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Designing surface structured substrates for use as negative electrodes in zero excess lithium metal batteries

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Summary:

In recent years, the ever-increasing demand for high energy density batteries has led to a renewed interest in lithium metal battery (LMB) technologies as an alternative to state of the art lithium ion batteries (LIBs). However, the significant challenges that occur when lithium metal is used as an anode material in such batteries need to be overcome. These challenges prominently include security issues and large volume changes during operation as well as continuous parasitic side reactions between the lithium metal and the electrolyte.

One approach to avoid these hindrances is the utilization of so-called anode-free concepts for lithium batteries, which replace the lithium metal anode with only a thin current collector substrate. During charging of the battery, the lithium ions from the cathode material are electrodeposited on the current collector to form a temporary lithium metal anode. During the subsequent discharging of the cell, this lithium metal layer is electrodissolved again and the lithium ions are integrated back into the cathode structure. This way, in theory, a constant presence of reactive lithium metal in the cell throughout all charging states is avoided, which increases the safety and reduces the occurrence of side reactions. Also, the volume and mass of the resulting cell are reduced when compared to both LIBs and LMBs, which increases its energy density. Since a cell based on this setup can only utilize the amount of lithium originally included in the cathode material, such batteries are also referred to as zero excess systems. Due to the lack of lithium reserves, the Coulombic efficiency of these batteries must be as high as possible to ensure a long cycle life. Therefore, among other factors like the reactivity of the used electrolyte with lithium metal, a highly reversible electrodeposition/-dissolution process is of key importance to high performance zero excess lithium metal batteries.

Increasing the surface area of the current collector by replacing a planar 2D surface with a 3D structured one lowers the local current density and therefore facilitates a more homogeneous lithium deposition. Furthermore, depositing the lithium in the free space inside a 3D structure instead of on a flat surface eliminates the problem of severe volume changes throughout battery operation. In this presentation, various methods for the preparation of 3D host structures for use as negative electrodes in zero excess lithium metal batteries are showcased with a focus on alloy corrosion and templated electrodeposition. We provide insights into the process of designing such structured anode host materials and assess the current state of development towards a usage in practical battery cells.

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