Spark plasma sintering plus heat-treatment of Ta-doped Li7La3Zr2O12 solid electrolyte and its ionic conductivity

Ta-doped Li7La3Zr2O12 (Ta-LLZO) solid electrolyte ceramics were synthesized by solid-state sintering, spark plasma sintering (SPS) and SPS plus heat-treatment (two-step method) using LiOH·H2O, La2O3, ZrO2 and Ta2O5 as the raw materials. The Ta-LLZO samples prepared from these three routes are all in cubic phase structure. However, the solid-state method results in Ta-LLZO samples with different particle sizes and porous morphology, while the samples prepared by SPS route contain fewer internal voids and smaller particle size. The two-step method not only retains the high compactness of SPS samples, but also the increment of grain size. The SEM results demonstrate that Ta-LLZO sintered by the two-step method shows optimal electrical properties. And the total ionic conductivity is 4.6 × 10−3 S cm−1 at 150 °C and the activation energy is calculated to be 0.38 eV.


Introduction
Lithium-ion battery has drawn great attention due to its high voltage, small volume, low weight, high energy density, non-memory effect, low self-discharge and long cycle life [1][2][3]. However, the safety issues remain a major challenge for lithium-ion batteries using traditional liquid electrolytes. Based on the advantages in safety and energy density, all solid-state batteries (ASSBs) have become the only way to develop lithium batteries in the future [4][5][6]. Compared with conventional lithium ion batteries, the potential advantages of ASSBs include their high energy density, simple structure, high safety and reliability [7,8]. However, there are still some limitations in the application of solid electrolytes, such as the solid-solid interface between electrodes and electrolytes, as well as the relatively low ionic conductivity.
Solid electrolytes can be divided into inorganic solid electrolytes and polymer electrolytes according to their composition. Garnet-type solid electrolyte has attracted extensive attention because of its promising electrochemical performance. Li 7 La 3 Zr 2 O 12 with a cubic crystal structure generally exhibits relatively high ionic conductivity. However, this material easily decomposes into tetragonal phase or La 2 Zr 2 O 7 during high temperature heat treatment [9][10][11]. The ionic conductivity of tetragonal phase is two orders of magnitude lower than that of cubic phase. However, the cubic phase can be effectively stabilized under elevated sintering temperature in the synthesis process. To obtain stabilized cubic phase Li 7 La 3 Zr 2 O 12 , the sintering temperature is generally higher than 1000°C, which also evokes serious lithium volatilization problems [12].
At present, the synthesis of solid electrolyte Li 7 La 3 Zr 2 O 12 is mainly conducted using solid-state and sol-gel routes [13][14][15]. Solid-state method is widely used because of its low cost and simple operation. Its disadvantages are the low reactivity of powder and high energy consumption. Due to the high temperature sintering in solidstate synthesis, the lithium volatilization will lead to the instability of crystal structure and poor sintering performance [9]. At the same time, the non-conductive La 2 Zr 2 O 7 will be produced, which will dramatically reduce the ionic conductivity [16]. On the other hand, to ensure the formation of cubic structure and Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. densification of Li 7 La 3 Zr 2 O 12 electrolyte samples, high temperature sintering under long time is needed and lithium volatilization can hardly be eliminated. On the contrary, the sol-gel method requires lower sintering temperature and uniform doping at the molecular level. Kotobuki et al have prepared Li 7 La 3 Zr 2 O 12 precursor powders by sol-gel method and garnet electrolyte after sintering at 1100°C for 6 h, which exhibits cubic structure and high ionic conductivity of 1.5×10 −4 S cm −1 [17]. However, the sol-gel method results in much higher cost of the raw reactants and much longer time of the synthesis process. Therefore, other innovative sintering techniques have be proposed to overcome these drawbacks, such as spark plasma sintering (SPS) [18][19][20], hot-press sintering [21] and chemical precipitation method [22,23]. Compared with conventional sintering route, SPS can be performed at a lower temperature and shorter time duration (within 1-10 min), which can successfully prepare garnet electrolyte without any sintering additives [24,25]. SPS has been widely used to prepare a variety of conducting and non-conducting materials including lithium ion conductors, such as NASICON-type LiTi 2 (PO 4 ) 3 and LiHf 2 (PO 4 ) 3 [19,20], as well as perovskite-type Li 3x La 2/3−x TiO 3 [26].
In this study, we have explored the synthesis of garnet electrolyte Ta-doped Li 7 La 3 Zr 2 O 12 (Ta-LLZO) using solid-state sintering, SPS and SPS plus heat-treatment routes. The heat-treatment process was introduced on the basis of SPS densification, which is beneficial to further promote the bulk density and grain size. It's known that voids and grain boundaries are very harmful for ion conductivity. Thus, the elevated grain size is expected to reduce the grain boundary surface and improve the ion conductivity. The effect of grain size on the ion conductivity was explored. The phase composition, microstructure and ionic conductivity analysis using different synthesis routes were investigated as well.

Preparation of Ta-LLZO pellets
Raw powders of LiOH, La 2 O 3 , Ta 2 O 5 and ZrO 2 were purchased from Aladdin Biochemical Technology Co. Ltd as the raw materials, which were mixed with the corresponding molar ratio to produce the target product (Li 7 La 3 Zr 2 O 12 ). In order to compensate for the loss of lithium under high temperature sintering, 10 wt% excess of lithium hydroxide was added. The weighed samples were ball-milled for 12 h with isopropanol as the dispersing reagent. The mixture was dried in oven and then pressed into cylinder pellets at 20 MPa, which were calcined at 900°C for 6 h to form the initially tetragonal LLZO phase. The calcined pellets were ground again and re-compressed into pellets for next calcination at different temperatures (1150°C-1200°C) and dwelling times (6-24 h). The whole calcination process was conducted with the pellets be imbedded in mother powder.
The two-step process includes spark plasma sintering (SPS) and heat-treatment in air. The preparation of pre-sintered powder is the same as that of the previous solid phase method. The different sintering schedule is shown in table 1. SPS sintering was carried out on the pre-sintered powder, in which the heating rate was 50°C/ min with 40 MPa pressure and 5 min holding time. The sintering temperature was fixed to be 800°C. The graphite on the sample surface was treated and then subjected to high temperature heat treatment, which was imbedded in mother powder under sintering in air. Compared with the two-step method, the solid phase method was conducted at 1175°C with 12 h for the preparation of LLZO pellets.

Characterizations
The structure and composition of calcined powders and pellets were analyzed using x-ray diffractometer (XRD, X' Pert PRO, PANalytical B V, Netherlands) with a copper anode (Cu Kα radiation, λ=1.54187 Å). The morphology of sintered pellets were observed by scanning electron microscope (SEM, UItra55, Carl zeiss, Germany). AC impedance spectroscopy measurement was conducted on an Agilent 4294 A electrochemical workstation to determine the ionic conductivity of the pellets. The polished pellets were pasted by Ag electrode on both surfaces and loaded into a Swagelok cell in Ar-glovebox to exclude the adverse influence of air/moisture on conductivity measurement. The measurements were performed by applying 10 mV in the frequency range of 1 MHz-100 Hz in the temperature range from 30°C to 150°C. The ionic conductivity of solid electrolytes is calculated using equation (1):

( )
where σ, l and D represent the ionic conductivity, thickness and diameter of solid electrolytes and R refers to the impedance value obtained from the impedance spectra, respectively. The activation energy of solid electrolytes is calculated by the following equation (2): where σ 0 , E a and K are pre-exponential ionic conductivity, activation energy and Boltzmann's constant, respectively.

Results and discussion
The sintering temperature exhibits an important influence on the densification of solid electrolytes. For garnet Li 7 La 3 Zr 2 O 12 solid electrolyte, the suitable sintering temperature also means the phase transformation from tetragonal phase to cubic phase. Figure 1 shows the XRD results of LLZO pellet after solid-state sintering at 1150°C, 1175°C and 1200°C for 6 h. It can be clearly seen from the diagram that the LLZO phase gradually transforms from tetragonal phase to cubic phase with the increase of sintering temperature. In order to compensate for lithium volatilization at high temperature, 10 wt% excessive lithium and burial sintering in mother powder were utilized in this experiment, resulting in the emergence of impurity Li 2 ZrO 3 phase. According to this result, 1175°C was selected as the optimal sintering temperature of LLZO. Figure 2 shows the AC impedance plots of LLZO at 30-150°C and its Arrhenius curve. The corresponding ionic conductivities were calculated as shown in table 2. The total impedance of electrolyte sheet decreases with the increase of testing temperature, which indicates that the increase of temperature makes the conductible lithium ion with higher energy and can break through the barrier needed for migration. However, when the temperature exceeds 120°C, the total impedance decreases slightly, which indicates that the lithium ion at this temperature has enough energy to migrate, and the temperature shows little effect on the ionic conductivity of the sample. The activation energy of 1175°C sintered LLZO pellet was calculated to be 0.41 eV.
Ta-doped Li 7 La 3 Zr 2 O 12 (Ta-LLZO) is deemed as a promising inorganic solid electrolytes. Figure 3 presents the XRD patterns of Ta-LLZO samples obtained by the three synthesis methods. It can be concluded that all the three sampls maintain the cubic phase structure. However, the impurity La 2 Zr 2 O 7 phase was formed in SPS method due to the volatilization of lithium. On the other hand, the impurity Li 2 ZrO 3 phase was produced in the two-step route, which may be related with the excessive Li content in the sintering process and burial sintering in mother powder. Li 2 ZrO 3 phase is also a conductive phase of lithium, which is not very harmful for the ion conductivity. This result shows that the formation of non-conductive phase La 2 Zr 2 O 7 can be inhibited by the burial sintering in mother powder and the existence of excessive lithium dopant.
The total impedance of SPS sintered LLZO samples is obviously divided into grain impedance and grain boundary impedance as shown in figure 4. The grain conductivity and total conductivity of Ta-LLZO samples sintered by SPS are measured to be 1.2×10 -5 S cm −1 and 7.6×10 −6 S cm −1 , respectively. The total     conductivity of the sample obtained by two-step method is 2.9×10 −4 S cm −1 . Compared with the Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 electrolyte sheet (6.9×10 −4 S cm −1 , 25°C) synthesized by SPS sintering [27], the ionic conductivity of Ta-LLZO electrolyte sheet obtained in this experiment is much lower. The decrement of ionic conductivity is mainly attributed to the formation of more non-conductive phase La 2 Zr 2 O 7 , which reduces the whole ionic conductivity dramatically. It is obvious that the ionic conductivity of SPS samples has been greatly improved after subsequent heat-treatment.
As can be seen from figure 5 and table 3, the ionic conductivity of the two-step synthesized Ta-LLZO sample exhibits a similar trend as that of the undoped LLZO sample mentioned above. The total ionic conductivity is significantly promoted as the 30°C value is 9.58×10 −5 S cm −1 . With the increase of testing temperature, the total resistance decreases and the ionic conductivity of the sample increases accordingly. The activation energy of Ta-doped LLZO was calculated to be 0.38 eV, which is lower than that of the undoped LLZO sample. The decrease of activation energy means that the energy required for lithium ion migration decreases and the ionic conductivity increases. Figure 6 shows the SEM images of Ta-LLZO under different sintering methods. The size of the samples sintered by solid-state method is uneven and the voids between the particles are large, which means the relative density is low. However, the sample sintered by SPS exhibits small and uniform grains with grain size of 3-6 μm and compact distribution among the particles. Obviously, the density of the samples sintered by two-step method is much higher than that of the samples sintered without pressure. The grain size of the samples sintered by two-step method is much larger than that of the samples sintered by single SPS method. The grain size is about 10-15 μm and arranged compactly with almost no pore between the particles. It can be concluded that the Ta-LLZO samples obtained by SPS sintering at 1175°C for 12 h have the advantages of large grain size, less porosity and high compactness in microstructure. This phenomenon indicates that the density does not always decrease with the increase of grain size, which has also been testified in the previous reports [28,29]. It can be concluded that the Ta-LLZO sample obtained by two-step method have the advantages of large grain size, less porosity and high compactness in microstructure.

Conclusions
Ta-LLZO ceramics were synthesized by three routes of high temperature solid-state method, SPS and SPS plus heat-treatment. From the XRD results, phase compositions of the three samples obtained from these routes are very similar. But the microstructure of solid-state sintered Ta-LLZO samples shows different particle size and more voids. The inner voids of the SPS sintered Ta-LLZO samples are much fewer, and the particle size is uniform and smaller. The two-step method not only retain the high compactness of SPS sample, but also promotes the grain size. These microscopic characteristics are also reflected by their ionic conductivity. With the  increase of density and grain size in the two-step method, the total ionic conductivity of Ta-LLZO samples is also elevated.