1. Introduction
In a society where the energy demand is constantly growing hand in hand with technological development and, at the same time, awareness of environmental issues is rising, studies of life cycle assessment (LCA) have become fundamental. Before 2015, when the EU Circular Economy Action Plan had not yet come into play [
1], the LCA was already a discussion topic but mainly in research environments. The outdated and most common way of approaching goods production was mainly focused on performances and costs in “linear life thinking”, which caused dramatic results in terms of raw material usage. The United Nations Environment Programme (UNEP) stated that, in the last 40 years, raw material consumption tripled, going from 27 billion tons in 1970 to 98 billion tons in 2018 [
2]. In the last decade, the LCA has generated more and more interest not only in the academic world but also among industry stakeholders who have begun to change their mindset, switching to the new “circular life thinking” or life cycle thinking (LCT) [
3]. Both private realities (industries, companies) and public ones (governments) have started to embrace green economy principles, establishing and regulating business models so that both materials and products can keep their highest value throughout their whole life cycles [
4,
5]. This new way of thinking about the product impact does not only consider its disposal stages [
6] but also all the production phases from the extraction of raw materials to the manufacturing processes, including the use of energy and water [
7,
8], wastes and transportation emissions [
9]. Finally, the disposal of the product is investigated, taking into account the possibility of recycling all or part of it [
10,
11]. This last consideration is the key-point that allows distinguishing two different approaches to the product impact: the cradle-to-grave (CTG) and the cradle-to-cradle (CTC).
The CTG approach dates from the 1990s [
6] and takes into account the whole life cycle of a product up to the disposal stage. It has increased producers’ awareness of the real consumption related to their production, teaching the LCT and motivating them not to waste resources. Regarding the CTC approach, it does not consider the analysis impact of a real final disposal phase as what is considered waste in CTG studies is “food” for CTC ones. It was in 2002 that the definition of CTC was given by the chemist Michael Braungart and the architect William McDonough: drawing a parallel with nature where “the waste of one system becomes food for another” [
12], they suggested a product impact approach where the products which arrived at the end of their lives would have become a sort of raw material to produce something of equal or even better quality. This innovative LCT implied recycling processes which, in recent decades, have been significantly developed and improved in every field. The International Organization for Standardization (ISO) includes widely recognized procedures to conduct valid and extensive LCA studies [
13]. In particular, ISO 14040 and ISO 14044 represent the basis of the International Reference Life Cycle Data System (ILCD), a technical guideline to carry on accurate LCAs considering product-specific criteria [
3].
To complete a detailed and useful LCA, ISO 14040 and ISO 14044 suggest to proceed by following four different phases which are often dependent on each other. Nevertheless, none of these stages should be considered as concluded until the entire study is complete. The aforementioned stages are the following: goal and scope; life cycle inventory (LCI); life cycle impact assessment (LCIA); and interpretation. During the goal and scope stage, the intended application and audience, the reason for the study and whether the results will be used in a comparative dissertation intended for public release must be defined. This is a fundamental preliminary phase which saves time during the following ones by focusing the attention on the real core of the investigation. The LCI is the data collecting phase resulting from an input and output analysis and quantification: this is the moment when a real inventory is compiled [
14] considering raw materials, energy and water requirements, atmospheric emissions, resource usage and all the other flows involved in a product or process life cycle. Flow charts, such as the Sankey diagram, are very helpful during this phase [
15]. Knowing the methods to extract or machine the materials, it is possible to work on the impact assessment. First, it is necessary to classify the kind of impact associated with the considered material (climate change, ozone depletion, or toxicity related to anthropogenic activity). Then, all the inventory items (flows) have to be characterized: this means that every flow related to a certain impact has to be classified under a common unit of comparison [
13]. For instance, considering the global warming potential (GWP) as the impact, the unit is the equivalent
weight which has to be assigned to all the items in the inventory. This procedure is usually performed with the help of databases. Finally, there is the interpretation of the collected data: the LCI and the LCIA are checked, evaluated and all the issues coming from the investigation of these two previous phases are identified and faced by choosing among a broad repertoire of methodological alternatives [
16]. A series of limitations and recommendations represents the final result of the interpretation phase.
Nowadays, LCA studies focus on extremely varied fields of services and products, such as packaging [
17], tourism [
18], textile industry [
19,
20], food processes [
21,
22,
23,
24], drink consumption [
25,
26], electronics [
27], and materials [
28]. Transportation is another huge sector in which LCA analysis is applied [
29], and particularly when considering the issue of sustainable urban and commercial development, many studies have investigated the benefits brought by the electrification of the public means compared to the impacts of more ordinary thermal-engine-powered vehicles [
30,
31,
32]; on the other hand, other studies have analyzed the positive environmental and economic impacts resulting from a ships electrification campaign, considering factors, such as the kind of fuel/propulsion, the chemistry of the BPs, and the average cruise speed [
33,
34,
35,
36]. A great deal of effort is also focused on the automotive field, studying the effects of car lightening [
37], the benefits of using recycled natural materials [
38], and, when it talks about green mobility [
39,
40,
41], the attention is often focused on the Li-ion BP. Several researches deal with the environmental impact of a whole electric vehicle (EV) BP [
42], other studies focus on the constituent materials [
43] or components [
44,
45,
46].
In this paper, LCA literature results are used to carry on a cradle-to-gate analysis regarding the
LiBER [
47] BP based on the modular integration of lithium-ion cylindrical cells. This prototype is based on an international patent [
48], whose main innovations are the modularity of the system, the thermal control of each cell in order to prevent thermal runaway events and some novel processes for the assembling of the cells and the bricks which avoid the use of wires. A custom-made battery pack based on the concept of the LiBER group has been installed on
Emilia 4 [
49], the solar car of the University of Bologna built by the sport association
Onda Solare.
Emilia 4 is a 4-seats competitive vehicle, fully made of carbon fibers, whose main characteristics are: two rear in-wheel electric motors, 5 m
2 of monocrystalline silicon solar panels and a 32 kWh Li-ion battery pack. Several studies have already been devoted to design, environmental, mechanical, and electrical aspects related to
Onda Solare solar cars [
50,
51,
52,
53,
54,
55,
56,
57,
58]. Taking into account
Emilia 4, the aim of this work is to deepen the knowledge regarding the environmental impact of its BP. This study deals with the raw materials extraction, their processing, the assembly of the components, and the products transportation. All these stages are evaluated in terms of energy demand (ED) and GWP impact. A final interpretation of the results proposes several energy saving solutions to lower the
ED and
GWP values.
2. Materials and Methods
Figure 1 shows the Li-ion battery (LIB) considered for this analysis, which is made of 2 modules, both of them composed of a stack of 15 bricks which, in turn, contains 48 cells each.
Figure 2 shows a detailed view of the LiBER brick: the 48 cells are inserted into the slots of the support structure, which is made of acrylonitrile butadiene styrene (ABS).
In the brick, as shown in
Figure 2, the cells are separated from the ABS by means of thin aluminum liners, locked in the right position by two bus-bar plates and electrically connected to the positive and negative bus-bars by means of laser welded tabs. Additional holes in the support structure host the cooling piping. A printed circuit board called BMS-brick is installed on the side of each brick and it is used for the acquisition of the cells voltage and temperature. The bus-bars host the power connections which allow the direct connection in series of the bricks forming a stack of bricks. At the two ends, the stack is completed with electrical terminations, BMS control boards, and ancillaries. The module is formed by inserting the complete stack into a container made of composite material. The container is closed at the two sides with two metal plates to guarantee mechanical protection and to seal the modules. Finally, the two modules are connected either in series or in parallel by using external power wiring to form the BP. The
LiBER battery design for simple disassembling and reassembly operations, combined with a distributed State of Health (SOH) estimator, allows an easy reuse of the bricks in second life applications. This analysis regarding the LCA over a longer life time that includes disassembly, reassembly, and reuse will be addressed in a next work.
The analysis is divided into five main research macro-topics usually taken into consideration in this genre of studies [
59]: the cells support, the Li-ion cells, the BMS system, the liquid-based BTMS and the container. The other parts of the BP, such as the electrical components (fuses, switches, connectors) are not considered in this LCA analysis. The LCA is conducted as a cradle-to-gate analysis (a partial cradle-to-grave without the study of the product disposal) and, concerning the kind of resultant impact, the
ED and the
GWP values are considered.
2.1. Cells Support
In
Figure 2, a section of the
LiBER cells support is shown: it is a plastic structure with 48 holes for the Li-ion cells arranged in a 6 × 8 configuration and one aluminum liner in each hole to better dissipate the heat generated by the cells. About the aluminum, it is assumed to be all primary aluminum as the majority of the literature data about the impact evaluations (ED and GWP) deal with that kind of metal. The LCA about the plastic part of the support involves the analysis of two different materials: the polyoxymethylene (POM) usually used in the manual production line and the ABS common in the automatic production line.
2.2. Li-Ion Cells
The cells chosen for this LCA analysis are the cylindrical
INR 21700 P42A produced by
Molicel [
60] and based on
cathode. This kind of cells is a quite common choice in the BP mass production when frequent charging/discharging cycles are required (energy storage, automotive): the reason is that the low cobalt content brings to lower costs [
61]. The
LiBER BP concept and assembly processes do not depend on the cells used. Then, the study presented in this paper is designed so that the section concerning the cradle-to-gate analysis of the Li-ion cells can be replaced with a new one focused on a different type of cells instead of the ones analyzed in this work, with the other sections remaining the same. It has to be specified that the following analysis does not take into account the environmental impacts coming from the use of the Li-ion cells (charging/discharging phases), as well as the battery post-processing. In addition to the anode and the cathode components, the study of a cell deals with many other materials: the constituents of the binder, the separator ones, the electrolyte salt and its additives, and, enclosing the whole assembly, the nickel-plated steel cell casing. Before starting with the impact evaluation some assumptions are completed: the binder is considered as made of polyvinylidene fluoride (PVDF) and represents a negligible mass percentage (<1%) [
60] in the selected NMC cells [
59]; among the data found in the literature, a separator composition equal to 50% of polypropylene (PP) and 50% of polyethylene (PE) is selected; the electrolyte solvent is considered as 50% dimethyl carbonate (DMC) made and 50% ethylene carbonate (EC) made as the other carbonates which usually compose the NMC cell solvent are in much lower and negligible percentages. Furthermore, the cell casing is not taken into account as in the overall
emissions balance its impact is lower than 1% [
59]. The resulting inventory includes: the anode (graphite coating on a copper foil), the cathode (cobalt(III) lithium oxide (
),
,
coating on an
foil), the electrolyte salt
(lithium hexafluorophosphate), and the electrolyte solvent (50% DMC, 50% EC) [
60]. Studies demonstrate that the ED and
emissions related to the cell assembly are much lower than the those coming from the cell processing (in particular, the processes involving the anode and the cathode) [
45]. Other studies highlight how reasonable this thesis is compared to the extremely low emissions for the entire BP assembly and those associated with the overall LCA (
GWPASSEM ≃ 0.7 ·
GWPTOT) [
62]. Finally, in this study the manufacturing phase is evaluated only concerning the Li-ion cells, instead this contribution is considered irrelevant regarding the cells support and the liquid-cooled BTMS.
2.3. BMS
Each brick is realized with a 1.8 mm thick FR4 layer and a 35 μm copper thick sheet.
2.4. Liquid-Cooled BTMS
The BTMS considered in this study is based on the real liquid cooling system installed on the
LiBER BP prototype: it is a water-based BTMS with a coolant circuit made of a silicone rubber tube passing through the holes of the brick. As seen in the brick scheme (
Figure 2), among the bigger holes for the cells there are smaller ones for the cooling circuit which passes through 12 of them while the coolant flows along it. The liquid duct holes have a diameter
dhole = 10 mm, so the chosen silicone rubber tube has an external and an internal diameter equal to
dtube,ext = 10 mm and
dtube,int = 8 mm, respectively, resulting in a thickness
stube = 1 mm. The LCA regarding the BTMS is here facilitated just considering the silicon rubber tube in terms of raw materials extraction, processing and transportation.
2.5. Container
The battery pack container is made of pultruded composite materials and it has also a structural function. The container cap is not considered as it gives a negligible contribution with modules composed of a high number of bricks.
2.6. LCA Approach
The system boundary of the study comes from a cradle-to-gate approach: it starts with the extraction of the raw materials and ends with the assembly of the BP module.
Figure 3 depicts the system boundary and shows the details of the sections present in this work.
4. Discussion
The results presented in the previous sections can be referred to the whole BP, visualizing the contributions of the five sub-systems, and then discussing the possible strategies to reduce the overall environmental impact. The results related to the entire BP are collected in
Table 12 and shown in
Figure 8 per unit of energy.
The contribution of the cells is the most relevant due to the results obtained for the extraction of the raw materials (
Table 5). For instance, cobalt extraction has a deep impact in terms of both
and
, followed by the nickel which has a high
value, too. These are two fundamental elements to produce the cathode powder for the NMC111 cells, so it is not possible to lower this contribution if the use of this kind of cells is mandatory for the application. However, when it is possible to use another type of NMC cells, the more innovative NMC622 or NMC811 batteries represent a better choice. The NMC622 cathode is composed of 60% of
, 20% of
, and 20% of
; the NMC811 cathode is composed of 80% of
, 10% of
, and 10% of
. Both of them have a lower weight (lower transport impact) and a higher energy density with respect to the NMC111. The extremely reduced quantity of
in NMC811 cells, their lowest weight and highest energy density may suggest that this is the best choice. However, on the other hand, the high
percentage is responsible for a more sensitive chemistry and, consequently, the need of additional post-processing phases and a more expensive manufacturing process [
94]. Plus, NMC811 cells are sensitive to moisture and air so, in an automotive application, a further sealed structure would be necessary to insulate them from the outside. All these additional processes and components would increase the
and
values, nullifying the benefits brought by the lower
percentage. From this point of view, the NMC622 represent a good compromise and this is the reason why the
Molicel cells have been chosen for the
LiBER BP.
About the processing of the cells materials (
Table 5), results show high values of
and
for several processed materials, but it is possible to lower only some of them. One of the initial assumptions is considering the separator as 50% PP-made and 50% PE-made, but looking at the related
and
, the results show that the use of a 100% PP-made separator brings to a decrease in both energy demand (
) and
emissions (
). A similar evaluation can be completed regarding the electrolyte solvent which is considered 50% DMC-made and 50% EC-made. The use of a 100% EC-made electrolyte solvent brings to a great decreasing of both energy demand (
) and
emissions (
). The processes regarding the anode and the cathode powders and collectors, as well as the electrolyte salt production are very energy-intensive as they include several thermal, chemical, and electrochemical phases. As these treatments are fundamental for the quality of the final components, the only possible choice to lower the energy demand is the exploitation of the most efficient available industrial plants. Although during the assembly phase there are stages with a higher impact than others, the final
and
values do not deeply influence the overall results of the LCA study. For this reason, and also because the assembly impact is extremely dependant from several hardly predictable factors, a more effective strategy to lower the whole cycle
and
is focusing on the other more manageable production steps. Considering the assumption of the cells assembled in Chile, the transportation impact includes both truck and cargo stages. As seen before, the sea transport is a better choice than the road one: although the total distance travelled by the truck (3694 km) represents the 50.68% of the distance travelled by the cargo (7289 km), the sea transport results in great
and
decreases equal to
and
. So, the more it is possible to exploit the sea transport, the better is in terms of impact.
Figure 7 shows that the impact of the cells support is about the 20% of the total ED.
Looking at the results obtained for the extraction and production of the aluminum needed to realize the cells support (
Table 1) it is clear that innovative technologies to process the aluminum could represent an efficient solution to lower the
value instead of proceeding with the ordinary and energy-intensive methods (Bayer and Hall–Héroult processes,
Section 3). Cutting-edge technologies would be also promising to reduce the
emissions. Regarding the plastic material for the cells support, the choice is driven by both the
and the
value. As a matter of fact, comparing
for POM and ABS in
Table 1, results show the values 18.17 kg
CO2/brick for POM and 4.25 kg
CO2/brick for ABS, with a percentage decreasing
choosing ABS. Considering
in
Table 1, the obtained values 415.52 MJ/brick for POM and 99.08 MJ/brick result in a percentage decreasing
with ABS. The ABS is definitely the best choice in terms of both
and
. Analyzing the transportation impact for the materials of the cells support, regarding the aluminum it clearly appears that choosing Al6063 instead of Al6065 represents the more convenient option considering both the energy demand and the emissions. Indeed, the overall Al6063
results to be 0.2 MJ/kg
Al6063 instead of 2.58 MJ/kg
Al6065 for Al6065, with a percentage decrease
and the overall Al60603
results to be 0.01 kg
CO2/kg
Al6063 instead of 0.36 kg
CO2/kg
Al6065 for Al6065, with a percentage decrease
. However, it has to be highlighted how convenient is the cargo transport with respect to the truck one. Focusing on the Al6065 impact values, the results show that the
for the cargo stage is higher by 50% than that related to the truck stage but the distance travelled by the truck is just the 8.01% of the cargo distance. Further, the
for the cargo stage is lower by 50% than that related to the truck stage. Hence, if it is possible to choose between a road or a sea transportation, it should be better to go for the second option. Finally, also the module composite container shows a relevant impact (see
Figure 7). To lower the environmental impact coming from the composite goods production, EoL recycling stages are fundamental and increasingly adopted as effective strategy [
106]. However, there is still a lot to do in the energy saving direction for composite materials. In particular, considering lower energy-intensive composites processes and the production of more sustainable matrices, such as bio-based resins, further investigation needs to be performed to optimize this production field, trying to overcome the existent drawbacks [
107].