## 1 Introduction
In addition, the sorbate mass transfer into the system will be hindered, causing issues such as high-pressure drops and ultimately system failure, but also corrosion issues arise due to the dripping of the salt solution to other metal components of the systems. Accordingly, several efforts were carried out to implement LiCl-based TES technologies and, more specifically, from the material perspective. The general idea is to embed the salt into porous matrices (such as carbon foams, expanded natural graphite, zeolite, vermiculite, silica gel, etc.) to prevent deliquescence and improve water transport into the materials [8; 16; 17]. [PERSON] et al. developed a LiCl-based composite on mesoporous alumina [18]. The material showed a TES density of up to 1040 J/g. However, due to the low pore volume of the alumina, only -15 wt.% of the salt could be loaded in the matrix, thus limiting the possibility of further increasing TES density. Multi-walled carbon nanotubes (MWCNTs), due to their extremely large and variable pore volume, were used as a matrix to host LiCl salt [19]. This composite, with a 44 wt.% content of LiCl salt, achieved a heat storage capacity of 1.6-1.7 kJ/g. More recently, a LiCl/ vermiculite, with \(\sim\)59 wt.% of LiCl, was developed by [PERSON] and coworkers [20]. The heat storage capacity of this composite is 1.8-2.6 kJ/g (224-253 Wh/m\({}^{3}\)). It has to be pointed out that, intrinsically, the porous structure of natural mineral vermiculite is non-reproducible and so is the composite. Additionally, vermiculite suffers from hydrothermal structural stability issues that are critical for the technology's long-term durability. [PERSON] et al. developed silica gel-LiCl composites by varyingthe amount of embedded salt between 10 and 40 wt.% [21]. An optimum compromise between the heat storage capacity and material stability was identified in the composition with 30 wt.% of embedded salt. For this composition, a heat storage capacity of 480 J/g was determined. The presence of salt deposited onto the external surface of the composite has clearly a detrimental effect. This issue was addressed specifically on mesoporous silica gel-LiCl composites with a post-treatment based on an adsorption phase followed by a slow desorption phase, which allowed the salt to move from the surface into the pores, likely due to a capillary effect [22]. The investigated matrices, being rigid structures, could limit the mechanical stability of the material over cycling due to the consecutive expansion and contraction of the salt grains during the hydration and dehydration steps. In this regard, the use of flexible polymeric macro-porous foam as matrices for salts was proposed in literature for other types of salts, e.g., MgSO\({}_{4}\cdot\)7H\({}_{2}\)O [23; 24]. The flexibility of these foams can accommodate the expansion and shrinkage of the salt hydrate volume during the hydration and dehydration reactions, thus enhancing the mechanical stability of the composite over the cycles. In addition, the foam should embed the salt, thus retaining the deliquescence that inevitably occurs due to the operating conditions. In situ studies on TES materials can certainly provide an in-depth understanding of the reaction mechanism and materials changes during the charging/discharging operations, thus leading to valuable information for the advancement of TES materials. Here, we investigate the structural and morphological modification during hydration/dehydration cycles on a LiCl-silicon foam (LiCl-PDMS) with a salt content of 40 wt.%. Specifically, a characterization protocol coupling temperature scanned X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analysis was proposed. The operando conditions of the dehydration/hydration cycle were reproduced while structural and morphological characterizations were performed, allowing for the evaluation of the interaction between the salt and the water vapor environment in the confined silicon matrix.