Double-Layer Protection for Lithium-Metal Anode

Lithium (Li) metal has been regarded as one of the most promising anodes to achieve a high-energy-density battery due to its ultrahigh theoretical specific capacity (3860 mAh g^–1) and low redox potential (– 3.04 V vs. standard hydrogen electrode). However, the practical usage of Li metal anode is hindered by several challenges including the generation of heterogeneous/non-uniform solid electrolyte interphase (SEI) layer, “dead” Li formation during repeated cycling causing volume change and loss of active Li, continuous consumption of electrolyte triggering low Coulombic efficiency (CE). To address these problems, we have developed a double-layer coating to protect Li metal anode. It is well known that PEO is stable with Li metal and has a high donor number for Li-ion and high chain flexibility, which are important for promoting ion transport and considered a suitable candidate for the ionic conductive protection layer for Li metal anode. However, PEO is dimensionally unstable in liquid electrolytes due to dissolution or swelling. To maintain the integrity of PEO in liquid electrolyte, a double layer (DL) concept is proposed. The DL consists of a PEO-based bottom layer and a top layer which is stable in organic solvent. A uniform double-layer (less than 1 μm thick) was obtained as shown by scanning electron microscope. DL-coated Li metal anode (DL@Li) enabled a long cycling lifespan of 1000 h without a significant increase in polarization of Li||Li symmetric cells when cycled in a capacity of 1 mAh cm^–1 at a current density of 1 mA cm^–1. In comparison, the bare Li anode is shorted at around 960 h under the same condition. Furthermore, the DL@Li showed smooth and stable Li deposition/stripping profiles at a high current density (2 mA cm^–1) while the bare Li presented the sudden voltage changes which were caused by continuously building up and breaking down of SEI layers at the electrolyte/Li interface. Therefore, the DL@Li may alleviate electrolyte consumption which induces interface reaction during cycling. In addition, the DL@Li enables stable deposition/stripping behavior with a slightly higher average Li Coulombic efficiency value of 99.5% than the bare Li (99.4%). Benefitting from the advantage of the DL protection, the DL@Li||Ni-rich cathode cell showed a much better long cycling stability than the bare Li||Ni-rich cathode at a high current density of 2.1 mA cm^–2 (C/2 rate). After cycling, the DL@Li exhibited a flat and smooth surface without Li dendrites while the bare Li anode illustrated a rough and porous surface with massive dendritic Li. In addition, the DL protection improved the fast-charging capability of the DL@Li||Ni-rich cathode cell (4.6 mAh cm^–1) and led to a stable Li deposition even at a current density of 6.9 mA cm^–2 based on its remarkable promotion of uniform Li-ion flux. This study provides an efficient approach to protect Li metal anode and enhance the performance of high-performance Li-metal batteries.