PHM of Flexible Li-ion Batteries Subjected to Sustained Elevated-Temperature Storage and Thermal Cycling

The capacity of a power source, dependent on electrode materials and cell design, degrades during its life as a function of time is also highly influenced by the operating and storage environments. Calendar aging is a term often used to describe long term storage at elevated temperatures. Effect of such storage environments on the health of ultra-thin flexible li-ion batteries is relatively unknown but needs to be studied as it is replicative of shelf life the power sources see between fabrication and in-field usage. Also, health monitoring techniques which can successfully predict remaining useful life for flexible lithium-ion power sources have yet to be fully studied. It is well known that cyclic aging can deteriorate the health of a power source ultimately resulting in capacity fade over its lifetime. Now, whether or not calendar aging and varying thermal loads (during storage) have an effect on the working of flexible li-ion battery is yet to be established. With the current state-of-the-art moving towards flexible electronics and flexible components, it is therefore of the utmost importance to establish test protocols, test standards, prognostic health management frameworks and test methodologies for flexible power sources which will be implemented as energy harvesting units in such electronic assemblies. In this research study, flexible lithium-ion batteries have been analyzed for effects of long term calendar aging and varying thermal loads by subjecting the test samples to: 1) unpowered aging at 50°C ranging from 10 days-120 days 2) unpowered exposure to thermal cyclic environment, 10°C-50°C, ranging from 50 loops to 550 loops. After the pre-conditional exposure the power sources were cycled through multiple full charge and full discharge cycles at ambient operating temperature under a constant C-rate. Output parameters such as efficiency, capacity and charge-discharge time have been studied for battery degradation. SEM and EDX techniques have also been utilized to study the effects on cathode microstructure and report if any visual evidence for film formation, electrolyte decomposition and/or lithium plating is observed.