Study On High Performance Foldable Lithium Ion Batteries Based On Macro - Membrane Of Carbon Nanotubes

The rapid development of mobile internet technology propels the rise of intelligent wearable device. High energy batteries which can be stretched, compressed, rolled, buckled and even folded are expected to support theses new devices. Lithium-ion batteries(LIBs) are regareded as the ideal option. Conventional LIBs use metallic foils(Aluminum and Copper) as current collectors on which the active materials are deposited. Such metal foils are unsuitable for flexible devices due to the problem of delamination of the active materials. Since these current collectors do not contribute to the capacity of the battery, replacing the metallic foils with a highly conductive, light-weight and flexible current collector could increase the energy density of the battery while also imparting flexibility to the device. Carbon nanotube(CNT) macro-film(CMF) is obviously a kind of aforementioned current collector.In this work, fully foldable LIBs(~170 Wh kg-1) with free-standing CMF as current collectors were studied. First, dynamic mechanical analysis, four-probe system, standard electrochemical instrumentation and Raman spectroscopy were performed to characterize the physical and chemical properties of CMF. Then, the electrochemical performances of the cathode and anode were studied after the CMF were applied, relavant informations of structure and suface were characterized by X-Ray Diffraction and X-ray photoelectron spectroscopy. The results demonstrated that good electrochemical performance can be obtained after the CMF was loading Li Co O2 and Li4Ti5O12 as cathode and anode, respectively. Based on these works, CMF-based battery was assembled and its relvant properties were tested. Finally, fully foldable battery was developed by optimizing process parameters. The main results for this study were as follows:(1) Bundles of high graphitization CNTs can manufacture a large-scale CMF(~3 m2) wih low density(~0.3 mg cm-2), high intensity(~80 Mpa), and fine conductivity(~ 300 S cm-1). CMF is not only durable to current but also provides more channels for electron to transport between the active material layer and current collector, and thereby reduces the interfacial resistance.(2) Choosing appropriate electrode materials is the key to obtain high reversibility of CMFbased electrode. There are no problems for most cathode materials because their potential is above 3.4(vs. Li/Li+). Main troubles come from anode unless the potential of anode materials exceed 0.9 V(vs. Li/Li+). Li4Ti5O12 is the ideal candidate in commercialized anode materials.(3) CMF-based battery in this work has long cycling life beyond 1000 times and its capacity at-20 °C is more than 80% of the capcatity at 25°C. Lightweight CMF can help LIBs with Li4Ti5O12 anode compensate the effect on energy desity causing by low working voltage, especially at low areal density. The CMF-based LIBs after optimal designing can be folded repeatedly, although the area reduced ~90%(folding 5 times), its structure remains intact and electrochemical properties have no obvious change.