Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator.

Prior to the practical application of rechargeable aprotic Li-O2 batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li2O2, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li2O2 reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li2O2 particles established that both growth and decomposition proceeded at the Li2O2/electrolyte interface in a system containing the Br-/Br3- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li2O2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li2O2/electrolyte interface is critical to improve the cycle life of Li-O2 batteries.

Synergistic Effect of Binary Electrolyte on Enhancement of the Energy Density in Li-O2 Batteries.

Enhancement of the discharge capacity of lithium-oxygen batteries (LOBs) while maintaining a high cell voltage is an important challenge to overcome to achieve an ideal energy density. Both the cell voltage and discharge capacity of an LOB could be controlled by employing a binary solvent electrolyte composed of dimethyl sulfoxide (DMSO) and acetonitrile (MeCN), whereby an energy density 3.2 times higher than that of the 100 vol % DMSO electrolyte was obtained with an electrolyte containing 50 vol % of DMSO. The difference in the solvent species that preferentially solvates Li+ and that which controls the adsorption-desorption equilibrium of the discharge reaction intermediate, LiO2, on the cathode/electrolyte interface provides these unique properties of the binary solvent electrolyte. Combined spectroscopic and electrochemical analysis have revealed that the solvated complex of Li+ and the environment of the cathode/electrolyte interface were the determinants of the cell voltage and discharge capacity, respectively.

Dynamic Changes in Charge Transfer Resistances during Cycling of Aprotic Li-O2 Batteries.

Various electrolyte components have been investigated with the aim of improving the cycle life of lithium-oxygen (Li-O2) batteries. A tetraglyme-based electrolyte containing dual anions of Br- and NO3- is a promising electrolyte system in which the cell voltage during charging is reduced because of the redox-mediator function of the Br-/Br3- and NO2-/NO2 couples, while the Li-metal anode is protected by Li2O formed via the reaction between Li metal and NO3-. To maximize the potential of this system, the fundamental factors that limit the cycle life should be clarified. In the present work, we used nondestructive electrochemical impedance spectroscopy to analyze the temporal change of the charge transfer resistances during cycles of Li-O2 batteries with dual anions. The charge transfer resistance at the cathode was revealed to exhibit good correlation with the reduction of the discharge voltage. These results, combined with the results of electrode surface inspections, revealed that irreversible accumulation of insulating deposits such as Li2O2 and Li2CO3 on the cathode surface was a major cause of the short cycle life. Furthermore, the analyses of the time course of the solution resistance suggested that diminished reactivity between the redox mediators and Li2O2 was a critical factor that led to the irreversible accumulation of the less-reactive Li2O2 on the cathode and eventually to a shortened cycle life. These findings indicated that increasing the reactivity between Br3- and Li2O2 is essentially important for improving the cycle stability of Li-O2 batteries and the reactivity can be nondestructively assessed by tracking the dynamic changes in the solution resistance.

Publisher Correction: Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygen batteries.

The original version of this Article contained an error in the title, which was previously incorrectly given as 'Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries'. The correct version states 'lithium-oxygen' in place of 'lithium-oxygene'. This has been corrected in both the PDF and HTML versions of the Article.

Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries.

In non-aqueous lithium-oxygen batteries, the one-electron reduction of oxygen and subsequent lithium oxide formation both occur during discharge. This lithium oxide can be converted to insulating lithium peroxide via two different pathways: a second reduction at the cathode surface or disproportionation in solution. The latter process is known to be advantageous with regard to increasing the discharge capacity and is promoted by a high donor number electrolyte because of the stability of lithium oxide in media of this type. Herein, we report that the cathodic oxygen reduction reaction during discharge typically exhibits negative differential resistance. Importantly, the magnitude of negative differential resistance, which varies with the system component, and the position of the cathode potential relative to the negative differential resistance determined the reaction pathway and the discharge capacity. This result implies that the stability of lithium oxide on the cathode also contributes to the determination of the reaction pathway.