Yu- Guo

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.

Binding SnO2 Nanocrystals in Nitrogen‐Doped Graphene Sheets as Anode Materials for Lithium‐Ion Batteries

Hybrid anode materials for Li-ion batteries are fabricated by binding SnO2 nanocrystals (NCs) in nitrogen-doped reduced graphene oxide (N-RGO) sheets by means of an in situ hydrazine monohydrate vapor reduction method. The SnO2NCs in the obtained SnO2NC@N-RGO hybrid material exhibit exceptionally high specific capacity and high rate capability. Bonds formed between graphene and SnO2 nanocrystals limit the aggregation of in situ formed Sn nanoparticles, leading to a stable hybrid anode material with long cycle life.

Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery.

Well-defined Li(4)Ti(5)O(12) nanosheets terminated with rutile-TiO(2) at the edges were synthesized by a facile solution-based method and revealed directly at atomic resolution by an advanced spherical aberration imaging technique. The rutile-TiO(2) terminated Li(4)Ti(5)O(12) nanosheets show much improved rate capability and specific capacity compared with pure Li(4)Ti(5)O(12) nanosheets when used as anode materials for lithium ion batteries. The results here give clear evidence of the utility of rutile-TiO(2) as a carbon-free coating layer to improve the kinetics of Li(4)Ti(5)O(12) toward fast lithium insertion/extraction. The carbon-free nanocoating of rutile-TiO(2) is highly effective in improving the electrochemical properties of Li(4)Ti(5)O(12), promising advanced batteries with high volumetric energy density, high surface stability, and long cycle life compared with the commonly used carbon nanocoating in electrode materials.

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.

Mono dispersed SnO2 nanoparticles on both sides of single layer graphene sheets as anode materials in Li-ion batteries

A two step programmed method is developed to load mono dispersed SnO2 nanoparticles onto single layer graphene sheets. The SnO2-G composite has near mono dispersion of the SnO2 nanocrystals as well as a high SnO2 content of over 60 wt%. These outstanding features are desirable and enable the composite material to be an excellent anode material for Li-ion batteries.

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.

Synthesis of CuO/graphene nanocomposite as a high-performance anode material for lithium-ion batteries

An optimized nanostructure design for electrode materials for high-performance lithium-ion batteries was realized by introducing three-dimensional (3D) graphene networks into transition metal oxide nanomicrostructures. A CuO/graphene composite was selected as a typical example of the optimized design. Self-assembled CuO and CuO/graphene urchin-like structures have been successfully synthesized by a simple solution method and investigated with SEM, TEM, XRD, and electrochemical measurements. The CuO/graphene nanocomposite exhibits a remarkably enhanced cycling performance and rate performance compared with pure CuO urchin-like structure when being used as anode materials in lithium-ion batteries. During all the 100 discharge-charge cycles under a current density of 65 mA g−1, the CuO/graphene electrode can stably deliver a reversible capacity of ca. 600 mA h g−1. At a high current density of 6400 mA g−1, the specific charge capacity of the CuO/graphene nanocomposite is still as high as 150 mA h g−1, which is three times larger than that of graphene (48 mA h g−1), while that of CuO is nearly null under the same current density. The enhancement of the electrochemical performance could be attributed to the 3D electrically conductive networks of graphene as well as the unique nanomicrostructure of the CuO/graphene nanocomposite in which the CuO nanomicroflowers are enwrapped by a thin layer of graphene as an elastic buffer.