Quantum Simulations of Battery Electrolytes with VQE-qEOM and SQD: Active-Space Design, Dissociation, and Excited States of LiPF$_6$, NaPF$_6$, and FSI Salts

Abstract

Accurate prediction of excited states in battery electrolytes is central to understanding photostability, oxidative stability, and degradation. We employ hybrid quantum-classical algorithms -- the Variational Quantum Eigensolver (VQE) for ground states combined with the quantum equation of motion (qEOM) for vertical singlet excitations -- to study LiPF$_6$, NaPF$_6$, LiFSI, and NaFSI. Compact active spaces were constructed from frontier orbitals, mapped to qubits, and reduced via symmetry tapering and commuting-group measurements to lower sampling cost. Within $\sim$10-qubit models, VQE-qEOM agrees closely with exact diagonalization of the same Hamiltonians, while sample-based quantum diagonalization (SQD) in larger active spaces recovers near-exact (subspace-FCI) energies. The spectra display clear anion and cation trends: PF$_6$ salts exhibit higher first-excitation energies (e.g., LiPF$_6$ $\approx$13.2 eV) and a compact three-state cluster at 12-13 eV, whereas FSI salts show substantially lower onsets ($\approx$8-9 eV) with a near-degenerate (S$_1$,S$_2$) followed by S$_3$ $\sim$1.3 eV higher. Substituting Li$^+$ with Na$^+$ narrows the gap by $\sim$0.4-0.8 eV within each anion family. Converting S$_1$ to wavelengths places the onsets in the deep-UV (LiPF$_6$ $\sim$94 nm; NaPF$_6$ $\sim$100 nm; LiFSI $\sim$141 nm; NaFSI $\sim$148 nm). All results pertain to isolated species or embedded clusters appropriate to the NISQ regime; solvent shifts can be incorporated a posteriori via classical $\Delta$-solvation or static embedding. These results demonstrate that current quantum algorithms can deliver chemically meaningful excitation and binding trends for realistic electrolyte motifs and provide quantitative baselines to guide electrolyte screening and design.