2025 Volume 93 Issue 2 Pages 027004
Amorphous powder with the composition of high-Al substituted Li1+xAlxGe2−x(PO4)3 (x = 1.0) [LAGP(x = 1.0)] were prepared by mechanical milling treatment of the starting materials. Crystalline compounds with LiGe2(PO4)3-based NASICON-type structure were mainly formed by low-temperature heat treatment of amorphous LAGP(x = 1.0) at 450–750 °C. NASICON-type solid electrolytes prepared by the heat treatment of pelletized-type amorphous LAGP(x = 1.0) at 600–750 °C exhibited relatively high conductivities on the order of 10−5 S cm−1 at room temperature, although they contained AlPO4 and Li9Al3(P2O7)3(PO4)2 compounds. The LAGP-type high-Al solid electrolytes would be a potential candidate as lithium-ion conductors for all-solid-state battery applications.
All-solid-state batteries using lithium-ion conducting inorganic solid electrolytes would be useful as rechargeable batteries for energy storage system and electric vehicles,1 because they are expected to show high energy density, rapid charge performance, and high reliability. Many researchers had reported that NASICON-type LiM2(PO4)3(M = Ge, Ti, etc.) crystalline and glass-ceramics solid electrolytes for oxide-based all-solid-state batteries exhibited high lithium-ion conductivities over 10−4 S cm−1 at room temperature.2–10 Such oxide-based all-solid-state batteries require sintering process due to the poor lithium-ion conduction at grain boundaries.11 However, the sintering process at high temperature of oxide-based inorganic lithium-ion conductors cause the loss of lithium elements with high volatility and poor charge/discharge performance for the all-solid-state batteries.12 In addition, when the solid electrolytes and electrode active materials are co-sintered at high temperatures above 800 °C, the various elements contained in both the solid electrolyte and electrode materials diffuse into each other. As the results, the formation of different phases with extremely low electrical conductivity is promoted, which directly leads to battery performance degradation. Sintering at lower temperatures than 800 °C may be preferable to prevent the formation of different phases.13
As one of the oxide-based solid electrolytes, Li1+xAlxGe2−x(PO4)3 (LAGP) solid electrolytes with partial substitution of Ge4+ for Al3+ in NASICON-type LiGe2(PO4)3 structure had high electrochemical reduction resistance and could be sintered at low temperature, which is expected to suppress side reactions for co-sintering with electrode materials.14–16 Solid solution limits of solid electrolytes in the LAGP system were the composition of x = 0.4 or 0.5 for high temperature heat treatment at 800–900 °C. The LAGP(x = 0.4 or 0.5) solid electrolytes have been reported as the composition with the maximum conductivity at room temperature.17,18
Recently, unique co-sintering type oxide-based all-solid-state batteries consisting of NASICON-type Li1+xAlxGe2−x(PO4)3 (LAGP) compounds as solid electrolytes, NASICON-type Li1+xAlxTi2−x(PO4)3 (LATP) additive olivine-type LiCoPO4 (LCP) compounds as positive electrodes, and LCP additive LCP compounds as negative electrodes were fabricated by co-sintering at low temperatures using the multilayer ceramic capacitors (MLCC) process and the local electrochemical reactions in the all-solid-state batteries during the cyclic voltammetry (CV) measurements have been investigated by the operando technique.19 In this case, the co-sintering process were performed at lower temperatures than 750 °C because LCP and LATP were known to react with each other during co-sintering at high temperature.20 Therefore low-temperature preparations of LAGP solid electrolytes are very importance to develop all-solid-state batteries obtained by co-sintering solid electrolytes with electrodes.
We have reported that the LAGP(x = 0.5) solid electrolytes prepared by the low-temperature heat treatment at 550–700 °C of amorphous LAGP(x = 0.5) in air exhibited relatively high conductivities on the order of 10−5 S cm−1 at room temperature.21 Although there have been reported on the crystalline structure of high Al-substituted LAGP-based solid electrolytes (e.g. LAGP(x = 1.0)) using XRD and NMR measurements, the relationship of preparation temperature and the lithium-ion conductivities of the high Al-substituted LAGP solid electrolytes has not been investigated.22 Development of high-Al substituted LAGP is effective in reducing the content of expensive Ge element in the electrolytes. Thus, LAGP(x = 1.0) solid electrolyte will provide low cost. Substitution of Ge in LAGP structure with light Al elements without spoiling the remarkable performance of LAGP will contribute to decrease in the weight of solid electrolyte layer, possibly leading to enhancement in energy density of the resulting all-solid-state batteries. As a result, weight loss of solid electrolyte layer will promote an increase of battery capacities per weight of all-solid-state batteries.
In this study, amorphous materials with the nominal composition of high-Al substituted Li1+xAlxGe2−x(PO4)3 (x = 1.0) (LAGP(x = 1.0)) were prepared by mechanical milling (MM) technique. LAGP-based lithium-ion conducting solid electrolytes were obtained by crystallization of the amorphous LAGP(x = 1.0) at low-temperature heat treatment process in air. The high-Al substituted effects on the lithium-ion conductivities, crystalline phase, and grain boundaries of the solid electrolytes were investigated. We tried the low-temperature preparation of newly solid electrolytes with high lithium-ion conductivities at room temperature for all-solid-state batteries. Comparing the LAGP(x = 1.0) compounds with the LAGP(x = 0.5) compounds,21 we discussed their properties as solid electrolytes.
Li2O (99 %, Kojundo Chemical Laboratory), Al2O3 (99.99 %, Kojundo Chemical Laboratory), GeO2 (99.997 %, Kojundo Chemical Laboratory), and P2O5 (99.9 %, Kojundo Chemical Laboratory) were used as starting materials. Each starting material was weighed with the nominal composition of Li1+xAlxGe2−x(PO4)3 (x = 1.0) in glove box under a dry Ar atmosphere. The conditions of mechanical milling (MM) treatment were based on our previous work as described below.21 MM treatment using a planetary ball mill (Fritsch, p-7) were carried out on mixtures of the starting materials at rotation speed 450 rpm for 20 h at room temperature under a dry Ar atmosphere. The volumes of the zirconia pot, number and diameter of balls were 45 cc, 10, and 10 mm, respectively.
2.2 Preparation of LAGP(x = 1.0) solid electrolytesThe amorphous LAGP(x = 1.0) [MM] powder was formed into pellets (diameter: 10 mm) by uniaxial pressing at a press pressure of 180 MPa using pellet molding die. The pelletized-type LAGP(x = 1.0) [MM] were obtained by heat treatment for 10 h at 600, 650, 700 and 750 °C in an electric furnace (Yamato Scientific, FP100), respectively. At that time, the temperature rise time was set to 3.5 h.
2.3 Characterization methods of amorphous LAGP(x = 1.0) [MM] powder and LAGP(x = 1.0) solid electrolytesPowder X-ray diffraction (XRD) measurements were performed to confirm the amorphization of the powder after MM treatment. The crystalline phase after the heat treatment of LAGP(x = 1.0) [MM] powder was also investigated by XRD measurements. X-ray diffractometer (Mini Flex600, Rigaku) with CuKα-rays (voltage: 40 kV, current: 15 mA) were used at diffraction angle of 5° ≤ 2θ ≤ 80° and sampling width of 0.01°.
The diameter, thickness and weight of the heat-treated pellets were then measured using calipers, micrometers, and electronic balances, respectively. The apparent density of the sintered pellets after the heat treatment was calculated by using Eq. 1.
| \begin{equation} \rho = m/(S\times L) \end{equation} | (1) |
where ρ is the apparent density of the pellet, m is the weight of the pellet, S is the area of the pellet, and L is the thickness of the pellet.
Sintered solid electrolytes for electrical conductivity measurements are obtained by gold sputtering on both sides of the heat-treated pellets. Gold sputtering was performed by using a sputtering apparatus (Sanyu Electronics, QUICK COATHER SC-701MKII) at 5 mA for 10 minutes per one side of the pellet.
Electrical conductivity measurements were performed by the AC impedance technique. The conductivity measurements were performed using an electrochemical analyzer (Bio-Logic, VSP-300) with an applied voltage of ±10 mV and a frequency range of 0.1 Hz to 1 MHz at the temperature range of 0–80 °C in air. The activation energy was calculated from the slope of the Arrhenius plot using Eq. 2.
| \begin{equation} \sigma = 1/R_{\text{i}}(L/S) = \sigma_{0} \exp (-E_{\text{a}}/RT) \end{equation} | (2) |
where σ is total conductivity, Ri is total resistance of bulk and grain boundary, L is pellet thickness, S is pellet area, σ0 is pre-exponential term (substance-specific constant), R is gas constant, Ea is activation energy required for ion transfer, and T is absolute temperature.
The lithium-ion transport number of LAGP(x = 1.0) solid electrolytes prepared by heat treatment of pelletized-type LAGP(x = 1.0) [MM] at 700 °C was measured by chronoamperometry at 25 °C. The applied voltage and time were 2.5 V and 5 h, respectively. The lithium-ion transport number: ti was calculated from Eq. 3.23
| \begin{equation} t_{\text{i}} = (I_{\text{i}} - I_{\text{f}})/I_{\text{i}} \end{equation} | (3) |
where Ii is initial current and If is stabilizing current.
Scanning electron microscopy (SEM) observations before and after heat treatment of the powder prepared by MM were performed by using a scanning electron microscope (JEOL, JSM-6510). SEM observations were also carried out on the cross-sectional of the heat-treated pelletized-type solid electrolytes.
Figure 1 shows XRD patterns of the LAGP(x = 1.0) [MM] powder and LAGP(x = 1.0) powder prepared at 350, 400, and 450 °C. It was found that LAGP(x = 1.0) [MM] powder became amorphous because XRD peak patterns were halo. Weak diffraction peaks of Al2O3 as the starting materials were also observed in the powder after MM treatment. These results may be caused by the substitution of high Al element. The weak peaks of LAGP(x = 1.0) at 450 °C were attributed to LiGe2(PO4)3(LGP) (COD 1008511) and Al2O3, indicating that the LAGP-based NASICON-type crystal structure began to form through low-temperature heat treatment of amorphous LAGP(x = 1.0) powder. The LAGP structure formation of amorphous LAGP(x = 0.5) occurred at 400 °C, whereas it shifted to higher temperatures for x = 1.0, suggesting that the amount of high-Al substitution will affect the formation of LAGP crystals from the amorphous phase.
XRD patterns of LAGP(x = 1.0) [MM] powder and LAGP(x = 1.0) powder prepared at 350 °C, 400 °C, and 450 °C.
Figure 2 shows SEM images of (a) LAGP(x = 1.0) [MM] powder and LAGP(x = 1.0) powder prepared at (b) 350, (c) 400, and (d) 450 °C. The average particle sizes were ca. 1.0 µm by the SEM observation, respectively. These results were similar to those for after heat treatment of amorphous LAGP(x = 0.5) obtained by MM method.21 It was suggesting that the amorphous materials of LAGP(x = 0.5 and 1.0) have been crystallized without the particle growth at 400 and/or 450 °C.
SEM images of (a) LAGP(x = 1.0) [MM] and LAGP(x = 1.0) powder prepared at (b) 350 °C, (c) 400 °C, and (d) 450 °C.
Figure 3 shows XRD patterns of LAGP(x = 1.0) solid electrolyte pellets prepared at 600, 650, 700 and 750 °C. The peaks attributed to LiGe2(PO4)3 (LGP) (COD 1008511) were mainly observed for all solid electrolytes after heat treatment. The peak intensity attributed to LiGe2(PO4)3 (LGP) (COD 1008511) increased with increasing the heat treatment temperature, suggesting the enhancements of crystallinity. Thus, the solid solution formation with LAGP-based NASICON-type crystal structure was also observed for the composition of LAGP(x = 1.0) beyond the solid solubility limit (composition: LAGP(x = 0.5)). The peaks attributed to AlPO4 were detected after heat treatment at 600 and 650 °C. On the other hand, the peaks attributed to both AlPO4 and Li9Al3(P2O7)3(PO4)2 were detected after heat treatment at 700 and 750 °C. It was found that the crystalline Li9Al3(P2O7)3(PO4)2 compound began to be formed by heat treatment of LAGP(x = 1.0) [MM] powder at around 700 °C. The peak intensity of Li9Al3(P2O7)3(PO4)2 increased with increasing the heat treatment temperature. In addition, after heat treatment at 750 °C, the weak peaks attributed to GeO2 were also detected. The peak intensities of AlPO4, Li9Al3(P2O7)3(PO4)2 and GeO2 except LAGP solid solutions were stronger with compared to those at 700 °C. Such these results suggested that the high Al-substituted LAGP(x = 1.0) solid electrolytes will promote the formation of LAGP-based NASICON-type solid electrolytes containing AlPO4 and Li9Al3(P2O7)3(PO4)2 compounds.
XRD patterns of LAGP(x = 1.0) solid electrolyte pellets prepared at 600 °C, 650 °C, 700 °C, and 750 °C.
Figure 4 shows cross-sectional SEM images of LAGP(x = 1.0) solid electrolyte pellets prepared at (a) 600 °C, (b) 650 °C, (c) 700 °C, and (d) 750 °C. There were no clear grain boundaries and the surface of particles was smooth. Voids were also observed in the fracture surface area. Such particle morphology would not appear to differ markedly with changes into heat treatment temperature. Softening and fusion of amorphous particles may be more likely to occur at high-Al substituted compositions. Morphological changes caused by heat treatment of the amorphous LAGP(x = 1.0) particles seemed to be more progressed than those of the amorphous LAGP(x = 0.5) particles shown in the SEM images of our previous paper.21 For this reason, the reduction of grain boundary resistance will be expected by low-temperature heat treatment of pelletized-type amorphous LAGP(x = 1.0).
Cross-sectional SEM images of LAGP(x = 1.0) solid electrolyte pellets prepared at (a) 600 °C, (b) 650 °C, (c) 700 °C, and (d) 750 °C.
Figure 5 shows Arrhenius plots of the temperature dependence of electrical conductivities calculated using the total resistance values of bulk and grain boundary resistances of Nyquist plot obtained from AC impedance measurements. Table 1 shows room temperature conductivities, activation energies, and relative densities of LAGP(x = 1.0) solid electrolytes pellets at 600, 650, 700, and 750 °C.
Arrhenius plots of LAGP(x = 1.0) solid electrolyte pellets at 600, 650, 700, and 750 °C.
| Heat treatment temperature/°C |
600 | 650 | 700 | 750 |
|---|---|---|---|---|
| σ25/S cm−1 | 1.18 × 10−5 | 2.10 × 10−5 | 2.69 × 10−5 | 2.79 × 10−5 |
| Ea/kJ mol−1 | 39.5 | 39.8 | 40.6 | 38.9 |
| Relative density/% | 67.6 | 67.3 | 66.3 | 69.3 |
The relative density of the heat-treated pellets was calculated from Eq. 4.
| \begin{equation} \rho' = (\rho/\rho_{\text{LAGP}})\times 100 \end{equation} | (4) |
where ρ′ is relative density, ρ is apparent density of the pellet, and ρLAGP (3.42 g cm−3)16 is the theoretical density of LAGP(x = 0.5). The relative densities of LAGP(x = 1.0) solid electrolytes were around 66–69 %. In this case, it was found that pellet densities varied hardly with heat treatment temperature. The relative densities of LAGP(x = 0.5) solid electrolyte pellets at 700 °C were less than 63 %.21 Sintered LAGP(x = 1.0) could promote densification as compared to sintered LAGP(x = 0.5). It suggested that reduction of grain boundary will be caused by low-temperature heat treatment of pelletized-type amorphous LAGP(x = 1.0).
Figure 6 shows the relationship between room temperature conductivity (σ25) and activation energy (Ea) of LAGP(x = 1.0) solid electrolyte pellets at 600, 650, 700, and 750 °C. The relationship between σ25 and Ea of LAGP(x = 0.5) solid electrolyte pellets at 600 and 700 °C of pelletized-type amorphous LAGP(x = 0.5) after MM was also shown in Fig. 6.21 The σ25 of LAGP(x = 1.0) solid electrolytes tended to decrease more than those with LAGP(x = 0.5) solid electrolytes. The Ea (ca. 40 kJ mol−1) of LAGP(x = 1.0) tended to increase more than those (ca. 37 kJ mol−1) with LAGP(x = 0.5). For the heat treatment temperature range of 600–700 °C, the relationship between electrical conductivity and heat treatment temperature was similar for the x = 0.5 and x = 1.0 compositions. All of LAGP(x = 1.0) solid electrolytes exhibited relatively high conductivities on the order of 10−5 S cm−1. Such results were similar to the room temperature conductivities of LAGP solid electrolytes obtained by high-temperature treatment (solid phase reaction process) for the mixture of the starting materials at 800–1000 °C.24 The room temperature conductivities of LAGP(x = 1.0) solid electrolytes were increased with increasing heat treatment temperature. On the other hand, their activation energy of LAGP(x = 1.0) solid electrolytes did not differ significantly with increasing heat treatment temperatures. The LAGP(x = 1.0) solid electrolyte pellets at 750 °C did not cause a decrease for room temperature conductivity and an increase for activation energy, although they will form the excess AlPO4, Li9Al3(P2O7)3(PO4)2, and GeO2 compounds. Even in the nominal composition of LAGP(x = 1.0) outside the solid solution in the LAGP system, relatively high conductivities will be achieved by low-temperature heat treatment processes of pelletized-type LAGP(x = 1.0) [MM] at around 650–700 °C. Such the results, the crystalline AlPO4 and Li9Al3(P2O7)3(PO4)2 compounds may be suggested to effectively provide the reduction of grain boundaries and lithium-ion conduction paths at grain boundary in sintered LAGP(x = 1.0) solid electrolytes.
The relationship between room temperature conductivity and the activation energy of LAGP(x = 1.0) solid electrolyte pellets at 600, 650, 700, and 750 °C. The relationship between σ25 and Ea for LAGP(x = 0.5) solid electrolyte pellets was also shown in this figure.
Figure 7 shows the current curve obtained by chronoamperometric measurement of LAGP(x = 1.0) solid electrolyte pellet prepared at 700 °C. Chronoamperometric curve up to 20 s is also shown as an enlarged view in Fig. 7. Lithium-ion transport number was calculated from the initial and stabilizing currents using Eq. 3. In the case, the lithium-ion transport number calculated from the initial current of 29.4 µA and stabilization current of 0.0144 µA was 0.999, indicating single ion conductivity with a lithium-ion transport number of approximately 1.
Chronoamperometric curve of LAGP(x = 1.0) solid electrolyte pellets at 700 °C.
LAGP(x = 1.0) solid electrolytes were prepared by the low-temperature heat treatment of pelletized-type amorphous powder with the nominal composition of high-Al substituted LAGP(x = 1.0) at 600–750 °C. Even in the nominal composition of LAGP(x = 1.0) outside the solid solution in the LAGP system, the LAGP(x = 1.0) solid electrolyte pellets exhibited relatively high conductivities on the order of 10−5 S cm−1 at room temperature. The reason for the excellent lithium-ion conducting properties can be suggested that heat treatment via amorphous materials promotes the formation of conduction paths by crystallization and crystal growth, which significantly contributes to the reduction of grain boundaries. High Al-substituted LAGP(x = 1.0) solid electrolytes can be significantly reduced the content of expensive Ge element and will be prepared at the low-temperature, comparing with LAGP(x = 0.5).
From the results described above, we think that high-Al substituted LAGP compounds enable the low-temperature preparation of newly solid electrolytes. The low-temperature process is also considered to be a useful benefit for the practical application of co-sintering type all-solid-state batteries with high safety and reliability. We are currently studying the densification of sintered LAGP(x = 1.0) solid electrolytes.
Nana Shinada: Data curation (Lead), Investigation (Lead), Writing – original draft (Lead), Writing – review & editing (Supporting)
Toi Shimoda: Data curation (Supporting), Formal analysis (Supporting), Investigation (Supporting)
Ryohei Kurihara: Data curation (Supporting), Formal analysis (Supporting), Investigation (Supporting)
Hideyuki Morimoto: Conceptualization (Lead), Writing – original draft (Supporting), Writing – review & editing (Lead)
The authors declare no conflict of interest in the manuscript.
A part of this paper has been presented in the 65th battery symposium in Japan in 2024 (Presentation 2G-17).
H. Morimoto: ECSJ Active Member
