Intercalation Voltage Research Articles

Unlocking the Multi-Redox (Nb5+/Nb4+/Nb3+) Reactions in Nasicon- Nb2(PO4)3 Anode for Li- and Na-Ion Batteries

NASICON (Natrium Super-Ionic CONductor) type materials are known for their high structural, and thermal stability and superior sodium ion mobility which make this framework an efficient electrode material for Na-ion batteries.1 In recent times NASICON-type compounds have been widely explored as Li- and Na-ion cathodes and solid-state electrolytes but are largely ignored as anodes due to their lower capacities and higher intercalation voltages, which reduce the overall energy densities of Li- and Na-ion batteries (LIBs and SIBs). NASICON structure has a general molecular formula of NaxMM′(PO4)3, where two MO6 octahedra corners are shared with three PO4 tetrahedra along the c-direction resulting in the “lantern units” and each lantern unit joins six other lantern units, generating a large interstitial space that can accommodate up to four alkali cations per formula unit.2 In the NASICON family usually all members contain Na/Li ions in terms of synthesis and electrochemical/chemical oxidation routes must be employed to remove the Na from the parent compound.Herein, for the first time we will present a comprehensive study on the structural and electrochemical properties of the empty NASICON-type (contains no Li or Na ions in its pristine state) mixed valance Nb2(PO4)3 and its potential application as an anode in LIBs and SIBs. Due to the multi-redox activity of Nb, it is possible to attain high insertion capacity (~ 150 mAh g-1 for SIBs and ~ 170 for LIBs) and relatively lower voltages (1.8 V vs. Li+/Li0 and 1.4 V vs. Na+/Na0) compared to Ti or V-based NASICON anodes.3,4 Our in-situ XRD measurements revealed multiple-phase transformations during Li and Na intercalation with the formation of short-range ordered Li3Nb2(PO4)3 and triclinic (P-1)-Na3Nb2(PO4)3 at the end of discharge. The distinct electrochemical Li and Na intercalation behavior of the Nb2(PO4)3 anode can be ascribed to the relative differences in size, filling of crystallographic sites, and chemical characters of Li and Na ions. Our density functional theory calculations are also in agreement with the in-situ XRD measurements in predicting a stable Na3Nb2(PO4)3 composition in the Na-Nb2(PO4)3 pseudo-binary system. X-ray absorption spectroscopy confirms the participation of multi-redox Nb5+/Nb4+/Nb3+ couples. Although the micron-sized anode showed moderate storage capacities (~106 and 84 mAh g-1 for Li and Na cells, respectively) at 1C rate after 200 cycles, further optimization of electrode and electrolytes is expected to produce better performance. In the future, pairing Nb2(PO4)3 anode with suitable cathodes can enable high-energy density batteries.5

Study of structural, electric, and electrochemical properties and storage mechanism of Na doped LiFePO4 cathode materials

Density functional theory (DFT) has been used to investigate the structural, electrochemical and electronic effects of the Na-doped Li site of the LiFePO4 cathode material. NaxLi1−xFePO4(x=0,0.25,0.5) materials are considered. The results of intercalation voltage are 3.79,3.28and3.52 for x=0,0.25,0.5 respectively. The electronic and thermal conductivity of Na0.5Li0.5FePO4 are the same as that of simple LiFePO4, with an exception at high temperature where the conductivities of LiFePO4 are greater than that of the others. The Na0.25Li0.75FePO4 material show a small electronic conductivity compared to the others.

Electrochemical Behaviors and Doping Rules of NaRhO2 Cathode Materials for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have great advantages for energy storage and conversion due to their low cost and large storage capacity. Currently, NaRhO2 is used as an electrode material for sodium-ion batteries. Doping first- and second-row transition metals has been carried out to comprehensively assess NaRhO2 as a cathode material. The geometric and electronic structures and electrochemical and doping behaviors of NaRhO2 cathode materials for SIBs have been investigated using density functional theory calculations. The results show that the bond lengths of Rh-O in NaRhO2 decrease during sodium deintercalation. The band gap of NaRhO2 with sodium extraction gradually reduces. The density of states of NaxRhO2 shows that the interaction between the Rh-4d and O-2p orbitals increases and the orbitals shift toward the right. The average intercalation voltage of NaxRhO2 cathode material increased from 2.7 to 3.9 eV. After doping with first- and second-row transition metal elements from Sc to Zn and Y to Cd, the changes in the band gaps of the doped NaRhO2 materials exhibit a W-type rule. In contrast, their magnetic moments show a reverse W-type rule. These findings on the pristine and doped NaRhO2 can provide theoretical guidance for the preparation of novel electrode materials suitable for sodium-ion batteries.

First Principles Calculations of Novel Binary Transition Metal Oxides NaCr0.5X0.5O2 (X = Y, Tc, Rh) for Na-Ion Batteries

Recently, layered transition metal oxides have demonstrated voltage windows favorable for use as cathodes in sodium-ion batteries, leading to increased specific capacity and energy density. Here, NaCr0.5Y0.5O2, NaCr0.5Tc0.5O2, and NaCr0.5Rh0.5O2 were studied by using first-principles calculations to elucidate their properties. There is a trigonal arrangement in the structure of both pure and substituted NaCrO2. Structural characteristics reveal the ferromagnetic behavior of these compounds (NaCr0.5X0.5O2; X = Y, Tc, Rh). We calculated the density of states and spin-polarized electronic band structures for the three compounds tested. NaCr0.5Y0.5O2 and NaCr0.5Rh0.5O2 exhibit diluted magnetic semiconductor (DMS) behavior, while NaCr0.5Tc0.5O2 has half-metallic (HM) behavior. The substitutional material’s ferromagnetic nature is confirmed by the negative values of the exchange constants (Nօ α and Nօ β). The fractional value of the magnetic moment also confirms the DMS nature of NaCr0.5Y0.5O2 and NaCr0.5Rh0.5O2, while the HM behavior is confirmed by the integral value of the magnetic moment for NaCr0.5Tc0.5O2. The thermoelectric characteristics were computed using the BoltzTraP code. Alectrochemical analysis of NaCr0.5Y0.5O2 showed a theoretical discharge capacity of 400 mhAg−1 and an average intercalation voltage of 4.87 V, calculated from the total energies of the optimized compounds and their de-sodianted phases. These theoretical computations demonstrate that the binary layered TM oxides studied are appropriate substances to employ in coin cell fabrication as cathodes.

Stabilizing Multi‐Electron NASICON‐Na1.5V0.5Nb1.5(PO4)3 Anode via Structural Modulation for Long‐Life Sodium‐Ion Batteries

AbstractMulti‐electron NAtrium SuperIonic CONductor (NASICON)‐Nb2(PO4)3 (N0NbP) is an attractive Na‐ion battery anode, owing to its low intercalation voltage (1.4 V vs Na+/Na0) and high capacity (≈150 mAh g−1). However, it suffers from poor capacity retention due to structural degradation. To overcome this issue, extra Na+ ions are introduced at the Na(1) sites, via V3+ substitution, which can act as stabilizing agents to hold lantern units together during cycling, producing NASICON‐Na1.5V0.5Nb1.5(PO4)3 (N1.5VNbP). The N1.5VNbP anode exhibits reversible capacities of ≈140 mAh g−1 at 1.4 V versus Na+/Na0 through Nb5+/Nb4+/Nb3+ and V3+/V2+ redox activities. The extra Na+ ions in the framework forms a complete solid‐solution during Na (de)intercalation and enhances sodium diffusivity, in agreement with first‐principles calculations. Further, N1.5VNbP demonstrates extraordinary cycling (89% capacity retention at 5C after 500 cycles) and rate performances (105 mAh g−1 at 5C). Upon pairing the N1.5VNbP anode with the NASICON‐Na3V2(PO4)3 cathode, the full Na‐ion cell delivers a remarkable energy density of 98 Wh kg−1 (based on the mass of anode and cathode) and retains 80% of its capacity at 5C rate over 1000 cycles. The study opens new possibilities for enhancing the electrochemical performance of NASICON anodes via chemical and structural modulations.

Theoretical insights into surface-phase transition and ion competition during alkali ion intercalation on the Cu4Se4 nanosheet.

The development of stable and efficient electrode materials is imperative and also indispensable for further commercialization of sodium/potassium-ion batteries (SIBs/PIBs) and new detrimental issues such as proton intercalation arise when utilizing aqueous electrolytes. Herein the electrochemical performance of the Cu4Se4 nanosheet was determined for both organic and aqueous SIBs and PIBs. By means of density functional theory calculation, Na+, K+ and H+ intercalations onto both sides of the Cu4Se4 nanosheet were revealed. The Cu4Se4 nanosheet well maintains its metallic electronic conductivity and the changes in lateral lattice parameters are within 4.66% during the whole Na+/K+ intercalation process for both SIBS and PIBs. The theoretical maximum Na/K storage capacity of 188.07 mA h g-1 can be achieved by stabilized second-layer adsorption of Na+/K+. The migration barriers of Na and K atoms on the Cu4Se4 nanosheet are 0.270 and 0.173 eV, respectively. It was discovered that Na/K- intercalation in the first layer is accompanied by a first-order surface phase transition, resulting in an intercalation voltage plateau of 0.659/0.756 V, respectively. The region of the two-surface phase coexistence for PIBs, is shifted toward a lower coverage when compared with that for SIBs. The partially protonated Cu4Se4 nanosheet (HxCu4Se4, x ≤ 10/9) was revealed to be structurally and thermodynamically stable. While the partially protonated Cu4Se4 nanosheet is favorable in acidic electrolytes (pH = 0) when protons and Na/K ions compete, we showed that Na+/K+ intercalated products may be preferred over H+ at low coverages in alkali electrolyte (pH = 14). However, the proton intercalation substantially decreases the battery capacity in aqueous SIBs and PIBs. Our work not only identifies the promising performance of Cu4Se4 nanosheets as an electrode material of SIBs and PIBs, but also provides a computational method for aqueous metal-ion batteries.

Layered CrO2·nH2O as a cathode material for aqueous zinc-ion batteries: ab initio study.

Aqueous zinc-ion batteries are considered potential large-scale energy storage systems due to their low cost, environmentally friendly nature, and high safety. However, the development of high energy density cathode materials and uncertain reaction mechanisms remains a major challenge. In this work, the reaction mechanism, discharge voltage and diffusion properties of layered CrO2 as a cathode material for aqueous zinc-ion batteries were studied using first-principles calculations, and the effect of pre-intercalated structural water on the electrochemical performance of CrO2 electrodes is also discussed. The results show that CrO2 exhibits high average discharge voltages (2.65 V for H insertion (pH = 7) and 1.97 V for Zn insertion) and medium theoretical capacities (319 mA h g-1 (H and Zn)). The H intercalation voltage strongly depends on the pH value of the electrolyte. The H/Zn co-insertion mechanism occurs at low hydrogen concentrations (c(H) ≤ 0.125), where the initial insertion of H reduces the total amount of subsequent Zn insertion. For the substrate containing structured water (CrO2·nH2O, n ≥ 0.5), the average voltage of Zn insertion is significantly increased, while the average voltage of H slightly decreases. In addition, the pre-intercalated water strategy significantly improved the diffusion properties of H and Zn. This study shows that layered CrO2·nH2O is a promising cathode material for aqueous zinc-ion batteries, and also provides theoretical guidance for the development of high-performance cathode materials for aqueous zinc-ion batteries.

Tuning electrolyte solvation structure with Co-solvent engineering to enable highly reversible and stable Zn-graphite dual-ion batteries

Graphite-based dual-ion batteries (GDIBs) have attracted ever-growing attentions due to its non-transition metal configuration and high working potential. However, the safety concerns associated with the use of organic electrolyte make it unpractical for large-scale energy storage system. Moreover, the anodic limit of aqueous electrolytes, even the improved anodic limit derived from highly concentrated aqueous electrolytes, is not sufficient to support high graphite intercalation voltage. Herein, a co-solvent-induced hybrid aqueous/non-aqueous electrolyte has been proposed to break this bottleneck through regulating the solvation structures around anions. A series of hybrid electrolytes are prepared by controlling dimethyl carbonate (DMC) ratio and the relation between the solvation structures and anion intercalation chemistry is explored, indicating that the mutual interaction between solvent molecules and anions determines the anion intercalation behaviour. The optimized DMC-dominated solvation structures with a mass ratio of DMC to water of 1.2:1 significantly expand the anodic limit of electrolyte up to 2.54 V (vs. Zn/Zn2+). The as-designed Zn-Graphite batteries exhibit an excellent cyclic stability and reversibility (99 % capacity retention after 200 cycles at 200 mA g−1, nearly 98 % Coulombic efficiency). This work provides guidance to achieve the balance between security capability and electrochemical performances through controllable solvation structures.

Experimental and computational study of binary transition metal oxides NaNi1/2 X1/2 O2 (X=Cr, Fe, Co and Cu) for energy storage applications

For novel applications as cathodes in sodium-ion batteries, layered transition metal oxides have been reported to have an appropriate voltage window that results higher specific capacity and energy density. In the present work, a series of binary layered transition metal oxides: NaNi1/2Cr1/2O2, NaNi1/2Fe1/2O2, NaNi1/2Co1/2O2, and NaNi1/2Cu1/2O2 were prepared using sol-gel technique. XRD analysis confirmed that all the synthesized materials stabilize a hexagonal structure with an R3m space group. SEM micrographs showed the formation of well-separated particles of varying shapes. FTIR spectra indicated the presence of metal oxygen absorption bands. Using density functional theory (DFT), crystal structure was generated using the lattice parameters worked out using XRD. The spin polarized electronic band structures and density of states (DOS) computed for all four compounds revealed half metallic character. Average intercalation voltages (AIV) were calculated theoretically from the total energies of the optimized compounds and their de-sodianted phases. Volume changes were also calculated to study its minimal values for the compounds, to identify them as of good structural stability. Both experimental findings and theoretical calculations show that the investigated binary layered transition metal oxides are suitable cathode materials for coin cell fabrications.

An ab initio investigation of LiCoBO3 as Li-ion battery cathode material

We give a first principles approach for the structural, electronic and magnetic properties of LiCoBO3 and CoBO3 in the monoclinic lattices. Along [001] direction, based on density functional theory approach and using Full Potential Linear Augmented Plane Wave (FP-LAPW) method, Polarized spin and spin–orbit coupling are included in calculations, and major spin was set up. The calculations were carried out using generalized gradient approximation (GGA) + an on-site Coulomb self-interaction correction potential (GGA + U). The magnetic and orbital moments are estimated. The U value was considered to be equal to 6 eV for Co atom. The average equilibrium voltage over a full cycle, of the LiCoBO3 battery is estimated from our FP-LAPW calculations. The capacity of a cell and energy density of LiCoBO3 were calculated. The compound LiCoBO3 is a semiconductor with average intercalation voltage 4.01 V. The voltage difference help promote electrochemical performance, which opens a window for Li-ion battery marketing application.

Effect of molybdenum disulfide doping with substitutional nitrogen and sulfur vacancies on lithium intercalation

Molybdenum disulfide (MoS2) nanomaterials were synthesized by rapid decomposition of ammonium tetrathiomolybdate in argon or ammonia at 600 °C and 700 °C. The change of environment had no effect on the morphology, but it affected the composition and electronic structure of the nanomaterial. The use of gaseous NH3 in the synthesis led to the incorporation of nitrogen and the formation of sulfur vacancies in the MoS2 lattice. An electrochemical study showed that this dual lattice modification significantly reduced the irreversible capacity in the first cycle and improved the lithium capacity and structural stability of MoS2 in the voltage range of 2.5–1.1 V. After 65 cycles of the operation of lithium-ion battery, the specific capacity of the defective MoS2 was 189 mAh g–1, which is higher than the theoretical capacity of ideal MoS2. An increase in electrical conductivity and a decrease in charge transfer resistance determined by using electrochemical impedance spectroscopy were associated with a decrease in the band gap in MoS2 due to the substitutional nitrogen atoms and sulfur vacancies, as shown by density functional theory calculations. In addition, these defects create new sites for lithium adsorption and increase the intercalation voltage.

First-principles approach to the structural, electronic and intercalation voltage of Prussian blue (KxFe[Fe(CN)6]) (x = 1, 2) as potential cathode material for potassium ion batteries

First-principles approach to the structural, electronic and intercalation voltage of Prussian blue (KxFe[Fe(CN)6]) (x = 1, 2) as potential cathode material for potassium ion batteries

Bulk substitution of F-terminations from Ti3C2Tx MXene by cation pillaring and gas hydrolysation

Applications of 2D MXenes are limited by the difficulty of controlling bulk termination groups after the initial HF etching step without forming surface oxides. Here, we report on gas hydrolysation using a continuous flow of Ar (g) with a controlled partial pressure of H2O (g) as a new method to change the terminations of multilayered Ti3C2Tx MXene particles (T = O, OH and F), and demonstrate pre-intercalation of cations as a necessity for successful hydrolysation as it enables water molecules to enter the Ti3C2Tx MXene structure. Hydrolysation of pristine HF-etched Ti3C2Tx shows no compositional change before oxidation into a TiO2/C composite starts at T > 300 ˚C. However, by pre-intercalating various cations into the MXene, a pillaring of the structure is achieved, which for certain cations (K+ and Na+) remains even after hydrolysation at 300 ˚C. By hydrolysing K-intercalated Ti3C2Tx at 300 ˚C, a significant bulk F reduction of 78 % was achieved, accompanied by a comparable increase in O content and insignificant surface oxidation of the particles. For other cations (Mg2+, Li+ and TBA+) the expanded interlayer spacing collapsed upon hydrolysation, resulting in no significant compositional changes. Moreover, hydrolysation is shown to give higher selectivity towards F removal compared to air annealing, which instead resulted in the oxidation of C to CO2 and the formation of TiOF2. In Li-ion battery half cells, the intercalation of K-ions reduces both the capacity and energy efficiency compared to pristine Ti3C2Tx. Nevertheless, hydrolysation increases the capacity and intercalation voltage, and is thus a feasible method to control the electrochemical performance of Ti3C2Tx MXene. In summary, gas hydrolysation is demonstrated as a selective and efficient method to substitute F terminations with O-related terminations in multilayered MXene particles and pave the way for utilisation on other MXene compositions.

Unveiling a high capacity multi-redox (Nb5+/Nb4+/Nb3+) NASICON-Nb2(PO4)3 anode for Li- and Na-ion batteries

Sodium superionic conductor (NASICON)-type materials are widely explored as Li- and Na-ion cathodes and solid-state electrolytes but are largely ignored as anodes due to their lower capacities and higher intercalation voltages, which reduce the overall energy densities of Li- and Na-ion batteries (LIBs and SIBs).

Enhanced electrochemical performance of NASICON-type sodium ion cathode based on charge balance theory

The variable components and relatively high Na + intercalation voltage makes sodium ion superconductors (NASICON) as ideal cathode materials for secondary sodium-ion batteries. Na 3 V 2 (PO 4 ) 3 , as one of the most typical NASICON-type cathode, needs urgent improvement to enable its wide application. Based on charge balance theory, via utilizing Al 3+ and SiO 4 4− to replace V 3+ and PO 4 3− in Na 3 V 2 (PO 4 ) 3 , respectively, Na 3+ x V 1.5 Al 0.5 (PO 4 ) 3- x (SiO 4 ) x (0≤ x ≤0.5) cathode materials were prepared in this work. In comparison with the initial state, the operation potential was apparently promoted because of the reversible access of V 4+ /V 5+ redox couple at 4.0 V vs. Na + /Na; the higher specific capacity can be achieved owing to much more conductive Na + involved in the redox reaction of V species. Furthermore, the structural stability and diffusion kinetics is dramatically enhanced, which are further validated by ex-situ XRD and GITT analysis. Specifically, Na 3.3 V 1.5 Al 0.5 (PO 4 ) 2.7 (SiO 4 ) 0.3 can offer highly reversible capacities of 127.3 and 181.5 mAh g −1 within the region of 2.5-4.4 V and 1.4-4.4 V, respectively, making itself a promising sodium ion cathode material. Na-ion full cells paired with Na 3.3 V 1.5 Al 0.5 (PO 4 ) 2.7 (SiO 4 ) 0.3 cathode and hard carbon anode are assembled, rendering a specific capacity of 154.3 mAh g −1 at 0.5 C based on the mass of cathode.

Exploration of NaSICON Frameworks as Calcium-Ion Battery Electrodes

The development of energy storage technologies that are alternative to state-of-the-art lithium-ion batteries but exhibit similar energy densities, lower cost, and better safety is an important step in ensuring a sustainable energy future. Electrochemical systems based on calcium (Ca)-intercalation or deintercalation form such an alternative energy storage technology but require the development of intercalation electrode materials that exhibit reversible Ca-exchange with reasonable energy density and power density performance. To address this issue, we use first-principles calculations to screen over the wide chemical space of sodium superionic conductor (NaSICON) frameworks, with a chemical formula of CaxM2(ZO4)3 (where M = Ti, V, Cr, Mn, Fe, Co, or Ni and Z = Si, P, or S) for Ca electrode materials. We calculate the average Ca2+ intercalation voltage, the thermodynamic stability (at 0 K) of charged and discharged Ca-NaSICON, and the migration barriers of (meta)stable Ca-NaSICON compositions. Importantly, our calculations indicate CaxV2(PO4)3, CaxMn2(SO4)3, and CaxFe2(SO4)3 Ca-NaSICONs to be promising as Ca-cathodes. We find all silicate Ca-NaSICONs to be thermodynamically unstable and hence unsuitable as Ca-cathodes. We report the overall trends in the ground state Ca-vacancy configurations, besides voltages, stabilities, and migration barriers. Our work contributes to unearthing strategies for developing practical calcium-ion batteries, involving polyanionic intercalation frameworks.

Accurate Electronic Properties and Intercalation Voltages of Olivine-Type Li-Ion Cathode Materials from Extended Hubbard Functionals

The design of novel cathode materials for Li-ion batteries would greatly benefit from accurate first-principles predictions of structural, electronic, and magnetic properties as well as intercalation voltages in compounds containing transition-metal elements. For such systems, density-functional theory (DFT) with standard (semi-)local exchange-correlation functionals is of limited use as it often fails due to strong self-interaction errors that are especially relevant in the partially filled $d$ shells. Here, we perform a detailed comparative study of the phospho-olivine cathode materials Li$_x$MnPO$_4$, Li$_x$FePO$_4$, and the mixed transition metal Li$_x$Mn$_{1/2}$Fe$_{1/2}$PO$_4$ ($x=0, 1/4, 1/2, 3/4, 1$) using four electronic-structure methods: DFT, DFT+$U$, DFT+$U$+$V$, and HSE06. We show that DFT+$U$+$V$, with onsite $U$ and intersite $V$ Hubbard parameters determined from first principles and self-consistently with respect to the structural parameters by means of density-functional perturbation theory (linear response), provides the most accurate description of the electronic structure of these challenging compounds. In particular, we demonstrate that DFT+$U$+$V$ displays very clearly "digital" changes in oxidation states of the transition-metal ions in all compounds, including the mixed-valence phases occurring at intermediate Li concentrations, leading to voltages in remarkable agreement with experiments. We show that the inclusion of intersite Hubbard interactions is essential for the accurate prediction of thermodynamic quantities, balancing the drive for localization induced by the onsite $U$ with intersite $V$ orbital hybridizations.

First‐Principles Study on the Electronic Properties and Mechanical Stabilities of Anion‐Cation Multiple‐Doped LiFePO4

AbstractIn this paper, N, Nb composite doped lithium iron phosphate structure was constructed, and its thermodynamic stability, intercalation voltages, volume change rate, electronic structure properties, and mechanical properties were systematically investigated using the first principles. The results of formation energy demonstrate that the N and Nb composite doping system meets the thermodynamic stability requirements and can exist consistently. In the process of de‐lithium, the volume change rate of the anion‐cation hybrid doping system is significantly decreased, and the intercalation voltage is increased, which indicates that the doping makes the cycling performance and energy density of the material improved. A radical shift in the material's electronic structure after doping is conducive to the enhanced conductivity of lithium iron phosphate. Besides, studies on the elastic properties of materials demonstrated that both N and Nb doping inhibited the generation of microcracks and diminished the occurrence of shear deformation.

Synthesis, Structural and Electrochemical Properties of Sodium Transition Metal Fluorosulfate Cathodes for Na-Ion Batteries

The development of low-cost and sustainable rechargeable batteries is attractive for storing energy from renewable sources. At present, the expansion of Na-ion batteries (NIBs) serves as an appealing solution from the perspective of both raw material abundance as well as the cost in comparison with the existing Li-ion batteries.1 However, the energy densities of NIBs are lower compared to their Li-ion counterparts due to thermodynamic reasons. To address this issue, researchers have turned their attention towards the development of high energy density cathodes, such as Na3V2(PO4)2F3 and Na3V2(PO4)3-type materials,2 in which the operation of multi-redox couples render high storage capacities. Moving forward, the replacement of PO4 3- by SO4 2- provides higher intercalation voltages (due to inductive effect), thereby further improving the overall energy density of cathode. 3 The inclusion of fluoride into the sulphate based polyanionic frameworks will help to increase the operating voltage through inductive effects.4 This will enhance cationic mobility by reducing electrostatic interactions along conduction channels, thereby, lead to increase in cell capacity.Herein, we report synthesis, structural and electrochemical properties of two classes of sodium transition metal fluorosulfates, namely Na3MF2(SO4)2 and jarosites NaM3(SO4)2(OH/F)6 (M= V and Mn etc.,). The Na2VF3(SO4)2 framework consists of chains of trans-VO2F4 octahedra linked to each other via vertex sharing of F atoms and bridged by SO4 tetrahedron as adjacent pairs. 5 Jarosite is a natural mineral to be found in acidic and sulfate-rich environments with a general formula of AM3(SO4)2(OH)6, where A = K+, Na+, and NH4 + and M = Fe3+, Cr3+, V3+, Ga3+, Al3+, and In3+. Jarosite crystals stabilize in a trigonal system with space group R3m. The structure is composed of 2D uneven layers formed by linkage of transition metal octahedra [MO2(OH)4] and sulfate tetrahedra (SO4), that are stacked along the c direction. 6 We unveil the details of growth mechanism of these materials in hydro- and solvo-thermal conditions using X-ray diffraction and microscopy techniques. Further, the mechanism of electrochemical (de)sodiation reactions in these materials will be presented using galvanostatic cycling and in-operando XRD measurements.

An Undoped Tri‐Phase Coexistent Cathode Material for Sodium‐Ion Batteries

AbstractThe layered transition‐metal‐oxide materials are one type of promising cathode materials for sodium‐ion batteries, which typically include P2, P3, and O3 phases, and each has its own advantages and challenges. Taking into account the complementarity of each single‐phase structure, constructing the composite structures is an efficient pathway to stabilize the structure and improve the electrochemical performance. Herein, three composite cathode materials owning the phase structures of P3/P2, P2/O3, and P3/P2/O3 are prepared by changing the calcinating conditions without introducing any doping element. Among them, the composite of P3/P2/O3 shows the best capacity retention of 80 mAh g–1 after 200 cycles and the highest rate performance of 100 mAh g–1 at a current density of 750 mA g–1. The improved electrochemical performance can be attributed to the staggered arrangement of different phase structures and the gradient Na‐extraction/intercalation voltages of different phases. The slip of the transition metal layer is subjected to the constraint of the adjacent phase structure, thus inhibiting the phase transformation for capacity fading. This work provides an easy way for the preparation of composite cathode structures and brings a clear case for understanding the advantage of composite structures on electrochemical performance.