Room-temperature sodium-ion batteries attract increasing attention for large-scale energy storage applications in renewable energy and smart grid. However, the development of suitable anode materials remains a challenging issue. Here we demonstrate that the spinel Li 4 Ti 5 O 12 , well-known as a 'zero-strain' anode for lithium-ion batteries, can also store sodium, displaying an average storage voltage of 0.91 V. With an appropriate binder, the Li 4 Ti 5 O 12 electrode delivers a reversible capacity of 155 mAh g À 1 and presents the best cyclability among all reported oxide-based anode materials. Density functional theory calculations predict a three-phase separation mechanism, 2Li 4 Ti 5 O 12 þ 6Na þ þ 6e À 2Li 7 Ti 5 O 12 þ Na 6 LiTi 5 O 12 , which has been confirmed through in situ synchrotron X-ray diffraction and advanced scanning transmission electron microscope imaging techniques. The three-phase separation reaction has never been seen in any insertion electrode materials for lithium-or sodium-ion batteries. Furthermore, interfacial structure is clearly resolved at an atomic scale in electrochemically sodiated Li 4 Ti 5 O 12 for the first time via the advanced electron microscopy.

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.

This article presents a comprehensive and critical review on the recent progress of titanium-based electrode materials for sodium-ion batteries.

Application of high-voltage spinel LiNi 0.5 Mn 1.5 O 4 cathode material is the closest and the most realistic approach to meeting the midterm goal of lithium-ion batteries for electric vehicles (EVs) and plug-in hybrid electric vehicles (HEVs). However, this application has been hampered by long-standing issues, such as capacity degradation and poor first-cycle Coulombic efficiency of LiNi 0.5 Mn 1.5 O 4 cathode material. Although it is well-known that the structure of LiNi 0.5 Mn 1.5 O 4 into which Li ions are reversibly intercalated plays a critical role in the above issues, performance degradation related to structural changes, particularly in the first cycle, are not fully understood. Here, we report detailed investigations of local atomic-level and average structure of LiNi 0.5 Mn 1.5 O 4 during first cycle (3.5−4.9 V) at room temperature. We observed two types of local atomic-level migration of transition metals (TM) ions in the cathode of a well-prepared LiNi 0.5 Mn 1.5 O 4 //Li half-cell during first charge via an aberration-corrected scanning transmission electron microscopy (STEM). Surface regions (∼2 nm) of the cycled LiNi 0.5 Mn 1.5 O 4 particles show migration of TM ions into tetrahedral Li sites to form a Mn 3 O 4 -like structure. However, subsurface regions of the cycled particles exhibit migration of TM ions into empty octahedral sites to form a rocksalt-like structure. The migration of these TM ions are closely related to dissolution of Ni/Mn ions and building-up of charge transfer impedance, which contribute significantly to the capacity degradation and the poor first-cycle Coulombic efficiency of spinel LiNi 0.5 Mn 1.5 O 4 cathode material. Accordingly, we provide suggestions of effective stabilization of LiNi 0.5 Mn 1.5 O 4 structure to obtain better electrochemical performance.

Lithium iodide (LiI) has garnered considerable attention in aprotic Li–O2 batteries. However, the reaction mechanism is still under hot debate and is attracting increasing controversy due to contrasting observations. Herein, on the basis of thorough evidence, a relevant mechanism has been systematically illustrated. LiI has been revealed to promote the superoxide-related nucleophilic attack toward electrolyte by catalyzing the decomposition of peroxide intermediate, resulting in the accumulation of LiOH and other parasitic products. Also, they refuse to be oxidized by not only triiodide (I3 –) but also iodine (I2), resulting in inevitable degradation. However, as a proton-donor, water can buffer the superoxide-related nucleophilic attack by reducing it to moderate hydroperoxide (HO2 –). More importantly, the catalysis of iodide toward speroxide is restrained with the increase of alkalinity in water-contained electrolyte, resulting in the formation of Li2O2. Turning LiOH into Li2O2, the newly proposed mechanism leads to revolutionary reunderstanding toward the role of iodide and water in Li–O2 battery systems.

As the rapid growth of the lithium‐ion battery (LIB) market raises concerns about limited lithium resources, rechargeable sodium‐ion batteries (SIBs) are attracting growing attention in the field of electrical energy storage due to the large abundance of sodium. Compared with the well‐developed commercial LIBs, all components of the SIB system, such as the electrode, electrolyte, binder, and separator, need further exploration before reaching a practical industrial application level. Drawing lessons from the LIB research, the SIB electrode materials are being extensively investigated, resulting in tremendous progress in recent years. In this article, the progress of the research on the development of electrode materials for SIBs is summarized. A variety of new electrode materials for SIBs, including transition‐metal oxides with a layered or tunnel structure, polyanionic compounds, and organic molecules, have been proposed and systematically investigated. Several promising materials with moderate energy density and ultra‐long cycling performance are demonstrated. Appropriate doping and/or surface treatment methodologies are developed to effectively promote the electrochemical properties. The challenges of and opportunities for exploiting satisfactory SIB electrode materials for practical applications are outlined.

The gliding of transition-metal slabs in layered Na containing oxides would limit the battery performance in several crucial aspects.

Direct observation of delithiated structures of LiCoO(2) 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 LiCoO(2) nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO(2), 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 LiCoO(2) to O1-type Li(x)CoO(2) (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li(x)CoO(2) (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li(x)CoO(2) in the phase diagram of O3-LiCoO(2), through which the two previously separated LiCoO(2) 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.