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 RuO_x nanodots decorated mesoporous boron-doped carbon nitride (RuO_x@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:

[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193

[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.