Researchers have proposed an efficient and stable dual-phase lithium metal anode for Li-S batteries, containing polysulfide-induced solid electrolyte interphase and nanostructured graphene framework at Tsinghua University, appearing on ("Dual-Phase Lithium Metal Anode Containing a Polysulfide-Induced Solid Electrolyte Interphase and Nanostructured Graphene Framework for Lithium–Sulfur Batteries"). 
A distinctive graphene framework structure coated by an in situ formed solid electrolyte interphase (SEI) with Li depositing in the pores as the anode of Li-S batteries. (© ACS) Among various promising battery candidates with high energy densities, Li-S batteries, with a high theoretical capacity of 1675 mAh g-1 (based on sulfur) and an energy density of 2600 Wh kg-1 (based on the Li-S redox couple), are highly considered. "The superior property prompts the tremendous potential of Li-S batteries in portable electronics, electric vehicles, and renewable energy harvest," said Dr. Qiang Zhang, an associate professor at Department of Chemical Engineering, Tsinghua University. "Despite these advantages, many obstacles still need to be overcome for practical applications of Li-S batteries, such as the low conductivity of sulfur, the shuttle of long-chain polysulfide intermediates in the sulfur cathode and Li dendrite issues in the Li metal anode. Relative to the wide research in the cathode and electrolyte, Li metal in the anode has obtained few attentions." The formation of Li dendrites is a primary issue for Li metal batteries including Li-S batteries, which always leads to serious safety concerns and low Coulombic efficiency. Li dendrites are among the toughest issues of Li metal anode, however, it is not the exclusive one. Researchers form Pacific Northwest National Laboratory discovered a novel failure mechanism of Li metal anodes, that the porous interphase of the anode grew inward toward the bulk (fresh) Li metal, which evolved into a messy and highly resistive layer and, thus, resulted in huge transfer resistance and a great amount of Li metal losing contact with electrons (dead Li) in the inert layer. Before the dendrite-induced short circuit, the impedance of the battery escalated sharply and the service life was terminated early. "In a Li-S cell, this phenomenon is more frequent and serious, because sulfur and lithium sulfide products are both ion- and electron-insulating and the cross-coupling effect will lead to a sharp decrease in the voltage and energy density. Consequently, it is critically important to design an anode structure with desirable electron and ion channels to improve transfer properties and recycle dead Li in a Li-S cell," Qiang told Nanowerk. 
Side-view optical image of bulk graphene framework on a flower. (© ACS) Based on this concept, Xin-Bing Cheng, a graduate student and the first author, proposed a nanostructured graphene framework with Li depositing to be a high-efficiency and high-stability Li metal anode for Li-S batteries. In a routine configuration of Li metal anode without graphene framework, Li dendrites easily grew on routine 2D substrates (such as Cu foil). As the root of dendrites can receive the electron easily and then dissolve earlier, Li dendrites easily fractured and were detached from the substrate to form dead Li. If there is a pre-existed conductive framework such as self-supported graphene foam, the deposited Li will be well accommodated. Free-standing graphene foam affords several promising features as underneath layer for Li anode, including (1) relative larger surface area than 2D substrates to lower the real specific surface current density and the possibility of dendrite growth, (2) interconnected framework to support and recycle dead Li, and (3) good flexibility to sustain the volume fluctuation during repeated incorporation/extraction of Li. "We hope that the wise combination of the nanoscale engineering and electrochemistry can help improve Coulombic efficiency and ion conductivity of Li metal anode for the applications of Li-S batteries," said Xin-Bing. Future research is required to investigate the diffusion of Li ions before and after crossing the SEI. The results indicated that nanoscale interfacial electrode engineering could be a promising strategy to tackle the intrinsic problems of lithium metal anodes and the concepts described herein shed a new light toward high-energy-density LMBs, such as Li-S and Li-O2 batteries.
read more "Robust graphene based dual-phase Li metal anode with polysulfide-induced solid electrolyte interphase"
A distinctive graphene framework structure coated by an in situ formed solid electrolyte interphase (SEI) with Li depositing in the pores as the anode of Li-S batteries. (© ACS) Among various promising battery candidates with high energy densities, Li-S batteries, with a high theoretical capacity of 1675 mAh g-1 (based on sulfur) and an energy density of 2600 Wh kg-1 (based on the Li-S redox couple), are highly considered. "The superior property prompts the tremendous potential of Li-S batteries in portable electronics, electric vehicles, and renewable energy harvest," said Dr. Qiang Zhang, an associate professor at Department of Chemical Engineering, Tsinghua University. "Despite these advantages, many obstacles still need to be overcome for practical applications of Li-S batteries, such as the low conductivity of sulfur, the shuttle of long-chain polysulfide intermediates in the sulfur cathode and Li dendrite issues in the Li metal anode. Relative to the wide research in the cathode and electrolyte, Li metal in the anode has obtained few attentions." The formation of Li dendrites is a primary issue for Li metal batteries including Li-S batteries, which always leads to serious safety concerns and low Coulombic efficiency. Li dendrites are among the toughest issues of Li metal anode, however, it is not the exclusive one. Researchers form Pacific Northwest National Laboratory discovered a novel failure mechanism of Li metal anodes, that the porous interphase of the anode grew inward toward the bulk (fresh) Li metal, which evolved into a messy and highly resistive layer and, thus, resulted in huge transfer resistance and a great amount of Li metal losing contact with electrons (dead Li) in the inert layer. Before the dendrite-induced short circuit, the impedance of the battery escalated sharply and the service life was terminated early. "In a Li-S cell, this phenomenon is more frequent and serious, because sulfur and lithium sulfide products are both ion- and electron-insulating and the cross-coupling effect will lead to a sharp decrease in the voltage and energy density. Consequently, it is critically important to design an anode structure with desirable electron and ion channels to improve transfer properties and recycle dead Li in a Li-S cell," Qiang told Nanowerk. Side-view optical image of bulk graphene framework on a flower. (© ACS) Based on this concept, Xin-Bing Cheng, a graduate student and the first author, proposed a nanostructured graphene framework with Li depositing to be a high-efficiency and high-stability Li metal anode for Li-S batteries. In a routine configuration of Li metal anode without graphene framework, Li dendrites easily grew on routine 2D substrates (such as Cu foil). As the root of dendrites can receive the electron easily and then dissolve earlier, Li dendrites easily fractured and were detached from the substrate to form dead Li. If there is a pre-existed conductive framework such as self-supported graphene foam, the deposited Li will be well accommodated. Free-standing graphene foam affords several promising features as underneath layer for Li anode, including (1) relative larger surface area than 2D substrates to lower the real specific surface current density and the possibility of dendrite growth, (2) interconnected framework to support and recycle dead Li, and (3) good flexibility to sustain the volume fluctuation during repeated incorporation/extraction of Li. "We hope that the wise combination of the nanoscale engineering and electrochemistry can help improve Coulombic efficiency and ion conductivity of Li metal anode for the applications of Li-S batteries," said Xin-Bing. Future research is required to investigate the diffusion of Li ions before and after crossing the SEI. The results indicated that nanoscale interfacial electrode engineering could be a promising strategy to tackle the intrinsic problems of lithium metal anodes and the concepts described herein shed a new light toward high-energy-density LMBs, such as Li-S and Li-O2 batteries.
Scanning electron microscope image of a self-assembled platinum lattice, false-colored to show the two-layer structure. Each inner square of the nanoscale grid is just 34 nanometers on each side. The scientists synthesized the materials at Brookhaven Lab's Center for Functional Nanomaterials (CFN) and characterized the nanoscale architectures using electron microscopy at CFN and x-ray scattering at the National Synchrotron Light Source—both DOE Office of Science User Facilities. The new technique relies on polymer self-assembly, where molecules are designed to spontaneously assemble into desired structures. Self-assembly requires a burst of heat to make the molecules snap into the proper configurations. Here, an intensely hot laser swept across the sample to transform disordered polymer blocks into precise arrangements in just seconds. "Self-assembled structures tend to automatically follow molecular preferences, making custom architectures challenging," said lead author Pawel Majewski, a postdoctoral researcher at Brookhaven. "Our laser technique forces the materials to assemble in a particular way. We can then build structures layer-by-layer, constructing lattices composed of squares, rhombuses, triangles, and other shapes." Laser-assembled nano-wires For the first step in grid construction, the team took advantage of their recent invention of laser zone annealing (LZA) to produce the extremely localized thermal spikes needed to drive ultra-fast self-assembly. To further exploit the power and precision of LZA, the researchers applied a heat-sensitive elastic coating on top of the unassembled polymer film. The sweeping laser's heat causes the elastic layer to expand—like shrink-wrap in reverse—which pulls and aligns the rapidly forming nanoscale cylinders. "The end result is that in less than one second, we can create highly aligned batches of nano-cylinders," said study coauthor Charles Black, who leads the Electronic Nanomaterials group at CFN. "This order persists over macroscopic areas and would be difficult to achieve with any other method." 
Scanning electron microscope image of a three-layer platinum mesh. The colored inset shows each distinct layer of the nanoscale grid. To make these two-dimensional grids functional, the scientists converted the polymer base into other materials. One method involved taking the nano-cylinder layer and dipping it into a solution containing metal salts. These molecules then glom onto the self-assembled polymer, converting it into a metallic mesh. A wide range of reactive or conductive metals can be used, including platinum, gold, and palladium. They also used a technique called vapor deposition, where a vaporized material infiltrates the polymer nano-cylinders and transforms them into functional nano-wires. Layer-by-layer lattice The first completed nano-wire array acts as the foundation of the full lattice. Additional layers, each one following variations on that same process, are then stacked to produce customized, crisscrossing configurations—like chain-link fences 10,000 times thinner than a human hair. "The direction of the laser sweeping across each unassembled layer determines the orientation of the nano-wire rows," Yager said. "We shift that laser direction on each layer, and the way the rows intersect and overlap shapes the grid. We then apply the functional materials after each layer forms. It's an exceptionally fast and simple way to produce such precise configurations." 
Illustration of the experiment showing the sweeping laser inducing intense heat that both accelerates polymer self-assembly and precisely aligns the nano-cylinders that form the foundation of the final grid. Study coauthor Atikur Rahman, a CFN postdoctoral researcher, added, "We can stack metals on insulators, too, embedding different functional properties and interactions within one lattice structure. "The size and the composition of the mesh make a huge difference," Rahman continued. "For example, a single layer of platinum nano-wires conducts electricity in only one direction, but a two-layer mesh conducts uniformly in all directions." LZA is precise and powerful enough to overcome interface interactions, allowing it to drive polymer self-assembly even on top of complex underlying layers. This versatility enables the use of a wide variety of materials in different nanoscale configurations. "We can generate nearly any two-dimensional lattice shape, and thus have a lot of freedom in fabricating multi-component nanostructures," Yager said. "It's hard to anticipate all the technologies this rapid and versatile technique will allow."
The MIT researchers' prototype for a chip measuring 3 millimeters by 3 millimeters. The magnified detail shows the chip's main control circuitry, including the startup electronics; the controller that determines whether to charge the battery, power a device, or both; and the array of switches that control current flow to an external inductor coil. This active area measures just 2.2 millimeters by 1.1 millimeters. (click on image to enlarge) Moreover, the researchers’ chip achieves those efficiency improvements while assuming additional responsibilities. Where its predecessors could use a solar cell to either charge a battery or directly power a device, this new chip can do both, and it can power the device directly from the battery. All of those operations also share a single inductor — the chip’s main electrical component — which saves on circuit board space but increases the circuit complexity even further. Nonetheless, the chip’s power consumption remains low. “We still want to have battery-charging capability, and we still want to provide a regulated output voltage,” says Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science and first author on the new paper. “We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels — 10 nanowatts to 1 microwatt — for the Internet of things.” The prototype chip was manufactured through the Taiwan Semiconductor Manufacturing Company's University Shuttle Program. Ups and downs The circuit’s chief function is to regulate the voltages between the solar cell, the battery, and the device the cell is powering. If the battery operates for too long at a voltage that’s either too high or too low, for instance, its chemical reactants break down, and it loses the ability to hold a charge. To control the current flow across their chip, El-Damak and her advisor, Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, use an inductor, which is a wire wound into a coil. When a current passes through an inductor, it generates a magnetic field, which in turn resists any change in the current. Throwing switches in the inductor’s path causes it to alternately charge and discharge, so that the current flowing through it continuously ramps up and then drops back down to zero. Keeping a lid on the current improves the circuit’s efficiency, since the rate at which it dissipates energy as heat is proportional to the square of the current. Once the current drops to zero, however, the switches in the inductor’s path need to be thrown immediately; otherwise, current could begin to flow through the circuit in the wrong direction, which would drastically diminish its efficiency. The complication is that the rate at which the current rises and falls depends on the voltage generated by the solar cell, which is highly variable. So the timing of the switch throws has to vary, too. Electric hourglass To control the switches’ timing, El-Damak and Chandrakasan use an electrical component called a capacitor, which can store electrical charge. The higher the current, the more rapidly the capacitor fills. When it’s full, the circuit stops charging the inductor. The rate at which the current drops off, however, depends on the output voltage, whose regulation is the very purpose of the chip. Since that voltage is fixed, the variation in timing has to come from variation in capacitance. El-Damak and Chandrakasan thus equip their chip with a bank of capacitors of different sizes. As the current drops, it charges a subset of those capacitors, whose selection is determined by the solar cell’s voltage. Once again, when the capacitor fills, the switches in the inductor’s path are flipped. “In this technology space, there’s usually a trend to lower efficiency as the power gets lower, because there’s a fixed amount of energy that’s consumed by doing the work,” says Brett Miwa, who leads a power conversion development project as a fellow at the chip manufacturer Maxim Integrated. “If you’re only coming in with a small amount, it’s hard to get most of it out, because you lose more as a percentage. [El-Damak’s] design is unusually efficient for how low a power level she’s at.” “One of the things that’s most notable about it is that it’s really a fairly complete system,” he adds. “It’s really kind of a full system-on-a chip for power management. And that makes it a little more complicated, a little bit larger, and a little bit more comprehensive than some of the other designs that might be reported in the literature. So for her to still achieve these high-performance specs in a much more sophisticated system is also noteworthy.”