I have devoted my recent research to the development of advanced solid electrolytes for power batteries used in new energy aircraft. The electrification of aviation is no longer a distant vision; it is a rapidly expanding engineering reality driven by the need for zero-carbon flight, low noise, high efficiency, and operational safety. In this context, the solid state cell has emerged as one of the most promising architectures for future aviation power sources because it can combine a lithium metal anode with a high-voltage cathode and a solid electrolyte, thereby offering high energy density, improved thermal stability, and reduced flammability. My work specifically addresses the sintering science of a NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte, and I compare hot-pressing (HP) sintering with conventional pressureless sintering (PLS). I show that the application of a constant uniaxial pressure during sintering increases the sintering driving force, promotes grain rearrangement, plastic flow, and diffusion creep, suppresses abnormal grain growth and gaseous defects, and thereby yields a dense, highly conductive LAGP electrolyte that is suitable for a solid state cell in aviation applications.
The motivation for this work is straightforward. The aviation industry requires power batteries with an energy density approaching 400 Wh kg−1 for mass production and 500 Wh kg−1 for small-scale validation. Such targets cannot be met by conventional liquid-electrolyte lithium-ion batteries without compromising safety. A solid state cell using an inorganic solid electrolyte can, in principle, reach these targets because it enables the use of lithium metal, which has a theoretical specific capacity of 3860 mAh g−1 and the lowest electrochemical potential. However, the commercialization of the solid state cell still faces significant obstacles, including high interfacial resistance, poor densification of the electrolyte, and insufficient ionic conductivity at room temperature. My research focuses on the densification step because it is the bridge between a promising powder and a functional solid state cell.
I selected LAGP because it belongs to the NASICON family and exhibits high bulk ionic conductivity, good thermal stability, excellent chemical stability against air and moisture, and a wide electrochemical window. These characteristics make LAGP especially attractive for aviation power batteries, where safety and reliability are non-negotiable. Nevertheless, conventionally sintered LAGP often suffers from high porosity, weak grain-to-grain contact, and low room-temperature ionic conductivity. I hypothesized that hot-pressing sintering could overcome these limitations by applying pressure during the densification process. The pressure acts directly on the ceramic compact, enhancing particle rearrangement, plastic deformation, and diffusional mass transport, which together eliminate closed pores and reduce porosity. In my experiments, I systematically varied the sintering temperature and pressure, characterized the microstructure and phase composition, measured the ionic conductivity and activation energy, evaluated the mechanical properties, and finally assembled symmetric and full solid state cell prototypes to demonstrate the practical benefits of hot-pressing.
In this article, I present my first-person account of the experimental design, the results, and the interpretation. I use tables and equations to summarize the key quantitative relationships. I also discuss how the sintering parameters affect the performance of the resulting solid state cell. My goal is to provide a clear, reproducible, and scalable methodology for producing advanced solid electrolytes for new energy aircraft.
1. Background and Motivation
The aviation transportation sector is undergoing a profound transformation. Electric vertical take-off and landing aircraft, electric fixed-wing aircraft, and hybrid-electric regional aircraft all require power sources that are simultaneously lightweight, safe, and long-lasting. The solid state cell is widely regarded as the most likely candidate to satisfy these requirements. Unlike a conventional lithium-ion cell, which contains a flammable organic liquid electrolyte, a solid state cell uses a solid electrolyte that is non-flammable and can physically suppress lithium dendrite growth. When paired with a lithium metal anode, the solid state cell can achieve a cell-level energy density above 400 Wh kg−1, which is essential for extending the range and payload of electric aircraft.
Among solid electrolytes, oxide ceramics such as LAGP, LATP, and garnet-type LLZO have attracted particular attention. LAGP has a rhombohedral NASICON structure with a three-dimensional framework of GeO4 and PO4 tetrahedra, which provides fast lithium-ion migration pathways. The substitution of Ge4+ by Al3+ and the resulting lithium excess improve the ionic conductivity. However, the preparation of dense LAGP ceramics remains challenging. Pressureless sintering often requires high temperatures and long dwell times, which can cause lithium volatilization, impurity phase formation, and exaggerated grain growth. These problems degrade the ionic conductivity and mechanical integrity of the solid electrolyte, ultimately limiting the performance of the solid state cell.
Hot-pressing sintering offers a solution. In hot pressing, a uniaxial pressure is applied to the powder compact while it is heated. The applied pressure increases the contact area between particles, reduces the activation energy for diffusion, and promotes densification at lower temperatures and shorter times. This is especially important for LAGP because lower sintering temperatures help retain lithium and avoid the formation of insulating impurity phases such as AlPO4 and GeO2. I therefore set out to determine the optimal hot-pressing temperature and pressure for LAGP and to compare the resulting electrolyte with a pressureless-sintered counterpart in terms of microstructure, ionic conductivity, activation energy, mechanical strength, and solid state cell cycling stability.
2. Experimental Design
I prepared LAGP powder by a high-energy ball-milling and solid-state reaction route. The starting materials were Li2CO3, Al2O3, GeO2, and NH4H2PO4. To compensate for lithium volatilization at high temperature, I added 10 wt% excess Li2CO3. The raw materials were mixed with ethanol and ball-milled at 350 rpm for 12 h, then dried at 60 °C overnight. The dried powder was pre-calcined at 700 °C for 4 h with a heating rate of 5 °C min−1 to release CO2, NH3, and H2O. Subsequently, the mixture was calcined at 800 °C for 6 h, ball-milled again for 6 h, and sieved through a 500-mesh screen. The resulting LAGP powder was stored in a dry box.
For pressureless sintering, I pressed 0.5 g of LAGP powder into a 15 mm diameter pellet at 13 MPa for 5 min. The green pellet was placed in an alumina crucible and sintered at 750, 800, 850, or 900 °C for 5 h. The samples were designated PLS-750, PLS-800, PLS-850, and PLS-900.
For hot-pressing sintering, I placed an appropriate amount of LAGP powder into a 15 mm cylindrical graphite die. A 0.1 mm thick carbon paper was used to wrap the pre-calcined powder to prevent adhesion and overflow. The hot-pressing schedule was as follows: heating from room temperature to 400 °C at 7 °C min−1 with an initial pressure of 17 MPa; then heating from 400 °C to the target temperature at 8 °C min−1; upon reaching the target temperature, the pressure was changed to the desired value and held for 1 h; finally, the sample was cooled naturally. After demolding, the cylindrical LAGP electrolyte was cut into 15 mm diameter, 1 mm thick discs using an automatic inner-circle cutting machine. The samples were designated HP-750, HP-800, HP-850, and HP-900 according to the sintering temperature, and HP20-800, HP25-800, HP30-800, and HP35-800 according to the applied pressure at 800 °C.
For cathode preparation, I mixed LiFePO4 (LFP), Super P carbon black, and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1. The LFP and Super P were ground for 30 min, then added to an N-methyl-2-pyrrolidone (NMP) solution containing PVDF and stirred magnetically for 12 h to form a homogeneous slurry. The slurry was coated onto an aluminum foil current collector using a 250 µm doctor blade, dried at 60 °C in a vacuum oven for 12 h, and punched into 12 mm diameter discs. All electrodes were stored in an argon-filled glovebox.
For solid state cell assembly, I used CR2032 coin cells. The symmetric cell configuration was negative shell, spring, stainless steel spacer, lithium foil, LAGP electrolyte, lithium foil, and positive shell. The full cell configuration was negative shell, spring, stainless steel spacer, lithium metal anode, LAGP electrolyte, LFP cathode, and positive shell. All assembly was performed inside an argon-filled glovebox with controlled oxygen and moisture levels below 0.1 ppm.
The phase composition was characterized by X-ray diffraction (XRD) with Cu Kα radiation. The microstructure was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Elemental mapping was performed by energy-dispersive X-ray spectroscopy (EDS). Raman spectra were collected to confirm the local structure of the phosphate units. The relative density was calculated from the measured geometric density and the theoretical density of LAGP. The ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using a frequency response analyzer over a frequency range of 1 Hz to 1 MHz with an AC amplitude of 10 mV. The activation energy was determined from variable-temperature EIS measurements. The Vickers hardness and flexural strength were measured by a microhardness tester and a three-point bending fixture, respectively. The electrochemical cycling of symmetric and full cells was performed on a battery test system at room temperature.
3. Key Equations and Definitions
The ionic conductivity of the solid electrolyte was calculated from the bulk resistance obtained from the high-frequency intercept of the Nyquist plot:
$$\sigma = \frac{L}{R_b A}$$
where σ is the ionic conductivity in S cm−1, L is the thickness of the electrolyte pellet in cm, Rb is the bulk resistance in Ω, and A is the electrode area in cm2.
The relative density was calculated as:
$$\rho_{rel} = \frac{\rho_{measured}}{\rho_{theoretical}} \times 100\%$$
where ρmeasured is the geometric density and ρtheoretical is the theoretical density of LAGP, taken as 3.56 g cm−3.
The temperature dependence of ionic conductivity follows the Arrhenius relationship:
$$\sigma T = \sigma_0 \exp\left(-\frac{E_a}{k_B T}\right)$$
which can be linearized as:
$$\ln \sigma = \ln \sigma_0 – \frac{E_a}{k_B T}$$
where σ0 is the pre-exponential factor, Ea is the activation energy in eV, kB is the Boltzmann constant (8.617 × 10−5 eV K−1), and T is the absolute temperature in K. When the horizontal axis is 1000/T, the activation energy can be obtained from the slope:
$$E_a = -k_B \times \text{slope} \times 1000$$
The Vickers hardness was calculated as:
$$HV = \frac{1.854 F}{d^2}$$
where F is the applied load in N and d is the average diagonal length of the indentation in mm.
The flexural strength was calculated from the three-point bending test:
$$\sigma_f = \frac{3FL}{2bd^2}$$
where F is the maximum load, L is the span length, b is the specimen width, and d is the specimen thickness.
The capacity retention after N cycles was defined as:
$$C_{ret} = \frac{C_N}{C_1} \times 100\%$$
where C1 is the initial discharge capacity and CN is the discharge capacity after N cycles.
The polarization voltage of the symmetric cell was evaluated as:
$$\eta = \frac{V_{max} – V_{min}}{2}$$
where Vmax and Vmin are the maximum and minimum voltages during a constant-current cycling step.
4. Microstructure and Phase Analysis of the LAGP Powder
I first examined the as-synthesized LAGP powder. The SEM images showed that the powder consisted of uniformly distributed grains with no obvious agglomeration. TEM analysis combined with EDS confirmed that the particle size was in the micrometer range and that the main elements were Al, Ge, O, and P, with no detectable impurity elements. The XRD pattern of the powder exhibited the typical NASICON-type crystal structure. The diffraction peaks matched the standard card PDF#80-1924 for LiGe2(PO4)3. I observed sharp and intense peaks at 2θ = 15.1°, 21.3°, 25.2°, 30.4°, 34.0°, 37.7°, 48.9°, 51.6°, 56.2°, and 59.1°, corresponding to the (012), (104), (113), (024), (116), (300), (128), (226), (2110), and (410) planes, respectively. The absence of impurity peaks indicated that the synthesized LAGP powder had high purity and good crystallinity.
| 2θ (°) | Plane (hkl) | Relative intensity |
|---|---|---|
| 15.1 | (012) | Medium |
| 21.3 | (104) | Strong |
| 25.2 | (113) | Strong |
| 30.4 | (024) | Very strong |
| 34.0 | (116) | Medium |
| 37.7 | (300) | Medium |
| 48.9 | (128) | Weak |
| 51.6 | (226) | Weak |
| 56.2 | (2110) | Weak |
| 59.1 | (410) | Weak |
The powder was then used for both pressureless sintering and hot-pressing sintering. I compared the phase evolution and microstructure as a function of temperature and pressure. The details are presented in the following sections.
5. Effect of Sintering Temperature on Microstructure and Phase Composition
I sintered LAGP pellets by both pressureless sintering and hot pressing at 750, 800, 850, and 900 °C for 1 h. In the hot-pressing experiments, the applied pressure was initially fixed at 20 MPa. The SEM images revealed a clear difference between the two methods. For pressureless sintering, the grains in PLS-750 and PLS-800 were irregular in shape, and noticeable voids existed between grains, leading to low density. As the temperature increased to 850 °C and 900 °C, the grains grew larger, but the density remained limited by the lack of external pressure. In contrast, the hot-pressed samples HP20-750 and HP20-800 exhibited intimate grain-to-grain contact, significantly reduced porosity, and a more homogeneous microstructure. At 850 °C, the grains continued to grow, and at 900 °C, thermal cracks appeared in the hot-pressed LAGP. These cracks likely originated from interfacial stress concentration, volume changes induced by phase transformations, or volatilization of minor components. Such cracks are detrimental to the mechanical strength and long-term stability of a solid state cell.
The XRD patterns of the pressureless-sintered and hot-pressed LAGP pellets confirmed that the main phase remained LiGe2(PO4)3 (PDF#80-1924) up to 850 °C. However, at 900 °C, both pressureless-sintered and hot-pressed samples developed impurity peaks corresponding to GeO2 and AlPO4. The formation of these impurity phases is undesirable because AlPO4 is electrically insulating and can block lithium-ion transport, thereby increasing the interfacial resistance of the solid state cell. Therefore, 800 °C was identified as the optimal hot-pressing temperature, balancing densification, phase purity, and grain size.
Raman spectroscopy of the hot-pressed LAGP electrolytes showed the typical NASICON structure with PO4 units as the main vibration mode. A sharp and intense peak at 458 cm−1 was assigned to the bending mode of PO43− ν2, confirming a high degree of local order in the phosphate framework. This structural order is beneficial for fast lithium-ion conduction in the solid state cell.
| Method | Temperature (°C) | Pressure (MPa) | Dwell time (h) | Sample designation |
|---|---|---|---|---|
| PLS | 750 | 0 | 5 | PLS-750 |
| PLS | 800 | 0 | 5 | PLS-800 |
| PLS | 850 | 0 | 5 | PLS-850 |
| PLS | 900 | 0 | 5 | PLS-900 |
| HP | 750 | 20 | 1 | HP20-750 |
| HP | 800 | 20 | 1 | HP20-800 |
| HP | 850 | 20 | 1 | HP20-850 |
| HP | 900 | 20 | 1 | HP20-900 |
6. Ionic Conductivity and Relative Density
I measured the AC impedance of the hot-pressed LAGP electrolytes at room temperature. All Nyquist plots consisted of a semicircle in the high-frequency region, which represents the grain-boundary impedance, and an inclined line in the low-frequency region, which represents the diffusion impedance. The bulk resistance was extracted from the high-frequency intercept, and the ionic conductivity was calculated using the equation given earlier. The relative density was calculated from the geometric density and the theoretical density of LAGP.
The results showed that both the relative density and the ionic conductivity increased first and then decreased as the sintering temperature increased from 750 to 900 °C. The maximum values were obtained at 800 °C, with a relative density of 97.34% and an ionic conductivity of 3.34 × 10−4 S cm−1. When the temperature was further increased to 850 and 900 °C, the samples maintained a high relative density, but the ionic conductivity decreased significantly. This decrease is attributed to the formation of the insulating AlPO4 impurity phase at high temperature, which blocks lithium-ion migration and increases the grain-boundary resistance. Therefore, although higher temperatures can improve densification, they also promote undesirable phase transformation and impurity formation, ultimately degrading the performance of the solid state cell.
| Sample | Relative density (%) | Ionic conductivity (S cm−1) | Activation energy (eV) |
|---|---|---|---|
| HP20-750 | 94.12 | 1.85 × 10−4 | 0.36 |
| HP20-800 | 97.34 | 3.34 × 10−4 | 0.33 |
| HP20-850 | 96.85 | 2.42 × 10−4 | 0.34 |
| HP20-900 | 95.21 | 1.58 × 10−4 | 0.35 |
The activation energy was determined from the Arrhenius plots of ln σ versus 1000/T. The linear relationship between ln σ and 1000/T confirmed that the ionic conduction is thermally activated and follows the Arrhenius law. The activation energy was 0.36 eV for HP20-750, 0.33 eV for HP20-800, 0.34 eV for HP20-850, and 0.35 eV for HP20-900. The lowest activation energy at 800 °C indicates that the lithium-ion migration barrier is minimized when the electrolyte has the highest density, the cleanest grain boundaries, and the highest phase purity. This is a critical result for the solid state cell because a lower activation energy translates to higher ionic conductivity at room temperature and better rate capability.
7. Effect of Applied Pressure on Densification and Ionic Conductivity
After identifying 800 °C as the optimal sintering temperature, I investigated the effect of applied pressure. I prepared hot-pressed LAGP electrolytes at 800 °C with pressures of 20, 25, 30, and 35 MPa, designated HP20-800, HP25-800, HP30-800, and HP35-800. The SEM images showed that as the pressure increased from 20 to 35 MPa, the grains grew larger, the grain boundaries became less distinct, and the pores almost disappeared. Elemental mapping by EDS confirmed a uniform distribution of O, Ge, Al, and P in the HP30-800 sample, indicating excellent chemical homogeneity.
The relative density increased monotonically with pressure. The values were 97.34%, 98.15%, 98.87%, and 98.98% for HP20-800, HP25-800, HP30-800, and HP35-800, respectively. The ionic conductivity also increased with pressure, reaching 3.34 × 10−4, 4.17 × 10−4, 5.56 × 10−4, and 5.68 × 10−4 S cm−1 for the same series. The improvement in ionic conductivity is attributed to the higher density and better grain-to-grain contact, which provide continuous pathways for lithium-ion transport and reduce the grain-boundary resistance. In a solid state cell, these microstructural improvements directly translate into lower internal resistance and higher power capability.
| Sample | Pressure (MPa) | Relative density (%) | Ionic conductivity (S cm−1) | Activation energy (eV) |
|---|---|---|---|---|
| HP20-800 | 20 | 97.34 | 3.34 × 10−4 | 0.33 |
| HP25-800 | 25 | 98.15 | 4.17 × 10−4 | 0.32 |
| HP30-800 | 30 | 98.87 | 5.56 × 10−4 | 0.31 |
| HP35-800 | 35 | 98.98 | 5.68 × 10−4 | 0.31 |
The activation energy decreased as the applied pressure increased. The values were 0.33, 0.32, 0.31, and 0.31 eV for HP20-800, HP25-800, HP30-800, and HP35-800, respectively. The lower activation energy at higher pressure is consistent with the higher ionic conductivity and higher density. When the electrolyte is dense and the grain boundaries are clean, lithium ions encounter fewer obstacles during migration, resulting in a lower activation energy. This is beneficial for the low-temperature performance of the solid state cell.
The mechanism by which pressure enhances densification can be summarized as follows. First, the applied pressure promotes particle rearrangement, which reduces the initial porosity. Second, the pressure induces plastic deformation of the particles, which fills the voids between them. Third, the pressure enhances diffusional mass transport along grain boundaries and through the lattice, which accelerates the closure of residual pores. These effects are especially pronounced in the early and intermediate stages of sintering. As a result, hot pressing can achieve high density at a lower temperature and in a shorter time than pressureless sintering.
8. Mechanical Properties
Mechanical integrity is essential for a solid electrolyte in a solid state cell because the electrolyte must withstand the stresses generated during cell assembly, stack pressure, and cycling. I measured the Vickers hardness and flexural strength of the hot-pressed LAGP electrolytes. At 800 °C and 20 MPa, the HP20-800 sample exhibited a Vickers hardness of 9.27 GPa and a flexural strength of 169.45 MPa. As the pressure increased to 25, 30, and 35 MPa, the Vickers hardness increased to 9.67, 10.01, and 10.04 GPa, and the flexural strength increased to 173.45, 179.57, and 179.58 MPa, respectively. The most significant improvement occurred when the pressure increased from 20 to 30 MPa. Beyond 30 MPa, the additional improvement was marginal.
| Sample | Pressure (MPa) | Vickers hardness (GPa) | Flexural strength (MPa) |
|---|---|---|---|
| HP20-800 | 20 | 9.27 | 169.45 |
| HP25-800 | 25 | 9.67 | 173.45 |
| HP30-800 | 30 | 10.01 | 179.57 |
| HP35-800 | 35 | 10.04 | 179.58 |
These results demonstrate that increasing the sintering pressure not only improves the density and ionic conductivity but also enhances the mechanical strength of the LAGP electrolyte. A stronger electrolyte is less likely to crack or deform during cell fabrication and operation, which is crucial for the durability and safety of a solid state cell in aviation applications. Based on the combined electrical and mechanical performance, I selected 30 MPa as the optimal hot-pressing pressure.
9. Electrochemical Performance of Symmetric Solid State Cells
To evaluate the practical benefit of hot-pressing, I assembled symmetric solid state cells using either the pressureless-sintered PLS-850 or the hot-pressed HP30-800 electrolyte, with lithium metal on both sides. The cells were cycled at a current density of 0.1 mA cm−2 at room temperature. The Li/PLS-850/Li symmetric cell exhibited an initial polarization voltage of about 0.5 V, whereas the Li/HP30-800/Li symmetric cell exhibited an initial polarization voltage of only 0.2 V. As cycling proceeded, the polarization voltage of the Li/PLS-850/Li cell increased significantly, and after 300 h, it began to rise sharply. After 500 h, the cell almost completely failed. In contrast, the polarization voltage of the Li/HP30-800/Li cell remained nearly constant throughout the test, indicating excellent interfacial stability and dendrite suppression.
I also performed EIS measurements before and after cycling. For the Li/PLS-850/Li cell, the initial total impedance was 1113 Ω. After 300 h of cycling, it increased to 1675 Ω, and after 350 h, it further increased to 2508 Ω. After 400 h, the total impedance increased sharply to 5017 Ω. This dramatic increase in impedance is consistent with the polarization voltage evolution and indicates severe interfacial degradation and eventual short circuit. In contrast, the Li/HP30-800/Li cell had an initial total impedance of 547 Ω, which increased to 597 Ω after 300 h, 696 Ω after 350 h, and only 750 Ω after 400 h. The much smaller impedance increase demonstrates that the hot-pressed LAGP electrolyte maintains better interfacial stability and ionic conductivity during long-term cycling. This is a key advantage for the solid state cell, especially in aviation applications where reliability and cycle life are critical.
| Cell configuration | Initial impedance (Ω) | Impedance after 300 h (Ω) | Impedance after 350 h (Ω) | Impedance after 400 h (Ω) |
|---|---|---|---|---|
| Li/PLS-850/Li | 1113 | 1675 | 2508 | 5017 |
| Li/HP30-800/Li | 547 | 597 | 696 | 750 |
10. Electrochemical Performance of Full Solid State Cells
I further assembled full solid state cells using lithium metal as the anode, LFP as the cathode, and either PLS-850 or HP30-800 as the solid electrolyte. The cells were cycled at 0.1C at room temperature. The initial discharge capacities of LFP/PLS-850/Li and LFP/HP30-800/Li were 161.9 and 162.6 mAh g−1, respectively. After 250 cycles, the LFP/PLS-850/Li cell decayed rapidly, retaining only 4.45% of its initial capacity, whereas the LFP/HP30-800/Li cell retained 85.35% of its initial capacity. This result clearly demonstrates that the hot-pressed LAGP electrolyte provides better interfacial stability and lower interfacial impedance, which effectively slows down capacity fading and extends the cycle life of the solid state cell.
I also evaluated the rate performance. The LFP/PLS-850/Li cell delivered 54.37 mAh g−1 at 1C and recovered to 120.11 mAh g−1 when the rate returned to 0.1C. In contrast, the LFP/HP30-800/Li cell delivered 110.65 mAh g−1 at 1C and recovered to 149.78 mAh g−1 at 0.1C. The superior rate capability of the hot-pressed cell is attributed to the higher ionic conductivity and lower interfacial resistance of the HP30-800 electrolyte, which enables faster lithium-ion transport across the electrolyte and across the electrolyte-electrode interfaces.
| Cell configuration | Initial capacity (mAh g−1) | Capacity retention after 250 cycles (%) | Capacity at 1C (mAh g−1) | Capacity recovery at 0.1C (mAh g−1) |
|---|---|---|---|---|
| LFP/PLS-850/Li | 161.9 | 4.45 | 54.37 | 120.11 |
| LFP/HP30-800/Li | 162.6 | 85.35 | 110.65 | 149.78 |
The voltage-capacity curves further confirmed these findings. For the LFP/PLS-850/Li cell, the discharge capacity decreased from 150.3 to 139.4 mAh g−1 after 50 cycles, and the polarization voltage increased significantly. After 250 cycles, the polarization became even more pronounced. In contrast, the LFP/HP30-800/Li cell exhibited a discharge capacity decrease from 159.8 to 156.7 mAh g−1 after 50 cycles and to 141.6 mAh g−1 after 250 cycles, with a stable discharge plateau around 3.25 V and no obvious polarization. These results highlight the superior stability of the hot-pressed electrolyte in a practical solid state cell.
11. Discussion
The results of this study provide a consistent picture of how hot-pressing sintering improves the performance of LAGP solid electrolytes for aviation power batteries. The application of uniaxial pressure during sintering increases the driving force for densification, promotes particle rearrangement, plastic flow, and diffusion creep, and suppresses abnormal grain growth and gaseous defect formation. As a result, the hot-pressed LAGP electrolytes achieve higher relative density, higher ionic conductivity, lower activation energy, and better mechanical strength than their pressureless-sintered counterparts.
The optimal hot-pressing temperature is 800 °C. At this temperature, the LAGP electrolyte exhibits a relative density of 97.34%, an ionic conductivity of 3.34 × 10−4 S cm−1, and an activation energy of 0.33 eV. When the temperature is increased to 850 or 900 °C, impurity phases such as AlPO4 and GeO2 form, which degrade the ionic conductivity. When the temperature is lower than 800 °C, the densification is insufficient, and the ionic conductivity is limited by porosity and poor grain-to-grain contact. Therefore, 800 °C represents a balance between densification and phase purity.
The optimal hot-pressing pressure is 30 MPa. At this pressure, the LAGP electrolyte achieves a relative density of 98.87%, an ionic conductivity of 5.56 × 10−4 S cm−1, an activation energy of 0.31 eV, a Vickers hardness of 10.01 GPa, and a flexural strength of 179.57 MPa. Further increasing the pressure to 35 MPa yields only marginal improvements in density and mechanical properties, suggesting that 30 MPa is a practical and efficient upper limit for the hot-pressing process.
The electrochemical tests of symmetric and full solid state cells confirm the practical benefits of hot-pressing. The Li/HP30-800/Li symmetric cell exhibits a low and stable polarization voltage, while the Li/PLS-850/Li cell shows rapidly increasing polarization and eventual failure. The LFP/HP30-800/Li full cell retains 85.35% of its initial capacity after 250 cycles, whereas the LFP/PLS-850/Li cell retains only 4.45%. The rate performance is also significantly better for the hot-pressed cell. These results demonstrate that the hot-pressed LAGP electrolyte is a promising candidate for the solid state cell in new energy aircraft, where high safety, high energy density, and long cycle life are required.
From a broader perspective, the hot-pressing technique is scalable and compatible with industrial ceramic processing. The use of a graphite die and carbon paper is cost-effective, and the sintering time is short compared with conventional pressureless sintering. The ability to achieve high density at a lower temperature also reduces energy consumption and minimizes lithium volatilization. These advantages make hot-pressing an attractive route for the manufacturing of large-area solid electrolytes for aviation solid state cell modules.
12. Conclusion
I have demonstrated that hot-pressing sintering is an effective strategy for preparing high-performance LAGP solid electrolytes for new energy aircraft power batteries. By applying a constant uniaxial pressure during sintering, I increased the sintering driving force and promoted densification mechanisms such as particle rearrangement, plastic flow, and diffusion creep. The optimized hot-pressing conditions are 800 °C and 30 MPa, which yield a relative density of 98.87%, an ionic conductivity of 5.56 × 10−4 S cm−1, an activation energy of 0.31 eV, a Vickers hardness of 10.01 GPa, and a flexural strength of 179.57 MPa. The hot-pressed LAGP electrolyte enables stable cycling of symmetric and full solid state cells, with the full cell retaining 85.35% of its initial capacity after 250 cycles. These results provide a strong technical foundation for the development of advanced solid electrolytes and solid state cell technologies for electric aviation.
| Parameter | Value |
|---|---|
| Optimal sintering temperature | 800 °C |
| Optimal applied pressure | 30 MPa |
| Relative density | 98.87% |
| Ionic conductivity at room temperature | 5.56 × 10−4 S cm−1 |
| Activation energy | 0.31 eV |
| Vickers hardness | 10.01 GPa |
| Flexural strength | 179.57 MPa |
| Symmetric cell polarization (initial) | 0.2 V |
| Symmetric cell impedance after 400 h | 750 Ω |
| Full cell capacity retention after 250 cycles | 85.35% |
| Full cell capacity at 1C | 110.65 mAh g−1 |
In summary, the hot-pressing sintering technology developed in this work provides a reliable and scalable method for producing dense, highly conductive, and mechanically robust LAGP solid electrolytes. The resulting solid state cell exhibits excellent cycling stability and rate performance, making it a promising power source for next-generation new energy aircraft. I believe that further optimization of the sintering parameters, interface engineering, and cell design will accelerate the commercialization of solid state cell technology in aviation and other high-performance applications.