Strategy of In-Situ Catalytic Grafting at the Solid Electrolyte Interface

A team of researchers, headed by Professor LI Chilin from Shanghai Institute of Ceramics of the Chinese Academy of Sciences, has suggested a new method of in-situ catalytic grafting at the interface.

This was in view of the challenges faced with regards to the poor flexibility of solid electrolyte interface (SEI) layer containing only the inorganic component and also the intricate procedure involved to develop an organic-inorganic hybrid SEI.

By employing liquid polydimethylsiloxane terminated by -OCH₃ group (PDMS-OCH₃) as a graftable additive, the “fragments” and “grafting” reactions on the surface of a lithium metal take place under the influence of electric field and electrochemical potential, which not only facilitates high-efficiency plating stability but also enables dendritic suppression of lithium metal battery anode. The study results have been reported in Advanced Functional Materials.

Thanks to its low redox electrochemical potential and high theoretical specific capacity, lithium metal has the ability to become the state-of-the-art anode material. When lithium metal is combined with conversion-type S, fluorides and sulfides, relatively higher energy densities can be achieved for the corresponding Li metal batteries, or LMBs.

The spread and growth of lithium dendrites at the anode side are likely to lead to poor cycling stability of LMBs, and thus raise their safety risks of short circuit.

One of the most standard methods to strengthen the SEI film and enhance the anode interface to inhibit the growth of lithium dendrites is to modify the SEI component by introducing a low-content electrolyte additive. Conversely, the strengthening effect of SEI relies on the additive’s degradation reaction with the reductive surface of the lithium.

Professor LI’s team has suggested an effective and facile method for in-situ catalytic grafting at the interface.

The surface of the lithium metal has a thin “skin” layer of LiOH and Li₂O that exist naturally. This layer is capable of catalyzing and triggering the PDMS-OCH₃ dissociation reaction under charge transfer. It is possible to graft the broken macromolecules onto the lithium metal surface, whereas the tinier molecules can be easily condensed into inorganic Li_xSiO_y moieties with fast ion conductivity property.

An organic-inorganic hybrid interfacial phase (that is, grafted SEI) like this can also be strengthened by administering a high concentration of LiF at the time of the electrochemical process. The combination of solid inorganic components of Li_xSiO_y and LiF offers fast-ion interfaces and channels for homogenization of Li-ion flowing as well as deposition of Li-mass, while the soft PDMS branches can improve the buffer effect and flexibility of the whole SEI.

When liquid PDMS-OCH₃ is added into the carbonate system, the protected Li anode with the grafted surface can be imparted with a low potential polarization of approximately 25 mV and a stable cycling of Li-Li symmetrical cells for 1,800 hours. The Li-Cu asymmetric cells allow a high Coulombic efficiency (CE) value of around 97% even under high areal capacity and high current density.

Hence, compared to other solid silicone additives having poor grafting capability, liquid PDMS additive demonstrates a considerable benefit in achieving the compaction of lithium metal and the stabilization of the SEI layer.

The researchers have made a series of developments on the anode interface modification of lithium metal batteries, particularly in implementing functional filler/additive and conformal coating techniques to create stable artificial SEI layers.

For instance, in 2019, the team proposed how a range of metal-organic frameworks, or MOFs can be used as solid additives to activate the in-situ injection of high-concentration LiF into strong Zr-O-C-based SEI for two-fold reinforcement (ACS Applied Materials & Interfaces), a conformal sericin protein coating to allow high-rate Li-S batteries and air-stable lithium metal anode (J. Power Sources), and the development of alloyable three-dimensional (3D) skeleton to guide unique coaxial and conformal lithium deposition (ACS Applied Energy Materials).