Capacity fade studies and high performance cathode materials for secondary lithium batteries

The capacity of a lithium-ion battery decreases during cycling. This capacity loss or fade occurs due to several different mechanisms which are due to or are associated with unwanted side reactions that occur in these batteries. These reactions occur during overcharge or overdischarge and cause electrolyte decomposition, passive film formation, active material dissolution and other phenomena and affects the cycle life and rate behavior of lithium-ion cells in a severe manner. One such process is the overcharge of the negative electrode causing lithium deposition, which can lead to capacity losses including a loss of active lithium and electrolyte and represents a potential safety hazard. A mathematical model is developed for a Li_xC₆!1 M LiPF₆, 2:1 EC/DMC, p(VdF-HFP)!LiMn₂O ₄ system to investigate the influence of lithium deposition on the charging behavior of intercalation electrodes. Model results can be used to establish operational and design limits within which safety hazards and capacity fade problems, inherent in these cells, can be minimized.; Computer simulations are compared with experimental data for Bellcore Plastic Lithium-Ion () cells having higher active material loadings, competitive energy densities and higher specific energies than liquid lithium-ion batteries. Cells with different electrode thicknesses, initial salt concentrations, and higher active material loadings were examined using the mathematical model to understand better the transport processes in the plasticized polymer electrolyte system. The model simulations indicate that solution-phase diffusion limitations are major limiting factor during high-rate discharges.; The second part of this dissertation concerns with synthesis and characterization of high performance cathode materials. These materials exhibit higher capacity, and lower capacity fade compared to prominent cathode materials presently used in commercial secondary lithium batteries. Chromium oxides and lithiated chromium oxides were synthesized by thermal decomposition of chromium trioxide (CrO₃) at high temperature and pressures. Synthesis temperature and pressure markedly affect the performance of these cathode materials. Higher pressures lead to higher O/Cr ratio and fewer impurities in the final product. These materials are stable intercalation hosts for lithium in the entire intercalation range and exhibit high capacity. A small amount of cobalt as dopant in the chromium oxides provides greater stability to the structure of CrO_x and helps in improving the high-rate behavior of these oxides. A wide range (y = 0.05 to y = 0.33) of cobalt-doped LiCo_yMn_2-yO ₄ spinels were synthesized and characterized electrochemically. Cobalt-doped spinels of LiCo_yMn_2-yO₄ type showed improved specific capacity and capacity retention over pure spinels.