The transition to electric vehicles (EVs) in place of internal combustion engines (ICEs) is ongoing throughout the developing world. Many environmental impact assessments have found that while the initial resource use of an EV battery is higher than that for ICEs (Dolganova et al., 2020; Verma et al., 2022; Xia and Li, 2022), overall, the use of EVs reduces the environmental impacts of cars and other vehicles. Various Li-ion-based batteries have been used in EVs and assessed environmentally, with lithium–iron–phosphate (LFP) often being seen as overall better for the environment than nickel–manganese–cobalt (NMC) batteries (Feng et al., 2022; Xia and Li, 2022). The use of an energy storage system (ESS) occupies a significant portion of the overall environmental impact of a vehicle, and it is essential to explore different options for electrifying our vehicles. A dual energy storage system (DESS), often called a hybrid energy storage system (HESS), in the context of an electric vehicle (EV), is an energy storage system composed of a high-energy (HE) component and a high-power (HP) component. The use of the term DESS is to avoid confusion with hybrid electric vehicles (HEV), which refer to a combination of electric and internal combustion power sources. There is also some use of a DESS within an HEV, which may cause further confusion of the definitions (Song et al., 2018a). Within a DESS, the high-energy component provides energy for cruising and other low-power maneuvers, and the high-power component provides additional (or all) power for acceleration along with being a sink for energy generated from braking—limiting the demand on the HE part and extending its lifetime (Lencwe et al., 2022). In this work, the life cycle assessment is topology-agnostic; however, a schematic of a typical DESS is shown in Figure 1.

Usually, the HE component is an energy-dense battery, such as lithium-ion, with the HP component being provided by supercapacitors (often called ultracapacitors). The use of DESSs in EVs is not widely adopted, with many discussions of use cases based on different driving styles and different vehicle applications, which are summarized in Table 1. There is a range of configurations for DESSs, depending on the specific scenario and vehicle scale, from a city bus driving on a normal route (Ostadi and Kazerani, 2015) to a sports car being driven aggressively (Zhu et al., 2020). While the adoption of DESSs is in its infancy, some parallels can be made with conventional hybrid (internal combustion engine—battery) systems (Sankarkumar and Natarajan, 2021). From Table 1, it is clear that the number of (1–38,406), voltage (136–444 V), and capacity (0.9–115 kWh) of the Li-ion cells as well as the capacitance of the SCs vary widely depending on application. Factors that can impact the configurations are the criteria that are optimized, such as minimizing weight, cost, and charge time. However, there may also be a desire to keep battery replacements to a minimum or reduce the number of charge/discharge cycles or state of charge optimization over the vehicle’s lifetime. The article’s focus is highlighted in Table 1. Ensuring the overall battery’s and DESS’s state of health is also a driving factor in system configuration (Song et al., 2015a; Zhang et al., 2018). Overall, only one study was found to use environmental impact as a key driver (Venkateswaran et al., 2020), instead of a reported output or “bonus” to the other optimizations. For example, two papers optimizing a DESS for the Tesla Model S report differences in power requirements of ∼120 kW in the supercapacitors and a ∼9-Wh difference in battery-provided energy. The higher power and energy demanding result considered optimizing for cost reduction (Zhu et al., 2018), whereas the other took a balanced look at cost, mass, and volume reduction to provide a different result (Zhu et al., 2020).

A further distinguishing factor is the ratio of HE:HP components: whether the HP component is being used purely for power-smoothing or as the main contributor to all high-power needs (such as accelerations). For a power-smoothing application, the HP component makes up a smaller proportion of the DESS. Both options show improvements in terms of the overall lifetime of the energy storage system and cost savings; however, there has not been an environmental assessment into which DESS version has the fewest environmental impacts over the lifetime of the vehicle used. This will be addressed as the focus of the assessment presented in this paper. There has been an initial life cycle assessment, LCA, looking at a DESS with two types of Li-ion batteries, a high-power and high-energy version, which showed environmental benefits compared to a single type of the Li-ion battery (Sanfélix et al., 2015); however, it was noted that the emissions produced through manufacturing were higher than that of a HE-only Li-ion battery pack (a 50% increase for CO₂ emissions and almost double for the impact on water acidification), and environmental benefits were only seen if the lifetime of the car could be doubled. Many LCAs have been performed for different Li-ion battery configurations (Peters et al., 2017; Ellingsen et al., 2018; Peters and Weil, 2018), as well as for capacitors and supercapacitors (SCs) (Smith et al., 2018; Cossutta et al., 2020; Sperling and Kivelä, 2022); however, these have not been evaluated within a DESS from an environmental perspective. The manufacturing process is a key source of emissions for Li-ions, and if emissions can be reduced at production by using different HP components, while providing the same lifetime extension, then the overall environmental impacts can be further reduced. Continued reliance on lithium batteries will put more pressure on the already politicized mining of associated minerals and may increase the costs beyond the lifetime extension benefits. Therefore, looking at non-lithium, lower cost, and easier-to-produce options for the high-power component of a DESS is crucial to making them accessible.

This paper specifically considers two case studies, a city bus (example 10) and an electric vehicle (EV) modeled as a 1,100 kg car (example 7), taking data from earlier studies (Song et al., 2018b; Lu and Wang, 2020). These DESSs have different output suggestions for the proportion of HE:HP components as well as the overall power and energy required. The city bus requires 150 kW from the HP component, while the EV requires 19.9 kW. Here, a lifecycle assessment approach is used to analyze the environmental impacts of dual energy storage systems proposed for each case study, namely, a supercapacitor/Li-ion (SC/Li-ion) based on the literature results reported. This study further evaluates the use of an aqueous Al-ion/Li-ion configuration as an emerging high-power battery technology. Aluminum is an attractive material for battery applications in EVs due to its high charge storage capacity of 2980 mA h g−1/8046 mA h cm⁻³ and relative abundance (Elia et al., 2021; Verma and Kumar, 2021). Aluminum ions are readily solvated in both aqueous and non-aqueous (organic or ionic liquid) solvents with a number of materials, including TiO₂, MoO₃, C, and Al being suggested as positive electrodes, while negative electrode materials include Fe-, Mn-, Cu-, and V-complexes (Melzack and Wills, 2022; Pan et al., 2022) Aqueous Al-ion batteries are one of a number of metal–ion systems being investigated utilizing water as the electrolyte (Wu et al., 2022). This aqueous Al-ion technology has already been identified as potentially having fewer environmental impacts per kW than some supercapacitors and, as such, is a good case study to test the methodology of comparing environmental impacts within DESS design (Melzack et al., 2021). The dual energy storage systems will be compared to a Li-ion-only configuration. The aim is to evaluate the environmental perspective alongside battery chemistry to link these two aspects during new energy storage device development. This LCA is for the cradle-to-gate section of the product lifecycle, using the methodology set out in Potocnik (2013) and Siret et al. (2018). Therefore, this assessment includes the mining and production of raw materials through to manufacture, but it does not include the use phase or any second-life applications or end-of-life activities (such as recycling).

Using openLCA (Ciroth, 2007) and the European Union’s Environmental Footprint (EUEF) midpoint analysis (Siret et al., 2018), a cradle-to-gate analysis was performed. The fuel consumption and usage of the systems during the car’s lifetime are not considered; only the procurement of raw materials and manufacturing of the energy storage systems are considered. The impacts of production for all required components (including replacement batteries) were assessed over 19 impact categories, including acidification, carcinogenic properties, climate impact, land usage, ozone impact, and energy resource. Respiratory particulates are also calculated as part of the EUEF; however, with regards to nanoparticles (TiO₂ in the Al-ion and graphene in the SCs modeled are nanopowders), this assessment criterion is not fully accurate, as the calculations do not take into account nanoparticulates currently and, therefore, do not consider the full range of particulates produced during manufacture (Salieri et al., 2018). For the purpose of this assessment, the main impact categories—climate change, acidification, and resource use—are presented in Tables 7, 8, with the impacts shown graphically in Figure 3 for the city bus and in Figure 4 for the EV. Full results of all impact categories are shown in the Supplementary Information. For the SC/Li-ion case modeled, no replacements are needed, and so BOL and EOL are plotted as a single value. The replacement of the Al-ion battery over the lifetime increases the environmental impacts; however, they are still consistently lower than those of the equivalent Li-ion case.

Initially, it is worth noting that the ratio of HP:HE in the DESSs described is different in both cases. The city bus has a mass ratio of ∼1:1200, and the car has a mass ratio of ∼1:131 for the SC: Li-ion. The differences in these outcomes are likely due to the driving use cases used to optimize the DESSs and the likely different uses of a DESS, with the car using the HP for all high-power cases and the bus for “topping up” with the higher demand accelerations.

When looking at environmental impacts, the results at BOL are within one order of magnitude for all the cases presented. Therefore, it is fair to say that for a vehicle with a short overall lifetime (defined as less than 5 years for the city bus and less than ∼5000 cycles for the EV), there is no strong environmental reason to use a DESS. Furthermore, a DESS is often more expensive than using just one type of a battery (Lu and Wang, 2020; Vukajlović et al., 2020; Zhu et al., 2020). Al-ion cells, although adding an appreciable mass to the DESS, have comparable environmental impacts per kW output (Melzack et al., 2021) to the SC. It should be noted that the added mass was not factored into the vehicle lifetime data. The resulting environmental impacts are roughly the same to one significant figure across all BOL cases. When looking at resource use, the Al-ion/Li-ion seems the worst option for the low-use case; however, as this study does not take into account recycling and re-use, it is likely that the ease of recycling of Al-ion compared with Li-ion batteries (Bi et al., 2019) would lead to a more sustainable result. However, this is beyond the scope of this study.

We see the impact of the lifetime extension provided by the DESS when assessing the EOL results. This impact is shown most strongly with the SC/Li-ion DESS for both case studies. For the city bus example, at EOL, the SC/Li-ion would have just under a third of the environmental impact of the Li-ion-only option. Specifically, looking at the climate change impact, for the Li-ion-only option at end-of-life, the emissions are around 25,446 kg CO₂ eq. compared to 7936 kg CO₂ eq. for the SC/Li-ion DESS. For the EV, there is a ∼50% reduction in all impact categories reported. This reduction in impacts is mostly true for both the Al-ion/Li-ion EOL cases. The city bus case study shows that the Al-ion/Li-ion at EOL has 35% of the emissions of the Li-ion EOL in climate change impacts and 70% of Li-ion EOL’s resource use (minerals and metals). This high use of minerals and metals comes from the contribution of the copper current collectors in the Al-ion cell, and while not considered within this study, the recycling and re-use of copper in the future would reduce this impact. For the EV case study, the resource use (minerals and metals) is higher for the Al-ion/Li-ion EOL than for the Li-ion EOL, with 0.063 and 0.039 kg Sb eq., respectively. Overall, the impacts reported for the Al-ion/Li-ion EOL for the EV are slightly lower than that for the Li-ion-only EOL case—the climate change impact is 72% that of the Li-ion-only case, while the impact of acidification is 90% that of the Li-ion-only case. In this example, we can see the impact of the ratio of HP:HE and the difference it makes on the environmental output for the DESS.

For the city bus case study, Al-ion could provide the HP component of a DESS in the future, as it provides similar environmental benefits to the SC but has the advantage of being cheaper, safer, and easier to produce (Melzack et al., 2021; Melzack and Wills, 2022). However, the electric vehicle assessed, with higher reliance on the HP component, would benefit environmentally from an SC/Li-ion DESS. For the EV example, it is useful to discuss the measures needed to make the Al-ion/Li-ion DESS a viable environmental option as this will inform future development of the technology. The first option is an increase in the lifetime of the Al-ion battery such that no replacements are needed over the vehicle lifetime—this would lead to the BOL = EOL results, as seen for the SC/Li-ion examples in both case studies. However, if this were not achievable, looking at the impact that is above that of Li-ion-only—minerals and metals—we can inspect the data for the key contributor to this category. From Melzack et al. (2021), we can see that the key contributors are the copper current collector and the battery casing, made mainly from PEEK (polyether ether ketone). The battery casing for the Al-ion cell is ∼11% wt compared to those of the SC and Li-ion casing, which are around 20% of the total mass. The casings for SC and Li-ion, however, are made of aluminum, a low-impact metal with many established routes for production and recycling, which minimizes the overall environmental impact of the case used. The copper current collectors, as mentioned previously, are assumed to be primary copper; a simple improvement in terms of the resource use here is to switch to secondary (or recycled) copper for the current collectors, which will minimize the impact and have no design implications. Changing the copper source within the openLCA software to 75% recycled copper reduces the impact of the minerals and metals to 0.047 kg Sb eq. at EOL—still higher than the 0.39 kg Sb eq. for the Li-ion-only EOL case. At 100% recycled copper, the impact becomes 0.043 kg Sb eq.—at which point the next highest contributor takes over.

Other options in terms of reducing the overall impact of the EV EOL case using Al-ion would be to look at overall improvements in terms of the active material used within the batteries (Melzack, 2022). Currently, only ∼0.6% of the mass of the battery is active material—the rest is support, binders, casing, etc. However, within a fully commercial cell, the mass percentage of the active material is closer to 30% (Ellingsen et al., 2014; Peters and Weil, 2018). With a higher active material %, the overall amount of support materials reduces, which in turn reduces the environmental impacts, maintaining the active material mass required of 66.24 kg for the Al-ion component. A 30% active material mass percentage in the Al-ion component reduces the overall mass of that component to 220.8 kg and the impact of the mineral and metal resource category to 0.054 kg Sb eq. Combining this active material increase with 75% recycled copper for the current collectors, we arrive at a value of 0.025 kg Sb eq.—below that of the Li-ion-only EOL. These changes require the development of the Al-ion battery electrode in order to accommodate and use more active material throughout the battery life and testing to ensure that the increased material loading does not degrade the battery performance (Melzack, 2022).

When assessing the environmental impacts of products, it is important to understand the limitations of the functional unit and approach used. For this case, the size of the ESSs was calculated and assessed based on values from literature. From looking purely at the energy storage systems, using any DESS provides an environmental advantage in terms of the city bus assessed—but only over a long enough lifetime. This holds true even as Li-ion battery development extends the useful lifetime since the DESS will always offer an advantage. However, for the electric car assessed, the SC/Li-ion had the least environmental impacts, with the Al-ion/Li-ion mostly reducing impacts compared to Li-ion-only, except for the resource use of copper in the current collector. For vehicles with high mileage requirements over their lifetime, a SC/Li-ion DESS appears to have a clear environmental (and financial since Li-ion batteries are expensive (Larcher and Tarascon, 2015)) advantage at both HP:HE ratios. However, for short mileage requirements, the benefit may not be seen in the lifetime of the vehicle—but may still preserve the DESS for second-life uses. This study does not take into account the growing field of second-life uses for car batteries once they have reached 80% capacity and are no longer fit for their initial function. Grid storage and home-batteries for photovoltaics are applications that are currently being studied (Shahjalal et al., 2022). A DESS would likely have a longer second lifetime compared to a Li-ion ESS; however, this has not yet been studied. Therefore, using these energy storage systems in EVs may not just depend on their use in the EV but over the whole lifetime of the DESS. This does add mass to the overall system—which is a non-trivial consequence.

A limitation of this study is that the full electricity use and fuel-saving impacts are not taken into account over the vehicle’s operational lifetime. This study only assesses the cradle-to-gate portion of the life cycle. A further limitation is that the Al-ion cells are still lab-based, and therefore there are many assumptions made in this scale-up—which are discussed in Melzack et al. (2021). Further uncertainties are introduced when modeling the use-case, such as electricity mixes, driving consistency, and temperature ranges (Onat and Kucukvar, 2022). Furthermore, as illustrated in this study, and shown in Table 1, there is a vast range in the make-up of proposed DESSs (varying numbers of cells and supercapacitors with varying overall performance needs), and the choice with the least environmental impact may depend on the overall design and use assumptions of the DESS.

It is important to understand that the Li-ion battery pack and supercapacitors are already commercial technologies compared to the Al-ion development stage. Given that the environmental improvements are already comparable with the SC DESS, this shows that there is potential for further improvements with development of cell design, as shown with the EV example—increasing the active material and using recycled metals in the current collector (Melzack, 2022). These are key areas of research that are strongly supported by this work.

A further consideration in developing DESS systems is balancing operational requirements with environmental impact and safety. Environmental and safety aspects are linked to some degree in terms of the impact on acidification and climate change. Operational safety of DESS systems needs to incorporate a discussion on control topologies, which is beyond the scope of this work. Using aqueous, non-toxic materials should provide advantages in reducing the likelihood and impact of thermal events, and further work should be done in this area during the development of such systems.

When looking at existing LCAs of Li-ion batteries, the functional units vary. For example, Peters et al. (2017) evaluates the kgCO₂ eq. per Wh produced over the batteries’ lifetime—these specific comparisons are assessed within previous work (Melzack et al., 2021; Melzack, 2022). Cossutta et al. (2020) reports on the impacts per five capacitors, while the functional units of this study are per DESS. However, we can convert results into per kg values if we make some assumptions. For supercapacitors, the production phase states that 1.06E-1 kgCO₂ eq. are produced per five capacitors. Assuming the capacitor assessed is a Maxwell 5F capacitor (Maxwell. 2, 2013), the mass of five capacitors is equal to 10 g. In this study, 4.32-kg SCs are required for the EV case; therefore, based on Cossutta et al. (2020), kgCO₂ eq. would be produced. Based on (19), which looked at capacitors (not SCs) per kg, a capacitor produced 3.8 kgCO₂ eq., which, scaled to this study’s 4.32 kg, gives 16.5 kgCO₂ eq. Isolating the 4.32 kg of SCs required for the EV in this study, the EUEF method used gives a total emission of 28.7 kgCO₂ eq. The value calculated in this study is intermediate to those it has been compared to, and there are many factors that can cause discrepancies, including the specific capacitors analyzed being different, there are differences in transportation calculations and electricity mixes used in manufacturing, as well as the different calculation methods and software used. It is important, therefore, to understand these limitations when comparing LCA works and use them to understand trends and ranges to aid our design development.

This study looked at the practical use case of a dual energy storage system (DESS) in a city bus and an EV of 1,100 kg. Supercapacitors and aqueous Al-ion cells are assessed as the HP components of the DESS. For the electric vehicles assessed, the use of a DESS extended the energy system’s overall lifetime and reduced the environmental impacts of the energy storage system, compared to a Li-ion-only ESS—apart from the Al-ion/Li-ion for the mineral and metal impact category, where it uses 163% of that of the Li-ion-only option within the EV. Overall, the SC version of the DESS has lower environmental impacts. Given the SC assessed is commercially available, the mostly comparable results of the lab-based aqueous Al-ion technology are promising in terms of its potential sustainable development—with clear development directions such as using recycled copper and redesigning the battery casing being attainable options. It is likely that as this technology develops, its overall mass will reduce and become a viable option for future DESSs. Further work should include looking at the whole life cycle, including use-phase and end-of-life, to gain a complete picture of the overall impacts of the DESS within EVs. This must be done in a way that is sensitive to locations of manufacture and use, with the electricity mix used during both the manufacture and the lifetime of the EV being analyzed and understood. Furthermore, the end-of-life and second-life usage must be explored, and recycling options for the different ESSs need to be realistic to build a pragmatic understanding of where we are and what infrastructure needs to be put in place for the future.

This study strongly argues for the development of the DESS for all EVs as the life-extension compared to a Li-ion-only ESS has been proven by multiple studies (Masih-Tehrani et al., 2013; Song et al., 2015a; Song et al., 2018b; Ruan et al., 2019; Yang et al., 2020; Zhu et al., 2020; Silva et al., 2021; Zheng et al., 2021) and directly links to a reduced environmental impact. Even for low-usage cars, this implementation of the DESS would increase the lifetime of both the second-hand electric car market and further second-life applications. We need to design our energy storage with a holistic, multi-use, and sustainable viewpoint. We should no longer design energy storage systems for an EV, but for a lifetime of uses, of which EVs are the first use—and a DESS is the best solution for that. Furthermore, the development of aqueous Al-ion HP components shows potential for a further reduced environmental impact as the technology advances and active material loading increases.