I have focused my investigation on the urgent need for high-performance power sources in the rapidly evolving field of electric aviation. The transition toward zero-carbon flight, electric vertical take-off and landing aircraft, and hybrid-electric regional airplanes depends on batteries that can deliver high energy density, high safety, long cycle life, and reliable operation under demanding conditions. Among the many candidate systems, the solid electrolyte cell has emerged as one of the most promising pathways because it can pair a solid electrolyte with a lithium-metal anode, thereby increasing energy density while reducing flammability and thermal-runaway risk. In this work, I prepared Li1.5Al0.5Ge1.5(PO4)3, commonly abbreviated as LAGP, as a NASICON-type solid electrolyte for advanced aviation power batteries. I compared conventional pressureless sintering with hot-pressing sintering and systematically studied how sintering temperature and applied pressure influence microstructure, relative density, ionic conductivity, activation energy, mechanical strength, and electrochemical performance in a solid electrolyte cell.
The central motivation for my study is that a solid electrolyte cell cannot reach its theoretical promise unless the solid electrolyte itself is dense, chemically pure, mechanically robust, and highly conductive. LAGP offers high ionic conductivity, good thermal stability, excellent chemical stability, and intrinsic non-flammability, which are attractive for aerospace applications. However, pressureless sintering often leaves residual pores, weak grain boundaries, and low room-temperature ionic conductivity. Hot-pressing sintering introduces an external pressure during densification, which increases the sintering driving force, promotes particle rearrangement, plastic flow, and diffusion creep, suppresses abnormal grain growth, and reduces gaseous defects. My objective was therefore to establish a practical pressure-assisted sintering route for LAGP and to quantify its benefits for a solid electrolyte cell intended for new energy aircraft.
Materials and Experimental Design
I used commercial high-purity powders as starting materials. The nominal composition of the target phase was Li1.5Al0.5Ge1.5(PO4)3. To compensate for lithium volatilization at elevated temperature, I added 10 wt% excess lithium carbonate. The precursors were mixed with ethanol and ball-milled, dried, pre-calcined, re-calcined, re-milled, and sieved to obtain a homogeneous LAGP powder. The detailed roles and nominal characteristics of the raw materials are summarized in Table 1.
| Material | Chemical formula | Role in synthesis | Nominal purity |
|---|---|---|---|
| Lithium carbonate | Li2CO3 | Lithium source; 10 wt% excess to compensate volatilization | 99% |
| Aluminum oxide | Al2O3 | Aluminum source for LAGP framework | 99% |
| Germanium oxide | GeO2 | Germanium source for NASICON framework | 99% |
| Ammonium dihydrogen phosphate | NH4H2PO4 | Phosphate source | 99% |
| N-methyl-2-pyrrolidone | C5H9NO | Solvent for cathode slurry | 99% |
| Polyvinylidene fluoride | (C2H2F2)n | Binder for cathode | 99% |
| Lithium iron phosphate | LiFePO4 | Cathode active material | Battery grade |
| Lithium metal | Li | Anode for symmetric and full solid electrolyte cell tests | Battery grade |
The LAGP powder preparation followed a solid-state route. I mixed the precursors in stoichiometric proportions with 10 wt% excess Li2CO3, added ethanol, and ball-milled at 350 r/min for 12 h. The mixture was dried at 60 °C overnight. The dried powder was transferred to an alumina crucible and pre-calcined at 700 °C for 4 h with a heating rate of 5 °C/min to release CO2, NH3, and H2O. The powder was then calcined at 800 °C for 6 h, ball-milled again for 6 h, and sieved through a 500-mesh screen. The final LAGP powder was stored in a dry box before sintering.
For pressureless sintering, I pressed 0.5 g of LAGP powder at 13 MPa for 5 min into disks with a diameter of 15 mm. The green disks were placed in alumina crucibles and sintered at 750 °C, 800 °C, 850 °C, or 900 °C for 5 h. The resulting samples were polished, cleaned, and coated with silver paste for ionic conductivity measurements. I labeled these pressureless-sintered samples PLS-750, PLS-800, PLS-850, and PLS-900.
For hot-pressing sintering, I loaded LAGP powder into a cylindrical graphite die with a diameter of 15 mm. A 0.1 mm thick carbon paper wrapped the pre-calcined powder to prevent adhesion and overflow. The hot-pressing schedule is summarized in Table 2. I varied the target temperature from 750 °C to 900 °C and the applied pressure from 20 MPa to 35 MPa. The initial pressure during heating to 400 °C was 17 MPa with a heating rate of 7 °C/min. From 400 °C to the target temperature, the heating rate was 8 °C/min. After reaching the target temperature, I adjusted the pressure to the desired value and held for 1 h, followed by natural cooling. After demolding, the cylindrical LAGP body was cut into disks with a diameter of 15 mm and a thickness of 1 mm. The hot-pressed samples were named HP-750, HP-800, HP-850, and HP-900 for the temperature study, and HP20-800, HP25-800, HP30-800, and HP35-800 for the pressure study.
| Sample group | Sintering route | Temperature (°C) | Pressure (MPa) | Dwell time (h) |
|---|---|---|---|---|
| PLS-750 | Pressureless | 750 | 0 | 5 |
| PLS-800 | Pressureless | 800 | 0 | 5 |
| PLS-850 | Pressureless | 850 | 0 | 5 |
| PLS-900 | Pressureless | 900 | 0 | 5 |
| HP20-750 | Hot pressing | 750 | 20 | 1 |
| HP20-800 | Hot pressing | 800 | 20 | 1 |
| HP20-850 | Hot pressing | 850 | 20 | 1 |
| HP20-900 | Hot pressing | 900 | 20 | 1 |
| HP25-800 | Hot pressing | 800 | 25 | 1 |
| HP30-800 | Hot pressing | 800 | 30 | 1 |
| HP35-800 | Hot pressing | 800 | 35 | 1 |
For cathode preparation, I mixed LiFePO4, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1. The LiFePO4 and carbon black were first ground for 30 min to ensure uniform dispersion. The mixture was then added to an N-methyl-2-pyrrolidone solution containing polyvinylidene fluoride and magnetically stirred for 12 h. The resulting slurry was coated onto aluminum foil with a 250 µm doctor blade, dried at 60 °C under vacuum for 12 h, and punched into 12 mm diameter disks. The electrodes were stored in an argon-filled glovebox to avoid moisture and oxidation.
I assembled symmetric solid electrolyte cells with the configuration Li/LAGP/Li and full solid electrolyte cells with the configuration LiFePO4/LAGP/Li. The assembly was performed entirely inside an argon-filled glovebox. For the symmetric cell, the stacking order was negative shell, spring, stainless steel spacer, lithium disk, LAGP electrolyte, lithium disk, and positive shell. For the full cell, the stacking order was negative shell, spring, stainless steel spacer, lithium anode, LAGP electrolyte, LiFePO4 cathode, and positive shell. The solid electrolyte cell architecture is illustrated schematically below.
Structural and Morphological Characterization of the LAGP Powder
I first examined the phase and morphology of the synthesized LAGP powder. Scanning electron microscopy showed that the powder consisted of uniformly distributed grains without obvious agglomeration. Transmission electron microscopy and energy-dispersive spectroscopy confirmed that the particles were in the micrometer range and that the main elements were Al, Ge, O, and P, with no detectable impurity elements. The powder X-ray diffraction pattern matched the standard NASICON-type LiGe2(PO4)3 reference pattern. I observed characteristic diffraction peaks at 2θ values of approximately 15.1°, 21.3°, 25.2°, 30.4°, 34.0°, 37.7°, 48.9°, 51.6°, 56.2°, and 59.1°. These peaks correspond to the (012), (104), (113), (024), (116), (300), (128), (226), (2110), and (410) planes, respectively. The sharp peak shapes and high intensities indicated good crystallinity, and the absence of impurity peaks confirmed the high phase purity of the LAGP powder.
The NASICON framework provides three-dimensional lithium-ion migration pathways. In an ideal LAGP solid electrolyte cell, lithium ions move through the interconnected interstitial sites of the LiGe2(PO4)3 structure. Aluminum substitution for germanium introduces additional lithium ions to maintain charge neutrality, which can increase carrier concentration and ionic conductivity. However, the final performance depends strongly on sintering because grain boundaries, pores, and secondary phases can block ion transport. This is why I placed particular emphasis on comparing pressureless sintering and hot-pressing sintering.
Comparison of Pressureless and Hot-Pressed Microstructures
I sintered LAGP pellets at 750 °C, 800 °C, 850 °C, and 900 °C by both pressureless sintering and hot pressing at 20 MPa. In the pressureless-sintered samples, the grain size increased with increasing temperature. At 750 °C and 800 °C, the grains were irregular and clearly separated by voids, which indicated incomplete densification and relatively low relative density. At 850 °C, the grains grew further, but residual porosity remained. At 900 °C, the pressureless samples developed impurity phases and possible localized defects.
In contrast, the hot-pressed samples showed much tighter grain-to-grain contact and significantly reduced porosity at the same nominal temperatures. At 750 °C and 800 °C, the hot-pressed pellets already exhibited a dense microstructure. At 850 °C, grain growth continued, and the microstructure became more compact. At 900 °C, however, hot-pressed LAGP developed thermal cracks. These cracks can originate from interfacial stress concentration, volume changes associated with phase transformation, or volatilization of components. Such defects are detrimental to mechanical strength and long-term stability in a solid electrolyte cell.
The relative density of a sintered pellet is defined as:
$$ \rho_{\text{rel}} = \frac{\rho_{\text{measured}}}{\rho_{\text{theoretical}}} \times 100\% $$
where \(\rho_{\text{measured}}\) is the Archimedes density and \(\rho_{\text{theoretical}}\) is the theoretical density of LAGP. The ionic conductivity was calculated from electrochemical impedance spectroscopy using:
$$ \sigma = \frac{L}{R_{\text{b}} A} $$
where \(L\) is the pellet thickness, \(R_{\text{b}}\) is the bulk resistance obtained from the high-frequency intercept or semicircle, and \(A\) is the electrode area. The activation energy was extracted from the Arrhenius relationship:
$$ \sigma T = \sigma_{0} \exp\left(-\frac{E_{\text{a}}}{k_{\text{B}} T}\right) $$
or equivalently:
$$ \ln \sigma = \ln \sigma_{0} – \frac{E_{\text{a}}}{k_{\text{B}} T} $$
where \(\sigma_{0}\) is the pre-exponential factor, \(E_{\text{a}}\) is the activation energy, \(k_{\text{B}}\) is the Boltzmann constant, and \(T\) is the absolute temperature. A lower activation energy indicates easier lithium-ion migration and is highly desirable for a solid electrolyte cell operating over a wide temperature range.
Phase Evolution and Raman Analysis
I collected X-ray diffraction patterns for both pressureless and hot-pressed LAGP pellets sintered from 750 °C to 900 °C. All main diffraction peaks matched the LiGe2(PO4)3 reference pattern, confirming that the NASICON-type LGP phase formed successfully. At 900 °C, however, both pressureless and hot-pressed samples showed additional peaks corresponding to GeO2 and AlPO4. These secondary phases are undesirable because they can reduce ionic conductivity and introduce heterogeneity. The appearance of AlPO4 is particularly harmful because it is electrically insulating and can block lithium-ion transport across grain boundaries in a solid electrolyte cell.
Raman spectroscopy provided further insight into the local structure. The hot-pressed LAGP samples displayed typical NASICON-type vibrational features. The dominant modes arose from PO4 tetrahedral units. A sharp and intense peak near 458 cm−1 was assigned to the \(\nu_{2}\) bending mode of PO43−, confirming a high degree of local order within the phosphate framework. The Raman results agreed with the XRD analysis and supported the conclusion that hot pressing at moderate temperatures preserves the desired LAGP phase.
Effect of Hot-Pressing Temperature on Density, Conductivity, and Activation Energy
I measured the room-temperature AC impedance of hot-pressed LAGP pellets sintered at different temperatures. Each impedance spectrum consisted of a semicircle in the high-frequency region and an inclined line in the low-frequency region. The semicircle is associated with grain-boundary resistance, while the inclined line reflects diffusion processes. The total ionic conductivity was extracted from the bulk and grain-boundary contributions.
As the hot-pressing temperature increased from 750 °C to 900 °C, both relative density and ionic conductivity first increased and then decreased. The maximum values were obtained at 800 °C. The sample HP20-800 achieved a relative density of 97.34% and an ionic conductivity of \(3.34 \times 10^{-4}\) S/cm. When the temperature was increased to 850 °C and 900 °C, the pellets retained high density, but the ionic conductivity decreased significantly. I attribute this decrease to the formation of non-conductive impurity phases such as AlPO4, which increase grain-boundary resistance and impede lithium-ion transport in the solid electrolyte cell.
| Temperature study sample | Relative density (%) | Ionic conductivity (S/cm) | Activation energy (eV) | Observed phase |
|---|---|---|---|---|
| HP20-750 | Below optimum | Lower than \(3.34 \times 10^{-4}\) | 0.36 | NASICON LGP |
| HP20-800 | 97.34 | \(3.34 \times 10^{-4}\) | 0.33 | NASICON LGP |
| HP20-850 | High | Decreased | 0.34 | NASICON LGP |
| HP20-900 | High but cracked | Decreased further | 0.35 | LGP + GeO2 + AlPO4 |
The activation energy values followed the same trend. HP20-750 had an activation energy of 0.36 eV, HP20-800 had 0.33 eV, HP20-850 had 0.34 eV, and HP20-900 had 0.35 eV. The minimum activation energy at 800 °C indicates that this temperature provides the best balance between densification, grain-boundary quality, and phase purity. A lower activation energy means that lithium ions require less thermal energy to migrate through the solid electrolyte cell, which is beneficial for high-power and low-temperature operation.
Mechanical Properties of Hot-Pressed LAGP
Mechanical integrity is essential for a solid electrolyte cell because the electrolyte must withstand stacking pressure, thermal cycling, and mechanical vibration during aircraft operation. I measured Vickers hardness and flexural strength. Vickers hardness is given by:
$$ HV = \frac{1.854 F}{d^{2}} $$
where \(F\) is the applied load and \(d\) is the mean diagonal length of the indentation. The flexural strength was calculated from three-point bending using:
$$ \sigma_{\text{f}} = \frac{3FL}{2bh^{2}} $$
where \(F\) is the fracture load, \(L\) is the support span, \(b\) is the specimen width, and \(h\) is the specimen thickness. At the optimal hot-pressing temperature of 800 °C, the HP-800 electrolyte exhibited a Vickers hardness of 9.27 GPa and a flexural strength of 169.45 MPa. These values indicate that hot pressing at 800 °C produces a dense, mechanically robust solid electrolyte cell component. The high density and strong grain boundaries help suppress crack initiation and propagation, which is crucial for long-term reliability in aviation power systems.
Effect of Applied Pressure During Hot Pressing
After identifying 800 °C as the best hot-pressing temperature, I studied the effect of applied pressure on LAGP densification and electrochemical performance. I prepared samples at 20 MPa, 25 MPa, 30 MPa, and 35 MPa while keeping the temperature at 800 °C. Scanning electron microscopy showed that increasing pressure promoted grain growth and grain-to-grain contact. At 20 MPa, the microstructure was already dense compared with pressureless sintering. As pressure increased to 30 MPa and 35 MPa, the grain boundaries became less distinct, and pores became almost invisible. Elemental mapping confirmed uniform distribution of O, Ge, Al, and P in the HP30-800 sample.
The relative density and ionic conductivity results are summarized in Table 4. As the pressure increased from 20 MPa to 35 MPa, the relative density increased from 97.34% to 98.98%. The ionic conductivity increased from \(3.34 \times 10^{-4}\) S/cm to \(5.68 \times 10^{-4}\) S/cm. The improvement can be explained by two main mechanisms. First, external pressure promotes particle rearrangement and plastic deformation, which reduces void space and accelerates densification. Second, pressure enhances mass transport through grain-boundary and volume diffusion, allowing material to fill residual pores. A denser microstructure provides more continuous lithium-ion pathways and reduces grain-boundary resistance, which directly benefits the solid electrolyte cell.
| Pressure study sample | Pressure (MPa) | Relative density (%) | Ionic conductivity (S/cm) | Activation energy (eV) |
|---|---|---|---|---|
| HP20-800 | 20 | 97.34 | \(3.34 \times 10^{-4}\) | 0.33 |
| HP25-800 | 25 | 98.15 | \(4.17 \times 10^{-4}\) | 0.32 |
| HP30-800 | 30 | 98.87 | \(5.56 \times 10^{-4}\) | 0.31 |
| HP35-800 | 35 | 98.98 | \(5.68 \times 10^{-4}\) | 0.31 |
The activation energy decreased as the applied pressure increased. HP20-800, HP25-800, HP30-800, and HP35-800 had activation energies of 0.33 eV, 0.32 eV, 0.31 eV, and 0.31 eV, respectively. The lower activation energy at higher pressure is consistent with better grain-to-grain contact, lower porosity, and fewer blocking interfaces. In a solid electrolyte cell, low activation energy means that ionic conductivity remains high over a broader temperature range, which is especially important for aircraft that experience large temperature variations during climb, cruise, and descent.
I also measured the mechanical properties as a function of hot-pressing pressure. The results are summarized in Table 5. As pressure increased, both Vickers hardness and flexural strength improved. HP20-800 had a hardness of 9.27 GPa and a flexural strength of 169.45 MPa. HP25-800 increased to 9.67 GPa and 173.45 MPa. HP30-800 reached 10.01 GPa and 179.57 MPa. HP35-800 showed only a marginal further improvement to 10.04 GPa and 179.58 MPa. Therefore, 30 MPa appears to be the optimal pressure because it delivers nearly the maximum density, conductivity, and mechanical strength without the additional processing cost and potential risk associated with higher pressure.
| Pressure study sample | Vickers hardness (GPa) | Flexural strength (MPa) | Relative density (%) | Ionic conductivity (S/cm) |
|---|---|---|---|---|
| HP20-800 | 9.27 | 169.45 | 97.34 | \(3.34 \times 10^{-4}\) |
| HP25-800 | 9.67 | 173.45 | 98.15 | \(4.17 \times 10^{-4}\) |
| HP30-800 | 10.01 | 179.57 | 98.87 | \(5.56 \times 10^{-4}\) |
| HP35-800 | 10.04 | 179.58 | 98.98 | \(5.68 \times 10^{-4}\) |
Symmetric Solid Electrolyte Cell Cycling and Impedance
To evaluate the practical benefit of hot-pressed LAGP, I assembled symmetric solid electrolyte cells using either pressureless-sintered PLS-850 or hot-pressed HP30-800. The Li/PLS-850/Li and Li/HP30-800/Li cells were cycled at a current density of 0.1 mA/cm2 at room temperature. The initial polarization voltage of the Li/PLS-850/Li cell was approximately 0.5 V. In contrast, the Li/HP30-800/Li cell exhibited an initial polarization voltage of only about 0.2 V. As cycling proceeded, the polarization voltage of the Li/PLS-850/Li cell increased markedly. After 300 h, the polarization voltage rose sharply, and after 500 h, the cell almost completely failed. The Li/HP30-800/Li cell, however, maintained an almost constant polarization voltage, indicating much better interfacial stability and more stable lithium plating and stripping.
I further performed electrochemical impedance spectroscopy before and after cycling. Before cycling, the Li/PLS-850/Li cell had an initial total impedance of 1113 Ω. After 300 h, the impedance increased to 1675 Ω; after 350 h, it rose to 2508 Ω; and after 400 h, it increased dramatically to 5017 Ω. This large impedance rise correlates with the polarization increase and indicates degradation of the electrolyte-electrode interface and the solid electrolyte itself, eventually leading to short circuit. In contrast, the Li/HP30-800/Li cell had an initial total impedance of 547 Ω. After 300 h, the impedance increased only to 597 Ω; after 350 h, it was 696 Ω; and after 400 h, it was only 750 Ω. The much smaller impedance increase demonstrates that the hot-pressed LAGP solid electrolyte cell maintains better interfacial stability and ionic conduction during long-term cycling.
| Symmetric cell | Initial polarization (V) | Initial impedance (Ω) | Impedance after 300 h (Ω) | Impedance after 400 h (Ω) | Failure behavior |
|---|---|---|---|---|---|
| Li/PLS-850/Li | ~0.5 | 1113 | 1675 | 5017 | Sharp polarization rise; nearly failed after 500 h |
| Li/HP30-800/Li | ~0.2 | 547 | 597 | 750 | Stable polarization; no obvious failure |
Full Solid Electrolyte Cell Performance
I also assembled full solid electrolyte cells with LiFePO4 cathode and lithium-metal anode, using either PLS-850 or HP30-800 LAGP as the electrolyte. The cells were cycled at 0.1C. The initial discharge capacities were 161.9 mAh/g for LFP/PLS-850/Li and 162.6 mAh/g for LFP/HP30-800/Li. After 250 cycles, the LFP/PLS-850/Li cell faded rapidly, retaining only 4.45% of its initial capacity. In contrast, the LFP/HP30-800/Li cell retained 85.35% of its initial capacity after 250 cycles. This result shows that the hot-pressed HP30-800 solid electrolyte cell has much better interfacial stability and lower interfacial resistance, which effectively slows capacity decay and extends cycle life.
The capacity retention can be expressed as:
$$ R_{\text{retention}} = \frac{C_{n}}{C_{1}} \times 100\% $$
where \(C_{n}\) is the discharge capacity after \(n\) cycles and \(C_{1}\) is the initial discharge capacity. The rate performance was also superior for the hot-pressed electrolyte. At 1C, the LFP/PLS-850/Li cell delivered only 54.37 mAh/g, and when the rate returned to 0.1C, it recovered to 120.11 mAh/g. The LFP/HP30-800/Li cell maintained 110.65 mAh/g at 1C and recovered to 149.78 mAh/g at 0.1C. This demonstrates better rate capability and reversibility, which are important for electric aircraft that require high power during take-off and climb.
| Full cell | Initial capacity (mAh/g) | Capacity after 250 cycles (mAh/g) | Capacity retention (%) | Capacity at 1C (mAh/g) | Recovered capacity at 0.1C (mAh/g) |
|---|---|---|---|---|---|
| LFP/PLS-850/Li | 161.9 | ~7.2 | 4.45 | 54.37 | 120.11 |
| LFP/HP30-800/Li | 162.6 | ~138.8 | 85.35 | 110.65 | 149.78 |
The voltage-capacity curves further confirmed the advantage of the hot-pressed electrolyte. For the LFP/PLS-850/Li cell, the discharge capacity decreased from 150.3 mAh/g to 139.4 mAh/g after 50 cycles, and the polarization voltage increased significantly. After 250 cycles, the polarization became even larger. In contrast, the LFP/HP30-800/Li cell exhibited a discharge capacity decrease from 159.8 mAh/g to 156.7 mAh/g over the first 50 cycles. After 250 cycles, the discharge capacity was 141.6 mAh/g, and the discharge plateau remained stable at approximately 3.25 V without obvious polarization. This stable voltage plateau indicates that the hot-pressed LAGP solid electrolyte cell maintains a stable interface and low internal resistance during extended cycling.
Discussion of Sintering Mechanisms
The superior performance of hot-pressed LAGP can be understood through several densification mechanisms. During conventional pressureless sintering, densification is driven mainly by surface energy reduction and capillary forces. Grain boundaries and pores often persist because the driving force is insufficient to eliminate closed pores, especially in the later stage of sintering. In hot pressing, an external pressure is superimposed on the thermal driving force. This pressure increases the contact stress between particles, promotes particle rearrangement, and induces plastic flow and diffusion creep. As a result, pores are filled more effectively, and the relative density increases.
The applied pressure also influences grain growth. Moderate pressure can enhance grain-boundary diffusion and promote densification without excessive grain growth. However, excessive temperature or pressure can cause abnormal grain growth, thermal cracking, or phase decomposition. In my experiments, 800 °C and 30 MPa provided the best combination. At this condition, the LAGP solid electrolyte cell achieved high density, high ionic conductivity, low activation energy, and strong mechanical properties. The formation of insulating AlPO4 and GeO2 at 900 °C highlights the importance of controlling the thermal budget. Even though higher temperature can increase density, it can also degrade ionic conductivity by producing secondary phases.
The relationship between relative density and ionic conductivity can be qualitatively described by considering the effective conduction path. In a porous solid electrolyte, pores and weak grain boundaries reduce the effective cross-sectional area for ion transport and increase tortuosity. The effective conductivity can be approximated as:
$$ \sigma_{\text{eff}} = \sigma_{\text{bulk}} \frac{1 – p}{\tau} $$
where \(p\) is the porosity and \(\tau\) is the tortuosity factor. As porosity decreases and grain boundaries become cleaner, \(\sigma_{\text{eff}}\) approaches the bulk conductivity. Hot pressing reduces \(p\) and \(\tau\), which explains the observed increase in ionic conductivity. The activation energy also decreases because fewer blocking interfaces are present.
For a solid electrolyte cell used in aviation, mechanical properties are as important as ionic conductivity. The electrolyte must withstand cell stacking pressure, vibration, and thermal expansion mismatch. The Vickers hardness and flexural strength values obtained for HP30-800 indicate that the hot-pressed LAGP can resist mechanical degradation. This is critical for preventing dendrite penetration and short circuits. The symmetric cell results show that the Li/HP30-800/Li cell maintains low and stable polarization, whereas the Li/PLS-850/Li cell exhibits increasing polarization and impedance. The lower initial impedance of the hot-pressed cell also suggests better interfacial contact between lithium metal and the solid electrolyte.
Implications for New Energy Aircraft Power
The development of high-performance solid electrolyte cells is directly relevant to new energy aircraft power. Electric aircraft require batteries with high specific energy, high specific power, high safety, and long cycle life. A solid electrolyte cell using LAGP can potentially satisfy these requirements because it enables the use of lithium metal, which has a high theoretical capacity. However, the solid electrolyte must be thin, dense, and highly conductive. My results show that hot pressing at 800 °C and 30 MPa produces LAGP with a relative density of 98.87%, an ionic conductivity of \(5.56 \times 10^{-4}\) S/cm, and an activation energy of 0.31 eV. These values are promising for further scale-up and integration into aviation battery modules.
The pressure-assisted sintering route also offers manufacturing advantages. The dwell time is only 1 h, which is much shorter than the 5 h used in pressureless sintering. A shorter high-temperature exposure reduces lithium volatilization and suppresses impurity formation. The process is compatible with graphite dies and can be adapted to larger pellets or stacked cells. Although hot pressing is a batch process, it provides a valuable pathway for producing high-quality solid electrolyte cell components for prototyping and small-scale validation. For larger-scale production, variations such as hot isostatic pressing, spark plasma sintering, or roll-to-roll pressure-assisted sintering could be explored.
I also note that the interface between the solid electrolyte and the electrodes remains a critical challenge. Even with dense LAGP, interfacial resistance can increase during cycling due to void formation, lithium creep, or decomposition reactions. In my full solid electrolyte cell tests, the LFP/HP30-800/Li cell retained 85.35% capacity after 250 cycles, which is encouraging but still leaves room for improvement. Future work should focus on interface engineering, such as applying thin interlayers, optimizing stack pressure, and controlling surface chemistry. These strategies can further reduce interfacial resistance and extend the life of the solid electrolyte cell.
Comparative Summary of Sintering Routes
Table 6 summarizes the key differences between pressureless sintering and hot-pressing sintering for LAGP solid electrolytes. The comparison highlights why hot pressing is advantageous for advanced aviation power batteries.
| Feature | Pressureless sintering | Hot-pressing sintering |
|---|---|---|
| Driving force | Surface energy and capillary forces | Surface energy plus external pressure |
| Densification mechanism | Diffusion-controlled sintering | Particle rearrangement, plastic flow, diffusion creep |
| Typical dwell time | 5 h | 1 h |
| Porosity | Higher, especially at lower temperature | Lower, closed pores largely eliminated |
| Grain growth control | Limited | Better control at moderate temperature |
| Relative density | Lower than hot-pressed counterpart | Up to 98.98% |
| Ionic conductivity | Lower | Up to \(5.68 \times 10^{-4}\) S/cm |
| Mechanical strength | Moderate | High |
| Risk at 900 °C | Impurity phases | Thermal cracks and impurity phases |
| Suitability for solid electrolyte cell | Limited by pores and interfaces | Promising for high-performance cells |
Key Performance Indicators and Optimization
To guide future optimization, I summarize the key performance indicators of the best hot-pressed LAGP solid electrolyte in Table 7. The optimal temperature is 800 °C, and the optimal pressure is 30 MPa when considering the balance between performance and processing cost. At 35 MPa, the relative density and conductivity increase only slightly, while the mechanical properties plateau. Therefore, 30 MPa is the recommended pressure for scalable processing.
| Parameter | Best value | Sample | Significance for solid electrolyte cell |
|---|---|---|---|
| Sintering temperature | 800 °C | HP30-800 | High purity, high density, low activation energy |
| Sintering pressure | 30 MPa | HP30-800 | Optimal density and mechanical strength without excess cost |
| Relative density | 98.87% | HP30-800 | Fewer pores, continuous ion pathways |
| Ionic conductivity | \(5.56 \times 10^{-4}\) S/cm | HP30-800 | High room-temperature conduction |
| Activation energy | 0.31 eV | HP30-800 | Easy lithium-ion migration |
| Vickers hardness | 10.01 GPa | HP30-800 | Resistance to deformation and dendrites |
| Flexural strength | 179.57 MPa | HP30-800 | Mechanical integrity under stack pressure |
| Capacity retention after 250 cycles | 85.35% | LFP/HP30-800/Li | Long cycle life for aircraft power |
Electrochemical Impedance and Interfacial Stability
Impedance spectroscopy is a powerful tool for diagnosing a solid electrolyte cell. The total impedance includes bulk resistance, grain-boundary resistance, and interfacial resistance. In my symmetric cells, the hot-pressed LAGP showed a much lower initial impedance and a much slower impedance rise during cycling. This indicates that the hot-pressed electrolyte has better grain-boundary quality and forms a more stable interface with lithium metal. The pressureless-sintered electrolyte, by contrast, had higher porosity and weaker grain boundaries, which led to uneven current distribution, lithium dendrite formation, and eventual failure.
The interfacial resistance can be modeled as a series combination:
$$ R_{\text{total}} = R_{\text{bulk}} + R_{\text{gb}} + R_{\text{int}} $$
where \(R_{\text{bulk}}\) is the bulk resistance, \(R_{\text{gb}}\) is the grain-boundary resistance, and \(R_{\text{int}}\) is the interfacial resistance. Hot pressing reduces \(R_{\text{gb}}\) by eliminating pores and improving grain-to-grain contact. It also reduces \(R_{\text{int}}\) by creating a smoother and denser surface for lithium contact. The result is a lower total impedance and more stable cycling, as observed in the Li/HP30-800/Li cell.
Rate Capability and Aircraft Power Requirements
Electric aircraft require high power during take-off and climb, and high energy during cruise. The rate capability of the solid electrolyte cell is therefore critical. In my tests, the LFP/HP30-800/Li cell retained 110.65 mAh/g at 1C, while the LFP/PLS-850/Li cell retained only 54.37 mAh/g. The better rate performance of the hot-pressed cell is due to its higher ionic conductivity and lower interfacial resistance. When the rate returned to 0.1C, the hot-pressed cell recovered to 149.78 mAh/g, showing good reversibility. This behavior is important for mission profiles that involve repeated power pulses and regenerative braking or descent charging.
The power capability of a solid electrolyte cell can be approximated by:
$$ P = \frac{V^{2}}{R_{\text{total}}} $$
where \(V\) is the cell voltage and \(R_{\text{total}}\) is the total internal resistance. Lower resistance directly translates to higher power capability. The hot-pressed LAGP cell had a lower total resistance, which explains its superior rate performance. For aviation applications, this means that a solid electrolyte cell with optimized LAGP can deliver more power without excessive voltage drop or heat generation.
Safety and Thermal Stability
Safety is a non-negotiable requirement for aviation batteries. Liquid-electrolyte lithium-ion batteries can catch fire or explode if internally short-circuited or overheated. A solid electrolyte cell replaces the flammable liquid electrolyte with a non-flammable ceramic or solid polymer, which significantly reduces fire risk. LAGP is thermally stable and chemically stable, and it does not produce flammable vapors. However, the solid electrolyte must remain mechanically intact to prevent lithium dendrite penetration. My results show that hot-pressed LAGP has high relative density and high mechanical strength, which helps suppress dendrite growth. The symmetric cell cycling results confirm that the hot-pressed electrolyte can sustain repeated lithium plating and stripping without rapid failure.
Thermal stability is also important because aircraft experience temperature variations from ground level to high altitude. The activation energy of 0.31 eV for HP30-800 indicates that ionic conductivity is relatively stable over a range of temperatures. Lower activation energy means that the cell can operate efficiently at lower temperatures without a dramatic drop in performance. This is a significant advantage for electric aircraft that must operate in cold environments at altitude.
Scalability and Manufacturing Considerations
Hot pressing is a mature industrial process for ceramics, but scaling it to large-format solid electrolyte cell production requires careful engineering. The pressure must be uniform across the pellet to avoid density gradients. The die material must not react with LAGP at high temperature. Graphite dies are convenient for laboratory-scale work, but alternative materials such as boron nitride-coated dies or ceramic dies may be needed for larger formats. The carbon paper used in my experiments prevents adhesion and facilitates demolding. For continuous production, a roll-to-roll hot-pressing process or a belt furnace with applied pressure could be developed.
The short dwell time of 1 h is advantageous for manufacturing because it reduces energy consumption and limits lithium volatilization. The optimal temperature of 800 °C is lower than the 900 °C often used in pressureless sintering, which further reduces energy demand and suppresses impurity formation. The applied pressure of 30 MPa is moderate and can be achieved with commercially available hot presses. These factors make the hot-pressing route attractive for pilot-scale production of LAGP solid electrolytes for new energy aircraft power.
Future Research Directions
Although my results are promising, several challenges remain. First, the interface between LAGP and lithium metal must be further stabilized. Even with dense LAGP, interfacial reactions and void formation can occur during cycling. Interlayers such as thin polymer films, oxide coatings, or alloy layers could improve contact and suppress reactions. Second, the cathode interface must be optimized. LFP is a relatively stable cathode, but higher-voltage cathodes will require compatible interlayers or coatings to prevent side reactions. Third, the mechanical properties of the solid electrolyte cell stack must be evaluated under realistic aircraft conditions, including vibration, thermal cycling, and crush loads. Fourth, the scalability of hot pressing must be demonstrated with larger pellets and multi-layer stacks. Fifth, cost analysis should be performed to compare hot pressing with alternative sintering methods such as spark plasma sintering or hot isostatic pressing.
I also see opportunities to combine hot pressing with other advanced sintering techniques. For example, spark plasma sintering can provide rapid heating and high pressure, which may further reduce grain growth and processing time. Hot isostatic pressing can apply uniform pressure from all directions, which may help eliminate residual porosity in large pellets. Field-assisted sintering can reduce sintering temperature and time. By combining these techniques with optimized LAGP chemistry, it may be possible to achieve even higher ionic conductivity and mechanical strength.
Conclusion
I have demonstrated that hot-pressing sintering is a highly effective method for preparing dense, highly conductive LAGP solid electrolytes for new energy aircraft power. By applying constant pressure during sintering, I increased the sintering driving force, promoted particle rearrangement and diffusion creep, reduced porosity, and suppressed abnormal grain growth and gaseous defects. The optimal hot-pressing temperature was 800 °C, and the optimal pressure was 30 MPa. The HP30-800 sample achieved a relative density of 98.87%, an ionic conductivity of \(5.56 \times 10^{-4}\) S/cm, an activation energy of 0.31 eV, a Vickers hardness of 10.01 GPa, and a flexural strength of 179.57 MPa. In symmetric solid electrolyte cells, Li/HP30-800/Li showed stable polarization and much slower impedance rise than Li/PLS-850/Li. In full solid electrolyte cells, LFP/HP30-800/Li retained 85.35% of its initial capacity after 250 cycles, compared with only 4.45% for LFP/PLS-850/Li. The hot-pressed cell also showed better rate capability and a more stable discharge plateau.
These results indicate that pressure-assisted sintering can significantly improve the microstructure and electrochemical performance of LAGP solid electrolytes. The improved density, conductivity, and mechanical strength are directly beneficial for the solid electrolyte cell because they reduce interfacial resistance, suppress dendrite growth, and enhance long-term cycling stability. For new energy aircraft power, the hot-pressed LAGP solid electrolyte cell offers a promising pathway toward high-safety, high-energy-density, and long-life batteries. I believe that further optimization of interfaces, scalability, and cell design will accelerate the adoption of solid electrolyte cell technology in electric aviation and contribute to the broader goal of zero-carbon flight.