Potential and Most Promising Second-Life Applications for Automotive Lithium-Ion Batteries Considering Technical, Economic and Legal Aspects

Abstract

Electric vehicle (EV) batteries, i.e., currently almost exclusively lithium-ion batteries, are removed from the vehicle once they no longer meet certain requirements. However, instead of being disposed of or recycled, the removed batteries can be used in another, less demanding application, giving them a “second life”. Research in the field of second-life batteries (SLBs) is still at an early stage and, to better understand the “second life” concept and the related challenges, potential second-life applications need to be identified first. Using a detailed study of the scientific literature and an interview with field experts, a list of potential second-life applications was drafted. Afterwards, a technical, economic, and legal evaluation was conducted to identify the most promising options. The findings of this research consisted of the identification of 65 different mobile, semi-stationary and stationary second-life applications; the applications selected as most promising are automated guided vehicles (AGVs) and industrial energy storage systems (ESSs) with renewable firming purposes. This research confirms the great potential of SLBs indicating that second-life applications are many and belong to a broad spectrum of different sectors. The applications identified as most promising are particularly attractive for the second-life use of batteries as they belong to fast-growing markets.

1. Introduction

Lithium-ion batteries (LIBs) are a common solution for powering electric motors in electric vehicles (EVs). During use, the batteries are subject to calendar and cyclic ageing [1,2] which leads to continuous degradation [3]. Therefore, the properties of the battery will change over time, and it will need to be replaced once certain automotive requirements are reached, e.g., 70–80% state of health (SOH) [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21].

The average lifetime of a battery used in an EV is typically estimated to be between 8 and 15 years [14,17,20,22,23,24,25,26,27,28,29,30,31,32,33]. Thus, considering the significant increase in the number of EVs sold [34], more than 200 GWh of batteries from EVs will have reached their end of first life (EOFL) by end of this decade and will need to be replaced [35]. Batteries removed from EVs still have a high value, even if they no longer meet automotive requirements. Thus, instead of disposing of or recycling them, the batteries can be reused in another, less demanding application, giving them a second life. Using batteries in a second life is a solution in line with the European Union’s (EU) goal of greater sustainability and a more circularly oriented competitive economy [36,37,38]. This solution is valuable because it prolongs the life of the battery and thus the resources and emissions required to produce the battery are spread over a longer period [39]. As a result, the environmental impact of the battery is reduced [40,41,42,43,44]. In addition to the environmental benefits, using batteries for a second life may also increase their economic efficiency and enable the emergence of new markets and business models (BMs) [41,45,46,47,48,49,50].

Research in the field of second-life batteries (SLBs) is still in its infancy, and there are still many challenges to be overcome, especially in terms of safety, to enable a successful transition from the first to the second life. As a first step, the potential applications for using SLBs (in this paper referred to as second-life applications) need to be identified and have to be critically assessed not only in technical terms but also by considering the economic and legal aspects.

From an economic point of view, the battery of an EV contributes about one-third of the total value of the vehicle [51]. Therefore, it is important to consider how this expensive component can be used beyond its EOFL. With the increasing number of EVs on the roads, the higher availability of SLBs leads to novel business opportunities that need to be explored, but which are strongly influenced, and in many cases also limited, by political regulations, profitability considerations as well as rising environmental awareness.

From a legal perspective, the issue of the second-life use of LIBs is a global one. The UNECE World Forum for Harmonization of Vehicle Regulations (WP.29), with its Working Party on Pollution and Energy (GRPE), is attempting to create global technical regulations (GTRs) for the standardisation of the legal framework in the automotive sector. In particular, a proposal for mandatory minimum battery durability guarantees for electric and plug-in hybrid vehicles has been published [52].

At the European level, the European Union is planning to enact specific, harmonized legal regulations in the future. In particular, the new European Battery Regulation [53] will be the most important legal text related to the second-life use of batteries. This harmonizing regulation is expected to be directly applicable in all European member states in 2023 (but latest by 2026 [54]) and will represent the central legal regulation. In addition, however, national regulations will also be adapted to second-life use cases. Regarding the UNECE GTRs, the European Union was authorized by the European Council to negotiate and adopt them on behalf of the member states, which gives these regulations—once adopted by the UNECE—a mandatory character for the entire Union [55].

There are several review papers covering the topic of SLBs [41,45,56,57,58,59,60,61], however, none of them focuses exclusively on potential second-life applications.

Summarising potential second-life applications in a cohesive manner provides a good basis for future studies on SLBs. Therefore, the research task of this paper is to collect potential applications with SLBs and then identify which ones are most promising in technical, economic and legal terms using a specific assessment methodology. The investigation findings not only enable a better understanding of the state of the art but also provide a targeted selection of the most promising applications, which can be analysed in more detail in future investigations.

2. Materials and Methods

The purpose of this research is to accomplish two findings. The first one is to draft a list of potential second-life applications, while the second one is to identify the most promising second-life applications. The workflow followed to achieve the purpose of this research is schematically shown in Figure 1. The list of potential second-life applications is derived from in-depth literature research and comprehensive expert interviews in the form of an online survey (see Appendix A). The most promising applications are obtained through the definition of a set of evaluation criteria derived from the literature research.

2.1. Potential Second-Life Applications

The literature research laid the groundwork and provided an overview of the second-life applications addressed by the scientific literature, while the interview with experts provided novel second-life applications. The online survey gathered feedback from 70 international experts in the fields of mobility, automotive, electrification and batteries.

A more specific insight into second-life applications was then made by selecting the most relevant applications. The selection was guided by the number of scientific publications concerning the application under consideration and the feedback obtained from the expert interviews in which experts were asked to evaluate the examined applications. The average of the two normalised scores was taken as the final score. After that, all applications that scored above the average of the grades plus a standard deviation were selected. Consequently, the list of potential second-life applications was narrowed down to focus on only eight applications that were currently considered the most relevant by the scientific and industrial actors.

2.2. Most Promising Second-Life Applications

The next step in this research was then to identify the most promising applications. The applications were assessed using an evaluation catalogue containing technical, economic and legal evaluation criteria. The focus of the technical aspects considered as evaluation criteria are on safety. Hence, technical aspects influencing the useful life of the battery (e.g., the number of cycles or state of charge (SOC) range) were not considered, however, they may be relevant for research in which the remaining useful life (RUL) is evaluated. The following criteria were considered to evaluate the applications:

• Maximum discharge rate. This criterion value depends on the type of application and is considered relevant because when the discharge current is too high, this results in lithium plating, capacity fade and internal resistance increase [62,63,64].
• Maximum charge rate. This criterion directly influences the charging time, which is a crucial aspect for certain applications (e.g., EVs). When the charging rate is too high, this leads to lithium plating, capacity and power fade, faster ageing and, in the worst case, thermal runaway [63,64,65,66,67,68].
• Required capacity. This criterion is relevant for both practical feasibility and safety reasons. On the one hand, it is inconvenient to use batteries for applications where the required capacity is less than that provided by a single module or even a single cell [69,70]. On the other hand, it is complicated to have a sufficient number of SLBs to meet the demands of applications where the overall capacity required is very high [41]. Furthermore, the safety issues (and consequent maintenance costs) associated with high energy densities confined to a geographically limited area must also be taken into account [41].
• Degree of mobility (stationary, semi-stationary, mobile). This criterion influences the likelihood of the battery being subjected to mechanical loads (e.g., mechanical shock, indentation) that could eventually lead to catastrophic consequences [71,72,73,74].
• Operating and storage temperature. Temperature is a major concern in terms of safety. There is a temperature safety window (25 °C–35 °C) in which the battery is intended to operate [75]. A battery that is thermally abused by elevated temperatures is subject to the decomposition of active material and, in the worst case, exothermic reaction and thermal runaway [63,64,76,77]. If, on the other hand, temperatures are too low, there is a decrease in the reaction rate, metallic lithium depositing, irreversible capacity loss and an increased risk of internal short-circuit [63,64,76,78]. The temperature depends not only on the battery and the cooling system but also on the surrounding environment.
• Applicable BM patterns. The 55 highly successful field-proven BM patterns published by Gassmann et al. [79] have been taken as a basis to build up consecutive analyses and trains of thought. A promising BM pattern in terms of second-life applications can either lead to a successful BM or can be combined with other promising BM patterns into a bundle forming a prosperous BM.
• Legal knock-out criteria. The legal assessment (analysing the thematically pertinent legal texts and case law including broad comprehensive research of the legal literature on European and Austrian/German national level to identify potential problems) was carried out to find possible knock-out criteria that would make certain applications unfeasible.

In order to narrow the scope of the investigation, only the eight most relevant applications were considered for the selection of the most promising applications. The relevant second-life applications determined previously are then assessed using the technical, economic and legal evaluation criteria given above. This evaluation method enables us to rank the investigated second-life applications and identify which applications are most promising. Finally, the most promising applications are validated by comparing the use of LIBs with alternative energy storage technologies.

3. Results

3.1. Potential Second-Life Applications

SLBs exhibit a higher state of degradation than fresh batteries, so applications with lower requirements than those of the first life must be selected. Potential second-life applications are those where capacity and density are not critical and fast and continuous charging and discharging are not expected. Furthermore, batteries removed from a vehicle at the EOFL can be used in a second-life application at different levels (i.e., pack, module and cell level). Yet, minimising operations to be performed on the battery also reduces costs, so it is generally more cost-effective to use the battery directly at the pack or module level, rather than at the cell level [69,70].

A significant number of potential second-life applications were found with the study of the scientific literature and innovative ideas were also gathered using expert interviews. For simplicity’s sake, the applications have been categorised into three categories according to their degree of mobility (i.e., mobile, semi-stationary or stationary). The list of potential second-life applications is summarised in

Table 1. Potential second-life applications.

Mobility Degree	Category		Application	Source
Mobile	Commercial EVs	1	Short-range EVs	[19,41,45,60,80]
		2	Hybrid trucks	[80]
	Industrial Vehicles	3	Forklifts	[19,20,20,60,81,82,83,84]
		4	Pallet trucks	[19,20,20,60,82]
		5	Tractors	[19,20,20,60,82]
		6	Transport trolleys	[19,20,20,60,82]
		7	Sweepers	[19,20,20,60,82]
		8	Automated guided vehicles (AGVs)	[19,20,20,60,82]
		9	Excavators	[19,20,20,60,82]
		10	Dumpers	[19,20,20,60,82]
		11	Wheel loaders	[19,20,20,60,82]
		12	Telescopic handlers	[19,20,20,60,82]
		13	Airport pushback tractors	Expert feedback
		14	Airport belt loaders	Expert feedback
		15	Airport passenger stairs	Expert feedback
	Micro-mobility	16	E-bikes	[19,20,20,80]
		17	E-scooters	[19,20,20]
		18	Electric wheelchairs	[19,20,20,81]
	Lightweight vehicles	19	Golf carts	[19,20,20,80,82,85]
		20	Three-wheel vehicles	[19,20,20,82,85]
	Lead–acid replacement	21	Automotive starting	[86]
		22	Automotive lighting	[86]
		23	Automotive ignition	[86]
		24	Industrial trucks	[86]
	Autonomous mobile robots	25	Robotic vacuum cleaners	[80]
		26	Robotic lawnmowers	Expert feedback
	Consumer electronics	27	Leisure time gadgets	[80]
		28	Kitchen appliances	Expert feedback
		29	Working tools	Expert feedback
	Marine applications	30	Full propulsion	[19,83,85]
		31	Hybrid propulsion	[19,80,83,85]
		32	Spinning reserve	Expert feedback
		33	Load-levelling	Expert feedback
		34	Shore-stations	[85]
		35	Peak shaving/transient load management	[85]
		36	Energy recapture	Expert feedback
	Rail transport	37	Trams power supply	[87]
		38	Trams backup system	Expert feedback
		39	Trains power supply	Expert feedback
		40	Trains backup system	Expert feedback
	FC-based transportation	41	Energy buffer for H2FC	Expert feedback
Semi- stationary	Mobile power supplies	42	Power-stations for construction sites	[20,20,82]
		43	Power-stations for major events	[20,20]
		44	Power-stations for outdoor camping	Expert feedback
		45	Power-stations for outdoor leisure office	Expert feedback
		46	Power-stations for outdoor emergency power supply	[20,20]
		47	Automotive mobile charging stations	[19,56,82]
	Other	48	Buffers for stationary traffic signs	Expert feedback
Stationary	Lead–acid replacement	49	Telecommunication backup power	[5,85,86]
		50	Uninterruptible power supplies	[85,86,88]
	EV chargers	51	On-grid buffer storages at charging station	[19,20,20,41,44,45,56,60,81,83,87,89,90,91,92]
		52	Off-grid buffer storages at charging station	[5,18,45,56,83,89,91,93,94]
	Special grids	53	Micro-grids	[20,20,41,45,56,60,85,88,91,95,96,97]
		54	Smart grids	[11,41,45,56,60,85,88,91,95,96,97,98,99]
	Residential ESS	55	Residential ESSs with load following purposes	[19,20,20,21,41,45,56,57,60,81,83,85,86,87,91,92,95,100,101,102,103,104,105,106]
		56	Residential ESSs connected to a RES	[8,15,16,19,20,20,21,41,43,44,45,56,57,81,89,92,100,101,102,103,104,105,106,107,108,109,110,111,112,113]
		57	Residential ESSs with backup purposes	[19,20,20,41,56,57,60,83,85,90,91,95,111]
	Commercial ESS	58	Commercial ESSs with peak shaving purposes	[5,19,41,43,45,56,57,60,80,82,83,86,88,90,91,93,95,100,101,102,103,111,114,115,116]
		59	Commercial ESSs with load following purposes	[5,20,20,21,41,45,46,56,57,60,86,88,104,105,111,116]
		60	Commercial ESSs with backup purposes	[19,20,20,41,45,57,60,80,90,91,92,95,111]
	Industrial ESS	61	Industrial ESSs with load levelling purposes	[5,19,20,20,21,41,45,57,60,82,111,117,118]
		62	Industrial ESSs with renewable firming purposes	[5,7,20,20,21,41,45,56,57,60,80,81,82,85,88,89,90,91,92,95,100,102,103,108,111,113,114,119,120,121,122,123]
		63	Industrial ESSs with spinning reserve/area regulation purposes	[5,7,19,21,41,44,45,60,86,89,90,91,95,103,111,114,124]
		64	Industrial ESSs with peak shaving purposes	[5,19,41,44,45,56,60,80,82,83,86,88,91,93,95,101,102,103,111,114,115]
		65	Industrial ESSs with transmission stabilisation purposes	[7,20,20,21,41,45,56,60,80,86,88,91,95,114,118]

.

The first major category of possible second-life applications is “mobile applications”, i.e., applications where the battery is expected to move during use. One possible application is to reuse the batteries in a short-range EV. In fact, although EOFL is commonly defined as when the battery reaches 70–80% capacity retention, the range provided by the battery is sufficient for most daily trips. For example, a battery with 60% capacity retention is able to meet the daily travel needs of over 75% of drivers [12,41]. Furthermore, this is a cost-effective solution as reprocessing the battery is hardly, or not at all, necessary [20]. Similarly, SLBs can be installed in hybrid trucks operating in urban areas to provide power at low speeds before the internal combustion engine (ICE) starts up and recharges the battery. Another possibility is to use the batteries for the propulsion of micro-mobility vehicles (e.g., e-bikes, e-scooters, electric wheelchairs), lightweight vehicles (e.g., golf carts, three-wheel vehicles) and industrial vehicles (e.g., forklifts, pallet trucks, tractors) or for the internal energy management of some vehicles [83] (e.g., food trucks).

The ability to withstand partial electrochemical cycles without degrading excessively and the higher energy density in terms of both weight and volume result in LIBs being more appealing than conventional lead–acid batteries. The main obstacle to using LIBs rather than lead–acid batteries is the difference in cost, which makes the use of lead–acid batteries more widespread than LIBs for certain types of applications. However, when considering the case of SLBs, the price difference flattens out, leaving room for the emergence of new second-life applications linked to the replacement of lead–acid batteries [60]. Possible replacements for mobile applications may be the batteries used for automotive starting, lighting and ignition or the ones used in industrial trucks.

Other examples of mobile second-life applications include the use of SLBs as buffer storage in fuel cell (FC) vehicles or different types of transportation technologies, e.g., rail (e.g., trams and trains propulsion) or marine applications (e.g., propulsion, backup, load levelling).

Another category of potential mobile second-life applications includes all consumer electronic applications (e.g., mobile home robots, household appliances, working tools, laptops and other leisure time gadgets) as further potential mobile second-life applications. However, it is not always considered an optimal solution because, in most applications, the battery provided by a single large-format automotive cell has a higher capacity than required for the specific consumer electronics application [86].

Second-life applications in which the batteries are not intended to operate while moving but are expected to be relocated frequently are defined as “semi-stationary”. Examples are power generators or power stations used in remote areas and automotive mobile charging stations [19,56,82].

Another major category of second-life applications includes stationary use cases, i.e., the battery is not expected to move during its operational cycle. In addition to the degree of mobility, the difference between stationary and mobile (or semi-stationary) second-life applications is that they tend to have less stringent weight and volume limits. Moreover, they have a reduced possibility of being mechanically abused, which could allow for less stringent safety requirements than those applied in the case of mobile applications. The first macro-category for stationary second-life applications is energy storage systems (ESSs). ESSs can be subdivided into smaller categories depending on the type of consumer, whether attached to the grid or not, and the functions to be fulfilled. In this paper, the first mentioned classification method is utilised.

The first type of consumer is at the residential level and therefore concerns individual households (e.g., private houses, flats). Residential second-life battery energy storage systems (SLBESSs) are a viable solution in terms of performance as the required power and capacity are provided by most SLBs extracted from EVs. Residential SLBESSs can store energy when consumption is low and release energy when higher consumption is reached, which results in smoothing the load and gives the possibility of participating in the energy arbitrage business; this not only provides economic benefit but also a more environmentally sustainable solution and reduces the strain on the electric grid. Furthermore, they can be used in combination with renewable energy sources (RESs), for example, photovoltaics (PV), encouraging RES use and promoting the decentralisation of the electricity production system. The SLBESSs can also act as an energy backup in the event of a power failure or blackout. Depending on the purpose, the scenarios to which the ESS is subjected are different. Considering a typical residential demand load profile (LP1—Urban Domestic 24-Hour [125]), in the case of load-following, a capacity of at least 3–4 kWh is required [111]. The expected load profile is characterised by one deep discharge and several small discharges during the day, and a typical discharge rate of C/3 (The C-rate refers to the current at which the battery is charged (or discharged) with respect to its nominal capacity. For example, a discharging C-rate of C/3 indicates that it ideally takes 3 h to discharge the battery from 100% to 0% SOC). In the case of the backup system, the capacity required is 25 kWh (in the case of an off-grid application) and moderate daily discharges are expected (depth of discharge < 50%) [41,111]. The use of SLBs also tackles the problem of the high costs of using new batteries, which is the main limitation for the installation of domestic ESSs [102]. Nevertheless, there are other limitations to consider, e.g., the needed amount of space and the high maintenance requirements, which suggest that residential ESSs are often not the best solution [85]; furthermore, the risk due to the proximity of the residence and people should also be addressed [41].

ESSs may also be intended for larger applications at the commercial or industrial level. These types of applications require a greater investment in storage size and tighter safety standards [41].

The commercial-level consumer is broad and includes different types of businesses and light industries. Commercial businesses, where the presence of ESS could be beneficial, are, for example, telecommunication companies, large offices and fresh food distribution centres. The commercial load is on average higher than the domestic load, making it more suitable for applications such as load following and peak shaving. When considering a typical commercial load (LP5—Non-Maximum Demand Non-domestic 24-h [125]), in the case of peak shaving, the required capacity is about 3000–4000 kWh, thus a considerable number of batteries is needed (approximately 178–238 reconditioned Nissan Leaf batteries [111]), which is why a hybrid solution with both new and SLBs is commonly chosen. The expected discharge rate ranges between C/2 and C and occurs daily. In the case of load following, the required capacity is 75–100 kWh (approximately 4–6 reconditioned Nissan Leaf batteries [111]), considerably less than that required by peak shaving, making this application more likely to be fully managed with SLBs [41,111]. In addition, a deep discharge and many small discharges are expected on a daily basis. The C-rate required for load following is typically C/3, which is easily sustained using LIBs. Another possible application concerns the use of SLBs as a backup system that comes into operation in the event of unusual scenarios, in this case, the expected discharge rate is approximately C/5 [41,111].

The power demand for industrial ESS applications (e.g., heavy industry) is even higher than for residential and commercial applications [41]. In the case of industrial ESS, there are several possible purposes: load levelling, renewable firming, spinning reserve or area regulation, peak shaving and transmission stabilisation.

When considering a typical industrial load (LP8—MD Load Factor > 30 to 50% [125]), the capacity required for industrial load levelling is around 100 MWh (approximately 6000 reconditioned Nissan Leaf batteries), the high number of batteries required makes this application impractical with SLBs [41,111]. A lower number of batteries is required in the case of renewable firming (1–10 MWh, approximately 60–595 reconditioned Nissan Leaf batteries), spinning reserve/area regulation (5–7.5 MWh) and peak shaving where the capacity required is similar to the one seen in the commercial sector (75–100 kWh, i.e., 4–6 reconditioned Nissan Lead batteries) [41,111]. Renewable firming is characterised by frequent discharge cycles with an intensity of C/5, spinning reserve/area regulation and peak shaving by discharge cycles varying from C/2 up to C, all of which are suitable for SLBs. Another application worth mentioning is transmission stabilisation (140 kWh, 500 MW), where short bursts of power are used for voltage and frequency regulation; however, the C-rate required in this application is higher than the capabilities of LIBs [41,111]. Another promising application is to replace grid-connected combustion turbine peakers with ESS to provide peak shaving services and thus improve the system efficiency [86,124].

SLBs can also be used in microgrids and smart grids to provide localised support for the production and distribution of energy from alternative power sources. Microgrids can be either attached to the main grid (on-grid) or belong to a stand-alone system (off-grid). Microgrids promote the decentralisation of energy storage and allow energy to be supplied in remote areas in combination with renewable energy production systems, ensuring increased stability and providing backup power [41,85,91].

EV batteries can also be used as buffer storage at charging stations to reduce the power demand while charging and to relieve the load on the public grid or decentralised buffer storages in combination with energy production from renewable sources allowing the charging of vehicles in remote areas where the public grid does not reach. The use of new batteries, however, is not cost-effective. In contrast, SLBs, in addition to meeting the required technical expectations, have a lower cost [41]. Furthermore, for this type of application, assuming the battery is used from 80% to 60% SOH, the estimated lifespan is 15 years [44].

Finally, as in the case of mobile applications, there are stationary examples of SLBs replacing lead–acid batteries in stationary applications. For example, for telecommunication backup power or uninterruptible power supply (UPS).

3.2. Most Promising Second-Life Applications

From the list of potential second-life applications, four mobile and four stationary applications were selected to be analysed in more detail. As mentioned above, the selection of these eight applications was based on their relevance in the scientific literature and the feedback obtained from the expert interviews. The selected applications are listed in the first column in

Table 2. Filled evaluation matrix with real values linked to the applications under investigation.

Application	Max Discharge	Max Charge	Required Capacity	Degree of Mobility	Min T	Max T	Promising BM Patterns *	Legal Knockout Criteria	Score
	[C-Rate]	[C-Rate]	[kWh]	[-]	[°C]	[°C]	[-]	[-]
Forklift	+ (0.71)	++ (0.89)	++ (34)	++ (Mobile)	- (0)	o (40)	o (6)	o (None)	5
Pallet truck	+ (0.56)	- (3.33)	++ (2)	++ (Mobile)	o (10)	o (30)	o (6)	o (None)	3
AGV	++ (0.13)	++ (0.50)	++ (10)	++ (Mobile)	o (10)	o (30)	o (6)	o (None)	7
Golf cart	x (6.90)	++ (0.35)	++ (3)	++ (Mobile)	- (0)	o (40)	o (6)	o (None)	Discarded
On-grid buffer storage at charging station	- (2.29)	+ (1.22)	+ (140)	+ (Stationary)	x (−30)	- (50)	+ (7)	o (None)	Discarded
Commercial ESS with peak shaving purposes	o (1.00)	+ (1.00)	o (4000)	+ (Stationary)	+ (20)	o (30)	+ (7)	o (None)	5
Industrial ESS with peak shaving purposes	o (1.00)	+ (1.00)	o (4000)	+ (Stationary)	+ (20)	o (30)	+ (7)	o (None)	5
Industrial ESS with renewable firming purposes	++ (0.20)	++ (0.20)	- (10000)	+ (Stationary)	+ (20)	o (30)	+ (7)	o (None)	7

* The list of the promising BM patterns for each application is shown in Appendix E.

. The table was then filled with the technical data from real cases or the scientific literature (see Appendix B). The comparison and ranking of the application were performed using an evaluation system (see Appendix C) which assigns a score (++, +, o, -, --, x) based on the value in brackets.

The analysis of the legal framework yielded that there is no law prohibiting the applications under investigation. Therefore, all applications are valid to be used with SLBs in the same way. For more details concerning the legal framework, see Appendix D.

The last column of Table 2 summarises the scores of the eight applications under investigation. Golf carts and on-grid buffer storage at charging stations were discarded because one of the parameters was outside the limits imposed by the evaluation knock-out criteria. All the remaining applications scored between three and seven. Due to their overall score, AGVs for mobile applications and industrial ESSs with renewable firming purposes for stationary applications are considered as most promising.

4. Discussion

The literature research combined with the expert interviews yielded a wide spectrum of possible stationary, semi-stationary and mobile applications that can be powered with SLBs. The fields of application vary extensively from transport to home applications, from buffer storage to ancillary services on the public power grid. This demonstrates the potential for SLBs to be versatile in a plethora of application areas, to serve existing markets and open new markets using the associated application of a wide range of BMs.

The subsequent analysis indicated two applications with different degrees of mobility as “most promising”, again suggesting that the complexities of the transition to second life are many and involve multidisciplinary studies.

4.1. Validation

AGVs and industrial ESS with renewable firming purposes are considered the two most promising second-life applications. However, it is necessary to consider that different types of energy storage technologies could be more advantageous than LIBs. It is therefore necessary to analyse the possible energy storage technologies for the two selected applications and confirm that LIBs are a viable solution.

In the case of AGVs, the main alternative to LIBs is lead–acid batteries [126]. Currently, the best solution depends on the type of application. Yet, as LIBs offer several advantages over lead–acid batteries (e.g., lighter and more compact, suitable for fast charging, longer runtimes and battery life) the AGV market is witnessing a gradual shift from lead–acid to LIBs. This is also connected to the fact that in most scenarios where AGVs are involved, high round-trip efficiency is required [127].

In the case of industrial ESS with renewable firming purposes, in addition to the use of batteries (e.g., LIBs, lead–acid batteries, high-temperature batteries, flow batteries), mechanical energy accumulators can also be utilised (e.g., pumped-storage hydroelectric, compressed air energy storage, flywheels). An analysis conducted by the International Renewable Energy Agency (IRENA), in which all these different technologies are compared, shows that the use of LIBs is the best solution [128]. In addition, another comparative study revealed that the cost-effectiveness of lithium-ion batteries is greater than that of lead–acid and flow batteries [129].

As a result, it emerged that in the case of both AGVs and industrial ESS with renewable firming purposes, the use of LIBs is not only a viable solution but is currently the best solution.

4.2. Limitations

More than 60 potential applications have been identified; however, the applicability must be studied on a case-by-case basis as there are some parameters (e.g., extreme geographical latitude and environmental conditions) that may result in an application being unprofitable or even unfeasible. The analysis of the most promising applications was conducted on eight specific second-life applications. However, the method used is suitable for studying a larger number of applications that could also be promising. Furthermore, it is important to consider that second-life applications belong to a rapidly evolving market that may witness significant changes in the coming years.

5. Conclusions

A multidisciplinary study was conducted to identify potential and relevant second-life applications of automotive lithium-ion batteries using a comprehensive study of the scientific literature and consultation with experts in the relevant fields of application, and to subsequently evaluate their basic feasibility/implementation potential (“most promising applications”) according to a specific set of evaluation criteria, taking into account economic and legal criteria as well as technical ones, ensuring that the study was not limited to a purely technical analysis.

From the investigation, more than 60 potential second-life stationary, semi-stationary and mobile applications were identified, with AGVs and industrial ESSs with renewable firming purposes being the most promising considering economic, legal and technical aspects.

The results show that SLBs can be used in a multitude of potential applications. In addition, novel second-life applications, not addressed in the scientific literature, emerged from the feedback obtained from the expert interview. The applications evaluated according to the evaluation criteria received different scores which allowed them to be ranked, thus demonstrating the importance of analysing each second-life application on a case-by-case basis given the variety of each application.

The findings obtained are of interest as they provide a valuable and informative summary of the potential and most promising second-life applications. The method used for the identification of the most promising applications can be adapted and used in similar research. The identified applications can be further researched by the scientific community or developed into novel products in the industry.

The authors are currently continuing research in the field of SLBs by analysing in more depth the applications identified as most promising to understand the actual feasibility of SLB use from an economic, legal, technical and safety perspective.