Li8MnO6: A Novel Cathode Material with Only Anionic Redox.

In Li-excess transition metal-oxide cathode materials, anionic oxygen redox can offer high capacity and high voltages, although peroxo and superoxo species may cause oxygen loss, poor cycling performance, and capacity fading. Previous work showed that undesirable formation of peroxide and superoxide bonds was controlled to some extent by Mn substitution, and the present work uses density functional calculations to examine the reasons for this by studying the anionic redox mechanism in Li8MnO6. This material is obtained by substituting Mn for Sn in Li8SnO6 or for Zr in Li8ZrO6, and we also compare this to previous work on those materials. The calculations predict that Li8MnO6 is stable at room temperature (with a band gap of 3.19 eV as calculated by HSE06 and 1.82 eV as calculated with the less reliable PBE+U), and they elucidate the chemical and structural effects involved in the inhibition of oxygen release in this cathode. Throughout the whole delithiation process, only O2- ions are oxidized. The directional Mn-O bonds formed from unfilled 3d orbitals effectively inhibit the formation of O-O bonds, and the layered structure is maintained even after removing 3 Li per Li8MnO6 formula unit. The calculated average voltage for removal of 3 Li is 3.69 V by HSE06, and the corresponding capacity is 389 mAh/g. The high voltage of oxygen anionic redox and the high capacity result in a high energy density of 1436 Wh/kg. The Li-ion diffusion barrier for the dominant interlayer diffusion path along the c axis is 0.57 eV by PBE+U. These results help us to understand the oxygen redox mechanism in a new lithium-rich Li8MnO6 cathode material and contribute to the design of high-energy density lithium-ion battery cathode materials with favorable electrochemical properties based on anionic oxygen redox.

Theoretical study on Y-doped Na2ZrO3 as a high-capacity Na-rich cathode material based on anionic redox.

First-principles calculations based on density functional theory were utilized to study the performance of Na2ZrO3 (NZO) and yttrium-doped Na2ZrO3 (Y-NZO) as cathode materials for sodium ion batteries (SIBs), including the stability of the desodiated structures, desodiation energy, redox mechanism, and the diffusion of Na. When 62.5% sodium is removed from NZO, its structure and volume change little and the layered structure is retained, whereas the structure starts to distort and shift to the ZrO3 phase with the extraction of more than 62.5% sodium. As desodiation proceeds, oxygen anions act as the only redox center for charge compensation, yielding a high initial voltage of 4.03 eV vs. Na/Na+ by PBE + U-D3 functional and 4.82 eV vs. Na/Na+ by HSE06-D3 functional. When the desodiation content is less than 31.25%, O23- is formed with an O-O distance of 2.38 Å. At the desodiation content of 31.25%, peroxide dimer O22- starts to form; at the desodiation content of 56.25%, the O-O bond distance is further shortened to 1.3 Å, corresponding to the formation of superoxide O2-. However, for Y-NZO, the redox reaction firstly involves O2-/O1-, which does not occur in NZO. Peroxides and superoxides appear when the sodium removal concentration is 59.38% and 75%, respectively. This indicates that the O-O dimers appear in Y-NZO at much deeper sodium removal. The calculations of diffusion paths and barriers of Na ions in NZO by PBE + U-D3 predict that the barrier of Na escaping from the mixed layer to the Na layer in NZO is 0.48 eV (the reverse barrier is 0.76 eV), smaller than those of other O3-type layered transition metal compounds, such as Na2IrO3 and Na2RuO3. After yttrium doping, the diffusion of Na ions becomes easier, indicating that the Y-doping improves the diffusion ability. This investigation interprets the mechanism of oxygen oxidation of NZO as a cathode for SIBs, and provides theoretical support for a better design of Na-rich layered oxide Na2MO3 (M represents the transition metal element) in the future research.

Defected vanadium bronzes as superb cathodes in aqueous zinc-ion batteries.

There is a growing need for fast, efficient, safe, and low-cost energy storage. Aqueous zinc-ion batteries (AZIBs) may be able to address this need, but suffer from fast capacity fade and poor ion diffusion kinetics due to unstable structures and non-optimised interspacing of layered cathode materials. Herein, we propose a structural engineering strategy by synergistically inducing anionic defects and cationic groups within vanadium bronze structures to improve kinetics and boost capacity. The materials discovered and used as the cathodes in AZIBs showed a high capacity of 435 mA h g-1 at a current density of 0.2 A g-1 and excellent stability with 95% capacity retention after 1500 cycles at 10 A g-1. This combined experimental and computational study systemically indicated that rapid Zn2+ storage was achieved from both a highly porous structure and enlarged d-spacing combined with improved electron conductivity as determined by density of states calculations. The modification of vanadium bronze-type cathodes achieved by controlled pre-intercalated species and tailored oxygen deficiency opens up an avenue for the realization of superior material design, whose feasibility is proved in this work.

Metal-organic framework-derived hollow structure CoS2/nitrogen-doped carbon spheres for high-performance lithium/sodium ion batteries.

A composite of hollow structure CoS2/nitrogen-doped carbon spheres derived from a Co-based metal-organic framework has been fabricated through a facile synthetic route and delivered superior rate capability and long-term cycling performance as an anode for lithium- and sodium-ion batteries.

Hierarchical Composite of Rose-Like VS2 @S/N-Doped Carbon with Expanded (001) Planes for Superior Li-Ion Storage.

In the present work, a hierarchical composite of rose-like VS2 @S/N-doped carbon (VS2 @SNC) with expanded (001) planes is successfully fabricated through a facile synthetic route. Notably, the d-spacing of (001) planes is expanded to 0.92 nm, which is proved to dramatically reduce the energy barrier for Li+ diffusion in the composite of VS2 @SNC by density functional theory calculation. On the other hand, the S/N-doped carbon in the composite greatly promotes the electrical conductivity and enhances the structural stability. In addition, the hierarchical structure of VS2 @SNC facilitates rapid electrolyte diffusion and increases the contact area between the electrode and electrolyte simultaneously. Benefiting from the merits mentioned above, the VS2 @SNC electrode exhibits excellent electrochemical properties, such as a large reversible capacity of 971.6 mA h g-1 at 0.2 A g-1 , an extremely high rate capability of 772.1 mA h g-1 at 10 A g-1 , and a remarkable cycling stability up to 600 cycles at 8 A g-1 with a capacity of 684.5 mA h g-1 , making it a promising candidate as an anode material for lithium-ion batteries.

Polydopamine-coated magnetic molecularly imprinted polymer for the selective solid-phase extraction of cinnamic acid, ferulic acid and caffeic acid from radix scrophulariae sample.

We describe novel cinnamic acid polydopamine-coated magnetic imprinted polymers for the simultaneous selective extraction of cinnamic acid, ferulic acid and caffeic acid from radix scrophulariae sample. The novel magnetic imprinted polymers were synthesized by surface imprinting polymerization using magnetic multi-walled carbon nanotubes as the support material, cinnamic acid as the template and dopamine as the functional monomer. The magnetic imprinted polymers were characterized by transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and vibrating sample magnetometry. The results revealed that the magnetic imprinted polymers had outstanding magnetic properties, high adsorption capacity, selectivity and fast kinetic binding toward cinnamic acid, ferulic acid and caffeic acid. Coupled with high-performance liquid chromatography, the extraction conditions of the magnetic imprinted polymers as a magnetic solid-phase extraction sorbent were investigated in detail. The proposed imprinted magnetic solid phase extraction procedure has been used for the purification and enrichment of cinnamic acid, ferulic acid and caffeic acid successfully from radix scrophulariae extraction sample with recoveries of 92.4-115.0% for cinnamic acid, 89.4-103.0% for ferulic acid and 86.6-96.0% for caffeic acid.