**1. Introduction**

Lithium ion (Li-ion) batteries were first developed in the 1970s [1–3]. After two decades of intensive materials development, Li-ion cells were commercialized by Sony in 1991 [4,5]. Constructed as the best compromise due to many excessive failures of rechargeable Li-metal cells beforehand, Li-ion cells have undergone a tremendous evolution in the last few decades and have been widely utilized for energy storage in different portable, computing, and telecommunicating devices as well as electrified transport vehicles. Clean electric conveyances are a possible way to reduce the environmental impact of private transport and abate about 10–20% of the emissions [6,7]. However, increasing the energy density of Li-ion batteries to accomplish the actual demand of electrified vehicles is of importance. As a solution, electrodes have to become thicker and denser. If the negative carbon-based electrode is manufactured with a higher thicknesses, more density, and less electrolyte uptake, then the occurrence of Li-plating becomes an unavoidable issue. Moreover, capacity retention, lifetime, fast and low temperature charging, and safety performance of the cells still require improvements. These challenging demands are all directly or indirectly influenced by lithium deposition [8–10]. The appearance of metallic lithium on the surface of carbon particles is a complex function of temperature, aging, and cycle loads.

Lithium deposition or lithium plating, which generally means the formation of metallic lithium on the negative electrode is an all-time undesirable phenomenon, contributing to cell performance degradation, reducing the cell durability and cyclability. Additionally, it significantly raises safety issues [11]. Avoiding or suppressing the lithium deposition reaction is essential to the reliability and also improvement of Li-ion cells. This paper introduces a way to investigate plating occurrence. Therefore, it is important to describe the phenomenon of Li-Plating in more details.

While charging, lithium ions are extracted from the cathode, diffuse through the electrolyte, and intercalate into the graphite structure of the negative electrode. The Li<sup>+</sup> charge transfer process (CTP) takes place on the surface of electrode particles, meaning the de-solvation of solvated lithium ion from the electrolyte, transferring through the solid electrolyte interface (SEI) layer and entering into the electrode particle, becoming an intercalated lithium. The intercalated lithium diffuses further inside the insertion electrode, i.e., the graphene planes of graphite, preventing the particle surface to reach saturation. This is called the lithium solid diffusion process (SDP). Depending on the operation conditions e.g., at a high C-rate, low temperature and high state of charge (SOC), the CTP and SDP will be the limiting factor, respectively. It means the speed of the lithium ion flow in the electrolyte is exceeding the charge transfer process or solid diffusion. Consequently, the lithium deposition reaction can occur instead or in parallel with intercalation. Thermodynamically, lithium plating is not favorable in comparison to intercalation since its reaction enthalpy is more positive [12,13]. Nevertheless, during a charge process, due to the deviation from equilibrium, an induced overpotential is formed [14] which may cause the anode potential to drop below 0 V vs. Li<sup>+</sup>/Li and consequently the lithium deposits on the surface of the graphite particle [15]. It should be noted that for intercalation, the potential range is from 200 to 65 mV vs. Li<sup>+</sup>/Li [13]. Lithium solid diffusion overpotential and charge transfer overpotential, which enable the lithium deposition reaction, are the kinetic causes of this phenomenon. Operating conditions like low temperature (below 25 ◦C), fast charging, and high SOCs increase the cell overpotentials [16,17].

According to the literature [18], a Li<sup>+</sup> ion which is transferred from electrolyte to the anode experiences discrete energy barriers at different regions. Diffusing in the electrolyte (considered a liquid) has a relatively low energy barrier while charge transfer through the SEI usually has the biggest activation energy (E*a*) and diffusion of Li into graphite has a moderate energy barrier in the range of 0.22–0.4 eV, increasing by x in Li*x*C6 [19,20]. According to Arrhenius, a low temperature slows down the reactions [14] which means less Li<sup>+</sup> ions can overcome the charge transfer energy barrier to intercalate. At the same time, the solid diffusion of Li happens more sluggish at high SOCs. Therefore, when the charging current is high enough to induce a big ion flux toward the anode, so that the Li solid diffusion rate cannot compete, lithium deposition happens with a higher probability [15,17,21].

There is some literature investigating factors influencing CTP kinetics. Some believe that the Li<sup>+</sup> de-solvation step is always slower than Li<sup>+</sup> transferring through the SEI [22,23]. Some other investigate different electrode as well as electrolyte materials and conclude that the kinetics of Li<sup>+</sup> charge transfer process is controlled by the chemistry of the electrode components and their interfacial layer. By having a SEI layer with a low energy barrier for conducting Li+, the de-solvation step is limiting and vice versa if the SEI layer is not conductive enough [24].

Numerical simulations provide quantitative information to further investigate the phenomenon. Firstly Arora et al. [25] introduced a numerical way for describing and predicting the lithium deposition at charge and overcharge. Years later Tang et al. [26] extended the previous work to 2D. Legrand et al. [13] investigated Li-P (lithium plating) through CTP limitations by an electrochemical model, ye<sup>t</sup> they have not examined the SPD process. Jiang et al. [27] proposed characteristic times to explain charge and species transport limitations in Li-ion batteries, but they did not cover the aging influences. Understanding the lithium plating phenomenon has been the focus of many studies, however there is still a lack of information on the mechanisms of transport-related performance limitations during charge/discharge operations over the life time of Li-ion cells. In the present model-based study we investigate the transport mechanisms behavior of Li-ion cell over 400 cycles. Aging mechanisms are included by a growing surface layer consisting of SEI and plated lithium. We introduce and explain five regions in the cell (A-E) which are contributing to the Li<sup>+</sup> charge transfer and Li solid diffusion processes. Ionic conductivities of these regions are calculated afterwards. The cell

aging behavior and appearance of Li plating from the 116th cycle of the simulated cell are explained and discussed with the help of ionic conductivity variation in the mentioned regions.

The paper is organized as follows. Section 2 explains the transport regions and implementation of our P2D (pseudo two-dimensional) model. In Section 3 we show the validation of our calculations, discuss the model results, and explain them with the help of ionic conductivity calculations. Section 4 summarizes the results.
