Anup Barai

A study of the effects of external pressure on the electrical performance of a lithium-ion pouch cell

The introduction of lithium-ion batteries for vehicle powertrain electrification has increased in recent years. They feature high energy density, high power density, long cycle life, and is also environmentally friendly compared with other types of batteries. A large number of Li-ion cells are usually required to meet the demand in capacity and power for automotive applications. Pouch cells have been favored by many manufacturers because of the high packaging efficiency and, therefore, a higher pack energy density. However, robust packaging is required for performance and safety criteria due to their low mechanical stability, which results in them being compressed in the module/pack. This paper describes research into the effects of external pressure on the electrical performance of lithium-ion pouch cells. The authors have adopted pulse power test, capacity test and electrical impedance spectroscopy test to characterize the effects and the test result indicates lithium-ion pouch cells performance changes under varying external pressures. Conclusions are drawn on how to make use of the results presented to influence and improve the design of automotive battery modules and packs to meet the challenges in the automotive industry.

A study of the influence of measurement timescale on internal resistance characterisation methodologies for lithium-ion cells

The power capability of a lithium ion battery is governed by its resistance, which changes with battery state such as temperature, state of charge, and state of health. Characterizing resistance, therefore, is integral in defining battery operational boundaries, estimating its performance and tracking its state of health. There are many techniques that have been employed for estimating the resistance of a battery, these include: using DC pulse current signals such as pulse power tests or Hybrid Pulse Power Characterization (HPPC) tests; using AC current signals, i.e., electrochemical impedance spectroscopy (EIS) and using pulse-multisine measurements. From existing literature, these techniques are perceived to yield differing values of resistance. In this work, we apply these techniques to 20 Ah LiFePO4/C6 pouch cells and use the results to compare the techniques. The results indicate that the computed resistance is strongly dependent on the timescales of the technique employed and that when timescales match, the resistances derived via different techniques align. Furthermore, given that EIS is a perturbative characterisation technique, employing a spectrum of perturbation frequencies, we show that the resistance estimated from any technique can be identified – to a high level of confidence – from EIS by matching their timescales.

Transportation Safety of Lithium Iron Phosphate Batteries - A Feasibility Study of Storing at Very Low States of Charge

In freight classification, lithium-ion batteries are classed as dangerous goods and are therefore subject to stringent regulations and guidelines for certification for safe transport. One such guideline is the requirement for batteries to be at a state of charge of 30%. Under such conditions, a significant amount of the battery’s energy is stored; in the event of mismanagement, or indeed an airside incident, this energy can lead to ignition and a fire. In this work, we investigate the effect on the battery of removing 99.1% of the total stored energy. The performance of 8Ah C6/LiFePO4 pouch cells were measured following periods of calendar ageing at low voltages, at and well below the manufacturer’s recommended value. Battery degradation was monitored using impedance spectroscopy and capacity tests; the results show that the cells stored at 2.3 V exhibited no change in cell capacity after 90 days; resistance rise was negligible. Energy-dispersive X-ray spectroscopy results indicate that there was no significant copper dissolution. To test the safety of the batteries at low voltages, external short-circuit tests were performed on the cells. While the cells discharged to 2.3 V only exhibited a surface temperature rise of 6 °C, cells at higher voltages exhibited sparks, fumes and fire.

The influence of temperature and charge-discharge rate on open circuit voltage hysteresis of an LFP Li-ion battery

Open circuit voltage (OCV) is a crucial parameter in an equivalent circuit model (ECM). The path dependence of OCV is a distinctive characteristic of a Li-ion battery; this is known as OCV hysteresis. In this manuscript the influence of temperature and charge/discharge rate on OCV hysteresis has been identified. OCV hysteresis was found to be 13mV higher at 0°C while remaining unchanged at 45°C compared to the 25°C result. In general, OCV hysteresis was found to be less dependent on charge/discharge rate than temperature. The potential explanations of these results have been reported.