）The surface of LiMn0.8Fe0.2PO4 was sequentially coated with GO and QS by mechanical stirring method,and the LiMn0.8Fe0.2PO4/C cathode material was obtained by pyrolysis.The cathode material exhibited the best electrochemical performance and structural stability when the additions of GO and QS are 1.5 wt.%and 4.9 wt.%,respectively.The discharge specific capacities at 0.2,1 and 5 C current densities are157.5,150.5 and 124.0 m Ah·g-1,respectively.At the current density of 1 C,the high specific capacity of 142.8 m Ah·g-1 can be maintained with 94.9%of capacity retention after 200 cycles. |
