Tuning the Nanoparticle Interfacial Properties and Stability of the Core-Shell Structure in Zn-Doped NiMoO4@AWO4.

The ability to tune the interfacial region in core-shell nanocomposites with a surface reconstruction as a source for surface energy (de)stabilization is presented. We consider Zn-doped nickel molybdate (NiMoO4) (ZNM) as a core crystal structure and AWO4 (A = Co or Mg) as a shell surface. Based on the density-functional theory method, the interfacial models of Zn-doped NiMoO4@AWO4 (ZNM@AW) core@shell structures are simulated and revealed to undergo surface reconstruction on the (-110) and (-202) surfaces of the AW shells, where the surface degradation of ZNM@MW(-110) is observed. The theoretical simulation is validated against the electrochemical performance of supercapacitor studies. To verify, we synthesize the hierarchical ZNM@AW core@shell semiconductor structured nanocomposites grown on a nickel foam conductive substrate using a facile and green two-step hydrothermal method. The morphology and chemical and electrochemical properties of the hierarchically structured nanocomposites are characterized in detail. The performance of the core@shell is significantly affected by the chosen intrinsic properties of metal oxides and exhibited high performance compared to a single-component system in supercapacitors. The proposed asymmetric device, Zn-doped NiMoO4@CoWO4 (ZNM@CW)||activated carbon, exhibits a superior pseudo-capacitance, delivering a high areal capacitance of 0.892 F cm-2 at a current density of 2 mA cm-2 and an excellent cycling stability of 96% retention of its initial capacitance after 1000 charge-discharge cycles. These fundamental theoretical and experimental insights with the extent of the surface reconstruction sufficiently explain the storage properties of the studied materials.

Zn Metal Atom Doping on the Surface Plane of One-Dimesional NiMoO4 Nanorods with Improved Redox Chemistry.

The effect of zinc (Zn) doping and defect formation on the surface of nickel molybdate (NiMoO4) structures with varying Zn content has been studied to produce one-dimensional electrodes and catalysts for electrochemical energy storage and ethanol oxidation, respectively. Zn-doped nickel molybdate (Ni1-xZnxMoO4, where x = 0.1, 0.2, 0.4, and 0.6) nanorods were synthesized by a simple wet chemical route. The optimal amount of Zn is found to be around 0.25 above which the NiMoO4 becomes unstable, resulting in poor electrochemical activity. This result agrees with our density functional theory calculations in which the thermodynamic stability reveals that Ni1-xZnxMoO4 crystallized in the ß-NiMoO4 phase and is found to be stable for x≤0.25. Analytical techniques show direct evidence of the presence of Zn in the NiMoO4 nanorods, which subtly alter the electrocatalytic activity. Compared with pristine NiMoO4, Zn-doped NiMoO4 with the optimized Zn content was tested as an electrode for an asymmetric supercapacitor and demonstrated an enhanced specific capacitance of 122 F g-1 with a high specific energy density of 43 W h kg-1 at a high power density of 384 W kg-1. Our calculations suggest that the good conductivity from Zn doping is attributed to the formation of excess oxygen vacancies and dopants play an important role in enhancing the charge transfer between the surface and OH- ions from the electrolyte. We report electrochemical testing, material characterization, and computational insights and demonstrate that the appropriate amount of Zn in NiMoO4 can improve the storage capacity (∼15%) due to oxygen vacancy interactions.

Defect Thermodynamics in Nonstoichiometric Alluaudite-Based Polyanionic Materials for Na-Ion Batteries.

Sodium iron sulfate in the form of alluaudite Na2+2xFe2-x(SO4)3 (or NFSx) has emerged as one of the most promising cathodes for Na-ion batteries due to its highest Fe2+/3+ redox potential, low cost, sustainability, and high rate capability. Unlike most of the other cathodes, NFSx generally crystalizes in its nonstoichiometric form with partial Na substitution for Fe sites and contains a small amount of impurities. However, profound explanations behind this inherent behavior including others, like phase stability, configurational structure, and defect formation are still ambiguous. We therefore performed first-principles calculations combined with a random swapping method to determine the minimum energy configurations of NFSx (with x = 0, 0.25, and 0.5) and find a correlation between the Na distribution pattern and energetics in which the site preference for Na+ ion is in a sequence of Na4 > Na1 > Na2 > Na3. Our result points out that the nonstoichiometry cannot be properly described under the framework of primitive structures. Moreover, we investigated phase stability diagrams and defect formations based on thermodynamic criteria. Our predicted phase diagrams can explain the inevitable impurity precipitation, which can be reduced as x diminishes. Defect formation analysis indicates an unlikely formation of channel blockage and identifies the dominant formation of FeNa + VNa and Nai + NaFe complexes. While the former can become spontaneous in a Na-deficient environment, the latter occurs mainly in NFS0 and accommodates the presence of nonstoichiometry.

Current computational trends in polyanionic cathode materials for Li and Na batteries.

A long-standing effort has been devoted for the development of high energy density cathodes both for Li- and Na-ion batteries (LIBs and SIBs). The scientific communities in battery research primarily divide the Li- and Na-ion cathode materials into two categories: layered oxides and polyanionic compounds. Researchers are trying to improve the energy density of such materials through materials screening by mixing the transition metals or changing the concentration of Li or Na in the polyanionic compounds. Due to the fact that there is more stability in the polyanionic frameworks, batteries based on these materials mostly provide a prolonged cycling life as compared to the layered oxide materials. Nevertheless, the bottleneck for such compounds is the weight penalty from polyanionic groups that results into the lower capacity. The anion engineering could be considered as an essential way out to design such polyanionic compounds to resolve this issue and to fetch improved cathode performance. In this topical review we present a systematic overview of the polyanionic cathode materials used for LIBs and SIBs. We will also present the computational methodologies that have become significantly relevant for battery research. We will make an attempt to provide the theoretical insight with a current development in sulfate (SO4), silicate (SiO4) and phosphate (PO4) based cathode materials for LIBs and SIBs. We will end this topical review with the future outlook, that will consist of the next generation organic electrode materials, mainly based on conjugated carbonyl compounds.

Structural prediction of host-guest structure in lithium at high pressure.

Ab initio random structure searching (AIRSS) technique is used to identify the high-pressure phases of lithium (Li). We proposed the transition mechanism from the fcc to host-guest (HG) structures at finite temperature and high pressure. This complex structural phase transformation has been calculated using ab initio lattice dynamics with finite displacement method which confirms the dynamical harmonic stabilization of the HG structure. The electron distribution between the host-host atoms has also been investigated by electron localization function (ELF). The strongly localized electron of p bond has led to the stability of the HG structure. This remarkable result put the HG structure to be a common high-pressure structure among alkali metals.

Effect of Transition Metal Cations on Stability Enhancement for Molybdate-Based Hybrid Supercapacitor.

The race for better electrochemical energy storage systems has prompted examination of the stability in the molybdate framework (MMoO4; M = Mn, Co, or Ni) based on a range of transition metal cations from both computational and experimental approaches. Molybdate materials synthesized with controlled nanoscale morphologies (such as nanorods, agglomerated nanostructures, and nanoneedles for Mn, Co, and Ni elements, respectively) have been used as a cathode in hybrid energy storage systems. The computational and experimental data confirms that the MnMoO4 crystallized in ß-form with α-MnMoO4 type whereas Co and Ni cations crystallized in α-form with α-CoMoO4 type structure. Among the various transition metal cations studied, hybrid device comprising NiMoO4 vs activated carbon exhibited excellent electrochemical performance having the specific capacitance 82 F g-1 at a current density of 0.1 A g-1 but the cycling stability needed to be significantly improved. The specific capacitance of the NiMoO4 electrode material is shown to be directly related to the surface area of the electrode/electrolyte interface, but the CoMoO4 and MnMoO4 favored a bulk formation that could be suitable for structural stability. The useful insights from the electronic structure analysis and effective mass have been provided to demonstrate the role of cations in the molybdate structure and its influence in electrochemical energy storage. With improved cycling stability, NiMoO4 can be suitable for renewable energy storage. Overall, this study will enable the development of next generation molybdate materials with multiple cation substitution resulting in better cycling stability and higher specific capacitance.

Synthesis, structural and electrochemical properties of sodium nickel phosphate for energy storage devices.

Electrochemical energy production and storage at large scale and low cost, is a critical bottleneck in renewable energy systems. Oxides and lithium transition metal phosphates have been researched for over two decades and many technologies based on them exist. Much less work has been done investigating the use of sodium phosphates for energy storage. In this work, the synthesis of sodium nickel phosphate at different temperatures is performed and its performance evaluated for supercapacitor applications. The electronic properties of polycrystalline NaNiPO4 polymorphs, triphylite and maricite, t- and m-NaNiPO4 are calculated by means of first-principle calculations based on spin-polarized Density Functional Theory (DFT). The structure and morphology of the polymorphs were characterized and validated experimentally and it is shown that the sodium nickel phosphate (NaNiPO4) exists in two different forms (triphylite and maricite), depending on the synthetic temperature (300-550 °C). The as-prepared and triphylite forms of NaNiPO4vs. activated carbon in 2 M NaOH exhibit the maximum specific capacitance of 125 F g(-1) and 85 F g(-1) respectively, at 1 A g(-1); both having excellent cycling stability with retention of 99% capacity up to 2000 cycles. The maricite form showed 70 F g(-1) with a significant drop in capacity after just 50 cycles. These results reveal that the synthesized triphylite showed a high performance energy density of 44 Wh kg(-1) which is attributed to the hierarchical structure of the porous NaNiPO4 nanosheets. At a higher temperature (>400 °C) the maricite form of NaNiPO4 possesses a nanoplate-like (coarse and blocky) structure with a large skewing at the intermediate frequency that is not tolerant of cycling. Computed results for the sodium nickel phosphate polymorphs and the electrochemical experimental results are in good agreement.

Synthesis, and crystal and electronic structure of sodium metal phosphate for use as a hybrid capacitor in non-aqueous electrolyte.

Energy storage devices based on sodium have been considered as an alternative to traditional lithium based systems because of the natural abundance, cost effectiveness and low environmental impact of sodium. Their synthesis, and crystal and electronic properties have been discussed, because of the importance of electronic conductivity in supercapacitors for high rate applications. The density of states of a mixed sodium transition metal phosphate (maricite, NaMn(1/3)Co(1/3)Ni(1/3)PO4) has been determined with the ab initio generalized gradient approximation (GGA)+Hubbard term (U) method. The computed results for the mixed maricite are compared with the band gap of the parent NaFePO4 and the electrochemical experimental results are in good agreement. A mixed sodium transition metal phosphate served as an active electrode material for a hybrid supercapacitor. The hybrid device (maricite versus carbon) in a non-aqueous electrolyte shows redox peaks in the cyclic voltammograms and asymmetric profiles in the charge-discharge curves while exhibiting a specific capacitance of 40 F g(-1) and these processes are found to be quasi-reversible. After long term cycling, the device exhibits excellent capacity retention (95%) and coulombic efficiency (92%). The presence of carbon and the nanocomposite morphology, identified through X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, ensures the high rate capability while offering possibilities to develop new cathode materials for sodium hybrid devices.