Ultrasonic Analysis for Monitoring Li-Ion Batteries

To seek for increasing accuracy in LIBs monitoring a new trend is to rely on data indicative of material properties whose variations are induced by Li ions intercalation and deintercalation. A wider variety of information can be achieved if data are referred to a specific layer of the battery. Since ultrasounds propagate at different speed according to materials properties and are reflected every time they hit an interface, ultrasonic analysis (UA) has the potential to improve the quality of LIB monitoring providing in operando and through thickness information. In this work NMC-graphite LIBs are cycled and monitored in operando via UA in pulse-echo and through-thickness modes with a temporal resolution of 7 ns (which to the best of our knowledge is the highest ever tested) and a 5 MHz frequency. First, the effect of (i) temperature and (ii) C-rate on the waveform evolution was investigated: (i) a 100% charged battery was kept at rest in a thermal chamber at fixed temperatures varying between 10 and 50°C, the ultrasonic signal was acquired after 30 min at rest; (ii) an initially 100% charged battery underwent two subsequent discharge-charge cycles at constant current with fixed C-rates varying between 0.2 and 2C. Then a series of cycles at 50°C aimed at causing battery degradation was performed while acquiring the ultrasonic signal every 2 min. The efficacy of the degradation protocol was assessed through EIS prior and post cycling the battery. Surprisingly, results show an opposite (but systemic) trend with respect to what is reported in literature, with an increasing time of flight (ToF) for increasing state of charge (SoC). Additionally, it is observed that most of the peaks increase their ToF with the number of cycles. Signal amplitude shows a periodical behavior as well but values vary as degradation has gone on. It is also observed that within a cycle some peaks increase their intensity during charge while others decrease, suggesting that they belong to opposite electrodes.