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We report the development of a dual-phase membrane for CO2 separation based on a molten hydroxide liquid phase and a nanoporous yttria-stabilized zirconia solid support phase, termed hydroxide/ceramic dual-phase (HCDP) membranes, that can operate in the flue gas temperature range of 250–650 °C. HCDP membranes demonstrate selectivity for CO2 over N2 greater than 1000 and CO2 permeability of 1.78 ± 0.16 × 10–10 mol m m–2 s–1 Pa–1 (5.32 × 105 barrer) at 550 °C with 20 vol% CO2, which is more than an order of magnitude greater than the best values for dual-phase and polymer membranes reported in the literature. We also demonstrate using three different experimental methods that CO2 absorption by molten hydroxides is reversible in the presence of water vapor at temperatures as low as 300 °C.

Microalga is a promising feedstock for biofuel, chemical, food, and animal feed; however, harvesting is a critical barrier for its commercial application. This communication demonstrates a new harvesting technology by utilizing the phase separation of thermoresponsive polymers and charged copolymers of N-isopropylacrylamide and allylamine. Chlorella protothecoides cells are separated from solution when the mixture of algae and polymers is heated above the lower critical solution temperature of polymers (~ 32 °C), where the polymer phase separates from the aqueous media and aggregates into a solid–gel phase. It was found that copolymer concentration, allylamine content (mol%) and charge (based on initial solution pH) affect the extent of polymer phase separation and alga separation efficiency. The copolymer containing allylamine with lower than 2.6 mol% displayed nearly complete algal cell separation at polymer concentrations of 25–50 mg/mL and pH 7. The results indicated that thermoresponsive polymers provide a promising technology for alga harvesting using recyclable and reusable materials.

Macromolecular ionomer solutions exhibiting macroscopic properties that change in response to temperature are referred to as thermally responsive polymer electrolytes (RPEs). Such materials provide a means to control electrochemical systems using an external stimulus that affects the polymer phase behavior and electrolyte properties. RPEs were synthesized with N-isopropylacrylamide, which governs the thermal properties, and varying fractions of acrylic acid, which provides ionic properties. These polymers undergo a thermally activated phase separation in aqueous solutions at a given temperature, thereby altering the ionic strength, pH, and conductivity of the electrolyte solution. In this article, we demonstrate how the molecular properties of RPEs, specifically the ionic composition, influence the temperature-dependent electrolyte properties and the extent to which these electrolytes can control the activity of redox electrodes. Materials with high ionic content provide the highest room temperature ion conductivity and redox activity; however, RPEs with low ionic content provide the highest “on–off” ratio in electrochemical activity at elevated temperatures.

For the purpose of realizing inherently safe high-power Li-ion batteries, a model Li4Ti5O12/LiFePO4 rechargeable battery is investigated using the thermally responsive polymer, poly(benzyl methacrylate), in an ionic liquid. At high temperature, battery operation is inhibited as a result of increased internal resistance caused by polymer and ionic liquid phase separation. Li-ion concentration is shown to affect the phase transition temperature and the extent to which batteries are deactivated.

Energy storage has emerged as a critical challenge due to the increasing demand to utilize energy from intermittent, renewable resources. Furthermore, growing interest in decreasing our dependence on petroleum necessitates the development of safe and efficient energy storage for transportation. Li-ion batteries have emerged as the premier candidate to meet these demands; however, such systems are limited by thermal hazards (runaway reactions, fires, explosions), which become increasingly dangerous in large-format batteries. We have developed an electrolyte system comprising poly(ethylene oxide), an ionic liquid and lithium salt that exhibits an intrinsic mechanism for inhibiting device operation when the temperature increases beyond a given threshold, which is attributed to polymer–ionic liquid phase separation. In the following article, we describe how the thermally activated phase separation causes a decrease in ion conductivity, thereby affecting the concentration of ions at the electrode, in addition to an increase in charge transfer resistance from the formation of a polymer coating on the porous electrode. Such mixtures provide a transformative approach to regulating electrochemical processes, which is necessary to achieve inherently safe operation of large format Li-ion batteries.

A thermally responsive copolymer is designed to modulate the properties of an electrolyte solution. The copolymer is prepared using pNIPAM, which governs the thermal properties, and acrylic acid, which provides the electrolyte ions. As the polymer undergoes a thermally activated phase transition, the local environment around the acid groups is reversibly switched, decreasing ion concentration and conductivity. The responsive electrolyte is used to control the activity of redox electrodes with temperature.

In recent years, remarkable progress in the development of Li-ion battery technology has led to devices that deliver records in power and energy density. The thermal instability in such batteries, however, creates a major roadblock to implementing these devices in large-format systems to support emerging applications in transportation and energy storage from intermittent sources, such as wind and solar. Efforts to mitigate safety issues pertaining to flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible and destructive safety devices, which prove problematic for high-power, large-scale systems. In this presentation, we will discuss a new approach to achieve thermal stability in Li-ion batteries using an electrolyte system that contains a polymer that phase separates from solution above a specific temperature. The phase separation causes the solid polymer to coat the electrode and separator leading to an increase in internal resistance, which prevents the flow of current within a specified voltage range. This approach is advantageous over existing methods because the polymer phase separation is a local process, that is, if hot spots form within the battery, the polymer within that area will phase separate and coat the electrode while the rest of the device continues to operate. Two responsive electrolyte systems will be described: poly(ethylene oxide) (PEO) in ionic liquids (ILs) and poly(benzyl methacrylate) PBMA in ILs. Both systems exhibit Li-ion concentration dependent phase behavior, where ion concentration affects the temperature at which the phase transition occurs and the reversibility of the phase transition. Two distinct differences exist between these electrolyte systems. The PEO-IL system exhibits a change in solution conductivity and charge transfer resistance above the phase transition temperature, while the PBMA-IL system only exhibits an increase in charge transfer resistance, but to a much greater extent. Solution and ion transport resistances are measured using electrochemical impedance spectroscopy (EIS) on stainless steel electrodes to determine changes in solution resistance with increasing temperature, and on porous carbons to measure ion-transport resistances. The influence of ion and polymer concentration on the phase transition temperature will be discussed, along with the extent to which these properties can changed with temperature. Li-ion batteries were fabricated using two electrode configurations, each including a high temperature stable separator (Dreamweaver, International) soaked in the responsive electrolytes. In one setup, lithium titanate (LTO) anodes were used with lithium iron phosphate (LFP) cathodes due to the stability of each material in various ionic liquids. With this cell, we investigated key battery characteristics as a function of temperature using EIS and constant current charge-discharge measurements. With properly formulated electrolytes, we showed that battery operation was inhibited above the phase transition temperature of the electrolyte, which fell in the range of 120-150 °C. Figure 1 shows the discharge characteristics of the LTO-LFP cell at a 4C discharge rate at 60 °C and at 150 °C. Above the phase separation temperature, the internal resistance of the cell increases leading to a rapid decrease in potential as the cell tries to deliver the specified current. Open circuit voltage measurements confirm the change in internal resistance with temperature. In another cell configuration, we investigate the performance of responsive electrolytes with lithium cobalt oxide (LiCoO2) cathodes and graphitic carbon anodes. These batteries represent state-of-the-art cells with high operating voltages and high power capabilities. The performance and stability of responsive electrolytes in these systems will be discussed and compared to LTO-LFP systems, and conventional batteries comprising electrolytes with lithium salt in propylene carbonate. Electrolytes with temperature-responsive polymers provide a new approach to achieve thermal stability in Li-ion batteries. Results presented in this work will provide the foundation for the development of advanced materials that enable large-format and safe lithium batteries capable of delivering high-power. Figure 1

In recent years, remarkable progress in the development of batteries, supercapacitors and Li-ion technology has led to devices that deliver new records in power and energy density. The thermal instability in batteries, particularly lithium ion, creates a major roadblock to implementing these devices into large scale systems to support emerging, intermittent energy sources such as wind and solar. Efforts to mitigate safety issues such as flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible safety devices, which prove problematic for efficient, large-scale energy systems. Before high performance devices can be applied to large format energy storage applications, thermal safety must be improved. “Smart materials” or stimuli-responsive materials are those with properties responding in a desired manner to an external stimulus or environmental change. While several classes of responsive materials exist, soft materials, particularly synthetic polymers, represent an opportunity to design systems with unique functionality to achieve a wide variety of chemical and physical properties. Here we present two systems utilizing thermally-responsive polymers capable of reducing lithium conductivity or lithium charge transfer ability in non-aqueous electrolytes with increasing temperature. These proposed systems would create self-limiting lithium reactions within a battery should the temperature increase beyond a target, thus inhibiting thermal runaway. Li-ion systems utilizing polymer gel electrolytes are commercially available and ionic liquids (IL) have gained attention as potential replacements for traditional electrolytes (low volatility, non-flammability, high conductivity, and a wide potential window). Poly(ethylene oxide) (PEO) gels have long been combined with lithium salts to create polymeric electrolytes, which while safe, possess diminished conductivity. These PEO systems have recently been combined with ILs to boost performance while retaining the safety and mechanical stability of conventional polymer gel electrolytes. In our first proposed system, we combine current research efforts utilizing polymer gel electrolytes and ILs, both of which provide improved thermal stability, while making use of the recently discovered lower critical solution temperature (LCST) between PEO and certain ILs, to modulate electrolyte conductivity with temperature [1,2]. We design the polymer electrolyte to decrease conductivity during a phase transition at elevated temperatures. As the temperature cools, the initial conductivity is reversibly restored as the electrolyte returns to its initial state. Additionally, we show how the PEO/IL composition and the addition of lithium salts affect both the magnitude of change in conductivity and the LCST in these thermally-responsive polymer electrolyte systems. In our second proposed system, we utilize similar LCST phase behavior in IL systems by modifying electrode surfaces with responsive polymer brushes consisting of poly(methacrylates) [3-5]. Electrode surfaces were modified using the grafting from approach so that the polymer collapses on the surface of the electrode during a phase transition at elevated temperatures. In this collapsed form, the electrode surface is coated with an electrically and ionically insulating layer that prevents double layer formation in supercapacitors and charge transfer (or redox behavior) in Faradaic systems. As in the previous system, the initial electrochemical activity is reversibly restored as the temperature cools and the responsive brushes return to their solvated state. The experiments performed to date show tremendous promise for controlling battery or supercapacitor operation using temperature. Further development of responsive electrolytes and electrode surfaces will lead to numerous opportunities associated with thermal safety and “smart” electrochemical systems. H.-N. Lee and T. P. Lodge, J. Phys. Chem. Lett., 1, 1962 (2010). H.-N. Lee, N. Newell, Z. Bai and T. P. Lodge, Macromolecules, 45, 3627 (2012). T. Ueki and M. Watanabe, Langmuir, 23, 988 (2006). K. Kodama et al., Langmuir, 25, 3820 (2009). H.-N. Lee and T. P. Lodge, J. Phys. Chem. B, 115, 1971 (2011).