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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Quantitative Analysis of Microstructures and Reaction Interfaces on Composite Cathodes in All-Solid-State Batteries Using a ThreeDimensional Reconstruction Technique Sungjun Choi,†,‡ Minjae Jeon,† Junsung Ahn,† Wo Dum Jung,† Sung Min Choi,† Ji-Su Kim,† Jaemin Lim,§ Yong-Jun Jang,§ Hun-Gi Jung,∥ Jong-Ho Lee,† Byoung-In Sang,‡ and Hyoungchul Kim*,† Downloaded via DURHAM UNIV on July 11, 2018 at 02:01:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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High-Temperature Energy Materials Research Center and ∥Center for Energy Storage Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea § Automotive Research & Development Division, Hyundai Motor Company, 150 Hyundaiyeonguso-ro, Namyang-eup, Hwaseong-si, Gyeonggi-do 18280, Republic of Korea S Supporting Information *

ABSTRACT: The composite cathode of an all-solid-state battery composed of various solid-state components requires a dense microstructure and a highly percolated solid-state interface different from that of a conventional liquidelectrolyte-based Li-ion battery. Indeed, the preparation of such a system is particularly challenging. In this study, quantitative analyses of composite cathodes by three-dimensional reconstruction analysis were performed beyond the existing qualitative analysis, and their microstructures and reaction interfaces were successfully analyzed. Interestingly, various quantitative values of structure properties (such as the volume ratio, connectivity, tortuosity, and pore formation) associated with material optimization and process development were predicted, and they were found to result in limited electrochemical charge/discharge performances. We also verified that the effective two-phase boundaries were significantly suppressed to ∼23% of the total volume because of component dispersion and packing issues. KEYWORDS: 3D reconstruction, composite cathode, microstructure, two-phase boundary, all-solid-state battery

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INTRODUCTION Next-generation Li-ion battery (LIB) applications such as electric vehicles, unmanned aerial vehicles, energy-storage systems, and integrated power devices for the Internet of Things require the development of technologies that allow the storage of electrical energy at safer and higher energy densities over a wider range of operating temperatures. In this context, all-solid-state battery (ASSB) technologies have the advantage of eliminating the organic electrolyte from the existing secondary battery in addition to incorporating solid-state materials for all components (i.e., the anode, cathode, and electrolyte), thereby meeting all requirements for nextgeneration LIB applications.1,2 Recently, a new solid-state electrolyte exhibiting high Li-ion conductivity (σ) of ≥1 mS/ cm was developed,3−6 and ASSBs have been actively studied because of an increase in battery applicability to related technologies. However, current ASSB technologies exhibit a number of technological difficulties in terms of material optimization,7,8 process development,9,10 and cell operation.11,12 In particular, © XXXX American Chemical Society

serious issues relating to the microstructure of the composite cathode13,14 and the electrochemical reaction interface are exacerbated during the solidification of all battery components.15−17 Although there are structural features (e.g., a robust conformal interface and a porous microstructure) that can be realized through uniform mixing of the liquid electrolyte and the solid-state cathode active material in conventional LIBs, the composite cathodes of ASSBs are prone to inhomogeneous mixing caused by the use of solid-state components or the formation of interface pore structures through point contacts.5,10 To improve cell performances, a highly percolated ionic/electronic pathway is also required, with high cathode active material loadings.13,18 Till date, only qualitative shape and distribution observations based on scanning electron microscopy (SEM)11,14 and energy dispersive X-ray spectroscopy18,19 have been possible for the microstructural analysis of Received: March 14, 2018 Accepted: June 27, 2018

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DOI: 10.1021/acsami.8b04204 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Detailed Processing Conditions for Our Wet-Casted ASSB Cella part

component

material

ratio (wt %)

supplier

composite cathode (79 μm thick)

active material solid electrolyte conductive additive binder solvent active material solid electrolyte conductive additive binder solvent solid electrolyte binder solvent

Nb/NCM622 LPN821 SUPER C65 ethyl acrylate o-xylene artificial graphite LPN821 SUPER C65 ethyl acrylate o-xylene LPN821 ethyl acrylate o-xylene

67.6 29.0 1.4 2.0

NCM622: Umicore (d50 = 4 μm), LiNbO3 coating: homemade homemade (d50 = 6 μm)6 TIMCAL Sigma-Aldrich Sigma-Aldrich Mitsubishi Chemical homemade (d50 = 6 μm)6 TIMCAL Sigma-Aldrich Sigma-Aldrich homemade (d50 = 6 μm)6 Sigma-Aldrich Sigma-Aldrich

composite anode (110 μm thick)

electrolyte (90 μm thick)

56.4 34.7 4.3 4.5 97.1 2.9

a

Nb/NCM622 and LPN821 denote LiNbO3-coated LiNi0.6Co0.2Mn0.2O2 and (Li2S)8(P2S5)2(Ni3S2)1, respectively.

Figure 1. (a) Schematic illustration of an ASSB cell showing the cell configuration, main component materials, and cell dimensions. (b) Charge/ discharge profiles of an ASSB cell for selected cycles (1, 5, and 10) at a current density of 0.02 C-rate. (c) Cycle performance of an ASSB cell at 0.02 C-rate. All electrochemical performance tests were carried out at 25 °C.

component, will also be performed. These quantitative analysis results are characterized in conjunction with material optimization and process development for the improvement of ASSB technologies.

ASSB composite cathodes, and only a limited understanding of the microstructure and reaction interface in the composite cathode has been achieved.20,21 However, such qualitative analytical techniques are limited in the context of improving ASSB performances through material optimization and process development. It is still difficult to develop conformal and robust interface between all-solid-state components because of such qualitative analysis, and there is no notable strategy for the improvements of the composite cathode through material optimization and process development. The introduction of a new comprehensive approach is therefore required based on a quantitative understanding of the microstructure and reaction interface to permit the development of advanced ASSBs. Along with the microstructures involved in material optimization, precise reaction interface calculations associated with process development can be achieved through a quantitative analysis of composite cathodes in ASSBs.22,23 Thus, we herein report the quantitative analysis of the microstructure and reaction interface of the composite cathodes in ASSBs prepared using a wet-casting process with a sulfide-based solid electrolyte. Focused ion beam (FIB) milling and image analysis techniques22−25 will be employed to identify the structural distribution of the individual components of the composite cathode, and the quantitative values related to the microstructure of each element will be derived. In addition, an analysis of the effective reaction sites, which are the active interfaces determined by the distribution of each

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EXPERIMENTAL SECTION

Cell Fabrication. Table 1 outlines the various materials employed during ASSB cell fabrication in addition to the weight ratios of all components. For the solid electrolyte and cathode active material, (Li2S)8(P2S5)2(Ni3S2)1 (hereafter, LPN821) electrolyte and LiNbO3coated LiNi0.6Co0.2Mn0.2O2 (Nb/NCM622) powder were used, and the detailed synthesis of the electrolyte can be referred in the literature.6 All slurries were prepared using a planetary mixer (Thinky Corp.) at 2000 rpm. The cathode slurry was prepared by adding a solid electrolyte to o-xylene and mixing for 5 min prior to the sequential addition of a binder, a conductive additive, and an active material and mixing for 5 min after the addition of each component. The resulting cathode slurry was cast on a Ni foil (the current collector) to prepare a sheet cathode. The anode slurry was prepared by dispersing the active material, the solid electrolyte, and the binder together in o-xylene for 1 min, prior to the addition of a conductive additive, mixing for 30 min, and casting on the main Ni foil. The electrolyte slurry was prepared by mixing the solvent and the solid electrolyte for 5 min prior to addition of the binder and mixing for 5 min. The resulting electrolyte slurry was cast on a dried anode tape. The obtained bilayer tape was cut into circular segments using a 15 mm diameter punch, and the cathode was subjected to similar treatment using a 14 mm diameter punch. The cathode and bilayer tapes were laminated at a compression pressure of 231 MPa, and this B


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ACS Applied Materials & Interfaces

Figure 2. Flow diagram for quantitative analysis using three-dimensional (3D) reconstruction technique. Detailed analysis steps and additional information are listed in Figures S1−S5. (a) 3D reconstruction analysis area corresponds to the region indicated by the dashed rectangle (left) and a trench-shaped sectioning region of the composite cathode (right). The arrow indicates the FIB milling and SEM imaging directions. All scale bars correspond to 10 μm. (b) Images showing the segmented region labeled by color: solid electrolyte (yellow), cathode active material (blue), conductive additive (red), and pore (purple). (c) Full 3D reconstructed structure of the composite cathode. In the equation, V, k, and S indicate volume, expansion coefficient, and spatial structure, respectively.

g−1 and the Coulombic efficiency was 64.38%. After the second cycle, the efficiency was increased to approximately 90%. In the context of the discharge capacity, a performance of ≥100 mA h g−1 was achieved in the initial five cycles, although an abrupt decrease in capacity was observed in the subsequent cycles. After 40 cycles, an approximately 75% decrease from the initial discharge capacity was noted. Indeed, as indicated by the electrochemical performance shown in Figure 1, the ASSB cell employed herein has a number of limitations in terms of its charge/discharge capacity and cycle performance. Thus, to identify the causes of such limitations, we quantitatively analyzed the microstructure of the composite cathode and the reaction interface, which are the most important factors in determining the charge/discharge performance behavior of LIBs. Furthermore, this paper aims to provide a clear understanding of the poor solid-to-solid interfacial structure of a composite cathode in ASSBs manufactured by the state-ofthe-art powder and wet-casting technology and to provide insights on material optimization and process development to overcome this problem. For the purpose of our studies on 3D cathode reconstruction analysis, a 2032 coin-type ASSB cell, prepared according to the same process as that employed for the above 100-cycle test cell, was disassembled in its fully discharged state after a single cycle. The 3D reconstruction technique sequence consisting of

multilayer structure was assembled into a 2032-type coin cell. A schematic illustration showing the cell configurations and dimensions is given in Figure 1a. Electrochemical Characterization. The prepared ASSB cell was subjected to 100 charge/discharge tests between 2.5 and 4.2 V (vs Li/ Li+). The theoretical capacity of the cathode active material was set at 178 mA h g−1, and the rate was calculated based on the quantity of the cathode active material (11.099 mg) present in the coin cell. Charging was performed in the constant current−constant voltage (CC−CV) mode, and discharging was performed in the CC mode at 25 °C. A current density of 0.026 mA cm−2 (i.e., 0.02 C-rate) was employed in the CC mode, whereas in the CV mode, charging was performed at 0.0052 mA cm−2 (20% of the current density employed in the CC mode current).

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RESULTS AND DISCUSSION An ASSB cell was fabricated via the wet-casting process, as described in the Experimental Section, and its electrochemical charge/discharge performance was confirmed. Figure 1a shows the schematic illustration of the cell configuration and the dimensions employed herein. Each wet-cast sheet was cut to an appropriate size and compacted by uniaxial pressing to produce an ASSB cell in the form of a 2032 coin with a 15 mm diameter. This ASSB cell was then subjected to 100 charge/discharge cycles, and its electrochemical performance was examined, as outlined in Figure 1b,c. As indicated, the initial discharge capacity of the test ASSB cell was 124.18 mA h C


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Table 2. Quantified Structural Properties Calculated from a 3D Reconstructed Composite Cathode of Our ASSB Cell reactive TPB material

volume ratio (%)

connectivity

tortuosity

second material

area (μm2)

density (μm−1)

electrolyte active material conductive additive pore

42.52 40.05 2.44 14.99

0.9959 0.9884 0.3331 0.9984

6.4208 6.1875

conductive additive electrolyte active material others

360 1120 194 5610

0.0684 0.213 0.0368 1.06

13.2344

Figure 3. 3D-reconstructed rendering images of the composite cathode showing the connectivity of each component: (a) the solid electrolyte, (b) the cathode active material, (c) the conductive additive, and (d) the pore. Individual colors represent structures with connectivity.

between such point contact interfaces with the micropores acts as a resistance element that interferes with Li-ion transport during battery charge and discharge and consequently causes a deterioration in the battery performance. However, such a qualitative analysis is insufficient for obtaining accurate microstructure comparisons and reliable process optimization. Furthermore, such qualitative methods do not provide innovative insights associated with material optimization and process development for advanced ASSBs. Thus, to confirm the microstructure and the reaction interface efficiently and precisely, it is necessary to quantitatively analyze the structural information for each element through a 3D reconstructed structure, as listed in Figure 2. Detailed analysis flow diagrams and additional information regarding each quantitative postprocessing are also presented in Figures S1−S5. Using this 3D reconstructed structure, the structural properties of the individual components, such as the volume ratio, the connectivity, and the tortuosity (τ), were calculated, and the results are summarized in Table 2. Initially, despite the requirement to prepare a dense electrode, the composite cathode contained a pore content of approximately 15%. Excluding this pore volume, the volume ratios occupied by the solid electrolyte, the cathode active material, and the conductive additive in the entire composite cathode were 50.01, 47.11, and 2.87%, respectively, which indicates that there were slight differences from the input material volume ratio employed during the cell manufacturing process (58.48, 38.37, and 3.15%). This quantitative error can be accounted for by limitations in the image analysis due to SEM resolution, differences in the gray-scale contrast, and the small

FIB milling, acquisition of consecutive SEM images, and subsequent image segmentation and reconstruction is outlined in Figure 2. More specifically, in a 3D reconstruction using dual beam equipment, an FIB was used to perform ion milling in the direction perpendicular to the prepared specimen, and SEM imaging of the milled surface was carried out at an angle of 52°. As shown in Figure 2a, the composite cathode surface was deposited with Pt prior to FIB milling to protect the surface from ion beam damage during milling. In addition, the reliability, efficiency, and consistency of the acquired SEM images were confirmed using the Π-shape trench structure during consecutive slicing and imaging processes. Slicing and imaging were performed continuously 100 times with 100 nm intervals, and the final data were obtained for the composite cathode analysis at a depth of 10 μm. Applying the image segmentation method based on the differences in the gray-scale contrast, image processing was performed by dividing each region into four composite cathode components (i.e., the solid electrolyte, active material, conductive additive, and pore), as shown in Figure 2b, and subsequent interface formation. Using the obtained two-dimensional (2D) SEM images, a composite cathode region with a total volume of 5263 μm3 (i.e., 20.4 μm × 25.8 μm × 10 μm) was successfully reconstructed in three dimensions, and the resulting reconstruction structure is shown in Figure 2c. Qualitative analysis of the 2D SEM image and the 3D reconstruction structure allowed us to confirm that the electrolyte and the active material were in close contact with one another. Unnecessary micropores were also widely distributed in various interfacial regions. The interaction D


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ACS Applied Materials & Interfaces reconstructed volume.22,26 The finely dispersed conductive additive can mix with the micropores, and accurate definition of the region can be challenging because of limits in SEM resolution and a weak image contrast. As a result, distribution of the conductive additive is underestimated and the pore region is overestimated, leading to an error in the quantitative properties. Furthermore, sampling errors due to the limited volume of the 3D reconstruction area must also be considered. Indeed, a large quantity of electrolyte powder was detected in the initial analysis volume, and the volume ratio of the solid electrolyte reached 61.45% of the total volume by the end of the analysis, later decreasing to 42.52%. The connectivity and tortuosity of each component constituting the composite cathode are outlined in Table 2 and Figure 3. Figure 3 shows the connectivity visually by expressing the individual structures connected together as a single color. In the case of the electrolyte, active material, or pore exhibiting a high connectivity, the entire region is represented by a structure of the same color. In particular, we note that the solid electrolyte and the cathode active material gave high connectivities of approximately 0.99 and 0.98, respectively, as a high volume ratio of >40% allows percolation to occur. By contrast, the conductive additive that determines the electron path is mainly an isolated structure in the 3D image and is represented by a wide variety of colors because of its low degree of connection (i.e., ∼0.33). Tortuosity is an indicator of how much a curve is twisted and is employed to explain the transport phenomenon occurring in a porous medium. It is mathematically expressed as the ratio of the length of the curve (L) to the distance between the ends (H), that is, L/H. As shown in Table 2, the electrolyte and the active material have tortuosities of ∼6, whereas the pore structure has tortuosity of ∼13. We note that the tortuosity value of the conductive additive, a nonpercolated (isolated) structure, is not listed because there is no physical meaning for the electron transport. Tortuosity is closely related to porosity (ε) as in the Bruggeman relation (generally, τ = ε−0.5) and represents the reduction in transport because of the composite microstructure composed of various solid-state components. For example, the effective conductivity is expressed as σeff = (ε/τ)σ, which shows well the effect of tortuosity in transport phenomena. As reported in the literature,27−29 tortuosity value of components such as solid-state electrolytes is very important for ionic transport and can seriously affect cell performance. This large tortuosity value observed in our sample is related to the formation of many interfacial pores, which in turn limits ion transport and results in low cell performance, as shown in Figure 1c. It is therefore of particular importance that the high connectivity and low tortuosity of all components are controlled to secure the carrier-transfer pathway and impart effective percolation in the entire cathode structure. The most important quantitative result of the microstructure analysis as mentioned above is that the pore network, a harmful structure of the ASSBs, is observed in a large percentage of the whole space. In this study, we first developed a pore decomposition process (PDP) that tracks the origins of pore structures as an example of a more practical problemsolving strategy, using quantitative values of a 3D reconstruction structure. Figure 4 and Table 3 show the 3D rendering images and microstructure information of each pore structure obtained by the PDP analysis, respectively. The cathode active material (NCM622) and the electrolyte (LPN821) have different effects on the formation of the pore structure because

Figure 4. 3D reconstructed rendering images of the decomposed pore structure derived from the (a) cathode active material and (b) solid electrolyte. Individual colors represent structures with connectivity.

Table 3. Decomposed Pore Structure Results Calculated from the Pore Analysisa

structure Spore,NCM Spore,LPN

pore volume (μm3)

pore surface area (μm2)

377.1 | 4725 | 59.10% 56.36% 261.0 | 3659 | 40.90% 43.64%

pore surface area from TPB (μm2) 3020 | 57.20% 2260 | 42.80%

pore connectivity

pore tortuosity

0.9960

19.93

0.9880

24.67

a

Spore,i indicates the pore structure derived from component i (i = LPN, NCM).

of the difference in the particle size (d50,NCM = 4 μm and d50,LPN = 6 μm) and powder elasticity (e.g., bulk modulus B), and differences of quantitative results (volume, surface area, connectivity, and tortuosity) were confirmed. As shown in Table 2, the volume ratio of the solid electrolyte and the cathode active material was similar, whereas the solid electrolyte and the cathode active material showed a noticeable difference in the ratio of induced pore volume: 43% of the total pore volume from LPN and 56% from NCM. Such difference is due to the fact that the solid electrolyte with a low bulk modulus (BLi3PS4 ≈ 20 GPa30) has excellent plastic deformation characteristics, and the cathode active material with poor deformability (BNCM ≈ 140 GPa31) is hard and induces more pore structure. Therefore, the pore formation, powder characteristics, and process optimization are closely related, and our quantitative results from 3D reconstruction process provide relevant insights. Finally, to identify more precisely the electrochemical reaction interface present in the composite cathode, the interface between the two different materials, otherwise known as the two-phase boundary (TPB), was quantitatively analyzed. We visualized in the TPB that the two components make contact with each other, as shown in Figure 5. It shows all twophase interfacial combinations of the electrolyte, active material, and conductive additive in our composite cathode. E


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Figure 5. 3D reconstructed rendering images of the composite cathode showing the effective TPBs of two components: (a) the solid electrolyte and the cathode active material (green), (b) the cathode active material and the conductive additive (orange), (c) the conductive additive and the solid electrolyte (purple), and (d) superimposition of all effective TPBs.

using the 3D reconstruction technique. The initial cell discharge capacity and Coulombic efficiency of the cell fabricated using the wet-casting process were 124.18 mA h g−1 and 64.38% at 0.02 C-rate, respectively. After 100 cycles, a degradation of ∼78% was also confirmed in the discharge performance. A total of 100 SEM images were obtained by FIB milling, and a 3D cathode structure segmented into four components through subsequent image analysis was reconstructed. This allowed a number of quantitative values for the microstructure and the reaction interface to be obtained, and new strategies for the improvement of this composite cathode were derived. The decomposition analysis of pore structure correlated with the pore formation, and powder characteristics confirmed that rigid cathode active materials induce more pores than sulfide electrolytes. In addition, the effective reaction area (i.e., the TPB between the cathode active material and the solid electrolyte) was ∼23% of the total volume because of the presence of interfacial micropores and agglomeration of the conductive additive. Furthermore, these two materials exhibited high connectivity (∼1) because of the high volume ratio (i.e., ∼40%) of the cathode active material and the solid electrolyte. Unlike conventional LIBs that contain liquid electrolytes, the ASSB cell has a unique structure in which all components are present in the solidstate phase. As a result, the microstructure and the reaction interface of the composite cathode responsible for the effective charge/discharge reaction require novel features differing from those of conventional LIBs. Because of the novel 3D analysis techniques employed herein and the obtained quantitative results, we expect that our study will contribute to improvements in the composite cathode structures for ASSBs in addition to the development of advanced ASSB cells by applying various strategies (e.g., minimization of the interfacial pore structure, percolated network with minimal electrolyte content, high loading of cathode active material, and well dispersion of the conductive additive) employing powder optimization (size, shape, and distribution control) and process development (dispersion, drying, and packing control).

As mentioned above, the micropores become a harmful structure in the solid-state ASSB composite cathode, thereby preventing the effective formation of heterogeneous interfaces. Upon considering the high electron conductivity of the cathode active material,32 three types of effective TPBs exist between the electron conductor and the ion conductor formed in the 3D reconstructed structure, and the areas and densities of these TPBs are summarized in Table 2. The largest area and density (i.e., 1120 μm2 and 0.213 μm−1, respectively) were obtained from a combination of the electrolyte and the active material. Upon considering that the total TPB area and density (including the pores) are approximately 7280 μm2 and 1.38 μm−1, respectively, the effective reaction interface is limited to ∼23% of the total TPB. This is due to the nonuniform dispersion of the conductive additive and the distribution of numerous micropores at the interface between the active material and the electrolyte, thereby resulting in failure of the effective reaction interface. As such, two major improvements are required to overcome the limitations of the composite cathode discussed herein. More specifically, powder optimization (e.g., sub-micrometer size, spherical shape, and weak agglomeration) and cell manufacturing processes (e.g., welldispersion slurry, gradient-free drying, multimodal packing, and low-temperature isotropic densification) that reduce the number of micropores between the two different phases are required. As a result, if the interface resistance induced by the point contact is reduced, the battery efficiency can be maximized. In addition, percolation of the ion and electron pathways should be achieved under conditions that maximize the volume ratio of the active material. Moreover, the uniform dispersion of the conductive additive and an improvement in the packing density of the high-loading active material using a fine solid electrolyte can enhance the cell capacity and stabilize the performance even at high C-rates.

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CONCLUSIONS Quantitative analyses of the microstructures and reaction interfaces of composite cathodes in ASSBs were performed F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04204.

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Five supporting figures showing the flow diagram and further information on 3D reconstruction process (phase segmentation, volume analysis, connectivity analysis, tortuosity analysis, and pore analysis) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hun-Gi Jung: 0000-0002-2162-2680 Jong-Ho Lee: 0000-0003-4481-6258 Hyoungchul Kim: 0000-0003-3109-660X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by grants from Hyundai NGV and Hyundai Motor Company, award no. 2I22810. This work also supported in part by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20152020106100). We are grateful to Yanghee Kim and Dr. Jae-Pyoung Ahn for valuable comments and discussion on FIB sampling.

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REFERENCES

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