Mapping Out Fast Charging Safe Limits for High‐Loading Lithium‐Ion Cells by High‐Fidelity Operando Microscopy

Lithium-ion batteries (LIBs) have become the dominating energy storage technology for electronic devices and electric cars. The demand for higher energy density and the desire for faster charging pose significant challenges to battery cycle life and safety, as lithium plating occurring during faster charging becomes dead lithium, causing irreversible capacity loss, or penetrates the porous separator to short the cell, inducing thermal runaway. To overcome these challenges, the direct and accurate detection of the onset of tiny lithium plating in realistic cell configurations is the key to success. Here, transparent micro-LIBs fabricated in glass capillaries retain the geometry of practical cells and enable direct observation of minute lithium plating, which is otherwise challenging to detect and free from beam-induced damage typical of electron or X-ray techniques. Using a promising ether-based electrolyte, our operando microscopy reveals up to a 54% improvement of the charging capacity before triggering lithium plating, which is also realized in coin cells using commercial graphite powder and electrodes. We identify a performance map that can be adopted by battery management systems to look up the safe fast-charging capacity as a function of temperature and charging rate, avoiding lithium plating that occurs before reaching the cut-off voltage.