Quantum Simulations of Battery Electrolytes Using Variational Quantum Eigensolver, Equation-of-Motion, and Sample-Based Diagonalization Methods: Active-Space Design, Dissociation, and Excited States of LiPF6 , NaPF6 , and FSI Salts

Abstract

Accurate prediction of excited states in battery electrolytes is crucial for understanding photostability, oxidative stability, and degradation. We employ hybrid quantum-classical algorithms-the Variational Quantum Eigensolver (VQE) for ground states and the quantum equation of motion (qEOM) for vertical singlet excitations to study LiPF6, NaPF6, LiFSI, and NaFSI. Compact active spaces from frontier orbitals were mapped to qubits and reduced via symmetry tapering and commuting-group measurements to lower sampling cost. Within similar to 10-qubit models, VQE-qEOM agrees closely with exact diagonalization, while sample-based quantum diagonalization (SQD) in larger spaces recovers near-exact (subspace-FCI) energies. Spectra show clear anion-cation trends within the VQE-qEOM framework: PF6 salts have higher first-excitation energies (LiPF6 approximate to 13.2 eV) and a compact threestate cluster at 12-13 eV, whereas FSI salts exhibit lower onsets (approximate to 8-9 eV) with nearly degenerate S-1 and S-2 states and a higher S-3 separated by similar to 1.3 eV. Independent TDDFT calculations yield systematically lower absolute excitation energies but reproduce the same anion- and cation-dependent trends, confirming that the relative ordering and physical interpretation of the quantum results are robust. Replacing Li+ with Na+ narrows the gap by similar to 0.4-0.8 eV per anion family. Converting S1 to wavelengths places onsets in the deep UV (LiPF6 94 nm; NaPF6 100 nm; LiFSI 141 nm; NaFSI 148 nm). Results for isolated species or embedded clusters are NISQ-feasible, with solvent shifts incorporable via classical Delta-solvation. Current quantum algorithms capture excitation trends, advancing electrolyte design.