ABSTRACT
Continuous lithium (Li) depletion shadows the increase in energy density and safety properties promised by zero-excess lithium metal batteries (ZELMBs). Guiding the Li deposits toward more homogeneous and denser lithium morphology results in improved electrochemical performance. Herein, a lithium nitrate (LiNO3 ) enriched separator that improves the morphology of the Li deposits and facilitates the formation of an inorganic-rich solid-electrolyte interphase (SEI) resulting in an extended cycle life in Li||Li-cells as well as an increase of the Coulombic efficiency in Cu||Li-cells is reported. Using a LiNi0.6 Co0.2 Mn0.2 O2 positive electrode in NCM622||Cu-cells, a carbonate-based electrolyte, and a LiNO3 enriched separator, an extension of the cycle life by more than 50 cycles with a moderate capacity fading compared to the unmodified separator is obtained. The relative constant level of LiNO3 in the electrolyte, maintained by the LiNO3 enriched separator throughout the cycling process stems at the origin of the improved performance. Ion chromatography measurements carried out at different cycles support the proposed mechanism of a slow and constant release of LiNO3 from the separator. The results indicate that the strategy of using a LiNO3 enriched separator instead of LiNO3 as a sacrificial electrolyte additive can improve the performance of ZELMBs further by maintaining a compact and thus stable SEI layer on Li deposits.
ABSTRACT
Li metal batteries (LMBs) containing cross-linked polymer electrolytes (PEs) are auspicious candidates for next-generation batteries. However, the wetting behavior of PEs on uneven Li metal surfaces has been neglected in most studies. Herein, it is shown that microscale defect sites with curved edges play an important role in a wettability-dependent electrodeposition. The wettability and the viscoelastic properties of PEs are correlated, and the impact of wettability on the nucleation and diffusion near the Li|PE interface is distinguished. It is found that the curvature of the edges is a key factor for the investigation of wetting phenomena. The appearance of microscale defects and phase separation are identified as main causes for erratic nucleation. It is emphasized that the implementation of stable and consistent long-term cycling performance of LMBs using PEs requires a deeper understanding of the "soft-solid"-solid contact between PEs and inherently rough Li metal surfaces.
ABSTRACT
The demand for high energy densities has brought rechargeable lithium metal batteries back into the research focus. Ionic liquids (ILs) are considered as suitable electrolyte components for these systems. In this work, the wetting behavior of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([C2MIm]TFSI), 1-butyl-3-methylimidazolium bis-((trifluoromethyl)sulfonyl)imide ([C4MIm]TFSI), 1-hexyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([C6MIm]TFSI), and N-butyl-N-methylpyrrolidinium bis((trifluoromethyl)sulfonyl)imide (Pyr14TFSI) on mechanically modified lithium electrodes, with and without lithium bis((trifluoromethyl)sulfonyl)imide (LiTFSI) conducting salt, is investigated and is compared to an organic carbonate-based electrolyte. Three different patterns were chosen for the lithium modification, enabling a surface area increase of 12%, 20%, and 56% for the modified lithium electrodes. Especially for pure ILs, the contact angle on lithium was significantly larger with higher surface areas of the lithium electrodes. Since the addition of LiTFSI remarkably decreased the contact angles of the ILs on the modified lithium surfaces, it could be shown that the effect of LiTFSI can be attributed to a decreased surface tension. This observation could be explained by an interruption of the ordering of ionic liquid cations and anions, which is supported by Raman spectroscopy and molecular dynamics (MD) simulations.
