Cu substitution in FePO₄ cathodes: Electronic structure and transport properties from DFT+U calculations

Copper substitution represents an effective strategy for tuning the electronic properties of phosphate-based materials. In this work, the effect of partial Cu substitution in orthorhombic FePO₄ was investigated using spin-polarized DFT+U calculations combined with semiclassical Boltzmann transport theory. Cu incorporation preserves the structural framework while introducing Cu 3d states near the Fermi level, reducing the band gap from 2.2 eV to 1.5 eV. These additional states increase the density of states near the band edges, suggesting enhanced electronic transport relative to pristine FePO₄. The electronic structure exhibits pronounced anisotropy, as revealed by constant-energy surface analysis, indicating direction-dependent band dispersion and transport behavior. BoltzTraP2 calculations performed within the constant relaxation time approximation (CRTA) predict an electrical conductivity ratio (σ/τ) of approximately 2.4 × 10¹⁹ S m⁻¹ s⁻¹ at 300 K, together with a Seebeck coefficient of −36 μV K⁻¹, consistent with n-type conduction. Because the absolute conductivity scales linearly with the relaxation time, transport properties are discussed in terms of relative trends within the CRTA framework. Overall, Cu substitution significantly modifies the electronic structure of FePO₄ by narrowing the band gap and increasing the density of states near the Fermi level, thereby promoting electronic transport. In addition, the elastic response of both compounds was investigated using a stress–strain approach: Cu substitution induces a pronounced, direction-dependent softening of the phosphate framework, reducing the principal elastic constants (C₁₁, C₂₂, C₃₃) by approximately 36–40% relative to pristine FePO₄. These findings provide insight into the role of Cu doping in tailoring the electronic properties of phosphate cathode materials. However, the present study focuses exclusively on electronic transport descriptors and does not address ionic diffusion or electrochemical interfacial processes, which require dedicated atomistic and kinetic investigations.