Lithium‐Metal Foil Surface Modification: An Effective Method to Improve the Cycling Performance of Lithium‐Metal Batteries

Abstract

Although the lithium-metal anode has the advantages of both high gravimetric and volumetric capacities (3862 Ah kg −1 and 2085 Ah L −1 ) [13] and is already successfully used in primary batteries, it is still plagued by a series of issues that limit its successful operation in rechargeable applications, when organic solvent based electrolytes are used. [14] One of them is the nature of the lithium-metal dissolution and redeposition in the discharge and charge process together with the composition of the solid electrolyte interphase (SEI) [15,16] that is formed immediately after electrolyte addition and continues to form, grow and alter during cycling, [17] which limits the rechargeability in these battery systems and decreases their safety. [15,18,19] The SEI, though not being a homo geneous single phase, varies in composition and thickness and these differences lead to inhomogeneous and thus locally different current densities during the discharge and charge process, which can ultimately cause the formation of high surface area lithium (HSAL) during lithium deposition (charging) and hole/pit formation during dissolution (discharging). [20][21][22] In the worst case, the HSAL morphology takes the form of dendrites, i.e., small needle like lithium deposits that can grow through the separator from the anode towards the cathode. This process can lead to an internal short circuit of the cell resulting in local overheating and possibly cause a cell fire due to an increased reactivity with the electrolyte and the low melting point of lithium (180.54 °C). [23] Practical approaches to improve the rechargeable lithiummetal anode from the electrode material's point of view concentrate on either using coated lithium powder [24,25] or foil [26] and lithium with surface micropatterning. [27,28] The main underlying principle is increasing the specific surface area thus decreasing the effective current density and the resulting overpotential. However, the behavior of the lithium-metal electrode is quite complex and electrolyte-dependent [22,[29][30][31] and there is a need to identify the optimal conditions under which lithium-metal electrodes can cycle with both an increased reversibility and low overpotentials. [32] As-received lithium-metal foil contains several contaminants, [33] particularly on the surface. [34] In addition, even a new lithium foil that is considered to be smooth shows a non-negligible surface roughness that Lithium metal as an electrode material possesses a native surface film, which leads to a rough surface and this has a negative impact on the cycling behavior. A simple, fast, and reproducible technique is shown, which makes it possible to flatten and thin the native surface film of the lithium-metal anode. Atomic force microscopy and scanning electron microscopy images are presented to verify the success of the method and X-ray photoelectron spectroscopy measurements reveal that the chemical composition of the lithium surface is also changed. Furthermore, galvanostatic measurements indicate superior c...

Guided Lithium Deposition by Surface Micro‐Patterning of Lithium‐Metal Electrodes

“…From the evolution of the Li deposit with increasing capacity, the preferential Li deposition site can be identified for the electrodes. For the control Li electrode, as shown in Figure a–c, the initial Li deposition was irregularly localized, which would be attributed to the preferential Li deposition on less resistive surfaces . As the capacity increased, the Li deposited area was expanded covering the whole electrode surface.…”

“…Furthermore, the surface roughness and defects were remarkably reduced through the lamination process indicating the occurrence of surface deformation. As suggested in a previous publication, the original native passivation film consisting of various lithium compounds such as Li 2 O, Li 2 CO 3 and LiOH can be deformed by the pressing. In this aspect, the native passivation film on the wall and bottom sides of the square hole would be different from that of the pristine surface and top surface of the MP‐Li electrodes.…”

“…The different Li deposition behaviors depending on the geometry of the pattern raises the question of how Li deposition is guided onto a specific site of the mechanically deformed surface depending on the structural feature of the patterned Li surface. The other important observation regarding this issue is that a more uniform Li deposition can be induced by a mechanical roll‐pressing of the Li‐metal electrode; the roll‐pressing induces a thinner passivation film resulting in a smaller interfacial resistance and consequently an improved cycling performance of the Li‐metal electrode . It suggests that the stress‐driven deformation of the initial passivation film should be considered as well as the geometry of the surface pattern to understand guided Li deposition on patterned Li surfaces.…”

Ionic Liquids and their Polymers in Lithium‐Sulfur Batteries

“…The higher storing temperature of M3FSI was required since M3FSI is solid at room temperature. Prior the storage, the Li‐metal anodes were roll‐pressed according to the method described by Becking et al . This method reduces in one pass both the surface roughness of pristine Li‐metal anodes as well as the thickness of the native surface film.…”

“…This includes implementing IL based electrolytes to reduce polysulfide solubility, coating of the Li‐metal anode with PILs to offer surface protection, and using PIL as a cathode binder to improve cycling stability. Much of this work is the result of a collaboration between partners within the HELIS (High energy lithium sulphur cells and batteries) consortium, an EU H2020 funded project for the development Li−S batteries for commercial applications …”