A New Technique for In Situ Determination of the Active Surface Area Changes of Li-Ion Battery Electrodes

Li-ion batteries have been of a great interest for over three decades. A geometric electrode surface area is generally used for Li-ion electrochemical parameters calculations. Since the real electrode is a complex system composed of the thick porous structure, the contact surface area between the active mass and electrolyte is far larger than geometrical. This approximation leads to a large deviation of obtained results, especially within different laboratories and for volume and surface changing materials, e. g., silicon. The article presents a new method of in situ analysis of active surface area variations applicable for Li-ion electrodes. The method relies on the electrochemical impedance spectroscopy measurement (EIS) performed at an arbitrarily chosen state of charge during superimposed DC current flow. The correlation between a local ion concentration with a charge transfer resistance allows to evaluate the differences of the active surface area. The presented method is not affected by the SEI layer presence, the material composition, nor the lithiation mechanism. Due to limited EIS frequency range the presented method can be performed with a relatively short time. Our new in situ surface area determination can greatly improve the accuracy of the electrochemical parameters evaluation and enable the proper result analysis. We believe that our method can become a standard procedure implemented in every research focusing on the electrochemical parameter determination of the volume changing active materials.