**Preface to "Composite Electrolyte & Electrode Membranes for Electrochemical Energy Storage & Conversion Devices"**

Electrolyte membranes have found large employment as separators in electrochemical energy systems, such as flow batteries, fuel cells, electrolyzers, etc., and as such have found increasing applications also in commercial batteries and supercapacitors. One of the most promising approaches for improving the safety and reliability of the commercial rechargeable lithium batteries is the adoption of ionically conducting polymer membranes. These membranes of the capacity to reduce/minimize the liquid leakage within a device. A large effort is currently devoted towards optimizing and improving the chemistry and formulation of the electrolyte separator as it plays a key role in the performance and safety of the electrochemical system. Similarly, the formulation of electrode membranes strongly influences power density, cycling behavior, and reliability. Therefore, these issues are currently under worldwide investigation and represent a key point for the development of the final electrochemical system. The goal of this Special Issue is to present a variegated and a detailed overview on the last findings and frontier approaches regarding these challenging topics. Moreover, it represents an optimal site for welcoming the latest innovations obtained by key laboratories covering the following issues: gel and solvent-free electrolyte membranes for lithium batteries, electrolyte membranes for fuel cells, and electrode membranes for lithium batteries.

> **Giovanni Battista Appetecchi** *Editor*
