2, UAE University, Abu Dhabi, , United Arab Emirates
3, United Arab Emirates Space Agency, Abu Dhabi, , United Arab Emirates
Lithium iron phosphates (LiFePO4 and Li3Fe2(PO4)3) are a group of structurally different cathode materials for lithium-ion batteries (LIBs). In particular, LiFePO4 was reported and proposed as possible alternatives to the LIB cathode material LiCoO2, which is the most popular cathode material for rechargeable consumer applications, such as phones, laptops, and electric vehicles. LiCoO2-based LIBs are however unsustainable; the raw materials are not widely available and, despite improvements in battery design, the batteries are prone to suffer from dangerous oxygen release upon overcharge, which can lead to fire hazards. LiFePO4 has a high theoretical energy density of 170 mAh/g, good chemical and physical stability and a discharge potential of 3.4-3.5 V vs. Li/Li+. However, LiFePO4 cathode suffer from poor electronic conductivity and low ionic diffusivity, which can lead to losses of high initial capacity and poor rate capabilities which can hinder its practical applications. Multi-walled carbon nanotubes (MWCNTs) providing tridimensional networks have been proven to be the most effective in reducing the resistance and improving the electrochemical performance of the nanocomposites composed of MWCNTs and LiFePO4 nanomaterials. In this work, nanocomposite of LeFePO4/MWCNTs was prepared as cathode materials for LIBs. First, a sheet of MWCNTs, so called ‘Buckypaper’ with uniform thickness was prepared by surface-engineered tape-casting (SETC) technique. SETC enables fabrication of strong stand-alone structured carbon nanotube (CNT) sheet with tunable thickness and composition. Secondly, nanometer-scale LiFePO4 particles were grown on the surface of the MWCNTs by hydrothermal process. A sheet of buckypaper was treated by O2 plasma to enhance hydrophilicity and at the same time generate nucleation sites in the MWCNT networks for growth of LiFePO4 particles. The precursors include stoichiometric amounts of LiCl, FeCl2, H3PO4 with ascorbic acid as reducing agent and Deionized water. The resulting mixture was introduced into a sealed Teflon-lined autoclave and heated at 160°C for 3 h. It was found that nanometer-scale LiFePO4 particles with good crystal quality were grown on the MWCNTs sheet and uniformly distributed. Size and distribution density of the LiFePO4 particles were controlled by the experimental conditions such as mixing ratio of precursor chemicals, temperature and time of hydrothermal process as well as pre-treatment of MWCNT sheets with O2 plasma. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy were used to characterize the nanocomposite composed of LiFePO4 nanoparticles on MWCNT sheet. The electrochemical characterization of the cathode of LiFePO4/MWCNTs is in progress using a CR2025-type coin cell with lithium foil and anode. This process could pave the way of enabling fabrication of ‘Flexible’ lithium-ion batteries with light-weight, high energy density and improved safety issue.