Solid-state batteries (SSBs) are widely viewed as a safer, higher-energy alternative to today’s Li-ion technology, particularly when paired with lithium metal anodes or anode-free designs. Yet, achieving high Coulombic efficiency and long calendar life remains difficult. Interfacial studies typically focus on the solid electrolyte interphase formed at the solid–solid boundary between the Li metal anode and the solid electrolyte. However, SSBs contain an often-overlooked feature: a gas phase trapped inside the cell, where freshly plated lithium can directly contact trace molecules such as O2, N2, CO2 and H2O. Now, Neil Dasgupta and colleagues from the University of Michigan and the Nissan Research Center, Yokosuka, reveal how these reactions form a distinct solid–gas interphase (SGI) that degrades lithium metal anodes in SSBs.
Using a customized environmental chamber, the team cycle anode-free SSBs under controlled gas compositions and pressures. They show that SGI formation depends strongly on plated lithium capacity, background gas species and ageing time, and that it directly influences Coulombic efficiency. Introducing higher N2 levels lowers Coulombic efficiency and leads to nitrogen-containing residues on the stripped surface, whereas CO2 atmospheres unexpectedly improve reversibility. To track these reactions in real time, the researchers employ operando X-ray photoelectron spectroscopy under ultrahigh vacuum, capturing the rapid chemical evolution of freshly deposited lithium even under trace gas exposure. Extending the study to solid-state pouch cells assembled in a dry-room environment, they further demonstrate that SGI growth impacts calendar life in practical configurations.
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