Mechanical stability and thermodynamic properties of GeP and GeP₃ as battery anode materials from first principles

Abstract The demand for high-capacity anode materials beyond conventional graphite has intensified research into alternative candidates for next-generation lithium-ion and sodium-ion batteries. Germanium phosphides emerge as promising materials, combining germanium’s high theoretical capacity with phosphorus’s structural versatility and potential for improved cycling stability. We employ first-principles density functional theory calculations to systematically investigate the mechanical, electronic, and thermodynamic properties of three GeP polymorphs (monoclinic, tetragonal, cubic) and rhombohedral GeP₃ as potential anode materials. Our comprehensive analysis reveals that polymorphism critically influences anode performance through distinct mechanical and electronic characteristics. GeP-cubic exhibits mechanical instability, rendering it unsuitable for practical applications. GeP-tetragonal shows the highest stiffness (bulk modulus 79. 4 GPa, Young’s modulus 170. 7 GPa) but pronounced brittleness (Pugh’s ratio K/G = 1. 06), potentially limiting cycling durability. GeP-monoclinic offers greater mechanical compliance (bulk modulus 32. 1 GPa) but suffers from extreme elastic anisotropy (universal anisotropy index A U = 7. 90), which may lead to non-uniform stress distribution and structural degradation during cycling. In contrast, GeP₃ demonstrates an optimal balance of properties with intermediate mechanical stiffness (bulk modulus 61. 0 GPa, Young’s modulus 121. 6 GPa), low elastic anisotropy (A U = 0. 77). Electronic structure calculations reveal metallic conductivity for GeP-tetragonal, GeP-cubic, and GeP₃, ensuring efficient charge transport during battery operation. These findings establish GeP₃ as the most promising candidate among the studied materials, offering balanced mechanical resilience, thermal robustness, and isotropic properties essential for stable long-term cycling performance in practical battery applications.