Kinetics Improvement through Cathode Designing for Aprotic Li-O2 batteries
Authors: Cui Zhonghui , Lou Peili , Guo Xiangxin , Li Hong 
Published: February 2020

The rechargeable aprotic lithium-air (Li-O2) battery is a promising potential technology to provide a safe and cost-effective secondary battery with several times higher energy density than state-of-the-art Li-ion batteries, and thus attracting extensive interest over the past decade. Despite their great promise, putting the Li-O2 batteries into practise are hindered by a range of fundamental challenges [1-2]. The greatest problem is lacking of stable materials that could endure the attack from the intermediates and/or the products of oxygen reduction/evaluation reaction (ORR/OER) during the operation of Li-O2 batteries. For example, masses of works have proven that many kind of materials undergo serious side reactions during the operation of Li-O2 batteries, which induce the formation and accumulation of carbonates on cathode and eventually kill the batteries. Moreover, the intrinsic poor kinetics of Li/O2 chemistry, slow kinetics of ORR (2Li+ + 2e- + O2 → Li2O2) and significantly sluggish kinetics of OER (Li2O2 → 2Li+ + 2e- + O2), are also of significant influence on cell performances. To address these issues, cathode designing toward a stable interface with improved activity on ORR/OER is of great promise. Recently, by virtue of the material and structure design for air cathodes, optimization of the operation protocols, and adding soluble catalysts/redox mediators into the electrolytes, many reports have shown that Li-O2 batteries could be run for hundreds or even thousands of cycles with considerably large specific capacities [2]. In spite of the rapid progress made on improving their cyclic performance and reducing their voltage polarization, lots of work related to thermodynamics and kinetics of Li/O2 chemistry as well as cathode designing are still needed prior to the realization of Li-O2 batteries.
The galvanostatic intermittent titration technique (GITT), which combines the transient and steady-state measurements, is a widely used tool to determine kinetic properties and thermodynamic data for a specific electrode [3]. In this report, the thermodynamic equilibrium voltages for Li-O2 reaction and overpotential variation was studied using GITT measurements, from which a zero voltage gap for the open circuit voltage (OCV) between charging and discharging, an asymmetrical polarization behavior at different current densities and temperatures, a continuous increase of overpotential during charging, as well as a negative temperature coefficient of the cell's thermodynamic equilibrium voltage were observed [4]. This work also gives a clue that the kinetics of Li/O2 reaction plays a major role on improving the performances of Li-O2 battery. Based on such clue, several kind of cathodes ranged from carbon based to carbon-free materials, such as MnO nanoparticles embedded N-doped carbon composites (MnO-m-N-C), Ru nanoparticles anchored carbon nanotubes with ionic liquid coating layer (Ru-IL-CNTs) and RuOx nanodots decorated mesoporous boron-doped carbon nitride (RuOx@m-BCN), have been developed for Li-O2batteries, all of which show positive effects on lowering charge overpotential and improving cycle performance and energy efficiency [5-7].
References:
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[2] Zhong Ma, Xianxia Yuan, Lin Li, Zi-Feng Ma, David P. Wilkinson, Lei Zhang and Jiujun Zhang, Energy Environ. Sci., 2015, 8, 2144
[3] W. Weppner and R. A. Huggins,J. Electrochem. Soc., 1977, 124, 1569
[4] Zhonghui Cui, Xiangxin Guo and Hong Li, Energy Environ. Sci., 2015, 8, 182
[5] Zhonghui Cui, Xiangxin Guo, J. Power Sources 2014, 267, 20
[6] Peili Lou, Chilin Li, Zhonghui Cui and Xiangxin Guo, J. Mater. Chem. A, 2016, 4, 241
[7] Zhonghui Cui, Peili Lou and Xiangxin Guo, In preparation.