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.

Reversible and irreversible charge exchange reactions of Li- and Mn-rich layered oxides (Li1.15Ni0.2Co0.1Mn0.55O2, LLO) are investigated with bulk and surface-sensitive near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the Ni L 2,3, Co L 2,3, Mn L 2,3, and O K edges; mass spectrometry (MS); and operando synchrotron X-ray powder diffraction (SXPD). The present work shows the relation between O–O formation in the bulk and Ni, Co, and Mn reduction/oxidation processes, which in turn deliver outstanding capacities of Li-rich layered oxides (LLO). Moreover, the reversibility of charge compensation reactions is discussed and differences between O–O formation and oxygen release, both occurring mainly upon the first charge (so-called activation), are identified.

The syntheses of Ni-poor (NCM111, LiNi 1/3 Co 1/3 Mn 1/3 O 2 ) and Ni-rich (NCM811 LiNi 0.8 Co 0.1 Mn 0.1 O 2 ) lithium transition-metal oxides (space group R3̅ m) from hydroxide precursors (Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 , Ni 0.8 Co 0.1 Mn 0.1 (OH) 2 ) are investigated using in situ synchrotron powder diffraction and near-edge X-ray absorption fine structure spectroscopy. The development of the layered structure of these two cathode materials proceeds via two utterly different reaction mechanisms. While the synthesis of NCM811 involves a rock salt-type intermediate phase, NCM111 reveals a layered structure throughout the entire synthesis. Moreover, the necessity and the impact of a preannealing step and a high-temperature holding step are discussed.

Mixed-metal oxides (MMOx), oxides with more than one (transition) metal, provide versatile structural and electrochemical properties well exploited in energy conversion and electrochemical energy storage. The preparation of MMOx from single-source precursors benefits from homogeneous composition and uniform metal distribution. Herein, we describe layered mixed-metal carbides (MXenes) as templates to prepare MMOx. Through thermal oxidation of TiNb-based MXenes in CO 2 , mixtures of Ti and Nb oxides were produced. The Ti-to-Nb ratio in the MXene significantly affects the derived oxide composition but does not show a direct stoichiometric relation between them. At higher Ti ratios, oxide mixtures of TiO 2 and titanium niobium oxide are obtained, while with Nb excess, only MMOx are produced. Multilayer MXenes retain carbon upon oxidation and produce TiNbO x /C hybrids, while delaminated MXenes lead to pure TiNbO x . When tested as Li-ion battery electrodes, the multilayer MXene-derived MMOx with Ti/Nb = 1:5 presented 226 mAh•g −1 at 10 mA•g −1 and 75% retention after 1000 cycles at 1 A•g −1 .