http://pubs.acs.org/journal/aelccp
Electrolyte Confinement Alters Lithium Electrodeposition Aashutosh Mistry,† Conner Fear,† Rachel Carter,‡,§ Corey T. Love,*,‡ and Partha P. Mukherjee*,† †
School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States Chemistry Division, U.S. Naval Research Laboratory, Washington, DC 20375, United States § NRC/NRL Cooperative Research Associate, U.S. Naval Research Laboratory, Washington, DC 20375, United States Downloaded via IOWA STATE UNIV on January 12, 2019 at 06:38:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: The metastability of lithium electrodeposition continues to be a scientific mystery. Local ionic depletion has been conventionally argued to be a root cause for nonlinear morphological manifestations. Given the bulk nature of electrolyte transport limitation, it should be absent for very small interelectrode separations; however, even under such conditions, sustained electrodeposition is not observed. We find that the passivating film formed due to lithium’s high reactivity alters the surface energies and in turn deposition preference for fresh lithium. This asymmetry in deposition preference leads to nonuniform surface structure growth and traps the electrolyte layer. Such electrolyte confinement causes polarization, even at subcritical currents. The existence of confined electrolyte and associated electrochemical complexations is proved through temperature-controlled electrodeposition experiments.
L
ithium metal electrode, despite its theoretical promise of the highest specific energy, lowest electric potential (zero volts vs Li), high reactivity, and electronic conductivity, remains a challenge for prolonged use.1−6 Past studies7−37 have identified dendritic growth38 as the root cause of electrochemical instability. Given the simplicity of the Li/Li+ redox couple, it is expected that a Li−Li cell would demonstrate high capacity (i.e., theoretically limited when all of the Li from one electrode deposits on the other), although such a symmetric construct behaves otherwise.36 In this regard, Sand’s time39 is often used to argue that plating terminates once the ionic concentration in the bulk electrolyte near the electrode being plated drops to zero. This criterion predicts a critical current density that locally depletes the ions being electrodeposited. Figure 1 demonstrates this regime where limitations arise due to bulk electrolyte transport. If Sand’s argument is valid, Li electrodeposition outside of this range should enable safe and sustained (until one electrode completely electrodissolves) operation. Surprisingly, even when electrodeposition is carried out in this “safe” regime based on Sand’s criterion, operational limitations are nonetheless observed (representative studies10,12,27,36 are identified, along with exemplar bounds for Li electrodeposition at 20 °C and 1 M liquid electrolyte). This regime of “unknown” operational limitation is of both scientific and practical interest as it covers the desirable range of currents and physical © 2018 American Chemical Society
Figure 1. Regimes of Li electrodeposition: Bulk electrolyte limitation identifies the currents and interelectrode spacings for which ionic concentration drops to zero near the electrode being plated. Even if electrodeposition is carried in the absence of bulk electrolyte limitations, the nonideal response is observed and points to the presence of previously unknown interactions. Representative studies are also identified on this map.
Received: October 19, 2018 Accepted: December 4, 2018 Published: December 4, 2018 156
DOI: 10.1021/acsenergylett.8b02003 ACS Energy Lett. 2019, 4, 156−162
Letter
Cite This: ACS Energy Lett. 2019, 4, 156−162
Letter
ACS Energy Letters
Figure 2. Electrodeposition surface structure growth: Extent of uniformity in deposition is correlated to surface structure growth. (a) Snapshots of surface structure growth; (b) Li electrode interfacial structure due to nonuniform electrodeposition and confined electrolyte layer; (c) resistance to ionic transport in the surface structure; abstracting the surface structure growth in terms of (d) critical species flux and (e) reaction area evolution.
from nonuniform electrodeposition (the electrolyte is lithiophilic; hence, it seeps into the new surface structure in response to capillary forces40 and makes a conformal contact). This electrolyte layer is different from that of the bulk electrolyte between the two electrodes and is referred to as “confined electrolyte” hereafter. Figure 2b demonstrates the electroplated electrode along with the corresponding confined layer. This confined electrolyte layer is further analyzed to characterize the ionic transport and reaction limitations. For example, Figure 2c presents a dimensionless ionic concentration profile for the corresponding critical flux. The concentration approaches the bulk value, Cf, at the interface of confined and bulk layers. The concentration at the confined electrolyte−lithium contact decreases as the flux increases and reaches zero for the critical molar flux.
dimensions. We identify interfacial energy differences as the origin of this otherwise perceived anomalous behavior. A distinct aspect of lithium electrochemistry is the formation of the solid electrolyte interphase (SEI).27 At the beginning of electrodeposition, the electrode surface is covered with an SEI (this surface is referred to as the pristine Li electrode in Figure 2a). At every instant of electrodeposition, Li can deposit on either this SEI-covered pristine Li or freshly deposited Li. Because the interfacial energies for both of these surfaces are, in general, disparate, energetics40,41 also contributes to electrodeposition dynamics. Geometrically, this manifests as the degree of nonuniformity. For example, a higher cohesive energy (i.e., new Li preferentially depositing on freshly deposited Li) results in greater nonuniformity. Figure 2a presents snapshots of a representative electrodeposition growth sequence for a particular choice of interfacial energies. As deposition takes place, the SEI-covered pristine Li surface area decreases and the freshly deposited Li surface area increases. The rates of interfacial area growth are related to the degree of nonuniformity. For example, the more nonuniform the deposition , the slower the reduction in the pristine area and the faster the increase in the fresh Li surface area. Note that this surface structure evolution is different from the morphological features observed in transparent cells.10,12 Such experiments have a relatively large interelectrode separation (millimeters in contrast to microns, Figure 1) and, consequently, represent a distinctly different length scale as compared to the interfacial structures studied in this work. Given this spatial inhomogeneity of the interfacial growth, ions do not reach the reaction sites as conveniently. This forms an electrolyte layer that is trapped in the asperities originating
• Transport: The irregular surface structure hinders the ionic mobilities, leading to a concentration drop in the confined electrolyte layer, Figure 2c. • Reaction: Nonuniform electrodeposition increases the lithium−electrolyte contact, i.e., the electrochemically active area. Comprehensive combinations of electrodeposition structures were studied with different interfacial energies and deposition thicknesses (refer to section S1 in the Supporting Information for specific details). The lateral dimensions of these structures are large enough to provide a representative area averaged description.42 The characterization results are abstracted in terms of the critical ionic flux, J* (expresses the transport efficacy) and reaction area, a*, and are summarized in Figure 2d,e. Both are dimensionless quantities and exhibit a 157
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Figure 3. Isothermal electrodeposition: (a) Schematic diagram of a Li−Li symmetric cell; (b) evolution of the electrode potential and contribution from interfacial resistance due to electrolyte confined in the surface structure; (c) evolution of ionic concentration in the bulk electrolyte during stage I; (d) comparison of concentration drops due to bulk and confined electrolyte; (e) critical current density changes in time in response to growth of the interfacial structure at the electrode undergoing plating; (f) transport limitations exacerbate as the operating current is increased. (b−e) Operation at 2 mA/cm2 and 20 °C with 20 μm spacing between the two electrodes.
events,38 and the nucleation site density (# of sites/area) provides an estimate for the length scale of interfacial features, l. Mathematically
dependence on the amount of deposition (i.e., deposition thickness, h* = h/l) as well as the extent of nonuniformity. The reaction area, a*, is a ratio of the effective electrochemical area (i.e., reaction area) to the cross-sectional area. The critical flux, J*, accounts for the confined electrolyte resistance and represents the molar flux at which the ionic concentration at the electrode surface becomes zero. It is the maximum species flux that can be sustained without starving the corresponding electrochemical reaction(s). It relates to cation (Li+) flux as follows
J = J *·
Df Cf l
l=
Iapp ∂C = (1 − t+) F ∂x
(3)
where N0 is the nucleation site density. Figure 2d reveals that as electrodeposition becomes more nonuniform J* decreases, which suggests an increased transport resistance. The nonuniform surface structure growth has a higher effective area than the uniform deposition (Figure 2e). A higher value of J* means more efficient transport, while higher a* implies improved kinetics. Microscopically, the electrodeposition current is made up of multiple growth events taking place at the electrochemically active surface. Because nucleation is a prerequisite for growth, current is distributed over nucleation sites. Thus, the active surface growth is intrinsically inhomogeneous. For largescale electrochemistry, e.g., metallurgical electrodeposition, this surface inhomogeneity is much smaller compared to the limiting bulk lengths, and the active surface can be treated as flat. However, this interfacial inhomogeneity becomes relevant for Li electrochemistry. At such smaller lengths where inhomogeneity is intrinsic, uniform electrodeposition refers to a dense lithium deposition structure with surface undulations being much smaller than the deposition thickness, and the nonuniformity increases as undulation heights become comparable to the deposition thickness (Figure S1).
(1)
where Df and Cf are salt diffusivity and concentration from the bulk electrolyte profile. The molar flux is further related to applied current density via the transference number as J = −D
1 N0
(2)
In the context of intercalation or the conversion electrode, the appropriate length scale is visibly present as the particle dimension43,44 or pore size;45 however, a representative length scale for electrodeposition is not readily apparent. The interelectrode spacing in a Li−Li symmetric cell is the length scale associated with the bulk electrolyte transport and does not serve as a representative length scale for the electrode− surface complexations. The aforementioned interfacial energyguided preference is closely tied in with the nucleation 158
DOI: 10.1021/acsenergylett.8b02003 ACS Energy Lett. 2019, 4, 156−162
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Figure 4. The thermal gradient effect on Li electrodeposition is closely associated with changes in the morphology of the interfacial structure when plated at different temperatures. Comparison of three operating scenarios (all at 2 mA/cm2; counter electrode held at 20 °C): cold plating at −5 °C, isothermal at 20 °C, and hot plating at 45 °C. (a) Potential evolution; quasi-steady (b) bulk concentration profiles and (c) salt diffusivities. The cells were dissembled after the operation, and the plated electrodes were visually examined. (d,e) Optical images demonstrate the correlation between surface uniformity and plating temperature. (f) The spacing between the two electrodes affects the origins of transport limitations. As spacing increases, transport within the bulk electrolyte becomes rate-limiting.
bulk concentration profile is sketched in Figure 3c, which evolves during stage I and then becomes quasi-static (here C* = C/C0, with C0 being the initial electrolyte concentration). For a Li/Li+ redox couple, the open-circuit potential is governed by the Nernst relation as (U0 = 0 V at C0 = 1000 mol/m3)
Lithium electroplating is studied here in a symmetric setup (Figure 3a) wherein passage of current strips one electrode of lithium (at x = 0), the current translates from electronic to ionic at the Li−electrolyte interface and generates Li+ in the electrolyte, ions travel to the other electrode (ionic current) and eventually plate at x = L. Plating/stripping events cause a bulk flow of the liquid electrolyte as it is an incompressible fluid. (Section S3 summarizes the relevant governing equations within a moving coordinate frame, which accounts for the advection, i.e., flow, of the electrolyte; here x represents the coordinate attached to the surface of the electrode being stripped.) Li is a good electrical conductor and shows a negligible potential drop. Also, given the high reactivity of Li, the kinetic overpotential is also quite small (this is also a distinct characteristic of Li electrodeposition as compared to other electrodeposition systems where kinetic overpotential affects deposition morphology38,46). Thus, ionic concentration evolution is the dominant cause of the overpotential. Figure 3b shows the evolution of potential ϕ = ϕsx=0 − ϕsx=L when Li electroplating is carried out at 2 mA/cm2 under isothermal conditions (20 °C). ϕs is the potential of the electrode and is related to local electrolyte phase potential ϕe via the kinetic overpotential. The potential profile exhibits three stages: (I) early time increase in the potential, ϕ, (II) a nearly constant potential, and (III) the eventual sudden rise that reflects the end of electrodeposition. In order to understand the origins of these features, the concentration profiles are analyzed. The
U = U0 +
RT log C C0 F
( ) ≈ RTF log( C C ) 0
(4)
+
During stage I, as the local Li concentration drops at the electrode being plated, U decreases below zero and results in a gradual increase in the potential, ϕ. Because the electrolyte concentration profile is quasi-static, ϕ stays almost invariant during stage II. Once sufficient electrodeposition has taken place, confined electrolyte limitations set in that cause the concentration at the electrode surface to drop rapidly and lead to the final sudden rise. The transport resistances in the two electrolyte layers are directly related to respective concentration drops in Figure 3d. During stages I and II, the concentration drop in the bulk electrolyte is dominant and dictates the potential evolution, while later in stage III, the confined electrolyte layer becomes limiting due to nonuniform growth. The contribution of the confined layer transport resistance can be defined as %R confined = 159
ΔCconfined × 100 ΔC bulk + ΔCconfined
(5)
DOI: 10.1021/acsenergylett.8b02003 ACS Energy Lett. 2019, 4, 156−162
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temperature-driven morphological variations, the electrodeposited electrodes were examined optically inside of a glovebox. Figure 4d,e confirms the mechanistic interpretation of the experiments, thus simultaneously proving the hypothesized confined electrolyte effect. The variation of the deposition nonuniformity (eq S3) with temperature suggests that at the interfacial scale the surface energies of the Li covered with SEI and freshly deposited Li scale differently with temperature, and the relative disparity grows at smaller temperatures (both surface energies change with temperature40). To assess the pertinence of Sand’s argument,39 the field evolutions are analyzed for different interelectrode spacings (all at 2 mA/cm2). Figure 4f shows the concentration drop in the bulk electrolyte and reveals that it monotonically grows with interelectrode spacing. Thus, the bulk electrolyte becomes ratelimiting at a larger interelectrode spacing (∼millimeters), which is in line with the conventional belief.10,12 However, Sand’s argument fails to account for surface structure growth and associated confined electrolyte limitations when the electrode spacing reduces to microns. Here we identify the existence of this new mechanism limiting Li electrodeposition based on theoretical analysis and controlled experiments. Electroplating of lithium has its unique set of complexities arising from interfacial metastability. The formation of an SEI changes the surface energy and equivalently the energy landscape for lithium depositions as compared to the fresh lithium. These energetic differences give rise to qualitatively different evolution of surface structures. The nonuniform electrodeposition forms an irregular electrochemically active interface. Ionic transport is hindered due to such geometrical nonuniformity and causes an additional concentration drop that scales with the extent of electrodeposition. The potential evolution correlates to this concentration drop. Note that the physical features of such structural evolution are quite smaller compared to the dendritic lengths often observed in Li metal electrodes. This hypothesized mechanism is confirmed via controlled experiments. It is found that the surface structure growth becomes more nonuniform at lower temperatures which exacerbates the electrolyte confinement. Because the surface structure growth and electrolyte confinement are dynamically coupled, the electrochemical surface should be further probed to identify factors furnishing reduced confinement.
Figure 3b presents this confined layer resistance, alongside the potential evolution profile delineating the dominance of confined electrolyte layer limitations in stage III. The rate of deposition is proportional to the applied current, Iapp, and equivalently, the deposition thickness grows linearly (Figure 3e). For a given degree of nonuniformity, the critical flux J* varies with deposition. The critical current, Icrit, is related to the critical flux as Icrit = F
Dx = L Cx = L J* (1 − t+)l
(6)
When the confined electrolyte is rate-limiting, the cell operation stops as Icrit approaches Iapp. As electrodeposition is carried out at higher currents, the general nature of Icrit remains the same, as shown in Figure 3e, and higher current suggests that Icrit → Iapp at an early time and explains the behavior in Figure 3f. The higher currents give rise to starker gradients and an equivalently higher concentration overpotential. Of various physicochemical interactions taking place during Li electrodeposition, ionic transport is rate-limiting (good electrical conductivity and high reactivity rule out other effects). Bulk ionic transport, i.e., in the region between two electrodes, scales down with the interelectrode spacing. In general, as the length scales become smaller, interfacial effects dominate over bulk behavior.40 Hence, a smaller length scale transport effect has to be the dominant factor affecting the electrodeposition dynamics. The confined electrolyte effect proposed here is a joint outcome of the geometrical evolution of the interface and resultant alteration of ionic transport to reaction sites. The growth characteristics of the surface structure are implicitly related to interfacial energies. The surface energies are, in general, a strong function of temperature.40 Also, note that the geometrical evolution signature is correlated to the electrochemical progression. Thus, a change in electroplating temperature should alter the surface structure and subsequently the electrochemical response.47 This hypothesis is tested by carrying out electroplating at three different temperatures −5 °C (cold plating), 20 °C (isothermal), and 45 °C (hot plating). The counter electrode is always kept at 20 °C so that the reference potential is identical for comparison across the three scenarios. Given the temperature dependence48 of electrolyte properties, the bulk transport effects have to be deconvolved meticulously. The bulk electrolyte profiles become time-invariant at the end of regime I (e.g., Figure 3b). Figure 4b compares the bulk concentrations across the three electrodeposition events, while the corresponding diffusivities are plotted alongside in Figure 4c. Despite the temperature dependence, the bulk concentration drop is negligible given the small interelectrode spacing. Figure 4a plots the potential evolution for these three different thermal conditions. Electrodeposition takes place more nonuniformly at lower temperatures, which promotes greater confined electrolyte limitation. In response, cold plating shows a reduced capacity as well as a larger potential (Figure 4a). The hot plating shows the opposite trend given improved electrolyte transport as well as more uniform growth during the plating operation. The theoretical predictions (Figure 4a curves) help infer the nonuniformity of the electrodeposition growth from electrochemical experiments (Figure 4a dots) carried out at identical currents. To visually identify the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b02003. Modes of surface structure growth, characterization of electrodeposition structure, details of electrodeposition dynamics, experiments with Li−Li symmetric cells, and discussion on physicochemical complexations (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (P.P.M.). *E-mail:
[email protected] (C.T.L.). ORCID
Aashutosh Mistry: 0000-0002-4359-4975 Rachel Carter: 0000-0001-6583-1049 160
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Corey T. Love: 0000-0003-2581-3625 Partha P. Mukherjee: 0000-0001-7900-7261 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (N00014-17-1-2942) as part of the NURP program. P.P.M., C.F., and A.M. thank Dr. Maria Medeiros at the Office of Naval Research for funding this work. Dr. Karen Swider-Lyons is acknowledged for support of this work. C.L. thanks Dr. Michele Anderson at the Office of Naval Research for funding. R.C. is funded through the National Academy’s NRC RAP program.
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