In situ and operando Raman spectroscopy is proposed to provide unique means for deeper fundamental understanding and further development of layered transition metal LiMO 2 (M = Ni, Co, Mn) oxides suitable for Li-ion battery applications. We compare several spectro-electrochemical cell designs and suggest key experimental parameters for obtaining optimum electrochemical performance and spectral quality. Studies of the most practically relevant LiMO 2 compositions are exemplified with particular focus on two experimental approaches: (1) lateral and axial Raman mapping of the electrode's (near-) surface to monitor inhomogeneous electrode reactions and (2) time-dependent singleparticle spectra during cycling to analyze the Li x MO 2 lattice dynamics as a function of lithium content. Raman Spectroscopy is claimed to provide a unique real-time probe of the M-O bonds, which are at the heart of the electrochemistry of LiMO 2 oxides and govern their stability. We highlight the need for further fundamental understanding of the relationships between the spectroscopic response and oxide lattice structure with particular emphasis on the development of a theoretical framework linking the position and intensity of the Raman bands to the local Li x MO 2 lattice configuration. The use of complementary experimental techniques and model systems for validation also deserve further attention. Several novel LiMO 2 compositions are currently being explored, especially containing dopings and coatings, and Raman spectroscopy could offer a highly dynamic and convenient tool to guide the formulation of high specific charge and long cycle life LiMO 2 oxides for next-generation Li-ion battery cathodes.

The local structure evolution of Li x Ni0.8Co0.15Al0.05O2 (NCA) is linked to its electrochemical response during cycling (and overcharge) by operando Raman spectroscopy with findings supported by complementary techniques, such as online electrochemical mass spectrometry (OEMS) and density functional theory (DFT) phonon calculations. The vibrational motion of lattice oxygens is observed to be highly dependent on the local Li x MO2 lattice environment, e.g. MO bonding strength/length and state of lithiation x. All vibrational modes generally harden upon delithiation due to MO bond character (ionic → covalent) evolution (disregarding an early bond softening due to Li+ vacancy formation) and evidence the important influence of the local structural lattice configuration on the electrochemical response of NCA. Although the intensities of all Raman active bands generally increase upon delithiation, a major inflection point at x = 0.2 marks the onset of a partly irreversible fundamental transition within NCA that is most likely related to electron removal from MO bonding states and partial oxidation of oxygen sublattice, which is also indicated by the observed concomitant O2 release from the particle surface. Operando Raman spectroscopy with higher time resolution provides unique possibilities for detailed studies of how chemical parameters (Li+ vacancy formation, transition metal cation concentration, and lattice doping, etc.) may govern the onset and nature of processes (such as bond character evolution and stability) that define the performance of the Li x MO2 class of oxides. The further insights thus gained can be exploited to guide the development of next-generation layered cathodes for Li-ion batteries operating stably at higher voltages and capacities.

Although layered transition metal oxides are the state-of-the-art cathode active materials for Li-ion batteries, many fundamental aspects of their operation are poorly understood; in particular how the local lattice structure and the transition metal composition influence their electrochemical activity. In this work, the local structure and redox activity of NCM111, NCM622 and NCM811 Li-ion cathodes are characterized under standard and overcharge operating conditions with a recently-developed operando Raman spectroscopy methodology. Supported by DFT phonon calculations and advanced data-analysis methods, we demonstrate that the Raman spectra of NCMs entail spectroscopic signatures of cation ordering phenomena, the sequential oxidation/reduction of nickel and the participation of bulk lattice oxygen in the charge-compensation process at low state of lithiation. Our methodology enables monitoring such processes during cycling and offers the potential for investigating the mechanisms by which certain strategies (i.e. doping, surface coatings, etc.) ameliorate electrochemical performance.

The solvation structure of several lithium and sodium based electrolytes are explored as a function of salt concentration over a wide range via a detailed PM7 computational study.The cation coordination shells are found to be well-defined and solvent rich for dilute electrolytes, while disordered and anion rich for the more concentrated electrolytes. The Nabased electrolytes display larger cation coordination shells with a more pronounced presence of fluorine as compared to the Li-based electrolytes. The origins of the structural differences are discussed as well as their consequences for properties of battery electrolytes and battery usage -especially targeting the current large interest in highly concentrated electrolytes.