Effects of Cesium Cations in Lithium Deposition via Self-Healing

Feb 5, 2014 - ... self-healing electrostatic shield (SHES) mechanism to achieve dendrite-free Li deposition by adding so-called non-Li+ SHES additives...
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Effects of Cesium Cations in Lithium Deposition via Self-Healing Electrostatic Shield Mechanism Fei Ding,†,‡ Wu Xu,*,† Xilin Chen,† Jian Zhang,† Yuyan Shao,† Mark H. Engelhard,§ Yaohui Zhang,†,∥ Thomas A. Blake,⊥ Gordon L. Graff,† Xingjiang Liu,‡ and Ji-Guang Zhang*,† †

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States National Key Laboratory of Power Sources, Tianjin Institute of Power Sources, Tianjin 300381, China § Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ∥ Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China ⊥ Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡

S Supporting Information *

ABSTRACT: Lithium (Li) dendrite formation is one of the critical challenges for rechargeable Li metal batteries. The traditional method of suppressing Li dendrites, by using highquality solid electrolyte interphase films, cannot effectively solve this problem. Recently, we proposed a novel self-healing electrostatic shield (SHES) mechanism to achieve dendritefree Li deposition by adding so-called non-Li+ SHES additives in electrolytes, which adsorb but do not deposit on the active sites of Li electrodes and thus force Li to be deposited in the region away from protuberant tips. In this paper, the electrochemical behavior of the cesium cation (Cs+) as the typical non-Li cation suitable for the SHES mechanism is further investigated in detail to reveal its effects on preventing the growth of Li dendrites. Typical adsorption behavior rather than chemical reaction is observed. The existence of Cs+ cations in the electrolyte does not change the components or structure of the Li surface film, which is consistent with what the SHES mechanism predicts. Various factors affecting the effectiveness of the SHES mechanism are also discussed. The morphologies of the deposited Li films are smooth and uniform during the repeated deposition−stripping cycles and at various current densities (from 0.1 to 1.0 mA cm−2) by adding just a small amount (0.05 M) of Cs+ additive in the electrolyte.

1. INTRODUCTION The development of electric vehicles requires energy storage devices to have much higher specific energy and energy density, lower cost, and higher safety than the state-of-the-art Li ion batteries. There are two ways to improve the energy density of a cell: one is to use high-capacity electrode materials and the other to increase the cell voltage by using high-voltage cathode materials. Li metal has an extremely high theoretical specific capacity (3860 mAh g−1) and the lowest negative electrochemical potential (−3.040 V vs the standard hydrogen electrode). It has been considered as the most attractive highenergy anode material for the next-generation rechargeable batteries, such as Li−sulfur batteries1,2 and Li−air batteries.3,4 Li metal has been studied as a rechargeable anode since the 1960s.5 Unfortunately, in spite of the extensive work on research and development of a Li metal anode, rechargeable Li metal batteries still have not found significant commercial applications due to Li dendrite formation and low Coulombic efficiency during repeated charging and discharging cycles.6 Although the low Li Coulombic efficiency can be partially compensated by using excess Li metal in a battery anode, the Li © 2014 American Chemical Society

dendrite formation has been a more formidable barrier that prevents practical application of rechargeable Li metal batteries. Li dendrite formation and growth is the root cause of not only the low cycling efficiency but also a safety issue: it induces the internal short circuit of a battery and may lead to thermal runaway or even fire and/or explosion of the battery, which is not acceptable for a commercial battery.6 In the past 40 years, there have been many literature reports about the observation,7−10 analysis,11−13 and simulation14−16 of Li dendrite formation. Several possible reasons for Li dendrite growth were suggested, for example, breaking of the solid electrolyte interphase (SEI) film,6 inhomogeneity of the substrate surface,7 and nonuniformity of Li+ ion concentration on the Li electrode surface.10 Actually, such causes are unavoidable in a practical rechargeable Li battery system because an initial protuberant tip would form sooner or later, and more Li+ ions would preferentially deposit on it to form a Received: December 30, 2013 Revised: January 29, 2014 Published: February 5, 2014 4043

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Figure 1. SEM images of the morphologies of Li electrodes after different repeated deposition/stripping cycles in the control electrolyte (a, b, c, and d) and electrolyte with Cs+-salt additive (e, f, g, and h): (a, e) after first deposition, (b, f) after second deposition, (c, g) after third deposition, and (d, h) after 10th deposition. The electrolytes were 1 M LiPF6 in PC with and without 0.05 M CsPF6. The current for Li deposition and stripping was 0.1 mA cm−2. Parts a and e are reprinted with permission from ref 26.

2. EXPERIMENTAL SECTION Lithium hexafluorophosphate (LiPF6), propylene carbonate (PC), and dimethyl carbonate (DMC) were procured in battery grade from Novolyte Technologies (currently BASF Battery Materials). Cesium iodide (CsI, 99.999%) and silver hexafluorophosphate (AgPF6, 98%) were obtained from SigmaAldrich. Cesium hexafluorophosphate (CsPF6) was synthesized by mixing stoichiometric amounts of AgPF6 and CsI in PC solution inside an argon-filled glovebox (MBraun), followed by the filtration of the formed AgI from the solution using 0.45 μm syringe filters. The CsPF6−PC solution was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP/AES) to confirm that no excess Ag+ was left as described in our previous report26 and by the addition of AgNO3− acetonitrile solution to confirm no excess I− remained. Li was deposited on copper (Cu) foil substrates (1 cm × 1 cm) in different electrolyte solutions at desired current densities for a certain period of time. The galvanostatic deposition method was used on a Solartron electrochemical interface (SI 1287). After deposition, the electrode was thoroughly washed with anhydrous DMC several times to remove the residual electrolyte and dried in the antechamber of the glovebox under vacuum. Then the electrode was analyzed with scanning electron microscopy (SEM) for the surface morphology, and X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy for the surface components. To avoid air contamination during the sample transfer, sample loading, and analyses, the Li electrode samples were transferred in airtight containers filled with argon to a nitrogen-purged glovebag for SEM or a nitrogen glovebox for XPS. SEM images were taken with a JEOL 5900 scanning electron microscope as described in a previous paper.27 XPS measurements were conducted on a Physical Electronics Quantera scanning X-ray microprobe as described previously.28 The Li deposition samples for FTIR analysis were prepared using the method described by Aurbach et al.29 and measured on a Bruker IFS66/S FTIR spectrometer with a Bruker A513 specular reflectance accessory. The incident beam was parallel (p) polarized to achieve greater sensitivity to species within a fraction of a wavelength of the Li surface,29 and the polarization-dependent reflectance measurements were made at 75° using Specac wire grid linear polarizers (Specac model 57011 on KRS-5 substrate).

dendrite. The traditional way to prevent Li dendrites is to form a stronger SEI film on the Li metal anode to avoid breakage of the SEI film and then to suppress the dendrite growth.6,16 A more uniform SEI film makes ion conduction more homogeneous, which decreases protuberant tip formation,17,18 and a stronger SEI film can effectively suppress Li dendrite growth.6,19 In previous studies, modifications of the SEI film on the Li surface were regarded as the most fundamental and effective ways to decrease the Li dendrite formation. These approaches included using different electrolyte solvents,6,20 salts,17,21 and additives,22,23 forming Li alloys,24 and increasing electrode surface area25 and Li surface pressure.7 However, as indicated by the experimental6 and modeling14 results, the modifications to the SEI film are not sufficient to suppress Li dendrite growth for a practically dendrite-free Li metal anode. A better understanding of the dendrite formation process can help us find the fundamental reasons behind the limited effectiveness of the traditional SEI film. Recently we have discovered several new electrolyte additives to control Li dendrite growth based on a newly proposed selfhealing electrostatic shield (SHES) mechanism.26 The SHES mechanism utilizes a physically electrostatic field effect instead of chemical reaction products to prevent dendrite growth on the Li electrode surface. The added non-Li cations, such as Cs+ and Rb+, have lower reduction potentials than that of Li+ ion when they are at low concentrations according to the Nernst equation so that they will preferentially adsorb and accumulate on the Li metal tips due to the electrostatic field and force the Li+ ions to be deposited in the valley instead of on the tip of the dendrite in the deposited film. This mechanism is different from the traditional SEI film and alloying mechanisms for suppressing Li dendrite formation and growth. The Cs+ cation is a promising non-Li cation for the SHES mechanism. Its effects on the smoothness of deposited Li film morphology and the improvement of the long-term cycling stability of Li anodes were reported in our previous paper.26 However, several factors may affect the effectiveness of the SHES mechanism. In this paper, the fundamental behavior of Cs+ cations during Li deposition has been further investigated. The effects of Cs+ cations on the physical properties of the deposited Li films, the morphologies of Li deposition films at different current rates, and repeated deposition−stripping processes have been further studied via several different approaches. 4044

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Electrochemical impedance spectroscopy (EIS) was conducted in a beaker cell with three-electrode configuration using a Solartron 1287 electrochemistry workstation coupled with a Solartron 1255 frequency analyzer. A piece of Li foil was used as the working electrode, another piece of Li foil as the counter electrode, and a Li strip as the reference electrode. The impedance of the Li electrode was tested at open-circuit voltage with an applied bias of 5 mV from 1 MHz to 1 Hz. In the electrochemical quartz-crystal microbalance (EQCM) measurement, a CHI404A EQCM workstation (CH Instruments) and its accessories were used. The gold foil on the quartz-crystal slide was used as the working electrode, and a Li foil and a Li strip were used as the counter electrode and the reference electrode, respectively. For this equipment, 1 Hz frequency decrease means a 1.34 ng weight increase. The values of the mass per mole electron (mpq) of each scan was calculated based on the equation mpq = (1.34 × Δf × F)/(I × t), where Δf is the frequency change from 2.86 to 2.60 V in this work, F is the Faraday constant (96 485 C mol−1), I is the average current, and t is the scan time.

Figure 2. SEM morphologies of Li films deposited at different current densities in the electrolyte of 1 M LiPF6 in PC with 0.05 M CsPF6 as additive: (a) 0.1, (b) 0.2, (c) 0.5, and (d) 1.0 mA cm−2. Part a is reprinted with permission from ref 26.

3. RESULTS AND DISCUSSION 3.1. Morphology Evolution with Continuous Li Deposition−Stripping Processes. Figure 1 compares the SEM images of the Li electrode morphologies after different deposition−stripping cycles in beaker cells containing 1.0 M LiPF6−PC electrolytes without (Figure 1a−d) and with (Figure 1e−h) 0.05 M CsPF6 additive. The surface morphology of the films after the first deposition, second deposition, third deposition, and tenth deposition are shown in Figure 1a,e, Figure 1b,f, Figure 1c,g, and Figure 1d,h, respectively. Obviously, the Li electrode morphologies obtained in the Cs+-containing electrolyte still look smooth and dendrite-free after 10 cycles, although there are some small Li bumps. In addition, with the continuous Li deposition−stripping cycles, more and more products from the non-Faradaic reactions between Li and electrolyte (solvent and salt) accumulate on the Li surface and increase the thickness of the SEI layer. Although the surface of the deposited Li film becomes rougher with increasing cycle number, no dendrite growth is observed (Figure 1f−h). This phenomenon can be attributed to the preferential stripping and redeposition of Li due to the initial surface fluctuation of the Li film, which will be amplified in subsequent cycles. Without the Cs+ additive, dendrite growth will soon be out of control as shown in Figure 1b−d. When Li was deposited in a beaker cell, no pressure was applied to the Li surface. Therefore, the morphologies shown in Figure 1 reveal the real dynamics of Li dendrite growth. The large robust Li dendrites and dark dead Li particles are clearly observed on the Li surface deposited from the control electrolyte without Cs+ additive. The morphology of the Li anode surface rapidly becomes worse with repeated deposition−stripping cycling, but it should be significantly improved when the repeated deposition−stripping cycling test is conducted in real Li metal batteries due to the pressure effect, as observed in Li|Li4Ti5O12 coin cells reported in our previous work.26 The consistent results indicate that the Cs+ cation as a SHES mechanism additive leads to a smooth Li surface during repeat cycles. 3.2. Effect of Current Density on Li Deposition Morphology. The SHES effect of the Cs+ additive on preventing Li dendrite formation during deposition at different current densities was also examined. Figure 2 shows the

morphologies of Li films deposited in the 1.0 M LiPF6−PC electrolyte containing 0.05 M CsPF6 at current densities from 0.1 to 1.0 mA cm−2. Lower current density results in smoother Li deposition films, which is consistent with previous reports.7,30−32 However, all samples show a dendrite-free surface although more Li bumps are formed with increasing current density. It is clear that Cs+ cation as the electrolyte additive could effectively smooth the Li surface at least at a current density of up to 1.0 mA cm−2. Lee and Rasaiah had used molecular dynamics simulation methods to calculate the diffusion coefficient and then the ionic mobility of alkali metal ions in water at 25 °C.33,34 They reported that the Cs+ ion has higher diffusion coefficient and mobility in water than the Li+ ion has. The diffusion coefficient values are 2.0 × 10−5 cm2 s−1 for Cs+ and 1.2 × 10−5 cm2 s−1 for Li+, while the ionic mobilities are 7.8 × 10−4 cm2 V−1 s−1 for Cs+ and 4.7 × 10−4 cm2 V−1 s−1 for Li+. In another previous work,35 Ohtaki reported that the solvation numbers of Li+ and Cs+ in PC are 3.4 and 1.6, respectively. This means Cs+ diffusion in PC will be much easier than Li+ diffusion because Cs+ will be associated with less PC molecules during the diffusion process. This is one of the reasons that the Cs+ additive can work well in the SHES mechanism and effectively prevent Li dendrite growth at reasonable current rates (up to 1.0 mA cm−2). When the deposition current increases, more and more fastgrowing protuberant tips will form. One reason for the increased film roughness is that there are not enough Cs+ cations to effectively cover the fast-growing protuberant tips at higher deposition current densities due to the limited concentration (≤0.05 M) of Cs+ cations in the LiPF6−PC electrolyte. On the other hand, a higher current density will lead to a larger internal resistance or IR drop in the cells. The basis of the SHES mechanism is the reversal of the effective reduction potential generated by the concentration difference between the main salt (1 M LiPF6) and the additive salt (CsPF6). In the case of 0.05 M CsPF6 additive, the theoretical voltage reversal generated by the concentration difference is 4045

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only 0.063 V.26 With increasing current density, the IR drop in the cell may well exceed 0.063 V and the SHES mechanism may become less effective. Therefore, more Li bumps are formed with increasing current density as shown in Figure 2. The specific limit of the current density depends on the properties of the additives (including their reduction potential, relative concentration, solvability, and diffusivity in the given solvent) and cell design. With the optimized selection of additive, solvent, and cell design, it is possible to make the SHES mechanism valid even at a current density much higher than 1.0 mA cm−2. 3.3. Adsorption of Cs+ on Li Anode during Li Deposition. Several papers in the literature reported the use of metal ion additives to reduce Li dendrite formation, including Na+,36 Mg2+,24 Al3+,37 Zn2+,38 and Sn2+.28 These metal ions are used to modify the SEI film formed during Li deposition and/or to form an alloy component in the deposited bulk metal because all these metal ions will be reduced and codeposited during the Li deposition. However, the SHES mechanism relies on the physical static electrical shielding of Li+ ions (where an additive such as Cs+ is electrostatically adsorbed on the Li surface) instead of the mechanical blocking of the SEI layer (where additives are reduced and deposited in SEI films). The non-Li cations used in the SHES mechanism would not change the SEI film or the bulk material component of the deposited Li film, at least within the limited current densities. Herein, two electrochemical methods were used to further investigate the electrostatic adsorption behavior of the Cs+ cations on Li surfaces during Li deposition. First, the electrostatic adsorption behavior of Cs+ cations on the Li surface was investigated by EIS analysis via the ac impedance difference of one Li electrode in the electrolytes without and with Cs+ additive. The Li electrode was first immersed in the control electrolyte (without Cs+ cations) for more than 2 days to allow the formation of a stable SEI film, and the impedance does not change much in a short period of time (1.5 h; see the green solid triangles and black open circles and curves in Figure 3). After the ac impedance was tested, the

impedance than those in the control electrolyte, including higher interface resistance and double layer capacitance. Normally, the ac impedance spectra of the Li electrodes include the impedances of the electrolyte, the SEI layer, and the charge transfer.18,39−41 It can be seen from Figure 3 that the interfacial impedance (including SEI and charge-transfer impedances) of the Li electrode in the control electrolyte only shows a minimal increase at the storage time from 51 to 52.5 h, indicating that a relatively stable SEI film has been formed during the 51 h pretreatment in the control electrolyte. The time for the Li electrode being immersed and tested in the Cs+-containing electrolyte was only several minutes so the SEI film may not have changed significantly in such a short time. Therefore, the significant increase in the impedance of the Li electrode (shown as the red line in Figure 3) should be attributed to the greatly increased ion transfer resistance in the Cs+-containing electrolyte. In this electrolyte, Cs+ cations adsorbed on the surface of the SEI film partially block the pathways of Li+ ions and thus increase the charge transfer resistance. This result reveals again that non-Li cations in the SHES mechanism would adsorb onto the Li surface and influence the electrochemical behavior of Li+ ions. Another sensitive technology for analyzing surface adsorption is EQCM. The adsorption of Cs+ cations on the working electrode can be detected by this method. Figure 4 shows the

Figure 4. LSV curves of the gold electrode in the electrolytes with and without 0.05 M CsPF6 additive after stabilization. The control electrolyte was 1 M LiPF6 in PC, and the scan rate was 20 mV s−1.

results of the linear sweep voltammetry (LSV) measurement of a gold working electrode in the electrolytes without and with Cs+ additive within the voltage range of 2.86−2.6 V vs Li/Li+. The change of the electrode weight was measured simultaneously by EQCM. As mentioned in the literature,42 there is no obvious reaction between a gold electrode and PC-based electrolyte in the voltage range from 3.0 to 2.5 V. Therefore, the current response between 2.86 and 2.6 V should be attributed to the capacitive adsorption of cations on the gold electrode surface. The values of the mpq in both electrolytes were calculated from the EQCM data during the cathodic adsorption process. The average mpq value from five parallel tests is 12.22 g mol−1 in the control electrolyte and 15.86 g mol−1 in the Cs+containing electrolyte. Since the mpq in the control electrolyte is higher than 6.94 g mol−1 (which is the atomic weight of Li), it is clear that some solvent molecules are adsorbed together with Li+ ions on the electrode surface. Based on the measured

Figure 3. Alternating current (ac) impedance curves of Li electrodes in electrolytes with and without 0.05 M CsPF6 additive. The control electrolyte was 1 M LiPF6 in PC.

Li electrode was quickly taken out of the control electrolyte, immediately put into the electrolyte with Cs+ additive, and the ac impedance was measured again. Obviously, the EIS Nyquist plot for the Li electrode in Cs+-containing electrolyte shown in Figure 3 (the red open diamonds) shows that it has higher 4046

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Figure 5. XPS spectra of (a) Li 1s, (b) F 1s, (c) O 1s, and (d) C 1s for the SEI films on Li electrodes formed in LiPF6−PC electrolytes with and without CsPF6 additive.

peak at 531.5 eV is for the O of Li2CO3.27 In the C 1s spectra, the peak at 290.0 eV is for CO and the one at 285.0 eV for C−O.27 Therefore, from the above XPS results it is clear that the main components and structure of the SEI film on the Li electrode from the electrolyte with Cs+ additive are very similar to those from the control electrolyte without Cs+ additive. There are slight differences in the peak area and peak height for the 531.5 eV peak in the O 1s and for the 290.0 eV in the C 1s spectra, which may indicate that the Cs+-containing electrolyte leads to formation of fewer compounds containing CO but more C−O compounds in the SEI film. A very small amount of Li2O (528.8 eV)18,43 is found on Li surface deposited in the Cs+-containing electrolyte, and it could be attributed to a slight change in the local conditions when the Cs+ cation was used. The FTIR spectra in reflectance mode of the deposited Li films from LiPF6−PC electrolytes without and with CsPF6 additive were also measured and are shown in Figure 6. No obvious differences in components or structure are observed for the surface films, and the main peaks for the surface components are very similar. The FTIR result shows again that the Cs+ additive does not participate in the SEI filmforming process. The above electrochemical studies and surface analyses indicate that Cs+ cations used in in the electrolyte only adsorb onto the Li electrode surface by electrostatic attraction during the designed Li metal deposition process, which is consistent with the assumption of the SHES mechanism proposed in the previous work.26 The addition of Cs+ additive leads to negligible change in the composition of the SEI layer on the surface of deposited Li electrode. It is anticipated that Cs+ cation as a typical SHES mechanism additive may also be

mpq value, it can be estimated that about 0.05 PC molecules per Li+ ion were adsorbed on the electrode surface in an equilibrium state. Apparently, this value is far different from the dynamic values reported in previous works.35 We notice that a higher mpq value is obtained when the gold electrode is tested in the electrolyte containing Cs+ additive. This implies that another kind of cation that is heavier than the Li+ cation has been adsorbed on the electrode surface as well. It is clear that the Cs+ ion (atomic weight 132.91 g mol−1) is the only second cation other than a Li+ ion in this electrolyte that can be adsorbed on the working electrode surface during the cathodic polarization process. Considering that the ratio of Cs+ over all the cations in the studied electrolyte was only 4.8% and assuming that the amounts of solvent molecules adsorbed per molecule were the same in both electrolytes when the system was in a static state, then the mpq value was calculated to be 18.26 g mol−1. This value is higher than the measured value of 15.86 g mol−1. The lower measured value can be attributed to the lower solvation number of Cs+ in PC,35 which means fewer PC molecules will be associated with each Cs+ cation adsorbed on the Li surface. Therefore, this result is consistent with the assumption of the SHES mechanism that Cs+ cations can be adsorbed onto the Li electrode during Li deposition. 3.4. Analysis of Li Metal Surface Layer. The SEI films on the Li electrodes obtained from the Cs+-containing electrolyte and the control electrolyte without Cs+ additive were analyzed by XPS as shown in Figure 5. The Li 1s spectra can be considered as being composed of a sharp peak at 56.0 eV and a small shoulder peak at about 54.5 eV, which respectively represent LiF (55.7 eV) and Li2CO3 (54.9 eV).17 The F 1s peak located at 685.5 eV matches well the value for LiF.27 The O 1s 4047

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applied to other rechargeable batteries, such as Li ion batteries, Na ion batteries, Li−sulfur batteries, Li−air batteries, and zinc− air batteries, where smoothness of all deposited films is required.

4. CONCLUSIONS The Cs+ cation is one of the most suitable additives to prevent Li dendrite growth based on the SHES mechanism, which relies on an effective physical adsorption of additives instead of chemical reduction of additives to achieve smooth Li deposition. Addition of Cs+ additive results in a smooth Li surface during the repeated deposition−stripping cycles and even at relatively high current densities. Electrochemical measurements and surface analyses clearly reveal that Cs+ cations adsorb on the Li electrode surface during the Li deposition process but do not change the composition of SEI films, as the SHES mechanism predicts. Because of the narrow window between the effective reductive potentials of Cs+ and Li+, Cs may be deposited on the Li surface at high current densities. Therefore, more work is needed to optimize the electrolyte composition to improve the effectiveness of the SHES mechanism at high current densities. ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. FTIR spectrum of a Li electrode surface formed in LiPF6− PC electrolytes with and without CsPF6 additive.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.X.). *E-mail: [email protected] (J.-G.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technology of the U.S. Department of Energy (DOE). The XPS measurements were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. 4048

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4127754 | J. Phys. Chem. C 2014, 118, 4043−4049