Dual Protective Layer Improves Cycle Stability of Lithium Metal Batteries

Lithium metal batteries feature higher capacity and specific energy than lithium ion batteries. However, as the lithium metal is very reactive, especially towards a liquid electrolyte, protective layers are essential on the lithium surface. They prevent parasitic reactions with the electrolyte. A team from MEET Battery Research Center at the University of Münster and Helmholtz Institute Münster of Forschungszentrum Jülich has now developed a dual protective layer (DPL). It consists of an intermetallic and an overlying inorganic layer and improves the cycle stability of lithium metal battery cells.

Analysis of Cross-sections Explains Improved Cycle Stability

In a previous study, the scientists had already demonstrated that the thickness of the intermetallic protective layer and the pre-treatment of the lithium electrode using roll-pressing had a positive effect on the cycle stability and lifetime of lithium metal batteries. However, the intermetallic protective layer still cracked and high surface area lithium (HSAL) continued to form during cycling, although to a smaller amount. Therefore, the research team has now developed an innovative dual protective layer based on the previous results. “The additional inorganic component is the ideal addition. It increases the stability because it is more resistant,” explains MEET scientist Dr Marlena Maria Bela. Both layers have a high conductivity of lithium ions and enable their homogeneous deposition on the entire lithium electrode surface.

Using electrochemical analyses, the researchers showed that the cycle stability of symmetrical cells increased by 80 percent when the electrodes had a dual protective layer and were not only pre-treated by roll-pressing. The stability of cells with lithium nickel manganese cobalt oxide based cathodes (NMC622||Li cells) improved by 50 percent. “The analysis of the cross-sections prepared under cryogenic conditions clarified that the combination of both layers leads to substantial improvements,” explains Bela. A single layer cracked after just 25 cycles at the areas where HSAL was growing. When using the dual protective layer, the intermetallic protective layer started to dissolve after 50 cycles. In addition, the inorganic protective layer remained intact and thus protected the underlying lithium.

Detailed Results Online Available

The entire study has been published by the authors Dr Marlena Maria Bela, Maximilian Mense, Sebastian Greiwe, Dr Simon Wiemers-Meyer and Dr Markus Börner, MEET Battery Research Center, Dr Marian C. Stan, Helmholtz Institute Münster of Forschungszentrum Jülich, as well as Prof. Dr Martin Winter, MEET Battery Research Center and Helmholtz Institute Münster, in the “Journal of Materials Chemistry A”.