Quantifying Inactive Lithium in Lithium Metal Batteries (1811.01029v3)

Abstract: Inactive lithium (Li) formation is the immediate cause of capacity loss and catastrophic failure of Li metal batteries. However, the chemical component and the atomic level structure of inactive Li have rarely been studied due to the lack of effective diagnosis tools to accurately differentiate and quantify Li+ in solid electrolyte interphase (SEI) components and the electrically isolated unreacted metallic Li0, which together comprise the inactive Li. Here, by introducing a new analytical method, Titration Gas Chromatography (TGC), we can accurately quantify the contribution from metallic Li0 to the total amount of inactive Li. We uncover that the Li0, rather than the electrochemically formed SEI, dominates the inactive Li and capacity loss. Using cryogenic electron microscopies to further study the microstructure and nanostructure of inactive Li, we find that the Li0 is surrounded by insulating SEI, losing the electronic conductive pathway to the bulk electrode. Coupling the measurements of the Li0 global content to observations of its local atomic structure, we reveal the formation mechanism of inactive Li in different types of electrolytes, and identify the true underlying cause of low Coulombic efficiency in Li metal deposition and stripping. We ultimately propose strategies to enable the highly efficient Li deposition and stripping to enable Li metal anode for next generation high energy batteries.

• The paper introduces Titration Gas Chromatography (TGC) for quantifying inactive lithium with 1 microgram sensitivity.
• It reveals that unreacted metallic lithium, not SEI compounds, predominantly drives Coulombic efficiency loss.
• The study employs cryo-FIB-SEM and cryo-TEM to link lithium deposit morphology with electrolyte properties and battery performance.

Quantifying Inactive Lithium in Lithium Metal Batteries: A Technical Analysis

The paper "Quantifying Inactive Lithium in Lithium Metal Batteries" presents a significant effort in addressing a fundamental issue in lithium metal batteries (LMBs): the formation of inactive lithium, which is a primary cause of low Coulombic efficiency (CE) and subsequent battery failure. This research advances the understanding of inactive lithium's contributions to capacity loss and proposes methodological improvements for its precise quantification and reduction.

Key Findings

The researchers developed a novel analytical method, Titration Gas Chromatography (TGC), which excels in quantifying metallic lithium constituting inactive lithium at a sensitivity of 1 microgram. This tool directly addresses the inadequacy of previous methods like X-ray photoelectron spectroscopy (XPS) and cryogenic transmission electron microscopy (cryo-TEM), which are limited in providing global quantification and mainly offer surface-level insights.

Significantly, the paper uncovers that unreacted metallic lithium, rather than lithium compounds within the solid electrolyte interphase (SEI), predominantly contributes to inactive lithium and resultant capacity loss. The linear relationship between inactive metallic lithium and CE loss underlines the need to reevaluate strategies aimed at improving the performance of Li metal anodes.

Experimental Approaches

Advanced characterization techniques, including cryo-focused ion beam scanning electron microscopy (Cryo-FIB-SEM) and cryo-TEM, provided complementary insights into the microstructural and nanostructural aspects of inactive lithium. The combination of these techniques elucidates the morphology of lithium deposits and the formation mechanisms of inactive lithium products in different electrolytes, highlighting the role of electrolyte properties on the microstructural evolution of lithium deposits.

Implications and Future Directions

The paper's findings challenge the prevailing assumption that SEI components are the primary contributors to capacity loss in LMBs. By accurately distinguishing between SEI-bound and unreacted metallic lithium, the research offers a nuanced understanding of the factors affecting CE and cycle life. It suggests that structural connectivity—as the capability of active lithium to sustain an electronic conductive network—is critical in reducing inactive lithium formation.

The research also discusses strategic directions for enhancing CE, such as developing lithium deposits with low tortuosity, adopting electrolytes that encourage columnar lithium microstructures, and implementing optimal external pressures to maintain conductive networks. Such strategies are projected to pave the way for higher efficiency and safer Li metal anodes suited for next-generation energy storage applications.

Conclusion

This research contributes a methodological advance in quantifying inactive lithium and offers unprecedented insights into its predominance over SEI lithium in causing capacity loss in LMBs. The employment of TGC and state-of-the-art characterization techniques has revealed foundational aspects of lithium deposition and dissolving mechanisms, providing a basis for further exploration into enhancing LMB performance. Future research could extend these methodologies to examine long-term cycling behaviors and varied operational conditions, potentially informing the optimization of other metal battery systems, including sodium and magnesium. Such extensions may significantly influence the development of more effective energy storage solutions.