Stationary energy storage systems are playing an increasingly important role in the energy revolution. By flexibly storing electrical energy, they enable the long-term integration of renewable energies while maintaining the stability and performance of the power grid. Battery-based energy storage systems, or so-called “Battery Energy Storage Systems” (BESS), are particularly suitable for these applications and are therefore the focus of Kyon Energy's activities. Battery cells form the core of such stationary energy storage systems. In this blog article, we will shed light on how they work and which technologies are suitable.
Batteries have become an integral part of modern life and are considered one of the most disruptive technologies in human history: we encounter them — in addition to stationary energy storage systems, of course — in smartphones and laptops, as well as in the increasing number of electric cars in road traffic. All of these applications work with rechargeable batteries, so-called “secondary cells” or accumulators, or “battery” for short. These must be differentiated from disposable batteries, so-called “primary cells”, which are still used in remote controls or flashlights. In the rest of this article, for the sake of simplicity, “battery” will be synonymous with rechargeable batteries. BESS normally uses battery modules, which consist of individual battery cells connected in series and in parallel.
The technological breakthrough in battery technology came over 30 years ago with the development of the now well-known lithium-ion battery (LIB), which was commercialized by Sony in 1991. For their fundamental contribution to this development, researchers J. B. Goodenough, M. S. Whittingham and A. Yoshino were honored with the Nobel Prize in Chemistry in 2019. Since then, lithium-ion cells have become the dominant battery technology on the market with an annual volume of around 700 gigawatt hours (GWh) and 80 billion euros. By 2030, Fraunhofer ISI predicts an annual global demand for lithium-ion batteries of over 3 terawatt hours (TWh).
Energy storage and energy supply in batteries is achieved through the reversible conversion of electrical and chemical energy. Redox reactions, i.e. the transfer of electrons between two substances, one of which is oxidized and the other reduced, play an important role here. The general structure of a battery is schematic in Figure 1 shown and consists of two different electrodes, which are immersed in an electrolyte and are connected via an external current-conducting circuit. A separator separates and insulates the electrodes from one another to prevent an internal electrical short circuit in the cell.
Here, the electrodes contain redox-active and electron-conducting materials, while the electrolyte is an ion-conducting medium, for example a liquid salt solution. Electrochemical reactions take place at the interface between electrodes and electrolyte, i.e. spatially separated redox reactions, which are accompanied by an ion current through the electrolyte. By definition, the electrode in which oxidation (electron release) takes place during discharge is referred to as an “anode,” while the electrode where reduction (absorption of electrons) takes place during discharge is referred to as a “cathode.” Electron balancing between the electrodes is carried out by the external circuit, which operates a load in the event of battery discharge or contains a current/voltage source in the case of battery charging. The materials used for the electrodes and the electrolyte are characteristic of the various battery technologies.
Battery technologies are typically classified using the following technical key performance indicators (KPIs):
· Cell voltage:Potential difference (volts, DC voltage) between cathode and anode. The higher the cell voltage, the more energy and power a battery cell can deliver.
· Energy density:storable energy in watt hours per weight or volume (Wh/kg andWh/liter)
· Power density:available power per weight or volume (represents in the form of the achievable C-rate, which is the reciprocal of the discharge time. A discharge time of 2 hours therefore corresponds to a C rate of 0.5)
· efficiency:Proportion of charged energy that can be recovered during discharging
· Stability:remaining energy over the operating period as a function of cycle degradation and calendar aging
· Self-discharge:remaining energy while not in use
In addition to the inherent specifics of a particular battery technology, the properties of battery cells can be optimized for specific applications by specifically optimizing the electrolyte and electrode design. However, it should be noted that the optimization of certain KPIs is usually at the expense of others, and the design of a battery cell must be adapted to the specific application. When looking at battery technologies with regard to stationary energy storage systems, it must also be considered that these KPIs depend heavily on the system level considered. They can therefore change significantly from the individual battery cell, through the various system levels from battery module to battery container, to the entire system. In addition to the KPIs, the various battery technologies have implications to be considered at plant level in terms of design and construction, as well as associated measures for safe operation. In addition, life cycle costs, resource availability, resilient and sustainable supply chains and production processes, as well as the recyclability of used batteries are playing an increasingly important role in evaluating the various technologies.
A selection of different battery technologies will be presented below with regard to use for stationary energy storage systems. Due to their long history and market penetration, the focus will be on lithium-based cell chemistries to explain general principles, while some interesting and innovative future alternatives will also be presented.
The lithium nickel-manganese cobalt oxide (NMC) cell is a development from the early 2000s from the original lithium cobalt oxide cell. Here, NMC is the active material of the cathode, while graphite (a special form of pure carbon) is used for the anode. By adding the so-called “transition metals” nickel and manganese, it was possible to optimize electrode properties in contrast to pure cobalt oxides and also to reduce the proportion of expensive and disputed cobalt. Common compositions include 8-1-1, 6-2-2 and or 1-1-1, where the numbers indicate the relative stoichiometric proportions of nickel, manganese and cobalt. Similarly, lithium nickel-cobalt aluminum oxide (NCA) cathodes and other cell chemistries with transition metal oxide cathode materials have also been developed. It should be mentioned here that the widely known lithium polymer batteries do not have any separate cell chemistry, but use common active materials such as NMC. In contrast to “normal” LIB, however, these rely on a special polymer-based electrolyte (instead of normal liquid electrolyte) and therefore offer degrees of freedom with regard to cell geometry.
In the last 20 years, the research and development of transition metal oxide cathode materials has been driven significantly by requirements and optimizations for mobile applications such as battery-electric cars, smartphones or laptops. Traditional lithium NMC batteries therefore have a very high energy density and high efficiency. As market-dominating cell chemicals, these also found their way into stationary energy storage systems, especially at the beginning, but are increasingly disappearing from the portfolio of suppliers and system integrators in this market segment for cost reasons.
From an electrochemical point of view, energy storage in modern lithium-ion batteries is based on the process of so-called “intercalation,” which describes the reversible storage of ions from the electrolyte into the solid structure of the electrode. This is exemplified in Figure 2 shown in LiBs for the graphite electrode: while charging the cell, lithium ions from the electrolyte are stored between the 2D carbon layers of the graphite, while electrons are absorbed from the external power circuit at the same time (reduction). When discharging, the process is reversed: electrons are released to the external circuit (oxidation) and the lithium ions are converted back into the electrolyte, which is known as deinterlation. An analogous process takes place on the crystal structure of the cathode's active material (e.g. NMC): lithium deintercalation and electron release during charging, lithium intercalation and electron absorption during discharge. The respective oxidation and reduction reactions at the cathode and anode have a specific electrochemical potential. Their difference is the voltage of the battery cell and is therefore an important characteristic of the various technologies.
Because of the high cell voltage of up to 4 volts, traditional lithium-based cell chemistries normally use organic (i.e. carbon-based) compounds, such as ethylene carbonate, as solvents for the lithium-containing conductive salt in the electrolyte. This is due to the fact that they are (largely) stable against decomposition by electrolysis at such high cell voltages or the respective electrode potential ranges.
However, the use of organic solvents also poses challenges that place special requirements on the layout and design of stationary energy storage systems. For example, the risk of combustibility is counteracted by extensive fire detection and fire fighting measures at various system levels. In the case of batteries with high energy densities, i.e. also lithium-based cell chemistries, the so-called “thermal runaway” must be addressed in particular. When a critical cell temperature is exceeded, irreversible chemical reaction processes start, which convert the energy stored in the cells into heat within a very short period of time. Technically, preventing thermal runaway places strict requirements on the quality of cell production and the battery management system (BMS) with regard to correct charge management, as well as sufficient heat removal through the cooling system. As a result, the technology has been made more and more secure in recent years. Further requirements for the housings of the battery modules and structural safety measures ensure that no chemical components of the battery cell can escape into the environment.
It should not go unmentioned that lithium, although only small quantities are actually required in a battery cell, is not a completely critical-free resource. The global availability of raw material is currently concentrated in a few countries, namely Australia, Chile and China. The extraction of lithium has a strong impact on the local environment and it is questionable whether its scaling can keep up with the growing demand for batteries in the long term. Europe currently has no direct access to the supply chain. The further processing of raw materials and cell production are now carried out almost exclusively in Asia, although growing activities aim to bring this production to Europe and North America in particular. Nevertheless, this important technology has so far been largely dependent on non-European market participants. Increasingly stringent sustainability requirements on the part of the EU are trying to further reduce the negative effects of the interventions.
Lithium iron phosphate (LFP) is an alternative active material for the cathode from the class of intercalation materials, while the anode material and electrolyte composition, as well as the mode of operation, are otherwise generally identical to the NMC-based LIB. While the lower energy density compared to lithium-NMC limits the range of LFP-based electric cars, this disadvantage is significantly more tolerable in stationary energy storage systems, which is why LFP batteries now dominate the portfolio of BESS providers. In addition to lower costs, LFP technology offers other significant advantages compared to NMC, namely a lower cycle degradation (after an initial drop in capacity) and thus a longer lifetime, as well as a lower fire risk during thermal runaway. Even when using LFP batteries, technical and structural requirements and measures are required for safe operation. Due to the cost advantage of the materials used (iron and phosphate instead of nickel and cobalt), LFP cells have taken over ever larger market shares of NMC cells in recent years. Nonetheless, this technology remains as heavily dependent on Asian suppliers for Europe as is the case with NMC.
Lithium solid-state batteries, so-called “all-solid-state batteries” (ASSB), are currently considered the “holy grail” of battery research. The underlying principle is to replace the liquid electrolyte in traditional LIB with a lithium-conducting solid, thus making it possible to use pure lithium metal instead of graphite for the anode. In liquid electrolytes (i.e. as in conventional Li-NMC or LFP cells), the use of lithium metal would result in the growth of so-called dendrites, i.e. needle-like metallic deposits of lithium on the anode, within a few cycles. These can result in the battery cell being short-circuited, which therefore precludes the use of metallic anodes. Lithium metal anodes in solid-state batteries in turn promise significant improvements in the energy density of batteries, which is why the automotive industry is focusing on the development and commercialization of this technology. Another advantage is higher safety, as no flammable organic liquid electrolyte is used, and potentially higher stability. Although many players from industry and research are working on solid-state batteries, the major breakthrough has not yet been achieved due to existing technical challenges with the relevant solid-state electrolytes and the implications for the design and manufacture of battery cells. In any case, it is expected that this technology — as soon as available — will initially be reserved for more high-priced premium markets in the automotive sector due to the expected high initial costs and will play no role for stationary applications, at least in the short and medium term.
Sodium-ion batteries are currently receiving great (media) interest and are considered the most promising medium-term alternative to lithium-ion batteries. Sodium is not only significantly more abundant on Earth than lithium, but is also significantly more geographically distributed. The mode of operation is almost identical to that in LIB, with the difference that sodium is used instead of lithium for the electrochemical reactions. This requires modifications of the electrolyte and another carbon variation, so-called “hard carbon,” instead of graphite for the anode, while similar intercalation materials can be used for the cathode. In particular, the material class of “Prussian blue analogs”, which is based on elements available worldwide such as iron, carbon and nitrogen, should be mentioned. By establishing a local supply chain and production, Europe's current dependence on this technology could potentially be reduced in the future.
The KPIs of sodium-ion batteries are heavily dependent on exact cell chemistry. Apart from the slightly lower energy density, these are largely comparable to lithium-ion batteries or even superior in some dimensions. Although optimization of cell materials is still necessary, it is largely possible to build on existing production processes in the LIB sector, making sodium-ion batteries a so-called “drop-in” technology. This in turn should enable rapid scaling to mass production. For these reasons, a large-scale commercialization of the technology could probably be imminent, which is driven by both large established manufacturers and new players on the market. This cell technology has great potential as a cheaper and more environmentally friendly alternative with a significantly more diversified supply chain and will compete primarily with LFP batteries in the small car sector and stationary energy storage market. In general, however, battery technologies with low energy density must always take into account that more volume, weight and therefore more material overall are required to store the same amount of energy than, for example, when using lithium-ion batteries. This in turn has an impact on resource and material consumption, logistics costs, production and recycling costs, among other things. New technologies should therefore go hand in hand with further optimizations of the supply chain and the entire life cycle.
At this point, the water-based sodium-ion batteries should be mentioned. This uses water instead of the organic solvent in the electrolyte, which means that there is no longer a risk of fire. In principle, it would be possible to use seawater as an electrolyte, so to speak. In addition to high safety, this technology potentially offers further advantages in terms of environmental friendliness, production, costs, lifetime, deep discharge, performance, as well as lower cooling requirements and thus sound emissions. However, the low electrochemical stability of aqueous electrolytes, i.e. the splitting of water into oxygen and hydrogen, as in an electrolyzer, significantly limits the cell voltage and the selection of active materials. As a result, the energy density in particular is extremely reduced. Nonetheless, this technology could be an exciting future candidate for stationary applications.
Similar concepts based on aqueous electrolytes are possible with zinc ions, although zinc, in contrast to sodium, could be used in metallic form as an anode due to the electrochemical potential range. This can have a positive effect on energy density and, together with the low costs and good availability of zinc, offer interesting future developments.
In contrast to intercalation materials, metal-sulfur battery technology is based on so-called conversion cathodes. During discharge, pure sulfur is electrochemically reduced at the cathode and reacts with metal cations to form metal sulfides. At the anode, this very metal is present in pure form, is oxidized during discharge and the metal ions migrate from there through the electrolyte to the cathode. Various ions such as lithium, sodium, potassium or magnesium can be considered for this reaction, of which sodium is seen as the most promising candidate for this technology. Although research is also being carried out on room temperature systems, sodium-sulfur technology has so far only been able to reach a level of maturity sufficient for commercialization when designed as a so-called “thermal battery” with high operating temperatures of over 300°C. In this form, all active materials are present in liquid form as a melt: liquid sulfur or sodium (poly) sulfide at the cathode and liquid sodium at the anode. The electrolyte used is a sodium-conducting, non-liquid solid-state ceramic whose conductivity behavior requires a high operating temperature.
Sodium-sulfur batteries have been available on the market for decades and promise a long calendar lifetime, good energy density, a high level of safety through lengthy testing and high resource availability of the core components. At the same time, the energy required to maintain operating temperature has a negative impact on system efficiency and requires good housing insulation and effective heat management. In addition, this battery technology only allows low discharge rates and is therefore only suitable for long-term storage (several hours). Their future relevance for stationary energy storage systems therefore depends, among other things, on the development of electricity markets and regulations and thus the associated use cases (performance-based design vs. capacity-based design).
In contrast to the other technologies presented here, redox-flow batteries (RFB) contain the redox-active material not in the electrodes but in the electrolyte. An RFB consists of two electrolyte tanks, each of which contains a redox pair in dissolved form as ions. The two solutions are circulated by pumps through the actual electrochemical cell, which consists of two electrodes separated by a separator. Energy is stored via the redox pairs contained in the two solutions, which are each washed past an electrode surface (cathode and anode) and are oxidized or reduced in the process. The dimensions of the electrode surfaces determine the power and volume of the tanks the energy, which means that this battery technology allows decoupled scaling.
Although the energy density and efficiency of RFB is significantly lower than LIB, they have a very long lifetime and good safety. As a result, this technology is definitely an interesting candidate for stationary energy storage systems. The simple scalability of the system's energy across electrolyte volumes could provide a cost advantage over lithium-ion batteries, especially with storage periods of several hours. In current RFB systems, redox pairs based on vanadium still play a very important role. The high price volatility and geographically restricted access to relevant deposits of this element are still reflected today in the economic disadvantages of this technology, which is why alternative redox pairs are being actively researched. Iron-containing and organic compounds are particularly interesting in terms of sustainability and resource aspects.
For the sake of completeness, the well-known lead-acid batteries should also be mentioned here. Due to its long history as a starter battery in cars, this battery technology has a very high degree of scaling as well as a high degree of development readiness and reliability, which is why it was also used in stationary energy storage systems right from the start. Although its low energy density could be sufficient for stationary applications, this technology has largely disappeared from the market today. This is due to their various disadvantages, such as the long charging time, environmental hazards due to lead and acid, and the shorter lifespan. Nonetheless, this technology has advantages over traditional LIBs and is therefore still very relevant for uninterruptible power supply systems, which largely remain in standby mode at high charge levels.
Although alternative technologies are already in the starting blocks today, the market for battery cells is still dominated by lithium-ion technology. This is primarily due to the development of this relatively young market so far, which is driven primarily by the automotive industry's needs for high energy densities. The market share of stationary battery storage systems today accounts for around 10% of the total volume and was therefore historically too low for dedicated alternative technologies to have been able to establish themselves in terms of cost. As a result, lithium-ion cells are still used today as “all-rounders” in all applications. In the future, however, the market for battery cells is expected to diversify into dedicated “specialists” for mobile and stationary applications, which are perfectly adapted to the respective requirement profile. With their market readiness and scaling of mass production, the cost advantages of alternative battery technologies should manifest themselves through the focus on available and cheaper materials and help them achieve a breakthrough. In the short term, it is expected that LFP cells will completely prevail over Li-NMC cells on the stationary battery storage market. However, they could soon face competition from sodium-ion technology, which promises greater sustainability and resource availability. Depending on the development of the electricity market, redox-flow batteries and sodium-sulfur thermal batteries could also play an increasingly important role. At Kyon Energy, we follow new technological developments very closely and are in constant contact with manufacturers and suppliers regarding the best solutions for our battery storage systems.
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