| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 101 === The performance of high-energy and/or high-power Li-ion batteries depends strongly on the architecture of the electrode over-layers. The main purpose of this research is to explore new anode materials based on germanium for lithium-ion batteries. Germanium possesses a high theoretical capacity (~1600 mAh/g) compared to graphite (~372 mAh/g). However, the dramatic volumetric expansion during cycling result in structural instability and poor cyclability. This study was initiated from the structural stability viewpoints, respectively.
First, a structure design of porous Ge particles has been achieved via solution lithiation/de-lithiation method. Porous Ge particles displayed the pore and crack structure. The electrode of porous Ge displays improved charge capacity retention of ~440 mAh/g after 30cycles in CCCP mode. The particle size is only ~185% expansion during first cycle lithiation/de-lithiation by in-situ transmission X-ray microscopy analysis (in-situ TXM). This structure reduced clearly the volume expansion of Ge electrode. The pitch carbon-coated porous Ge (C-Ge) materials have been synthesized by thermal pyrolysis process. The electrode of pitch carbon coated porous Ge displays improved charge capacity retention of ~680 mAh/g after 30 cycles in CCCP mode.
The Ge/Cu3Ge composites were synthesized with different weight ratio of Ge and Cu2O by high energy ball-milling (HEBM) and 400℃ calcined process. It has been found that the cycle performance is prominently enhanced by Cu3Ge alloy. The structure of alloy is helpful to accommodate the volume expansion during charge/discharge process. For instance, the composites of the weight ratio of Ge and Cu2O is 1:1 that displays improved charge capacity retention of ~500 mAh/g after 50 cycles. This is better than one of using pure Ge. The particle size did not change a lot during first cycle lithiation/de-lithiation by in-situ transmission X-ray microscopy analysis (in-situ TXM). This composite reduced clearly the volume expansion of Ge electrode.
In addition, using different binder (SCMC, PVDF, Na-Alg) to investigate the effect on Ge/Cu3Ge system and reduce the SEI layer formation. The electrode which used Na-Alg as binder exhibited much reduced thickness expansion and remarkably enhanced cycling performance, as compared with that of other binder. The expansion of the electrode using Na-Alg is ~200% after 70 cycles which is better than the one using SCMC which is ~600% after 90th cycle by SEM analysis. The improvement have been attributed to the success in reducing the SEI layer formation after cycling. A better cycle performance is achieved by reducing the amount of Na-Alg. For instance, the capacity still have 400mAh/g and the capacity retention is about 62% after 120 cycles in constant current constant potential (CCCP) mode charge/discharge process. In rate performance test, the composite shows the excellent performance at high charge current.
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