The Na-ion battery is currently the focus of much research interest due to its cost advantages and the relative abundance of sodium as compared to lithium. Olivine NaMPO4 (M = Fe, Fe0.5Mn0.5, Mn) and layered Na2FePO4F are interesting materials that have been reported recently as attractive positive electrodes. Here, we report their Na-ion conduction behavior and intrinsic defect properties using atomistic simulation methods. In the olivines, Na ion migration is essentially restricted to the [010] direction along a curved trajectory, similar to that of LiMPO4, but with a lower migration energy (0.3 eV). However, Na/M antisite defects are also predicted to have a lower formation energy: the higher probability of tunnel occupation with a relatively immobile M2+ cation – along with a greater volume change on redox cycling – contributes to the poor electrochemical performance of the Na-olivine. Na+ ion conduction in Na2FePO4F is predicted to be two-dimensional (2D) in the interlayer plane with a similar low activation energy. The antisite formation energy is slightly higher; furthermore, antisite occupation would not be predicted to impede transport significantly owing to the 2D pathway. This factor, along with the much lower volume change on redox cycling, is undoubtedly responsible for the better electrochemical performance of the layered structure. Where volume change and structural effects do not incur impediments, Na-ion materials may present significant advantages over their Li counterparts.

Na-ion mobility in layered Na2FePO4F and olivine Na[Fe,Mn]PO4

Conduction mechanism study by overlapping large-polaron tunnelling model in SrNiP2O7 ceramic compound

Electric and dielectric studies of the [N(CH3)3H]2CuCl4 compound at low temperature

A noble-metal-free amorphous Ni2P2O7 modified g-C3N4 nanocomposite (a-Ni2P2O7/g-C3N4) has been successfully prepared by a facile calcined process. The experimental results suggest that a-Ni2P2O7/g-C3N4 displays the enhanced visible light absorption and improved charge separation relative to pristine g-C3N4, and shows smaller particle sizes and more negative conduction band edge potential than that of the crystalline Ni2P2O7 modified g-C3N4 nanocomposite (c-Ni2P2O7/g-C3N4), resulting in large amount of active sites and enhanced reduction capacity. The resultant a-Ni2P2O7/g-C3N4 is evaluated as an excellent catalyst for photocatalytic hydrogen (H2) generation under visible light irradiation, whose H2 generation rate is almost 5 and 37 times higher than that of c-Ni2P2O7/g-C3N4 and pristine g-C3N4, respectively, even better than that of Pt/g-C3N4. The improved activity of a-Ni2P2O7/g-C3N4 is mainly ascribed to the good visible light utilization, efficient electron-hole pairs’ separation and transfer, small particle sizes and high reduction ability of electron.

Amorphous nickel pyrophosphate modified graphitic carbon nitride: an efficient photocatalyst for hydrogen generation from water splitting

at an average storage voltage of 3.2 V with long cycle life. When coupled with hard carbon anode, a prototype rechargeable sodium-ion battery offers an energy density of 210 Wh kg −1 , a round-trip energy effi ciency of 90%, high rate capability, and excellent cycling stability. These desired performances make this system to be closer to the level of practical applications. The Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (Na 0.9 [Cu II 0.22 Fe III 0.30 Mn III 0.16 Mn IV 0.32 ]O 2 ) material was synthesized by a simple solid-state reaction at 850 °C in air atmosphere using precursors of Na 2 CO 3 , CuO, Fe 2 O 3 , and Mn 2 O 3 . The crystal structure of the as-synthesized material was determined by X-ray diffraction (XRD) as shown in Figure 1 a together with its refi nement results by the Rietveld method (see Table S1, Supporting Information). It can be seen that all the Bragg diffraction peaks are in excellent agreement with the JC DS No. 01-0821495 (O3-type α-NaFeO 2 ) and can be indexed to a hexagonal layered structure with a space group of 3 R m − , indicative of a typical O3-type layered structure (note that the letter “O” refers to the Na coordination environment of octahedral site whereas the number “3” refers to the number of MO 2 slab according to Delmas’ notation. [ 13 ] A schematic illustration of the O3-type structure is also shown in Figure 1 b. The structure refi nement gives the lattice parameters a = 2.9587(7) A, c = 16.3742(6) A. The lattice parameter of c -axis is slightly larger than that of other O3-type materials [ 5f–h , 6b , 8a ] because the Na content is less than 1. The inductively coupled plasma (ICP) result confi rms the composition of Na 0.89 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (see Table S2, Supporting Information). The morphology of the resulting sample is shown in Figure 1 c. The distribution of the particle size is in the range of 10–30 μm with about 3 μm sized primary particle agglomerations together (Figure 1 d). Most importantly, unlike other O3-type materials, [ 4–9,12 ] this material is very stable against water. In order to confi rm this, we intentionally design an accelerated aging experiment as described in the Experimental Section which was verifi ed by LiMO 2 as shown in Figure S1 (Supporting Information). We placed the as-synthesized material in deionized water for 3 d and then dried the material at 100 °C for overnight. The obtained material was checked by XRD again. It can be seen that the XRD pattern is nearly identical to that of the as-synthesized material, which is very different from other O3-type materials as shown in Figure S2 (Supporting Information). These results suggest that the O3-Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 is very stable against water. Furthermore, after the material was stored in air for one month Large-scale electrical energy storage systems are one of the core technologies in renewable energies and smart grid, among which sodium-ion batteries show great promise due to the abundant sodium resources. Layered metal oxides (of general formula: A x MO 2 , where A = Li, Na; M = Co, Ni, Mn, Cr, Fe, etc.) with alternating alkali metal layer and transition metal layer have long been of particular interest since the early 1980s as an important class of cathode materials for rechargeable batteries due to their easy synthesis and high energy density. [ 1 ] One of the most successful examples is LiCoO 2 , [ 1a ] which is commonly used as a cathode in lithium-ion batteries with the highest volumetric energy density for portable electronic devices. Its metal substituted materials (LiCo 1− x − y − z Ni x Mn y Al z O 2 ) are being used in power batteries for electric vehicles. In the case of sodiumion batteries operated at room temperature which are proposed for large-scale electrical energy storage owing to the naturally abundant sodium resources in recent years. [ 2,3 ] Na x CoO 2 that can electrochemically and reversibly intercalate Na is the fi rst example, [ 1b ] then a large number of layered metal oxides have been extensively exploited. [ 4–12 ] However, the practical applications have been hindered by two major challenges. First, unlike LiMO 2 , almost all the Na x MO 2 are not stable against moisture (either they can be oxidized by water or water/carbon dioxide molecules can be intercalated into alkali metal layer). [ 4–9,11,12 ]

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Prototype Sodium-Ion Batteries Using Air-Stable and Co/Ni-Free O3-Layered Metal Oxide Cathode

The complex impedance of the Ag2ZnP2O7 compound has been investigated in the temperature range 419–557 K and in the frequency range 200 Hz–5 MHz. The Z′ and Z′ versus frequency plots are well fitted to an equivalent circuit model. Dielectric data were analyzed using complex electrical modulus M* for the sample at various temperatures. The modulus plot can be characterized by full width at half-height or in terms of a non-exponential decay function $$ \phi \left( {\text{t}} \right) = \exp {\left( { - {\text{t}}/\tau } \right)^\beta } $$
 . The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law: $$ \sigma \left( \omega \right) = {\sigma_{\text{dc}}} + {\text{A}}{\omega^n} $$
 . The conductivity σ
 dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of M″, conductivity data, and equivalent circuit confirms that the transport is through ion hopping mechanism dominated by the motion of the Ag+ ions in the structure of the investigated material.

Dielectric relaxation and ionic conductivity studies of Ag2ZnP2O7

Ferroelectric properties of the [N(CH3)4]2CoCl2Br2 compound

The [N(CH3)4][N(C2H5)4]ZnCl4 compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P21212 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M * for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.

Electrical characterization of the [N(CH3)4][N(C2H5)4]ZnCl4 compound

In the last few years, new photovoltaic compounds have been sought out for the conversion of renewable and clean solar energy with a view to reducing the production cost of electricity and enhancing the conversion efficiency with a simple manufacturing process. Hybrid organic–inorganic perovskites have emerged among the most promising materials with potential applications in the optoelectronic and photovoltaic fields. These photovoltaic materials draw such a great deal of attention that they are marching towards commercialization thanks to their excellent optical properties, high-power conversion efficiency and low-processing cost. In this paper, we introduce the optical properties of a new lead-free perovskite CH3NH3CdCl3 (MACdCl3) which adopts a tetragonal symmetry (space group P4mm) at ambient conditions. Optical measurements proved that this highly crystalline perovskite absorber has a broad and total visible to near-infrared absorption as well as a tunable indirect band gap (1.1 eV). Besides, recent studies have proposed that the indirect gap is due to the Rashba spin–orbit coupling. These excellent results definitely present a path to high-power conversion efficiency in a two-junction device such as the perovskite-perovskite tandem solar cells.

CH3NH3CdCl3: A promising new lead‐free hybrid organic–inorganic perovskite for photovoltaic applications

Sodium-ion batteries have been dominating as a power source in digital cameras, laptop computers, mobile phones and electric/hybrid electric vehicles. This can be explained by their high life cycle and power density which differentiate them from other battery types. Sodium phosphate is considered among the favored solid electrolyte materials for the sodium-ion battery because of its high ionic conductivity. In this study, Na3PO4 was obtained using the classic ceramic method and characterized by X-ray powder diffraction patterns, infrared spectroscopy, differential scanning calorimetry (DSC) and electrical impedance. The Na3PO4 compound crystallized at room temperature in the tetragonal system with a P 4 ‾ 2 1 c space group. The morphology and composition of Na3PO4 were studied by a scanning transmission electron microscope coupled with the energy dispersive X-ray spectroscopy (STEM-EDS). The phase transition at T1 ≈ 603/605 K was conﬁrmed by the differential scanning calorimetry (DSC). Infrared spectroscopy confirmed the presence of the (PO4)3− group and its vibrations. The electrical technique was measured in the 10–106 Hz frequency range and 540–700 K temperature intervals. The frequency dependence of alternative current conductivity was explained using Jonscher law. The alternative current electrical conduction in Na3PO4 was interpreted through several processes, which could be associated with two different models: the overlapping large polaron tunneling (OLPT) model in phase I and the non-overlapping small polaron tunneling (NSPT) model in phase II. The conduction mechanisms of Na3PO4 were explained by Elliott's theory and consequently the Elliott's parameters were calculated.

Abdallah Ben Rhaiem

Papers

Dielectric relaxation and ionic conductivity studies of Ag2ZnP2O7

Ferroelectric properties of the [N(CH3)4]2CoCl2Br2 compound

Electrical characterization of the [N(CH3)4][N(C2H5)4]ZnCl4 compound

CH3NH3CdCl3: A promising new lead‐free hybrid organic–inorganic perovskite for photovoltaic applications

The sodium-ion battery: Study of alternative current conduction mechanisms on the Na3PO4 - Based solid electrolyte