In-situ catalytic grafting of liquid polydimethylsiloxane to achieve dendrite-free lithium plating and highly reversible lithium metal batteries. [Image: Prof. Li Chilin’s Group]

Facing the challenges of the poor flexibility of solid electrolyte interface (SEI) layers composed of a 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, Chinese Academy of Sciences proposes 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 surfaces occur under the impact of electrochemical potential and an electric field. It enables high-efficiency plating stability and dendrite inhibition of lithium metal battery anode. The result was published in Advanced Functional Materials (2019, 1902220, DOI:10.1002/adfm.201902220).

Lithium metal has the potential to become the next-generation anode material due to its high theoretical specific capacity (3860 mAh g^-1) and low redox electrochemical potential (-3.04 V vs. standard hydrogen electrode). When coupled with conversion-type S, sulfides and fluorides, much higher energy densities (500 ~ 900 Wh kg^-1) can be obtained for the corresponding Li metal batteries (LMBs).

However, the growth and spread of lithium dendrites on the anode side tends to cause poor cycling stability of LMBs, and increase their risks of short circuits, a safety issue. The extruded lithium dendrites may also destroy the SEI layer or cause the formation of “dead lithium”. With the increase of specific surface area and porosity of lithium metal anodes, the electrolyte consumption would be correspondingly increased. Moreover, the SEI accumulates and becomes thicker, which results in electrode passivation. These unfavorable factors lead to the increase of cell impedance and voltage polarization, as well as the degradation and fluctuation of Coulombic efficiency (CE), which severely frustrates the development of LMBs.

Therefore, 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. 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 to 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 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, the liquid PDMS additive shows significant advantages in realizing lithium metal compaction and SEI stabilization.

In addition, the research group made progress in recent research 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, they firstly proposed a two-dimensional carbon-nitrogen polymer (C₃N₄) reinforced electrolyte to achieve the effective inhibition of lithium dendrite growth (ACS Appl. Mater. Interfaces 2017, 9, 11615). They proposed in-situ construction of porous magnesium metal networks to stabilize the reversible plating of lithium metal anode (ACS Appl. Mater. Interfaces 2018, 10, 12678). They firstly proposed a solution to a category of lithium-rich open framework solid electrolytes of fluorides with high-ionic conductivity and their homogenization effect on Li-ion flowing (Energy Storage Mater. 2018,14,100; ACS Appl. Mater Interfaces 2018, 10, 34322). 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 2019, 11, 3869). They proposed a conformal coating of sericin protein to enable air-stable lithium metal anode and high-rate Li-S batteries (J. Power Sources 2019, 419, 72). They proposed the construction of an alloyable three-dimensional skeleton to guide unusual conformal and coaxial lithium deposition (ACS Appl. Energy Mater. 2019, DOI:10.1021 / acsaem.9b00573).

For more information, please contact:

Prof. Li Chilin

E-mail: chilinli@mail.sic.ac.cn

Shanghai Institute of Ceramics

Source: Shanghai Institute of Ceramics, Chinese Academy of Sciences