New Insights into how Cathodes Store Energy

Led by Dr. Galo J. Páez Fajardo and Professor Louis F. J. Piper, the paper addresses a fundamental question in battery science: how exactly do lithium-ion battery cathodes store and release energy at the sub-nanoscale?.

Páez Fajardo, G.J., Dogaru, D.E., Banerjee, H. et al. Direct evidence of metal–ligand redox processes in positive electrodes during lithium-based battery operation. Nat. Nanotechnol. (2026). https://doi.org/10.1038/s41565-026-02189-y

Describing lithium-based battery positive electrodes based on different transition metal or oxygen-redox regimes can cause confusion in understanding metal–ligand hybridization, oxygen dimerization and degradation processes. 

Overview and Objective

As the automotive industry pushes toward cell-level energy density targets exceeding 400 Wh/kg, traditional nickel-rich cathodes are hitting their limits. The classical understanding of battery operation relies on a simple ionic model where electrons are exclusively extracted from metal ions (such as nickel or iron) during charging, while oxygen ions act merely as passive bystanders. However, in highly covalent, next-generation materials like nickel-rich and lithium-excess cathodes, this conventional model fails to explain the excess capacity observed, leading to conflicting theories. Most notable is the debate over whether oxygen undergoes dimerisation (forming O₂ bonds) to store charge. This paper seeks to definitively resolve that debate.

Methodology

To reconcile these conflicting models, the researchers employed a methodology combining advanced experimental and theoretical techniques:

Experimental: They utilised cutting-edge X-ray techniques, specifically X-ray resonance photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS), to directly resolve the character of the redox-active orbitals during electrochemical cycling.

Theoretical: The experimental data was correlated with Single-Impurity Anderson (SIA) models, Dynamical Mean-Field Theory (DMFT), and the Zaanen–Sawatzky–Allen (ZSA) electronic framework, an established scheme for classifying charge-transfer behaviour in transition metal compounds.

Materials: They compared two contrasting positive electrode materials: LiMn₀.₆Fe₀.₄PO₄ (LMFP64) and LiNiO₂ (LNO), tested in non-aqueous coin cells under realistic operating conditions.

Key Findings

The study establishes that the degree of metal–oxygen hybridisation determines the charge compensation mechanism, demonstrating two distinct redox pathways:

1. The Classical Metal-Centred Redox (LMFP64): For the phosphate-based LMFP64, the researchers found a lack of strong metal-ligand hybridisation. The material behaves as the classical ionic model predicts, categorised as a Mott-Hubbard material with positive charge-transfer in the ZSA scheme. During charging, charge storage is driven entirely by the depopulation of electrons from the metal’s 3d states (specifically the Fe²⁺/Fe³⁺ transition), with oxygen showing little to no participation.
2. The Ligand-Hole Redox Mechanism (LNO): In stark contrast, the layered oxide LNO operates in a negative charge-transfer regime. Because of the strong hybridisation between nickel and oxygen, redox does not occur through simple nickel oxidation. Instead, charge compensation is dominated by the progressive formation and elimination of holes within the oxygen 2p band states that arise directly from that hybridisation. This confirms that in highly covalent materials, significant electron extraction occurs from the oxygen ions, in some cases more than from the metal sites themselves.

Significance and Impact

The central achievement of this paper is that it explains how charge and discharge capacities are linked to oxygen-dominated states without invoking oxygen dimerisation or the formation of trapped O₂ gas.

By applying the ZSA framework to directly resolve these mechanisms, the authors provide strong experimental validation for ligand-hole redox as a distinct charge compensation pathway in covalent oxides. This is not a rewriting of the rules so much as definitive evidence that settles a long-running debate about which mechanism actually operates in these materials.

It is worth noting that RPES and XAS are surface-sensitive techniques, and questions remain about whether the mechanisms observed at or near the electrode surface fully represent bulk behaviour during cycling. Further work probing bulk redox states will be needed to confirm the findings hold across the full electrode volume.

For battery engineers, the practical implication is that cathode design can no longer treat metal and oxygen redox as independent phenomena. Composition and dopant selection now need to account for how covalency tunes the hybridisation between metal d-states and oxygen 2p states, opening a more targeted route to higher capacity materials for next-generation EV cells.

See also: Lithium Ion Chemistry