Polymer Engineering for Improved Performance in Li-Ion Batteries

Lithium (Li) ion batteries (LIB) are an important energy storage technology, especially for the development of electric vehicles. However, two key challenges remain: (i) increasing the energy density and (ii) reducing the cost. To enable the ubiquitous utilization of electric vehicles, it is necessary to move forward to active materials that have increased Li storage capability. Silicon (Si) has a theoretical specific charge capacity ten times higher than the graphite anodes used in commercial cells. However, these materials experience extreme expansion and contraction during cycling, which leads to rapid deterioration of the electrode (e.g. cracks, electrical isolation of particles, pulverization, etc.) and dramatically reduces the battery lifetime. In recent work, our group has demonstrated improved cycling performance of Si negative electrodes for lithium ion (Li-ion) batteries though the use of a self-healing polymer (SHP) binder. Our work was successful in utilizing inexpensive micron-sized particles to produce cells that were stable for up to 500 cycles.

To further understand the materials properties that contributed to the SHP binder’s success, we investigated the effects of varying the SHP’s rheological properties on its performance in the Si microparticle electrodes. Crosslinking of the supramolecular polymer was systematically varied using a combination of di-functional and tri-functional starting materials. Frequency sweeps and stress relaxation experiments were used to examine the rheology of the synthesized polymers and quantitative relaxation times were extracted using spring and dashpot modeling. Cell cycling performance was correlated to this data and polymers with a relaxation time on the order of ~0.1s were found to give optimal cycling stability. Using this information, it is now possible to rationally design new polymer binders with these mechanical properties for further enhancement of Si cycling stability and eventual commercial production of Si negative electrodes.

To examine the effects of static crosslinks as compared to the dynamic hydrogen bonding of the SHP, a covalent crosslinker was used create a self-healing elastomer that also helped to prevent capacity fade in Si electrodes. Using this elastomer, we were able to increase the cycling stability of a carbon/Si foam electrode, and, along with my developed elastomer coating, the electrode became stretchable up to 88%. This is the first demonstration of a low potential, high capacity electrode for stretchable LIBs. This technique can easily be extended to other electrode systems using different polymer materials for further improvements leading to the development of next generation stretchable energy storage. Additionally, the developed ‘neat’ polymerization conditions and inexpensive commercially available starting materials allow for easy and cost effective scale up for the materials preparation. We are currently producing these materials on the 10s of grams scale and collaborating with industry to move these self-healing Si electrodes onto a production scale.