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The micrometer-scale reticulated structure of highly sulfur-philic nitrogen-doped graphene oxide (NrGO) that is achieved by chemically controlling the nitrogen-doping degree improves the chemical and electrochemical stability of the active sulfur while providing sufficient sulfur/electrolyte interface for a facile electrochemical process so that a high specific energy of 325 W h kg-1 can be achieved.

To date, Li2S has drawn significant attention as a positive electrode active material for rechargeable lithium cells due to its high theoretical specific capacity and capability of pairing with a lithium-free anode which can obviate any safety concern of the lithium metal anode when using sulfur. In recent years, various approaches have been employed to develop Li/Li2S rechargeable cells for commercialization that meet the performance goals for high energy/power applications. It is expected that high lithium sulfide-loading cells with long cycle life, an excellent capacity delivery and low electrolyte:sulfur weight ratio (E/S ratio) can be achieved. Here, we report a Li2S electrode comprised of a novel Li2S/KB@Cf nanocomposite which delivers an areal capacity of 7.56 mAh cm-2 and good cycling stability with a mass loading of 11.29 mg cm-2 and a robust 3-dimensional (3D) aluminum foam current collector with a high open area. The high conductivity and scalability of the active material, the availability of 3D current collection for the active material and the control of the electrolyte/sulfur ratio offer the potential of realization of practical Li/S cells.

Rational design of sulfur electrodes is exceptionally important in enabling a high-performance lithium/sulfur cell. Constructing a continuous pore structure of the sulfur electrode that enables facile lithium ion transport into the electrode and mitigates the reconstruction of sulfur is a key factor for enhancing the electrochemical performance. Here, we report a three-dimensionally (3D) aligned sulfur electrode cast onto conventional aluminum foil by directional freeze tape casting. The 3D aligned sulfur-graphene oxide (S-GO) electrode consisting of few micron thick S-GO layers with 10-20 μm interlayer spacings demonstrates significant improvement in the performance of the Li/S cell. Moreover, the freeze tape cast graphene oxide electrode exhibits homogeneous reconfiguration behavior in the polysulfide catholyte cell tests and demonstrated extended cycling capability with only 4% decay of the specific capacity over 200 cycles. This work emphasizes the critical importance of proper structural design for sulfur-carbonaceous composite electrodes.

Lithium nitrate (LiNO3) has been the most studied electrolyte additive in lithium-sulfur (Li-S) cells, due to its known function of suppressing the shuttle effect in Li-S cells, which provides a significant increase in the cell's coulombic efficiency and cycling stability. Previous studies indicated that LiNO3 participated in the formation of a passive layer on the lithium electrode and thus suppressed the redox shuttle of the dissolved polysulfides. However, the effects of the LiNO3 on the positive electrode materials have rarely been investigated. By combining scanning electron microscopy, element-selective X-ray absorption spectroscopy, and electrochemical characterizations, we performed a comprehensive study of how the LiNO3 altered the properties of the sulfur electrode/electrolyte interface in Li-S cells and thus influenced the cell performance. We found that LiNO3 is a double-edged sword in the Li-S cell: on one hand, it increased the consumption of the active sulfur; on the other hand, it promoted the survival of the carbon matrix constituent in the sulfur electrode. These two competitive effects indicated that a proper moderate concentration of LiNO3 is required to achieve an optimized cell performance.

Recently, 2D layered transition metal dichalcogenides (TMDs) have attracted great scientific interest in electrochemical energy storage research. Vanadium disulfide (VS2) as an important family member of TMDs, is a promising electrode material for lithium-ion cells because of its remarkable electrical conductivity and high Li+ diffusion rate, but its electrochemical reaction mechanism is still poorly understood. Herein, we have prepared VS2 nanosheets as the electrode and systematically investigated its structural and chemical evolution during the electrochemical processes by employing both in situ and ex situ x-ray spectroscopy. The VS2 undergoes intercalation and conversion reactions in sequence during discharge and this process is found to be partially reversible during the subsequent charge. The decreased reversibility of the conversion reaction over extended cycles could be mainly responsible for the capacity fading of the VS2 electrode. In addition, the hybridization strength between S and V shows a strong dependence on the states of charge, as directly illustrated by the intensity change of the V-S hybridized states and pure V states. We have also found that the solid electrolyte interphase on the electrode surface is dynamically evolved during cycling, which may be a universal phenomenon for the conversion-based electrodes. This study is expected to be beneficial for the further development of high-performance VS2-based electrodes.

With the increasing interest in transition metal chalcogenides, sulfide minerals containing the disulfide unit (S22-) have gained intensive attention for potential applications in energy storage devices, such as lithium-ion batteries (LIBs). Vanadium tetrasulfide (VS4) possesses a unique linear-chain structure with a Peierls distortion and shows great promise for application in LIBs. However, its electrochemical reaction mechanism is still controversial, mainly due to the amorphous nature of the intermediates and final products. Here, by applying multiple X-ray spectroscopies, we reveal that VS4 undergoes lithium intercalation and conversion reactions sequentially during the first discharge process, which are partially reversible in the subsequent charge process. However, an anomalous intercalation/conversion mixed reaction mechanism is dominant for the second cycle, mainly owing to the amorphization of the VS4 electrode during the first cycle. In addition, the sulfur atoms are also involved in the redox reaction during cycling, with the anionic contribution of S22- ↔ 2S2- transformation. Furthermore, we find that the formation process of the solid electrolyte interphase is highly dynamic during the discharge and charge processes. The present study provides deeper insights into the complex reaction mechanism of VS4. This knowledge can accelerate the development of high-performance VS4-based electrode materials for LIBs.

As the lightest and cheapest transition metal dichalcogenide, TiS2 possesses great potential as an electrode material for lithium batteries due to the advantages of high energy density storage capability, fast ion diffusion rate, and low volume expansion. Despite the extensive investigation of its electrochemical properties, the fundamental discharge-charge reaction mechanism of the TiS2 electrode is still elusive. Here, by a combination of ex situ and operando X-ray absorption spectroscopy with density functional theory calculations, we have clearly elucidated the evolution of the structural and chemical properties of TiS2 during the discharge-charge processes. The lithium intercalation reaction is highly reversible and both Ti and sulfur are involved in the redox reaction during the discharge and charge processes. In contrast, the conversion reaction of TiS2 is partially reversible in the first cycle. However, Ti-O related compounds are developed during electrochemical cycling over extended cycles, which results in the decrease of the conversion reaction reversibility and the rapid capacity fading. In addition, the solid electrolyte interphase formed on the electrode surface is found to be highly dynamic in the initial cycles and then gradually becomes more stable upon further cycling. Such understanding is important for the future design and optimization of TiS2 based electrodes for lithium batteries.

A novel ionic liquid-containing electrolyte has been demonstrated to improve charging efficiency, lifetime and safety of Li/S cells. However, the high viscosity of the ionic liquid can reduce the kinetics of the electrochemical process and degrade the wettability the S electrode by the electrolyte, which limits electrochemical utilization of the active S. To realize the advantages of the ionic liquid in the electrolyte, while maintaining good cell performances, we have investigated the critical properties of polymeric binders such as polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA) and lithium polyacrylate (LiPAA) for the S electrode operated in ionic liquid-containing electrolytes. The PAA binder electrode showed the most promising cell performance which is attributed to its good physical stability and wettability. Moreover, the cell performance of the PAA binder electrode was further improved by combining the PAA binder with the PVDF binder, which enhances the specific capacity up to 1.3 times higher than that of the PAA binder electrode.

As a typical transition metal dichalcogenide, MoS2 offers numerous advantages for nanoelectronics and electrochemical energy storage due to its unique layered structure and tunable electronic properties. When used as the anode in lithium-ion cells, MoS2 undergoes intercalation and conversion reactions in sequence upon lithiation, and the reversibility of the conversion reaction is an important but still controversial topic. Here, we clarify unambiguously that the conversion reaction of MoS2 is not reversible, and the formed Li2S is converted to sulfur in the first charge process. Li2S/sulfur becomes the main redox couple in the subsequent cycles and the main contributor to the reversible capacity. In addition, due to the insulating nature of both Li2S and sulfur, a strong relaxation effect is observed during the cycling process. This study clearly reveals the electrochemical lithiation-delithiation mechanism of MoS2, which can facilitate further developments of high-performance MoS2-based electrodes.

Lithium/Sulfur (Li/S) cells are a promising chemistry with potential to deliver a step-change in energy density compared to state-of-the-art Li-ion batteries. To minimize the environmental impact of the Li/S cell manufacturing and to compete with Li-ion cells in both performance and cost, electrodes cast using an aqueous process are highly desirable. Here we describe the discovery and application of a lithiated redox-mediating supramolecular binder based on the well-known n-type semiconductor, perylene bisimide, that forms high-fidelity sulfur electrodes from water-processed slurries. A 1.4-fold improvement in sulfur utilization at 3.0 C and 58% increase in capacity retention after 250 cycles at 1.5 C are reported for the prelithiated, supramolecular binder compared to control samples. These improvements are attributed to the self-assembly of lithiated perylene bisimide binders in water to yield nanowire web morphologies that increase interfacial area between electrode components and exhibit enhanced electrode-current collector adhesion.

High capacity and long cycle life are both desirable features for practical secondary cells. In this study, we compare two different synthesis processes for preparing active materials for lithium/sulfur cells using polysulfide and graphene oxide (GO) showing high sulfur utilization and cycling stability. The key factor determining cell performance is the oxygen-containing functional groups on GO and the fine structures. This study shows the possibility of application of GO for enhanced capacity and cycling stability.

The ambient-temperature rechargeable lithium/sulfur (Li/S) cell is a strong candidate for the beyond lithium ion cell since significant progress on developing advanced sulfur electrodes with high sulfur loading has been made. Here we report on a new sulfur electrode active material consisting of a cetyltrimethylammonium bromide-modified sulfur-graphene oxide-carbon nanotube (S-GO-CTA-CNT) nanocomposite prepared by freeze-drying. We show the real-time formation of nanocrystalline lithium sulfide (Li2S) at the interface between the S-GO-CTA-CNT nanocomposite and the liquid electrolyte by in situ TEM observation of the reaction. The combination of GO and CNT helps to maintain the structural integrity of the S-GO-CTA-CNT nanocomposite during lithiation/delithiation. A high S loading (11.1 mgS/cm2, 75% S) S-GO-CTA-CNT electrode was successfully prepared using a three-dimensional structured Al foam as a substrate and showed good S utilization (1128 mAh/g S corresponding to 12.5 mAh/cm2), even with a very low electrolyte to sulfur weight ratio of 4. Moreover, it was demonstrated that the ionic liquid in the electrolyte improves the Coulombic efficiency and stabilizes the morphology of the Li metal anode.

Lithium sulfide (Li2S) is a promising cathode material for lithium-sulfur (Li/S) cells due to its high theoretical specific capacity (1166 mAh g-1) and ability to pair with nonmetallic lithium anodes to avoid potential safety issues. However, when used as the cathode, a high charging voltage (∼4 V versus Li+/Li) is always necessary to activate Li2S in the first charge process, and the voltage profile becomes similar to that of a common sulfur electrode in the following charge processes. In this report, we have prepared an electrode of nanosphere Li2S particles and investigated its charging mechanism of the initial two charge processes by in situ and operando X-ray absorption spectroscopy. The results indicate that Li2S is directly converted to elemental sulfur through a two-phase transformation in the first charge process, while it is oxidized first to polysulfides and then to sulfur in the second charge process. The origin of the different charging mechanisms and corresponding charge-voltage profiles of the first and second charge processes is found to be related to the remaining polysulfides at the end of the first discharge process: they can not only facilitate the charge-transfer process at the Li2S/electrolyte interface but also chemically react with Li2S and act as the polysulfide facilitator for the electrochemical oxidation of Li2S in the following charge processes. Our present study provides a new fundamental understanding of the charging mechanism of the Li2S electrode, which should be of help for the further development of high-performance Li/S cells.

The electrolyte, a key component for the successful operation of energy materials, is greatly affected by its solvents. The influence of solvents on the electrochemical performance of a LiFePO4/C composite cathode was investigated at various operating temperatures. The reaction kinetics of the LiFePO4/C composite electrode, including changes of rate capability, redox potential, polarization degree, electrode reaction process, exchange current densities, and activation energies, were evaluated using various techniques. The composition and volume ratio of solvents greatly affect the electrode kinetics. In the mixed solvents of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), EMC is beneficial for the room temperature performance, while the substitution of 20 vol % of EMC by ethyl acetate (EA) is good for the low temperature performance. When 30 vol % of DMC is substituted by 10 vol % of EMC and 20 vol % of EA, the exchange current density increases from 0.022 to 0.038 mA cm−2 at −20 °C, while the activation energy of the charge-transfer process decreases from 48.36 to 33.01 kJ mol−1. Possible mechanisms for improving the electrochemical performance using different solvents have been analyzed. These results are significant for the exploration of appropriate electrolytes for the extensive applications of LiFePO4/C composite electrodes.

A surfactant material plays a significant role in creating a sulfur/carbon composite for lithium/sulfur cells. CTAB (cetyltrimethylammoniumbromide) is a widely used surfactant material in various fields. In this study we identified the functions of CTAB in a sulfur-graphene oxide composite used as the cathode material for lithium/sulfur cells, and the key features for enhancing the capacity of the cells. The CTAB added in the synthesis procedure reacts with polysulfides during the 155 °C heat treatment and produces cetylamine, cetylmethylamine, cetyldimetylamine, and dimethylpolysulfide as products. Furthermore the composite produced by the reactions between cetyldimetylamine and sulfur cover the surface of the sulfur. These work to enhance the performance of the cells. The findings presented here can be applied to various kinds of sulfur/carbon composites to enhance the performance of the sulfur electrode.

π-Stacked perylene bisimide (PBI) molecules are implemented here as highly networked, redox-active supramolecular polymer binders in sulfur cathodes for lightweight and energy-dense Li-S batteries. We show that the in operando reduction and lithiation of these PBI binders sustainably reduces Li-S cell impedance relative to nonredox active conventional polymer binders. This lower impedance enables high-rate cycling in Li-S cells with excellent durability, a critical step toward unlocking the full potential of Li-S batteries for electric vehicles and aviation.

In this study, we propose a modified mathematical model for Lithium/Sulfur (Li/S) cells and present a detailed numerical analysis showing the effects of (1) the presence of solid sulfur species (S8(s), Li2S2(s), Li2S(s)), (2) the solubility and diffusivity of polysulfides in the electrolyte, and (3) reaction rate constants of polysulfide reduction reactions at Li electrode on the electrochemical performance characteristics of Li/S cells including the cycling performance. The cell potential profiles predicted from numerical analysis were compared with experimental discharge and charge curves with emphasis on the possibility of the presence of the intermediate solid phase Li2S2(s). Numerical results suggest that the cell potential obtained without the consideration of the intermediate solid phase Li2S2(s) is in the best agreement with experimental results. Also, the polysulfide shuttling phenomenon has been numerically analyzed with electrolytes of different polysulfide solubility and compared with experimental cell discharge and charge curves. Our model clearly shows that the electrolyte of high polysulfide solubility and unprotected negative (anode) electrode can lead to unwanted reduction reactions of high-order polysulfides on the Li electrode, resulting in a significant overcharge problem. This agrees well with our experimental results for identical experimental conditions. Furthermore, the cycling performance of a Li/S cell was predicted, including the effect of the polysulfide shuttle on the subsequent discharge/charge curves. In the simulation, it was found that the high voltage plateau is reduced markedly in the discharge process after incomplete charge because sulfur (S8) could not be recovered when the polysulfide shuttle problem is significant. Also, it is suggested that cycling performance of Li/S cells would be improved by limiting both the polysulfide diffusivity and the reduction reaction rate of high-order polysulfides at Li electrode during charge.

The lithium-sulfur cell has shown great prospects for future energy conversion and storage systems due to the high theoretical specific capacity of sulfur, 1675 mAh g−1. However, it has been hindered by rapid capacity decay and low energy efficiency. In this work, polypyrrole (PPy)/TiO2 nanotubes with coaxial heterogeneous structure as the substrate of the cathode is prepared and used to improve the electrochemical performance of sulfur electrodes. TiO2 nanotubes decorated with PPy provide a highly ordered conductive framework for Li+ ion diffusion and reaction with sulfur. This architecture also is helpful for trapping the produced polysulfides, and as a result attenuates the capacity decay. Furthermore, the heat treatment temperature used in the sulfur loading process has been confirmed to have an important impact on the overall performance of the resultant cell. The as-designed S/PPy/TiO2 nanotube cathode using an elevated heating temperature shows excellent cycling stability with a high discharge capacity of 1150 mAh g−1 and average coulombic efficiency of 96% after 100 cycles.

Fully lithiated lithium sulfide (Li2S) has become a promising cathode material for Li/S cells due to its high theoretic capacity (1166 mA h g-1) and specific energy (2600 W h kg-1). However, low utilization of sulfur and poor rate capability still hinder the practical application of Li/S cells. In this paper, a carbon coated Li2S/graphene composite (Li2S/G@C) was developed by incorporating Li2S nano spheres with single-layered graphene and further forming a durable protective carbon layer on the surface of the Li2S particles using a facile CVD method. The high rate capability and remarkable cycle life of the Li2S/G@C cathode were demonstrated, which was mainly attributed to the unique structure of the Li2S/G@C that can significantly improve not only the electrical conductivity, but also the mechanical stability of the sulfur cathode.

We have investigated the chemical bonding interaction of S in a CTAB (cetyltrimethylammonium bromide, CH3(CH2)15N+(CH3)3Br-)-modified sulfur-graphene oxide (S-GO) nanocomposite used as the cathode material for Li/S cells by S K-edge X-ray absorption spectroscopy (XAS). The results show that the introduction of CTAB to the S-GO nanocomposite and changes in the synthesis recipe including alteration of the S precursor ratios and the sequence of mixing ingredients lead to the formation of different S species. CTAB modifies the cathode materials through bonding with Na2Sx in the precursor solution, which is subsequently converted to C-S bonds during the heat treatment at 155 °C. Moreover, GO bonds with CTAB and acts as the nucleation center for S precipitation. All these interactions among S, CTAB, and GO help to immobilize the sulfur in the cathode and may be responsible for the enhanced cell cycle life of CTAB-S-GO nanocomposite-based Li/S cells.

Lithium-ion batteries dominate the battery field, particularly for electric and hybrid vehicles. Monoclinic Li3V2(PO4)3 has emerged as one of the most promising candidates for the cathode in lithium-ion batteries, offering better environmental safety and lower cost than competing materials. We have used in situ X-ray absorption spectroscopy to characterize the evolution of the vanadium in a Li3V2(PO4)3 cathode as it is cycled electrochemically. These data demonstrate the presence of significant kinetic effects such that the measured electrochemical behavior does not represent the bulk vanadium. When the cell is cycled between 3 and 4.5 V, there are two distinct vanadium species. When the potential is raised above 4.5 V, a third species is observed, consistent with formation of V5+. XANES data for the cathode after 3-4.8 V cycling are consistent with a severely distorted vanadium site, suggesting that lithium-vanadium antisite mixing may be responsible for the electrochemical irreversibility that is seen above 4.5 V.

This study introduces a new flexible cathode that contains no binder, conductive additive and current collector, but instead consists solely of a sulfurized polyacrylonitrile nanofiber (SPAN) web which is prepared by a simple pyrolysis process with low cost raw materials. This not only exhibits good electrochemical properties, but also a high flexibility, rollability, and bendability to 180° without fracture. Its feasibility as a cathode for a low cost and flexible Na/S battery is subsequently evaluated on the basis that S, PAN, and Na are cheap materials. The SPAN web delivers a high first discharge capacity of 604 mAh g-1 - electrode (1473 mAh g-1 - sulfur) at 0.01 C based on sulfur content. In cycle performance at 0.1 C, a first discharge capacity of 342 mAh g-1 - electrode is obtained and remains over 266 mAh g-1 - electrode after 200 cycles along with the coulombic efficiency near 100% from the second cycle. In terms of rate capability, it is shown to be capable of delivering a capacity of as high as 71 mAh g-1 at 1 C. The reversible electrochemical reaction of the SPAN web with Na is related to a reversible bond between the C-S and S-S bonds of the SPAN web.

The X-ray absorption spectroscopy technique has been applied to study different stages of the lithium/sulfur (Li/S) cell life cycle. We have investigated how speciation of S in Li/S cathodes changes upon the introduction of CTAB (cetyltrimethylammonium bromide, CH₃(CH₂)15N⁺(CH₃)₃Br-) and with charge/discharge cycling. The introduction of CTAB changes the synthesis reaction pathway dramatically due to the interaction of CTAB with the terminal S atoms of the polysulfide ions in the Na₂Sx solution. For the cycled Li/S cell, the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur reaction products on the cathode surface during the charge/discharge processes make the capacity decay. A modified coin cell and a vacuum-compatible three-electrode electro-chemical cell have been introduced for further in-situ/in-operando studies.

Current lithium-ion batteries are predicted to be unable to provide the specific energy required to meet the ever-increasing demands of rapidly emerging technologies. Due to a high theoretical specific capacity of 1675 mAh/g, sulfur has gained much attention as a promising positive electrode material for high specific energy rechargeable batteries. Although the lithium/sulfur cell has been studied for many years and continues to receive much attention today as an alternative power source for zero-emission vehicles and advanced electronic devices, the realization of this novel cell's promise as a commercial product has yet to be successful. The major problems with sulfur electrodes involve: (1) the dissolution of sulfur (as polysulfides) and the resulting diffusion of dissolved polysulfides and (2) the deposition of insulating products (including Li2S) on both the negative and the positive electrodes. These solid deposits can physically block the electrode reaction sites, thus passivating the electrode surfaces. Another important problem is the large volume change that occurs with the conversion of S to Li2S. It is important to understand that the performance of Li/S cells is hampered by linked chemical and mechanical degradations and both degradation mechanisms must be correctly alleviated in order to markedly improve current-technology Li/S cells. In this study, improved cycling performance via the reactive functional groups on graphene oxide to successfully immobilize sulfur and lithium polysulfides during operation has been demonstrated. The use of a new electrolyte and binder leads to improved cell performance in terms of high-rate capability (up to at least 2 C) and good reversibility (S ↔ Li2S), yielding at least 800 cycles have also been demonstrated.

Adam Heller, Ernest Cockrell Sr. Chair in Engineering Emeritus of the John J. McKetta Department of Chemical Engineering at The University of Texas at Austin, recalls his childhood in the Holocaust and his contributions to science and technology that earned him the US National Medal of Technology and Innovation in a conversation with Elton J. Cairns, Professor of Chemical and Biomolecular Engineering at the University of California, Berkeley. Dr. Heller, born in 1933, describes the enslavement of his father by Hungarians in 1942; the confiscation of his family's home, business, and all its belongings in 1944; and his incarceration in a brick factory with 18,000 Jews who were shipped by the Hungarians to be gassed by Germans in Auschwitz. Dr. Heller and his immediate family survived the Holocaust and arrived in Israel in 1945. He studied under Ernst David Bergmann at the Hebrew University, and then worked at Bell Laboratories and GTE Laboratories, where he headed Bell Lab's Electronic Materials Research Department. At GTE Laboratories, he built in 1966 the first neodymium liquid lasers and in 1973 with Jim Auborn conceived and engineered the lithium thionyl chloride battery, one of the first to be manufactured lithium batteries, which is still in use. After joining the faculty of engineering of The University of Texas at Austin, he cofounded with his son Ephraim Heller TheraSense, now a major part of Abbott Diabetes Care, which produced a microcoulometer that made the monitoring of glucose painless by accurately measuring the blood glucose concentration in 300 nL of blood. He also describes the electrical wiring of enzymes, the basis for Abbott's state-of-the-art continuous glucose monitoring system. He discusses his perspective of reducing the risk of catastrophic global warming in a wealth-accumulating, more-energy-consuming world and provides advice for students entering careers in science or engineering.

In recent years, lithium/sulfur (Li/S) cells have attracted great attention as a candidate for the next generation of rechargeable batteries due to their high theoretical specific energy of 2600 W·h kg(-1), which is much higher than that of Li ion cells (400-600 W·h kg(-1)). However, problems of the S cathode such as highly soluble intermediate species (polysulfides Li2Sn, n = 4-8) and the insulating nature of S cause poor cycle life and low utilization of S, which prevents the practical use of Li/S cells. Here, a high-rate and long-life Li/S cell is proposed, which has a cathode material with a core-shell nanostructure comprising Li2S nanospheres with an embedded graphene oxide (GO) sheet as a core material and a conformal carbon layer as a shell. The conformal carbon coating is easily obtained by a unique CVD coating process using a lab-designed rotating furnace without any repetitive steps. The Li2S/GO@C cathode exhibits a high initial discharge capacity of 650 mA·h g(-1) of Li2S (corresponding to the 942 mA·h g(-1) of S) and very low capacity decay rate of only 0.046% per cycle with a high Coulombic efficiency of up to 99.7% for 1500 cycles when cycled at the 2 C discharge rate.

Alkaline fuel cell technology has been reinvigorated since the recent rapid development and deployment of anion exchange membranes. Without the "acid-stability" requirement in low pH environments such as that of proton exchange membrane fuel cells, a much wider range of materials including noble metals, non-noble transition metals, and even metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media have been developed due to both thermodynamic and kinetic reasons. As compared to the rapidly increasing number of reports on the development of novel catalyst materials, the understanding of the reaction mechanisms of the various ORR electrocatalysts is quite insufficient, and the application and investigation in real alkaline anion exchange membrane fuel cells (AAEMFCs) is even scarcer. By reviewing the compositions, preparation methods, physiochemical properties and ORR performance of different categories of cathodic electrocatalysts that have emerged in the past few years, some common and intrinsic properties and factors that account for the superior activity of these materials may be extracted and summarized, which may further help to identify the reasons for the kinetic facility of the ORR in alkaline media. Some practical issues of utilization of the promising novel replacement materials for the state-of-the-art Pt-based cathodic electrocatalysts in AAEMFCs are pointed out. In addition to the progress on the development of novel materials with outstanding ORR activity, many and varied compositions and morphologies in one, two and three dimensions, scalable preparation technologies, low cost, and other unique properties, some feedback on the performance and especially the problems of their use as cathodes in AAEMFCs is urgently needed. Such feedback should provide guidelines for the design and manufacture of next-generation electrocatalysts and accelerate the application of AAEMFCs.

Lithium/sulfur (Li/S) cells have a theoretical specific energy five times higher than that of lithium-ion (Li-ion) cells (2600 vs. ~500Whkg-1). The conventional Li/S cells that use an organic liquid electrolyte are short-lived with low coulombic efficiency due to the polysulfide shuttle. We herein design carbon-coated NanoLi2S (NanoLi2S@carbon) composites, which consist of Li2S nanoparticles as the core and a carbon coating as the shell. The carbon shell prevents the NanoLi2S core from directly contacting the liquid electrolyte, which improves the performance of Li/S cells to provide longer cycle life and high sulfur utilization. The cyclability of Li/S cells is further enhanced by mixing the core-shell NanoLi2S@carbon composites with graphene oxide, which chemically immobilizes polysulfides in the cathode through their functional groups. The resulting Li/S cell shows an initial specific discharge capacity of 1263mAhg-1 (normalized to sulfur) at the C/10 rate and a capacity retention of 65.4% after 200 cycles. The capacity decay mechanism during cycling is also characterized in detail using near edge X-ray absorption fine structure (NEXAFS) spectra. •Li2S@C core-shell nano particle active material developed for sulfur cathodes.•Improved cycle life of Li/S cells demonstrated with Li2S@C cathodes.•High utilization obtained with Li2S@C active material.

Lithium/sulfur (Li/S) cells have attracted much attention due to their higher theoretical specific capacity and energy compared to those of current lithium-ion cells. However, the application of Li/S cells is still hampered by short cycle life. Sulfur-graphene oxide (S-GO) nanocomposites have shown promise as cathode materials for long-life Li/S cells because oxygen-containing functional groups on the surface of graphene oxide were successfully used as sulfur immobilizers by forming weak bonds with sulfur and polysulfides. While S-GO showed much improved cycling performance, the capacity decay still needs to be improved for commercially viable cells. In this study, we attempt to understand the capacity fading mechanism based on an ex situ study of the structural and chemical evolution of S-GO nanocomposite cathodes with various numbers of cycles using scanning electron microscopy (SEM), near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). It is found that both the surface morphologies and chemical structures of the cathode materials change considerably with increasing number of cycles. These changes are attributed to several unexpected chemical reactions of lithium with S-GO nanocomposites occurring during the discharge-charge processes with the formation of Li2CO3, Li2SO3, Li2SO4, and COSO2Li species. These reactions result in the loss of recyclable active sulfur on the surface of the electrode, and thus capacity fades while coulombic efficiency is near 100%. Moreover, the reaction products accumulate on the cathode surface, forming a compact blocking insulating layer which may make the diffusion of Li ions into/out of the cathode difficult during the discharge-charge process and thus lead to lower utilization of sulfur at higher rates. We think that these two observations are significant contributors to the capacity and rate capability degradation of the Li/S-GO cells. Therefore, for the rechargeable Li/S-GO cells, we suggest that the content of oxygen-containing functional groups on GO should be optimized and more stable functional groups need to be identified for further improvement of the cycling performance. The information we gain from this study may provide general insights into the fundamental understanding of the degradation mechanisms of other rechargeable Li/S cells using similar oxygen-containing functional groups as sulfur immobilizers.

A newly formulated four-component modified Butler-Volmer model has been developed to evaluate global oxidation kinetic parameters for the various types of carbon monoxide adsorbates (COads) on a nanoparticle Pt surface determined by the type of bonding as well as the local structure of the adsorption site. Partial coverages of COads were prepared by potentiostatic adsorption of methanol followed by potentiostatic partial oxidation at various elevated potentials and for various durations. Anodic linear sweep voltammetry was then performed, and the COads oxidation peaks were fitted with the model to analyze the kinetics. According to the model, preferential oxidation with respect to COads bonding and Pt substrate structure can be achieved dependent upon the potential and extent of oxidation. Partial oxidation at 450 mV vs. RHE for 60 min. resulted in a majority population of linearly bonded COads on cubic-packed Pt sites; whereas partial oxidation at 650 mV vs. RHE for 220 sec. resulted in a majority population of bridged-bonded COads on close-packed Pt sites. © 2014 Elsevier Ltd.

Lithium-oxygen (Li-O2) cells are a promising power source, and carbons are an attractive non-metal catalyst for air electrodes. To improve the electrochemical performance, various carbons are heated in an inert atmosphere. It is found that heat treatment at 900 C can differently improve the electrochemical performance of multiwalled carbon nanotubes (CNTs), acetylene carbon black (AB) and activated carbon (AC), but the improvement of CNTs is the most obvious. After heat treatment, the peak current density of the oxygen reduction reaction (ORR) and the 1st discharge capacity of CNTs increase ∼30% and ∼125%, respectively, while the charge transfer reaction resistance and the Warburg diffusion resistance decrease ∼7.0% and ∼11.1%, respectively. AC has the highest charge capacities and capacity retention ratio in spite of little influence by heat treatment. The possible mechanism and reasons are analyzed using different techniques. Microstructure is superior to conductivity for enhancing the rechargeability and the cyclability, and heat treatment is effective for some carbon materials in improving the electrochemical performance of Li-O2 cells. © 2014 Elsevier Ltd.

Lithium sulfide (Li2S) is an attractive cathode material with a high theoretical specific capacity (1166 mAh g(-1)). However, the poor cycle life and rate capability have remained significant challenges, preventing its practical application. Here, Li2S spheres with size control have been synthesized for the first time, and a CVD method for converting them into stable carbon-coated Li2S core-shell (Li2S@C) particles has been successfully employed. These Li2S@C particles with protective and conductive carbon shells show promising specific capacities and cycling performance with a high initial discharge capacity of 972 mAh g(-1) Li2S (1394 mAh g(-1) S) at the 0.2C rate. Even with no added carbon, a very high Li2S content (88 wt % Li2S) electrode composed of 98 wt % 1 μm Li2S@C spheres and 2 wt % binder shows rather stable cycling performance, and little morphology change after 400 cycles at the 0.5C rate.

Lithium/sulfur (Li/S) cells are receiving significant attention as an alternative power source for zero-emission vehicles and advanced electronic devices due to the very high theoretical specific capacity (1675 mA·h/g) of the sulfur cathode. However, the poor cycle life and rate capability have remained a grand challenge, preventing the practical application of this attractive technology. Here, we report that a Li/S cell employing a cetyltrimethyl ammonium bromide (CTAB)-modified sulfur-graphene oxide (S-GO) nanocomposite cathode can be discharged at rates as high as 6C (1C = 1.675 A/g of sulfur) and charged at rates as high as 3C while still maintaining high specific capacity (~ 800 mA·h/g of sulfur at 6C), with a long cycle life exceeding 1500 cycles and an extremely low decay rate (0.039% per cycle), perhaps the best performance demonstrated so far for a Li/S cell. The initial estimated cell-level specific energy of our cell was ~ 500 W·h/kg, which is much higher than that of current Li-ion cells (~ 200 W·h/kg). Even after 1500 cycles, we demonstrate a very high specific capacity (~ 740 mA·h/g of sulfur), which corresponds to ~ 414 mA·h/g of electrode: still higher than state-of-the-art Li-ion cells. Moreover, these Li/S cells with lithium metal electrodes can be cycled with an excellent Coulombic efficiency of 96.3% after 1500 cycles, which was enabled by our new formulation of the ionic liquid-based electrolyte. The performance we demonstrate herein suggests that Li/S cells may already be suitable for high-power applications such as power tools. Li/S cells may now provide a substantial opportunity for the development of zero-emission vehicles with a driving range similar to that of gasoline vehicles.

Thin films of Cu2Sb, prepared on stainless steel and copper substrates with a pulsed laser deposition technique at room temperature, have been evaluated as electrodes in lithium cells. The electrodes operate by a lithium insertion/copper extrusion reaction mechanism, the reversibility of which is superior when copper substrates are used, particularly when electrochemical cycling is restricted to the voltage range 0.65-1.4 V vs. Li/Li+. The superior performance of Cu2Sb films on copper is attributed to the more active participation of the extruded copper in the functioning of the electrode. The continual and extensive extrusion of copper on cycling the cells leads to the isolation of Li3Sb particles and a consequent formation of Sb. Improved cycling stability of both types of electrodes was obtained when cells were cycled between 0.65 and 1.4 V. A low-capacity lithium-ion cell with Cu2Sb and LiNi0.8Co0.15Al0.05O2 electrodes, laminated from powders, shows excellent cycling stability over the voltage range 3.15 - 2.2 V, the potential difference corresponding to approximately 0.65-1.4 V for the Cu2Sb electrode vs. Li/Li+. Chemical self-discharge of lithiated Cu2Sb electrodes by reaction with the electrolyte was severe when cells were allowed to relax on open circuit after reaching a lower voltage limit of 0.1 V. The solid electrolyte interphase (SEI) layer formed on Cu2Sb electrodes after cells had been cycled between 1.4 and 0.65 V vs. Li/Li+ was characterized by Fourier-transform infrared spectroscopy; the SEI layer contributes to the large irreversible capacity loss on the initial cycle of these cells. The data contribute to a better understanding of the electrochemical behavior of intermetallic electrodes in rechargeable lithium batteries.

Mg2Si films, prepared by pulsed laser deposition (PLD), were amorphous, as prepared, and nanocrystalline following annealing. Their micro-structure and electrochemical characteristics were studied by high resolution transmission electron microscopy (HRTEM) and electrochemical cycling against lithium. HRTEM analysis revealed that some excess Si was present in the films. The more amorphous thinner film exhibited excellent cyclability. However, when the film becomes crystalline, the irreversible capacity loss was more significant during the initial cycling and after *50 cycles. Interpretations of the superior stability of the amorphous films are examined.

Room-temperature impedance measurements of a thin-film LiMn2O4/LiPF6-EC-DMC interface have been used to identify the spontaneous formation Li2Mn2O4 at the interface at room temperature at voltages of 3.7 and higher. The impedance of the LiMn2O4 films exhibited two time constants: at about 14 kHz and 60 to 200 Hz. The high frequency loop is dependent on film morphology and was attributed to the substrate/oxide interface. The low frequency behavior was dependent on both state-of-charge (SOC) and time at a given SOC. At full charge the impedance in this electrolyte was stable at room temperature over several days. At high lithium contents, film OCV and impedance tended to grow logarithmically with time, with lower rates for lower Mn3+ content in the film. The increased impedance was removed by oxidation of the film to 4.5 V vs. Li/Li+. The observations are consistent with a reversible disproportionation of part of the LiMn2O4 into Li2Mn2O4 and a lithium-deficient spinel. With extended constant current cycling part of the Li2Mn2O4 degrades to the Mn2O3 and the process is no longer reversible.

Room temperature impedance measurements of the LiMn2O4/LiPF6-EC-DMC interface have been used to identify a previously unreported step in the formation of the SEI layer on this cathode. The low frequency impedance and potential of pure dense LiMn2O4 films was found to depend logarithmically on time in the end-of-discharge (EOD) state. The rate of the impedance rise decreased with Mn3+ content. The increased impedance was removed by oxidation of the film to 4.5 V vs. Li/Li+. The observations are consistent with a reversible disproportionation of part of the LiMn2O4 into Li2Mn2O4 and l-Mn2O4. Analyses of cyclic voltammograms and impedance spectra at intervals during constant current cycling of the LiMn2O4 films suggest that Li2Mn2O4 on the surface also plays a major role in the capacity fade.

The impedance of pure dense Li1+yMn2-yO4 films in LiPF6-EC-DMC electrolyte exhibited two time constants: at about 14 kHz and 60 to 200 Hz. The high frequency loop is dependent on film morphology and was attributed to the substrate/oxide interface. The low frequency behavior was dependent on both state-of-charge (SOC) and time at a given SOC. At full charge the impedance in this electrolyte was stable at room temperature over several days. At high lithium contents, film OCV and impedance tended to grow logarithmically with time, with a rate three times higher at EOD than at a mid-SOC. This impedance rise was reversible by cycling the film between 4.5 and 3.7 V vs. Li/Li+. The observations are consistent with an ion exchange reaction between impurity protons in the electrolyte and Li+ in the oxide. A similar step involving the removal of Mn from the oxide has already been proposed in the literature.