392218 Foreign Metal Ions as Dendrite Suppressing Additives for Lithium Metal Anodes
Dendrites have prevented serious consideration of lithium metal anodes because their growth short-circuits the battery, leading to short life and undesired thermal effects. A lithium metal anode is still an intriguing possibility because it represents the maximum achievable energy density for a lithium-based battery, 3861 mAh/g instead of 329 mAh/g for commercial graphite anodes.
We used a lithium-sodium co-deposition from quaternary ammonium bis(trifluoromethansulfonyl)imide (TFSI) to mitigate this dendrite growth. Our modified electrolyte allowed lithium to plate in beads on the surface rather than dendrites. Lithium dendrites likely grow due to certain crystal faces being more electrochemically active than others. Sodium may deposit preferentially on these sites, resulting in a dimpled morphology that blocks dendrite growth.
This study was expanded to include the remaining alkali metal ions, as well as the group 2 alkaline earth metal ions. The morphology of lithium metal deposited from a quaternary ammonium bis(trifluoromethansolfonyl)imid (TFSI) ionic liquid was studied with each metal salt added in as an additive to the lithium electrolyte. While lithium-sodium was the only co-deposition observed, the alkaline earth metals prevented dendrites without becoming part of the metal deposit. This is thought to occur through an adsorption mechanism that results in an additional rate-limiting step in the reduction of lithium. Such a rate-limiting step would limit the fast directional growth that leads to dendrites.
The concentration of the metal salt additives has an effect on both the deposit morphology and the redox efficiency of the system. According to the Nernst equation, it is possible to shift the reduction potential, which may affect an ions efficacy in reducing dendrites; however it is not possible to reduce all metal ions from TFSI salts so the importance of this potential shift is called into question.
Despite claims that dendrites are a major cause of lithium metal cycling inefficiency, adding metal ions to prevent dendrites did not yield an increased efficiency. In most cases, while adding metal ions positively affected the morphology, the cycling efficiency decreased slightly. This could be due to irreversible reduction of some metal ions
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