Pristine Li-rich layered cathodes, such as Li(1.2)Ni(0.2)Mn(0.6)O(2) and Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2), were identified to exist in two different structures: LiMO(2)R3[overline]m and Li(2)MO(3)C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO(2)R3[overline]m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li(2)MO(3)C2/m to spinel involves removal of Li(+) and O(2-), which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF(3)-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF(3)-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.

Aqueous Zn‐ion batteries (ZIBs) have received incremental attention because of their cost‐effectiveness and the materials abundance. They are a promising choice for large‐scale energy storage applications. However, developing suitable cathode materials for ZIBs remains a great challenge. In this work, pioneering work on the designing and construction of aqueous Zn//Na0.33V2O5 batteries is reported. The Na0.33V2O5 (NVO) electrode delivers a high capacity of 367.1 mA h g−1 at 0.1 A g−1, and exhibits long‐term cyclic stability with a capacity retention over 93% for 1000 cycles. The improvement of electrical conductivity, resulting from the intercalation of sodium ions between the [V4O12]n layers, is demonstrated by single nanowire device. Furthermore, the reversible intercalation reaction mechanism is confirmed by X‐ray diffraction, Raman, X‐ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analysis. The outstanding performance can be attributed to the stable layered structure and high conductivity of NVO. This work also indicates that layered structural materials show great potential as the cathode of ZIBs, and the indigenous ions can act as pillars to stabilize the layered structure, thereby ensuring an enhanced cycling stability.

more than four decades ago with a TiS 2based cathode prototype battery, [ 3 ] which was followed shortly thereafter by Moli Energy's brief commercialization of a Li/ MoS 2 battery. Unfortunately, prodigious battery capacity losses were observed when Li metal was used as the anode, especially for high current density charging, which resulted in rapid cell failure and safety concerns. Li metal was therefore replaced with carbon coke and later graphitic carbon as an anode. Subsequently, intercalation cathode materials, such as LiCoO 2 and LiFePO 4 , were then discovered and these, in concert with graphitic carbon, now form the foundation of today's Li-ion batteries. [ 4,5 ] In general, however, Li metal continues to be used in three different categories for battery systems: 1) as a counter electrode in half-cells to evaluate the properties of cathode or anode materials such as LiNi 1/3 Mn 1/3 Co 1/3 O 2 or Si, respectively; 2) as an anode to study cathode materials such as V 2 O 5 , which have no Li source in the lattice; and 3) as an anode for next-generation, high-energy storage technologies such as Li-S and Li-O 2 batteries, as well as Li-S hybrid redox fl ow batteries. 7-9 For these high-energy systems, the Li metal is indispensable, thus marking the importance of obtaining a fundamental understanding of the Li metal failure mechanism during cell cycling.When compared with the original pristine, dense Li metal, the redeposited Li always displays a signifi cantly different morphology, i.e., mossy Li. In addition, some of the redeposited Li may gradually or suddenly lose electrical contact with the bulk material thus becoming inactive in the cell after repeated cycling. [ 6 ] The morphological transformation from dense to porous Li metal also leads to the uneven distribution of the electric fi eld in the Li anode resulting in an evolution in the electrochemical reactions during subsequent electrode cycling, further accelerating the inhomogeneous Li deposition. The end result is generally reported to be the growth of dendritic Li metal, which protrudes from the anode surface leading to cell shorting when contact is made with the cathode. [ 7,8 ] Much effort has been devoted to preventing this dendrite growth. A few common strategies can be identifi ed, including the 1) formation of Li-Al or Li-Mg alloys, [ 2,9 ] 2) use In recent years, the Li metal anode has regained a position of paramount research interest because of the necessity for employing Li metal in nextgeneration battery technologies such as Li-S and Li-O 2 . Severely limiting this utilization, however, are the rapid capacity degradation and safety issues associated with rechargeable Li metal anodes. A fundamental understanding of the failure mechanism of Li metal at high charge rates has remained elusive due to the complicated interfacial chemistry that occurs between Li metal and liquid electrolytes. Here, it is demonstrated that at high current density the quick formation of a highly resistive solid electrolyte interphase (SEI) entangled with Li metal,...

Voltage and capacity fading of layer structured lithium and manganese rich (LMR) transition metal oxide is directly related to the structural and composition evolution of the material during the cycling of the battery. However, understanding such evolution at atomic level remains elusive. On the basis of atomic level structural imaging, elemental mapping of the pristine and cycled samples, and density functional theory calculations, it is found that accompanying the hoping of Li ions is the simultaneous migration of Ni ions toward the surface from the bulk lattice, leading to the gradual depletion of Ni in the bulk lattice and thickening of a Ni enriched surface reconstruction layer (SRL). Furthermore, Ni and Mn also exhibit concentration partitions within the thin layer of SRL in the cycled samples where Ni is almost depleted at the very surface of the SRL, indicating the preferential dissolution of Ni ions in the electrolyte. Accompanying the elemental composition evolution, significant structural evolution is also observed and identified as a sequential phase transition of C2/m → I41 → Spinel. For the first time, it is found that the surface facet terminated with pure cation/anion is more stable than that with a mixture of cation and anion. These findings firmly established how the elemental species in the lattice of LMR cathode transfer from the bulk lattice to surface layer and further into the electrolyte, clarifying the long-standing confusion and debate on the structure and chemistry of the surface layer and their correlation with the voltage fading and capacity decaying of LMR cathode. Therefore, this work provides critical insights for design of cathode materials with both high capacity and voltage stability during cycling.