Commercial lithium-ion batteries only show us the total voltage, hiding what is actually happening inside each electrode during charging, discharging, and aging. This project aims to develop and use a very thin, minimally invasive "micro-sensor" that can be safely placed inside a real commercial battery to separately monitor the behavior of the anode and cathode without dismantling the cell. By doing so, the student will gain direct insight into why batteries lose capacity over time, which electrode limits fast charging, and how operating conditions such as temperature and state of charge affect battery health. The work combines hands-on experimentation with real batteries and simple data analysis, and its outcomes are directly relevant to electric vehicles, fast-charging strategies, and next-generation battery design.