| Description |
This dissertation work can be divided into two parts: (1) an experimental section involving synthesis and electrochemical testing of (a) Li-Mg and (b) columnar Si anodes for high capacity Li-ion batteries, and (2) a theoretical modeling section in which analytical modeling frameworks are developed to predict (a) electrochemical charge/discharge behavior of amorphous Si thin film anodes, and (b) the discharge characteristics of a graphite/LiFePO4 cell. In the first part of the experimental work, two Li(Mg) alloys with nominal compositions, Li-60 wt.% Mg (Li7Mg3) and Li-30 wt.% Mg (Li8Mg) were synthesized by direct alloying. These alloys showed electrochemical discharge characteristics that were comparable with those of pure Li. A phase transition, from the BCC Li(Mg) P-phase to the HCP Mg(Li) a-phase, was found to occur during the discharge. The Li(Mg) electrodes also showed some degree of reversibility when cycled against LiCoO2 and Li. In the second part of the experimental work, four different columnar Si structures were obtained by electrochemical etching. Si electrodes with porosities between 50-65% and with clear columnar structures showed reversible lithiation/delithiation capacities greater than 1000 mAh/g after 20 cycles, which is higher than the reported values in literature for Si anodes with similar structures. The Si electrode with wide interconnected pores showed poor electrochemical performance. The Si columns in the cycled electrodes appeared to be largely intact after 20 cycles. In the theoretical modeling work, we first developed an analytical modeling framework to predict the lithiation/delithiation behavior of amorphous Si (a-Si) thin film electrodes. Li transport through the electrode (by diffusion) and electrochemical charge transfer at the electrode-electrolyte interface were described mathematically in this model. The simulated charge/discharge characteristics agreed well with the experimental data of a-Si thin film anodes at different C-rates. An integrated model was developed to predict the discharge characteristics of a full cell consisting of graphite anode and LiFePO4 cathode. In this model, the phase boundary movement within the LiFePO4 electrode upon lithiation was described. When the simulation results for LiFePO4/Li and LiC6/Li half cells were coupled together, they were found to predict successfully the discharge behavior of a capacity-matched graphite/LiFePO4 cell. |
