A two-step AFM test is developed to measure multiple mechanical properties of SEIThe substrate (anode) interference is corrected for the first timeThe maximum elastic deformation energy (U) of SEI indicates its cyclic stabilityStabilization of Sb and Sn microparticle anodes is realized by building high-U SEI The inability of solid electrolyte interphase (SEI) to accommodate anode expansion reduces the Coulombic efficiency, induces safety issues, and greatly hinders the development of batteries. The lack of links between mechanical properties of SEI and its cyclic stability renders efforts aimless and inefficient. Here, multiple mechanical properties of SEI are accurately characterized by continuous indentation tests with different loads applied. The maximum elastic deformation energy (U) of SEI is found to best correlate with its cyclic stability. A high U allows SEI to fully consume the energy exerted by anode expansion through reversible elastic deformation. The indicator U effectively explains the behaviors of K/Li metal anodes in different electrolytes and facilitates the rapid screening of promising electrolyte formulations for K/Li ion batteries. This work unravels fundamental mechanisms and provides an entirely new direction to explore high-U SEIs for next-generation batteries. The capability of a solid electrolyte interphase (SEI) in accommodating deformation is critical to the electrode integrity. However, the nanoscale thickness and environmental sensitivity of SEI make it challenging to characterize multiple mechanical parameters and identify an appropriate predictor for such capability. Here, we develop a feasible atomic force microscopy (AFM)-based nanoindentation test that circumvents the interference of the anode substrate and allows accurate probing of Young’s modulus and elastic strain limit of SEI. The “maximum elastic deformation energy” (U) that an SEI can absorb is proposed to predict the stability of the SEI, as successfully demonstrated in Li/K metal anodes. We show that another asset of U is to provide a rapid and effective means to screen proper electrolytes for the stabilization of Sn and Sb microparticle anodes through building high-U SEIs. Overall, this new indicator, U, offers future directions toward rational design of robust SEIs for advanced anodes. The capability of a solid electrolyte interphase (SEI) in accommodating deformation is critical to the electrode integrity. However, the nanoscale thickness and environmental sensitivity of SEI make it challenging to characterize multiple mechanical parameters and identify an appropriate predictor for such capability. Here, we develop a feasible atomic force microscopy (AFM)-based nanoindentation test that circumvents the interference of the anode substrate and allows accurate probing of Young’s modulus and elastic strain limit of SEI. The “maximum elastic deformation energy” (U) that an SEI can absorb is proposed to predict the stability of the SEI, as successfully demonstrated in Li/K metal anodes. We show that another asset of U is to provide a rapid and effective means to screen proper electrolytes for the stabilization of Sn and Sb microparticle anodes through building high-U SEIs. Overall, this new indicator, U, offers future directions toward rational design of robust SEIs for advanced anodes. The development of a stable solid electrolyte interphase (SEI) holds the key to the successful commercialization of Li-ion batteries (LIBs).1Choi J.W. Aurbach D. Promise and reality of post-lithium-ion batteries with high energy densities.Nat. Rev. Mater. 2016; 1: 1-16Crossref Scopus (2502) Google Scholar A SEI layer is formed on the anode surface because of the electrolyte reduction during the initial cycles. This layer allows the diffusion of Li ions but blocks the electron transfer to prevent further decomposition on subsequent cycling.2Li S. Jiang M. Xie Y. Xu H. Jia J. Li J. Developing high-performance lithium metal anode in liquid electrolytes: challenges and Progress.Adv. Mater. 2018; 30: e1706375Crossref PubMed Scopus (258) Google Scholar, 3Stephan A.K. Completing the Picture of the Solid Electrolyte Interphase.Joule. 2019; 3: 1812-1814Abstract Full Text Full Text PDF Scopus (2) Google Scholar, 4Heiskanen S.K. Kim J. Lucht B.L. Generation and Evolution of the Solid Electrolyte Interphase of Lithium-Ion Batteries.Joule. 2019; 3: 2322-2333Abstract Full Text Full Text PDF Scopus (206) Google Scholar The mechanical failure of the SEI because of the repeated volume expansion of the anode leads to a continuous consumption of the electrolyte and therefore a rapid decay in its capacity, which is among the greatest challenges in developing metal and alloy anodes that experience large volume expansion. This challenge spans essentially to all existing and future battery chemistries, becomes more acute with the upcoming Na and K batteries, and mainly suffers from the lack of an appropriate indicator to determine the capability of the SEI against deformation. To improve the mechanical properties of the SEI, many researchers have designed various approaches to tailor its microstructure, which has significantly improved the cyclic stability of the electrodes.5Zhang B. Rousse G. Foix D. Dugas R. Corte D.A.D. Tarascon J.M. 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Therefore, here, our motivation is to target the specific mechanical property of SEI so as to efficiently boost the cycling performance of batteries. Among the various mechanical properties, the Young’s modulus and elastic strain limit are most widely examined.11Gu Y. Wang W.-W. Li Y.-J. Wu Q.-H. Tang S. Yan J.-W. Zheng M.-S. Wu D.-Y. Fan C.-H. Hu W.-Q. et al.Designable ultra-smooth ultra-thin solid-electrolyte interphases of three alkali metal anodes.Nat. Commun. 2018; 9: 1339Crossref PubMed Scopus (175) Google Scholar, 12Koerver R. Zhang W. de Biasi L. Schweidler S. Kondrakov A.O. Kolling S. Brezesinski T. Hartmann P. Zeier W.G. Janek J. Chemo-mechanical expansion of lithium electrode materials - on the route to mechanically optimized all-solid-state batteries.Energy Environ. Sci. 2018; 11: 2142-2158Crossref Google Scholar, 13Kuznetsov V. Zinn A.-H. Zampardi G. Borhani-Haghighi S. La Mantia F. Ludwig A. Schuhmann W. Ventosa E. 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Commun. 2018; 9: 2340Crossref PubMed Scopus (66) Google Scholar It is generally accepted that an SEI with a high Young’s modulus (E) enhances the cyclic performance.17Stone G. Mullin S. Teran A. Hallinan D. Minor A. Hexemer A. Balsara N. Resolution of the modulus versus adhesion dilemma in solid polymer electrolytes for rechargeable lithium metal batteries.J. Electrochem. Soc. 2012; 159: A222-A227Crossref Scopus (310) Google Scholar, 18Liu Y. Lin D. Yuen P.Y. Liu K. Xie J. Dauskardt R.H. Cui Y. An artificial solid electrolyte interphase with high Li-ion conductivity, mechanical strength, and flexibility for stable lithium metal anodes.Adv. Mater. 2017; 29: 1605531Crossref Scopus (654) Google Scholar, 19Monroe C. Newman J. The effect of interfacial deformation on electrodeposition kinetics.J. Electrochem. Soc. 2004; 151: A880-A886Crossref Scopus (215) Google Scholar, 20Monroe C. Newman J. The impact of elastic deformation on deposition kinetics at lithium/polymer interfaces.J. 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USA. 2018; 115: 1156-1161Crossref PubMed Scopus (322) Google Scholar Generally, it is difficult to find a material that reunites both a high Young’s modulus and a large elastic strain limit. Finding the proper trade-off between these two parameters is becoming essential to regulate the mechanical property of SEI; however, we are missing explicit theoretical guidelines. More critically, the SEI has a rather complex chemical composition and nanoscale microstructure and is sensitive to electron beams and air.23Huang J. Guo X. Du X. Lin X. Huang J.-Q. Tan H. Zhu Y. Zhang B. Nanostructures of solid electrolyte interphases and their consequences for microsized Sn anodes in sodium ion batteries.Energy Environ. Sci. 2019; 12: 1550-1557Crossref Google Scholar,24v Cresce A. Russell S.M. Baker D.R. Gaskell K.J. Xu K. 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Power Sources. 2015; 277: 169-179Crossref Scopus (86) Google Scholar which is simpler, more efficient, and more reproducible than the tensile test of free-standing SEIs. Given that the thickness of SEI is on the same scale as the penetration depth of the indentation test, the elastic field under the probe is not limited to the SEI itself but also extends to the underlying anode.28Saha R. Nix W.D. Effects of the substrate on the determination of thin film mechanical properties by nanoindentation.Acta Mater. 2002; 50: 23-38Crossref Scopus (1229) Google Scholar As such, the mechanical response of SEI-anode in the nanoindentation test is more reminiscent of a coating-substrate system rather than a bulk material. Ignoring the substrate effect could lead to inaccurate test results. Surprisingly, heretofore, the effect of the underlying anode on the accurate evaluation of the mechanical properties of the SEI has not been reported. In this work, we deviate from the classical approach relying on monitoring the individual values of E and εY, and propose to use the “maximum elastic deformation energy” (U) that an SEI can absorb to predict its stability during cycling. We optimize the AFM-based nanoindentation test by conducting two consecutive tests at the same position of the SEI to separately determine its elastic and plastic properties. The interference due to the anode is eliminated by adopting a coating-substrate model. The reliability of U in predicting the stability of the SEI during cycling is verified in both K and Li metal battery systems. We then construct SEIs having high U to stabilize K-Sb and Li-Sn micro-particle anodes, which validate the benefit of the proposed indicator U in screening promising electrolytes. Clarifying the stress state of the SEI during charge/discharge cycles is the first step in unraveling the mechanical origin of stable SEI. To demonstrate our concept, we focus on the high-capacity metal and alloy anodes such as Sn or Sb, which suffer from severe volume expansion (nearly ∼300%) during cycling. It is worth noting that the stress state and deformation behavior on the SEI during cycling is complex and diverse, even on the same anode. For example, differences in the roughness and thickness of the SEI can result in the whole SEI receiving different kinds and degrees of stresses and deformations. This situation becomes even more complicated when summarizing different types of anodes. In order to reveal a uniform principle of SEI mechanical stability, we would like to find a stress state that most SEIs experience and compare the mechanical stability of the SEIs formed in different electrolyte systems under this same stress condition. In metal anodes, such as Li and K, the ions diffuse through the SEI and deposit on the current collector (Figures 1A and 1B ). Having a confocal uniform and smooth deposition is a coveted goal, but it is difficult to achieve in most cases. Under the internal pressure due to the battery encapsulation,29Louli A.J. Ellis L.D. Dahn J.R. Operando Pressure Measurements Reveal Solid Electrolyte Interphase Growth to Rank Li-Ion Cell Performance.Joule. 2019; 3: 745-761Abstract Full Text Full Text PDF Scopus (87) Google Scholar,30Wang M.J. Choudhury R. Sakamoto J. Characterizing the Li-Solid-Electrolyte Interface Dynamics as a Function of Stack Pressure and Current Density.Joule. 2019; 3: 2165-2178Abstract Full Text Full Text PDF Scopus (155) Google Scholar the non-uniformly deposited atom clusters and islands (in metal anodes, Figure 1B) or the electrochemically driven swelling of the electrode particles (in alloy anodes, Figures 1C and 1D) subject the SEI to a stress state akin to local indentation (inset of Figure 1E). In principle, either a larger E or εY helps resist or accommodate elastic deformation and favors uniform swelling19Monroe C. Newman J. The effect of interfacial deformation on electrodeposition kinetics.J. Electrochem. Soc. 2004; 151: A880-A886Crossref Scopus (215) Google Scholar,20Monroe C. Newman J. The impact of elastic deformation on deposition kinetics at lithium/polymer interfaces.J. Electrochem. Soc. 2005; 152: A396-A404Crossref Scopus (920) Google Scholar and therefore contributes to the stability of the SEI (Figure S1). However, the introduction of a parameter that could reflect the collective effects of E and εY will be more than welcome in broadly addressing the SEI mechanical stability. We here propose to use the “maximum elastic deformation energy” (U) that an SEI can store to predict its stability. The value of U corresponds to the area under the force-displacement (F-d) curve up to the elastic limit (Figure 1E; see Note S6):U=81545π5⋅r3⋅1−ν24⋅E⋅εY5(Equation 1) where ν is the SEI Poisson’s ratio, and the r radius of the rigid indenter is assumed the same (ν = 0.3, r = 1 μm) for all systems. In practice, values of E and εY of the SEI are obtained from AFM-based nanoindentation tests (Figures 2A and 2B ) in which the anode material is always bonded as a substrate to the SEI (because it is impossible to obtain a stand-alone SEI for testing). We developed a two-step nanoindentation test with the AFM in order to characterize separately both the Young’s modulus and elastic strain limit of the SEI. Two consecutive indentations are performed at the same position on the surface of the SEI. The results are usually plotted as the force (F) versus displacement (d) curves with two segments, showing the approach and retract process of the indenter. The first indentation is conducted with a small force (<30 nN) such that the deformation of the SEI is limited to the elastic region, as evidenced by the coinciding “approach” and “retract” curves (Figure 2A). From point 1 to point 2 in the approach segment of Figure 2A, the AFM probe moves closer to the sample surface driven by the extension of the piezo. The negative peak force between point 1 and point 2 is due to the attractive forces (Van der Waals force) between the sample surface and the AFM probe. The sample starts to deform at point 2 and continues to deform under the downward force exerted by the bending cantilever up to point 3. After reaching the pre-set maximum load, the piezo retracts and lifts the AFM probe (reflected as the retract segment in Figure 2A from point 3 to point 2, then to point 1). The disturbances of surface roughness, surface contaminants, agglomeration of small particles or sliding usually manifest themselves as irreversible processes in the indentation test, which are reflected in the mechanical curves as irregular perturbations in the loading section. The interference of these factors can be greatly eliminated by screening curves with fully coincided loading and unloading portions to ensure elastic deformation only. The Young’s modulus of the SEI is then evaluated by best fitting the unloading curve to the Hertz contact model (see Note S1)31Hertz H. On the contact of elastic solids.Z Reine Angew Mathematik. 1881; 92: 156-171Google Scholar taking into account the effect of the underlying anodes (see detailed discussion later). In the subsequent indentation, a larger load (∼900 nN) is applied to intentionally fail the SEI (as evidenced by Note S3 and Figure S2) and then to obtain data on its elastic strain limit. A representative force-displacement curve is shown in Figure 2B. Point b of the approach curve is located at the first discontinuity point corresponding to plastic yielding32Page T.F. Hainsworth S.V. Using nanoindentation techniques for the characterization of coated systems: a critique.Surf. Coat. Tech. 1993; 61: 201-208Crossref Scopus (154) Google Scholar,33Christensen R.M. Mechanics of composite materials. Courier Corporation, 2012Google Scholar of the soft organic phase in the SEI,34Seltzer R. Cisilino A.P. Frontini P.M. Mai Y.-W. Determination of the Drucker–Prager parameters of polymers exhibiting pressure-sensitive plastic behaviour by depth-sensing indentation.Int. J. Mech. Sci. 2011; 53: 471-478Crossref Scopus (38) Google Scholar which is used to determine the elastic strain limit of the SEI (Note S2).35Gao Y. Shi S.-Q. Zhang T.-Y. Adhesion contact deformation in nanobridge tests.Nanoscale. 2017; 9: 6033-6040Crossref PubMed Google Scholar It can be experimentally located as the first deviation point from the fitting curve (the blue dot line in Figure 2B) of Hertz’s contact model starting from point a. From point b to point c (point c is located as the first point that deviates from the fitting curve of the Hertz contact model starting from point d), the SEI below the probe undergoes plastic deformation, crack growth, and/or densification. As shown in Figure 2B, the distance between point a and point c can be approximated as the thickness t of the SEI.11Gu Y. Wang W.-W. Li Y.-J. Wu Q.-H. Tang S. Yan J.-W. Zheng M.-S. Wu D.-Y. Fan C.-H. Hu W.-Q. et al.Designable ultra-smooth ultra-thin solid-electrolyte interphases of three alkali metal anodes.Nat. Commun. 2018; 9: 1339Crossref PubMed Scopus (175) Google Scholar For each electrode sample, around 100 test positions are indented for calculations of the Young’s modulus and the elastic strain limit. The anode materials beneath the SEI that are generally harder and much thicker than the SEI itself serve as a substrate during indentation36Manika I. Maniks J. Effect of substrate hardness and film structure on indentation depth criteria for film hardness testing.J. Phys. D Appl. Phys. 2008; 41: 074010Crossref Scopus (69) Google Scholar,37Westbrook J.H. The science of hardness testing and its research applications. American Society for Metals, 1973Google Scholar (inset of Figure 2A). We thus conduct finite element (FE) simulations (with ANSYS) to explore the substrate effects in extracting the Young’s moduli from the nanoindentation tests (see Note S4). Figure 2C displays one typical FE simulation result, where the indentation depth (d) is one-tenth the SEI thickness (t = 30 nm). Referring to the legend of Figure 2C, the displacement degree of each point is indicated by its color. The deformed region extends beyond the SEI to the substrate, demonstrating clearly the non-negligible effect of the substrate. Figure S3C summarizes the FE simulation results and illustrates that omission of the substrate effect from the mechanics analysis significantly overestimates E by up to 200%. Such an over-estimation exacerbates with increasing indentation depth and widening difference between E and Es (that is the Young’s modulus of substrate). For a soft coating on a hard substrate, it is generally accepted that the substrate interference on hardness evaluation can be ignored when d/t < 0.1.36Manika I. Maniks J. Effect of substrate hardness and film structure on indentation depth criteria for film hardness testing.J. Phys. D Appl. Phys. 2008; 41: 074010Crossref Scopus (69) Google Scholar,37Westbrook J.H. The science of hardness testing and its research applications. American Society for Metals, 1973Google Scholar Our FE simulation results, however, show that the substrate has a more severe effect on the measurement of Young’s modulus. Even when d/t = 0.05, ignoring the substrate effect leads to a 50% overestimation of E when E/Es = 0.014, and a ∼30% overestimation when E/Es = 0.57. These results stress further that the substrate influence must be considered to accurately probe the Young’s modulus of the SEI by using indentation test. Bearing in mind the effect of the substrate, the Young’s modulus of the SEI, E, is determined from the recast equation:28Saha R. Nix W.D. Effects of the substrate on the determination of thin film mechanical properties by nanoindentation.Acta Mater. 2002; 50: 23-38Crossref Scopus (1229) Google Scholar,38Doerner M.F. Nix W.D. A method for interpreting the data from depth-sensing indentation instruments.J. Mater. Res. 1986; 1: 601-609Crossref Scopus (2414) Google Scholar,39King R.B. Elastic analysis of some punch problems for a layered medium.Int. J. Solids Struct. 1987; 23: 1657-1664Crossref Scopus (724) Google ScholarEr=[1−νtip2Etip+1−ν2E(1−e−αta)+1−νs2Es(e−αta)]−1(Equation 2) in which Er is the reduced modulus determined by fitting the unloading curve of the first AFM test to the Hertz contact model (Equation S1); νtip,νs, and ν are the Poisson’s ratios of AFM probe, substrate, and SEI, respectively; Etip and Es are Young’s moduli of the AFM probe and substrate, respectively; a (=πRd)is the square root of the projected contact area, where R is the radius of AFM probe; and α is a scaling parameter depending on a/t (see Note S5 and Figure S4). The values of d and t (Figure S6) are directly obtained from the force-displacement curves of the first and the second AFM tests, respectively, as marked in Figures 2A and 2B. Equation 2 is next used to experimentally probe the effect of the substrate on the modulus evaluation of the SEI in K metal anodes. We fabricated Cu/K cells by using Cu and K metal as the working and counter electrodes, respectively, where K is reversibly plated and stripped on Cu. SEI layers for mechanical characterization are in the discharged state after ten rounds of cycling in a wide variety of common electrolytes, including 1 M KPF6/ethylene glycol dimethyl ether (DME), 1 M potassium bis(fluorosulfonyl)imide (KFSI)/ethylene glycol diethyl ether (EGDE), 1 M KFSI/DME, 1 M KFSI/tetrahydrofuran (THF), and 1 M KFSI/ethylene carbonate (EC)-diethyl carbonate (DEC). The corresponding Young’s moduli of the SEI layers obtained with and without considering the substrate effect are plotted in Figure 2D. Regardless of the electrolytes, the Young’s moduli of the SEIs are overestimated by 41%–97% when the substrate effect is ignored. Having determined the correct Young’s moduli and elastic strain limits of the SEI layers, we next search for a possible correlation between the mechanical properties and the stability of the SEI. Figure S5A compares the Coulombic efficiency (CE) of the K metal anode in the above five electrolytes as a function of cycle number. For the 5 samples, the CE values are below 96% in the first cycle as a result of the SEI formation. Afterward, robust SEIs prevent the continuous consumption of electrolytes, leading to augmented CEs with cycling (Figures S5C–S5F), whereas a poor SEI fails (Figure S5B). The CE eventually reaches 99.15%, 98.88%, 98.57%, 94.24%, and 68.89% in the 10th cycle for 1 M KFSI/EGDE, 1 M KFSI/DME, 1 M KFSI/THF, 1 M KFSI/EC-DEC, and 1 M KPF6/DME systems, respectively (Figure S5A). This is in agreement with Xiao et al.,40Xiao N. McCulloch W.D. Wu Y. Reversible dendrite-free potassium plating and stripping electrochemistry for potassium secondary batteries.J. Am. Chem. Soc. 2017; 139: 9475-9478Crossref PubMed Scopus (276) Google Scholar who reported the benefit of ether-DME over carbonate-based electrolyte to improve the electrochemical performance of K metal batteries. Moreover, we observe that EGDE with a different linear ether structure from DME and THF with a cyclic ether structure also deliver promising performances. In addition, the SEIs formed in the first 3 systems display no obvious variation in their CEs over more than 100 cycles, whereas those in the last 2 systems fall quickly in the first 50 cycles. Thus, we use the average Coulombic efficiency (ACE) in the first 50 cycles to represent the stability of the SEI. To interrogate a possible correlation between the two mechanical parameters and the ACE, we construct (Figure 3A) the εY-E diagram within which each electrolyte is positioned with their respective ACE. The SEI in 1M KFSI/EC-DEC electrolyte having the highest E (642.04 MPa) among the five (Figure 2D) shows an inferior performance than its counterparts having lower E (Figures S5 and 3A). In short, there is no significant correlation between ACE and the individual values of εY or E. In contrast, we observe that the stable systems are located in the upper right region of the εY-E diagram. This indicates that the U (defined in Equation 1) could be a compelling indicator to represent the SEI stability given that it embodies the synergistic effect of εY and E. Further exploiting this direction, we plot the ACE against U in Figure 3B and observe a pronounced correlation between these two parameters. The U values of SEIs in the two poorly cycling systems are only about 70% of that in 1 M KFSI/EGDE. This is because an S