Cathode For Na-ion Batteries Research Articles

Design of Perovskite-Type Fluorides Cathodes for Na-ion Batteries: Correlation between Structure and Transport

Transition metal-based sodium fluoro-perovskite of general formula NaMF3 (M = Fe, Mn, and Co) were investigated as cathode materials for rechargeable Na-ion batteries. Preliminary results indicated Na-ion reversible intercalation but highlighted the need to find optimization strategies to improve conductivity and to modulate the operating voltages within experimentally accessible electrolytes’ stability windows, in order to fully exploit their potential as high-voltage cathodes. In this study, we combined experimental and computational techniques to investigate structures, defects, and intercalation properties of the NaFe1-xMnxF3 and NaCo1-xMnxF3 systems. Through the use of a simple solvothermal synthesis, we demonstrated the possibility to modulate the sample’s morphology in order to obtain fine and dispersed powder samples. The structural results indicated the formations of two solid solutions with a perovskite structure over the entire compositional range investigated. Atomistic simulations suggested that Na-ion diffusion in these systems was characterized by relatively high migration barriers and it was likely to follow three-dimensional paths, thus limiting the effect of anti-site defects. The correlation between structural and computational data highlighted the possibility to modulate both ionic and electronic conductivity as a function of the composition.

Gradational anionic redox enabling high-energy P2-type Na-layered oxide cathode

Anionic-redox-based layered oxide materials are considered promising cathodes for Na-ion batteries because of their high energy densities. However, the anionic redox reaction at high voltage results in structural instability of the layered oxides, leading to not only poor electrochemical properties but also structural degradation after prolonged cycling. Herein, through combined studies using first-principles calculation and various experimental techniques, we investigate the role of the combination of earth-abundant Mn, Fe, and Mg in enabling a stable and gradational anionic redox reaction in a P2-type Na-layered oxide cathode during charge/discharge, resulting in outstanding electrochemical performance. At 10 mA g−1, P2-type Na0.67[Mg0.22Mn0.55Fe0.23]O2 delivers a large specific capacity of ∼207 mAh g−1, corresponding to ∼0.8 mol Na+ de/intercalation via both cationic and anionic redox reactions. The outstanding cycle performance, well-retained crystal structure, and morphology after prolonged cycling indicate that the anionic redox reaction of O2−/O− stably occurred in the P2-type Na0.67[Mg0.22Mn0.55Fe0.23]O2 structure despite the charging process in the high-voltage region. Furthermore, the use of earth-abundant Mn, Fe, and Mg is beneficial in terms of the economic feasibility for low-cost and high-energy Na-ion batteries. These intensive investigations provide key knowledge for understanding anionic-redox-based cathode materials with high structural stability for Na-ion batteries.

Rate-dependent electrochemical strain generation in composite iron phosphate cathodes in Li-ion batteries

The performance of battery electrodes is significantly impacted by chemo-mechanical instabilities at faster charge/discharge rates. This study reports rate-dependent mechanical deformations in the LiFePO4 cathodes during battery cycling by synchronizing in situ digital image correlation and electrochemical techniques. The electrode undergoes larger mechanical deformations in the early cycles and irreversible strains become negligible in the subsequent cycles. Cumulative irreversible strains show a linear relationship with the square root of cycling time, and the slope of the cumulative irreversible strains is greater at faster rates. The study compares the irreversible strains in LiFePO4 for Li-ion batteries with its analogous NaFePO4 cathodes for Na-ion batteries. Rate-dependent mechanical deformations are reported as the LiFePO4 electrode undergoes larger strains per capacity at faster rates. Pulsed current charge/discharge experiments coupled with strain measurements suggest a delay in the phase transformations at faster rates. The study provides new insights into rate-dependent chemo-mechanical deformations in the LiFePO4 electrodes.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures.

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

Enhanced rate performance and specific capacity in Ti-substituted P2-type layered oxide enabled by crystal structure and particle morphology modifications

In recent years, P2-type layered oxides have received considerable attention as potential cathodes for Na-ion batteries owing to their compositional diversity, good specific capacity, and cyclability. However, poor rate performance and low-capacity retention at high discharge rates have limited their use in commercial battery applications. This study aims to mitigate this issue by synthesizing a series of structurally engineered P2-type cathode materials, through Ti substitution, with high cyclability and improved rate performance. P2-type Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 were prepared through a sol–gel route and were characterized for their structural, electrical, and electrochemical properties In the Ti-substituted samples, the Rietveld refinement of XRD data revealed an increased size of the bottleneck area of the Na-O6 prism planes, while the SEM images showed a decrease in the aspect ratio of hexagon-type morphology of particles which facilitates faster Na-ion conduction through the material. These changes in the crystal structure and particle morphology induced by Ti substitution have significantly improved electrical and electrochemical performance compared to the parent material. The sample with x = 7.5% exhibited a specific capacity of 126 mAh/g at a discharge rate of 0.1C in the 2.00–4.25 V window, which was about 25% more than that of the undoped material. At a discharge rate of 1C, the specific capacity of Na0.70Ni0.20Cu0.15Mn0.575Ti0.075O2 was 97 mAh/g compared to 74 mAh/g for Na0.70Ni0.20Cu0.15Mn0.65O2 sample. Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 samples also exhibited excellent cyclability, with over 95% of original capacity retained after 300 cycles. Complex impedance measurements corroborated the improved Na-ion conductivity in Ti-doped samples and an associated increase in Na-ion transference number from 0.86 for the sample with x = 0 to 0.97 for the sample with x = 0.075.

Perspective: Design of cathode materials for sustainable sodium-ion batteries

Manufacturing sustainable sodium ion batteries with high energy density and cyclability requires a uniquely tailored technology and a close attention to the economical and environmental factors. In this work, we summarized the most important design metrics in sodium ion batteries with the emphasis on cathode materials and outlined a transparent data reporting approach based on common metrics for performance evaluation of future technologies.Sodium-ion batteries are considered as one of the most promising alternatives to lithium-based battery technologies. Despite the growing research in this field, the implementation of this technology has been practically hindered due to a lack of high energy density cathode materials with a long cycle-life. In this perspective, we first provide an overview of the milestones in the development of Na-ion battery (NIB) systems over time. Next, we discuss critical metrics in extraction of key elements used in NIB cathode materials which may impact the supply chain in near future. Finally, in the quest of most promising cathode materials for the next generation of NIBs, we overlay an extensive perspective on the main findings in design and test of more than 295 reports in the past 10 years, exhibiting that layered oxides, Prussian blue analogs (PBAs) and polyanions are leading candidates for cathode materials. An in-depth comparison of energy density and capacity retention of all the currently available cathode materials is also provided. In this perspective, we also highlight the importance of large data analysis for sustainable material design based on available datasets. The insights provided in this perspective, along with a more transparent data reporting approach and an implementation of common metrics for performance evaluation of NIBs can help accelerate future cathode materials design in the NIB field.Graphical abstract

Rapid and solvent-free mechanochemical synthesis of Na iron hexacyanoferrate for high-performance Na-ion batteries

Sodium iron hexacyanoferrate (NaFeHCF) containing Fe as the only transition metal element has attained extensive interest for Na-ion batteries due to the low raw material cost and high resource abundance; however, the traditional solution-based synthesis methods, including coprecipitation and hydrothermal reaction, are usually time-consuming and difficult to control the interstitial water and Na content in the lattice, which puts a challenge on the high-efficiency and controllable preparation of NaFeHCF. Herein, we demonstrate a rapid and solvent-free mechanochemical route under the Ar atmosphere to synthesize Na1.59Fe[Fe(CN)6]0.95□0.051.92H2O cathode, which delivers an initial discharge capacity of ∼130 mA h g−1 at 0.2 C and 118 mA h g−1 at a high rate of 10 C. Besides, ex situ XRD confirms that NaFeHCF cathode experienced a phase transition from the initial cubic phase to Na-free cubic structure during the first charging process and followed by a reversible phase change between the monoclinic and Na-free cubic structures in the subsequent cycles. More impressively, the optimized NaFeHCF/NaTi2(PO4)3 full cell reveals a decent capacity retention of 80% over 1000 cycles at 1 C, exhibiting a promising prospect in the practical application. Our current work sheds light on the high-efficient scalable production of high-performance NaFeHCF cathodes for Na-ion batteries via mechanochemical approach.

Tuning anionic redox activity to boost high-performance sodium-storage in low-cost Na0.67Fe0.5Mn0.5O2 cathode

Na-based layered iron-manganese oxide Na0.67Fe0.5Mn0.5O2 containing only low-cost elements is a promising cathode for Na-ion batteries used in large-scale energy storage systems. However, the poor cycle stability restricts its practical application. The capacity decay of Na0.67Fe0.5Mn0.5O2 mainly originates from the irreversible anionic redox reaction charge compensation due to the high-level hybridization between oxygen and iron. Herein, we rationally design a surface Ti doping strategy to tune the anionic redox reaction activity of Na0.67Fe0.5Mn0.5O2 and improve its Na-storage properties. The doped Ti ions not only enlarge the Na migration spacing layer but also improve the structure stability thanks to the strong Ti–O bond. More importantly, the d0-shell electronic structure of Ti4+ can suppress the charge transfer from the oxidized anions to cations, thus reducing the anionic redox reaction activity and enhancing the reversibility of charge compensation. The modified Na0.67Fe0.5Mn0.5O2 cathode shows a reversible capacity of 198 mA h g−1 and an increased capacity retention from 15% to 73% after about 1 month of cycling. Meanwhile, a superior Na-ion diffusion kinetics and rate capability are also observed. This work advances the commercialization process of Na-based layered iron-manganese oxide cathodes; on the other hand, the proposed modification strategy paves the way for the design of high-performance electrode materials relying on anionic redox reactions.

Organic Solvothermal Method Promoted Monoclinic Prussian Blue as a Superior Cathode for Na-Ion Batteries

Organic Solvothermal Method Promoted Monoclinic Prussian Blue as a Superior Cathode for Na-Ion Batteries

Expanded solid-solution behavior and charge-discharge asymmetry in NaxCrO2 Na-ion battery electrodes

Sodium chromium oxide, NaCrO2, has received significant attention as positive electrode (cathode) for Na-ion batteries mainly due the high capacity retention of the material. In this paper, we use operando powder X-ray diffraction to investigate the structural evolution of O3–NaCrO2 during Na-ion extraction and insertion with use of both liquid and solid-state electrolytes. This reveals a previously overlooked phase transition, causing formation of an unexplored O′3-NaxCrO2 intermediate during Na-ion extraction (charge). The phase transitions within the electrode are investigated by sequential Rietveld refinement and distortion analysis, which shows that the discovered O′3-NaxCrO2 intermediate accommodates the increasing interlayer repulsion and in-plane contraction with less distortion of the [NaO6]-polyhedra as compared to the other observed phases. Our structural findings are corroborated by analysis of the Na-ion diffusion coefficients through the galvanostatic intermittent titration technique. Furthermore, the operando powder X-ray diffraction reveals substantial charge-discharge asymmetry with a significant preference for solid-solution and two-phase behavior during charge (Na-extraction) and discharge (Na-insertion), respectively.

Achieving High Stability and Performance in P2-Type Mn-Based Layered Oxides with Tetravalent Cations for Sodium-Ion Batteries.

P2-type sodium-manganese-based layered cathodes, owing to their high capacity from both cationic and anionic redox, are a potential candidate for Na-ion batteries (NIBs) to replace Li-ion technology in certain applications. Still, the structure instability originating from irreversible oxygen redox at high voltage remains a challenge. Here, a high sustainability cobalt-free P2-Na0.72 Mn0.75 Li0.24 X0.01 O2 (X = Ti/Si) cathode is developed. The outstanding capacity retention and voltage retention after 150 cycles are obtained in half-cells. The finding shows that Ti localizes on the surface while Si diffuses to the bulk of the particles. Thus, Ti can act as a protective layer that alleviates side reactions in carbonate-based electrolyte. Meanwhile, Si can regulate the local electronic structure and suppress oxygen redox activities. Notably, full-cells with hard carbon (≈300-335Whkg-1 based on the cathode mass) deliver the capacity retention of 83% for P2-Na0.72 Mn0.75 Li0.24 Si0.01 O2 and 66% for P2-Na0.72 Mn0.75 Li0.24 Ti0.01 O2 after 500 cycles; this electrochemical stability is the best compared to other reported cathodes based on oxygen redox at present. The superior cycle performance also stems from the ability to inhibit microcracking and planar gliding within the particles. Altogether, this finding offers a new composition for developing high-performance low-cost cathodes for NIBs and highlights the unique role of Ti/Si ions.

Mg-doped layered oxide cathode for Na-ion batteries

Na-ion batteries (NIBs) are regarding as the optimum complement for Li-ion batteries along with the rapid development of stationary energy storage systems. In order to meet the commercial demands of cathodes for NIBs, O3-type Cu containing layered oxide Na0.90Cu0.22Fe0.30Mn0.48O2 with good comprehensive performance and low-cost element components is very promising for the practical use. However, only part of the Cu3+/Cu2+ redox couple participated in the redox reaction, thus impairing the specific capacity of the cathode materials. Herein, Mg2+-doped O3-Na0.90Mg0.08Cu0.22Fe0.30Mn0.40O2 layered oxide without Mn3+ was synthesized successfully, which exhibited improved reversible specific capacity of 118 mAh/g in the voltage range of 2.4–4.0 V at 0.2 C, corresponding to the intercalation/deintercalation of 0.47 Na+ (0.1 more than that of Na0.90Cu0.22Fe0.30Mn0.48O2). This work demonstrates an important strategy to obtain advanced layered oxide cathodes for NIBs.

Enhanced stability and rate performance of zinc-doped cobalt hexacyanoferrate (CoZnHCF) by the limited crystal growth and reduced distortion

Cobalt hexacyanoferrate (CoHCF) is a potential cathode for aqueous Na-ion batteries due to its high theoretical specific capacity (170 mAh g−1); however, its lower rate capability and cyclability limit its applications. Structural distortion at a weak N-coordinated crystal field during cycling disintegrates Co, yielding an irreversible reaction. Different Zn amounts ranging 0–1 were added to the Co site to suppress the structural irreversibility of CoHCF, yielding Co1-xZnxHCF powder; this Zn (x ≤ 0.09) addition reduced the powder’s dimension because the lower four coordination of Zn–N, not the six coordination of Co–N, limits the powder growth. Simultaneously, a small lattice parameter and interaxial angle (∼90°) are obtained, implying that a narrower Co1−xZnxHCF inner structure is formed to accommodate Na ions. Moreover, the electronic conductivity of Co1−xZnxHCF gradually increased within 0–0.09 range. A smaller particle size with a high surface area leads to a near-surface-limited redox process, similar to a capacitive reaction. Both the surface-limited reaction and electronic conductivity enhances the reversibility due to the smaller charge transfer resistance at the electrode/electrolyte interface caused by Zn addition. Replacing redox-active Co with non-active Zn amount of 0.07 (Co0.93Zn0.07HCF) slightly reduces the specific capacity from 127 to 119 mAh g−1 at 0.1 A g−1 due to the shrunken Co charging sites. Rate performance is enhanced by compromising the capacity and reduced distortion, resulting in 81% retention at a 20-times-faster charging rate. Notably, the Co0.93Zn0.07HCF sample exhibited the good stability while preserving 74% of the initial capacity at 0.5 A g−1 after 200 cycles.

Regulated Synthesis of α-NaVOPO4 with an Enhanced Conductive Network as a High-Performance Cathode for Aqueous Na-Ion Batteries.

The low-cost and profusion of sodium reserves make Na-ion batteries (NIBs) a potential candidate to lithium-ion batteries for grid-scale energy storage applications. NaVOPO4 has been recognized as one of the most promising cathodes for high-energy NIBs, owing to their high theoretical capacity and energy density. However, their further application is hindered by the multiphase transition and conductivity confinement. Herein, we proposed a feasible, one-step hydrothermal synthesis to regulate the synthesis of α-NaVOPO4 with controlled morphologies. The electrochemical properties of the NaVOPO4 electrode can be significantly enhanced taking Ketjen black (KB) as the optimized conductive carbon. Besides, combining with the nanocrystallization and construction of the conductive framework via high-energy ball milling, taking KB as the conductive carbon, the as-prepared NaVOPO4/5%KB exhibits superior Na-storage performance (140.2 mA h g-1 at 0.1 C and a capacity retention of 84.8% over 1000 cycles at 10 C) to the original NaVOPO4 (128.5 mA h g-1 at 0.1 C and a capacity retention of 83.1% over 1000 cycles at 10 C). Moreover, the aqueous full cell with NaTi2(PO4)3 as the anode delivers a capacity of 114.7 mA h g-1 at 0.2 C (141 W h kg-1 energy density) and 80.6% capacity retention over 300 cycles at 5 C. The excellent electrochemical performance can be attributed to the nanosized structural and enhanced interfacial effect, which could be rewarding to construct electron transportation tunnels, thus speeding up the Na+-diffusion kinetics. The modified strategy provides an efficient approach to intensify the electrochemical performance, which exhibits potential application of the NaVOPO4 cathode for NIBs.

Carbon-coated FePO4 nanoparticles as stable cathode for Na-ion batteries: A promising full cell with a Na15Pb4 anode

The cheap and non-toxic FePO4, which can be obtained from LiFePO4 recovered from spent Li-ion batteries, provides a reversible capacity of 103 mAh/g in coin cells, corresponding to 76% of theoretical capacity, with very good rate capability and stability. The study of the electrochemical mechanism during sodiation carried out by in situ X-ray diffraction (XRD) reveals a pure biphasic transition during Na+ (de)insertion. The chemical diffusion coefficient (DNa) determined from galvanostatic intermittent titration technique and electrochemical impedance spectroscopy varies in the range 10−16–10−10 cm2/s at room temperature depending on the degree of sodiation. As a proof of concept, a full cell fabricated using a carbon-coated FePO4@C cathode and a Na15Pb4 anode obtained by electrochemical sodiation of recyclable Pb, maintained 76% of the initial capacity over 100 cycles at constant C/10 rate. This outstanding electrochemical performance, based on the combination of the low-cost FePO4@C cathode with a recyclable lead-based anode, makes this new technology a promising real-world alternative for future commercial Na-ion systems.

Enhanced Electrochemical Properties of Na0.67MnO2 Cathode for Na-Ion Batteries Prepared with Novel Tetrabutylammonium Alginate Binder

Both the binder and solid–electrolyte interface play an important role in improving the cycling stability of electrodes for Na-ion batteries. In this study, a novel tetrabutylammonium (TBA) alginate binder is used to prepare a Na0.67MnO2 electrode for sodium-ion batteries with improved electrochemical performance. The ageing of the electrodes is characterized. TBA alginate-based electrodes are compared to polyvinylidene fluoride- (PVDF) and Na alginate-based electrodes and show favorable electrochemical performance, with gravimetric capacity values of up to 164 mAh/g, which is 6% higher than measured for the electrode prepared with PVDF binder. TBA alginate-based electrodes also display good rate capability and improved cyclability. The solid–electrolyte interface of TBA alginate-based electrodes is similar to that of PVDF-based electrodes. As the only salt of alginic acid soluble in non-aqueous solvents, TBA alginate emerges as a good alternative to PVDF binder in battery applications where the water-based processing of electrode slurries is not feasible, such as the demonstrated case with Na0.67MnO2.

Fabrication and Characterization of Conductive Polyaniline-Decorated Na3v2(Po4)3 Cathode for Na-Ion Batteries

Fabrication and Characterization of Conductive Polyaniline-Decorated Na3v2(Po4)3 Cathode for Na-Ion Batteries

High-throughput design of Na–Fe–Mn–O cathodes for Na-ion batteries

Over 448 compositions of Na–Fe–Mn–O sodium-ion battery cathodes were made and characterized to determine structure, electrochemical performance, and air stability. Two materials (P2 and P3) are found to have particularly significant improvements.

Fabrication and Characterization of Conductive Polyaniline-Decorated Na3v2(Po4)3 Cathode for Na-Ion Batteries

Fabrication and Characterization of Conductive Polyaniline-Decorated Na3v2(Po4)3 Cathode for Na-Ion Batteries

Large Scale One-Pot Synthesis of Monodispersed Na 3 (VOPO 4 ) 2 F Cathode for Na-Ion Batteries

Na-ion batteries (NIBs) have received significant interest as potential candidates for large-scale energy storage owing to the widespread distribution of sodium and superior low-temperature performance. However, their commercial application is usually hindered by the high production cost and inadequate performance for electrode materials, particularly for cathodes. Na 3 (VOPO 4 ) 2 F (NVOPF) has been recognized as one of the most promising cathodes for high-energy NIBs owing to the high working voltage and energy density. Here, we report a facile highly efficient room-temperature solution protocol for large-scale synthesis of NVOPF cathode for NIBs. By simply regulating pH, NVOPF can be obtained, which delivered a discharge capacity of 120.2 mAh g -1 at 0.1 C and 72% capacity retention over 8000 cycles at 25 C. Besides, the kilogram-level NVOPF products have been synthesized, and 26650 cylindrical cells were fabricated, which exhibit excellent cycling stabilities, remarkable low-temperature performance with comparable safety features. We hope our findings could provide insights on the industrial application of NVOPF in NIBs.