# Recognition of Driving Behavior in Electric Vehicle's Li-Ion Battery Aging
## 2 Battery Degradation Mechanism  
There are three varieties of EVs available on the market: BEV, hybrid electric vehicle (HEV), and plug-in hybrid electric vehicle (PHEV) [2]. They all have different batteries (i.e., capacity, chemical characteristics, and degradation mechanisms). For BEV, HEV, and PHEV, the capacity is typically 20 to 80 kWh, 1.3 to 1.6 kWh, and 4.5 to 10 kWh, respectively. The electrochemistry of different EVs and the associated battery types also differ. For example, the Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\) (LTO) anode material in the battery can provide stronger electrolyte stability than graphite, although it has a lower capacity. Despite having a higher energy density, the \(Li[Ni_{x}Mn_{y}Co_{z}]O_{2}\) (NMC) with graphite material in the battery has a limited lifespan [66]. Only the BEV, the most common EV, and the most widely used battery, which uses the NMC material will be the subjects of this study [67]. [PERSON] et al. [66] showed the main elements and degradation mechanisms inside the lithium-ion battery (LiB). Generally, anode and cathode material sheets are intercalated by the current collectors (i.e., copper and aluminum) and isolated by a sheet of separator in the middle. Inside the battery shell, the electrolyte is filled to allow the full conduction of the materials. The structure and NMC materials can provide a high energy density, long cycle life, and exceptional thermal stability, but also poor safety [68]. This is because the electrochemical characteristics of these materials are not stable. They will continue to produce chemical reactions, discharging and charging over time, which will affect the life of the battery. Meanwhile, the battery's minuscule internal short circuit and a rapid spike in heat generation at various SOCs hinder efforts to maximize battery utilization [69; 70]. All the electrochemical processes that have been identified as contributing to LiB degradation are outlined in Figure 1. Degradation comes in two forms: capacity fading and power fading. Capacity fading would be the SOH, which can be assessed in a variety of ways [71]. Power fading is determined by [PERSON]'s law and the electric power formula \(P=UI=U^{2}/R\). The power decreases as the battery's internal resistance rises. The next subsections will go into further detail on the reactions. Graphite currently makes up the majority of anode active material. The fundamental problem causing the battery's deterioration is the solid electrolyte interphase (SEI) film [72], as illustrated in Table 1. When the LiB is charged for the first time, 5% to 10% of the Li-ions are used up to create a layer between the electrolyte and the graphite that prevents the graphite from coming into touch with the electrolyte and gives the Li-ions a high degree of permeability [73]. This layer is prone to corrosion and will eventually develop fissures. Additionally, cracks appear during charging due to the volume expansion up to 10% of the graphite [74]. According to [PERSON] et al. 2021 [2], the SEI layer cracks would grow larger and faster when the battery was under high DOD and SOC conditions. These fissures will continue to interact with the electrolyte, consuming the Li-ions and generating gas. As a result, the SEI layer's thickness will continue to rise with the capacity loss caused by the reduction of Li-ions. Figure 1: The degradation reactions overview. The active NMC cathode was first suggested in the 1990s [77]. Currently, many electrochemical processes that contribute to LiB aging have been identified over the decades. The main processes are summarized as following Table 2. Due to the large reduction in Li-ions, cathode-electrolyte interface (CEI) film is one of the processes that contributes most to degradation. The electrolyte maintains the LiB's cycle stability, capacity, and safety between the anode and the cathode [83; 84]. Table 3 is a breakdown of the precise electrochemical processes. The mainstream material for BEV is lithium hexafluorophosphate (LiPF\({}_{6}\)).