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Three Year's Perseverance Spells Success

Ivan Song, School of Engineering
23, 2022

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School of Engineering

In 2018, Rui Lin, a new Ph.D. student who entered Westlake University and became a member of Prof. Jianhui Wang's group. He cannot imagine that in the next three years or more he would devote himself to working on a small training project.

Around the same time, Prof. Wang left The University of Tokyo and joined Westlake University in hopes to advance his research in energy storage. For Prof. Wang, Ni-MH Batteries, Hydrogen Storage Materials, and Lithium Batteries are all his fields of research. But which would be the next breakthrough was beyond his prediction.

Just like Prof. Wang's motto, "Scientific research — the coexistence of belief and questioning as well as occasionality and inevitability — shows appalling challenges but also great opportunities! Let's work for new possibilities!" There is a research story about them.

Why Water-Based Battery

The importance and ubiquity of batteries are self-evident. From the deep-sea exploration to our daily life, the usage of batteries is already a part of our life.

From the world's first "volt stack" to the current battery we use, the technology of batteries has gone through tremendous changes in the past 200 years. In recent years, with the global goals of carbon peaking and carbon neutrality being determined, the development of large-scale efficient energy storage technologies for renewable energies and the popularization of electric vehicles has become an inevitable trend. Safe, environmental friendly, high-energy-density, and low-cost batteries are becoming urgent in demand, thus, stimulating scientists to explore a new generation of batteries.

Taking the most common lithium-ion battery as an example, it is lightweight and high-energy-density, not to mention its compact size and long service life. However, one of its major shortcomings is its high reactivity. Lithium reacts instantly in the presence of trace amount of water. Lithium-ion batteries can only be produced in a rigorously controlled drying room and work in a narrow temperature range between -20°C to 50°C. Once some internal part of the battery is overheated, a series of exothermic reactions can be triggered, potentially causing fire and even explosion. According to partial statistics, around 3,000 electric vehicles were reported fire-related incidents nationwide in 2021. At the same time, Electronic Bike (E-Bike) fire-related incidents soar to a whopping 18,000. Such related incidents have caused large numbers of injuries and property losses.

In the face of such unsolved safety issues, scientists have turned their attention to aqueous electrolytes. Compared to organic electrolytes currently used in lithium-ion batteries, the "safety factor" of aqueous electrolytes is much higher. It is not difficult to understand that aqueous solutions are not flammable, which greatly reduces the risk of the battery fire and explosion.

High safety and low cost have always been the eye-catching advantages of aqueous electrolytes. But its bottleneck is equally prominent — the narrow voltage window limits the energy density of the battery. For example, the voltage of conventional aqueous batteries, such as lead-acid batteries and nickel-cadmium batteries, is 1-2 V, and the energy density is only about 30-50 Wh/kg. This is much lower compared to that of nonaqueous lithium-ion batteries (3-4 V, 150-250 Wh/kg). With the energy density of water-based batteries reaching only 1/3 of the capabilities of lithium-ion batteries, there is near zero competitive advantage. In order to overcome the bottleneck of aqueous electrolytes, Prof. Wang and Rui Ling stood up to the challenge and strove themselves into it. After three years of painstaking work, they finally had a breakthrough.

In January 2022, an article entitled "Asymmetric donor-acceptor molecule regulated core-shell-solvation electrolyte for high-voltage aqueous batteries" cooperated by the teams of Prof. Jianhui Wang and Prof. Shi Liu from the Westlake University, was published on Joule as the Cover, with Rui Lin as the first author.


The New Recipe's Secret Ingredient: Methyl Urea

In the new aqueous electrolyte formula, methyl urea molecules were added. Methyl urea is a low-cost chemical substance mainly used in organic synthesis and pharmaceutical industries. More importantly, it is a non-flammable and low-toxic substance, making it a near-perfect material for the aqueous electrolyte.

Through various in-situ/ex-situ characterizations and repeated experiments, the addition of methyl urea to the electrolyte can effectively inhibit side effects such as oxidation/reduction of water under high/low potential conditions. Under the same test conditions, the research team compared with the most representative nine reported high-voltage aqueous electrolytes. The results show that methyl urea aqueous electrolyte has the widest electrochemical stability window of 4.5 V, which is two times as large of the conventional aqueous electrolyte. Based on this result, the energy density of aqueous battery can be greatly improved. It is expected to be developed as a new type of battery that is comparable to a nonaqueous lithium-ion battery with increased safety and reduction in cost.

Material structure determines its property. Prof. Wang and Rui Lin from the School of Engineering were not satisfied with a simple progress just on properties. They wanted to dig deeper and "know why". They found Prof. Shi Liu's team from the School of Science for cooperation, applying the molecular level simulated calculation to gain a deep understanding of the material composition-structure-property relationship. Based on the analysis of a large number of data and the observation at different spatial scales, the team was surprised to discover a unique solution structure that is significantly different from the ordinary aqueous solutions.

On the cover of the February issue of Joule, a drop-shaped pattern is presented, in which the yellow and green "salt core" is tightly wrapped by the blue and purple "organic/aqueous shell", forming a unique solution structure called “core-shell". Prof. Wang said that this is the "most common" structure in solid-state nanomaterials but finding such a similar structure in liquids was unexpected. This research provides a new insight for manipulating the solution structure, designing new electrolytes, and subsequent research on various possible applications.

Three Years' Perseverance

In January 2022, Rui Lin finally received the message from the editorial office of Joule that the paper was accepted. This is his first paper since entering Westlake University. After three years of research on aqueous electrolytes, his research is finally recognized by his peers.

At first, this was just a small subject for "training". After getting some preliminary results, Rui Lin completed the first draft in early 2020. However, Prof. Wang keenly noticed that many preconditions (such as the use of specific current collectors, various pretreatments on the electrode surface, excess lithium sources, and flooded electrolyte dosage) were applied in prior work of aqueous battery, which deviates from the practical requirements. In addition, for the stable operation of the battery, flammable organic materials are introduced, which ignores the safety—the original intention of using aqueous electrolytes.

Thus, a small project became something big. With the encouragement of his mentor, Rui Lin struggled for another two years and finally came up with a simple and effective strategy to obtain an aqueous electrolyte with a wide electrochemical window. This electrolyte is low-cost and safe, boosting the development of safe, inexpensive, and high-energy-density aqueous batteries.

The moment he saw the cover of Joule, the only thought that was racing through Rui Lin’s mind was his perseverance finally paid off. He could have chosen to submit his preliminary results to some journals that are easy to publish in the first year, or he could have chosen to change the direction of his research to topics with less demanding and greater opportunities, which means more possibilities. However, he believed that he should stick it out and do a deeper and more instructive research since he has chosen this subject.

"If publishing articles is the only purpose of doing scientific research, then the research itself will lose its significance", Rui Lin said.

Prof. Wang and His Team

In the future, Rui Lin will continue his work in the field of aqueous electrolytes. He is looking forward to finding low-cost, green, and safe electrolytes to put into practical use in batteries.

Restraining inner anxiety, thinking calmly, believing in the meaning of persistence, questioning authoritative answers, the scientific research results are between occasionality and inevitability.