High-Frequency Electron Paramagnetic Resonance Analysis of the Oxidation State and Local Structure of Ni and Mn Ions in Ni,Mn-Codoped LiCoO₂

Abstract

High-frequency electron paramagnetic resonance (HF-EPR) spectroscopy was employed to examine the oxidation state and local structure of Ni and Mn ions in Ni,Mn-codoped LiCoO(2). The assignment of EPR signals was based on Mg,Mn-codoped LiCoO(2) and Ni-doped LiCoO(2) used as Mn(4+) and low-spin Ni(3+) EPR references. Complementary information on the oxidation state of transition-metal ions was obtained by solid-state (6,7)Li NMR spectroscopy. For slightly doped oxides (LiCo(1-x)Ni(x)Mn(x)O(2) with x < 0.05), nickel and manganese substitute for cobalt in the CoO(2) layers and are stabilized as Ni(3+) and Mn(4+) ions. The local structure of Mn(4+) ions was determined by modeling of the axial zero-field-splitting parameter in the framework of the Newman superposition model. It has been found that the local trigonal distortion around Mn(4+) is smaller in comparison with that of the host site. To achieve a local compensation of Mn(4+) charge, several defect models are discussed. With an increase in the total dopant content (LiCo(1-x)Ni(x)Mn(x)O(2) and 0.05 <or= x <or= 0.10), a saturation in the Ni(3+) amount (up to x < 0.05) is attained, while the Mn(4+) content increases gradually. Ni(3+) ions are surrounded by Co(3+) ions only in the whole concentration range (0 < x <or= 0.10). The first metal coordination sphere of Mn(4+) ions undergoes a transformation with an increase in the total Ni and Mn contents due to a progressive replacement of Co(3+) by Mn(4+) and Ni(2+) ions. For highly doped oxides (LiCo(1-x)Ni(x)Mn(x)O(2) with x = 0.10), nickel and manganese achieve, with respect to the local charge compensation, their usual oxidation states of 2+ and 4+.

Resolving chemical and spatial heterogeneities at complex electrochemical interfaces in Li ion batteries

“…In contrast, lithiation (discharge) of NMC811 is expected to increase paramagnetic interactions between the substrate and the CEI, leading to signal dephasing in SSNMR. Accordingly, both EPR in the active material, 61,[64][65][66][67][68] which is supported by control experiments comparing NMC811 with NMC532, NMC111, and LNMO (LiNi0.5Mn1.5O4) (Figures S1-S2).…”

Resolving Chemical and Spatial Heterogeneities at Complex Electrochemical Interfaces in Li-Ion Batteries

“…Co 3+ (S = 0) 303 is diamagnetic and thus, EPR inactive. 64,65 304 Therefore, we assign the broad signal observed at 305 3.0 V to Mn 4+ -Ni 2+ exchange coupling interactions 306 This is a provisional file, not the final typeset article in the active material, 61,[64][65][66][67][68] which is supported by control experiments comparing NMC811 with NMC532, NMC111, and LNMO (LiNi0.5Mn1.5O4) (Figures S1-S2).…”

“…Decreased EPR signal from the active material in the cathode composite is consistent with the oxidation of Ni 2+ /Ni 3+ (paramagnetic) to Ni 4+ (S = 0, diamagnetic) that occurs during delithiation. 22,65,66 Since the Mn 4+ (S = 3/2) and Co 3+ (S = 0) are not redox active, 31,69,70 the EPR signal from NMC811 disappears as the bulk oxidation state of the transition metal centers are oxidized to form diamagnetic Ni 4+ . Paramagnetic Mn 4+ cannot couple with diamagnetic Ni 4+ , decreasing the observable EPR signal.…”

Rechargeable Batteries from the Perspective of the Electron Spin

“…doped with paramagnetic TM species. [19][20][21][22][23][24][25] In such systems, large g-tensor anisotropies (Dg = g^ -g| |)…”

“…For systems containing more than one paramagnetic spin environment or species, differentiating between individual g-tensors is complicated by signal overlap at X-band frequencies. This limitation can be overcome with high field EPR, where larger Zeeman splittings increase signal resolution, as demonstrated by Stoyanova et al 25 In paramagnetically concentrated systems, EPR enables the detection of TM clusters through dipole-dipole and magnetic exchange interactions. Departure from a statistical distribution of paramagnetic species can be identified using eq.…”