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During Li-ion battery operation, (electro)chemical side reactions occur within the cell that can promote or degrade performance. These complex reactions produce byproducts in the solid, liquid, and gas phases. Studying byproducts in these three phases can help optimize battery lifetimes. To relate the measured gas-phase byproducts to species dissolved in the liquid-phase, equilibrium proprieties such as the Henry's law constants are required. The present work implements a pressure decay experiment to determine the thermodynamic equilibrium concentrations between the gas and liquid phases for ethylene (C 2 H 4 ) and carbon dioxide (CO 2 ), which are two gases commonly produced in Li-ion batteries, with an electrolyte of 1.2 M LiPF 6 in 3:7 wt/wt ethylene carbonate/ethyl methyl carbonate and 3 wt % fluoroethylene carbonate (15:25:57:3 wt % total composition). The experimentally measured pressure decay curve is fit to an analytical dissolution model and extrapolated to predict the final pressure at equilibrium. The relationship between the partial pressures and concentration of dissolved gas in electrolyte at equilibrium is then used to determine Henry's law constants of 2.0 × 10 4 kPa for C 2 H 4 and k CO 2 = 1.1 × 10 4 kPa for CO 2 . These values are compared to Henry's law constants predicted from density functional theory and show good agreement within a factor of 3., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Interfacial structural and chemical evolution underpins safety, energy density, and lifetime in batteries and other electrochemical systems. During lithium electrodeposition, local nonequilibrium conditions can arise that promote heterogeneous lithium morphologies but are challenging to directly study, particularly at the nanoscale. Here we map chemical microenvironments at the active copper/electrolyte interface during lithium electrodeposition, presenting operando freezing cryogenic electron microscopy (cryo-EM), a new method, to lock in structures arising in coin cells. We find local ion depletion is correlated with lithium whiskers but not planar lithium, and we hypothesize that depletion stems from root-growing whiskers consuming ions at the growth interface while also restricting ion transport through local electrolyte. This can allow dangerous lithium morphologies to propagate, even in concentrated electrolytes, as ion depletion favors dendritic growth. Operando freezing cryo-EM thus reveals local microenvironments at active electrochemical interfaces to enable direct investigation of site-specific, nonequilibrium conditions that arise during operation of energy devices., Competing Interests: The authors declare the following competing financial interest(s): N.S.D. and K.J. are inventors on a U.S. Patent Application 2024/0060864 A1 related to this research., (© 2024 The Authors. Published by American Chemical Society.)