Development of nano-structured metal oxides/graphene for energy applications

Qi Liu, Purdue University

Abstract

Six parts consist of my dissertation work centered on the investigation of V2O5 and LiFePO4 as the cathode materials for Li-ion batteries. The first part is about the investigation of structure evolution of vanadium oxide (V2O5) nanocrystals during the Li+ ion intercalation and deintercalation processes using in operando high-energy x-ray diffraction (HEXRD) and in operando x-ray adsorption near edge spectroscopy (XANES). The second part is about development of novel method of incorporating graphene sheets into V2O5 nanoribbons via the so-gel process. The graphene modified nanostructured V2O 5 Hybrid has been developed with extraordinary electrochemical performance, 438 mAh/g, almost achieving the theoretical specific capacity, 443 mAh/g, with only 2% graphene in the composite. An in operando high energy synchrotron XRD revealed that such performance is the result of the enhanced thermal stability of the V2O5 xerogel. The graphene sheets help to preserve the V2O5 xerogel structure and keep the xerogel from collapsing by maintaining 0.3 water molecules per V2O5 (water molecules serve as a pillar between the V2O5 layers) during the annealing process. The AC impedance indicates that the electric conduction, vanadium redox reaction, and Li+ diffusion in the graphene modified nanostructured V2O5 hybrid have been greatly improved, resulting in a significant improvement on rate performance and cycle life. The third part is about the investigation of Li-ion insertion/deinsertion behavior of LiFePO4 cathodes in commercial 18650 LiFePO4 cells. In this study, we have performed operando synchrotron high-energy x-ray diffraction (XRD) to obtain non-intrusive, real-time monitoring of the dynamic chemical and structural changes in commercial 18650 LiFePO 4/C cells under realistic cycling conditions. The results indicate a non-equilibrium lithium insertion and extraction in the LiFePO4 cathode, with neither the LiFePO4 phase nor the FePO4 phase maintaining a static composition during lithium insertion/extraction. Based on our observations, we propose that the LiFePO4 cathode simultaneously experiences both a two-phase reaction mechanism and a dual-phase solid-solution reaction mechanism over the entire range of the flat voltage plateau, with this dual-phase solid-solution behavior being strongly dependent on charge/discharge rates.The fourth part of the dissertation is about the study of the capacity fade mechanism of the commercial LiFePO4 18650 cells via in-situ high energy synchrotron X-ray Diffraction techniques. The fifth part is about the development of a new synchrotron-based in-situ X-ray near-edge adsorption spectroscopy (XANES) to study the evolution of the vanadium ion valence state of both the positive and negative electrolytes in the all vanadium redox flow battery (VRB) under realistic cycling conditions. The results indicate that using the common widely used charge-discharge profile during the 1st charge process (charging the VRB to 1.65 V under a constant current density), the vanadium ion valence is far from V(V) and V(II) in positive electrode and negative electrode, respectively. And the state of charge (SOC) is only about 82%, which is far from 100% SOC. Such charged mix electrolytes can not only waste part of the electrolytes but also significantly decrease the cell performance in the following cycles. Based on our observations, we proposed a new designed charge-discharge profile and an almost 100% SOC is achieved after initial charge process. Consequently, the columbic efficiency (CC) and energy efficiency (EC) in the following cycles are higher than that obtained from the conventional charge-discharge profile (1.5% higher of CC and 2.5% higher of EC). Such proposed new charge-discharge profile can increase the charge capacity to the theoretical value and achieve the full utilization of electrolytes, and thus significantly reduces the cost of electrolytes of all VRB. The sixth part of my dissertation work is focused on the storing hydrogen using enhanced physical adsorption through polarizing hydrogen molecules", hydrogen storage would be achieved using a high electric field to polarize the hydrogen molecules and enhance the hydrogen adsorption over sorbents. To prove this concept, the hydrogen molecule adsorption under the high electric field has been measured. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Son, Purdue University.

Subject Area

Chemical engineering|Mechanical engineering|Materials science

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