Analysis of Degradation Mechanisms in Quinone-Based Electrodes for Aqueous Electrolyte System via in Situ XRD Measurements

Organic materials are promising electroactive components of energy storage devices such as lithium-ion batteries and electrochemical capacitors. Among them, low-molecular-weight organics have attracted attention as higher-energy-density, environmentally friendly, and inexpensive electrode materials, but their poor cycle performance is the main drawback. Using in situ XRD measurement in aqueous electrolyte system, here we investigated the capacity fading mechanism of an organic electrode based on low-molecular-weight quinones. Although the capacity fading of such organic electrodes is generally attributed to their elution into the electrolyte, our structural analysis reveals that the capacity fading is also associated with the expansion of an electrochemically inactive region, which persists in the electrode but does not take part in the reversible redox reactions. Moreover, the detailed analysis of the XRD patterns suggests that the capacity fading of the electrode is accompanied by the crystal growth of organic component, which occurs through dissolution-reprecipitation processes taking place during charge-discharge cycling. The association between capacity fading and the increased size of these crystalline domains suggests that the elongated electrical/ionic conduction paths in the growing organic crystals (leading to the expansion of the electrochemically inactive region of the electrode) can be a possible capacity fading mechanism in organic electrodes.