Boosting the Electrochemical Performance of Li-S Batteries with a Dual Polysulfides Confinement Strategy.

Lithium-sulfur (Li-S) batteries have attracted more and more attention because they represent one of the most promising candidates to satisfy emerging energy storage demands. The biggest challenge regarding the application of the Li-S battery is to suppress the polysulfide shuttle while maintaining a high sulfur loading mass. Here, a dual polysulfide confinement strategy is designed by confinement of sulfur in polydopamine-coated MXene nanosheets (denoted as S@Mxe@PDA) that performs as a high-performance cathode for Li-S cells owing to their inherently high underlying metallic conductivity and chemical bonding and strong chemical adsorption to lithium polysulfides (LPs). This dual LPs confinement strategy is supported by the results of density functional theory calculations. It is demonstrated that the S@Mxe@PDA cathode exhibits outstanding electrochemical properties, including high reversible capacity (1044 mAh g-1 after 150 cycles at 0.2 C), superior rate capability (624 mAh g-1 at 6 C) and excellent cycling stability (556 mAh g-1 after 330 cycles at 0.5 C with 4.4 mg cm-2 sulfur loading). This work offers a facile and effective method for boosting Li-S batteries into practical applications.

Carbon nanofiber interlayer: a highly effective strategy to stabilize silicon anodes for use in lithium-ion batteries.

Silicon (Si) possesses the highest theoretical capacity as an anode material for lithium-ion batteries, and many efforts have been made to address the poor cycling stability issue that is associated with its huge volume changes during Li-Si alloying/de-alloying processes, mostly through the design of nanostructured materials. Herein, we report a simple cell configuration approach to improve the lithium storage performance of commercial nano-Si through the insertion of carbon nanofiber films (CNFs) as interlayers between the Si electrodes and separators. For this advanced cell configuration, commercial Si nanoparticle (Si NP) electrodes demonstrate a significantly improved reversible capacity (2700 mA h g-1 after 40 cycles at 50 mA g-1) and an ultralong cycle life (1250 mA h g-1 after 430 cycles at 1500 mA g-1). Even when cycled at 4 A g-1, the material still demonstrates a very high capacity of 870 mA h g-1. The excellent electrochemical performance of the Si NPs is attributed to the novel cell configuration. Macropores between the carbon nanofibers provide good access of the electrolyte to the Si NP electrodes. The 3D interconnected networks of the CNF interlayer not only decrease the internal charge transfer resistance and enhance the electron transport rate but also offer electron pathways along the CNF interlayer for cracked and disconnected Si NPs after cycling.

Amorphous Red Phosphorus Embedded in Sandwiched Porous Carbon Enabling Superior Sodium Storage Performances.

The red P anode for sodium ion batteries has attracted great attention recently due to the high theoretical capacity, but the poor intrinsic electronic conductivity and large volume expansion restrain its widespread applications. Herein, the red P is successfully encapsulated into the cube shaped sandwich-like interconnected porous carbon building (denoted as P@C-GO/MOF-5) via the vaporization-condensation method. Superior cycling stability (high capacity retention of about 93% at 2 A g-1 after 100 cycles) and excellent rate performance (502 mAh g-1 at 10 A g-1 ) can be obtained for the P@C-GO/MOF-5 electrode. The superior electrochemical performance can be ascribed to the successful incorporation of red P into the unique carbon matrix with large surface area and pore volume, interconnected porous structure, excellent electronic conductivity and superior structural stability.

Carbon and Carbon Hybrid Materials as Anodes for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have attracted much attention for application in large-scale grid energy storage owing to the abundance and low cost of sodium sources. However, low energy density and poor cycling life hinder practical application of SIBs. Recently, substantial efforts have been made to develop electrode materials to push forward large-scale practical applications. Carbon materials can be directly used as anode materials, and they show excellent sodium storage performance. Additionally, designing and constructing carbon hybrid materials is an effective strategy to obtain high-performance anodes for SIBs. In this review, we summarize recent research progress on carbon and carbon hybrid materials as anodes for SIBs. Nanostructural design to enhance the sodium storage performance of anode materials is discussed, and we offer some insight into the potential directions of and future high-performance anode materials for SIBs.

Expanding pore sizes of ZIF-8-derived nitrogen-doped microporous carbon via C60 embedding: toward improved anode performance for the lithium-ion battery.

Porous carbon and nanocarbons have been extensively applied as anode materials for high-energy density lithium-ion batteries (LIBs). However, as another representative nanocarbon, fullerenes, such as C60, have been scarcely utilized in LIBs because of their poor electrochemical reversibility. Herein, we designed a novel C60-embedded nitrogen-doped microporous carbon material (denoted as C60@N-MPC), which was derived from a zeolitic imidazolate framework-8 (ZIF-8) precursor, demonstrating its promising application as a superior anode material for LIB. We first embedded C60in situ into a ZIF-8 matrix via a facile solid-state mechanochemical route, which acted as a precursor and was transformed to C60@N-MPC after carbonization. The C60@N-MPC was applied as a novel anode for LIBs, showing an improved reversible specific capacity of ≈1351 mA h g-1 at 0.1 A g-1 and a better rate capacity (≈1077 mA h g-1 at 1 A g-1 after 400 cycles) relative to those based on the unmodified N-MPC anode. The role of C60 in the superior lithium storage performance of C60@N-MPC was elucidated, revealing that C60 functioned as a pore expander for N-MPC with 3-20 nm mesopores (versus sub-1 nm micropores for the unmodified N-MPC), which facilitated the rapid diffusion of the organic electrolyte.

Carbon-Coated Na3V2(PO4)3 Anchored on Freestanding Graphite Foam for High-Performance Sodium-Ion Cathodes.

Na3V2(PO4)3 (NVP) has been considered as a most promising cathode material for sodium-ion batteries (SIBs), but NVP usually exhibits poor cycling stability and rate performance due to the low intrinsic electrical conductivity. Herein, we prepared carbon-coated Na3V2(PO4)3 anchored on freestanding graphite foam (denoted as NVP@C-GF) as a cathode for SIBs. The NVP@C-GF exhibits superior sodium-ion storage performance, including rate capability (56 mAh g-1 at 200 C) and long cycle life (54 mAh g-1 at 100 C after 20 000 cycles). The resulting NVP@C-GF inherits the advantages of 3D free-standing graphite that possesses high electrical conductivity and porous structure for the electrolyte to soak in. Furthermore, carbon-coated NVP particles anchored on the surface of GF not only accommodate the volume change of NVP during charge/discharge but also reduce the diffusion distance of the Na+ ion.

Amorphous Red Phosphorus Embedded in Highly Ordered Mesoporous Carbon with Superior Lithium and Sodium Storage Capacity.

Red phosphorus (P) have been considered as one of the most promising anode material for both lithium-ion batteries (LIBs) and (NIBs), because of its high theoretical capacity. However, natural insulating property and the large volume expansion of red P during cycling lead to poor cyclability and low rate performance, which prevents its practical application. Here, we significantly improves both lithium storage and sodium storage performance of red P by confining nanosized amorphous red P into the mesoporous carbon matrix (P@CMK-3) using a vaporization-condensation-conversion process. The P@CMK-3 shows a high reversible specific capacity of ∼ 2250 mA h g(-1) based on the mass of red P at 0.25 C (∼ 971 mA h g(-1) based on the composite), excellent rate performance of 1598 and 624 mA h g(-1) based on the mass of red P at 6.1 and 12 C, respectively (562 and 228 mA h g(-1) based on the mass of the composite at 6.1 and 12 C, respectively) and significantly enhanced cycle life of 1150 mA h g(-1) based on the mass of red P at 5 C (500 mA h g(-1) based on the mass of the composite) after 1000 cycles for LIBs. For Na ions, it also displays a reversible capacity of 1020 mA h g(-1) based on the mass of red P (370 mA h g(-1) based on the mass of the composite) after 210 cycles at 5C. The significantly improved electrochemical performance could be attributed to the unique structure that combines a variety of advantages: easy access of electrolyte to the open channel structure, short transport path of ions through carbon toward the red P, and high ionic and electronic conductivity.

General Strategy for Fabricating Sandwich-like Graphene-Based Hybrid Films for Highly Reversible Lithium Storage.

We report a general strategy for the fabrication of freestanding sandwich-like graphene-based hybrid films by electrostatic adsorption and following reduction reaction. We demonstrate that by rational control of pH value in precursors, graphene oxide (GO) sheets can form three-dimensional (3D) sandwich frameworks with nanoparticles decorated between the layers of graphene. In our proof-of-concept study, we prepared the graphene/Si/graphene (G@Si@G) sandwich-like films. When used as negative electrode materials for lithium-ion batteries, it exhibits superior lithium-ion storage performance (∼1800 mA h g(-1) after 40 cycles at 100 mA g(-1)). Importantly, with this simple and general method, we also successfully synthesized graphene/Fe2O3/graphene and graphene/TiO2/graphene hybrid films, showing improved electrochemical performance. The good electrochemical property results from the enhanced electron transport rate, and the 3D flexible matrix to buffer volume changes during cycling. In addition, the porous sandwich structure consisting of plate-like graphene with high surface area provides effective electrolyte infiltration and promotes diffusion rate of Li(+), leading to an improved rate capability.

Carbon-Coated Germanium Nanowires on Carbon Nanofibers as Self-Supported Electrodes for Flexible Lithium-Ion Batteries.

A hybrid structure with carbon-coated germanium nanowires grown on the surface of carbon nanofibers is fabricated using an in situ vapor-liquid-solid process. It is used as a self-supported and flexible anode for Li-ion batteries.

Free-standing porous carbon nanofibers-sulfur composite for flexible Li-S battery cathode.

Flexible and free-standing sulphur/(PCNFs-CNT) composite (S@PCNFs-CNT) electrode was successfully prepared by infiltrating sulfur into microporous carbon nanofibers-carbon nanotube (PCNFs-CNT) composite. When used as a cathode material for Li-S batteries, the S@PCNFs-CNT exhibits much better cycle performance and rate performance compared to CNT-free S@PCNFs. It delivers a reversible capacity of 637 mA h g(-1) after 100 cycles at 50 mA g(-1) and a rate capability of 437 mA h g(-1) at 1 A g(-1). The improved electrochemical performance is attributed to synergistic effect of the 3D interconnected structure, the additive of CNT, and the uniform distribution of micropores (<2 nm) in the PCNFs-CNT matrix. Our results indicate the potential suitability of PCNFs-CNT for efficient, free-standing, and high-performance batteries.

Superior lithium storage in a 3D macroporous graphene framework/SnO2 nanocomposite.

A three-dimensional (3D) interconnected graphene framework (GF)-based SnO2 nanocomposite (3D SnO2/GFs) was prepared using self-assembly of polystyrene (PS)@SnO2 nanospheres and graphene oxide (GO) nanosheets under suitable pH conditions, followed by a thermal treatment. The electroactive material (SnO2) is anchored to the wall of electrochemically and ionically conductive 3D interconnected GFs. When used as anodes for LIBs, the 3D SnO2/GFs deliver an excellent reversible capacity (1244 mA h g(-1) in 50 cycles at 100 mA g(-1)) and outstanding rate capability (754 mA h g(-1) in 200 cycles at 1000 mA g(-1)). The ultra-small size of SnO2 (sub 10 nm) and dimensional confinement of SnO2 nanoparticles by the wall of GFs limit the volume expansion upon lithium insertion, and the 3D interconnected porous structures serve as buffered spaces during charge-discharge and result in superior electrochemical performance by facilitating the electrolyte to contact the entire nanocomposite materials and reduce lithium diffusion length in the nanocomposite.

Germanium nanoparticles encapsulated in flexible carbon nanofibers as self-supported electrodes for high performance lithium-ion batteries.

Germanium is a promising high-capacity anode material for lithium ion batteries, but still suffers from poor cyclability due to its huge volume variation during the Li-Ge alloy/dealloy process. Here we rationally designed a flexible and self-supported electrode consisting of Ge nanoparticles encapsulated in carbon nanofibers (Ge-CNFs) by using a facile electrospinning technique as potential anodes for Li-ion batteries. The Ge-CNFs exhibit excellent electrochemical performance with a reversible specific capacity of ∼1420 mA h g(-1) after 100 cycles at 0.15 C with only 0.1% decay per cycle (the theoretical specific capacity of Ge is 1624 mA h g(-1)). When cycled at a high current of 1 C, they still deliver a reversible specific capacity of 829 mA h g(-1) after 250 cycles. The strategy and design are simple, effective, and versatile. This type of flexible electrodes is a promising solution for the development of flexible lithium-ion batteries with high power and energy densities.