In the present work, the extent and the role of oxygen release during the first charge of lithium-and manganese-rich Li 1.17 [Ni 0.22 Co 0.12 Mn 0.66 ] 0.83 O 2 (also referred to as HE-NCM) was investigated with on-line electrochemical mass spectrometry (OEMS). HE-NCM shows a unique voltage plateau at around 4.5 V in the first charge, which is often attributed to a decomposition reaction of the Li-rich component Li 2 MnO 3 . For this so-called "activation", it has been hypothesized that the electrochemically inactive Li 2 MnO 3 would convert into MnO 2 while lattice oxide ions are oxidized and released as O 2 (or even CO 2 ) from the host structure. However, qualitative and quantitative examination of the O 2 and CO 2 evolution during the first charge shows that the onset of both reactions is above the 4.5 V voltage plateau and that the amount of released oxygen is an order of magnitude too low to be consistent with the commonly assumed Li 2 MnO 3 activation. Instead, the amount of released oxygen can be correlated to a structural rearrangement of the active material which occurs at the end of the first charge. In this process, oxygen depletion from the HE-NCM host structure leads to the formation of a spinel-like phase. This phase transformation is restricted to the near-surface region of the HE-NCM particles due to the poor mobility of oxide ions within the bulk. From the evolved amount of O 2 and CO 2 , the thickness of the spinel-like surface layer was estimated to be on the order of ≈2-3 nm, in excellent agreement with previously reported (S)TEM data. Since the discovery of the positive electrode material LiCoO 2 and its commercialization in the Li-ion technology by Sony in 1991, 1 analogous layered oxides (LiMeO 2 , Me = Ni, Co, Mn, Al, etc.) were studied, aiming at higher intrinsic specific capacity, energy, stability, and lower costs.2-7 Among others, Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 (NCM-111) showed very interesting performances in terms of specific capacity and stability. 8,9 Recently, materials characterized by an increase of exploitable Li + charge drew a lot of attention. 10,11 These so-called Lirich compounds result from the substitution of part of the transition metal ions by Li + , in a structural arrangement closely related to the layered structure. 11-14Li 2 MnO 3 (or Li[Li 1/3 Mn 2/3 ]O 2 ) is the simplest structure in this category and crystallizes in the monoclinic system (space group C2/m), while the common LiMeO 2 -based layered structures crystallize in the hexagonal system (space group R-3m). 11,13,14 The two structures are very close to each other despite this difference in symmetry, related simply to the Li + ordering in the transition metal sites. This similarity is evident in the structure of the Li-rich NCM Li 1+x Me 1-x O 2 (Me = Ni, Co, Mn), also referred to as high-energy NCM (HE-NCM), where the hexagonal symmetry of the layered structure is broken by the superstructure of Li + in the transition metal sites, shown by the superlattice reflections in the diffractograms. 15,16 This s...

Ni-rich layered oxides, like NCM-811, are promising lithium-ion battery cathode materials for applications such as electric vehicles. However, pronounced capacity fading, especially at high voltages, still lead to a limited cycle life, whereby the underlying degradation mechanisms, e.g. whether they are detrimental reactions in the bulk or at the surface, are still controversially discussed. Here, we investigate the capacity fading of NCM-811/graphite full-cells over 1000 cycles by a combination of in situ synchrotron X-ray powder diffraction, impedance spectroscopy, and X-ray photoelectron spectroscopy. In order to focus on the NCM-811 material, we excluded Li loss at the anode by pre-lithiating the graphite. We were able to find a quantitative correlation between NCM-811 lattice parameters and capacity fading. Our results prove that there are no considerable changes in the bulk structure, which could be responsible for the observed ≈20% capacity loss over the 1000 cycles. However, we identified the formation of a resistive surface layer, which is responsible for (i) an irreversible loss of capacity due to the material lost for its formation, and (ii) for a considerable impedance growth. Further evidence is provided that the surface layer is gradually formed around the primary NCM-811 particles.

High-energy Li1.17Ni0.19Co0.10Mn0.54O2 (HE-NCM) is a lithium-rich layered oxide with alternating Li- and transition-metal (TM) layers in which excess lithium ions replace transition metals in the host structure. HE-NCM offers a capacity roughly 50 mAh g–1 higher compared to that of conventional layered oxides but suffers from capacity loss and voltage fade upon cycling. Differential capacity plots (taken over 100 cycles) show that the origin of the fading phenomenon is a bulk issue rather than a surface degradation. Although previous studies indicate only minor changes in the bulk material, long duration in situ synchrotron X-ray powder diffraction measurements, in combination with difference Fourier analysis of the data, revealed an irreversible transition-metal motion within the host structure. The extensive work provides new insights into the fading mechanism of the material.

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The degradation of LiNi0.8Co0.15Al0.05O2 (LNCAO) is reflected by the electrochemical performance in the fatigued state and correlated with the redox behavior of these cathodes. The detailed electrochemical performance of these samples is investigated by galvanostatic and voltammetric cycling as well as with the galvanostatic intermittent titration technique (GITT). Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to investigate the oxidation state of all three materials at the Ni L2,3, O K, and Co L2,3 edges at five different states of charge. Surface and more bulklike properties are distinguished by total electron yield (TEY) and fluorescence yield (FY) measurements. The electrochemical investigations revealed that the changes in the cell performance of the differently aged materials can be explained by considering the reaction kinetics of the intercalation/deintercalation process. The failure of the redox process of oxygen and nickel at low voltages leads to a significant decrease of the reaction rates in the fatigued cathodes. The accompanied cyclic voltammogram (CV) peaks appear as two peaks because of the local minimum of the reaction rate, although it is one peak in the CV of the calendarically aged LNCAO. The absence of the oxidation/reduction process at low voltages can be traced back to changes in the surface morphology (formation of a NiO-like structure). Further consequences of these material changes are overpotentials, which lead to capacity losses of up to 30% (cycled with a C/3 rate).

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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.

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