Driven by the ever-growing concerns toward safety issues in lithium-ion batteries (LIBs), the study devoting to battery aging has drawn significant attention. An in-depth understanding of aging mechanisms is essential for developing LIBs with enhanced operating stability and improved life-span. Solid-electrolyte-interphase (SEI) formation on the anode is considered one of the main factors that give rise to the irreversible capacity loss in the battery. Two layers, thin inner-SEI and thick outer-SEI have been detected via experimental methods. However, due to the complex chemical componence of SEI, the understanding of SEI formation, especially for lithium immobilization in SEI, remain a challenge and is rarely investigated experimentally.
In the present work, an electron-tunneling-based model is extended to systematically describe the SEI formation in C6/LiFePO4 (C6/LFP) batteries under different aging conditions. Various cycling currents, storage state-of-charge (SOC), and temperatures are included. Additionally, it is well known that metal ions can diffuse from the cathode side in an aged battery and subsequently reduce on the anode surface, resulting in the aggregation of metal clusters. Consequently, the deposited metal cluster will provide extra electrons pathways and speed up the growth of SEI film. This process will be accelerated at elevated temperatures. Thus, to describe the effect of cathode diffusion on SEI formation quantitively, Fe plating-induced aging model is also proposed. SEI formation on graphite surface in C6/LFP batteries can be classified into three circumstances: (a) under storage and no significant volume change; (b) under cycling condition, SEI forming on freshly exposed graphite surface (Afr) due to structural fracture, and on initially covered graphite surface (Acov); (c) under cycling at elevated temperatures, SEI formation accelerating by deposited Fe cluster.
The irreversible capacity losses (ΔQir) under various aging conditions are modeled based on proposed aging models. Excellent agreement between experimental and simulation results is achieved in all cases. According to the results of simulation and experiments, the capacity loss is strongly dependent on storage SOC and cycling current. During cycling, the additional generated cracks will exacerbate performance decay in the batteries by facilitating SEI growth on freshly exposed graphite surfaces. Nearly a logarithmical relation between capacity loss and calendar aging time is observed. For the SEI formation under elevated temperatures, the development of inner and outer SEI layers, as well as the surface area of deposited Fe cluster on graphite surface, have been accurately determined via the proposed aging models. It can be concluded that both cathode dissolution and metal deposition will induce higher capacity losses and lead to severer degradation in C6/LFP batteries.
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