Synthesis and Characterization of Surface Modification of Anode Materials for Lithium-ion Batteries
Date Issued
2014
Date
2014
Author(s)
Li, Fu-Sheng
Abstract
Firstly, this research presents a new concept for forming an artificial solid-electrolyte-interphase (A-SEI) for Li-ion battery (LIB) electrodes based on the rational design of multifunctional polymer blend to achieve favorable solid/electrolyte interfacial properties. It leads to the successful development of a mechanically robust and high-performance polymeric electrode coating containing two polymers, namely polyethylene glycol tert-octylphenyl ether (C14H22O(C2H4O)n) (PEGPE) and poly(allyl amine) ((C3H5NH2)n ), with four tailored functional groups for graphite and graphite-Si composite anodes. Because the ether and amine functional groups of the coating can chelate Li ions, the A-SEI coating can act as a buffer zone providing a gradual change in the coordination state of Li ions transferring between electrolytes and graphite. This change lowers the energy barrier to the solvation/de-solvation processes and thus accelerates the ion transfer rate. Moreover, the hydrogen bridge bonding between the two polymer components and the pi-pi interaction between the aromatic structure of PEGPE and the graphite surface yield sufficient mechanical strength and interfacial adhesion to protect graphite anodes from exfoliation and graphite/Si composite particles from disconnection during cycling. Therefore, the developed A-SEI considerably enhanced the capacity reversibility, cycle stability, and, in particular, high-rate performance of all of the studied anode materials, including natural graphite, artificial graphite, and a Si nanoparticle-graphite composite. Secondly, a structure with well-dispersed conductive additives as Li-ion battery anode material is manufactured by the combination processes of substrate-induced coagulation (SIC) and polymer coating by electrostatic self-assembly (ESA), which are implemented experimentally by the facile and scalable processes. This results in the enhancement of the dispersion stability of conductive additives such as carbon black (CB) or carbon nanotube (CNT) and the improvement of interconnection between active materials due to a continuous three-dimensional network for electron conduction. Moreover, the electrode porosity in this uniform dispersion structure forms the complementary network for Li-ion transport in the electrolyte as compared to those distributed with agglomeration. Electrochemical experiments indicated that the rate capabilities and cycle performance of the electrodes are consistent with dispersion properties of conductive additives. Furthermore, the contact resistance and charge transfer resistance for Li-insertion and Li-extraction are smaller due to the homogeneous distribution of conductive particles, which leads to a less significant electrode polarization. Finally, Si/graphite composites are ideal anode materials for LIBs due to the enhancement of energy density for next generation LIBs. Advanced commercial products comprising graphite anodes modified with a few percent of Si or Si oxide to achieve high capacity have been announced. In this research, we demonstrate an electrostatic self-assembly dispersion technique and a hybrid surface modification method as the facile and scalable processes to prepare hierarchical micro-sized composites of Si nanoparticles encapsulated in natural graphite and layers of carbon and polymer. In addition, the as-synthesized composite is remarkably desirable for a superior anode because it not only facilitates the conduction of electron and the diffusion of Li ion but also renders both elastic and mechanical buffer to accommodate the large volume change of Si nanoparticles during Li insertion and extraction process.
Subjects
Li-ion batteries
Graphite
Silicon
SEI film
Surface modification
SDGs
Type
thesis
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