The loss of sulfur cathode material as a result of polysulfide dissolution causes significant capacity fading in rechargeable lithium/sulfur cells. Here, we use a chemical approach to immobilize sulfur and lithium polysulfides via the reactive functional groups on graphene oxide. This approach enabled us to obtain a uniform and thin (around tens of nanometers) sulfur coating on graphene oxide sheets by a simple chemical reaction-deposition strategy and a subsequent low-temperature thermal treatment process. Strong interaction between graphene oxide and sulfur or polysulfides enabled us to demonstrate lithium/sulfur cells with a high reversible capacity of 950-1400 mA h g(-1), and stable cycling for more than 50 deep cycles at 0.1C (1C = 1675 mA g(-1)).

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The electrocatalytic activity of well-characterized Pt-Ru alloy electrodes toward the electrooxidation of CO in acidic electrolyte at room temperature was measured on alloy surfaces prepared in UHV (ultrahigh vacuum).Clearly defined surface composition was determined via LEIS (low-energy ion scattering). Electrocatalytic activities were measured by CO stripping voltammetry as well as by potentiostatic oxidation of adsorbed CO.It was found that the property of Ru atoms to nucleate oxygen-containing species at low potentials produced a strong enhancement in the catalytic activity of sputter-cleaned Pt-Ru alloy electrodes compared to pure Pt, thereby supporting the concept of the bifunctional character of the oxidation process of these alloys. A further synergistic effect of the alloy with a Ru surface composition of 4 0 atom % Ru was observed, with a catalytic shift in the CO electrooxidation current of -0.25 and -0.15 V compared to those of pure Pt and pure Ru surfaces, respectively. This synergism was attributed to a uniquely active state of OH& on Pt-Ru pair sites. The different electrocatalytic activities of sputter-cleaned uersus annealed Pt-Ru alloy electrodes with essentially identical Ru surface compositions are discussed in terms of Ru clustering during annealing.

The kinetics of methanol electro‐oxidation on well‐characterized Pt‐Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to ≈10 atomic percent (a/o) Ru at 25°C to a value in the vicinity of ≈30 a/o at 60°C. The shift in optimum composition with temperature was attributed to a shift in the rate‐determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate‐determining step.

Sulfur (S) encapsulated in porous carbon nanofibers (CNFs) was synthesized via electrospinning, carbonization and solution-based chemical reaction-deposition method. The chemical reactiondeposition strategy provides intimate contact between the S and the CNFs. This would not necessarily be the case for other reported methods, such as ball milling and thermal treatment. These novel porous carbon nanofiber-sulfur (CNF-S) nanocomposites with various S loadings showed high reversible capacity, good discharge capacity retention and enhanced rate capability when they were used as cathodes in rechargeable Li/S cells. We demonstrated here that an electrode prepared from a porous CNF-S nanocomposite with 42 wt% S maintains a stable discharge capacity of about 1400 mA h g À1 at 0.05 C, 1100 mA h g À1 at 0.1 C and 900 mA h g À1 at 0.2 C. We attribute the good electrochemical performance to the high electrical conductivity and the extremely high surface area of the CNFs that homogeneously disperse and immobilize S on their porous structures, alleviating the polysulfide shuttle phenomenon. SEM measurements showed that the porous CNF structures remained nearly unchanged even after 30 cycles' discharging/charging at 0.05 C.

TitleA long-life, high-rate lithium/sulfur cell: A multifaceted approach to enhancing cell performance Permalink https://escholarship.org/uc/item/97z815sv Journal Nano Letters, 13(12)

In this review, we begin with a brief discussion of the operating principles and scientific/technical challenges faced by the development of lithium/sulfur cells. We then introduce some recent progress in exploring cathodes, anodes, and electrolytes for lithium/sulfur cells. In particular, several effective strategies used to enhance energy/power density, obtain good efficiencies, and prolong cycle life will be highlighted. We also discuss recent advancements in techniques for investigating electrode reactions in real time and monitoring structural/morphological changes of electrode materials under cell operating conditions to gain a better understanding of the mechanistic details of electrode processes. Finally, the opportunities and perspective for future research directions will be discussed.

The cycling behavior and electrochemical characteristics of the sulfur electrode in the lithium/sulfur rechargeable cell were investigated using galvanostatic cycling and cyclic voltammetry. The cycling performance of the sulfur electrode was characterized in different solvents (triglyme, PEGDME 250 and 500). The variations of capacity and capacity fade rate were also measured as a function of the content of sulfur, carbon and binder. The capacity fade rate of the sulfur electrode was lower with a higher molecular weight solvent such as PEGDME 250 or 500. The content of carbon in the sulfur electrode plays an important role in capacity improvement. The best cell cycled for 600 deep cycles above 100 mAh/g of electrode, and 400 cycles above 160 mAh/g of electrode (80% of original capacity) at room temperature. © 2002 The Electrochemical Society. All rights reserved.