1606422(1 of 8) carbon-based anode, which does not participate in redox reactions and only serves as supporting materials for ion intercalation, Li-metal anode undergoes a striping/ plating process during cycling and is unable to act as a "host" for ion deposition, thus the morphology of the redeposited Li metal in subsequent cycles is generally hard to control. [3,5,6] The charge-discharge of an Li-metal anode also accompanies a virtually relative infinite volume change (anode volume at completely charged state vs its volume at completely discharged state), which would cause internal pressure change and interface fluctuation. [7] Once uneven Li electrodeposits nucleate and shoot out from the anode surface, the solid electrolyte interface (SEI) could continuously break and form. Its instability in turn accelerates the growth of Li dendrite. [8] As a consequence, an additional amount of electrolytes is consumed and "dead Li" accumulates, hence reducing the cell Coulombic efficiency and causing capacity fading. The sharp dendritic structures of Li could pierce the separator and cause short circuit and even explosion. [9,10] These drawbacks of lithium-metal anode hinder the practical applications of rechargeable Li-metal batteries including Li-sulfur [11][12][13][14] and Li-air [15,16] batteries over the past 40 years.To regulate the Li-metal anode, plenty of efforts have been made on all the essential components of a battery cell in order to keep the growth of Li dendrites under control and to stabilize the SEI formation. [10,[17][18][19] In terms of electrolytes, conventional liquid electrolytes consisting of compatible solvents and Li salts have been intensively studied to improve the stability of electrolytes/electrode interfaces. [20][21][22] Using solid or gel electrolytes with relatively high mechanical strength [23][24][25] to replace the conventional liquid electrolytes also gained great achievement in cycling stabilization, and electrolyte additives such as lithium fluoride (LiF), [19] lithium nitrate (LiNO 3 ) , Cs + , Rb + , [26] and lithium polysulfide (Li 2 S x ) [27] have been revealed to be beneficial for forming better SEI. More recently, attention has been moved to the design of Li anodes and current collectors, in order to inhibit the growth of Li dendrites and to regulate the cycling behavior of lithium. Lowering the local current density along the anode surface with high surface area current collector (i.e., 3D carbon nanofibers [28] and 3D porous Cu foil [29] ), anode host (i.e., porous polyimide membrane, [30] polyimide particles, [31] and layered reduced graphene oxide [7] ), Lithium-metal batteries are of particular interest for next-generation electrical energy storage because of their high energy density on both volumetric and gravimetric bases. Effective strategies to stabilize the Li-metal anode are the prerequisite for the progress of these exceptional storage technologies, such as Li-S and Li-O 2 batteries. Various challenges, such as uneven Li electrodeposition, anode volume expansion...
Carbon nanotubes (CNTs) are of great interest for many potential applications because of their extraordinary electronic, mechanical and structural properties. However, issues of chaotic staking, high cost and high energy dissipation in the synthesis of CNTs remain to be resolved. Here we develop a facile, general and high-yield strategy for the oriented formation of CNTs from metal-organic frameworks (MOFs) through a low-temperature (as low as 430 °C) pyrolysis process. The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources. The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process. This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs, which further homogeneously incorporate heteroatoms into the CNTs. Specifically, nitrogen-doped CNT-assembled hollow structures exhibit excellent performances in both energy conversion and storage. On the basis of experimental analyses and density functional theory simulations, these superior performances are attributed to synergistic effects between ideal components and multilevel structures. Additionally, the appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. The viable synthetic strategy and proposed mechanism will stimulate the rapid development of CNTs in frontier fields.
Li-rich layered oxides have attracted much attention for their potential application as cathode materials in lithium ion batteries, but still suffer from inferior cycling stability and fast voltage decay during cycling. How to eliminate the detrimental spinel growth is highly challenging in this regard. Herein, in situ K(+)-doped Li1.20Mn0.54Co0.13Ni0.13O2 was successfully prepared using a potassium containing α-MnO2 as the starting material. A systematic investigation demonstrates for the first time, that the in situ potassium doping stabilizes the host layered structure by prohibiting the formation of spinel structure during cycling. This is likely due to the fact that potassium ions in the lithium layer could weaken the formation of trivacancies in lithium layer and Mn migration to form spinel structure, and that the large ionic radius of potassium could possibly aggravate steric hindrance for spinel growth. Consequently, the obtained oxides exhibited a superior cycling stability with 85% of initial capacity (315 mA h g(-1)) even after 110 cycles. The results reported in this work are fundamentally important, which could provide a vital hint for inhibiting the undesired layered-spinel intergrowth with alkali ion doping and might be extended to other classes of layered oxides for excellent cycling performance.
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