Electrochemical performance of All-Solid-State Lithium-Oxygen Batteries

Introduction

The lithium-air batteries have attracted much more attentions because of its high theoretical energy density, which is over 3500 Wh kg^-1.  However the lithium-air batteries still have a lot of problems and the problem related with the decomposition of organic liquid electrolytes is the most serious and one of the main research topics in the present stage.  Thus, we have developed all-solid-state lithium-air batteries using the inorganic solid electrolytes, which are more stable than the liquid electrolytes. The application of the solid electrolytes to the lithium-air batteries can solve not only the decomposition of electrolytes but also the safety problem and some other problems specific to the liquid such as leakage and volatilization. 

In this study, an all-solid-state cell consisting of Li anode, Li_1+xAl_yGe_2-y(PO₄)₃ (LAGP) solid electrolyte, and LAGP-carbon nanotube (CNT) composite cathode was operated in the pure oxygen atmosphere (so-called Li-O₂ cell) and the fundamental electrochemical performance was examined.

Experimental

The all-solid-state Li-O₂ cells were fabricated.    The LAGP solid electrolyte was synthesized by the conventional solid-phase method.  The air electrode was composed of LAGP and multi-walled CNT particles.  The composite electrode was fabricated on the LAGP pellet by sintering and Li anode was fused on another side of the LAGP pellet.  Li anode was sealed by the plastic film.  The cell was put into the bottles connecting to the oxygen gas flow channel.

Results and discussion

The electrochemical properties of the solid-state lithium-oxygen battery were examined.  First of all, the potential for the electrochemical reactions in the Li / LAGP / LAGP-CNT-O₂ cell was investigated using cyclic voltammetry (CV) as shown in Fig. 1a.  The measurement was conducted at a scan rate of 10 mV s^-1 at room temperature in the O₂ atmosphere.  The cell showed the cathodic peak at the potential less than 3 V and anodic peak between 3.1 V and 4.0 V. 

Then the cell was discharged and charged under the constant current density of 10 mA g-1 in the voltage range of 2.0 - 4.8 V at room temperature (Fig. 1b).  The cell showed the voltage of 2.4 V at the beginning of discharge and then the discharge voltage gradually decreased to 2.0 V.  The discharge capacity was about 1420 mAh g^-1 and the recharge capacity of about 1130 mAh g^-1 was obtained in the cell charged up to 4.8 V.  It is noteworthy that the charge process starts from 3.0 V and proceeds in the voltage below 4.2 V.  This voltage profile in the constant current condition matches the result obtained by CV measurement.

Fig. 1c shows the cycle performance under the current density of 10 mA g^-1 at room temperature in the voltage region of 1.6 - 4.8 V.  The measurement was conducted with the capacity limit of 500 mAh g^-1.  Although the discharge voltage decreased with the cycling, the discharge capacity was retained during 10 cycles.  The charge capacity was smaller than the discharge capacity and efficiencies of all cycles were 80 ~ 90 %.  It results in increasing the amount of remained discharge products after charging, which would caused the decrease of the discharge voltage with cycling.

The redox potentials in the all-solid-state Li-O₂ cells are consistent with the potentials for the oxygen reduction reaction and oxygen evolution reaction reported by many researchers.  Especially, the cell using the glyme electrolyte, which are well-known as relatively stable liquid electrolytes, shows similar CV results [1].  In addition, the all-solid-state cell showed the repeatedly same charge curve and the rise at the end of the charging.  It means that the continuous decomposition of electrolyte and electrode materials does not occur in this voltage region.  Therefore, it can be stated that the solid electrolyte is quite stable in the all-solid-state Li-O₂ cells.

References

[1] H.-G. Jung et al., Nature Chem., 4, 579 (2012).

Acknowledgements

We are thankful to Kato Foundation for Promotion of Science for supporting the present work.