Synthesis Of LiFePO₄/C,Proposed Strategy,and Related Mechanisms For Enhancing Its Ageing Resistance

As one of cathode materials for lithium-ion batteries?LIBs?,polyanion-type LiFePO₄?LFP?has been subjected to extensive research due to its high theoretical capacity(170 mA h g^-1),relatively higher discharge plateau?3.45 V?,high thermal stability,environmental friendliness,and low cost.However,pristine LFP suffered from low electronic conductivity and low ion diffusivity,thereby limiting its applications in scalable energy storage systems.So far,several strategies have been developed,and carbon coating in composition with ion conductors is well accepted as a simple and effective method for addressing the aforementioned issue.In this thesis,hierarchically structured carbon-coated LFP?LFP/C?was prepared through a freeze drying method in combination with anneal treatment,using hydroxyethyl cellulose?HEC?as an in-situ carbon source and structure directing agent,FeSO₄?7H₂O as an iron source,phytic acid as a phosphorous source,and LiOH?H₂O as a lithium source.The effect of anneal temperature on the electrochemical performance was investigated.The samples were characterized with XRD,SEM,TEM,Raman,BET,and TG/DSC techniques,and further tested in the role of cathode material for LIBs.The results show:?1?The LFP/C samples are featured with a hierarchical structure.LFP nanoparticles are completely embedded in the carbon matrix for LFP/C samples obtained by anneal treatment at 700?LFP/C-700?and 750??LFP/C-750?,whereas LFP nanoparticles are protruded on the carbon matrix for LFP/C sample obtained at780??LFP/C-780?.?2?LFP/C-750 exhibits superior electrochemical performance to LFP/C-700 and LFP/C-780.This result is attributed to the complete coating of LFP nanoparticles by carbon with relatively higher degree of graphitization.At 0.1 and 1 C(1 C=170 mA g^-1),the discharge capacity values of LFP/C-750 are 162.7 and 27.3 mA h g^–1.After100 cycles,the capacity retains 151.9 and 124.6 mA h g^-1,presenting capacity retention of 93.4%and 97.9%,respectively.The issue concerning the lwo electronic and ionic conductivity of LFP has been well addressed;however,the issue concering the ageing resistance of LFP upon exposure to ambient atmosphere has been ignored by researchers.LFP and even LFP/C exhibit deteriated electrochemical performance after long-term exposure to ambient atmosphere.To solve this issue,we designed a simple and environment-friendly strategy by vapor deposition of trace amount of pyrrole onto LFP/C powders under ambient temperature and pressure.The captured pyrrole molecules on the surface of LFP/C in situ polymerize into polymers of pyrrole,which are helpful for isolating the uncoated LFP from ambient air.As a result,the ageing resistance and electrochemical performance of LFP/C can be improved.The LFP/C samples without and without surface modification by polymers of pyrrole were characterized with XRD,SEM,TEM,TG/DSC,BET,XPS,GPC,TOF-MS,M?ssbauer spectroscopy,and magnetization-measurement techniques.The adsorption behaviors of pyrrole molecules upon the carbon layers were calculated by molecular dynamics simulations.The electrochemical properties of the samples were tested.The results show:?1?It is an energy-favored process for pyrrole adsorption on the concaved carbon layer compared with pyrrole adsorption on the planar carbon layer.This result implies pyrrole molecules tend to be captured by the pores of the carbon layer.?2?The confined pyrrole molecules in the carbon pores can be oxidized by the synergistic effect of Fe²⁺,H₂O,and O₂,resulting in the formation of oligomers of pyrrole.The oligomers of pyrrole are featured with atomic thickness and p-type conductance,and can finely tune the pores of the carbon layer.?3?LFP/C without surface modification exhibits deteriated discharge capacity(C_dis),rate capability,and cycling stability after exposure to ambient atmosphere for 2months.With prolonging exposure time to 4 months,the deteriation of the electrochemical properties is aggravated.?4?LFP/C with surface modification with trace amount of pyrrole,whose weight accounts for 0.05-0.2 wt%relative to that of LFP/C,exhibits dramatic increase in ageing resistance and electrochemical performance.After 100 cycles at 1 C,the C_disis values of cLFPM-10-2,cLFPM-20-4,cLFP,cLFP-2,and cLFP-4 retain 137.1,127.3,116.2,97.0,and 94.1 mA h g^-1,presenting capacity retention of 92.7%,90.9%,84.8%,73.4%,and 77.6%.?5?It is beneficial to improve the ageing resistance of LFP/C as the amount of pyrrole increases.Nevertheless,20?L of pyrrole relative to 10 g of LFP/C is enough for LFP/C to resist 4-month exposure to ambient atmosphere.