S6C5: Batteries & Energy Storage

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a) Principles of the voltaic cells

We are nearing the end of the Knowledge Series by now and this is the final chapter that will be dealing with anything physics or chemistry in a technical way. Since the beginning of this sixth series, we have looked at energy generation, then transmission and distribution in Chapter 4 on the electricity grid, and mentioned a few times the importance of storage solutions in order to optimize the entire ecosystem from generation to consumption and avoiding the need to build an expensive base load capacity to meet peak demand of power.

Back in S1 Section 7.e on electrochemical reactions, the concept of the electric battery was explained at a very high level, let’s describe it in more detail now. The process is called electrochemical because chemical energy is converted into electrical energy in the form of current, which is the consistent flowing of electrons in one direction. For batteries, this would be direct current (DC) because the direction doesn’t reverse in phases. In a prototypical voltaic cell (also called galvanic cell), there would be two half-cells: one with a zinc bar surrounded by an electrolyte such as zinc sulphate (ZnSO₄) and the other with a copper bar and another electrolyte such as copper sulphate (CuSO₄) that splits into two ions Cu²⁺ and SO₄^2- when dissolved in water. Because the electrons in the zinc metal are not as strongly bound compared to the pull of electrons from Cu²⁺, if we connect the two half-cells then the electrons will flow from the zinc anode towards the copper cathode – this is our electric current. At the anode, oxidation takes place and the zinc atoms losing a pair of electrons will detach from the solid metal as Zn²⁺ and find their way into the electrolyte and, at the cathode, the copper cations will reduce into copper metal. As a result, the zinc bar loses mass and the copper bar gains mass. Nonetheless, there is a missing part here since these would result in a build-up of positive charges at the anode and negative charges at the cathode. Thus, in order to allow the charges to flow back from one side to the other without mixing the two solutions, we can use a salt bridge with Na⁺ and Cl^– ions and those will move through plugs or filters to neutralize the electrical charge on each side: Cl^– towards the anode with excess of Zn²⁺ and the Na⁺ will make its way towards the cathode with excess SO₄^2-. Reaction in the battery will cease when the zinc bar has fully been oxidized or the salt bridge is emptied of ions.

The reactions of a galvanic cell are spontaneous and only occur in one direction because they are exothermic. However, some batteries can be recharged by applying direct electric current, which provides additional energy into the system and can yield the reverse endothermic electrochemical reaction. When amortized over the life of the rechargeable battery, this significantly reduces the cost for the consumer, and in most cases, for the environment as well.

An example of rechargeable battery most readers will be familiar with is the lead-acid battery we can find in many cars powered by an internal fuel-combustion engine (S5 Section 2.b). When charged, the positive plate is lead dioxide (PbO₂), the negative one is pure lead, and in between is the sulfuric acid solution (H₂SO₄). During the discharge process, the lead reacts with the acid into lead sulphate (PbSO₄) and hydrogen (H₂) plus 2 electrons and those move to the positive plate where they facilitate the reaction of the lead oxide plus the sulfuric acid plus the hydrogen into lead sulphate and water (H₂O). Accordingly, in a discharged state, the negative and positive plates consist of lead sulphate and the electrolyte acid solution has been diluted with water. And the reverse set of reactions occurs when the electrons are made to flow the other way around.

Other popular types of batteries include:

Alkaline batteries using zinc (Zn) and manganese dioxide (MnO₂) with an electrolyte having a pH value above 7.0, such as potassium hydroxide (KOH). Although some models can be recharged a few times, this formulation is widespread among disposable batteries sold to consumers in AA or AAA and even some button formats.
Nickel-cadmium batteries (Ni-Cd) were once popular among rechargeable batteries but the toxicity of Cd means they have been superseded by Nickel-metal hydride batteries (NiMH). Metal hydrides is a generic name for a metal cation bound to a hydrogen anion.
Lithium-ion batteries are quite different and lithium ions will separate from graphite at the anode during discharge and migrate through an electrolyte consisting of lithium salts (e.g. LiPF₆ called lithium hexafluorophosphate) to the cathode to create a compound such as lithium cobalt oxide (LiCoO₂) – while the electrons move in the same direction but via the electric circuit. These batteries are prevalent in consumer electronics and electric vehicles.

b) Key properties of electric batteries

Different types of batteries means different types of chemical reactions and therefore properties. First among those is energy density expressed in Megajoule or watt-hours per litre (MJ/L or Wh/L) – alternatively we can consider this concept as a ratio to mass rather than volume – this is called specific energy in the jargon and it is measured in energy density in watt-hours per kilogram. For a given size or mass, the energy density will thus determine how much energy can be stored and made available during discharge. On the other hand, power density is the speed at which the stored energy can be released and specific power is the power-to-mass ratio. All of these are a function of the compounds used at the anode and the cathode, and therefore the energy released in each half-cell during the reaction, as well as other parameters such as the ionic conductivity of the electrolytes and the nature and weight of the other elements necessary to make a functioning battery, including the separators and casing. To give a sense of proportion, a lead-acid battery may have an energy density of 35Wh/kg, an alkaline one might reach in excess of 150Wh/kg, Nickel Metal Hydride would be around 100Wh/kg and the Lithium cobalt oxide mentioned earlier would be the highest among these at 195Wh/kg.

Then there are many practical aspects that may become important depending on the applications. For instance, the lead-acid batteries can cope with the elevated input surge current drawn from a starter to initiate the engine’s operation. Or the chemical processes taking place within lithium-ion batteries is significantly impacted by low temperatures, so they effectively lose some of their charge in such environments.

Beyond the type of application, other practical aspects include whether or not a battery can be recharged or needs to be disposed after a one-time use and its charging time, a function of the size of the battery and total energy stored of course, but also of the nature of the reactions. The self-discharge rate and shelf life are also aspects to consider since they can vary widely. The latter can be as high as 30 or even 50 years for Nickel-iron, around 20 years for lithium-ion batteries and 5 to 10 years for alkaline types. And the self-discharge rate would be around 5% per month for Li-ion, less than 1% for alkaline and possibly more than 10% per month for lead-acid.

Finally, cost needs to be considered. This can be environmental and would depend on the toxicity of the chemicals, the disposal and recycling processes, as well as the energy intensity of the manufacturing process. Then there is the financial cost, which should be amortized over the lifecycle of the batteries, and for rechargeable versions the evaluation should take into account the maximum number of charge cycles – there is an upper limit because each round of chemical reaction is never 100% efficient so there is an increasing percentage of compounds that no longer reacts at the time of charge and discharge.

c) Fuel cells

A battery can be thought of as an isolated or self-contained system consisting of a pair of redox reactions, being oxidation at the anode and reduction at the cathode, set up in such a way that electrons can be made to flow from one side to the other through a circuit and the build-up of charges can be neutralized through an electrolyte, together with a given quantity of fuel and oxidizing agent. In other words, we have a process and content all in one place and limited quantity. In the case of a rechargeable battery, it is possible to “reinitialize” the quantities by supplying direct current to reverse the chemical reactions.

When seen from this perspective, we can then describe a fuel cell as a partially open system where the process is unidirectional but the volume of fuel and oxidizer can be topped up ad infinitum, subject to tank storage space. In this sense a fuel cell, like an internal combustion engine, relies on the flow of those two compounds to generate electric current and as long as fuel and oxidizing agents are provided the fuel cell can release energy.

The most common oxidizer for fuel cells is oxygen from our atmosphere and it can be treated as a limitless supply in most applications. As for the fuel, it is often hydrogen, and you may recall fuel cells were mentioned in S6 Section 3.d titled “the hydrogen markets”. There are several ways this can happen, and the two most common are called proton-conducing and the alkaline fuel cells. In this former instance a hydrogen molecule (H₂) loses 2 electrons to become 2 hydrogen cations (H⁺), also known as protons, and four of those H⁺ react with oxygen molecules (O₂) and the electrons coming in at the cathode to yield two water molecules (H₂O). In the latter case, there is an alkaline solution in a porous substrate situated between the two electrodes and the hydrogen gets oxidized by hydroxide ions (OH-) at the anode which are produced by the reduction of oxygen with water at the cathode. I include a link to the Wikipedia entry for alkaline fuel cell at the end of this chapter if you wish to know more.

The main advantages of fuel cells are their reliability, because there is no internal combustion or moving parts, their specific energy is very high compared to electric batteries, and they can be coupled with fuel storage to provide electricity. This makes them an ideal candidate for markets such as electricity generation next to hydrogen production sites and weight but non-volume sensitive propulsion engines such as spacecrafts. Unfortunately, its widespread adoption is partially tied to the issues faced by hydrogen as a fuel generally, as discussed in S6 Section 3.f.

d) Mechanical energy storage

Batteries provide a relatively energy-dense way to store energy and release it in electrical form and so do fuel cells when coupled to stored fuels (and to an oxidizer, if it is not present in the immediate surroundings). However, these are relatively expensive fuels and equipment and, even though the purpose of energy storage is often to produce electricity, like the water reservoirs used for hydropower, this is not always the case. For that matter, several types of energy storage existed before the concept of electricity had been worked out, and among these the two main types are mechanical and thermal in nature.

Some of the benefits of this type of storage can be the low cost of material, the high energy efficiency, meaning there is little loss in the process of back-and-forth conversion, the long-life cycle of the materials and systems, and the limited “self-discharge” over time, what we can think of as energy leakage from the system. Furthermore, the materials employed can often be selected to have low environmental footprint and no or low toxicity. The major drawback however, is the low energy density and specific energy, so that a lot of volume and materials are required in comparison with electric batteries or fuel cells. In this section we will look at three examples of mechanical energy storage technologies and in the next one we will shift our attention to thermal energy storage solutions.

We have already partially looked at the first one in the list, springs, in S4 Section 10.a on the mechanical watch and saw how they provide a continuous, gradual release of energy through physical pressure and the way this allows for the maintenance of an oscillation frequency and the subsequent driving of the cogs. A spring can loosely be defined as a device made of an elastic material, so it can undergo compression and yet return to its original shape when compression ceases. Accordingly, a bow would qualify though, for long-term storage of meaningful energy amounts, coiled metallic springs would be preferred. The force deployed over the distance of physical deflection experienced by the spring is called the spring’s extension or compression rate and is expressed in newton per metre (N/m). It works until the elasticity limit is reached and it is additive, so if there are 10 springs next to each other with the same compression rate then the whole set can store 10 times as much energy as a single spring would. Torsion springs are partially different because they store energy in the form of torque when twisted, so the amount of energy stored is a function of the angle by which it has been twisted around its axis, and its torsion rate or coefficient is measured in newton-meters per radian (2π radians is equal to 360 degrees).

Since we are on the topic of torque and angles, angular momentum can also be used to store energy: take a heavy object, deploy torque to spin it, keep it spinning until you need energy, and then convert the angular momentum into another form of energy, typically electricity via a generator. This technology is called flywheel energy storage and, because materials with low density and high strength such as carbon-fibre composites are used, they can exhibit good specific energy, in the 100-130 Wh/kg range and high energy efficiency, as elevated as 90% for a round-trip. The main challenge is to avoid dissipation during storage and the key in that respect is the nature of the bearings, air friction, and the alignment with the axis of rotation of our planet. In a vacuum and with magnetic bearings, the loss can be minimized.

The main applications of flywheels in the context of power supply and management are for load levelling on the grid or business consumer side, temporary storage coupled to renewable energy assets such as wind turbines with large intraday variation of electricity output and, because it can deliver very high levels of powers for a short duration, the fusion experiments using tokamak technologies mentioned at the end of S6 Section 3.d.

Onto our third example of mechanical energy storage. We have seen that whether it is wind or compressed steam in a steam engine, an area with a high gas pressure will exert a force on a foil or other device where the other side has a lower pressure, and it will do so until the pressure has been equalized. Therefore, energy can be stored as compressed gas, as long as the pressure can be maintained without requiring additional energy input. This method is called compressed-air-energy storage (CAES) and it has historically been used to power mine locomotives (no spark involved, which is much safer in an underground coal mine) or to help initiate the motion of the crankshaft in large combustion engines such as those of freight ships.

The major concern with such process is the amount of energy dissipated as heat since a rise of pressure is by definition an increase in kinetic energy of the molecules and atoms within the fluid and thus induces higher temperatures. Consequently, the energy of CAES can be greatly enhanced when the system is supplemented with heat recovery and storage equipment. Which provides the perfect segue to the next section.

e) Thermal energy storage

Like mechanical energy storage, the thermal version can be used to reduce the installed electricity generation requirement within the electric grid ecosystem by supplying some of the peak power demand. The technology we are the most familiar with is the isolating of materials that are kept either hot or cold until required to avoid the need for cooling or heating equipment. Water ice from the mountains is a prime example and has been used for several centuries. At industrial scale, the dominant material used is molten salts that are solid at room temperature but become liquid when the temperature increases (this would be 450°C for potassium chloride for instance) and, as fluids, they can then be used for transferring heat into another system. For example, concentrated solar power (refer to S6 Section 2.e) can use molten salts to store heat by focusing sunlight and raise their temperature. This heated liquid is then kept in isolation until the energy is needed, at which point it can be converted to superheated steam driving steam turbines to produce electricity.

In S1 Section 2.e we had covered the nature of phase transitions from one state of matter to another, such as liquefaction, freezing or vaporization under the effect of increased kinetic energy brought about by heat, cold, pressure or lack thereof. This means that, even though the temperature during a phase transition doesn’t change, the energy stored in the system does vary. Accordingly, systems can be engineered where heat will induce a phase transition at a certain temperature and, without this temperature changing, heat can be released later one when the phase transition is being reversed. This process is called latent heat storage and is not unidirectional, as suggested earlier. For example, energy can be used to liquefy air or other gases, and later on their vaporization will extract heat from the surrounding environment, thus acting as a cooling medium.

The change of states of matter could be thought of as a reaction resulting in the severing or creation of intermolecular bonds and, if this can either absorb or release energy, so can chemical reactions where it is intramolecular bonds that are being re-arranged, as we have seen many times by now. Hence, a chemical reaction can be described as endothermic or exothermic, meaning it requires or releases energy, respectively, and it is possible to temporarily store energy by inducing an endothermic chemical reaction, storing the compounds, and then release energy by reversing the reaction, which this time around is exothermic.

We call this thermo-chemical storage and one technology we can bring up to illustrate this process is the use of adsorbents and refrigerants to cool a space. The refrigerant is the working fluid and the process of adhesion of the molecules to the surface of the adsorbent is exothermic while the drying up of the adsorbent and re-formation of the cooling liquid is endothermic, which has a cooling effect on the surrounding air since it extracts heat from it. Conversely, exposing the adsorbent to dry hot air will store up heat that can later be released by letting humidified air flow through it and the water be adsorbed. A popular adsorbent is zeolites, these are microporous crystalline minerals also regularly used as catalysts. I am including the Wikipedia entry for zeolites in the last section if you wish to read more about them.

f) Trivia – Charging towards the future

As we progressed through this chapter and the previous one, it should have become clear the overall efficiency of a power generation system should often not be considered solely as a matter of energy conversion but should also take into account the energy storage aspect. This more holistic approach opens new options and make others look either uneconomical or impractical. This explains why so many investments are directed towards more efficient energy storage systems, and towards electrical batteries in particular.

Here I will briefly highlight three promising technologies that are already known to be feasible but have not yet been commercially scaled up or widely adopted.

Instead of the liquid electrolytes found in “standard” batteries such as lead-acid or Li-ion, solid-state batteries allow for ions’ transfer through a solid electrolyte. The materials used vary and include polymers or ceramics and they offer comparatively higher energy density, faster charging time, more thermal stability and the ability to operate at higher temperatures. The main challenges at this stage are the manufacturing costs, inefficiencies or degradation at the interface between the electrodes and the solid electrolyte, and their poor performance in cold temperatures.
In Sodium-ion batteries (Na-ion), it is Na+ which serves to close the circuit in terms of charge rather than the Li+ of the lithium-ion batteries. The overall principle is otherwise the same and the choice of sodium should not be surprising since they belong to the same group in the periodic table of the elements (refer to S1 Section 2.c on chemical elements). The main advantage, of course, is cost since sodium is plentiful in seawater and cheaper, more abundant materials such as iron and manganese oxides can be used for the redox reactions. Compared to Li-ion technology, it offers a much better power-to-weight ratio but lower volumetric energy density and specific energy. More importantly, current technologies are still hampered by a lack of cycling stability due to reactions taking place at the interface between the electrolyte and the electrodes.
Finally, iron-air batteries. These are only one type within the broader category of metal-air batteries and the main raw material used is iron oxide (FeO), also known as rust! The oxidation of iron (Fe) alongside water (H₂O) requires electricity and yields iron oxide and hydrogen (H₂) and this hydrogen can then be used in a fuel cell to produce electricity. It is the combination of both that creates a rechargeable battery. The main drawbacks of this technology are the slow charging and discharging time, the limited round-trip efficiency (in the 50-60% range) and the degradation of the materials, thereby limiting their lifecycle.