Degradation and Stabilization of Lithium Cobalt Oxide in Aqueous Electrolytes

Abstract

Lithium cobalt oxide (LCO) has long been proven to be an excellent material for cathodes in organic electrolytes [1]. It has shown high volumetric capacity and good stability in non-aqueous environments of commercial lithium-ion (Li-ion) batteries. Unfortunately, the flammability of organic electrolytes in combination with a special propensity for batteries constructed with LCO to experience thermal runaway creates safety concerns [2]. Here we discuss the exceptional electrochemical performance characteristics and stability of LCO in aqueous electrolytes. Aqueous electrolytes offer greatly improved safety and lower cost. In addition, they exhibit higher ionic motilities for the Li+ions and thus can be potentially used for faster charging batteries [3] or batteries with thicker electrodes. While LCO has been demonstrated to cycle for 20-100 cycles in aqueous environments [4, 5], the causes of its degradation have not been investigated in detail. In our previous work we have demonstrated promising characteristics of lithium iron phosphate (LFP) cathodes in aqueous electrolytes [6]. Here we further expanded our studies and demonstrated LCO cathodes with remarkably stable performance showing only 13 % fading after over 1500 cycles. Post mortem analysis of the electrodes was conducted to understand the effect of cycling and the causes of degradation. A powerful combination of X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) have been utilized. Our systematic studies and post-mortem analyses of LCO cells reveal that the primary mechanism of LCO degradation in aqueous electrolytes is the formation of a resistive layer of cobalt (II) oxide on the particles’ surfaces. Electrolyte composition was found to have a dramatic impact on the electrochemical performance and stability of LCO in aqueous environments. These results were recently accepted for publication in the Energy and Environmental Science Journal [7]. This talk will provide an overview of our findings and provide guidance for the future designs of aqueous electrochemical cells with stable cathodes.