Surface dissolution of manganese is a long-standing issue hindering the practical application of spinel LiMn2O4 cathode material, while few studies concerning the crystal structure evolution at the surface area have been reported. Combining X-ray photoelectron spectroscopy, electron energy loss spectroscopy, scanning transmission electron microscopy, and density functional theory calculations, we investigate the chemical and structural evolutions on the surface of a LiMn2O4 electrode upon cycling. We found that an unexpected Mn3O4 phase was present on the surface of LiMn2O4 via the application of an advanced electron microscopy. Since the Mn3O4 phase contains 1/3 soluble Mn2+ ions, formation of this phase contributes significantly to the Mn2+ dissolution in a LiMn2O4 electrode upon cycling. It is further found that the Mn3O4 appears upon charge and disappears upon discharge, coincident with the valence change of Mn. Our results shed light on the importance of stabilizing the surface structure of cathode material, especially at the charged state. The understanding of the manganese dissolution reaction that occurs in the LiMn2O4 can certainly be extended to other oxide cathodes.

Direct observation of delithiated structures of LiCoO2 at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO2 nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO2, the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO2 to O1-type LixCoO2 (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type LixCoO2 (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-LixCoO2 in the phase diagram of O3-LiCoO2, through which the two previously separated LiCoO2 phases, i.e. O2 and O3 systems, are connected. These new investigations shed new insight into the lithium storage mechanism in this important cathode material for Li-ion batteries.

A highly ordered interface between LiFePO4 phase and FePO4 phase with staging structure along the a axis and perpendicular to the b axis direction has been observed for the first time, in a partially chemically delithiated Li0.90Nb0.02FePO4 by advanced aberration-corrected annular-bright-field (ABF) scanning transmission electron microscopy (STEM).

Titanium niobium oxide (TiNb2O7) with a monoclinic layered structure has been synthesized by a solid state reaction method as an anode candidate for Li-ion batteries. The TiNb2O7 electrode shows a lithium storage capacity of 281 mAh g−1with an initial coulombic efficiency as high as 93% at a current density of 30 mA g−1 (ca. 0.1C). The average lithium insertion voltage is about 1.64 V vs.Li/Li+ at a voltage range of 0.8–3.0 V. The electrodes exhibit small voltage hysteresis (c.a. 0.1 V at 30 mA g−1) and good capacity retention. Such superior electrochemical performance of TiNb2O7 makes it one of the most promising anode materials to replace spinel Li4Ti5O12 for applications in hybrid vehicles and large scale stationary Li-ion batteries. In addition, we demonstrate crystal structures of TiNb2O7 and lithiated TiNb2O7 using advanced spherical-aberration-corrected scanning transmission electron microscopy (STEM), to picture the lattice sites occupied by the Li, Ti, Nb and O atoms at atomic-scale. Possible lithiation/delithiation processes and reaction mechanisms are revealed in consistence with first-principles prediction.

A highly ordered interface between LiFePO4 phase and FePO4 phase with staging structure along the a axis and perpendicular to the b axis direction has been observed for the first time, in a partially chemically delithiated Li0.90Nb0.02FePO4 by advanced aberration-corrected annular-bright-field (ABF) scanning transmission electron microscopy (STEM).