There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Li-ion batteries have transformed portable electronics and will play a key role in the electrification of transport. However, the highest energy storage possible for Li-ion batteries is insufficient for the long-term needs of society, for example, extended-range electric vehicles. To go beyond the horizon of Li-ion batteries is a formidable challenge; there are few options. Here we consider two: Li-air (O(2)) and Li-S. The energy that can be stored in Li-air (based on aqueous or non-aqueous electrolytes) and Li-S cells is compared with Li-ion; the operation of the cells is discussed, as are the significant hurdles that will have to be overcome if such batteries are to succeed. Fundamental scientific advances in understanding the reactions occurring in the cells as well as new materials are key to overcoming these obstacles. The potential benefits of Li-air and Li-S justify the continued research effort that will be needed.

Li-O2 and Li-S batteries with high energy storage.

2.1. Solvents 4307 2.1.1. Propylene Carbonate (PC) 4308 2.1.2. Ethers 4308 2.1.3. Ethylene Carbonate (EC) 4309 2.1.4. Linear Dialkyl Carbonates 4310 2.2. Lithium Salts 4310 2.2.1. Lithium Perchlorate (LiClO4) 4311 2.2.2. Lithium Hexafluoroarsenate (LiAsF6) 4312 2.2.3. Lithium Tetrafluoroborate (LiBF4) 4312 2.2.4. Lithium Trifluoromethanesulfonate (LiTf) 4312 2.2.5. Lithium Bis(trifluoromethanesulfonyl)imide (LiIm) and Its Derivatives 4313

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Nonaqueous liquid electrolytes for lithium-based rechargeable batteries.

In the previous paper Ralph Brodd and Martin Winter described the different kinds of batteries and fuel cells. In this paper I will describe lithium batteries in more detail, building an overall foundation for the papers that follow which describe specific components in some depth and usually with an emphasis on the materials behavior. The lithium battery industry is undergoing rapid expansion, now representing the largest segment of the portable battery industry and dominating the computer, cell phone, and camera power source industry. However, the present secondary batteries use expensive components, which are not in sufficient supply to allow the industry to grow at the same rate in the next decade. Moreover, the safety of the system is questionable for the large-scale batteries needed for hybrid electric vehicles (HEV). Another battery need is for a high-power system that can be used for power tools, where only the environmentally hazardous Ni/ Cd battery presently meets the requirements. A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an electrolyte. The anode, in the case of a lithium battery, is the source of lithium ions. The cathode is the sink for the lithium ions and is chosen to optimize a number of parameters, discussed below. The electrolyte provides for the separation of ionic transport and electronic transport, and in a perfect battery the lithium ion transport number will be unity in the electrolyte. The cell potential is determined by the difference between the chemical potential of the lithium in the anode and cathode, ∆G ) -EF. As noted above, the lithium ions flow through the electrolyte whereas the electrons generated from the reaction, Li ) Li+ + e-, go through the external circuit to do work. Thus, the electrode system must allow for the flow of both lithium ions and electrons. That is, it must be both a good ionic conductor and an electronic conductor. As discussed below, many electrochemically active materials are not good electronic conductors, so it is necessary to add an electronically conductive material such as carbon * To whom correspondence should be addressed. Phone and fax: (607) 777-4623. E-mail: stanwhit@binghamton.edu. 4271 Chem. Rev. 2004, 104, 4271−4301

http://www.chemie.uni-regensburg.de/Anorganische_Chemie/Vortrag/StudentenWS06-07/Vortrag_Amereller.pdf

Lithium Batteries and Cathode Materials

The Li-S battery has been under intense scrutiny for over two decades, as it offers the possibility of high gravimetric capacities and theoretical energy densities ranging up to a factor of five beyond conventional Li-ion systems. Herein, we report the feasibility to approach such capacities by creating highly ordered interwoven composites. The conductive mesoporous carbon framework precisely constrains sulphur nanofiller growth within its channels and generates essential electrical contact to the insulating sulphur. The structure provides access to Li+ ingress/egress for reactivity with the sulphur, and we speculate that the kinetic inhibition to diffusion within the framework and the sorption properties of the carbon aid in trapping the polysulphides formed during redox. Polymer modification of the carbon surface further provides a chemical gradient that retards diffusion of these large anions out of the electrode, thus facilitating more complete reaction. Reversible capacities up to 1,320 mA h g(-1) are attained. The assembly process is simple and broadly applicable, conceptually providing new opportunities for materials scientists for tailored design that can be extended to many different electrode materials.

http://dns2.asia.edu.tw/~ysho/YSHO-English/1000%20WC/PDF/Nat%20Mat8,%20500.pdf

A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteries

The loss of sulfur cathode material as a result of polysulfide dissolution causes significant capacity fading in rechargeable lithium/sulfur cells. Embodiments of the invention use a chemical approach to immobilize sulfur and lithium polysulfides via the reactive functional groups on graphene oxide. This approach obtains a uniform and thin (˜tens of nanometers) sulfur coating on graphene oxide sheets by a chemical reaction-deposition strategy and a subsequent low temperature thermal treatment process. Strong interaction between graphene oxide and sulfur or polysulfides demonstrate lithium/sulfur cells with a high reversible capacity of 950-1400 mAh g −1 , and stable cycling for more than 50 deep cycles at 0.1 C.

/pdf/graphene-oxide-as-a-sulfur-immobilizer-in-high-performance-1zkmxq7wnu.pdf

Graphene oxide as a sulfur immobilizer in high performance lithium/sulfur cells

The catalytic activity of single-phase polycrystalline Pt-Ru bulk alloys toward the electrooxidation of methanol in sulfuric acid electrolyte at room temperature was measured for the first time on well-characterized alloy surfaces, in terms of surface composition and true surface area. Sputter-cleaned and annealed alloys were prepared in ultrahigh vacuum (UHV) and the surface composition of oxygen-free surfaces was assessed via low-energy ion scattering (LEIS). Pt-Ru alloy surfaces annealed in UHV were strongly enriched in Pt, whereas the surface composition of sputter-cleaned alloys was essentially identical to their bulk composition. It was shown that the sputter-cleaned alloy with a Ru surface of [approx equal] 10 atomic % gave the highest catalytic enhancement (over pure Pt) for the steady-state electrooxidation of methanol at 25[degree]C and 0.5 V, viz., by a factor of 12 in 5.0 mM CH[sub 3]OH solution and by a factor of 30 in 0.5 M CH[sub 3]OH. It was found that the balance between the rate of methanol adsorption and the rate of the oxidative removal of dehydrogenated fragments determines the activities of Pt-Ru alloys with different Ru surface compositions. 51 refs., 9 figs., 2 tabs.

/pdf/methanol-electrooxidation-on-well-characterized-platinum-2q1ht27nkt.pdf

Methanol electrooxidation on well-characterized platinum-ruthenium bulk alloys

The electrocatalytic activity of well-characterized Pt-Ru alloy electrodes toward the electrooxidation of CO in acidic electrolyte at room temperature was measured on alloy surfaces prepared in UHV (ultrahigh vacuum). Clearly defined surface composition was determined via LEIS (low-energy ion scattering). Electrocatalytic activities were measured by CO stripping voltammetry as well as by potentiostatic oxidation of adsorbed CO. It was found that the property of Ru atoms to nucleate oxygen-containing species at low potentials produced a strong enhancement in the catalytic activity of sputter-cleaned Pt-Ru alloy electrodes compared to pure Pt, thereby supporting the concept of the bifunctional character of the oxidation process of these alloys

Carbon monoxide electrooxidation on well-characterized platinum-ruthenium alloys

Sulfur (S) encapsulated in porous carbon nanofibers (CNFs) was synthesized viaelectrospinning, carbonization and solution-based chemical reaction–deposition method. The chemical reaction–deposition strategy provides intimate contact between the S and the CNFs. This would not necessarily be the case for other reported methods, such as ball milling and thermal treatment. These novel porous carbon nanofiber–sulfur (CNF–S) nanocomposites with various S loadings showed high reversible capacity, good discharge capacity retention and enhanced rate capability when they were used as cathodes in rechargeable Li/S cells. We demonstrated here that an electrode prepared from a porous CNF–S nanocomposite with 42 wt% S maintains a stable discharge capacity of about 1400 mA h g−1 at 0.05 C, 1100 mA h g−1 at 0.1 C and 900 mA h g−1 at 0.2 C. We attribute the good electrochemical performance to the high electrical conductivity and the extremely high surface area of the CNFs that homogeneously disperse and immobilize S on their porous structures, alleviating the polysulfide shuttle phenomenon. SEM measurements showed that the porous CNF structures remained nearly unchanged even after 30 cycles' discharging/charging at 0.05 C.

Porous carbon nanofiber–sulfur composite electrodes for lithium/sulfur cells

The kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to [approximately]10 atomic percent (a/o) Ru at 25 C to a value in the vicinity of [approximately]30 a/o at 60 C. The shift in optimum composition with temperature was attributed to a shift in the rate-determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate-determining step.

Elton J. Cairns

Papers

Graphene oxide as a sulfur immobilizer in high performance lithium/sulfur cells

Methanol electrooxidation on well-characterized platinum-ruthenium bulk alloys

Carbon monoxide electrooxidation on well-characterized platinum-ruthenium alloys

Porous carbon nanofiber–sulfur composite electrodes for lithium/sulfur cells

Temperature‐Dependent Methanol Electro‐Oxidation on Well‐Characterized Pt‐Ru Alloys