Optimized Concentration of Redox Mediator and Surface Protection of Li Metal for Maintenance of High Energy Efficiency in Li–O₂ Batteries

Abstract

this problem, solid [4][5][6][7][8][9] and liquid [10][11][12][13][14][15][16] catalysts have been introduced in Li-O 2 batteries. However, solid catalysts such as noble metals have been reported to promote not only the reversible decomposition of Li 2 O 2 but also electrolyte decomposition. [17][18][19] In addition, when direct contact between the solid catalyst and Li 2 O 2 is prevented or the surface of the catalyst is covered with Li 2 O 2 , no catalytic activation effect is expected. [10,11] Alternatively, liquid catalysts, called redox mediators (RMs), [10][11][12][13][14][15][16] have been added to the electrolyte as electron-hole transfer agents to promote reversible Li 2 O 2 formation/ decomposition. RMs constitute a more efficient solution for charge-discharge reversibility, resulting in a reduction of the charge overpotential without severe electrolyte decomposition. However, it has recently been reported that RMs can be deactivated during cycling through chemical reduction at the Li metal electrode (self-discharge of the electrochemically oxidized RM) and attack by oxygen species at the cathode. [20][21][22] Thus, more stable RMs are still required for practical application of Li-O 2 batteries with longer cycle life and higher efficiency.To alleviate the aforementioned problems associated with RMs in Li-O 2 batteries, Li metal, which is an essential anode component for obtaining a high energy density, should be protected through surface treatment and the employment of functional separators. [20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] However, reported Li protection materials and Li-protective layers mainly consist of polymers, which have insufficient stiffness to suppress the growth of Li dendrites. These polymer materials can also be decomposed by reactive oxygen radicals that are produced as intermediates during the formation and decomposition of Li 2 O 2 . [29,41] Herein, we attempted to solve these problems by controlling the concentration of the RM (LiBr) in the electrolyte and using a stable and stiff Li metal protective layer (a graphenepolydopamine composite layer, GPDL), which can help maintain high efficiency over prolonged cycling in Li-O 2 batteries. An appropriate concentration of LiBr in diethylene glycol dimethyl ether (DEGDME) was used as the RM based on previous reports. [15,42] A graphene-polydopamine composite was developed as a protective layer and was uniformly coated onto the Li metal electrode to suppress undesired reactions with LiBr in the electrolyte, as well as with oxygen and moisture. Using these approaches, we successfully demonstrated high-efficiency Recently, various approaches for adding redox mediators to electrolytes and introducing protective layers onto Li metal have been suggested to overcome the low energy efficiency and poor cycle life of Li-O 2 batteries. However, the catalytic effect of the redox mediator for oxygen evolution gradually deteriorates during repeated cycling owing to its decomposition at the surfaces of both t...

Safe Lithium‐Metal Anodes for Li−O2 Batteries: From Fundamental Chemistry to Advanced Characterization and Effective Protection

“…Additionally, uncontrollable compositional evolution, ultrathin thickness and other features also make in‐situ SEI vulnerable to dendrite penetration and volume change upon cycling. As revealed in Figure a, Sun and co‐workers fabricated a stiff and uniform graphene‐polydopamine composite layer (GPDL) on Li anode by drop‐casting method. With the protective layer, Li−O 2 battery maintains over 150 cycles while delivering a high energy efficiency of 80 % that achieve the objective of obtaining high‐efficiency and long‐lifetime Li−O 2 batteries thanks to the barrier property of GPDL in suppressing undesired reactions between Li anode with redox mediators as well as with oxygen and moisture.…”

Promoting Li–O2 Batteries With Redox Mediators

“…Br 3 À /Br À has ah igherr edox potential of 3.5 Va nd showedahigh rate of oxidizing Li 2 O 2 to O 2 . [25] The discharge products of the cells were mainly Li 2 O 2 when using different ether solvents containing LiBr and water contaminations. [25b] Liang and Lu proved that charging with LiBr largely reduces parasitic gas evolution and therefore improveso xygen recovery using high temporal resolutiono nline electrochemical mass spectrometry analysis.…”

An Extremely Simple Method for Protecting Lithium Anodes in Li‐O2 Batteries

“…Lee and co‐workers reported a composite protective layer comprising Al 2 O 3 and polyvinylidene fluoride‐hexafluoro propylene for Li metal anodes, which resulted in dramatic enhancement of cycling stability of Li‐O 2 batteries . Similar work has been done in Sun group, and they used a graphene‐polydopamine composite as the layer to protect Li metal anodes from the attack of LiBr (the redox mediator) . Interestingly, a tissue could be even applied to protect Li anodes in Li‐O 2 batteries.…”