| Olivine-structured Li Fe PO4(LFP) is considered as the most potential cathode material for power lithium ion batteries(LIBs) due to its good thermal safety, environmental compatibility, cycling stability and abundant raw material etc. However, because of the inherent defects including poor electronic conductivity(10-10 S·cm-1) and low Li+ diffusion rate(one dimensional diffusion channels, 10-14 cm2·s-1) have highly limited its practical application. Recently, constructed three dimensional(3D) LFP-based composite, such as: composited with graphene to build three 3D porous composite, has been demonstrated as an efficient way to improve its electrochemical performances. Exploring effective preparation method to realize the efficient combination of LFP with other functional components is the key as well as the hotspot and difficult for the research of LFP-based composite.In this research, we designed and prepared a series of 3D structured LFP-based composites and realized the in-situ effectively compound between LFP and graphene. Further, we explored and studied the synthesis process, physical properties, electrochemical performances and the synergistic effect of the composites. This work not only provides efficient technological approaches for developing new type phosphate cathode materials for power LIBs, but also open s up a new research method for deep studying the interaction and synergistic effect between different components in the composite electrode materials.Employing a rheological phase-carbothermal method synthesized a carbon coated LFP/graphene composite(C@LFP/RGO) using the thermal reduced graphene(RGO), in which, the LFP nanoparticles with average size of ~80 nm unfirmly anchored on the surface or inserted in interlamination of RGO. At the meantime, LFP particles were coated with amorphous carbon layers of ~3 nm thickness. Further, through improving the synthesis method-using graphene oxide(GO) and introducing a high-energy ball-milling process as well as Mg source, prepared a 3D mesoporous carbon-coated LFP nanocrystals co-modified by graphene and Mg doping(C@M-LFMP/RGO). After this treatment, the mass transport process was further improved and the electronic conductivity and Li+ diffusion rate inside LFP crystal were also enhanced. The testing results demonstrated that C@M-LFMP/RGO exhibited excellent electrochemical performances: at 20 C, the discharge capacity was 124 m Ah·g-1; fast charging for 160 s at a test system by charging at 20 C and discharging at 1 C(20 C/1 C), the charged capacity was reached 140 m Ah·g-1; cycling after 1000 times at 10 C, the capacity retention was 95 %.Employing a solvothermal method synthesized a(010) facet contact mode LFP/graphene composite(LFP/GNs), in which(010) facet orientated LFP nanoplatelets in situ grew on the surface of graphene and realized the most efficient “face-to-face” electronic contact. LFP/GNs displayed good electrochemical performances, which can exhibited a discharge capacity of 76 m Ah·g-1 at 60 C; fast charging for 60 s at 60 C/1 C, the charged capacity was reached 143 m Ah·g-1; cycling after 1000 times at 10 C, the capacity retention was 90 %. In order to further improve its cycling performance, we selectively coated the exposed LFP surface in LFP/GNs with a carbon layer, and after this treatment, the cycling stability was much enhanced with a capacity retention as high as 98 % after cycling for 1000 times at 10 C.Employing a electrostatic adsorption-hydrothermal self-assembly combined method synthesized a LFP/nitrogen-doped graphene aerogel composite(LFP/N-GA), in which the(010) facet orientated LFP nanoplatelets were surrounded by a N-GA constructed bicontinuously(electronic and ionic) channels. Such a special composite could simultaneously optimize the four basic steps of electrochemical reaction, and thus, exhibit superior electrochemical performances: at 100 C, the discharge capacity was 106 m Ah·g-1; fast charging for 60 s at 100 C/1 C, the charged capacity was reached 145 m Ah·g-1.At last, we took(010) facet contact LFP/GNs for example compared with similar sized S-LFP and graphene(GNs) material to deeply study the synergistic effect from physical characterization, electrochemical test and theoretical calculation three aspects and explored its mechanism preliminarily. Physical characterization confirmed by compounding S-LFP with GNs could improve electronic conductivity, relieve the agglomeration of single component and construct porous structure. Electrochemical analysis found a serial electron channel between LFP and GNs, and the high specific area and fast-capacity-response property of GNs could reduce the ohmic and charge-transfer resistance and improve the apparent lithium ion diffusion coeffecient of the electrode. DFT theoretical calculation found, by compositing with GNs, changed the bond structure and density of states of S-LFP, increased the possibility for electron appearing at Fermi level, moved the electron cloud to the compound interface; at the meantime, lithium ions also gathered at the compound interface, resulting in a higher electrochemical activity at the interface area. |
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