Electronic conductivity, high theoretical capacity, and mechanical durability are among the useful characteristics of graphene-based tin dioxide electrodes for use in flexible batteries. Writing in the journal Energy Storage Materials, a team from China discusses how this composite material provides the requisite mechanical integrity and electrochemical performance. An understanding of these factors in designing flexible electrodes has not until now been available and will help guide the development of this novel technology for wearable and other devices where flexibility would be a boon. [Liu, X. et al., Energy Storage Mater. (2020); DOI: 10.1016/j.ensm.2020.09.012]

To this end, the team has carried out the reported mechanical simulation of lithium intercalation induced stress in a pristine tin oxide anode and an anode modified with a soft, double coating of graphene and also a jacket made from hard, amorphous carbon. The simulations show that a double coating is far more effective in reducing the charging-induced stresses and avoiding mechanical failure. Pristine tin oxide and anodes coated with amorphous carbon are much more susceptible to damage.

Based on the simulations, the team next created a double-jacketed flexible tin oxide composite electrode. The core-shelled carbon on tin oxide anode is embedded in graphene nanosheet. The graphene was shown experimentally to suppress crack formation, without it cracks form readily in the unprotected and the hard-protected electrodes. The cycling between lithiation and delithiation induces stress in batteries, which leads to mechanical failure so side-stepping that issue to some degree with alternative composites for some components is a key area of research into rechargeable batteries. There is also the additional advantage of utilizing oxides of metals that are far more natural abundant and readily available than lithium.

The team adds that their jacketed anode, as they anticipated shows a high specific capacity of 836 milliamp-hours per gram at 100 milliamps per gram charging. It also has an excellent rate capability of 506 milliamp-hours per gram at 2 Amps per gram. Moreover, its stability under repeated charge-discharge cycling is very high.

They suggest that their simulations neatly pointed the way to a solution to the problem of protecting electrodes without requiring rare elements or materials. They explain that the unique hierarchical structure of a hard carbon shell and soft graphene sheet around the tin oxide electrode gives them all the properties such an electrode needs to cope with repeated lithium insertion and extraction. "This work suggests a strategy to fabricate flexible graphene-based composite paper electrodes with various large-volume-change materials which could be used to improve the electrochemical performance of flexible lithium ion batteries," the team concludes.