Fabrication of All-Solid-State Lithium-Ion Cells Using Three-Dimensionally Structured Solid Electrolyte Li7La3Zr2O12 Pellets

Introduction

In recent years, the application of lithium-ion batteries has been expanded from portable electronic devices to large devices such as electric vehicles and energy storage systems on an industrial scale. According to this movement, higher battery performance, particularly safety is being required (Tarascon and Armand, 2001). However, it is basically difficult to realize high safety in current lithium-ion batteries because of the use of liquid electrolytes including flammable organic solvents, which also demands the massive battery package to avoid the electrolyte leakage. Therefore, the replacement of liquid electrolytes with solid electrolytes is an important issue to contribute the improvement of both safety and energy density of lithium-ion batteries. Actually, these advantages can be confirmed in polymer lithium-ion batteries, however, in which gel-type electrolytes including liquid electrolytes have been still used owing to low conductivity of true polymer electrolytes. On the other hand, inorganic solid electrolytes have attracted much attention in recent decade since those materials are non-flammable and thermally stable (Knauth, 2009). So far, many kinds of inorganic solid electrolytes including both glass- and crystal-based materials have been developed. They are categorized into two major groups. One is sulfide-based solid electrolyte group, which mainly consists of glass materials with relatively high plasticity. In the recent development, the Li⁺-ion conductivity of sulfide materials is approaching the value of liquid electrolytes. For example, 1.2 × 10⁻² S cm⁻¹ at room temperature has been reported for the composition of Li₁₀GeP₂S₁₂ by Kanno et al. (Kamaya et al., 2011). However, there is a serious problem to be solved. The sulfide-based solid electrolytes, particularly those with high Li⁺-ion conductivity, easily react with water and generate toxic hydrogen sulfide gas. The other group consists of oxide-based solid electrolytes such as Li_0.35La_0.55TiO₃ (LLT), Li_1+xAl_xTi_2−x(PO₄)₃ (LATP), and Li₇La₃Zr₂O₁₂ (LLZ). Compared to the sulfide-based solid electrolytes, these kinds of solid electrolytes have good thermal and chemical stability and high mechanical strength. LATP has a NASICON-type (Na super ion conductor) structure and exhibits a total (bulk + grain-boundary) ionic conductivity of 7 × 10⁻⁴ S cm⁻¹ at 25°C when the composition is Li_1.3Al_0.3Ti_1.7(PO₄)₃ (Aono et al., 1989, 1990). LLT is a perovskite-type oxide Li⁺-ion conductor with a high bulk ionic conductivity of 1 × 10^–3 S cm⁻¹ at 25°C, but its total ionic conductivity is as low as 7.5 × 10^–5 S cm⁻¹ (Inaguma et al., 1993). These are not stable against Li-metal due to the reduction of Ti⁴⁺ to Ti³⁺ at 1.8 V vs. Li/Li⁺, resulting in the appearance of electronic conduction (Knauth, 2009). Therefore, if lithium-metal is used as an anode in all-solid-state lithium-ion batteries to realize high energy density, a Li⁺-conducting protective layer such as poly(methyl methacrylate) (PMMA) gel-electrolyte has to be formed as a buffer layer to prevent the direct contact of those solid electrolytes with Li-metal (Hoshina et al., 2005).

Recently, many research groups including our group are focusing on Li₇La₃Zr₂O₁₂ (LLZ) as one of promising solid electrolytes for all-solid-state lithium-ion batteries using Li-metal anode (Murugan et al., 2007a). LLZ has a wider electrochemical window than LLT due to the stability against Li-metal. The total ionic conductivity of LLZ with a cubic structure is 7.7 × 10⁻⁴ S cm⁻¹ at 25°C, which is larger than that of LLT and also those of other Li⁺-ion conducting garnet-like oxides (Thangadurai et al., 2003; Murugan et al., 2007b). In addition to the cubic phase, LLZ has a tetragonal phase, whose ionic conductivity is as low as ~10⁻⁶ S cm⁻¹. Therefore, it is necessary to use cubic LLZ for battery applications. The cubic phase is stable above 640°C and is easily transformed to the tetragonal phase as decreasing the temperature. Therefore, many approaches have been done to stabilize the cubic phase at room temperature. The Al³⁺ substitution of Li⁺ sites in LLZ (Al-doping) is an effective way to improve the stability of cubic phase at room temperature. For example, Li_6.25Al_0.25La₃Zr₂O₁₂ (x = 0.25 in Li_7–3xAl_xLa₃Zr₂O₁₂) has been known as a stable cubic LLZ (Matsuda et al., 2015). These kinds of Al-doped LLZs are usually denoted as LLZAls and show a relatively high bulk ionic conductivity such as 3.1 × 10⁻⁴ S cm⁻¹ at 25°C in Li_6.25Al_0.25La₃Zr₂O₁₂. Although the Li⁺-ion conductivity of oxide-based solid electrolytes is still one order of magnitude lower than that of liquid electrolytes, a practical Li⁺-ion conductivity similar to that of liquid electrolytes in a separator can be realized if those solid electrolytes are formed in the thickness of less than 50 μm since their Li⁺-ion transport number is almost unity. In order to fabricate such thin solid electrolyte membranes with both high density and high mechanical strength, special fabrication techniques are required.

In our previous study, we fabricated a LiCoO₂ cathode layer on a LLZ pellet by a simple sintering process using LiCoO₂ powder at 800°C for 1 h. However, the electrochemical performance was very poor with the specific discharge capacity of as small as 0.27 mAh g⁻¹ (Kotobuki et al., 2010). From the scanning electron microscopic observation, it was found that the contact between LiCoO₂ cathode and LLZ solid electrolyte was poor. This problem can be improved by applying the precursor sol of LiCoO₂. When the mixture of LiCoO₂ particles and its precursor sol is applied, the better electrode–electrolyte contact can be formed, resulting in the lower internal resistance of all-solid-state lithium-ion batteries (Kotobuki et al., 2011). A similar approach to improve the electrode–electrolyte contact in all-solid-state lithium-ion batteries has been carried out by Ohta et al. (2013). They applied Li₃BO₃ as a supporting electrolyte to improve the contact between LiCoO₂ cathode and LLZ solid electrolyte, in which the mixture of Li₃BO₃ and LiCoO₂ was sintered onto a LLZ pellet with the composition of Li_6.75La₃(Zr_1.75Nb_0.25)O₁₂ optimized by them (Ohta et al., 2011). Consequently, the total resistance including LiCoO₂/LLZ and Li-metal/LLZ interfacial resistances was improved from 700 to 230 Ω cm⁻². This result suggests that the use of Li⁺-ion conducting supporting electrolytes is effective to improve the solid (electrode)–solid (electrolyte) contact. However, the capacity of all-solid-state lithium-ion batteries cannot be increased only by the improvement of interfacial contact. It is basically needed to make thick electrodes to obtain high capacity in batteries. The Li⁺-ion conductivity of cathode materials used in lithium-ion batteries are not so high. Therefore, thicker cathode layer for higher capacity cannot be used due to limit of the lithium-ion migration length. One of the ideas to realize practically high capacity in all-solid-state lithium-ion batteries is to make 3D-structured electrodes. Of course, this idea is applicable to current lithium-ion batteries using liquid electrolytes to improve their electrochemical performance. For example, the energy density of battery can be increased without losing the power density in the combination of interdigitated pillar-arrays of anode and cathode since the Li⁺-ion diffusion length between anode and cathode can be maintained even when the electrodes are made thicker (higher) to increase the battery capacity. Namely, the trade-off problem between energy and power density in batteries can be solved. Actually, we had reported this advantage by making some 3D-structured electrodes for current lithium-ion batteries (Izumi et al., 2012). The similar approach is also effective to increase the practical capacity of all-solid-state lithium-ion batteries. However, unlike liquid electrolytes, solid electrolytes have no fluidity, which makes it difficult to introduce them into the space between 3D-strucutrued electrodes. Oxide-based solid electrolytes have high mechanical strength compared to other electrolytes, so that their various 3D structures can be formed. When a solid electrolyte membrane with a hole-array structure is formed, 3D batteries can be fabricated by introduction of electrode active materials into the holes. In 3D-structured solid electrolyte membranes, the electrode–electrolyte contact area per unit volume is increased. Therefore, the application of 3D structures is particularly effective to improve the electrochemical performance of all-solid-state lithium-ion batteries. In addition, the electrolyte thickness in the bottom of holes can be thin for short Li⁺-ion diffusion length since the other parts maintain the mechanical strength of electrolyte membrane. So far, we have tried to prepare LLZ solid electrolyte membranes with hole-array structures (Figure 1) by several methods. One is a micro-stereolithography using a LLZ slurry including a photo-curable resin. This method can make LLZ membranes with several tenths of micrometer-scale patterns precisely. However, their densification was difficult by simple sintering process (Figure 2). Consequently, the resulting LLZ membranes were porous and had high resistance. More simply, we recently have applied a press method using a die with a pillar-array structure to make LLZ pellets with a hole-array structure (Shoji et al., 2014). Compared to the micro-stereolithography, the fineness of patterns is slightly lost but dense LLZ pellets with 3D hole-array structures can be formed reproducibly. In this study, we apply this press method to make LLZAl pellets with 3D hole-array structures and present their effectiveness on improvement of electrochemical performance of all-solid-state lithium-ion batteries.

FIGURE 1

Figure 1. Schematics of 3D-structured solid electrolyte membrane.

FIGURE 2

Figure 2. SEM image of 3D-structured LLZ membrane.

Materials and Methods

LLZAl powder was synthesized by a solid state reaction (Kotobuki et al., 2011). LiOH∙H₂O, La(OH)₃, ZrO₂ (7.7:3:2: in molar ratio) were mixed by planetary ball-milling and then calcined at 900°C for 6 h. The calcined powder was mixed with γ-Al₂O₃ powder by planetary ball-milling, and then a binder was added to make a pellet with a hole-array structure, i.e., 3D-structured pellet, by using a special punch, which has many small and short pillars (Figure 3). For comparison, a flat LLZAl pellet was also fabricated using binder-free powder. The 3D-structured pellet was sintered by two sintering steps of 900°C for 3 h and then 1200°C for 12 h. Although a sintering mechanism of LLZ is complicate, the two sintering steps can prevent to remain impurity phases such as La₂Zr₂O₇ and Li₂CO₃, which behave as an inhibitor of densifying LLZ (Chen et al., 2014).

FIGURE 3

Figure 3. Schematics of pelletization for 3D-structured pellet.

The characterization of sintered 3D-structured LLZAl pellets was performed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and impedance measurement from 0.1 to 10⁶ Hz at 30°C. To estimate Li⁺-ion conductivity of LLZAl, the 3D-structured LLZAl pellet was polished to be a flat pellet for removing the effect of 3D structure from the estimation of its Li⁺-ion conductivity.

For the assembly of all-solid-state battery using the 3D-structured LLZAl pellet, LiCoO₂ cathode with and without Li₃BO₃ was filled into the holes of the 3D-structured LLZAl pellet. The detailed procedures are as follows: LiCoO₂ sol was prepared by mixing LiNO₃, Co(NO₃)₂∙6H₂O, CH₃COOH, 2-propanol, H₂O, and polyvinylpyrrolidone (PVP) in a molar ratio of 1.1:1:10:1:0.06. Li₃BO₃ powder was synthesized by a solid state reaction using Li₂CO₃ and B₂O₃ in a molar ratio of 3:1 (Ohta et al., 2013). For the case of LiCoO₂/Li₃BO₃ composite cathode, the mixture of LiCoO₂ sol and Li₃BO₃ in a weight ratio of 7:3 was injected into the holes of the 3D-structured LLZAl pellet. Then, it was calcination at 400°C for 5 min and then 700°C for 30 min to form an intermediate layer for making good contact between LiCoO₂/Li₃BO₃ composite cathode and LLZAl solid electrolyte. After that, the mixture of Li₃BO₃ powder and commercially available LiCoO₂ powder in a weight ratio of 3:7 was filled in holes of the 3D-structured LLZAl pellet, and then the Li₃BO₃-added LiCoO₂ sol denoted above was also dropped into the mixed powder-filled holes of the 3D-structured LLZAl pellet. This pellet was finally heated at 400°C for 5 min and then 700°C for 30 min. For comparison, Li₃BO₃-free sample was also prepared according to the same procedure except the use of Li₃BO₃. Their characterizations were performed by XRD, SEM, energy dispersive X-ray spectrometry (EDS), impedance measurement, and charge-discharge test.

Results and Discussion

Figure 4 shows a photograph of sintered 3D-structured LLZAl pellet with a diameter of about 10 mm. The XRD pattern of the 3D-structured LLZAl pellet shown in Figure 5 is attributed to the cubic crystal. From an impedance spectrum, the ionic conductivity of LLZAl estimated from its polished flat pellet was 3.5 × 10⁻⁴ S cm⁻¹ at 30°C, which is a typical ionic conductivity of LLZ-type electrolytes. Figure 6 shows surface and cross sectional SEM images of the 3D-structured LLZAl pellet. A pore size of holes on the 3D-structuerd LLZAl pellet was estimated to be about 700 μm × 700 μm. A thickness of the thick parts of 3D-structuerd LLZAl pellet and a depth of holes was about 1 mm and 700 μm, respectively. The electrolyte thickness in the bottom of holes was approximately 300 μm. This hole-array structure was very coarse. However, the surface area of the 3D-structuerd LLZAl pellet was approximately 2.5 times as large as flat pellet. In the 3D-structuerd LLZAl pellet, some large pores of about 100 μm and many small pores of about 10 μm were observed. The relative density of the 3D-structured LLZAl pellet was estimated to be 88.2%. A flat LLZAl pellet prepared from binder-free calcined powder had no large pores. Therefore, these large pores were formed by the use of calcined powder containing the binder. To get the large pores to disappear and improve the relative density, it is required to decrease the binder containing calcined powder and optimize a dewaxing process during the sintering. Only a few grain boundaries were observed in the 3D-structured LLZAl pellet. In our previous study (Shoji et al., 2014), we prepared a 3D-structured LLZAl pellet that contained many grain boundaries, and the all-solid-state battery using the 3D-structured LLZAl pellet did not work as a rechargeable battery. A two-sintering step process has not been adopted in the previous study. Therefore, the two-sintering step process has an effect on the decrement of grain boundaries by preventing impurity phases to remain. As a result, prepared 3D-structured LLZAl pellet in this study had no penetration. It was regarded that the 3D-structured LLZAl pellet can be used to fabricate the all-solid-state battery.

FIGURE 4

Figure 4. Photograph of sintered 3D-structured LLZAl pellet with diameter of 10 mm.

FIGURE 5

Figure 5. XRD pattern of sintered 3D-structured LLZAl pellet and simulated cubic LLZ.

FIGURE 6

Figure 6. (A) Surface and (B) cross sectional SEM images of 3D-structured LLZAl pellet.

For improvement of contact between the LiCoO₂ layer and the 3D-structured LLZAl pellet, we used a Li₃BO₃ additive as a supporting electrolyte. Cross sections of each pellet were observed by SEM and EDS. These results are shown in Figure 7. By comparing these, large voids were not observed in the Li₃BO₃-added LiCoO₂ layer. However, the volumes of Li₃BO₃-added LiCoO₂ or Li₃BO₃-free LiCoO₂ layer except the voids were estimated to about the same, which were estimated from the amount of weight increment of the 3D-structured LLZAl pellets. The Li₃BO₃ additive has no work in densifying the LiCoO₂ layer. However, the Li₃BO₃-added LiCoO₂ layer was more uniformly distributed in the holes than Li₃BO₃-free LiCoO₂ layer. These results have indicated that the fluidity of LiCoO₂ layer emerged due to melting Li₃BO₃ around 700°C during the heat treatment, and then, LiCoO₂ in the holes was uniformly distributed. SEM images shown in Figures 8B,C clearly indicated that clay-like adhering material is attached around LiCoO₂ particles. Probably, this adhering material is Li₃BO₃ that is melted once during the heat treatment, and the Li₃BO₃ was connected to each LiCoO₂ particle and between LiCoO₂ particles and LLZAl. To confirm an effect of the Li₃BO₃ additive, an impedance measurement of each sample was carried out. Impedance spectra are showed in Figure 9. These spectra indicated a resistance of LLZAl (bulk and grain boundary) and interfacial resistances between LLZAl and electrodes (Li-metal and LiCoO₂), LLZAl and Li₃BO₃, and LiCoO₂ and Li₃BO₃. The resistances of bulk and grain boundary of LLZAl had no significant difference between each sample, about 0.2 kΩ. On the other hand, the interfacial resistances of the 3D-structured LLZAl pellet injected Li₃BO₃-added LiCoO₂ was lower than injected Li₃BO₃-free LiCoO₂. Since a Li-metal was not involved with Li₃BO₃, an interfacial resistance between LLZAl and Li-metal had no difference between each sample. Thus, LiCoO₂ and the Li₃BO₃ additive attributed to the decrement of the interfacial resistances, because the Li₃BO₃ additive can form a better contact in interface (Ohta et al., 2013).

FIGURE 7

Figure 7. SEM images and EDS mappings of 3D-structured LLZAl pellets injected Li₃BO₃-free LiCoO₂ (A,B) and LiCoO₂-Li₃BO₃ composite (C,D), respectively.

FIGURE 8

Figure 8. SEM images (A) Li₃BO₃-free LiCoO₂, (B) Li₃BO₃-added LiCoO₂, and (C) interface between Li₃BO₃-added LiCoO₂ and LLZAl.

FIGURE 9

Figure 9. Impedance spectra of LiCoO₂/flat LLZAl/Li-metal and LiCoO₂/3D-structured LLZAl/Li-metal batteries.

Charge-discharge tests of all-solid-state lithium-ion batteries using the 3D-structured LLZAl pellets injected Li₃BO₃-free LiCoO₂ or Li₃BO₃-added LiCoO₂ were performed under a constant current density of 14 μA cm⁻² in cutoff voltages of 2.5 and 4.2 V at 60°C. Figure 10 shows charge–discharge curves of each sample. Both all-solid-state lithium-ion batteries were worked as rechargeable batteries, exhibiting very low discharge capacities between 0.6 ~ 7 mAh g⁻¹. On the other hand, the utilization of Li₃BO₃-added LiCoO₂ was larger than Li₃BO₃-free LiCoO₂. In the SEM images of Figure 8, each LiCoO₂ particle and LiCoO₂ and LLZAl pellet were connected by the Li₃BO₃ additive. It is considered that the improvement of the utilization was attributed to Li₃BO₃, which worked as ionic migration path. However, the utilization was still very low. It is attributed that the contact area between LLZAl and LiCoO₂ was still small. To improve the utilization, a denser Li₃BO₃-added LiCoO₂ layer is required. In addition, holes of the 3D-strucutred LLZAl pellet were too large for the Li⁺-ion diffusion length. LiCoO₂ around the center of this large size holes could not be utilized as active materials due to too long of a distance between LiCoO₂ and LLZAl. It is necessary to design finer hole with size of about 100 μm or less.

FIGURE 10

Figure 10. Charge–discharge curves of (A) LiCoO₂/flat LLZAl/Li-metal and (B) LiCoO₂/3D-structured LLZAl/Li-metal batteries.

Conclusion

In this report, we reported about a concept of 3D-structuring of LLZAl solid electrolyte to realize high capacity density and high mechanical strength of all-solid-state lithium-ion batteries. Since densification of LLZAl membranes was difficult by simple sintering process, we made dense LLZAl pellets with 3D-structure (a hole-array structure). An ionic conductivity of the 3D-structured LLZAl was estimated to 3.5 × 10⁻⁴ S cm⁻¹, which is a sufficient ionic conductivity as a LLZ electrolyte. And it was confirmed that the all-solid-state battery using the 3D-structured LLZAl pellet was operated as a rechargeable battery. Also, addition of Li₃BO₃ to LiCoO₂ layer was an effective way to improve contact between each LiCoO₂ particles or LiCoO₂ and LLZAl. However, this battery could not exhibit high utilization of LiCoO₂ due to low density of LiCoO₂ layer and coarse holes of the 3D-structured LLZAl pellet. Finer hole-array structure with size of about 100 μm or less is required to improve a utilization of LiCoO₂. Of course, high mechanical strength is also needed. However, to realize the 3D-structured LLZAl membranes, it is necessary to establish techniques to mold using the punch with many finer pillars. Moreover, the optimization of sintering processes for preparation of dense LLZAl membranes must be studied.

Author Contributions

KK conceived the conception and design of the study, MS performed the experiments and acquired the data, MS, HM and KK analyzed the data, MS and HM drafted and revised the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was supported by Advanced Low Carbon Technology Research and Development Program – Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) from Japan Science and Technology Agency (JST).

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