A paper published in the journal ACS Applied Energy Materials has reported the development of a low-cost inorganic room-temperature molten salt electrolyte with an improved electrochemical window for the design of safer, more efficient lithium-ion batteries. The research has been conducted by a team from the Huazhong University of Science and Technology in China.
Study: Room Temperature Molten Salt”-Based Polymer Electrolyte Enabling a High-Rate and High-Thermal Stability Hybrid Li/Na-Ion Battery Image Credit: Immersion Imagery/Shutterstock.com
Addressing the Challenges of Anthropogenic Climate Change
Conventional fossil fuel-based energy generation emits vast amounts of harmful greenhouse gases, which have been recognized as the main driver of increasing global temperatures. Extreme weather events, more intense wildfires, increased acidification of the world’s oceans, and the loss of biodiversity have all been linked to anthropogenic climate change.
Electrification of society and industry has been highlighted as a central strategy to address the issues with human industrial activities and keep global temperature rises within internationally agreed limits. Renewable energy generation technologies such as photovoltaic solar cells, wind, and hydroelectric power, biofuels, carbon capture and storage, and clean, sustainable energy storage technologies have been developed in recent decades.
Improving Lithium-Ion Batteries
Lithium-ion batteries are a key technology in the drive toward full electrification. These devices have the benefits of long cycle lives and high energy density, but serious safety issues persist with lithium-ion batteries that use nonaqueous electrolytes. For this reason, aqueous electrolytes have been extensively explored in recent years, especially in large-scale stationary energy storage applications.
Aqueous electrolytes are hindered by the narrow electrochemical window, which limits their energy density. To overcome this issue, water-in-salt electrolytes were developed in 2015 by Suo et al. The electrochemical window of these electrolytes reached ~3.0 V. However, the process developed in this research involved the use of lithium salts, which can be corrosive and expensive, which presents serious issues with safety and cost.
Developing RTMS Electrolytes
The authors conducted previous studies on the development of a room-temperature molten salt (RTMS) electrolyte, which demonstrated favorable results. This low-cost electrolyte was composed of NaNO3 and LiNO3.3H2O and the team incorporated this novel RTMS electrolyte in a hybrid lithium-sodium ion battery.
The research produced an aqueous electrolyte with an expanded electrochemical window of ~3.1 V. This was mainly caused by decreased water activity. Water hydrolysis was suppressed in the novel electrolyte; however, solidification and ion mobility loss were caused by the loss of crystalline water during the electrochemical process. This also occurred at elevated temperatures due to increased kinetic energy. The release of lithium ions was promoted by strong bonding between NO3- and Na+ ions.
The new study has improved on the author’s previous work with RMTS electrolytes. The authors have incorporated a hydrophilic polymer electrolyte based on RMTS. By incorporating this element, crystalline water loss and hydrogen evolution are suppressed. Previous works have demonstrated the benefits of polymer-water interactions in hydrogels, with reduced dynamics of water molecules around polymer chains in these materials. Water molecules are tightly bound to the polymer matrix.
Electron beam irradiation was used to provide rapid polymerization in the electrolyte material. Polymerization is initiated in this process by the interaction of the high-energy electron irradiation with substances within the material. Molecules are ionized and excited by the electron beam. Electron beam polymerization provides rapid cross-linking without the need for initiators, which gives it significant advantages over processes such as UV irradiation-polymerization.
RTMS was combined with a methacrylic polymer to create the novel flexible RTMS-polymer electrolyte. PEGDMA was selected as the monomer for the polymer matrix. BEMA was used as the cross-linking agent, and HMPP was used as a photoinitiator. Hydrophilic groups in the material immobilized and stabilized the water molecules, thereby reducing the water activity and consequently expanding the novel electrolyte’s electrochemical window.
Aside from the widened electrochemical window at room temperature, the electrolyte also possessed a high ionic conductivity. The electrochemical performance of the novel electrolyte was highly favorable, with a high capacity and an 85% capacity retention over 100 cycles.
Thermal stability was also improved by the incorporation of the polymer material, enhancing its high-temperature performance, with high capacity and capacity retention of 92% over one hundred cycles at 60 °C. The electrolyte possessed 100% coulombic efficiency.
The study has reported the development of a superior, low-cost hybrid RTMS-polymer electrolyte with improved energy density, an outstanding rate, and high performance at elevated temperatures. This electrolyte is intrinsically safer compared with conventional aqueous lithium-ion batteries. The research has benefits for manufacturing reliable, stable, safe, and efficient wearable devices and large-scale stationary energy storage applications.
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Wang, Z et al. (2022) “Room Temperature Molten Salt”-Based Polymer Electrolyte Enabling a High-Rate and High-Thermal Stability Hybrid Li/Na-Ion Battery ACS Applied Energy Materials [online] pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsaem.2c00483