"Opting for a PhD to improve one's quality of life is utterly misguided!" Liu Ping has dissuaded many students from pursuing this path.
During his 8 years teaching at the University of California, San Diego, Liu Ping has encountered numerous students who opt for doctoral studies not out of a genuine interest in research. Coming from a rural background himself, he understands the importance of a high income but is even more aware of the negative chain reactions that stem from pursuing scientific research without real passion.
For Liu Ping, conducting research should come with a sense of relaxation. It is this balance of strictness and flexibility that has seen his students make significant strides in their research capabilities, leading to numerous major breakthroughs by his team.
Recently, Liu Ping's team made groundbreaking progress by solving a longstanding issue with solid-state lithium-sulfur batteries, developing a self-healing version. Their findings were published in Nature.
Liu Ping
Cracking a Long-Standing Conundrum in Academia
The surge in new energy vehicles has fueled the robust growth of lithium batteries, from electric vehicles to grid storage; everyone is seeking breakthroughs in solid-state batteries, a success that would be tremendously impactful.
Solid-state lithium-sulfur batteries, a type of rechargeable battery made of solid electrolytes, lithium metal anodes, and sulfur cathodes, are poised to be the best alternatives to current lithium-ion batteries due to their higher energy density and lower cost.
"Sulfur is a simple material, devoid of complex chemical properties, and abundantly available worldwide, making it very cost-effective. The most significant advantage of solid-state lithium-sulfur batteries is that, theoretically, they can store at least twice the energy per kilogram compared to traditional lithium-ion batteries. In other words, they could double the driving range of electric vehicles without increasing the battery weight," Liu Ping, the corresponding author of the study, explained in an interview with China Science Daily.
Globally, numerous teams are researching solid-state lithium-sulfur batteries, but many have been stuck on the same issues for years, hindered by the inherent characteristics of sulfur.
Liu Ping started his research on solid-state lithium-sulfur batteries using polymer sulfides 15 years ago, a long and challenging journey with few breakthroughs.
Sulfur itself is non-conductive, and the sulfur cathode significantly expands during charging and discharging, with volume changes up to 80%, leading to structural damage and reduced contact with the solid electrolyte. These issues can impair the overall performance and lifespan of solid-state batteries and affect the stable transmission of charge. Without solving these two problems, the industry of all-solid-state lithium-sulfur batteries would be severely hampered.
This time, Liu's team used "iodine" to break these shackles, changing everything.
In chemistry, hardly anyone talks about thioiodide compounds. "However, we discovered that by inserting iodine molecules into the crystalline structure of sulfur, the conductivity of the cathode material increased by 11 orders of magnitude, making the conductivity of the crystal formed by sulfur and iodine a hundred billion times higher than that of crystals made solely of sulfur. We discovered a new compound and solved the issue of sulfur being an insulator," said Liu.
What's more, the melting point of sulfur is over 100 degrees Celsius, similar to iodine, but the melting point of this new compound is around 65 degrees Celsius, lower than the temperature of a cup of hot coffee.
This is quite intriguing.
Currently, the electrolytes used in lithium-ion batteries are liquid, so it doesn't matter if the cathode expands or contracts; the liquid can flow and maintain perfect contact with the solid. However, solid-state batteries must replace the liquid electrolyte with a solid one, which behaves entirely differently. When the interface between the cathode and the solid electrolyte is damaged due to expansion, gaps form, ions cannot be transferred to the electrode material, leading to increased resistance and battery damage.
This low-melting-point new compound can perfectly address this issue. When the battery operates normally, the temperature can sometimes reach over 60 degrees Celsius, easily melting the cathode and allowing the gaps to heal, thereby self-repairing the damaged interface.
To verify the effectiveness of this new cathode material, researchers constructed a test battery and subjected it to repeated charging and discharging cycles. The battery remained stable over 400 cycles while retaining 87% of its capacity.
The invention of the thioiodide cathode material has significantly advanced the research on solid-state lithium-sulfur batteries, clearing the main hurdle to its commercialization and potentially ushering in the era of high-energy-density solid-state batteries.
Currently, the team is attempting to further accelerate the development of solid-state lithium-sulfur battery technology by improving battery design and increasing battery size.
"Entering academia was never about income" Born in a rural area of Jiangsu province, without the influence of a scientific atmosphere or the legacy of research genes—his father only attended elementary school for a few years, and his mother never went to school—Liu Ping had only a vague impression of scientists from what his teachers said and what he read in books.
His genuine interest in research began when he struggled to gain admission to Fudan University. "Fudan has a strong research culture. Even as undergraduates, we often had the opportunity to experience the lab and attend various academic reports. I was well nurtured during my undergraduate years," Liu said.
Because he loved the atmosphere so much, Liu continued at Fudan to pursue his master's and doctorate degrees. In the chemistry department, battery was a niche specialty, but Liu was very interested in electrochemistry.
It turned out he had a good eye. In 1991, Sony launched the first commercial lithium-ion battery, sparking a research boom in lithium-ion batteries. His doctoral thesis focused on researching anode materials for lithium-ion batteries.
After earning his doctorate, Liu went to the National Renewable Energy Laboratory in the United States for postdoctoral work to broaden his exposure in the field of electrochemistry, focusing on thin-film lithium batteries and securing a permanent position.
Six years later, Liu resigned due to family reasons and moved to California, where he served as a senior scientist and manager at HRL Laboratories (an industrial laboratory jointly owned by General Motors and Boeing) in the Energy Technology department, managing a research department with an annual budget of over $5 million, dedicated to innovating various energy storage technologies: rechargeable batteries, fuel cells, hydrogen storage, and electrochemical capacitors.
Nine years later, he joined the Advanced Research Projects Agency-Energy (ARPA-E) at the U.S. Department of Energy as a program director, initiating and leading research projects on electric vehicle energy storage and thermal management technologies to improve energy efficiency, overseeing a total investment of over $100 million.
For over two decades in his career, although he changed jobs several times, his focus never strayed from energy materials and batteries.
It wasn't until 2016, nearing the age of 50, that Liu joined the University of California, San Diego, as an associate professor, becoming a "latecomer" to teaching. However, the university highly recognized his contributions to research, awarding him a tenured professorship. After circling back to academia, Liu Ping still carries a fervent passion. Despite stints in industry and government, he found his heart lies in academia, particularly within the vibrant atmosphere of university campuses.
"Entering academia was never about income for me. The salaries from my previous jobs were much higher than what I get as a university professor. But I simply love teaching, it's a gratifying experience. And the university is such a dynamic place, always with new students and new ideas," Liu Ping expressed.
For Liu Ping, getting published in Nature isn't the end goal; rather, his research aims to directly impact production and daily life. In 2021, Liu Ping co-founded Tyfast, aiming to commercialize batteries. Prior to this, his team published research on lithium-ion battery anode materials in Nature in September 2020. Tyfast is now driving its industrialization forward. And this recent study on solid-state lithium-sulfur batteries will open up even more avenues for development.
"As a teacher, the most important work isn't papers, it's the students," influenced deeply by his mentor during his PhD studies, Liu Ping places great importance on fostering students' independent capabilities.
His mentor was Wu Haoqing, an academician of the Chinese Academy of Sciences and a professor in the Department of Chemistry at Fudan University, who is also one of the pioneers in the field of electrochemistry in China.
"Wu never assigned us menial tasks. After honing our fundamentals, he simply let us loose. With ample resources at our disposal, we could pursue whatever we wished. More importantly, he was ahead of his time in advocating for research on carbon anode materials. This establishment of a major direction had a significant impact on my subsequent research career," Liu Ping remarked.
Zhou Jianbin (left) and Liu Ping (right) received excellent mentorship, with Liu Ping placing great emphasis on nurturing students. Besides requiring students to have independent capabilities, he also emphasizes the importance of having an interest in the research topics.
Liu Ping often tells his students, "I'm not doing work for the professor; the articles we write are ours." This viewpoint opposes the idea of treating research as a task. Students should have control over their research direction. Learning to conduct research independently is the "key" to entering academia and lays the foundation for becoming competent researchers in the future.
The first author of this Nature paper, Zhou Jianbin, is a student with strong autonomy. Liu Ping jokingly refers to him as the "perpetual motion machine" of their team. Although his mentor never demanded anything specific from him, Zhou could work on experiments tirelessly, displaying remarkable perseverance. "I admire his enthusiasm for research. But excessive effort can be counterproductive in research. Research requires a sense of relaxation, so I allow Zhou to take breaks when necessary. It's okay for him to stop working for a few days and relax."
Coming from a humble background, Liu Ping can empathize with the difficulties students face in their lives and work. Many students in his lab have families, each facing different challenges. He understands that when a student's mental state is not good, it may not necessarily be related to research. "I have advised students with poor mental states to completely stop their work for the time being. They can even take a week off, or if it's serious, they can take a semester off. Some difficulties cannot be overcome by simply soldiering on."
Liu Ping has firsthand experience in handling relationships with students. "During your PhD, avoid internal conflicts and don't hesitate to trouble your advisor. If you don't voice your ideas, how can your professor know? I always tell every student who joins my team that even if your research yields negative results, you must inform me. What you consider bad news might be good news to me. Students who actively seek help from me when encountering difficulties will always find a solution in the end."
Liu Ping believes that mentors have a significant responsibility. He measures his effectiveness with an unusual criterion. "Whether students go into industry or academia after graduation, as long as they still have a passion for research, it means the teacher has fulfilled their duty. If a student decides they never want to engage in research again after graduating from your lab, then we have failed."
"As a teacher, the most important achievement is not papers but students!"
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