Direct Observation of Reductive Coupling Mechanism between Oxygen and Iron/Nickel in Cobalt‐Free Li‐Rich Cathode Material: An in Operando X‐Ray Absorption Spectroscopy Study

Abstract

Development of positive electrode (cathode) materials for Li-ion batteries, possessing high capacity is necessary to meet current energy requirements and hence to facilitate their commercial applications. The layered Li-rich materials with a general composition xLi 2 MnO 3 •(1−x) LiMO 2 (M = Mn, Co, Ni, Fe) are attractive candidates as high-capacity cathode materials as they can deliver long term reversible capacities close to 200 mAh g −1 when cycled within the voltage range 2.0-4.8 V. [1][2][3][4][5][6] In the Li-rich layered materials, mixed transition metal/Li layers exist unlike in pure layered materials like LiCoO 2 . The former layer will be separated from Li-only layers by oxygen layers. Due to the presence of Li on the transition metal layer, a superlattice ordering of the transition metal and lithium takes place, which lowers the symmetry of the Li-rich material to mono clinic C2/m from R3m. [3] It was first demonstrated in 1999 by Kalyani et al., that Li 2 MnO 3 could be activated electrochemically in a Li-half cell by increasing the upper cut off voltage to 4.5 V. [7] When the initial research works were focused on purely manganese-containing layered Li-rich chemistries, later works mainly focused on Co and Ni-substituted layered Li-rich materials. [4,[8][9][10][11][12][13] Additionally, Fe-substituted layered materials also received considerable attention from the '90s onwards, due to the low cost and wide abundance of Fe compared to Co and Ni. [6,14,-20] In 1993, Reimers et al., reported a work on a Fe and Ni containing layered material, LiFe y Ni 1−y O 2 , which was found isostructural with LiNiO 2 in a selected composition range of 0 ≤ y ≤ 0.23. [21] In another composition range of 0.23 ≤ y ≤ 0.48, a coexistence of hexagonal (LiFe 0.23 Ni 0.77 O 2 , with a cation mixing between lithium layers and transition metal, particularly Fe) and cubic phases (LiFe 0.48 Ni 0.58 O 2 ) were observed. The electrochemical cycling conducted between voltage range of 2.0-4.2 V reveals a decrease in the reversible capacity with increase in the iron content "y". [21] Fe-substituted Li 2 MnO 3 was investigated as a 4 V cathode in lithium-ion cells in 2002, by Tabuchi et al. [22] The capacity delivered by the material was found to be influenced by the synthesis method, annealing temperature and voltage range applied. In situ Moessbauer spectroscopy investigations reveal the oxidation of Fe 3+ Li-rich cathodes possess high capacity and are promising candidates in next-generation high-energy density Li-ion batteries. This high capacity is partly attributed to its poorly understood oxygen-redox activity. The presentLi-rich cathodes contain expensive and environmentally-incompatible cobalt as a main transition metal. In this work, cobalt-free, iron-containing Li-rich cathode material (nominal composition Li 1.2 Mn 0.56 Ni 0.16 Fe 0.08 O 2 ) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g −1 ) and cycling stability. In operando, X-ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its elec...

Facilitating Reversible Cation Migration and Suppressing O2 Escape for High Performance Li‐Rich Oxide Cathodes

“…Studies have demonstrated that lattice oxygen oxidation can cause charge transfer from oxygen to TM, accompanied by lattice oxygen escape. [ 45 ] The reduction of Mn reduces the density of electronic holes in O2p‐Mn3d, thus weakens the O K‐edge pre‐edge peaks. While due to the inhibition of lattice oxygen redox, the intensity of pre‐edge peaks for charged LMNPO‐V O are almost the same with those for pristine LMNPO‐V O .…”

Regulation of 3d‐Transition Metal Interlayered Disorder by Appropriate Lithium Depletion for Li‐Rich Layered Oxide with Remarkably Enhanced Initial Coulombic Efficiency and Stability

“…When further charging to 4.8 from 4.36 V, it is noted that the "white line" absorption peak remains constant, whereas the Ni K-edge absorption shifts slightly to lower energy, similar to the trend observed in ex situ XAS results in several previous works. [46][47][48] This phenomenon is related to the reduction of a small fraction of Ni 4+ , which is most likely caused by reduced coupling of Ni with oxygen, accompanied by oxidization of oxygen species in the vicinity. [48,49] In contrast, the absorption edge of n-LMNO does not coincide with the pristine state when discharged to 2.0 V (Figure S11, Supporting Information), whereas they overlap exactly in d-LMNO (Figure 4b).…”

Configuration‐dependent anionic redox in cathode materials

“…To improve the structural stability of Li 2 MnO 3 , the LMR analogues were proposed. [71][72][73][74][75] However, the origin of the high charge capacity and the reason for the oxygen redox are still unclear. Because of the intrinsic complexity of LMR in composition and structure, Tarascon et al 43 4).…”