A Conversation with Yuegang Zhang

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If everything works well, a lithium–sulfur battery can store five times as much energy by mass as current lithium-ion batteries. Because sulfur is lightweight, these batteries have very high potential to be used in applications such as drones. And if we increase the lifetime of these batteries to be comparable to lithium-ion batteries, they can also be used in electric vehicles. Let’s assume we can keep the size of the battery about the same, but we can double or triple the energy stored. That will extend the mileage of electric vehicles significantly.

For the sulfur cathode, three difficulties prevent its application. One is that sulfur is not conductive; it’s insulating. So we have to mix sulfur with conductive carbon. Another thing is that sulfur expands as it takes up lithium ions when the battery discharges. The volume can expand a lot—up to 76%. That requires you to design a cathode that’s very mechanically stable so it won’t pulverize over cycles of charging and discharging. The third challenge is the intrinsic difficulty of the lithium–sulfur chemistry. When lithium ions reach the sulfur cathode, they react to form unwanted compounds like lithium polysulfide. These intermediate reaction products can easily dissolve into most of the organic solvents used in electrolytes, corroding the anode over time so that the battery can store less and less energy.

Wanting to commercialize lithium–sulfur batteries was one of the most important motivations for me to move to China in 2012. I first moved to the Chinese Academy of Sciences. In 2016 I founded my company, Monta Vista Energy Technology in Suzhou. And this May I officially joined Tsinghua University in Beijing. Tsinghua has a good atmosphere for commercialization; the university encourages professors to transfer their technologies to market.

There are still challenges. Now that we have a good cathode material, we have found a problem on the lithium metal anode side: the pulverization of lithium. When you discharge the battery cell, most of the lithium atoms move to the cathode side. So on the anode side, there’s not much lithium left. When you charge the cell, the lithium is supposed to come back to the anode. But this process is not uniform. The ions come back but in some places they don’t redeposit on the anode uniformly, so after 30 or 40 cycles the anode is destroyed. We are still working on solutions to this problem.