Solid electrolytes capable of operating at high temperatures above 100°C are critical for applications where thermal stability and safety are non-negotiable. Among the most promising candidates are lithium aluminum titanium phosphate (Li1.3Al0.3Ti1.7(PO4)3, LATP) and doped ceria-based materials, which exhibit ionic conductivity and structural integrity under extreme conditions. These materials must overcome challenges such as thermal expansion mismatch, phase decomposition, and interfacial degradation to be viable for demanding environments like aerospace propulsion systems or grid-scale thermal energy storage.

LATP belongs to the NASICON-type solid electrolyte family, known for its three-dimensional lithium-ion conduction pathways. At room temperature, its ionic conductivity ranges between 10^-4 and 10^-3 S/cm, but this improves at elevated temperatures due to enhanced lithium-ion mobility. However, above 300°C, LATP faces partial decomposition, leading to the formation of secondary phases like AlPO4 and TiO2, which degrade long-term performance. Doping strategies, such as partial substitution of Ti with Ge or Zr, have been shown to improve phase stability up to 500°C while maintaining conductivity above 10^-2 S/cm. The thermal expansion coefficient of LATP is approximately 10 x 10^-6 K^-1, which creates challenges when interfaced with electrodes having dissimilar expansion properties, such as lithium cobalt oxide (14 x 10^-6 K^-1).

Doped ceria electrolytes, particularly gadolinium-doped ceria (GDC, Ce0.9Gd0.1O1.95), exhibit oxygen ion conductivity dominant above 400°C, making them suitable for dual-ion conduction systems. GDC demonstrates a conductivity of 0.1 S/cm at 800°C with negligible electronic leakage, a critical feature for preventing self-discharge in high-temperature batteries. The thermal expansion coefficient of GDC is around 12 x 10^-6 K^-1, closely matching common cathode materials like lanthanum strontium cobalt ferrite (LSCF). However, under reducing atmospheres at high temperatures, partial reduction of Ce4+ to Ce3+ occurs, leading to electronic conductivity and volumetric instability. Co-doping with praseodymium or samarium has been demonstrated to suppress this reduction while maintaining ionic transport properties.

Interfacial reactions between solid electrolytes and electrodes accelerate at high temperatures. For LATP, titanium reduction at the anode interface forms a resistive layer, increasing interfacial impedance by up to 50% after 100 thermal cycles between 25°C and 150°C. Applying buffer layers such as lithium phosphorus oxynitride (LiPON) or lithium borohydride reduces this degradation, with reported interfacial resistance stabilization below 20 Ω cm^2 after cycling. In doped ceria systems, interdiffusion of cations like Gd3+ into adjacent electrodes occurs above 600°C, requiring diffusion barriers such as yttria-stabilized zirconia (YSZ) layers of at least 2 μm thickness to prevent performance decay.

Thermal cycling performance is a key metric for aerospace applications, where batteries may experience rapid transitions between -50°C and 200°C during high-altitude missions. LATP-based cells subjected to 500 cycles under these conditions retain 85% of initial capacity when paired with thermally stable cathodes like lithium iron phosphate (LFP). In contrast, doped ceria electrolytes show better retention above 400°C but require preheating systems below their activation temperature, adding system complexity. For grid storage collocated with concentrated solar power plants, where operating temperatures reach 150-200°C daily, LATP-graphite composite electrolytes demonstrate stable operation over 5 years with capacity fade rates below 0.5% per year.

Manufacturing these electrolytes for high-temperature use requires specialized processing. Spark plasma sintering (SPS) of LATP at 900°C produces densities exceeding 95% with grain boundary conductivity optimized by controlled cooling rates. Tape casting of doped ceria achieves thin films below 50 μm thickness necessary for low ohmic losses, with co-firing temperatures carefully balanced below 1300°C to prevent lithium volatilization from adjacent layers. For large-scale grid storage applications, roll-to-roll manufacturing of LATP-polymer composites has achieved 1 m^2/min production rates with less than 5% thickness variation.

Safety testing under thermal abuse conditions reveals distinct advantages of these materials. LATP cells subjected to external heating at 10°C/min up to 300°C show no thermal runaway, with heat generation rates below 1 W/Ah. Doped ceria systems exhibit similar stability up to 800°C but require hermetic sealing to prevent oxygen exchange with the environment during cooling. In nail penetration tests at 150°C, LATP-based cells maintain open-circuit voltage within 5% of initial values, whereas conventional liquid electrolytes fail catastrophically under identical conditions.

System integration challenges include current collector selection and stack design. For LATP, aluminum current collectors are unsuitable above 150°C due to alloying with lithium, necessitating nickel or stainless steel alternatives. Doped ceria systems require precious metal electrodes like platinum or gold above 600°C to minimize polarization losses. In stack designs for aerospace applications, compressive loading of at least 1 MPa is maintained to counteract differential thermal expansion during cycling, with compliant mica-based gaskets providing gas-tight seals.

Emerging developments include nanocomposite approaches combining LATP with boron nitride nanotubes, showing a 40% reduction in thermal expansion mismatch while doubling fracture toughness. For doped ceria, infiltration of ionic liquids into nanopores extends operational range down to 200°C while maintaining high-temperature stability. These hybrid systems are being evaluated for supersonic aircraft auxiliary power units requiring 10,000 cycles between -40°C and 250°C.

Performance metrics under realistic conditions demonstrate progress toward commercialization. Prototype LATP-based 18650 cells for geothermal well monitoring operate continuously at 175°C with energy densities of 180 Wh/kg, outperforming molten salt batteries by 30% in specific energy. Doped ceria micro-tubular cells for spacecraft thermal management deliver 50 mW/cm^2 at 700°C with zero degradation over 1,000 hours in vacuum conditions.

Material supply considerations favor LATP due to abundant aluminum and titanium reserves, with projected costs below $20/kg at scale. Doped ceria relies on rare earth elements like gadolinium, creating supply chain vulnerabilities unless recycling rates exceed 90%. Both systems benefit from absence of flammable components, eliminating fire suppression requirements in energy storage installations.

Standardization efforts are underway for testing protocols specific to high-temperature solid electrolytes, including revised ionic conductivity measurement techniques accounting for thermal gradient effects and accelerated aging tests with controlled atmosphere cycling. These developments will enable direct comparison between emerging materials and established systems.

Continued research focuses on interfacial engineering to extend operational lifetimes beyond 10,000 cycles at extreme temperatures, with atomic layer deposition of interfacial layers showing particular promise. Parallel efforts optimize manufacturing scalability to meet growing demand from renewable energy integration and electrified aviation sectors requiring reliable energy storage under thermally challenging conditions.