Silicon anodes can deliver 3~5-times higher capacities compared with current graphite anodes in lithium-ion batteries. However, their volume expands enormously during each charge-discharge cycle, leading to fractures of the electrode particles or delamination of the electrode film, both resulting in rapid capacity decay. Typical charge-discharge numbers for microparticle-size Si anodes are less than 100.
Numerous approaches have been tried to address this problem. Now, a KAIST research team led by Professors Jang Wook Choi and Ali Coskun have integrated molecular pulleys, called polyrotaxanes, into the electrode binder to enable stable cycle life for silicon microparticle anodes at commercial-level areal capacities. A paper on their work appears in the journal Science.
In a polyrotaxane, rings are threaded into a polymer backbone and can freely move along the backbone. The free moving of the rings in polyrotaxanes can follow the volume changes of the silicon particles.
The rings’ sliding motion can efficiently hold Si particles without disintegration during their continuous volume change. Even pulverized silicon particles can remain coalesced because of the high elasticity of the polyrotaxane binder.
The functionality of the new binders is in sharp contrast with existing binders (usually simple linear polymers) with limited elasticity since existing binders are not capable of holding pulverized particles firmly. Previous binders allowed pulverized particles to scatter, and the silicon electrode thus degrades and loses its capacity.
The authors said, “This is a good example of showing the importance of fundamental research. Polyrotaxane received Nobel Prize last year, based on the concept called ‘mechanical bond.’ The ‘mechanical bond’ is a newly identified concept, and can be added to classical chemical bonds in chemistry, such as covalent, ionic, coordination, and metallic bonds. The long fundamental study is now expanding in an unexpected direction that addresses longstanding challenges in battery technology.”
The authors are currently collaborating with a major battery maker to get the molecular pulleys integrated into real battery products.
Mechanical bonds have come to the rescue for the first time in an energy storage context. KAIST team’s ingenious use of mechanical bonds in slide-ring polyrotaxanes—based on polyethylene glycol threaded with functionalized alpha-cyclodextrin rings—marks a breakthrough in the performance of marketable lithium-ion batteries. This important technological advance provides yet more evidence that when pulley-like polymers carrying mechanical bonds displace conventional materials based on chemical bonds alone, the unique influence of this physical bond on the properties of materials and the performance of devices can be profound and game-changing.—Sir Fraser Stoddart of Northwestern University, 2016 Noble Laureate in Chemistry
The Nobel Prize in Chemistry 2016 was awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Bernard L. Feringa for their design and production of molecular machines. They developed molecules with controllable movements, which can perform a task when energy is added.
The first step towards a molecular machine was taken by Jean-Pierre Sauvage in 1983, when he succeeded in linking two ring-shaped molecules together to form a chain, called a catenane. Normally, molecules are joined by strong covalent bonds in which the atoms share electrons, but in the chain they were instead linked by a freer mechanical bond. For a machine to be able to perform a task it must consist of parts that can move relative to each other. The two interlocked rings fulfilled exactly this requirement.
The second step was taken by Fraser Stoddart in 1991, when he developed a rotaxane. He threaded a molecular ring onto a thin molecular axle and demonstrated that the ring was able to move along the axle. Among his developments based on rotaxanes are a molecular lift, a molecular muscle and a molecule-based computer chip.
Bernard Feringa was the first person to develop a molecular motor; in 1999 he got a molecular rotor blade to spin continually in the same direction. Using molecular motors, he has rotated a glass cylinder that is 10,000 times bigger than the motor and also designed a nanocar.
Sunghun Choi, Tae-Woo Kwon, Ali Coskun, Jang Wook Choi (2017) “Highly elastic binders integrating polyrotaxanes for silicon microparticle anodes in lithium ion batteries” Science Vol. 357, Issue 6348, pp. 279-283 doi: 10.1126/science.aal4373