From the point of view of thermodynamics, polymer-in-salt electrolyte should be treated as a metastable system. In case of PD 153035 matrices representing weak interaction with salt, like in the studied system, the concentration of salt required to form polymer-in-salt electrolyte may exceed solubility limit of salt in polymer. During preparation by solvent-casting method, homogenization of electrolyte is obtained by the use of a large amount of solvent. After removal of solvent, the presence of polymer chains prevents aggregation of salt. However, the polymer matrix can be considered as a dynamic system in which rearrangement of molecules can take place. This can possibly lead to segregation of polymer and salt, and eventually precipitation of crystalline salt. For a rechargeable battery based on polymer-in-salt electrolyte, this process would increase internal resistance of the device, and layers of salt formed during subsequent charge/discharge cycles would eventually block ion transport at the electrodes.