Lithium-Silver Alloys: Solid-Solution Ranges for High Capacity Foil Electrodes

Abstract

Silver is a promising electrode material for advanced lithium-based batteries, however it remains relatively unexplored due in part to the complexity of the lithium-silver phase diagram. [1] The larger opportunity is to realize the high capacity of lithium-rich phases with limited volume changes in a lithium-silver foil electrode.[2] ,[3] In order to accomplish this we must first understand the correlation between the equilibrium diagram and dynamic electrochemical phase formation.The lithium-silver equilibrium phase diagram consists of many solid solutions of varying lithium content and capacity. Notable phases (and their approximate compositions and gravimetric capacities based on total mass) are α (Li0-0.5Ag), β (Li1Ag, 230 mAh/g), γ3 (Li2Ag, 450 mAh/g), γ2 (Li4Ag, 800 mAh/g), γ1 (Li9Ag, 1500 mAh/g), and δ (Li50Ag, 3000 mAh/g). Early electrochemical studies of lithium-silver focused on sputter deposited silver films.[4] ,[5] These studies demonstrated the formation of three lithium-silver phases (β, γ3, and γ2) and exhibited poor capacity retention. The effect of deposition conditions (e.g. oxygen incorporation, rapid quenching) on the silver film and its electrochemical behaviour were unclear. More recently, lithium-rich (Li20Ag) composite foils were created by incorporating silver powder in molten lithium and then cooling. The resulting foils were then reversibly cycled between Li20Ag and Li5Ag (reversible capacity of 1600 mAh/g).2 Li20Ag has not been identified as an equilibrium phase but was also not identified as a composite of multiple Li-Ag phases. Each phase identified in the equilibrium diagram exhibited a wide, temperature-dependent composition range. The field needs conclusive identification of lithium-silver phases (including solid solution ranges) as a function of electrochemical conditions for reversible high capacity lithium-silver foil electrodes to be realized.Here we report on the electrochemical lithiation of silver foil using a suite of in-situ and operando methods over a wide range of temperatures and current densities. Two types of pure silver foil (99.99%, ESPI, 25 μm thick; and 99.9%, Shizendo, 2 μm thick) were galvanostatically cycled against lithium metal using Conflat cells[6] and standard electrolytes. Thick foils were used as a pure reference to understand the effects of diffusion; thin foils enabled higher rates and more meaningful x-ray diffraction studies.Potential vs. composition data is provided in Fig. 1a). Results from thick foils are very complicated. In most cases the highest composition observed was approximately Li3Ag. This does not align with a phase boundary in the established equilibrium phase diagram. A solid solution is likely present at intermediate temperatures as a plateau at approximately 10 mV vs. Li/Li+ is apparent. Ex-situ x-ray diffraction of thick lithiated foils (not shown) indicated β and γ3 Li-Ag phases were present on the lithium-facing surface of the electrode. However, the backside of the electrode was nearly pristine Ag. Such discrepancies can be attributed to slow diffusion, low foil porosity, and the penetration depth of our 8 keV X-rays (a few μm).Thin silver films exhibited more complex electrochemical behaviour at high temperatures compared to previous reports of sputter deposited thin films.4,5 A plateau at approximately 150 mV vs. Li/Li+ can be attributed to the growth of the β phase of Li-Ag. A second plateau at approximately 60 mV vs. Li/Li+ corresponds with the expected formation of γ3 Li-Ag. Two additional features loosely correlate with the expected composition of γ2 and γ1Li-Ag. However, only γ2 is expected as γ1 is only thought to be stable to 120°C. Operando X-ray diffraction patterns collected from a thin silver foil lithiated at 110°C are provided in Fig. 1b). β, γ3, and γ2 Li-Ag phases, and broad solid solution ranges, are present. There is no clear structural or electrochemical evidence of the formation of γ1 Li-Ag. Given that γ1 has an approximate composition of Li10Ag and the high capacity of lithium-silver foil electrodes depend on even more lithium rich compositions (e.g. Li20Ag) extensive studies on the formation range of all Li-Ag phases are required. Electrochemically-accessible phase diagrams, including solid-solution ranges, based on ex-, in-situ, and operando electrochemical and structural measurements will be provided.[1] Okamoto, J. Ph. Eq., 38, 70 (2017).[2] Jin et al, J. Am. Chem. Soc., 142, 8818 (2020).[3] Wu et al., Adv. Energy Mater., 11, 2003082 (2021).[4] Taillades and Sarradin, J. Power Sources, 125, 199 (2004).[5] Xie et al., J. Solid State Electrochem., 15, 2031 (2011).[6] Fleischauer et al., J. Electrochem. Soc., 168, A398 (2019). Figure 1. a) Potential vs. composition curves for the insertion of lithium into silver foils. Electrochemical conditions are indicated. b) Operando electrochemical and x-ray diffraction patterns of the lithiation of silver foil. Figure 1