The increasing interest in energy storage for the grid can be attributed to multiple factors, including the capital costs of managing peak demands, the investments needed for grid reliability, and the integration of renewable energy sources. Although existing energy storage is dominated by pumped hydroelectric, there is the recognition that battery systems can offer a number of high-value opportunities, provided that lower costs can be obtained. The battery systems reviewed here include sodium-sulfur batteries that are commercially available for grid applications, redox-flow batteries that offer low cost, and lithium-ion batteries whose development for commercial electronics and electric vehicles is being applied to grid storage.

Electrical Energy Storage for the Grid: A Battery of Choices

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.

Energy storage using batteries offers a solution to the intermittent nature of energy production from renewable sources; however, such technology must be sustainable. This Review discusses battery development from a sustainability perspective, considering the energy and environmental costs of state-of-the-art Li-ion batteries and the design of new systems beyond Li-ion. Images: batteries, car, globe: © iStock/Thinkstock.

Towards greener and more sustainable batteries for electrical energy storage

Electrolytes and interphases in Li-ion batteries and beyond.

Energy density is the main property of rechargeable batteries that has driven the entire technology forward in past decades. Lithium-ion batteries (LIBs) now surpass other, previously competitive battery types (for example, lead–acid and nickel metal hydride) but still require extensive further improvement to, in particular, extend the operation hours of mobile IT devices and the driving mileages of all-electric vehicles. In this Review, we present a critical overview of a wide range of post-LIB materials and systems that could have a pivotal role in meeting such demands. We divide battery systems into two categories: near-term and long-term technologies. To provide a realistic and balanced perspective, we describe the operating principles and remaining issues of each post-LIB technology, and also evaluate these materials under commercial cell configurations. Post-lithium-ion batteries are reviewed with a focus on their operating principles, advantages and the challenges that they face. The volumetric energy density of each battery is examined using a commercial pouch-cell configuration to evaluate its practical significance and identify appropriate research directions.

Promise and reality of post-lithium-ion batteries with high energy densities

Global energy and climate-change concerns have accelerated the electrifi cation of vehicles, aided by advances in battery technology. It is now recognized that low-cost, scalable energy storage will also be key to continued growth of renewable energy technologies (wind and solar) and improved effi ciency of the electric grid. While electrochemical energy storage remains attractive for its high energy density, simplicity, and reliability, existing battery technologies remain limited in their ability to meet many future storage needs. Here we propose and demonstrate a new storage concept, the semi-solid fl ow cell (SSFC), which combines the high energy density of rechargeable batteries with the fl exible and scalable architecture of fuel cells and fl ow batteries. In contrast to previous fl ow batteries, energy is stored in suspensions of solid storage compounds to and from which charge transfer is accomplished via dilute yet percolating networks of nanoscale conductors. These novel electrochemical composites constitute fl owable semi-solid ‘fuels’ that are here charged and discharged in prototype fl ow cells. Potential advantages of the SSFC approach include projected system-level energy densities that are more than ten times those of aqueous fl ow batteries, and the simplifi ed low-cost manufacturing of large-scale storage systems compared to conventional lithiumion batteries. Demand for batteries of higher energy and power has driven several decades of research in electrochemical storage materials, resulting recently in signifi cant improvements in the stored energy of cathodes and anodes. [ 1 , 2 ] However, most batteries have designs that have not departed substantially from Volta’s galvanic cell of 1800, and which accept an inherently poor utilization of the active materials. [ 3 ] Even the highest energy density lithium ion cells currently available, e.g., 2.8‐2.9 Ah 18650 cells having > 600 Wh L − 1 , have less than 50 vol% active material. The reduced energy density, along with higher cost, result because the high-energy-storage compounds are diluted by inactive and costly components necessary to extract power (e.g., currentcollector foils, tabs, separator fi lm, liquid electrolyte, electrode binders and conductive additives, and external packaging). Further dilution of energy density, by about a factor of two, occurs between the cell and system level. [ 4 ] Electrode designs that minimize inactive material, bio- and self-assembly, and 3D architectures are new approaches that promise improved design effi ciency but have yet to be fully realized. [ 5 ‐ 9 ] Decoupling power components from energy-storage components so that stored energy can be scaled independently of power is a strategy for improving system-level energy density. Redox fl ow batteries have such a design, in which active materials are stored within external reservoirs and pumped into an ion-exchange/electron-extraction power stack. [ 10 ] As the system increases in capacity, its energy density may asymptotically approach that of the redox active solutions. Aqueous-chemistry fl ow batteries are of much current interest for stationary applications due to their scalability, relative safety, and potentially low cost. However, they currently use low energy density chemistries limited by electrolysis to ≈ 1.5 V cell voltage and have low ion concentrations (typically 1‐2 M ), yielding ≈ 40 Wh L − 1 energy density for the fl uids alone. [ 10 ] Furthermore, the large fl uid volumes that must be pumped produce parasitic mechanical losses that detract signifi cantly from round-trip effi ciency. The fl ow-cell’s design advantages are therefore offset by the use of low-energy-density active materials. In a new system we call a semi-solid fl ow cell (SSFC, Figure 1 ), the inherent advantages of a fl ow architecture are retained while dramatically increasing energy density by using suspensions of energy-dense active materials in a liquid electrolyte. [ 11 ] This approach to fl owable electrodes can produce more than 10 times the charge storage density of typical fl ow-battery solutions, due to the much greater energy density inherent to solidstorage compounds. For example, in molarity units the concentration of reversibly-stored lithium in lithium-ion cathodes such as LiCoO 2 , LiFePO 4 , LiNi 0.5 Mn 1.5 O 4 , and 0.3 Li 2 MnO 3 ‐0.7 Li M O 2 ( M = Mn, Co, Ni), and anodes such as Li 4 Ti 5 O 12 , graphite, and Si (assuming ≈ 1000 mAh g − 1 reversible capacity), is 51.2, 22.8, 24.1, 39.2, 22.6, 21.4, and 87 M , respectively. [ 2 ] Assuming a solids content of 50% (up to 70 vol% solids have been achieved in fl suspensions of other materials), the volumetric capacity of the semi-solid suspensions is 5‐20 times greater (e.g., 10 to 40 M ) than that of aqueous redox solutions ( ≈ 2 M). The semi-solid approach may be applied to aqueous chemistries, in which case the volumetric energy density is also 5‐20 times greater since cell voltages remain limited by electrolyte hydrolysis to ≈ 1.5 V. [ 12 , 13 ] When applied to nonaqueous Li-ion chemistries, however, energy density is further increased by another factor of 1.5‐3, in direct proportion to cell voltage. (Energy density is the product of volumetric charge capacity, e.g., in molarity or Ah l − 1 units, and the cell voltage. Specifi c values for the systems studied are given later.) In this work, we demonstrate working prototype SSFCs using fl owable suspensions having up to ≈ 12 M concentration. We show that in addition to energy density advantages, the SSFCs can operate at low fl ow rates with very low mechanical energy dissipation. The design fl exibility inherent in the SSFC approach may enable new use-models for electrical storage, such as rapid refueling

Semi‐Solid Lithium Rechargeable Flow Battery

Redox flow devices are described including a positive electrode current collector, a negative electrode current collector, and an ion-permeable membrane separating said positive and negative current collectors, positioned and arranged to define a positive electroactive zone and a negative electroactive zone; wherein at least one of said positive and negative electroactive zone comprises a flowable semi-solid composition comprising ion storage compound particles capable of taking up or releasing said ions during operation of the cell, and wherein the ion storage compound particles have a polydisperse size distribution in which the finest particles present in at least 5 vol % of the total volume, is at least a factor of 5 smaller than the largest particles present in at least 5 vol % of the total volume.

High energy density redox flow device

Soft robotics represents a new set of technologies aimed at operating in natural environments and near the human body. To interact with their environment, soft robots require artificial muscles to actuate movement. These artificial muscles need to be as strong, fast, and robust as their natural counterparts. Dielectric elastomer actuators (DEAs) are promising soft transducers, but typically exhibit low output forces and low energy densities when used without rigid supports. Here, we report a soft composite DEA made of strain-stiffening elastomers and carbon nanotube electrodes, which demonstrates a peak energy density of 19.8 J/kg. The result is close to the upper limit for natural muscle (0.4–40 J/kg), making these DEAs the highest-performance electrically driven soft artificial muscles demonstrated to date. To obtain high forces and displacements, we used low-density, ultrathin carbon nanotube electrodes which can sustain applied electric fields upward of 100 V/μm without suffering from dielectric breakdown. Potential applications include prosthetics, surgical robots, and wearable devices, as well as soft robots capable of locomotion and manipulation in natural or human-centric environments.

https://www.pnas.org/content/pnas/116/7/2476.full.pdf

Realizing the potential of dielectric elastomer artificial muscles.

A novel method for the fabrication of dielectric elastomer actuators (DEAs) combines acrylic polymers and single wall carbon nanotube network electrodes. DEAs made using this technique do not require prestretching, have extremely thin electrodes, and can be actuated at low voltage. The method is applied to create a multimorph device with nine actuation modes based on just four inputs.

Multilayer Dielectric Elastomers for Fast, Programmable Actuation without Prestretch.

The design and fabrication of a rolled dielectric elastomer actuator is described and the parametric dependence of the displacement and blocked force on the actuator geometry, elastomer layer thickness, voltage, and number of turns is analyzed. Combinations of different elastomers and carbon nanotube electrodes are investigated and optimized to meet performance characteristics appropriate to tactile display applications, namely operation up to 200 Hz with a combination of a 1 N blocked force and free displacement of 1 mm, all within a volume of less than 1 cm3. Lives in excess of 50 000 cycles have been obtained. Key to meeting these objectives is control of the multilayering fabrication process, the carbon nanotube electrode concentration, the selection of a soft elastomer with low viscous losses, and a proof-testing procedure for enhancing life cycle reliability.

Mihai Duduta

Papers

Semi‐Solid Lithium Rechargeable Flow Battery

High energy density redox flow device

Realizing the potential of dielectric elastomer artificial muscles.

Multilayer Dielectric Elastomers for Fast, Programmable Actuation without Prestretch.

Compact Dielectric Elastomer Linear Actuators