Polymer Stabilized Liquid Metal Nanodroplets As an Artificial Interphase Layer for Lithium Metal Anodes
Authors: Liu Tong , Lorandi Francesca , Whitacre Jay , Matyjaszewski Krzysztof 
Published: July 2021

Lithium (Li) metal batteries are considered to be the next-generation high energy density batteries because of the low reduction potential of metallic Li (−3.04 V) and its high specific capacity (3860 mA h g−1). However, there are challenges to implement lithium metal as an anode material. Especially the brittle and unstable solid electrolyte interface (SEI) formed by reaction of highly reactive lithium metal with liquid electrolytes, which can lead to low Coulombic efficiency and short cycle life.1 The design of an artificial SEI, i.e. ex-situ building of a protective layer on Li metal has shown to be effective for stabilizing Li metal anode and interphase. Especially, Li alloys can enable stable Li metal anodes by i) providing fast ion transport at the interphase, ii) suppressing the Li dendrites growth, iii) generating a stable interface between Li alloy anode and electrolyte, and iv) exhibiting high exchange currents, overall resulting in stable long-term performance.2 In 2017, Nazar et al. first reported a facile and scalable procedure to fabricate thin alloy layers onto Li metal foil via the in situ reduction of metal chlorides by Li.3 Two-metal component alloy and ternary alloy systems (based on Ga, Sn, In, Zn, Mg, Ag etc.) have also been investigated in Li-ion and alkali metal batteries. Goodenough et al. reported a lithium alloyed Ga-In nanoparticle/carbon/binder electrode. The liquid Ga-In system exhibited self-healing feature and high diffusivity in liquids, and the composite anode realized a high capacity and excellent stability in both Li-ion and Na-ion batteries.4

Herein, EGaIn consisting of 75 wt% Ga and 25 wt% In is chosen as a model liquid metal. An artificial SEI design based on poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA) diblock copolymer was exploited to stabilize EGaIn nanodroplets. The attachment of polymer brushes onto EGaIn nanodroplets can greatly improve the dispersibility of nanoparticles, therefore creating a more uniform protection layer than bulk EGaIn. PAA chains are designed as anchoring blocks to attach onto the EGaIn “skin” layer via multiple coordination interactions, while PEO chains should ensure the ionic conductivity for Li ions transport. The molecular weight and molecular weight distribution of PAA can be precisely controlled by atom transfer radical polymerization (ATRP) (Figure 1a). Under ultrosonication, PEO-b-PAA act as polymeric surfactant to fabricate polymer-grafted-EGaIn nanodroplets through a “grafting onto” method via the PAA anchoring groups. This provides a general and robust strategy to achieve high colloidal stability and good dispersibility (Figure 1b). The artificial SEI was then fabricated on Cu current collector by spray coating, which is a facile strategy to form uniform and thin layers onto large surface area substrates. Galvanostatic Li electrodeposition voltage profiles for asymmetric Li cells (Cu|Li, Cu@EGaIn-PEO-b-PAA|Li) were conducted. The lower nucleation overpotentials on coated Cu compared to bare Cu illustrates that the EGaIn/PEO-b-PAA artificial SEI help Li to stably nucleate and uniformly grow the Li nuclei via interlayer gaps above and under the SEI. The artificial SEI regulate lithium deposition and enabes faster Li-ion transport and diffusion. In conclusion, polymer stabilized EGaIn nanodroplets used as artificial SEI can effectively protect Li metal anodes, further improving the capacity retention, Coulombic efficiency, and lifespan of Li metal batteries.
Figure 1. a) Synthesis of PEO-b-PAA copolymer. b) Preparation of polymer stabilized EGaIn nanodroplets. c) Nucleation behavior of Li metal on artificial SEI modified Cu substrate.

Figure 1