Lithium-ion batteries (LIBs) have become a major choice for electric vehicles and renewable energy storage systems owing to their high energy density and high charging efficiency. While LIBs play a crucial role in meeting global energy demand, during long-term operation they undergo undesirable effects that cause battery degradation. Battery aging is caused by side reactions and degradation processes at various points within the battery. Several factors such as the chemistry, charge/discharge conditions, and temperature influence the rate of degradation. More importantly, the growth of the parasitic solid-electrolyte-interface (SEI) layer and lithium plating (Li-P) on graphite anodes play a vital role in LIB degradation, causing capacity fade, increased resistance, possible internal short circuits, and eventually leading to enhanced thermal runaway hazards. This study investigates the effects of SEI growth and Li-P on battery degradation through a physics-based pseudo-two-dimensional (P2D) model, which is based on the Doyle-Fuller-Newman (DFN) framework. Results from the model indicate that SEI and Li-P have a great impact on the performance and aging of LIBs, with SEI growth triggering the onset of lithium plating. This degradation model provides valuable insights into the performance and aging behavior of LIBs under varying operational conditions and thus can help define operational and design boundaries to reduce capacity loss and safety risks.