Nanocarbon-containing High Power Cathode for Rechargeable Hybrid Aqueous Battery
Aqueous batteries have been considered to be one of the most suitable large-scale energy storage systems that require excellent safety, low cost and high power. A new secondary aqueous Zn/LiMn2O4 battery system, called Rechargeable Hybrid Aqueous Battery (ReHAB), has been developed by our group recently. Compared to lead-acid batteries that have been widely used as large-scale energy storage systems, this system is more environmentally friendly and shows higher Coulombic efficiency as well as longer cycle life. However, the rate capability of the LiMn2O4 cathode is intrinsically low, which leads to the low capacity at a high current density operation. One strategy to increase the rate capability of the LiMn2O4 cathode is adding conductive additives (e.g., acetylene black (AB), carbon nanotubes (CNTs) and graphene) to the LiMn2O4 cathode to improve its electrical conductivity. However, traditional conductive additives, like AB, give limited conductivity improvement and can provide neither long-range conductivity nor multiple Li ion pathways. Besides, AB and LiMn2O4 nanoparticles are easily aggregated during charge/discharge processes. CNTs with a large aspect ratio can provide long-range conductivity, better interfacial contact between LiMn2O4 nanoparticles and conducting pathways. At the same time, they can create more stable network structures for dispersing LiMn2O4 nanoparticles. Hence, CNTs are an ideal material to improve the electrical conductivity of the LiMn2O4 cathode. In addition to CNTs, graphene with superior electrical conductivity and high surface area can provide a highly conductive matrix and offer a high contact area between electrolyte and electrode, facilitating the transportation of Li ions and electrons in the electrode. Graphene is also an ideal template to construct a hybrid material with good dispersion of LiMn2O4 nanoparticles and improved electrical conductivity. This thesis is focused on preparation of new carbon materials (CNTs and graphene) as conductive additives to improve the rate performance of the LiMn2O4 cathode and includes two parts. The first part is focused on the preparation of CNTs by a chemical vapor deposition (CVD) method and its use as a new conductive additive to increase the rate performance of the LiMn2O4 cathode. Firstly, a three-dimensional hierarchically structured CNT/AB network was fabricated to increase the conductive contacts among LiMn2O4 nanoparticles and provide conductive pathways for fast electron transfer. The 3.3wt%CNT/AB/LiMn2O4 electrode exhibited excellent rate capability (specific discharge capacity of 105 and 73 mAh g−1 at 10 C and 20 C, respectively) and cyclability (Coulombic efficiency of almost 100% over 300 charge/discharge cycles at 4 C), in comparison with the cathodes prepared previously. Additionally, the initial specific discharge capacity researched 139 mAh g-1, which has never been reported in the literature. When the content of CNTs reduced to 2 wt%, the 2wt%CNT/AB/LiMn2O4 electrode still exhibited at least 23% higher rate capability than that of the traditional 10wt%AB/LiMn2O4 electrode. Then, highly robust, binder-free flexible LiMn2O4/CNT electrodes were synthesized and used in the aqueous battery for the first time. The excellent electrical conductivity of the electrodes facilitates electron transport, resulting in a high rate capability. Besides, it can be bent and twisted under mechanical stress, which makes it possible to be used in flexible devices. The second part of the thesis is focused on the preparation of graphene and its use as new conductive additives to increase the rate performance of the LiMn2O4 cathode. Firstly, three different kinds of graphene were prepared and used as conductive additives of the LiMn2O4 cathode of the ReHAB, which are exfoliated graphene (EG), reduced graphene oxide (RGO) and porous graphene (PG). PG prepared from the CVD method exhibited excellent conductivity and porous structure, resulting in significantly better rate capability and cyclability of the ReHAB than those using EG and RGO. In order to further improve the rate capability of the ReHAB, PG/AB conductive networks were prepared by simple mechanical mixing. The rate performance of the LiMn2O4 cathode was obviously improved when PG and AB coexisted as conductive additives. The PG/AB conductive network can provide: (1) conductive contacts between LiMn2O4 and AB nanoparticles; (2) conductive contacts between LiMn2O4 nanoparticles and PG nanosheets; (3) physical contacts between AB nanoparticles and PG nanosheets. The PG/AB conducting network facilitated the transfer of electron, resulting in an excellent rate performance of the ReHAB. When the content of the PG reduced to 1 wt%, the 1wt%PG/AB/LiMn2O4 electrode still exhibited at least 19% higher rate capability than the traditional 10wt%AB/LiMn2O4 electrode. It is worth mentioning that the CNT/AB/LiMn2O4 and PG/AB/LiMn2O4 electrodes were prepared by a simple mechanical mixing method, which is effective and inexpensive in comparison with other complicated preparation approaches, paving the way for easy scale-up of the process.
Cite this version of the work
Xiao Zhu (2017). Nanocarbon-containing High Power Cathode for Rechargeable Hybrid Aqueous Battery. UWSpace. http://hdl.handle.net/10012/12129