Stationary energy storage systems are playing an increasingly important role in the energy transition. Through the flexible intermediate storage of 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 (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 explain 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 - alongside stationary energy storage systems, of course - in smartphones and laptops, as well as in the increasing number of electric cars on the road. All of these applications work with rechargeable batteries, so-called "secondary cells" or accumulators, or "batteries" for short. These are to be distinguished from disposable batteries, so-called "primary cells", which are still used in remote controls or flashlights. For the sake of simplicity, "battery" will be used synonymously with rechargeable batteries in the rest of this article. In BESS, battery modules are normally used, which consist of individual battery cells connected in series and parallel.
The technological breakthrough in battery technology came over 30 years ago with the development of the now widely 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, the lithium-ion cell has become the dominant battery technology on the market with an annual volume of around 700 gigawatt hours (GWh) and 80 billion euros. The Fraunhofer ISI forecasts an annual global demand for lithium-ion batteries of over 3 terawatt hours (TWh) by 2030.
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 shown schematically in Figure 1 and consists of two different electrodes which are immersed in an electrolyte and connected via an external current-conducting circuit. A separator separates and insulates the electrodes from each other to prevent an internal electrical short circuit in the cell.
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 a flow of ions through the electrolyte. By definition, the electrode where oxidation (electron release) takes place during discharge is referred to as the "anode", while the electrode where reduction (electron acceptance) takes place during discharge is referred to as the "cathode". Electrons are balanced between the electrodes by the external circuit, which operates a load in the case of battery discharging 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 and Wh/liter)
- Power density:available power per weight or volume (represented in the form of the achievable C-rate, which is the reciprocal of the discharge duration. 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 discharge
- Stability:remaining energy over the operating period as a function of cycle degradation and calendar ageing
- Self-discharge:remaining energy during non-use
In addition to the inherent specifics of a particular battery technology, the properties of battery cells can be optimized for specific applications through targeted optimization of 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 considering battery technologies with regard to stationary energy storage systems, it must also be taken into account that these KPIs are heavily dependent on the system level under consideration. They can therefore change significantly from the individual battery cell, through the various system levels from the battery module to the battery container, right up to the entire system. In addition to the KPIs, the various battery technologies have implications to be considered at the system level with regard to design and construction, as well as associated measures for safe operation. Furthermore, 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 the evaluation of the various technologies.
In the following, a selection of different battery technologies will be presented with regard to their 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 further development from the early 2000s of 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 the electrode properties in contrast to pure cobalt oxides and also reduce the proportion of expensive and controversial cobalt. Common compositions are 8-1-1, 6-2-2 and or 1-1-1, with the numbers indicating 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 a separate cell chemistry, but use common active materials such as NMC. In contrast to "normal" LIBs, however, these rely on a special polymer-based electrolyte (instead of the normal liquid electrolyte) and therefore offer a degree of freedom in terms of cell geometry.
Over the last 20 years, the research and development of transition metal oxide cathode materials has been significantly driven by the requirements and optimizations for mobile applications such as battery electric cars, smartphones or laptops. As a result, traditional lithium NMC batteries have a very high energy density and a high degree of efficiency. As a market-dominating cell chemistry, these also found their way into stationary energy storage systems, especially at the beginning, but are increasingly disappearing from the portfolios 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 in the solid-state structure of the electrode. This is shown as an example in Figure 2 for the graphite electrode in LIBs: during charging of the cell, lithium ions from the electrolyte are stored between the 2D carbon layers of the graphite, while at the same time electrons are absorbed from the external circuit (reduction). When discharging, the process is reversed: electrons are released into the external circuit (oxidation) and the lithium ions are transferred back into the electrolyte, which is known as deintercalation. An analogous process takes place at the crystal structure of the active material of the cathode (e.g. NMC): Lithium deintercalation and electron release during charging, lithium intercalation and electron acceptance during discharging. The respective oxidation and reduction reactions at the cathode and anode have a specific electrochemical potential. The difference between them forms the voltage of the battery cell and is therefore an important feature of the various technologies.
Due to the high cell voltage of up to 4 volts, organic (i.e. carbon-based) compounds, such as ethylene carbonate, are normally used as solvents for the lithium-containing conducting salt in the electrolyte in classic lithium-based cell chemistries. This is because these 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 demands on the layout and design of stationary energy storage systems. For example, the risk of flammability is countered by extensive fire detection and firefighting measures at the various system levels. In the case of batteries with high energy densities, including lithium-based cell chemistries, the so-called "thermal runaway" must be addressed in particular. If 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 time. Technically, the prevention of thermal runaway places strict demands on the quality of cell production and the battery management system (BMS) in terms of correct charge management and sufficient heat dissipation through the cooling system. This has made the technology increasingly safe in recent years. Further requirements for the housing 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 uncritical resource. The global availability of the raw material is currently concentrated in just 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 pace 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 currently take place almost exclusively in Asia, although growing activities are aimed at bringing this production to Europe and North America in particular. Nevertheless, this important technology is still largely dependent on non-European market participants. The EU is attempting to further reduce the negative effects of these interventions by imposing increasingly strict sustainability requirements.
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 NMC-based LIBs. While the lower energy density in direct comparison to lithium NMC limits the range of LFP-based electric cars, this disadvantage is much more tolerable in stationary energy storage systems, which is why LFP batteries dominate the portfolio of BESS providers today. In addition to lower costs, LFP technology offers other significant advantages compared to NMC, namely lower cycle degradation (after an initial drop in capacity) and therefore a longer service life, as well as a lower risk of fire during thermal runaway. Technical and structural requirements and measures for safe operation are also necessary when using LFP batteries. Due to the cost advantage of the materials used (iron and phosphate instead of nickel and cobalt), LFP cells have taken over increasing market shares from NMC cells in recent years. Nevertheless, this technology remains just as dependent on Asian suppliers for Europe as NMC.
Lithium solid-state batteries, so-called "all-solid-state batteries" (ASSB), are currently regarded as the "holy grail" of battery research. The underlying principle is to replace the liquid electrolyte in traditional LIBs with a lithium-conducting solid and thus enable the use of 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 lead to 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 a short circuit in the battery cell, which rules out the use of metallic anodes. Lithium metal anodes in solid-state batteries 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 the higher level of safety, as no flammable organic liquid electrolyte is used, as well as 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 solid-state electrolytes in question and the implications for the design and manufacture of the battery cells. In any case, it is expected that this technology - once available - will initially tend to be reserved for high-priced premium markets in the automotive sector due to the expected high initial costs and will not play a role for stationary applications, at least in the short and medium term.
Sodium-ion batteries are currently attracting a great deal of (media) interest and are seen as the most promising medium-term alternative to lithium-ion batteries. Sodium is not only much more common than lithium on earth, but is also much more geographically distributed. The mode of operation is virtually identical to that of LIBs, with the difference that sodium is used for the electrochemical reactions instead of lithium. This requires modifications to the electrolyte as well as a different carbon variation, so-called "hard carbon", instead of graphite for the anode, while similar intercalation materials can be used for the cathode. The material class of "Prussian blue analogs", which is based on globally available elements such as iron, carbon and nitrogen, should be mentioned here in particular. 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 highly dependent on the exact cell chemistry. Apart from the slightly lower energy density, they are largely comparable to lithium-ion batteries or even superior in some dimensions. Although the cell materials still need to be optimized, existing production processes from the LIB sector can be used for the most part, making sodium-ion batteries a drop-in technology. This in turn should enable rapid scaling up to mass production. For these reasons, large-scale commercialization of the technology could be imminent, driven by both large established manufacturers and new players in the market. This cell technology has great potential as a cheaper and more environmentally friendly alternative with a much more diversified supply chain and will primarily compete with LFP batteries in the small car and stationary energy storage market. In general, however, it must always be borne in mind that battery technologies with a low energy density require more volume, weight and therefore more material overall 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 be accompanied by further optimization of the supply chain and the entire life cycle.
At this point, the water-based sodium-ion batteries should be mentioned. Here, water is used instead of the organic solvent in the electrolyte, which eliminates the risk of fire. In principle, it would be possible to use seawater as the electrolyte. In addition to the high level of safety, this technology potentially offers further advantages in terms of environmental friendliness, production, costs, service life, deep discharge, performance and lower cooling requirements and therefore noise emissions. However, the low electrochemical stability of aqueous electrolytes, i.e. the splitting of water into oxygen and hydrogen, as in an electrolyser, significantly limits the cell voltage and the choice of active materials. As a result, the energy density in particular is extremely reduced. Nevertheless, this technology could become an exciting future candidate for stationary applications.
Similar concepts based on aqueous electrolytes are possible with zinc ions, whereby zinc, in contrast to sodium, could be used as an anode in metallic form due to the electrochemical potential range. This could have a positive effect on the energy density and, together with the low costs and good availability of zinc, could lead to interesting future developments.
In contrast to intercalation materials, the technology of metal-sulphur batteries is based on so-called conversion cathodes. During discharge, pure sulphur is electrochemically reduced at the cathode and reacts with metal cations to form metal sulphides. This metal is present in pure form at the anode, 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, the sodium-sulphur technology has so far only reached a sufficient degree of maturity for commercialization in the design as a so-called "thermal battery" with high operating temperatures of over 300°C. In this form, all active materials are available for commercialization. In this form, all active materials are present in liquid form as a melt: liquid sulphur or sodium (poly)sulphide at the cathode and liquid sodium at the anode. The electrolyte is a sodium-conducting, non-liquid solid ceramic whose conductivity behavior requires the high operating temperature.
Sodium-sulphur batteries have been available on the market for decades and promise a long calendar life, good energy density, high safety thanks to long testing and high resource availability of the core components. At the same time, the energy required to maintain the operating temperature has a negative impact on system efficiency and requires good housing insulation and effective thermal 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 the 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, the redox-active material in redox-flow batteries (RFB) is not located in the electrodes, but in the electrolyte. An RFB consists of two electrolyte tanks, each containing 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 flushed past an electrode surface (cathode and anode) and oxidized or reduced in the process. The dimensioning of the electrode surfaces determines the power and the volume of the tanks determines the energy, which means that this battery technology allows decoupled scaling.
Although the energy density and efficiency of RFBs is significantly lower than that of LIBs, they have a very long service life and good safety. This makes this technology an interesting candidate for stationary energy storage systems. The simple scalability of the system's energy via the electrolyte volumes could provide a cost advantage over lithium-ion batteries, especially for storage periods of several hours. Redox couples based on vanadium still play a very important role in current RFB systems. The high price volatility and geographically limited access to relevant deposits of this element are still reflected in the economic disadvantages of this technology, which is why alternative redox pairs are being actively researched. Iron-containing and organic compounds are of particular interest from a sustainability and resource perspective.
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 scalability as well as a high level of development maturity and reliability, which is why it was also used in stationary energy storage systems at the beginning. Although its low energy density could be sufficient for stationary applications, this technology has now largely disappeared from the market. This is due to its various disadvantages, such as the long charging time, the environmental hazard of lead and acid, and the shorter service life. Nevertheless, this technology has advantages over classic LIBs and therefore remains highly relevant for uninterruptible power supply systems, which largely remain in standby mode at high states of charge.
Although alternative technologies are already in the starting blocks, the market for battery cells is still dominated by lithium-ion technology. This is mainly due to the development of this relatively young market to date, which is primarily driven by the needs of the automotive industry for high energy densities. The market share of stationary battery storage systems currently accounts for around 10% of the total volume and has therefore historically been too small 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 that are perfectly adapted to the respective requirement profile. Once they are ready for the market and mass production is scaled up, 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, LFP cells are expected to completely prevail over Li-NMC cells in the market for stationary battery storage systems. However, these 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-sulphur thermal batteries could also play an increasingly important role. At Kyon Energy, we closely follow new technology developments and are in constant communication with manufacturers and suppliers regarding the best solutions for our battery storage systems.
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