title: Investigation on the Electrochemical Performance of the Silicon and Germanium Based Lithium-ion Batteries
Creators: name: Liu, Chenguang
Publisher: name: University of Liverpool Repository
Year: 2020

description: Lithium ion batteries (LIBs) have currently dominated the commercial market owing to the environmental benignity, suitable energy density, and long cycle lifetime. The commercial LIBs are commonly using graphite as anode materials, however, it has become clear that the theoretical capacity (~372 mAh g-1) of graphite has nearly reached the bottlenecks with little room for further exploration, and also the energy density and rate performance of existing LIBs are not sufficient for some advanced electronics equipment such as smart watch, and micro implantable biosensor system. With increasing demand and market potential, the worldwide academia researches and industrial community have been focused on investigating anode materials to achieve desirable power density, high rate performance, and long-term stability energy storage system, generating further impetus on flexible electrochemical applications, such as wearable devices, portable electronic devices especially for implant biological equipment. Alternative anode materials such as metal (Si, Ge and Sn) and metal oxide (Co3O4, SnO2 and GeO2) have been considered. Among them, the Si and germanium oxide have the highest theoretical gravimetric capacity in the elementary substance and oxide-based anode material respectively, which have been proposed as the best candidates for rechargeable battery anode. However, some challenges for these anode materials are also obvious due to the low conductivity and large volume expansion (&gt; 300%) during the usage of LIBs. This expansion problem causes the pulverization of active materials and the repeated formation of the solid electrolyte interface (SEI) on that, resulting in the loss of interparticle electrical contact, and consequently deteriorating the battery cycle lifetime and capacity performance. In this work, we firstly demonstrated a facile method to fabricate a flexible alloyed copper/silicon core-shell nanoflowers structure anchored on the three-dimensional graphene foam as a current collector. In electrochemical testing, the resulting copper/silicon core-shell nanoflowered electrode demonstrates a high initial capacity of 1869 mAh g-1 at 1.6 A g-1, with a high retention rate of 66.6 % after 500 cycles. More importantly, at a high current density of 10 A g-1, this anode remains a high capacity retention &gt; 63% (compared with the highest capacity 679 mAh g-1), offering enormous potential for energy storage applications. Secondly, we introduced a facile method to synthesize an amorphous GeOx-coated MXene nanosheet structure as the anode in lithium-ion batteries. For electrochemical performance, this GeOx/MXene nanosheet exhibited a reversible capacity of 950 mA h g-1 at 0.5 A g-1 after 100 cycles. It is indicated that the GeOx/MXene nanosheet structure can significantly improve the stability during the lithiation/delithiation prosses, with the enhanced capacity by the improvement of processes' kinetics. Thirdly, we built up a facile equipment to measure the high frequency capacitance change of silicon composite electrode. As this high frequency situation, the hypothesis circuit of the coin cell could be seemed as a combination of geometrical capacitance and resistance. For the alloy anodes which exhibited huge volume expanse during the lithiation/delithiation processes, the change of geometrical capacitance could be ascribed to the stress evolution and pulverization effect. Thereby the variation trend of the stress and pulverization could be determined by the change geometrical capacitance change. To conclude, this project mainly focused on the pulverization and stress effect of the anode materials with alloying lithiation type. The strategies of first and second work were using the nanostructure engineering and 2D materials to release the stress and prevent the pulverization in the electrode. The results from these electrodes exhibited a stable electrochemical performance. Meanwhile, the rate performance of these electrodes was also improved by the additive of highly conductivity materials (e.g., copper, graphene, and MXene). To further investigate the consequence of severe volume expansion, we also built a high-frequency capacitance characterization system to perform the in-situ measurement of stress evolution and pulverization in coin cell with composite Si anode. That demonstrated the expected behavior corresponding to the electrode in the different states of charging.