Electrochemical Performance of All-Solid-State Li-Ion Batteries Based on Garnet Electrolyte Using Silicon as a Model Electrode Giulio Ferraresi,† Mario El Kazzi,† Lukas Czornomaz,§ Chih-Long Tsai,‡ Sven Uhlenbruck,‡ and Claire Villevieille*,† †
Electrochemical Energy Storage Section, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland IBM Research-Zürich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland ‡ Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany §
S Supporting Information *
ABSTRACT: Owing to improved safety, all-solid-state batteries based on the garnet Ta-substituted Li7La3Zr2O12 solid electrolyte are promising alternatives to conventional Li-ion batteries with organic electrolytes. However, to date, the viability of such all-solid-state batteries is uncertain because their performance is limited by the problematic electrode− electrolyte interface. Herein, we report the viability to use Si anodes facilitated by enhancement of this interface. Before depositing Si as a thin film electrode (50 nm) on the smooth surface of the solid electrolyte, we treated the electrolyte surface by argon plasma etching to reduce the amount of resistive species. This approach enabled the cycling of Si/ garnet/Li all-solid-state cells, achieving an initial capacity of ∼2700 mAh/g followed by partial fading and stabilization for more than 100 cycles. Electrochemical measurement, coupled with morphological and chemical investigations, demonstrate that Si is a viable anode in combination with garnet electrolyte and emphasize the importance of controlling the solid/solid interface.
A
bulk electrolyte development, only a few papers have reported the electrochemical performance of garnet-based SSBs in combination with active materials. Ohta et al. showed the possibility to cycle LiCoO2 electrodes with capacities close to the theoretical value, if the electrode is deposited as a thin film on bulk Li6.75La3Zr1.75Nb0.25O12.12 The results obtained are remarkable as the reported electrochemical performance is comparable to conventional (liquid electrolyte) Li-ion batteries. Other than this example, the electrochemical performances of garnet-based SSBs are unsatisfactory, and the interface remains the main challenge. Control over the electrolyte/active material interface is crucial because no optimal contact between these components causes high interfacial resistance and is hence problematic for Li+ transformation. Van der Broek et al. proposed several interesting methods for engineering electrode−electrolyte interfaces by roughening the electrolyte surface or increasing the compression of the stack, which improved the contact between a Li4Ti5O12 (LTO) electrode
ll-solid-state batteries (SSBs) based on ceramic electrolytes are one of the most promising and emerging technologies for future electrochemical energy storage devices. The main advantage of SSBs over other batteries (e.g., Li-ion, Li−S) is the safety enhancement due to the shift from a flammable organic electrolyte to a solid electrolyte.1 Among the existing options for solid electrolytes, ceramic materials have attracted much attention and are extensively investigated.2,3 Despite their lower ionic conductivity at room temperature (0.3 mS/cm) compared to a standard organic electrolyte (1 mS/cm), oxide-based electrolytes with the composition Li7La3Zr2O12 (LLZ) have a great advantage of a wide electrochemical stability window, which enables, in theory, the use of high-energy anodes with low operating potential. To improve the ionic conductivity of garnet materials, Li+, La3+, or Zr4+ ions are partially substituted by supervalent cations such as Al3+, Ga3+, or Ce4+, leading to stabilization of the cubic structure over the tetragonal structure. Following this approach, Ta-substituted LLZO (LLZTa) solid electrolytes have attracted the most interest thanks to the lower cost of Ta compared to that of Ga or Sc, higher stability vs Li metal of Ta compared to Nb, and shorter sintering time.4−11 Despite the efforts made in © 2018 American Chemical Society
Received: February 14, 2018 Accepted: March 22, 2018 Published: March 22, 2018 1006
DOI: 10.1021/acsenergylett.8b00264 ACS Energy Lett. 2018, 3, 1006−1012
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Cite This: ACS Energy Lett. 2018, 3, 1006−1012
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ACS Energy Letters
Figure 1. SEM images of (a) fractured and (b) ion-milled LLZTa cross sections; (c) schematic of the 50 nm Si film coating procedure; (d) SEM top-view of the Si-coated LLZTa; (e) SEM image of the Si/LLZTa interface.
and the solid electrolyte.13,14 These approaches, however, yielded only slight improvements in the electrochemical performance as the obtained capacity of 75 mAh/g was only maintained for a few cycles (Qtheo = 150 mAh/g). This low electrochemical performance and large capacity fading might result from fast interfacial degradation caused by the presence of resistive species (Li2CO3) formed by the reaction between the garnet electrolyte and air/moisture,15 which leads to high polarization during cycling.16,17 Research targeting the use of high-energy-density anodes in SSBs is also pursued; thus, the feasibility of alloy-based anodes must also be considered.18−20 Si is one of the best alloy materials as it can deliver a capacity up to 3400 mAh/g (theoretical capacity: 4200 mAh/g) at an average operating potential of ∼0.3 V vs Li+/Li;21 furthermore, Si is inherently safe, abundant and unlike metallic Li, which suffers from Lidendrite formation.22 Unfortunately, Li-alloys, Si included, suffer from large volume changes upon lithiation/delithiation that generally hinder the mechanical stability of electrodes and might affect directly the interface in SSBs;23 however, when used as a thin film, the mechanical issue and charge-transfer resistance at the interface are greatly mitigated.24 Recent works have achieved promising results using alloy thin films (Si, Ge) as an intermediate buffer layer for the LLZO/Li interface, but the application as an anode material was not investigated.25,26 In this context, we combine garnet LLZTa and a Si thin film negative electrode as an electrochemical proof-of-concept to investigate a high-energy SSB. To ensure the interface stability, we demonstrate that via an electrolyte pretreatment step the presence of Li-carbonate species can be suppressed, thus improving the electrode−electrolyte interface. The electrolyte pretreatment step involved polishing the LLZTa surface and plasma etching by sputtering with Ar+ immediately before Si film deposition, which allowed the formation of a sharp and firm interface. The adhesion of Si to the solid electrolyte was assessed by scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), and X-ray photoelectron
spectroscopy (XPS), prior to electrochemical investigation of the Si/LLZTa/Li stack at 80 °C. The Si thin film electrode was efficiently cycled for more than 100 cycles, with specific charges ranging from 2700 to 1200 mAh/g depending on the cycle number and applied current rate. First, we started by optimization of the garnet solid electrolyte. Garnet Li6.45La3Zr1.45Ta0.55O12 (LLZTa) samples were prepared starting from the corresponding powder (synthesis reported elsewhere) and molded into dense pellets by a combination of uniaxial compression and sintering.27 The X-ray diffraction (XRD) pattern shown in Figure S1 (Supporting Information) confirmed the formation of the desired cubic phase with crystallinity (ICSD card n°183687). Two additional unassigned diffraction peaks were attributed to impurities; however, they had low intensities and the solid electrolyte had the expected conductivity, indicating that the impurities have no impact on the ionic transport of the electrolyte. The relative packing densities of the prepared LLZTa samples were 93−94%, with ionic conductivity values ≥ 0.3 mS/cm at 25 °C and 3 mS/cm at 80 °C (Figure S2, Supporting Information). The compactness and homogeneity of the as-prepared LLZTa pellets were evaluated using SEM. Figure 1 shows two different cross-sectional images: the “fractured” cross section (Figure 1a) obtained by manually breaking the pellet and the ion-milled cross section (Figure 1b). The former shows highly packed grains, indicating that the pellet has a high density, confirmed by the little porosity observed. The latter gives insight into intergranular transport, evidencing a continuous network for Li+ transfer with limited influence from resistance at grain boundaries.28 Afterward, the sintered samples underwent a polishing step to decrease the final thickness of the pellets to ∼550 μm and to remove surface imperfections, thus obtaining a “clean” surface, which is crucial for deposition of the final Si film. The effectiveness of the polishing step is demonstrated in Figure S3 (Supporting Information), with distinct sintered grains observed on the pellet surface. Although polishing effectively creates a smooth 1007
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Figure 2. (a) Optical microscope image of the pristine Si thin film deposited on a LLZTa pellet; (b) top-view with an overlapping EDX line scan at the edge of the Si-coated and masked LLZTa interface; (c) EDX analysis of the cross section of the Si/LLZTa interface tilted at a 20° angle; (d) XPS spectrum of the Si 2p core level for the 50 nm Si-coated LLZTa.
also confirmed by the above XPS analyses. The remaining traces of carbon on the etched surface might be due to the difficulty of completely removing carbonates by anisotropic etching from the open porosity, as previously reported for this material.29 It is also noteworthy to mention the presence of a mask shadowing effect at the very edge of the coated area, which limits the accessibility of this surface both for etching of carbonates and for Si deposition. The Si/LLZTa interface was also characterized by EDX in Figure 2c (supported by detailed SEM investigation in Figure S7a,b), showing the boundary between Si (pink) and LLZTa (gray). XPS surface analysis of the as-deposited Si film (Figure 2d) revealed the presence of two main peaks for the Si 2p core level at 99.65 and 103.85 eV, which are related to Si0 and SiIVO2 (native oxide), respectively. A third component observed at 102.7 eV was identified as a substoichiometric SiO2−x species formed naturally upon air exposure. After thorough electrode−electrolyte optimization, the viability of the deposited Si film as an anode with garnet LLZTa must be proven. To identify the reversible processes in this configuration, cyclic voltammetry measurement (CV, Figure 3a) compares the initial cycle of the Si-coated LLZTa and the bare LLZTa (not Ar+ etched). CV was conducted for both measurements at a scan rate of υ = 50 μV/s in the potential range of 50 mV to 1.5 V (all potentials are given vs Li+/Li). The Si-coated LLZTa cell shows barely any electrochemical activity above 0.3 V during the first cathodic sweep, indicating that electrolyte decomposition or side reactions did not occur on the current collector during the first lithiation process, in contrast to the behavior of liquid electrolytes in conventional Li-ion batteries.31 Focusing on the inset plot in Figure 3a, an irreversible peak with negligible intensity is observed at 0.5 V only during first lithiation, not related to the solid electrolyte as it is not present in the bare LLZTa plot, while major electrochemical activity was recorded at lower potentials and related to the lithiation of Si to form LixSi alloys, analogous to conventional Li-ion batteries.21,23,32 During the
surface, this step was inevitably executed in air; therefore, the formation of surface species (such as LiOH and Li2CO3) via the reaction of LLZTa with air/moisture was not able to be prevented.17,29,30 Such surface species are undesired at the electrode−electrolyte interface and hence must be removed before deposition of the film electrode. XPS analyses confirmed their presence, as shown in Figure S5. The presence of peaks related to Li 1s, C 1s, and O 1s and the absence of peaks related to La and Zr confirm undoubtedly the reactivity of the LLZTa pellet with air/moisture, which generates a surface film of more than 8 nm with Li depletion from the LLZTa structure. Additionally, a depth profile analyses was conducted in the XPS chamber using Ar+ gun sputtering until the signals related to La and Zr could be detected. This XPS screening reveals that it is possible to “clean” the surface of the solid electrolyte before the deposition of a Si thin film electrode. Thus, we used in situ plasma etching treatment (Ar+ etching) directly in the sputtering deposition chamber before depositing the Si film (Figure 1c). A mask was developed to protect the LLZTa edges from side deposition and allow better control of the coated area during plasma etching and the subsequent deposition of amorphous Si thin films. Successful etching and deposition of a 50 nm Si film were verified by SEM, EDX, and XPS. Figure 1d,e displays top-view and cross-sectional SEM images of the deposited Si thin film. The pellet was homogeneously coated by the thin film without altering the surface morphology. Open porosity was visible on the surface of the pellet, corresponding to the defects formed upon surface polishing. As shown in Figure 2a, Si thin film deposition led to the formation of a dense gray/pink layer (Si-coated area) on a pale white/yellow pellet (masked area). Figure 2b shows top-view EDX measurements at the boundary between the Si-coated and masked areas to clarify the efficiency of plasma etching for removing C-rich surface contamination. The amount of carbon species in the Si-coated area was drastically reduced compared to the masked area that was not exposed to the plasma etching treatment. The species on the surface are related to Li2CO3, as 1008
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Figure 3. (a) Cyclic voltammogram of a 50 nm Si film/LLZTa/Li metal at 50 μV/s rate (the inset image shows a magnified view of the comparison with a bare LLZTa pellet); (b) cycling performance at a C/18 rate; (c) impedance spectra recorded after the 1st, 2nd, and 100th lithiation at 80 °C in the range of 1 MHz−100 mHz (the inset figure shows magnification at low ZRe); (d) impedance spectra recorded after the 1st, 2nd, and 100th delithiation at 80 °C in the range of 1 MHz−100 mHz (the inset figure shows magnification at low ZRe); (e) normalized specific charge for selected cycles at a C/18 rate; (f) normalized specific charge for the 2nd cycle at different C rates (C/18, C/9, and C/5).
first anodic scan, two distinct peaks were observed at 0.3 V and 0.4 V, and additional activity was seen at around 0.7 V, in agreement with the LixSi dealloying activity. The second cycle showed similar activity in the cathodic and anodic scans, confirming the reversibility of the initial reactions. Theoretical calculations have predicted that no electrochemical activity will be observed in the 0.05−2.9 V window for garnet-based electrolytes.33,34 Our experimental results are consistent with this prediction as the observed electrochemical activity was directly related to the alloying reaction. Galvanostatic cycling was performed to assess the electrochemical performance of the Si/LLZTa/Li stack at different
cycling rates. The cell cycled at a C/18 rate showed an initial delithiation capacity of 2702 mAh/g (54% initial Coulombic efficiency), which was maintained for four cycles (Figure 3b). Afterward, the delithiation capacity started to decrease rapidly to 2000 mAh/g after the 20th cycle before stabilizing above 1200 mAh/g between the 25th and 100th cycles. This initial capacity decrease followed by stabilization suggests an evolution in the electrochemical reaction pathway during the first 20 cycles, which does not evolve further up to the 100th cycle. Electrochemical impedance spectroscopy (Figure 3c,d) was conducted after the 1st, 2nd, and 100th lithiation/ delithiation to evaluate the cell resistance evolution. In all 1009
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the lithiation step was the only kinetically limited. Detailed surface investigations are necessary to clarify this latter observation and the composition of the interface. As per our understanding, at high rates, mass transport of Li is limited by the high resistance when the electrode is unlithiated, as confirmed by impedance measurements, thus leading to charge accumulation and overpotential. Thus, continuous cycling at the C/5 rate has showed a rather early battery failure after only the 18th cycle. The postmortem SEM images reveal that the cause of the failure is probably linked to film detachment from the LLZTa surface (Figure S8), which resulted from elevated stress at the interface. In conclusion, we propose the use of a 50 nm Si thin film as a model anode material for garnet-based SSBs. We have demonstrated that an electrolyte pretreatment step is beneficial for removing Li2CO3 species from the LLZTa surface, allowing effective contact at the electrode−electrolyte interface. Reversible lithiation and delithiation of the Si film in half-cell configuration is achieved and investigated. Chemical, electrochemical, and morphological characterizations reveal firm deposition of the film and an initial capacity of 2700 mAh/g followed by a rapid drop and stabilization at around 1200 mAh/g for at least 100 cycles at a C/18 rate. The investigation suggests a change in electrochemical reaction, as supported by EIS analysis. Tests at different C rates revealed an increased overpotential during lithiation but not during delithiation. Although the route toward practical application of the Si/LLZTa stack requires further optimization, these results offer important enhancements in SSBs by improving the electrode−electrolyte interface and providing a unique prospect to cycle high-energy alloy anodes using a garnet solid electrolyte. With the aim to commercialize bulk SSBs, solutions to increase the loading of Si in the electrode and its combination with a high-energy cathode material are necessary to compete with LIB technology, while commercialization of thin film SSBs require the successful deposition of LLZTa thin films, which is the remaining challenge. Once these issues are resolved, both technologies will benefit from the insights revealed in the presented research.
cases, a depressed semicircle was observed at high/mid frequency, followed by a straight line at lower frequency for lithiation or by an additional semicircle at low frequency. The high/mid frequency behavior was related to the interfacial resistances, while the low frequency was related to the bulk electrodes. 16,35 During lithiation, the overall interfacial resistances appeared much reduced compared to those observed during delithiation, and they decreased also from the 1st to the 100th cycle.36 This latter phenomena is related to the volume expansion caused by the Li−Si alloy, which creates enhanced surface contact. The overall interfacial resistances intercept at the low frequency of the semicircle decreased after every cycle although appearing at the same frequency (0.4 Hz). During delithiation, the interfacial resistances increased as a consequence of a rise of the charge-transfer resistance compared to full lithiation, caused by the volume contraction upon dealloying. The charge-transfer resistance recorded after each delithiation step decreased after consecutive cycles, as already observed for lithiation. The significant drop in resistance observed at high frequency after long cycling is likely related to the different lithiation pathway starting after 20 cycles, in agreement with the extra peak appearing at 0.26 V during lithiation in Figure S6. As for the bulk electrode processes, the several LixSi alloys improved the Li diffusion inside of the electrode, as already demonstrated in conventional Li-ion batteries.37 To better evidence what type of changes appear upon cycling, normalization of selected cycles are shown in Figure 3e. During the first cycle, initial irreversible activity at ∼0.4 V is detected and followed by two lithiation steps at 0.23 and 0.1 V, in complete agreement with the CV measurement. The first delithiation resulted in a sloping potential plateau evolving smoothly from 0.2 to 0.5 V. From the 1st to 10th galvanostatic cycle, there were no relevant changes except for the disappearance of the activity at 0.4 V and a decrease of the peak intensity at 0.25 V during lithiation, as better depicted in the derivative curve in Figure S6. From the 10th to 20th, there is the appearance of extra electrochemical activity at 0.26 V during lithiation and 0.47 V during delithiation, suggesting a modified electrochemical pathway. After the 20th cycle, a slight overpotential and peak broadening are observed during both lithiation and delithiation up to the 100th cycle that are due to intensive charge/discharge repetition with large volume change. From a morphological point of view, the electrode turns from an initial homogeneous layer (Figure S7a,b) into an island-like film after the 100th cycle with an island width in the range of 0.1−1 μm (Figure S7c,d). Despite the local roughening due to the island formation, Figure S7d suggests that the film is still properly anchored to the LLZTa solid electrolyte, declining film detachment. Rate capability tests (C/18, C/9, and C/5 rates) were also performed to investigate the reaction kinetics with a CCCV protocol (constant charge constant voltage). For a fair comparison, we reported in Figure 3f the normalized galvanostatic curves of the second cycle. At a C/9 rate, a slight overpotential was observed to build up during lithiation, but no overpotential was observed during delithiation. At a C/5 rate, the electrode could not be discharged galvanostatically due to the high overpotential as the potential directly dropped to the cutoff value of 0.05 V, indicating that the interfacial resistance is too high for the applied current. However, most of the capacity could be recovered in the constant voltage mode adopted at the cutoff potential. Surprisingly, the delithiation step proceeded without an overpotential at C/9 and C/5 rates, indicating that
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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.8b00264. Experimental section; XRD and EIS of pristine LLZTa; optical images before and after pellet polishing; EDX analysis of Si-coated LLZTa; XPS analysis on uncoated LLZTa; derivative curves for galvanostatic cycling up to the 100th cycle; SEM of the 1st cycle and postmortem SEM/EDX images after the 100th cycle; and postmortem SEM/EDX after cycling at a C/5 rate (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Giulio Ferraresi: 0000-0003-2436-6093 Mario El Kazzi: 0000-0003-2975-0481 Chih-Long Tsai: 0000-0001-8103-3514 Sven Uhlenbruck: 0000-0003-0334-0425 1010
DOI: 10.1021/acsenergylett.8b00264 ACS Energy Lett. 2018, 3, 1006−1012
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ACS Energy Letters
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Claire Villevieille: 0000-0001-8782-4800 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.F. and C.V. acknowledge gratefully the financial support extended by the Centre of Competence for Energy and Mobility and Swiss Electric Research (Project No. 911). G.F., M.E.K., and C.V. also thank Prof. Dr. Petr Novák for fruitful discussions on the topic. C.-L.T. and S.U. acknowledge financial support by Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. under grant “Elektrochemische Speicher im System − Zuverlässigkeit und Integration” and by “Bundesministerium für Bildung und Forschung” (Federal ministry of education and research), Germany, under Project 03X4634C.
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DOI: 10.1021/acsenergylett.8b00264 ACS Energy Lett. 2018, 3, 1006−1012
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DOI: 10.1021/acsenergylett.8b00264 ACS Energy Lett. 2018, 3, 1006−1012
