Polyoxometalates and ionic liquid enhance solid-state lithium-ion electrolyte performance
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THE GREEN POLYOXOMETALATE-BASED LITHIUM SALT (LPM) IS SANDWICHED BETWEEN TWO LAYERS OF AN IONIC LIQUID (ORANGE) THAT PROMOTES THE DISSOCIATION OF LITHIUM IONS FROM LPM, INCREASING ION CONDUCTIVITY OF THE COMPOSITE SOLID ELECTROLYTE MEMBRANE.
view moreCREDIT: POLYOXOMETALATES, TSINGHUA UNIVERSITY PRESS
Polyoxometalates (POMs) containing charged lithium ions combined with ionic liquids, increase the ion conductivity of a solid-state electrolyte membrane.
Solid-state lithium-ion batteries depend on the movement of ions (charged atoms) in the solid, rather than liquid, state to either charge or discharge the battery. These solid-state electrolytes are safer, more cost efficient and capable of higher energy densities than batteries that rely on liquid electrolyte solutions, but suffer from low ionic conductivity, or movement of ions, and poor thermal stability. A new composite solid electrolyte (CSE) membrane was synthesized using lithium salts and an ionic liquid to improve the dissociation, and therefore conductivity, of charged lithium atoms in a solid-state electrolyte battery.
Polyoxometalates (POMs) are clusters of metal and oxygen atoms with properties that are determined by the well-defined structure of the POM atom cluster. Researchers from Northeast Normal University recently introduced a POM-based lithium salt, Li6P2Mo18O62 (LPM) into a solid-polymer electrolyte (SPE) made up of the polymer polyethylene oxide (PEO), an inexpensive and stable chain of many ethylene oxide subunits. PEO suffers from low ionic conductivity, and the addition of LPM salt alters the properties of the polymer and enhances ion movement. The research team also incorporated an ionic liquid (IL) to free lithium ions from LPM, further improving the conductivity of the composite electrolyte material.
The team published their results in the journal Polyoxometalates on September 28, 2023.
“Solid-state electrolytes (SSEs) are considered… the most promising candidates for next-generation energy storage devices due to their excellent thermal and electrochemical stability. Although SPEs have excellent flexibility and viscosity, they are severely limited due to their low ionic conductivity, poor mechanical strength and low thermal stability at room temperature. In contrast, inorganic solid electrolytes (ISEs) like LPM... usually have high ionic conductivity. By incorporating ISEs like LPM into SPEs to form composite polymer electrolytes, we leverage their respective properties… to achieve optimized mechanical properties and improve their ionic conductivity,” said Hong-Ying Zang, senior author of the paper and professor in the Key Laboratory of Polyoxometalate and Reticular Material Chemistry at Northeast Normal University in Changchun, China.
“Currently, inorganic electrolyte fillers include nanoparticles… and ionic-conductive inorganics. As a class of metal-oxygen clusters, the application of polyoxometalates in solid-state batteries is hampered by the difficulty of moving lithium ions. In this paper, we promote the dissociation of lithium ions from polyoxometalates with ILs… to impart LPM and IL composites (LPM-IL) with good electrical conductivity,” said Zang.
The team characterized the ion conductivity and mobility of the composite membrane by measuring the AC impedance, or the resistance of current flow in a circuit. The team found that electrolyte membranes containing the optimal concentration of LPM and IL demonstrated three times higher conductivity than membranes prepared without IL.
In a similar manner, the team determined that the conductivity of composite membranes generated with polyvinylidene fluoride (PVDF), a non-reactive thermoplastic filler material, in conjunction with PEO increased conductivity ten times compared to LPM-IL membranes synthesized without PVDF. The composite membrane also demonstrated good stability over 12 hours at a temperature of 80℃.
“The results of these experiments demonstrate that polyoxometalates can be used as inorganic solid electrolytes,” said Zang. IL effectively increased the dissociation of lithium ions from LPM and improved the ionic conductivity of the composite solid electrolyte membrane. The incorporation of PVDF also created a PEO-PVDF conductive network in the membrane that further promoted lithium ion movement, enhancing conductivity.
The research team believes their unique, PEO-based composite membrane containing PVDF, POM-based lithium salt and IL provides a practical means of increasing ionic conductivity in solid-state electrolytes for use in lithium-ion batteries. “Our next step is to improve the performance of polyoxometalates to create better solid-state lithium-ion batteries,” said Zang.
Other contributors include Qianqian Liu, Yunzuo Cui, Lijie Zhu, Dongming Cheng, Chen Wang, Siqi Lu, Bo Li and Xinyu Chen from the Key Laboratory of Polyoxometalate and Reticular Material Chemistry at the Ministry of Education at Northeast Normal University in Changchun, China.
This work was supported by the National Natural Science Foundation of China (Grant No. 22302102, 21871042, 21471028, 22073094), the Fundamental Research Funds for the Central Universities-Excellent Youth Team Program (2412023YQ001), the Natural Science Foundation of Jilin Province (Grant No. 20200201083JC) and the Natural Science Foundation of Department of education of Jilin Province (JJKH20201169KJ).
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About Polyoxometalates
Polyoxometalates is a peer-reviewed, international and interdisciplinary research journal that focuses on all aspects of polyoxometalates, featured in rapid review and fast publishing, sponsored by Tsinghua University and published by Tsinghua University Press. Submissions are solicited in all topical areas, ranging from basic aspects of the science of polyoxometalates to practical applications of such materials. Polyoxometalates offers readers an attractive mix of authoritative and comprehensive Reviews, original cutting-edge research in Communication and Full Paper formats, Comments, and Highlight.
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JOURNAL
Polyoxometalates
ARTICLE TITLE
Ionic liquid-mediated PEO-based solid-state electrolyte membrane modified with Dawson-type polyoxometalates
Stronger lithium batteries may need ‘weaker’ solvation structure, researchers report
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EMERGING ELECTROLYTES WITH A DIFFERENT MICROSTRUCTURE TO CONVENTIONAL ELECTROLYTES MAY HELP ADVANCE LITHIUM BATTERIES TO KEEP PACE WITH FUTURE TECHNOLOGIES, ACCORDING TO A REVIEW PAPER FROM RESEARCHERS AT THE HONG KONG POLYTECHNIC UNIVERSITY. THEIR ANALYSIS CONCLUDED THAT BETTER KINETICS, REDUCED SOLVENT POLARIZATION, REDUCED DESOLVATION ENERGY AND MORE INORGANIC-RICH INTERPHASES MAY IMPROVE LITHIUM BATTERIES.
view moreCREDIT: ENERGY MATERIALS AND DEVICES, TSINGHUA UNIVERSITY
Lithium batteries power our phones, computers, many of our cars and so much more — even the drill and weedwhacker. But as technology advances, can they keep up in their current format? No, but there is a way forward, according to a new review paper from researchers at Hong Kong Polytechnic University, by further developing the electrolytes that allow for energy storage and discharge.
The team published their work in Energy Materials and Devices on September 18, 2023.
“Lithium batteries have revolutionized our modern life,” said first author Zhijie Wang, a Postdoctoral Research Fellow in the Department of Applied Physics and the Research Institute of Smart Energy at Hong Kong Polytechnic University. “The performance of lithium batteries — including energy density, lifespan, safety and more — is greatly determined by the recipe and microstructure of the electrolytes.”
Lithium batteries consist primarily of two current collectors, a negatively charged electrode, a positively charged electrode, an electrolyte and a separator. The electrolyte carries positively charged lithium ions from the positively charged electrode through the separator to the other electrode and negatively charged ions in the other direction. As the lithium ions move through the electrolyte, the anode collects free electrons, leading to a positive charge at one current collector. That positive current discharges to power a device, flowing through and returning to the battery as a negative current.
Electrolytes most commonly comprise a solution of lithium salt in a polar solvent, or a substance that uses a tiny electrical charge produced by the shape of its constituent molecules to dissolve the salts and free the lithium ions. In conventional electrolyte, lithium ions separate with the salt anions; in the (localized) high-concentration electrolytes developed in recent years, salt anions are solvated with Li ions, and they have unique functions in improving battery performance.
However, Wang said, a type called weakly solvating electrolytes first proposed for this use three years ago may offer a pathway to more robust batteries. In this type of electrolyte, the lithium ions are weakly coordinated with the solvent molecules, and they also coordinate with the negatively charged salt anions.
“Such a structure can be obtained in a relatively dilute salt concentration and without the use of non-solvating diluters,” Wang said. “This makes it different from conventional electrolytes and (localized) high-concentration electrolytes. It can improve the low-temperature, fast-charging, safety and cycling properties of lithium batteries, so it has attracted intensive research interest in recent years.”
In this study, Wang and corresponding author Biao Zhang examined recent research papers on weakly solvating electrolytes and concluded that basic design principles and future research directions were lacking. Zhang is an associate professor in the Department of Applied Physics and the Research Institute of Smart Energy at Hong Kong Polytechnic University.
“We found, from the current literature, that the key to constructing weakly solvating electrolytes is creating a balance between the interactions of lithium ions with solvent and anions,” Wang said. “We also determined that the design concepts of weakly solvating electrolytes can be extended to other batteries, such as systems made with sodium, potassium, magnesium or zinc.”
The researchers also recommended that future work in the field should focus on simplifying the synthesis procedures, improving yields and reducing costs of the electrolyte components.
“The goal of this paper is to promote the understanding of both research and industrial communities on the functions, design principles and recent research progress of the emerging weakly solvating electrolytes,” Wang said. “We hope our insights contribute to developing next-generation lithium batteries with robust properties and thus boost their share of the power source market, especially in electric vehicles.”
The Hong Kong Polytechnic University supported this work.
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About Energy Materials and Devices
Energy Materials and Devices is launched by Tsinghua University, published quarterly by Tsinghua University Press, aiming at being an international, single-blind peer-reviewed, open-access and interdisciplinary journal in the cutting-edge field of energy materials and devices. It focuses on the innovation research of the whole chain of basic research, technological innovation, achievement transformation and industrialization in the field of energy materials and devices, and publishes original, leading and forward-looking research results, including but not limited to the materials design, synthesis, integration, assembly and characterization of devices for energy storage and conversion etc.
About SciOpen
SciOpen is a professional open access resource for discovery of scientific and technical content published by the Tsinghua University Press and its publishing partners, providing the scholarly publishing community with innovative technology and market-leading capabilities. SciOpen provides end-to-end services across manuscript submission, peer review, content hosting, analytics, and identity management and expert advice to ensure each journal’s development by offering a range of options across all functions as Journal Layout, Production Services, Editorial Services, Marketing and Promotions, Online Functionality, etc. By digitalizing the publishing process, SciOpen widens the reach, deepens the impact, and accelerates the exchange of ideas.
JOURNAL
Energy Materials and Devices
ARTICLE TITLE
Weakly solvating electrolytes for next-generation lithium batteries: design principles and recent advances
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