Study On Synthesis And Electrochemical Properties Of LiFePO₄-C Cathode Materials For Lithium Ion And Lithium Polymer Batteries

In this study, LiFePO₄ and LiFePO₄-C composite were synthesized by a hydrothermal method followed by ball-milling and heat treating. Different carbon conductive additives including nano-sized acetylene black (AB) and multi-walled carbon nanotubes (MWCNTs) were used to enhance the electronic conductivity of LiFePO₄. In the meantime, different electrolytes including liquid electrolyte (LE) and solid polymer electrolyte (SPE) were used. The physical performance of LiFePO₄ and LiFePO₄-C composite was analysed by electronic conductivity, BET, SEM, TEM HR-TEM and XRD. The electrochemical performance of Li/LE/LiFePO₄, Li/LE/LiFePO₄-AB, Li/LE/LiFePO₄-MWCNTs, Li/SPE/LiFePO₄ and Li/SPE/LiFePO₄-MWCNTs batteries was analysed by AC impedance, CV and charge/discharge tests.LiFePO₄ was synthesized by a hydrothermal method at different temperatures. LiFePO₄ synthesized at 170℃and subsequent 500℃can be indexed to a single-phase material having an orthorhombic olivine-type structure with a space group of Pnma and the average particle size of 200 nm. No impurity such as Fe2O3, Li₃Fe₂(PO₄)₃ and Li3PO4 is found in the LiFePO₄ powders. The discharge capacity of 167 mAh g-1 (98% of the theoretical capacity) at discharge rate of 0.1 C is the largest compared to other samples. CV results demonstrate that the reversibility and reactivity of lithium extraction/insertion reactions at the electrode/electrolyte interface in LiFePO₄ synthesized at 170℃and subsequent 500℃are the best. The charge/discharge tests at different C rates indicate that the battery may operate at relatively high rates, confirming the improved kinetics of LiFePO₄ cathode materials.LiFePO₄-AB composite was synthesized by a hydrothermal method followed by ball-milling and heat treating. The tests demonstrate that the electronic conductivities are 5.86×10-9 S cm-1 for LiFePO₄, 8.00×10-5 S cm-1 for LiFePO₄-AB with 5 wt%, 7.85×10-4 S cm-1 for LiFePO₄-AB with 10 wt% and 2.71×10-2 S cm-1 for LiFePO₄-AB with 15 wt%. SEM and TEM observations indicate that LiFePO₄ is mixed well with AB and make the elctronic conductivity of LiFePO₄-AB composite improve. XRD results demonstrate that LiFePO₄-AB composite can be indexed to a single-phase material having an orthorhombic olivine-type structure with a space group of Pnma. No impurity such as Fe2O3, Li₃Fe₂(PO₄)₃ and Li3PO4 is found in the LiFePO₄-AB powders. There is no evidence for amorphous carbon. It is demonstrated that the added nano-sized AB does not change crystal structure of LiFePO₄. The charge/discharge tests demonstrate that the discharge capacities of Li/LE/LiFePO₄-AB batteries with 5 wt%, 10 wt% and 15 wt% AB hardly change upon cycling and cycling performance of Li/LE/LiFePO₄-AB battery with 10 wt% is the best and its discharge capacity is 123 mAh g-1.LiFePO₄-MWCNTs composite was synthesized by a hydrothermal method followed by ball-milling and heat treating. The tests demonstrate that the electronic conductivities are 5.86×10-9 S cm-1 for LiFePO₄, 1.08×10-1 S cm-1 for LiFePO₄-MWCNTs with 5 wt%. SEM observations show that the MWCNTs intertwine with LiFePO₄ particles together to form a three-dimensional network. The dispersed MWCNTs provide pathways for electron transference and make the elctronic conductivity of LiFePO₄-MWCNTs composite obviously improve. XRD results demonstrate that LiFePO₄-MWCNT composite can be indexed to a single-phase material having an orthorhombic olivine-type structure with a space group of Pnma. No impurity such as Fe2O3, Li₃Fe₂(PO₄)₃ and Li3PO4 is found in the LiFePO₄-MWCNTs powders. There is no evidence for amorphous carbon. It is demonstrated that the added nano-sized MWCNTs do not change crystal structure of LiFePO₄. The charge/discharge tests demonstrate that the discharge capacities of Li/LE/LiFePO₄-MWCNTs batteries with 2.5 wt%, 5 wt% and 10 wt% MWCNTs hardly change upon cycling and cycling performance of Li/LE/LiFePO₄-MWCNTs battery with 5 wt% is the best and its discharge capacity is 142 mAh g-1. In the meantime, it is demonstrated that the cycling performance of Li/LE/LiFePO₄-MWCNTs battery with 5 wt% MWCNTs is better than that of Li/LE/LiFePO₄-AB battery with 5 wt% AB.The lithium-ion diffusion coefficient of LiFePO₄-MWCNTs composite with 5 wt% MWCNTs is improved from 2.41×10-15 cm2 s-1 for pure LiFePO₄ to 1.50×10-14 cm2 s-1 for LiFePO₄-MWCNTs composite with 5 wt% MWCNTs. It is demonstrated that the resistance of Li/SPE/LiFePO₄-MWCNTs battery with 5 wt% MWCNTs does not change upon cycling and is 280 ?. CV results demonstrate that the redox peak profile of Li/SPE LiFePO₄-MWCNTs battery with 5 wt% MWCNTs is more symmetric and spiculate than that of Li/SPE/LiFePO₄, demonstrating that the reversibility and reactivity of Li/SPE/LiFePO₄-MWCNTs battery with 5 wt% MWCNTs are enhanced due to improvement of electronic conductivity and the reduced diffusion length resulting from a decrease in the crystallite size by MWCNTs. The initial charge/discharge curves of Li/SPE/LiFePO₄-MWCNTs batteries with different MWCNTs contents at a discharge rate of 0.25 C demonstrate that Li/SPE/LiFePO₄ -MWCNTs battery with 5 wt.% MWCNTs exhibits the highest specific capacity with a charge capacity of 122 mAh g-1 and a discharge capacity of 115 mAh g-1. The rate capability of Li/SPE/LiFePO₄-MWCNTs batteries with different MWCNTs contents at various C rates ranging from 0.1C to 3C rate (1C=170 mA g-1) at room temperature demonstrate that the discharge rate capability of Li/SPE/LiFePO₄-MWCNTs battery with 5 wt% MWCNTs is better than that of Li/SPE/LiFePO₄. The cycling performance of Li/SPE/LiFePO₄-MWCNTs batteries with different MWCNTs contents at various C rates ranging from 0.1C to 3C rate (1C=170 mA g-1) at room temperature demonstrates that the high-rate discharge performance of Li/SPE/LiFePO₄-MWCNTs battery with 5 wt% MWCNTs is better than that of Li/SPE/LiFePO₄.