Charging processes in lithium-oxygen batteries unraveled through the lens of the distribution of relaxation times

Abstract

•Identifying the origins of overpotential of the charging process of Li-O2 batteries •Decoupling the entangled processes/reactions by their time constants •Explaining the evolution of overpotential during the charging process using EIS Charging lithium-oxygen batteries is characterized by large overpotentials and low Coulombic efficiencies. Charging mechanisms need to be better understood to overcome these challenges. Charging involves multiple reactions and processes whose specific timescales are difficult to identify. Electrochemical impedance spectroscopy (EIS) is suited for this task, but its interpretation is ambiguous. Here, we combine the distribution of relaxation times (DRT) with the distribution of capacitive times (DCT) to identify the timescales of lithium-oxygen battery charging through EIS. In situ differential electrochemical mass spectrometry (DEMS) is used to validate the impedance results. The results show that the overpotential is mainly due to Li2O2’s charge transfer resistance at the initial and final stages of charging, whereas at mid-charging, Li2CO3 passivation becomes dominant. Mid-charging is particularly critical as Li2CO3 decomposition leads to electrolyte degradation and byproducts clogging the pores, inhibiting the diffusion process in the composite electrode. These methods provide a framework for studying other electrochemical systems. Charging lithium-oxygen batteries is characterized by large overpotentials and low Coulombic efficiencies. Charging mechanisms need to be better understood to overcome these challenges. Charging involves multiple reactions and processes whose specific timescales are difficult to identify. Electrochemical impedance spectroscopy (EIS) is suited for this task, but its interpretation is ambiguous. Here, we combine the distribution of relaxation times (DRT) with the distribution of capacitive times (DCT) to identify the timescales of lithium-oxygen battery charging through EIS. In situ differential electrochemical mass spectrometry (DEMS) is used to validate the impedance results. The results show that the overpotential is mainly due to Li2O2’s charge transfer resistance at the initial and final stages of charging, whereas at mid-charging, Li2CO3 passivation becomes dominant. Mid-charging is particularly critical as Li2CO3 decomposition leads to electrolyte degradation and byproducts clogging the pores, inhibiting the diffusion process in the composite electrode. These methods provide a framework for studying other electrochemical systems.