Objectively Evaluating the Cathode Performance of Lithium‐Oxygen Batteries

Abstract

Anode : Li Li ↔ + + − e (1) Cathode : 2Li O 2e Li O 2 22 + + ↔ + − (2) Total : 2Li O Li O 2 2 2The open circuit voltage is 2.96 V vs Li/Li + . Based on the molecular weight of the discharge product Li 2 O 2 , the theoretical specific energy can be calculated as high as 3460 W h kg −1 , which is 10 times higher than state of the art LIB. It should be noticed that there is another kind of LAB which uses aqueous electrolyte at the cathode side. [22][23][24][25] But the capacity of such LAB is limited by the solubility of the discharge products in the aqueous electrolyte and the heavy weight of anode protection membrane, [26][27][28][29] it is generally believed that this type of LAB has lower specific energy than non-aqueous LOBs. [17,30] So in this report, we mainly focus on the non-aqueous LOB.In the last decade, the non-aqueous LOB has attracted worldwide attention and achieved great progress. Many literatures have reported LOB cathodes with specific capacity higher than 1,00 mA h g −1 and cyclability of more than 100 cycles in half-cell configuration. [31][32][33][34][35] These values are very attractive by considering that the specific capacity of LIB cathode is usually below 300 mA h g −1 . [36,37] However, some scientists believe that such performance is largely overestimated. [38,39] Estimation of LOB performance is usually based on half-cell test, which refers to the experimental measurement that is specialized to characterize the half-reaction on cathode side. The half-cell usually contains large excess of anode material which has high exchange current density, and the potential of the anode maintains stable during testing. However, on prototype cell level, high specific energy LOBs are still irreversible. [40,41] For example, in 2016, Samsung Electronics Inc. reported their latest progress on LOB prototype cell. [42] Their 1 A h cell with zigzag folding structure realized a specific energy of 500 W h kg −1 but only worked for 3 cycles. Apparently, conventional half-cell testing results cannot objectively reflect the cathode performance in LOB prototype cells.In this report, the particularity of the LOB cathode is extensively analyzed. We point out that some conventional parameters to evaluate the performances of LIB cathode are not suitable for LOB. In some cases, higher "specific capacity" cathodes may lead to lower prototype cell specific energy, and longer "cycle life" in limited capacity cycling tests may actually due to more unwanted side reactions. To avoid the above misleading Lithium-oxygen battery is a promising high specific energy electrochemical system for next generation energy storage. Many literatures have reported lithium-oxygen battery cathodes with specific capacity higher than 1000 mA h g −1 and cyclability of several hundred cycles in half-cell tests. However, on the prototype cell level, high specific energy lithium-oxygen batteries are still irreversible. In this report, the huge discrepancy between the half-cell result and the prototype cell performance is carefully analyzed. Some m...

Nature‐Inspired Tri‐Pathway Design Enabling High‐Performance Flexible Li–O2 Batteries

“…As shown in Figure 5A-F, we observed a layer of material coated on the top surface and inside the surface of the channels; however, this layer was not thick enough to totally block the channels, ensuring the efficient transport of O 2 gas during the cycling process. XRD patterns of the two electrodes ( Figure 5O) further confirmed the formation of Li 2 O 2 upon discharging and its disappearance after charging, based on the following redox reaction: [31][32][33][34][35] (1) Figure S25 in the Supporting Information and Figure 5N graphically illustrate the breathable triphase redox reaction process involving electrons, Li + ions, and O 2 gas, and their transport in the F-Wood-based cell. At the end of the charging process, the product almost totally disappears, exposing the CNT/Ru-coated surface both on the top and inside surfaces of the channels ( Figure 5G-L).…”

“…The corresponding initial discharge/charge curves of the CNT/Ru‐coated F‐Wood electrode is illustrated in Figure M. XRD patterns of the two electrodes (Figure O) further confirmed the formation of Li 2 O 2 upon discharging and its disappearance after charging, based on the following redox reaction:…”

Performance and Sustainability Tradeoffs of Oxidized Carbon Nanotubes as a Cathodic Material in Lithium‐Oxygen Batteries

“…For example, while 4 of the 5 processed MWCNTs had higher CED/discharge and GWP/discharge ratios than the base case for the FB‐CVD scenario, only one (CT−O) has similar ratios to the base case when MWCNTs are synthesized by HW‐CVD (Figure 4b). This indicates that, under a circumstance where MWCNT synthesis is more optimized, the more environmentally friendly approach to increase total discharge capacity would be to use a higher mass of pristine MWCNTs in the cathode, rather than using a similar mass of an oxidized or annealed set of MWCNTs, since total discharge capacity increases with increasing cathode mass [67] …”

Strategies Toward Stable Nonaqueous Alkali Metal–O2 Batteries

“…Especially in the cycling test with limited capacity, the catalyst related side reactions may lead to a fake “long cycle life”. Distinguishing the “reusability of the catalyst” and the “reversibility of the reaction” is important, as we highlighted before …”

“…Distinguishing the "reusability of the catalyst" and the "reversibility of the reaction" is important, as we highlighted before. [25]…”