A research team led by Prof. Quan-Hong Yang at Tianjin University and Prof. Wei Lv at Tsinghua University has developed a wax-PEO coating on lithium metal surface by a simple dip coating method to realize an air-stable and waterproof lithium metal anode. Wax as a commonly-used inert sealing material is easily coated on the surface of lithium metal.

Li et al.

Lithium metal anodes offer the potential to boost the energy density of lithium-ion batteries due to high specific capacity (3800 mAh g^-1) and low voltage (-3.04 V vs. Li/Li⁺). However, the safety issues caused by dendrite growth and instability in air caused by lithium’s high chemical activity have limited its large-scale use as an electrode material.

Lithium metal is highly sensitive to moisture and oxidative components in the air, leading to the generation of insulating products such as lithium hydroxides on its surface and the resultant deterioration of the electrochemical performance.

Moreover, when lithium contacts with water, combustion and explosion might occur due to the production of hydrogen and heat. The sensitivity of lithium metal raises demanding requirements for the transport, storage and process of the lithium metal anode. It is hence highly desired to develop an air-stable and waterproof lithium metal anode for real use in the future.

In the electronics field, packaging technology protects electronic components from physical damage and corrosion in humid air and water by using a coating; this provided the design thought for the protection of lithium metal.

The obtained wax-based composite coating prevents the adverse reactions of lithium metal in the air and water. In batteries, the coating retards the etching of electrolyte to the surface of lithium metal anodes while the homogeneously distributed PEO guarantees the uniform lithium ion conduction at the interface and inhibits the growth of lithium dendrites.

Under the protection of wax-PEO coating, the lithium surface kept unchanged in the air with high relative humidity of 70% for 24h, with high capacity retention of 85%. There was no combustion or capacity decay even after direct contact with water.

The coated lithium metal anode keeps stable for as long as 500 h in symmetric cells and the lithium sulfur batteries assembled with the coated lithium metal anode show a low capacity decay rate of 0.075% per cycle for 300 cycles.

… we have developed a method of improving both the stability in air and even in water and electrochemical performance of lithium metal by applying a composite coating of wax and PEO. This dense thin coating is hydrophobic and ionically conductive, for the combination of properties of wax and PEO. It packages the lithium metal to avoid adverse surface-related reactions, and this makes it possible for lithium metal anodes with this coat- ing to remain stable in air with relative humidity of 70% for more than one day, and no combustion happens even in contact with water.

From the perspective of electrochemical performance, wax inhibits adverse interfacial reactions between the electrolyte and the LMA, while the PEO provides conducting paths for the Li⁺ and regulates the ion flux. Therefore, in symmetric cells, increased cycling stability of more than 500 h was achieved.

When paired with a sulfur cathode, the wax-PEO coating allows Li+ ions to access the LMA while inhibiting polysulfides from corroding it, and as a result the capacity decay of the cells was as low as 0.075% per cycle after 300 stable cycle tests.

This simple method of modifying the surface of lithium metal also has potential for use with other air-sensitive electrode materials or structures in future research and large-scale production.

—Li et al.

Resources

Yunbo Zhang, Wei Lv, Zhijia Huang, Guangmin Zhou, Yaqian Deng, Jun Zhang, Chen Zhang, Boyu Hao, Qi Qi, Yan-Bing He, Feiyu Kang, Quan-Hong Yang (2019) “An air-stable and waterproof lithium metal anode enabled by wax composite packaging.” Science Bulletin, 64 (13) 910-917 doi: 10.1016/j.scib.2019.05.025