On the Origin of Reversible and Irreversible Reactions in LiNi_xCo_(1−x)/2Mn_(1−x)/2O₂

Abstract

Bond formation and breakage is crucial upon energy storage in lithium transition metal oxides (LiMeO2, Me = Ni, Co, Mn), i.e., the conventional cathode materials in Li ion batteries. Near-edge x-ray absorption finestructure spectroscopy (NEXAFS) of the Me L and O K edge performed upon the first discharge of LiNixCo(1-x)/2Mn(1-x)/2O2 (x = 0.33: NCM111, x = 0.6: NCM622, x = 0.8: NCM811) in combination with charge transfer multiplet calculations provide unambiguous experimental evidence that redox reactions in NCMs proceed via a reversible oxidation of Ni associated with the formation of covalent bonds to O neighbors, and not, as widely assumed, via pure cationic or more recently discussed, pure anionic redox processes. Correlating these electronic changes with crystallographic data using operando synchrotron X-ray powder diffraction shows that the amount of ionic Ni limits the reversible capacity - at states of charge where all ionic Ni is oxidized (above 155 mAh/g), the lattice parameters collapse, and irreversible reactions are observed. Yet the covalence of the Ni-O bonds also triggers the electronic structure and thus the operation potential of the cathodes.

Cycle-Induced Interfacial Degradation and Transition-Metal Cross-Over in LiNi0.8Mn0.1Co0.1O2–Graphite Cells

“…Instead, we attribute this to changes in the composition of the NMC surface, particularly the transition-metal ratios. Similar variations have previously been observed in Co x Mn 3– x O 4 compounds as the ratio between Co and Mn changes and between different NMC compositions . The observed changes in ratio with cycle number are thus consistent with the dissolution of transition metals from the NMC surface in a nonstoichiometric ratio, although they could alternatively reflect preferential migration of certain transition metals between the NMC bulk and surface.…”

“…Similar variations have previously been observed in Co x Mn 3– x O 4 compounds as the ratio between Co and Mn changes 66 and between different NMC compositions. 67 The observed changes in ratio with cycle number are thus consistent with the dissolution of transition metals from the NMC surface in a nonstoichiometric ratio, although they could alternatively reflect preferential migration of certain transition metals between the NMC bulk and surface. Given that the 529.8 eV peak is attributable to Mn 4+ and that significant Mn incorporation into the anode SEI is observed with cycling (see below), preferential dissolution of Mn appears the most likely explanation.…”

Unraveling the State of Charge-Dependent Electronic and Ionic Structure–Property Relationships in NCM622 Cells by Multiscale Characterization

“…With increasing covalence (Ni 3+ configuration), the Ni–O hybrid peaks at 529 eV and 530 eV increase (Figure b,d, purple circles). ,, At the end of charging, the Ni 2+ /Ni 3+ ratios of the TEY and FY data are nearly aligned with each other, although the ratio decreases significantly upon charging in both cases (Figure a,c).…”

“…The 003 and 101 reflections shifting in opposite directions (Figure a) imply that anisotropic changes occur in the crystallographic structure of NCM622 during delithiation/lithiation. The a and b lattice parameters decrease with an increasing SOC (Figure c), which is due to the increasing covalence of the Me–O bonds …”

“…NEXAFS measurements of Ni L 2.3 , Co L 2.3 , Mn L 2.3 , and O K of the polished samples were carried out in fluorescence yield (FY) detection mode at the Karlsruhe synchrotron light source KARA (Germany). The photon energy resolution was set to 0.2–0.4 eV, and energy calibration (using a NiO reference), dark current subtraction, division by I 0 , background subtraction, data normalization, and absorption correction were performed. ,, …”