Ya-Xia Yin

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.

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.

High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries

High-quality Prussian blue crystals with a small number of vacancies and a low water content are obtained by employing Na4Fe(CN)6 as the single iron-source precursor. The high-quality Prussian blue shows high specific capacity and remarkable cycling stability as the cathode material for Na-ion batteries because of its excellent ion storage capability and impressive structure stability.

Accommodating lithium into 3D current collectors with a submicron skeleton towards long-life lithium metal anodes

Lithium metal is one of the most attractive anode materials for electrochemical energy storage. However, the growth of Li dendrites during electrochemical deposition, which leads to a low Coulombic efficiency and safety concerns, has long hindered the application of rechargeable Li-metal batteries. Here we show that a 3D current collector with a submicron skeleton and high electroactive surface area can significantly improve the electrochemical deposition behaviour of Li. Li anode is accommodated in the 3D structure without uncontrollable Li dendrites. With the growth of Li dendrites being effectively suppressed, the Li anode in the 3D current collector can run for 600 h without short circuit and exhibits low voltage hysteresis. The exceptional electrochemical performance of the Li-metal anode in the 3D current collector highlights the importance of rational design of current collectors and reveals a new avenue for developing Li anodes with a long lifespan.

An Artificial Solid Electrolyte Interphase Layer for Stable Lithium Metal Anodes

A Li3PO4 solid electrolyte interphase (SEI) layer is demonstrated to be stable in the organic electrolyte, even during the Li deposition/dissolution process. Thus, the Li-conducting Li3PO4 SEI layer with a high Youngs modulus can effectively reduce side reactions between Li metal and the electrolyte and can restrain Li dendrite growth in lithium-metal batteries during cycling.

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.

Sulfur Encapsulated in Graphitic Carbon Nanocages for High-Rate and Long-Cycle Lithium–Sulfur Batteries

Hybrid sp2 carbon with a graphene backbone and graphitic carbon nanocages (G-GCNs) is demonstrated as an ideal host for sulfur in Li-S batteries, because it serves as highly efficient electrochemical nanoreactors as well as polysulfides reservoirs. The as-obtained S/(G-GCNs) with high S content exhibits superior high-rate capability (765 mA h g-1 at 5 C) and long-cycle life over 1000 cycles.

Insight into the effect of boron doping on sulfur/carbon cathode in lithium-sulfur batteries.

To exploit the high energy density of lithium-sulfur batteries, porous carbon materials have been widely used as the host materials of the S cathode. Current studies about carbon hosts are more frequently focused on the design of carbon structures rather than modification of its properties. In this study, we use boron-doped porous carbon materials as the host material of the S cathode to get an insightful investigation of the effect of B dopant on the S/C cathode. Powder electronic conductivity shows that the B-doped carbon materials exhibit higher conductivity than the pure analogous porous carbon. Moreover, by X-ray photoelectron spectroscopy, we prove that doping with B leads to a positively polarized surface of carbon substrates and allows chemisorption of S and its polysulfides. Thus, the B-doped carbons can ensure a more stable S/C cathode with satisfactory conductivity, which is demonstrated by the electrochemical performance evaluation. The S/B-doped carbon cathode was found to deliver much higher initial capacity (1300 mA h g(-1) at 0.25 C), improved cyclic stability, and rate capability when compared with the cathode based on pure porous carbon. Electrochemical impedance spectra also indicate the low resistance of the S/B-doped C cathode and the chemisorption of polysulfide anions because of the presence of B. These features of B doping can play the positive role in the electrochemical performance of S cathodes and help to build better Li-S batteries.