Large LiMn 1.5 Ni 0.5 O 4 single crystals in plate shape with (112) surface facets and octahedral shape with (111) surface facets were obtained by molten-salt synthesis. The presence of transition-metal ordering in both samples was independently confirmed by SAED, FTIR, NMR, and electrochemical studies, demonstrating the excellent capability of each technique in distinguishing the ordered and disordered phases. The apparent chemical diffusion minima during Li extraction and insertion were correlated with the occurrence of the first-order phase transition, implying that phase boundary movement limits Li transport in the spinel cathodes. Despite a more ordered structure, nearly ten times less Mn 3+ content, and increased two-phase boundary movement during delithiation and relithiation, the octahedral crystals exhibited superior rate capability and a larger chemical diffusion coefficient, suggesting the kinetic preeminence of (111) surface facets over (112). The dominating effect of particle morphology and the importance of morphology design in achieving optimal performance of the LiMn 1.5 Ni 0.5 O 4 spinel are clearly demonstrated for the first time.

Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects.

Recent reports on high capacities delivered by Li-excess transition-metal oxide cathodes have triggered intense interest in utilizing reversible oxygen redox for high-energy battery applications. To control oxygen electrochemical activities, fundamental understanding of redox chemistry is essential yet has so far proven challenging. In the present study, micrometer-sized Li 1.3 Nb 0.3 Mn 0.4 O 2 single crystals were synthesized for the first time and used as a platform to understand the charge compensation mechanism during Li extraction and insertion. We explicitly demonstrate that the oxidation of O 2− to O n− (0 < n < 2) and O 2 loss from the lattice dominates at 4.5 and 4.7 V, respectively. While both processes occur in the first cycle, only the redox of O 2− /O n− participates in the following cycles. The lattice anion redox process triggers irreversible changes in Mn redox, which likely causes the voltage and capacity fade observed on this oxide. Two drastically different redox activity regions, a single-phase behavior involving only Mn 3+/4+ and a two-phase behavior involving O 2− /O n− (0 ≤ n < 2), were found in Li x Nb 0.3 Mn 0.4 O 2 (0 < x < 1.3). Morphological damage with particle cracking and fracturing was broadly observed when O redox is active, revealing additional challenges in utilizing O redox for high-energy cathode development. Recently, approaches to enable high-energy cathodes by utilizing redox reactions of both TM cations and oxygen anions have triggered intense interest. 5−7 One of the most studied examples is the lithium and manganese-rich (LMR) layered 49 oxides with a general formula of Li 1+x Mn 1−x−y−z Ni y Co z O 2. 8−10 50 Our recent work showed that, contrary to the common notion 51 of a nanocomposite structure, the oxide has a single monoclinic 52 phase (C2/m) with a large number of domains corresponding 53 to different variants. 11 To involve the O 2p electrons in the 54 following electrochemical reactions, the material typically 55 undergoes an initial activation process signaled by a unique 56 charging voltage profile that is much different from those of the 57 subsequent cycles. Recent studies by Luo et al. suggested the 58 formation of O − holes in the intermediates, as evidenced by the 59 progressive growth of a new peak on the O K-edge X-ray 60 absorption spectroscopy (XAS) along with the use of a number 61 of other characterization techniques, including isotopically 62 labeled differential electrochemical mass spectroscopy 63 (DEMS), X-ray absorption near edge structure (XANES), 64 and resonant inelastic X-ray scattering (RIXS). 9 However, this 65 remains controversial as experimental evidence is difficult to

The rate capability of Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC) electrode is studied in this paper at the particle scale. Experimental results obtained on thin electrodes show that NMC is an extremely high-rate material capable of charge and discharge at rates exceeding 100C. The high capacity retention has not been previously reported in the literature. Even higher rate capability was seen on charge. The transport properties of the material were explored by combining experiments on thin electrodes with a continuum model of a single spherical particle. The use of thin electrodes minimized porous electrode effects and allowed the assumption of a uniform current distribution in the electrode. A qualitative estimate of the lithium diffusion coefficient in the NMC particle was obtained by comparing the experimental and simulated potentials during open-circuit relaxation at various states of charge. The fitting results show that the lithium diffusion coefficient increases with increasing state of charge. The value ranges from 10 −16 m 2 /s when completely discharged to 10 −14 m 2 /s when completely charged, suggesting that the use of a varying diffusion coefficient is necessary for studying the transport processes in this material and for further application to the macroscopic porous electrode models.

The chemical phase distribution in hydrothermally grown micrometric single crystals LiFePO4 following partial chemical delithiation was investigated. Full field and scanning X-ray microscopy were combined with X-ray absorption spectroscopy at the Fe K- and O K-edges, respectively, to produce maps with high chemical and spatial resolution. The resulting information was compared to morphological insight into the mechanics of the transformation by scanning transmission electron microscopy. This study revealed the interplay at the mesocale between microstructure and phase distribution during the redox process, as morphological defects were found to kinetically determine the progress of the reaction. Lithium deintercalation was also found to induce severe mechanical damage in the crystals, presumably due to the lattice mismatch between LiFePO4 and FePO4. Our results lead to the conclusion that rational design of intercalation-based electrode materials, such as LiFePO4, with optimized utilization and life requires the tailoring of particles that minimize kinetic barriers and mechanical strain. Coupling TXM-XANES with TEM can provide unique insight into the behavior of electrode materials during operation, at scales spanning from nanoparticles to ensembles and complex architectures.