The long cycle stability of electrode materials is required for lithium-ion batteries used in electric vehicles. The effective conductivity and the stable structure of these electrode materials are critical to the cycle stability. The olivine-type lithium iron phosphate is one such electrode material, that is able maintain its crystal structure stability, which in turn minimizes volumetric changes in the charge-discharge process. Advances geared towards improving the effective conductivity have been made by increasing the electrical conductivity of lithium iron phosphate electrode materials through applying carbon nanotubes in these materials. However, the preparation of uniformly distributed carbon nanotubes in the lithium iron phosphate faces many obstacles that some synthetic techniques have tried to address.
In a recent paper published in Electrochimica Acta and led by Professor Tong De Shen and Professor Yu Qing Qiao at Yanshan University developed an innovative technique of coating carbon nanotubes with polyvinylpyrrolidone, which effectively combines the carbon nanotubes and lithium iron phosphate to produce a nanocomposite that exhibits an excellent ultralong cycling stability and high-rate capacity.
The research team made polyvinylpyrrolidone-carbon nanotube-water dispersions which were subjected to repeated freezing and thawing to produce modified carbon nanotubes. They then prepared lithium iron phosphate particles which were then combined with the modified carbon nanotubes to produce lithium iron phosphate-carbon nanotube electrode material. The polyvinylpyrrolidone acts as a dispersant, surfactant and binder. The nanocomposite was heated at 600 OC in the presence of nitrogen to eliminate the polyvinylpyrrolidone, after which the carbon nanotubes were manipulated to form uniform three-dimensional conductive networks in lithium iron phosphate electrode.
The authors deduced that the polyvinylpyrrolidone coating process reduces the amount of disordered and defected carbon atoms. It was observed that the synthesized lithium iron phosphate exhibits a single phase of orthorhombic olivine-type structure. The observed average crystallite size of the lithium iron phosphate was about 30 nm.
The research team observed that the discharge capacity of the lithium iron phosphate electrode material with 3% carbon nanotubes, is about 9.4% greater than the lithium iron phosphate without any carbon nanotubes.
From the impedance spectra, it was deduced that there was a lower charge-transfer resistance in the electrode with 3% carbon nanotubes, which was about a third of the charge transfer resistance of the electrode without any carbon nanotubes. This shows that about 3% carbon nanotubes can develop a conductive network that is highly efficient, and therefore the lithium iron phosphate electron conduction is significantly improved.
The authors noted that the lithium iron phosphate electrode material with 3% carbon nanotubes had a lithium ion diffusion coefficient that was 25 times faster as compared with that without any carbon nanotubes, which shows that the carbon nanotubes effectively improve the diffusion of lithium ions. Also, the conductivity of the former is about 7.5 times higher as compared with the latter.
Further analysis showed excellent cycle stability of the lithium iron phosphate electrode containing about 3% carbon nanotubes, such that after about 1000 charge/discharge cycles at a discharge rate of 10C, there was only 1.6% loss in capacity, and a discharge capacity that was as high as about 123.0 mAhg-1. Additionally, the nanocomposite was observed to have a cycling lifetime of 3400 cycles as compared with 750 cycles for the commercially available lithium iron phosphate electrode materials.
Yu Qing Qiao, Wei Liang Feng, Jing Li, Tong De Shen. Ultralong cycling stability of carbon-nanotube/LiFePO4 nanocomposites as electrode materials for lithium-ion batteries. Electrochimica Acta 232 (2017) 323-331.Go To Electrochimica Acta