Sen Xin

Lithium–Sulfur Batteries: Electrochemistry, Materials, and Prospects

With the increasing demand for efficient and economic energy storage, Li-S batteries have become attractive candidates for the next-generation high-energy rechargeable Li batteries because of their high theoretical energy density and cost effectiveness. Starting from a brief history of Li-S batteries, this Review introduces the electrochemistry of Li-S batteries, and discusses issues resulting from the electrochemistry, such as the electroactivity and the polysulfide dissolution. To address these critical issues, recent advances in Li-S batteries are summarized, including the S cathode, Li anode, electrolyte, and new designs of Li-S batteries with a metallic Li-free anode. Constructing S molecules confined in the conductive microporous carbon materials to improve the cyclability of Li-S batteries serves as a prospective strategy for the industry in the future.

Smaller Sulfur Molecules Promise Better Lithium-Sulfur Batteries

The lithium-sulfur battery holds a high theoretical energy density, 4-5 times that of todays lithium-ion batteries, yet its applications have been hindered by poor electronic conductivity of the sulfur cathode and, most importantly, the rapid fading of its capacity due to the formation of soluble polysulfide intermediates (Li(2)S(n), n = 4-8). Despite numerous efforts concerning this issue, combatting sulfur loss remains one of the greatest challenges. Here we show that this problem can be effectively diminished by controlling the sulfur as smaller allotropes. Metastable small sulfur molecules of S(2-4) were synthesized in the confined space of a conductive microporous carbon matrix. The confined S(2-4) as a new cathode material can totally avoid the unfavorable transition between the commonly used large S(8) and S(4)(2-). Li-S batteries based on this concept exhibit unprecedented electrochemical behavior with high specific capacity, good cycling stability, and superior rate capability, which promise a practicable battery with high energy density for applications in portable electronics, electric vehicles, and large-scale energy storage systems.

Nanocarbon Networks for Advanced Rechargeable Lithium Batteries

Carbon is one of the essential elements in energy storage. In rechargeable lithium batteries, researchers have considered many types of nanostructured carbons, such as carbon nanoparticles, carbon nanotubes, graphene, and nanoporous carbon, as anode materials and, especially, as key components for building advanced composite electrode materials. Nanocarbons can form efficient three-dimensional conducting networks that improve the performance of electrode materials suffering from the limited kinetics of lithium storage. Although the porous structure guarantees a fast migration of Li ions, the nanocarbon network can serve as an effective matrix for dispersing the active materials to prevent them from agglomerating. The nanocarbon network also affords an efficient electron pathway to provide better electrical contacts. Because of their structural stability and flexibility, nanocarbon networks can alleviate the stress and volume changes that occur in active materials during the Li insertion/extraction process. Through the elegant design of hierarchical electrode materials with nanocarbon networks, researchers can improve both the kinetic performance and the structural stability of the electrode material, which leads to optimal battery capacity, cycling stability, and rate capability. This Account summarizes recent progress in the structural design, chemical synthesis, and characterization of the electrochemical properties of nanocarbon networks for Li-ion batteries. In such systems, storage occurs primarily in the non-carbon components, while carbon acts as the conductor and as the structural buffer. We emphasize representative nanocarbon networks including those that use carbon nanotubes and graphene. We discuss the role of carbon in enhancing the performance of various electrode materials in areas such as Li storage, Li ion and electron transport, and structural stability during cycling. We especially highlight the use of graphene to construct the carbon conducting network for alloy anodes, such as Si and Ge, to accelerate electron transport, alleviate volume change, and prevent the agglomeration of active nanoparticles. Finally, we describe the power of nanocarbon networks for the next generation rechargeable lithium batteries, including Li-S, Li-O(2), and Li-organic batteries, and provide insights into the design of ideal nanocarbon networks for these devices. In addition, we address the ways in which nanocarbon networks can expand the applications of rechargeable lithium batteries into the emerging fields of stationary energy storage and transportation.

Improving the Electrode Performance of Ge through Ge@C Core–Shell Nanoparticles and Graphene Networks

Germanium is a promising high-capacity anode material for lithium ion batteries, but it usually exhibits poor cycling stability because of its huge volume variation during the lithium uptake and release process. A double protection strategy to improve the electrode performance of Ge through the use of Ge@C core-shell nanostructures and reduced graphene oxide (RGO) networks has been developed. The as-synthesized Ge@C/RGO nanocomposite showed excellent cycling performance and rate capability in comparison with Ge@C nanoparticles when used as an anode material for Li ion batteries, which can be attributed to the electronically conductive and elastic RGO networks in addition to the carbon shells and small particle sizes of Ge. The strategy is simple yet very effective, and because of its versatility, it may be extended to other high-capacity electrode materials with large volume variations and low electrical conductivities.

Cu-Si nanocable arrays as high-rate anode materials for lithium-ion batteries.

There is a surge in developing rechargeable lithium-ion batteries (LIBs) with higher energy densities and higher rate performance for application in powering future advanced communications equipment and electric vehicles (EVs). [ 1–6 ] The development of the electrode materials is essential for the improvement of the electrochemical properties of LIBs. [ 7–10 ] Among various anode materials tested for LIBs, Si has triggered signifi cant research effort because of its low Li-uptake potential and the high theoretical capacity (4200 mA h g − 1 ). [ 6 , 11–19 ] However, the main disadvantage that restricts the application of Si is the large volume changes of Si during Li + insertion and extraction, which results in a pulverization of the Si particles, a peeling off the current connection network, and, consequently, a rapid capacity decline upon cycling. [ 11–17 ] To overcome this issue, Si nanostructures, such as Si nanowires and nanotubes, have been fabricated. [ 6 , 11 , 18–23 ] The procedures for the fabrication of the Si nanostructures have also been well developed. [ 24–26 ] These nanostructures can provide spaces to accommodate the large volume variation during charge and discharge processes and thus allow for facile strain relaxation, which prevents pulverization upon lithium insertion. [ 11–19 , 27 ] The cycle stability of the Si anode has been signifi cantly improved by using these nanostructures. [ 11–17 , 27 ] Nevertheless, the rate capability of these materials highly needed for EVs is still not satisfying. This is possibly due to the lack of favorable electronic conductivity and the continuous growth of the unstable solid electrolyte interphase (SEI) at the Si/electrolyte interface upon cycling. Therefore, a new design for the structure of the Si anode is in high demand to achieve both longer cycling life and higher rate capability. Our previous work suggested that the application of nanocable structures in LIBs electrodes can signifi cantly improve the batteries’ electrochemical performance, especially the high

Carbon Nanofibers Decorated with Molybdenum Disulfide Nanosheets: Synergistic Lithium Storage and Enhanced Electrochemical Performance

Traditional lithium-ion batteries that are based on layered Li intercalation electrode materials are limited by the intrinsically low theoretical capacities of both electrodes and cannot meet the increasing demand for energy. A facile route for the synthesis of a new type of composite nanofibers, namely carbon nanofibers decorated with molybdenum disulfide sheets (CNFs@MoS2), is now reported. A synergistic effect was observed for the two-component anode, triggering new electrochemical processes for lithium storage, with a persistent oxidation from Mo (or MoS2) to MoS3 in the repeated charge processes, leading to an ascending capacity upon cycling. The composite exhibits unprecedented electrochemical behavior with high specific capacity, good cycling stability, and superior high-rate capability, suggesting its potential application in high-energy lithium-ion batteries.

An Advanced Selenium–Carbon Cathode for Rechargeable Lithium–Selenium Batteries†

The rapidly developing market for mobile electronics and hybrid electric vehicles (HEVs) has prompted the urgent need for batteries with high energy density, long cycle life, high efficiency, and low cost. Recently, rechargeable lithium-sulfur (Li–S) batteries have attracted considerable attention because of their high theoretical gravimetric (volumetric) energy density of 2570 Wh kg 1 (2200 Whl ), and low cost. However, the use of S as cathode material for Li–S batteries suffers from two major issues. One is the insulating nature of S, which results in low active-material utilization and limited rate capability. The other is the formation of electrolytesoluble polysulfides; these polysulfide intermediates, which are generated in the discharge/charge process, dissolve in the electrolyte and migrate to the Li anode, a process known as the shuttle effect. Consequently, the S cathode suffers a significant loss of S during cycling, resulting in a rapid capacity decrease. Many strategies have been used to address these problems, such as the impregnation of S into various conductive porous matrixes, surface coating of S, and the use of suitable electrolytes and additives. Although remarkable improvements have been achieved, the application of Li–S batteries is still hindered by the intrinsic drawbacks of S. Therefore, it is of great importance to explore and develop new high-energy cathode materials with improved electronic conductivity and cycling stability, to cover the shortfalls of S and provide alternative choices for practical applications. From this perspective, selenium, an element belonging to the same group in the periodic table as sulfur, is a prospective candidate for cathode materials. Although Se has a lower theoretical gravimetric capacity (675 mAhg ) than S (1675 mAh g ), its higher density (ca. 2.5 times that of S) offsets the deficiency and provides a high theoretical volumetric capacity density (3253 mAh cm ), comparable to that of S (3467 mAh cm ). It has been reported that Li–Se batteries deliver a high output voltage, so Li–Se batteries are also expected to have a high volumetric energy density. It is known that for applications in portable devices and HEVs, volumetric energy density is more important than gravimetric energy density because of the limited battery packing space. Moreover, the electronic conductivity of Se (1 10 3 Sm ) is considerably higher than that of S (5 10 28 Sm ), which suggests that Se could have higher utilization rate, better electrochemical activity, and faster electrochemical reaction with Li. Therefore, the advantages of Se promise an attractive alternative cathode material for building high-energy batteries for specific applications, including consumer electronics and transportation. However, at present, research on Li–Se batteries is still at a very early stage. Recently, Abouimrane et al. conducted pioneering work on the use of Se as a cathode material. The results show that, even bulk Se has an active material utilization of ca. 45% upon cycling, which is not commonly observed in Li–S batteries with a bulk S cathode. This suggests that Se cathode has a much better activity and a weaker shuttle effect than S. Nevertheless, bulk Se cannot completely deliver the theoretical capacity. Moreover, given the weak interaction between bulk Se and the conductive substrate, the polyselenide species generated during the Li uptake/release process cannot be effectively restrained on the cathode side. Thus, the shuttle effect of Se is not eliminated, which deteriorates the cycling performance of the Se cathode. To address these issues, encapsulation of Se molecules into a conductive porous carbon matrix may greatly improve the electrochemical performance of Se. However, this assumption has not yet been demonstrated, and the mechanism of the electrochemical reaction between Se molecules and Li remains unclear to date. Herein, we report a Se composite cathode material, in which Se is confined as cyclic Se8 molecules in the mesopores of an ordered mesoporous carbon (CMK-3) matrix. When assembled into Li–Se batteries with the water-soluble binder sodium alginate (SA), the Se/CMK-3 composite exhibits novel electrochemical behavior with a single plateau in the discharge/charge process. Data from ex situ Raman and X-ray diffraction (XRD) analysis suggest that this behavior is due to the conversion of cyclic Se8 molecules into chain-like Sen molecules in the carbon channels. Given the high electrochemical activity of the chain-like Sen molecules and the strong interaction between them and the carbon mesopores, this Se cathode shows a high capacity that approaches the theoretical value of Se, and exhibits favorable capacity retention upon cycling. The Se/CMK-3 composite was synthesized through a facile melt-diffusion process from a ball-milled mixture of [*] C.-P. Yang, S. Xin, Dr. Y.-X. Yin, H. Ye, J. Zhang, Prof. Y.-G. Guo CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190 (P. R. China) E-mail: ygguo@iccas.ac.cn

Ionothermal synthesis of sulfur-doped porous carbons hybridized with graphene as superior anode materials for lithium-ion batteries.

Sulfur-doped porous carbons hybridized with graphene (SPC@G) have been synthesized via a simple ionothermal method. The obtained SPC@G nanocomposite exhibits both high capacity and excellent rate performance, making it a promising anode material for lithium-ion batteries.

Plating a Dendrite-Free Lithium Anode with a Polymer/Ceramic/Polymer Sandwich Electrolyte

A cross-linked polymer containing pendant molecules attached to the polymer framework is shown to form flexible and low-cost membranes, to be a solid Li(+) electrolyte up to 270 °C, much higher than those based on poly(ethylene oxide), to be wetted by a metallic lithium anode, and to be not decomposed by the metallic anode if the anions of the salt are blocked by a ceramic electrolyte in a polymer/ceramic membrane/polymer sandwich electrolyte (PCPSE). In this sandwich architecture, the double-layer electric field at the Li/polymer interface is reduced due to the blocked salt anion transfer. The polymer layer adheres/wets the lithium metal surface and makes the Li-ion flux at the interface more homogeneous. This structure integrates the advantages of the ceramic and polymer. With the PCPSE, all-solid-state Li/LiFePO4 cells showed a notably high Coulombic efficiency of 99.8-100% over 640 cycles.

Superior radical polymer cathode material with a two-electron process redox reaction promoted by graphene

Poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) displays a two–electron process redox reaction, high capacity of up to 222 mA h g−1, good rate performance and long cycle life, which is promoted by graphene as cathode material for lithium rechargeable batteries.