Synthesis Of Fe-based Compounds And Their Properties As Electrode Materials For Li-ion Batteries

Lithium-ion batteries have received the most attention in the development of new energy in the past two decades.Currently,lithium iron phosphate（Li Fe PO₄）and Li（Ni Co Mn）O₂（NMC）occupy the main market in commercial fields such as consumer electronics,power batteries,and energy storage batteries.Li Fe PO₄materials are stable,safe,and low in cost but have a low specific capacity,specific energy and short battery life;NMC has good energy density and specific power but poor safety and expensive.Both materials have insurmountable shortcomings and cannot meet the higher requirements of the next-generation new lithium-ion batteries.Therefore,there is urgent to find an electrode material with a high specific capacity,low cost and environmental friendliness to replace the existing commercial products.Among the options,iron-based compounds have great application prospects due to their high specific capacity,low cost,and safety and stability.In this paper,several novel iron-based compounds were synthesized and modified by ion doping and graphene modification,then used as cathode or anode active materials for lithium-ion batteries,and their structures and electrochemical performance were systematically studied.The specific research content and progress are as follows:1.A fluorine-doped and graphene-incorporated iron-rich lithium iron silicate（F-LFSO/G）nanoparticle was synthesized via the hydrothermal method.The incorporation of graphene into LFSO can significantly improve the electrical conductivity,while fluorine doping can change the O bond structure to realize the O redox,thus realizing the two-electron process.Meanwhile,the excess Fe in the system can generate a local Fe-O-Fe configuration,narrow the band gap of LFSO and lower the energy barrier for iron to migrate to Li sites,thereby accelerating the electrochemical conversion.As the result,at room temperature under 0.1 C,F-LFSO/G delivers a specific capacity as high as 328.43 m A h g^-1,which is almost 99%of the theoretical specific capacity of Li₂Fe Si O₄(331 m A h g^-1)and the retention is more than80%after more than 40 cycles.At last,spin-polarized density functional theory（DFT）calculations were investigated to reveal the possible chemical environment changes of O,which verified the existence of O redox during the cycling process.This is the first time to detect the O redox in polyanion cathodes（LFSOs）at room temperature,which has great significance for the exploration of next-generation high-capacity lithium-rich cathodes.2.Amorphous iron vanadate（Fe VO₄,labeled as FVO）cathode materials were synthesized through a facile and efficient ion exchange-liquid phase precipitation method.Compared with crystalline FVO,amorphous FVO can provide a specific capacity as high as 275 m A h g^-1 at room temperature under 0.1 C,which is about 95%of the theoretical specific capacity of Fe VO₄.The systematical kinetics analysis demonstrates that the amorphous FVO nanoparticle cathode material is superior to the crystalline FVO in terms of specific capacity and rate performance,one of the reasons is that there are both pseudocapacitance contributions and diffusion behaviors during the charge-discharge process,and the pseudocapacitance contribution in amorphous FVO is much larger than that in the crystalline state.At a scan rate of 8 m V s^-1,the pseudocapacitance contribution of Fe VO₄ is as high as 67.92%,which is more than twice that of the diffusion behavior.3.A F-doped and graphene-incorporated anode material（F-Fe Mo O₄/Mn₂Mo₃O₈/G,labeled as F-FMO/MMO/G）including both iron molybdate and manganese molybdate crystal nanocomposite were synthesized by hydrothermal method.The results show that F-FMO/MMO/G exhibits good specific capacity and high-rate performance,the retention is 91.8%after 150 cycles at 0.1 C.After multiple cycles at 10 C,when the rate returns to 0.1 C,the capacity retention can be restored to 95%of the initial cycle.The electrochemical impedance results show that the impedance of the material will be significantly reduced after F doping and graphene modification,which is also confirmed by the resistivity results of the Hall effect test and EPR test.Finally,we designed an orthogonal experiment to find out the optimal level combination conditions for the material to release the highest specific capacity.When the coating thickness is150μm,the mass ratio of active material,super P and PVDF is 7.5:1.25:1.25,and the amount of electrolyte added in the battery is 100μL,the material can provide a stable specific capacity of up to 1086 m A h g^-1.4.Developed a novel nanostructured multi-valence cathode material,graphene-modified iron oxyfluoride（Fe OF-G）,with extremely high specific capacity(621 m A h g^-1)and specific energy(1124 Wh kg^-1)（3 times higher than current commercial cathode materials）.Such performance is a consequence of synergistic efforts associated with（1）making the（de）lithiation reversible by immobilizing Fe OF nanoparticles and the discharge products of Fe nanoparticles and Li F over the graphene surface,and sandwiching Fe OF,Fe,and Li F between graphene sheets to prevent them from migration and dissolution into the electrolytes and（2）providing the inter-particle electric conductivity.The new structure not only leads to a significant improvement of the electrochemical performance,such as the initial specific capacity,rate capability,and cycle life,but also results in a new morphology and chemical composition.In situ high energy synchrotron XAS and other characterizations revealed reversible valence state changes and mass transfer during cycling of the new Fe OF-G material,and a reversible reaction mechanism was also derived.Importantly,it demonstrates that introducing small amounts of graphene can greatly improve the performance of the new material,so it provides a new approach for the development of more high-capacity reversible lithium battery materials to solve the problem of short battery life of rechargeable devices such as electric vehicles.