Mitigating “Electrostrictive” Coupled‐Disruption and Crafting Endogenous Solid‐Liquid Interface Toward Mixed Phosphate Cathode for Sodium‐Ion Batteries

ABSTRACT

Na₄Fe₂Mn(PO₄)₂(P₂O₇) has received widespread attention due to high energy density and less structural variations. However, its rate capability and cycling performance are far inferior to expectations. This study unveils underlying failure mechanisms for the performance degradation: “electrostrictive” coupled‐disruption driven by charge changes causes the Na⁺ channels closure, while irregular cathode electrolyte interphase (CEI) growth hinders interface Na⁺ diffusion and causes transition metal dissolution. Therefore, halogen elements (F, Cl, Br) are introduced into the material through defect‐engineering. The strong electronegativity of F and the spatial effects of Cl/Br effectively regulate the coordination environment to suppress the coupled‐disruption. Furthermore, the surface halogen elements spontaneously combine with Na⁺, ultimately forming uniform, surface organic‐rich and interior inorganic‐rich CEI layers. Based on this, the modified material shows high‐rate performance (55.0 mAh g⁻¹ at 200 C) with ultra‐long cycle stability (98% after 20000 cycles at 50 C) and exhibits excellent electrochemical performance in full‐cell and all‐solid‐state battery applications.

This study unveals additional phase transition processes in NFPP through in situ XRD, leading to the discovery of “electrostrictive” coupled‐disruption in iron/manganese‐based mixed phosphates. A modification strategy by introducing halogen elements through defect‐engineering is proposed to ensure a stable bond structure and uniform and stable CEI layers, resulting in sodium‐ion batteries with high energy density and ultra‐long cycle life.

1. Introduction

To address the growing energy demand, renewable energy sources‐including wind, hydro, and solar power‐are undergoing rapid expansion. The effective storage of these renewable energy is crucial for achieving sustainable development [1, 2, 3]. Notably, sodium‐ion batteries have emerged as a promising candidate for large‐scale energy storage systems (ESSs), owing to the virtually inexhaustible supply of sodium resources and their electrochemical similarities to lithium‐ion batteries [4]. Nevertheless, the substantial size of Na⁺ and their comparably modest standard operating potential necessitate the pursuit of positive electrode materials for sodium‐ion batteries endowed with elevated energy density, prolonged cycling longevity, and an open lattice structure. Its pursuit holds pivotal significance in propelling the advancement of large‐scale energy storage systems [5, 6].

In recent years, propelled by the fluctuating prices of lithium resources, sodium‐ion batteries have undergone rapid development. Layered oxide and polyanion cathode materials have emerged as robust contenders for the forthcoming generation of commercial sodium‐ion battery cathode materials [7, 8, 9, 10]. Notably, mixed‐phosphate, with the chemical formula Na₄M₃(PO₄)₂P₂O₇ (M = Mn, Fe), has garnered significant interest on account of its inherent open Na⁺ transport channels and stable crystal structure [11, 12, 13].

In view of the low working potential exhibited by the Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) electrode [14, 15], as well as the inadequate structural stability observed in the Na₄Mn₃(PO₄)₂(P₂O₇) electrode [12], Na₄Fe_{3− z} Mn _z (PO₄)₂(P₂O₇) composite has been widely studied by researchers due to its combination of the advantages of both [16, 17, 18, 19]. Among them, Na₄Fe₂Mn(PO₄)₂(P₂O₇) (NFMPP) has the smallest volume change during the charge and discharge, but its overall electrochemical performance is still limited by various factors [20]. However, the majority of studies predominantly concentrate on doping modifications [21, 22], with scant exploration of the failure mechanisms of materials. Such a narrow focus hampers our comprehensive comprehension of the structural evolution and modification mechanisms of materials.

Previous research [23, 24, 25] have traditionally posited the presence of three phase transition processes in NFPP. However, this study successfully identifies a fourth phase transition process in NFPP, thereby unveiling the “electrostrictive” coupled‐disruption in NFMPP through comparison. The “electrostrictive” coupled‐disruption driven by charge changes leads to abnormal phase transitions, resulting in the closure of Na⁺ channels and structural collapse. The subtle internal variations in the crystal structure are difficult to be observed by characterizations such as conductivity, Na⁺ diffusion kinetics, and unit cell volume. Therefore, they are often overlooked by researchers. Furthermore, this study unveils the irregular cathode electrolyte interface growth in NFMPP. The formation of an uneven, weak cathode electrolyte interface (CEI) layer due to side reactions between the electrode and electrolyte impedes the diffusion of Na⁺ at the interface and results in transition metal (TM) dissolution and electrolyte loss. The dual failure mechanism contributes to voltage decay and capacity loss during cycling and voltage polarization at high rates. Inspired by the inorganic salt components in the solid electrolyte interface layer [26], this study proposes a modification strategy based on halogen element doping. Oxygen vacancies are innovatively introduced through defect engineering, promoting the occupation of halogen elements X (X = F, Cl, Br). The surface F, Cl, and Br elements spontaneously form a pre‐formed CEI layer during charge and discharge processes. It eventually leads to form a uniform and stable surface organic‐rich and interior inorganic‐rich CEI layers. As the CEI layer at the material interface is enriched with inorganic components such as NaF, NaCl, and NaBr, it significantly enhances the material interface diffusion kinetics and effectively suppresses the TM dissolution. Additionally, the Mn‐O coordination environment is enhanced due to the strong electronegativity of F, restraining the lattice disorder induced by “electrostrictive” coupled‐disruption. Simultaneously, Br and Cl, as weak‐field ligands, effectively mitigate electrostatic repulsion in materials during charge–discharge processes through their spatial effects. The synergistic regulation among halogen elements can be described as a balance of rigidity and flexibility. As a result, the Na₄Fe₂Mn_0.91(PO_3.91(F, Br, Cl)_0.03)₂P₂O₇/C cathode exhibits a breakthrough in electrochemical performance, with astonishing rate performance (55.0 mAh g⁻¹ at 200 C) and an ultra‐long cycle life (98% capacity retention after 20000 cycles with minimal capacity and voltage deterioration). Additionally, it exhibits excellent electrochemical performance in full‐cell and all‐solid‐state battery applications.

2. Results and Discussion

2.1. Origin of Structural Degradation and Structural Verification

As is well‐known, in Na₄Fe₂Mn(PO₄)₂(P₂O₇) (NFMPP), the P₂O₇ groups undergo distortion during the extraction of the last Na⁺ [27]. It is due to electrostatic repulsion, leading the Mn1O₆ (Mn1 sites) octahedron and Fe3O₆ (Fe3 sites) octahedron to transfer from a shared edge connection to a shared angle connection. In addition, the redox process of Mn²⁺/Mn³⁺ proceeds simultaneously with the extraction of the last Na⁺ [12, 20]. The Jahn–Teller distortion of Mn³⁺ induces a contraction in the horizontal direction of the MnO₆ octahedron, thereby shortening the distance between Mn1 and Fe3 sites, intensifying the electrostatic repulsion among them. Thus, it further exacerbates the P₂O₇ distortion. The synergistic distortion driven by charge changes can be referred as “electrostrictive” coupled‐disruption. The irreversible transformation progresses throughout the charging and discharging process, leading to the closure of Na⁺ channels (Figure 1a). Figure 1b describes the irregular CEI growth. The CEI layer not only fails to effectively protect the electrode, leading to the TM dissolution, but also hinders the transport of Na⁺ at the interface [23, 28]. Therefore, exploring modification strategies that can simultaneously regulate internal structure and facilitate the formation of a stable CEI layer is essential. Inspired by the inorganic components in the CEI layer [26], the introduction of halogen element X (X = F, Cl, Br) into the internal structure of the material is considered. Utilizing the strong electronegativity of F and the spatial effects of Cl and Br, a combination of rigidity and flexibility is employed to suppress the lattice disorder and Na⁺ channel closure. In addition, according to the BVEL results (Figure S1), there are diffusion channels in the material for the migration of halogen elements. Therefore, surface halogen elements can combine with Na⁺ during the charge–discharge process, pre‐generating the CEI layer [10]. However, due to the large ionic radii of Cl and Br, they are typically difficult to dope. Drawing inspiration from previous research [29], defect engineering can be utilized to induce the formation of oxygen vacancies, facilitating the “squeezing” of Cl and Br ions into the lattice.

FIGURE 1.

Failure mechanism and crystallographic structural information. (a) Schematic diagram of Na⁺ channel closure caused by the synergistic effect of P₂O₇ and Mn Jahn–Teller distortion. (b) Schematic illustration of the formation of an uneven CEI Layer at the interface. (c) Synchrotron radiation XRD Rietveld refinement pattern and (d) HRTEM image of F2MX‐δ. (e) Fe K‐edge and (f) Mn K‐edge XANES spectra of F2M and F2MX‐δ. Fe K‐edge WT images of (g) F2M and (h) F2MX‐δ; Mn K‐edge WT images of (i) F2M and (j) F2MX‐δ.

In order to reveal the failure mechanism and the effectiveness of the modification strategy, Na₄Fe₃(PO₄)₂(P₂O₇)/C (NFPP), Na₄Fe₂Mn(PO₄)₂(P₂O₇)/C (F2M), Na₄Fe₂Mn(PO_3.91(F, Br, Cl)_0.03)₂P₂O₇/C (F2MX), Na₄Fe₂Mn_0.91 (PO_3.91F_0.09)₂P₂O₇/C (F2MF‐δ), Na₄Fe₂Mn_0.91(PO_3.91(F, Cl)_0.045)₂ P₂O₇/C (F2MFCl‐δ) and Na₄Fe₂Mn_0.91(PO_3.91(F, Br, Cl)_0.03)₂P₂O₇/C (F2MX‐δ) are prepared through mechanical activation method. The crystalline structure is characterized using synchrotron X‐ray diffraction (XRD). The XRD patterns align closely with the standard card (Figure 1c and Figures S2 and S3). It demonstrates that defect engineering coupled with halogen element doping does not affect the synthesis of the pure phase. The refinement results (Table S1) further demonstrate the changes in the unit cell volume and lattice parameters of the material. Defect engineering substantially reduces the unit cell volume, which adversely affects sodium‐ion diffusion within the lattice. The introduction of Cl and Br ions with larger ionic radii partially mitigates the lattice contraction. For a more comprehensive understanding, detailed structural information for the samples is listed in Tables S2–S6. Additionally, the high‐resolution transmission electron microscopy (HRTEM) image of F2MX‐δ reveals lattice fringes corresponding to the (3 0 1) plane of F2M (Figure 1d).

Moreover, Energy Dispersive X‐ray (EDX) mapping (Figure S4) illustrated the distribution of various elements, which demonstrates the successful incorporation of halogen elements. The C‐S analysis shows that the carbon content in F2M, F2MX, and F2MX‐δ, as detailed in Table S7. Furthermore, X‐ray photoelectron spectroscopy (XPS) analysis is conducted on different samples to substantiate the driving effect of defects on halogen element doping (Figure S5). A comparison of the XPS spectra of F2MX and F2MX‐δ (Figure S6) reveals that the peaks of Br and Cl in F2MX are relatively weaker compared to F2MX‐δ in the absence of defect engineering, which demonstrates that halogen elements with larger ionic radii face challenges in doping into the crystal lattice. However, the incorporation of halogen elements is effectively promoted by strategically creating oxygen vacancies through defect engineering. The valence states of Fe and Mn in F2MX‐δ are further investigated by X‐ray absorption near‐edge structure (XANES). As shown in Figure 1e,f, the Fe and Mn K‐edge spectra of F2MX‐δ overlap with those of F2M, which affirms that the introduction of halogen elements, facilitated by defect engineering, upholds charge conservation without affecting the valence states of Fe and Mn within the material. Furthermore, the wavelet transform (WT) plots further elucidated the impact of the strategy on the coordination environment of materials. By comparing the WT plots of F2M and F2MX‐δ at the Fe and Mn K‐edge edges, it can be observed that with defect engineering and the introduction of halogen elements, the strength of Fe‐O and Mn‐O peaks is significantly enhanced, which signifies an enhancement in the material's ordering and coordination strength of M‐O [23, 24]. It is attributed to the robust electronegativity of fluorine as a potent ligand, fostering more robust bonding with transition metals and oxygen, fortifying the localized coordination milieu of Fe and Mn. Meanwhile, the incorporation of chlorine, bromine, and defects offers structural support and stress alleviation, mitigating lattice strain in the synthesis process and diminishing the disorder of material structure. These observations align with the refinement results from synchrotron XRD analysis.

2.2. Unveiling “Electrostrictive” Coupled‐Disruption and Abnormal Phase Transition

In order to identify the “electrostrictive” coupled‐disruption of F2M, in situ XRD characterization of NFPP, F2M, and F2MX‐δ is conducted (Figure 2a–c and Figure S7). Different from viewpoints found in other literatures [23, 24, 25], Figure 2a,d reveals that the structural evolution of NFPP can be delineated into four phases. The phases τ₁' and τ₂' correspond to the Fe²⁺/Fe³⁺ reaction induced by PO₄ ³⁻, while phases τ₃' and τ₄' correspond to the Fe²⁺/Fe³⁺ reaction induced by P₂O₇ ⁴⁻. The transition from τ₂' to τ₄' phase corresponds to the occurrence of P₂O₇ distortion. Furthermore, as illustrated in Figure 2b,e and Figure S7a, processes τ₁ and τ₂ in F2M also correspond to the Fe²⁺/Fe³⁺ reaction induced by PO₄ ³⁻. However, due to the enhanced solid solution effect brought about by the introduction of Mn, only one phase τ₃ exists during the oxidation of Mn²⁺/Mn³⁺ induced by P₂O₇ ⁴⁻, leading to a smaller lattice volume change in F2M compared to NFPP. Nevertheless, it is noteworthy that during the transition from τ₂ to τ₃, anomalous shifts in the characteristic peaks of the (0 0 4) and (0 2 2) crystal planes are observed in Figure 2b. It signifies a significant contraction of the c‐axis during τ₂ to τ₃ transition (Figure S7a).

FIGURE 2.

Structural evolution during charging and discharging processes. In situ XRD patterns of (a) NFPP, (b) F2M, and (c) F2MX‐δ electrodes during the charge–discharge process. Schematic diagram of structural evolution of (d) NFPP and (e) F2M. Fe and Mn K‐edge FT‐EXAFS spectra of (f,h) F2M and (g,i) F2MX‐δ in R space at different charge/discharge states. (j) Fe and (k) Mn K‐edge FT‐EXAFS spectra of F2M and F2MX‐δ in R space before and after 2000 cycles at 20 C. Mn K‐edge WT images of (l) F2M and (m) F2MX‐δ.

The abnormal phase transition can be attributed to the “electrostrictive” coupled‐disruption driven by charge changes due to the introduction of Mn. The Mn Jahn–Teller effect causes the MnO₆ octahedron to contract horizontally, bringing the Mn1 site closer to the Fe3 site, which further intensifies the electrostatic repulsion between them, thereby exacerbating the P₂O₇ distortion. Furthermore, the distortion of the MnO₆ octahedron caused by the Mn Jahn–Teller effect exacerbates the structural degradation. Therefore, despite the minor volume changes in the unit cell of F2M during charge–discharge cycles, it undergoes a remarkable structural evolution internally. It results in the gradual closure of Na⁺ channels during cycling, ultimately leading to voltage decay throughout the cycling process. To further delve into the failure mechanism, the ex situ Fourier transform EXAFS (FT‐EXAFS) spectra (Figure 2f–i and Figures S8 and S9) of F2M and F2MX‐δ in R space unveil the anomalous phase transition within the material. When the SOC (stage of charge) of materials increases from 3.4 to 4.3 V, the M─O and M─P (M═Fe, Mn) bonds undergo abnormal elongation, which is attributed to the transition of adjacent MnO₆ and FeO₆ octahedra from shared edge connections to shared angular connections, resulting in the distortion of phosphate ions. Moreover, the distortion of the MnO₆ octahedron induced by the Jahn–Teller effect of Mn³⁺ further exacerbates the structural distortio. During the charging process, the unit cell volume decreases with the extraction of Na⁺, while the MnO₆ and FeO₆ octahedra undergo expansion. It results in the obstruction of Na⁺ channels, inhibiting diffusion within the material. Furthermore, the distortion is irreversible, accumulating continuously during the cycling process and ultimately leading to structural degradation within the material's crystal lattice (Figure S10). However, F2MX‐δ demonstrates smaller volume changes during charging and discharging, exhibiting a ΔV of 2.81% in contrast to 4.83% in F2M (Figure S7). In addition, as depicted in Figure 2c,g,i, the anomalous phase transition (abnormal variations of M‐O and characteristic peaks of (0 2 2) and (0 0 4) crystal plane) is effectively suppressed in F2MX‐δ. It can be attributed to two aspects. First, due to the strong ligand properties of F, it enhances the M‐O coordination, effectively suppressing the transition of the FeO₆ and MnO₆ octahedra connection form, as well as the Mn Jahn–Teller distortion. Additionally, the spatial effect of Cl and Br ions with larger ionic radius effectively alleviates the electrostatic repulsion between the Fe3 and Mn1 sites. The approach, which couples rigidity with flexibility fundamentally inhibits the “electrostrictive” coupled‐disruption, thereby opening up the Na⁺ transport channel. Furthermore, F2MX‐δ still maintains a stable crystal structure even after 2000 cycles at 20 C (Figure S10). Further analysis (Figure 2j–m) is conducted on the FT‐EXAFS and WT spectra of F2M and F2MX‐δ after 2000 cycles at 20 C. F2MX‐δ exhibits a more stable coordination environment and a higher degree of order. Furthermore, the variations in M─O and M─P of F2MX‐δ are reduced after cycling, indicating effective suppression of “electrostrictive” coupled‐disruption. Moreover, GITT testing is carried out to characterize the diffusion of Na⁺ within the materials after cycles. The results (Figures S11 and S12) indicate that the migration of Na⁺ in cycled F2M is hindered during the charging and discharging process. It is attributed to the continuous accumulation of “electrostrictive” coupled‐disruption during cycling, leading to the blockage of the migration pathway for the last Na⁺, consequently resulting in a severe lag in the reaction.

2.3. Identifying CEI Formation and Endogenous Interface Design

The composition and morphology of the CEI in the materials are analyzed with a time‐of‐flight secondary ion mass spectrometer (TOF‐SIMS), XPS depth analysis, Atomic Force Microscope (AFM), and HRTEM tests (Figure 3). As shown in Figure 3a,b, TOF‐SIMS 3D images of the anion depth profile and the corresponding anion mapping indicate that the CEI layer formed in F2M has an uneven distribution of components, with little to no presence of inorganic constituents such as NaF and NaCl. In contrast, F2MX‐δ forms a uniform, inorganic‐rich CEI layer on the surface. In addition, XPS depth analysis (Figure 3c,d and Figures S13 and S14) further analyzed the distribution of different components in CEI. As shown, the proportion of inorganic components in the CEI layer of F2MX‐δ gradually increases with etching time. It signifies the formation of a surface organic‐rich and interior inorganic‐rich CEI layer in F2MX‐δ. Such a structure is conducive to promoting Na⁺ diffusion at the CEI‐material interface and effectively inhibiting side reactions with the electrolyte and internal strains during cycling processes.

FIGURE 3.

Properties and growth behavior of CEI after cycles. The anion depth profile 3D images of the ionic fragments of (a) F2M and (b) F2MX‐δ. XPS depth analysis spectra of C 1s and F 1s in (c) F2M and (d) F2MX‐δ. (e) AFM and HRTEM images of F2M and F2MX‐δ. (f) The diagram of the structure and composition of the CEI layer.

Figure 3e and Figure S15 show the morphology and surface roughness of the CEI layer of F2M and F2MX‐δ. The electrode surface of F2M exhibits unevenness and high roughness. Furthermore, TEM images (Figure S16) indicate the presence of particle damage in the particle of F2M after cycling, with an uneven coating layer on the surface, resembling fin‐like and island‐like structures. Combined with the HRTEM image (Figure 3e), it reveals the localized overgrowth of the CEI layer on F2M due to electrolyte side reactions with the electrode. Such an uneven and excessively thick CEI layer not only fails to provide effective protection for the electrode but also hinders Na⁺ diffusion. In contrast, the electrode surface of F2MX‐δ appears significantly more uniform, with HRTEM observation (Figure 3e and Figure S16b), revealing a uniform and thin CEI layer on the particle surface. The phenomenon arises from the combination of surface halogen elements and Na⁺ during the Na⁺ extraction/insertion process, leading to the pre‐formation of an inorganic‐rich interface layer. Furthermore, the pre‐formed interface and some organic components formed by the electrolyte decomposition jointly contribute to the formation of a surface‐rich organic and interior‐rich inorganic CEI layer (The composition and morphology of the CEI are illustrated in Figure 3f).

To further investigate the dissolution of transition metal ions throughout cycling, batteries undergoing 20 C cycling are disassembled to be analyzed. The characteristic peaks of Mnkα and Fekα observed by EDX mapping, as illustrated in Figure S17, signifies the presence of a higher concentration of transition metal elements in F2MX‐δ. Combining ICP test results (Table S8), the dissolution of TM is effectively inhibited. In summary, the thin, uniform, and stable CEI layer of F2MX‐δ not only effectively suppresses electrolyte side reactions, lattice strain, and TM dissolution but also provides excellent interface Na⁺ diffusion kinetics. Furthermore, the LST/QST approach is utilized to determine the Na energy barrier and the Mn dissolution kinetic barriers in the CEI layer (Figures S18–S21). Figure S19 demonstrates that NaX (X = F, Cl, Br) showcases a smaller diffusion barrier than Na₂CO₃ and RCONa. It further proves that the formation of NaF, NaCl, and NaBr promotes the sodium‐ion diffusion within the CEI layer. Moreover, Figure S21 presents the diffusion energy barrier for Mn ions migrating through the NaX lattice. The high diffusion energy barrier indicates that the CEI layer can effectively prevent Mn dissolution.

2.4. Charge Transfer and Na⁺ Diffusion Kinetics

The investigation further delved into the Na⁺ diffusion dynamics and charge transfer process in the material through a combination of experimental analysis and computational simulations. The Bond Valence (BV) methodology has demonstrated its efficacy in visually illustrating Na⁺ pathways [30, 31]. BV Site Energy (BVSE) computations are carried out to elucidate the routes of Na⁺ migration in the F2M and F2MX‐δ (Optimized crystal structures are shown in Figure S22).

As illustrated in Figure 4a–d, the modification has significantly broadened the Na⁺ BV routes in F2MX‐δ, in sharp contrast to the restricted and impeded pathways noted in F2M. Furthermore, defect engineering offers additional routes for Na⁺ migration, thereby enhancing the Na⁺ diffusion kinetics within the material. Considering that the distortion of P₂O₇ predominantly affects the migration of Na⁺ along the c‐axis direction, the LST/QST approach is utilized to determine the energy barrier related to Na⁺ diffusion along the spherical diffusion pathway (from Na1 to Na3 as depicted in Figure 4e). Figure 4f demonstrates that F2MX‐δ showcases the minimal diffusion barrier of 0.622 eV. Furthermore, Galvanostatic Intermittent Titration Technique (GITT) is utilized to precisely ascertain the Na⁺ diffusion coefficients of the material (Figure 4g,h and Figure S23). The calculated data indicate that the average Na⁺ diffusion coefficients (D_Na+) for F2M and F2MX‐δ are 3.34 × 10⁻¹² and 2.03 × 10⁻¹¹ cm² s⁻¹, respectively. It indicates that the introduction of halogen elements enhances the diffusion capability of Na⁺, especially effectively alleviating the closure of sodium ion channels caused by “electrostrictive” coupled‐disruption during deep charging. In addition, Figure S23 also indicates that both single doping and co‐doping can effectively alleviate this phenomenon. Additionally, DOS analysis reveals that F2MX‐δ exhibits a smaller bandgap (1.17 eV) compared to F2M (2.75 eV), as depicted in Figure 4i,j.

FIGURE 4.

Na⁺ diffusion kinetics and charge transfer. The iso‐surfaces of constant bond valence site energy in F2M and F2MX‐δ from (a), (b) ac plane, and (c), (d) bc plane. (e) The diffusion pathway of Na⁺. (f) Na⁺ migration energy barriers for F2M and F2MX‐δ. (g) GITT curves of Na‐ion for F2M and F2MX‐δ. (h) Comparison of D _Na ⁺. PDOS of (i) F2M and (j) F2MX‐δ. (k) Schematic diagram of band structure. The charge density differences of (l) F2M and (m) F2MX‐δ. (n) In situ EIS spectra of F2MX‐δ.

It is attributed to defect engineering and halogen element doping, which introduce new, lower‐energy impurity levels at the bottom of the conduction band. Furthermore, strong hybridization occurs between the F 2p the M 3d orbitals, generating new bonding and anti‐bonding states, leading to the downward shift of the conduction band and the narrowing of the bandgap. The transformation renders the material more inclined toward an n‐type semiconductor, which has a higher electron concentration, promoting electron transition (Figure 4k). The 3D isosurfaces of charge density depicted in Figure 4l,m unveil alterations in electron distributions. The difference observed in charge density suggests a redistribution of the electron cloud density surrounding the Mn and Fe atoms due to the modification. In contrast, F2MX‐δ exhibits a more consistently enriched electron cloud density encircling the Mn and Fe atoms. (The blue and red regions represent electron cloud loss and gain). Additionally, to gain insights into the electrochemical kinetic processes and distinguish between them, in situ electrochemical impedance spectroscopy (EIS) measurements and distribution of relaxation time (DRT) analysis are performed on F2M and F2MX‐δ, as depicted in Figure 4n and Figures S24–S26. As shown in Figures S25 and S26, a comparative analysis reveals that F2MX‐δ exhibits a similar charge transfer process to F2M, yet with a lower charge transfer resistance. It is noteworthy that according to in situ DRT analysis, the charge transfer resistance of the material can be divided into CEI resistance and charge transfer resistance in the lattice. The results indicate that F2M has a larger CEI resistance, which further underscores the exceptional charge transfer performance of F2MX‐δ. In summary, the synergistic modification effectively promotes charge transfer and Na⁺ diffusion in the material.

2.5. Electrochemical Performance

The electrochemical performance is characterized by assembling the materials with sodium metal as the negative electrode in a half‐cell. As shown in Figure 5a, F2MX‐δ demonstrates a better specific capacity (132.3 mAh g⁻¹ at 0.2 C). The phenomenon of voltage hysteresis has been significantly suppressed. Furthermore, F2MX‐δ demonstrates outstanding rate performance (Figure 5b and Table S9), exhibiting discharge specific capacities of 120.9, 103.5, 85.1, 71.2, 64.1, 55.0 mAh g⁻¹ at 0.5, 2, 10, 50, 100, 200 C, respectively. The charge–discharge curves at various current densities (Figure 5c) elucidate that the strategy substantially improves the material's rate capability, concurrently addressing the voltage attenuation concern at higher rates. Moreover, Charge–discharge curves of F2M and F2MX‐δ (Figure 5d,e) during 1000 cycles at 5 C unveiled that F2MX‐δ exhibits reduced capacity loss and voltage drop. Figure S27 further reveals that F2MX‐δ exhibits a higher operating voltage at 5 C and exhibits reduced voltage deterioration during cycling. In addition, the cycling performance of all samples is depicted in Figure 5f and Table S10. Notably, F2MX‐δ demonstrates remarkable durability, with 94.6% capacity retention after 2000 cycles. Impressively, even after enduring 20 000 cycles at 50 C, F2MX‐δ exhibits an approximate 98% capacity retention with minimal capacity or voltage deterioration, coupled with a significantly high coulombic efficiency of around 99.9% (Figure 5g and Figure S28). Even compared to NFPP, its performance still has significant advantages (Figure S29).

FIGURE 5.

Enhancement of electrochemical performance. (a) Charge–discharge curves of F2M and F2MX‐δ at 0.2 C. (b) Rate performance. (c) Discharge curves at various C‐rates. Charge–discharge curves of (d) F2M, and (e) F2MX‐δ for the first 1000 cycles at 5C. (f) Cycling performance after 2000 cycles at 20 C of F2M, and F2MX‐δ. (g) Long cycling performance of F2MX‐δ at 20 C (The inner figure shows the discharge curve for every 1000 cycles). (h) Comparison of decay rate between F2MX‐δ and other phosphate SIB cathodes [15, 19, 21, 22, 32, 33, 34, 35, 36, 37, 38] (Table S11). (i) Charge–discharge curves and (j) cycling performance of F2MX‐δ in the full cell. (k) Charge–discharge curves of F2M and F2MX‐δ at 0.1 C in all‐solid‐state battery.

To further elucidate the efficacy of the three halogen element combinations, the impact of a single halogen element (F2MF‐δ) and dual halogen element doping (F2MFCl‐δ) on the material's electrochemical performance is also compared (Figure S30). Among them, F2MX‐δ still exhibits the optimal electrochemical performance. Besides, compared to previously reported mixed phosphate cathode materials, F2MX‐δ exhibits significant advantages (Figure 5h and Table S11). To further evaluate the practical application potential of F2MX‐δ, the full cell is assembled using F2MX‐δ as the cathode and self‐fabricated hard carbon (HC) as the anode for electrochemical testing (Figure S31). Figure 5i shows the charge and discharge curves of the full cell for the initial two cycles, within a voltage window of 4.2–1.6 V. The full cell exhibits excellent rate performance (Figure S32), exhibiting discharge specific capacities of 119.6, 101.7, 90.0, 82.3, 76.2, 68.4, and 64.6 mAh g⁻¹ at 0.2, 1, 2, 5, 10, 20, and 50 C, respectively. Furthermore, the full cell demonstrates impressive capacity retention rates of 93.5% after 1000 cycles at 2 C and 93.3 after 2000 cycles at 5 C, respectively (Figure 5j). In addition, the application of mixed phosphates in solid‐state batteries is explored for the first time (an all‐solid‐state batteries are prepared using NaTaOCl₄ and NaBH₄ as the solid‐state electrolyte and Na‐Sn alloy as the negative electrode). Due to the poor Na diffusion kinetics, the charge–discharge curve of the F2M all‐solid‐state battery hardly exhibits a discharge plateau characteristic of the cathode material (Figure 5k). In contrast, the all‐solid‐state battery of F2MX‐δ demonstrates a typical discharge plateau, significantly enhancing the average voltage (elevated from 2.96 to 3.28 V). It is due to the occurrence of coupling distortion corresponding to the oxidation reaction of Mn²⁺/Mn³⁺, which limits the diffusion of sodium ions and hinders the oxidation reaction of Mn²⁺/Mn³⁺. In addition, due to the slower diffusion of sodium ions in solid electrolytes, it ultimately leads to the disappearance of the discharge plateau characteristics in charge–discharge curve of F2M. Additionally, the F2MX‐δ all‐solid‐state battery exhibits good cyclic stability (Figure S33). To further investigate the interface between the solid electrolyte and the cathode, the cycled solid‐state batteries are disassembled, and the interface between the electrolyte and the electrode is analyzed using SEM. The composite cathodes are synthesized from F2M/F2MX‐δ, NaTaOCl₄, and VGCF, and NaTaOCl₄ and NaBH₄ are serving as the electrolyte. As shown in Figures S34–S36, EDS elemental mappings indicate that there are significant cracks at the interface between the F2M composite electrode and the solid electrolyte. These cracks are attributed to strain within the material, which causes a mismatch at the electrode/electrolyte interface, significantly hindering sodium ion transport across the interface. However, there is a continuous interface between F2MX‐δ and the solid electrolyte, which facilitates the transfer of sodium ions between the interfaces and effectively improves the performance of its solid‐state battery. It is due to the introduction of halogen elements, which greatly alleviates the “electrostrictive” coupled‐disruption within the material, effectively suppressing the lattice strain generated during the cycling process. It further proves the superiority of this modification.

2.6. Analysis of “Electrostrictive” Coupled‐Disruption and Irregular CEI Growth

Based on the above results, the dual failure mechanism in NFMPP has been successfully revealed. As illustrated in Figure 6, within the lattice, the increase in repulsive forces between the Mn1 and Fe3 sites due to charge variations leads to a transition in the connection of their octahedral structure (from edge‐sharing to angle‐sharing) during the final sodium extraction process. The asymmetric distribution of Mn³⁺ electrons leads to Jahn–Teller effect, shortening the distance between Mn1 and Fe3 sites, thereby intensifying the electrostatic repulsion. In addition, the MnO₆ octahedra distortion further exacerbates the closure of Na⁺ channels within the lattice. The “electrostrictive” coupled‐disruption results in an abnormal phase transition in the material, exacerbating structural damage and hindering Na⁺ transport. The phenomenon leads to the disappearance of discharge plateau characteristics in the material during the cycling process, ultimately resulting in performance degradation. Furthermore, due to the irregular growth of the CEI layer, the material fails to receive effective protection, resulting in the TM dissolution and structural damage caused by side reactions with the electrolyte. Moreover, the excessively thick CEI layer hinders the interface diffusion of Na⁺. It leads to voltage attenuation and capacity loss during the cycling process. In order to address the dual failure mechanism, halogen elements are successfully introduced into the material through defect engineering. Internally, the highly electronegative F effectively enhances the orderliness and coordination strength between M‐O. It acts as a chain, locking the MnO₆ and FeO₆ octahedra tightly, maintaining their edge‐sharing connections.

FIGURE 6.

Schematic diagram of dual failure mechanism and modification strategy.

Furthermore, due to their larger ionic radii, Cl and Br ions increase the distance between the Mn1 and Fe3 sites, effectively buffering the electrostatic repulsion. The strategy of combining rigidity and flexibility effectively suppresses the “electrostrictive” coupled‐disruption, improving the structural stability. Externally, halogen elements at the interface combine with Na⁺ during the charging and discharging process, pre‐generating an inorganic‐rich layer at the interface. It synergizes with the partial decomposition of the electrolyte, ultimately forming a surface organic‐rich and interior inorganic‐rich CEI layer. The endogenous interface is more uniform and stable, effectively preventing further side reactions with the electrolyte and the dissolution of transition metals. Furthermore, it exhibits excellent Na⁺ diffusion kinetics due to the rich inorganic content (NaF, NaCl, NaBr). The strategy of “Inside‐Out” stabilization significantly improves rate and cycling performance.

3. Conclusions

In conclusion, this study unveils additional phase transition processes in NFPP through in situ XRD, leading to the discovery of “electrostrictive” coupled‐disruption in NFMPP. In addition, the irregular SEI growth in NFMPP is revealed through multi‐scale characterization. To address the dual failure mechanism, halogen elements are innovatively introduced into the material through defect engineering. By regulating the coordination environment and lattice structure internally, the abnormal phase transition caused by the “electrostrictive” coupled‐disruption is effectively suppressed, enhancing the structural orderliness during the cycling process and broadening the Na⁺ transport pathways. Furthermore, the surface halogen elements can spontaneously bind with Na⁺ during charge and discharge processes, ultimately forming a surface organic‐rich and interior inorganic‐rich CEI layer which effectively inhibits the TM dissolution during cycling, and significantly promotes the interface Na⁺ diffusion kinetics. As a result, F2MX‐δ has achieved a breakthrough in electrochemical performance, exhibiting astonishing rate performance (55.0 mAh g⁻¹ at 200 C) and strong structural stability (98% capacity retention after 20000 cycles with minimal capacity and voltage deterioration). In addition, the application of mixed phosphates in all‐solid‐state batteries is explored for the first time, further demonstrating the effectiveness of the strategy. This work offers guidance for future researchers to delve into the phase transitions and all‐solid‐state battery application in Fe/Mn‐based mixed‐phosphate cathode materials.

Author Contributions

L.W, L.Z., and Y.S. conceived the idea and supervised this project. Y.W. and M.D. designed and conducted the experiments and wrote the manuscript. C.T., W.F., and X.Z. analyzed the data and revised the manuscript. H.G. and Z.H. performed DFT and BVSE calculations.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73900‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.