(668c) Effect of Li Salt Concentration in the Electrolyte on Internal Heating of a Lithium-Ion Battery

Rise in temperature of a battery during operation can trigger a chain of events that can possibly lead to thermal runaway. Electrolytes are designed to offer high ionic conductivity (κ) for minimizing internal heating and reduce the safety risk in batteries. However, amidst its non-isothermal operation, significant concentration gradients develop within the battery and the stand-alone electrolyte design falls short to account for these conditions. System level studies can be instrumental to include the concentration and temperature conditions existing within a live battery and help minimize internal heating. Here we systematically investigate the influence of Li-ion concentration in the electrolyte and initial battery temperature on various heat source terms during charging/discharging a lithium ion battery. The study is aimed to draw insights for selecting the electrolyte concentration for a battery that ensures minimum internal heating and mitigates the risk of thermal runaways in batteries under different operating conditions.

We have used the P2D (Pseudo 2Dimensional) model to simulate the electrochemical-thermal characteristics of the lithium ion battery. The variables of interest of the model are - potential of electrode (Φ₁) and electrolyte (Φ₂), Li-ion concentration of electrodes (c₁) and electrolyte (c₂). The electrode and electrolyte potentials are obtained by solving the charge conservation equations in the electrode and electrolyte phase respectively. The Li-ion concentration of the electrodes and the electrolyte is obtained by solving the mass conservation equations in the electrode and electrolyte phase respectively. The charge and mass conservation equations are coupled by the Butler-Volmer kinetics equation, which relates the flux of Li-ions to the electrode and electrolyte potentials. To model the thermal characteristics of the battery, heat balance equation is introduced, and the temperature dependencies of the physico-chemical properties are considered. The heat source terms in the battery include Ohmic heat, reaction heat and reversible heat generated due to intercalation/deintercalation of lithium ions at the electrodes. The cell considered in this study consists of LiCoO₂/Graphite electrodes and the electrolyte is LiPF₆ salt in PC/EC/DMC mixture. In the simulations, the battery was charged using Constant Current Constant Current (CCCV) method and discharged at constant C-Rates.

We have considered two scenarios in this study. In the first one, we vary the Li-ion concentration in the electrolyte and study the heat generated at different charge and discharge rates. Fig. 1(a) shows the temperature attained by the battery at the end of charge as a function of electrolyte concentration and for different C-rates. This result show minimum internal heating of the battery for electrolyte concentration of around 1.5-2M. These minima correspond to the electrolyte concentration that offers maximum electrolyte conductivity (κ) as heat generation inversely depends on κ. The optimum electrolyte concentration for achieving high conductivity as suggested by experimental studies or molecular simulations may not necessarily hold true amidst the non-isothermal operating conditions of the battery. Even a slight reduction in battery temperature attained by changing the electrolyte concentration may prove vital. This difference in temperature attained, if on the undesirable side, can get amplified via positive feedback resulting in additional heat generation. This can get more pronounced at higher C-rates observed in fast charging applications. Hence, it is important to find out the electrolyte concentration which cause minimum heating in the given operating conditions.

In the second scenario, the initial battery temperature was varied along with the electrolyte concentration to study the temperature attained after 1C charge and discharge. Fig. 1(b) shows the temperature attained by battery at the end of 1C charge against various initial battery temperatures and electrolyte concentrations. It is found that higher the initial temperature, higher is the final temperature attained by the battery after charge. However, the temperature rise, (ΔT = T_final-T_initial), decreases with increase in initial battery temperature (inset Fig. 1(b)). After charging, around 60-70 K temperature rise is seen for initial temperature 273 K, whereas only 20-30 K rise is observed for initial temperature 348 K. Arrhenius temperature dependencies leads to increased conductivity, which in turn leads to reduced Ohmic heating and charging time. Also, for each value of initial temperature, the temperature rise shows a minimum at around 1.5-2M electrolyte concentration as observed in the earlier section. A high initial battery temperature boosts physicochemical properties and may be advantageous in fast charging applications that require cutting down transport and reaction times.

Li ion battery is a highly coupled electrochemical-thermal system and such physics-based modelling studies can be used at different operating conditions to identify optimal material properties for improved battery design. Recently, highly concentrated electrolytes have been investigated to offer better stability and performance in lithium-based batteries. Studies such as this can aid in testing the performance of the novel concentrated electrolytes and provide useful insights for better design.

Image Caption: (a) Battery temperature attained as a result of heat generated due to CCCV charging for different electrolyte concentrations; C-Rates: 0.5C, 1C, 1.5C and 2C; Cut-off Voltage = 4.0V (b) Battery temperature attained as a result of heat generated due to CCCV charging at 1C, keeping the cell at different initial temperatures; Cut-off voltage = 4.2V