China team outlines 5 key areas of future research to realize Li-air batteries

In an open access paper published in the International Journal of Smart and Nano Materials, researchers from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences review significant developments and remaining challenges of practical Li–air batteries and the current understanding of their chemistry.

The energy density of the lithium–air battery with respect to the anode could reach 13,000 Wh kg⁻¹—quite close to the 13,200 Wh kg⁻¹ of gasoline, they note. Although researchers have made significant progress, the Li–air battery is still at an embryonic stage, with numerous scientific and technical challenges that must be overcome if the promise is to be realized. From the perspective of the authors, the key areas for future research are as follows:

1. Porous carbon-based air cathode. The oxygen cathode is the key component related to the performance of a Li–air battery, in which the electrons are confined inside the electrode material while the oxygen is in both the gaseous and solution phases and the lithium ions are contained in the electrolyte solution.

   Design and synthesis of a novel porous carbon material with high conductivity, which would ensure sufficient pores to store discharge products, channels to diffuse oxygen and good electrolyte wettability. This would provide an adequate and suitable three-phase interface (solid–liquid–gas) for the charge/discharge process.

   This three-phase interface is very important for aprotic Li–air batteries, because only where the liquid electrolyte with Li⁺ ions, O₂ from the environment and the insoluble solid products, i.e. lithium oxides, coexist can they react simultaneously.

   —Zhang et al.

2. Screening bifunctional cathode catalysts with improved activity for both the ORR (oxygen reduction reaction) during discharge and the OER (oxygen evolution reaction) during charge, achieving a high round-trip efficiency.

3. Development of stable electrolytes with high O₂ solubility, excellent lithium ionic conductivity, low viscosity and vapor pressure. The organic electrolyte in an aprotic Li–air system stabilizes the anode, conducts Li⁺ ions, dissolves O₂ and provides a reaction interface. Properties of the electrolyte such as ionic conductivity, O₂ solubility, viscosity and contact angle strongly influence the cell discharge performance.

4. Developing a high lithium ionic conducting separator and a high throughout oxygen-breathing membranes used at the cathode to block H₂O, CO₂ and other air components except O₂.

   As an indispensable and important part of a Li–air battery, an ideal separator should be a determined block for gases, excellent penetrator for lithium ions, good reservoir for electrolyte, etc.

   —Zhang et al.

5. Understanding of the complex chemical reaction mechanisms that occur during charge and discharge.

   Up to now, no less than five different mechanisms for O₂ reduction in Li+ electrolytes have been proposed over the last 40 years based on electrochemical measurements alone. The mechanism for cathode reactions are complex and uncertain because they are electrolyte-, catalyst- and sometimes even battery operation environment-dependent. It is still a great challenge and intensive research is urgently needed to clarify the cathode reactions.

   —Zhang et al.

There are four basic chemical architectures of Li–air batteries being pursued worldwide, they note. Three versions use liquid electrolytes: a fully aprotic liquid electrolyte; an aqueous electrolyte; and a mixed system with an aqueous electrolyte immersing the cathode and an aprotic electrolyte immersing the anode. The fourth approach is an all-solid-state battery with a solid electrolyte. In their paper, the authors focused principally on the aprotic configuration of a Li–air battery as the one showing the promise of rechargeability, and the one that has attracted the most effort worldwide to date.

Source: Green Car Congress