Cathode Candidate For Sodium-ion Batteries Research Articles

Achieving Superior Cyclability Pouch Cells with Oxygen Vacancy-Moderated P'2/P3 Hybrid Layered Sodium Cathode Materials.

Layered P2-type sodium manganese oxide has emerged as a promising cathode candidate for sodium-ion batteries due to its appealing cost-effectiveness and high discharge voltage. However, its practical capacity within the voltage range of 2.0-4.0 V (vs Na+/Na) is relatively low, and its rate capability is hampered by the adverse charge/vacancy ordering during charge/discharge. In this study, a layered P'2/P3 mixed-phase Na0.8-aMn0.675Ni0.225Li0.1O2-x cathode with high (003) crystal plane intensity was designed by introducing oxygen vacancies to P2-structured materials. Aided by these advantages, the hybrid cathode material demonstrates impressive structural and thermal stability and faster Na-ion diffusion kinetics compared to a regular P2 material. Half-cell shows an initial discharge capacity retention of 101 mA h/g at 12 mA/g and 92.25% retention after 500 cycles at 120 mA/g. In combination with a hard carbon anode, the 0.5 A h pouch cell achieved a prevailing capacity retention of 95.2% after 2600 cycles at 36 mA/g. This work opens new dimensions for layered cathode materials with the aim of achieving superior cyclabilities.

Synergistic Strain Suppressing and Interface Engineering in Na4MnV(PO4)3/C for Wide-Temperature and Long-Calendar-Life Sodium-Ion Storage.

A Na4MnV(PO4)3 (NMVP) cathode is regarded as a promising cathode candidate for sodium-ion batteries (SIBs). However, issues such as low electronic conductivity and partial cation dissolution contribute to high polarization and structure distortion. Herein, we engineered the local electron density and reaction kinetic properties of NMVP cathodes with varying oxygen vacancies by introducing varying amounts of Zr doping and carbon coating. The optimized sample exhibited a high-rate capacity of 71.8 mAh g-1 at 30 C (83.1% capacity retention after 1000 cycles) and excellent performance over a wide temperature range (84.1 mAh g-1 at 60 °C and 61.4 mAh g-1 at -30 °C). In situ X-ray diffraction technology confirmed a redox solid solution and a two-phase reaction mechanism, revealing minor changes in cell volume and slight strain variations after Zr doping, effectively suppressing the structural distortion. Theoretical calculations illustrated that Zr doping largely shrinks the band gap of NMVP, enriches local electron density, and slightly alters the local element distribution and bond lengths. Moreover, full-cells have shown high energy density (259.9 Wh kg-1) and outstanding cycling stability (200 cycles). The work provides fresh insights into the synergistic effect of strain suppressing and interface engineering in promoting the development of wide temperature range and long-calendar-life SIBs.

Unraveling the functioning mechanism of fluorine-doping in Mn-based layered oxide cathodes toward enhanced sodium-ion storage performance

Manganese-based layered oxides with anionic redox activity are considered as one of the most promising cathode candidates for sodium-ion batteries (SIBs) owing to their abundant resources and high theoretical specific capacities. However, the severe Jahn-Teller (J-T) effect of Mn3+ and irreversible lattice oxygen loss result in rapid structural degradation and electrochemical performance deterioration. Herein, the functioning mechanism of F-doping in regulating the local and electronic structures of Mn-based layered oxides is unraveled. The introduction of the more electronegative F ions on one hand breaks the electronic symmetry of the MnO6 octahedra and effectively alleviates the J-T distortion, on the other hand suppresses the Zn ions migration through the strong Zn-F bonds and stabilizes the oxygen redox chemistry and facilitates the Na+ diffusion. The above reaction mechanisms are systematically validated by in-situ/ex-situ analyses and theoretical computations. As a result, the optimum P2-Na0.75Zn0.28Mn0.72O1.93F0.07 cathode demonstrates significantly improved rate capability (178.6 mAh g−1 at 0.1 C with 64.4 mAh g−1 at 10 C) and enhanced cycling durability (83.1 % capacity retention over 400 cycles at 3 C) compared to the un-doped P2-Na0.75Zn0.28Mn0.72O2 material. This study clarifies the F-doping mechanism in layered oxides and provides new perspectives for designing high-energy and high-stability cathodes for SIBs.

Anion-doped Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode with improved cyclability and air stability for low-cost sodium-ion batteries

Polyanion-type Fe-based sulfates are one of the most promising cathode candidates for sodium-ion batteries (SIBs) due to the low cost and high voltage. However, their practical application is still hindered by the poor air stability. Herein, we report a novel anion-doped Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode material with higher air stability and faster ion/electron transport kinetics compared to pristine Na2.6Fe1.7(SO4)3 cathode as evidenced by first-principles calculations. The obtained Na2.9Fe1.7(SO4)2.7(PO4)0.3 not only manifests a high discharge capacity of 100.4 mAh g−1 with an operating voltage of ∼3.57 V (Na+/Na), but also exhibits excellent rate performance (∼86.7 mAh g−1 at 30 C) and cycle stability over 6000 cycles (capacity retention of 76.3%). Particularly, Na2.9Fe1.7(SO4)2.7(PO4)0.3 cathode still maintain its original crystal structural and sodium-storage property after exposure to air for one week. The high-voltage, long-life, and air-stable Na2.9Fe1.7(SO4)2.7(PO4)0.3 could be a competitive cathode candidate for low-cost sodium-ion batteries.

Achieving rapid sodiation/desodiation kinetics in NASICON-structured Na3MnTi(PO4)3 via Cr doping

Na3MnTi(PO4)3 is one of the most promising cathode candidates for sodium-ion batteries owing to its low cost, high operating voltage, and large reversible capacity. However, its electrochemical performance is significantly impeded by its deteriorative structure and sluggish Na+ diffusion kinetics. Herein, Cr-doping is introduced to tailor the structure of Na3MnTi(PO4)3, which helps to lengthen the Na2-O bond and expand the Na ion diffusion channels. X-ray diffraction, cyclic voltammetry, galvanostatic intermittent titration technique, and first principle calculations confirm the broadened diffusion channel for Na+ and facilitated Na+ diffusion rate after Cr-doping. As a result, the Na3.1MnTi0.9Cr0.1(PO4)3 electrode exhibits superior electrochemical performance with high discharge capacity (167.5 mAh g–1 at 0.1 C), outstanding rate capability (127.5 mAh g–1 at 10 C), and remarkable cyclability (70.56 % retention after 1400 cycles at 5 C). We believe that the results are useful for the design of high-performance NASICON-structured cathode materials for practical sodium-ion batteries.

Boosting sodium-ion battery performance using Na3(VO)2(PO4)2F microrods self-embedded in a 3D conductive interpenetrated framework

Na3(VO)2(PO4)2F (NVPOF) is considered as a promising cathode candidate for sodium-ion batteries (SIBs) owing to the high theoretical capacity and operating voltage. However, the challenges of its rate capability and cyclability limit its widespread application. Herein, the reduced graphene oxide decorated NVPOF microrods (NVPOF@G) are successfully synthesized by a facile hydrothermal-induced self-assembly procedure. It is found that the graphene oxides added act as a crystal nucleation growth regulator and provide rapid Na+ and electron transport in the electrodes. The prepared NVPOF@G microrods exhibit a superior rate capability of 82.2 mAh g−1 at 50 C (fast discharge in 72 s) as well as considerable long-term cycle stability with a capacity retention of 87.6% over 1000 cycles at 0.5 C. Even at 50 °C, the NVPOF@G still achieves a high reversible capacity of 109.4 mAh g−1 with a capacity retention of 87.7% after 1000 cycles. This microrod structure shows superior structural stability, even at high temperatures (50 °C). Such a cathode incorporates the benefits of batteries and supercapacitors. This work gives a way to fabricate microrod-structure phosphate-based materials for commercial applications in high-performance SIBs.

Promoting Na-ion storage in P2-Na0.67Ni0.33Mn0.67O2 cathode via synergetic Ti and Zn Co-incorporation

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is regarded as a promising cathode candidate for sodium-ion batteries due to its high energy density. However, the electrochemical performance is hindered by Na+/vacancy order, irreversible P2–O2 phase transition at high voltage (>4.2 V), and harmful oxygen evolution. Herein, a synergetic Zn and Ti co-incorporation tactic is proposed for designing a Na0.67Ni0.29Zn0.04Mn0.63Ti0.04O2 (NNZMTO) cathode to overcome the above-mentioned challenges. First, the incorporated Ti heteroatom could break down Na+/vacancy order of NNMO by taking advantage of a similar ionic radius and substantially different Fermi levels with host Mn atom. Subsequently, the introduced Zn heteroatom could induce local Na–O–Zn configurations, buffer interlayer O2−–O2− electrostatic repulsion, as well as inhibit unfavorable phase transition. Moreover, the d10 band of Zn is lower than the oxygen states, and the Zn behaves like an s/p metal with oxygen, thus avoiding O2 release. Notably, in comparison with highly oxidized (Ni4+/Mn4+O6)δ− octahedron, the partial Na+ for charge neutrality in alkali metal layers could be well maintained in the as-designed (Zn2+/Ti4+O6)δ′−, which could be served as “pillars” to avoid layer gliding and structural collapse in the c-direction. As a result, an excellent electrochemical performance with high specific capacity of 90.9 mA h g−1 at 7 C could be retained for NNZMTO thanks to the synergetic effect from Ti and Zn incorporation. This study provides deep insights for designing superior layered cathode via conducting a rational cations co-incorporation strategy.

Ti4+ substitution suppressing P2-O2 phase transition to construct stable P2-Na0.67Ni0.33Mn0.67O2 cathode for long-term durable sodium-ion batteries

Due to high safety and cost-effective resources, sodium-ion batteries show great prospect in large-scale energy storage systems. Owing to wide structural framework and high theoretical capacity, Na-based layered oxide with prismatic P2-type structure is an ideal cathode candidate for sodium-ion batteries. However, Na-based layered compounds always suffer from detrimental multiple phase transitions during cycle period, resulting in structure deterioration and rapid capacity decay. Herein, stable P2-Na0.67Ni0.33Mn0.67-xTixO2 cathodes are constructed through substituting Mn with Ti. In-situ XRD demonstrates that Ti substitution effectively successfully suppresses the undesired phase transition of P2-O2, and in turn appears a stable Z-symbiotic intermediate phase with minimal volume change. Hence, designed P2-Na0.67Ni0.33Mn0.37Ti0.3O2 cathode shows better structural stability and long-term cycling reliability, yielding a high reversible capacity of 123 mAh g−1 and a high capacity retention of 89 % after 100 cycles at 0.2C. More importantly, a high working voltage of 3.5 V and a boosted energy density of 400 Wh kg−1 is achieved in the NNMTO-3 || HC full cell, which further strengthens the competence of Ti-substituted Na-based layered oxide cathodes. This work provides a feasible design insight for long-life, high-voltage sodium-ion batteries, which can be applied to construct more stable layered compounds and further improves their electrochemical properties.

Developing an abnormal high-Na-content P2-type layered oxide cathode with near-zero-strain for high-performance sodium-ion batteries.

Layered transition metal oxides (NaxTMO2) possess attractive features such as large specific capacity, high ionic conductivity, and a scalable synthesis process, making them a promising cathode candidate for sodium-ion batteries (SIBs). However, NaxTMO2 suffer from multiple phase transitions and Na+/vacancy ordering upon Na+ insertion/extraction, which is detrimental to their electrochemical performance. Herein, we developed a novel cathode material that exhibits an abnormal P2-type structure at a stoichiometric content of Na up to 1. The cathode material delivers a reversible capacity of 108 mA h g-1 at 0.2C and 97 mA h g-1 at 2C, retaining a capacity retention of 76.15% after 200 cycles within 2.0-4.3 V. In situ diffraction studies demonstrated that this material exhibits an absolute solid-solution reaction with a low volume change of 0.8% during cycling. This near-zero-strain characteristic enables a highly stabilized crystal structure for Na+ storage, contributing to a significant improvement in battery performance. Overall, this work presents a simple yet effective approach to realizing high Na content in P2-type layered oxides, offering new opportunities for high-performance SIB cathode materials.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking.

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.

Fe-rich pyrophosphate with prolonged high-voltage-plateaus and suppressed voltage decay as sodium-ion battery cathode

Fe-based polyanionic materials are potential cathode candidates for sodium-ion batteries, however, voltage decay and cycling stability become main challenges for practical applications. Besides, the mechanism of phase transition during charge/discharge processes is still unclear. Herein, the Fe-rich phase Na1.4Fe1.3P2O7 with Na/Fe atom vacancies is constructed, showing robust voltage decay suppression and a larger ratio of the plateau length (6:1) at 3.0 and 2.5 V than that of the conventional Na-rich phase Na2FeP2O7 (2.5:1). After 650 cycles at 1 C, the capacity and average voltage retentions of Fe-rich phase are 84 % and 95 %, respectively, much higher than that of Na-rich phase (12 % and 61 %). Furthermore, Mössbauer spectra unveil that, during discharge processes at high voltages, more Fe3+reduction in Fe-rich phase is the intrinsic reason for larger plateau length and lower voltage decay. Density functional theory (DFT) calculation results demonstrate that the higher reduction ability of Fe3+ in Fe-rich phase at high voltages is due to the better reversibility of the two-phase transition between β-NaFeP2O7 and Fe-rich phase than that between β-NaFeP2O7 and Na-rich phase. These allow Fe-rich phase with higher energy densities, lower voltage decay and better cycling stability at high voltage plateaus. Our work provides a new idea for safe design, low-cost and long cycle life sodium ion cathode battery materials.

Solvation-enhanced electrolyte on layered oxide cathode tailoring even and stable CEI for durable sodium storage

Layered transition-metal oxide materials are ideal cathode candidates for sodium-ion batteries due to high specific energy, yet suffer severe interfacial instability and capacity fading owing to strongly nucleophilic surface. In this work, the interfacial stability of layered NaNi1/3Fe1/3Mn1/3O2 cathode was effectively enhanced by electrolyte optimization. And the interfacial chemistry between the cathode and four widely used electrolytes (EC/DMC, EC/EMC, EC/DEC and EC/PC) was elucidated through experiments and theoretical calculations. The Na+ solvation structures at cathode-electrolyte interface in all four electrolytes exhibited enhanced coordination due to high electron density and strong nucleophilicity of oxide surface, which promoted the electrolytes’ decomposition with decreased oxidation stability. Among them, the EC/DMC electrolyte showed the tightest solvation structure due to smaller molecular chains and stable electrochemistry, which derived an even and robust cathode electrolyte interphase. It effectively protected the cathode and facilitated the reversible Na+ transport during long cycles, enabling the batteries with a high capacity retention of 83.3% after 300 cycles. This work provides new insights into the role of electrode surface characteristics in interface chemistry that can guide the design of advanced electrode and electrolyte materials for rechargeable batteries.

Construction of superior performance Na3V2-xCrx(PO4)2F3/C cathode by homovalent doping strategy toward enhanced sodium ion storage

Na3V2(PO4)2F3, with robust 3D structural framework and high operating potential, is regarded as a promising cathode candidate for sodium-ion batteries. However, the low electrical conductivity caused by poly-anionic polyhedron leads to slow kinetics and poor rate performances. In this work, Cr3+ doped Na3V2(PO4)2F3/C is prepared by using chromium as homovalent dopant ion. The synergistic effect of Cr3+ doping and carbon coating promotes the rapid transportation of electrons in Na3V2(PO4)2F3/C particles. The Rietveld refinements show that Cr3+ doping can adjust the crystal structure and accelerate the diffusion kinetics of Na+. The Cr3+ doping improves the electrical conductivity of Na3V2-xCrx(PO4)2F3/C. In-situ electrochemical impedance spectroscopy measurements show that Cr3+ doping effectively reduces the charge transfer impedance and plays an important role in improving the charge transfer kinetics. Consequently, the optimized Na3V1.98Cr0.02(PO4)2F3/C exhibits excellent rate capability of 91.2 mAh·g−1 at 30 C and long cyclic stability, in which the capacity decay per cycle is 0.0029% over 1000 cycles at 10 C. The results provide a facile strategy and beneficial way to design advanced Na3V2(PO4)2F3 cathode toward sodium-ion batteries.

Unprecedented Cyclability and Moisture Durability of NaCrO2 Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr2O3 Coating.

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO2 remains one of the most attractive materials due to its reasonable level of capacity, nearly flat reversible voltages, and high thermal stability. However, the cyclic stability of NaCrO2 needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr2O3-coated and Al-doped NaCrO2, which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr2O3 shell and a Na(Cr1-2xAl2x)O2 core, rather than xAl2O3/NaCrO2 or Na1/1+2x(Cr1/1+2xAl2x/1+2x)O2, through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr2O3-coated NaCrO2 without Al dopants or Al-doped NaCrO2 without shells because of their synergistic contributions. As a result, Na(Cr0.98Al0.02)O2 with a thin Cr2O3 layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO2. In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr2O3-coated Na(Cr1-2xAl2x)O2.

Monoclinic Na2VOP2O7: A 4V-class cost-effective cathode for sodium-ion batteries

It is definitely not enough to realize the large-scale application of sodium-ion batteries (SIBs) only depending on the earth abundance and low cost of Na resources. Low-cost synthesis methods and high performance electrode materials are also imperative. In this work, a simple and low-cost synthesis method is introduced to synthesize both monoclinic and tetragonal Na2VOP2O7, which are the cathode candidates for SIBs. The results of our calculation based on bond valence sum show that monoclinic Na2VOP2O7 has high Na + conductivity. More importantly, monoclinic Na2VOP2O7 presents a high operation voltage about 4.0 V (vs Na+/Na) and a moderate discharge capacity of ∼90 mAh g−1, resulting in a remarkable energy density of 360 W h kg−1.

Boosted electrochemical performance of Na3V2(PO4)3 at low temperature through synergistical F substitution and construction of interconnected nitrogen-doped carbonaceous network

Benefitting from its unique NASICON-type framework, the Na3V2(PO4)3 (NVP) cathodes have aroused extensive interest and have been deemed as the promising cathode candidate for sodium-ion batteries (SIBs). Unfortunately, the poor electronic conductivity, combined with the undesirable volume variations, seriously hinders the practical application of NVP cathode, especially at low temperatures. Herein, a dual-strategy, F substitution accompanied by V vacancies and the construction of three-dimensional (3D) nitrogen-doped carbonaceous frameworks (NC), were employed for the NVP cathode (F-NVP/C@3DNC). The former can remarkably decrease the particle size and enhance Na+ migration capability, increasing the ionic conductivity. Meanwhile, the electronic connection and effective buffering can be obtained from the latter, strengthening the electrode integrity. Consequently, up to 100 cycles at 0.1 A g–1, a reversible capacity of 113.8 mAh g–1, approaching the theoretical value (117 mAh g–1), is demonstrated, accompanied by impressive capacity retentions at 1.0 (93.75% after 4800 cycles) and 20.0 A g–1 (92.7% after 1000 cycles). More importantly, even at –20 °C, a superior specific capacity (102.6 mAh g–1 after 100 cycles at 0.1 A g–1) and high capacity retention (86.6% at 20.0 A g–1 up to 1000 cycles) can still be obtained simultaneously. Significantly, the design of F-NVP/C@3DNC provides insights for the fabrication of polyanion cathodes for applications at low temperatures with modified structure stability and reaction kinetics.

Regulating the local chemical environment in layered O3-NaNi0.5Mn0.5O2 achieves practicable cathode for sodium-ion batteries

The O3-type layered oxides featuring sufficient Na reservoir have been extensively investigated as promising cathode candidates for sodium-ion batteries. However, the complicated phase transitions derived fast capacity decay has severely impeded its practical applications. Herein, we present a scalable yet feasible strategy to tune the local chemical environment in O3-NaNi0.5Mn0.5O2 via Al3+ doping for dramatically enhancing its electrochemical properties. Specifically, the modulated O3-NaNi0.45Al0.1Mn0.45O2 cathode raises the capacity retention rate from 40.9% to 86.2% within 200 cycles at a current density of 85 mA g−1. Combining theoretical calculation and experimental characterizations, it is confirmed that the Al3+ doping induced reinforcement of transition metal-oxygen bonds and enlargement of Na layer distance can simultaneously inhibit the complicated phase transitions and decrease the Na+ diffusion energy barrier, which are responsible for the impressive performance improvement. Importantly, the full-cell device based on this unique cathode and commercial hard carbon anode can deliver a high energy density of 213.5 Wh kg−1, suggesting its great potential of practicability. This work provides a new insight in finely designing high performance layered oxide cathode for sodium ion batteries, which advances the next generation of cost-effective yet grid-level energy storage systems.

Sodium Composite Oxide Cathode Materials:Phase Regulation, Electrochemical Performance and Reaction Mechanism

AbstractThe abundance of sodium resources provides the basis for the large‐scale application of sodium‐ion batteries (SIBs) in the field of energy storage. Among the cathode candidates for SIBs, oxide materials stand out because of their ease of synthesis, environmental friendliness and high capacity. According to the crystal structure, oxide cathode materials can be mainly classified as P2, P3, O3 and tunnel phases. Benefiting from the combined advantages of different phases, composite oxide cathode materials have been widely studied due to the enhanced both cycling and rate performance. In this review, we analyzed the three key issues of phase regulation, electrochemical performance and reaction mechanism based on the recent progress for composite oxide cathode materials. The composite oxide cathode materials will be one of the most competitive and attractive cathodes for SIBs.

Na-Rich Layered Transition Metal Oxides with Core/Shell Structures for Improved Performance of Sodium-Ion Batteries

While P2-type layered transition metal oxides are regarded as potential cathode candidates for sodium-ion batteries because of the high theoretical capacity and a wide operating voltage window, they still suffer from low capacity and sluggish sodium-ion kinetics. Here, we report a Na0.67Mn0.66Co0.17Ni0.17O2 cathode material with a core/shell structure. P2-Na0.67Mn0.66Co0.17Ni0.17O2 has exhibited excellent discharge specific capacity (266 mAh g–1 at 0.1 C and 89 mAh g–1 at 5 C) and ultralong cycle stability, which are among the best state-of-the-art values for Na-based Na-ion batteries. The superior stability results from core/shell structures composed of nanosheets, and the space between the core and shell effectively relieves the volume expansion caused by the deintercalation of sodium ions. Transition metal doping effectively improves the structural distortion caused by the Jahn–Teller effect and provides high sodium mobility, creating higher specific capacity. These findings provide new ideas for the design and development of high-performance sodium-ion battery cathode materials.

Air degradation and rehealing of high-voltage Na0.7Ni0.35Sn0.65O2 cathode for sodium ion batteries

Layered transition metal oxides (NaxTMO2, TM = transition metal) are the most promising cathode candidate for sodium-ion batteries (SIBs) due to their two-dimensional ion diffusion channel that enables easy ion intercalation/deintercalation, superior specific capacity, environmental benignity, and feasibility for mass production. Among the reported NaxTMO2, Na0.7Ni0.35Sn0.65O2 has the highest working plateau and an average voltage as high as 3.7 V and thus has great potential as a high-energy cathode candidate for SIBs. However, its air sensitivity upon exposure and storage, which would greatly affect the cost and feasibility of its practical application, remains unclear. Here, the degradation mechanism of Na0.7Ni0.35Sn0.65O2 upon air exposure was investigated by ex-situ X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that Na0.7Ni0.35Sn0.65O2 suffers from severe air sensitivity and completely transforms to its hydrated phase within 4 h under 25°C and 80% relative humidity. Further, the deteriorated materials were repaired by reheating under different conditions. In-situ variable temperature XRD, infrared, Raman, and thermogravimetry analyses revealed that the recovery of the electrochemical properties and crystal structure of Na0.7Ni0.35Sn0.65O2 depends on the reinsertion of sodium ions into the lattice; these sodium ions have been extracted by H2O upon air exposure. This work provides new insights into the air degradation of electrode materials for SIBs and a deep understanding of their reheating recovery strategy.