Strategies to improve the electrochemical performance of electrodes for lithium-ion batteries
Abstract
Lithium-ion batteries are widely used in consumer market because of their lightweight and rechargeable property. However, for the application as power sources of hybrid electric vehicles (HEVs), which need excellent cycling performance, high energy density, high power density, capacity, and low cost, new materials still need to be developed to meet the demands. In this dissertation work, three different strategies were developed to improve the properties of the electrode of lithium batteries. First, the voltage profile and lithium diffusion battier of LiM1/2Mn 3/2O4 (M=Ti, V, Cr, Fe, Co, Ni and Cu) were predicted by first principles theory. The computation results suggest that doping with Co or Cu can potentially lower Li diffusion barrier compared with Ni doping. Our experimental research has focused on LiNixCuyMn 2-x-yO4 (0<x<0.5, 0<y<0.5) and we found that the amount of Cu will affect the lattice parameters, the cation disorder in the spinel lattice, the particle morphology, as well as the electrochemical properties. With detailed electrochemical measurements and in situ XAS experiments of LiNi0.25Cu0.25Mn1.50 O4, the proposed explanation of the voltage profile by the first principles computation was proven, a second plateau at 4.2V originates from the oxidation of Cu2+ to Cu3+ and the plateau at 4.95V originates from extra electrons provided by oxygen ions. Although the reversible discharge capacity decreases with increasing the Cu amount, optimized composition such as LiCu0.25Ni0.25Mn 1.5O4 exhibits high capacities at high rates. Second, titanium dioxide flakes have been synthesized through a simple spreading method that is easily scalable. The calcined titanium dioxide flakes exhibit larger reversible charge/discharge capacity, better rate capability and excellent cycling stability compared to anatase titanium dioxide nanoparticles. The smaller grain size in the flakes most likely enables the formation of the new LiTiO2 phase during the lithiation process attributing to the improved reversible charge/discharge capacity. The larger surface area of the flakes leads to a larger contact area between electrode and electrolyte, shorter diffusion lengths for the transfer of ions and electrons results in the better rate capability. The cycling performance was significantly improved by the porous structure of the calcined titanium dioxide flakes. Finally, the all-solid state thin film batteries were deposited by pulsed laser deposition method to explore the intrinsic properties of electrode itself. The cathode of LiNi0.5Mn1.5O4 thin film was fabricated at 300mtorr 600°C on SiO2/Si and show good electrochemical properties. The thin film electrodes offer the ability to probe the surface of the material without the need of a conductive agent and polymer binder, typically used in composite electrodes. The results suggest that neither oxidation of PF6 to POF5 nor the decomposition of ethylene carbonate or dimethylene carbonate occurs on the surface of the spinel material. These results confirm the enhanced cycling stability and rate capability associated with the high voltage spinel material and suggests that the SEI layer forms due to the extra electrochemically inactive components in the composite electrode. LVSO ceramic was chosen as the solid electrolyte and deposited by PLD technique. The analysis results show the LVSO thin film exhibit uniform element distribution with exact stoichiometry and amorphous phase that is suitable for the application of all-solid state thin film batteries. Finally, two different thin films were fabricated as the anodes for all solid state thin film batteries. The Li4Ti5O12 thin film/Au deposited at 700°C shows excellent electrochemical properties but high temperature deposition limits its application because there are differences in thermal expansion coefficients along with inter-layer diffusion during the final layer deposition process. The TiO2 thin film deposited at 350-450°C show high capacity coupled with excellent cycling performance. The smaller grain size in the thin films most likely enables the formation of the new LiTiO2 phase during the lithiation process. The porous thin film deposited at 350°C leads to a larger electrode/electrolyte contact area results in the better rate capability. Finally, the all solid state thin film battery with TiO2/LiNi0.5Mn1.5 O4/L3.4V0.6S0.4O4/SiO 2/Si stack was fabricated successfully only by PLD process.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2012
- Bibcode:
- 2012PhDT.......105Y
- Keywords:
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- Chemistry, Inorganic;Nanotechnology;Engineering, Materials Science