Graphene wrapped LiFePO4/C composites as cathode materials for Li-ion batteries with enhanced rate capability

Abstract

To reduce the reaction time, electrical energy consumption, and cost, LiFePO 4 /C/graphene has been synthesized by a rapid, one-pot, microwave-assisted hydrothermal method within 15 min at a temperature of 200 C, followed by sintering at 600 C for 2 h under a H 2 /Ar (5 : 95, v/v) atmosphere.The microstructure and morphology of the LiFePO 4 /C/graphene products were characterized by means of X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The carbon coated LiFePO 4 /C nanoparticles, around 200 nm in size, are thoroughly wrapped by crumpled micrometer-size graphene sheets. In this kind of structure, the bridging graphene nanosheets can form an effective conducting network and provide interconnected open pores that favor electrolyte absorption and reduce the diffusion path of the lithium ions. The cyclic voltammograms, charge/discharge profiles, and AC impedance measurements indicated that the kinetics of the LiFePO 4 /C/graphene was better than that of LiFePO 4 /C. The LiFePO 4 /C/graphene composite exhibited a discharge capacity of 165 mA h g À1 at 0.1 C and 88 mA h g À1 at 10 C, respectively.Therefore, the LiFePO 4 /C/graphene composite is a promising candidate for the development of highperformance, cost-effective lithium batteries for the hybrid vehicle and electric vehicle markets.

Graphene-modified LiFePO4 cathode for lithium ion battery beyond theoretical capacity

“…The EG-wrapped cLFP cathodes provide the capacity from 187 to 208 mAh g À 1 , depending on the weight percentage of EG (from 0.8 to 2 wt%). These values are higher than the reported values of 120-160 mAh g À 1 for commercially available or synthetic LFP materials in research laboratories 16,17,21,22 . Surprisingly, these values are even in excess of the theoretical value of 170 mAh g À 1 for LFP.…”

“…Alternatively, significant effort has been devoted to coating the LFP surfaces with electrically conductive materials, such as amorphous carbon and conducting polymers to ARTICLE enhance the electrical conductivity of LFP surfaces [27][28][29][30][31][32] . Recent studies have adopted rGO or GO to cover the LFP surfaces [16][17][18][19] . However, studies so far have only shown certain improvements in the rate capability but no report has exceeded the theoretical limit of the capacity.…”

“…The most popular approach to synthesize graphene is to reduce the chemically exfoliated graphene oxide (GO) obtained by Hummer's method, owing to that the method is potentially scalable. Several reports have demonstrated some enhancement in rate capacity using the reduced GO (rGO)/LFP composite material as the cathode, where the cathode was prepared by pyrolyzing the GO or rGO together with either LFP precursors or LFP particles [16][17][18][19] . The capacity increase was attributed to that the rGO sheets helped to enhance the electron transport rate in cathodes.…”

Research Progress in Improving the Rate Performance of LiFePO4 Cathode Materials

“…The composite with a low content of graphene exhibited a high initial discharge capacity of 163. performance compared to the conventional graphene/LiFePO 4 composite and bare LiFePO 4 . The as-obtained sample delivered 73 mAh g −1 of discharge capacity at 25 C. Shi and co-workers [101] described an advanced microwave-assisted hydrothermal route for preparation of a highly ordered LiFePO 4 /C/graphene nano-composite. LiFePO 4 /C nanoparticles were embedded in the conductive and interconnected graphene networks, and exhibited a discharge capacity of 88 mAh g −1 at 10 C.…”

A Review on Nanocomposite Materials for Rechargeable Li-ion Batteries

“…Utilizing graphene in LiFePO4 composites resulted in achieving near theoretical specific capacity (i.e., 170 mAh/g), exceptional rate capability and improved the cycle life due to intimate contact between non-aqueous electrolyte and the LiFePO4 active materials as well as enhanced the charge and mass transfer [110,111]. The schematic of the graphene wrapped LiFePO4 particles are shown in Figure 11 [111]. Hu et al reported that electrochemically exfoliated graphene wrapped commercial LiFePO4 particles can deliver 208 mAh/g capacity which is more than the theoretical capacity of LiFePO4.…”