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Edited by Peter J. Rossky of Rice University in Houston, Texas, approved on July 3, 2021 (review received on March 16, 2021)

At high ion concentrations, the distribution of ions at the charged solid-water interface (known as the electric double layer (EDL)) is poorly understood, partly because of the lack of a molecular-level description of the interactions between adsorbed hydrated ions. Here, the direct visualization of the salinity-dependent evolution of the EDL structure reveals the molecular origin of the non-classical transformation of EDL, where charge overscreening and heterogeneous nucleation are driven by ion-ion correlation at the interface. This manifestation of the atomic basis of non-classical behavior provides a much-needed understanding of the effects of ion synergy at charged interfaces to develop predictive models of element transmission in natural environments and advanced technologies for material growth and synthesis in salt environments.

The classic electric double layer (EDL) model is the basis for representing the atomic structure and reactivity of the charged interface. An important limitation of these models is that they rely on a mean field approximation that is strictly valid for dilute aqueous solutions. Due to the lack of visualization of the structure over a wide range of ion concentrations, theoretical efforts to overcome this limitation have been severely hampered. Here, we report the salinity-dependent evolution of the EDL structure at the negatively charged mica-water interface, revealing the transition from Langmuir-type charge compensation in dilute salt solutions to non-classical charge sieving in high-concentration solutions . The EDL structure in this overcharged state is characterized by the lateral positional correlation between adsorbed ions and the development of alternating cations and anions vertical stratification, which is reminiscent of the structure of strongly correlated ionic liquids. These EDL ions can spontaneously grow into nanocrystalline nuclei of ionic compounds at a threshold ion concentration significantly lower than the bulk solubility limit. These results reveal the impact of ionic synergy, which drives the heterogeneous non-classical behavior of EDL under high salinity conditions.

Our understanding of the behavior of ions at the charged-solid-water interface is based on the theory of electric double layer (EDL), which represents the charge shielding that occurs through two phenomena: ions interact with specific surface sites in the Stern (or inner Helmholtz) layer. And the interaction of ions with the average electrostatic field in the diffusion (or outer Helmholtz) layer (ie, using the linearized Poisson-Boltzmann equation) (1). A well-known weakness of these classical theories is that they explicitly ignore the energy contribution of the interaction between adsorbed ions (2⇓ –4) and hydration effects (5⇓ ⇓ –8). Therefore, they are often unable to describe the various interface phenomena that occur under high salinity, which are common in geological and biological systems (9⇓-11) and chemical and engineering processes (12⇓⇓-15). For example, the recent interest in using solvate melt electrolytes in electrochemical energy storage systems is caused by the enhancement of the electrolyte stability window, which is speculated to be related to the formation of solid electrolyte interfaces on the electrodes, but the detailed mechanism has not yet been determined. Be fully understood (16⇓ –18). These gaps in our understanding of the molecular scale have been a bottleneck in the development of environmental remediation prediction models and advanced technologies for material growth and synthesis.

Here, we observed the atomic details of the EDL structure in situ at the charged solid-water electrolyte interface to visualize the molecular origin of non-classical behavior as a function of salinity. In situ high-resolution X-ray reflectance (XR) was used to study the distribution of ions adsorbed on the surface of muscovite negatively charged substrates, which is a clear interface system with a fixed charge density (8, 19, 20). 21) Determine the atomic scale structure (ie electron density distribution) as a function of distance from the interface (22, 23). Our research focuses on simple Rb-based aqueous electrolytes (ie RbCl and RbI solutions), in which the distribution of Rb adsorbed on the mica-water interface is directly detected by element-specific resonance abnormality XR (RAXR) (24, 25). These structures were compared with those derived from molecular dynamics (MD) simulations using methods previously developed for alkali metal halide systems (26). This combination of experimental and computational methods provides unexpected insights that affect how we view and understand the ion-ion correlation (2⇓ –4) and ion hydration (5⇓ ⇓ –8) near the charged interface. Non-classical interface phenomenon. Specifically, the results reveal the way in which the surface structure and charge distribution control ion adsorption under high salinity, where specific interactions between adsorbed ions lead to strong deviations from the classic EDL model, including charge sieving, multilayer ion adsorption And, finally, the heterogeneous nucleation and growth of ionic crystals.

The sample used for XR measurement was prepared by contacting freshly cracked muscovite crystals with RbCl or RbI solutions in thin-film batteries (22) (Figure 1A, SI appendix, Figure S1). All solutions are prepared at a pH close to neutral (~6) to minimize the influence of hydronium and hydroxide ions in the system. The XR signal, defined as the ratio of reflected to incident X-ray intensity (21), is collected as a function of the momentum transfer (q) along the surface normal direction, where q is defined as 4πsin(2θ/2)/λ 2θ, the incident sum The angle between the reflected X-rays, λ = 0.689 Å, and the X-ray wavelength is 18.0 keV (materials and methods). The RAXR spectrum is measured as a function of the photon energy E, near the edge energy of the X-ray K absorption of Rb under a series of fixed q values. As mentioned earlier (27), the ion concentration in thin-film batteries increases over time because water evaporates slowly through the permeable Kapton membrane. The fluorescence yield (FY) of Rb Kα measured as a function of time (t) and position (x) transverse to the scattering plane shows the greatest increase in the concentration of central ions on the sample surface (Figure 1B). As far as we know, in most previous XR studies, this chemical change has been neglected: otherwise, its effect can be mitigated by measuring the data away from the center of the substrate surface or frequently flushing the solution in the battery. In this In this study, we used this time change of solution composition to gain insight into the relationship between solution chemistry and interface structure. In other words, we use Rb Kα FY to continuously monitor the changes in the composition of dissolved ions, especially Rb, while optimizing the measurement scheme (ie angle range and counting time) to complete an XR scan in a shorter period of time (~ 3 minutes Rather than the typical time of >30 minutes), thereby minimizing the change in ion concentration within each scan. This method allows us to obtain multiple XR data sets spanning a wide range of Rb concentrations in a single experiment (Figure 1C). By using eight different mica crystals (purchased from Asheville-Shoonmaker Mica Company) (SI appendix, Table S1), repeated measurements were used to test and confirm the reproducibility of the XR data.

In-situ high-resolution X-ray reflectance and MD simulation were used to observe the EDL structure at the muscovite (001)-salt solution interface. (A) Experimental device for X-ray reflectivity. Single crystal muscovite mica in contact with RbCl or RbI solution is encapsulated in a thin-film X-ray cell (22). (B) The position-dependent evolution of the Rb concentration ([Rb]) in the cell recorded as a function of time. Collect data by translating the mica sample laterally to the X-ray scattering plane (initial [Rb] is 0.3 M). The black arrow with four circles schematically shows the way to collect a series of XR data sets during the evolution of the solution composition in the cell. (C) A series of XR data sets as a function of [Rb]. Collect data with an average [Rb] of 0.36, 0.76, 1.6, and 2.6 M as a function of the momentum transfer (q) along the surface normal, and 10−2, 1, 102, and 104 respectively for clarity. (D) MD simulation snapshot at [Rb] = 0.5 M (yellow = Si; pink = Al; red = O; white = H; cyan = K; purple = Rb; blue = Cl). The water molecules are displayed using a stick model.

A series of XR data sets measured under various RbCl components are shown in Figure 2A and the SI appendix. S2-S4. These data are drawn after normalization to the general crystal truncated rod shape (22) to visually enhance the fine details related to the change of the interface structure. The most significant change was observed in the q range of 0.8 to 1.7 Å-1, where the single concave pattern at [Rb] ≤ 0.05 M gradually transformed into a bimodal pattern with the increase of [Rb] (Figure 2A). This concentration-dependent change is different from our previous results at lower [Rb] (ie 0.001 to 0.020 M), where the XR data is largely independent of salinity (22, 28, 29). In this lower concentration range, the maximum Rb surface coverage matches the cationic charge density required to compensate for the negative charge density on the mica surface. Compared with these observations, our observations at higher [Rb] reveal the beginning of the secondary evolution of the EDL structure outside of the diluted electrolyte system.

Structural changes at the muscovite (001)-RbCl interface measured by X-ray reflectance and MD simulation. (A) Normalized XR as a function of [Rb] (0.022, 0.050, 0.090, 0.36, 0.49, 0.76, 0.84, 1.3, 2.1, 2.6, 3.7, and 6.7 M). These data sets are scaled by a factor of 10n–1, where n (= 1, 2, 3, ...) increases from bottom to top in the order of increasing [Rb]. (B) The total electron density profile derived from the best fit model of the XR data. The result is normalized to the density of a large amount of water and expanded by the spatial resolution of the data (π/qmax = ∼0.8 Å, where qmax is the maximum q of each XR data set). The thickness of the line reflects the uncertainty of the two standard deviations. The profile at the lowest [Rb] (= 0.022 M) is offset by 3, and each of the subsequent profiles is offset by 1 from the previous profile. Rb specific contours derived from RAXR data in [Rb] = 0.003 and 0.4 M (ρRb0.003 and ρRb0.4) are drawn without vertical offset. (C) Total electron density profile derived from MD simulation as a function of [RbCl] (= 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0 M). The results are calculated based on the density distributions of the nuclei, which are weighted by the number of electrons associated with each nucleus and broadened by the spatial resolution of the corresponding XR data. The profile offset at [Rb] = 0.1 M is 5 to match the profile offset determined experimentally at similar [Rb] (= 0.09 M), and each subsequent profile is offset by one from the previous profile. The Rb profile determined by MD at the lowest [RbCl] limit and 3.0 M (ρRb0.0 and ρRb3.0) has no vertical offset, while the Cl determined by MD in the same solution (ρCl0.0 and ρCl3.0) Sectional view. 0) Draw as a dashed line with a vertical offset of 4. The black and green hollow arrows indicate the positions of Cl− and secondary Rb adsorbed on the mica (001)-water interface, respectively.

The best fit model of the XR data (SI Appendix, Tables S2-S4) attributed this unexpected change in reflectance mode to the excessive adsorption of Rb at the mica-water interface. Each total electron density distribution from the best fit model has a prominent solution species adsorption peak, adsorbed at a height (z) of about 2 Å (measured from the top oxygen plane of the mica surface) (Figure 2B and SI appendix, figure S2-S4). The electron density of this peak is significantly higher than that of bulk water, and its height matches the height of the inner sphere (IS) Rb adsorbed on the double-triangular cavity of the mica surface (25). As [Rb] increases, the integrated area under the peak (hereinafter referred to as the occupancy factor) increases, indicating that the adsorption of IS Rb increases. In addition to IS Rb absorption, other changes in the interface structure include the appearance and growth of electron density peaks at z ∼ 4 and 6.5 Å (indicated by the black and green hollow arrows in Figure 2B, respectively).

The RAXR measured in 0.003- and 0.4-M RbCl solutions was used to determine the salinity-dependent change of the precise distribution of Rb adsorbed on the mica-water interface (Figure 2B, SI appendix, Figures S5 and S6). In the 0.003-M solution, the derived specific distribution of Rb (ρRb0.003) contains IS Rb at z ∼ 2 Å, which is consistent with the position of the highest electron density peak of the solution distribution. The coverage of IS Rb (ΓRbIS) is ~0.8 Rb /AUC, where AUC = 46.72 Å2 is the area of ​​the unit cell in the mica (001) plane. The distribution map also has a small part of Rb (coverage range ∼0.1 Rb/AUC) adsorbed further away from the surface (concentrated at ∼6.5 Å, indicated by the green arrow at the bottom of Figure 2B). These two adsorbed Rb species compensated for about 90% of the negative charge (~1e-/AUC) on the surface of the mica (22), which is consistent with the expectations of the classical EDL theory. In a solution with a higher ion concentration (ie [Rb] = 0.4 M, Figure 2B and SI Appendix, Table S5), the derived Rb distribution (ρRb0.4) is shown to be 0.003-M RbCl (ρRb0.003). The IS Rb coverage rate of ρRb0.4 is about 1.5 Rb/AUC, which is almost twice that of ρRb0.003. This element-specific analysis also revealed two additional Rb species that are farther from the surface (approximately 6.5 and 11 Å) than IS Rb (SI appendix, Table S5) and have a smaller coverage area (approximately 0.5 and 0.3, respectively). Rb /AUC)). In general, the total coverage of Rb adsorbed on the surface of mica (~2.3 Rb /AUC) far exceeds the amount required for charge compensation on the surface of mica (~1 Rb /AUC), indicating a significant deviation from the charge screening Classic EDL model.

MD simulations were used to verify the salinity-dependent changes in the EDL structure observed by XR and RAXR. In short, the simulation represents a piece of muscovite mica whose 2M1 structure has a unit cell formula KAl2(Si3Al)O10(OH)2 based on Catti et al. (30), in a simulated pool of 52.1 × 45.2 × 180.0 Å3 periodically replicated with solutions of various concentrations (Figure 1D and SI appendix, Figure S7; materials and methods). A sophisticated force field (31⇓ ⇓ –34) selected based on our previous work (26) and additional verification (Materials and Methods and SI Appendix, Figure S8) is used to describe the interaction between atoms. Each system is balanced for 1 ns, and then simulated 30 ns at 298 K in a specification ensemble with fixed atomic number N, volume, V, and temperature T (NVT), with a time step of 1-fs.

The electron density distribution derived from the MD simulation in the RbCl solution is usually consistent with the experimental observations of our XR measurement (for example, the comparison of Figure 2C and Figure 2B). The simulation confirmed that the increase in electron density at z ∼ 2 Å is due to IS Rb adsorption (as shown by ρRb0.0 and ρRb3.0, obtained from the Rb atomic density distribution weighted by the number of electrons). They also showed that the electron density peak at 4 Å, which is not seen by Rb-RAXR, originates from Cl- adsorption above the IS Rb plane (indicated by black arrows in Figures 2B and C). Finally, they predicted the presence of additional Rb species at heights of ∼6.3 Å and ∼10 Å (as shown at the bottom of Figure 2C), but only at significantly higher concentrations (ie [RbCl] ≥ 2 M and 5 M, Respectively) instead of RAXR (ie [RbCl] = 0.4 M; Figure 2B). We note that this alternating layer structure is similar to the oscillating structure of ionic liquids observed at charged interfaces (35, 36), where the formation of discrete charge layers is controlled by the balance between the space between ionic substances and the electrostatic force (37, 38).

We also explored the role of anions by comparing XR data measured in RbCl and RbI aqueous solutions (SI Appendix, Figures S9 and S10, and Tables S6 and S7). When ~0.02 ≤ [Rb] <~1 M, the IS Rb coverage in RbI solution increases as [Rb] increases beyond the level required for charge compensation, which is consistent with the overcharge phenomenon observed in RbCl solution Consistent (Figure 3A). In the RbI solution, the electron density peak at z ∼ 4 Å, which is attributed to the co-adsorbed anion simulated by MD (Figure 2C), has a larger occupancy factor and height, which is related to the adsorption of more electron density and greater The I− is consistent. Cl- (Figure 3 B and C and SI appendix, Figure S9). However, when [Rb] ≥ 1 M, the distribution in the RbI solution shows obvious atomic stratification beyond z = 10 Å (SI appendix, Figure S10), in sharp contrast to those in the RbCl solution, which only shows Generally, the overall electron density is enhanced within the same height range, and there is no discernible change in the fine structure. These complex interface structures in RbI are due to the heterogeneous nucleation and growth of RbI crystals on the surface of mica (39, 40). When [RbI] ≥ 1 M, XR data shows that new Bragg peaks appear at q = 2.24 and 4.48 Å-1, corresponding to the (111) and (222) reflections of cubic RbI crystals (SI appendix, Figure 2). S10). Specifically, these scattering signals, such as the scattering signal of RbI (111), appear as well-defined peaks instead of arcs (the latter is an X-ray diffraction (XRD) pattern of randomly oriented salt powder), indicating these orders of structure The arrangement of the grade crystals relative to the mica (001) plane (SI appendix, Figure S11). It is worth noting that these solutions are unsaturated with respect to the solid phase (SI appendix), which means that this nucleation is caused by the structural similarity between the (001) and (111) planes of the mica surface (that is, the epitaxy ) Promoted RbI crystals. Previously, the heterogeneous nucleation of RbI crystals on the surface of mica has been observed (39, 40) and explained as the promotion of epitaxy, which is consistent with our explanation. In contrast, no similar behavior was observed for RbCl, presumably because the lattice spacing in the RbCl(111) plane is about 10% smaller than the lattice spacing on the mica surface (SI appendix, Figure S11).

Ion synergistic control of the interface energy and structure at the muscovite-salt solution interface. (A) [Rb] dependent changes in the coverage of IS Rb (ΓRbIS) in RbCl and RbI from XR. The solid black line passing through the data points comes from the two-K isotherm model (SI appendix). The deviation of the data from the classic Langmuir isotherm (solid green line) is due to overcharging. The theoretical limits related to the official charge on the surface of mica are also shown for comparison. The changes in IS Rb coverage obtained from MD simulations are also plotted (open circles). The simulated data showed a similar trend to the experimental data. However, the adsorption edge moved to a higher [Rb] by ~10, indicating that the simulated adsorption of Rb during overcharging was weaker than experimentally determined. (B and C) Comparison of the interface electron density distribution between RbCl and RbI solutions. The total electron density distribution measured in a solution containing [Rb] = ∼0.05, ∼0.5, and ∼1 M is shown as a dot, a dotted line, and a solid line, respectively. When [Rb] ≥ 1 M (ie lower than the saturation concentration of the ionic crystal [shown as the yellow area in A]), additional stratification was observed in the RbI solution (SI Appendix, Figures S10 and S11). (D and E) Snapshots of the mica-salt solution interface at 1 and 20 ns [RbI] = 3 M, respectively, showing IS Rb and the first layer co-adsorbed anions. (FH) Large-scale simulation of the mica-RbI solution interface at 3 M. Side view snapshots taken at 1, 18, and 30 ns show the evolution of heterogeneous nucleation events. Note that the large number of heterogeneous nucleation crystals at an earlier time (ie 18 ns) contrasts with the rare uniform nucleation (blue arrow, single event observed in the 30 ns simulation; movie S2).

The systematic changes in the absorption behavior of Rb provide a window through which the atomic details of the overcharging phenomenon at the mica-water interface can be observed (Figure 3A). In a [RbCl] solution containing [Rb] ≤ 0.02 M, ΓRbIS increases with the increase of [Rb] until it reaches a plateau of ∼0.9 Rb/AUC [This is better than the ΓRbIS determined by RAXR at [Rb] = 0.003 M About 10% higher (22)]. For [Rb]> 0.02 M (for RbCl and RbI solution systems), the Rb coverage gradually increases, beyond this platform, indicating overcharge. According to previous analysis (41), this overcharging occurred at a much lower salinity than expected (Figure 3A; adsorption edge concentration of Rb = 0.48 ± 0.11 M; SI appendix). For example, the transition from classical charge screening to non-classical charge screening in RbCl or RbI bulk aqueous electrolytes is expected to occur at [Rb] ∼ 3 M (41). The difference between this theoretical calculation and our experimental observations indicates that the mica surface plays a key role in promoting the emergence of non-classical effects. For [Rb] ≥ 1 M, the coverage of Rb is close to the number density of double-triangular cavity sites (= 2 sites/AUC) on the mica surface. Compared with these XR results, MD simulations that independently predict the same overcharge phenomenon underestimate the IS Rb coverage rate for a given [Rb]. This underestimation indicates that charge sieving is sensitive to one or several phenomena that are not fully or indirectly represented in our simulations. These phenomena may include atomic polarization effects, relaxation of mica surface structure, or details of mica structure charge distribution (materials and Method and SI appendix, Figure S7).

In order to clarify the atomic details of these non-classical phenomena at the mica-water interface, we simulated the time evolution of the RbI system at 3 M using a horizontally extended unit (260.5 Å × 225.0 Å in the appendix of DH and SI in Figure 3). S12 and film S1). In the early stage of the simulation (<∼2 ns, Movie S1), the coverage of IS Rb gradually increased, exceeding the amount required to fully compensate for the negative charge on the mica surface, indicating the beginning of overcharging. At this stage, the simulation trajectory shows that excessive Rb adsorption occurs at the double-triangular cavity site, where at least one pre-existing Rb is adsorbed in the adjacent cavity site, followed by the adsorption of anions (ie I-) (Figure 3D). These adsorbed anions are usually located near the centerline between two adjacent IS Rbs (Figure 3D and movie S1), and the adsorbed <Rb...I...Rb> spans two adjacent cavity positions Balanced the local charge density on the mica surface. As the simulation time increases, the coverage of IS Rb increases, resulting in IS Rb ions appearing more and more frequently in the three adjacent cavities of the triangular configuration. In this geometry, the co-adsorbed anions are stabilized above the center of the triangle formed by the three IS Rbs (Figure 3E and Movie S1). These simulations clearly show that the synergy between adsorbed cations and anions, and the transformation of the related ion structure, promotes the gradual evolution of the arrangement of adsorbed ions in the Stern layer.

Our simulation also shows the additional adsorption of Rb on the pre-established adsorption anion plane (that is, beyond the expectation of the classic Stern layer structure). In the RbI system, this secondary Rb adsorption only occurs at a single site (SI appendix, Figure S13), resulting in an alternating layered structure of Rb and I-, which eventually evolves into the nucleation and growth of crystal structures in multiple locations Surface (figure 3 FH and movie S2). Using the set of force fields used in this study, although there are metastable RbI and RbCl ion pairs at [Rb] ≥ 1 M (SI appendix, Figure S12 and SI appendix), the growth is mainly based on the ion-by-ion classical nucleation model The attachment mode described in (42, 43). The resulting crystal structure is in the form of a cubic RbI crystal, with its (111) plane aligned parallel to the (001) plane of the mica substrate, as confirmed by our in-situ observation of the Bragg peak when the RbI concentration increases (SI Appendix, Figures S10 and S11). In contrast, the MD simulation in the RbCl system shows that the secondary Rb ions dynamically exchange their positions between two different states of equivalent energy, and before [RbCl] approaches the salt saturation level (SI), no observations are made Appendix for heterogeneous nucleation of RbCl crystals, Figure S13 and SI Appendix).

We use a combination of experimental and computational methods to visualize the atomic-scale details in the evolution of the EDL structure at the surface of the muscovite negatively charged substrate in contact with the concentrated salt solution. The observed transition from classical Langmuir-type full charge compensation of adsorbed ions to non-classical charge over-screening is controlled by the positional correlation between adsorbed ions, rather than chemical complexation as commonly assumed in current physical and chemical theories ( For example, ion pair formation). The charge sieving at the mica-saline solution interface is described by discrete atomic layers formed by alternating monovalent cations and anions. For the ion-ion correlation theory (2⇓ –4) of aqueous solutions, this observation is unexpected, where the positional correlation between monovalent ions is usually assumed to be negligible. The observed multilayer structure is similar to the oscillating structure of the ionic liquid formed on the charged interface of the solvent-free system (35⇓ ⇓ –38). Here, the correlation between monovalent ions is enhanced at elevated ion concentrations near the highly charged surface (ie, low water content). In natural water, these ion-ion correlations are expected to be affected by other ions, including hydronium and hydroxide ions in acidic and alkaline solutions, through competition for adsorption sites on the surface of mica or changes in ion morphology ( For example, ion pair formation and hydrolysis) in solution (28, 29, 44). Due to stronger ion-ion correlation (2⇓ –4) and increased ionic hydration (5⇓ ⇓ –8), these interactions are expected to be more pronounced and complex for multivalent cations. In the RbI system, the ions adsorbed in this multilayer structure are continuously rearranged in the two-dimensional lattice structure, promoted by epitaxy, and eventually evolve into the nucleation and growth of ionic crystals, even if it is relatively solid. This is also true in solutions with unsaturated phases. These results clarify the way in which the surface structure and charge distribution control the specific interaction of adsorbed ions beyond the Stern layer. More generally, the performance of the atomic basis of non-classical behavior in EDL provides a predictive model for the development of elemental transport in natural environments (including nutrients and heavy metals) and advanced material growth and synthesis technologies, including energy storage materials, for the synergy of charged interface ions. .

The experimental solutions were prepared by dissolving RbCl (99.8% from Sigma-Aldrich) and RbI (99.9% from Sigma-Aldrich) salts in deionized water (DIW) (18.2 MΩ·cm). Each salt solution is prepared with a series of Rb concentrations [Rb], ranging from 0.01 to 3.0 M (molar concentration, mol/L). The prepared solution is stored in a high-density polyethylene (HDPE) bottle before use. Split the gem-grade single crystal muscovite from Asheville-Schoonmaker Mica Company to expose the fresh (001) split surface, and rinse with DIW. Before injecting the Rb-containing solution, the wet crystal is installed in the X-ray film cell (21).

X-ray data is collected at the beam line 33-ID-D of the advanced photon source. The monochromatic incident X-ray beam of 18.00 keV (that is, the wavelength λ is 0.689 Å) is vertically focused by the Kirkpatrick-Baez mirror. The size of the focused beam is 0.03 mm (v) × 1.0 mm (h), and the flux at the sample position is ~5 × 1012 photons/sec. Collect in-situ crystal truncated rod (CTR) data in mirror geometry as a function of momentum transfer q, defined as 4πsin(2θ/2)/λ = 2πL/d, where 2θ is incident and reflected X-rays and L is muscovite The Bragg index of (001) reflection, d of ~19.96 Å is the (001) interlayer spacing (45). Each data set was collected in the q range from ~0.15 to ~4.0 Å-1. During this measurement, the change of [Rb] is monitored by measuring Rb Ka FY at a constant incident angle (α = 1.6°, corresponding to momentum transfer q = 0.48 Å-1). At this angle, the X-ray beam irradiates the mica surface of ∼1.0 mm × 1.0 mm. The CTR data was analyzed using the least squares regression algorithm described in the SI Appendix, Supplementary Methods.

The in-situ RAXR spectrum is collected by scanning the X-ray K of Rb under a series of fixed q to absorb the photon energy (E) around the edge energy (ERb = ∼15.2 keV). The evolution of [Rb] is also monitored by measuring Rb Ka FY. When [Rb]> 0.6 M, we rinse the battery with fresh 0.1-M RbCl solution, and the excess solution is drained from the battery by gravity. At the beginning and during a series of data collection in one setting, the stability of the interface structure was monitored by periodically measuring the RAXR spectra at q = 0.48 Å-1, where the RAXR signal was sensitive to the coverage of adsorption and the average highly sensitive cations (46) . The analysis only includes data measured in the range 0.2 M ≤ [Rb] ≤ 0.6 M. The detailed information of RAXR data analysis is described in the SI Appendix, Supplementary Methods.

Except for the hydroxyl structure H atoms and interlayer K ions, the muscovite structure is simulated as a rigid body, and the SHAKE algorithm (47) is used for water molecules to maintain their rigidity. Solve the interatomic interactions up to 12 Å in real space. The particle-particle-particle-grid (PPPM) technology is used to solve long-range Coulomb interactions over 12.0 Å in k-space with an accuracy of 99.99%. All MD simulations are performed on the supercomputer of the National Energy Research and Scientific Computing Center (NERSC) using the code Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) (48).

According to the ideal muscovite unit cell chemical formula KAl2(Si3Al)O10(OH)2 (30), following the Löwenstein circumvention rule (that is, there is no adjacent isostructural Al substitution), a quarter of the Si atoms are replaced by Al, where each The substitution site of each tetrahedral sheet is randomly determined. The inhomogeneity of aluminum distribution (ie, charge distribution) in the mica model produced patches with higher or lower surface charge density (SI appendix, Figure S7). We noticed that the ideal formula for simulating muscovite may not exactly match the unit cell formula of natural muscovite used in the XR experiment, which may explain the slight difference between the predicted and measured IS Rb coverage.

For the RbCl and RbI systems, we simulated a large amount of liquid water with a length of 1 ns for various concentrations. The simulation is run in an isothermal-isobaric ensemble with fixed atomic number N, pressure, P = 1 bar, and temperature T = 298 K (NPT), in a 31 × 31 × 50 Å3 unit with periodic boundary conditions. Plot the resulting density as a function of ion concentration (in molar concentration, mol/kg) and compare it with the experimental data (SI Appendix, Figure S8 A and B). The experimental data of RbI solution is limited, so the choice of force field is mainly notified by the RbCl system. The results show that the parameters of Dang (33) and Dang and Garrett (32), combined with the extended simple point charge (SPC/E) water model (31), underestimate the salinity dependence of water density by about 2 to 10%. From experiments Data (49, 50). The interaction parameters recently proposed by Joung and Cheatham (51) produced improved predictions of RbCl density. Unfortunately, as far as our XR data is concerned, the compatibility of Joung and Cheatham parameters with the CLAYFF force field (34) is not satisfactory, as shown in Figure S8C, where the density enhancement due to IS Rb adsorption is predicted to be 1.7 Å (offset by 0.3 Å), as the concentration increases, the second and third density peaks tend to move away from the mica surface.

All research data is included in the article and/or supporting information.

This work was supported by the Office of Science, Basic Energy Science, Chemical Science, Earth Science, and Biological Sciences of the U.S. Department of Energy. According to the contract DE-AC02-06CH11357, UChicago Argonne, LLC acts as the operator of Argonne National Laboratory (Argonne) ( For SSL and PF for XR data measurement and analysis) and DE-SC0018419 for Princeton University (for AK and ICB for calculation simulation and data analysis). X-ray data is collected at beamline 33-ID-D, an advanced photon source. The use of advanced photon sources is supported by the Office of Science and Basic Energy Sciences of the U.S. Department of Energy. According to the contract DE-AC02-06CH11357, UChicago Argonne, LLC acts as the operator of the Argonne National Laboratory. The MD simulation was performed using NERSC resources, which was supported by the US Department of Energy's Office of Science in accordance with the DE-AC02-05CH11231 award. The submitted manuscript was created by UChicago Argonne, LLC, the operator of Argonne. Argonne is a laboratory of the Office of Science of the U.S. Department of Energy and operates under contract DE-AC02-06CH11357. The U.S. government reserves for itself and others acting on its behalf the non-exclusive, irrevocable global license paid in the article to reproduce, prepare derivative works, distribute copies to the public, and perform and display publicly, by or On behalf of the government.

↵2 Current address: Department of Energy and Geosciences, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

Author contributions: SSL, AK, ICB and PF design research, conduct research, analyze data and write papers.

The author declares no competing interests.

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