Direct monitoring of trace water in Li-ion batteries using operando fluorescence spectroscopy.

The direct monitoring of trace water in real time during electrochemical cycles is of vital importance because water impurities are one of the causes of reduced lifetimes and capacity fading in Li-ion batteries. However, the most common Karl-Fischer titration lacks the ability to perform real-time monitoring of trace water while the battery is operating. Here, we demonstrate the use of nanosized coordination polymers as a sensing platform for the rapid and highly sensitive detection of water molecules, which gives a distinguishable turn-on fluorescence (FL) response toward water with a quantifiable detection range from 0 to 1.2% v/v, offering a novel opportunity to monitor trace water during electrochemical cycles. To demonstrate the practical value of our platform, we designed an in situ measurement system using nanosized coordination polymers as an electrolyte additive. Within the platform, the findings indicate that trace water is indeed generated during the first discharge process, in which the FL intensity shows a linear increase over time along with the gradual formation of water. We believe that this strategy provides new insights into the in situ monitoring of complex electrochemical processes, and it may help to pave the way for the development of new operando analytical techniques for lithium-ion batteries.