Enabled by the reversible conversion between Li2O2 and O2, Li-O2 batteries promise theoretical gravimetric capacities significantly greater than Li-ion batteries. The poor cycling performance, however, has greatly hindered the development of this technology. At the heart of the problem is the reactivity exhibited by the carbon cathode support under cell operation conditions. One strategy is to conceal the carbon surface from reactive intermediates. Herein, we show that long cyclability can be achieved on three dimensionally ordered mesoporous (3DOm) carbon by growing a thin layer of FeO(x) using atomic layer deposition (ALD). 3DOm carbon distinguishes itself from other carbon materials with well-defined pore structures, providing a unique material to gain insight into processes key to the operations of Li-O2 batteries. When decorated with Pd nanoparticle catalysts, the new cathode exhibits a capacity greater than 6000 mAh g(carbon) (-1) and cyclability of more than 68 cycles.
As a promising high-capacity energy storage technology, Li-O2 batteries face two critical challenges, poor cycle lifetime and low round-trip efficiencies, both of which are connected to the high overpotentials. The problem is particularly acute during recharge, where the reactions typically follow two-electron mechanisms that are inherently slow. Here we present a strategy that can significantly reduce recharge overpotentials. Our approach seeks to promote Li2O2 decomposition by one-electron processes, and the key is to stabilize the important intermediate of superoxide species. With the introduction of a highly polarizing electrolyte, we observe that recharge processes are successfully switched from a two-electron pathway to a single-electron one. While a similar one-electron route has been reported for the discharge processes, it has rarely been described for recharge except for the initial stage due to the poor mobilities of surface bound superoxide ions (O2(-)), a necessary intermediate for the mechanism. Key to our observation is the solvation of O2(-) by an ionic liquid electrolyte (PYR14TFSI). Recharge overpotentials as low as 0.19 V at 100 mA/g(carbon) are measured.
Hierarchical
stannosilicate molecular sieves with ordered mesoporosity
and MFI topology (three dimensionally ordered mesoporous imprinted
(3DOm-i) Sn-MFI) were successfully synthesized within the confined
space of three dimensionally ordered mesoporous (3DOm) carbon by a
seeded growth method. The obtained 3DOm-i Sn-MFI consisting of 30
nm spherical elements forming an opaline structure contains highly
ordered mesopores ranging from 4 to 11 nm. Compared with conventional
Sn-MFI, 3DOm-i Sn-MFI exhibits superior catalytic performance for
the isomerization of cellulosic sugars. No diffusion limitation was
observed for the isomerization of a triose sugar, dihydroxyacetone
(DHA), into methyl lactate (ML). The presence of weak Brønsted
acid in the 3DOm-i Sn-MFI catalyst facilitates the reaction by catalyzing
the formation of an intermediate, pyruvaldehyde (PA). 3DOm-i Sn-MFI
offers significant improvements for the isomerizations of C5 and C6 sugars, such as xylose and glucose, by greatly
enhancing molecular transport. The reaction rate of xylose on 3DOm-i
Sn-MFI is at least 20 times higher than that on conventional bulky
sized Sn-MFI. The reaction rate for glucose is also enhanced by using
3DOm-i Sn-MFI, but to a lesser extent as compared with the reaction
of xylose, possibly because glucose cannot diffuse into the 10-membered-ring
pore of MFI, and the reaction is catalyzed only on the external surface
of the Sn-MFI catalysts. Moreover, the combination of seeded growth
with confined synthesis allows us to synthesize hierarchical Sn-MFI
using commercially available carbon materials, such as carbon black
and activated carbon, indicating that the synthesis strategy is a
versatile and reliable method for tailoring the structure of hierarchical
zeolites.
Formation of branched glucan chains by co-impregnation with glucose can greatly improve efficiency of mechano-catalytic depolymerization of crystalline cellulose.
Enabled by the reversible conversion between Li 2 O 2 and O 2 ,L i-O 2 batteries promise theoretical gravimetric capacities significantly greater than Li-ion batteries.T he poor cycling performance,however,has greatly hindered the development of this technology.A tt he heart of the problem is the reactivity exhibited by the carbon cathode support under cell operation conditions.O ne strategy is to conceal the carbon surface from reactive intermediates.Herein, we show that long cyclability can be achieved on three dimensionally ordered mesoporous (3DOm) carbon by growing athin layer of FeO x using atomic layer deposition (ALD). 3DOm carbon distinguishes itself from other carbon materials with well-defined pore structures,providing aunique material to gain insight into processes key to the operations of Li-O 2 batteries.W hen decorated with Pd nanoparticle catalysts,t he new cathode exhibits ac apacity greater than 6000 mAh g carbon À1 and cyclability of more than 68 cycles.
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