A heteroatom-doped, single crystal, cobalt-free p2- type layered oxide cathode material for elevated cycling life of sodium-ion batteries, synthesis process thereof

Abstract

A synergistic stabilization effect in a Nb-doped P2-type single crystal cobalt-free layered oxide cathode material, offering remarkable cycling stability and high-power performance for Na-ion batteries have unveiled in this study. The introduction of Nb in the transition metal layer not only reduces the electronic band gap but also enhances electronic conductivity and mitigates ionic diffusion energy barriers.

Description

[0001] A HETEROATOM-DOPED, SINGLE CRYSTAL, COBALT- FREE P2-TYPE LAYERED OXIDE CATHODE MATERIAL FOR ELEVATED CYCLING LIFE OF SODIUM-ION BATTERIES, SYNTHESIS PROCESS THEREOF TECHNICAL FIELD The present invention relates to a heteroatom-doped, single crystal, cobalt-free P2-type layered oxide cathode material for elevated cycling life of Sodium-ion batteries and synthesis process thereof. Particularly, the invention relates to Nb- doped P2-type single crystal cobalt-free layered oxide cathode material for elevated cycling life of Sodium-ion Batteries. BACKGROUND AND PRIOR ART The worldwide shift towards sustainable energy generation has significantly highlighted the need for exploration of renewable sources for electrochemical energy storage applications. Notably, Na-ion batteries have surfaced as a highly propitious solution in this quest. Their potential stems from the abundance and cost-effectiveness of Na resources, combined with redox chemistry akin to Li-ion batteries [J-M. Tarascon, Na-ion vs. Li-ion batteries: complementarity rather than competitiveness, Joule 4(8) (2020) 1616-1620; D. Larcher, J.-M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nature Chemistry (2015) 719-29; A. Kumar, A. Ghosh, A. Ghosh, A. Ahuja, A. Sengupta, M. Forsyth, D.R. MacFarlance, S. Mitra, Sub-zero and room- temperature sodium–sulfur battery cell operations: A rational current collector, catalyst and sulphur-host design and study, Energy Storage Materials (2021) 42 608-617; A. Kumar, A. Ghosh, A. Ghosh, A. Ahuja, M. Forsyth, D.R. MacFarlane, S. Mitra, Approach to Increase the Utilization of Active Material in a High Sulfur- Loaded Cathode for High Areal Capacity Room-Temperature Sodium-Sulfur Batteries, ACS App. Energy Mater. 41 (2021) 384-393; A. Ahuja, A. Kumar, A. Sengupta, M. Gautam, H. Lohani, P. Kumari, S. Mitra, Single‐crystal spinel Li1.08Mn1.92O4 octahedra cathode covered with Li-ion permeable robust NMC thin-layer protection for high voltage lithium‐ion batteries, Energy Storage Materials 52 (2022) 169-179.]. In comparison to lithium-ions, the bigger size of Na-ions presents a formidable obstacle in the pursuit of suitable host electrode materials for Na-ion batteries [J. Nongkynrih, A. Sengupta, B. Modak, S. Mitra, A.K. Tyagi, D.P. Dutta, Enhanced electrochemical properties of W-doped Na3V2(PO4)2F3@C as cathode material in sodium ion batteries, Electrochimica Acta 415 (2022) 140256; Y. Yang, D. Ning, Q. Li, A. Franz, L. Zheng, N. Zhang, G. Ren, G. Schumacher, X. Liu, Revealing the anionic redox chemistry in O3-type layered oxide cathode for sodium-ion batteries, Energy Storage Materials 38 (2021) 130-140; R. Usiskin, Y. Lu, J. Popovic, M. Law, P. Balaya, Y-S. Hu, J. Maier, Fundamentals, status and promise of sodium-based batteries, Nature Reviews Materials 6 (2021) 1020-1035.]. Host materials for sodium-ion batteries need to exhibit both high capacity and rapid kinetics to ensure the reversible insertion and de-insertion of Na-ions. This process is impeded by the slower transport and stronger bonding inherent in this firm oxide frameworks [L. Zhao, T. Zhang, W. Li, T. Li, L. Zhang, X. Zhang, Z. Wang, Engineering of Sodium-Ion Batteries: Opportunities and Challenges, Engineering 24 (2023) 172-183]. Despite extensive study on a variety of host materials, including phosphate complex, transition-metal oxides, and Prussian blue equivalents, their cycle stability, reversible capacity, and working potential are insufficient for real-world battery applications. Therefore, the search for novel cathode materials continues to be an important and challenging project in the development of sustainable sodium-ion batteries. [J-Y. Hwang, S-T. Myung, Y-K. Sun, Sodium-ion batteries: present and future. Chem. Soc. Rev. 46 (2017) 3529-3614; A. Rudola, A.J.R. Rennie, R. Heap, S.S. Meysami, A. Lowbridge, F. Mazzali, R. Sayers, C.J. Wright, J. Barker, Commercialisation of high energy density sodium-ion batteries: Faradion's journey and outlook, J. Mater. Chem. A 9 (2021) 8279-8302]. The P2-type Na0.67Ni0.33Mn0.67O2 positive electrode material stands out as a foremost choice for sodium-ion batteries due to its eco-friendliness, easy synthesis, open prismatic framework and exceptional properties, including high specific capacity and operating voltage [Q. Shen, Y. Liu, L. Jiao, X. Qu, J. Chen, Current state-of-the-art characterization techniques for probing the layered oxide cathode materials of sodium-ion batteries, Enenrgy Storage Materials 35 (2021) 400- 430; D. Kundu, E. Talie, V. Duffort, L.F. Nazar, The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew Chem. Int. Edition 54 11 (2015) 3431-3448; R-M. Gao, Z-J. Zheng, P-F. Wang, C-Y,. Wang, H. Ye, F-F. Cao, Recent advances and prospects of layered transition metal oxide cathodes for sodium-ion batteries, Energy Stoarge Materials 30 (2020) 9-26]. Moreover, its cobalt-free composition and resistance to air-moisture make it highly attractive for practical applications

[0015] . However, challenges hinder its full potential utilization, such as rapid capacity decay at higher voltages, substantial volume changes during phase transitions, and irreversible capacity loss during the P2-to- O2 phase transition

[0016] . Efforts to mitigate these issues, like limiting the cut-off potential, have come at the expense of voltage, capacity, and energy density [17- 19]. Cation doping has shown promise in suppressing the P2-to-O2 phase transition, enhancing cycling stability, and rate performance [20-22]. To address these constraints, inventive approaches are essential to formulate cathode materials that hinder the P2-to-O2 phase transition at elevated voltages, while concurrently enhancing sodium-ion reaction kinetics. This, in turn, enhances cycling stability and rate performance in sodium-ion batteries [23-24]. Numerous investigations have been conducted to address the phase transition issues, aiming to enhance sodium-ion batteries' performance. One such study demonstrated the effectiveness of incorporating magnesium as a substitution for nickel ions, yielding Na0.67Mn0.67Ni0.33-xMgxO2 (0 ≤ x ≤ 0.10), which significantly alleviated the problem

[0025] . Utilizing a clever in-situ coating technique with Na2Ti3O7 to inhibit Na+ / vacancy ordering and the P2-to-O2 phase transition was another cutting-edge strategy that produced remarkable overall performance and cycling stability

[0026] . Moreover, a thorough first-principles analysis demonstrated the impact of vanadium (V) or niobium (Nb) doping on the anode's performance in Na2Ti3O7 for sodium-ion batteries. Phase stability, transport characteristics, and interphase behaviour are all influenced by the larger radius of Na+ ions than Li+ ions

[0027] . Furthermore, a second study examined the complex interactions between Mg1-xNixO coating and P2-type Na0.67Ni0.33Mn0.67O2, resulting in the creation of extremely stable cathodes for sodium-ion batteries [28-31]. All of these investigations point to the significant potential of various strategies, including coating, doping, and substitution, in reducing the phase transition in P2- type Na0.67Ni0.33Mn0.67O2 and enhancing sodium-ion batteries' overall performance. Reference is made to “Investigation on the Air Stability of P2-Layered Transition Metal Oxides by Nb Doping in Sodium Ion Batteries”, authored by Yanyan Chen et.al., Batteries 2023, 9(3),183; https: / / doi.org / 10.3390 / batteries9030183. This document teaches that Nb-induced surface preconstructed layer inhibited the surface dissolution of the P2 material in the electrochemical reaction and formed a stable and thin (cathode–electrolyte interphase) CEI film, which prevented water molecules from entering the P2-NaxTMO2lattice. Na0.67Mn0.67Ni0.33Nb0.03O2 could exhibit superior rate performance (a reversible capacity of 72.5 mAh g−1at 20 C) and outstanding cycling performance (84.43% capacity retention after 1000 cycles at 5 C) in a half cell after exposed in a moisture atmosphere (RH93%) for 20 days. Further reference is made to CN116525810A. This document teaches a preparation method and application of and O3-type ternary ferro-manganese- nickel sodium-ion battery positive electrode active material. The method for producing the O3-type layered single crystal structure involves a complex process of mixing and reacting a mixed aqueous solution of ferric salt, manganese salt and M salt, a precipitator, a complexing agent and a dispersing agent. Moreover, the multiple structural phase-transition in O3-type layered oxide has been tackled by doping multiple elements, thereby making the synthesis quite difficult and time consuming. Further reference is made to CN113972367A. The inventors suggest a method to manufacture a layered metal oxide high-performance sodium-ion battery. The preparation presented by them involves taking the oxides of the precursors in a stoichiometric ratio, ball milling them and then subjecting the treated material to calcination and finally forming an electrode out of it and evaluating its performance in a sodium-ion battery. Another reference is made to CN112103483A where the inventors have disclosed the method of co-precipitation or sol-gel fabrication technique and the performance of a two-phase NaxAyNbzMn1-y-zO2-NaNbO3 P2-phase sodium ion battery positive electrode material for low-temperature operation. They have incorporated metal Nb to create a two-phase structure, which takes the shape of a NaxAyNbzMn1-y-zO2 phase structure and a NaNbO3 phase structure thereby, enhancing the material's electronic conductivity, long-cycle stability, and outstanding high-rate performance. In CN115863559A, the inventors prepare a NaTMO3 / Na0.67Ni0.33- xTMxMn0.67O2 positive electrode material, where TM is Nb, Ta, and Mo, and x is greater than 0 and less than or equal to 0.1 using a straightforward high- temperature solid-phase approach. They suggest that while the active material is coated and protected with NaTMO3, the doping of high-valence TM ions could increase the lamellar spacing, control and regulate the local electronic structure, improve the stability of the material during the cyclic process, and the material system's electrochemical performance. In CN116062807A, the inventors developed a high-entropy doped manganese- based layered oxide, having a chemical formula of NaxMnyMzO2, where Na stands for sodium, Mn for manganese, M for doping, and n for an integer between 1 and 3. In the third to fifth periods of the periodic table of elements, the doping elements are chosen from among more than five metal elements other than sodium and manganese: x is greater than or equal to 0.6 and less than 1, y is greater than or equal to 0.8 and less than 1, z is greater than 0 and less than or equal to 0.2, and the sum of y and z equals 1. The invention describes the process of applying high- entropy doping to over five low-content metal elements in order to create a novel high-entropy doped manganese-based layered oxide. However, there remain a need to develop an energy efficient and facile microwave-assisted solid-state method to efficiently produce layered single crystal cobalt-free P2-type sodium nickel manganese niobium oxide and a Niobium doped single crystal, cobalt-free P2-type layered oxide prepared by the method to be used as a cathode material for sodium-ion batteries. OBJECTS OF THE INVENTION One of the objects, among others, of the present invention is to provide a highly proficient cathode material for Na-ion battery through a quick, easy, energy efficient and an inexpensive microwave irradiated solid-state synthesis technique. Another object of present invention is to provide an alternate method of preparing niobium (Nb) doped, hexagonal single crystal cobalt-free P2-type layered material with clear facets, easier electrolyte percolation so that the electrochemical performance is greatly enhanced even at higher C-rate. SUMMARY OF THE INVENTION The following disclosure presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key / critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later. According to one aspect of present invention, there is provide a process of synthesis of a cathode material including heteroatom doped, single crystal, cobalt- free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2 for Na-ion batteries wherein x is preferably 0.02, comprising: ^ mixing vigorously precursor materials in a certain ratio in a ball miller at a 300 rpm for 2 hours; ^ pre-calcining the resulting powder from the ball miller at 450 °C for 6 hrs in a muffle furnace; ^ hand grinding the decomposed product obtained from the muffle furnace in a mortar-pestle for an hour; ^ calcining the product by microwave irradiation at 900 °C with a reaction time of 12 minutes; ^ cooling slowly and transferring into a vacuum atmosphere within a desiccator to prevent any reactions with moisture and carbon dioxide. In an embodiment, said Na0.67Ni0.33-xMn0.67NbxO2 layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2. In another embodiment, said precursor materials comprises anhydrous sodium carbonate (Na2CO3), Nickel acetate ((Ni(OCOOCH3)2.4H2O), Mn acetate ((Mn(O(COOCH3)3.2H2O) and niobium oxide (Nb2O5). In a further embodiment, said precursor materials are vigorously mixed in the molar ratio of 0.67:0.31:0.67:0.02. According to another aspect of present invention, there is provided a battery comprising: a cathode material including heteroatom doped, single crystal, cobalt- free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2for Na-ion batteries wherein x is preferably 0.02 and an anode material including pre- sodiated hard carbon. In another embodiment, the Na0.67Ni0.33-xMn0.67NbxO2 layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2. In a further embodiment, wherein the battery is a sodium ion battery. The focal point of present invention is the elucidation of the synergistic impact arising from the introduction of Nb dopant and the attainment of single crystallinity on the P2-to-O2 phase transition, cycling stability and rate performance of this cathode material in sodium-ion batteries. This study is the first to demonstrate doping of Nb in the bulk structure with homogenous distribution throughout the single crystal instead of accumulating on the surface and causing atomic surface reorganization. Notably, the incorporation of Nb as a heteroatom dopant manifests a substantial reduction in the electronic band gap and ionic diffusion energy barrier, thereby facilitating rapid electron and sodium ion transfer—a phenomenon also meticulously elucidated through density functional theory (DFT) investigations. Furthermore, the pronounced hybridization observed between Ni-d states and O-p states near the Fermi level imparts a more covalent bonding character and enhanced delocalization of O- states. In contrast, the interaction between Mn4+and O2-ions exhibits more ionic bonding, leading to the localization of O-states. The single crystal Na0.67Ni0.31Mn0.67Nb0.02O2indicates ionic bonding involving Mn-O and Nb-O orbitals, while Ni-O exhibits polar covalent bonding, thereby suppressing the phase transition and oxygen gas loss at deeply desodiated state, proved through electron delocalization function (ELF) calculations. Beyond the intrinsic electrochemical advancements, the creation of a Nb-doped single crystal structure emerges as a strategic measure to circumvent P2-to-O2 phase transition and intragranular cracking, thereby upholding the material's bulk integrity during high-voltage cycling, an assertion substantiated through an array of advanced analytical techniques. Through enhanced conductivity, lower ionic diffusion barriers, and accelerated charge transfer, Nb doping significantly improves the efficiency of the charge-discharge cycles even at higher C-rate. The synergistic effect of Nb5+doping at the Ni2+site and the single crystal design in Na0.67Ni0.31Mn0.67Nb0.02O2(SC_Nb-NMNO) demonstrates encouraging performance, with SC_Nb-NMNO exhibiting an initial discharge capacity of 149.8 mA h g-1, retaining >95% of its capacity after 100 cycles at 0.1 C, maintaining a capacity retention of >90% even after an extensive cycling of 2000 cycles at 1 C, all within the full operating voltage range of 2.5 – 4.2 V, while preserving its well-defined microstructural properties, unlike the pristine Na0.67Ni0.33Mn0.67O2 cathode material which could only retain a capacity of ~86% after 100 cycles at 0.1 C with severe exfoliation of the active material. The niobium is doped in the bulk at the nickel site and is homogeneously distributed throughout the crystal instead of inducing atomic scale surface reorganization, thereby establishing chemical changes on the surface layer of the material. This builds the nodes that holds the integrity of the entire bulk structure even when cycling at higher voltage for prolong cycling. This innovation, coupled with the stabilization of the cathode structure, the elimination of kinetic factors, and extended cycle life, establishes Nb-doped P2-type cathodes as a revolutionary advancement, poised to transform the landscape of sustainable energy solutions in sodium-ion batteries. Additionally, harnessing the distinctive characteristics of single crystals enhances multiple kinetic factors influenced by particle size distribution, crystal facet, grain boundary, Na-ion diffusion, strain, and minimal O2 gas release. This allows to vividly illustrate the robust correlation between a material's structural defects and chemical proliferation and their pivotal influence on electrode performance and stability. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS Fig. 1. Illustrates structural characterization of the single crystal cathode material according to one embodiment of present invention. (a) X-ray Diffraction (XRD) patterns of the single crystal Na0.67Ni0.31Mn0.67Nb0.02O2(SC_Nb-NMNO), where the orange spheres indicate the experimental data, the violet spheres denotes the Rietveld refinement data, the pink line denotes the Bragg’s position, and purple line denotes the difference between the experimental and refinement results. (b) Schematic diagram of the effect of Nb-doping on the crystal structure of NMNO, where the yellow sphere represents the Na-ion, the green one indicates the transition metal, and the red sphere denotes the oxygen anion. (c) Electron localization function (ELF) in Nb-doped NMNO indicates ionic bonding involving Mn-O and Nb-O orbitals, while Ni-O presents polar covalent bonding. (d) Low magnification scanning electron microscopy (SEM) micrograph of SC_Nb-NMNO, scale bar, 10 µm. (e) Magnified view of the single crystal of SC_Nb-NMNO cathode material through SEM, scale bar, 200 nm. (f) Transmission electron microscopy (TEM) image of the as-synthesized SC_Nb- NMNO hexagonal single crystal, scale bar, 300 nm. (g) High resolution TEM (HR-TEM) image showing the lattice fringes with an interlayer spacing of 0.558 nm of the plane d(002), scale bar, 5 nm. The inset on the left confirms the plane of (002) through a selective area electron diffraction pattern (SAED) of the single crystal SC_Nb-NMNO, and the inset on the right shows the FFT pattern. (h) HAADF-STEM image of the SC_Nb-NMNO single crystal, scale bar, 200 nm. (i- m) Elemental mapping of the as-prepared SC_Nb-NMNO single crystal powders via STEM mode. (i) The alkali-ion, Na (j) The active transition metal, Ni (k) The inactive transition metal-ion, Mn (l) The heteroatom dopant, Nb (m) A comprehensive view of the homogenous elemental distribution of all the elements present both in line scanning and elemental mapping. (n) Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements for probing the distribution of niobium (Nb) in the bulk of SC_Nb-NMNO. Figure 2 illustrates electrochemical performance of the NMNO and SC_Nb- NMNO Cathode Materials against Na-metal at 20 °C. (a) Typical galvanostatic charge-discharge profile of SC_Nb-NMNO at 0.1C within the potential window of 2.5 V – 4.2 V against Na / Na+. (b) Theoretically calculated voltage profile of pristine and single crystal Nb-doped NNMO at different Na-concentrations (calculated voltage profiles shown in are shifted up by 0.8 V to match with experimental results). (c) Rate capability test at 2.5 V – 4.2 V against Na / Na+ of SC_Nb-NMNO cathode material cycled against Na-metal at varying C-rate. (d) Comparison of the performance of cathodes: Charge-discharge capacity and coulombic efficiency versus cycle number of NMNO and SC_Nb-NMNO at the current rate of 0.1 C. (e) Charge transfer resistance of SC_Nb-NMNO at different cycling stages; at OCV and after 1st cycle. (f) Evaluation of Na-ion diffusion coefficient (DNa+) through cyclic voltammetry collected at different voltage scanning rates from 0.05 mV s-1 to 1.0 mV s-1 at 20 °C. (g) Cycling performance analysis through charge-discharge capacity and Coulombic efficiency versus cycle number plot of SC_Nb-NMNO at a high current rate of 1 C. Figure 3 illustrates in-situ operando characterization of both the Pristine NMNO and the Single Crystal SC_Nb-NMNO Cathode. (a) In-situ X-ray Diffraction (XRD) patterns of SC_Nb-NMNO using Swagelok cell corresponding to the charge and discharge curves between 2.5 – 4.3 V at C / 15, 20 °C focussing on the (002) plane. (b) Energy difference between P2 andO2 phases for Na0.0 in pristine (blue) and Nb-doped (orange) NNMO (ΔE=EP2-EO2) estimated through DFT. More negative ΔE values indicate more stable P2 phase. (c) Operando Online electrochemical mass spectroscopy of both SC_Nb-NMNO and NMNO (pristine) electrode collected during the first galvanostatic charge-discharge at C / 15 within the potential window of 2.5 V – 4.3 V. Figure 4 illustrates investigation of the morphology in the Cycled Electrodes through Microscopy. (a) Schematic illustration of the possible mechanism through microscopy. (b) Ex-situ SEM of pristine NMNO electrode obtained after 100 cycles at 0.1 C, showing cracks and delamination of the layers, scale bar, 400 nm. (c and d) Ex-situ HR-TEM of pristine NMNO showing the phenomena of exfoliation and cracking parallel to the lower indexed lattice plane, scale bar, 50 nm and 10 nm. The inset shows a magnified view of the HR-TEM image to clearly show the stressed and strained lattice fringes, scale bar, 5 nm. (e) Ex-situ SEM of single crystal SC_Nb-NMNO electrode obtained after 100 cycles at 0.1 C, showing no evidence of cracks and delamination of layers, scale bar, 600 nm. (f and g) Ex-situ HR-TEM of single crystal SC_Nb-NMNO with no exfoliation and cracking parallel to the lower indexed lattice plane, scale bar, 50 nm and 10 nm. The inset shows a magnified view of the HR-TEM image indicating preserved lattice fringes as that of the initial cathode material, scale bar, 5 nm. (h) STEM- HAADF image of SC_Nb-NMNO electrode obtained after 100 cycles of charge- discharge at C / 10 with clear facets, scale bar, 200 nm. (i-m) Elemental mapping of the as-prepared SC_Nb-NMNO single crystal powders via STEM mode. (i) The alkali-ion, Na (j) The active transition metal, Ni (k) The inactive transition metal- ion, Mn (l) The heteroatom dopant, Nb (m) The oxygen anion. Figure 5 illustrates sodium-ion battery with Single Crystal SC_Nb-NMNO as Cathode and Hard Carbon as Anode. (a) Charge-discharge profile of SC_Nb- NMNO and Hard carbon in terms of areal capacity for full cell balancing. (b) Cycling stability performance analysis of Hard carbon as anode (loading = 4.33 mg cm-2) and SC_Nb-NMNO as cathode (loading = 9.04 mg cm-2) against sodium metal. (c) Electrochemical performance of the SC_Nb-NMNO / / Hard carbon full cell at 0.1 C within the potential window of 2 – 4.2 V, maintaining ~1.1 N / P ratio. (d) Cycling performance of the full cell at 0.13 mA cm-2 for 100 cycles. (e) Digital image of the pouch cell fabricated using SC_Nb-NMNO as a cathode and Hard carbon as an anode. (f) Cycling performance of the fabricated pouch cell at 0.109 mA cm-2 for 100 cycles. Figure 6 illustrates schematic illustration of the preparation of pure-phase single crystal Nb doped Na0.67Ni0.33Mn0.67O2 (The original hexagonal morphology of SC_Nb-NMNO single crystal from SEM micrographs were used for making the schematics). Figure 7 illustrates 3D-column plot of the as-synthesized pristine NMNO and SC_Nb-NMNO to confirm the stoichiometric ratio. Figure 8 illustrates structural characterization of pristine NMNO. (a) Transmission electron microscopy (TEM) image of the as-synthesized NMNO material, scale bar, 200 nm. (b) High resolution TEM (HR-TEM) image showing the lattice fringes with an interlayer spacing of 0.55 nm of the plane d(002), scale bar, 5 nm. (c) Scanning electron microscopy (SEM) micrograph displaying the particles of NMNO at a low magnification, scale bar, 20 μm. (d) Magnified view of the primary crystal of NMNO cathode material through SEM, scale bar, 200 nm. (e) HAADF-STEM image of the NMNO single crystal, scale bar, 500 nm. (f-i) Elemental mapping of the as-prepared NMNO cathode powders via STEM mode. (f) The alkali-ion, Na (g) The active transition metal, Ni (h) The inactive transition metal-ion, Mn (i) The oxygen anion. Figure 9 illustrates morphology analysis of single crystal Sc_Nb-NMNO. (a) Scanning electron microscopy (SEM) micrograph displaying the particles of SC_Nb-NMNO, scale bar, 2 μm. (b) SEM image of single crystal SC_Nb-NMNO particles at a low magnification, scale bar, 1 μm. (c) Single crystals of Nb doped NMNO cathode material at slightly magnified scale through SEM, scale bar, 200 nm. (d) High magnification image of single crystal SC_Nb-NMNO via SEM micrograph, scale bar, 200 nm. Figure 10 illustrates schematic of the 2032coin cell architecture in a half-cell configuration SC_Nb-NMNO as a cathode and Na-metal as an anode plated on the stainless steel disc. Figure 11 illustrates XPS spectra of Ni 2p, Mn 2p and Nb 3d for the SC_Nb- NMNO single crystal cathode material charged till 4.2 V. Figure 12 illustrates equivalent circuit model used for fitting the impedance spectra of the EIS obtained for SC_Nb-NMNO. Figure 13 illustrates the corresponding fitting curve between the peak currents (ip) and the scan rates (V) for SC_Nb-NMNO. (a) Oxidation peak at 3.8 V (b) Reduction peak at 3.6 V (c) Oxidation peak at 3.4 V (d) Reduction at 3.2 V. Figure 14 illustrates in-situ x-ray diffraction (XRD) patterns of NMNO using swagelok cell corresponding to the charge and discharge curves between 2.5 – 4.3 V at C / 15, 20 °C Figure 15 illustrates Galvanostatic charge-discharge profile of hard carbon anode against Na metal in half cell configuration Figure 16 illustrates schematic of the configuration of the pouch cell constructed with both side coated SC_Nb-NMNO as a cathode and hard carbon as an anode on aluminium current collector. Fig.17. FIB-TEM Analysis of SC_Nb-NMNO Lamella (a) STEM image of the FIB pre-treated SC_Nb-NMNO lamella (b-f) Energy dispersive Spectroscopy through STEM mode of the FIB treated SC_Nb-NMNO lamella. (b) Oxygen (O) elemental mapping (c) Sodium (Na) mapping (d) Nickel (Ni) mapping (e) Manganese (Mn) mapping (f) Niobium (Nb) mapping (g) Line scanning of the FIB treated SC_Nb-NMNO lamella taken through the STEM mode. DETAILED DESCRIPTION OF THE INVENTION The present inventors have strategically designed a mechanically robust hierarchical nanostructured Ti4+doped P2-type Na2 / 3Ni1 / 3Mn1 / 2Ti1 / 6O2 as a highly proficient cathode material for Na-ion battery through a quick, easy, energy efficient and an inexpensive microwave irradiated solid-state synthesis technique. According to one aspect of present invention, there is provide a process of synthesis of a cathode material including heteroatom doped, single crystal, cobalt- free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2 for Na-ion batteries wherein x is preferably 0.02, comprising: ^ mixing vigorously precursor materials in a certain ratio in a ball miller at a 300 rpm for 2 hours; ^ pre-calcining the resulting powder from the ball miller at 450 °C for 6 hrs in a muffle furnace; ^ hand grinding the decomposed product obtained from the muffle furnace in a mortar-pestle for an hour; ^ calcining the product by microwave irradiation at 900 °C with a reaction time of 12 minutes; ^ cooling slowly and transferring into a vacuum atmosphere within a desiccator to prevent any reactions with moisture and carbon dioxide. In an embodiment, said Na0.67Ni0.33-xMn0.67NbxO2layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2. In another embodiment, said precursor materials comprises anhydrous sodium carbonate (Na2CO3), niobium oxide (Nb2O5), Nickel acetate ((Ni(OCOOCH3)2.4H2O) and Mn acetate ((Mn(O(COOCH3)3.2H2O). In a further embodiment, said precursor materials are vigorously mixed in the molar ratio of 0.67:0.31:0.67:0.02. According to another aspect of present invention, there is provided a battery comprising: a cathode material including heteroatom doped, single crystal, cobalt- free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2 for Na- ion batteries wherein x is preferably 0.02 and an anode material including pre- sodiated hard carbon. In another embodiment, the Na0.67Ni0.33-xMn0.67NbxO2 layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2. In a further embodiment, wherein the battery is a sodium ion battery. Synthesis and characterization Phase-pure Na0.67Ni0.31Mn0.67Nb0.02O2 (SC_Nb-NMNO) hexagonal single crystals were synthesized through a rapid solid-state synthesis technique. As illustrated in Fig. 6, initially, the precursors, sodium carbonate (Na2CO3), niobium oxide (Nb2O5), and acetates of nickel (Ni(OCOOCH3)2.4H2O) and manganese (Mn(O(COOCH3)3.2H2O)), underwent dry mixing in a planetary ball miller. This was followed by pre-calcination process in a muffle furnace at 450 ºC for 6 hours. Subsequently, the pre-calcined mixture decomposed underwent further grinding in a mortar and pestle before being subjected to microwave irradiation in a microwave furnace at 900 ºC for 30 minutes, with a ramp rate of 5 ºC / min. This process resulted in the formation of a phase-pure single crystal cathode material denoted as SC_Nb-NMNO. The resulting SC_Nb-NMNO crystals underwent comprehensive characterizations to evaluate their physical properties. Similarly, the pristine NMNO powder was prepared using an identical procedure. A comprehensive account of the experimentation details has been provided in the experimental section. The crystal structures of the pristine NMNO and the heteroatom doped, single crystal SC_Nb-NMNO were determined using Rietveld refinements of a long scan X-Ray diffraction (XRD) pattern. A hexagonal lattice of P63 / mmc space group, where Mn, Ni, and Nb atoms occupying the 2a site of the transition metal layer, may fully index all of the diffraction peaks in the XRD pattern. Tables S1 and S2 contain a list of the structural parameters, which include atom location, lattice parameters derived from Rietveld refinement, occupancies of Na-ions, and their standard derivations. Using Rietveld refinement of NMNO and SC_Nb-NMNO, as seen in Fig. 1a and 1b, respectively, an excellent agreement between the observed and calculated XRD pattern is produced. Fig. 1c shows the stacked hexagonal lattice structure along the ‘a’ and ‘c’ axes. For an ideal close-packed structure, the radius ratio (r / r) for octahedral voids in an O2 or O3-type layered cathode material is approximately 0.414, and for prismatic voids in a P2 or P3- type, it is around 0.732, giving us a general idea of the relative difference in the void’s sizes [R.J. Clément, P. G. Bruce, C. P. Grey, Review—Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials, J. Electrochem. Soc.162 (2015) A2589-A2604]. Hence, the sodium-ions are able to be accommodated due to the ample gap in the prismatic voids between the MO6octahedral planes. The Na layer has two types of prismatic sites, Nafand Nae, which share two faces and two edges within the MO6 upper and lower octahedral, respectively. As previously documented in the literature, SC_Nb-NMNO exhibits a favoured occupancy at Naesites (Nae: 38%, Naf: 28%, as determined by DFT calculations discussed below), which lessens the electrostatic repulsion between Na-ions. According to the Bragg equation [Q. Shi, R. Qi, X. Feng, J. Wang, Y. Li, Z. Yao, X. Wang, Q. Li, X. Lu, J. Zhang, Y. Zhao, Y, Niobium-doped layered cathode material for high power and low-temperature sodium-ion batteries, Nature Comm. 13 (2022) 3205], the doping with Nb results in a minor displacement of the (002) and (004) peaks to a lower angle, which increases the d-spacing. When Ni is replaced with Nb, the bond lengths of TM-O and Na-O are regulated, which promotes high Na+ mobility and lowers activation energy barriers, especially in the highly desodiated state towards the end of the charging [C.G. Pope, X-ray Diffraction and the Bragg Equation, J. Chem. Educ. 74 (1997) 129]. This has been further proved through the electron localization function (ELF) calculated through density functional theory as has been illustrated in Fig.1d. This indicates ionic bonding involving Mn-O and Nb-O orbitals in SC_Nb-NMNO, with Ni-O exhibiting polar covalent bonding. The advantageous hybridization of Nb-O orbitals clearly suggests may play a crucial role in impeding oxygen loss, particularly in deeply charged states. The inductively coupled plasma atomic emission spectrometry (ICP-AES) method, as shown in Fig. 7, was used to estimate the elemental ratios of Na, Ni, Mn in NMNO and Na, Ni, Mn combined with Nb in SC_Nb-NMNO. The results were 0.67:0.33:0.67 (Na: Ni: Mn) and 0.67:0.31:0.67:0.02 (Na: Ni: Mn: Nb), respectively. Transmission electron microscopy (TEM) and field-emission gun scanning electron microscopy (FEG-SEM) were used to investigate the morphological properties of pristine NMNO and SC_Nb-NMNO. A low-magnification TEM micrograph of SC_Nb-NMNO's single crystal, which has a shape resembling a hexagonal plate, is displayed in Fig. 1e. The strong crystallinity of the SC_Nb- NMNO is demonstrated by the clear lattice fringes as seen in the high-resolution transmission microscopy (HRTEM) image (Fig.2f). For the (002) plane of the P2 phase, the interplanar distance determined from the HRTEM picture is 0.558 nm, indicating a 3.22% increase in interlayer distance due to Nb5+doping. The low magnification TEM and HRTEM of NMNO show that the spacing between the lattice fringes is approximately 0.55 nm, which is consistent with the reported data [H. Wang, B. Yang, X-Z. Liao, J. Xu, D. Yang, Y-Sh. He, Z-F. Ma, Electrochemical properties of P2-Na2 / 3[Ni1 / 3Mn2 / 3]O2 cathode material for sodium ion batteries when cycled in different voltage ranges, Electrochemica Acta 113 (2013) 200-204; L. Wang, A. Mukherjee, C-Y. Kuo, S. Chakrabarty, R. Yemini, A.A. Dameron, J.W. DuMont, S.H. Akella, A. Saha, S. Taragin, H. Aviv, D. Naveh, D. Sharon, T-S. Chan, H-J. Lin, J-F. Lee, C-T. Chen, B., Liu, X. Gao, S. Basu, Z. Hu, D. Aurbach, P.G. Bruce, M. Noked, High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures, Nature nanotechnology (2023)], and also depicted in the low magnification TEM and HRTEM of NMNO as shown in Fig. 8a and 8b. Selected area electron diffraction (SAED) was utilized to explore the single crystalline structure of the SC_Nb-NMNO in more detail. The P2 phase's

[0002] zone axis can be used to index the diffraction pattern found in the inset of Fig. 2f. The as- prepared cathode material SC_Nb-NMNO, comprises of numerous individual single crystals, as evidenced by low-magnification scanning electron microscopy (SEM) images (Fig. 2g). These single crystals take the form of hexagonal plates, exhibiting a diameter of 538.37 nm and a thickness of 36.04 nm (Fig.2h). Fig.8c and d show the morphology of the pristine NMNO as seen in low- and high- magnification SEM micrographs. A single crystal hexagonal plate-like particle captured at the

[0002] zone axis in a low magnification TEM image is clearly in accordance with the low magnification high-angle annular dark field image depicted in Fig. 2i. When analysed using energy-dispersive X-ray spectroscopy (EDS) mapping in scanning electron transmission microscopy (STEM) mode, the elemental mappings of Na, Ni, Mn, and Nb reveal a uniform element distribution in the SC_Nb-NMNO single crystal, as shown in Fig. i-n. As seen in Fig. 1n, the homogenous distribution of the elements, including oxygen, was further examined using line scanning and overlapped mapping of the elements. We observe that the niobium is doped in the bulk and is uniformly distributed throughout the crystal instead of inducing atomic scale surface reorganization, thereby establishing chemical changes on the surface layer of the material. This builds the nodes that holds the integrity of the entire bulk structure even when cycling at higher voltage for prolong cycling. The HAADF image and the elemental distribution of NMNO by EDX in STEM mode are shown in Fig. 8e-i, which also illustrates the uniform distribution of the elements. Additionally, to provide a detailed visualization of the acquired single crystals, scanning electron microscopy (SEM) micrographs at both low and high magnifications are presented in Fig. 9a-d. The SEM images reveal a distinctive hexagonal morphology in the obtained single crystals. At low magnification (Fig. 9a-b), the overall hexagonal shape is discernible, suggesting uniformity in crystal structure. Upon closer inspection at high magnification (Fig. 9c-d), the intricate details of the hexagonal facets become more apparent, indicating a well-defined and well-formed single crystal structure with sharp edges and clear facets. This hexagonal morphology is indicative of the crystallographic arrangement and growth characteristics of the single crystals. Na-storage performance of SC_Nb-NMNO in half-cell configuration The sodium-storage performance of SC_Nb-NMNO was evaluated through galvanostatic charge-discharge cycling within the potential range of 2.5 V – 4.2 V. 2032 coin cells in the half-cell configuration, the architecture of which has been displayed in Fig. 10 were assembled using a Na-metal anode and NaPF6salt electrolyte (1 M NaPF6 in ethylene carbonate and propylene carbonate (EC:PC) in a 1:1 ratio with 2 vol% FEC) in a coin cell format. In order to discern the impact of Nb doping on the performance of NMNO as a viable cathode for Na-ion batteries, we also prepared and tested an undoped NMNO cathode material as a control. The capacity-voltage profiles of the first three charge-discharge profiles obtained at 0.1 C (1 C = 171 mA h g-1) during the aforementioned potential window (2.5 V – 4.3 V) are shown in Fig. 3a. The cathode only showed a brief charging voltage plateau at 3.2 V, followed by a smooth curve, suggesting that a phase transition happened at the start of the Na+ extraction process and then a solid-solution reaction

[0024] , which is covered in more detail below. At the conclusion of the first cycle, a reversible discharge capacity of 149.8 mA h g-1 was attained, which results in an acceptable initial Coulombic efficiency of around 90%. The average discharge voltage is approximately 3.4 V at 20 ºC. It should be noted that the introduction of niobium (Nb) doping at a level of 3.2% plays a pivotal role in facilitating significant nickel participation in the electrochemical activity of the cathode material. This effective doping strategy is crucial for maintaining stable voltage plateaus corresponding to the Ni2+ / Ni3+ / Ni4+ redox reactions within the favourable range of 2.5 V – 4.2 V [Q. Shi, R. Qi, X. Feng, J. Wang, Y. Li, Z. Yao, X. Wang, Q. Li, X. Lu, J. Zhang, Y. Zhao, Y, Niobium-doped layered cathode material for high power and low-temperature sodium-ion batteries, Nature Comm. 13 (2022) 3205]. The confirmation of this electrochemical behaviour was further substantiated through ex-situ X-ray Photoelectron Spectroscopy (XPS) analysis of the single crystal Nb-doped P2-type sodium nickel manganese oxide (SC_Nb- NMNO) electrode after charging the cathode material up to 4.2 V. The obtained spectroscopic data, illustrated in Fig. 11, highlights that, under these conditions, nickel is the sole active transition metal, demonstrating its redox activity, while manganese (Mn) and niobium (Nb) remain in their original states. Therefore, the approach of doping with Nb serves as a foundation for achieving improved electrochemical performance and voltage stability within the designated voltage range. The galvanostatic charge-discharge curves of SC_Nb- NMNO exhibited remarkable reversibility, particularly after the initial cycle, showcasing excellent stability even during cycling at higher C-rates. This kind of cathode material is offering high operational and high capacity is very appealing for high energy density batteries. The augmentation of the experimental voltage profile was meticulously substantiated through advanced theoretical calculations, leveraging the precision of Density Functional Theory (DFT) computations with the Perdew-Burke- Ernzerhof exchange-correlation functional augmented by dispersion corrections (PBE-D3). The calculated voltage profiles, as illustrated in Fig. 2b and detailed by Equation 3 in the theoretical section of the experimental procedure, underscore the significant impact of niobium (Nb) doping on the electrochemical performance of the P2-type Na0.67Ni0.33Mn0.67O2 (NMNO) cathode material [A. Chakraborty, S. Kunnikuruvan, S. Kumar, B. Markovsky, D. Aurbach, M. Dixit, D.T. Major, Layered Cathode Materials for Lithium-Ion Batteries: Review of Computational Studies on LiNi1–x–yCoxMnyO2 and LiNi1–x–yCoxAlyO2, Chemistry of Materials 32 (3) (2020) 915-952; M. K. Aydinol, A. F. Kohan, G. Ceder, K. Cho, J. Joannopoulos, Ab initio study of lithium intercalation in metal oxides and metal dichalcogenides, Physical Review B 56 (3) (1997) 1354-1365; M. Dixit, M. Kosa, O.S. Lavi, B. Markovsky, D. Aurbach, D.T. Major, Thermodynamic and kinetic studies of LiNi0.5Co0.2Mn0.3O2 as a positive electrode material for Li-ion batteries using first principles, Physical Chemistry Chemical Physics 18 (9) (2016) 6799-6812]. A rigid shift of 0.8 V was applied for both pristine and single crystal Nb-doped NMNO, unveiling distinctive features in the voltage response. The presented data in Fig. 2b distinctly reveals that Nb- doping induces a markedly elevated voltage profile, surpassing that of pristine NMNO. Notably, this enhancement extends to the provision of a broader operational voltage window, a critical parameter for the efficient functioning of sodium-ion batteries. Theoretical insights garnered from these DFT calculations align seamlessly with the experimental voltage profiles, as showcased in Fig. 2a. This concordance between theoretical predictions and experimental observations serves as a compelling validation of the accuracy and reliability of the computational model in capturing the intricacies of the electrochemical behaviour induced by Nb-doping. The calculated voltage profiles not only offer a quantitative representation but also provide valuable qualitative insights into the electrochemical characteristics, further enriching our understanding of the tailored electrochemical performance conferred by Nb-doping in P2-type NMNO. The SC_Nb-NMNO cathode's rate performance is shown in Fig. 2c, where it is shown to consistently produce over 120 mA h g-1 when the rate is increased step- by-step from C / 10 to 1 C. Remarkably, the cathode could deliver more than 50% of its initial capacity recorded at the low rate (C / 10) even in the harsh conditions of a rate as high as 5C. The initial discharge capacity was recorded to be 94.7 mA h g-1. Thus, SC_Nb-NMNO is demonstrated to be capable of promoting faster diffusion kinetics of Na+ ions. When subjected to cycling at a rate of C / 10, the SC_Nb-NMNO cathode exhibited enduring cyclability, manifesting a remarkable capacity retention surpassing >95% after 100 cycles (Fig. 2d). This performance was complemented by highly uniform galvanostatic charge-discharge profiles throughout the testing period. However, the NMNO electrode when cycled within the same potential window could only deliver a capacity retention of ~86% after 100 cycles, significantly reduced stability compared to SC_Nb-NMNO as shown in Fig.2d. Electrochemical impedance spectroscopy (EIS) of the cells was conducted at different stages: before cycling (i.e., at open-circuit voltage), after the initial charge, and post 100 cycles. This was undertaken to scrutinize alterations in charge transfer resistance within the single crystal SC_Nb-NMNO, as depicted in Fig. 2e. The corresponding circuit employed for fitting the impedance spectra is illustrated in Fig. 12. According to the EIS data, the charge transfer resistance (Rct) of the SC_Nb-NMNO electrode measures 185.3 Ohms at open-circuit voltage (OCV) and escalates to 306.6 Ohms after the third cycle. Numerous parameters, including phase composition, microstructure, and stoichiometry, affect the Rct value. The charge transfer resistance value changes as dopants are added to the cathode material, demonstrating how well doping works to enhance the battery's electrochemical performance [I. Hasa, S. Passerini, J. Hassoun, Toward high energy density cathode materials for sodium-ion batteries: investigating the beneficial effect of aluminum doping on the P2-type structure, J. Mater. Chem. A 5 (2017) 4467–4477]. Incorporation of Niobium in the NMNO cathode material simultaneously lowers the ionic diffusion energy barrier and the electronic band gap. This allows for rapid electron and Na+ transfer, particularly in the desoidated state, which ultimately lowers the charge transfer.24 This suggests that Nb-doping in NMNO is preventing phase transitions and cracks from forming during cycling because it improves cyclability and rate capability by decreasing the total cell impedance and expanding the Na+ diffusion path. Moreover, as illustrated in Fig. 2f, cyclic voltammetry (CV) profiles were acquired at varying sweep rates ranging from 0.05 mV s-1 to 1.0 mV s-1 to elucidate the average diffusion coefficient (DNa+) of sodium ions within SC_Nb- NMNO. The estimation of DNa+ through CV analysis was conducted utilizing the Randles-Sevcik equation, as outlined in a prior scholarly publication [A. Sengupta, A. Kumar, G. Barik, A. Ahuja, J. Ghosh, H. Lohani, P. Kumari, T.K. Bhandakkar, S. Mitra, Lower Diffusion-Induced Stress in Nano-Crystallites of P2- Na2 / 3Ni1 / 3Mn1 / 2Ti1 / 6O2 Novel Cathode for High Energy Na-ion Batteries, Small 1912 (2023) 2206248]. Fig. 13a-d portrays the fitted curves correlating ip and v1 / 2 of the redox peaks. The application of the Randles-Sevcik equation revealed average DNa+ values of approximately 10-9 cm2 s-1 for both the oxidation and reduction peaks of SC_Nb-NMNO, which is lower than the diffusion coefficient of the pristine Na 2 0.67Ni0.33Mn0.67O2 in the range of 10-11 cm s-1according to the previous reports [S. Sun, X. Li, L. Yan, W. Chen, X. Lu, Y. Bai, Constructing Magnetic Ion Accelerator at Na2 / 3Ni1 / 3Mn2 / 3O2 Surface for Sodium Ion Batteries. ACS Energy Lett.810 (2023) 4349-4356]. It is noteworthy that with increasing scan rates, the redox currents exhibit noticeable escalation, and the potential differentials between the oxidation and reduction peaks undergo a subtle widening. This phenomenon signifies a minor degree of polarization in the SC_Nb-NMNO cathode material. In extended cycling tests at 1C, the single crystal SC_Nb-NMNO cathode material granted a capacity retention of >90% after 2000 cycles (calculated from 3rd cycle), as observed in Fig. 2g. We further compared the performance of our SC_Nb-NMNO with various P2-type cathode materials reported in the literature in terms of its capacity retention, number of cycles and C-rate in Table 8 of the supporting information. These electrochemical results portray the effectiveness of Nb-doped SC_Nb-NMNO owing to it being less susceptibility to cracking and exfoliation during cycling along with suppressed P2-to-O2 phase transition. Understanding the Stabilization Mechanism of the Bulk Structure The preceding statement indicates that the unfavourable phase transition at 4.2 V against Na / Na+, which generates a considerable lattice distortion, is the primary cause of the P2 type cathode's poor electrochemical stability [I. Hasa, S. Passerini, J. Hassoun, Toward high energy density cathode materials for sodium- ion batteries: investigating the beneficial effect of aluminum doping on the P2- type structure, J. Mater. Chem. A 5 (2017) 4467–4477]. The better performance of our SC_Nb-NMNO is thought to be caused by the suppressed high-voltage phase transition, as it does not show a plateau at around 4.2 V. In order to bolster this theory, we performed in-situ X-ray diffraction (XRD) studies on SC_Nb- NMNO electrodes, providing information about the structural changes that occurred during the first charge-discharge cycle, as depicted in Fig. 3a. It is important to note that the pristine state of the in-situ holder exhibits several peaks attributed to components like the Be / BeO window, Al foil, separator, and cell parts. However, these peaks remain unchanged throughout the charge-discharge process. Therefore, in the interest of clarity, we have chosen to focus on the peak relevant to the phase transition. During the initial charging process at C / 15 within the potential window of 2.5 V – 4.3 V, we observe a gradual shift of the (002) peaks towards lower angles. As sodium-ions deintercalate, this shift shows a progressive expansion of the lattice parameter ‘c’ and a concurrent contraction of the lattice parameter ‘a’, which translates to an increase in the interlayer distance and a decrease in the intraplanar distance of the cathode material. Throughout the charge process, the P2 phase's lattice volume continued to increase while the O2 phase's lattice volume continued to decrease due to these conflicting trends in the ‘a’ and ‘c’ axes. Interestingly, beyond a charge of 4.2 V, a distinct new peak emerges at approximately 16.5°, attributed to the O2 structure [Q. Shi, R. Qi, X. Feng, J. Wang, Y. Li, Z. Yao, X. Wang, Q. Li, X. Lu, J. Zhang, Y. Zhao, Y, Niobium-doped layered cathode material for high power and low-temperature sodium-ion batteries, Nature Comm. 13 (2022) 3205; S-G. Hwang, C-H. Kim, S- H. Choe, K-C. Ri, C-J. Yu, Revealing the effect of Nb or V doping on anode performance in Na2Ti3O7 for sodium-ion batteries: a first-principles study. RSC Adv. 13 (2023) 16749-16757]. Subsequently, during the discharge process, it gradually reverts to its pristine state. This dynamic behaviour clearly illustrates the reversible P2-O2 phase transition within our SC_Nb-NMNO electrode. It's worth noting that the O2 phase consists of O-type stacking faults, closely resembling the P2 structure, emphasizing the intricate nature of the phase transition. This in-situ XRD analysis offers valuable insights into the structural changes occurring during charge and discharge, shedding light on the role of Nb ion doping in facilitating this reversible phase transition. On the contrary, during the charging of pristine NMNO, the (002) peak initiates a shift to a lower angle until 3.8 V, causing an expansion in the 'c' direction due to increased repulsion between the transition metal layers caused by the weakening electrostatic effect of sodium-ion release. Upon further charging to 4.3 V, a new peak, identified as the O2 phase, emerges due to the sliding of the P2 phase in the transition metal layer under high pressure. This results in constant volume changes during charge and discharge, leading to the destruction of the crystal structure. Conversely, upon charging to 4.3 V, the undoped NMNO cathode material displays sequential transitions from the P2 phase to the O2 phase, as depicted in Fig. 14, which matches well with the reported data [S. Sun, X. Li, L. Yan, W. Chen, X. Lu, Y. Bai, Constructing Magnetic Ion Accelerator at Na2 / 3Ni1 / 3Mn2 / 3O2 Surface for Sodium Ion Batteries. ACS Energy Lett.810 (2023) 4349-4356]. It is commonly known that P2-to-O2 phase transitions occur in Na intercalation cathodes. ABAC stacking occurs when trigonal prismatic sites are replaced by octahedral sites due to sheering of oxygen layers to stabilize the trigonal prismatic coordination in the absence of Na+ ions between the oxygen layers [Z. Lu, J.R. Dahn, In Situ X-Ray Diffraction Study of P2 Na2 / 3[Ni1 / 3Mn2 / 3]O2, Journal of The Electrochemical Society 148 (11) (2001) A1225]. The effect of Nb5+doping on phase-transition was also calculated by density functional theory (DFT) through the energy difference of P2- and O2 phases for both the undoped and Nb-doped NMNO at Na0.0. In Fig, 3b, we see that the P2-to-O2 phase transition is more likely to occur in pristine NMNO, while such phase transition could be suppressed in Nb-doped NMNO as the difference in energy between the P2 and O2-type phase formation at the completely desodiated state in -0.07885 eV for pristine NMNO and -0.14978 eV for SC_Nb-NMNO. Hence, this proves that the transition of phase from P2 to O2-type would require more energy in SC_Nb- NMNO, thereby making the transition difficult. This correlates quite well with our observation from in-situ XRD analysis of suppressed phase transition in SC_Nb- NMNO. To theoretically evaluate the phase transition mechanism, the formation energy for P2 and O2 phase for both NMNO and SC_Nb-NMNO were calculated separately and then the energy difference was found out. The phase transition from P2 to O2 in P2-type layered oxide cathode materials is related with the evolution of oxygen or carbon dioxide gas, which can have implications for the performance and stability of sodium-ion batteries. The phase transition from P2-to-O2 occurs at high charge potentials (>4.2 V) when the Na+ ions are extracted from the layer, leading to an energetically unfavourable state [Y. Wang, Z. Feng, P. Cui, W. Zhu, Y. Gong, M-A. Girard, G. Lajoie, J. Trttier, Q. Zhang, L. Gu, Y. Wang, W. Zuo, Y. Yang, J.B. Goodenough, K. Zaghib, Pillar- beam structures prevent layered cathode materials from destructive phase transitions, Nature Comm.12:13 (2021)]. This phase transition is related with the evolution of oxygen gas, which can result in gassing behaviour and safety concerns, ultimately affecting the cycling performance and stability of the battery. The released O2 is highly reactive and can lead to the formation of CO2, which is indicative of the oxygen evolution process and can further contribute to safety issues and reduced battery performance [C. Zhao, C. Li, H. Liu, Q. Qiu, F. Geng, M. Shen, W. Tong, J. Li, B. Hu, Coexistence of (O2)n− and Trapped Molecular O2 as the Oxidized Species in P2-Type Sodium 3d Layered Oxide and Stable Interface Enabled by Highly Fluorinated Electrolyte, J. Am. Chem. Soc., 14344 (2021) 18652-18664]. Therefore, understanding and controlling the phase transition from P2-to-O2 is crucial for mitigating the adverse effects of oxygen release and ensuring the long-term stability and safety of sodium-ion batteries. Hence, to further investigate the potential release of oxygen gas and carbon dioxide from NMNO and its Nb-doped counterpart, we employed in-operando Online Electrochemical Mass Spectrometry (OEMS). This technique allows real- time monitoring of gas emissions during the electrochemical processes, providing crucial insights into the material's behaviour during charge-discharge cycles. Fig. 3c presents the outcome of this analysis within the voltage range of 2.5–4.3 V (vs. Na / Na+) cycled at C / 15. Remarkably, we observed only minimal evidence of O2 gas release in SC_Nb-NMNO, indicating a substantial reduction in oxygen evolution compared to NMNO. In particular, the measured quantity of CO2 gas that resulted from electrolyte and trace Na2CO3 breakdown was extremely small, only 0.05 mol per mole of active materials for SC_Nb-NMNO. In contrast, NMNO exhibited a significantly higher CO2 gas generation at 0.09 mol per mole. This notable reduction in gas evolution further underlines the efficacy of Nb doping (3.8%) in mitigating the release of oxygen and associated irreversible phase transformations, ultimately contributing to enhanced cycling stability [A. Joshi, S. Chakrabarty, S.H. Akella, A. Saha, A. Mukherjee, B. Schmerlin, M. Ejgenberg, R. Sharma, M. Noked, High-Entropy Co-Free O3-Type Layered Oxyfluoride: A Promising Air-Stable Cathode for Sodium-Ion Batteries, Adv. Mater. (2023) 2304440; S.H. Akella, S. Taragin, A. Mukherjee, O-L. Shalev, H. Aviv, M. Zysler, D. Sharon, M. Noked, Tailoring Nickel-Rich LiNi0.8Co0.1Mn0.1O2 Layered Oxide Cathode Materials with Metal Sulfides (M2S:M = Li, Na) for Improved Electrochemical Properties, J. Electrochem. Soc. 168 (2021) 080543]. Operando OEMS provides critical information on the material's electrochemical behaviour, allowing us to comprehend the mechanisms underlying phase transitions and gas evolution during charge-discharge cycles. In the case of SC_Nb-NMNO, the reduction in O2 gas release can be attributed to the conversion of the irreversible P2-to-O2 phase transition, predominant in NMNO, to a more reversible P2-to-O2 phase due to Nb doping. This structural transformation significantly influences the cathode material's cycling stability, and the observed minimal gas evolution implies a suppressed phase transition, supporting the enhanced stability of SC_Nb-NMNO. In-operando OEMS proves to be a valuable tool in understanding the electrochemical performance of materials, shedding light on the role of Nb5+doping in suppressing oxygen and carbon dioxide gas release, and irreversible phase transitions. The insights gained from this analysis underscore the potential of SC_Nb-NMNO as a stable electrode material for sodium-ion batteries, paving the way for further advancements in battery technology through controlled doping strategies. The pristine NMNO faces challenges such as the irreversible P2-to-O2 phase transition, substantial volume change, exfoliation, and cracking, leading to rapid capacity decay and irreversible capacity loss. Our approach involves doping Nb into the NMNO structure, effectively reducing the electronic band gap and lowering the ionic diffusion energy barrier. Moreover, the single crystal preparation through a microwave-irradiated solid-state synthesis process offers advantages, including the homogeneous distribution of Nb throughout the crystal, absence of atomic surface reorganization, preservation of bulk structure integrity during prolonged cycling, and improvement in various kinetic factors such as particle size distribution, crystal facets, grain boundary, Na-ion diffusion, strain, and low O2 gas release. A schematic representation of our hypothesis is delineated in Fig. 4a, portraying the effect of Na-ion deintercalation / intercalation on the exfoliation and intragranular cracking in pristine NMNO and the positive consequences of Nb doping. However, to delve deeper into the intricate mechanisms governing the robust structural stability of the single crystal cathode material, the morphologies of cycled electrodes from both pristine NMNO and SC_Nb-NMNO were meticulously characterized through ex-situ field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) at a 0.1 C rate post-100 cycles of charge / discharge. A comprehensive summary of the post- cycling morphological examination is elucidated in Fig.4b-m. In Fig. 4b, the SEM micrograph of the NMNO electrode after 100 cycles vividly reveals the presence of microcracks and layer exfoliation. High-resolution TEM in Figure 5C exposes the delamination of layers parallel to the low-indexed lattice planes

[0010] [S. Guo, Q. Li, P. Liu, M. Chen, H. Zhou, Environmentally stable interface of layered oxide cathodes for sodium-ion batteries, Nat. Commun. 8 (2017) 1–9.]. Further magnification in Fig. 4d and its inset accentuates the exacerbation of exfoliation phenomena and the microstructural damage arising from the inhomogeneous cathode composition and dimensional changes during cycling. Contrastingly, the cycled SC_Nb-NMNO single crystal cathode conspicuously retains its pristine morphology, exhibiting minimal signs of exfoliation, microcracks, and pulverization, as evident in both the SEM and HR- TEM images in Fig. 4e, 4f, 4g, and its inset. This substantiates the commendable structural stability of the cathode material during (de)intercalation of sodium. Fig. 4h showcases the high-angle annular dark-field (HAADF) image, confirming the crystallinity of the single crystal even after 100 cycles of charge-discharge at 0.1 C. Additionally, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) were employed on the cycled SC_Nb-NMNO single crystal to analyse the elemental distribution, as illustrated in Fig.4i-m. Integrated full-cell assembly of SC_Nb-NMNO / / Hard carbon Capitalizing on the remarkable performance exhibited by the P2-type SC_Nb- NMNO in a half-cell configuration, we sought to assess its practicality in a sodium-ion full-cell setup with SC_Nb-NMNO as the cathode material and pre- sodiated hard carbon as the anode, the performance of which is summarized in Fig. 5. To mitigate the adverse impact of the first cycle low Coulombic efficiency inherent in hard carbon anodes, a strategic approach of pre-sodiation was adopted [H. Lohani, A. Kumar, P. Kumari, A. Ahuja, M. Gautam, A. Sengupta, S. Mitra, Artificial Organo-Fluoro-Rich Anode Electrolyte Interface and Partially Sodiated Hard Carbon Anode for Improved Cycle Life and Practical Sodium-Ion Batteries, ACS Appl. Mater. Interfaces 14 (2022) 37793−37803]. The process of pre- sodiating the hard carbon has been discussed in the experimental procedure. Utilizing the P2-type layered oxide cathode materials in conjunction with pre- sodiated hard carbon derived from biomass is a promising strategy in sodium-ion battery full-cell fabrication. This approach leverages the favorable characteristics of P2-type cathodes, such as high reversible capacity and enhanced cycling stability. In the realm of sodium-ion battery technology, constructing a viable full- cell involves careful consideration of both cathodic and anodic components. The sodium storage behavior of both SC_Nb-NMNO and the pre-sodiated hard carbon was meticulously examined using capacity vs voltage profiles of half cells for SC_Nb-NMNO cathode and pre-sodiated hard carbon, shown in Fig.5a. The half- cell of hard carbon was cycled between 0.01 V – 1 V, exhibiting a first charge / discharge capacity of 1.9 / 2.2 mA h cm-2at 85% Coulombic efficiency. Notably, an extra focus was directed towards ensuring sufficient reversible capacity in the anode, achieved through excess anode reversible capacity. This approach serves to create an abundant reservoir of active sites for effective Na-ion storage within the hard carbon anode. In essence, the optimization of the anode's properties in this manner contributes to a more efficient full-cell design, enhancing the overall performance and stability of the sodium-ion battery system [H. Lohani, A. Kumar, P. Kumari, A. Ahuja, M. Gautam, A. Sengupta, S. Mitra, Artificial Organo-Fluoro-Rich Anode Electrolyte Interface and Partially Sodiated Hard Carbon Anode for Improved Cycle Life and Practical Sodium-Ion Batteries, ACS Appl. Mater. Interfaces 14 (2022) 37793−37803]. The first three cycles of sodiation / desodiation have been represented in Fig. 15. The half-cell configuration of the SC_Nb-NMNO single crystal underwent cycling between 2.5 V – 4.2 V, manifesting a first charge / discharge capacity of 1.497 / 1.354 mA h cm-2at approximately 91% Coulombic efficiency. The specific areal capacities of both hard carbon and SC_Nb-NMNO single crystal were assessed based on their half- cell performance against a Na metal anode. As depicted in Fig. 5b, the cycling performance of the SC_Nb-NMNO single crystal cathode and pre-sodiated hard carbon anode is illustrated. In the case of pre-sodiated hard carbon, a consistent specific capacity of approximately 346.7 mA h g-1and an areal capacity of 1.501 m Ah cm-2were observed with a loading of 4.33 mg cm-2at a current density of 0.15 mA cm-2. In contrast, the SC_Nb-NMNO single crystal electrode demonstrated a stable specific capacity of around 149.8 mA h g-1and an areal capacity of 1.354 mA h cm-2, exhibiting a loading of 9.039 mg cm-2at a current density of 0.13 mA cm-2. The construction of a full cell involves electrode mass balancing, which is based on the half-cell performance of the anode and cathode shown in Fig.5b. Anode to cathode (N / P) areal capacity ratio of about 1.1 is kept constant. The extra anode is used to supply plenty of sites for Na-ion intercalation in hard carbon without plating and to make up for the loss of anode active material during the creation of the solid electrolyte interface (SEI). As shown in Fig. 5c, the cell is charged at a current density of 0.13 mA cm-2within the potential range of 2.0 V – 4.2 V after being fully discharged after manufacturing. A first cycle charge / discharge capacity of 1.414 / 1.277 mA h cm-2with 90.33% Coulombic efficiency and a second cycle charge / discharge capacity of 1.285 / 1.271 mA h cm-2with 98.9% Coulombic efficiency were both displayed by the SC_Nb-NMNO / / Hard carbon 2016-coin cell. In Fig. 5d, the long-term cycle stability of the full cell, comprising the SC_Nb- NMNO single crystal cathode and pre-sodiated hard carbon anode, is presented. The system demonstrates a capacity retention exceeding 93% after 100 cycles at 0.13 mA cm-2, affirming its commendable cyclability for practical applications in Na-ion batteries. A digital representation of a Na-ion pouch cell with both side-coated SC_Nb- NMNO single crystal cathode and a hard carbon anode is shown in Fig. 5e. The pouch casing is sealed with a Celgard polypropylene separator; a schematic representation of the cell is shown in Fig.16. Charge-discharge procedures for the pouch cell were similar to those of the complete cell based on the 2016 coin cell. With a current density of 0.109 mA cm-2 / 1.0985 mA h cm-2, around 89% of the capacity was retained after 100 cycles, as shown in Fig.6f. Table S4 provides the average stable areal capacity, Coulombic efficiency, and standard deviation of the hard carbon sodium-ion full-cell SC_Nb-NMNO. Hence, combining the as-synthesized Nb-doped single crystal NMNO cathode material with pre-sodiated hard carbon, enables high cycle life practical Na-ion batteries. Additionally, the utilization of biomass-derived hard carbon aligns with sustainable and eco-friendly practices in energy storage technologies. Experimental Section Materials In the synthesis of both the pristine P2-type Na0.67Ni0.33Mn0.67O2 and Nb-doped Na0.67Ni0.31Mn0.67Nb0.02O2, all precursor materials were utilized without additional purification. Sodium carbonate (Na2CO3) was procured from Sigma Aldrich, India, while nickel acetate (Ni(OCOOCH3)2.4H2O), manganese acetate (Mn(O(COOCH3)3.2H2O)), niobium dioxide was acquired from Aldrich Chemistry, India. The conducting carbon, Super P, was obtained from Timcal, Switzerland, and the polyvinylidene fluoride (PVDF) binder was sourced from TOB, China. For electrolyte preparation, sodium hexafluorophosphate (NaPF6) salt was purchased from TCI Chemicals, India. The solvents ethylene carbonate (EC), propylene carbonate (PC), and the additive fluoroethylene carbonate (FEC) were procured from Aldrich, India. Synthesis of Nb-doped single-crystal cobalt free P2-type Na0.67Ni0.31Mn0.67Nb0.02O2(SC_Nb-NMNO) In the synthesis of single crystal Nb-doped Na0.67Ni0.31Mn0.67Nb0.02O2 (SC_Nb- NMNO), Na2CO3, along with acetates of Ni and Mn, and niobium dioxide, in the molar ratio of 0.72:0.31:0.67:0.02, underwent thorough dry mixing in a ball miller operating at 300 rpm for 2 hours. To compensate for volatilization during calcination, an additional 5% excess sodium was incorporated. The resultant powder underwent pre-calcination at 450 ºC for 6 hours in a muffle furnace. The decomposed product was subjected to hand grinding in a mortar pestle for an hour. Subsequently, it was irradiated in a microwave furnace at 900 ºC for a mere 12 minutes, slowly cooled, and then transferred into a vacuum atmosphere within a desiccator to prevent any reactions with moisture and carbon dioxide. The process has been schematically illustrated in Fig. 6. The pristine P2-type Na0.67Ni0.33Mn0.67O2(NMNO) was prepared in a similar way except that the final calcination was done in a tubular furnace at 900 ºC for around 14 hours. Materials Characterizations The synthesized sample's phase confirmation was documented through powder X- Ray Diffraction (XRD) patterns using a Rigaku Japan Smart Lab diffractometer equipped with a 3 kW X-ray generator and Cu Kα radiation (λ = 1.5418 Å). Scanning occurred at a rate of 0.7º min-1over an angular range of 10 – 90º at 20 ºC. The determination of lattice parameters was achieved through Rietveld refinement using the Full Prof suite software. To confirm stoichiometry, i.e., the chemical composition of the prepared samples, Inductive Coupling Plasma – Atomic Emission Spectroscopy (ICP-AES) from Spectro Analytical Instruments (GmbH) was employed. The as-prepared powders underwent digestion in a microwave digester utilizing hydrofluoric acid and nitric acid with a 0.5 mg mL-1suspension. Morphological studies of the samples were conducted using an Ultra 55 Carl Zeiss Field Emission Scanning Electron Microscope (FEG-SEM) with an impressive 0.8 nm resolution. Transmission Electron Microscopy (TEM) from Thermo-Fischer Scientific provided high-resolution images, and High-Angle Annular Dark Field (HAADF) imaging with Energy Dispersive X-Ray (EDX) analysis via Scanning Transmission Electron Microscopy (STEM) mode facilitated elemental mapping. The specific surface areas of the samples were measured from N2 adsorption–desorption isotherms obtained by the BET method using a surface area analyser (Nova 1000e, Quantachrome, USA) located at National Institute of Technology Durgapur, West Bengal. Electrochemical measurements Electrochemical performance of the P2-type NMNO and single crystal SC_Nb- NMNO cathode materials were tested in CR 2032-type coin cells for cycling against Na-metal and CR 2016-type coin cell when cycled against pre-sodiated hard carbon. The electrodes were prepared by dispersing the active material, conducting carbon (Super P) and the polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone solvent to form a homogenous slurry. The prepared slurry was then casted on an aluminium current collector through the doctor’s blade technique and dried overnight at 60 ºC in a vacuum oven. The mass loading of active materials on each electrode slide was calculated to be 2 – 3 mg cm-2for the half-cell testing and for the full cell the active material mass loading utilized for hard carbon was 4.33 mg cm-2and single crystal SC_Nb- NMNO was 9.04 mg cm-2. All the cells were assembled in an argon-filled glovebox (Unilab Plus, MBraun, Germany) with O2and H2O levels maintained at ≤ 0.5 and < 0.5 ppm, respectively. For the half-cell, the prepared cathode, used as a working electrode was assembled against the negative electrode composed of sodium metal disc pressed on stainless steel current collector assembled in a 2032- coin cell, whereas, for the full-cell pre-sodiated hard carbon electrode was utilized as the negative electrode and assembled in a 2016-coin cell. Glass fibre was used as a separator in case of the half-cell while a Celgard polypropylene separator was used for the full cell. For pouch cell fabrication, the electrodes of (3 * 5.5 cm) 16.5 cm2area were prepared with SC_Nb-NMNO single crystal cathode and hard carbon anode coated on both side of the aluminium foil. The electrode stack was separated by Celgard polypropylene separator enclosed in a pouch cell casing. A 1 M solution of NaPF6in an ethylene carbonate (EC) / propylene carbonate (PC) mixture (1:1 by weight %) along with 2% fluoroethylene carbonate (FEC) as an additive was used as an electrolyte (approximately 100 μl for the coin cell). The galvanostatic charge-discharge of the Na-ion half and full cells were tested in an Arbin battery testing instrument (BT-2000 model Arbin Instruments, College Station, TX) within a voltage range of 2.5 – 4.3 V at varied current rates. Bio- Logic potentiostat (VMP-3 model, Bio-Logic Science Instruments, France) was used to carry out cyclic voltammetry (CV) experiments at 0.1 mV s-1constant scan rate. Bio-Logic potentiostat (VMP-3 model, Bio-Logic Science Instruments, France) was used to carry out cyclic voltammetry (CV) experiments. All the electrochemical tests were performed at controlled temperature of 20 ºC. Sample preparation for post cycling analysis To assess single crystal SC_Nb-NMNO's efficacy in mitigating exfoliation and particle cracking due to phase transition, relative to NMNO, electrodes subjected to cycling at 0.1 C were scrutinized through ex-situ Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). The cycled electrodes were retrieved from utilized 2032 coin cells within an argon-filled glove box, subsequently cleansed with the solvent (propylene carbonate), and dried within the glove box environment. Post-treatment, the samples were carefully transferred into vials and preserved in a vacuum prior to the initiation of post-cycling analysis experiments. In-Situ Experimentation X-ray diffraction (XRD): XRD is conducted for the NMNO and SC_Nb-NMNO using Bruker D8 Advanced X-ray diffractometer, using Cu Kα radiation source. Powder samples were used for recording the intensities within the 2θ range from 10° to 80° varying at a step of ~ 0.0194°. In-situ cells were assembled in glove boxes filled with highly pure argon as reported previously. These measurements were performed upon the 1st charge / discharge, galvanostatically at approximately the C / 10 rate, assuring synchronization of recordings from the XRD device and the battery tester. This facility was availed at the Bar-Ilan University, Israel. Online electrochemical mass spectroscopy: The in-situ gas evolution during the charge–discharge cycles was monitored using an online electrochemical mass spectrometer (OEMS) from Hiden Analytical at the Bar-Ilan University, Israel. OEMS is conducted in an in-house designed cell as reported previously, we monitored the O2 and CO2 evolution with respect to the voltage profiles for the prisitne, and doped electrodes. OEMS cells were prepared in an argon-filled glovebox by combining the cathode electrode (ø 11 mm) used in combination with Na as the anode with two polypropylene separators (ø 29 mm), and 150 μL NaPF6salt-based electrolyte solution. The cell outlet was connected to the OEMS capillary. The electrochemical measurements were conducted using a VSP potentiostat (BioLogic Science Instruments) within a potential window of 2.5–4.3 V and with a first cycle at C / 15 followed by 0.1C charge–discharge cycles. The partial pressures of the evolved gases were plotted against time. Pre-sodiation of Hard Carbon The issue of low initial Coulombic efficiency in hard carbon anodes for sodium- ion batteries is linked to the declining capacity and unstable cycling associated with the ongoing development and breakdown of the solid electrolyte interface that forms on the surface of hard carbon, thereby degrading the full-cell perfromance. In our previous work, we've shown a strategy to develop an extremely thin, strong, long-lasting, and organic-fluoro-rich solid-electrolyte interface (SEI) on hard carbon electrodes, which was followed by a partial pre- sodiation process including the incubation of hard carbon electrodes wetted with a warm electrolyte rich in fluoroethylene carbonate (FEC) in close proximity to sodium (Na) metal. The artificial SEI that was formed on the surface of partially sodiated hard carbon could almost entirely offset the Na losses that occur during the first sodiation cycle. Lohani et al., had further documented a comparison between SEIs produced electrochemically by incubating hard carbon in direct contact with Na metal and galvanostatically discharging a hard carbon half-cell. After the process of pre-sodiation, the hard carbon underwent initial sodiation / charging and subsequent desodiation / discharging cycles against Na metal prior to its integration into our full-cell setup. Supporting Information Table S1. Crystallographic parameters of P2– Na0.67Ni0.31Mn0.67O2 (NMNO) refined by the Rietveld method. Table S2. Crystallographic parameters of P2– Na0.67Ni0.31Mn0.67Nb0.02O2(SC_Nb- NMNO) refined by the Rietveld method. Table S3. The comparison of the electrochemical performance of P2-type layered oxide cathode material in half-cell configuration with existing literature. Sl. Layered oxide cathode Capacity retention Cycle C- Reference No. active material (%) number rate 1. Na2 / 3Mg1 / 3Mn2 / 3O2 80 100 1 C Dai et al. (2019). High Reversibility of Lattice Oxygen Redox Quantified by Direct Bulk Probes of Both Anionic and Cationic Redox Reactions. Joule, 3, 518–541. Fang et. al. A Fully Sodiated NaVOPO4 with Layered Structure for High-Voltage and Long-Lifespan Sodium-Ion Batteries. (2018). Chem, 4, 1167– 1180. Yang et. al. (2023). Boosting the Ultrastable High-Na-Content P2-type Layered Cathode Materials with Zero- Strain Cation Storage via a Lithium Dual-Site Substitution Approach. ACS Nano, 17, 18, 18616–18628 Cao et. al. (2019). Restraining Oxygen Loss and Suppressing Structural Distortion in a Newly Ti-Substituted Layered Oxide P2- Na0.66Li0.22Ti0.15Mn0.63O2. ACS Energy Lett. 4, 10, 2409–2417. Su et al. (2023). Ca / Mg dual-doping P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode material for sodium ion batteries. Materials Letters, 331, 133425 Na0.67Ni0.31Mn0.67Nb0.02O2 >90 2000 1 C Our work (SC_Nb-NMNO) Na0.67Ni0.31Mn0.67Nb0.02O2 >95 100 0.1 Our work (SC_Nb-NMNO) C Table S4. Average stable areal capacity and Coulombic efficiency, and their standard deviation of SC_Nb-NMNO / / Hard carbon sodium-ion full-cell. When transitioning from Ti to Nb as a dopant in the synthesis of nanocrystals, the change in crystal structure from polycrystalline to single crystal is because of the following; Influence of Nb as a Dopant: The presence of niobium (Nb) as a dopant influences the crystal growth mechanism of the crystals during synthesis. Nb doping alters the kinetics and thermodynamics of crystal growth, promoting the formation of larger, more ordered single crystals instead of multiple smaller crystallites. Also, the presence of Nb atoms within the transition metal oxide lattice alters its electronic structure and bonding characteristics, leading to improved crystallinity and a preference for single-crystal formation. Enhanced Crystallinity: The substitution of Ti with Nb enhances the structural order within the nanocrystals. By modifying the crystal lattice through doping, the arrangement of atoms becomes more uniform and aligned, favouring the growth of larger, well- defined single crystals with reduced defects and grain boundaries. Synthesis Optimization: Adjusting synthesis parameters such as the pre-calcination temperature, which is 450 °C for the Nb doped cathode material and doping levels, which is 2% of Nb with respect to 16% of Ti can further optimize the formation of single crystals with Nb doping. Fine-tuning these factors allows for precise control over crystal growth and orientation. In summary, changing the dopant from Ti to Nb in nanocrystal synthesis induces a shift towards single-crystal formation by modifying the crystal growth mechanism, enhancing crystallinity, and optimizing synthesis conditions. This transition is driven by the influence of Nb doping on structural properties and growth dynamics, ultimately leading to the production of high-quality nanocrystals with improved single-crystal characteristics. In comparison to the previous study published in [Nat Commun 13, 3205 (2022)], which explored a similar material with the same structure and similar elemental composition, but with niobium showing segregation on the surface. In contrast, present study reveals a homogeneous distribution of niobium throughout the single crystal, specifically doped at the nickel site within the bulk, without inducing atomic-scale surface reorganization. This distinction could be attributed to variations in synthesis technique and parameters, including precursor composition, reaction conditions, and thermal treatment, which can significantly influence the distribution of dopants within the material. In our case we have used a microwave assisted solid-state method, whereas, they have used co-precipitation and conventional solid-state method for their synthesis. Unlike conventional heating, which requires around 12-20 hours and high energy consumption for synthesis of similar composition, microwave irradiation induces rapid solid-state reaction as it heats the material on a molecular level leading to a time-efficient and uniform heating while retaining the crystallinity of the structure. The reduced synthesis time could also be a great practical method to commercially synthesize and develop layered transition metal oxide cathode for Na-ion batteries at a low cost. Further to support the doping of Nb in the bulk, present inventors have included additional characterization techniques such as transmission electron microscopy with FIB pre-treatment and Time-of-Flight Secondary Ion Mass Spectrometry analysis, as detailed in the revised manuscript and supplementary information. Moreover, present invention focuses on elucidating a synergistic stabilization effect in a Nb-doped P2-type single crystal cobalt-free layered oxide cathode material, demonstrating remarkable cycling stability and high-power performance for Na-ion batteries. We show that the introduction of Nb in the transition metal layer not only reduces the electronic band gap but also enhances electronic conductivity and mitigates ionic diffusion energy barriers, contributing to the stabilization of the host structure. Notably, the presence of single crystals in present material improved various kinetic factors, demonstrating the profound correlation between structural defects and chemical proliferation, thereby reducing the evolution of oxygen gas. Practical assessments in half and complete cell setups, including pouch cell configurations, were conducted to validate the material's performance under realistic battery operating conditions. The present inventors observed the superior performance of present synthesized SC_Nb- NMNO at a very high C-rate of 5 C, as shown in Fig. 2c and in fact, present invention reveals a capacity retention of >90% after a prolonging cycling of 2000 cycles at a high C-rate of 1 C, as displayed in Fig.2g. Therefore, the combination of novel doping strategies, comprehensive characterization, and practical assessments distinguishes present invention and underscores its significant contribution to advancing sodium-ion battery technology.

Claims

CLAIMS:

1. A process of synthesis of a cathode material including heteroatom doped, single crystal, cobalt-free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2 for Na-ion batteries wherein x is preferably 0.02, comprising: ^ mixing vigorously precursor materials in a certain ratio in a ball miller at a 300 rpm for 2 hours; ^ pre-calcining the resulting powder from the ball miller at 450 °C for 6 hrs in a muffle furnace; ^ hand grinding the decomposed product obtained from the muffle furnace in a mortar-pestle for an hour; ^ calcining the product by microwave irradiation at 900 °C with a reaction time of 12 minutes; ^ cooling slowly and transferring into a vacuum atmosphere within a desiccator to prevent any reactions with moisture and carbon dioxide.

2. The process as claimed in claim 1, wherein said Na0.67Ni0.33-xMn0.67NbxO2 layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2.

3. The process as claimed in claim 1, wherein said precursor materials comprises anhydrous sodium carbonate (Na2CO3), niobium oxide (Nb2O5), Nickel acetate ((Ni(OCOOCH3)2.4H2O), and Mn acetate ((Mn(O(COOCH3)3.2H2O).

4. The process as claimed in claim 1, wherein said precursor materials are vigorously mixed in the molar ratio of 0.67:0.31:0.67:0.

02.

5. A electrochemical cell comprising: a cathode material including heteroatom doped, single crystal, cobalt-free P2-type layered oxide represented by Na0.67Ni0.33-xMn0.67NbxO2 for Na-ion batteries wherein x is preferably 0.02 and an anode material including pre-sodiated hard carbon.

6. The electrochemical cell as claimed in claim 5, wherein the Na0.67Ni0.33-xMn0.67NbxO2 layered oxide is Na0.67Ni0.31Mn0.67Nb0.02O2.

7. The electrochemical cell as claimed in claim 5, wherein the electrochemical cell is a sodium ion battery.

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