Determination of High‐Temperature Float Charge Failure Mechanisms in Lithium‐Ion Batteries by Quantifying Active Lithium Loss

ABSTRACT Lithium‐ion batteries (LIBs) suffer from float charge failure in the grid‐scale storage market. However, the lack of a unified descriptor for the diverse reasons behind float charge failure poses a challenge. Here, a quantitative analysis of active lithium loss is conducted across multiple temperatures into float charge of Li(Ni0.5Co0.2Mn0.3)O2–graphite batteries. It is proposed that the active lithium loss can be used as a descriptor to describe the reasons for float charge quantitatively. Approximately 6.88% and 0.96% of active lithium are lost due to solid electrolyte interphase thickening and lithium deposition, which are primary and secondary failure reasons, respectively. These findings are confirmed by X‐ray photoelectron spectroscopy depth profiling, scanning electron microscope, and accelerating rate calorimeter. Titration‐gas chromatography and nuclear magnetic resonance are utilized to quantitatively analyze active lithium loss. Additionally, electrolyte decomposition at high temperatures also contributes to active lithium loss, as determined by Auger electron spectrum and nondestructive ultrasound measurements. Notably, no failure is detected in the cathode due to the relatively low working voltage of the float charge. These findings suggest that inhibiting active lithium loss can be an efficient way of delaying failure during high‐temperature float charge processes in LIBs.