Electrochemical behavior of lithium cobalt oxide in aqueous electrolytes

Abstract

Lithium-ion (Li-ion) batteries are the most popular energy devices for almost all electronics today. From cell-phones and laptops, to advanced uses in automotive and aircraft applications, lithium-ion batteries have slowly taken over the market. Unfortunately, today’s lithium-ion batteries are also highly unsafe. They rely heavily on organic solvents for electrolytes in the battery. These organic solvents are inherently flammable in nature and have caused several fires reported in batteries over the past few years. In this research, I aimed to investigate changes in the electrochemical behavior of electrodes if we replace flammable organic solvents with a safer alternative such as water. Water-based batteries may offer greatly improved safety and lower cost (from lower raw material cost to reduced manufacturing costs). In addition, water-based electrolytes may exhibit dramatically higher ionic mobility for Li ions and thus can be potentially used for faster charging batteries or batteries with thicker electrodes, which are easier and cheaper to construct. Lithium cobalt oxide (LCO) has long been proven to be an excellent material for cathodes in conventional organic electrolytes. It has shown high volumetric capacity and good stability in non-aqueous environments of commercial Li-ion batteries. Unfortunately, the flammability of organic electrolytes in combination with a propensity for batteries constructed with LCO to experience thermal runaway creates safety concerns. Due to extensive knowledge accumulated on LCO and its structural similarity with many other common cathode materials, LCO may serve as a model material for studying electrochemical interactions of layered lithium transition metal oxides with aqueous electrolytes. While LCO had previously been demonstrated to cycle for 20-100 times in aqueous environments, the causes of its degradation had not been investigated in detail. Our studies demonstrated that in certain aqueous electrolytes LCO cathodes could cycle with a remarkable stability showing only 13% fading after over 1,500 cycles. Post mortem analysis of the electrodes was conducted to understand the effect of cycling and the causes of degradation. Electrolyte composition was found to have a dramatic impact on the electrochemical performance and stability of LCO in aqueous environments. The temperature range for aqueous electrolytes at sub-zero temperatures was also investigated in detail. We showed that Li-ion batteries with aqueous electrolytes can be excellent candidates for battery applications at low temperatures. In contrast to a common misconception, aqueous Li-ion batteries can operate at several tens of degrees below the freezing point of water when high concentration electrolyte solutions are utilized. By leveraging the colligative properties of water, I demonstrated that aqueous electrolytes can function much below the freezing point of water down to -40oC. The performance of water-based electrolyte systems with three low-cost inorganic salts (LiNO3, Li2SO4, and LiCl) was extensively studied to understand the rate-limiting step in battery performance at sub-zero temperatures. It was found that the charge transfer resistance is the largest contributor to impedance at low temperatures, until the complete solidification of the aqueous electrolytes takes place. In sharp contrast, it was found that common organic electrolytes do not support any cycling below -20oC. The contributions from the various resistances that affect low temperature cycling from the perspective of the electrode as well as the electrolyte were investigated in detail.