Structural and electrochemical properties of Na2FeSiO4 polymorphs for sodium-ion batteries

Electronic, mechanical and piezoelectric properties of glass-like complex Na2Si1-x Ge x O3 (x = 0.0, 0.25, 0.50, 0.75, 1.0)

Motivated by our previous work on pristine Na2SiO3, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na2Si1-x Ge x O3 (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT). Interestingly, the optimized bond lengths and bond angles of Na2SiO3 and Na2GeO3 resemble each other with high similarity. On doping we report the negative formation energy and feasibility of transition of Na2SiO3 → Na2GeO3 while the structural symmetry is preserved. Analyzing the electronic profile, we have observed a reduced band gap on increasing x = Ge concentration at Si-sites. All the systems are indirect band gap (Z-Γ) semiconductors. The studied systems have shown mechanical stabilities by satisfying the Born criteria for mechanical stability. The calculated results have shown highly anisotropic behaviour and high melting temperature, which are a signature of glass materials. The piezoelectric tensor (both direct and converse) is computed. The results thus obtained predict that the systems under investigation are potential piezoelectric materials for energy harvesting.

Progress on Fe-Based Polyanionic Oxide Cathodes Materials toward Grid-Scale Energy Storage for Sodium-Ion Batteries

The development of large-scale energy storage systems (EESs) is pivotal for applying intermittent renewable energy sources such as solar energy and wind energy. Lithium-ion batteries with LiFePO₄ cathode have been explored in the integrated wind and solar power EESs, due to their long cycle life, safety, and low cost of Fe. Considering the penurious reserve and regional distribution of lithium resources, the Fe-based sodium-ion battery cathodes with earth-abundant elements, environmental friendliness, and safety appear to be the better substitutes in impending grid-scale energy storage. Compared to the transition metal oxide and Prussian blue analogs, the Fe-based polyanionic oxide cathodes possess high thermal stability, ultra-long cycle life, and adjustable voltage, which is more commercially viable in the future. This review summarizes the research progress of single Fe-based polyanionic and mixed polyanionic oxide cathodes for the potential sodium-ion batteries EESs candidates. In detail, the synthesized method, crystal structure, electrochemical properties, bottlenecks, and optimization method of Fe-based polyanionic oxide cathodes are discussed systematically. The insights presented in this review may serve as a guideline for designing and optimizing Fe-based polyanionic oxide cathodes for coming commercial sodium-ion batteries EESs.

Electronic, mechanical, optical and piezoelectric properties of glass-like sodium silicate (Na2SiO3) under compressive pressure

The structural, mechanical, electronic, optical and piezoelectric properties of Na2SiO3 are studied under varying compressive unidirectional pressure (0-50 GPa with a difference of 10 GPa) using density functional theory (DFT). The calculated structural properties agree well with previously reported results. At 12 GPa, our calculation shows a structural phase transition from orthorhombic Cmc21 to triclinic P1. The mechanical profile of Na2SiO3 structures under different compressive unidirectional pressures are analysed by calculating the elastic moduli, Poisson's ratio and eigenvalues of stiffness matrix. Our study shows the mechanical stability of the system up to a pressure of 40 GPa. Herein, we have obtained an indirect band gap of 2.97 eV at 0 GPa. Between 0-50 GPa, the band gaps are within the range 2.62 to 3.46 eV. The system in our study possesses a wide band gap and high optical absorption in the UV-Vis range of electromagnetic radiation. The calculated static refractive indices η x,y,z (0) are close to unity suggesting its transparency. For piezoelectric properties, we have reported the total Cartesian polarization. Our calculations have revealed that Na2SiO3 is a promising candidate for optoelectronic devices while its application in ferroelectric and piezoelectric devices could be improved with further research.

Intrinsic Defects, Diffusion and Dopants in AVSi2O6 (A = Li and Na) Electrode Materials

The alkali metal pyroxenes of the AVSi2O6 (A = Li and Na) family have attracted considerable interest as cathode materials for the application in Li and Na batteries. Computer modelling was carried out to determine the dominant intrinsic defects, Li and Na ion diffusion pathways and promising dopants for experimental verification. The results show that the lowest energy intrinsic defect is the V–Si anti-site in both LiVSi2O6 and NaVSi2O6. Li or Na ion migration is slow, with activation energies of 3.31 eV and 3.95 eV, respectively, indicating the necessity of tailoring these materials before application. Here, we suggest that Al on the Si site can increase the amount of Li and Na in LiVSi2O6 and NaVSi2O6, respectively. This strategy can also be applied to create oxygen vacancies in both materials. The most favourable isovalent dopants on the V and Si sites are Ga and Ge, respectively.

The dielectric relaxation behavior induced by sodium migration in the Na2CoSiO4 structure within a three-dimensional Co-O-Si framework

The disodium cobalt(ii) orthosilicate material (NCS) has been synthesized using improved solid-state (NCS-SS) and co-precipitation (NCS-CP) methods of synthesis. The Rietveld refinement of the XRD pattern of Na₂CoSiO₄ has demonstrated an orthorhombic crystal system with the space groups Pna2₁ and Pbca for NCS-SS and NCS-CP respectively. The elemental mapping of microstructures by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) showed the porous morphology and the homogenous particles of the Na₂CoSiO₄ powders. Their dielectric properties were measured in the frequency and temperature ranges of 0.1-10⁶ Hz and 383-613 K respectively. Different dielectric relaxation phenomena associated with the Na⁺-ion migration through different paths were displayed in relation with the temperature and frequency. The decrease and increase in the dielectric properties were found to be dependent on the formation of short-range ordered structure formed after the migration of Na⁺-ions. In the present work, an attempt has been made to study the relation between the structural properties and the dielectric process. Thus, interesting insights into the transport behavior of Na⁺-ions in different chemical environments were obtained. This in turn provides an effective procedure to probe the relationship between the diffusion pathway of Na⁺-ions and the dielectric response.

Benefits of Ga, Ge and As substitution in Li2FeSiO4: a first-principles exploration of the structural, electrochemical and capacity properties

Herein, the feasibility of Fe substitution by Ga, Ge and As in Li2FeSiO4 in modulating its structural, mechanical, electrochemical, capacity and electronic properties was systematically studied via first-principles calculations based on density functional theory within the generalized gradient approximation with Hubbard corrections (GGA+U). The calculated results show that Ga, Ge and As doping can effectively reduce the range of the cell volume change during Li+ removal, improving the Li+ detachment ability and cycle stability of the system. Meanwhile, the calculated mechanical properties including modulus ratio, B/G, and Poisson ratio, ν, indicate that the doped systems of Ga, Ge and As exhibit excellent mechanical properties. In addition, besides the increase in theoretical average deintercalation voltage induced by the Ga dopant when more than one Li+ ion is removed in the formula unit, the doping of Ga, Ge and As all reduce the theoretical average deintercalation voltage in the process of Li+ extraction. Especially in the case of doping of Ge, when 0.5 Li+ is removed from LiFe0.5Ge0.5SiO4, the theoretical average deintercalation voltage only increases by 0.19 V compared with the case of the removal of one Li+ in Li2Fe0.5Ge0.5SiO4, which causes the cathode material to have a longer and more stable discharge platform. Moreover, in the process of Li+ removal, the doping of Ga, Ge and As can effectively participate in the charge compensation of the system, and Ge and As can provide further charge, increasing the capacity of the Li2FeSiO4 cathode material considerably.

Polyanion-type cathode materials for sodium-ion batteries

This review summarizes the recent progress and remaining challenges of polyanion-type cathodes, providing guidelines towards high-performance cathodes for sodium ion batteries.

Studies on the Kinetic Behaviors of Na Ions Insertion/Extraction in Na2FeSiO4/C Cathode Material at Various Desodiation States

Na₂FeSiO₄, as one of the promising cathode materials in sodium-ion batteries, has attracted great interests. However, studies on the kinetic behaviors of Na ions insertion/extraction in Na₂FeSiO₄ composite electrode have been barely considered, until now. Importantly, the specific capacity and rate capability of Na₂FeSiO₄ cathode materials are greatly correlated with the kinetics of Na⁺ transfer in the host material. Herein, on the basis of the characterizations of microstructure and morphologies (i.e., Rietveld refinement, FESEM, HRTEM, etc.), the electrochemical kinetics of Na ions extraction in Na₂FeSiO₄/C electrode are first studied in detail via two electrochemical techniques (EIS and GITT), establishing the rate-controlling steps of Na⁺ transport in Na₂FeSiO₄/C, evaluating series of kinetic parameters, as well as calculating the Na⁺ diffusion coefficient at various state-of-desodiation. Changes of impedance response of Na₂FeSiO₄/C electrode depending on the different levels of desodiation show that a serial features of electrode process for Na ions migration have tremendous discrepancies, indicating that the kinetics of Na⁺ extraction from Na₂FeSiO₄/C electrode are largely influenced by different electrode reaction processes. These results provide useful insight into the inner properties of Na₂FeSiO₄/C electrode, and it is significant to optimize the electrochemical performance of Na₂FeSiO₄/C. Moreover, two models of equivalent circuits are also constructed to simulate the electrode processes and describe the behaviors of Na ions transfer in Na₂FeSiO₄/C.