Probing the account of phase transition upon electrochemical cycling of the P2-Na0.67Ni0.15Fe0.2Mn0.65O2 layered oxide cathodes for sodium-ion batteries

Layered P2-Na0.67Ni0.15Fe0.2Mn0.65O2 (P2-NFM) cathode material has attracted great attention in sodium-ion batteries due to its high theoretical capacity, low cost, and environmental friendliness. However, P2-NFM exhibits irreversible phase transition and slip of transition metal layers in the high voltage range during charging process, leading to a gradually declined performance of the cathode material. It is therefore necessary to investigate the mechanism of phase transition of P2-NFM as well as the effect of phase transition on its performance. Herein, utilizing ex situ x-ray diffraction spectroscopy and x-ray photoelectron spectroscopy, the crystal structure and TM (transition-metal) bonding changes caused by phase transition are elucidated. It is found that P2-NFM is prone to undergo an irreversible P2-O2 phase transition at high voltage, causing changes in lattice parameters and rapid capacity decay. The irreversible phase transition is mainly due to he dynamic transformation of valence states of Fe and Ni in P2-NFM materials at high voltage. It is this process that results in irreversible fluctuations in the bond lengths between these elements and oxygen, consequently instigating interlayer slip within the material. Besides, the charge compensation mechanism of P2-NFM has been elucidated based on the study of its initial charging process. Results show that the charge compensation is mainly contributed by Ni and Fe in the high voltage range, while by a small amount of Mn in the low voltage range. It reveals the essential cause of the adverse phase transition of P2-NFM materials and points out the direction for improving the cycling stability of these layered oxide materials.


Introduction
In contrast to lithium, sodium is ubiquitously distributed and abundant.Sodium-ion batteries (SIBs) are considered as one of the most promising systems to replace Lithium-ion batteries [1].The electrochemical metrics of SIBs are predominantly shaped by the attributes of the cathode material, encompassing considerations such as energy density and cycle longevity [2].Layered transition-metal oxides (Na x TMO 2 , TM = transition metal), especially for P2-type, recognized by their substantial specific capacity, superior ionic conductivity, and facile synthesis procedures, stand out as among the preeminent choices for the cathode material [3].
Due to the abundant natural resources and easily development of Fe and Mn, as well as the advantages of the Fe 3+ /Fe 4+ and Ni 2+ / 3+ / 4+ redox couples providing high capacity, Ni/Fe/Mn-based layered cathode materials have become a popular research topic [4].However, several challenges still need to be addressed.Firstly, after charging to approximately 4 V, the material undergoes an irreversible P2-O2 phase transition and the transition metal layer slip, causing a gradually decreased performance of the layered structure [5].Secondly, there is a large amount of Mn 3+ present in the low voltage region, which causes distortion in the crystal structure due to the Jahn-Teller effect [6].Finally, the complexity of the Ni/Fe/Mn-based layered cathode materials system has hindered thorough investigation into the local evolution of transition metal ions (TM) regarding their valence and bonding structures.Consequently, the phase transition mechanism remains poorly understood, lacking clarity [7].
The phase transition issue of layer-structured cathode materials based on P2-type nickel-manganese and nickel-iron-manganese has been extensively studied by numerous scholars both domestically and internationally.Zheng et al investigated the phase transition of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 material during the first charge and discharge process using in situ x-ray diffraction technique [8].The results showed that the electrode maintained a P2 structure at a low charging voltage range.At the voltage of 4.1 V, the characteristic peak (002) of the P2 phase began to shift towards lower angles, while the peaks (100), (102), and (103) shifted towards higher angles, indicating the expansion of the c-axis.When the voltage reached around 4.2 V, the characteristic peak (002') of the O2 phase appeared, and the electrode material existed in a mixed form of P2 and O2 phases.The volume of the unit cell changed up to 19.4% during the entire P2-O2 phase transition process.Compared to the irreversible structural evolution of P2-O2 phase, materials capable of generating OP4 or Z-phase structures exhibit improved cycling performance [9].Feng et al found that P2-Na 0.67 Ni 0.1 Mn 0.8 Fe 0.1 O 2 material began to transform from the P2 phase to P2+'Z' composite phase when charged above 4.1 V [10].When the material charged to the cut-off voltage of 4.5 V, the 'Z' phase contained more O phase, but it still did not completely transform into the O2 phase.Hence, the P2-Na 0.67 Ni 0.1 Mn 0.8 Fe 0.1 O 2 material had a first discharge capacity of 172.4 mAh g −1 within the voltage range of 2-4.5 V with a capacity retention rate of 83.5% after 100 cycles.Additionally, when the P2-phase material discharged to a lower potential, additional Na + were embedded into the system structure.In particular, in Mn-based materials, these additional embedded Na + further promoted the reduction of Mn 4+ to Mn 3+ , causing the Jahn-Teller effect, and leading to lattice distortion of the transition metal octahedron and the transformation of P2 phase to P′2 phase [11].However, in order to address the underlying issues with materials, further research is needed on the phase transition process and sodium storage mechanisms of the material.Transition metals (TMs) play a crucial role in layered cathode materials in terms of charge compensation, average electrode potential, air stability, and structural stability.Firstly, TMs act as redox couples to provide capacity in the electrode through charge compensation mechanisms [12].Secondly, the electrode potential is dependent on the valence state, ionic radius, electronegativity, and local environment of the cations in the electrode material [13].The more energy released or consumed during electrons inserting into or promoting from TM orbitals, generally represents the higher potential of the electrode material.Thirdly, the air stability of layered cathodes can be significantly improved by substituting some TMs with cation doping [14].Water molecules in the air can easily enter the sodium layer of the layered cathode, leading to an increased interlayer distance and impurities, making the layered electrode unstable in air [15].The structural stability of the electrode is significantly affected by Jahn-Teller distortion or TM migration, for example, Ni 3+ , Mn 3+ and Fe 3+ ions exhibit strong Jahn-Teller effects, causing crystal distortion and hindering Na + diffusion.Furthermore, previous studies have found that the migration of Fe from octahedral positions in TM layers to the sodium layer may lead to unfavorable structural changes and capacity degradation in layered cathodes [14,16].
That is, a significant theoretical capacity is inherent in the majority of Ni/Fe/Mn-based layered materials drawing on the aforementioned researches.However, the large-scale application is impeded by the occurrence of irreversible phase transitions and the subsequent manifestation of inadequate structural stability.Based on the influence exerted by TMs on the phase transitions, charge compensation, and air stability of layered cathode materials, this paper systematically analyzes the electrochemical performance and phase transition mechanism of the P2-Na 0.67 Ni 0.15 Fe 0.2 Mn 0.65 O 2 (P2-NFM) material.Through the utilization of potential-resolved in situ electrochemical impedance spectroscopy (PRIs-EIS) and ex situ x-ray diffraction (XRD) analysis, we discovered that the bond length variation between transition metals and oxygen is irreversible during the charge-discharge process.This irreversible change leads to interlayer sliding and consequently triggers irreversible phase transitions.Furthermore, we meticulously examined the profound impact of these phase transitions on material performance, laying a solid foundation for a deeper comprehension of the underlying mechanisms and the interplay with material behavior.This study provides valuable insights into the phase transition mechanisms of layered cathode materials, serving as a guiding reference for further research in this field.

Experimental section 2.1. Materials synthesis
A co-precipitation technique was employed to fabricate the P2-NFM layered cathode material.A transition metal salt solution, with a concentration of 0.5 mol l −1 , was prepared by dissolving Mn( in a specific quantity of deionized water at a predetermined chemical molar ratio.A precipitant was then prepared by dissolving 9.42 g of Na 2 CO 3 and 0.44 g of NaOH in 200 ml of deionized water.The precipitant was added dropwise into the transition metal salt solution, followed by a reaction period of 6 h, and subsequent washing, filtration, and drying steps to obtain the precursor powder.The first calcination step was conducted at 450 °C for 6 h with a heating rate of 5 °C min −1 .The resultant powder was then ground, dissolved in 100 ml of anhydrous ethanol with Na 2 CO 3 , and stirred in an oil bath at 50 °C for preliminary drying.The sample was further dried at 120 °C under vacuum to remove moisture.The second calcination step was carried out at 900 °C for 12 h with a heating rate of 3 °C min −1 .The final product, P2-NFM, was obtained by natural cooling and stored in an argon-filled glove box [17,18].

Materials characterization
The crystal structures of cathode materials were characterized by XRD (Empyrean-100) from 10°to 80°at scan rate of 2°min −1 .XRD refinements were executed by the Rietveld method using the General Structure Analysis System (GSAS-EXPGUI) software [19].For the ex situ XRD measurements, the charged or discharged electrode materials were soaked in the dimethyl carbonate (DMC) solvent for 4 h and dried for 1 h in the vacuum oven [20].The microstructure and morphology were observed by scanning electron microscope (SEM, ZEISS Gemini 300) and transmission electron microscopy (TEM, JEC-2100).The surface elemental compositions and valence states were analyzed by the x-ray photoelectron spectroscopy (XPS) spectra (AXIS UltraDLD x-ray photoelectron spectrometer) [21,22].For the ex situ XPS measurements, the charged electrode materials were soaked in the dimethyl carbonate (DMC) solvent for 4 h and dried for 1 h in the vacuum oven.The elemental content was characterized by inductively coupled plasma mass spectrometry (ICP-OES, PerkinElmer NexION 300X).The Energy Dispersive Spectroscopy (EDS) was used to analyze the type and content of the elements in the microarea of materials.

Electrochemical measurements
Coin half cells (CR2032) of P2-NFM/Na were assembled in an argon-filled glovebox.The working electrode was prepared by coating the mixture of P2-NFM active material (80 wt%), Super P (10 wt%) and polytetrafluoroethylene (10 wt%) in N-methyl-2-pyrrolidone onto an Al foil.The loading mass of active material in each electrode pellet of 14.0 mm in diameter was 3.2 mg cm −2 .The counter electrode was sodium foil.The electrolyte was 1.0 M NaClO 4 in the mixed solvent of ethylene carbonate/diethyl carbonate (1:1, v/v), and the separator was glass fiber filter [17,23].The electrochemical performance tests of the P2-NFM/Na half cells were conducted on a LAND CT2001A tester (Wuhan, China) in voltage ranges of 1.5-4.3V.The cycle performance was tested at 0.1 C (1 C = 1.95 × 10 −3 mA cm −2 ), and the rate performance was conducted from 0.1 to 10 C taking five cycles at an interval.Cyclic voltammetry (CV) and PRIs-EIS tests of the P2-NFM/Na half cells were carried out with a CHI660E electrochemical workstation.For CV test, the voltage range is from 1.5 V to 4.3 V, and the scan rate is 0.1 mV s −1 .For PRIs-EIS test, in situ EIS procedure is superimposed on staircase potential ramp (increased by 5 mV each time) over the potential range from open-circuit voltage (OCV) to 4.3 V in the frequency range of 10 5 -10 −1 Hz with the sinusoidal AC perturbation of 5 mV [24].

Results and discussion
P2-NFM cathode materials obtained by co-precipitation method were subjected to XRD diffraction tests, and the crystal structure of the material was further analyzed using GSAS/EXPGUI software.The XRD refinement results in figure 1(a) show that characteristic peaks of the P2 phase are significantly present in the XRD diffraction spectrum, such as the ∼16°(002) peak, ∼39°(012) peak and ∼49°(104) peak, all of which point to a hexagonal lattice with the space group of P63/mmc [25].The atomic occupancy information and unit cell parameters obtained from the refinement are presented in table S1 (Supporting Information), and the unit cell parameters of P2-NFM are a = b = 2.9079 Å, c = 11.2091Å, and V = 82.307Å 3 , which are in good agreement with those reported P2-type materials [19].Figure 1(b) shows a schematic of the crystal structure of P2-NFM, where Ni, Fe and Mn ions mainly occupy octahedral sites in the transition metal layer, and O (red spheres) combines with Na + (white spheres) to form a trigonal prism of the sodium layer.The oxygen layers are stacked in an ABAB sequence, accumulating and repeating in this way to form the crystal structure of P2-NFM.
Figures 2(a) and (b) show the SEM images of P2-NFM.The primary particles of the sample exhibit a hexagonal plate-like structure with an average size of 1-2 μm, and the particle surfaces are smooth with clear edge structures, indicating the good crystallinity of the prepared P2-NFM.These primary particles stack in a layered manner to form secondary particles.TEM characterization further confirms the spatial structure, space group, and lattice fringes of P2-NFM.The interplanar spacing of the material is 0.56 nm, corresponding to the (002) plane of the P63/mmc space group, as shown in figures 2(c) and (d) [26].The regular diffraction spots of P2-NFM also indicates its good crystallinity.The uniform distribution of elements in the material is crucial for its electrochemical performance.EDS testing reveals that the elements of Na, Mn, Fe, Ni, and O are uniformly distributed in P2-NFM (figure 2(e)).Subsequently, the elemental composition of P2-NFM is determined by ICP-OES, and the molar ratio of Na: Ni: Fe: Mn is found to be 0.673: 0.151: 0.214: 0.652, consistent with the designed stoichiometry.The above results indicate that the P2-NFM with uniformly elements distribution can be synthesized by co-precipitation method as expected.
The charge-discharge tests were carried out at the rate of 0.1 C within the voltage range of 1.5-4.3V.The result, showed in figure 3(a), reveals that the P2-NFM material exhibits an impressive initial charge capacity of 205.9 mAh g −1 and an initial discharge capacity of 195.6 mAh g −1 , with a first-cycle coulombic efficiency of 94.9%.It indicates that Na + could not be fully reversibly intercalated into the host structure after being extracted from the material, possibly due to the irreversible phase transition caused by the electrochemical extraction process [27].Furthermore, two distinct long voltage plateaus are observed near 4.1 V in the charge curve and 3.5 V in the discharge curve, indicating a phase transition of the material at these voltages.This phase transition is also evident across different cycling numbers (figure 3   material under higher discharge rates.The dQ/dV curve of the first charge-discharge cycle is illustrated in figure 3(e).The peaks at 4.1 V and 3.8 V are corresponding to the charge and discharge plateau, respectively.These peaks is in accordance with the voltage positions of the irreversible phase transition of P2-O2, further supporting the notion that the irreversible phase transition of P2-O2 is a major contributor to the capacity decay of P2-NFM.
The investigation of the degradation mechanism in P2-NFM materials concerning their capacity decay can be conducted by means of CV testing.Figure 3(f) shows the CV curves of P2-NFM cathode material in a voltage range of 1.5 V to 4.3 V and a scan rate of 0.1 mV s −1 .The main characteristic oxidation-reduction peaks can be observed at 2.4/1.9V and 4.2/3.4V.Moreover, the broad peak at 3.7 V is caused by the loss of electrons from Ni 2+ to Ni 3+ during the charge compensation process.Generally, Ni 3+ is oxidized to Ni 4+ during the subsequent charge compensation process, coupled with the oxidation process of Fe 3+ /Fe 4+ redox couple.Therefore, the peak at 2.4/1.9V corresponds to the oxidation and reduction processes of the Mn 3+ /Mn 4+ couple, while the peak at 4.2/3.4V corresponds to the oxidation and reduction processes of the Fe 3+ /Fe 4+ and Ni 3+ /Ni 4+ couples.During cycling, the position of the oxidation-reduction peaks and the peak current of the P2-NFM cathode materials show slight changes.Meanwhile, the oxidation-reduction peak at 4.2/3.4V corresponds to the P2-O2 phase transition of the material [28].Since the P2 to O2 phase transition is not completely reversible, this process is an important cause of the irreversible capacity loss of the material.Additionally, the small voltage shift of the oxidation peak in the CV curve during cycling indicates that the material undergoes partial lattice distortion, which reduces its cycling capacity retention [29].
The phase transition during charging and discharging directly affect the migration and diffusion of Na + within the material, leading to changes in the rate performance of the material.The Na + diffusion coefficient (D Na + ) of P2-NFM material has been calculated by its CV curves (Figure S1, Supporting Information) at different scan rates (0.1, 0.2, 0.3, 0.5 and 1.0 mV s −1 ).The oxidation-reduction peak in the CV curve is related to the phase transition process.The peak intensity increases as the scan rate increases, and the accordingly changed peak current and shape is closely related to the Na + transport rate in the material.Linear fitting result of the peak current (i p ) and the square root of the scan rate (v 1/2 ) shows good linear relationships for both the Na + extraction and insertion processes, indicating significant diffusion control behavior of the material during charging and discharging.The D Na + can be calculated using the Randles-Sevcik equation [30]: where n is the charge transfer number, A is the area of the electrode (current collector), C is the Na + concentration, and D corresponds to the diffusion coefficient of Na  s −1 , respectively.These results strongly indicate that P2-NFM has good Na + conductivity, thereby exhibiting excellent kinetic performance.
As reported, impedance information can be dynamically displayed throughout the entire charging process by employing PRIs-EIS testing and a scatter plot [31].The differential Nyquist top diagramt (DNTD) and 3D Bode-phase graphs of P2-NFM material in the NaClO 4 electrolyte system were obtained through PRIs-EIS testing, as depicted in figures 4(a), (b).The color gradient bands in the DNTD plot corresponds to the impedance magnitude of the corresponding kinetic process, with the width of the color gradient band representing the diameter of the half circle in the traditional Nyquist curve.The color gradient bands appeared in the highfrequency region are related to the charge transfer impedance (R ct ) of Na + at the electrode/electrolyte interface, while the color gradient bands appeared in the mid-frequency region are related to the impedance (R cei ) of Na + passing through the cathode solid electrolyte interface membrane.The red color band appeared in the lowfrequency region is related to the diffusion impedance (R w ) of Na + in the material bulk phase [24].Notably, the appearance of several color gradient bands in the DNTD plot indicates the presence of multiple time constants, which manifest as peaks in figure 4(b).By monitoring the number of characteristic time constants, it is possible to determine whether a new interface has formed at the electrode/electrolyte interface.Additionally, changes in impedance can reflect the Na + transport resistance at the cathode/electrolyte interface.
As shown in figure 4(a), the color gradient band I in the high frequency region persists throughout the entire voltage increase process, which gradually widens as the voltage above 4.2 V.The color gradient band I primarily reflects the impedance dynamic changes of Na + at the electrode/electrolyte interface in the high frequency region.The main reason for its widening is that as the voltage increases, ion reconstruction occurs on the surface, resulting in a higher activation energy and an increase in charge transfer resistance.At a voltage of 2.68 V, the color gradient band I in the high frequency region does not change significantly, while the red color band in the low frequency region gradually widens, indicating an increase in the impedance (R cei ) of Na + passing through the cathode solid electrolyte interface membrane.The main reason for this change is the valence state change of Mn ions, which causes crystal distortion, as evidenced by the Mn 3+ /Mn 4+ oxidation peak in the above CV curve.When the voltage increases to 3.0 V, a new color gradient band II appears in the mid-frequency region, indicating the production of a new phase in this process.However, this gradient band disappears when the voltage reaches 4.2 V, mainly because the material undergoes a P2-O2 phase transition in this voltage range.Above 4.2 V, the increased R cei impedance is possibly due to the presence of O2 phase, which increases the diffusion barrier of Na + and slows down its diffusion rate.The variation in R ct and R cei values at the important voltage points is shown in figure 4(c), and it can be confirmed that the fitted impedance value changes can also prove our impedance change analysis.Generally, by correlating the changes in the material's structure and impedance, it can be concluded that the changes in diffusion impedance are caused by the valence state changes of the transition metal and the changes in Na + diffusion rate caused by irreversible phase transitions.The three important voltage points (2.68 V, 3.0 V, and 4.2 V) with significant changes in impedance in the in situ impedance diagram also provide data references for subsequent ex situ XRD studies.
In order to elucidate the intricate mechanism and structural evolution of Na + de-intercalation and intercalation in P2-NFM material crystal during the charge and discharge processes, ex situ XRD characterization was conducted for the first charge and discharge cycle of P2-NFM cathode material, as depicted in figures 5(a), (b) [32,33].The XRD pattern of the uncharged material electrode displays a similar pattern to that of the initial synthesized material, suggesting a P2-type structure.As the initial charging process commenced, Na + is released from the structure, leading to the gradual shift of the (002) characteristic diffraction peak of P2 phase to a lower angle.Until the high voltage of 3.9 V is reached, the characteristic peak starts to shift towards higher angles.The (100) and (012) peaks shifting to higher angles is mainly due to the oxidation reaction of the active transition metal elements with increasing voltage, resulting in an ion radius reduction, and causing a contraction of the crystal plane and a decrease in the lattice parameter of a [34,35].According to Bragg's law, 2dsinθ = nλ, the interlayer spacing and lattice parameter of c gradually increases as Na + releases from the structure, primarily due to the increased electrostatic repulsion between transition metal layers and resulting in a displacement slip between the layers [36].When charged to 4.1 V, a new peak emerges in the XRD spectrum at around 20.1°, and gradually shifts to lower angles with the increasing voltage, which is ascribed to the characteristic peak of the O2 structure in previous literature [37].Compared with the P2-phase triclinic configuration, the octahedral configuration of O2-phase significantly reduces the crystal volume, which poses a threat to the structural stability of the material.Furthermore, it is observed that some characteristic peaks of P2 phase, such as (004), (100), (003), ( 012) and (104), become indistinct during the transition from the initial Narich state to the high voltage Na-poor state, signifying a coexistence of P2 and O2 phases.During discharging, the remained characteristic peak of O2 phase indicates that irreversible phase transition occurs between the P2 and O2 phases during the de-intercalation and intercalation of Na + [38,39].
The lattice parameter curves for lattice parameters of a and c, layer spacing of d, unit cell volume of V (figure 6), as well as the bond length change curves for transition metals and oxygen (figure 7) were given by analyzing the XRD data of P2-NFM material with varying degrees of sodium removal.The cell parameter of a and the volume of V of P2-NFM material decreases gradually with the increasing voltage, exhibiting change rates of 1.42% for a and 3.03% for V.The pronounced reduction in V is attributed to the irreversible P2-O2 phase transition, which distorted the P2 structure from a trigonal prism to an octahedron along the a axis [40].Both of the interlayer spacing of d and the lattice parameter of c of P2-NFM material increase gradually with the increasing voltage, displaying smooth curves and small changes.The bond length curves of Fe-O and Ni-O exhibits inflection points in the voltage range of 2.8-4.2V and 3.6-4.2V, respectively, with an increasing rate of bond length reduction, indicating a change in valence state of Fe and Ni.This resulted in a contraction in the crystal surface and a decrease in lattice parameter of a, as depicted in the corresponding curve [41].The bond lengths of Fe-O and Ni-O also exhibit fluctuations in the discharge process, consistent with the production of O2 phase in the crystal structure when the material discharged to 1.5 V.The bond length of Mn-O displayed a larger change in the low voltage range (1.5-2.4V), primarily due to the transformation of Mn 3+ with highly electrochemical activity to stable Mn 4+ with lower electrochemical activity, as evidenced by the Mn 3+ /Mn 4+ redox peak cutoff voltage range.Subsequently, Mn 4+ become the dominant valence state in P2-NFM material, reducing the Jahn-Teller distortion caused by Mn 3+ and increasing the reversibility of the change in Mn-O bond length [42].
To investigate the charge compensation mechanism of P2-NFM material, we have performed ex situ XPS characterization on P2-NFM material at different cut-off voltages during the initial charging process.The Avantage software has been used to analyze and fit the valence states of TM elements on the electrode surface under various test conditions, in order to elucidate the dynamic changes of TM elements valence states during the charging process of P2-NFM material [43].

Conclusion
In summary, this study elucidates the phase transition mechanism of P2-NFM cathode material and explains the effect of changes in material structure and bond lengths of transition metal ions on electrochemical performance.It has been confirmed that although the P2-NFM cathode material demonstrates a substantial discharge capacity, its cycling performance is compromised by the emergence of an irreversible P2-O2 phase transition throughout the charge-discharge cycle.The characterization of XRD combined with PRIs-EIS shows that P2-NFM undergoes an irreversible P2-O2 phase transition at high voltage, leading to changes in the crystal cell volume and rapid capacity decay.Furthermore, we demonstrate that the change in bond length of Fe and Ni in P2-NFM at high voltage ranges is a significant factor causing the material to undergo irreversible phase transition.It reveals the essential cause of the irreversible phase transition of P2-NFM materials and points out the direction for improving the cycling stability of these layered oxide materials.For example, the interaction between the TM layer and the oxygen layer can be strengthened by doping low-valence metal ions (such as Mg 2+ and Li + ) which have stronger bonding effects with oxygen.
(b)), indicating that the impact of this irreversible phase transition on the material structure persists in each electrochemical cycling process.The constant current cycling performance of the P2-NFM cathode at a rate of 0.1 C for 50 cycles is displayed in figure3(c).The capacity dropping from 193 mAh g −1 to 116 mAh g −1 and the capacity retention rate of only 60.7% further demonstrates the rapid capacity decay of the material after charging to 4.3 V due to structural degradation.The rate performance of P2-NFM shown in figure 3(d) displays that after cycling at rates of 0.1, 0.2, 0.5, 1, 2, 5 and 10 C and reverting to 0.1 C. It is evident that the cell maintains a capacity close to 70 mAh g −1 at 2 C.However, as the rate escalates to 5 C and 10 C, the discharge capacity of the material diminishes drastically, nearing zero.Moreover, upon reverting to lower current densities, it barely sustains an average discharge specific capacity of 86 mAh g −1 .These observations collectively underscore the occurrence of irreversible structural damage to the
+ during the extraction and insertion process.The value of D Na + can be calculated from the curve fitting results in figure S1.For P2-NFM material, the D Na + values during the Na + extraction and insertion processes are 3.83 × 10 −10 cm 2 s −1 and 1.78 × 10 −10 cm 2

Figure 4 .
Figure 4. (a) Differential Nyquist top diagramt from PRIs-EIS, (b) 3D Bode-phase graphs and (c) R ct and R cei values measured at selected potentials during the charging process of P2-NFM.

Figure 5 .
Figure 5. Evolution of the electrochemical behavior of P2-NFM compounds during the initial charge-discharge process: (a) The ex situ XRD patterns collected during the 1st cycle of cathode electrode under 0.1 C, (b) The image plots of the (002) peak reflection.

Figure 6 .
Figure 6.(a) Lattice parameter of a, (b) Lattice parameter of c, (c) Layer spacing of d and (d) Unit cell volume of V of P2-NFM under different charge/discharge states.