Thermally activated spin fluctuations in stoichiometric LiCoO2clarified by electron paramagnetic resonance and muon-spin rotation and relaxation measurements

Abstract

Lithium cobalt dioxide (LiCoO 2) belongs to a family of layered CoO 2-based materials and has considerable interests in both fundamental physics and technological applications in lithium-ion batteries. We report the results of structural, electrochemical, magnetic susceptibility (χ), electron paramagnetic resonance (EPR), and muon-spin rotation and relaxation (μSR) measurements on powder Li x 0 CoO 2 samples, where the nominal Li/Co ratios (x 0) were 0.95, 1.00, 1.02, 1.05, and 1.10, respectively. Structural, electrochemical, and χ measurements suggested that the sample with x 0 = 1.02 is very close to single stoichiometric LiCoO 2 (ST-LCO) phase and that the Co ions in the x 0 = 1.02 sample are in a nonmagnetic low-spin state with S = 0 (t 6 2g). However, both EPR and μSR revealed that the x 0 = 1.02 (ST-LCO) sample includes a large amount of nonordered magnetic phase in the temperature (T) range between 100 and 500 K. The volume fraction of such magnetic phase was found to be ß45 vol% at 300 K by μSR, indicating an intrinsic bulk feature for ST-LCO. In fact, structural and photoelectron spectroscopic analyses clearly excluded the possibility that the nonordered magnetism is caused by impurities, defects, or surfaces. Because EPR and μSR sense static and dynamic nature of local magnetic environments, we concluded that Co spins in ST-LCO are fluctuating in the EPR and μSR time-windows. We also proposed possible origins of such nonordered magnetism, that is, a spin-state transition and charge disproportionation.

Antisite occupation induced single anionic redox chemistry and structural stabilization of layered sodium chromium sulfide

“…The Cr 3+ ion in NaCrS 2 has the 3 d 3 electron configuration, which is EPR-active owing to existence of unpaired electrons, while S 2 4− in NaCrS 2 is EPR-inactive, because all electrons are paired 26 . All spectra consist of single Lorentzian line, and g factor is calculated using the following relationship 27 : …”

Stabilizing Co4+ Ions in Ultrathin Cobalt Oxide Nanosheets for Efficient Oxygen Evolution Reaction

“…HV‐LCO‐NS shows a relatively strong signal at about g=2.12 instead of the characteristic value for oxygen vacancy (g=2.00), providing the direct evidence of absence of oxygen vacancy . According to previous reports, the g factor of 2.12 could be attributed to the spin‐state transition or the existence of Co 4+ ,. Obviously, HV‐LCO‐NS shows a relatively strong EPR signal comparing to other two counterpart samples (LCO‐NS and LCO‐BULK), demonstrating the existence of Co 4+ ions in HV‐LCO‐NS.…”

Charging of lithium cobalt oxide battery cathodes studied by means of magnetometry

“…On the one hand, an initial voltage plateau, corresponding to a coexistence of two structural hexagonal phases (see above), can be seen at the beginning of the charging curve. On the other hand, the characteristic voltage behavior around 4.2 V, which is due to the structural phase transition as observed by XRD, appears at a Licontent of x = 0.50 (see [17]). …”

“…In fact, the magnetic susceptibility changes by several 100% with Li de-/intercalation and, therefore, serves as sensitive fingerprint of the charging state. Over the last few years, various groups were concerned with magnetic measurements on Li x CoO 2 cathodes [4,7,[9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Studies over a certain range of concentrations were performed by extracting Li either chemically or electrochemically: chemical Li extraction by Kellerman et al [7] (0.98 ≥ x ≥ 0.60, 6 samples), Hertz et al [9] (1≥ x ≥ 0.51, 9 samples), or Miyoshi et al [10] (0.99 ≥ x ≥ 0.25, 6 samples), electrochemical Li extraction by Sugiyama et al [11] (1 ≥ x ≥ 0.70, 3 samples), Mukai et al [12] (1 ≥ x ≥ 0.10, 8 samples) or Motohashi et al [13] (1≥ x ≥ 0, 9 samples).…”