Effect of Carbon Surface Area on First Discharge Capacity of Li-O₂Cathodes and Cycle-Life Behavior in Ether-Based Electrolytes

Abstract

In this study we report on the direct proportionality between cathode surface area and first discharge capacity of non-aqueous Li-O 2 cells using ether-based electrolytes. Seven different highly structured carbon blacks, characterized by different surface areas and porosities, were used to prepare non-catalyzed cathodes. Surface measurements and porosity analyzes were carried out on both raw materials and electrodes in order to estimate the fraction of cathode surface accessible by the electrolyte. The first specific discharge capacity of different cathodes was then normalized over its specific surface and a strong correlation between the two quantities was found. This result strongly supports a discharge mechanism for Li-O 2 batteries wherein the main factor limiting the capacity is the formation of a passivating layer of products on the surface of the cathode material, impeding ORR at carbon active sites. Also the cyclability of the cells was considered, demonstrating the effect of electrolyte degradation on the increased capacity upon cycling of ethereal electrolytes (e.g. DEGDME). The first discharge specific capacity and cyclability using TEGDME, commonly used in Li-air research, was found similar to reacting or degraded electrolytes, suggesting a much higher reactivity toward superoxide ion radical in comparison to its lighter homologues DEGDME and DME.

The Influence of Water and Protons on Li2O2Crystal Growth in Aprotic Li-O2Cells

“…Furthermore, it is noteworthy that cells without additional water cycled with the low-surface carbon-paper cathodes show only a slight increase in capacity during cycling, which is strikingly different from the up to six fold increase in capacity observed from the first to the third cycle in our previous study employing high surface porous carbon electrodes. 5,7 As the absolute current at high rates is only a factor of three lower than for our Vulcan electrodes, we believe that not only the current, but also the ratio of electrolyte to active surface area might play a role for the amount of electrolyte decomposition, which leads in our opinion to the remarkable enhancement of discharge capacity during the first cycles for high-surface-area-carbon electrodes. 7 This might sound inconsistent to the effect explained above that a lower Li 2 O 2 yield was found for the carbon paper electrodes compared to the Vulcan electrodes.…”

“…2 In our earlier work, we found a direct correlation between the measured external surface area of a variety of carbon-based cathodes and their first discharge capacity, suggesting that a ≈2 monolayer thick Li 2 O 2 film is sufficient to block the discharge reaction. 5 On the other hand, we observed discharge capacities corresponding to more than 2 monolayers only when using electrolytes which contain species reactive towards the oxygen reduction reaction intermediate superoxide O 2 •− , such as propylene carbonate 6 or non-purified tetraglyme. 7 In contrast, Linda Nazar's group reported recently a substantial capacity increase when discharging at extremely low rates.…”

The Lithium/Air Battery: Still an Emerging System or a Practical Reality?

“…GDLs can be further "Tefl onized" to make them almost impermeable to the electrolyte, thereby allowing current densities up to 2.4 mA cm −2 and extended lifetimes. [ 137 ] Other materials used are carbon cloths, [ 95 ] stainless-steel and Ti meshes, [ 94,202 ] inks and slurries directly cast on the separator, [ 203,204 ] aluminum foil, [ 27 ] graphene nanosheets, [ 205 ] and porous Ni meshes/foams, [ 17,26 ] although the latter have been reported to lead to the electrocatalytic decomposition of carbonate-based electrolytes. [ 206 ] Ein-Eli and co-workers have proposed an interesting approach of enhancing the O 2 transport by impregnating the GDL with perfl uorocarbons; [ 207 ] this class of chemicals can dissolve a signifi cant amount of oxygen while not mixing it with the electrolyte, due to their low polarity.…”

Synthesis optimization of carbon-supported ZrO2 nanoparticles from different organometallic precursors

“…where q ZrO 2 ¼ 6:1 g=cm 3 is the density of tetragonal zirconia, L ZrO 2 is the loading of ZrO 2 on the catalyst powder in percent, A carbon = 800 m 2 /g for Ketjenblack E-type [39], and D average has the same meaning as defined before. If one calculates the AID for the samples shown if Fig.…”