Optofluidic Raman analysis and capacity recovery in Li-ion cells

Battery performance degradation arises from multifactorial and complex electrochemical processes involving all the battery components. Therefore, identifying and mitigating/suppressing the main degradation mechanisms is essential for deploying these technologies. Such a holistic understanding requires advanced and dedicated analytical tools as well as effective strategies to recover battery capacity, extend battery life, and facilitate second-life use and recycling of devices. Here, we develop a set of optical and microfluidic methodologies that enable operando chemical analysis of liquid electrolytes using Raman spectroscopy. We demonstrate that our methods have minimal perturbation on device operation and can be readily embedded in standard manufacturing processes. A bi-directional microfluidic access port allows the extraction and reinjection of carbonate-based electrolytes for analysis over hundreds of electrochemical cycles. Our data show that lithium salt anion (PF6-) degradation is a major source of electrolyte degradation and capacity loss in industrially relevant Li-ion pouch cells (Ni-rich layered cathodes (NMC811), and graphite anodes). The effects are particularly strong when cycled to an upper-cut voltage of 4.3V. Finally, we demonstrate that battery capacity loss can be partially reversed by re-infusing pristine, uncycled electrolyte via an integrated microfluidic access port. This strategy not only mitigates degradation and extends battery life but also offers a practical pathway toward second-life applications and adaptable cell chemistries for future battery technologies.