**2. The Assessed Documents**

The literature review was realized by searching in Google Scholar for the following keywords: Automotive batteries life cycle assessment, Automotive batteries life cycle, Battery life cycle, Electric vehicle batteries environmental impacts.

From the search results, we selected only the works that presented the following features:


According to these criteria, seventeen documents were suitable for the current literature review (Table 2).


### **Table 2.** Documents analyzed within this bibliographic review.

Most of these documents were published between 2014 and 2019, and only three of them between 2009 and 2013. Thirteen documents are papers published in scientific journals, three documents are research centers' technical reports and one work is a technical guide.

### **3. Goal and Scope**

The goal and scope phase describes and defines the LCA study and the processes involved in the life cycle: targets, functional unit, system boundaries, impact categories, possible allocation procedures and cut-off rules. In the present paragraph, we analyze the main methodological choices made by different authors for this relevant part of an LCA study and we make some recommendations for future LCAs application on traction batteries.

## *3.1. Functional Unit*

ISO14040 [23] and ISO14044 [24] standards define the LCA functional unit as the quantified performance of a product system, to be used as a reference unit. The functional unit has to be consistent with the goal and scope of the study and must provide a reference for normalizing the input and the output data. It is important to remind that thanks to the choice of a proper functional unit, it is possible to compare different systems and products offering similar services. In our analysis, despite the relatively limited number of analyzed papers, we found several different functional units. In three studies, [7,9,16] the functional unit is the battery pack. This kind of functional unit does not seem to be particularly appropriate, since it does not refer to the service offered by the systems (as requested by the ISO 14040 norm) and does not allow us to easily compare the environmental performances of different batteries. Although in reference [15], the suggested functional unit is 1 kWh of delivered energy over the service life of batteries, in five works [10,12,18,21,22], the functional unit is the battery unit storage capacity (e.g., 1 kWh, except for [21] where it is 50 MJ). As the assessed batteries are housed in electric vehicles, the authors in [11] and [4] decided to choose 1 km traveled as the functional unit, whereas in references [14] and [17], the distance traveled by the vehicle during its entire lifetime is considered. The functional unit is not clearly defined only in references [8,13,19] due to the nature and contents of these documents. As discussed, 1 kWh of battery capacity is the most used functional unit as it allows the comparison of different batteries' systems in an easy way [7]. Nevertheless, this functional unit seems more suitable for cradle to grave studies, and other functional units relating to the distance travelled, such as 1 km or the distance travelled in the battery life time, can be used, as far as transport is the service provided by the system. However, assumptions concerning the life of the battery itself must be well clarified. Although sometimes it is useful to express results also per unit of mass (kg of battery) [10], mass-based functional units are not related to the performance of the analyzed systems and we do not recommend to use them in traction batteries LCA studies.
