An enhancement of two orders of magnitude in the ambient temperature ionic conductivity of poly(ethylene oxide)-based nanocomposite polymer electrolyte (NCPE) membranes is here fundamentally achieved by the incorporation of specific amounts of aluminium-based metal organic framework (MOF) as the filler. Thorough characterization, particularly solid-state NMR and FT-IR studies, shed light on the specific role of the defective MOF frameworks in greatly enhancing the Li⁺ ion mobility inside the polymeric matrix. The prepared NCPEs are highly stable towards lithium metal even after prolonged storage time, and an excellent cycling profile is evidenced even at moderate temperatures, which has never been reported so far for an all-solid-state lithium polymer cell composed of Li/NCPE/LiFePO₄.

The shuttling of polysulfides between the electrodes in a lithium–sulfur battery (Li–S) system remains a challenge to be addressed in order to realize the full potential of this promising technology. In order to overcome this issue several strategies have been adopted. In the present work, UiO-66-NH₂@SiO₂ was successfully synthesized and coated on a commercial Celgard 2320 membrane. The coating of UiO-66-NH₂@SiO₂ on a Celgard 2320 membrane has not only enhanced the thermal stability and wettability but also other electrochemical properties such as ionic conductivity, compatibility and charge–discharge behavior. The Li–S cell with the UiO-66-NH₂@SiO₂-coated membrane delivered higher discharge capacity than the Li–S cell with SiO₂-coated and uncoated membranes. The enhanced discharge capacity was attributed to the electrostatic and/or H-bonding interactions between the polysulfide and UiO-66-NH₂@SiO₂ as evidenced by its positive zeta potential (+56.42 mV). More importantly, the permselective properties of the membrane significantly play against the self-discharge of Li–S cells in which 98.5% of its capacity was retained even after 40 h which is superior to earlier reports.

Organic redox compounds are a fascinating class of active materials used in energy storage applications. The structural diversity as well as ability to be molecularly tailored assists in fine-tuning of their electrochemical properties at the molecular level, which is highly desired for performance improvement. In reality, it is very challenging to achieve the desired physicochemical properties. Viologens represent a unique class of redox-active molecules that undergo two one-electron reductions in steps. Their redox properties have been highly explored in various fields such as electrocatalysis, electrochromism, photochromism, herbicides, and to some extent in energy storage systems. Their redox behavior as well as cationic charges are highly useful for improving device performance in energy storage systems. Different energy systems require different states of viologens, i.e., dicationic, monocation radical, or redox couple. For example, viologen dication shows electrostatic attraction towards polysulfides in Li–S batteries, and similarly, dication radical prevents Li dendrite growth in Li-ion batteries. In the case of redox flow batteries, such as supercapacitors and Li–air batteries, the redox behavior of viologens is useful for improving the performance. Despite these advantages, there are some challenges existing in viologen-based systems that must be conquered at the molecular level. In this regard, a systematic study on structure–activity relationships becomes crucial to address the bottlenecks. This review comprehensively analyses the role of viologens in various energy storage systems and the mode of chemical interactions responsible for enhancing the overall performance of electrochemical storage devices.

Poly(ethylene oxide) (PEO)-based composite polymer electrolytes (CPEs) containing the amine-functionalized, zirconium-based metal–organic framework @silica (UiO-66-NH₂@SiO₂) and lithium, LiN(CF₃SO₂)₂ salt (LiTFSI) are prepared using a simple hot press method. The electrochemical properties such as compatibility of the electrolyte with the Li metal anode, Li transference number, and ionic conductivity are investigated for the different systems containing different relative concentrations of the additives. The incorporation of UiO-66-NH₂@SiO₂ in the PEO-LiTFSI matrix not only enhanced ionic conductivity by one order of magnitude but also offered better compatibility and suppressed the formation of lithium dendrites appreciably. X-ray photoelectron spectroscopy studies on post-cycled materials revealed the formation of lithium alkoxide (RO–Li) on the cathode and Li₂O on the anode. The coin cell (2032-type) consisting of LiFePO₄/CPE/Li with UiO-66-NH₂@SiO₂ as filler provided a discharge capacity of 151 mA h g^–1 at 0.1 C-rate at 60 °C, measurably higher than control experiments utilizing SiO₂ and UiO-66-NH₂. The notable enhancement of electrochemical properties when incorporating the UiO-66-NH₂@SiO₂ at the CPE was attributed to formation of more uniform ion conduction pockets and channels within the PEO matrix, facilitated by the presence of the microporous UiO-66-NH₂@SiO₂. The enhanced distribution of microporous channels, where Li ions are assumed to percolate through within the matrix, is assumed to desirably reduce formation of Li dendrites by increasing diffusion channels and therefore reducing crystallization and growth of dendrites at the electrode surface.