Electrode Materials For Na-ion Batteries Research Articles

Obtaining V2(PO4)3 by sodium extraction from single-phase NaxV2(PO4)3 (1 < x < 3) positive electrode materials.

We report on single-phase NaxV2(PO4)3 compositions (1.5 ≤ x ≤ 2.5) of the Na super ionic conductor type, obtained from a straightforward synthesis route. Typically, chemically prepared c-Na2V2(PO4)3, obtained by annealing an equimolar mixture of Na3V2(PO4)3 and NaV2(PO4)3, exhibits a specific sodium-ion distribution (occupancy of the Na(1) site of only 0.66(4)), whereas that of the electrochemically obtained e-Na2V2(PO4)3 (from Na3V2(PO4)3) is close to 1. Unlike conventional Na3V2(PO4)3, when used as positive electrode materials in Na-ion batteries, the NaxV2(PO4)3 compositions lead to unusual single-phase Na+ extraction/insertion mechanisms with continuous voltage changes upon Na+ extraction/insertion. We demonstrate that the average equilibrium operating voltage observed upon Na+ deintercalation from single-phase Na2V2(PO4)3 is increased up to an average value of ~3.70 V versus Na+/Na (thanks to the activation of the V4+/V5+ redox couple) compared to 3.37 V versus Na+/Na in conventional Na3V2(PO4)3, thus leading to an increase in the theoretical energy density from 396.3 Wh kg-1 to 458.1 Wh kg-1. Electrochemical and chemical Na+ deintercalation from c-Na2V2(PO4)3 enables complete Na-ion extraction, increasing energy density.

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

Na2.64Mn1.64(MoO4)3 and Na2.62Ni1.69(MoO4)3: Physicochemical investigations and electrochemical properties as 3.5 V class positive electrode material for Na-batteries

Powders of two new mixed molybdates Na2.64Mn1.64(MoO4)3 (1) and Na2.62Ni1.69(MoO4)3 (2) have been prepared via a solid-solid reaction. They are isostructural, both crystallizing in monoclinic system (S. G.: C2/c), Z = 1, and the unit cell parameters a = 12.74109 (18) Å, b = 13.69390 (15) Å, c = 7.20028 (8) Å, β = 112.4314 (7) °, V = 1161.22 (2) Å3 (1), and a = 12.6368 (2) Å, b = 13.3633 (2) Å, c = 7.08501 (10) Å, β = 111.9248 (10) °, V = 1109.91 (3) Å3 (2). The structure consists of a 3D network of corners and edges sharing [MoO4] and [MO6] polyhedral, delimiting tunnels in which the Na ions partly site. The samples were investigated by powder X-ray diffraction, infrared spectroscopy, and scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM-EDX). The vibrational study confirms the existence of the MoO42− functional groups. Laser Raman spectroscopy showed that the MoO42− tetrahedra in the two compounds are more regular, suggesting high symmetry for the compound. The as-synthesized compounds are fiber-like grains with an average length of 10−1 μm. FT-IR spectra showed the presence of all the functional groups present in the compounds. Magnetisation measurements showed their paramagnetic behaviour. The title compounds were characterized by cyclic voltammetry (CV), electrochemical performance and limiting factors are also applied as electrode material for Na-ion batteries at room temperature. Benefiting from these advantages, between 1.5 V and 4.6 V, (1) and (2) deliver high specific capacities of 230 and 160 mA h g−1 after 60 cycles, and good rate performances, respectively. It can also be inferred that the prepared electrocatalyst showed for both phases excellent Hydrogen Evolution Reaction (HER) performance with an overpotential of 531 mV for Mn and 512 mV for Ni, required to afford a current density of 10 mA cm−2.

First principle evaluations on perovskite-type sodium trifluorides, NaMF3 (M= V, Mn, Fe, Co, Ni, and Cu), as electrode materials for Na-ion batteries

This ab initio study investigates the structural, Na diffusion channels, electrochemical, and electrical properties of NaMF3 (M= V, Mn, Fe, Co, Ni, and Cu) for as novel electrode materials for Na-ion batteries, using density functional theory (DFT) calculations. The stability and structural changes are evaluated during (de)sodiation. Considering electrochemical potential of the materials by Fermi and internal energy approaches, NaNiF3 and NaVF3 have the highest and lowest voltages, respectively. The electrical properties of the materials are examined in terms of intrinsic- and extrinsic-like band gaps. For comprehensiveness, the electrical rate-capability is estimated using two different approaches, i.e. Delta and CCTB. Consequently, utilizing various DFT approaches and evaluating properties, each considered material has its own advantages and drawbacks to be used as Na-ion electrode. Using a general perspective, this paper ranks the materials as NaCuF3, NaCoF3, NaVF3, NaMnF3, NaFeF3, and NaNiF3, respectively. This study opens a new landscape for future investigations about these promising intercalation electrode materials and also the other analogous systems.

Designing a 3D framework NaVOPO4 as a high-power, low-strain and long-life positive electrode material for Na-ion batteries

Advances in sodium-ion batteries hugely rely on perfecting the performance of active electrode materials. In this paper, we offer a new NaVOPO4 polymorph adopting a KTiOPO4-type framework as a promising high-rate, low-strain and long-life positive electrode material for sodium-ion batteries. NaVOPO4 is prepared via a facile hydrothermally-assisted solid-state ion exchange. The crystal structure is refined based on synchrotron X-ray powder diffraction (XRD) and validated by X-ray absorption spectroscopy (XAS), transmission electron microscopy analysis and density functional theory (DFT) calculations. The electrochemical performance of NaVOPO4 is evaluated through galvanostatic charge/discharge tests, cyclic voltammetry, potentiostatic intermittent titration and electrochemical impedance spectroscopy. A carbon-coated NaVOPO4 demonstrates a specific capacity of ∼110 mAh g–1 at a C/10 rate at an average potential of ∼3.93 V vs. Na+/Na. The material exhibits decent capacity retention and rate capability, maintaining over 74 % of the initial capacity after 1000 cycles at a C/2 rate and around 87% at a 2C charge/discharge rate. Diffusion coefficients of 10–11 cm2 s–1 and DFT-calculated energy barriers of ∼0.15–0.35 eV indicate fast Na+ ion diffusion within the NaVOPO4 framework. The vanadium oxidation state and charge compensation mechanism in NaVOPO4 are studied by XAS and DFT. Moreover, the operando XRD analysis coupled with DFT elucidates several phase transitions occurring in NaVOPO4 with an exceptionally low volume variation of 2.4% in total, highlighting the reversibility and structural stability during Na+ de/insertion. Overall, NaVOPO4 exhibits attractive electrochemical performance and stability, making it a potential candidate for sodium-ion battery cathode materials.

Decoupling electrolyte strategy for a high-voltage Na-ion hybrid capacitor based on NaFeO2 and activated carbon

Sodium iron dioxide (NaFeO2) with high specific capacity, has been widely used as a kind of electrode materials for Na-ion batteries. When NaFeO2 is used as the battery-type electrode for aqueous Na-ion hybrid capacitors, there are a lot of technical challenges to meet, especially the narrow voltage window limited by the thermodynamic decomposition of water. We propose a novel decoupling electrolyte strategy to suppress the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on the NaFeO2 anode and the activated carbon (AC) cathode respectively, in order to expand the voltage window of the aqueous AC//NaFeO2 Na-ion hybrid capacitor. The alkaline anolyte (2 M NaOH) and the neutral catholyte (1 M Na2SO4) can be decoupled by the cation exchange membrane, which selectively allows the Na+ ions through the membrane to carry the electric currents in the charge/discharge process. The NaFeO2 sample with the stable crystal structure and oxygen vacancy defects, can provide a gravimetric specific capacitance of 101.1 F g−1 at 1 A g−1. Electrochemical kinetic analysis indicates that the capacitor-type behaviour is predominant in the electrochemical energy storage process of the AC//NaFeO2 Na-ion capacitor at the scan rates higher than 40 mV s−1. The AC//NaFeO2 Na-ion capacitor with the stable voltage window of 2 V, can provide an energy density of 12.6 Wh kg−1 at 1 kW kg−1. Due to its inhibiting effects on HER and OER, this decoupling electrolyte strategy holds great potential for constructing high-voltage aqueous energy storage devices.

A first-principles study of TiA2P4 (A = Si and Ge) monolayers as excellent electrode materials for Na-ion batteries

Two-dimensional (2D) materials have shown excellent application prospects in energy storage technologies due to the extraordinary physical and chemical properties. In this work, TiA2P4 (A = Si or Ge) monolayers, that members of MA2Z4 family, are selected as electrode materials for lithium/sodium-ion batteries. Their structural stabilities, mechanical properties, electronic structures, and energy storage performances are investigated by using first-principles calculations. Satisfactorily, TiA2P4 monolayers have good thermodynamic stability and mechanical properties. Meanwhile, low diffusion barriers, suitable open circuit voltages and significant theoretical capacities prove that TiA2P4 are suitable electrode materials for Li/Na storage, except for TiGe2P4 that lack the ability to store lithium. The theoretical capacity of TiSi2P4 monolayer is up to 470.33/705.50 mA h g−1 for Li/Na storage, and that of TiGe2P4 monolayer is 507.24 mA h g−1 for Na-ions, which are better than many metal-like 2D layered materials. Our research indicates that both of TiA2P4 are potential electrode material for sodium-ion batteries, and inspire the future design of other MA2Z4 family in energy storage technologies.

Structure and evolution of disordered deep-charge phases in NaxCrO2 Na-ion battery electrodes

Layered sodium chromium oxide, O3–NaCrO2, is a promising electrode material for Na-ion batteries due to the high thermal stability, high rate-capability, and good coulombic efficiency of the material. However, it is well-known that the material loses crystallinity and the ability to store Na-ions reversibly when taken to high states of charge. While the structural identity of the disordered state has been investigated, a bulk structural model has so far been lacking, as has the mechanism for the order-disorder transition from the P′3-NaxCrO2 (the last crystalline phase observed during charge) to the disordered state. In this work, we combine pair distribution function analysis from total X-ray scattering and powder X-ray diffraction to obtain a complete structural model for the disordered O3-like NaxCrO2 forming at high states-of-charge, and from operando pair distribution function analysis and X-ray absorption spectroscopy, we elucidate the structural evolution of the order-disorder transition and shed new light on the Cr-migration in the disordered state, which is the cause of the irreversibility of the Na-ion storage.

Phase-engineering modulation of Mn-based oxide cathode for constructing super-stable sodium storage

The Mn-based oxide cathode with enriched crystal phase structure and component diversity can provide the excellent chemistry structure for Na-ion batteries. Nevertheless, the broad application prospect is obstructed by the sluggish Na+ kinetics and the phase transitions upon cycling. Herein, we establish the thermodynamically stable phase diagram of various Mn-based oxide composites precisely controlled by sodium content tailoring strategy coupling with co-doping and solid-state reaction. The chemical environment of the P2/P'3 and P2/P3 biphasic composites indicate that the charge compensation mechanism stems from the cooperative contribution of anions and cations. Benefiting from the no phase transition to scavenge the structure strain, P2/P'3 electrode can deliver long cycling stability (capacity retention of 73.8 % after 1000 cycles at 10 C) and outstanding rate properties (the discharge capacity of 84.08 mA h g−1 at 20 C) than P2/P3 electrode. Furthermore, the DFT calculation demonstrates that the introducing novel P'3 phase can significantly regulate the Na+ reaction dynamics and modify the local electron configuration of Mn. The effective phase engineering can provide a reference for designing other high-performance electrode materials for Na-ion batteries.

In Operando Strain Evolution in Sodium Chromium Oxide Cathode for Na-Ion Batteries during Battery Cycling

Layer-structured Na intercalation compounds such as NaxMO2 (M=Co, Mn, Cr) have attracted much attention as cathode materials for sodium-ion batteries due to their high volumetric and gravimetric energy densities. Among them, NaCrO2 with layered rock salt structure is one of the promising cathodes since NaCrO2 has a desirable flat and smooth charge/discharge voltage plateau.1 In addition, NaCrO2 has the highest thermal stability at charged state which makes it a potentially safer cathode material.2 The NaCrO2 exhibits a reversible capacity of 110 mAh g-1 with good cycling performance.3 However, the transition metal oxide (TMO) cathode materials in NIBs undergo severe chemo-mechanical deformations which leads to capacity fade and poor cycling and is the limiting factor of NIBs. The electrochemical characterization and examination of the electrode structure were the primary focus of several investigations. To improve the lifespan and performance of electrode materials for Na-ion batteries, it is vital to comprehend how Na ions impact the chemo-mechanical stability of the electrodes.In this talk, we will discuss the driving forces behind the structural and interfacial deformations on NaCrO2 cathodes. Digital image correlation measurements were conducted to probe strain evolution in the electrode during cycling. The free-standing composite NaCrO2 electrode was used for stain measurements in custom-cell assembly. The battery was cycled against Na metal in 1 M NaClO4 in PC. The first part of the study involves structural and interfacial deformations in the lower voltage range of 2.3 V to 3.5 V where x<0.5 in NaxCrO2. And the second part focuses on the structural and interfacial deformations in the voltage range of 2.3 V to 4.7 V where x>0.5 in NaxCrO2. In the preliminary studies, we observed that the initial insertion of Na ions leads to negative strain evolution (contraction) in the electrode, followed by expansions in the electrode at a higher state of discharge. Similar phenomena are also observed during charge cycles, where extraction of Na results in an initial contraction in the electrode, followed by expansion at a higher state of charge. Understanding the mechanisms behind chemo-mechanical deformations will allow to tune structure & material property for better electrochemical performance.Acknowledgement: This work is supported by National Science Foundation’s (NSF) Faculty Early Career Development (CAREER) Program, 2142726.

LCA and C-LCC Indicator as Tools for Sodium-Ion Batteries’ Eco-Design

Sodium-ion batteries are considered promising alternatives to lithium-ion technology; however, the diffusion on a commercial scale is hindered by the struggle to identify materials with high electrochemical performances. Studies available in the literature are mainly focused on electrochemical performance and neglect aspects related to the environmental sustainability. In fact, the current state-of-the-art (presented in this study) shows that life cycle assessment (LCA) studies related to the production processes of electrode materials for Na-ion batteries are still very limited. The LCA methodology applied during the development of a technology phase can constitute a valid support for an eco-oriented design and, therefore, to the choice of solutions characterized by a lower environmental impact with the same electrochemical performance. In this context, a life cycle-based environmental–economic assessment was performed to evaluate the environmental impacts of the production process of cathode and anode materials for sodium-ion batteries. The study is focused on the cathodic active material Na0.66MnO2, considering two synthesis paths, and the anodic material consisting of tin (Sn) and Sn-carbon nanofiber (Sn-Cn) active material, binder, and other additives. Results illustrate the environmental performance of the different materials and constitute a useful input for their selection within an eco-design view.

Thermal Stability of Na3–xV2(PO4)2F3–yOy: Influence of F– for O2– Substitution and Degradation Mechanisms

The Na-ion battery technology appears as a reliable, sustainable, and environmentally friendly alternative to the Li-ion one, especially for stationary energy storage. As for the Li-ion technology, the safety aspect is of high importance to ensure large-scale development. In this work, we studied the thermal stability and decomposition mechanisms of carbon-coated Na3V2(PO4)2F3 and two fluorine-rich phases belonging to the solid-solution Na3V3+2–yV4+y(PO4)2F3–yOy (y = 0.07 and y = 0.12), that family of compounds being often considered among the most promising positive electrode materials for Na-ion batteries. This study shows the good thermal stability of these polyanionic materials and reveals that a low O2– for F– substitution has a very limited effect on the thermal stability of fully reintercalated materials recovered in the discharged state of the battery, whereas it has a beneficial impact for highly deintercalated ones, obtained by in-depth charges. Furthermore, whatever the state of charge and the oxygen content in NaxV2(PO4)2F3–yOy (1 ≤ x ≤ 3 and y = 0, 0.07 and 0.12), the thermal degradation leads, quite unexpectedly, to the formation of crystalline Na3V3+2(PO4)2F3 in addition to an amorphous phase. The fluorination of the partial oxygen for fluorine substituted material was clearly demonstrated by X-ray diffraction (XRD) and solid state nuclear magnetic resonance spectroscopy (NMR) on materials recovered after differential scanning calorimetry (DSC) analyses. The formation of a fully sodiated crystalline phase from the thermal degradation of the material obtained in charged states of the battery, with or without the presence of electrolyte, was never reported before.

The synergistic effect of Ti and Nb in TiNbC leads to enhanced anode performance for Na-ion batteries - first-principles calculations

Currently, MXenes have been identified as promising candidate electrode materials for Na-ion batteries because of their excellent energy storage and electrical conductivity. Among them, dual transition metal MXenes have attracted attention due to their excellent properties as anode materials for Na-ion batteries. In order to explore the reason why dual transition metal MXenes outperform single transition metal MXenes, we systemically investigated and compared the performance of TiNbC-based dual transition metal MXenes (TiNbC, TiNbCF2, and TiNbCO2) with that of Nb2C-based single transition metal MXenes (Nb2C, Nb2CF2, and Nb2CO2) as anode materials for Na-ion batteries based density functional theory calculations. The results showed that TiNbC, TiNbCO2, Nb2C, and Nb2CO2 are promising anode materials for Na-ion batteries due to the low diffusion barrier and high capacity of Na on their surfaces. Compared with the Nb2C-based single transition metal MXenes, the TiNbC-based dual transition metal MXenes have better adsorption performance, diffusion rate, and theoretical storage of Na atoms. The higher adsorption ability of TiNbC and TiNbCO2 to Na atoms was attributed to the synergistic effect of Ti and Nb which increases the interaction between the substrate and Na. This conclusion provides a new insight for the development of other high-performance MXenes-based anode materials for Na-ion batteries.

Lower Diffusion-Induced Stress in Nano-Crystallites of P2-Na2/3 Ni1/3 Mn1/2 Ti1/6 O2 Novel Cathode for High Energy Na-ion Batteries.

P2-type Na2/3 Ni1/3 Mn1/2 Ti1/6 O2 (NMTNO) cathode is a preeminent electrode material for Na-ion batteries owing to its open prismatic framework, air-moisture stability, inexpensiveness, appealing capacity, environmental benignity, and Co-free composition. However, the poor cycling stability, sluggish Na-ion kinetics induced in bulk-sized cathode particles, cracking, and exfoliation in the crystallites remain a setback. To outmaneuver these, a designing strategy of a mechanically robust, hexagonal nano-crystallites of P2-type Na2/3 Ni1/3 Mn1/2 Ti1/6 O2 (NMTNOnano ) electrode via quick, energy-efficient, and low-cost microwave-irradiated synthesis is proposed. For the first time, employing a unified experimental and theoretical approach with fracture mechanics analysis, the mechanism behind the enhanced performance, better structural stability, and lower diffusion-induced stress of NMTNOnano compared to micro-sized Na2/3 Ni1/3 Mn1/2 Ti1/6 O2 is unveiled and the electrochemical shock map is predicted. The NMTNOnano cathode provides 94.8% capacity retention after 100 cycles at 0.1 C with prolonged performance for 1000 cycles at 0.5 C. The practical viability of this cathode, tested in a full cell against a hard carbon anode delivered 85.48% capacity retention at 0.14mA cm-2 after 200 cycles. This work bridges the gap in correlating the microstructural and electrochemical properties through experimental, theoretical (DFT), and fracture mechanics analysis, thereby tailoring efficient cathode with lower diffusion-induced stress for high-energy Na-ion batteries.

Electrochemical capacitance properties of pre-sodiated manganese oxide for aqueous Na-ion supercapacitors.

Mn-based oxides are widely investigated as electrode materials for electrochemical supercapacitors, because of their high specific capacitance in addition to the high abundance, low cost, and environmental friendliness of Mn. The pre-insertion of alkali metal ions is found to improve the capacitance properties of MnO2. While the capacitance properties of MnO2, Mn2O3, P2-Na0.5MnO2, and O3-NaMnO2etc. are reported, there is no report yet on the capacitive performance of P2-Na2/3MnO2, which has already been studied as a potential positive electrode material for Na-ion batteries. In this work, we have synthesized sodiated manganese oxide, P2-Na2/3MnO2 by a hydrothermal method followed by annealing at a high temperature of about 900 °C for 12 h. For comparison, manganese oxide Mn2O3 (without pre-sodiation) is synthesized by following the same method, but annealing at 400 °C. While P2-Na2/3MnO2 exhibits a high specific capacitance of 234 F g-1, Mn2O3 can deliver only 115 F g-1 when cycled at 0.4 A g-1 in an aqueous electrolyte of 1.0 M Na2SO4 in a three-electrode cell. An asymmetric supercapacitor Na2/3MnO2‖AC is assembled, which can exhibit a SC of 37.7 F g-1 at 0.1 A g-1 with an energy density of 20.9 W h kg-1, based on the total weight of Na2/3MnO2 and AC with an operational voltage of 2.0 V and possesses excellent cycling stability. This asymmetric Na2/3MnO2‖AC supercapacitor can be cost-effective considering the high abundance, low-cost and environmental friendliness of Mn-based oxides and aqueous Na2SO4 electrolyte.

First-principles design of nanostructured electrode materials for Na-ion batteries: challenges and perspectives.

Post-lithium batteries are emerging as viable solutions for sustainable energy transition. Effective deployment in the market calls for great research efforts in the identification of novel component materials and the assessment of related working principles. Computational modelling can be a key player in boosting innovation and development by enabling rational strategies for the design of appropriately tuned materials with optimized activity towards battery operating processes. By gaining access to the structural and electronic features of functional electrodes, state-of-the-art DFT methods can unveil the subtle structure-property relationship that affects the uptake, transport, and storage efficiency. Hereby, we aim at reviewing the research status of theoretical advances in the field of Na-ion batteries (NIBs) and illustrating to what extent atomistic insights into sodiation/desodiation mechanisms of nanostructured materials can assist the development of effective anodes and cathodes for stable and highly performing devices. Thanks to increasing computer power and fruitful cooperation between theory and experiments, the route for effective design methodologies is being paved and will feed the upcoming developments in NIB technology.

First-principles investigation of phase stability in layered NaxCrO2

Layered oxide intercalation compounds continue to attract interest as electrode materials for Na-ion batteries. However, many of these materials undergo complex phase transitions during cycling that influence battery performance but are still not completely understood. We have conducted a first-principles study of layered ${\mathrm{Na}}_{x}{\mathrm{CrO}}_{2}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$ to assess phase stability between various Na-vacancy ordered phases in the O3 and P3 host structures. We predict that many of the low-energy phases belong to families of Na orderings that follow specific patterns. At high $x$, we identify families of vacancy row orderings in O3, which may also couple to magnetic orderings of the Cr spins. We predict similar orderings at intermediate $x$ in P3 that contain antiphase boundaries between regions of the $x=1/2$ ordering. In both cases, the average spacing between rows/boundaries is set by the overall composition. At $x=0$, we find a strong preference for charge disproportionation and migration of Cr to tetrahedral sites in the intercalation layers. We obtain generally good agreement with experimental observations and rationalize key discrepancies.

Nano-Crystallites of P2-Type Layered Transition Metal Oxide High Voltage Cathode for Sodium-Ion Battery

In this era of transitioning from conventional sources of energy to non-conventional, sodium-ion battery research has been burgeoning as an indigenous solution to energy storage applications, considering the sustainability, cost effectiveness, high availability and a familiar redox chemistry 1. P2-type Na2/3Ni1/3Mn2/3O2 is one of the preeminent cathode for sodium-ion batteries because of their environmental friendliness, open framework, superior specific capacity, higher operating voltage and air-moisture stability. However, rapid capacity decay on charging it to a higher voltage because of P2 to O2 phase transition and a large volume change leading to exfoliation of the layers have impeded the practicability of this as an electrode material for Na-ion battery 2,3,4.Here in this work, we report the preparation of hexagonal nanocrystals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, which is titanium doped in pristine Na2/3Ni1/3Mn2/3O2 via a novel and quick microwave synthesis technique. This provides sharp and clear facets that allows accelerated sodium-ion migration within the crystal during extraction and insertion of Na-ions, making this material a highly efficient cathode. Unlike conventional heating, which requires around 12-20 hours of synthesis time and high energy consumption, microwave radiation induces rapid solid-state reaction that heats the material on molecular level leading to uniform heating, thus retaining the nanocrystallinity of the structure 5. The distinctive hierarchical nanostructure having large surface area could efficiently facilitate the transportation of Na+ ions, fast utilization of active materials, overcome the effect of internal strain generated inside and reduces the pulverization of active materials, thereby restraining the P2 to O2 phase transition at higher voltage 6. Aiding from the combined effect of titanium doping at manganese site and designing hierarchical nanocrystrals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, we obtained a rate capability of 145 mAh g-1 at 0.1 C and a prolonged cycling life (87.3% capacity retention after 500 cycles at 1C) within a voltage range of 2.5 – 4.2 V, restraining the P2 to O2 type phase transition at higher potential. The combined analysis of X-ray diffraction, scanning electron microscopy and transmission electron microscopy along with density functional theory (DFT) calculations demonstrated the optimization of the structure and the physical properties of pristine Na2/3Ni1/3Mn2/3O2 and Ti doped structure along with their Bader charge analysis and electronic properties. Further the mechanical integrity of the nano Na2/3Ni1/3Mn1/2Ti1/6O2 and micro Na2/3Ni1/3Mn1/2Ti1/6O2 were analyzed through micro-compression test of the as prepared pellets. The underlying mechanism for the suppression of phase transition in Na2/3Ni1/3Mn1/2Ti1/6O2 was elucidated by ex-situ X-ray diffraction (XRD) and ex-situ Transmission electron microscopy (TEM). The electrochemical kinetics regarding Na+ diffusion coefficient was further studied through galvanostatic intermittent titration technique (GITT). An analytical model was established to probe deeper into the reason for exfoliation and thus, support our hypothesis. In addition, a sodium-ion full cell was constructed by pairing the as-prepared P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 with a hard carbon anode. This modification of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 nanocrystallites with comprehensive electrochemical performance can be a path breaking, highly efficient cathode material for large-scale energy storage applications.

Extended conjugated carbonyl-containing polymer as a negative electrode material for Na-ion batteries

Extended conjugated carbonyl-containing polymer as a negative electrode material for Na-ion batteries

Large-scale CuS nanotube arrays@graphdiyne for high-performance sodium ion battery

Copper sulfide (CuS) is emerged as a promising electrode material for Na-ion batteries due to its high theoretical specific capacity, environmental friendliness and abundant resource. However, the conversion type mechanism for storing the Na+ causes huge volume variations, irreversible phase changes and disintegration of the structure during charge and discharge processes. These intractable issues lead to the fast capacity degradation, and sluggish reaction kinetics, and performances are hard to fulfill the criteria in practical applications. Herein, the large-scale free-standing CuS nanotube arrays sandwiched by successive graphdiyne (GDY) nanofilm are readily prepared using a simple and efficient strategy. The in-situ protection of GDY nanofilm successfully enhances the structural stability and the network conductivity of the free-standing electrode. The as-obtained CuS electrode protected by GDY exhibits excellent electrochemical sodium storage capability, in the term of long-term reversibility (After 1000 cycles at 2 C, the capacity retention is 78.7%.) and rate performance.