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Summary and Opinion by Hunaid Nulwala Ph.D.

Introduction: Lithium-sulfur (Li-S) batteries have emerged as promising candidates for next-generation energy storage systems due to their high specific energy capacity and density. However, challenges such as sulfur’s insulating nature and the large volume change of S/Li2S during discharge/charge processes hinder the full utilization of sulfur in Li-S batteries.

halide on lithium sulfur batteries

Figure 1: The addition of halides in the MOS₂ results in improved Lithium Sulfur battery performance.

To overcome these limitations, researchers have been exploring various strategies to improve the Li2S/S reaction and enhance the overall performance of Li-S batteries. One such approach is the incorporation of bromine (Br) and iodine (I) into Li₂S with carbon black. This work was quite interesting because halides are known to be found in many ionic liquids. It makes me think that not all halide is likely detrimental, and certainly, there is some synergy. However, the control of halide content is important. The work performed by Wang et al,¹ caught my attention. After ball-milling carbon black and MoS2 with LiI-LiBr, the particle size of MoS2 with LiI-LiBr is reduced. Overall, they observed enhanced electronic conductivity and improved MoS2 reversibility and kinetics.

My thoughts stem from varied research results found in the battery space by various research groups. I firmly believe that halides play a significant role in all energy storage devices that use ionic liquids. I have come to believe that the incorporation of various halides as additives in ionic liquids can significantly improve the performance of lithium-ion batteries.

A note of caution: Significant side reactions can occur. One must think about the whole system, not just the components.

Potential implication of a small amount of halide in a Li-S battery:

Complex Defect Formation: When Br/I is introduced into Li2S, complex defects are formed, including Li vacancies, along with Br-on-S or I-on-S substitutions. These arise due to valence inconsistencies between Br/I and S. The formation of these defects plays a crucial role in enhancing the Li2S/S reaction, this can enhance kinetics.
Reduced Li-S Bond Strength: The complex defects formed by Br/I incorporation reduce the bonding strength of the Li-S bond. In this paper, Crystal Orbital Hamilton Population (COHP) analysis reveals that the bonding state of the Li-S bond around complex defects decreases, leading to a weakened Li-S bond. This weakened bond facilitates the delithiation of Li2S during the discharge process.
Enhanced Li-Ion Conductivity: Incorporating Br/I into Li2S significantly improves ionic conductivity. Li2S typically exhibits low ionic conductivity (10^-13 S cm^-1). However, introducing LiI alone increases the ionic conductivity to 6.4 × 10^-8 S cm^-1 in the Li2S@LiI composite. Furthermore, incorporating the LiI-LiBr compound further enhances the ionic conductivity, resulting in 7 orders of magnitude increase (1.08 × 10^-6 S cm^-1) compared to Li2S. One can think about the size of halides. But there is not enough information to guess or provide insight.
Unknown side reactions: Halides are reactive, and if not properly handled, they can result in unwanted side reactions that harm the device and can be a major safety concern. One has to use halides as an additive in a controlled manner.

The authors in this publication also performed Density Functional Theory (DFT) calculations to support the experimental findings. They show that LiI-LiBr incorporation reduces the formation energy of complex defects in Li2S. The lower formation energy indicates the stability and effectiveness of these complex defects in improving the Li2S/S reaction.

Conclusion: Br/I incorporation in Li2S is a promising strategy for enhancing the Li2S/S reaction in lithium-sulfur batteries. The formation of complex defects, the weakened Li-S bond strength, and the improved Li-ion conductivity contribute to the overall improvement in the electrochemical performance of Li-S batteries. These advancements bring us closer to realizing the full potential of Li-S batteries as a high-capacity and energy-dense energy storage solution.

Numerous strategies can be incorporated to improve lithium-based batteries. Indeed, controlled impurities of halides other than fluoride can significantly improve battery performance.

At RoCo, we are developing carbon specifically enhanced with ionic liquids to improve battery performance. These materials will be added to our store to be purchased shortly for improving your battery research.

Reference:

(1) Wan, H.; Zhang, B.; Liu, S.; Zhang, J.; Yao, X.; Wang, C. Understanding LiI-LiBr Catalyst Activity for Solid State Li2S/S Reactions in an All-Solid-State Lithium Battery. Nano Lett 2021, 21 (19), 8488–8494. https://doi.org/10.1021/acs.nanolett.1c03415.

Blog Materials World

The ABCs of Ionic Liquids

A is for…Additives. Ionic Liquids have multiple uses as an additive for polymer processing, electrolytes, and antimicrobials.

B is for…Batteries. Ionic liquids hold the potential to overcome the high voltage problems associated with Dual-Ion Batteries and can also be used in the development of metal-free energy storage devices.

C is for…Corrosion inhibitor. Our partner, IoLiTec GmBH, has tested several ionic liquids as promising corrosion inhibitors. In this work, IoLiTec obtained optimal results with several acetate-based ILs.

D is for…Desalination. New research shows that ionic liquids can be used to desalinate seawater. This is critical for solving global water shortages.

E is for…Energy storage applications. At RoCo®, we have worked with industry giants in battery chemistry and developed electropolymerized ionic liquids to form SEI (Solid Electrolyte Interphase) layers. These materials can improve the SEI layer and significantly improve the safety of lithium-ion batteries.

F is for…Fuel cells. Multiple ionic liquids have found uses as an electrolyte in fuel cells.

G is for…Green Solvent. Ionic liquids are considered more environmentally friendly alternatives to traditional solvents due to their low toxicity and ability to dissolve a wide range of compounds. They can be used as greener replacements for volatile organic solvents in industries such as pharmaceuticals, chemicals, and materials. ILs can be made from amino acids, which can be non-toxic.

H is for…Hydrophobic or Hydrophilic. ILs can be either hydrophobic or hydrophilic, depending on their chemical structure. The number of possible ionic liquids that can be accessed synthetically is estimated to be 10¹⁸. That is a huge number! Commercially, you can buy ~1000 ILs.

I is for…Imidazolium. This class of Ionic Liquids is just one of many, but one of the most popular, due to their wide variety of applications.

J is for…Joule. It is the unit of energy in the International System of Units (SI). The use of ILs can save significant Joules in terms of energy consumption. That directly translates to $$

K is for…Potassium (K). Ionic Liquids have found applications in Potassium-Ion Batteries(KIB). Potassium-ion batteries are noteworthy for their abundant raw materials, high energy density, fast ion transport in the electrolyte, and lower cost. An advantage of potassium-ion batteries is that they can use more inexpensive and abundant materials such as potassium, iron, and aluminum instead of expensive ones like lithium, cobalt, and copper. Additionally, KIBs have a lower fire risk than lithium batteries, making them a safer option.

L is for…Lubricants. In recent years, ILs have found new applications as an additive to lubricants due to their strong adsorption properties.

M is for…Metal Salts. Metal Salts, specifically Lithium, are becoming very popular. These products have a variety of uses, including battery applications and energy storage.

N is for…Non-Volatile. One property of ILs is that they are non-volatile, meaning they do not disperse VOC (volatile organic compounds) into the atmosphere. ILs are being used to develop to produce O₂ in space applications using lunar regolith.

O is for…Organic. The structure of an Ionic Liquid is an organic cation combined with either an organic or inorganic anion. ILs have found use in organic synthesis.

P is for…Polarity. ILs are polar and non-polar groups coupled together. This coupling of polar and non-polar groups together gives them unique solubility properties.

Q is for… Quinine. Quinine is a medication used to treat malaria and babesiosis. There have been six ionic liquids synthesized from a Quinine base. Who knows what applications they may bring?

R is for…RoCo. Our company has years of expertise in researching and developing ionic liquids, and we can create customizable ILs and Poly ILs. We are also the North American supplier of IoLiTec GmbH.

S is for…Sensors. Ionic liquid-based skin sensors are an emerging type of pressure sensor capable of perceiving external stimuli of pressure, strain, and torsion and turning them into electrical signals. These ionic liquid sensors can be encapsulated in silicone and worn directly. When connected with smart devices, it effectively expands the ability of human beings to perceive and evaluate the external environment.

T is for…Thermal stability. Their thermal stability makes them ideal for various industries, such as energy storage, catalysis, and electrochemistry.

U is for…Upcycling. RoCo® is committed to finding a solution to Polypropylene (PP) Recycling by developing innovative technologies that will aid in upcycling PP. Over 80 million metric tons of PP were made in 2021, and less than 2% was recycled. We are currently working on an ionic compatibilizer that is halide-free, non-toxic, and miscible with polyolefins, which make PP resilient to contaminants. This results in the upcycling of PP upon recycling.

V is for…Vapor pressure. Traditional solvents can easily evaporate into the air, while ionic liquids remain in the liquid state even under atmospheric conditions due to their negligible vapor pressure.

W is for…Wide liquid range. Unlike most substances that have a specific melting and boiling point, ionic liquids can exist as liquids over a broad temperature range. Their liquid range can extend from below -100°C to well above 200°C, depending on the specific composition of the ionic liquid.

X is for… X-ray scattering. Small-angle X-ray scattering is an ideal technique for examining the structures of ILs due to its ability to review a relatively large sample volume. Scientists have found nano-scale segregation in ionic liquids using X-ray techniques.

Y is for…Ylide. Ylides are neutral dipolar molecules containing a negatively charged atom directly attached to a positively charged heteroatom. Phosphonium ylides are used in the Wittig reaction, a method used to convert ketones and especially aldehydes to alkenes. Ylides are a bit crazy 😊

Z is for…Zwitterion. A zwitterion is a molecule that contains both positively and negatively charged functional groups. These molecules are being used in a novel class of ILs for use in Lithium-Sulfur Batteries.

Blog Materials World

Rethinking Power: The Rise of Sodium and Potassium Batteries and the Need for U.S. Engagement

Rethinking Power: The Rise of Sodium(Na-ion) and Potassium (PIB) Batteries and the Need for U.S. Engagement

Blog By Hunaid Nulwala, Ph.D

Lithium-ion batteries have long been the standard for energy storage, powering a vast array of devices from smartphones to electric vehicles and the electric grid. However, lithium’s limited availability and exorbitant cost are causing significant concerns in the supply chain. Fortunately, promising alternatives such as Sodium-ion (Na-ion) and Potassium-ion (PIB) batteries can effectively address lithium shortages and be a dependable option for large-scale energy storage.

Sodium-Ion Batteries: A Bright Prospect

Na-ion batteries are a more affordable and widely available alternative to lithium-ion batteries. Due to their similar electrochemistry, they are becoming an increasingly popular choice. The abundance of sodium makes these batteries an even better option. Over the past ten years, Na-ion batteries have made significant progress and can replace lithium-ion batteries in specific applications, such as stationary storage.

Chinese companies and automakers have made impressive progress in developing sodium-ion battery technology. JAC Motors, a Chinese automaker, recently revealed a vehicle powered by a 25-kilowatt-hour sodium-ion battery produced by HiNa Battery. The car can travel up to 250 kilometers (155 miles) on a single charge. CATL, China’s biggest EV (Electric Vehicle) battery maker, has also developed a sodium-ion battery for use in a Chery vehicle.

Scientists are working on developing anode-free sodium (Na) metal batteries that are both cost-effective and environmentally friendly. The goal is to create energy storage systems comparable to lithium-ion batteries in terms of energy density and safety while reducing environmental impact. With their low cost, high safety, broad temperature range, and sustainability, these Na metal batteries have the potential to become a viable commercial option.

Potassium-Ion Batteries: Full of Potential

Potassium-ion batteries are noteworthy for their abundant raw materials, high energy density, fast ion transport in the electrolyte, and lower cost. An advantage of potassium-ion batteries is that they can use more inexpensive and abundant materials such as potassium, iron, and aluminum instead of expensive ones like lithium, cobalt, and copper. Additionally, PIBs have a lower fire risk than lithium batteries, making them a safer option.

To give you a visual, here is a handy comparison of ionic and conductivity among potassium, sodium, and lithium:

Table 1: Difference in ionic radius, stokes radius, and ionic conductivity. 1

Adopting sodium-ion and potassium-ion batteries could significantly lessen the demand for lithium, cobalt, and nickel, freeing up these resources for next-generation high-energy-density lithium-ion batteries. Potassium (K+) is larger than sodium (Na+) and lithium (Li+); therefore, it has a lower charge density. However, let’s consider its transport behavior, as shown in Table 1. K+ has the lowest energy of solvation/desolvation of the three, as reported by Okoshi.2 This lower energy enables a fast desolvation process at the electrode-electrolyte interface with beneficial effects on rate capability. In LIBs, Abe et al. demonstrated that one of the critical rate-limiting processes of Li intercalation into the graphite electrode is the desolation process, suggesting that potassium ion batteries may be more suitable for high-power applications.3 Also, K+ in solution typically has a smaller stokes radius due to weaker interactions with solvent molecules which may facilitate higher ion transport.

The United States must prioritize improving its sodium and potassium-ion battery technology to keep up with China’s advancements in non-lithium battery technologies. There are numerous sodium and potassium salts and ionic liquids that show great promise for the development of these batteries. With advances in battery chemistry, cell engineering, and system integration, sodium and potassium batteries could become widely used within the next decade. These batteries could drastically transform the energy landscape, from portable electronics to electric vehicles and grid energy storage. Promising materials and ionic liquids such as Potassium trifluoromethanesulfonamide, sodium trifluoromethanesulfonamide, Sodium tricyanomethanide, Sodium dicyanamide, N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI), 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMim FSI), and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14 TFSI) can be used for developing such batteries.

Sodium and potassium batteries have the potential to be widely used in various applications such as portable electronics, electric vehicles, and grid energy storage. With the proper support, including advancements in battery chemistry, cell engineering, and system integration, these batteries could be put into operation within the next ten years. This is expected to impact our society’s energy landscape significantly.

References:

(1) Hosaka, T.; Kubota, K.; Hameed, A. S.; Komaba, S. Research Development on K-Ion Batteries. Chem Rev 2020, 120 (14), 6358–6466. https://doi.org/10.1021/acs.chemrev.9b00463

(2) Okoshi, M.; Yamada, Y.; Komaba, S.; Yamada, A.; Nakai, H. Theoretical Analysis of Interactions between Potassium Ions and Organic Electrolyte Solvents: A Comparison with Lithium, Sodium, and Magnesium Ions. J Electrochem Soc 2017, 164 (2), A54–A60. https://doi.org/10.1149/2.0211702jes

(3) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. Solvated Li-Ion Transfer at Interface Between Graphite and Electrolyte. J Electrochem Soc 2004, 151 (8), A1120. https://doi.org/10.1149/1.1763141

Blog Materials World

Role of Polymers in Packaging: Ensuring Sustainability for the Future

Blog by Hunaid Nulwala, PhD

Packaging materials have become essential to our daily lives, from plastic bags at the grocery store to tetra packs holding our favorite drinks. The development of polymers has revolutionized packaging materials, making it possible for food to have a longer shelf life and enabling us to handle and store necessary and unnecessary items efficiently. Polymers have also led to the creation of numerous products that have transformed our world, and imagining where we would be without them is difficult.

Polymers have numerous advantages compared to other packaging materials. They have exceptional barrier properties to water and oxygen, which are crucial in preserving perishable items like food. Moreover, plastics are transparent, enabling us to see the contents of the package. They are versatile as they can be colored and printed on. The lightness of plastics makes them suitable for all packaging purposes, including shipping and storage. The advent of plastics has significantly revolutionized various industries worldwide.

Throughout history, packaging materials have been an essential component of human society. From the early days of woven baskets and earthenware pots to the industrial revolution’s development of tin cans, the need for packaging has always existed. However, the packaging industry underwent a significant transformation in the mid-20th century by introducing synthetic polymers, which have become a vital component of modern-day packaging. These versatile materials have revolutionized the industry, providing unparalleled strength, durability, and flexibility to support the needs of an ever-changing world. Today, synthetic polymers continue to play an indispensable role in packaging, enabling us to transport and store goods safely and efficiently. However, they lack polymer sustainability.

I remember vividly from childhood when I was around 4-5 years old, standing in a classic 1978- Honda 50 motorcycle basket to get milk from the Nagori milk shop in Karachi, Pakistan. My siblings would sometimes join me in the basket, where there was a stainless-steel canister to hold the milk in this “bring milk home” joy ride. It was a daily chore for my dad to get milk. As soon as the milk arrived, my mom would boil it right away, and the cream would rise above the milk (something that doesn’t happen in pasteurized milk), and we would eat it with honey and parathas. However, plastic bags and tetra packs took over by the late 20th century, and my early morning trips to the milk shop became just a memory.

Plastic bottles made it extremely easy and convenient for the dairy industry to deliver milk to every household. It curbed spoilage, simplified the supply chain, and brought the product to consumers in a way that was never possible. The famous line from the movie “The Graduate,” “Plastics” certainly rang true.

Despite their numerous advantages, plastic polymers present significant environmental challenges. Plastic waste is a major problem worldwide, and much of it ends up in the oceans and landfills, where it can take hundreds of years to decompose. This has led to a growing movement towards more sustainable packaging solutions, such as recycling, upcycling, and biodegradable and compostable materials. As we progress, we must think beyond convenience and focus on sustainability. From an application perspective, an item used in agriculture (such as ag films) can become a part of the soil. Plastics used in shampoos and personal hygiene should be targeted to be upcycled into higher-value components, such as automotive parts and durable products.

The good news is that sustainable packaging is seeing many exciting innovations that offer hope for the future. Some companies now use bioplastics made from renewable resources, such as cornstarch, which can be composted after use. Others are utilizing plant-based materials like mushrooms or seaweed to create eco-friendly packaging options. Additionally, advanced recycling (upcycling) methods are being developed to generate higher-value products. It is essential to consider the end-use application perspective, and we need to develop innovations specifically for food packaging made entirely from starch and starch-based polymers that can be easily segregated and composted. We should also look into creating 100% edible food packaging, which can become part of the environment even when thrown away. On the recycling side, we need to use computational science, artificial intelligence, and chemical intuition to make more informed decisions about the types of polymers used in future products to make component recycling more accessible.

As individuals, we hold the power to promote sustainability by demanding eco-friendly packaging and supporting companies that prioritize sustainability. We can also urge legislators to pass laws that impose carbon taxes and prioritize environmental values, especially on products that are difficult to recycle or decompose. Additionally, researchers and scientists can contribute to sustainability efforts by developing innovative materials and processes for sustainable packaging. Making small changes in our daily lives, such as using reusable bags and containers, can also significantly reduce our environmental impact. By taking these steps, we can all play a vital role in ensuring a sustainable future.

It is imperative that we work together to maintain the advantageous contributions of polymers and packaging toward our societal and personal advancements. However, we must also guarantee that we do so in a sustainable manner that ensures the prosperity and well-being of future generations.

Blog Materials World

Polymer Upcycling is a Solution to Circular Economy

Polymer Upcycling is a Solution to Circular Economy

Blog by Hunaid Nulwala, Ph.D.

The concept of a circular economy involves designing products and systems to minimize waste and maximize the use of resources, to create a closed-loop system in which waste is eliminated or repurposed into new products.

One of the biggest challenges in transitioning to a circular economy is overcoming the significant inertia and resistance to change that exists in many companies and individuals. Many businesses and consumers are used to the linear model of take, make, and waste, and switching to a circular model requires a shift in mindset and business practices. In addition, the economic incentives of the linear economy often prioritize short-term profits over long-term sustainability and social responsibility.

Another challenge is the need for more infrastructure and technology to support a circular economy. Efficient recycling and waste management systems and innovative technologies to transform waste into valuable resources are essential for a circular economy. However, these systems and technologies still need to be implemented in many regions of the world.

Furthermore, regulatory frameworks and policies are often not designed to support a circular economy. Tax incentives, for example, may need to be put in place to encourage companies to adopt circular business models or prioritize sustainable practices. In addition, many consumers need to be made aware of the benefits of a circular economy or how they can participate in it, which can limit demand for sustainable products and services.

Addressing these challenges will require collaboration among all stakeholders, including government, industry, and consumers. Governments can implement policies and regulations encouraging circular practices, such as tax incentives or extended producer responsibility programs. Industries can invest in research and development to create new technologies that support a circular economy and communicate the benefits of circular practices to consumers. Consumers can also play a role by demanding sustainable products and services and trying to recycle and dispose of waste responsibly.

Using post-consumer resin (PCR) content in plastic packaging is one effective way to move towards a circular economy. This approach can help to build and stabilize recycled markets by creating a demand for recycled materials and improving innovation. It can also level the competitive playing field by creating a need for recycled materials that compete with virgin materials. Additionally, using PCR content in plastic packaging can provide an environment for end-market investment, innovation, and growth, as companies can develop new products and processes that utilize recycled materials.

Designing packaging and materials to be easily recyclable is crucial to creating a more sustainable economy. Plastics that can be easily comingled, for example, make sorting and separating materials easier and more efficient, which can help overcome some of the challenges associated with recycling and reduce contamination of the recycling stream. Selecting additives, fillers, and polymer treatments that improve the recyclability of plastic products can also increase the value of recycled materials and make them more attractive to end markets.

Incorporating these design strategies into the development of packaging and materials can help to create a more sustainable and circular economy. By designing for recyclability, we can reduce waste and support the growth of recycling programs while creating new opportunities for innovation and investment in the recycling industry. Ultimately, transitioning to a circular economy will require a significant shift in mindset and practices, but it is necessary for a more sustainable future.

Blog Materials World Functional Materials Ionic Liquids

Metal Free Battery: Ionic Liquids

It is estimated that there are about 15 billion mobile devices today. The number of devices will increase to 50 billion by 2030. As we become more connected, use more electric vehicles, and integrate renewable energy into our daily lives, demand will increase for energy storage. When it comes to portable energy, lithium-ion batteries (LIB) are the king. LIBs have found their way in all sorts of new devices with a net improvement in our lives. In fact, LIBs have made widespread adoption possible for smartphones. In the past 25 years, LIB improvements have focused on continuously increasing their specific energy (Wh kg−1) and energy density (Wh L−1). The use of LIBs in automotive is now the hottest growth area.

Figure 1: Future Battery Performance Parameters

Moving away from fossil energy absolutely necessary. However, we need to be cognizant of environmental hazards with battery technologies, especially LIBs. We certainly don’t want to develop new technology with dire long-term environmental consequences. LIBs are not hazard-free, and the public notion that they are environmentally friendly is wrong. It is important to note that 99.5% of lead-acid batteries are recycled. However, no real recycling effort exists in batteries such as LIBs for consumer electronics, and the exact combination and number of chemicals inside a battery vary drastically. Many batteries include metals like cadmium, nickel, cobalt, copper, iron, and lithium. The newer generation of battery electrolytes also contains fluorinated electrolytes, which can be toxic if not disposed of properly. When batteries are thrown into household trash, they end up in landfills, ultimately leaching chemicals into the soil and making their way into our water and food supply. If the batteries are burned, they can release highly toxic and hazardous compounds.

Depending on the application, battery types change. For example, power tools vs. portable electronics have different needs in terms of energy output and sustained performance. Battery manufacturers are aware of the energy needs of the specific device. However, as the number of battery power devices grows, it is now clear that we need to think about additional key parameters. These include 1) the CO2 footprint of the battery in its production and use. 2) Reducing the use of metals such as Cd, Li, Co, or Ni 3) Using more sustainable or “green” materials such as sodium and potassium 4) the design of batteries needs to be such that they are 100% recyclable (lead-acid batteries are almost 99% recycled). 5) additional incentives such as tax credits towards green batteries.

Indeed, there are emerging technologies that utilize much lower toxicity and abundant materials. Specifically, the dual-ion battery (DIB) is safer and much easier on the environment (compared to mining cobalt and lithium). DIB uses positive and negatively charged ions that are active in energy storage, as shown in Figure 2.

Figure 2: Schematic illustration of the dual-graphite battery (DGB) system

during the charging process, using ionic liquids as the electrolyte.

DIBs have strong potential for high energy density and low-cost, safe materials, but problems with stability and electrolyte performance at higher voltages have been challenging. DIBs can be charged and discharged at much larger currents and are very suitable for high-power applications such as moving large, heavy objects at high speed.

Ionic liquids hold the potential to overcome the high voltage problems associated with DIBs and can also be used in the development of metal-free devices. The high working voltage of DIBs makes ionic liquids, especially feasible as electrolytes with high working voltages. Recently, Wang et al. reported a novel dual-graphite battery system using pure 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid. The DIB developed in this paper has no metallic elements and was investigated in great detail about its performance. This work opens up new possibilities for making batteries without any metals and the possibility for easy recycling and reuse.

Are you looking for an ideal partner to help you rapidly advance various technologies? Perhaps you want to design and test custom gas separation membranes for your application, an improved battery electrolyte, or an additive to reduce viscosity?

Contact RoCo® today to learn more about our Research & Development Services and how we can help you meet, and exceed, your goals.

Blog Materials World

Ionic Liquids in Industrial Applications

Research in the field of ionic liquids (ILs) has increased since their initial discovery in 1914 by Paul Walden. Ionic liquids, salts in liquid state, are frequently described as being liquid below an arbitrary value of 100°C. However, one should not use this constraint for a substance to be considered an IL. We think of any organic salt which melts without decomposition as an ionic liquid at that temperature. The organic nature of ILs is attributed to the bulky and unsymmetrical structure of the constituent ions, which can be paired together in different combinations to target specific properties such as electrical, chemical, and thermal.

Some examples of common cations and anions are shown above with together with their preferred names.

ILs have been investigated in an extensive range of applications including solvents, catalysts, lubricants, and in a number of electrochemical applications such as batteries, super capacitors etc.

Due to the wide range of tunable properties, ILs are deemed as material of the future. ILs have been slowly but surely finding their way in several industrial processes which heavily rely on toxic, flammable and highly volatile systems. The low vapor pressure of ILs has been of particular interest due to their ability to lower the atmospheric pollution. However, not all ILs are environmentally friendly. Depending on the chemical structure, some ILs can be extremely toxic to humans and the environment. IL toxicity, particularly toward aquatic organisms, has been identified as an emerging problem as the use of ILs increases.

ILs are thermal stability and due to their ionic nature they are generally non-flammable. Their low vapor pressure at ambient temperature has also garnered quite a bit of interest in decreasing volatile organic content (VOC). These properties have been quite useful in increasing the safety of high temperature applications. One of the areas which ILs have had a major impact is in energy storage resulting in improved performance and safety.

The unique properties of ILs have opened up new applications specifically in ultra-low vacuum and in a number of outer space applications. Additionally, as ILs consist largely of ions, they also have higher thermal and electrical conductivities in comparison to common solvents, and much larger electrochemical windows. This has meant that they have found quite a few uses, see figure 2.

Adoption of ILs in a number of industrial applications has bloomed since their discovery (Figure 3). Today, ionic liquids are found in cell phones, tablets and in various displays. ILs are now found in a number of commercial plastics specifically high end applications such as an anti-static agents, to improve dispersibility etc. Figure 3 shows some industrial application of ionic liquids by various large organizations.

The possibilities for IL application seem endless. It’s important to understand the structure property relationships of ILs to maximize their performance in a specific application while minimizing their environmental impact. Are you looking for an ideal partner to help you rapidly advance material initiatives? Learn more how can we help you with material application by using ionic liquids as additives in your application. Contact RoCo Global today to learn more about our Research & Development Services and how we can help you meet, and exceed, your goals.

Blog Materials World Carbon dioxide capture Gas separation membrane

High performance polyphosphazene mixed matrix membranes for gas separations

Polyphosphazenes-based membranes have shown considerable promise for post-combustion CO₂ capture mainly because of their excellent CO₂/N₂ gas separation properties. Polyphosphazenes are inorganic-organic hybrid polymers with an inorganic backbone of alternating phosphorus and nitrogen atoms, while the organic groups are usually the two side groups attached to each phosphorus atom. The polymers are synthesized via macromolecular substitution reactions generally by the replacement of chlorine atoms in poly(dichlorophosphazene) by various nucleophiles such as alkoxides, aryloxides, and primary or secondary amines.

Figure 1: Polyphosphazenes offer many unique advantages over traditional polymers.

The phosphorus-nitrogen bonds in the polymers are extremely flexible. Mainly due to the low torsional barrier within the backbone thus making them suitable for high performance elastomers. Other uses include fire retardant materials, electrolytes, optical polymers, and biomedical materials.

If the macromolecular substitution is carried out with polar substituents the resulting polymer has an extremely high affinity for CO₂. The base materials selected in this work had both selectivity and permeability. The polyphosphazene base material chosen for this study had a CO₂ selectivity of 50(CO₂/N₂) and a permeability of 500 Barrer. To overcome the needed mechanical properties, we converted the polyphosphazenes into crosslinked interpenetrated networks resulting in significantly improved mechanical properties to cast thin films and with excellent selectivity of 46 (CO₂/N₂) and permeability of 500. The overall approach for the development of tougher films is outlined in figure 2.

Figure 2: Top) synthesis procedure of interpenetrating network using UV-crosslinking. Bottom) SEM image showing the resulting thin polyphosphazene dense layer on top of the porous support.

However, it should be noted that within polymeric membrane materials, there is a tradeoff between improving selectivity and permeability. This tradeoff manifests itself in the Robeson upper bound, which establishes upper-limit combinations of permeability and selectivity for the best-performing membranes. And within a polymer class, an increase in permeability is almost always met with a decrease in selectivity. Conversely, some classes of inorganic membranes have selectivities that are many times higher than traditional polymeric materials, but less straightforward fabrication makes these membranes economically infeasible for large-scale applications with ceramic, glass, and zeolitic membranes, sometimes costing several orders of magnitude more than comparable polymer membranes.

One way to enhance gas transport properties and fabricate economically viable membranes which exceed the Robeson upper bound involves forming composite membranes from polymeric materials and inorganic filler particles, yielding mixed matrix membranes (MMM). In theory, the advantages of both the polymer (ease of processing, low cost) and the inorganic material (favorable separation properties) can be realized in MMMs. MMMs have traditionally employed rigid hydrophilic zeolites or carbon molecular sieve particles as the inorganic filler phase, which are usually incompatible with polymers. This incompatibility of surfaces results in a gap (also known as an envelope) between the surfaces of the polymer and inorganic filler particles (Figure 3). Since the gap tends to be much larger than the pores of the inorganic particle, unselective diffusion in the gap dominates selective transport through the particle, failing to realize the performance potential of the membrane. Many other problems may also arise at the polymer/inorganic interface.

Figure 3: Gas transport in mixed-matrix membranes (MMMs) with good material compatibility (left) and poor material compatibility (right).

Most previous research assumed that to surpass the Robeson upper bound, the structure of the MMM had to eliminate the gap at the polymer/filler interface. This “ideal” morphology is impossible due to poor polymer/filler adhesion. We have previously performed work on metal-organic framework (MOF) based MMMs, which were focused on improving the compatibility between the MOF and polymer to eliminate the interfacial gap.

We, along with many other researchers, believed that MOFs’ organic/inorganic hybrid structure should make them intrinsically more compatible with polymers and lend them great promise as fillers compared to inorganic materials. Even making use of these unique materials; however, it has yet proven impossible to fully eliminate the particle envelopment effect and realize the separation potential of the MOFs. The addition of CO₂ selective particles does improve the permeability and selectivity, but these improvements tend to be quite small than what is theoretically possible.

During our work on the compatibility of polymers and MOFs, a great deal of understanding was gained on how CO₂ diffuses through composite membrane materials. This understanding has led us to conclude that the “ideal” MMM morphology may not be the most desirable for the selective separation of CO₂ from N₂. We believe that a better solution is to carefully control and take advantage of the gap between the polymer and filler particle rather than trying to eliminate it completely. The interfacial interactions between the polymer and particle will be precisely controlled to accomplish the morphology necessary to create the desired transport. We have coined the term as Interfacially-Controlled Envelope (ICE) membranes to describe the resulting specialized MMMs.

Our approach utilized inexpensive scalable silica nanoparticles commercially available from Nissan Chemical and Evonik corporation (10-15 nm) followed by a simple surface modification reaction to yield material that can allow us to control the interface and thus provide us with excellent membrane properties. We evaluated a few functional groups on the silica particles but found that nanoparticles modified by the cyclohexyl group worked the best in our case. We also found that 40% (TGA) surface modification was optimal. The general synthesis route to make surface-modified silica nanoparticles is outlined in figure 4.

Figure 4: General synthesis for the surface modification of silica nanoparticles.

A loading of 30-40% surface-modified nanoparticles was found to be optimal, yielding a selectivity of 44 and permeability of 1600 barriers which is significant as it meant a 3X improvement in permeability without the loss in selectivity.

Figure 5: Membrane results for the ICE membrane; note that the ICE membrane supposed the upper bound limits.

In short, polyphosphazenes are excellent materials for gas separation membranes when coupled with surface-modified filler particles. We saw excellent improvement in permeability without the loss of selectivity (selectivity of 44 and permeability of 1600 barriers). The crosslinked interpenetrating approach is a highly useful methodology to improve the toughness of these materials. This work was funded by US-Department of Energy ‘s National Energy Technology Laboratory under the award number DE-FE0026464. A full report on this work can be accessed online at https://www.osti.gov/servlets/purl/1484714.

The work presented here is a joint effort between Prof. Harry Allcock’s Laboratory @ Penn State and RoCo global.

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