Organic supramolecular protective layer with rearranged and defensive Li deposition for stable and dendrite-free lithium metal anode

Abstract

Lithium (Li) metal anode is proposed to take on the heavy responsibility for high-energy Li metal batteries (LMBs). However, some severe obstacles such as uncontrolled growth of Li dendrites, infinite volume variation of Li deposition and low Coulombic efficiency are still big challenges. Herein, a copper foam covered with dense and rigid organic supramolecular protective layer (OSPL) with rearranged and defensive Li deposition is reported to effectively inhibit the formation of dendritic Li and suppress the depletion of Li metal and electrolyte, which is evidenced by optical/electron microscopy, in situ FTIR spectra, electrochemical probing and theoretical calculation. The composite matrix performs high Coulombic efficiencies of 98% for 250 cycles at 1 ​mA ​cm⁻² and enhanced cycling lifespan of 1300 ​h with low potential polarization (20 ​mV) for symmetric cell. Moreover, small nucleation overpotential (27.1 ​mV) benefits from attracting Li ions by massive polar functional groups in the organic supramolecular structure. When coupled with LiFePO₄, full-cells display ultrahigh retention ratio (96.2%) and slow decay of specific capacity after 250 cycles at 0.5 ​C. Low voltage hysteresis reflects the improved kinetics and fast charge-transfer behavior during Li charging/discharging process. The strategy using OSPL provides a new insight for exploring high performance LMBs.

Introduction

Rechargeable lithium (Li)-ion battery technology is revolutionizing daily life of human beings from all aspects. However, the current battery system cannot fulfill the requirements of future vehicle and electronic devices with energy density [1,2]. Lithium metal batteries (LMBs) combined with Li metal anode (LMA) have been regarded as the savior of the next generation of batteries, considering the outstanding ultra-high theoretical specific capacity (3860 ​mA ​h ​g⁻¹) and the lowest redox potential (−3.04 ​V vs. standard hydrogen electrode) of metallic Li. High gravimetric energies of 650 and 950 ​Wh kg⁻¹ can be delivered by LMBs paired with sulfur and air cathodes respectively [3,4]. However, LMA exhibits unstable chemical activity during charging/discharging process, such as dendritic Li growth, huge volume fluctuation, loss of the deposited Li, electrolyte consumption, and low Coulombic efficiency (CE) [5,6]. These drawbacks extremely hindered the practical commercial applications for LMA.

Based on the aforementioned issues, researchers have paid great attention to overcoming the instability of LMA. Modified liquid electrolytes by adding additives like LiF, LiNO₃, and metal ions are usually used to guide uniform Li deposition and stabilize the solid electrolyte interface (SEI) [[7], [8], [9], [10]]. Artificial protective film and various solid-state electrolytes have been also investigated to suppress Li dendrite formation for high mechanical strength [[11], [12], [13]]. Li metal-based composite electrodes prepared by melting lithium into porous lithiophilic frameworks are also considered as effective methods to alleviate volume fluctuation and achieve stable cycling [[14], [15], [16], [17]]. Meanwhile, constructing three-dimensional (3D) porous host as the current collector for loading deposited metallic Li is also attractive [[18], [19], [20]]. Porous Cu foil decorated with massive vertical ordered microchannels fabricated by laser etching can regulate the current density distributions to relieve lithium deposition [21]. Honeycomb-like hierarchical 3D porous Ni@Cu matrix was constructed by galvanostatic electrochemical deposition on Cu foil. The unique structure accompanied with good lithiophilicity of Li acted as an excellent scaffold for the plated Li, inhibiting the directional Li growth and leading to an extremely stable cycling performance [22]. Carbon nanotubes (CNTs)-decorated carbon sponges via thermal annealing realized uniformly-distributed local current density, resulting in uniform Li nucleation on matrix [23]. Although these excellent structural designs facilitate homogeneous Li ion flux and charge distribution, the commercial realization of LMA still faces great obstacles because of the vulnerable SEI of the fresh plated Li and excessive uncontrollable electrolyte consumption. In addition, Li preferentially deposited on the top of conductive framework may grow continuously and would cause short circuit. It is a challenge to design a 3D porous framework with excellent functionalized protective layer, which can effectively decrease the corrosion of electrolyte on fresh Li and enhance the stability of LMA.

Herein, a porous and stable organic network is reported as a superior protective layer for LMA, which is inspired by the natural tree bark and exhibits great defense for internal materials. The solid and dense organic supramolecular protective layer (OSPL) by the polycondensation of melamine and cyanuric acid is uniformly grown on the surface of copper foam (CF@OSPL), whose structure is a resemblance to branches and trunks of a tree. OSPL, acting like a fence, is capable of relieving intense Li ion attack induced by electrons and ensures defensive Li deposition. Numerous polar groups (e.g., amino group, carbonyl and triazine) from melamine and cyanuric acid can disperse concentrated Li ion flux by attracting Li ions and rearrange Li deposition. Plenty of hydrogen bonds formed among the polar groups offer OSPL good elasticity, which is difficult to break and can alleviate the volume expansion of Li plating. Moreover, the electrolyte is prevented from direct contact with freshly-deposited Li with the assistance of such a protective layer (OSPL). Based on these positive effects, the CF@OSPL electrode exhibits improved Coulombic efficiency (CE) of 98% for 250 cycles and long lifetime of 1300 ​h with low voltage hysteresis of ~20 ​mV for the symmetric cell at 1 ​mA ​cm⁻². When served as the anode for LiFePO₄ (LFP)-based full cell, the CF@Li@OSPL|LFP cell delivers the high capacity retention ratio of 96.2% at 0.5 ​C after 250 cycles. Sluggish capacity decay demonstrates that OSPL makes a great contribution to the inhibition of Li dendrites and the decrease of extra consumption of electrolyte during cycling.