Question

Explain how grid-level energy storage makes renewable energy sources much more practical and feasible.

Answer 1

Batteries store a large amount of energy, but are relatively slow in discharging it and they have a limited lifespan or cycle-life, than their counterparts electrochemical capacitors, which are commonly called supercapacitors or ultracapacitors. Conventional supercapacitors provide a high power output with minimal degradation in performance for as many as 1,000,000 charge-discharge cycles. The capacitor can rapidly store and discharge energy, but only in small amounts compared to the battery. The obstacle in the way of using either a battery or a supercapacitor to store energy in the grid is that energy storage ability is inextricably tied to the size of the battery or the supercapacitor being used. Supercapacitors, similar to lithium-ion batteries, are manufactured in fairly small cells ranging in size from a coin to a soda can. Large amounts of expensive material, such as metal current collectors, polymer separators and packaging, would be required to construct a battery or supercapacitor of the size necessary to function effectively in the energy grid. “Packing together thousands of conventional small devices to build a system for large-scale stationary energy storage is too expensive,” said Dr. Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute and the lead researcher on the project. “A liquid storage system, the capacity of which is limited only by the tank size, can be cost-effective and scalable.” The team’s research yielded a novel solution that combines the strengths of batteries and supercapacitors while also negating the scalability problem. The “electrochemical flow capacitor” (EFC) consists of an electrochemical cell connected to two external electrolyte reservoirs - a design similar to existing redox flow batteries which are used in electrical vehicles. This technology is unique because it uses small carbon particles suspended in the electrolyte liquid to create a slurry of particles that can carry an electric charge.Uncharged slurry is pumped from its tanks through a flow cell, where energy stored in the cell is then transferred to the carbon particles. The charged slurry can then be stored in reservoirs until the energy is needed, at which time the entire process is reversed in order to discharge the EFC.The main advantage of the EFC is that its design allows it to be constructed on a scale large enough to store large amounts of energy, while also allowing for rapid disbursal of the energy when the demand dictates it.“By using a slurry of carbon particles as the active material of supercapacitors, we are able to adopt the system architecture from redox flow batteries and address issues of cost and scalability,” Gogotsi saidIn flow battery systems, as well as the EFC, the energy storage capacity is determined by the size of the reservoirs, which store the charged material. If a larger capacity is desired, the tanks can simply be scaled up in size. Similarly, the power output of the system is controlled by the size of the electrochemical cell, with larger cells producing more power.“Flow battery architecture is very attractive for grid-scale applications because it allows for scalable energy storage by decoupling the power and energy density,” said Dr. E.C. Kumbur, director of Drexel’s Electrochemical Energy Systems Laboratory. “Slow response rate is a common problem for most energy storage systems. Incorporating the rapid charging and discharging ability of supercapacitors into this architecture is a major step toward effectively storing energy from fluctuating renewable sources and being able to quickly deliver the energy, as it is needed.”This design also gives the EFC a relatively long usage life compared to currently used flow batteries. According to the researchers, the EFC can potentially be operated in stationary applications for hundreds of thousands of charge-discharge cycles.“This technology can potentially address cost and lifespan issues that we face with the current electrochemical energy storage technologies,” Kumbur said.“We believe that this new technology has important applications in [the renewable energy] field,” said Dr. Volker Presser, who was an assistant research professor in the Department of Materials Science and Engineering at the time the initial work was done. “Moreover, these technologies can also be used to enhance the efficiency of existing power sources, and improve the stability of the grid.”This concept for energy storage was recently published in a special issue of Advanced Energy Materials focused on next-generation batteries. The team’s ongoing work is focused on developing new slurry compositions based on different carbon nanomaterials and electrolytes, as well as optimizing their flow capacitor design. The group is also designing a small demonstration prototype to illustrate the fundamental operation of the system.“We have observed very promising performance so far, being close to that of conventional packaged supercapacitor cells,” Gogotsi said. “However, we will need to increase the energy density per unit of slurry volume by an order of magnitude, and achieve it using very inexpensive carbon and salt solutions to make the technology practical.”

The latest lithium-ion batteries on the market are likely to extend the charge-to-charge life of phones and electric cars by as much as 40 percent. This leap forward, which comes after more than a decade of incremental improvements, is happening because developers replaced the battery’s graphite anode with one made from silicon. Research from Drexel University and Trinity College in Ireland now suggests that an even greater improvement could be in line if the silicon is fortified with a special type of material called MXene.This adjustment could extend the life of Li-ion batteries as much as five times, the group recently reported in Nature Communications. It’s possible because of the two-dimensional MXene material’s ability to prevent the silicon anode from expanding to its breaking point during charging — a problem that’s prevented its use for some time.

Silicon anodes are projected to replace graphite anodes in Li-ion batteries with a huge impact on the amount of energy stored,” said Yury Gogotsi, PhD, Distinguished University and Bach Professor in Drexel’s College of Engineering and director of the A.J. Drexel Nanomaterials Institute in the Department of Materials Science and Engineering, who was a co-author of the research. “We’ve discovered adding MXene materials to the silicon anodes can stabilize them enough to actually be used in batteries.”In batteries, charge is held in electrodes — the cathode and anode — and delivered to our devices as ions travel from anode to cathode. The ions return to the anode when the battery is recharged. Battery life has steadily been increased by finding ways to improve the electrodes’ ability to send and receive more ions. Substituting silicon for graphite as the primary material in the Li-ion anode would improve its capacity for taking in ions because each silicon atom can accept up to four lithium ions, while in graphite anodes, six carbon atoms take in just one lithium. But as it charges, silicon also expands — as much as 300 percent — which can cause it to break and the battery to malfunction.Most solutions to this problem have involved adding carbon materials and polymer binders to create a framework to contain the silicon. The process for doing it, according to Gogotsi, is complex and carbon contributes little to charge storage by the battery.

By contrast, the Drexel and Trinity group’s method mixes silicon powder into a MXene solution to create a hybrid silicon-MXene anode. MXene nanosheets distribute randomly and form a continuous network while wrapping around the silicon particles, thus acting as conductive additive and binder at the same time. It’s the MXene framework that also imposes order on ions as they arrive and prevents the anode from expanding.“MXenes are the key to helping silicon reach its potential in batteries,” Gogotsi said. “Because MXenes are two-dimensional materials, there is more room for the ions in the anode and they can move more quickly into it — thus improving both capacity and conductivity of the electrode. They also have excellent mechanical strength, so silicon-MXene anodes are also quite durable up to 450 microns thickness.”MXenes, which were first discovered at Drexel in 2011, are made by chemically etching a layered ceramic material called a MAX phase, to remove a set of chemically-related layers, leaving a stack of two-dimensional flakes. Researchers have produced more than 30 types of MXene to date, each with a slightly different set of properties. The group selected two of them to make the silicon-MXene anodes tested for the paper: titanium carbide and titanium carbonitride. They also tested battery anodes made from graphene-wrapped silicon nanoparticles.

All three anode samples showed higher lithium-ion capacity than current graphite or silicon-carbon anodes used in Li-ion batteries and superior conductivity — on the order of 100 to 1,000 times higher than conventional silicon anodes, when MXene is added.

“The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change but also well resolves the mechanical instability of Si,” they write. “Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.”Chuanfang Zhang, PhD, a post-doctoral researcher at Trinity and lead author of the study, also notes that the production of the MXene anodes, by slurry-casting, is easily scalable for mass production of anodes of any size, which means they could make their way into batteries that power just about any of our devices.“Considering that more than 30 MXenes are already reported, with more predicted to exist, there is certainly much room for further improving the electrochemical performance of battery electrodes by utilizing other materials from the large MXene family,” he said.The study was led by Zhang, a post-doctoral researcher at Trinity College who was a PhD student in Gogotsi’s lab. It was a collaborative effort between Gogotsi and Trinity professors Jonathan N. Coleman and Valeria Nicolosi, recognized European leaders in the field of 2D materials. Sang-Hoon Park, Andrés Seral-Ascaso, Sebastian Barwich, Niall McEvoy, Conor S. Boland, from Trinity College, also contributed to this research.