As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

Lithium-ion batteries have witnessed meteoric advancement the last two decades. The anode area has seen unprecedented research activity on Si and Sn, the two anode alternatives to currently used carbon following the initial seminal work by Fuji on tin oxide nanocomposites. Recent reports on silicon nanowires, porous Si, and amorphous Si coatings on graphite nanofibers (GNF) have been very encouraging. High capacity and long cycle life anodes are still, however, elusive and much needed to meet the ever increasing energy storage demands of modern society. Herein, we report for the first time the synthesis of novel 1D heterostructures comprising vertically aligned multiwall CNTs (VACNTs) containing nanoscale amorphous/nanocrystalline Si droplets deposited directly on VACNTs with clearly defined spacing using a simple two-step liquid injection CVD process. A hallmark of these single reactor derived heterostructures is an interfacial amorphous carbon layer anchoring the nanoscale Si clusters directly to the VACNTs. The defined spacing of nanoscale Si combined with their tethered CNT architecture allow for the silicon to undergo reversible electrochemical alloying and dealloying with Li with minimal loss of contact with the underlying CNTs. The novel heterostructures thus exhibit impressive reversible stable capacities approximately 2050 mAh/g with very good rate capability and an acceptable first cycle irreversible loss approximately 20% comparable to graphitic anodes indicating their promise as high capacity Li-ion anodes. Although warranting further research, particularly with regard to long-term cycling, it can be envisaged that optimization of this simple approach could lead to reversible high capacity next generation Li-ion anodes.

http://ma.ecsdl.org/content/MA2010-01/5/352.full.pdf

Nanostructured Hybrid Silicon/Carbon Nanotube Heterostructures: Novel Reversible High-Capacity Lithium-Ion Anodes

The ever-increasing demands for batteries with high energy densities to power the portable electronics with increased power consumption and to advance vehicle electrification and grid energy storage have propelled lithium battery technology to a position of tremendous importance. Carbon nanotubes (CNTs) and graphene, known with many appealing properties, are investigated intensely for improving the performance of lithium-ion (Li-ion) and lithium-sulfur (Li-S) batteries. However, a general and objective understanding of their actual role in Li-ion and Li-S batteries is lacking. It is recognized that CNTs and graphene are not appropriate active lithium storage materials, but are more like a regulator: they do not electrochemically react with lithium ions and electrons, but serve to regulate the lithium storage behavior of a specific electroactive material and increase the range of applications of a lithium battery. First, metrics for the evaluation of lithium batteries are discussed, based on which the regulating role of CNTs and graphene in Li-ion and Li-S batteries is comprehensively considered from fundamental electrochemical reactions to electrode structure and integral cell design. Finally, perspectives on how CNTs and graphene can further contribute to the development of lithium batteries are presented.

The Regulating Role of Carbon Nanotubes and Graphene in Lithium‐Ion and Lithium–Sulfur Batteries

Lithium, the lightest and most electronegative metallic element, has long been considered the ultimate choice as a battery anode for mobile, as well as in some stationary applications. The high electronegativity of Li is, however, a double-edged sword—it facilitates a large operating voltage when paired with essentially any cathode, promising a high cell-level energy density. It is also synonymous with a high chemical reactivity and low reduction potential. The interfaces a Li metal anode forms with any other material (liquid or solid) in an electrochemical cell are therefore always mediated by one or more products of its chemical or electrochemical reactions with that material. The physical, crystallographic, mechanical, electrochemical, and transport properties of the resultant new material phases (interphases) regulate all interfacial processes at a Li metal anode, including electrodeposition during battery recharge. This Review takes recent efforts aimed at manipulating the structure, composition, and physical properties of the solid electrolyte interphase (SEI) formed on an Li anode as a point of departure to discuss the structural, electrokinetic, and electrochemical requirements for achieving high anode reversibility. An important conclusion is that while recent reports showing significant advances in the achievement of highly reversible Li anodes, e.g. as measured by the coulombic efficiency (CE), raise prospects for as significant progress towards commercially relevant Li metal batteries, the plateauing of achievable CE values to around 99 ± 0.5% apparent from a comprehensive analysis of the literature is problematic because CE values of at least 99.7%, and preferably >99.9% are required for Li metal cells to live up to the potential for higher energy density batteries offered by the Li metal anode. On this basis, we discuss promising approaches for creating purpose-built interphases on Li, as well as for fabricating advanced Li electrode architectures for regulating Li electrodeposition morphology and crystallinity. Considering the large number of physical and chemical factors involved in achieving fine control of Li electrodeposition, we believe that achievement of the remaining ∼0.5% in anode reversibility will require fresh approaches, perhaps borrowed from other fields. We offer perspectives on both current and new strategies for achieving such Li anodes with the specific aim of engaging established contributors and newcomers to the field in the search for scalable solutions.

Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries

Understanding and suppression strategies toward stable Li metal anode for safe lithium batteries

Unique 3D nanoporous/macroporous structure Cu current collector for dendrite-free lithium deposition

The development of silicon-based anode materials is important for improving the energy density of current lithium ion batteries. However, there are still strong demands for these materials with better cycle stability and higher reversible capacity. Here, a kind of dual bond restricted MXene-Si-CNT composite anode materials with enhanced electrochemical performance is reported. These dual bonds have been clearly revealed by an X-ray photoelectron spectroscopy technique and also proven by theoretical calculations with spontaneous reaction energy values (-0.190 and -0.429 eV/atom for Ti-Si and C-Si bonds, respectively). The cycle stability of the composites, prepared by a facile ball-milling synthetic method, can obviously be improved because of the existence of these dual bonds and the multidimensional constructed architecture. The MXene-Si-CNT composite with 60 wt % silicon possesses the best overall performance, with ∼80% capacity retention after 200 cycles, and achieves 841 mAh g-1 at 2 A g-1. This approach demonstrates a promising strategy to exploit high-performance anode materials and lessens the immanent negative effect of silicon-based materials. Furthermore, it is significant to extend this method to other anode materials with serious volumetric change problems during the cycling process.

Dual Bond Enhanced Multidimensional Constructed Composite Silicon Anode for High-Performance Lithium Ion Batteries.

Despite the fact that the silicon-based anode has attracted immense attention with extremely high theoretical capacity, practical applications have been impeded by severe capacity fading during cycling processes and high preparation cost. In this work, a mass-produced and low-cost hierarchical mesoporous/macroporous silicon-based composite material with an ample porous structure and dual carbon protective layers has been rationally designed and constructed. Through adjusting the phase ratio of intermediate products (Mg2Si and MgO), the traditional magnesiothermic reduction method based on the low cost silicon source of diatomaceous earth (DE) has been precisely optimized to fabricate a controlled mesoporous structure on the original macroporous structure of DE. Furthermore, dual carbon protective layers on the hierarchical mesoporous/macroporous structure silicon-based composite material have also been constructed using the vacuum adsorption technique, showing that both the porous channel and the composite material surface are wrapped with carbon. Electrochemical performance tests show that both the controlled mesoporous/macroporous structure and dual carbon protective layers have enhanced the cycle stability of the Si/SiO2@C composite anode material for lithium-ion batteries. The capacity retention of the hierarchical mesoporous/macroporous Si/SiO2@C composite material with 13% carbon can reach 99.5% after 200 electrochemical cycles, and the reversible capacity can reach 534.3 mA h g−1 even at 500 mA g−1. This paper not only provides a low-cost and high electrochemical property silicon-based composite anode material for lithium-ion batteries, which possesses important significance in both academic and industrial worlds, but also opens up a way on how to design the hierarchical mesoporous/macroporous structure with precision control on the phase ratio.

Designing of hierarchical mesoporous/macroporous silicon-based composite anode material for low-cost high-performance lithium-ion batteries

Amorphous nanosized silicon with hierarchically porous structure for high-performance lithium ion batteries

A phase structure proportionally and gradiently changed lithium-enriched layered oxide material and a preparation method thereof belongs to the field of lithium ion battery positive electrode materials and electrochemistry. The phase structure proportionally and gradiently changed lithium-enriched layered oxide material is prepared through a precipitation-solid-phase synthesis method. The phase structure proportionally and gradiently changed lithium-enriched layered oxide material is spherical particles; from the center to the surface of the particles, the number of monoclinic Li2MnO3 structural units decrease gradually, and the number of rhombic LiTMO2 structural units increases gradually. Through proportional control over the structural units of monoclinic Li2MnO3 and rhombic LiTMO2 fromthe center to the surface of the particles, properties such as the cycling stability, the specific discharge capacity and safety of the lithium-enriched layered oxide material in lithium ion batteries can be regulated. The preparation method of the phase structure proportionally and gradiently changed lithium-enriched layered oxide material is simple and easy to control, low in costs of raw materials, environmentally friendly, applicable of large-scale industrialization and capable of achieving a good application prospect.

Lin Wang

Papers

Unique 3D nanoporous/macroporous structure Cu current collector for dendrite-free lithium deposition

Dual Bond Enhanced Multidimensional Constructed Composite Silicon Anode for High-Performance Lithium Ion Batteries.

Designing of hierarchical mesoporous/macroporous silicon-based composite anode material for low-cost high-performance lithium-ion batteries

Amorphous nanosized silicon with hierarchically porous structure for high-performance lithium ion batteries

Phase structure proportionally and gradiently changed lithium-enriched layered oxide material and preparation method thereof