Chapter 12
Conclusions and Future Trends
Energy storage in the future will remain as a critical part of our society. There is no doubt that humans will continue to explore more efficient and reliable ways to store and carry energy. Although we can store energy relatively successfully through a wide range of techniques and mechanisms, electrochemistry will continue to offer one of the most attractive methods in the future. In this chapter, we will discuss several emerging and future trends in energy storage and provide concluding remarks.
12.1 Future of Energy Storage
Electrochemical cells will continue to provide the most useful and convenient energy storage method, especially for mobile and portable applications. In other words, electrochemical devices like batteries are here to stay for a very long time. For this reason, we will focus our attention on emerging and future trends that are mostly related to the electrochemical cells.
One of the basic advantages of the electrochemical energy storage devices is their fundamentally high thermodynamic efficiency. Since electrochemical processes are not heat engines, the Carnot efficiency limitation is avoided. The efficiency, η, of a heat engine, a device for the conversion of thermal energy to mechanical or electrical work output, is expressed as follows:
(12.1)
where Th and Tc are the high and low temperatures of the thermal reservoirs between which the engine is operating. This efficiency is considered an ideal or maximum theoretical efficiency.
However, as in the case of all proffered solutions to any problem, there are negative aspects. Probably the single largest drawback to electrochemical cells is their relatively low energy densities compared to fuel cells and other higher density energy sources. The second limitation is their relatively short useful life and consequent high cost, compared to other long cycle life and lower cost devices. Double-layer supercapacitors can achieve high cycles since no major chemical reduction or oxidation at the electrode takes place, however, other problems like self-discharge may persist.
Overall, specific applications need to be matched with suitable energy storage devices that provide compatible performance and properties. Some degree of compromise in properties and performance may be necessary when selecting the optimum energy storage method for specific applications with multifaceted operational requirements.
12.2 Flexible and Stretchable Energy Storage Devices
Conventional energy storage devices like batteries are often packaged in rigid casing such as coin cell, cylindrical and prismatic packages. They can be relatively heavy and bulky, and are generally filled with liquid organic electrolytes. A recent trend that will continue well into the future is the emergence of the mechanically flexible energy storage devices. This is a transformation that involves the replacement of the conventional flammable organic liquid electrolyte with solid or quasi-solid electrolyte.
A flexible thin film energy storage device (Figure 12.1) can be made in any shape or design, and most importantly, it is capable of mechanical bending, twisting or folding. These flexible energy storage devices like batteries and supercapacitors are considered multifunctional devices. In other words, they provide simultaneous mechanical, electrochemical and other functions as required.
Figure 12.1 Flexible lithium ion battery lighting an LED (Ardebili’s Lab at the University of Houston).
Another class of multifunctional energy storage devices is stretchable devices. They are capable of not only bending but also stretching or contraction as depicted in Figure 12.2. They can simultaneously store (or release) energy while mechanically deforming under tension or compression. Flexible, stretchable, and foldable energy storage devices can accommodate various design and dimensional constraints. They are especially suitable for wearable applications and implantable biomedical sensors, and can be integrated in various soft and hard materials and structures.
Figure 12.2 Stretchable spiral lithium ion battery lighting an LED (Ardebili’s Lab at the University of Houston).
In general, flexible and stretchable devices are designed to be electrochemically and thermally safer and more stable compared to their conventional rigid counterparts. They can be used for more intimate interaction with human organs, and can be embedded and integrated in materials and structures that require higher safety and stability in electrochemical, mechanical, and thermal properties. This enhanced safety is mainly attributed to the use of solid electrolyte including ion conductive ceramic, glass and polymer materials that are much safer and more stable compared to the organic flammable liquid electrolytes. For example, thermal runaway that involves self-heating and potential catastrophic events in organic liquid based batteries, may be suppressed in solid electrolyte batteries.
“Structural energy storage” devices, as the term implies, are designed to provide structural support and mechanical compliance. They can be embedded inside materials including composites and polymers and can offer mechanical support while simultaneously storing and providing electrochemical power and energy. Therefore, the mechanical properties of a structural energy storage device and its components need to be carefully designed to match the application and operational requirements.
Materials used in flexible and stretchable batteries, in general, must be solid, or at minimum, quasi-solid. Mechanical manipulation of the device such as bending, twisting, folding or stretching can dramatically increase the risk of battery leakage if liquid electrolyte is used. A small amount of liquid electrolyte that is well trapped inside a solid matrix may be tolerable (i.e., quasi-solid) for certain applications. The device based on solid electrolyte (i.e., ion conductive ceramic or polymer) must be mechanically stable and robust while providing energy and power.
One important aspect regarding a flexible energy storage device is its thickness. In general, these devices can be made very thin (e.g., micron scale) allowing mechanical flexibility and the ability to bend. In fact, any material, whether it’s metallic, composite, ceramic or polymer that is made sufficiently thin, becomes mechanically flexible and is capable of bending. Therefore, flexible devices such as thin-film batteries can still utilize the existing mature and optimized chemistry and technologies associated with conventional electrodes and current collectors. In other words, conventional anode and cathode materials used in rigid cased batteries can also be used in flexible lithium ion batteries if they are made sufficiently thin. The main distinction between the flexible batteries and the rigid conventional batteries is generally the electrolyte. The liquid organic electrolyte in conventional battery must be replaced with solid or quasi-solid electrolyte for enhanced mechanical stability, safety, and thin-film manufacturability.
We can also further enhance the materials in flexible energy storage devices to optimize the performance of the multifunctional device. For example, nanosized materials like ceramic nanoparticles, carbon nanotubes, graphene or graphene oxide nanosheets, and hybrid 3D nanomaterials [Tang et al., 2012], can be added to the conventional electrodes, current collectors or solid (or quasi solid) electrolytes to enhance interfacial, mechanical and conductive properties of the materials. Ion conductive and electrically insulating nanomaterials are preferred for solid electrolyte. For electrode materials, nanomaterial additives that are both ionic and electrically conductive (allowing both ion and electrons to transport) are ideal, but enhancing one type of conductivity can still be effective to improve the device performance.
Let’s now shift our attention from flexible energy storage devices to stretchable devices. How do we make a device stretchable? We can enable the device to deform or stretch through several different mechanisms. For example, serpentine (spring-like) interconnects can be attached between segmental batteries, supported and encapsulated by stretchable material [Xu et al., 2013]. In this case, the battery segments do not need to be stretchable, and the ability to deform is achieved through the stretchable spring-like interconnects. Another design that can provide stretching capability is the Kirigami-based design [Song et al., 2015]. Multiple devices or segments (not necessarily flexible or stretchable) can be connected through foldable interfaces. Folding and unfolding in various design configurations and shapes, allows the system of devices or segments to expand and contract in different directions.
Another method to create a stretchable energy storage device is to design a spiral configuration onto the thin-film device. For example, a spiral thin-film lithium ion battery can be made to stretch out-of-plane as shown in Figure 12.2 [Kammoun et al., 2016]. A device can also be fabricated in the form of a cable [Kwon et al., 2012]. A central wire with consecutive material coatings is generally needed to create such a cable-shaped device. The cable battery can be turned into a coil, and can be uncoiled as needed.
Fabric-based flexible and stretchable devices are also attracting much attention in recent years, because of the promise of better compatibility with wearable applications and more effective integration with smart textiles and clothing. Electrochemically active electrode materials can be deposited and coated onto stretchable fabrics, and the electrode can stretch along with the fabric (Figure 12.3). Studies of flexible, stretchable, foldable energy storage devices based on different materials can be found in [Kammoun et al., 2015] [Kelly et al., 2016] [Berg et al., 2017] [Qu et al., 2014] [Koo et al., 2012] [Kammoun et al., 2016] [Li et al., 2013].
Figure 12.3 Stretchable fabric-based LiCoO2 positive electrode for stretchable lithium ion batteries (Ardebili’s Lab at the University of Houston).
12.3 Self-Charging Energy Storage Devices
Another emerging and future trend is the development of integrated energy harvesting and storage devices. Imagine if you could have a battery that charges itself! This has been the focus of researchers for some time and many innovative techniques have been proposed and explored.
We can harvest, generate and convert energy through various methods and sources such as solar, photovoltaic, thermal, piezoelectric, thermoelectric, chemical, and other. There are two main approaches for combining energy harvesting and storage. One approach is to externally connect the individual devices (i.e., energy harvesting and storage) that are compatible in materials and design, into one coherent system. For example, both devices can be mechanically flexible and bendable. Another approach is to integrate the energy conversion and storage components internally into one single device. The latter approach requires common materials and mechanisms that are inherent in both energy conversion and storage methods. A precise and efficient integration strategy is often required in such combined energy harvesting/storage systems. Several studies of integrated energy devices are listed in the bibliography.
12.4 Recovering Wasted Energy
As the energy storage methods utilized by humans date back to thousands of years ago, so does energy wastage. For as long as humans have been harvesting, converting, storing and consuming energy, they have also been effectively wasting it. Reports show that in the United States and other countries around the globe, every year, a proportion of energy is wasted or “rejected” (Figure 12.4). In the last few decades, we have become especially more aware of the energy wastage issue and have made attempts to circumvent the problem through innovative recovery approaches and solutions.
Figure 12.4 Energy consumption in the United States in 2017 (Lawrence Livermore National Laboratory).
There are many reasons why the problem of wasted energy should be addressed. First, wastage of any type causes inefficiencies in the system at all levels that can devastate the economy over time, and adversely impact various sectors of the society. Secondly, we are faced with the crisis of depletion of non-renewable energy sources and it’s imperative that we conserve energy. As such, any type of energy wastage is counteractive to our primary objectives and must be eliminated or minimized. Third, we have better measurement tools and estimation methods for the quantitative amount of the wasted energy. This can lead to more effective and precise solutions and strategies. Finally, new innovative techniques have emerged to revitalize various forms of energy that are being wasted. As our technology advances, more opportunities can be realized for combatting energy wastage, thus motivating higher level of activities in this field of research and development.
The energy flow chart for the United States, created by the Lawrence Livermore National Laboratory, is shown in Figure 12.4. This chart provides an estimation of energy services and rejected energy. Such charts are also available for individual states in the United States as well as other countries around the world.
The term “energy services” in the flow chart in Figure 12.4 represents the energy that was actually used, such as the energy that drives the car wheels forward on the road. The “rejected energy” is the energy that went into a system or process, but was wasted. For example, the energy that dissipated as heat from the coolant or the exhaust of an automobile vehicle is the rejected (wasted) energy.
According to the flow chart of energy consumption in the United States in 2017 (Figure 12.4), the estimated amount of energy services (used energy) was 31.1 quads. In the same year, the rejected energy was about 66.7 quads. This indicates that close to 70% of the total energy in 2017 was wasted. This is a relatively high amount of energy wastage, and it signals significant inefficiencies in our energy consumption devices and systems.
To combat energy wastage and inefficiencies in the energy processes and systems in the United States and around the world, several innovative approaches have been proposed. Figures 12.5 and 12.6 show methods of converting wasted energy in a vehicle into useful energy.
Figure 12.5 Energy recovery methods in vehicle systems
[Adapted from Gabriel Buenaventura et al., 2015].
Figure 12.6 The kinetic energy in the vehicle is recovered using an elastomer unit located between the engines and the wheels. The elastomer deforms as it stores elastic potential energy and can release it at a later time
[Adapted from Gabriel-Buenaventura et al., 2015].
Energy recovery approaches can include mechanical, chemical, and thermal methods including elastomer/spring, flywheels, and thermoelectrics as shown in Figures 12.5 and 12.6. More information on wasted (rejected) energy and respective recovery approaches and innovations can be found in [Gabriel-Buenaventura et al., 2015] and [Orr, 2016].
12.5 Recycling Energy Storage Devices
There is a strong impetus towards designing and fabricating environmentally friendly energy storage materials and devices that can be fully recycled. It is expected that innovations in recycling of energy storage devices will continue in the future. Here, we will focus our discussions on the recycling of lithium ion batteries (LIBs), even though many of the general processes involved can be applicable to the other energy storage devices.
Currently, only a small percentage of spent batteries are processed in recycling facilities. This is mainly due to the lack of proper development and implementation of environmental regulations, in the United States and worldwide, with respect to the end-of-life of batteries. Unfortunately, most batteries end up in the landfills and their materials are wasted. It is important to emphasize that in addition to the environmental benefits, there is a strong economic incentive to reuse the expensive and rare metals in a spent lithium ion battery.
To recycle lithium ion batteries, many different approaches can be used. They all belong to two main categories: physical and chemical processes, as depicted in Figure 12.7. Physical processes include mechanical separation of the device or material components, thermal treatment, mechanochemical process and dissolution. Chemical processes consist of acid leaching, bioleaching, electrochemical process, chemical precipitation, and solvent extraction.
Figure 12.7 Recycling processes for energy storage devices such as lithium ion batteries.
In general, a single type of recycling process such as thermal treatment, acid leaching or chemical precipitation may not be sufficient for a complete recycling of LIBs. Often, different types of processes should be combined to extract the main metal species in spent LIBs that consists of aluminum, copper, manganese, cobalt and lithium. Figure 12.8 shows an example of combined processes for recycling lithium ion batteries. More information on recycling of lithium ion batteries can be found in [Xu et al., 2008], [Dewulf, 2010], [Georgi-Maschler et al., 2012], [Huang et al., 2018], [Mayyas et al., 2019].
Figure 12.8 Metal recovery process for lithium ion battery.
12.6 New Chemistry for Electrochemical Cells
Exploring new chemistry for energy storage devices including new electrode and electrolyte materials will continue to play a critical role in enhancing the performance of the devices. New ion type technologies as alternative to lithium ion (e.g., Na+, Al+, K+, etc.) may provide economic and environmental advantages compared to lithium ion batteries. For example, sodium and potassium are much more abundant on Earth than lithium and can provide advantages for large-scale energy storage. However, there are still several challenges that need to be resolved in these new chemistry-based energy storage devices.
Transformative and disruptive technologies can now and then arise that offer new and unconventional advantages. In the last several decades much work has been directed toward new chemistry research and development of electrochemical cells, and it is expected to continue in the future. New chemistry that can satisfy simultaneous goals of safety, environmental friendliness, and recyclability of energy storage materials and devices will be pursued.
Resolving the dendrite growth issue in lithium ion batteries, with lithium metal as the anode, will continue to be an important objective. Lithium metal anode has the promise of high theoretical capacity (3,860 mAh g–1); however, it’s prone to dendrite growth. Dendrite generally grows from the lithium metal anode surface, through the liquid electrolyte in the cell. As the dendrite grows into branches and expands, it can pierce through the separator membrane (located between the anode and cathode) and reach the other side of the cell. The lithium metal dendrite eventually makes contact with the other electrode (cathode) causing electrical shorting in the battery cell. Preferably lithium dendrite should not be allowed to initiate. If it does, the growth should be suppressed or slowed down considerably, through various approaches, to achieve a reasonable battery lifetime.
Novel material designs and fabrications of electrode and electrolyte, and new modeling and experimental approaches will continue until a comprehensive and successful solution to the dendrite growth problem is fully demonstrated. A schematic of dendrite growth and several related issues in a lithium metal anode battery are depicted in Figure 12.9 [adapted from Cheng et al., 2017].
Figure 12.9 Lithium metal dendrite growth
(adapted from Cheng et al., 2017).
Machine learning can be used to more effectively identify materials with optimum properties. Big data analysis and artificial intelligence based research enables an accelerated materials design process and it is a trend that will continue in all data driven fields in future. Studies involving machine learning related to batteries can be found in the literature including [Fujimura, et al., 2013] [Sendek, et al., 2018].
Advanced and high precision in situ characterizations will continue to provide more insight into physical and chemical phenomena related to energy storage. For example, monitoring the lithium intercalation process of electrode in a lithium ion electrochemical cell, in real time and under transmission electron microscopy (TEM), will provide more precise observations of the fundamental mechanisms involved. More discussions on in situ TEM investigations related to batteries, can be found in the literature, including [Liu et al., 2011] [Wang et al., 2012] [Yuan et al., 2017].
12.7 Non-Electrochemical Energy Storage
Non-electrochemical energy storage devices and systems have also important roles in the future. They have their niche applications and special use in various sectors of our society. For example, NASA uses flywheel energy storage due to its very high reliability and long lifetime. Molten salt systems storing thermal energy have also gained popularity due to their environmental friendliness.
As we step into the future, materials innovations and process enhancements will continue to benefit all types of energy related fields. Non-electrochemical energy storage devices are no exception. Devices such as flywheels (mechanical energy storage) and thermal molten salt ponds (thermal energy storage) will continue to improve in performance and reliability as they incorporate and adopt more advanced materials and processes.
Chemical energy storage through hydrogen, biofuels and others will also benefit from materials and process enhancements, either directly or indirectly. For example, hydrogen fuel cell or proton exchange membrane fuel cell (PEMFC) requires hydrogen fuel. Hydrogen needs to be stored in tanks and transported to the fuel cell. If PEMFC is used in a vehicle, then hydrogen needs to be ideally transported to and available at the fuel stations.
If we consider the storage and transportation of the hydrogen itself, the hydrogen tanks require special material properties to ensure reliable and efficient storage. When we discuss all aspects of hydrogen fuel cells, we sometimes inadvertently exclude the storage and processes related to the fuel itself (i.e., hydrogen and oxygen). There are many intricacies related to any energy storage device or system at multiple levels that need to be considered for an overall efficient, effective and reliable energy storage method, especially for large mass consumption.
The materials for flywheels have been enhanced over a long time. Innovations in composites and superconducting materials have led to significant improvements in flywheels, and such enhancements will continue in future. One example is the ongoing research and development in the superconductive magnetic bearing used in flywheels. More information on flywheels can be found in the literature including [Mukoyama et al., 2017] [Miyazaki, 2016] [Faraji et al., 2017].
Superconductive magnetic energy storage system (SMES) has also promises in the future. SMES is considered for integration with large energy harvesting and consumption systems. Studies related to SMES can be found in the literature, including integration strategies with various energy methods, such as SMES-wind power integration [Zhao, 2015] and hybrid SMES-batteries for electrical microgrids [Cansiz et al., 2018].
12.8 Concentration Cells
Perhaps the most intriguing aspect of the concentration cell approach is its utter simplicity, at least in terms of its basic operation. The fact that it is possible to store usable amounts of energy with only a single element or compound in merely a rarified or compressed state in the immediate presence of an electrode is remarkable. That a potential can be obtained in such a fashion between the same materials simply at different concentrations holds a certain fascination fact. Furthermore, that it requires no sensible difference in pressure to maintain these concentration differentials is also quite attractive. The easiest way to understand this notion is to relate it to the compression of a gas as an analogous means of storing energy in mechanical form.
12.8.1 Pros and Cons of Concentration Cells
The search for hardware simplicity, long operating life, and low cost has led to the exploration of the concentration cell as a means of storing energy in electrical form and returning it in the same form. Probably the single greatest attraction of this class of device is the possibility of an extremely long operating life and very high energy densities. Since such cells are entirely symmetrical physically as well as in materials, it offers promise of a sustained and easily regenerated format. The concentration cell is symmetrical around an axis at the center. In other words, the two electrodes are identical in materials of construction as well as physical size and shape. An electrolyte is also the same at the time of construction.
Cycling the cell merely changes the concentrations of the same materials. A completely discharged cell has electrolytes identical at each electrode. These concentrations change during charging but will return to the initial value when discharged. The main motivation for pursuing this approach is its independence of specific materials properties. In principle, there is no limitation to energy density as there is in electrochemical couples where voltages are direct functions of the materials. Since energy is stored as “compressing and rarefying” a single substance, the capacity limitation, though real, is due only to physical material storage capacity.
The second attraction is the low cost and easily managed electrolytes and electrodes. The electrolytes, usually a compound of iron or sulfur, are chemically well behaved. Carbon in various forms is employed as the electrode material, and it is exceedingly inert at the temperatures at which cells are operated. Depending on cell design and the materials employed, it is possible to construct well functioning cells without the necessity of expensive or special property ion transfer membranes.
On the negative side, there are a number of cell characteristics around which one must design in order to obtain a power source and practical device. Perhaps the most serious of these inconvenient characteristics are the self-discharge rates and the non-constant load voltage. Cell potential is directly proportional to its state of charge. Hence, these types of concentration dependent devices will have “sloping voltages,” or voltages that change with time. Such undesirable characteristics necessitate the use of external circuitry to “flatten out” the voltage versus time, or voltage versus “state-of-charge” curves, in order to make them more compatible with practical load and power supply situations.
If there is improvement in employing the reagents in such cells in the solid form, as was described in some detail in previous chapters, then the concentration cell might very well be the leading contender for energy storage on a large scale and for electric vehicles of various kinds. An energy density of 100 WH/lb at the very least or 200+ WH/Kg is needed for any electric car to become practical even on a limited basis.
12.8.2 Future Performance and Limitations
The energy density of just the reagents can be estimated by assuming a nominal operating cell voltage. It is necessary to make such assumptions because a concentration cell has no specific voltage in the same sense that a “conventional cell” has due to its different, specific electrode materials. Again, let us continue with the sulfur/sulfide cell employing sodium as cations as a means for making such projections.
Looking again at the simplest version of this concentration cell based upon sulfur chemistry, we see that at full discharge the composition of electrolytes in both chambers must be the same. When such a cell is in the discharged state, symmetry dictates that the concentration of ionic components must be identical. Thus, when discharged the chemical compositions on either side of the cell are
(12.2)
and when totally charged its electrolyte form is
(12.3)
Considering this as the normalized minimal electrolyte condition, the charge available for each mole is 52 ampere-hours. A one mole-perconstituent cell would have a total weight of reactants equal to about 220 grams. About 52 ampere-hours are minimally required to charge such a cell with one mole of reactants on each side of the separator. If we assume an average working voltage of 1.0 volt, then the energy density of the cell exclusive of weight contributions of water, case, or electrodes becomes 215 watt-hours per pound of reagents – a respectable number, but hardly impressive.
Replacing sodium with lithium can contribute to an increase in ED. The atomic weight of lithium is about 7, as compared to 23 for sodium.
In order to significantly raise the energy density of these cells, it is necessary to resort to non-aqueous solvents, thus enabling cell voltages that are much higher than the maximum 1.2 volts. If this is done, then the upper limits of energy density can be many times higher.
Regardless of how high the ED can be raised, concentration cells’ advantages of low cost, dependability, and long life make such an approach to energy storage attractive enough to be pursued further and with some intensity in the future.