(760d) Improved Lithium Battery Performance through Well Dispersion of Iron Phosphate Nanoparticles and Carbon Nanotubes

Improved lithium battery performance through well dispersion of iron phosphate nanoparticles and carbon nanotubes

Tongbao Zhang, Yangcheng Lu*, Guangsheng Luo

Despite substantial effort for developing cathode material for high performance Li ion battery, problems still remain with respect to their availability, cost and safety.  Amorphous iron phosphate is a fascinating cathode material due to its high capacity (178 mAh/g theoretically), abundant resources and low price.^[1] Besides, it’s also a potential host for various charge carrier ions, such as sodium, potassium, which is believed could relief the huge demand for lithium. ^[2-3]

Like other phosphate-based cathode material, amorphous iron phosphate suffers from two main drawbacks: one is the low conductivity, the other is the low lithium mobility. The wise solution to solve the low lithium mobility is reducing particle size to nanoscale. However, different from other crystalline phosphate-based cathode material, improving the conductivity of amorphous iron phosphate could not be easily achieved through carbon coating by themolysis of organics under inert atmosphere. Because it needs at least 600 °C to transfer organics to carbon, however amorphous iron phosphate could only be stable below 460 °C, above which amorphous iron phosphate would transfer to electrochemical inert crystalline iron phosphate. The only way to improve the conductivity is through structure design of electrode, specifically, through well dispersion of active material and conducting material. Thus, firstly, amorphous iron phosphate nanoparticles and carbon nanotubes themselves must be high dispersion. Secondly, the interaction between them should be introduced to further improve the conductivity of electrode.

In this work, well dispersion iron phosphate nanoparticles were generated through coupling a microreactor technology and step-by-step washing technology. Specifically, through regulating solution environment, Fe³⁺ and HPO₄^2- were used as continuous and disperse fluid, well dispersion and high pure iron phosphate nanoparticles was  generated when they contacted in the microreactor. Then in order to avoiding the aggregation of these hydrophilic nanoparticles during removing solvent, water-alcohol-petroleum ether was sequentially used to wash the product. Finally, the product was dried at 105°C overnight and high dispersion iron phosphate nanoparticles were obtained. In order to improve the dispersion of carbon nantubes in water, they were firstly treated in H₂SO₄ and HNO₃ mixed solution to generate –COOH and make them hydrophilic. Then, in order to introduce the interaction between them, iron phosphate nanoparticles were further modified by aminopropyltrimethoxysilane. Thus, the interaction was introduced between modified iron phosphate nanoparticles and oxidized carbon nanotubes through charge attract and spontaneously formed amide bond. Finally, the electrode was tested in lithium battery, which delivered nearly theoretical capacity at 0.2C, excellent rate capacity performance and long-term stability (stable more than 200 times at 1C and 5C).

* Corresponding author: luyc@tsinghua.edu.cn

REFERENCE:

[1] Yun Jung Lee, Hyunjung Yi, Woo-Jae Kim, etal. Fabricating genetically engineered high-power lithium-ion batteries using multiple virus genes. Science, 2009, 324, 1051.

[2] Vinod Matthew, Sungjin Kim, Jungwon Kang, etal. Amorphous iron phosphate: potential host for various charge carrier ions. NPG Asia Materials, 2014, 6, e138.

[3] Yongjin Fang, Lifen Xiao, Jiangfeng Qian,et al. Mesoporous amorphous FePO₄ nanospheres as high-performance cathode material for sodium-ion batteries. Nano Lett. 2014, 14, 3539.