Effect of Relaxation Periods over Cycling Performance of a Li-Ion Battery

Abstract

Various operational parameters such as charge/discharge rates, relaxation periods, and depth of charge and discharge play an important role in enhancing the cycling life of Li-ion batteries. Providing batteries with a relaxation period after discharging and charging might be essential for removing concentration gradients generated due to passage of current. In the present work, the effect of providing open-circuit time durations after completion of each charge and discharge over the performance of Li-ion cells has been analyzed and quantified. It is shown that relaxing the cell after discharge has significant influence over cell performance, whereas relaxation after charge has a marginal effect. In the former case, a relatively thicker film forms at the solid-electrolyte interface in the negative electrode. Moreover, providing a sufficiently long relaxation to the cell at the end of discharge results in (a) a higher concentration of lithium in the solid matrix of the negative electrode and (b) a lower concentration of lithium in the positive electrode, both leading to a higher cell potential during the discharge phase of the subsequent cycle. Charging the cell following a relaxation period of more than one hour at the end of discharge results in a better utilization of cyclable lithium. Diminishing fossil fuel reserves have made it imperative to develop alternative energy sources to power automotive vehicles. Many researchers have focused their attention toward sources such as biomass, solar, wind, fuel cells and batteries [1][2][3] to accomplish this requirement. By virtue of their lower fuel consumption, silent operation and zero emissions, battery powered vehicles (such as hybrid and electric) offer a substitute to gasoline-fuelled vehicles. 4 In the last decade, Li-ion batteries (LIBs) have been used extensively as power sources for many small and portable electronic devices such as mobiles, camcorders, PDAs, laptops, digital cameras and other communication devices. However, these portable electronic devices require a very low current. For high power applications, such as electric, hybrid-electric and plug-in hybrid-electric vehicles, a higher current is required. This puts limitations on the use of LIBs as power sources in these scenarios since their capacity decreases rapidly at high discharge rates. 5Several studies have focused their attention toward enhancement of battery performance in terms of energy and power density. In this regard, various combinations of electrode/electrolyte materials have been used with different operational and design parameters to minimize the capacity fading of Li-ion cells.5-9 Mesocarbon Microbead (MCMB) has been used commercially as an anode material due to its high reversible capacity, while it has limitation of film formation at solid electrolyte interface (SEI).10,11 Whereas in common cathode materials, manganese oxide is used as it is abundant, cheaper and environmentally friendly in nature. Moreover, use of manganese oxide as cathode material minimizes safety issues a...

Physics‐based model of lithium‐ion batteries running on a circuit simulator

“…We postulate that this was due to the fact that, in our model, we used single active material particle approximation for the electrode structure and, as such, the transport of the Li ions in the porous electrodes could not be fully expressed. For example, when discharging, the Li concentration in the positive electrode pores declines greatly . It is known that a decrease in the Li ion concentration leads to a decrease in the ionic conductivity of the electrolyte .…”

“…In addition, the handling of transport of Li ions in a porous electrode in which two phases, namely the electrode active material and the electrolyte, coexist is largely different from that of a bulk electrolyte. As such, in existing physics models, in many cases, the effective ion conductivity, obtained by applying Bruggeman's correction to the ionic conductivity of the electrolyte, is used …”

“…we used parameters estimated based on general electrochemical measurements of an LIB as the model parameters. When modeling the electrode reactions, for example, in existing LIB physical models, an electrode reaction rate constant is used . Information about such parameters derived from the electrode materials is not readily available to LIB users.…”

“…For example, when discharging, the Li concentration in the positive electrode pores declines greatly. [7][8][9] It is known that a decrease in the Li ion concentration leads to a decrease in the ionic conductivity of the electrolyte. 22,23 In addition, the handling of transport of Li ions in a porous electrode in which two phases, namely the electrode active material and the electrolyte, coexist is largely different from that of a bulk electrolyte.…”

“…5,6 Existing well-known LIB models include physical models (electrochemical models) and RC equivalent circuit models. The physical model [7][8][9][10][11][12][13] expresses the variety of electrochemical phenomena that occur in the active material particles (solid phase), the electrolyte (liquid phase), and the active material particle/electrolyte interface by governing equations that correspond to the physical mechanism thereof, and are mathematical models that simulate the behavior of LIBs by solving these governing equations simultaneously. In this model, theoretical calculation of the output voltage of the LIB is possible.…”