Lithium expulsion from solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12

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Lithium expulsion from solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12 by controlled electron injection in SEM Xiaowei Xie, Juanjuan Xing, Dongli Hu, Hui Gu, Cheng Chen, and Xiangxin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17276 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

Lithium expulsion from solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12 by controlled electron injection in SEM Xiaowei Xie1, Juanjuan Xing1, Dongli Hu1, Hui Gu*,1, Cheng Chen2,3, Xiangxin Guo*,2,4 1 School of Materials Science and Engineering, Materials Genome Institute, Shanghai University, Shanghai, 200444, China 2 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China. 3 University of the Chinese Academy of Sciences, Beijing, 100049, China 4 College of Physics, Qingdao University, Qingdao, 266071, China

Abstract: The garnet ionic conductor is one of promising candidate electrolytes for all-solid-state secondary lithium batteries, thanks to its high lithium ion conductivity, good thermal and chemical stability. However, its microstructure is difficult to approach since it is very sensitive to the inquisitive electron beam. In this study based on scanning electron microscope, we found that the electron beam expulses the lithium out of Li6.4La3Zr1.4Ta0.6O12 (LLZTO), and the expulsed zone expands to where a stationary beam could extend and penetrate. The expulsion of metallic lithium was confirmed by its oxidation reaction after nitrogen inflow into SEM. This phenomenon may provide us an effective probe to peer into the conductive nature of this electrolyte. A frame-scan scheme is employed to measure the expulsion rate by controllable and more uniform incidence of electrons. Lithium accumulation processes are continuously recorded and classified in four modes by fitting their growth behaviors into the dynamic equation that is mainly related to the initial ion concentration and ion migration rate in the electrolyte. These results open novel possibility of using SEM probe to gain dynamic information on ion migration and lithium metal growth in solid materials. Keywords: lithium ion solid-state electrolyte, cubic garnet Li6.4La3Zr1.4Ta0.6O12, electron beam irradiation, lithium expulsion, ion migration rate

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1. Introduction The lithium-ion conductor Li7La3Zr2O12 (LLZO) is a key solid electrolyte that attracts a lot of attentions because of its high lithium ion conductivity, good thermal and chemical stability as well as the availability of dense materials, which are all beneficial factors for all-solid-state secondary lithium batteries.1 LLZO may exist in two structures subject to the sintering temperature,2-3 and its ionic conductivity could vary by two orders of magnitude from 1.63×10-6 to 3×10-4 S/cm, depending on whether it is in tetragonal or cubic forms.1, 3 Although both structures are composed of dodecahedral LaO8 and octahedral ZrO6, the location of lithium ion and its transmission path are rather different.3-5 The cubic LLZO possesses isotropic three-dimensional ion transport channels and high concentration of vacancies to favor the migration of lithium ion, hence high ionic conductivity.6 To stabilize the cubic phase, substitution of Zr by Al, Si, Ta, W, or Nb has been extensively studied.7-12 In particular, Lai et al found that, by partial substitution of Ta for Zr, fully cubic phase was achieved via transformation from tetragonal phase, which increased the total conductivity to 6.9×10−4 S/cm.10 Despite of these advantages, chemical stability of LLZO is susceptible towards moisture, air and under electron beam. Cheng et al found that the lithium content was very high on the surface of LLZO exposed to air for two months as revealed by the laser-induced breakdown spectroscopic cross section mapping,13 and the surface component was detected as Li2CO3 by X-ray photoelectron spectroscopy (XPS). The effect of beam irradiation on lithium battery materials has recently been reported by several theoretical and experimental studies,14-18 but this still remains empty for its connection to the high ion conductive LLZO. There was a HRTEM study for LLZO, but little information on lithium distribution and behavior was obtained due to its quick amorphization, which is probably due to easy decomposition induced by the electron beam.19 This limits the research on its structural nature and microstructure behaviors. It is recently reported that suitable Ta doping in LLZO, i.e. Li7−xLa3Zr2−xTaxO12 (x=0.6) or LLZTO, could significantly improve the lithium discharge and cyclic performance to make it a better solid electrolyte and support for electrodes, while its 2


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ion conductivity could reach 1.6×10-3 S/cm.20 To avoid or minimize the beam irradiation on this garnet-type lithium ion conductor, we attempt to investigate the structural behavior of LLZTO in scanning electron microscope (SEM), and find that certain substance could be expulsed from the surface of the bulk electrolyte material even under relatively moderate electron beam, which is induced by requirement of electrochemical neutrality. Elucidation of the relationship between the controllable electron probe and the rate of expulsion in this electrolyte material might help us to better understand the mechanism of ion conductivity and lithium metal growth in LLZTO as well as for other solid-electrolyte materials.

2. EXPERIMENTAL SECTION LLZTO ceramics were prepared via conventional solid-state reactions, from stoichiometric LiOH (Alfa Aesar, 99.995%), La(OH)3 (Alfa Aesar, 99.95%), ZrO2 (Aladdin Reagent, 99.99%), Ta2O5 (Aladdin Reagent, 99.99%), and a 15 wt.% excess of LiOH to compensate volatile Li during synthesis. The powders were ball-milled for 12 h before heated in air at 900 ºC for also 12 h to ensure the formation of cubic LLZTO phase, and then ball-milled for 12 h again. Afterward, the resultant powders were pressed into pellets with a diameter of 12 mm at 100 MPa, and sintered at 1140 ºC for 9 h in oxygen. During sintering the pellets were covered with mother powders and put in alumina crucibles in a tube furnace.21To reach high surface flatness of the solid electrolyte pellet for microstructural observation and analysis, 3μm, 1μm and 0.5μm of the corundum emery papers are used in succession during the polishing, which leaves also very few scratches on the smooth sample surfaces. Not only the high-purity alcohol (99.97%) was pouring intermittently onto the emery papers to lubricate and protect the polishing surfaces, they were also cleaned by alcohol-soaked cotton swabs under the roasting lamp to avoid moisture that could react with the naked surface, before immediately placed onto SEM stage to start venting. Microstructures of LLZTO were characterized by SEM (Model Supra55, G300, Carl Zeiss, Germany) in secondary electron (SE) and backscattered electron (BSE) 3



imaging modes. Elemental analysis was carried out with an energy-dispersive spectrometer (EDS; model AZtec, Oxford Instrument, UK) attached to SEM with electron beam accelerating voltage at 8 kV, which covers all the involved elements except the undetectable lithium. The Monte Carlo simulation of electron trajectory was made by the commercial program Casino.

3. RESULT AND DISCUSSION 3.1. Expulsion of substance by stationary electron beam

Figure 1. BSE imaging of LLZTO at 10kV.

Although XRD analysis had exhibited monolithic cubic phase for this LLZTO material,21 SEM imaging at 10 kV in BSE mode reveals not only rather dense ceramic body to reveal it as good solid-state electrolyte, but also non-uniformity in local composition for grains in size of 5 μm, as shown in Figure 1. The latter is shown as intra-granular contrast that reflects the inhomogeneous distribution of relative mass within the LLZTO phase, and the white contrast should come from regions with high concentration of heavy elements, i.e. lanthanum and tantalum. Such chemical inhomogeneity is most likely the elemental segregation during preparation. When irradiating by a stationary electron beam, white substance in SE mode always expulses out from surface of LLZTO, one case is given in Figure 2 with a moderate probe current of 285.4×10-12 A at 10 kV. The expulsed substance starts to accumulate on the surface surrounding the electron beam (red spot) within one or few seconds of irradiation, and 4


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the accumulated substance grows while its visible area enlarges by continuing irradiation until pile-up as shown in Figure 2a. Areas of the accumulated substance are measured against the irradiation time as plotted in Figure 2b, indicating that the expulsion volume increases in decreasing rate to follow an exponential or power-law behavior. Monte Carlo simulation of this incident electron beam 10 kV reveals that the peering probe penetrates into the bulk of LLZTO in a depth and size of about 500 nm and 700 nm, respectively, as shown in Figure 2c. This indicates that the injection of electrons into the solid-state electrolyte creates a driving force in an extended area to expulse substance out from this LLZTO structure.

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Figure 2. Expulsion of substance from polished LLZTO surface induced by irradiation of a stationary electron beam at 10 kV; (a) SE images of expulsion sequence with the probe position marked as red; (b) accumulation area of the expulsed materials to increase with time, which fits into a parabolic curve; (c) intensity distribution of electron probe inside LLZTO bulk, simulated by Mont Carlo method.

3.2. Confirmation of the expulsed substance as metal lithium Because of high mobility of lithium ions in the electrolyte, the expulsed substance may just be lithium ions reduced directly to metallic lithium by the incident electrons. 6


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Although EDS method could not directly detect lithium, we employed an indirect method to measure the change of other elements, especially oxygen that may come out together with lithium which would easily react with lithium, hence blocking nitration or further in-take of oxygen. Nitrogen was introduced into the SEM chamber after the expulsion occurred, hence the residual oxygen brought by flowing nitrogen is sufficient to oxidize the expulsed substance during the venting of SEM chamber in a reasonably slow rate without affecting the native surface of LLZTO. EDS spectra were collected from the expulsed substances before and after nitrogen inflow, chosen in random over more than 20 cases since their shapes have changed due to the reaction. As exemplified in Figure 3, the difference spectra reveal always the in-take of oxygen by the expulsed substance after nitrogen inflow that bring the dry air. Except the increase of oxygen intensity, all the detectable cation remains intact through the inflow process, indicating that the expulsed substance reacted with oxygen should be the undetectable metal lithium. There is also no detection of nitrogen in this analysis, indicating that the nitration reaction was either absent, or occurred at a much slower rate than the oxidation for the expulsed lithium. It is interesting to find out the expulsed substance also collapsed after oxidation, indicating the oxidation has collapsed the initial structure of expulsion. Further EDS analysis reveals no change in the LLZTO composition induced by the nitrogen inflow (see Figure S1), hence the observed increase of oxygen originates only from its reaction with the expulsed substance, unaffected for the nearby bulk. In other words, all remaining components in the electrolyte were not involved in oxidation reaction, except the expulsed metal lithium that could not be directly analyzed. Nonetheless, the expulsed substance may still constitute of low or trace amount of other elements beside the dominant lithium, in the limit that their expulsion will not significantly affect the structure of LLZTO in any apparent way.

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Figure 3. EDS spectra taken from the expulsed substance (a) before and (b) after nitrogen inflow, their difference (c) exhibits excessive oxygen due to oxidation from this substance.

3.3. Delayed lithium expulsion at different rates by frame-scan The stationary probe interacts strongly with LLZTO electrolyte to expulse lithium once the beam is placed on any position of sample surface (Figure 2b). To understand the expulsion behavior as well as the underlining mechanism, it is necessary to lower the electron irradiation rate to expose details in expulsion process. We employ the fast frame-scan mode in SEM to record continuous images every 10 seconds in video mode without interruption, which also enlarges the irradiated area and make the electron intensity more uniform to minimize the effect of probe shape. Lithium expulsion from various locations at different rates and behaviors could be recorded in a same videoed series: the growth processes of many lithium expulsion from flat LLZTO surface are presented in Figure 4a, with intervals every 50 seconds between successive frames. It is revealed that the expulsion of lithium occurs from inside the grains rather than at grain-boundaries or from triple points, which decorate often with small holes in black contrast where no expulsion was found. In meanwhile, each expulsion starts at a different time and grows into its own shape in certain rate. As marked in Figure 4b, all these individual lithium expulsion processes could be categorized into four groups according to their final sizes and shapes. The first group of lithium expulsion is encircled in dark lines: they all reach micron size and grow rapidly, starting quite late after 5 min of irradiation. For the second and third groups of expulsion encircled in blue and green lines respectively, they grow correspondingly into sub-micron and nano-scale sizes in increasingly slower rates as compared to the first group, starting almost half times earlier. It is interesting to find a very fast expulsion of lithium into a strip shape, or effectively and ejection of lithium, which is marked in red line and categorized into the fourth group. All these four representative behaviors of lithium expulsion are plotted against time in Figure 4c, corresponding to the numbered processes in Figure 4b. These curves demonstrate not only that the accumulation rates of lithium expulsion are remarkably 8


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different, but the starting time of lithium expulsion is also variable among these groups: it is earlier for the slow accumulation rate while delayed for the rapid growth, which is also true even for the last group where the sudden ejection is effectively a very rapid expulsion to exceed even the frame recording rate. A higher rate of accumulation corresponds to a later starting time of expulsion, this indicates that the internal driving force for expulsion process should be stronger.

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Figure 4. Lithium expulsion behaviors recorded under continuous frame-scan every 10 seconds: (a) frames excerpt every 50 seconds; (b) four categories of expulsion behaviors (color coded) distinguished by their shapes and sizes; (c) The representative growth curve of the four categories marked in (b).

3.4. Dynamic equation to describe lithium accumulation 10


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In order to understand the underlying mechanism that drives the lithium expulsion, the rate of expulsed metal accumulation can be written as below to follow Saito et al:22 dS  k1E  k3 dt

(1)

where S is the accumulated area of expulsion to measure the quantity of expulsed substance, t is the time of irradiation, σ is the ionic conductivity, E is the strength of the electrical field in the electrolyte created by injected electrons from the beam, k1 is the coefficient of expulsion which is assumed as a constant. k3 is the evaporation rate of the accumulated metal, depending on its vapor pressure in SEM, which can be ignored due to good stability of lithium in vacuum. For lithium conductivity in LLZTO, we can use the general expression:

σ  nZeu

(2)

where n, Ze and u represent the mobile lithium ion concentration, number of charge per ion which is 1, and the ionic mobility, respectively. As the actual ion concentration during irradiation when it is affected by the accumulated metal:

n  n0  k2 S

(3)

where n0 is the initial concentration of the mobile ion, k2 is a coefficient, taking again as a constant. Therefore, we have dS

dt

 k1n0 uE  k1k 2uES  k1 0 E  k1k 2ES / n

(4)

where 0 is the initial ionic conductivity. When the quantity of metal accumulation S is small as compared to the irradiated region within electrolyte, the strength of the electrical field E could be considered as constant. Then, the logarithm integration of formula (4) turns it into:

In(k1 0 E  k1k 2ES / n )  k1k 2Et / n  C

(5)

where C is a constant from integration. It is reasonable to set for S = 0 at t = 0 when S is estimated from the accumulation area in the SEM images, which leads to: S  a1  exp( bt ) a  n0 / k 2 11


(6) (7)


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b  k1k 2σE/n  k1k 2uE

(8)

Therefore, time dependent accumulation of the lithium expulsion process could be quantitatively described, where the exponential constant b representing both the effects of lithium ion mobility and the expulsion filed created by the injected electrons.

Table 1. Lithium expulsion parameters resulted by fitting the expulsion curves of Figure 4c into Formula (6) Position

a

b×103

t0/s

Fitting correlation factors

①

0.9340

10

302

0.9881

②

0.3292

5.36

256

0.9931

③

0.0971

3.72

138

0.9926

④

0.2331

13.81

380

0.9796

The phenomenon of lithium expulsion fundamentally reflects the ionic migration characteristics of LLZTO as a solid electrolyte. Table 1 summarized that the expulsion constants and correlation factors obtained by fitting the representative accumulation curves in Figure 4c into Formula (6). The parameter a is the destined area of accumulation at infinite time according to Formula (6), hence the larger for the observed expulsion area, the bigger a will be, except for the fourth case where bending of the nano-strip onto surface has over-counted the expulsion area. For the parameter b, the larger value corresponds to faster rate of accumulation, which is even true for the fourth case with the ejection rate exceeding the frame recording speed. In addition, the parameter b is correlated inversely to the starting time of expulsion, or t0, as also listed in Table 1. Such a correlation between t0 and b indicates that, to initiate lithium expulsion, the driving electric field needs to build up by the incident electron beam till certain strength: the stronger expulsion with higher accumulation rate, the longer time (t0) it will require to build up stronger electric filed inside LLZTO electrolyte. Differentiating lithium expulsion into four modes under fast-framing electron beam equivalent to uniform electron irradiation, indicating that the different expulsion processes are associated with certain microstructural characters, either the variation in 12


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grain orientation, the compositional inhomogeneity, unknown surface defects or invisible interfaces, or any combination of these. To further explore and identify the microstructural origin, future study is necessary especially with elaborate modeling and simulation to verify and rationalize these experimentally discovered phenomena.

Figure 5. The schematic of lithium expulsion from LLZTO after electron irradiation in (a) stationary beam, (b) frame-scan mode.

The expulsion of lithium driven by internal electric fields created from both the stationary and fast-scan electron probes is schematic described in Figure 5(a) and 5(b), respectively. Accelerated by 10kV electric field, the incident electrons could penetrate deep and wide into the bulk electrolyte to leave themselves inside the electronically insulating LLZTO lattice, and some of them may combine with the lithium ions to create neutral lithium atoms. After certain time of accumulation, the excessive electrons create an internal electron field that attracts nearby lithium ions to move towards the charged zones, which expel the concentrated lithium to channel out the electrolyte from certain surface defects, or from grains with certain crystalline orientation or chemical inhomogeneity. This phenomenon of metal lithium expulsion from the electrolyte bears strong analogue to the creation of metal lithium dendrites in lithium batteries. The former case is initiated by fast electron beam penetrating deep into the solid electrolyte to create an internal driving field, and the latter is driven by polarization with large current at the interface between electrolyte and electrode during galvostatic cycling, while both are controlled by the unexpected electric fields. Effect of such internal electric fields and the way to control the lithium expulsion should be further studied in future work, which 13



may bring this analogue even closer and truer.

4. CONCLUSION To investigate the influence of beam irradiation on structural behavior of LLZTO, we have found that the lithium metal could be expulsed from the surface of the bulk electrolyte material under moderate electron beam in SEM. The fast scanning beam reveals different behaviors of lithium expulsion, which could be fitted into the dynamic equation to gain parameters that are related to the initial ion concentration and ion migration rate in the electrolyte. The injection of electrons into the bulk of LLZTO forms a negative electric field to create lithium metal out of the materials surface, this might help us to better understand the mechanism of lithium metal growth during polarization with great current densities in LLZTO as well as for other solid-electrolyte materials.



AUTHOR INFORMATION

Corresponding Author Hui Gu, Email: [email protected].

*

Xiangxin Guo, Email: [email protected].

*

 ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant number 51532002, 51532006 and 51771222) and by Science and Technology Commission of Shanghai Municipality under grant number 16DZ2260600. The authors would like to thank Prof. Bing-kun Guo and Prof. Si-qi Shi for beneficial discussion. 14


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 Supporting Information The Supporting Information is available free of charge on the ACS Publications website: EDS spectra of the sample surface before and after nitrogen inflow, and before and after the occurrence of accumulation. A full-footage video from which all the frames in Figure 4(b) were intercepted, and it could clearly be seen how each expulsion event, including the ejection of a strip, was proceeded, as described in the corresponding text.

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Lithium expulsion from solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12

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