Rooting MnO2 nanosheet on carbon nanoboxes as efficient catalytic host for lithium–sulfur battery

Crystal Facet Engineering of MXene-Derived TiN Nanoflakes as Efficient Bidirectional Electrocatalyst for Advanced Lithium-Sulfur Batteries

The design of nanomaterials with grain orientation structure by crystal facet engineering is of great significance for boosting the catalytic ability and electrochemical properties, but the controllable synthesis is still a challenge. Here, TiN nanoflakes with exposed (001) facets are prepared using 2D Ti₃ C₂ MXene as the initial reactant and applied as a bidirectional electrocatalyst for the reduction and oxidation process in lithium-sulfur batteries (LSBs). The (001) facet-dominated TiN nanoflakes have a strong adsorption capacity for soluble lithium polysulfides (LiPSs). More importantly, theoretical calculations and experiment results confirm the (001) facet-dominated TiN nanoflakes catalyze the conversion of soluble LiPSs to Li₂ S₂ /Li₂ S to induce the Li₂ S uniform deposition in the discharge process and decrease the delithiation barrier of Li₂ S in the charge process. Therefore, the excellent electrochemical properties of LSBs are achieved, which demonstrates a high discharge capacity of 949 mAh g^-1 at 1 C and maintains high capacity reversibility with a decay rate of 0.033% per cycle after 800 cycles.

A conductive framework embedded with cobalt-doped vanadium nitride as an efficient polysulfide adsorber and convertor for advanced lithium-sulfur batteries

The industrialization and commercialization of Li-S batteries are greatly hindered by several defects such as the sluggish reaction kinetics, polysulfide shuttling and large volume expansion. Herein, we propose a heteroatom doping method to optimize the electronic structure for enhancing the adsorption and catalytic activity of VN that is in situ embedded into a spongy N-doped conductive framework, thus obtaining a Co-VN/NC multifunctional catalyst as an ideal sulfur host. The synthesized composite has both the unique structural advantages and the synergistic effect of cobalt, VN, and nitrogen-doped carbon (NC), which not only improve the polysulfide anchoring of the sulfur cathode but also boost the kinetics of polysulfide conversion. The density functional theory (DFT) calculations revealed that Co doping could enrich the d orbit electrons of VN for elevating the d band center, which improves its interaction with lithium polysulfides (LiPSs) and accelerates the interfacial electron transfer, simultaneously. As a result, the batteries present a high initial discharge capacity of 1521 mA h g^-1 at 0.1 C, good rate performance, and excellent cycling performances (∼876 mA h g^-1 at 0.5 C after 300 cycles and ∼490 mA h g^-1 at 2 C after 1000 cycles, respectively), even with a high areal sulfur loading of 4.83 mg cm^-2 (∼4.70 mA h cm^-2 at 0.2 C after 100 cycles). This well-designed work provides a good strategy to develop effective polysulfide catalysis and further obtain high-performance host materials for Li-S batteries.

Self-induced matrix with Li-ion storage activity in ultrathin CuMnO2 nanosheets electrode

Conversion anode materials such as Mn₃O₄ draw much attention due to their considerable theoretical capacity for lithium-ion batteries (LIBs). However, poor conductivity, slow solid-state Li-ion diffusion, and huge volume expansion of the active materials during charge/discharge lead to unsatisfied electrochemical performance. Despite several strategies like nanocrystallization, fabricating hierarchical nanostructures, and introducing a matrix are valid to address these crucial issues, the achieved electrochemical performance still needs to be further enhanced. What is worse, the matrix with less or no Li-ion storage activity may lower the achieved capacity of the electrodes. Herein, ultra-thin CuMnO₂ nanosheets with the thickness of 5-8 nm were evaluated for LIBs. The ultra-thin sheet-like nanostructure offers sufficient contact areas with electrolyte and shortens the Li-ion diffusion distance. Moreover, the in-situ generated Mn and Cu along with their oxides could play the role of matrix and conductive agent in turn at different stages, relieving the stress brought by volume expansion. Therefore, the as-prepared ultra-thin CuMnO₂ nanosheets electrode displays a remarkable reversible capacity, long cycling stability, and outstanding rate capability (a reversible capacity of 1160.5 mAh g^-1 at 0.1A g^-1 was retained after 100 cycles with a capacity retention of 95.1 %, and 717.8 mAh g^-1 at 2.0 A g^-1 after 400 cycles).

TiN@C nanocages as multifunctional sulfur hosts for superior lithium-sulfur batteries

The lithium polysulfide (LiPS) shuttle effect and low redox kinetics are the key problems that hinder performance improvement and prevent achieving the commercial requirements for lithium-sulfur batteries (LSBs), and the reasonable construction of sulfur hosts is one effective strategy to relieve the polysulfide shuttle effect and improve redox kinetics. Herein, N-doped carbon nanocages decorated with homogeneously dispersed TiN nanoparticles (TiN@C NCs) as multifunctional sulfur hosts are designed for superior LSBs. Carbon nanocages provide space to mitigate volume expansion and provide additional physical adsorption to trap the LiPSs. Polar TiN nanoparticles not only exhibit the chemisorption capacity for LiPSs, but also catalyze and promote the conversion of long-chain LiPSs to Li₂S₂/Li₂S in the reduction process as well as the decomposition of Li₂S in the oxidation reaction, which significantly boosts electron/ion transport and decreases the potential barrier. Therefore, the S/TiN@C NC cathode has an excellent electrochemical capacity of 1485.7 mA h g^-1 at 0.1 C. In particular, the cathode demonstrates high capacity reversibility after 500 cycles at 3 C with a retention of about 73.1%, which is equivalent to a slow capacity decay rate of 0.053% per cycle.

A vanadium-based oxide-nitride heterostructure as a multifunctional sulfur host for advanced Li-S batteries

The commercial application of lithium-sulfur (Li-S) batteries is obstructed by the inherent dissolution/shuttling of lithium polysulfides (LiPSs) in a sluggish redox reaction. Here, a heterophase V₂O₃-VN yolk-shell nanosphere encapsulated by a nitrogen-doped carbon layer has been designed to address the problems of the short cycle life and rapid capacity decay of Li-S batteries synchronously. The structural merits comprise efficient polysulfide anchoring (V₂O₃), rapid electron transfer (VN) and a reinforced frame (N-doped carbon). The assembled cathode based on the V₂O₃-VN@NC sulfur host delivered a high initial capacity of 1352 mA h g^-1 at 0.1C with excellent rate performance (797 mA h g^-1 at 2C) and favorable cycle stability with a low capacity-decay rate of only 0.038% per cycle over 800 cycles at 1C. Even with a high sulfur loading of 3.95 mg cm^-2, an initial capacity of 954 mA h g^-1 at 0.2C could be achieved, along with a good capacity retention of 75.1% after 150 cycles. Density functional theory computations demonstrated the crucial role of the V₂O₃-VN@NC heterostructure in the trapping-diffusion-conversion of polysulfides. This multi-functional cathode is very promising in realizing practically usable Li-S batteries owing to the simple process and the prominent rate and cyclic performances.