Effect of sodium content on the electrochemical performance of P2‐Na2Ni2TeO6 layered oxide cathode for sodium‐ion batteries

Abstract

Sodium‐ion batteries (SIBs) employ P2‐type layered transition metal oxides as promising cathode materials, primarily due to their abundant natural reserves and environmentally friendly characteristics. However, structural instability and complex phase transitions during electrochemical cycling pose significant challenges to their practical applications. Employing cation substitution serves as a straightforward yet effective strategy for stabilizing the structure and improving the kinetics of the active material. In this study, we introduce a Ni‐rich honeycomb‐layered Na_2+xNi₂TeO₆ (NNTO) cathode material with variable sodium content (x = 0, 0.03, 0.05, 0.10). Physicochemical characterizations reveal that excess sodium content at the atomic scale modifies the surface and suppresses phase transitions, while preserving the crystal structure. This results in enhanced cyclic performance and improved electrochemical kinetics at room temperature. Furthermore, we investigate the performance of the NNTO cathode material containing 10% excess sodium at a relatively high temperature of 60°C, where it exhibits 71.6% capacity retention compared to 60% for the pristine. Overall, our results confirm that a preconstructed surface layer (induced by excess sodium) effectively safeguards the Ni‐based cathode material from surface degradation and phase transitions during the electrochemical processes, thus exhibiting superior capacity retention relative to the pristine NNTO cathode. This study of the correlation between structure and performance can potentially be applied to the commercialization of SIBs.

INTRODUCTION

Lithium-ion batteries (LIBs) have become an integral component of daily life, especially with the proliferation of portable electronic devices and electric vehicles.^1–3 Despite their utility, the high cost and limited availability of lithium resources necessitate the exploration of alternative energy storage technologies.^4–8 In this context, sodium-ion batteries (SIBs), which share similar energy storage mechanisms with LIBs and benefit from abundant sodium reserves, have emerged as promising candidates for next-generation energy storage systems.^9–13 However, the cathode material remains a critical factor affecting both the performance and manufacturing cost of batteries. Furthermore, undesirable phase transitions during electrochemical processes compromise cycle life and energy density. Therefore, the development of high-performance cathode materials is essential for the practical implementation of SIBs.^12,14–24

In the quest to find cathode materials that offer high energy density and exceptional electrochemical performance, honeycomb-layered structures with the composition Na_x(MM′)O₂ (where M = Ni, Co, Zn and M′ = Sb, Te, Nb, etc.) have garnered considerable attention^25–30 and are considered optimal candidates for SIBs. It is thought that this family of honeycomb-layered oxides can achieve higher energy density than conventional Na_xMO₂ cathodes.^27,31–34 Partial substitution of transition metals (M) with high-valency metals (M′) enhances both high-voltage performance and thermal stability. This substitution renders the M oxide bonds more ionized, thereby requiring additional energy for the oxidation of M cations during Na-ion removal. As a result, P2-type honeycomb-layered structures such as Ni-rich Na₂Ni₂TeO₆ (NNTO) can operate at high voltages, approximately 4.5 V, compared to Na_xMO₂ cathodes.^31–33 NNTO has been successfully employed in SIBs as a host framework for the reversible insertion of Na ions. Significant advancements were reported by Gupta et al., who observed a smooth charge and discharge profile with a capacity of 90 mAh g⁻¹ at 0.05 C.³⁵ Conversely, Pati et al. noted a stair-like galvanostatic profile in NNTO electrodes, attributing it to stacking disorders in the honeycomb-layered structure; these disorders were correlated with phase transitions, capacity decay, and structural collapse.^36,37 The effects of such structural anomalies on the functionality of layered materials can be either detrimental or beneficial.^36,38–41 Consequently, both theoretical and experimental investigations are warranted to elucidate the relationship between atomic-scale structural disorder, electrochemical performance, and phase transitions.

To address the challenges of structural defects or stacking disorders and to enhance the electrochemical performance of these layered structures, various strategies such as surface coating, morphology design, and cation substitution have been proposed.^27,31,32 Among these, cation substitution stands as a straightforward and effective method to improve Na⁺ kinetics and structural stability. Nevertheless, existing cathodes such as Na₂Ni₂TeO₆ ³⁰ and Na₂Ni_2−xCo_xTeO₆²⁷ continue to experience detrimental phase transitions and poor capacity retention. In this context, we synthesized P2-Na_2+xNi₂TeO₆ (NNTO) with varying levels of excess sodium content (x = 0, 0.03, 0.05, and 0.10) and examined the impact of excess sodium on morphology, crystal structure, and cyclic performance at temperatures of 25°C and 60°C. Our investigation revealed the presence of structural disorders in the P2-NNTO material at the higher sodium content (i.e., at x = 0.05 and 0.1). Detailed analysis confirmed that these defects are advantageous for the NNTO crystal structure. Specifically, they aid in mitigating adverse phase transitions, minimizing Na⁺-vacancy ordering, and thereby enhancing reversible capacity, rate performance, and cycle stability. Our study showed that the NNTO cathode with x = 0.10 exhibited remarkable stability during electrochemical cycling. The notable findings from our comprehensive analyses highlight its exceptional diffusion kinetics and its potential utility as a stable electrode material.

RESULTS AND DISCUSSION

P2-type Na₂Ni₂TeO₆ (NNTO-0) free from impurity phases was successfully synthesized using the high-temperature solid-state synthesis method. Its X-ray diffraction (XRD) pattern is presented in Figure 1A (black line). XRD patterns of sodium-enriched honeycomb-layered oxides Na_2+xNi₂TeO₆ for x = 0.03, 0.05, and 0.10, designated as NNTO-3 (red line), NNTO-5 (blue line), and NNTO-10 (green line), respectively, are also illustrated in Figures 1A and S1. All the XRD patterns indicate that these NNTO compounds possess a P2-type layered structure with a hexagonal phase; the majority of their peaks are in close alignment with the P6₃/mcm space group (PDF # 00-58-0052).³⁷ It is noteworthy that in the XRD pattern of NNTO-0, reflections with even l indices exhibit higher intensities compared to those with odd l indices. This pattern implies that the atoms (Ni and Te) are arranged at repeated distances of c/2, with Ni situated above Ni and Te above Te,^36,42 as illustrated in Figure 1C. In contrast, for NNTO-3, NNTO-5, and NNTO-10, an additional weak peak (101) located at 21.2° is also discernible in the XRD patterns (Figures 1B and S1). While this peak is forbidden in the P6₃/mcm space group, it is only permissible in the P6₃22 space group. This additional peak suggests the occasional presence of structural disorders or stacking faults along the c-axis.^36,42 It is important to note that a sodium content higher than x = 0.1 leads to a phase change from the P6₃/mcm to the P6₃22 space group, as shown in Figure S1. Moreover, the (002) diffraction peak at 2θ = 15.9° for sodium-enriched NNTO compounds shifts toward a lower angle (Figure 1B), indicative of increased interlayer spacing within the layered structure (Figure S2). Alterations in the lattice parameters of P6₃/mcm, originating from the different sodium contents, are documented in Table S1. This table confirms an expansion in the c-axis lattice parameter values, attributed to a rise in the d-spacing between the metal oxide layers (Figure S2). Concurrently, a reduction in the lattice parameter “a” for NNTO-10 also signifies a shortened path for Na⁺ ions, potentially facilitating improved ion diffusion within the bulk phase.^43–45

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To investigate the impact of sodium content on the morphology of NNTO powders, scanning electron microscopy (SEM) was employed. As shown in Figure 1D–G, NNTO exhibits varying morphologies with increasing sodium concentrations. The well-ordered cathode material NNTO-0, depicted in Figure 1D, exhibits particle sizes ranging from 2 to 11 μm, with an average particle size of 5.02 μm (Figure 1E). Further, horizontal striations are visible on the facets of these larger, hexagonal-shaped NNTO-0 platelets, separated by distances ranging from tens to hundreds of nanometers (Figure S3). In contrast, NNTO-10 (Figures 1F,G and S4) has an average particle size of 1.14 μm, while NNTO-3 and NNTO-5 have average particle sizes of 1.42 and 1.28 μm, respectively (Figure S5). SEM imaging also indicates that the thickness of smaller particles has increased in NNTO-10 (Figure S6) relative to NNTO-0, likely owing to the expansion of the c-axis in these structures.⁴⁶ It should be emphasized that these alterations in lattice parameters and particle sizes are solely induced by the increased sodium content. This variation in the stoichiometry may reasonably alter the surface energy of the crystal planes, which influence the growth rate of different facets, promote the formation of layered structures with more {010} facets, and increase the thickness of particles along the c-axis,^47,48 as shown in Figure S6b for NNTO-10. Additionally, energy-dispersive spectrometry (EDS) mappings of NNTO-0 and NNTO-10, presented in Figures S3 and S4, respectively, affirm the uniform distribution of Na, Ni, Te, and O throughout the lattice in the bulk material.

The influence of excess sodium content on the local structures and chemical distributions of NNTO is further elucidated using transmission electron microscopy (TEM), combined with selected area diffraction (SAD) patterns, fast fourier transform (FFT) and high-resolution (HR) images (Figure 1H–K). Large bright grains can be observed in the HR-TEM image (Figure 1J), which, when projected along the [1$<math xmlns="http://www.w3.org/1998/Math/MathML" altimg="urn:x-wiley:10973923:media:cey2658:cey2658-math-0001" wiley:location="equation/cey2658-math-0001.png"><mrow><mover><mn>1</mn><mo>\unicode{x000AF}</mo></mover></mrow></math>$0] zone axis, represent columns of transition-metal atoms (Ni and Te) in NNTO-0. These columns are consistent with the P2-type honeycomb layered oxide structure within the P6₃/mcm space group.³⁶ Moreover, the HR-TEM image of NNTO-0 reveals a lattice fringe distance of 0.556 nm corresponding to (002) planes, which is consistent with our XRD results. Conversely, the HR-TEM image of NNTO-10 (Figure 1I) shows the presence of a disordered, preconstructed surface layer on the active NNTO material in the presence of excess sodium. The thickness of this preconstructed surface layer is about 6.9 nm for NNTO-10 and is highly dependent on the content of sodium in the NNTO crystal structure, as shown in Figure S7. The SAD pattern of NNTO-0 (Figure 1J) displays a well-ordered structure with Te atoms vertically aligned above or below adjacent slabs, as can be seen in Figure 1C. In contrast, in NNTO-10 (Figure 1K), the presence of excess sodium ions in the ab plane causes deviation from vertical alignment, signifying the coexistence of structural disorders or stacking faults along the stacking direction (i.e., the c-axis).³⁶ The excess sodium not only increases the interatomic distance within the NNTO-10 bulk to 0.559 nm—favorable for Na-ion de/insertion during cycling—but also forms a beneficial preconstructed layer that enhances the structural stability of the cathode material against atmospheric conditions.^49,50 Subsequent stability tests for NNTO-0 and NNTO-10 were also conducted, and their ex-situ XRD patterns were analyzed after 1 day of exposure to air and moisture. Figure S8 confirms the structural reconstruction in NNTO-0 due to reactive Na and Ni species, whereas NNTO-10 displays excellent stability, attributed to its preconstructed layer that also acts as a protective surface layer.

The electrochemical performance of the NNTO series was evaluated using a half-cell configuration within a voltage range of 3.0–4.5 V versus Na⁺/Na (Figure 2). Figure 2A–C presents a comparison of galvanostatic charge–discharge (GCD) profiles for NNTO cathodes at a rate of 0.1 C. NNTO-0, NNTO-3, NNTO-5, and NNTO-10 samples with differing sodium contents exhibit first-charge capacities of 102, 104.8, 106.2, and 109.2 mAh g⁻¹, respectively, as shown in Figure 2A. In the subsequent discharge process, capacities of 80.4, 84.1, 85.6, and 88.5 mAh g⁻¹ were achieved, with Coulombic efficiencies of 78.8%, 80.2%, 80.6%, and 81.0%, respectively. While the charge–discharge capacity follows a trend similar to sodium content, all cathodes experience a significant irreversible loss of capacity following the initial charge. This loss may be attributed to kinetic barriers that impede sodium reinsertion into the structure.³⁷ The multiple plateaus in the voltage profile of NNTO-0, especially the stair-like features, indicate complex phase transitions that could contribute to this irreversible capacity loss.^32,37,45 It is also observed that the plateaus within the 3.6–4.2 V range for NNTO-0 (Figure 2B) diminish and eventually vanish with increasing sodium content, as shown for NNTO-5 (Figures 2A and S9) and NNTO-10 (Figure 2C). This alteration could be due to a volume expansion in the NNTO crystal structure. Additionally, the transition in particle size from micro- to sub-micro–sized particles and the formation of the surface layer could play a significant role in NNTO-10, as smaller particles offer a larger specific surface area.⁴⁵ In addition, the comparison of the voltage hysteresis for both NNTO-0 and NNTO-10 (Figure S10) confirmed that the formation of preconstructed surface layer also decreases the energy losses caused by the effects of polarization produced at high voltage,^51,52 thereby improving its electrochemical performance compared to other Ni-rich cathode materials (Table S2).^53–62

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To investigate and compare the mechanisms of sodium insertion and extraction in NNTO cathodes, cyclic voltammetry (CV) was performed in a half-cell configuration. Three CV cycles for both NNTO-0 and NNTO-10 are presented in Figure 2D,E, at a scan rate of 0.1 mV s^–1, displaying multiple redox peaks within the voltage range of 3.0–4.5 V. The principal oxidation peaks for NNTO-0 (Figure 2D) appear at 3.73, 3.98, 4.16, and 4.32 V, and the corresponding reduction peaks are observed at 3.45, 3.64, 3.83, and 3.98 V. These peaks signify multi-electron redox reactions associated with the Ni²⁺/Ni³⁺ oxidation and reduction processes.^35,37 Notably, splitting in some of the anodic and cathodic peaks is evident in the CV curves, indicating either interlayer sliding of transition metal layers or varying degrees of sodium disorder in NNTO-0.³⁷ Electrochemical sodium extraction results in slab gliding within the honeycomb structure, leading to modifications in the crystal structure during sodium atom rearrangement.^40,41,63 These rearrangements induce phase transitions, resulting in a stair-like voltage profile.⁴⁵ Interestingly, major oxidation peaks attributed to inter-slab gliding in the CV of NNTO-0 diminish and eventually disappear in the CV profiles of NNTO-3 (Figure S9c), NNTO-5 (Figure S9d), and NNTO-10 (Figure 2E). Specifically, two notable anodic peaks at 3.69 and 4.4 V, along with a significant cathodic peak at 3.69 V, emerge for the sodium-excess NNTO-10. The peaks between 3.6 and 4.2 V are ascribed to the Ni²⁺/Ni³⁺ redox reactions during sodium-ion extraction and insertion, while the anodic peak at 4.4 V may be related to high-voltage electrolyte oxidation.^32,35,37 The reduction in plateaus upon extraction and insertion of Na⁺ for cathode materials reflects the decrease in diffusion path due to changes in the lattice parameters and the size of particles, as shown in Figure 1.^21,33,45

The rate performances of all NNTO cathodes at various C-rates are shown in Figure 2F. NNTO-10 exhibits the highest discharge capacity and superior cyclic stability, affirming the improved kinetics originating from the reduced diffusion pathways along the a-axis (Table S1). A comparison of cyclic performance for all NNTO samples at a rate of 0.1 C is presented in Figure 2G. Discharge capacities reveal that NNTO-10 has superior capacity retention compared to NNTO-0, NNTO-3, and NNTO-5. After 100 cycles, capacity retention is 71.3% for NNTO-0 and 72.1%, 74.2%, and 76.7% for NNTO-3, NNTO-5, and NNTO-10, respectively. Hence, these observations confirm that a reduced diffusion path in NNTO-10, along with an increase in c/a ratio after reducing the particle size from micro to sub-micro meter in the presence of excess sodium ions, could effectively improve the sodium-ion diffusion, resulting in enhanced cycle stability and rate performance of the NNTO-10 cathode material. Additionally, the emergence of a preconstructed surface structure may contribute to maintaining structural stability.

To elucidate the impact of excess sodium on the oxidation states of nickel, both X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed. Figure 3A displays the Ni K-edge XANES spectra of NNTO-0, NNTO-3, NNTO-5, and NNTO-10, all of which resemble that of the reference material, indicating the divalent oxidation state of Ni in NNTO. In NNTO-0 (represented by the black line), a slightly higher oxidation state of Ni was observed compared to NNTO-5 (blue line) and NNTO-10 (green line). A minor shift toward lower energy levels was noticed with an increase in sodium content, as illustrated in the inset of Figure 3A. Notably, XPS results (Figure S11) corroborate these findings, demonstrating that the excess sodium content induces only a nominal shift in the Ni 2p core-level spectra toward lower binding energy. This suggests that the oxidation states of Ni remain largely unaffected with increased sodium content in the NNTO structure.

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Further insights into the effects of sodium excess on NNTO structure were gleaned through Fourier transforms (FTs) of the Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, as shown in Figure 3B. The peak at shorter distances (between 1 and 2 Å) is attributed to the Ni-to-O single scattering path, corresponding to the first shell of metal–oxygen bonds.³¹ Peaks at 2.6 Å are ascribed to the second shell involving Ni-to-Ni/Te bonds within the same ab plane.³¹ Prior studies have reported that a minor increase in bond length (Ni–O) is directly related to the reduction in particle size,⁴⁴ a phenomenon evidenced in NNTO-10 (green line). Such minor changes in bond lengths, induced by excess sodium in the NNTO crystal structure, could potentially contribute to residual stress and the fragmentation of larger particles into smaller counterparts. To elucidate the charge compensation mechanism, ex-situ X-ray absorption spectroscopy (XAS) was performed on both NNTO-0 and NNTO-10 during the charge–discharge cycles. Shifts in the Ni K-edge toward higher energy regions were observed, indicative of the oxidation of Ni²⁺ to Ni³⁺ during the charging process, as presented in the XANES spectra in Figure 3C. Conversely, the Ni K-edge shifted back during the discharge process, as evident in Figure 3D. Therefore, it can be confirmed that in both NNTO-0 and NNTO-10 cathodes, charge compensation is facilitated exclusively through the Ni²⁺/Ni³⁺ redox couple, a finding that aligns closely with the CV data presented in Figure 2.

To gain a deeper insight into the effect of excess sodium on structural transformations during electrochemical cycling, in situ XRD analyses were conducted on NNTO cathodes at a rate of 0.1 C. Figure 4 shows the comparative in situ XRD patterns for NNTO-0 and NNTO-10, each corresponding to distinct GCD profiles previously illustrated in Figure 2. Detailed XRD patterns for these materials are provided in Figure S12. For NNTO-0, the (002) diffraction peak manifests a consistent shift toward lower angles during the charging process (Figure 4A). This behavior signifies an elongation in the c-axis and a contraction of the ab-plane, a phenomenon attributable to increased electrostatic repulsion between adjacent oxygen layers (Figure 4). Interestingly, this peak exhibits reversibility upon reinsertion of sodium during the discharge process. Similar structural evolution was also performed for NNTO-10 (Figures 4B and S12b) during the charge–discharge process. The peaks corresponding to the (002), (004), and ($<math xmlns="http://www.w3.org/1998/Math/MathML" altimg="urn:x-wiley:10973923:media:cey2658:cey2658-math-0002" wiley:location="equation/cey2658-math-0002.png"><mrow><mn>2</mn><mover><mn>1</mn><mo>\unicode{x000AF}</mo></mover><mn>0</mn></mrow></math>$) planes underwent minor initial shifts but stabilized in successive charge and discharge cycles, affirming the stability of the lattice parameters (Figures 4D and S12d). This suggests that sodium extraction from NNTO-0 follows a quasi-solid-solution reaction mechanism, whereas NNTO-10 shows complete solid-solution behavior consistent with its GCD profile.

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To gain a better understanding of this process, a galvanostatic intermittent titration technique (GITT) was employed (Figure 4E). The GITT curve for NNTO-0 (represented by the red line) displays multiple plateaus at 3.6, 3.9, and 4.2 V, indicating high polarization during charging. In stark contrast, NNTO-10 (green line) revealed a single-phase reaction across a potential range of 3.0–4.25 V, which can be ascribed to reduced diffusion pathways and the presence of a pre-formed surface structure facilitated by excess sodium. GITT analysis was also used to calculate the sodium diffusion coefficient (D_Na+) for both NNTO materials across various potentials (Note S1). The observed values of D_Na+ for both NNTO-0 and NNTO-10 were approximately 10⁻¹² cm² s⁻¹ across the measured open-circuit voltages. NNTO-0 exhibits a notable decrease in D_Na+ at around 3.6 V (Figure 4F), likely attributed to sluggish electrochemical kinetics resulting from either phase transitions or metal layer gliding post sodium-ion extraction.^64,65 Upon further extraction, both materials display increased polarization with voltage, reaching up to 4.5 V; this is thought to be linked to electrolyte decomposition at elevated voltages.^35,37 Consequently, there is an increase in over-potential (η) at this stage (Figure 4G). A significant reduction in η was confirmed for NNTO-10 at these high cut-off voltages, affirming the decreased charge-transfer resistance owing to the protective surface-layer formation. During discharge, a substantial drop in D_Na+ occurs at 3.8 V for both materials (Figure 4H), corresponding to redox reactions within the active material and signaling poor diffusion kinetics upon sodium reinsertion.^66,67 Finally, the lower internal resistance observed for NNTO-10 relative to NNTO-0 (Figure 4I) suggests that NNTO-0 experiences inhibited diffusion kinetics, either due to phase transitions or due to increased disorder during cycling. Conversely, the diffusion kinetics in NNTO-10 are conducive to significant capacity retention over multiple cycles. Furthermore, the comparison of the EIS spectra for NNTO cathodes (Figure S13) confirms that the formation of a protective surface layer in the presence of excess sodium suppresses the structural disorder and interfacial degradation during cycling, thereby, decreasing the kinetic barrier for reversible sodium-ion insertion and enhancing the cycling stability and rate capability.

In summary, the comparative investigation between NNTO-0 and NNTO-10 strongly indicates that the presence of a preconstructed surface layer significantly contributes to structural stability and mitigates the propensity for side reactions at elevated cut-off voltages, specifically 4.5 V (Figure 5A). Previous studies on Ni-rich active materials have shown that cathode surface degradation due to electrolyte decomposition and oxidation can result in transition-metal dissolution and surface reconstruction.^68,69 As a result, surface modification emerges as an effective strategy for preventing transition-metal dissolution from cathodes, as it provides a protective layer that guards against interfacial degradation.^70,71 Herein, a straightforward, one-step synthesis incorporating such preconstructed surface modifications on sodium-excess NNTO has demonstrably improved the electrochemical performance of the NNTO cathode material.

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Furthermore, to elucidate the relationship between electrochemical performance and operating temperature, GCD profiles were examined within a voltage window of 3.0–4.5 V, at a rate of 0.1 C and an elevated temperature of 60°C (Figure 5B–D). At this temperature, NNTO-0 maintains its characteristic stair-like voltage profile, indicative of electrochemically driven quasi-phase transitions, with charge and discharge capacities of 141.2 and 72.6 mAh g⁻¹, respectively. In contrast, NNTO-10 displays a sloping voltage profile at 60°C, accompanied by superior charge (161.4 mAh g⁻¹) and discharge (73.8 mAh g⁻¹) capacities, confirming the significant impact of excess sodium ions in the structure. NNTO-10 achieves 71.6% capacity retention after 10 cycles, compared to 60% for NNTO-0. These results strongly indicate that the preconstructed surface layer not only offers effective protection against undesirable side reactions at elevated temperatures but also improves the electrochemical performance of the NNTO cathode material.

CONCLUSION

In summary, we have successfully synthesized a P2–Na_2+xNi₂TeO₆ cathode material characterized by high sodium content and modulated bulk and surface structures. The elevated sodium content in NNTO serves to regulate the Ni–O bond length, thus facilitating enhanced Na⁺ mobility, particularly in sodium-deficient states. Notably, the particle size of active material is decreased from 5.02 to 1.14 μm with an increase in the sodium content. Moreover, the excess sodium induced a reconstruction of the surface, forming a stable, protective layer that inhibited phase transformation and structure degradation. This contributed to the structure stability upon Na⁺ insertion and extraction during charge–discharge cycling. The P2–Na_2.1Ni₂TeO₆ (NNTO-10) delivered a reversible discharge capacity of approximately 88.5 mAh g⁻¹ at a rate of 0.1 C within the voltage range of 3.0–4.5 V at 25°C. Additionally, NNTO-10 exhibited a discharge capacity of 73.8 mAh g^–1 at 60°C, showing a capacity retention of 71.6%, in comparison to the pristine NNTO-0 cathode material, which could only maintain a 60% capacity retention. These encouraging results highlight the considerable potential of sodium- and nickel-rich NNTO materials for practical application in SIBs.

EXPERIMENTAL SECTION

Methods

Honeycomb Na₂Ni₂TeO₆ layered material was synthesized without any impurities using^53–62 a solid-state high-temperature route. Na₂CO₃, NiO, and TeO₂ were used in a stoichiometric ratio as starting materials. Before use, all precursors were heated to 200°C overnight to remove any residual moisture. The precursors were first ground to fine powder and then heated to 650°C for 12 h, followed by annealing at 900°C in air for another 12 h. For the preparation of NNTO with an excess content of sodium, 3%, 5%, and 10% excess of Na₂CO₃ were used to synthesize NNTO-3, NNTO-5, and NNTO-10, respectively. After the sintering process, the powders were kept in a dry room to prevent them from absorbing moisture.

Electrochemistry

NNTO electrodes were prepared with the active material, acetylene black carbon as a conducting network, and polyvinylidene fluoride as a binder in a mass ratio of 70:20:10. To prepare a uniform slurry, N-methyl-2-pyrrolidone was utilized as the solvent. The slurry was coated on an Al-foil, and was dried in a vacuum oven at 120°C overnight to remove the solvent and residuals. Sodium-metal foil was used as a counter electrode, while the electrolyte comprised 1 M of NaClO₄ in propylene carbonate and fluoroethylene carbonate in a ratio of 98:2 by wt.%. All electrochemical properties were tested using a multichannel battery tester (Maccor version 4000), performed at either room temperature (25°C) or high temperature (60°C). The cut-off voltages for the NNTO cathode in the half-cells were 3.0–4.5 V. To ensure complete electrolyte absorption into the electrode, the sealed cells were aged for 6 h. CV curves were acquired through a Biologic potentiostat/galvanostat model VMP3 (BioLab, Inc.) at a scan rate of 0.1 mV s⁻¹. All the CR2032 coin-half-cells were fabricated in an Ar-filled glove box with H₂O and O₂ of less than 0.01 ppm.

Material characterization

Ex-situ XRD of the NNTO cathode series was characterized by R-AXIS IV⁺⁺, Rigaku, with Mo-Kα radiation (wavelength of 0.7107 Å) to monitor changes in the crystal lattice and its phase. All powder samples were prepared inside a dry room and covered with Kapton tape before measurement. To study the structure evolution during cycling, in-situ XRD was performed on the samples. The obtained XRD patterns were converted to Cu-Kα radiation for a quick comparison with the literature. The lattice parameters of crystal structures were optimized using Match powder diffraction software. For the stability test, powder samples were kept in open air outside the dry room with a humidity level of approximately 60% for 1 day, and ex-situ XRD was measured at Ultima IV X-ray diffractometer (Rigaku) with monochromatic Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 10–80° at a scan rate of 0.02° s⁻¹. The surface morphology of the synthesized powders and EDS mapping were acquired using a field emission scanning electron microscope (FE-SEM; S-4200, Hitachi, Japan). Atomic-scale high-angle annular dark field transmission electron microscopy (HAADF-TEM) images, along with HR-TEM images and SAD patterns, were obtained using an FEI Titan TM 80 − 300 microscope at 300 kV. XAS analyses were performed at Ni K-edge with the 1D KIST-PAL bending magnet beamline at the Pohang Accelerated Laboratory. Total electron yield mode was used to obtain all the spectra at a base pressure of 3 × 10⁻¹⁰ Torr and 0.01 eV. To observe the oxidation states of nickel, a PHI 5000 Versa Probe (Ulvac-PHI) equipped with a monochromator of Al-Kα with an energy of 1486.6 eV was used. GITT measurements were performed with a current density of 0.1 C-rate, where an electrode was subjected to a 10-min current pulse before an hour-long open circuit relaxation. EIS tests were conducted using a Bio-Logic VMP3 from 1 MHz to 0.1 Hz. VESTA software was used to draw the NNTO crystal structure.

ACKNOWLEDGMENTS

This research was supported by the KIST Institutional Program (Project No. 2E33270) and the National Research Foundation of Korea (NRF- 2020M3H4A3081889), funded by the Ministry of Science and ICT.

CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.

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