Self-healing, the ability to repair damage spontaneously, is a ubiquitous feature in nature and plays a vital role in increasing the life expectancy of organisms. This inspires many promising applications toward smart electronics, artificial skin, and energy storage. For example, the self-healing characteristic is extremely desirable for long-life energy storage devices. On one hand, there are growing interests in adopting new battery chemistry to replace conventional lithium-ion batteries that are almost approaching the theoretical limit. On the other hand, the high-capacity electrode materials such as silicon and sulfur always suffer from rapid capacity fading and short life. This dilemma is primarily ascribed to the mechanical fracture and structural collapse. If damage of these electrode materials can be spontaneously healed, their huge potential will be well translated into reality.

… In this contribution, we propose a general scheme to explain the mechanism of self-healing electrolytes (SHEs) preloaded with polysulfides and to engineer extrinsic healing agents toward self-healable Li−S batteries. The healing mechanism mimics biological processes, fibrinolysis, within blood vessels. The uncontrolled deposition and accumulation of deactivated solid products are analogous to coagulation of thrombus that undesirably obstructs flow of blood to healthy vessels. Polysulfides, which mimic the basic function of an enzyme called plasmin to solubilize the thrombus, are capable of transferring these solid compounds into solution phase and enabling their subsequent reparticipation in electrochemical cycles. More importantly, similar to plasmin, the ability of polysulfides to self-regenerate guarantees their persistency to work in Li−S batteries. Additionally, a classical crystal growth model is first introduced to explain how polysulfides mediate the phase transfer and heal the sulfur cathode. Spatial heterogeneity plays a vital role in the conversion-type electrochemical reactions, agreeing with the insertion-type ones coincidently.

—Peng et al.

Schematic of healing mechanism for SMiPs. (A) top, simplified schematic of coagulation cascade toward formation of thrombus (blood clot) and fibrinolysis of thrombus into soluble fibrin fragments, which are modulated by two trypsins: thrombin and plasmin; bottom, schematic of deposition/dissolution of micrometer-sized sulfur/Li2S particles with limited electrical contact, hindering full electrochemical utilization. To enable the stable and reversible utilization, a complementary route is introduced, through chemical reactions between an extrinsic healing agent in solution and insoluble sulfur compounds. (B) Schematic of how spatial heterogeneity impacts the solution−solid phase transfer. Credit: ACS, Peng et al. Click to enlarge.

There are two types of self-healing, the researchers noted: intrinsic and extrinsic. The Tsinghua method is extrinsic because it is dependent on external healing chemicals. The healing agent must possess the following attributes, they added: (1) the possibility of being regenerated; (2) the capability to mediate solution-to-solid phase transfer controllably; and (3) the ability to restore deactivated solid compounds.

Even after a static operation for 72 h, the capacities did not decay, but manifested a slight increase at both low and high current densities, further indicating the unique self-healing feature, the researchers said. Furthermore, a cycled Li−S cell with SMiP and a conventional electrolyte exhibited restored capacity and stable cycling after being injected with the self-healing electrolyte, suggesting an unambiguous healing capability of the SHE.

If a highly efficient “sulfiphilic” host was employed to support SMiP/SHE, further enhancement in sulfur utilization can be expected.… Although it is extremely challenging to mimic the high efficiency, specificity, or precise controllability of biological processes, novel healing agents that are smart, sustainable, and rapidly responsive hold future promises. This innovative approach can also be feasibly and facilely implemented in other high-energy electrochemical storage/conversion systems such as metal−O₂ batteries and fuel cells.

—Peng et al.

Resources

• Hong-Jie Peng, Jia-Qi Huang, Xin-Yan Liu, Xin-Bing Cheng, Wen-Tao Xu, Chen-Zi Zhao, Fei Wei, and Qiang Zhang (2017) “Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control” Journal of the American Chemical Society doi: 10.1021/jacs.6b12358