Improving the Performance of Lithium-ion Batteries Through a Multi-Objective Investigation and Optimization of the Solid Electrolyte Interphase Microstructure

The rapid development of lithium-ion batteries (LIBs) has changed the world. However, LIBs suffer from performance degradation due to undesired chemical reactions, ageing, corrosion, compromised structural integrity, and thermal runaway. This degradation occurs during both calendar and cycling lifespans and reduces the longevity of LIBs. The main degradation mechanisms in LIBs vary with different active materials, however, it is well known that a carbonaceous lithium-intercalation electrode in contact with electrolyte solution becomes covered by a passivation layer called a solid electrolyte interphase (SEI). While this SEI layer can inhibit further electrolyte decomposition, SEI layer growth can also cause battery capacity fade and increase cell internal resistance. Therefore, the study of the SEI layer plays a key role in battery degradation and other related performance improvement research. The objective of this dissertation is to improve the performance of lithium-ion batteries through the investigation of the SEI layer and electrode microstructure. The investigations include the numerical simulation of the formation, morphology evolution, and crack propagation of the solid electrolyte interphase, and the experimental study of developing the porosity graded electrode in mitigating battery degradation. The phase field method is applied to investigate the SEI layer formation, crack propagation and dissolution. The simulation results prove that SEI layer formation, cracking and dissolution are location dependent. To improve the adverse impact of the SEI layer on LIBs performance, porosity graded electrodes are designed to mitigate LIBs degradation. Charging and discharging cycling tests show that porosity-graded cells reduce the capacity fade about 8.285% in full cell and 5.29% in half-cell, respectively. The porosity increase can improve the conductivity and diffusivity of lithium-ions through the electrode