Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes.

Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the battery's life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs.

High performance Li-ion sulfur batteries enabled by intercalation chemistry.

The unstable interface of lithium metal in high energy density Li sulfur (Li-S) batteries raises concerns of poor cycling, low efficiency and safety issues, which may be addressed by using intercalation types of anode. Herein, a new prototype of Li-ion sulfur battery with high performance has been demonstrated by coupling a graphite anode with a sulfur cathode (2 mA h cm(-2)) after successfully addressing the interface issue of graphite in an ether based electrolyte.

Energetics of defects on graphene through fluorination.

Functionalized graphene sheets (FGSs) comprise a unique member of the carbon family, demonstrating excellent electrical conductivity and mechanical strength. However, the detailed chemical composition of this material is still unclear. Herein, we take advantage of the fluorination process to semiquantitatively probe the defects and functional groups on graphene surface. Functionalized graphene sheets are used as substrate for low-temperature (<150 °C) direct fluorination. The fluorine content has been modified to investigate the formation mechanism of different functional groups such as C-F, CF2, O-CF2 and (C=O)F during fluorination. The detailed structure and chemical bonds are simulated by density functional theory (DFT) and quantified experimentally by nuclear magnetic resonance (NMR). The electrochemical properties of fluorinated graphene are also discussed extending the use of graphene from fundamental research to practical applications.

Manipulating surface reactions in lithium-sulphur batteries using hybrid anode structures.

Lithium-sulphur batteries have high theoretical energy density and potentially low cost, but significant challenges such as severe capacity degradation prevent its widespread adoption. Here we report a new design of lithium-sulphur battery using electrically connected graphite and lithium metal as a hybrid anode to control undesirable surface reactions on lithium. Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface layer to actively control the electrochemical reactions and minimize the deleterious side reactions, leading to significant performance improvements. Lithium-sulphur cells incorporating this hybrid anodes deliver capacities of >800 mAh g(-1) for 400 cycles at a high rate of 1,737 mA g(-1), with only 11% capacity fade and a Coulombic efficiency >99%. This simple hybrid concept may also provide scientific strategies for protecting metal anodes in other energy-storage devices.

Dendrite-free lithium deposition via self-healing electrostatic shield mechanism.

Rechargeable lithium metal batteries are considered the "Holy Grail" of energy storage systems. Unfortunately, uncontrollable dendritic lithium growth inherent in these batteries (upon repeated charge/discharge cycling) has prevented their practical application over the past 40 years. We show a novel mechanism that can fundamentally alter dendrite formation. At low concentrations, selected cations (such as cesium or rubidium ions) exhibit an effective reduction potential below the standard reduction potential of lithium ions. During lithium deposition, these additive cations form a positively charged electrostatic shield around the initial growth tip of the protuberances without reduction and deposition of the additives. This forces further deposition of lithium to adjacent regions of the anode and eliminates dendrite formation in lithium metal batteries. This strategy may also prevent dendrite growth in lithium-ion batteries as well as other metal batteries and transform the surface uniformity of coatings deposited in many general electrodeposition processes.

Enhanced Li+ ion transport in LiNi0.5Mn1.5O4 through control of site disorder.

High voltage spinel LiNi(0.5)Mn(1.5)O(4) is a very promising cathode material for lithium ion batteries that can be used to power hybrid electrical vehicles (HEVs). Through careful control of the cooling rate after high temperature calcination, LiNi(0.5)Mn(1.5)O(4) spinels with different disordered phase and/or Mn(3+) contents have been synthesized. It is revealed that during the slow cooling process (<3 °C min(-1)), oxygen deficiency is reduced by the oxygen intake, thus the residual Mn(3+) amount is also decreased in the spinel due to charge neutrality. In situ X-ray diffraction (XRD) demonstrates that the existence of a disordered phase fundamentally changes the spinel phase transition pathways during the electrochemical charge-discharge process. The presence of an appropriate amount of oxygen deficiency and/or Mn(3+) is critical to accelerate the Li(+) ion transport within the crystalline structure, which is beneficial to enhance the electrochemical performance of LiNi(0.5)Mn(1.5)O(4). LiNi(0.5)Mn(1.5)O(4) with an appropriate amount of disordered phase offers high rate capability (96 mAh g(-1) at 10 °C) and excellent cycling performance with 94.8% capacity retention after 300 cycles. The fundamental findings in this work can be widely applied to guide the synthesis of other mixed oxides or spinels as high performance electrode materials for lithium ion batteries.

Conductive rigid skeleton supported silicon as high-performance Li-ion battery anodes.

A cost-effective and scalable method is developed to prepare a core-shell structured Si/B(4)C composite with graphite coating with high efficiency, exceptional rate performance, and long-term stability. In this material, conductive B(4)C with a high Mohs hardness serves not only as micro/nano-millers in the ball-milling process to break down micron-sized Si but also as the conductive rigid skeleton to support the in situ formed sub-10 nm Si particles to alleviate the volume expansion during charge/discharge. The Si/B(4)C composite is coated with a few graphitic layers to further improve the conductivity and stability of the composite. The Si/B(4)C/graphite (SBG) composite anode shows excellent cyclability with a specific capacity of ∼822 mAh·g(-1) (based on the weight of the entire electrode, including binder and conductive carbon) and ∼94% capacity retention over 100 cycles at 0.3 C rate. This new structure has the potential to provide adequate storage capacity and stability for practical applications and a good opportunity for large-scale manufacturing using commercially available materials and technologies.

High-performance LiNi0.5Mn1.5O4 spinel controlled by Mn3+ concentration and site disorder.

The complex correlation between Mn(3+) ions and the disordered phase in the lattice structure of high voltage spinel, and its effect on the charge transport properties, are revealed through a combination of experimental study and computer simulations. Superior cycling stability is achieved in LiNi(0.45)Cr(0.05)Mn(1.5)O(4) with carefully controlled Mn(3+) concentration. At 250th cycle, capacity retention is 99.6% along with excellent rate capabilities.

Investigation of local environments in Nafion-SiO(2) composite membranes used in vanadium redox flow batteries.

Proton conducting polymer composite membranes are of technological interest in many energy devices such as fuel cells and redox flow batteries. In particular, polymer composite membranes, such as SiO(2) incorporated Nafion membranes, are recently reported as highly promising for the use in redox flow batteries. However, there is conflicting reports regarding the performance of this type of Nafion-SiO(2) composite membrane in the redox flow cell. This paper presents results of the analysis of the Nafion-SiO(2) composite membrane used in a vanadium redox flow battery by nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier Transform Infra Red (FTIR) spectroscopy, and ultraviolet-visible spectroscopy. The XPS study reveals the chemical identity and environment of vanadium cations accumulated at the surface. On the other hand, the (19)F and (29)Si NMR measurement explores the nature of the interaction between the silica particles, Nafion side chains and diffused vanadium cations. The (29)Si NMR shows that the silica particles interact via hydrogen bonds with the sulfonic groups of Nafion and the diffused vanadium cations. Based on these spectroscopic studies, the chemical environment of the silica particles inside the Nafion membrane and their interaction with diffusing vanadium cations during flow cell operations are discussed. This study discusses the origin of performance degradation of the Nafion-SiO(2) composite membrane materials in vanadium redox flow batteries.

Hierarchically porous graphene as a lithium-air battery electrode.

The lithium-air battery is one of the most promising technologies among various electrochemical energy storage systems. We demonstrate that a novel air electrode consisting of an unusual hierarchical arrangement of functionalized graphene sheets (with no catalyst) delivers an exceptionally high capacity of 15000 mAh/g in lithium-O(2) batteries which is the highest value ever reported in this field. This excellent performance is attributed to the unique bimodal porous structure of the electrode which consists of microporous channels facilitating rapid O(2) diffusion while the highly connected nanoscale pores provide a high density of reactive sites for Li-O(2) reactions. Further, we show that the defects and functional groups on graphene favor the formation of isolated nanosized Li(2)O(2) particles and help prevent air blocking in the air electrode. The hierarchically ordered porous structure in bulk graphene enables its practical applications by promoting accessibility to most graphene sheets in this structure.

Electrochemical performances of LiMnPO4 synthesized from non-stoichiometric Li/Mn ratio.

In this paper, the influences of the lithium content in the starting materials on the final performances of as-prepared Li(x)MnPO(4) (x hereafter represents the starting Li content in the synthesis step which does not necessarily mean that Li(x)MnPO(4) is a single phase solid solution in this work.) are systematically investigated. It has been revealed that Mn(2)P(2)O(7) is the main impurity when Li < 1.0 while Li(3)PO(4) begins to form once x > 1.0. The interactions between Mn(2)P(2)O(7) or Li(3)PO(4) impurities and LiMnPO(4) are studied in terms of the structural, electrochemical, and magnetic properties. At a slow rate of C/50, the reversible capacity of both Li(0.5)MnPO(4) and Li(0.8)MnPO(4) increases with cycling. This indicates a gradual activation of more sites to accommodate a reversible diffusion of Li(+) ions that may be related to the interaction between Mn(2)P(2)O(7) and LiMnPO(4) nanoparticles. Among all of the different compositions, Li(1.1)MnPO(4) exhibits the most stable cycling ability probably because of the existence of a trace amount of Li(3)PO(4) impurity that functions as a solid-state electrolyte on the surface. The magnetic properties and X-ray absorption spectroscopy (XAS) of the MnPO(4)·H(2)O precursor, pure and carbon-coated Li(x)MnPO(4) are also investigated to identify the key steps involved in preparing a high-performance LiMnPO(4).

LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for Li-ion battery cathode.

Electrochemically active LiMnPO(4) nanoplates have been synthesized via a novel, single-step, solid-state reaction in molten hydrocarbon. The olivine-structured LiMnPO(4) nanoplates with a thickness of approximately 50 nm appear porous and were formed as nanocrystals were assembled and grew into nanorods along the [010] direction in the (100) plane. After carbon coating, the prepared LiMnPO(4) cathode demonstrated a flat potential at 4.1 V versus Li with a specific capacity reaching as high as 168 mAh/g under a galvanostatic charging/discharging mode, along with an excellent cyclability.

Ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage.

Surfactant or polymer directed self-assembly has been widely investigated to prepare nanostructured metal oxides, semiconductors, and polymers, but this approach is mostly limited to two-phase materials, organic/inorganic hybrids, and nanoparticle or polymer-based nanocomposites. Self-assembled nanostructures from more complex, multiscale, and multiphase building blocks have been investigated with limited success. Here, we demonstrate a ternary self-assembly approach using graphene as fundamental building blocks to construct ordered metal oxide-graphene nanocomposites. A new class of layered nanocomposites is formed containing stable, ordered alternating layers of nanocrystalline metal oxides with graphene or graphene stacks. Alternatively, the graphene or graphene stacks can be incorporated into liquid-crystal-templated nanoporous structures to form high surface area, conductive networks. The self-assembly method can also be used to fabricate free-standing, flexible metal oxide-graphene nanocomposite films and electrodes. We have investigated the Li-ion insertion properties of the self-assembled electrodes for energy storage and show that the SnO2-graphene nanocomposite films can achieve near theoretical specific energy density without significant charge/discharge degradation.

Oriented nanostructures for energy conversion and storage.

Recently, the role of nanostructured materials in addressing the challenges in energy and natural resources has attracted wide attention. In particular, oriented nanostructures demonstrate promising properties for energy harvesting, conversion, and storage. In this Review, we highlight the synthesis and application of oriented nanostructures in a few key areas of energy technologies, namely photovoltaics, batteries, supercapacitors, and thermoelectrics. Although the applications differ from field to field, a common fundamental challenge is to improve the generation and transport of electrons and ions. We highlight the role of high surface area to maximize the surface activity and discuss the importance of optimum dimension and architecture, controlled pore channels, and alignment of the nanocrystalline phase to optimize the transport of electrons and ions. Finally, we discuss the challenges in attaining integrated architectures to achieve the desired performance. Brief background information is provided for the relevant technologies, but the emphasis is focused mainly on the nanoscale effects of mostly inorganic-based materials and devices.