Although being considered as one of the most promising cathode materials for Lithium-ion batteries (LIBs), LiNi1/3Co1/3Mn1/3O2 (NCM) is currently limited by its poor rate performance and cycle stability resulting from the thermodynamically favorable Li+/Ni2+ cation mixing which depresses the Li+ mobility. In this study, we developed a two-step method using fluffy MnO2 as template to prepare hierarchical porous nano-/microsphere NCM (PNM-NCM). Specifically, PNM-NCM microspheres achieves a high reversible specific capacity of 207.7 mAh g−1 at 0.1 C with excellent rate capability (163.6 and 148.9 mAh g−1 at 1 C and 2 C), and the reversible capacity retention can be well-maintained as high as 90.3% after 50 cycles. This excellent electrochemical performance is attributed to unique hierarchical porous nano-/microsphere structure which can increase the contact area with electrolyte, shorten Li+ diffusion path and thus improve the Li+ mobility. Moreover, as revealed by XRD Rietveld refinement analysis, a negligible cation mixing (1.9%) and high crystallinity with a well-formed layered structure also contribute to the enhanced C-rates performance and cycle stability. On the basis of our study, an effective strategy can be established to reveal the fundamental relationship between the structure/chemistry of these materials and their properties.
Improved conductivity and suppressed dissolution of lithium polysulfides is highly desirable for high‐performance lithium‐sulfur (Li‐S) batteries. Herein, by a facile solvent method followed by nitridation with NH3, a 2D nitrogen‐doped carbon structure is designed with homogeneously embedded Co4N nanoparticles derived from metal organic framework (MOF), grown on the carbon cloth (MOF‐Co4N). Experimental results and theoretical simulations reveal that Co4N nanoparticles act as strong chemical adsorption hosts and catalysts that not only improve the cycling performance of Li‐S batteries via chemical bonding to trap polysulfides but also improve the rate performance through accelerating the conversion reactions by decreasing the polarization of the electrode. In addition, the high conductive nitrogen‐doped carbon matrix ensures fast charge transfer, while the 2D structure offers increased pathways to facilitate ion diffusion. Under the current density of 0.1C, 0.5C, and 3C, MOF‐Co4N delivers reversible specific capacities of 1425, 1049, and 729 mAh g−1, respectively, and retains 82.5% capacity after 400 cycles at 1C, as compared to the sample without Co4N (MOF‐C) values of 61.3% (200 cycles). The improved cell performance corroborates the validity of the multifunctional design of MOF‐Co4N, which is expected to be a potentially promising cathode host for Li‐S batteries.
One key challenge facing room temperature Na-ion batteries lies in identifying earth-abundant, environmentally friendly and safe materials that can provide efficient Na storage sites in Na-ion batteries. Herein, we report such a material, polyoxometalate NaH[MnVO] (NMV), with entirely different composition and structure from those cathode compounds reported before. Ex-situ XPS and FTIR analyses reveal that NMV cathode behaves like an "electron/Na-ion sponge", with 11 electrons/Na acceptability per mole, which has a decisive contribution to the high capacity. The extraordinary structural features, evidenced by X-ray crystallographic analysis, of NaH[MnVO] with a flexible 2D lamellar network and 1D open channels provide diverse Na ion migration pathways, yielding good rate capability. First-principle calculations demonstrate that a super-reduced state, [MnVO], is formed with slightly expanded size (ca. 7.5%) upon Na insertion compared to the original [MnVO]. This "ion sponge" feature ensures the good cycling stability. Consequently, benefiting from the combinations of "electron/ion sponge" with diverse Na diffusion channels, when revealed as the cathode materials for Na-ion batteries, NaH[MnVO]/G exhibits a high specific capacity (ca. 190 mA h/g at 0.1 C), associates with a good rate capability (130 mA h/g at 1 C), and a good capacity retention (81% at 0.2 C). Our results promote better understanding of the storage mechanism in polyoxometalate host, enrich the existing rechargeable SIBs cathode chemistry, and enlighten an exciting direction for exploring promising cathode materials for Na-ion batteries.
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