Study On Preparation And Properties Of Multilayer Graphene/Polyanionic Phosphate Porous Film Composites As Cathode

Polyanionic compounds（LiFePO₄,Na₄Fe₃（PO₄）₂P₂O₇）as cathode materials for batteries have gained significant attention worldwide due to their excellent cycling stability and unique ion deintercalation mechanisms.However,the inherent limitations of polyanionic compounds,such as low electronic conductivity and slow ion diffusion rates,greatly restrict their application and development in high-power electrode materials.Composite with graphene is one of the most effective methods to improve electrochemical energy storage.However,the graphene used in the current report is mostly graphene oxide,which not only increases the production cost but also easily causes environmental pollution during the oxidation process of graphene.In this work,mechanically exfoliated multilayer graphene（MLG）was used as a substrate to prepare highly electrochemically active multilayer graphene/polyanionic compound cathode composites.This approach significantly reduces the production cost and ensures a green and pollution-free synthesis process.The main research objectives of this study are as follows:（1）Preparation and carbon coating of LiFePO₄/MLG composite.The LiFePO₄/MLG composite with the optimal molar ratio was prepared by Li H₂PO₄ and Fe₂O₃/MLG.On this basis,the effect of carbon coating on its micro-morphology and electrochemical performance was investigated.Fe₂O₃ nanoparticles grown on the surface of MLG were prepared by chemical bath deposition method.Li H₂PO₄ was uniformly attached to the surface of MLG and combined with Fe₂O₃ by adding liquid grinding method.LiFePO₄/MLG composite was obtained by high-temperature annealing at 600℃for 10 h.The results showed that the LiFePO₄/MLG composite prepared with a Fe₂O₃/MLG to Li H₂PO₄ molar ratio of 1:1.08 and a glucose mass fraction of6%exhibited high crystallinity and uniform distribution of particles on the surface of MLG.The initial discharge specific capacity is 124.4 m Ah g^-1 at 0.2 C,and the capacity retention rate is 78.8%after 200 cycles.（2）Mn²⁺doping in LiFePO₄/MLG composite.The electrochemical properties of LiFePO₄/MLG composites were improved by using manganese chloride tetrahydrate as metal Mn²⁺doping.The effect on the lithium storage performance of the composite after adding Mn²⁺was investigated.The results show that the LiFePO₄/MLG composite doped with 5%Mn²⁺in the liquid grinding stage exhibite a discharge specific capacity of 129.9 m Ah g^-1 at 0.2 C,the capacity retention rate is up to 97.6%after 100 cycles,demonstrating significantly enhanced cycling stability.（3）Study on sodium storage performance of Na₄Fe₃（PO₄）₂P₂O₇/MLG composite.Fe₂O₃/MLG prepared by chemical bath deposition method was used as iron source,and Na H₂PO₄ provided sodium and phosphorus source for the preparation of Na₄Fe₃（PO₄）₂P₂O₇/MLG composite.After adding liquid,grinding and drying,the Na₄Fe₃（PO₄）₂P₂O₇/MLG composite was obtained by annealing at 500℃for 10 h.The study investigated the effect of different rations of raw materials on the sodium storage performance of Na₄Fe₃（PO₄）₂P₂O₇.The results showed that the Na₄Fe₃（PO₄）₂P₂O₇/MLG composite with a molar ratio of Fe₂O₃/MLG to Na H₂PO₄ of 1:1.3 exhibited an initial discharge specific capacity of 118.6 m Ah g^-1 at 0.2 C and 96.3 m Ah g^-1 at 2 C,respectively.The capacity retention rate was87.9%after 200 cycles at 0.2 C and 97.7%after 1000 cycles at 2 C,demonstrating the optimal electrochemical performance.