Studying the Influence of Carbon Sources Added on a Cost Effective Route to Synthesize LiFePO4/C in a Quasi-Open Environment
Authors: Gu Fei , Jung Kichang , Martinez-Morales Alfredo A. 
Published: February 2020

With the increase in energy demand and a greater push for green energy, rechargeable batteries are playing a critical role in the energy industry. The development of Li-ion cathode materials is moving towards more affordable, safe, and efficient energy storage solutions. Lithium iron phosphate (LiFePO4, LFP) has proven to be a reliable cathode material for the battery industry. This is mainly driven by its stable operating voltage (3.5 V vs. Li/Li+) and high theoretical capacity (170 mAh/g). Additionally, LFP has excellent cycling performance, high safety (high thermal stability), environmental friendliness, and low raw material cost in comparison to traditional cathodes based on Co, Ni, and Mn. To further increase the competitiveness of LFP, in this work, we investigate the development of a synthesis process with the potential to reduce production costs. Our approach is based on the synthesis of carbon coated LFP (LFP/C), via lithiation in a quasi-open air environment. In contrast to the generic solid state synthesis for LFP, our approach is more time efficient. The lithiation process is completed under 30 minutes. Furthermore, our approach is a cost-effective process since the reaction is conducted under atmospheric pressure in a quasi-open environment.

Graphite and Super P are studied and compared as the carbon source and are added before the synthesis process. The carbon released by the decomposition of either graphite or Super P provides a reducing environment to protect the synthesized LFP from oxidation, while also forming a carbon coating on the surface of LFP. The carbon coating enhances the electrochemical performance of the assembled Li-ion batteries by improving electric conductivity of the LFP cathode. The combination of the synthesis and the carbon coating of LFP into a single step, further improves the efficiency of our existing quasi-open synthesis approach.

Depending on the crystal structure and chemical composition of the carbon source, the efficiency in preventing the synthesized LFP from oxidizing is different. These differences are mainly reflected in the carbon source decomposition temperature and rate, type of carbon formed, and byproduct released, such as CO2 and H2O. As allotropes of carbon, applying graphite or Super P eliminates the need to consider the thermal decomposition process of the carbon source and points to the crystal structure of the carbon source as the only factor that impacts the LFP synthesis. The results shed lighting into understanding the theory behind the LFP oxidation protection during our quasi-open synthesis method. Furthermore, graphite and super P have the highest carbon content, producing LFP/C with a highly efficient coating, at a low fabrication cost.

Overall, this work investigates the effect of the amount of carbon source added, reaction temperature, and reaction time for the time efficient and the synthesis of high quality LFP. The mechanism of carbon source assisted LFP synthesis are investigated. The reaction processes are studied by Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (TGA), respectively. Several techniques are used to characterize the properties of synthesized LFP/C. The crystal structure and chemical composition of the synthesized material are characterized by X-ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The grain size of synthesized materials is determined by Scanning Electron Microscopy (SEM). The degree of carbonization is characterized by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The electrochemical performance of synthesized cathode is investigated by Cyclic Voltammetry (CV) and the performance of assembled batteries is tested via an Arbin Tester.