Nature of Li2O2 and its relationship to the mechanisms of discharge/charge reactions of lithium-oxygen batteries.

Lithium-air batteries (LABs) are considered one of the most promising energy storage devices because of their large theoretical energy density. However, low cyclability caused by battery degradation prevents its practical use. Thus, to realize practical LABs, it is essential to improve cyclability significantly by understanding how the degradation processes proceed. Here, we used online mass spectrometry for real-time monitoring of gaseous products generated during charging of lithium-oxygen batteries (LOBs), which was operated with pure oxygen not air, with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) tetraethylene glycol dimethyl ether (TEGDME) electrolyte solution. Linear voltage sweep (LVS) and voltage step modes were employed for charge instead of constant current charge so that the energetics of the product formation during the charge process can be understood more quantitatively. The presence of two distinctly different types of Li2O2, one being decomposed in a wide range of relatively low cell voltages (2.8-4.16 V) (l-Li2O2) and the other being decomposed at higher cell voltages than ca. 4.16 V (h-Li2O2), was confirmed by both LVS and step experiments. H2O generation started when the O2 generation rate reached a first maximum and CO2 generation took place accompanied by the decomposition of h-Li2O2. Based on the above results and the effects of discharge time and the use of isotope oxygen during discharge on product distribution during charge, the generation mechanism of O2, H2O, and CO2 during charging is discussed in relation to the reactions during discharge.

Material balance in the O2 electrode of Li-O2 cells with a porous carbon electrode and TEGDME-based electrolytes.

This work figures out the material balance of the reactions occurring in the O2 electrode of a Li-O2 cell, where a Ketjenblack-based porous carbon electrode comes into contact with a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity. The ratio of electrolyte weight to cell capacity (E/C, g A h-1) is a good parameter to correlate with cycle life. Only 5 cycles were obtained at an areal capacity of 4 mA h cm-2 (E/C = 10) and a discharge/charge current density of 0.4 mA cm-2, which corresponds to the energy density of 170 W h kg-1 at a complete cell level. When the areal capacity was decreased to half (E/C = 20) by setting a current density at 0.2 mA cm-2, the cycle life was extended to 18 cycles. However, the total electric charge consumed for parasitic reactions was 35 and 59% at the first and the third cycle, respectively. This surprisingly large amount of parasitic reactions was suppressed by half using redox mediators at 0.4 mA cm-2 while keeping a similar cycle life. Based on by-product distribution, we will propose possible mechanisms of TEGDME decomposition and report a water breathing behavior, where H2O is produced during charge and consumed during discharge.