Conditioning the Surface and Bulk of High-Nickel Cathodes with a Nb Coating: An In Situ X-ray Study.

Surface coating is commonly employed by industries to improve the cycling and thermal stability of high-nickel (Ni) transition metal (TM) layered cathodes for their practical use in lithium-ion batteries. Niobium (Nb) coating or substitution has been shown to be effective in stabilizing LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes; in addition, the electrochemical performance of the final products varies depending on the postprocessing. In this follow-up study, we use in situ synchrotron X-ray diffraction to investigate the kinetic processes and the involved structural evolution in Nb-coated NMC811 upon heat treatment. Quantitative structure analysis reveals thermally driven concurrent changes in the bulk and surface, in particular, the phase evolution of the coating layer and Nb/TM interdiffusion that facilitates penetration of Nb into the bulk and particle growth at the increased temperatures. Findings from this study highlight the new opportunities for the intended control of the structure and surface properties of high-Ni cathodes through surface coating in conjunction with postprocessing.

Nanocrystal Conversion-Assisted Design of Sn-Fe Alloy with a Core-Shell Structure as High-Performance Anodes for Lithium-Ion Batteries.

Sn-based alloy materials are strong candidates to replace graphitic carbon as the anode for the next generation lithium-ion batteries because of their much higher gravimetric and volumetric capacity. A series of nanosize Sn y Fe alloys derived from the chemical transformation of preformed Sn nanoparticles as templates have been synthesized and characterized. An optimized Sn5Fe/Sn2Fe anode with a core-shell structure delivered 541 mAh·g-1 after 200 cycles at the C/2 rate, retaining close to 100% of the initial capacity. Its volumetric capacity is double that of commercial graphitic carbon. It also has an excellent rate performance, delivering 94.8, 84.3, 72.1, and 58.2% of the 0.1 C capacity (679.8 mAh/g) at 0.2, 0.5, 1 and 2 C, respectively. The capacity is recovered upon lowering the rate. The exceptional cycling/rate capability and higher gravimetric/volumetric capacity make the Sn y Fe alloy a potential candidate as the anode in lithium-ion batteries. The understanding of Sn y Fe alloys from this work also provides insight for designing other Sn-M (M = Co, Ni, Cu, Mn, etc.) system.

Li-Nb-O Coating/Substitution Enhances the Electrochemical Performance of the LiNi0.8Mn0.1Co0.1O2 (NMC 811) Cathode.

High-nickel layered oxides, such as NMC 811, are very attractive high energy density cathode materials. However, the high nickel content creates a number of challenges, including high surface reactivity and structural instability. Through a wet chemistry method, a Li-Nb-O coated and substituted NMC 811 was obtained in a single step treatment. This Li-Nb-O treatment not only supplied a protective surface coating but also optimized the electrochemical behavior by Nb5+ incorporation into the bulk structure. As a result, the 1st capacity loss was significantly reduced (13.7 vs 25.1 mA h/g), contributing at least a 5% increase to the energy density of the full cell. In addition, both the rate (158 vs 135 mA h/g at 2C) and capacity retention (89.6 vs 81.6% after 60 cycles) performance were enhanced.

A New Intermetallic NiSn5 Phase: Induced Synthesis, Crystal Structure Resolution, and Investigation of Its Mechanism.

Benefiting from the nanoscale effect, some metastable compounds can be synthesized in nanoparticles under normal conditions. The new intermetallic NiSn5 phase is synthesized by us for the first time by using a seed crystal induction method. This tetragonal phase in the P4/ mcc space group has stoichiometric Ni atom defects, yielding Ni0.62Sn5. A study of the growth mechanism reveals that the FeSn5/CoSn5 seed crystal plays a vital role in the formation of the NiSn5 phase. An investigation of the phase evolution during lithiation/delithiation processes indicates the irreversibility of NiSn5 as an anode for lithium ion batteries.

ε- and ß-LiVOPO4: Phase Transformation and Electrochemistry.

ε- and ß-LiVOPO4 were synthesized from the same precursor at different temperatures in an air atmosphere. ε-LiVOPO4 is obtained at 400 and 700 °C. The 700 °C sample has better purity and crystallinity, but the 400 °C sample has a little better electrochemical performance due to its smaller particle size and the conducting carbon residue in the sample. ß-LiVOPO4 is formed between the above two temperatures, which gives slightly lower capacity than that of the ε-LiVOPO4 sample, indicating higher kinetics of the lithium reaction for the ε phase than those of the ß one. The phase transformation from ε to ß then back reversibly to ε was also observed by ex situ X-ray diffraction. This thermal study verifies that ε-LiVOPO4 is the more stable phase for LiVOPO4; however, reaction kinetics control the phases formed at lower temperatures.

Hollow silica-copper-carbon anodes using copper metal-organic frameworks as skeletons.

Hollow silica-copper-carbon (H-SCC) nanocomposites are first synthesized using copper metal-organic frameworks as skeletons to form Cu-MOF@SiO(2) and then subjected to heat treatment. In the composites, the hollow structure and the void space from the collapse of the MOF skeleton can accommodate the huge volume change, buffer the mechanical stress caused by lithium ion insertion/extraction and maintain the structural integrity of the electrode and a long cycling stability. The ultrafine copper with a uniform size of around 5 nm and carbon with homogeneous distribution from the decomposition of the MOF skeleton can not only enhance the electrical conductivity of the composite and preserve the structural and interfacial stabilization, but also suppress the aggregation of silica nanoparticles and cushion the volume change. In consequence, the resulting material as an anode for lithium-ion batteries (LIBs) delivers a reversible capacity of 495 mA h g(-1) after 400 cycles at a current density of 500 mA g(-1). The synthetic method presented in this paper provides a facile and low-cost strategy for the large-scale production of hollow silica/copper/carbon nanocomposites as an anode in LIBs.

Enhanced Electrochemical Performance of Fe0.74Sn5@Reduced Graphene Oxide Nanocomposite Anodes for Both Li-Ion and Na-Ion Batteries.

The recently found intermetallic FeSn5 phase with defect structure Fe0.74Sn5 has shown promise as a high capacity anode for lithium-ion batteries (LIBs). The theoretical capacity is as high as 929 mAh g(-1) thanks to the high Sn/Fe ratio. However, despite being an alloy, the cycle life remains a great challenge. Here, by combining Fe0.74Sn5 nanospheres with reduced graphene oxide (RGO) nanosheets, the Fe0.74Sn5@RGO nanocomposite can achieve capacity retention 3 times that of the nanospheres alone, after 100 charge/discharge cycles. Moreover, the nanocomposite also displays its versatility as a high-capacity anode in sodium-ion batteries (SIBs). The enhanced cell performance in both battery systems indicates that the Fe0.74Sn5@RGO nanocomposite can be a potential anode candidate for the application of Li-ion and Na-ion battery.