Rolf Hempelmann

Phosphonium ionic liquids are emerging electrolyte platforms for safer and more durable electrochemical energy‐storage systems because they combine low volatility, reduced flammability, thermal robustness, broad electrochemical stability windows, and highly tunable cation–anion chemistry. Their performance emerges from coupled cation–anion structure, salt concentration, electrolyte formulation, solvation environment, and electrode chemistry. This Review develops a structure–property–interphase framework for phosphonium‐based electrolytes in lithium, sodium, zinc, magnesium, and aluminum batteries, as well as supercapacitors and quasi‐solid‐state systems. For phosphonium‐based electrolytes, we analyze how alkyl chain length, branching, ether functionalization, and anion identity govern viscosity, conductivity, ion coordination, metal plating/stripping, corrosion, gas evolution, and solid–electrolyte and cathode–electrolyte interphase formation. The analysis shows that smaller, asymmetric, branched, or ether‐functionalized cations generally promote faster ion transport, whereas long‐chain hydrophobic cations are better suited for thermally robust, low‐volatility, gel, or deep‐eutectic‐type formulations. Finally, we propose practical design rules emphasizing low‐viscosity cation design, fluorine‐reduced anion chemistry, standardized benchmarking, full‐cell validation, and surface‐sensitive/operando interphase characterization.

Lithium-ion batteries (LiBs) represent a state-of-the-art electrochemical energy storage technology, enabling applications from portable electronics and electric transportation to large-scale grid energy storage systems. Despite their widespread adoption, LiBs face challenges like performance decrease, reduced lifespan, and safety risks, all closely tied to battery degradation. This review systematically examines the factors influencing LiB degradation, dividing them into intrinsic and extrinsic categories. Intrinsic factors include material chemistry, manufacturing inconsistencies, and design limitations, while extrinsic factors cover operational conditions such as temperature, charge–discharge rates, state of charge, and depth of discharge. The degradation factors influence the degradation mechanisms, and these correlations are discussed in detail. Additionally, this study highlights mitigation strategies and tradeoffs, including advanced electrode designs, surface modifications, and optimized battery management systems. By consolidating current research, this review provides insights into overcoming the challenges associated with LiB degradation, aiming to guide future developments in achieving safer, more durable, and efficient energy storage systems.

Phosphorous is an essential element for the life of organisms, and phosphorus-based compounds have many uses in industry, such as flame retardancy reagents, ingredients in fertilizers, pyrotechnics, etc. Ionic liquids are salts with melting points lower than the boiling point of water. The term “polymerized ionic liquids” (PILs) refers to a class of polyelectrolytes that contain an ionic liquid (IL) species in each monomer repeating unit and are connected by a polymeric backbone to form macromolecular structures. PILs provide a new class of polymeric materials by combining some of the distinctive qualities of ILs in the polymer chain. Ionic liquids have been identified as attractive prospects for a variety of applications due to the high stability (thermal, chemical, and electrochemical) and high mobility of their ions, but their practical applicability is constrained because they lack the benefits of both liquids and solids, suffering from both leakage issues and excessive viscosity. PILs are garnering for developing non-volatile and non-flammable solid electrolytes. In this paper, we provide a brief review of phosphonium-based PILs, including their synthesis route, properties, advantages and drawbacks, and the comparison between nitrogen-based and phosphonium-based PILs. As phosphonium PILs can be used as polymer electrolytes in lithium-ion battery (LIB) applications, the conductivity and the thermo-mechanical properties are the most important features for this polymer electrolyte system. The chemical structure of phosphonium-based PILs that was reported in previous literature has been reviewed and summarized in this article. Generally, the phosphonium PILs that have more flexible backbones exhibit better conductivity values compared to the PILs that consist of a rigid backbone. At the end of this section, future directions for research regarding PILs are discussed, including the use of recyclable phosphorus from waste.

Dynamics and structures of protic ionic liquids from the superbase guanidinium are analyzed in detail. The H-bond strength correlates with the proton affinity of the constituting acid and can be monitored by the¹H or¹⁵N chemical shifts.

Phosphorus-containing catalyst supports synthesized by coating and pyrolysis of phosphonium ionic liquids lead to improved fuel cell properties.

In this study, we address the fundamental question of the physicochemical and electrochemical properties of phosphonium-based ionic liquids containing the counter-ions bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) and bis(fluorosulfonyl)imide ([FSI]⁻). To clarify these structure–property as well as structure–activity relationships, trimethyl-based alkyl- and ether-containing phosphonium ILs were systematically synthesized, and their properties, namely density, flow characteristics, alkali metal compatibility, oxidative stability, aluminum corrosivity as well as their use in Li-ion cells were examined comprehensively. The variable moiety on the phosphonium cation exhibited a chain length of four and five, respectively. The properties were discussed as a function of the side chain, counter-ion and salt addition ([Li][TFSI] or [Li][FSI]). High stability coupled with good flow characteristics were found for the phosphonium IL [P1114][TFSI] and the mixture [P1114][TFSI] + [Li][TFSI], respectively.