锂氧（Li?O2）电池因其理论能量密度高达3500 Wh kg–1而受到广泛关注，是目前最具应用潜力的电池体系之一。但受限于锂氧电池中放电产物过氧化锂（Li2O2）形成和分解的缓慢动力学过程，锂氧电池仍面临循环寿命短和充电过电位高等挑战。在众多催化剂中，可溶性的氧化还原介质（Redox mediator, RM），因其成本低和适当的氧化还原电位受到广泛研究。虽然RM的引入可以有效地降低锂氧电池的充电过电位，但可溶性的RM会向锂负极迁移并与其发生副反应，造成锂负极的腐蚀和RM自身的消耗，即穿梭效应。因此，缓解RM的穿梭对改善Li?O2电池的电化学性能并实现商业化具有重要意义。 本文聚焦于研究4A分子筛在抑制Li?O2电池中RM的穿梭效应中的应用，充分利用了4A分子筛的?级孔径来抑制RM的穿梭。一方面，通过简单的溶液搅拌和涂覆方法制备了一种基于锂化分子筛的保护层，该保护层可以通过物理孔径实现对2、2、6、6-四甲基哌啶氧化物（TEMPO）穿梭效应的抑制。同时，该保护层可以与电解液组分相互作用，提高锂离子的迁移能力。应用于Li?O2全电池后，锂化分子筛保护层可以将电池的全放电容量从2708 mA h g?1提升至3429 mA h g?1，并在250 mA g–1的电流密度以及500 mA h g–1的截止容量下稳定循环265圈，是不含保护层的十倍以上。此外，在1000 mA h g–1的高截止容量以及在使用2,5-二叔丁基-1,4-苯醌（DBBQ）作为RM的情况下，锂化分子筛保护层都展示出了优异的循环稳定性。 另一方面，将分子筛直接引入双层电解液体系中，可以提高分层电解液的稳定性并抑制不同RM的穿梭，从而提升锂氧电池的循环寿命。当使用TEMPO作为RM时，与单层电解液相比，含有4A分子筛的双层电解液可以将电池循环寿命从17圈提升至84圈；使用5,10-二甲基二氢化吩嗪（DMPZ）作为RM时，电池循环寿命从20圈提升至61圈；使用DBBQ作为RM时，电池循环寿命从16圈提升至75圈，证明了含有4A分子筛的双层电解液可以有效地提高Li?O2电池的循环寿命，具有普适性。 本文的研究工作表明，使用分子筛来抑制氧化还原介质的穿梭效应是一种成本低廉，简单且可扩展的方法。
Lithium–oxygen (Li–O2) batteries with extremely high energy density (3500 Wh kg–1) have attracted widespread attention and are currently one of the most promising battery systems for application. However, the slow kinetic processes associated with the formation and decomposition of the discharge product Li2O2 lead to challenges such as poor cycle life and high charge overpotential. Among various catalysts, soluble redox mediators (RMs) have been extensively studied due to their relatively low cost and appropriate redox potential. Although the introduction of RMs effectively decrease the charge overpotential of lithium–oxygen batteries, RMs can migrate towards the lithium anode and undergo spontaneous side reactions, causing corrosion of the lithium anode and consumption of the RMs, i.e. shuttle effect. Therefore, mitigating the shuttle effect of RMs is of significant importance for enhancing the electrochemical performance and realizing the commercialization of Li–O2 batteries. This study focuses on the application of 4A molecular sieves in inhibiting the shuttle effect of RMs in Li–O2 batteries, taking full advantage of the ?-level pore size of 4A molecular sieves to inhibit the movement of RMs. On the one hand, a protective layer based on lithiated molecular sieves was prepared through a simple solution stirring and dripping method. The protective layer can inhibit the shuttle effect of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) by physical aperture restriction and interact with electrolyte components to enhance the migration ability of Li+. When applied into Li–O2 cells, it increased the full discharge capacity from 2708 mA h g–1 to 3429 mA h g–1. Also, it enabled stable cycling for more than 265 cycles at a current density of 250 mA g–1 and a cut-off capacity of 500 mA h g–1, which is more than ten times that of cells without the protective layer. Additionally, at a higher cut-off capacity of 1000 mA h g–1 or when using 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) as the RM, the lithiated molecular sieve based protective layer demonstrated excellent long-term cycle stability. On the other hand, a bilayer electrolyte with better stability enhanced by molecular sieves was proposed to suppress the shuttle effect of different RMs and prolong the cycle life of Li–O2 batteries. With TEMPO as the RM, the cycle life was prolonged from 17 cycles to 84 cycles; with 5,10-dimethylphenazine (DMPZ) as the RM, the cycle life was extended from 20 cycles to 61 cycles; and with DBBQ as the RM, the cycle life was improved from 16 cycles to 75 cycles. The inclusion of 4A molecular sieves in the bilayer electrolyte effectively increased the cycle life of Li–O2 batteries, demonstrating universal applicability. The research work presented in this paper indicates that using molecular sieves to suppress the shuttle effect of redox mediators is a cost-effective, simple, and scalable method.
