## 1 Introduction
Compared to the configuration without the storage, its presence increases the acidification, the particulate matter, the photochemical oxidant formation, and climate change significantly but decreases the eutrophication, the toxicity, and the mineral resource depletion. Adiabatic CAES moderately increases the environmental impacts across all assessed impact categories, and impacts are sensitive to the required capacity of the thermal energy storage. In comparison with the impacts from the average European electricity mix, both adiabatic and diabatic CAES configurations have lower impacts per kWh of electricity delivered to the grid, except for mineral resource depletion. A study by [PERSON] et al. [17] reports the results of the calculation of the net energy ratios and life cycle GHG emissions of adiabatic and conventional CAES and PHES. The greenhouse gas emissions associated with construction, operation, and decommissioning life-cycle stages of the energy storage systems are evaluated. The emissions are highly dominated by the operational stage in all the energy storage systems, mainly due to electricity consumption, while the contributions due to the use of material are minimal. Starting from the above-mentioned published works, some main considerations can be drawn for the environmental assessment of energy storage:  
* Greater achievable exploitation of renewable energy does not always guarantee a lower environmental impact. * Not only the effects on climate change must be evaluated, but a more complete analysis needs to be performed. * The whole life must be considered, including raw material extraction, materials and products processing, assembly, transport, operation, and decommissioning stages. * Mainly for the operation stage, the results are greatly affected by the energy sources and the conversion technologies of the system in which the energy storage is integrated. For this reason, a proper analysis must consider the entire system, not only the storage devices. In this regard, an appropriate definition of the functional unit, i.e., the reference to which the results of the analysis refer, is very important. In this study, the use of electrochemical storage in hybrid energy systems is assessed by means of a life-cycle assessment analysis. Batteries are the most widespread technology in terms of the number of storage devices and do not suffer from geographical limitations, unlike CAES and PHES. They can therefore be adopted in many different places and applications. Lastly, among the other technologies, batteries are the center of public attention regarding the impact of their disposal. An LCA approach [18; 19] from cradle-to-grave is selected, since it represents a standardized methodology, well-known and internationally recognized. It allows the assessment and quantification of environmental impacts and damage related to the development of a product, a procedure, or goods or services in a specific context defined in the analysis. The analysis is integral because all the processes and procedures necessary for the implementation, development, use, and end-of-life phase of the activity are examined. Several examples of application of LCAs to energy systems can be found in the literature [20; 21; 22; 23]. The batteries are supposed to operate on two different small islands not interconnected to the national grid, where, at the time of writing, the energy production system consists of diesel engines and small renewable plants. Despite the small size of RES plants, their production can not be entirely used due to the lack of storage devices. For each case, the analysis regards the overall energy system and compares its current configuration with the one that comprises the storage unit made by the addition of batteries. The possibility to enlarge the size of RES plants with an additional increase of the number of batteries is also studied. An original focus of this study is on the influence on the results of the definition of the functional unit used in the analysis. The rest of the work is organized as follows.