Revealing the Influence of Film-forming Electrolyte Additives on Lithium Metal Batteries

Summary:

With high specific capacity, lithium metal is a promising material and might be an option for next generation of battery technology. However, the application of lithium metal batteries (LMBs) remains challenging due to low coulombic efficiency and safety issues arising from the formation of ‘dead’ lithium, unstable SEI layers on lithium metal surfaces and eventually dendrite formation. In this work, a powerful analytical approach based on solid state NMR was employed to rationalize the impact of film forming additives on observable lithium losses, demonstrating their impact on resulting (in)homogeneities of lithium deposits, SEI stability and occurrence of ‘dead lithium’ fractions. Herein, the film-forming additives FEC and LiPO2F2 (LiPF) are further discussed by not only applying in-situ 7Li NMR of pouch-type-cells, but also 19F 1D MRI profiling along with operando optical microscopy. Various electrolyte formulations are compared in view of different relevant cell configurations, including lithium symmetric cells, Li||Cu, Cu||NMC and Li||NMC cell systems, establishing the origin and contributions of irreversible capacity losses while systematically evaluating different active materials. Upon addition of an additive mixture (FEC and LiPO2F2), beneficial effects on achieving homogeneous lithium deposition were demonstrated, where the over-voltage during galvanostatic measurements (0.5 mA cm-2) remained at 20 mV even after 900 cycles. The capacity retention reached 98.3% even after 100 cycles of Li||NMC622 cells. In summary, the mixture of film-forming additives FEC and LiPO2F2 afforded ‘better’ interfacial properties and homogeneous lithium deposition, paving ways for better control of lithium losses and hence achieving long-term cycling of the cells.