Preparation And Performance Study Of Lithium Iron Phosphate/Graphene-type Composites

Olivine-type lithium iron phosphate?LiFePO₄?is used as cathode material of lithium ion batteries?LIBs?because of its good electrochemical performance,such as high open-circuit voltage?3.45 V vs.Li⁺/Li?and high theoretical discharge capacity(¹70 mA h g^-1),steady structure during the charge/discharge process.What is more,it has low cost,high safety,environmental-friendliness,as well as long cycle life and natural abundant raw materials.However,there is an instinct drawback of structure that baffles the marketization of LiFePO₄in the field of electrical vehicle,and that is the poor rate performance.The main methods to improve rate performance of LiFePO₄ include ion doping,surface coating,nanocrystallization,ect.In this research,we designed and prepared both 3D structured LFP-based composites which are 3D LiFePO₄/graphene nanocomposites and 3D LiFePO₄/fluorine-doped graphene-like carbon nanocomposites,and realized the in-situ effectively compound between LiFePO₄ and graphene or graphene-like carbon.Further,we explored and studied the synthesis methods,physical properties,electrochemical performances and the synergistic effect of the composites.This work provides efficient technological approaches for developing new type phosphate cathode materials for power LIBs.Composites of graphene-decorated carbon-coated LiFePO₄ nanospheres are sythesized using a high-energy ball milling-assisted rheological phase method combined with a solid-state reaction.The experimental results show that unfolded physical exfoliated multilayer graphene films are decorating carbon-coated LiFePO₄ nanospheres without stacking,which results in an abundance of mesopores constituting a unique 3D“sheets-in-pellets”and“pellets-on-sheets”conducting network structure.This structure highlights the incereses of the rate and cyclic performance as a cathode material of lithium-ion batteries,because the highly conductive and plentiful mesopores promote electronic and ionic transport.As a result,the hybrids with approximately 3 wt.%unfolded physical exfoliated multilayer graphene exhibit an outstanding rate capability with an initial discharge capacity of 163.8 and 147.1 mA h g^-1 at 0.1 and 1 C,and the capacity is retained at81.2 mA h g^-1 even at 20 C.Moreover,the composites also reveal an excellent cycling stability with the capacity retention is 92%at 10 C after 500 cycles.Polyvinylidene fluoride is used as a novel carbon and fluoride source for coating LiFePO₄to obtain LiFePO₄@fluoride-doped graphene-like carbon nanocomposites.The fluoride-doped graphene-like carbon that is in situ coated on nanometer-sized LiFePO₄particles constituting a 3D“core-shell”conducting network structure,which can effectively enhance the electrical conductivity and provide fast Li⁺transport paths.When used as a cathode material for LIBs,the LiFePO₄@fluoride-doped graphene-like carbon nanocomposite?97.2 wt.%of LiFePO₄?exhibits a favorable rate performance with an initial discharge capacity of 166.7 and 159.3 mA h g^-1 at 0.2 and 1 C,and the capacity is retained at100.2 mA h g^-1 even at 20 C.Moreover,the nanocomposite also shows an excellent cycling stability with only 8%capacity decay at 10 C after 1000 cycles.