Facing the challenges of the poor flexibility of solid electrolyte interface (SEI) layer composed of sole inorganic component as well as the complicated procedure to construct an organic-inorganic hybrid SEI, a research group led by Prof. LI Chilin from Shanghai Institute of Ceramics of the Chinese Academy of Sciences proposed a new strategy of in-situ catalytic grafting at interface.

By using liquid polydimethylsiloxane terminated by -OCH₃ group (PDMS-OCH₃) as a graftable additive, the “grafting” and “fragments” reactions on lithium metal surface occur under the impact of electrochemical potential and electric field, which enables high-efficiency plating stability and dendrite inhibition of lithium metal battery anode. The study was published in Advanced Functional Materials.

Lithium metal has the potential to become the next-generation anode material due to its high theoretical specific capacity and low redox electrochemical potential. When coupled with conversion-type S, sulfides and fluorides, much higher energy densities can be obtained for the corresponding Li metal batteries (LMBs).

The growth and spread of lithium dendrites at the anode side tend to cause poor cycling stability of LMBs, and increase their safety risks of short circuit. Adjusting the SEI component by adding a low content electrolyte additive is a most common way to reinforce the SEI film and improve the anode interface for the suppression of lithium dendrite growth. However, the reinforcement effect of SEI depends on the degradation reaction of additive with the reductive Li surface.

Prof. LI 's group proposes a facile and effective strategy of in-situ catalytic grafting at interface.

The thin “skin” layer of Li₂O and LiOH naturally existing on the surface of lithium metal can catalyze and activate the dissociation reaction of PDMS-OCH₃ under charge transfer. The broken macromolecules can be grafted onto the surface of lithium metal, while the smaller molecules can be condensed into inorganic Li_xSiO_y moieties with fast ion conductivity property.

Such an organic-inorganic hybrid interfacial phase (i.e. grafted SEI) is further reinforced by the injection of high concentration of LiF during the electrochemical process. The combination of hard inorganic components of LiF and Li_xSiO_y provides fast-ion channels and interfaces for homogenization of Li-ion flowing and Li-mass deposition, while the soft PDMS branches can enhance the flexibility and buffer effect of the entire SEI.

By adding liquid PDMS-OCH₃ in the carbonate system, the protected Li anode with grafted surface can be endowed with a stable cycling of Li|Li symmetrical cells for 1800 h and a low potential polarization of ~25 mV. The Li|Cu asymmetric cells enable a high Coulombic efficiency (CE) value up to 97% even under high current density and high areal capacity.

Thus, compared with other solid silicone additives with poor grafting capability, liquid PDMS additive shows significant advantage in realizing the lithium metal compaction and SEI stabilization.

The research group has made a series of progresses on the anode interface modification of lithium metal batteries, especially in adopting functional additive/filler and conformal coating methods to design stable artificial SEI layers. For example, in 2019, they proposed a series of metal-organic frameworks (MOFs) as solid additives to trigger the in-situ injection of high-concentration LiF into robust Zr-O-C-based SEI for dual reinforcement (ACS Appl. Mater. Interfaces), a conformal coating of sericin protein to enable air-stable lithium metal anode and high-rate Li-S batteries (J. Power Sources), and the construction of alloyable three-dimensional skeleton to guide unusual conformal and coaxial lithium deposition (ACS Appl. Energy Mater.).