The accumulation of lithium dendrites during charge-discharge cycles banned the development of lithium metal batteries, because the dendrites can account electrical shorting of the cells. A cardinal of tricks accept been acclimated to try to anticipate dendrite formation. Li et al. took the adverse access (see the Perspective by Mukhopadhyay and Jangid). They operated their beef at college accustomed densities, beneath which one would apprehend dendrites to anatomy attributable to the college nucleation rates. However, beneath these conditions, the dendrites that started to anatomy acrimonious up and annealed, arch to their disappearance.
Science, this affair p. 1513; see additionally p. 1463
Lithium (Li) metal electrodes are not adaptable in rechargeable batteries because electrochemical plating and stripping consistently leads to advance of dendrites that abate coulombic ability and eventually abbreviate the battery. It is about accustomed that the dendrite botheration is affronted at aerial accustomed densities. Here, we address a administration for dendrite change in which the about-face is true. In our experiments, we begin that aback the plating and stripping accustomed body is aloft aloft ~9 milliamperes per aboveboard centimeter, there is abundant self-heating of the dendrites, which triggers all-encompassing apparent clearing of Li. This apparent circulation heals the dendrites and smoothens the Li metal surface. We actualization that again doses of high-current-density healing assay enables the safe cycling of Li-sulfur batteries with aerial coulombic efficiency.
The appeal for higher-energy-density array systems (1, 2) has created a renewed absorption in the assay of brownish lithium (Li) as an anode material. This is because the packing body of Li atoms is the accomplished in its brownish form, constant in an ultrahigh (3) abstract specific accommodation of 3861 mAh g−1. However, the capital barrier block to deployment of Li metal anodes is the nucleation and advance of “dendritic” projections (4, 5) during the electrochemical plating-stripping action that occurs aback the array is actuality answerable or discharged. These Li dendrites are ambiguous in abounding respects. They access irreversible accommodation loss, abate the coulombic ability (CE), and advance the abasement and dehydration of the electrolyte (6, 7). However, the best acute affair is that the aciculate blooming projections can bore through the array separator and electrically abbreviate the array (8–10). This after-effects in a astringent thermal delinquent situation, which could aftereffect in the array communicable fire.
Researchers accept over the years devised assorted avant-garde approaches (11–17) to ascendancy and aish the advance of Li dendrites, but 100% aishment is challenging. This is because kinetically, the nucleation and advance of Li dendrites is awful favorable during electrochemical plating and stripping reactions. In general, it is taken for accepted that college accustomed densities in a Li metal array will crop accelerated advance of dendrites. In this Report, we authenticate a audible administration for Li dendrite change in which the adverse holds true. For our array system, we begin that aback the plating-stripping accustomed body is aloft aloft ~9 mA cm−2, there is abundant self-heating of the dendrites. The bounded heating gives alteration and breeze to Li, triggering all-encompassing apparent diffusion, which smoothens the dendrites and enables the calm collapsed agreement to be accustomed quicker. The self-heating is at safe levels, beneath any crisis of electrolyte breakdown or thermal accident to the separator.
To annotate the basal science abaft the dendrite healing, we activated Li-Li symmetric beef (fig. S1A). The tests were performed over a advanced ambit of operating accustomed densities, alignment from low (~0.75 mA cm−2) to actual aerial ethics (~15 mA cm−2). The potential-time and current-time profiles during galvanostatic charge-discharge cycling in Li-Li symmetric beef beneath altered accustomed densities are apparent in fig. S1, D and E. In the ideal thermodynamic condition, the voltage of Li stripping and plating should be at 0 V (versus Li-Li ). However, in convenance an over-potential exists, which increases about with the operating accustomed body (figs. S1, D and E, and S2). Electrochemical impedance spectra (EIS) of the Li-Li symmetric beef were recorded afore and afterwards 50 charge-discharge cycles at altered accustomed densities, and the after-effects are abbreviated in fig. S1, B and C. A arresting ascertainment is that the electrolyte attrition of the Li-Li balanced corpuscle cycled at ~15 mA cm−2 charcoal ~8 ohms both afore and afterwards 50 cycles, admitting the agnate attrition of the corpuscle cycled at ~0.75 mA cm−2 increases by ~75% from ~10 ohms afore cycling to ~17.5 ohms afterwards alone 50 charge-discharge steps. The accelerated access in electrolyte attrition empiric in fig. S1, B and C, is a signature of blooming growth. The connected accumulation of solid electrolyte interface (SEI) on the proliferating dendrites will account electrolyte drying, which increases the attrition to Li transport. The electrochemical affirmation in fig. S1, B and C, accordingly suggests that Li dendrite nucleation and advance appears to curl at accustomed densities of ~0.75 mA cm−2, admitting it seems to be suppressed aback the corpuscle is operated at accustomed densities of ~15 mA cm−2, an adjustment of aftereffect higher.
To approve the electrochemical evidence, we agitated out ex situ scanning electron microscopy (SEM) imaging of the surfaces of the Li metal foils acclimated in the experiments. Afore cycling, the Li foils displayed a about bland actualization and were absolutely chargeless of dendrites (fig. S3). Apparent in Fig. 1, A to D, are SEM images of the Li metal electrode apparent afterwards 50 cycles of allegation and acquittal at accustomed densities of ~0.75, ~4.5, ~9.0, and ~15 mA cm−2. Beneath low accustomed body (~0.75 mA cm−2) (Fig. 1A), a few abandoned but large-diameter blooming particles can be observed. With the access of accustomed body (Fig. 1B), the bore of Li dendrites decreases, but the packing body of the protrusions has added substantially. However, aback the accustomed body was added all the way up to ~15 mA cm−2 (Fig. 1D), the dendrites tend to agglutinate (merge) together. As a consequence, the apparent of the Li electrode becomes abundant smoother, which about lowers the accident of dendrite assimilation through the separator. At an boilerplate accustomed body of ~9 mA cm−2, there is fractional healing (Fig. 1C) of dendrites. This was additionally corroborated with cross-sectional SEM imaging of the Li metal foils (Fig. 1, E to H). As the accustomed body is aloft from ~0.75 to ~9 mA cm−2, the blooming band becomes added prominent, accretion in array from ~14 to ~37% of the Li antithesis thickness. However, at ~15 mA cm−2 accustomed body the blooming band undergoes a arresting transformation. The dendrites agglutinate through the absolute array of the blooming band (~48% of the Li antithesis thickness), transforming into a about bland and bunched (dense) layer.
(A and E) ~0.75 mA cm−2. (B and F) ~4.5 mA cm−2. (C and G) ~9 mA cm−2. (D and H) ~15 mA cm−2.
High-magnification SEM images of the dendrite particles in Fig. 1 are provided in fig. S4. Direct microscopy affirmation for fusing of the dendrites is provided in fig. S5. X-ray diffraction (XRD) and Raman spectroscopy confirms that these particles are Li dendrites and not apparent contaminants (figs. S6 and S7). SEI compositional assay indicates that the actual agreement of the SEI is agnate over a advanced ambit of accustomed densities (fig. S8). To verify that electrical shorting is suppressed in beef with the healed Li surface, we performed continued cycling of the Li-Li balanced beef (fig. S9). We begin that at the lower accustomed densities (such as ~4.5 mA cm−2), the Li-Li corpuscle shorts in ~500 hours because of the blooming projections that advance on the surfaces of the Li electrodes. By contrast, beef that were cycled at ~15 mA cm−2 showed no adumbration of electrical shorting alike afterwards 2000 hours (fig. S9).
To access a bigger compassionate of the dendrite nucleation and advance process, in situ ascertainment was agitated out by application an optical microscope (figs. S10 and S11). As apparent in fig. S11, aback altered accustomed densities are applied, audible nucleation patterns emerge. Aback low accustomed body (fig. S11, A to D) is applied, there are about no dendrite nuclei empiric on the Li electrode in the antecedent 20 min. At added times, several ample Li nuclei gradually appear but are able-bodied spaced apart. In adverse to this, at aerial accustomed body (fig. S11, E to H), densely arranged blooming nuclei (dark spots) bounce out over the absolute Li electrode apparent aural aloof a few minutes. Additional microscopy affirmation of bigger and densely busy dendrites at college accustomed densities is provided in fig. S12. We acclimated classical nucleation and advance approach to archetypal this abnormality (supplementary text). The predictions (fig. S11K) announce that the electrochemical over-potential increases with access in the accustomed density. This college over-potential reduces the nucleation ambit and increases the nucleation amount and the nucleation armpit body of the dendrites (fig. S11, J and L). The dense-packing of dendrites implies that the majority of the accustomed charge collectively breeze through the dendrites to ability the Li electrode. The constant Joule heating of the dendrites could activate all-encompassing apparent circulation of Li, enabling the carefully arranged blooming particles to be alloyed together. In accession to aerial packing density, the ample over-potentials additionally accomplish bigger dendrites, which in affiliation with the aerial accustomed body should accomplish abundant cocky (Joule) heating.
To appraise the temperature acceleration in the dendrites, we agitated out abundant calefaction alteration simulations (Fig. 2A and added text). We begin that the predicted temperature acceleration (Fig. 2B) in the dendrites at accustomed densities of ~15 mA cm−2 are on the adjustment of 40 to 60°C. The electrolyte and film separator (18) acclimated in our abstracts did not abase at these temperatures. Our thermal archetypal indicates that alike such abstinent temperature increases are not achievable at low accustomed densities (such as ~0.75 mA cm−2) (fig. S13). Although temperature increases on the adjustment of 40 to 60°C are acutely not ample abundant to cook dendrites (the melting temperature of Li is ~180.5°C, and the dendrites are ample abundant to aphorism out any admeasurement aftereffect on the melting temperature), they may be abundantly aerial to advance circulation of Li atoms on the blooming surfaces. To investigate this, we agitated out atomic dynamics (MD) simulations of the Li circulation action at assorted temperatures from allowance temperature (20°C) to 80°C (temperature acceleration of ~60°C). The atomic archetypal and adjustment of the Li dendrite particles acclimated in our simulation is apparent in Fig. 2C. To quantitatively appraise the accumulation carriage as a action of temperature, we focused on the atom clearing from the dendrite apparent to the basin formed amid two adjoining dendrite particles. The after-effects for the clearing accumulation breeze amount (atoms per second) of Li atoms as a action of temperature is apparent in Fig. 2E. Because the active force for apparent clearing is about the aberration in actinic abeyant because of curvature (very agnate to Ostwald ripening), a nonzero alteration was empiric alike at allowance temperature. However, the apparent circulation of Li is acerb temperature-dependent, with a aciculate access in the clearing accumulation breeze amount aloft 40°C. Apparent in Fig. 2D is the MD anticipation for the assay of the blooming particles afterwards annealing at ~80°C for ~50 ps. We acutely empiric amalgamation and fusing of the dendrite particles, which is constant with our beginning observations. Although the time scales in the MD simulations are actual altered from the experiments, the capital physics abide the same.
(A) Array diagram of the bread cell. Because of a repetitive geometry, a assemblage corpuscle is acclimated for thermal clay of a distinct dendrite with appropriate bore (dd), acme (hd), and agreement (sd). As apparent in the attrition diagram, calefaction generated in the dendrite can either be conducted to the substrate anon or via the surrounding electrolyte. Rconv, Rcase, Rspring, and Rspacer denote thermal resistances to calefaction transfer, agnate to accustomed convection, casing, bounce and spacer, respectively. Rden, Relec, and Rint denote thermal resistances to calefaction transfer, agnate to the dendrite, electrolyte and their interface, respectively. (B) Maximum dendrite temperatures at a accustomed body of ~15 mA cm−2. For the archetypal ambit of interfacial thermal resistances, dendrite temperatures in the ambit of 60 to 80°C are predicted. (C and D) Screenshots of atomic models acclimated in MD simulation, (C) antecedent accompaniment and (D) afterwards ~50 ps at ~80°C. (E) Clearing accumulation breeze amount of Li atoms at altered temperatures as predicted with MD. (F) SEM angel of Li dendrites in Li-Li balanced beef operated at a accustomed body of ~4.5 mA cm−2 for 50 charge-discharge cycles. (G) Healing of the dendrites in (F) by agency of thermal annealing in an apathetic Ar ambiance at ~70°C for 3 days.
To affirm that the healing is a thermal effect, we agitated out a dendrite annealing ascendancy experiment. In this test, a Li-Li balanced corpuscle was cycled at a accustomed body of ~4.5 mA/cm2 for 50 charge-discharge cycles (~200 hours). Then, the corpuscle was thermally annealed at a temperature of ~70°C for ~3 days. SEM images of the Li metal electrode afore and afterwards annealing are apparent in Fig. 2, F and G, respectively. The annealed Li antithesis displays a about bland film-like morphology, agnate to the electrochemically activated Li foils that were cycled at aerial accustomed densities (Fig. 1D). To annihilate the achievability that burden accretion in a bankrupt bread corpuscle contributes to the healing, we additionally performed an annealing agreement in an open-cell configuration. For this test, a Li-Li corpuscle was cycled at ~4.5 mA/cm2 for 50 charge-discharge accomplish in adjustment to anatomy the dendrites. Again the corpuscle was opened in an apathetic argon (Ar) environment, central a cuff box. The Li antithesis was removed from the corpuscle and placed on a hot bowl (within the cuff box) and thermally annealed at ~70°C for ~3 days. Agnate to the closed-cell case, SEM imaging (after annealing) of the Li antithesis in the open-cell agreement additionally appear a about bland apparent (fig. S14), advertence healing of the dendrites.
We advised the atom admeasurement administration of the Li dendrites based on angel assay of Fig. 1, B and D. At moderately ample accustomed densities (~4.5 mA cm−2), the boilerplate atom sizes are ~5.7 ± 2 μm forth the longest ambit and ~2.7 ± 0.7 μm forth the beeline ambit (fig. S15). However, at ultrahigh accustomed densities (~15 mA cm−2), it is axiomatic that the dendrite assay has afflicted because the grains get abundant larger—~8.8 ± 3 μm forth the longest ambit and ~5.2 ± 2 μm forth the beeline dimensions—and are added broadly broadcast (fig. S15). These after-effects are constant with the account of apparent diffusion–driven dissolution and amalgamation (coarsening) of the dendrites. Because the aerial accustomed body is activated during both plating and stripping, we accepted the healing to action during both the plating and stripping step. This was corroborated by a allegory of the Li foils afterwards plating and stripping at a accustomed body of ~15 mA cm−2 (fig. S16), advertence a agnate (flat) assay with commensurable atom sizes.
So far, we accept appear dendrite healing in Li-Li balanced cells. The catechism remained whether such furnishings can be reproduced in activated situations. To abstraction this, we activated a Li-sulfur (Li-S) abounding corpuscle (fig. S17A) (19). For this configuration, the anode is a Li metal foil, admitting the cathode is a S-carbon blended (18). Unlike Li-Li balanced cells, for Li-S batteries, alike beneath a low accustomed body (~0.75 mA cm−2), the Li dendrites are densely arranged (fig. S17D). This can be accepted from the voltage contour for the Li-S array (fig. S17B). On the base of the thermodynamics of the Li anode and sulfur cathode, the electrochemical acknowledgment (20) takes abode at ~2 V. Because of this ample over-potential (related to Li plating and stripping), the dendrites are densely arranged irrespective of the accustomed body at which the corpuscle is operated. Of course, accustomed densities of ~0.75 mA cm−2 are bereft to alleviate the dendrites through Joule heating; however, aback we added the accustomed body to ~9.0 mA cm−2, we acutely empiric healing of the dendrites into a about bland (film-like) assay (fig. S17E). The galvanostatic charge-discharge cycling and CE after-effects for the Li-S array are provided in fig. S17C. A key indicator of the severity of dendrites is low CE; as adumbrated in fig. S17C, the CE of the corpuscle cycled beneath aerial accustomed body is aing to 100%, admitting the CE is continuously abbreviating aback the corpuscle is cycled at low accustomed density. To authenticate that the healing apparatus in this case is identical to the ahead advised Li-Li balanced cells, we additionally annealed the Li anode cycled at ~0.75 mA cm−2 in the Li-S array at a temperature of ~70°C for ~3 canicule in an Ar environment. The about bland assay of the thermally annealed Li antithesis (fig. S17F) carefully matches the antithesis (fig. S17E) that was activated at a accustomed body of ~9.0 mA cm−2 in the Li-S cell. EIS assay (fig. S18 and tables S1 and S2) adumbrated that alike afterwards alone 50 cycles, the electrolyte and interface attrition added abundantly aback the Li-S corpuscle was cycled at ~0.75 mA cm−2 as compared with ~9.0 mA cm−2, which is constant with dendrite healing at the aerial accustomed density. The dendrite healing was additionally absolute by agency of array SEM of the Li electrode (fig. S19).
We added advised whether the Li-S array could accomplish in abbreviate bursts at aerial accustomed densities (with the purpose of healing the blooming projections) and again backslide aback to operation at accustomed (low) accustomed densities. This is approved in Fig. 3A, advertence that alternate operation at aerial accustomed densities (~9 mA cm−2) for a bound continuance is acceptable to alleviate the dendrites and about advance the CE (Fig. 3B) and to a bottom amount the accommodation assimilation (fig. S20) achievement of the array beneath approved (~0.75 mA cm−2 accustomed density) operating conditions. The bit-by-bit accommodation achromatize over continued cycling (300 charge-discharge steps) empiric in Fig. 3A was acquired by dissolution of boilerplate Li polysulfides into the electrolyte on the cathode (S) side. Such accommodation achromatize is archetypal of Li-S batteries afterwards polysulfide immobilizers (19) or specialized barrier coatings (20) to anticipate polysulfide dissolution.
(A) Aeon adherence and CE (highlighted in yellow) of Li-S batteries with and afterwards alternate doses of healing treatment. Blue abstracts credibility announce achievement of the baseline Li-S corpuscle cycled at ~0.75 mA cm−2 afterwards healing treatment. The red abstracts credibility are for alternate healing assay (at accustomed body of ~9 mA cm−2) activated amid the 21st and 50th, 151st and 180th, and 311th and 360th cycles. For the actual cycles, a accustomed body of ~0.75 mA cm−2 was applied, which is identical to the baseline corpuscle afterwards healing. (B) Allegory of boilerplate CE of the Li-S array aback operated at a accustomed body of ~0.75 mA cm-2 with and afterwards the healing treatment. (C) Digital photo of film separator in the cycled Li-S batteries afterwards 250 cycles of charge-discharge at a accustomed body of ~0.75 mA cm−2 afterwards the healing treatment. The abject amphitheater indicates the attendance of blooming Li particles that are abounding in the separator. (D) Agnate photo of the film separator taken afterwards 360 cycles of charge-discharge with the healing treatment. (E) Assay of the Li metal electrode apparent afterwards the third healing date [after the 360th charge-discharge aeon adumbrated in (A) by the abject circle].
We performed post-cycling assuming of the Li metal electrodes and the film separators acclimated in these tests. SEM imaging of the Li anode apparent at the end of the third healing footfall in Fig. 3A (after the 360th charge-discharge cycle) is apparent in Fig. 3E. The after-effects announce a healed (relatively smooth) apparent assay afterwards any aciculate blooming features. It is doubtful that such a about collapsed apparent could access and bite through the separator membrane. This was accepted with the optical microscopy images (Fig. 3, C and D) of the separator film in the post-cycled state. The film from the corpuscle that was cycled at ~0.75 mA cm−2 for 250 charge-discharge accomplish afterwards any healing assay showed the attendance of arresting blooming Li deposits that were abounding into the separator (Fig. 3C). Such Li metal deposits will eventually electrically abbreviate the Li-S array (fig. S21). By contrast, the separator taken from the Li-S corpuscle that was apparent to alternate doses of healing assay (at ~9 mA cm−2) (Fig. 3A) showed no adumbration (Fig. 3D) of such Li metal deposits alike afterwards continued cycling for 360 charge-discharge steps.
An important activated application for the healing action presented actuality is the time taken for the healing to be complete. Aback aerial accustomed body is applied, there is an antecedent bead (fig. S22A) in the CE as the blooming apparent and SEI reconstructs. Subsequently, as the apparent smoothens (fig. S22B) and stabilizes, the CE rises aback up to a aerial value. The healing time can be accepted from the time (~2 to 3 hours in fig. S22A) that it takes for the CE to balance aback to a aerial amount afterwards the antecedent dip. This healing time is accepted to alter as a action of the activated accustomed density. In accession to continuance of healing, the abundance at which the healing assay is again is additionally an important parameter, abnormally for aspersing the dehydration of the electrolyte. Our after-effects advertise a able aisle against the able deployment of Li metal electrodes in rechargeable batteries.
J. O. M. Bockris, A. K. Reddy, M. E. Gamboa-Adelco, Modern Electrochemistry, 2A: Fundamentals of Electrodics (Springer Science & Business Media, 2000).
F. Incropera, D. P. DeWitt, T. L. Bergman, A. S. Lavine, Fundamentals of Calefaction and Accumulation Alteration (John Wiley & Sons (2007).
Acknowledgments: Funding: N.K. acknowledges abutment from the U.S. National Science Foundation (awards 1234641, 1435783, 1510828, and 1608171) and the John A. Clark and Edward T. Crossan able Chair Professorship at the Rensselaer Polytechnic Institute. J.S. acknowledges the abutment from U.S. National Science Foundation beneath accolade CMMI 1635520. Author contributions: L.L. performed electrochemical testing and abstracts characterization; Y.W. contributed to interpreting the healing mechanism; L.L. and N.K. performed clay of dendrite nucleation; S.B., N.K., and Y.S. agitated out MD simulations; L.L., Y.W., Z.C., and J.S. agitated out in situ optical imaging; L.L., P.H., and B.W. agitated out x-ray photoelectron spectroscopy and XRD analysis; N.K. and S.N. agitated out computational thermal modeling; and L.L., S.B., Y.S., S.N., and N.K. wrote the paper. The activity was directed and supervised by N.K. Aggressive interests: Authors accept no aggressive interests. Abstracts and abstracts availability: All abstracts are accessible in the arrangement or the added materials.
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