CUHK Research: Changing the world

been prohibited by the crossover of the tiny molecules of sulphur. The sensitivity of the Nafion ion-selective membrane is insufficient to prevent severe crossover of molecules and self-discharge, leading to a rapid capacity decay and poor lifetime. To address the problem, the team added several layers to the commercial membrane, not only to prevent the crossover, but also to complete the chemical reaction before the crossover. One of the layers added is carbon bounded by a hydrophobic polymer, which helps to absorb sulphur and blockwater migration. It took the team three years to invent the charge-reinforced ion-selective (CRIS) membrane for polysulfide-based flow batteries. Their breakthrough findings were published in Nature Energy in 2021, the first time a highly stable polysulfide flow battery was demonstrated. With the patented CRIS membrane, the polysulfide-iodide flow batteries have revealed an ultralow capacity decay rate (0.005% per day) over 2.9 months, or 1,200 cycles. It has a calendar lifetime with over 2,000 hours cycling, in comparison to only 160 hours cycling with the commercial membrane. On fully charged, the battery with CRIS can run for 15 consecutive hours. Luquos Energy, a startup set up by Professor Lu in 2020 with the support of CUHK, is developing a prototype with a larger CRIS membrane aiming for commercialisation, expanding the size from 10cm x 10cm to 10 or 20 times larger. The team plans to create a five-kilowatt prototype by the end of 2023. If t he CR I S membrane i s successf u l , Professor Lu believes it can be adapted to the vanadium flow battery, which also has a crossover issue. Turning the Li-ion battery inherently safe The lithium-ion battery, though extensively used, has posed serious safety concerns with its flammable electrolytes. While replacing the electrolytes with an aqueous one cou l d be a so l u t i on , t he vo l t age window is limited by the instability of water (electrolysis will occur and break down water into hydrogen and oxygen when voltage is over 1.23 volts). Professor Lu’s team has discovered a way to improve the aqueous Li-ion battery. In 2020, her team introduced a novel molecular crowding electrolyte for aqueous Li-ion battery . It used the water-miscible polyethylene glycol – commonly used in skin cream and food additives – to decrease water activity and achieving a wider voltage window. The material is 30 to 100 times cheaper than toxic battery salts commonly used in Li-ion batteries. The new electrolyte has proved to be non- flammable and with an expanded voltage window of 3.2 volts. It enables the use of many electrode materials that cannot be used in the conventional aqueous electrolytes, providing a new platform for designing safe batteries with large voltage window and high stability. The team is working hard to raise the energy density of the aqueous battery by about 50% to match that of the non-aqueous one. “I hope this battery and chemistry will allow us to use massive renewable energy without hesitation. This is my dream,” she says. Our grand challenge is tomake energystorage technologyboth safe andenergy-dense so that it canbewidelyused to store renewable energy. 63