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Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques
Journal Pre-proof Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques Zhongting Wang, Masashi Kotobuki, Li Lu, Kaiyang Zeng PII:
S0013-4686(19)32425-9
DOI:
https://doi.org/10.1016/j.electacta.2019.135553
Reference:
EA 135553
To appear in:
Electrochimica Acta
Received Date: 11 July 2019 Revised Date:
9 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Z. Wang, M. Kotobuki, L. Lu, K. Zeng, Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques, Electrochimica Acta (2020), doi: https:// doi.org/10.1016/j.electacta.2019.135553. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques
Zhongting Wang, Masashi Kotobuki, Li Lu, Kaiyang Zeng* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576, Singapore
* Corresponding author: Dr. K.Y.Zeng, E-mail: [email protected]; Tel: (+65) 65166627; Fax: (+65) 6779 1459.
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Abstract All-solid-state battery is potentially for the next generation lithium ion battery. In allsolid-state battery, the solid electrolyte plays the most important role. However, the nanoscale electrochemical properties of the solid electrolyte are not well understood yet. In this study, by using Scanning Probe Microscopy (SPM) based techniques, including Atomic Force Microscopy (AFM) and Electrochemical Strain Microscopy (ESM), the electrochemical deformation within grains and grain boundaries in the NASICON-structured (sodium (Na) Super Ionic CONductor) Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte is studied at the nanoscale first time. The different electrochemical responses at grains and grain boundaries of the LAGP solid electrolyte are observed clearly due to different lithium ion conductivities insides the grains and along the grain boundaries. These results provide the new insights on the nanoscale electrochemical mechanisms and development of the solid electrolyte materials.
Keywords: solid electrolyte, electrochemical strain microscopy, lithium ion diffusion
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1. Introduction In current information-rich society, energy storage devices such as batteries are essential components. Large-scale batteries are usually used for stationary applications and the technologies have been maturely developed.[1] However, portable energy storage technologies have still developing in order to meet the increasing demand of miniature sized electronic devices such as handphone, laptops, digital cameras, and even for electric vehicles in terms of power and energy density. Among all portable energy storage devices, lithium ion batteries (LIBs) are currently used as the most common power source, due to relatively high energy density and good cyclability.[2] Current LIBs are mainly based on liquid electrolytes due to their high ionic conductivity. The liquid electrolytes are solutions of organic solvents, such as EC (ethyl carbonate), DMC (dimethyl carbonate) and DEC (diethyl carbonate) as well as their mixtures with a Li salt like LiPF6. These organic solvents are flammable and explosive, which are the major concerns on the safety issue in the development of LIBs. Moreover, the electrochemical window of the organic-based liquid electrolytes is usually in the range of 1 V to 4.5 V, hence some cathode materials with high operation voltage cannot be used. This restricts the improvement of the energy density of LIBs. The other type of liquid electrolytes is ionic liquids, which are more thermally and electrochemically stable. However, the performance of the ionic liquids is affected significantly by the temperature due to high sensitivity of the viscosity to the temperature.[3, 4] To avoid these disadvantages of the liquid electrolytes, solid electrolytes, especially non-flammable inorganic ceramic electrolytes, have been developed and become good candidates for the next generation LIBs. The solid electrolytes possess not only wide electrochemical window but also good thermal stability, which can prevent the risk of the fire hazards and explosions.[5] However, compared to liquid electrolytes, the main disadvantage of the solid electrolyte is the relatively low ionic conductivity (~10-4 S/cm) at room 3
temperature, comparing with the high ionic conductivity of the currently used liquid electrolytes ( ~10-2 S/cm).[6] Moreover, the resistance at the interfaces between solid electrolyte and electrodes (solid/solid interface) is usually higher than that between the liquid electrolyte and electrodes (liquid/solid interface), and this can lead to poor performance of LIBs with the solid electrolytes.[7, 8] Currently, four types of inorganic solid electrolytes have been intensively investigated, including: 1) superionic conducting sulfides, 2) perovskite-type oxides, 3) garnet-type oxides, and 4) NASICON-type (sodium (Na) Super Ionic CONductor) glass ceramics. Among all of these four types of solid electrolytes, NASICON-type materials show relatively high ionic conductivity at room temperature.[9, 10] Li1+xAlxGe2-x(PO4)3(LAGP), as one of the NASICONtype ceramics, has been expected to be the solid electrolyte for the next generation LIBs due to its high ionic conductivity and wide electrochemical window as well as the high thermal stability.[11, 12] Generally-speaking, the ionic conduction in ceramic material is composed of two components, one is the conduction inside the grain (grain conductivity, σg), and the other is the conduction at the grain boundary (grain boundary conductivity, σgb).[18,19] The total conductivity which determines the performance of the solid electrolyte can be affected by both grain and grain boundary conductivities. Comparing with the grain conductivity, the grain boundary conductivity of the LAGP has been considered to two or three orders lower.[12] Hence, one effective way to improve the total conductivity of the LAGP is to enhance the grain boundary conductivity. The glass-ceramic structure of the LAGP through the processes of melt-quenching with post-crystallization shows relatively high total conductivity, this is because the glass phase surrounding the crystal grains can improve the grain boundary conductivity.[13] The LAGP with glass-ceramic structure has been studied by several traditional characterization techniques, such as XRD (X-ray diffraction), NMR (Nuclear Magnetic 4
Resonance), SEM (Scanning Electron Microscope) and EIS (Electrochemical Impedance spectroscopy). Generally-speaking, XRD, NMR and SEM can provide the structure information of the material, while EIS measures the electric properties of the material.[13] However, all of these techniques cannot be used to investigate the nanoscale properties of the solid electrolyte. In order to study the local properties of the solid electrolytes, especially, the differences between the grains and grain boundaries conductivities or electrochemical performance, SPM (Scanning Probe Microscope)-based technique can be applied. Electrochemical Strain Spectroscopy (ESM) is one of the advanced SPM-based techniques developed in 2010.[14] This technique has been used to study the diffusion and electrochemical deformation of the cathode materials used for LIBs, in which the electrochemical strain of cathode materials under external bias was characterized.[14-16] However, this method has been rarely applied to characterize the electrochemical performance of the solid electrolytes.[20,21] In this study, the SPM-based techniques are applied to investigate local electrochemical deformation of the LAGP under external bias, especially the differences between the deformations at grains and grain boundaries on the nanoscale. The results clearly show the electrochemical strain is induced by applied bias through accumulation of lithium ions and formation of the vacancies. In addition, at the grain boundary, the strain induced by the bias is found to be larger than that in the grains. Furthermore, the strain is much lower than that of the electrode materials which are characterized by SPM under the same conditions. 2. Materials and Characterization 2.1 Preparation of LAGP The LAGP sample was prepared following the procedures in the literature.[13] Li2CO3, Al2O3, GeO2, NH4H2PO4 were ball-milled in ethanol for overnight. Furthermore, 10 mol % 5
excess Li2CO3 was added to compensate for Li evaporation during the heating process. Then, the milled powder was heated at 380 °C for 2 h to decompose the ammonia and remove volatile components. The mixture was ground by an agate mortar and pestle. The ground powder was put into a platinum crucible and melted at 1350 °C for 2 h. The melt was poured onto a stainless steel plate, which was preheated at 500 °C. After solidification, the cast sheets were annealed at 500 °C for 2 h to release the thermal stress. Finally, the pre-anneal sheets were crystallized at 950 °C for 16 h with a heating rate at 3 °C/min. The preparation procedures were exactly the same as those described in the previous work,[13] therefore, the structure of this sample should be the same as that described before.[13] As the preparation of glass-ceramic LAGP was through melt-quench and crystallization, the sample was first quenched to obtain the sample with glassy phase and then heated to a relative high temperature to achieve recrystallization. Figure 7 in the reference [13] has clearly illustrated the recrystallized grains grew within the glassy phase matrix, while the sample showed porous structure.[13] The surface roughness of the as-prepared sample was too large to meet the requirement for SPM measurement due to these processes. Therefore, the surface of the sample was polished by sandpapers. As the topography features were damaged during the polishing, the polished sample was then thermally etched at 950 °C for 5 minutes in order to view the features of grain and grain boundary. 2.2 Characterization 2.2.1 XRD Characterization Crystal structure of the sample was characterized by X-ray diffraction (XRD) (XRD6000, Shimadzu, Japan) with Cu kα radiation and Bragg-Brentano geometry. The scan was performed in a scan range of 2θ = 20 to 60o with a step of 0.02o and scan rate of 1o/min. The
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XRD pattern (Figure S1, Supplementary information (SI)) shows the dominant phase in the sample is LiGe2(PO4)3 with rhombohedral structure. A small amount of GeO2 was also observed. Standard diffraction peaks for LAGP is not available. Hence, LiGe2(PO4)3 peaks are used to assign LAGP, as the LAGP and LiGe2(PO4)3 possess the same crystal structure and Al-contained phases are not detected in the sample. Therefore, dominant phase can be assigned to LAGP. The formation of secondary phase such as GeO2 is almost inevitable in this study as shown in XRD pattern. However, it is not easy to use AFM to distinguish the secondary phase. The purpose of this paper is therefore to clarify the differences between the phenomena in grain and grain-boundary in the LAGP solid electrolyte. The influence of the secondary phase is not discussed here. 2.2.2 SEM Observations Surface morphology of the sample was observed by Scanning Electron Microscope (SEM, JEOL-6010 PLUS/LV, JEOL, Japan). Thin layer of Au was sputtered on the sample surface to provide electronic conductivity to the sample to eliminate the charge effect and then obtain better quality of images. The SEM images (Figure S2, SI) shows the surface morphology of the sample after the thermal etching. A square shape of the polycrystalline grains could be clearly observed in the whole surface of the sample. In addition, the edges of the grains were very sharp, indicating the sample was highly crystallized. 2.2.3 Electrochemical Strain Microscopy (ESM) Measurements ESM is one of the advanced SPM based techniques. The principle of ESM is schematically shown in Figure 1. In ESM operation, an AC bias is induced on the probe, which provokes redistribution of the local lithium ion concentration in the sample. The
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redistribution of the lithium ion changes the molar volume of the sample, and this change is defined as electrochemical strain, which can be measured by the same SPM probe. In the ESM mode, an AC bias is employed to measure the electrochemical strain. The amplitude of the AC bias is usually very small and can only affect the local area. Switching Spectroscopy ESM (SS-ESM) is coupling an AC bias (blue sinusoids) with increasing step DC bias (red steps) (Figure 1(d)). The AC bias is with too small amplitude to polarize the sample, and it is used to measure the electrochemical strain, while the DC bias is used to polarize the material with increasing value. Therefore, after each poling process, the electrochemical strain can be measured and the effect of DC bias in every step can be recorded. In this study, the ESM study is conducted on an commercial SPM system (MFP-3D, Asylum research, Oxford Instruments, CA, USA), the experiments were conducted in a closed cell with argon gas (purity of 99.9991%, with an oxygen content of <0.01 ppm and water content<0.02 ppm) environment to protect the sample from surface contamination by gas adsorption, especially moisture, oxygen and carbon dioxide. Moreover, during the experiment, as high voltage was applied, both the sample and tip might be burned if oxygen is present, hence, the experiments was conducted in the argon environment. The probes used in the experiments were Si probes coated by Platinum for conductive measurements (AC 240 PP, OPUS, Bulgaria) with a spring constant 2 N/m and resonance frequency of 70 kHz. The probe is calibrated before the experiment to ensure that spring constant and resonance frequency was within manufacturer’s specification ranges. The calibration procedure includes three steps. First step is to correct for virtual deflection effects in the AFM hardware. Second step is to calibrate the relationship between cantilever deflection and vertical cantilever motion. The last step is to withdraw tip and perform a
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thermal to determine the cantilever’s resonant frequency, and an algorithm computes the spring constant using the equi-partition theorem. There are several possible parasitics effects in the experiments. Firstly, the Vegards effect depends on the AC frequency, therefore the frequency of the AC voltage used in the experiment was kept at the contact resonance between the probe and the sample which is about 270 kHz. DART (dual AC resonance tracking) mode was used to track the contact resonance frequency during the scanning. The effects of frequency on Vegards strain are not discussed in this study. In addition, the main effect is from static electric charges, hence during the experiments, the sample is grounded to remove these static charges. However, simple grounding of a sample may not immediately remove all static charges, as the solid electrolyte is pure ionic conductor and poor electric conductor. Generally-speaking, it is not easy to remove all the static charges efficiently during the ESM scanning when tip is biased. Therefore, we first conduct a control experiment to study the relative contribution of the electrostatic force in the ESM signal of the solid electrolyte. 3. Results and Discussion First of all, the sample is measure by ESM mode at single point with small DC bias superposed by 1 V AC drive. The role of the small DC bias applied here is to vary the electrostatic force while maintaining the sample unchanged. Figure 2 shows the typical ESM amplitude against DC bias curves measured from the four randomly chosen positions in the sample. It is clear that these four curves are similar, implying the responses of these four different points are almost the same, so the results can represent the general responses of the sample. Taking Figure 2(a) as an example, at the minimum point N, the electrostatic force is zero since the DC bias equals to the surface potential here, thus the amplitude AN is the real 9
ESM amplitude of the sample.[22] At point M, the applied DC bias is 0 V, which is the scanning condition for all the ESM images, so AM represents the practical experimental amplitude in the ESM measurement. The difference δ between AM and AN approximately reflects the contribution from electrostatic force. Even though the effect of electrostatic force cannot be completely eliminated in all of the cases, the ratio δ/AM is very small, which means that the real ESM amplitude does play a dominate role in the ESM measurements of the sample. Therefore, it is believed that the ESM results in this work are governed by the electrochemical Vegard strain while the effect of electrostatic force can be neglected. The samples are then scanned by the ESM mode. Figures 3(a) and 3(b) show topography and electrochemical strain of the LAGP solid electrolyte, respectively. It is clear that the electrochemical strain at grain boundaries is larger than that within the grain. A line analysis is performed to show the relationship between topography and electrochemical strain in detail (Figure 3c). The topography curve changes gradually from left to right along the line, forming a valley which corresponds to the grain boundary. Contrary, the strain curve shows different features with three clear regions: in regions 1 at the left side and 3 at the right side, which are grains of the LAGP, the electrochemical strain is low and almost constant; in region 2, which corresponds to the grain boundary, the electrochemical strain is also low and constant at the left and right sides. At the middle of the region 2, a sharp peak appears, which corresponding to the surrounding glass phase. One possibility for the high electrochemical strain appearing in the glass phase may be due to the topographic cross talk, i.e., due to the lower position of the grain boundary rather than the intrinsic properties of the glass phase. To determine whether it is due to the effects of cross-talk with the topographic information, another area with high roughness is scanned, and the result is shown in Figure S3 (SI). The line analysis is also performed to illustrate the appearance of the topography and electrochemical strain. It is clear that there is a low valley 10
region between the coordinate 3 µm and 6 µm in the topography image. However, there were still several positions with higher electrochemical strain signals within this valley region. Moreover, two lines in Figure S3(c) (SI) show the topography and electrochemical strain are not negatively correlated. Therefore, it is believed that the high electrochemical strain at the grain boundary is induced by the external bias from the probe and it is intrinsic property of the glass phases in LAGP. Assuming that the lithium ion transport processes are diffusion-limited and the contribution of migration is minimal, the amplitude of the electrochemical strain (A) can be expressed by:[17] = 2(1 + )
√
√
(1)
where υ is the Poisson’s ratio, Vac is the AC voltage amplitude, ω is the frequency of the applied electric field, D is the lithium diffusion coefficient, η describes the linear relationship between the applied field and chemical potential, and β is the Vegards coefficient, which can be expressed as an empirical linear relationship between the lattice parameter and lithium concentration. Since υ, β, Vac, and ω should be constant in one experiment, hence A is proportional to the quantity of √/. However, a clear relationship between strain amplitude and lithium ion conductivity directly cannot be determined from this experiment. Hence, the ratio between the values of electrochemical strain within the grain and the glass phase can be
obtained as: = ×
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Taking Figure 3 as example, the peak value of electrochemical strain in glass phase is around Aglass=1.63×10-11 m, while the average electrochemical strain in region 1 is around Agrain=6.3×10-12 m, hence,
×
=
=
. × !"## . × !"#$
= 6.7 × )
+
*
= 2.59, and
(2)
The chemical potential between the grain and glass phases are different but their values cannot be determined from current experiments. However, the difference should be small because both phases have similar compositions. Moreover, these two phases are directly contacted each other and co-exist stably in the sample. Therefore, if we assume the chemical potential of the grain and glass phase are approximately the same, i.e., assuming ,-./0 ≈ ,2.33 for simplification purposes. The diffusion coefficient with glass phase Dglass is then 6.7 times higher than the diffusion coefficient within the grain Dgrain according to equation 2. Hence, it is believed that the diffusion of the lithium ions in the glass phase is much faster than that within the grains. Comparing to the crystal phase, there are much more defects in glass phase, such as vacancies embedded in the glass phases due to the quenching processes, which can provide the diffusion path for lithium ions. Therefore, the lithium ions should diffuse more easily in the glass phase than that within the grains. As the redistribution of the lithium ions in the sample causes the electrochemical strain, the observed different electrochemical strains in the grain boundary and in the grain indicate that the local lithium ion concentration in the grain boundary is different from that within the grains. However, it is still not clear that the large strain in the grain boundary is caused by accumulation of lithium ions or lack of lithium ions. To further study the relationship between electrochemical strain and lithium ion concentration, poling experiments are therefore conducted. 12
Two poling processes are used to study the relationship between electrochemical strain and lithium ion concentration. During the poling process, the whole area (256×256 points) was uniformly poled. After each poling step, an AC bias with small amplitude is immediately applied to measure the electrochemical strain with scanning points of 256×256. Then the area was poled again with a different voltage. One poling process (process I) is: applying the DC voltage in the order of + 10, + 20, + 30, -10, - 20 and - 30 V (with scanning frequency 1 Hz, 256 s in each step) (Figure 4). The other poling process (process II) is: the DC voltages are applied in the order of -10, -20, -30, +10, +20 and +30 V (Figure S4, SI). Figure 4 shows the evolution of the electrochemical strain in each step in the process I. Figure 4(a) reveals the topographies of the sample while other figures show the change of electrochemical strain after each poling step. There is a slight difference between the Figures 3 and 4 due to the different scanning areas and different scale bars. The average electrochemical strain of 256×256 points in the whole area from Figure 4 and Figure S4 (SI) in each step is plotted in Figure 5. Figure 5 shows both the positive and negative poling biases induce significant electrochemical strain. During poling by negative biases, the external bias attracted the positively charged-lithium ions and hence lithium ions were accumulated under the probe, while during the poling by positive biases, the external bias pushed away the lithium ions and hence the lithium vacancies would be created. Comparing the strain after poling by negative biases (shadow part in Figure 5a) with those by positive biases (solid part), it is clear the negative poling (accumulation of lithium ions) causes larger electrochemical strain than that under the positive poling, this indicates the electrochemical strain caused by the accumulation of lithium ions is larger than that induced by lithium vacancies.
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Hence, the large electrochemical strain at the grain boundaries can be attributed to the accumulation of lithium ions (Figure 3). The inhomogeneous lithium ion concentration under the external bias may be caused by the different diffusion characteristics of the lithium ion at grain and grain boundary, in other words, the different lithium ion diffusivities. Therefore, the accumulation of lithium ions at the grain boundaries implies the low ionic conductivity at the grain boundaries, which agree with the EIS results. [18, 19] In the earlier discussion, we have calculated the ratio of the diffusion coefficients of the grain and glass phase, this ratio indicates that the diffusion of lithium ion in glass phase is much faster than that within the grain. However, it is also found that the ionic conductivity at the grain boundary is lower than that within grains by EIS measurements. This discrepancy is caused by the oversimplifications of the grain boundary structure in the earlier analysis. Generally-speaking, in most of polycrystalline materials, the grain boundary is a very small region with several angstroms between two adjacent grains. However, in this glass-ceramic type of solid electrolyte, there are glass phases surrounding the grains. Therefore, there are interfacial region between the grain and the glass phase, in which has certain thickness. Therefore, there are four possible diffusion paths for lithium ions diffusing (Figure 6). Pg represents the lithium ions diffused within the grain and Pgb2 represents the lithium ions only diffused within the glass phase. Previous calculation estimated their ratio. However, there are still two more lithium ion diffusion paths: from the grain to the grain boundary (Pgb1) and from the grain boundary to the grain (Pgb3). The total resistance of the grain boundary region should include the resistance of path Pgb1, Pgb2 and Pgb3. In paths Pgb1 and Pgb3, the interfaces play a key role even though the scale of the interfaces are too small to be characterized. Hence, although the diffusion of lithium ion within the grain boundary is fast, the lithium ions may be accumulated at the grain boundaries due to slow diffusion of lithium ion from the grain boundary to the grains, which may suggest that it is necessary and important to 14
further reduce the effect of the interface region in LAGP to improve the total ionic conductivity. Comparing the two different poling processes (Figure 5b), it is observed that the strain evolutions are different in these two processes. The average electrochemical strain in process I is larger than that in process II, especially at ±30 V. In addition, for process I, the strain increases with the bias. On the other hand, although the same behavior is observed in the negative bias in the process II, the strain is almost constant and independent of the applied bias if it is positive. In the process I, positive bias is applied first (Figure 7a). Hence, positively charged lithium ions are pushed away from the contact point and vacancies are formed under the probe. By increasing the applied bias from +10 V to +30 V, more vacancies are created under the probe, which induces larger strain. Then, the external bias is changed into negative, in which will attract lithium ions. The increasing bias from -10 V to -30 V attracts more and more lithium ions at the vicinity of the probe and the accumulation of the ions provokes the larger strain. In the process II (Figure 7b), the negative bias is applied first. Hence, the lithium ions accumulate under the probe. When the voltages are increased from -10 V to -30 V, more and more lithium ions are accumulated, and strain also became larger and larger. However, the strain is still smaller than that in the process I. This may be attributed to the less availability of the vacancies for accumulation of the lithium ions compared with that in the process I. In process I, the positive bias can induce more vacancies before the negative bias is applied. Whereas in the process II, when applying the positive bias, it is found the strain is almost constant regardless the magnitude of the applied bias. This may be due to trap effect of the lithium ions at the grain boundaries. The negative bias first pushes some of lithium ions from grain into grain boundary and due to the low lithium ion conductivity at the grain boundary,
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these lithium ions are trapped even the positive bias is applied. Therefore, the strain is almost constant regardless the magnitude of the positive bias. To further study the effect of the external bias on electrochemical strain, Switching Spectroscopy ESM (SS-ESM) technique is applied. There are two parts of external bias in SS-ESM measurements, a DC bias and an AC bias. In this experiment, the amplitude of the triangular DC bias is set to 30 V with a sweep frequency of 0.3 Hz, while the driving amplitude of the AC bias is set to 3 V to measure the electrochemical strain. The results of SS-ESM are shown in Figure 8. The amplitude represents the electrochemical strain of the LAGP sample, while the bias reveals the external DC bias. The observed loop in Figure 8 is very similar to the hysteresis loop of the ferroelectric materials, which is composed of 4 stages. In stage 1, the external bias increases from 0 to + 30V, while the amplitude of ESM response slightly decreases from 15 pm to 5 pm. In stage 2, when the external bias decreases from +30 V to 0, the amplitude increases abruptly from 5 pm to 70 pm. Then, the strain increases gradually to around 80 pm with decreasing the external bias to -30 V (stage 3), whereas in stage 4, the strain decreases sharply to 15 pm while the external bias returns to 0 V. In stage 1, the ESM response decreases gradually when the positive external bias increases, and this can be attributed to the formation of vacancies under the probe. The positive bias pushes away the local lithium ions, hence only the frame (vacancies) is left there, which is not sensitive to ESM measurements as shown in the Figure 5. Therefore, lower ESM response is observed in the stage 1. On the contrary, in the stage 2 of negative bias, a lot of lithium ions can move to the probe because many vacancies are created in the stage 1 and the lithium ions are more sensitive to the ESM voltage applied, leading to sharp increase of the amplitude (electrochemical strain). The ESM response in the stage 3 is almost constant. This may be due to saturation of lithium ions at the vicinity of the probe. In the positive bias of
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stage 4, accumulated lithium ions are pushed away again and hence the decreased ESM response is observed. In this work, the electrochemical strain in LAGP solid electrolyte is studied using ESM and SS-ESM techniques. The electrochemical strain is uniformly distributed on the surface of LAGP solid electrolyte, where the grain boundaries reveals larger strain than that within grains due to the low ionic conductivity at the grain boundary areas. The ESM results clearly show accumulation of lithium ions and formation of vacancies under the probe at negative and positive biases, respectively, leading to the induced electrochemical strains. Additionally, the hysteresis loop caused by the accumulation of lithium ions and formation of vacancies is observed in SS-ESM. It is found that the average electrochemical strain of LAGP is about 6 pm within the grains and around 15 pm at grain boundary, while that of electrode materials are ~ 3 pm.[23, 24] Therefore, in an all-solid-state battery, an interface between the electrode and solid electrolyte may be unstable in long term of charge and discharge cycles due to the difference of the electrochemical strain between solid electrolyte and electrode materials. Furthermore, the LAGP solid electrolyte itself may be unstable because of the strain mismatch between the grain and grain boundary. For long-term operation of the all-solidstate battery, a mechanism release and/or absorption of the mismatched strain is needed. 4. Conclusions The ESM and SS-ESM are employed to investigate the electrochemical strain of the LAGP solid electrolyte under the external bias. ESM images clearly illustrate the different responses between grain and grain boundaries. The grain boundary reveals higher electrochemical strain due to the accumulation of lithium ions, which is consistent with the low ionic conductivity of the grain boundary in the previous reports. The SS-ESM loop shows the DC poling effects on LAGP solid electrolyte. Positive DC bias decreases the electrochemical strain due to a formation of lithium vacancies, while negative DC bias 17
increases the electrochemical strain by accumulation of lithium ions. The hysteresis phenomenon is also clearly observed because of the accumulation of lithium ions and formation of lithium vacancies under the ESM probe. The electrochemical strain induced by the external bias would cause the strain mismatch between electrode and electrolyte, resulting in significant influence on the performance of the all-solid-state battery in long-term operation.
Acknowledgements This work is supported by Ministry of Education (Singapore) through National University of Singapore under Academic Research Grant (R-265-000-406-112 and R-265000-596-112). One of the authors (ZTW) also thanks the postgraduate scholarship from National University of Singapore.
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[18] M. Zhang, K. Takahashi, N. Imanishi, Y. Takeda, O. Yamamoto, B. Chi, J. Pu, J. Li, Preparation and Electrochemical Properties of Li1+xAlxGe2-x(PO4)3 Synthesized by a Sol-Gel Method, Journal of The Electrochemical Society, 159 (2012) A1114-A1119. [19] J. Fu, Fast Li+ ion conducting glass-ceramics in the system Li2O-Al2O3-GeO2-P2O5, Solid State Ionics, 104 (1997) 191-194. [20] Schön, N., Gunduz, D. C., Yu, S., Tempel, H., Schierholz, R., Hausen, F, Correlative electrochemical strain and scanning electron microscopy for local characterization of the solid state electrolyte Li1.3Al0.3Ti1.7 (PO4)3. Beilstein journal of nanotechnology 9.1 (2018): 1564-1572. [21] Sasano, S., Ishikawa, R., Sugiyama, I., Higashi, T., Kimura, T., Ikuhara, Y. H., Ikuhara, Y. Relative Li-ion mobility mapping in Li0.33La0.56TiO3 polycrystalline by electron backscatter diffraction and electrochemical strain microscopy. Applied Physics Express, 10(6), 061102. [22] C. Harnagea, M. Alexe, D. Hesse, A. Pignolet, Contact resonances in voltagemodulated force microscopy, Applied Physics Letters, 83 (2003) 338-340. [23] Q.N. Chen, Y. Liu, Y. Liu, S. Xie, G. Cao, J. Li, Delineating local electromigration for nanoscale probing of lithium ion intercalation and extraction by electrochemical strain microscopy, Applied Physics Letters, 101 (2012) 063901. [24] S. Yang, B. Yan, J. Wu, L. Lu, K. Zeng, Temperature-Dependent Lithium-Ion Diffusion and Activation Energy of Li1.2Co0.13Ni0.13Mn0.54O2 Thin-Film Cathode at Nanoscale by Using Electrochemical Strain Microscopy, ACS Applied Materials & Interfaces, 9 (2017) 13999-14005.
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Figure Caption: Figure 1 Schematic of the principle of ESM: (a) topography of the sample without tip bias, (b) topography of the sample with tip bias, (c) cross-section of the sample with tip bias, and (d) schematic of the principle of SS-ESM Figure 2 ESM amplitude against DC bias at four different points. Figure 3 The ESM image of LAGP solid electrolyte: (a) topography, (b) electrochemical strain, and (c) a line analysis of topography and electrochemical strain. Figure 4 (a) Topography image and (b-h) ESM images in process I: (b) before poling, and poling by (c) +10 V, (d) +20 V, (e) +30 V, (f) -10 V, (g) -20 V and (h) -30 V, respectively. Figure 5 The average electrochemical strains in process I (in the order of + 10, + 20, + 30, 10, - 20 and - 30 V DC voltage) and process II (in the order of - 10, - 20, - 30, +10, + 20 and + 30 V) illustrated in (a) bar chart and (b) line graph. Figure 6 Schematic image of diffusion processes of lithium ions in grain and grain boundary Figure 7 Schematic image of diffusion processes of lithium ions and vacancies: (a) process I, and (b) process II Figure 8 Hysteresis loop of LAGP solid electrolyte.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Author Contributions:
Mr. Zhongting Wang conducted all SPM-based experiments and analysis, he also writes most of this manuscript. Dr. Masashi Kotobuki synthesis the solid electrolyte materials, also contributes the discussion to this manuscript. Dr. Li Lu has contributed to the discussion of the manuscript as well as to the synthesis of the solid electrolyte, Dr. Kaiyang Zeng oversees the whole project, experiments, discussion, writing and revision of the manuscript.
Kaiyang Zeng Corresponding author.
Declaration of Interest Statement
The authors declare that no interest conflict reported in this manuscript.
Kaiyang Zeng Corresponding author.
S0013-4686(19)32425-9
DOI:
https://doi.org/10.1016/j.electacta.2019.135553
Reference:
EA 135553
To appear in:
Electrochimica Acta
Received Date: 11 July 2019 Revised Date:
9 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Z. Wang, M. Kotobuki, L. Lu, K. Zeng, Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques, Electrochimica Acta (2020), doi: https:// doi.org/10.1016/j.electacta.2019.135553. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Nanoscale characterization of solid electrolyte by Scanning Probe Microscopy techniques
Zhongting Wang, Masashi Kotobuki, Li Lu, Kaiyang Zeng* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576, Singapore
* Corresponding author: Dr. K.Y.Zeng, E-mail: [email protected]; Tel: (+65) 65166627; Fax: (+65) 6779 1459.
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Abstract All-solid-state battery is potentially for the next generation lithium ion battery. In allsolid-state battery, the solid electrolyte plays the most important role. However, the nanoscale electrochemical properties of the solid electrolyte are not well understood yet. In this study, by using Scanning Probe Microscopy (SPM) based techniques, including Atomic Force Microscopy (AFM) and Electrochemical Strain Microscopy (ESM), the electrochemical deformation within grains and grain boundaries in the NASICON-structured (sodium (Na) Super Ionic CONductor) Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte is studied at the nanoscale first time. The different electrochemical responses at grains and grain boundaries of the LAGP solid electrolyte are observed clearly due to different lithium ion conductivities insides the grains and along the grain boundaries. These results provide the new insights on the nanoscale electrochemical mechanisms and development of the solid electrolyte materials.
Keywords: solid electrolyte, electrochemical strain microscopy, lithium ion diffusion
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1. Introduction In current information-rich society, energy storage devices such as batteries are essential components. Large-scale batteries are usually used for stationary applications and the technologies have been maturely developed.[1] However, portable energy storage technologies have still developing in order to meet the increasing demand of miniature sized electronic devices such as handphone, laptops, digital cameras, and even for electric vehicles in terms of power and energy density. Among all portable energy storage devices, lithium ion batteries (LIBs) are currently used as the most common power source, due to relatively high energy density and good cyclability.[2] Current LIBs are mainly based on liquid electrolytes due to their high ionic conductivity. The liquid electrolytes are solutions of organic solvents, such as EC (ethyl carbonate), DMC (dimethyl carbonate) and DEC (diethyl carbonate) as well as their mixtures with a Li salt like LiPF6. These organic solvents are flammable and explosive, which are the major concerns on the safety issue in the development of LIBs. Moreover, the electrochemical window of the organic-based liquid electrolytes is usually in the range of 1 V to 4.5 V, hence some cathode materials with high operation voltage cannot be used. This restricts the improvement of the energy density of LIBs. The other type of liquid electrolytes is ionic liquids, which are more thermally and electrochemically stable. However, the performance of the ionic liquids is affected significantly by the temperature due to high sensitivity of the viscosity to the temperature.[3, 4] To avoid these disadvantages of the liquid electrolytes, solid electrolytes, especially non-flammable inorganic ceramic electrolytes, have been developed and become good candidates for the next generation LIBs. The solid electrolytes possess not only wide electrochemical window but also good thermal stability, which can prevent the risk of the fire hazards and explosions.[5] However, compared to liquid electrolytes, the main disadvantage of the solid electrolyte is the relatively low ionic conductivity (~10-4 S/cm) at room 3
temperature, comparing with the high ionic conductivity of the currently used liquid electrolytes ( ~10-2 S/cm).[6] Moreover, the resistance at the interfaces between solid electrolyte and electrodes (solid/solid interface) is usually higher than that between the liquid electrolyte and electrodes (liquid/solid interface), and this can lead to poor performance of LIBs with the solid electrolytes.[7, 8] Currently, four types of inorganic solid electrolytes have been intensively investigated, including: 1) superionic conducting sulfides, 2) perovskite-type oxides, 3) garnet-type oxides, and 4) NASICON-type (sodium (Na) Super Ionic CONductor) glass ceramics. Among all of these four types of solid electrolytes, NASICON-type materials show relatively high ionic conductivity at room temperature.[9, 10] Li1+xAlxGe2-x(PO4)3(LAGP), as one of the NASICONtype ceramics, has been expected to be the solid electrolyte for the next generation LIBs due to its high ionic conductivity and wide electrochemical window as well as the high thermal stability.[11, 12] Generally-speaking, the ionic conduction in ceramic material is composed of two components, one is the conduction inside the grain (grain conductivity, σg), and the other is the conduction at the grain boundary (grain boundary conductivity, σgb).[18,19] The total conductivity which determines the performance of the solid electrolyte can be affected by both grain and grain boundary conductivities. Comparing with the grain conductivity, the grain boundary conductivity of the LAGP has been considered to two or three orders lower.[12] Hence, one effective way to improve the total conductivity of the LAGP is to enhance the grain boundary conductivity. The glass-ceramic structure of the LAGP through the processes of melt-quenching with post-crystallization shows relatively high total conductivity, this is because the glass phase surrounding the crystal grains can improve the grain boundary conductivity.[13] The LAGP with glass-ceramic structure has been studied by several traditional characterization techniques, such as XRD (X-ray diffraction), NMR (Nuclear Magnetic 4
Resonance), SEM (Scanning Electron Microscope) and EIS (Electrochemical Impedance spectroscopy). Generally-speaking, XRD, NMR and SEM can provide the structure information of the material, while EIS measures the electric properties of the material.[13] However, all of these techniques cannot be used to investigate the nanoscale properties of the solid electrolyte. In order to study the local properties of the solid electrolytes, especially, the differences between the grains and grain boundaries conductivities or electrochemical performance, SPM (Scanning Probe Microscope)-based technique can be applied. Electrochemical Strain Spectroscopy (ESM) is one of the advanced SPM-based techniques developed in 2010.[14] This technique has been used to study the diffusion and electrochemical deformation of the cathode materials used for LIBs, in which the electrochemical strain of cathode materials under external bias was characterized.[14-16] However, this method has been rarely applied to characterize the electrochemical performance of the solid electrolytes.[20,21] In this study, the SPM-based techniques are applied to investigate local electrochemical deformation of the LAGP under external bias, especially the differences between the deformations at grains and grain boundaries on the nanoscale. The results clearly show the electrochemical strain is induced by applied bias through accumulation of lithium ions and formation of the vacancies. In addition, at the grain boundary, the strain induced by the bias is found to be larger than that in the grains. Furthermore, the strain is much lower than that of the electrode materials which are characterized by SPM under the same conditions. 2. Materials and Characterization 2.1 Preparation of LAGP The LAGP sample was prepared following the procedures in the literature.[13] Li2CO3, Al2O3, GeO2, NH4H2PO4 were ball-milled in ethanol for overnight. Furthermore, 10 mol % 5
excess Li2CO3 was added to compensate for Li evaporation during the heating process. Then, the milled powder was heated at 380 °C for 2 h to decompose the ammonia and remove volatile components. The mixture was ground by an agate mortar and pestle. The ground powder was put into a platinum crucible and melted at 1350 °C for 2 h. The melt was poured onto a stainless steel plate, which was preheated at 500 °C. After solidification, the cast sheets were annealed at 500 °C for 2 h to release the thermal stress. Finally, the pre-anneal sheets were crystallized at 950 °C for 16 h with a heating rate at 3 °C/min. The preparation procedures were exactly the same as those described in the previous work,[13] therefore, the structure of this sample should be the same as that described before.[13] As the preparation of glass-ceramic LAGP was through melt-quench and crystallization, the sample was first quenched to obtain the sample with glassy phase and then heated to a relative high temperature to achieve recrystallization. Figure 7 in the reference [13] has clearly illustrated the recrystallized grains grew within the glassy phase matrix, while the sample showed porous structure.[13] The surface roughness of the as-prepared sample was too large to meet the requirement for SPM measurement due to these processes. Therefore, the surface of the sample was polished by sandpapers. As the topography features were damaged during the polishing, the polished sample was then thermally etched at 950 °C for 5 minutes in order to view the features of grain and grain boundary. 2.2 Characterization 2.2.1 XRD Characterization Crystal structure of the sample was characterized by X-ray diffraction (XRD) (XRD6000, Shimadzu, Japan) with Cu kα radiation and Bragg-Brentano geometry. The scan was performed in a scan range of 2θ = 20 to 60o with a step of 0.02o and scan rate of 1o/min. The
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XRD pattern (Figure S1, Supplementary information (SI)) shows the dominant phase in the sample is LiGe2(PO4)3 with rhombohedral structure. A small amount of GeO2 was also observed. Standard diffraction peaks for LAGP is not available. Hence, LiGe2(PO4)3 peaks are used to assign LAGP, as the LAGP and LiGe2(PO4)3 possess the same crystal structure and Al-contained phases are not detected in the sample. Therefore, dominant phase can be assigned to LAGP. The formation of secondary phase such as GeO2 is almost inevitable in this study as shown in XRD pattern. However, it is not easy to use AFM to distinguish the secondary phase. The purpose of this paper is therefore to clarify the differences between the phenomena in grain and grain-boundary in the LAGP solid electrolyte. The influence of the secondary phase is not discussed here. 2.2.2 SEM Observations Surface morphology of the sample was observed by Scanning Electron Microscope (SEM, JEOL-6010 PLUS/LV, JEOL, Japan). Thin layer of Au was sputtered on the sample surface to provide electronic conductivity to the sample to eliminate the charge effect and then obtain better quality of images. The SEM images (Figure S2, SI) shows the surface morphology of the sample after the thermal etching. A square shape of the polycrystalline grains could be clearly observed in the whole surface of the sample. In addition, the edges of the grains were very sharp, indicating the sample was highly crystallized. 2.2.3 Electrochemical Strain Microscopy (ESM) Measurements ESM is one of the advanced SPM based techniques. The principle of ESM is schematically shown in Figure 1. In ESM operation, an AC bias is induced on the probe, which provokes redistribution of the local lithium ion concentration in the sample. The
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redistribution of the lithium ion changes the molar volume of the sample, and this change is defined as electrochemical strain, which can be measured by the same SPM probe. In the ESM mode, an AC bias is employed to measure the electrochemical strain. The amplitude of the AC bias is usually very small and can only affect the local area. Switching Spectroscopy ESM (SS-ESM) is coupling an AC bias (blue sinusoids) with increasing step DC bias (red steps) (Figure 1(d)). The AC bias is with too small amplitude to polarize the sample, and it is used to measure the electrochemical strain, while the DC bias is used to polarize the material with increasing value. Therefore, after each poling process, the electrochemical strain can be measured and the effect of DC bias in every step can be recorded. In this study, the ESM study is conducted on an commercial SPM system (MFP-3D, Asylum research, Oxford Instruments, CA, USA), the experiments were conducted in a closed cell with argon gas (purity of 99.9991%, with an oxygen content of <0.01 ppm and water content<0.02 ppm) environment to protect the sample from surface contamination by gas adsorption, especially moisture, oxygen and carbon dioxide. Moreover, during the experiment, as high voltage was applied, both the sample and tip might be burned if oxygen is present, hence, the experiments was conducted in the argon environment. The probes used in the experiments were Si probes coated by Platinum for conductive measurements (AC 240 PP, OPUS, Bulgaria) with a spring constant 2 N/m and resonance frequency of 70 kHz. The probe is calibrated before the experiment to ensure that spring constant and resonance frequency was within manufacturer’s specification ranges. The calibration procedure includes three steps. First step is to correct for virtual deflection effects in the AFM hardware. Second step is to calibrate the relationship between cantilever deflection and vertical cantilever motion. The last step is to withdraw tip and perform a
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thermal to determine the cantilever’s resonant frequency, and an algorithm computes the spring constant using the equi-partition theorem. There are several possible parasitics effects in the experiments. Firstly, the Vegards effect depends on the AC frequency, therefore the frequency of the AC voltage used in the experiment was kept at the contact resonance between the probe and the sample which is about 270 kHz. DART (dual AC resonance tracking) mode was used to track the contact resonance frequency during the scanning. The effects of frequency on Vegards strain are not discussed in this study. In addition, the main effect is from static electric charges, hence during the experiments, the sample is grounded to remove these static charges. However, simple grounding of a sample may not immediately remove all static charges, as the solid electrolyte is pure ionic conductor and poor electric conductor. Generally-speaking, it is not easy to remove all the static charges efficiently during the ESM scanning when tip is biased. Therefore, we first conduct a control experiment to study the relative contribution of the electrostatic force in the ESM signal of the solid electrolyte. 3. Results and Discussion First of all, the sample is measure by ESM mode at single point with small DC bias superposed by 1 V AC drive. The role of the small DC bias applied here is to vary the electrostatic force while maintaining the sample unchanged. Figure 2 shows the typical ESM amplitude against DC bias curves measured from the four randomly chosen positions in the sample. It is clear that these four curves are similar, implying the responses of these four different points are almost the same, so the results can represent the general responses of the sample. Taking Figure 2(a) as an example, at the minimum point N, the electrostatic force is zero since the DC bias equals to the surface potential here, thus the amplitude AN is the real 9
ESM amplitude of the sample.[22] At point M, the applied DC bias is 0 V, which is the scanning condition for all the ESM images, so AM represents the practical experimental amplitude in the ESM measurement. The difference δ between AM and AN approximately reflects the contribution from electrostatic force. Even though the effect of electrostatic force cannot be completely eliminated in all of the cases, the ratio δ/AM is very small, which means that the real ESM amplitude does play a dominate role in the ESM measurements of the sample. Therefore, it is believed that the ESM results in this work are governed by the electrochemical Vegard strain while the effect of electrostatic force can be neglected. The samples are then scanned by the ESM mode. Figures 3(a) and 3(b) show topography and electrochemical strain of the LAGP solid electrolyte, respectively. It is clear that the electrochemical strain at grain boundaries is larger than that within the grain. A line analysis is performed to show the relationship between topography and electrochemical strain in detail (Figure 3c). The topography curve changes gradually from left to right along the line, forming a valley which corresponds to the grain boundary. Contrary, the strain curve shows different features with three clear regions: in regions 1 at the left side and 3 at the right side, which are grains of the LAGP, the electrochemical strain is low and almost constant; in region 2, which corresponds to the grain boundary, the electrochemical strain is also low and constant at the left and right sides. At the middle of the region 2, a sharp peak appears, which corresponding to the surrounding glass phase. One possibility for the high electrochemical strain appearing in the glass phase may be due to the topographic cross talk, i.e., due to the lower position of the grain boundary rather than the intrinsic properties of the glass phase. To determine whether it is due to the effects of cross-talk with the topographic information, another area with high roughness is scanned, and the result is shown in Figure S3 (SI). The line analysis is also performed to illustrate the appearance of the topography and electrochemical strain. It is clear that there is a low valley 10
region between the coordinate 3 µm and 6 µm in the topography image. However, there were still several positions with higher electrochemical strain signals within this valley region. Moreover, two lines in Figure S3(c) (SI) show the topography and electrochemical strain are not negatively correlated. Therefore, it is believed that the high electrochemical strain at the grain boundary is induced by the external bias from the probe and it is intrinsic property of the glass phases in LAGP. Assuming that the lithium ion transport processes are diffusion-limited and the contribution of migration is minimal, the amplitude of the electrochemical strain (A) can be expressed by:[17] = 2(1 + )
√
√
(1)
where υ is the Poisson’s ratio, Vac is the AC voltage amplitude, ω is the frequency of the applied electric field, D is the lithium diffusion coefficient, η describes the linear relationship between the applied field and chemical potential, and β is the Vegards coefficient, which can be expressed as an empirical linear relationship between the lattice parameter and lithium concentration. Since υ, β, Vac, and ω should be constant in one experiment, hence A is proportional to the quantity of √/. However, a clear relationship between strain amplitude and lithium ion conductivity directly cannot be determined from this experiment. Hence, the ratio between the values of electrochemical strain within the grain and the glass phase can be
obtained as: = ×
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Taking Figure 3 as example, the peak value of electrochemical strain in glass phase is around Aglass=1.63×10-11 m, while the average electrochemical strain in region 1 is around Agrain=6.3×10-12 m, hence,
×
=
=
. × !"## . × !"#$
= 6.7 × )
+
*
= 2.59, and
(2)
The chemical potential between the grain and glass phases are different but their values cannot be determined from current experiments. However, the difference should be small because both phases have similar compositions. Moreover, these two phases are directly contacted each other and co-exist stably in the sample. Therefore, if we assume the chemical potential of the grain and glass phase are approximately the same, i.e., assuming ,-./0 ≈ ,2.33 for simplification purposes. The diffusion coefficient with glass phase Dglass is then 6.7 times higher than the diffusion coefficient within the grain Dgrain according to equation 2. Hence, it is believed that the diffusion of the lithium ions in the glass phase is much faster than that within the grains. Comparing to the crystal phase, there are much more defects in glass phase, such as vacancies embedded in the glass phases due to the quenching processes, which can provide the diffusion path for lithium ions. Therefore, the lithium ions should diffuse more easily in the glass phase than that within the grains. As the redistribution of the lithium ions in the sample causes the electrochemical strain, the observed different electrochemical strains in the grain boundary and in the grain indicate that the local lithium ion concentration in the grain boundary is different from that within the grains. However, it is still not clear that the large strain in the grain boundary is caused by accumulation of lithium ions or lack of lithium ions. To further study the relationship between electrochemical strain and lithium ion concentration, poling experiments are therefore conducted. 12
Two poling processes are used to study the relationship between electrochemical strain and lithium ion concentration. During the poling process, the whole area (256×256 points) was uniformly poled. After each poling step, an AC bias with small amplitude is immediately applied to measure the electrochemical strain with scanning points of 256×256. Then the area was poled again with a different voltage. One poling process (process I) is: applying the DC voltage in the order of + 10, + 20, + 30, -10, - 20 and - 30 V (with scanning frequency 1 Hz, 256 s in each step) (Figure 4). The other poling process (process II) is: the DC voltages are applied in the order of -10, -20, -30, +10, +20 and +30 V (Figure S4, SI). Figure 4 shows the evolution of the electrochemical strain in each step in the process I. Figure 4(a) reveals the topographies of the sample while other figures show the change of electrochemical strain after each poling step. There is a slight difference between the Figures 3 and 4 due to the different scanning areas and different scale bars. The average electrochemical strain of 256×256 points in the whole area from Figure 4 and Figure S4 (SI) in each step is plotted in Figure 5. Figure 5 shows both the positive and negative poling biases induce significant electrochemical strain. During poling by negative biases, the external bias attracted the positively charged-lithium ions and hence lithium ions were accumulated under the probe, while during the poling by positive biases, the external bias pushed away the lithium ions and hence the lithium vacancies would be created. Comparing the strain after poling by negative biases (shadow part in Figure 5a) with those by positive biases (solid part), it is clear the negative poling (accumulation of lithium ions) causes larger electrochemical strain than that under the positive poling, this indicates the electrochemical strain caused by the accumulation of lithium ions is larger than that induced by lithium vacancies.
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Hence, the large electrochemical strain at the grain boundaries can be attributed to the accumulation of lithium ions (Figure 3). The inhomogeneous lithium ion concentration under the external bias may be caused by the different diffusion characteristics of the lithium ion at grain and grain boundary, in other words, the different lithium ion diffusivities. Therefore, the accumulation of lithium ions at the grain boundaries implies the low ionic conductivity at the grain boundaries, which agree with the EIS results. [18, 19] In the earlier discussion, we have calculated the ratio of the diffusion coefficients of the grain and glass phase, this ratio indicates that the diffusion of lithium ion in glass phase is much faster than that within the grain. However, it is also found that the ionic conductivity at the grain boundary is lower than that within grains by EIS measurements. This discrepancy is caused by the oversimplifications of the grain boundary structure in the earlier analysis. Generally-speaking, in most of polycrystalline materials, the grain boundary is a very small region with several angstroms between two adjacent grains. However, in this glass-ceramic type of solid electrolyte, there are glass phases surrounding the grains. Therefore, there are interfacial region between the grain and the glass phase, in which has certain thickness. Therefore, there are four possible diffusion paths for lithium ions diffusing (Figure 6). Pg represents the lithium ions diffused within the grain and Pgb2 represents the lithium ions only diffused within the glass phase. Previous calculation estimated their ratio. However, there are still two more lithium ion diffusion paths: from the grain to the grain boundary (Pgb1) and from the grain boundary to the grain (Pgb3). The total resistance of the grain boundary region should include the resistance of path Pgb1, Pgb2 and Pgb3. In paths Pgb1 and Pgb3, the interfaces play a key role even though the scale of the interfaces are too small to be characterized. Hence, although the diffusion of lithium ion within the grain boundary is fast, the lithium ions may be accumulated at the grain boundaries due to slow diffusion of lithium ion from the grain boundary to the grains, which may suggest that it is necessary and important to 14
further reduce the effect of the interface region in LAGP to improve the total ionic conductivity. Comparing the two different poling processes (Figure 5b), it is observed that the strain evolutions are different in these two processes. The average electrochemical strain in process I is larger than that in process II, especially at ±30 V. In addition, for process I, the strain increases with the bias. On the other hand, although the same behavior is observed in the negative bias in the process II, the strain is almost constant and independent of the applied bias if it is positive. In the process I, positive bias is applied first (Figure 7a). Hence, positively charged lithium ions are pushed away from the contact point and vacancies are formed under the probe. By increasing the applied bias from +10 V to +30 V, more vacancies are created under the probe, which induces larger strain. Then, the external bias is changed into negative, in which will attract lithium ions. The increasing bias from -10 V to -30 V attracts more and more lithium ions at the vicinity of the probe and the accumulation of the ions provokes the larger strain. In the process II (Figure 7b), the negative bias is applied first. Hence, the lithium ions accumulate under the probe. When the voltages are increased from -10 V to -30 V, more and more lithium ions are accumulated, and strain also became larger and larger. However, the strain is still smaller than that in the process I. This may be attributed to the less availability of the vacancies for accumulation of the lithium ions compared with that in the process I. In process I, the positive bias can induce more vacancies before the negative bias is applied. Whereas in the process II, when applying the positive bias, it is found the strain is almost constant regardless the magnitude of the applied bias. This may be due to trap effect of the lithium ions at the grain boundaries. The negative bias first pushes some of lithium ions from grain into grain boundary and due to the low lithium ion conductivity at the grain boundary,
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these lithium ions are trapped even the positive bias is applied. Therefore, the strain is almost constant regardless the magnitude of the positive bias. To further study the effect of the external bias on electrochemical strain, Switching Spectroscopy ESM (SS-ESM) technique is applied. There are two parts of external bias in SS-ESM measurements, a DC bias and an AC bias. In this experiment, the amplitude of the triangular DC bias is set to 30 V with a sweep frequency of 0.3 Hz, while the driving amplitude of the AC bias is set to 3 V to measure the electrochemical strain. The results of SS-ESM are shown in Figure 8. The amplitude represents the electrochemical strain of the LAGP sample, while the bias reveals the external DC bias. The observed loop in Figure 8 is very similar to the hysteresis loop of the ferroelectric materials, which is composed of 4 stages. In stage 1, the external bias increases from 0 to + 30V, while the amplitude of ESM response slightly decreases from 15 pm to 5 pm. In stage 2, when the external bias decreases from +30 V to 0, the amplitude increases abruptly from 5 pm to 70 pm. Then, the strain increases gradually to around 80 pm with decreasing the external bias to -30 V (stage 3), whereas in stage 4, the strain decreases sharply to 15 pm while the external bias returns to 0 V. In stage 1, the ESM response decreases gradually when the positive external bias increases, and this can be attributed to the formation of vacancies under the probe. The positive bias pushes away the local lithium ions, hence only the frame (vacancies) is left there, which is not sensitive to ESM measurements as shown in the Figure 5. Therefore, lower ESM response is observed in the stage 1. On the contrary, in the stage 2 of negative bias, a lot of lithium ions can move to the probe because many vacancies are created in the stage 1 and the lithium ions are more sensitive to the ESM voltage applied, leading to sharp increase of the amplitude (electrochemical strain). The ESM response in the stage 3 is almost constant. This may be due to saturation of lithium ions at the vicinity of the probe. In the positive bias of
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stage 4, accumulated lithium ions are pushed away again and hence the decreased ESM response is observed. In this work, the electrochemical strain in LAGP solid electrolyte is studied using ESM and SS-ESM techniques. The electrochemical strain is uniformly distributed on the surface of LAGP solid electrolyte, where the grain boundaries reveals larger strain than that within grains due to the low ionic conductivity at the grain boundary areas. The ESM results clearly show accumulation of lithium ions and formation of vacancies under the probe at negative and positive biases, respectively, leading to the induced electrochemical strains. Additionally, the hysteresis loop caused by the accumulation of lithium ions and formation of vacancies is observed in SS-ESM. It is found that the average electrochemical strain of LAGP is about 6 pm within the grains and around 15 pm at grain boundary, while that of electrode materials are ~ 3 pm.[23, 24] Therefore, in an all-solid-state battery, an interface between the electrode and solid electrolyte may be unstable in long term of charge and discharge cycles due to the difference of the electrochemical strain between solid electrolyte and electrode materials. Furthermore, the LAGP solid electrolyte itself may be unstable because of the strain mismatch between the grain and grain boundary. For long-term operation of the all-solidstate battery, a mechanism release and/or absorption of the mismatched strain is needed. 4. Conclusions The ESM and SS-ESM are employed to investigate the electrochemical strain of the LAGP solid electrolyte under the external bias. ESM images clearly illustrate the different responses between grain and grain boundaries. The grain boundary reveals higher electrochemical strain due to the accumulation of lithium ions, which is consistent with the low ionic conductivity of the grain boundary in the previous reports. The SS-ESM loop shows the DC poling effects on LAGP solid electrolyte. Positive DC bias decreases the electrochemical strain due to a formation of lithium vacancies, while negative DC bias 17
increases the electrochemical strain by accumulation of lithium ions. The hysteresis phenomenon is also clearly observed because of the accumulation of lithium ions and formation of lithium vacancies under the ESM probe. The electrochemical strain induced by the external bias would cause the strain mismatch between electrode and electrolyte, resulting in significant influence on the performance of the all-solid-state battery in long-term operation.
Acknowledgements This work is supported by Ministry of Education (Singapore) through National University of Singapore under Academic Research Grant (R-265-000-406-112 and R-265000-596-112). One of the authors (ZTW) also thanks the postgraduate scholarship from National University of Singapore.
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Figure Caption: Figure 1 Schematic of the principle of ESM: (a) topography of the sample without tip bias, (b) topography of the sample with tip bias, (c) cross-section of the sample with tip bias, and (d) schematic of the principle of SS-ESM Figure 2 ESM amplitude against DC bias at four different points. Figure 3 The ESM image of LAGP solid electrolyte: (a) topography, (b) electrochemical strain, and (c) a line analysis of topography and electrochemical strain. Figure 4 (a) Topography image and (b-h) ESM images in process I: (b) before poling, and poling by (c) +10 V, (d) +20 V, (e) +30 V, (f) -10 V, (g) -20 V and (h) -30 V, respectively. Figure 5 The average electrochemical strains in process I (in the order of + 10, + 20, + 30, 10, - 20 and - 30 V DC voltage) and process II (in the order of - 10, - 20, - 30, +10, + 20 and + 30 V) illustrated in (a) bar chart and (b) line graph. Figure 6 Schematic image of diffusion processes of lithium ions in grain and grain boundary Figure 7 Schematic image of diffusion processes of lithium ions and vacancies: (a) process I, and (b) process II Figure 8 Hysteresis loop of LAGP solid electrolyte.
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Author Contributions:
Mr. Zhongting Wang conducted all SPM-based experiments and analysis, he also writes most of this manuscript. Dr. Masashi Kotobuki synthesis the solid electrolyte materials, also contributes the discussion to this manuscript. Dr. Li Lu has contributed to the discussion of the manuscript as well as to the synthesis of the solid electrolyte, Dr. Kaiyang Zeng oversees the whole project, experiments, discussion, writing and revision of the manuscript.
Kaiyang Zeng Corresponding author.
Declaration of Interest Statement
The authors declare that no interest conflict reported in this manuscript.
Kaiyang Zeng Corresponding author.