Lithium-based electrolytes are, at least from a thermodynamic standpoint, the most suitable ion-transport materials for energy storage systems. However, lithium-based ionic conductors suffer from safety concerns, and the limited availability of lithium in the Earth’s crust is at the root of the need to consider alternative metal ions. Notably, sodium stands out as the sixth most-prevalent element; therefore, when considering mineral reserves, it as a very attractive candidate as an alternative to the status quo. Even if the specific energy and energy density of sodium are indeed inferior with respect to those of lithium, there is substantial economic appeal in promoting the use of the former metal in stationary energy storage applications. For these reasons, the promise of sodium is likely to extend to other commercial applications, including portable electronics, as well as hybrid and electric vehicles. Widely used organic liquid electrolytes, regardless of their chosen metal cation, are disadvantageous due to leakage, evaporation, and high flammability. Polymer electrolytes are acknowledged as the most effective candidates to overcome these obstacles and facilitate the advancement of next-generation energy storage applications. In this contribution, an in-depth and comprehensive review of sodium polymer electrolytes for primary and secondary batteries is proposed. The overarching goal was to gain insight into successful synthetic strategies and their implications for conduction parameters and conductivity mechanisms. The focus lies on solid, gel, and composite polymer electrolytes. Our hope is that the proposed discussion will be helpful to all operators in the field, whether in tackling fundamental research problems or resolving issues of practical significance.

We propose a physical mechanism of conformation-induced proton pumping in mitochondrial Complex I. The structural conformations of this protein are modeled as the motion of a piston having positive charges on both sides. A negatively charged electron attracts the piston, moving the other end away from the proton site, thereby reducing its energy and allowing a proton to populate the site. When the electron escapes, elastic forces assist the return of the piston, increasing proton site energy and facilitating proton transfer. We derive the Heisenberg equations of motion for electron and proton operators and rewrite them in the form of rate equations coupled to the phenomenological Langevin equation describing piston dynamics. This set of coupled equations is solved numerically. We show that proton pumping can be achieved within this model for a reasonable set of parameters. The dependencies of proton current on geometry, temperature, and other parameters are examined.