Stationary, Second Use Battery Energy Storage Systems and Their Applications: A Research Review

: The global demand for electricity is rising due to the increased electriﬁcation of multiple sectors of economic activity and an increased focus on sustainable consumption. Simultaneously, the share of cleaner electricity generated by transient, renewable sources such as wind and solar energy is increasing. This has made additional buffer capacities for electrical grids necessary. Battery energy storage systems have been investigated as storage solutions due to their responsiveness, efﬁciency, and scalability. Storage systems based on the second use of discarded electric vehicle batteries have been identiﬁed as cost-efﬁcient and sustainable alternatives to ﬁrst use battery storage systems. Large quantities of such batteries with a variety of capacities and chemistries are expected to be available in the future, as electric vehicles are more widely adopted. These batteries usually still possess about 80% of their initial capacity and can be used in storage solutions for high-energy as well as high-power applications, and even hybrid solutions encompassing both. There is, however, no holistic review of current research on this topic. This paper ﬁrst identiﬁes the potential applications for second use battery energy storage systems making use of decommissioned electric vehicle batteries and the resulting sustainability gains. Subsequently, it reviews ongoing research on second use battery energy storage systems within Europe and compares it to similar activities outside Europe. This review indicates that research in Europe focuses mostly on “behind-the-meter” applications such as minimising the export of self-generated electricity. Asian countries, especially China, use spent batteries for stationary as well as for mobile applications. In developing countries, off-grid applications dominate. Furthermore, the paper identiﬁes economic, environmental, technological, and regulatory obstacles to the incorporation of repurposed batteries in second use battery energy storage systems and lists the developments needed to allow their future uptake. This review thus outlines the technological state-of-the-art and identiﬁes areas of future research on second use battery energy storage systems.


Introduction
Europe and the rest of the world are forging a path towards a low-carbon future by establishing targets for drastically reduced environmental impacts as described in the Paris Agreement [1]. Clean, renewable sources like wind and solar energy must be used, in combination with the electrification of sectors of activity, to reach international sustainability targets [2][3][4]. However, the transition from fossil fuels to low-carbon energy sources is hindered by the ever-increasing global demand for energy [5] as well as the lack of reliability of renewable sources. Apart from the obvious transition on the supply side, the transition to a low-carbon society will also require that the traditional processes on the demand side, like transportation, be electrified ( Figure 1) [6,7]. As increased electrification causes high stress on the electrical grid, there is a need for either an expansion or upgrade of the electrical grid or measures to compensate for imbalances between supply and The key to a sustainable future for Europe is improving the European electrical grid and buffer capacities. This involves making the grid (1) more flexible to integrate increased electricity generation from renewable sources [11] and (2) more efficient and reactive to meet future demands from increased electrification, especially from the transport sector. Investing in new grid infrastructure or upgrading it is costly [12]. Buffer capacities, especially when close to generation and demand, help smooth supply and demand profiles and relieve grid fluctuations. This means that new infrastructure or expensive upgrades to the grid can be partially avoided by adding buffer capacities [13]. Buffer capacities are mainly found as centralised, large-scale power plants, such as pumped storage hydropower plants. Unfortunately, such power plants are limited by infrastructural considerations [14]. Therefore, additional modular systems are needed to compensate for supply and demand imbalances [15]. Battery energy storage systems (BESSs) have been investigated as an alternative to solve the grid and buffer capacity challenges of the future [16][17][18]. By using batteries, it is possible to balance demand and thus ensure that transient renewable energy, such as wind and solar energy, can be used when needed, not just when generated [16]. BESSs normally do not reach the storage capacities of large-scale units and are mostly found as distributed, small-scale units. They can work as small-scale, standalone units or large capacity, aggregated units [19]. Results from the large research project MONA (Merit Order Netz-Ausbau) in Germany [20] proved that storage systems in general, and BESSs in particular, can reduce the need for such expansion. However, this requires cost-effective BESSs [21].
A solution to reduce costs for stationary applications is to rely on decommissioned electric vehicle (EV) batteries [17,22]. In this paper, the term EV refers to full battery electric vehicles (BEVs) as well as plug-in hybrid electric vehicles (PHEVs). BEVs mostly refer to vehicles that run entirely on electricity, while PHEVs are hybrid electric vehicles whose batteries can be recharged by external electricity sources as well as by their internal combustion engines. The average battery capacity of BEVs and PHEVs is currently around 50 kWh and 11 kWh, respectively [23]. In 2019, the total stock of EVs exceeded 7.2 million units. Based on the Sustainable Development Scenario, a global market share of 30% could be reached by 2030 [24]. BEVs and PHEVs combined now represent 3% of all vehicles sold in the European Union (EU) (Figure 2a). Norway is the global leader in EV sales based on the market share (Figure 2b), where nearly 56% of new vehicle purchases in 2019 were either hybrid or fully electric [25]. This could be a model for the EU to follow. With the rising deployment of EVs, a large number of decommissioned batteries will be available for the foreseeable future. These, in turn, offer an economic and sustainable source for stationary BESSs.

Supply Side
Demand Side Increased Electrification Figure 1. The pathway towards a low-carbon society utilizing cleaner production and consumption of electric energy.
The key to a sustainable future for Europe is improving the European electrical grid and buffer capacities. This involves making the grid (1) more flexible to integrate increased electricity generation from renewable sources [11] and (2) more efficient and reactive to meet future demands from increased electrification, especially from the transport sector. Investing in new grid infrastructure or upgrading it is costly [12]. Buffer capacities, especially when close to generation and demand, help smooth supply and demand profiles and relieve grid fluctuations. This means that new infrastructure or expensive upgrades to the grid can be partially avoided by adding buffer capacities [13]. Buffer capacities are mainly found as centralised, large-scale power plants, such as pumped storage hydropower plants. Unfortunately, such power plants are limited by infrastructural considerations [14]. Therefore, additional modular systems are needed to compensate for supply and demand imbalances [15]. Battery energy storage systems (BESSs) have been investigated as an alternative to solve the grid and buffer capacity challenges of the future [16][17][18]. By using batteries, it is possible to balance demand and thus ensure that transient renewable energy, such as wind and solar energy, can be used when needed, not just when generated [16]. BESSs normally do not reach the storage capacities of large-scale units and are mostly found as distributed, small-scale units. They can work as small-scale, standalone units or large capacity, aggregated units [19]. Results from the large research project MONA (Merit Order Netz-Ausbau) in Germany [20] proved that storage systems in general, and BESSs in particular, can reduce the need for such expansion. However, this requires cost-effective BESSs [21].
A solution to reduce costs for stationary applications is to rely on decommissioned electric vehicle (EV) batteries [17,22]. In this paper, the term EV refers to full battery electric vehicles (BEVs) as well as plug-in hybrid electric vehicles (PHEVs). BEVs mostly refer to vehicles that run entirely on electricity, while PHEVs are hybrid electric vehicles whose batteries can be recharged by external electricity sources as well as by their internal combustion engines. The average battery capacity of BEVs and PHEVs is currently around 50 kWh and 11 kWh, respectively [23]. In 2019, the total stock of EVs exceeded 7.2 million units. Based on the Sustainable Development Scenario, a global market share of 30% could be reached by 2030 [24]. BEVs and PHEVs combined now represent 3% of all vehicles sold in the European Union (EU) (Figure 2a). Norway is the global leader in EV sales based on the market share (Figure 2b), where nearly 56% of new vehicle purchases in 2019 were either hybrid or fully electric [25]. This could be a model for the EU to follow. With the rising deployment of EVs, a large number of decommissioned batteries will be available for the foreseeable future. These, in turn, offer an economic and sustainable source for stationary BESSs. In EVs, the entire battery is often referred to as a battery pack. The battery pack typically consists of several battery modules which are comprised of individual battery cells in either a pouch, cylindrical, or prismatic cell format [26] (Figure 3). EV batteries are replaced before they reach their physical end of life, typically when they reach 70-80% of their initial capacity due to limited cruising range. Depending on the cell chemistry and the cell design or cell assembly, such batteries can still be used for less-demanding applications such as stationary BESSs [27]. The ageing of the battery is influenced by several factors such as temperature and driving style [28]. The literature is limited when it comes to the differences in capacity fade between BEVs and PHEVs. Second use also reduces the ecological footprint by reducing the need for new batteries (and thus new materials) for storage systems as well as by extending the lifespan of existing batteries and slowing the flow of used batteries that have to be recycled [29]. As an example, the calendar life for lithium-ion batteries is nearly 20 years [27,30] and the EV use phase is just about 8 years [31] (dependent on driving and charging behaviour [32]).
The second use storage system potential can be estimated by simulation. Different types of models are used to investigate the battery design, estimate the performance, and simulate electrical circuits. M. Chen et al.
[1] differentiated between (1) electrochemical models to understand the battery fundamentals and optimise the design, (2) mathematical models to predict system-level behaviour such as efficiency or capacity, and (3) electrical models (equivalent circuit models) to investigate the storage system integration. Electrical models can further be classified as (1) simple models, (2) Thevenin-based models, (3) impedance-based models, (4) runtime-based circuit models, (5) combined electrical circuitbased models, and (6) generic-based models [33]. In addition, model extensions can be added to describe the state-of-health/lifetime [34][35][36] and the optimal electrical components/equipment replacement [37,38]. The models can be used in optimisation algorithms to determine the battery operation for different applications. To verify the simulation results and further assess the BESS potential, field tests are performed. To test the BESSs in  In EVs, the entire battery is often referred to as a battery pack. The battery pack typically consists of several battery modules which are comprised of individual battery cells in either a pouch, cylindrical, or prismatic cell format [26] (Figure 3). In EVs, the entire battery is often referred to as a battery pack. The battery pack typically consists of several battery modules which are comprised of individual battery cells in either a pouch, cylindrical, or prismatic cell format [26] (Figure 3). EV batteries are replaced before they reach their physical end of life, typically when they reach 70-80% of their initial capacity due to limited cruising range. Depending on the cell chemistry and the cell design or cell assembly, such batteries can still be used for less-demanding applications such as stationary BESSs [27]. The ageing of the battery is influenced by several factors such as temperature and driving style [28]. The literature is limited when it comes to the differences in capacity fade between BEVs and PHEVs. Second use also reduces the ecological footprint by reducing the need for new batteries (and thus new materials) for storage systems as well as by extending the lifespan of existing batteries and slowing the flow of used batteries that have to be recycled [29]. As an example, the calendar life for lithium-ion batteries is nearly 20 years [27,30] and the EV use phase is just about 8 years [31] (dependent on driving and charging behaviour [32]).
The second use storage system potential can be estimated by simulation. Different types of models are used to investigate the battery design, estimate the performance, and simulate electrical circuits. M. Chen et al.
[1] differentiated between (1) electrochemical models to understand the battery fundamentals and optimise the design, (2) mathematical models to predict system-level behaviour such as efficiency or capacity, and (3) electrical models (equivalent circuit models) to investigate the storage system integration. Electrical models can further be classified as (1) simple models, (2) Thevenin-based models, (3) impedance-based models, (4) runtime-based circuit models, (5) combined electrical circuitbased models, and (6) generic-based models [33]. In addition, model extensions can be added to describe the state-of-health/lifetime [34][35][36] and the optimal electrical components/equipment replacement [37,38]. The models can be used in optimisation algorithms to determine the battery operation for different applications. To verify the simulation results and further assess the BESS potential, field tests are performed. To test the BESSs in  EV batteries are replaced before they reach their physical end of life, typically when they reach 70-80% of their initial capacity due to limited cruising range. Depending on the cell chemistry and the cell design or cell assembly, such batteries can still be used for less-demanding applications such as stationary BESSs [27]. The ageing of the battery is influenced by several factors such as temperature and driving style [28]. The literature is limited when it comes to the differences in capacity fade between BEVs and PHEVs. Second use also reduces the ecological footprint by reducing the need for new batteries (and thus new materials) for storage systems as well as by extending the lifespan of existing batteries and slowing the flow of used batteries that have to be recycled [29]. As an example, the calendar life for lithium-ion batteries is nearly 20 years [27,30] and the EV use phase is just about 8 years [31] (dependent on driving and charging behaviour [32]).
The second use storage system potential can be estimated by simulation. Different types of models are used to investigate the battery design, estimate the performance, and simulate electrical circuits. M. Chen et al.
[1] differentiated between (1) electrochemical models to understand the battery fundamentals and optimise the design, (2) mathematical models to predict system-level behaviour such as efficiency or capacity, and (3) electrical models (equivalent circuit models) to investigate the storage system integration. Electrical models can further be classified as (1) simple models, (2) Thevenin-based models, (3) impedance-based models, (4) runtime-based circuit models, (5) combined electrical circuit-based models, and (6) generic-based models [33]. In addition, model extensions can be added to describe the state-of-health/lifetime [34][35][36] and the optimal electrical components/equipment replacement [37,38]. The models can be used in optimisation algorithms to determine the battery operation for different applications. To verify the simulation results and further assess the BESS potential, field tests are performed. To test the BESSs in the field, research and industrial stationary storage units have been devel-oped, built, deployed, and operated. Industrial storage systems mostly focus on one of the few economically feasible applications of BESSs: the provision of primary frequency control [39][40][41][42][43]. Such storage systems have been deployed and are operating in several EU countries, mostly as a joint venture between vehicle manufacturers and utility companies. This review paper investigates research activities on second use stationary BESSs in Europe. After describing how literature for the review was selected and clarifying the most important terms and definitions around second use, suitable battery applications are reviewed. A mapping of research-based second use BESSs in Europe is used to give an indication about current activities and is followed by a brief discussion about the trends outside of Europe. Finally, barriers and future developments are examined.

Second Use: Terms and Definitions
Various terms and definitions are used to describe the process of using spent batteries in a new, different application. For example, the terms "second use" [44], "second life" [45], and "reuse" [31] have all been used for such a purpose. In addition, these terms have been used in various EU legislation, but no proper definitions are available [46]. Those phrases can have different meanings even though they describe the same goal, which is to extend the lifetime of a product or its components in order to reduce or eliminate waste. There are two approaches to achieve this goal: (1) a product or its components are used again for the same purpose, application, function, or context for which they were initially developed, and placed on the market, or, (2) a product or its components are used for a different purpose, application, function, or context.

Reuse
Reuse means that a product or its components are reviewed (diagnosed), repaired, restored, refurbished, reconditioned, replaced, rebuild, remanufactured, or reassembled with original new or used components resulting in second-hand products or components which are used again for the same purpose, application, function, or context. It is a common procedure for internal combustion engines in vehicles, since broken engines can be remanufactured and reused for much lower environmental and economic costs than manufacturing new ones [47]. Similarly, in the case of an EV, it means that the battery or its components are used again in an EV as a traction battery.

Second Use
Second use, also called second life, means repurposing a product or some components used for a different purpose, application, function, or context. Second use also means that the legal liability lies with the new producer. Using internal combustion engines from a vehicle in a generator unit would be considered second use. Similarly, using an EV battery or its components in a stationary energy storage system would be considered second use.

Method
This work is based on a structured literature review and a consultancy of academic, legislative, and industrial stakeholders. The research articles, reports, documents, etc. this review is based on, were found using databases of academic publishers such as Elsevier, MDPI, and IEEE, and in general, using search engines such as Google. The terms and definitions listed in Section 2 were used as keywords in combination with "stationary battery energy storage" and "applications". Project owners were contacted to back up the information given in Section 2. The verified information was stored, sorted, and structured in a database before the most relevant data was extracted for this review paper. In addition, the members of the European Technology and Innovation Platform-BatteRIes Europe [48] provided valuable input for each Section of this paper.

Battery Energy Storage Systems: Applications
G. Fitzgerald et al. [49] identified three different stakeholder groups and 13 different application areas for BESS in general, whereas G. Reid et al. [50] identified four stakeholder groups and 14 possible application areas, in particular for second use BESSs. G. Fitzgerald et al. [49] identified transmission system operators, utility companies, and customers as stakeholders while G. Reid et al. [50] identified utilities companies (including transmissions system operators), commercial, industrial and residential consumers, and off-grid systems as stakeholders. G. Reid et al. [50] also included off-grid systems under applications. Based on these reports, three combined stakeholder groups can be identified, namely "in-front-ofthe-meter", "behind-the-meter", and "off-grid". "In-front-of-the-meter" refers to power that must pass through an electric meter before reaching a customer, while "behind-themeter" refers to power that can be used on-site without passing through a meter. With such a classification, the stakeholder groups and corresponding application areas are listed in Table 1. Table 1. Stakeholder groups and corresponding application areas [49,50].

Stakeholder Groups Applications Description
In-front-of-the-meter Energy arbitrage Electrical energy is purchased and stored when the energy prices are low and sold or used when the energy prices are high.

Frequency control
Frequency control ensures that the grid frequency is held within a defined tolerance band to avoid grid instability. Primary, secondary, and tertiary frequency control, which act in different time domains, are done to balance supply and demand.
Spinning/Non-spinning reserve Spinning reserves are online generation capacities that can compensate for unexpected events like generation outages. Non-spinning reserves can compensate for such unexpected events within a short period of time.
Voltage support The grid voltage is maintained within a defined range to ensure that real and reactive power generation matches demand.
Black start Black start generation units are needed to restart generation at larger power stations, mainly thermal power stations, after a blackout, to recover the grid operation.

Resource adequacy
Existing power plants can be combined with energy storage units to manage peak demand without adding generation capacity, thus reducing investment costs and associated risks.

Transmission/Distribution deferral
Reduces utility investments in transmission/distribution system upgrades which are necessary to meet future demands.

Transmission congestion relief
In order to reduce congestion along transmission lines with high demand and the resulting requests for redispatch [51], battery storage units can be installed downstream of the transmission lines.

Behind-the-meter
Time-of-use bill management Shifting energy purchases to periods of low prices and using stored energy when the prices are high (price incentivised demand optimisation [52,53]).

Increased self-consumption
Minimising the export of locally generated electricity, e.g., by photovoltaic systems, while increasing self-consumption, or by using locally generated electricity to provide an auxiliary source for power demands such as EV fast-charging.

Demand charge reduction
Demand reduction (peak shaving) during times of peak demand to reduce the quantity of power bought at premium prices. This applies mostly to commercial customers but, based on regional/national tariffs, also to residential customers.
Backup power Backup power can be obtained from storage systems for short or medium time periods if a grid failure occurs.
Off-grid Off-grid Off-grid systems are systems that are not connected to the main electrical grid and are mostly small. Such systems need generation units, consumers, as well as buffer capacities to balance supply and demand.

Second Use Storage Systems in Europe
Second use storage systems based on EV batteries are being developed, implemented, and tested by industries, private consumers, and research projects. Industrial-scale second use storage systems are mostly business-to-business solutions based on battery packs, with a capacity ranging from several kWh up to several MWh [43,[54][55][56][57][58][59], and internal research and development projects with a capacity of several kWh [60,61]. These storage systems are often built by industry consortia consisting of EV manufacturers, utility companies and technology developers, and are mostly used for primary frequency control and demand charge reduction. Second use storage systems for private consumers are often used in combination with a photovoltaic system to increase their self-consumption. Such systems are mainly based on battery modules and reach a capacity of up to several kWh. Such storage systems are available on the market as out of the box solutions [62,63] or may be custom built. Global, ongoing, custom second use projects are listed in the second life storage database [64]. In Europe, second use, especially on the battery cell level, is uncommon due to the high effort and labour costs for repurposing [65].
The applications and properties of storage systems in research are not quite as transparent. In Table 2, second use storage systems being researched in the EU and European Free Trade Association are mapped and classified based on Table 1. It shows that nearly all storage systems are based on lithium-ion batteries from BEV, mostly making use of spent batteries from Nissan, Renault, or Volkswagen which mainly use NMC (nickel-manganesecobalt, the cathode composition) based battery chemistry. Some storage systems combine old and new cells, but always with the same chemistry. The capacities and power in-/outputs are rather low compared to industrial storage systems. Further, Table 2 shows that most storage systems in research are developed for "behind-the-meter" applications, most commonly to increase self-consumption. Often, several applications are investigated, also partly by simulation (sim), but not simultaneously.

Trends and Developments Outside of Europe
As the Global EV Outlook 2020 [24] shows, the worldwide stock of EVs (BEVs and PHEVs) has passed 7.2 million units in 2019. This is an increase of 40% from the previous year. In particular, there were 3.3 million EVs in China in 2019, which is about 47% of the total EV fleet. After China and Europe, the United States (US) are the third largest EV market [24]. Thus, substantial numbers of decommissioned EV batteries will be available in the near future for second use applications. Globally, second use storage systems may serve different applications than the ones in Europe. Therefore, the trends and developments are mapped for developed countries in North America and Australia (including New Zealand) and for developing countries in South America and Africa, and Asian countries, especially for China, Japan, and South Korea.

Second Use in North America and Australia
The US Department of Energy (DOE) published a report on Solving Challenges in Energy Storage which describes the critical need for energy storage in the electrical grid [104]. It mentions that advanced energy storage systems such as second use BESSs built from spent EVs provide a solution to some of the most critical issues associated with all stakeholder groups, as defined in Table 1. The National Research Council of Canada considers stationary, second use storage systems as a cost-efficient solution in the electrical grid [105]. The governments of Australia and New Zealand have discussed and support repurposing EV batteries for second use storage systems [106,107]. Various projects and applications can be found across North America and Australia. The applications are similar to the ones in Europe, listed in Section 4, and are discussed in detail in [49,50]. BESSs are used for commercial home applications [108,109], research storage system applications [110,111], and industrial applications [112,113]. Ongoing projects show that most second use projects rely on entire battery packs or battery modules (depending on the storage size) due to the high repurposing effort and labour costs.

Second Use in South America and Africa
In South America and Africa, most storage projects are based on backup power and off-grid applications to power critical infrastructure such as hospitals, schools, and electric lighting. Backup power systems are deployed locally in urban areas to counter frequent power outages. In areas where suitable grid infrastructure is lacking, off-grid solutions, comprised of an energy source such as a photovoltaic system and a battery storage system, offer remote electricity supply [114]. Several projects, supported by EU and US DOE grants, aim to establish off-grid solutions with second use batteries which are generally based on battery modules in the low kWh capacity range [114][115][116]. On account of their high initial prices and due to limited electrical grid infrastructure, EVs are less common and cannot compete on prices with vehicles with an internal combustion engine at present [117]. Therefore, developing countries are currently dependent on second use batteries and technologies from Europe, North America, Australia, New Zealand, or Asia.

Second Use in Asia
Second use of EV batteries in Asia is dominated by China, South Korea, and Japan. The application areas are from private consumer applications [118] to industrial applications [119,120] supported by research activities on second use storage systems [121][122][123]. Unfortunately, there is limited information available about ongoing activities with respect to second use and recycling of batteries, especially for China, even though they are the largest battery manufacturer and consumer in the world [124]. In China, the second use market is developing very quickly, mainly due to government policies and the rapidly growing EV market [125]. The application range is enormous compared to other countries. Even in mobile applications such as utility vehicles, repurposed batteries from EVs are considered [126]. GEM, Chinas largest recycling centre for waste batteries, among others, is working on battery pack regeneration [127] and has developed its own second use modules which can be used in various applications. Battery cells are being repurposed for consumer applications such as small power banks due to low labour costs compared to the battery costs [128]. A large amount of second use EV batteries are deployed in telecommunication infrastructure to back up transmission stations. Thus, high-quality, repurposed EV batteries are replacing lead-acid based backup power systems [129].

Barriers and Outlook
Although research in stationary BESSs has delivered promising results, some barriers remain to the widespread adoption of such systems on a commercial basis.

Economic Barriers
Stationary BESSs remain expensive for energy management [130], independent of whether they are first use or second use storage systems, which slows their implementation [131]. Although battery prices have been dropping due to technological improvements, especially for lithium-ion batteries [132], BESSs are economically challenging for widespread deployment. Even if investment costs drop further, the cycle stability and lifespan of the battery play a decisive role in the economics of such systems. It is therefore a strategic goal of the EU to reduce energy storage costs by 2030 to under 0.05 €/kWh/cycle to be competitive. Earlier research projects have shown the feasibility of second use batteries as stationary BESS. B. Bai et al. [133] give an overview of the economic performance of different BESSs in combination with photovoltaic systems to increase self-consumption. The second use of EV batteries has been the subject of an EU Joint Research Centre (JRC) Technical Report [134]. The report concluded that second use of batteries for stationary applications should be feasible, but that more in-depth research and demonstration sites needed to be developed. The European-funded ELSA (Energy Local Storage Advanced System) project developed several stationary BESSs using second use batteries. The results showed that it was possible to extend the lifespan of batteries in such applications since the economic benefits were two to three times superior to the costs of the systems, and such systems had a clear environmental advantage over other systems [135]. Prices for BESSs are expected to decrease further due to dropping battery cell and auxiliary component costs and economy-of-scale effects [39]. This leads to one of the biggest challenges for second use BESSs: the gap between the price of first use and second use batteries. Since prices for first use batteries are still dropping, future second use costs need to be in line with the falling costs for first use batteries [136]. Furthermore, current BESSs with their control systems are developed for one specific application (e.g., peak shaving) in most cases and cannot handle complex grid management and multiple grid applications [137]. This means that they are often in idle mode, which in turns means that their potential is not fully exploited. As a result, the research focus is moving towards advanced battery controllers for BESSs [138]. In addition, research is also being conducted on BESSs combining different battery technologies based on different battery chemistries and, thus, batteries with different power and energy densities (hybrid BESSs). Consequently, they can serve different grid applications simultaneously and maximise their potential [137,139]. To the author's knowledge, there is currently no control system that can handle hybrid BESSs allowing for complex grid management and multiple grid applications. Developments regarding hybrid storage systems that can simultaneously serve multiple applications are needed to increase the economic value of stationary BESSs.

Environmental Barriers
Harper et al. [26] estimated that, based on the global EV sales in 2017, 250,000 tons of batteries will need to be recycled just for that year. Discarded batteries will become a substantial waste problem in the future. Ballinger et al. [140] mentioned that lithiumion batteries are currently the most common battery technology in EVs. Based on the share of different types of chemistries used in lithium-ion batteries, they identified that there are potential supply risks for the critical raw materials such as natural graphite, lithium, and cobalt. Continuous developments of battery chemistry reduce the use of critical materials such as cobalt, making it feasible to recycle spent, high-cobalt-containing batteries to use the material in the next generation of batteries [141]. Unfortunately, the current process for recycling EV batteries is time consuming, dangerous, and inefficient, as recycling technologies have not been able to keep up with the complex chemistry of today's battery cells [142]. Thus, critical materials such as cobalt are partly lost during the recycling process. Manahan [143] stated that as much as 50% of cobalt is lost to tailing, slag, or other wastes, indicating that there is a significant potential to improve its recovery. Lithium-ion recycling processes are currently not efficient since only the most valuable materials such as copper, nickel, and cobalt are recovered [144,145]. Currently, the main recycling methods are pyrometallurgy, hydrometallurgy, and biometallurgy, and they all result in additional environmental issues regarding wastewater, residue, and exhaust gas, which require further downstream treatment [146]. By applying pyrometallurgical processes, materials such as nickel, cobalt, and copper can be recovered effectively today, while aluminium, graphite and lithium are typically lost. Other hydrometallurgical processes can include pre-treatment such as leaching, which allows for the recovery of lithium and aluminium, but at a high cost and high energy consumption [146]. Further processing usually involves burning the remnants of the battery. The modules and/or cells are often sent to Asia for further recycling [147]. Once burned or exported from Europe, these materials are permanently removed from the European battery value chain, thereby reducing material independence in Europe. Current recycling technologies are therefore not sufficient for full reutilisation of critical raw materials and do not meet EU Circular Economy strategies for eliminating waste and pollution in the battery value chain [148]. Second use of batteries, as part of the EU Circular Economy strategies, would reduce the need for extraction of virgin raw materials for new BESSs, extend the life cycle of EV batteries, and thereby reduce the demand for critical raw materials. Furthermore, it would also help to gain time for developing appropriate battery recycling technologies. This would lead to a more efficient use of available resources and address short-term battery waste issues. Finally, it is important not to consider batteries as waste even if they reach their end of life since they contain valuable materials. In addition, public awareness that batteries are valuable at any stage (in use or after reaching their end of life) would help with second use and further recycling.

Technological Barriers
All steps involved in repurposing EV batteries such as dissembling, handling, characterisation, and reassembly are still mostly done manually and thus have high labour costs. Automation will help reduce costs and make battery handling safer. Efficient automation requires that battery packs, modules, cells, and auxiliary components be designed for disassembly [149]. In addition, proper non-destructive, non-discriminating diagnosis methods to characterise spent batteries after their first life need to be developed [150]. This is critical to allow for fast and efficient sorting of potentially reusable battery packs, modules, or cells with sufficient reliability. Proper diagnoses would make it possible to classify spent batteries as safe and prognosticate their remaining lifetime. In addition, barriers also exist with respect to the auxiliary components needed to run a battery storage system. As one of the most critical parts of the battery setup, the battery management system (which ensures the optimal operation of the battery and performs safety-relevant measurements) is mostly not built to be used in second use applications. Importantly, the possibility of second use should be considered when designing a battery management system. This would reduce electronic waste and allow taking over battery history information as well as the battery control system models (algorithms). This, in turn, would help estimate battery parameters such as state-of-charge and state-of-health [45]. This would ensure the optimal usage of the batteries within the storage system. In addition, open access to the battery management software would make it possible to reconfigure storage system variables, such as capacity and power input/output. Developments in terms of second use for auxiliary electronics such as switches, fuses, AC/DC converters, heating, and cooling systems would help to further reduce BESS costs and electronic waste. Furthermore, increasing transparency and considering second use when developing software and hardware would allow combining batteries from the same type of vehicle with slightly different conditions such as state-ofhealth. For example, combining small battery packs from PHEV to form larger BESSs. In addition, software and hardware should be capable of combining and operating efficiently with different battery chemistries to form hybrid BESSs.

Regulatory Barriers
To overcome economic, environmental, and technical barriers, standards and regulations need to be in place. Simple measures such as battery labelling giving indications about the battery chemistry are still missing. This would help to handle and sort decommissioned EV batteries according to their properties and would further improve the recycling process [151]. Labelling would also help to keep track of the batteries and their components to ensure and control optimal circularity [152]. Standardised battery packs, modules, and cells would reduce the complexity across different (vehicle) manufacturers and thus help with the automated sorting for second use as well as for recycling [26]. Since second use storage systems will be sold as new, or separate products, legal liability measures to ensure safety, quality, and warranty from the producer of such storage systems will be needed. Policies regulating handling (especially battery transportation), ownership of waste, recycling, etc. are critical for fair and transparent market conditions and to guarantee the circular economy of batteries in the European battery value chain. This will also lead to a higher reutilisation of batteries and increased battery recycling rates. Reinhardt et al. [46] investigated legislation on second use EV batteries as stationary storage systems in the EU. They concluded that a harmonised policy framework needs to be developed based on existing automotive and energy binding legislation. One important step has been taken by the European Commission by updating the Batteries Directive (2006/66/EC) [153], which aims to minimise the negative impacts of batteries on the environment. A working document of the European Commission [154] showed some of the shortcomings of the current Battery Directive such as not addressing second use, labelling measures, extended producer responsibility, and recycling efficiencies of lithium-ion based batteries. To this end, the European Commission engaged in an Inception Impact Assessment [155] to modernise the EU's batteries legislation, in particular the Batteries Directive.

Discussion and Conclusions
Stationary, second use battery energy storage systems are considered a cost-efficient alternative to first use storage systems and electrical energy storage systems in general. Second use reduces the ecological footprint by reducing the need for new batteries (and thus new materials) for storage systems as well as by extending the lifespan of existing batteries and slowing the flow of used batteries that have to be recycled. The remaining capacity of spent electric vehicle batteries is sufficient for less-demanding stationary applications to balance supply and demand in the electrical grid and act as complementary storage for transient, renewable sources such as wind and solar energy. Thus, not only would second use battery energy storage systems make battery life cycles more sustainable, but they would also foster the development of cleaner energy production and consumption. To address the lack of a holistic review of current research activities on the topic, the author conducted a structured literature review and consulted academic, legislative, and industrial stakeholders to (1) identify potential applications for battery energy storage systems, (2) review second use storage systems in Europe, (3) map trends and developments outside of Europe, and (4) investigate barriers and outlook.
There is a range of potential applications for second use battery energy storage systems. These can be assigned to three potential stakeholder groups, namely "in-front-of-themeter", "behind-the-meter", and "off-grid" stakeholders. While "in-front-of-the-meter" applications mainly focus on maintaining and upgrading the grid, "behind-the-meter" applications focus on personal needs optimisation. "Off-grid" applications enable electrification and thereby replace fossil-based alternatives such as gas lightning. It is in the interest of each of these stakeholders to provide environmentally sustainable yet commercially viable solutions for these applications while simultaneously integrating larger amounts of clean electrical energy. In this regard, second use batteries offer cost-efficient as well as sustainable alternatives to first use batteries.
While in Europe the majority of battery energy storage systems in industrial applications are "in-front-of-the-meter" applications, research focuses on "behind-the-meter" applications. Most research projects try to increase energy self-sufficiency by minimising the export of self-generated, clean electricity, e.g., by photovoltaic systems. Often, different applications are investigated individually, but there is a lack of research on storage systems that can simultaneously serve multiple applications, which would further increase the economic value of second use, stationary storage systems. Such an approach would require hybrid battery systems, which combine different kinds of batteries that are optimised for high-energy and high-power densities. By replacing many battery systems for different applications with one hybrid solution, the sustainability of battery systems can be improved significantly due to the reduced number of batteries and electrical components needed.
The literature review on trends and developments outside of Europe shows that applications in developed countries such as Australia, New Zealand, and those in North America are similar. They focus mostly on optimising existing electrical grids and services while developing countries focus mostly on establishing and maintaining electrical grids and services. While in developing countries off-grid applications dominate, Asian countries, especially China, use spent batteries for all kinds of second use applications, even mobile ones. The repurposing activities show that there is a global increase in the second use of batteries, indicating that there are benefits such as sustainability and reduced costs.
Finally, the paper investigated the barriers to the uptake of second use battery energy storage systems as a sustainable product to foster the deployment of cleaner energy production and consumption. The review shows that the uptake of second use storage systems is heavily dependent on overcoming barriers related to economic and environmental considerations, technological developments, and regulatory factors. Current battery prices, the harsh mining and battery production conditions, as well as limited recycling opportunities, are challenges that need to be addressed. Furthermore, technological challenges related to the manufacture and assembly of battery hardware as well as those related to standardisation of control software need to be overcome to foster the deployment of second use battery energy storage systems. Additionally, there is a need for regulations on national and international levels directly related to the standardisation, transferability, and recyclability of electric vehicle batteries since these factors currently limit the second use of such batteries. Only the combined efforts of research, industry, policymakers, and the general public will make it possible to break through these barriers and thus lead to a more sustainable and environmentally friendly future. Acknowledgments: I would like to thank Anupam Akolkar whose comments, suggestions, and critical reading helped to significantly improve and clarify this review paper. Willy Tomboy's input for Section 2 was highly appreciated. I am particularly grateful for the assistance given by Verena Elisabeth Lechner for helping me with the illustrations. Special thanks to Maud Pélissier and Malin Toften Mangersnes for proofreading.

Conflicts of Interest:
The author declares no conflict of interest.