"If you're pursuing a Ph.D. to improve your quality of life, then you're completely mistaken!" Liu Ping has discouraged many students.
During his eight years teaching at the University of California, San Diego, Liu Ping has encountered numerous students who aren't passionate about research opting for a Ph.D. Coming from a rural background himself, he understands the importance of high income, but he also knows that research driven solely by financial motives can lead to a series of negative consequences.
Research requires a sense of relaxation, something Liu Ping highly values. It's under his supportive yet demanding guidance that students' research abilities have made significant leaps, and his team has achieved many breakthroughs in succession.
Recently, Liu Ping's team has made a groundbreaking advancement by solving a long-standing issue in solid-state lithium-sulfur batteries, developing self-repairing solid-state lithium-sulfur batteries. Their research findings were published in Nature.
Liu Ping Solves Long-standing Puzzle Plaguing Academia
The surge in popularity of new energy vehicles has propelled the vigorous development of lithium batteries. From electric cars to grid energy storage, everyone is seeking breakthroughs in solid-state batteries, knowing that success could have a tremendous impact.
Solid-state lithium-sulfur batteries, composed of a solid electrolyte, a lithium metal negative electrode, and a sulfur positive electrode, are poised to become the best alternative to current lithium-ion batteries. They offer higher energy density and lower costs.
"Sulfur is a simple material with no complex chemical properties, and global sulfur resources are abundant, making its cost very low. The biggest advantage of solid-state lithium-sulfur batteries is that, theoretically, they can store at least twice as much energy per kilogram as traditional lithium-ion batteries. In other words, they can double the range of electric vehicles without increasing the weight of the battery pack," explained Liu Ping, the corresponding author of the paper, in an interview with the Chinese Science Bulletin.
Teams worldwide are researching solid-state lithium-sulfur batteries, but many have been stymied by the inherent characteristics of sulfur for years.
Fifteen years ago, Liu Ping began researching solid-state lithium-sulfur batteries using polymer sulfides. It was a long and difficult journey with few results.
Because sulfur itself is not conductive, and the sulfur positive electrode significantly expands during charging and discharging, with volume changes of up to 80%, leading to structural damage and reduced contact with the solid electrolyte. These issues compromise the overall performance and lifespan of solid-state batteries, affecting stable charge transfer. Without solving these two problems, the solid-state lithium-sulfur battery industry faces significant challenges.
This time, Liu Ping's team broke through these barriers with "iodine," changing everything completely.
There's almost no mention of sulfur iodide compounds in chemistry. "But we found that by inserting iodine molecules into the crystal sulfur structure, the conductivity of the positive electrode material increased by 11 orders of magnitude. The crystal made of sulfur and iodine conducts electricity a billion times better than one made only of sulfur. We discovered a new compound and solved the problem of sulfur being an insulator," Liu Ping said.
More importantly, sulfur has a melting point of over 100 degrees Celsius, similar to iodine. However, this new compound's melting point is around 65 degrees Celsius, lower than the temperature of a cup of hot coffee.
This is quite intriguing.
Currently, lithium-ion batteries use liquid electrolytes, so it's okay if the positive electrode expands or contracts; the liquid flows, maintaining perfect contact with the solid. But solid-state batteries must replace liquid electrolytes with solid ones, completely changing their properties. When the interface between the positive electrode and solid electrolyte is damaged due to expansion, gaps are created, preventing ion transfer to the electrode material, leading to increased resistance and battery damage.
This low-melting-point compound happens to solve this problem. When the battery operates normally, temperatures can sometimes reach over 60 degrees Celsius, causing the positive electrode to melt easily, thus allowing gaps to heal and repairing damaged interfaces automatically.
To verify the effectiveness of this new positive electrode material, researchers constructed a test battery and subjected it to repeated charge and discharge cycles. The battery remained stable over 400 cycles while retaining 87% of its capacity.
The invention of sulfur iodide positive electrode material has propelled research on solid-state lithium-sulfur batteries forward, clearing the main obstacles to commercialization and potentially ushering in the rise of high-energy-density solid-state battery industry.
Currently, the team is attempting to further advance solid-state lithium-sulfur battery technology by improving battery structure design and increasing battery volume.
"In academia, it's never been about income."
Born in rural Jiangsu, Liu Ping didn't grow up in a research atmosphere, nor did he inherit a research gene. His father only attended primary school for a few years, and his mother never went to school. But he formed a vague impression of scientists from his teachers' stories and the content of books.
His real interest in research began after he struggled to get into Fudan University. "Fudan has a strong research atmosphere, and undergraduate students often have the opportunity to experience laboratory work, as well as various academic presentations. I received excellent guidance during my undergraduate studies," said Liu Ping.
Because he liked the atmosphere there, Liu Ping stayed at Fudan for his master's and doctoral degrees. In the chemistry department, battery research was a niche field, but Liu Ping was very interested in electrochemistry.
His choice proved to be prescient. In 1991, Sony released the first commercial lithium-ion battery, sparking a surge in lithium-ion battery research. His doctoral thesis topic was studying negative electrode materials for lithium-ion batteries.
After obtaining his doctoral degree, Liu Ping went to work as a postdoctoral fellow at the U.S. National Renewable Energy Laboratory to broaden his knowledge in the field of electrochemistry, focusing on thin-film lithium batteries, and eventually secured a permanent position.
After six years, Liu Ping resigned due to family reasons and moved to California, where he became a senior scientist and manager in the Energy Technology Division at HRL Laboratories (an industrial laboratory jointly owned by General Motors and Boeing), overseeing a research department with an annual budget exceeding $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. The total investment he was responsible for exceeded one billion dollars.
In over 20 years of his career, his experience has been rich and varied. Although he changed jobs frequently, his research focus never strayed from energy materials or batteries.
It wasn't until 2016, when Liu Ping was nearly 50 years old, that he joined the University of California, San Diego, as an associate professor, becoming a "latecomer" to the teaching profession. However, the school highly valued his research achievements and granted him tenure. After circling back to academia, Liu Ping's passion still burns bright. Despite stints in industry and government, he finds himself drawn back to the academic realm, particularly the vibrant atmosphere of university campuses.
"Entering academia was never about the paycheck for me. The salaries from my previous jobs were much higher than what I earn as a university professor. But I simply love teaching; it's a gratifying experience. And universities are incredibly dynamic places, always teeming with new students and fresh ideas," says Liu Ping.
For Liu Ping, getting published in Nature isn't the ultimate goal. His research endeavors aim to have a direct impact on production and daily life.
In 2021, Liu Ping co-founded Tyfast with others, with the primary goal of commercializing battery technology. Prior to this, Liu Ping's team published research on lithium-ion battery anode materials in Nature in September 2020. Tyfast is now driving towards industrialization based on this research, with the study on solid-state lithium-sulfur batteries poised for further development.
"As a teacher, your most important work isn't papers; it's your students."
During his PhD studies, Liu Ping was deeply influenced by his advisor, Professor Wu Haoqing, a member 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 always refrained from assigning us menial tasks. After laying a solid foundation, he entrusted us to work independently. With ample resources at our disposal, we were free to pursue our interests. More importantly, he was ahead of his time in advocating for research on carbon anode materials. This visionary direction had a significant impact on my subsequent research career," says Liu Ping.
Zhou Jianbin (left) and Liu Ping (right) received excellent mentorship, and Liu Ping also highly values 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 just working for the professor, and the articles we write are not just for the professor." He fears students treating research as a task to be executed. Students should have control over their research direction, and learning to independently conduct research is the "key" to entering academia, laying the groundwork for becoming independent 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 the mentor never demanded anything from him, Zhou could work day and night on experiments with extraordinary perseverance. "I admire his enthusiasm for research. But too much effort can be detrimental to research. Research requires a sense of relaxation, so I allow Jianbin to take breaks and rest when needed. It's okay to take a few days off."
Coming from a humble background, Liu Ping finds it easier to empathize with the difficulties students encounter in their lives and work. Many students have families, and everyone has their own private matters, so their difficulties vary. He understands that when a student's mental state is not good, it may not necessarily be related to research. "I've advised students with poor mental states to completely stop their current work for a while, even take a week off, or if it's severe, take a semester off. Some difficulties cannot be overcome by sheer force."
Regarding the relationship between students and mentors, Liu Ping has firsthand experience. "When pursuing a Ph.D., never engage in internal conflicts, and don't be afraid to trouble your advisor. If you have any ideas, speak up, how would the professor know otherwise? I tell every student who joins my team that if they encounter negative results in their research, they must inform me. Maybe what you consider bad news could be seen as good news to me. Students who actively seek help when facing difficulties are the ones who eventually succeed."
Liu Ping believes that mentors have a significant responsibility, and he measures whether he is competent with an unusual standard. "Regardless of whether students go into industry or academia after graduation, as long as they still have enthusiasm for research, it means the teacher has fulfilled their responsibility. If a student graduates from your supervision and never wants to do research again in their lifetime, then we have failed."
"As a teacher, the most important achievement is not the papers but the students!"
Reference links: Link to Nature article 1 Link to Nature article 2 All images in the article are provided by the interviewees.
