Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNiMnCoO (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni (active) and Co (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive Li nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R3[combining macron]m structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R3[combining macron]m → Fm3[combining macron]m), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.

Discerning the arrangement of transition metal atoms in Li[Ni x Mn y Co z O 2 ] cathode materials has remained an open problem for many years despite the commercial importance of some stoichiometries and the even more promising characteristics of others. We present a method for structural determination in this class of cathode materials. A simple definition of the total energy, based on the chemical principle of electroneutrality, is used in combination with a simulated annealing algorithm to generate model structures. The method reproduces the well known structure of Li[Li 1/3 Mn 2/3 O 2 ] and produces structures of the disordered Li[Ni x Mn x Co 1-2x O 2 ] phases (where x = 0.02, 0.1, 0.33) that are verified by detailed 7 Li NMR spectra. For each Li[Ni x Mn x Co 1-2x O 2 ] phase, the solution is found to be heavily disordered, yet retaining significant ion pairing. Since the underlying notion of favoring charge-neutral regions is generic, we anticipate its utility in a much broader family of materials.

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6 Li solid-state nuclear magnetic resonance (ssNMR) is used here to evaluate a series of Li 2 Mn 1−y Fe y P 2 O 7 cathode materials in an effort to quantify ion exchange rates and diffusion pathways. Magic angle spinning (MAS) NMR of the series of mixed metal pyrophosphates reveals a trade-off between electrochemical performance and well-resolved NMR spectra resulting from the change in electronic structure of the transition metal redox center. In addition, 1D 6 Li selective inversion NMR is employed to characterize Li ion dynamics in the fully Mn substituted member of the pyrophosphate series, where three of the four unique Li resonances are well resolved and labeled AB, C, and D, with AB corresponding to Li ions within one tunnel, and C and D Li ions residing in another. Despite limited inversion efficiency it is found that the utility of this experiment is not compromised so long as the initial magnetization conditions are well-defined. Initial fitting procedures involved the inclusion of all possible exchange pairs, a process which gave rise to consistently negative rate constants for C−AB or D−AB exchange, suggesting negligible exchange between these Li ions. Upon limiting the exchange model to ion exchange processes between the pairs of high and low frequency sites, rate constants of 45 ± 25 and 100 ± 30 Hz were obtained for C−D exchange at room temperature and 350 K respectively. Ion exchange pathways that are revealed by the exchange experiments imply limited mobility across distinct two-dimensional tunnels and slow exchange for within-tunnel ions. These exchange results provide corroboration for the geometrically determined site assignment in the 1-D spectrum, as well as support the notion of limited ion mobility in the Mn-phase resulting in poor electrochemical capability.