Incorporating cyclized-Polyacrylonitrile with Li4Ti5O12 Nanosheet for High Performance Lithium Ion Battery Anode Material
Introduction
Lithium ion batteries have been acknowledged to possess long cycle life, high energy density and elevated safety and have been used as promising power sources for various energy applications [1], [2]. Among its component, anode is always associated with the oxidation or release of electrons into external circuit and thus is seen as one of the key parts in lithium ion battery. In past two decades, various materials e.g. graphite, [3] silicon [4], [5], graphene [6], [7] have been investigated. Among all the candidates, spinel Li4Ti5O12 (LTO) [8], [9], [10] has received a great deal of attention due to its excellent cycling performance, easy and low cost fabrication, unbeatable safety. Specifically, the high operating voltage of LTO eliminates the development of lithium dendrites growth and the formation of SEI film [11], typical issue found in carbon based anodes. Like every coin has two sides, LTO suffers from low theoretical capacity (175 mAhg−1), low electronic conductivity (∼10−13 Scm−1) and reduced lithium diffusion especially at high charge–discharge rates. To tackle these problems, miscellaneous methods were employed. For example, Wang [12] reported the synthesis of porous LTO nanofiber that is capable of maintaining 120 mAhg−1 at high discharge rate(10C) under room temperature. Researchers at UW-Seattle enabled remarkable rate performance and long cycle life by combining the benefits of nanowire structure and hydrogen doping [13]. Huang and his co-authors [14] prepared monodispersed LTO hollow spheres of average outer diameter 1 um using carbon spheres as a template and the material exhibits very favorable rate performance. Although much progress have been obtained, the need of larger reversible capacity, higher energy and power density, along with more stringent safety concern under cruel environment are still in much demand. More importantly, how to balance outstanding battery performance with LTO synthesis/fabrication cost and easiness is becoming an imperative issue for the purpose of mass production.
In recent year, attention has been shifted beyond active component of the electrode material and toward overall electrode structure optimization to develop batteries with higher energy and power density. For example, searching for an appropriate polymeric material that acts as both conductive and binder in the electrode structure would ideally prevent large energy capacity loss and pulverization as a result of design integration, especially on Si-based electrode. In fact, a series of conducting polymers or bipolymers such as PEDOT:PSS [15], alginate [16], PANI [17], PPY [18] were employed in various LIB materials and all these polymers showed synergistic interaction with the electrode material to achieve favorable improvement in electrode lifetime, rate performance, elevated operating voltage and good mechanical resiliency and thermal stability.
In this work, we reported incorporation of a conjugated polymer cyclized PAN to LTO nanosheet we synthesized by hydrothermal approach to form high performance lithium ion battery anode composite material. Cyclized PAN is expected to function both as a good conductive due to delocalized sp2-π bonding during its conjugation and a strong adhesion by virtue of its excellent inherent resiliency. Our preliminary results show remarkable electrochemical performance out of cyclized-PAN/LTO composite electrode compared to pure LTO electrode in all tested conditions. The ability of cyclized PAN to improve battery performance and to be used as a binder alternative in the electrode fabrication process, along with the ease of its realization renders this simple method ideal for creating low-cost facile high-performance lithium ion battery anode material.
Section snippets
Experimental Section
The full synthesis of LTO nanosheet and cyclized-PAN/LTO composite samples are given in Fig. 1 and as follows.
Results and Discussion
Fig. 2a shows X-ray diffraction (XRD) patterns of synthesized pristine LTO nanosheet, uncyclized-PAN/LTO nanosheet and cyclized-PAN/LTO nanosheet samples. All diffraction peaks match well with the standard spinel phase Li4Ti5O12 (JCPDS #49-0207) in all samples regardless of PAN coating and only a tiny peak at 25.2°appears on pure LTO pattern. By cross-referencing diffraction database and examining our experimental procedure, this impurity peak highly likely represents (101) plane of anatase TiO2
Conclusions
The cyclized-PAN/LTO nanosheet composite material was synthesized by one-pot hydrothermal method and followed by a straightforward cyclization process for high-performance lithium ion battery anode material. Multiple material characterizations such as Raman, TG-DSC and XPS confirm the incorporation and turbostratic transition of polymer in the electrode material. Electron microscopy and EDS results tell that cyclized PAN is not only formed a conformal coating onto LTO particle, but it is formed
Acknowledgment
This work is supported by Shanghai Leading Academic Discipline Project (B502), Shanghai Nanotechnology Special Foundation (No. 11nm0500900) and Shanghai Key Laboratory Project (08DZ2230500).
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