*3.3. Allocation System*

The allocation process splits the input and output flows of a multiple product process between the analyzed system product and one or more other system products. The inputs and outputs have to be allocated to the different products following clear rules, defined at the beginning of the analysis. Most of the assessed documents do not specify any allocation rules, whereas only two studies mention the allocation: [11] and [15]. The first document [11] declares that there is no allocation regarding recycled products in the end of life. As a consequence, all the charges that may arise from material production are assigned to the first life of the product, even if the same can be reused, for example in a stationary domestic storage system. The second document [15] states that no accurate indication

for a possible allocation is provided, as there are no co-product cases identified during batteries' production phase. However, if it is necessary to allocate impacts of any co-products linked to the battery manufacturing process, to solve multi-functionality problems, authors recommend to apply a predefined hierarchic approach: division or system expansion; allocation based on a relevant physical relationship; allocation based on some other relationships. In batteries' LCAs, allocation seems relevant only when considering recycling and second life scenarios.

## *3.4. Cut-O*ff *Rules*

Cut-off rules define material or energy flows, associated with the process unit, which are excluded from the study. Eight studies use a cut-off system, whereas nine studies do not use it or it is not possible to deduce the following criteria [4,7,8,13,14,16,18,19,21]. In five studies [9,10,12,20,22], authors do not include in their analyses impacts and benefits linked to material recycled during end of life phase. This choice is due to a high uncertainty of data and information about recycling, reuse and substitution of primary raw material with secondary material. In reference [11], materials and processes are excluded when their potential contribution is negligible. This choice is based on a mass, energy demand and expected impacts per unit of mass or energy. Of course, given the growing importance of recycling materials in the framework of the circular economy and the use of critical materials in battery production, we encourage the inclusion of end of life and recycling in future LCA studies. As in reference [15], which uses a 1% cut-off rule, neglecting all phases which have impacts lower than this threshold, we suggest that negligible phases are: batteries distribution during the end of life, infrastructure and equipment for batteries assembly and recycling.

### *3.5. Impact Categories and Methods*

As described in the European Commission ILCD Handbook recommendations [25], impact categories selection must be consistent with the goal of the study. Furthermore, impact categories choice has to be complete, and should cover all the main environmental issues related to the system. In order to compare the results from different studies, it is certainly needed that studies use the same metrics: i.e., the same impact categories and the same impact method for their quantification. In our analysis, only eight studies [7,9,10,17,18,20–22] clearly explain the impact method used for quantifying midpoint impact categories [25], while six studies [4,11,12,14–16] declare the impact categories used but they do not specify the impact method followed. Finally, three studies [8,13,19] do not report any results evaluable with usual LCA impact categories or methods proposed by the ILCD Handbook [25]. In Figure 1, the impact categories used in the assessed studies are resumed.

The most used impact category is global warming (fourteen studies out of seventeen), followed by acidification (ten out of seventeen) and eutrophication (nine out of seventeen). Seven works use the impact categories ozone depletion and particulate matter, whereas six documents use the impact categories CED—cumulated energy demand, abiotic depletion, human toxicity and ecotoxicity. Other categories considered less frequently are: photo oxidant formation and resource depletion (five studies), fossils depletion (four studies), ionizing radiation (three studies), land use and water use (two studies). On the basis of these results, considering the impact categories used in almost 40% of the assessed studies, and taking into account the lesson learnt from reference [20], in an automotive battery LCA, it is suggested to use the following impact categories: global warming, acidification, eutrophication, ozone depletion, particulate matter, abiotic depletion, human toxicity, ecotoxicity and CED (Cumulated Energy Demand).

**Figure 1.** Impact categories used in the assessed studies.
