One of the tools at our disposal for understanding how lithium-ion batteries charge, discharge, and 'age' is the incremental energy (IE) analysis curve, a concept that may sound complex but is fundamental to battery health monitoring.
Think of filling a balloon with air. You want to know how much air goes in at each moment/blow and how the balloon's shape changes. Similarly, when we charge a lithium-ion battery, we are filling it with electrical energy, and the IE curve helps us track this process in detail.
The IE curve represents the relationship between the amount of energy a battery gains during charging and its voltage. Voltage in this context is like the pressure inside the balloon, which tells us how 'full' the battery is. As a battery charges, its voltage increases, but not linearly. This is where the IE curve becomes essential; it shows us how the energy storage changes at different voltage levels.
Looking at the image, we see two curves. One for charging at a rate of 0.1C and the other at 0.2C. The lower graph translates the data into a clear interpretation of the energy-versus-voltage relationship shown in the upper graph. The vertical axis, labeled 'dE/dV', represents the change in energy (dE) with respect to the change in voltage (dV), measured in watts per hour per volt (Wh/V). This tells us how much energy is added to or removed from the battery for a small change in voltage.
Notice how at specific points labeled as I peak, II peak, III peak, and IV peak, there are sharp changes in the slope of the curves. These peaks correspond to points where the energy storage rate changes significantly. Understanding these peaks is like listening to the battery's heartbeat; they give us insights into what's happening inside the battery chemically and electrically.
Why are these peaks and the IE curve important? Because they help us predict the battery's health and how it is aging. For example, if we observe that over time, these peaks shift or their magnitude changes, it can indicate that the battery's ability to hold charge is diminishing or that internal chemical reactions are changing, which might not be apparent just by looking at the battery's voltage or capacity.
The information from the IE curve is crucial, and it allows for early detection of battery degradation, which means we can take steps to prolong battery life, ensure safety, and maintain performance. It is like having a window into the battery's life, letting us keep a check on its health and anticipate when it might need attention.