Energy storage: electric battery technologies

Electric batteries are electrochemical energy storage devices. This means that in these devices energy is stored or discharged through chemical reactions. Batteries always have a very similar structure, being composed of cells made up of two electrodes, an electrolytic substance (also called an electrolyte, usually a liquid solution) and a separator, as shown in Figure 1.

What differentiates the various battery technologies are the materials used in the electrodes, the electrolytic substances and the construction aspects. The most common batteries are lead-acid (Pb), lead-carbon, lithium-ion (Li-ion), nickel-cium (NiCd), nickel-metal hydride (NiMH), nickel-sodium chloride (NaNiCl2), sodium-sulfur (NaS) and flow batteries (which can use different chemical elements). The subject of electrochemical batteries is quite rich and extensive.

The objective of this article is to carry out a brief review of electrochemical batteries, with emphasis on the technologies currently most widely used or most promising for use in Photovoltaic systems and electrical energy storage systems in general.

Among the technologies most used in energy systems, lead-acid and lithium-ion batteries stand out. Lead-acid batteries are an old technology and are still widely used, especially in off-grid energy systems.

As lithium ion batteries are currently the big stars of the storage market, with many applications in energy systems and electric mobility. The need for more efficient, more durable, more compact, lighter and cheaper batteries has led to the relentless search for new materials and new electrochemical storage technologies.

Recently, the growing interest in electric mobility has strongly driven research in the area of ​​solid electrolytes, which provide lithium batteries with greater durability, greater energy density and greater safety. Among the alternative technologies, flow batteries and liquid salt batteries stand out, still little known, but very promising for large-scale storage applications.

Battery characteristics

Batteries of different technologies can have very different characteristics. Batteries may have high storage capacity and high energy density, but their cycle life may be short. On the other hand, they may be designed to have high durability, but they may be heavy and bulky. They may also have high capacity and long durability, but their cost may be prohibitive for most applications.

Batteries can be evaluated according to specific energy (Wh/kg), energy density (Wh/L), charge capacity (Ah), acceptable depth of discharge (DOD), lifespan (related to the number of charge and discharge cycles that the battery s), the ability to withstand high temperatures (which affect lifespan and operating safety), specific power (W/kg) and C-rate, which determines the speed at which batteries can be charged or discharged.

C-Rate: the charging speed of battery systems

There are determining characteristics depending on the type of application. For stationary applications in renewable energy and electrical systems, the main characteristics are generally cost, number of work cycles, efficiency and robustness – the latter associated with the ability to withstand high temperatures, overloads or deep discharges.

For mobile applications, specific energy (mass), energy density (volume) and charge and discharge speed are important characteristics. There is no ideal battery, but there is the most suitable battery for each application, taking into technical, economic, logistical and even social aspects. For example, lead-acid batteries are very suitable for off-grid photovoltaic systems used to serve less privileged populations in remote and difficult-to-access locations, almost always in hot regions of the planet.

Lead-acid batteries withstand high operating temperatures (up to 60°C), are inexpensive, can withstand occasional deep discharges, are easy to recycle and are robust, requiring no specialized maintenance or complex electronic monitoring and control systems. The same cannot be said for lithium batteries, which are highly sought after for modern storage systems and electric vehicles, but are very complicated to install in locations where technical assistance is difficult or often unfeasible.

Figure 2 illustrates the relationship between specific energy (Wh/kg) and energy density (Wh/L) of some common battery technologies. The mass and volume disadvantage of lead-acid batteries compared to lithium batteries can be seen.

This disadvantage, however, does not take into other factors that can make lead-acid batteries more attractive, as mentioned in the previous paragraph. On the other hand, if we are looking for more compact and lighter systems, lithium technology is unmatched – which is why it currently dominates the energy storage market.

Figure 3 illustrates the relationship between charge/discharge time and the power of the systems in which these batteries can be used, including mature and widely used technologies (such as lead acid and lithium ion) and technologies still under development or little known (such as flow batteries).

Interpreting Figure 3, we can see that flow and liquid salt batteries are only viable in large storage systems, where their technical complexity and high response time are not limiting factors. Lithium-ion batteries have a moderate response speed and occupy an important position, being suitable for both small and large storage systems.

Lead acid batteries, due to their reduced cost, their robustness and their satisfactory response speed, are still preferred in many small power systems, and can also reach large-scale applications, although they are preferably replaced by lithium batteries or others with ability to perform a greater number of operation cycles.

Lead acid batteries

Lead-acid batteries are the oldest rechargeable batteries on the market. They first appeared in the early 1900s and are still the preferred choice for many applications due to their robustness and low cost. Their main disadvantages are their low energy density (they are heavy and bulky) and their short lifespan, which means they cannot withstand a large number of charge and discharge cycles.

They also have the disadvantage of low discharge depth, which is typically limited to 80% in extreme cases or 20% in regular operation, for greater longevity. Excessive discharge degrades the battery's electrodes, which reduces its ability to store energy and limits its useful life.

Lead acid batteries require constant maintenance of their state of charge and must always be stored at their maximum state of charge using the floating technique (maintaining the charge with a small electrical current, sufficient to cancel the effect of self-discharge).

These batteries can be found in several versions. The most common are the ventilated ones, which use liquid electrolyte, the valve-regulated gel batteries (VRLA) and the batteries with electrolyte embedded in a fiberglass mat (known as AGM – absorbent glass mat), which have intermediate performance and lower cost compared to gel batteries.

Valve-regulated batteries are virtually sealed, which prevents leakage and drying out of the electrolyte. The valve releases gases in cases of overcharging. Some lead-acid batteries are designed for stationary industrial applications and can accept deeper discharge cycles. There is also a more modern version, the lead-carbon battery.

Carbon-based materials added to the electrodes provide higher charge and discharge currents, greater energy density and longer service life. One advantage of lead-acid batteries (in any of their variations) is that they do not require a sophisticated charge management system (as is the case with lithium batteries, which we will see below).

Lead-acid batteries are much less likely to catch fire and explode when overcharged, because their electrolyte is not flammable like that of lithium-ion batteries. Furthermore, a slight overcharge is not dangerous for these types of batteries. Some charge controllers even have an equalization function that slightly overcharges the battery or battery bank, bringing all the cells to a full state of charge.

During the equalization process, the cells that eventually become fully charged before the others will have their voltage slightly increased, without risk, while the current flows normally through the series association of elements. In this way, we can say that lead batteries have the ability to equalize naturally and small imbalances between the cells of a battery or between the batteries in a bank do not pose a risk.

Figure 4 illustrates modern versions of lead-acid batteries, with the addition of carbon to the electrodes, among other features, that make them more durable and more suitable for application in stationary energy systems.

The sealed battery, shown on the left (in Figure 4), has the advantage of being maintenance-free and has a service life of over 1.000 cycles under normal temperature conditions and when operated with a depth of discharge of around 20% (the recommended limit for this type of battery). The vented version, shown on the right, s over 6.000 operating cycles under the same conditions, with the need for periodic replenishment of the electrolyte level.

Lithium-Ion Batteries

Currently, the most commercially successful batteries are lithium-ion batteries. After being adopted in portable electronic devices, lithium-ion technology has reached industrial applications, electrical power systems and electric vehicles. Lithium-ion batteries outperform many other types of rechargeable batteries in several aspects, such as energy storage capacity, number of operating cycles, charging speed and cost-effectiveness.

Currently, its only problem is safety, which is compromised by the flammable electrolyte, which can ignite at high temperatures, requiring the use of electronic control and monitoring systems. Lithium is the lightest of all metals and has the highest electrochemical potential, providing the highest energy density per volume and mass compared to other known battery technologies, as shown in Figure 2.

Lithium-ion technology has enabled the advancement of the use of energy storage systems, mainly associated with intermittent renewable sources (solar and wind), and is also responsible for the popularization of electric vehicles. Lithium-ion batteries applied to electric power systems and electric vehicles are liquid type.

Energy efficiency in PV systems: advances with lithium batteries

These batteries employ the traditional construction of an electrochemical battery, with two electrodes immersed in a liquid electrolyte solution, as shown in Figure 1. A separator (porous insulating material) is used to mechanically distance the electrodes, while allowing the free circulation of ions through the liquid electrolyte.

The main characteristic of the electrolyte solution is the ability to allow the conduction of ionic current (formed by ions, which are atoms with an excess or lack of electrons), while not allowing the age of electrons (as occurs in conductive materials). The exchange of ions between the positive and negative electrodes is the basis of how electrochemical batteries work.

Research into lithium batteries dates back to the 1970s, and the technology became mature and began to be used commercially around the 1990s. Lithium polymer batteries (with polymer electrolyte) are now used in cell phones, computers and all types of mobile devices, having replaced the old nickel-cium batteries, which had as their main problem the “memory effect”, which gradually reduced storage capacity when the batteries were recharged before they were fully discharged.

Compared to their older nickel-cium and, especially, lead-acid rivals, lithium-ion batteries have higher energy density (store more energy per volume), have a low self-discharge coefficient and a greater number of batteries. charge and discharge cycles, which translates into a prolonged useful life.

Around the early 2000s, lithium-ion batteries began to be used in the automotive industry. Around 2010, lithium-ion batteries gained interest in electrical energy storage, both in residential applications and in large ESS (energy storage systems), largely due to the increasing, worldwide, use of intermittent renewable sources (solar and wind).

Lithium-ion batteries can have different performances, lifespans and costs depending on how they are manufactured. Several materials have been proposed, especially with regard to the electrodes. Typically, a lithium battery consists of a lithium metal electrode forming the positive terminal of the battery and a carbon (graphite) electrode forming the negative terminal.

The lithium-based electrode can come in different configurations, depending on the technology used. The most commonly used materials in the manufacture of lithium cells and the main characteristics of these batteries are listed below:

• Lithium cobalt oxide (LCO): High specific energy (Wh/kg), with good storage capacity and satisfactory lifetime (number of cycles) for applications in electronic equipment, with the disadvantage of low specific power (W/kg), which reduces charge and discharge speeds;
• Lithium manganese oxide (LMO): Allows high charge and discharge currents, with low specific energy (Wh/kg), which translates into reduced storage capacity;
• Lithium, nickel, manganese and cobalt (NMC): Combines characteristics of LCO and LMO batteries. In addition, the presence of nickel in the composition helps to increase specific energy, providing greater storage capacity. Nickel, manganese and cobalt can be used in different proportions, according to the type of application (to prioritize one or another characteristic). In general, the result of this combination is a battery with good performance, good storage capacity, good service life and moderate cost. This type of battery has been widely used in electric vehicles and is also suitable for stationary energy storage systems;
• Lithium, iron and phosphate (LFP): The LFP combination provides batteries with good dynamic performance (charge and discharge speed), increased service life and greater safety due to their good thermal stability. The absence of nickel and cobalt in their composition reduces the cost and increases the availability of these batteries for mass production. Although their storage capacity is not the highest, they have been adopted by manufacturers of electric vehicles and energy storage systems due to their various advantageous characteristics, especially their low cost and good robustness;
• Lithium-titanium (LTO): This designation refers to batteries that have titanium and lithium in one of the electrodes, replacing carbon, while the second electrode is the same as that used in one of the other types (such as NMC – lithium, manganese and cobalt). Despite the low specific energy (which translates into reduced storage capacity), this combination has good dynamic performance, good safety and a significantly increased service life. Batteries of this type can accept more than 10.000 operating cycles with 100% depth of discharge, while other types of lithium batteries accept around 2.000 cycles.

Current research into lithium-ion batteries is looking for new materials and manufacturing methods that can increase battery life, energy density, safety, and charging speed – while reducing production costs. Recent developments have been toward solid-state lithium batteries, in which the liquid electrolyte is replaced by a solid material that has the property of conducting ions.

Solid electrolyte is a big bet for the electric vehicle industry in the coming years, with the promise of practically doubling the autonomy of current vehicles. The electrical energy sector, in which the use of storage systems has become increasingly intense, will also benefit from this new technology, with a reduction in the size and cost of battery banks.

Some advantages of solid electrolyte are increased battery durability, reduced mass and volume and increased safety, since solid electrolyte is not flammable and does not present a risk of explosion, as occurs in liquid batteries.

All of the above advantages are accompanied by reduced manufacturing costs, as the solid electrolyte eliminates the use of separators between electrodes and eliminates the need for sealing coatings (to prevent liquid leakage) and protective casings (to prevent accidental punctures, necessary concern in electric vehicles).

Lithium batteries are commercially available in cells, blocks and banks. Cells are the basic units, which have storage capacities of the order of 1 to 5 Ah, with a nominal output voltage of 3,7 V. For practical applications, these cells need to be organized in blocks or packs, as shown in Figure 5, where they are connected in series. The blocks, in turn, can be connected in parallel to form battery banks.

For application in energy systems, smart battery banks with one or more lithium cell packs associated and integrated with BMS (battery management system) circuits are already found on the market, produced by companies such as Tesvolt, BYD and Dyness – brands commercially available in Brazil.

Flow Batteries

Flow batteries are a technology that is slightly different from other electrochemical batteries that we know. Instead of the traditional structure shown in Figure 1, in which the electrodes are immersed in an electrolyte solution, flow batteries use electrolytes stored in tanks. For the cells to work, it is necessary to pump the electrolytes so that they come into with two electrodes separated by a membrane.

This type of battery is viable in large storage systems and is very advantageous due to its high longevity (with more than 10.000 charge and discharge cycles). Its specific energy (Wh/kg) is similar to that of lead-acid batteries. This fact, associated with its construction complexity and low response speed, makes its use unlikely in mobile systems.

Also known as redox (reduction and oxidation) batteries, this family can have different chemical compositions. There are already commercial versions of flow batteries, while they are still the subject of research and have not become as well-known as other technologies already well established on the market.

Liquid Salt Batteries

This type of battery uses electrodes composed of salt in a liquid state. For this to be possible, it is necessary to keep the salt heated to high temperatures (around 350 oC) through an internal heating system. This is not very practical for mobile applications, but for stationary power systems the technology is viable and promising.

This type of battery is very advantageous due to its high longevity, and can be stored for many years at room temperature, when its electrodes acquire a solid state. When in operation, keeping the electrodes in a liquid state, they can achieve a large number of charge and discharge cycles.

The best-known variations are the sodium-sulfur battery (NaS) and the nickel-chloride battery (NiCl2). Their specific energy (Wh/kg) is comparable to that of lithium-ion batteries, with the advantage of a long useful life – reaching 4.500 cycles, with a life expectancy of up to 20 years.

Although they are not as well known, and are still classified as alternative batteries, there are already commercial applications of this technology in large storage systems. The good longevity, satisfactory specific energy and reduced cost are characteristics that have attracted the attention of this type of battery in applications BESS (battery energy storage systems) large scale.

Nickel Batteries

Nickel-cium (NiCd) technology has long been widely used in portable electronic equipment. Known for its robustness, it was later replaced by nickel-metal-hydride (NiMH) technology, which has similar characteristics and the main advantage of not using cium, a toxic material that makes it difficult to dispose of batteries at the end of their useful life. , in addition to being less subject to the memory effect, which reduces the storage capacity of batteries.

NiMH technology is quite mature and has a long lifetime, high discharge capacity and is economically viable for use in consumer electronics. NiMH batteries are currently found on the market in the format of common batteries and also in other formats for portable and industrial applications. Its use in storage and electric mobility systems is restricted, as its characteristics do not overcome the advantages of lithium-ion batteries.

References

• A review of electrochemical storage technologies for photovoltaic application. Tatiane Silva Costa, Maria de Fatima Rosolem, Marcelo Gradella Villalva (in press)
• Types of Lithium-ion, Battery University, Cadex Electronics
• Batteries in a portable world, Cadex Electronics
• Overview of rechargeable lithium battery systems. Peter Kurzweil, Klaus Brandt. In: Electrochemical power sources: fundamentals, systems and applications, Elsevier, 2019
• Handbook of batteries. David Linden, Thomas B. Reddy. McGraw Hill
• Electric Energy Storage Technology Options: A White Paper Primers on Applications, Costs, and Benefits. Electric Power Research Institute
• Electricity Storage Handbook, Sandia National Laboratories, SAND2013-5131

This article was published in the 2nd edition of the Magazine Canal Solar