Structure and Charge Regulation Strategy Enabling Superior Cyclability for Ni-Rich Layered Cathode Materials.

Ni-rich layered oxides are significantly promising cathode materials for commercial high-energy-density lithium-ion batteries. However, their major bottlenecks limiting their widespread applications are capacity fading and safety concerns caused by their inherently unstable crystal structure and highly reactive surface. Herein, surface structure and bulk charge regulation are concurrently achieved by introducing high-valence Ta5+ ions in Ni-rich cathodes, which exhibit superior electrochemical properties and thermal stability, especially a remarkable cyclic stability with a capacity retention of 80% for up to 768 cycles at a 1C rate versus Li/Li+ . Due to the partial Ta enrichment on surface, the regulated surface enables high reversibility of Li+ insertion/extraction by preventing surface Ni reduction in deep charging. Moreover, bulk charge regulation that boosts charge density and its localization on oxygen remarkably suppresses microcracks and oxygen loss, which in turn prevents the fragmentation of the regulated surface and structural degradation associated with oxygen skeleton. This study highlights the significance of an integrated optimization strategy for Ni-rich cathodes and, as a case study, provides a novel and deep insights into the underlying mechanisms of high-valence ions substitution of Ni-rich layered cathodes.

High Capacitive Energy Storage of Nest-Like Porous Graphene Microspheres Electrode with High Mass Loading.

Nest-like porous graphene microspheres (NPGMs) are grown by using a chemical vapor deposition (CVD) method in a fluidized bed reactor from methane and basic magnesium carbonate microspheres (synthesized by a stirring-induced crystallization approach) as carbon source and template, respectively. The CVD-derived NPGMs have a few-layer structure and high electrical conductivity, as well as a three-dimensional individual macroarchitecture accompanied with well-developed pore channels and great structural integrity. As the electrode for a symmetric supercapacitor, the effect of different mass loadings for NPGMs-based electrodes on the capacitive energy-storage performance is investigated. Superior electrochemical properties with respect to gravimetric, areal, and total capacitances, rate capability, and durability are shown by the NPGMs-based symmetric supercapacitors, even at mass loadings up to 10 mg cm-2 . Moreover, the electrochemical behavior of the NPGMs-based electrode is much superior to those of two-dimensional lamella-like graphene and commercial activated carbon.

MnO@graphene nanopeapods derived via a one-pot hydrothermal process for a high performance anode in Li-ion batteries.

Although transition metal oxide-carbon (TMO-C) composites exhibit high Li storage capacity, the weak bonding between TMO particles and carbon mainly via van der Waals' force and the limited internal void space result in poor rate capability and cycling performance. Herein, MnO@graphene nanopeapods are produced by calcination of hydrothermally-synthesized MnO2-C composites. The flexible graphene shells provide superior conductivity and excellent structural stability to the MnO cores, and the enough internal void space can significantly buffer the drastic volume expansion. The MnO@graphene nanopeapods exhibit high Li storage capacity (1168 mA h g-1 at 50 mA g-1 and 945 mA h g-1 at 500 mA g-1) at a voltage platform of ∼1.2 V, excellent rate capability (728 mA h g-1 at 1000 mA g-1 and 505 mA h g-1 at 3000 mA g-1), high initial coulombic efficiency (85.9%) and remarkable long-life cycling performance (undiminished after 1000 cycles). The MnO@graphene nanopeapods have been successfully used as the anode to assemble a full battery with LiFePO4 as the cathode. Our results provide a useful and rational strategy to design high performance graphene-supported MnO composites for Li ion batteries.

Enhancing the Li storage capacity and initial coulombic efficiency for porous carbons by sulfur doping.

Here, we report a new approach to synthesizing S-doped porous carbons and achieving both a high capacity and a high Coulombic efficiency in the first cycle for carbon nanostructures as anodes for Li ion batteries. S-doped porous carbons (S-PCs) were synthesized by carbonization of pitch using magnesium sulfate whiskers as both templates and S source, and a S doping up to 10.1 atom % (corresponding to 22.5 wt %) was obtained via a S doping reaction. Removal of functional groups or highly active C atoms during the S doping has led to formation of much thinner solid-electrolyte interface layer and hence significantly enhanced the Coulombic efficiency in the first cycle from 39.6% (for the undoped porous carbon) to 81.0%. The Li storage capacity of the S-PCs is up to 1781 mA h g(-1) at the current density of 50 mA g(-1), more than doubling that of the undoped porous carbon. Due to the enhanced conductivity, the hierarchically porous structure and the excellent stability, the S-PC anodes exhibit excellent rate capability and reliable cycling stability. Our results indicate that S doping can efficiently promote the Li storage capacity and reduce the irreversible Li combination for carbon nanostructures.

Phosphorus and nitrogen dual-doped few-layered porous graphene: a high-performance anode material for lithium-ion batteries.

Few-layered graphene networks composed of phosphorus and nitrogen dual-doped porous graphene (PNG) are synthesized via a MgO-templated chemical vapor deposition (CVD) using (NH4)3PO4 as N and P source. P and N atoms have been substitutionally doped in graphene networks since the doping takes place at the same time with the graphene growth in the CVD process. Raman spectra show that the amount of defects or disorders increases after P and N atoms are incorporated into graphene frameworks. The doping levels of P and N measured by X-ray photoelectron spectroscopy are 0.6 and 2.6 at %, respectively. As anodes for Li ion batteries (LIBs), the PNG electrode exhibits high reversible capacity (2250 mA h g(-1) at the current density of 50 mA g(-1)), excellent rate capability (750 mA h g(-1) at 1000 mA g(-1)), and satisfactory cycling stability (no capacity decay after 1500 cycles), showing much enhanced electrode performance as compared to the undoped few-layered porous graphene. Our results show that the PNG is a promising candidate for anode materials in high-rate LIBs.