How Improper Use of Activated Carbon Leads to Biased Data Interpretation in Post-Li Battery Systems

Abstract

Accurate electrochemical evaluation is critical for the development of post-lithium batteries based on cations such as K+, Mg2+, or Ca2+. Because metal counter electrodes in these systems often suffer from unstable interphases or surface passivation, activated carbon is increasingly employed as an alternative counter electrode. However, its improper use can introduce severe biases and lead to erroneous interpretation of electrochemical data. Here, we systematically analyze how activated carbon behavior is affected by key parameters, including mass and capacity balancing, electrolyte composition, current density, activated carbon type, electrode processing, and self-discharge. Using a Ca-Zn-PTtSA as positive (working) electrode material, we show that inadequate control of these factors results in artificial voltage shift, distorted estimation of diffusion coefficient, and misattribution of electrolyte decomposition as intrinsic redox activity. Based on these insights, we suggest practical guidelines for the correct use and reporting of activated carbon counter electrodes. Although demonstrated for calcium-ion storage, these principles are broadly applicable to post-lithium battery chemistries.