High Performance Li₂Ru_1–yMn_yO₃ (0.2 ≤ y ≤ 0.8) Cathode Materials for Rechargeable Lithium-Ion Batteries: Their Understanding

Abstract

Understanding the origin of the high capacity displayed by Li2MnO3–LiMO2 (M = Ni, Co) composites is essential for improving their cycling and rate capability performances. To address this issue, the Li2Ru1–y Mn y O3 series between the iso-structural layered end-members Li2MnO3 and Li2RuO3 was investigated. A complete solid solution was found, with the 0.4 ≤ y ≤ 0.6 members showing sustainable reversible capacities exceeding 220 mAh·g–1 centered around 3.6 V vs Li+/Li. The voltage–composition profiles display a plateau on the first charge as compared to an S-type curve on subsequent discharge which is maintained on the following charges/discharges, with therefore a lowering of the average voltage. We show this profile to evolve upon long cycling due to a structural phase transition as deduced from XRD measurements. Finally we demonstrate, via XPS measurements, the oxidation and reduction of ruthenium (Ru5+/Ru4+) during cycling together with a partial activity of the Mn4+/Mn3+ redox couple. Moreover, we provide direct evidence for the reversibility of the O2– → O– anionic process upon cycling, hence accounting for the high capacity displayed by these materials. This work, by capturing the main redox processes pertaining to these materials, should facilitate their development.

Relationship between Cyclic Properties and Charge-discharge Condition for Li₂Mn_0.4Ru_0.6O₃ and Li₂RuO₃

“…The irreversible capacity is partially ascribed to a loss of oxygen from the lattice. 22,23 In contrast to the initial charge, the second charge curve showed simple sloping profile. A remarkable decrease in an irreversible capacity was observed in the second cycle.…”

“…Two distinct peaks at 3.65 V and 3.8 V are attributed to biphasic regions with the structural transitions 17 and a peak around 4.2 V is most likely associated with the oxidation of oxygen. 23 Moreover, a reduction peak appeared at 3.4 V and 3.3 V under a cutoff voltage of 4.2 V and 4.8 V, respectively, in the initial discharge. Li 2 RuO 3 exhibits distinct features in the dQ/dV plot in the subsequent cycles depending on the applied cut-off voltage, although capacity and voltage fading were nearly independent of the applied cut-off voltage.…”

“…22 A plateau-like region was observed around 4.2 V most likely associated with the oxidation of oxygen, 23 because the Ru L 3 -edge XANES spectra analysis has revealed that the valence state of Ru is invariant in the plateau region. 22 The discharge voltages exhibited sloping profiles.…”

“…14,18-24 Sathiya et al conducted a systematic study on the charge compensation mechanism of Li 2 Ru 1¹x M x O 3 (M = Mn, Sn) by using X-ray photoelectron spectroscopy and electron paramagnetic resonance, 23,24 and revealed that a reversible anionic redox process is the origin of the high capacity in these systems. 24 In our previous study, the charge compensation mechanism for Li 2 Mn 1¹x Ru x O 3 in the initial cycle under a charge cut-off voltage of 4.8 V has been investigated by Mn K-edge and Ru L-edge X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) measurements, indicating that the Ru 5+…”

Tuning charge–discharge induced unit cell breathing in layer-structured cathode materials for lithium-ion batteries

“…Recently, it was reported that Li 2 Ru 0.75 Sn 0.25 O 3 shows expanded parameter a(b) and contracted parameter c during charge 16 , while Li 2 Ru 0.5 Mn 0.5 O 3 shows unchanged lattice parameters before 1.2 Li was extracted but contracted parameters a, b and c during further delithiation 22 . In the case of Li 2 MoO 3 we studied here, all the three lattice parameters a, b and c were increased during delithiation.…”