**1. Introduction**

Lithium-based batteries are dominating the battery market because of their high energy density and rapidly decreasing manufacturing cost per kWh. While these batteries have many advantages, they also have disadvantages such as safety and recycling. Recycling of lithium ion batteries is a threefold process that involves pyrometallurgy (treatment with heat), hydrometallurgy (treatment with acid/liquid) and recycling through physical processes such as separation by weight. Recycling was mainly performed to recover the rare-earth metals that are hard or expensive to find and mine and hazardous materials that are toxic for the environment [1]. By recycling the used or spent lithium ion batteries, it is possible to recover up to 70% of the cathode material that is made up of rare earth metals [2]. However, the cost of recycling lithium ion batteries is increasing because of the increase in the complexity of lithium ion battery chemistries to ensure stability and improved tolerance to charging rates and temperatures [3]. Safety is a major concern in lithium ion batteries because they are designed to have highly combustible agents (such as organic solvents in electrolytes) and combustion inducing agents (electrochemical reactions that generate heat) in a sealed container. When operating normally, the electrochemical reactions generate very little heat and therefore prevent any kind of combustion or explosion. However, if subjected to extreme operating conditions (e.g., high charging/discharging currents, high temperatures, etc.) these agents can react violently and result in explosions [4]. Inappropriate operation can also lead to dendrite formations that can cause an internal short between the cathode and the anode and result in explosive reactions [5]. The electrolyte is one of the important components in the battery where heat or gas generation can cause problems. This is because most electrolytes in lithium ion batteries are dissolved in organic solvents that are highly flammable [6]. To ensure that these batteries operate as they are designed to, battery management/monitoring systems (BMS) are developed to continuously monitor the states of the battery such as the State of Charge (SOC), State of Health (SOH), Remaining Useful Life (RUL), State of Function (SOF) and temperature of operation. The BMS also monitors the charging and discharging operations of the battery to ensure that the operating currents are within the rated specifications of the battery and that the upper and lower cut-off voltage limits are not exceeded [7]. The temperature of operation of the battery also has a very important role in its operation and performance. When the battery is operated at higher temperatures, the electrolytic resistance decreases initially and then begins to dissociate resulting in an increase in the resistance between the electrodes. Similarly, when the temperature of the battery drops below the operating range, the electrolyte begins to coagulate, resulting in an increase in the resistance to the flow of lithium ions between the electrodes [8]. The C-rate or charging/discharging rate plays a crucial role in the degradation of the battery as well. Using very high C-rates can lead to deposition of lithium ions instead of intercalation. Deposition of the lithium ions will result in loss of active material and lithium inventory and cause the battery to degrade faster [9]. To better understand the operation of lithium ion batteries, a physics-based modeling approach is used to represent the lithium ion battery and its components [9]. Most batteries have four major components—electrodes, electrolytes, separators and current collectors. The electrodes, positive and negative, are the regions where electrochemical reactions take place that generate electrons. The electrolyte acts as a charge transportation medium between the positive and negative electrode and vice versa based on the mode of operation (i.e., charging or discharging) [10]. The electrolyte in lithium ion batteries is often lithium salts such as lithium hexafluorophosphate (LiPF6) dissolved in an organic solvents usually ethylene carbonate (EC) and di-methyl carbonate (DMC) [11]. The ratio of the EC and DMC is determined by the dielectric property and the viscosity requirements of the electrolyte. EC contributes to the dielectric property while DMC makes the electrolyte less viscous [12]. The dielectric property contributes to the charge holding capability and the viscosity determines the resistance to the flow of ions between the electrodes. The separator provides electrical isolation between the electrodes and is doused in the electrolyte to enable movement of ions through the separator. To reduce the effect of self-discharge, the electrolyte is designed to have very high ionic conductivity and minimal electronic conductivity, which means the electrolyte offers low resistance to lithium ion movement and very high resistance to the flow of electrons. This high ionic conductivity and low electronic conductivity is achieved by dissolving the LiPF6 in an organic solvent—the EC and DMC combination. The organic solvent ensures that the electrolyte offers high impedance to electron flow, and the dissolved LiPF6 ensures that it offers a low resistance to lithium ion flow. The electrolyte of the battery in this paper is LiPF6 dissolved in a 2:1 EC:DMC solution [13].

This paper discusses the physical manifestations of side reactions that happen in the electrolyte and the impact these manifestations have on the terminal voltage of the battery. It simulates degradation of the electrolyte through degradation of the salt diffusion coefficient and the transference number and generates the voltage profile when the battery model is subjected to a constant discharge current of 0.4C for a fixed duration of 4500 s or until the lower limit of the terminal voltage (3.5 V) is reached. The degradation of the electrolyte parameters was simulated in intervals of 10% so as to determine the sensitivity of the terminal voltage to the degradation of the parameter in consideration.

This article is organized as follows: Section 2 discusses the side reactions that take place in the battery, Section 3 presents the electrolyte salt diffusion coefficient degradation and the impact it has on the terminal voltage of the battery, Section 4 describes the impact of electrolyte transference number degradation on the voltage across the terminals of the battery and Section 5 concludes the paper and provides a discussion on the future work planned for this research area. The abbreviations, units and initial values of all the parameters used in the simulation are described in Table A1.

#### **2. Side Reactions**

The side reactions in a battery are highly reliant on the battery's operating parameters. Based on operating conditions such as temperature and charging and discharging C-rates, there are three areas where side reactions can occur—at the electrode–electrolyte interface [14], at the electrode–current collector interface and in the electrolyte itself.

#### *2.1. Electrode–Electrolyte Interface*

At the electrode–electrolyte interface, the side reaction causes an increase in the thickness of a solid electrolyte interface. The solid electrolyte interface (SEI) is a passivation layer that is designed by the manufacturer during the creation of the battery to isolate the electrode from the electrolyte as shown in Figure 1 [15]. If the battery is exposed to temperatures outside it's nominal operating range (higher or lower) or high charging/discharging rates, there is a significant rise in the loss of lithium inventory because of changes in the electrochemical reactions inside the battery. When operated under high charging/discharging rates, the lithium ions react with the SEI and cause a chemical reaction that results in the depletion of the SEI. Upon undergoing this kind of reaction, the anode is exposed to the electrolyte. The anode exposure to the electrolyte causes chemical reactions that produce compounds that inhibit the charge-producing electrochemical reactions [16]. The battery also undergoes different kinds of stress: charging/discharging stress, mechanical stress, temperature-based stress, etc. [17–19]. This paper focuses on the mechanical stress that the battery's electrodes undergo and the resulting side reactions. This kind of stress can cause the battery's electrode to separate from the current collector and cause a barrier to the current flow between the current collector and the electrode. This phenomenon is electrically represented as a growth in the internal resistance of the battery [20]. Exceeding the upper and lower cut-off voltage by forcing the battery to charge or discharge beyond the manufacturer's specification results in a chemical reaction between the current collectors, the electrode and the electrolyte. This corrosive reaction causes a passivation layer to form between the electrode and current collector. The passivation layer hinders the transfer of electrons from the electrode to the current collector and thus causes the resistance of the battery to increase [21].

**Figure 1.** Structure of the battery with manufacturer designed SEI.

#### *2.2. Electrode–Current Collector Interface*

The electrode–current collector interface is where the battery is able to accept the electrons from the external circuit to complete the electrochemical reaction. During the design of the battery, the electrode is deposited on the current collector, made of highly conductive metals such as aluminum and copper, to prevent any loss of contact. However, either due to improper design or wear and tear of the electrode or current collector, there may be loss of contact between the current collector and the electrode. This loss of contact between the two surfaces can result in an increase in the internal impedance because of the gap between the surfaces and thus cause energy loss [22].
