Synthesis Of Polyaniline-based And Polypyrrole-based Composite Materials And Their Applications In Lithium-ion Batteries

With the extensive applications of electric vehicles and many new types of portableelectronic devices, the demand for sustainable and clean energy storage and conversion isbecoming more and more critical. Lithium-ion batteries （LIBs） have been considered asone of the most effective and practical technologies for energy conversion and storageowing to the long cycle life, low self-discharge, high operating voltage, and no ‘‘memoryeffect’’. However, LiFePO₄, the promising cathode material for the next generation oflithium-ion batteries, shows some intrinsically disadvantages. Low electronic conductivityand slow lithium ion diffusion coefficient due to its1D channel for lithium insertion andextraction result in a poor rate capability. Moreover, the commercially-used anode materialgraphite has a low specific capacity （theoretically372mAh g1）, which cannot meet thegrowing need for LIBs with higher capacity and power. The research in this dissertationmainly focused on the synthesis of polyaniline-based and polypyrrole-based compositematerials for enhancing the lithium-stroage properties of LiFePO₄cathode material ormetal oxide anode material, respectively. The conditions for the preparation of thesecomposite materials were systematically explored and the possible mechanisms forenhanced lithium-ion storage performances were discussed in detail.LiFePO₄is believed to be a potential cathode material for the high-power batteriesdue to its low cost, high safety, non-toxicity, a competitive theoretical capacity, and aconstant open-circuit voltage of3.4V vs. Li⁺/Li. However, the main weakness of LiFePO₄lies in the intrinsically slow kinetics of lithium ions diffusion and low electronicconductivity, resulting in the poor rate capability. Electrochemically-active polymerpolyaniline （PANI） is an important conducting polymer due to its facile synthesis,environmental stability, and controllable physical and electrochemical properties byoxidation and protonation. The PANI polymer is electrochemically active in the range of2.03.8V, which overlaps the operative redox couple of LiFePO₄. Therefore, conductingpolymer PANI incorporated with LiFePO₄can not only efficiently enhance theconductivity of cathode material and rate capability, but also improve the reversible capacity due to the capacity contribution of PANI.An efficient strategy was employed to synthesis carbon-coated LiFePO₄（C-LFP）with PANI composites via a polymerization reaction in situ. Inorganic acids, such as HCl,H₂SO₄and H₃PO₄, are used as dopants for PANI. The effects of acidic dopants on theconductivity and electrochemical performance of the C-LFP composites were investigated.Owing to the enhancement of conductivity and capacity contribution of PANI, theC-LFP/PANI composite cathodes doped with HCl and H₃PO₄show a remarkableimprovement in capacity and rate capability, especially the C-LFP/PANI-HCl compositeexhibits the best electrochemical performance.Oxidant is considered as an important factor to influence the structure andelectrochemical properties of PANI and C-LFP/PANI composite. We systematicallyanalyzed the effect of oxidant ammonium persulfate （APS） on the C-LFP/PANI compositeby changing the ratio of [aniline] to [APS]. The electrochemical measurement shows thatthe composite containing7wt%PANI synthesized at [An]:[APS] ratio of1:1.5reveals thebest electrochemical performance, which gives a discharge capacity of165mAh g1at0.2C, and123mAh g1at10C. Moreover, the composite exhibits remarkably improvedcyclability as compared with the parent C-LFP. The mechanism has been carefullyinvestigated that the charge transfer impedance decreases significantly and the cathodesurface becomes much smooth over cycling with modification of conductive PANI. Theincorporated PANI can work not only as an additional host for lithium-ioninsertion/extraction, but also as a binder to modify the electrode surface and improve thecycling stability.Among anode materials, MnO has recently received much attention due to its hightheoretical capacity （755mAh g1）, relatively low electromotive force （emf） value （1.032V vs. Li⁺/Li）, abundant source and environmental friendliness. However, practicalapplication of the MnO anode is still hindered by several disadvantages such as poorcycling stability, large volume change, and unsatisfactory rate capability. Withinterconnected polypyrrole （PPy） fiber webs as precursor, we have successfully fabricateda novel hierarchical composite, carbon-encapsulated nano-MnO loaded on N-dopedcarbon webs （CMNCWs）. We found that the nano-sized MnO particles （¹0nm） were uniformly loaded on the N-doped carbon webs and coated with thin carbon shells （¹nm）.Benefited from the nano-sized MnO particles and multiple structures, the CMNCWsexhibit a superhigh reversible capacity and excellent rate capability, delivering a capacityas high as1268mAh g^-1even after700cycles at a current density of1.0A g^-1and386mAh g^-1at a high current density of10A g^-1. This facile strategy may be extended tofabricate other metal oxides and N-doped carbon composite materials with highperformance for energy storage.