摘要/Abstract
锂氧（Li-O2）电池因具有超高的理论能量密度而受到人们的关注，但仍面临实际比容量较低、过电势较高和循环稳定性较差等挑战.以具有高比表面积、分级孔结构、丰富缺陷和高电导率等特征的3D分级结构碳纳米笼（hCNC，hierarchical carbon nanocages）为正极材料，构建出具有高放电容量（14080 mAh·g-1）和良好循环稳定性的Li-O2电池；当在电解质中添加可溶性乙酰丙酮亚铁（Fe（acac）2）氧化-还原介质后，其放电容量、倍率性能和良好循环稳定性显著提升，过电势明显下降，如完全放电容量可达23560 mAh·g-1（XC-72的7.82倍），在0.5 A·g-1电流密度和800 mAh·g-1截止比容量下可稳定循环138圈（远高于未加Fe（acac）2的68圈和XC-72的13圈），在5.0 A·g-1高电流密度下仍可稳定循环63圈（远高于未加Fe（acac）2的21圈）.优异的电化学性能可归因于：hCNC的特征结构能有效地促进电子传输和2Li++O2+2e-⇆Li2O2（s）的可逆转化，为放电产物Li2O2提供足够分散和容纳空间；可溶性氧化-还原介质Fe（acac）2能有效地催化Li2O2放电产物形成均匀分散的小尺寸颗粒堆积多孔形貌和随后的充电分解，进而降低过电势和提升电池的循环稳定性.本研究提供了通过设计新型碳基正极材料和添加高效可溶性氧化-还原介质提高锂氧电池性能的新思路.
关键词: 锂氧电池, 分级碳纳米笼(hCNC), 乙酰丙酮亚铁(Fe (acac)2), Li2O2, 放电容量, 循环稳定性
Li-O2 batteries have received much attention due to the high theoretical energy density. However, they still suffer from many challenges such as unsatisfactory practical specific capacity, cycle stability, and relatively high overpotential. The electrochemical performance of Li-O2 batteries is closely related to the reversibility of the discharge (oxygen reduction reaction, ORR) and charge (oxygen evolution reaction, OER) processes. During the discharge process, non-conductive Li2O2 product is formed and gradually covers on the surface of the positive electrode material, leading to the deactivation of battery. The charging process is accompanied by the electrochemical decomposition of Li2O2 products. Therefore, how to achieve the highly reversible formation and decomposition of the Li2O2 product is the key to improve the electrochemical performance of Li-O2 batteries. To date, two strategies have been developed:(i) sp2 carbon materials with large specific surface area, suitable pore structure and high conductivity are used as cathode materials to disperse/accommodate the Li2O2 product and promote the electron transfer; (ii) the soluble redox mediators with ORR/OER bifunctionally catalytic activities are adopted as the electrolyte additive to promote the formation and decomposition of the Li2O2 product and lower the overpotentials. Recently, we reported a novel 3D hierarchical carbon nanocages (hCNC) featuring on the ultrahigh specific surface area, multiscale pore structure (micro-meso-macropore coexistence), high conductivity, and abundant defects, which demonstrated the excellent electrochemical performances in energy conversion and storage. Herein, taking advantages of hCNC, the high performances of Li-O2 batteries were fabricated, showing high full discharge specific capacity (14080 mAh·g-1) and good cyclability. After adding acetylacetone ferrous (Fe(acac)2) as the redox mediator to electrolyte, the electrochemical performances are further promoted. Namely, the discharge capacity reaches to 23560 mAh·g-1 at the current density of 0.1 A·g-1 (7.82 times of XC-72), and the cycle numbers are up to 138 cycles at the current density of 0.5 A·g-1 and the discharge/charge depth of 800 mAh·g-1 (far higher than 68 cycles of hCNC without Fe(acac)2 and 13 cycles of XC-72). Especially, at the high current density of 5.0 A·g-1, the cycle numbers still reach to 63 cycles, far higher than 21 cycles of hCNC without Fe(acac)2. Such excellent electrochemical performances can be ascribed to:the unique structure of hCNC facilitating electron transfer, reversible conversion of 2Li++O2+2e-⇆Li2O2(s), and dispersion/accommodation of the insulating Li2O2 product; the soluble redox mediator of Fe(acac)2 effectively catalyzing the discharge products of Li2O2 to form uniformly dispersed small-sized particles and decompose completely during the charge process. This provides a promising strategy for improving the performance of Li-O2 batteries via designing novel carbon-based positive electrode materials and adding efficient soluble redox mediators.
Key words: Li-O2 batteries, 3D hierarchical carbon nanocages (hCNC), acetylacetone ferrous (Fe(acac)2), Li2O2, discharge capacity, cycle stability
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