Anionic F-doping-induced engineering of P2-type layered cathode materials for high-performance potassium-ion batteries

Abstract

P2-type layered oxides have emerged as promising cathode candidate materials for potassium-ion batteries. Nevertheless, unsatisfactory cycling stability hinders their practical application, chiefly arising from deleterious phase transitions and the Jahn–Teller distortion of Mn³⁺. Herein, an anion-doping strategy where F⁻ is incorporated into P2-K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O₂ (KTMO) cathode materials is proposed. Raman spectroscopy was employed to investigate the local chemical environment of these materials. The results revealed a slight shift to higher wavenumbers in the E_g and A_1g peaks, which was ascribed to the shortening of the average TM–O bond length triggered by the addition of F. Ex situ XRD analysis revealed that the material K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_1.93F_0.07 effectively suppresses undesirable phase transitions. Moreover, the maximum variation in the lattice parameter c is only 2.2% during potassium insertion/extraction, which fully demonstrates the outstanding performance of this material in terms of structural stability. This strategy brings about excellent cycling stability with a reversible capacity of 131.8 mA h g⁻¹ and capacity retention of 76.8% after 100 cycles, within a voltage range of 2.0–4.0 V. These findings offer novel insights into the design of cathode materials possessing optimized structures and enhanced performance for potassium-ion batteries.

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Introduction

Among the numerous choices of cathode materials for potassium-ion batteries (PIBs), P2-type layered transition metal oxides have emerged as prominent candidates and are widely regarded as being key to the future development of PIBs.^1–4 The main advantage of these materials lies in their unique structural characteristics, specifically possessing two alkali metal storage sites.^5,6 The ingenuity of this structural design lies in ensuring some potassium ions remain relatively stable during the potassium-ion insertion and extraction cycle in the material, which can inhibit slippage of the transition metal layer, stabilize the crystal structure, and maintain the performance of the battery.^7–9 However, manganese-based materials in P2-type layered transition metal oxides also have problems that cannot be ignored. First, there is the Jahn–Teller effect of Mn³⁺ in manganese-based materials. This effect is like a “time bomb” that will cause extremely serious damage to the crystal structure of the material, greatly reducing the overall stability of the material. Even more troublesome is that Mn³⁺ has unstable chemical properties and undergoes a disproportionation reaction generating Mn²⁺ and Mn⁴⁺.¹⁰ Among these, Mn²⁺ dissolves in the electrolyte, further consuming the active material, like a “bottomless pit”, continuously devouring the capacity of the battery, which inevitably leads to further attenuation of the battery's capacity.^11,12 In addition, the unique ionic radius of K⁺ also presents additional challenges for PIBs. Due to the particularity of its ionic radius, during the process of K⁺ insertion and extraction into and from the cathode material, it is prone to cause an irreversible structural phase change, that is, from the P2 phase to the O2 phase.^13,14 Once this structural phase change occurs, it greatly changes the original structure and performance of the material, making the original ordered atomic arrangement of the material become chaotic and disordered, leading to further decline in the capacity of the material and becoming another key factor hindering performance improvement in PIBs.

Facing these severe challenges, researchers are actively exploring various modification strategies. Among them, the introduction of different cationic elements into the Mn site has become one of the current hot research directions.^15–18 By precisely controlling the introduction of appropriate amounts of electrochemically active or inert elements, the structure and performance of the material can be effectively altered, significantly suppressing the irreversible structural phase change of the material and the Jahn–Teller effect of Mn³⁺, bringing new hope and possibilities for solving the above problems.^19,20 In fact, existing results have shown that after some researchers successfully doped different electrochemically inert elements into the Mn site, the crystal structure of the material was significantly improved, thereby optimizing the cycling stability of the battery, enabling it to maintain a performance that is relatively stable after multiple charge–discharge cycles. Unfortunately, this cation doping method is not perfect and often requires sacrificing part of the initial capacity in exchange for an improvement in the capacity retention rate and rate performance. This means that during the material development process, in order to achieve the same energy output effect, more raw materials may be needed or more complex preparation processes may be adopted, thus increasing the material development cost to a certain extent. Therefore, from the perspective of cost-effectiveness and comprehensive performance optimization, it is particularly urgent and of great practical significance to actively explore other sites for element doping without sacrificing the initial capacity, which will open up new pathways and directions for improving the performance of cathode materials for potassium-ion batteries.

At the same time, in the research field of sodium-ion batteries (SIBs), anion doping has emerged as a promising modification measure and has been widely studied and verified.^21–25 Researchers believe that F ions can partially reduce Mn⁴⁺ to Mn³⁺ through charge compensation, improving the specific capacity and cycling stability. It is particularly noteworthy that Mn³⁺ obtained through this reduction method will undergo a site migration phenomenon and preferentially occupy other transition metal sites that are closer to its ionic radius. This redistribution of sites breaks the original ordered structure of the transition metals in the material, and this disordered transformation of the structure can effectively suppress the Jahn–Teller effect of Mn³⁺, fundamentally solving the structural stability problem caused by this effect. In addition, a stable chemical bond can form between the transition metal and fluorine, further enhancing the electrochemical performance of the material.^26–28

Although anion doping has shown significant advantages in SIBs, its application in PIBs remains underexplored. Currently, only Xu et al.²⁹ have carried out preliminary exploration in this regard, achieving enhanced redox activity of transition metals by partially substituting oxygen with fluorine. This improvement directly boosts the discharge capacity and cycling stability of PIB materials. However, existing studies lack systematic and comprehensive investigations into the effects of varying F doping content levels on PIB performance. Addressing these gaps through further research will provide a theoretical foundation and provide practical guidance for optimizing anion doping strategies in PIBs, ultimately advancing the development of high-performance, stable, and cost-effective PIB technologies to meet future energy storage demands.

Herein, we demonstrate a strategy to modify K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O₂ by F substitution to improve discharge capacity and structural stability by suppressing phase transitions of the materials and the Jahn–Teller effect of Mn³⁺. The resulting layered P2-K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_1.93F_0.07 (KTMO-F7) cathode is synthesized via a simple high-temperature solid-state method. The incorporation of F can enhance hybridization with the transition metal and eliminate the undesirable P2–O2 phase transition. Meanwhile, the K layer distance is extended, which helps to improve the diffusion dynamics of the material.

Experiments section

Materials synthesis

A series of F-doped K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O₂ cathode materials, denoted as K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_2−xF_x with x values of 0.05, 0.07, and 0.10, and abbreviated as KMTO-F5, KTMO-F7, and KTMO-F10, respectively, were prepared by the high-temperature solid-state reaction method. The specific synthesis steps are elaborated on by taking the KTMO-F7 sample as an example: first, stoichiometric amounts of K₂CO₃ (98%, Aladdin, 5 mol% excess), ZnO (99%, Aladdin), Mn₂O₃ (98%, Aladdin), TiO₂ (99%, Aladdin), Al₂O₃ (99.9%, Aladdin), and KF (99%, Aladdin) were ball-milled with absolute ethanol for 6 h at 400 rpm. The precursors were dried at 80 °C for 12 h and then transferred to an agate mortar for fine grinding. Subsequently, they were placed in a muffle furnace and calcined at 1000 °C for 10 h. After cooling to 200 °C, the target products were immediately transferred into a glove box filled with argon.

Materials characterization

The instrument used for the XRD test was the Rigaku Smartlab X-ray diffractometer (Cu Kα). The specific test parameters were set as follows: the scanning rate was controlled at 5° min⁻¹, the tube current was maintained at 200 mA, and the tube voltage was set at 45 kV. In the phase refinement stage, the step-scanning mode was adopted, with the step length precisely set at 0.02°, and the scanning angle range covered 10–80°. Rietveld refinements were conducted using GSAS II software. Prior to ex situ XRD testing, the battery was disassembled in a glovebox and the cathode electrode was extracted. It was cleaned with dimethyl carbonate 2–3 times, vacuum-dried at 60 °C for 12 hours, and stored in the glovebox for later use. For the observation of the microscopic morphology, the Zeiss SUPRA55 SAPPHIRE (Germany) field emission scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) was employed. Operating at a working voltage of 15 kV, this instrument was capable of acquiring high-resolution image information. The HRTEM images and SAED patterns were obtained using a transmission electron microscope (JEM 2100F). The XPS data were collected with the help of the Thermo Scientific ESCALAB 250Xi X-ray spectrometer produced by Thermo Fisher Scientific Inc. Raman spectroscopy analysis was performed with the Thermo Fisher 250XI Raman spectrometer.

Electrochemical measurements

Electrochemical studies were performed using CR2032-type cells, assembled in an argon-filled glovebox. The working electrodes were prepared by mixing the active materials (70 wt%), acetylene black (20 wt%) and polyvinylidene fluoride (PVDF, 10 wt%) binder and spreading the slurry onto Al foil; these electrodes were then dried at 80 °C under vacuum for about 12 h so as to remove the absorbed water and solvent. Potassium metal foil, 0.8 M KPF₆/EC-DMC (1 : 1 volume ratio) and glass fiber (GF/D1823-025, approximately 675 μm in thickness) were used as the counter electrode, electrolyte and separator, respectively.

The assembled battery was then left to stand for 12 h. The Land CT2001A battery test system (Wuhan Land Electronic Co. Ltd) was used for electrochemical testing with a voltage window of 1.5–4.0 V and a C-rate of 0.1 C, 0.2 C, 0.3 C, 0.5 C, 1 C, 2 C and 3 C according to specific testing requirements. The galvanostatic intermittent titration technique (GITT) was conducted under the conditions of 0.1 C charging for 15 min and open-circuit relaxation for 3 h. Cyclic voltammetry (CV) was performed on a CHI1000C electrochemical workstation (Shanghai, China) within a voltage range of 1.5–4.0 V at different scan rates from 0.1 to 1.0 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range from 0.1 Hz to 100 kHz (Energy-lab XM electrochemical workstation).

Results and discussion

Fig. 1a shows the XRD patterns of K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_2−xF_x (x = 0, 0.05, 0.07, 0.10). All diffraction peaks correspond to the orthorhombic structure of K_0.314Mn_0.985O₂ (PDF#97-015-6080, Ccmm space group).³⁰ Notably, no KF-related impurity peaks are observed, indicating that the F doping does not cause any alteration to the material's crystal structure. Fig. 1b shows that the (002) peak moves to the small-angle region when O is substituted by F, indicating increased interplanar spacing, as confirmed by the Bragg equation (2d sin θ = nλ) and refined crystallographic data. Fig. 1c–f display the refined patterns, with Rietveld refinement (Table S1, ESI†) showing R_p and R_wp values below 10%, indicating a good fit. As shown in Table S1,† the lattice parameter c increases with the substitution of O by F, which may be attributed to the doping of F leading to the partial reduction of Mn⁴⁺ to Mn³⁺, causing changes in the lattice spacing of the material. Since the ionic radius of Mn⁴⁺ is smaller than that of Mn³⁺, the diffraction peaks shift to lower angles. On the other hand, it is also possible that the F source used is KF, and the increase in K content might influence the interlayer distance of the material. Therefore, as the F content increases, the (002) diffraction peak of the material continuously shifts to the left. However, it should not be overlooked that an excessively high K content means that the material is quite sensitive to H₂O in the air, making it easier for K₂CO₃ to form on the surface of the material, which negatively affects its electrochemical performance. Overall, the expansion of the K layer can reduce the diffusion energy barrier of K⁺ and effectively facilitate the migration of K⁺ ions in the bulk phase. Meanwhile, the increase in the spacing of the K layer alleviates the increase in the repulsive force of the O–O layer during the extraction process of K, suppressing the slippage of the oxygen layer. The strengthened binding of the TM–O layer can stabilize the octahedral structure of TMO₆ and inhibit the slippage of the O layer during the cycling process.^31,32

Fig. 1 (a) XRD patterns of K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_2−xF_x (x = 0, 0.05, 0.07, 0.10), and (b) enlarged view of the (002) diffraction peaks. (c) The crystal structure model of the P2-phase material. Rietveld refinements of the XRD patterns for (d) KTMO-F5, (e) KTMO-F7, and (f) KTMO-F10. (g) Raman spectrum of K_0.6Zn_0.1Ti_0.05Al_0.05Mn_0.8O_2−xF_x (x = 0, 0.05, 0.07, 0.10).

To investigate the influence of varying K content in the F source on the basicity of the material, this research conducted pH value tests on three types of materials. The specific procedure involved uniformly dispersing the samples in deionized water and then using a pH indicator for testing. The test results are presented in Table S2.† From the table, it is observed that as the doping amount increases, the basicity of the material shows an increasing trend. To ascertain the impact of varying F doping amounts on the local chemical environment of the material, Raman spectroscopic tests were carried out on materials with different F doping levels, as depicted in Fig. 1g. It is evident in the figure that two distinct Raman bands emerged at 472 cm⁻¹ (E_g) and 583 cm⁻¹ (A_1g), which are respectively ascribed to the bending vibration of the O–TM–O bond and the asymmetric stretching vibration of the TM–O bond, characteristic of layered transition metal oxides. Concurrently, it was observed that the characteristic peak positions of E_g and A_1g manifested a subtle shift in the direction of higher wavenumbers. This occurrence robustly implies a modification in the inter-atomic distances within the material, which circumstantially validates the successful incorporation of F into the material system. The immediate consequence is a discernible shortening of the TM–O bond length. Through meticulous analysis and deduction, it is hypothesized that this could be attributed to the occupation, by Mn³⁺ generated during the reduction reaction, of the lattice sites formerly held by other transition metal elements. Such an occupation scenario subsequently led to a reduction in the average TM–O bond length of the material, ultimately culminating in the aforementioned slight shift of the characteristic peaks of E_g and A_1g towards the higher wavenumber range.³³

The morphology of the as-prepared samples was investigated by scanning electron microscopy (SEM). As shown in Fig. S1 and S2 (ESI),† nearly all the particles in the four samples exhibited an irregular flake-like structure, indicating that the substitution of F for O had a minimal impact on the morphology. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed that K, Mn, Zn, Ti, Al, O, and F were uniformly distributed throughout the material, as depicted in Fig. S1 and S2 (ESI).† Furthermore, Fig. 2a–c shows the HRTEM images and corresponding SAED pattern of the KTMO-F7 sample. TEM images (Fig. 2a and b) reveal a well-ordered crystalline P2-type layered structure with lattice stripes of 0.23 nm and 0.64 nm, which are consistent with the (112) and (002) crystal planes, respectively. The corresponding SAED pattern (Fig. 2c) obtained from the [−200] crystallographic axis further confirms orthorhombic spot patterns. The energy spectrum analysis under transmission mode further confirms that the elements in the KTMO-F7 material are uniformly distributed (Fig. 2d).

Fig. 2 (a) and (b) HRTEM images of the (002) and (112) crystal planes. (c) SAED pattern, and (d) EDS mapping for KTMO-F7. XPS spectra of (e) Mn 2p, (f) O 1s, and (g) F 1s of the cathodes with different F doping content.

The X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the oxidation states of Mn, O, and F ions in the F-doped samples and the results are shown in Fig. 2e–g. The two peaks with binding energies at 653.8 eV and 642.2 eV precisely correspond to the Mn 2p_1/2 and Mn 2p_3/2 energy levels of Mn⁴⁺, while the two peaks with binding energies of 652.4 eV and 640.8 eV clearly correspond to the Mn 2p_1/2 and Mn 2p_3/2 energy levels of Mn³⁺. Through in-depth quantitative analysis of Fig. 2e, as the content of the doping element F gradually changes, the area ratio of Mn³⁺/Mn⁴⁺ will also show a corresponding trend in dynamic change (Table S3, ESI†). According to previous research reports, the introduction of F can play a regulatory role in the content of Mn³⁺, prompting part of Mn⁴⁺ to be reduced to Mn³⁺, and thus changing the proportional distribution of manganese elements in different valence states. It is worth noting that this part of Mn³⁺ generated through reduction does not necessarily stay in its original lattice positions. Since the ionic radius of Mn³⁺ (with an ionic radius of 0.645 Å) is close to that of Ti⁴⁺ (with an ionic radius of 0.605 Å), some Mn³⁺ may occupy the original positions of Ti⁴⁺. Therefore, the increase in the amount of this part of Mn³⁺ does not necessarily mean that the Jahn–Teller effect will be enhanced accordingly. The substitution of Mn³⁺ into other transition metal sites is highly likely to alter the cation ordering within the material, which will ultimately manifest as significant variations in the electrochemical performance. As shown in Fig. 2f, the O 1s XPS spectrum reveals an additional characteristic peak at 531.0 eV upon 10% fluorine doping, corresponding to the metal hydroxide species formed in the material.³⁴ This is because the selected F source is KF, and with the increase in F doping, the K element is also introduced into the material system. The increase in K content will affect the interaction between the material and water and oxygen in the air. Specifically, it enhances the sensitivity of the material to water and oxygen, resulting in the appearance of such a characteristic peak at this energy position. The lattice oxygen (O_L, with a binding energy of 530.2 eV) plays an important role in stabilizing the structure of the material. By comparison, it is found that with the increase in F content, the binding energy of the O_L in the material shows a trend of shifting to higher energy. Among the samples, the binding energy of the O_L in the KTMO-F7 material shifts toward the higher binding energy direction most significantly. This phenomenon strongly indicates that stronger chemical bonds between transition metals and oxygen are formed in the KTMO-F7 material, enabling a more stable oxygen structure to be constructed on the surface of the material. From a practical application perspective, this means that the material has the ability to slow down side reactions with H₂O, CO₂, and electrolytes. It is speculated that the possible reason is that the F ion has a relatively large electronegativity. This characteristic can effectively enhance the binding force between TM and O, thereby improving the stability of the material. However, when the F content is further increased to 10%, the binding energy of the lattice oxygen shifts to the right again. After comprehensive analysis and judgment, this is likely to be an abnormal change caused by the K element contained in the F source. In Fig. 2g, it can be clearly observed that as the F doping amount increases, the intensity of the corresponding F 1s signal becomes more obvious, and the signal corresponding to TM–F (684.6 eV) becomes stronger. The peak with a binding energy of 682 eV is attributed to KF. The presence of two signals related to F in the F 1s spectrum indicates that F is not entirely in the form of KF in the material, but has been successfully incorporated into the material.

In the potassium-ion half-cell system, electrochemical performance tests were carried out on four different samples. The tests were conducted under constant current charge–discharge conditions within a voltage range of 1.5–4.0 V at a rate of 0.1 C (1 C = 100 mA g⁻¹), and the corresponding curves of the first three cycles are shown in Fig. 3a, b and Fig. S3 (ESI).† Compared with pure KTMO, the initial specific capacities of the F-doped samples all exceeded 120 mA h g⁻¹. Notably, the initial specific capacity of the KTMO-F7 sample was as high as 131.8 mA h g⁻¹, demonstrating its superior performance. Cyclic voltammetry tests were performed on the four cathode materials, and the obtained curves all exhibited four pairs of redox peaks (Fig. 3c and Fig. S4, ESI†). The redox peaks near 2.37 V/2.10 V and 2.6 V/2.4 V can be attributed to the redox reaction process of Mn³⁺/Mn⁴⁺. The two pairs of redox peaks above 3 V are most likely caused by the ordered rearrangement of K⁺ vacancies.

Fig. 3 The charge and discharge curves of (a) KTMO and (b) KTMO-F7. (c) The CV curves for the first three cycles of KTMO-F7. Cycling performances at (d) 0.5 C and (e) 1 C. (f) Rate performances. (g) CV curves of KTMO-F7 at various scan rates and (h) the linear fitting of peak current with the square root of sweeping rates (υ^1/2).

Furthermore, excellent long-term cycling stability and rate capability can also be attained for the as-prepared KTMO-F7. Fig. 3d shows that after 100 cycles at 0.5 C, the capacity retention rate of KTMO-F7 is 73.6%, while the capacity retention rate of KTMO is only 67.5%. In addition, when the cycling condition became 1 C and the number of cycles was also 100 (Fig. 3e), the capacity retention rate of KTMO-F7 was as high as 76.8%, much higher than that of KTMO (65.8%), indicating that the introduction of F ions improves the cycling stability and energy density retention rate of KTMO. Fig. 3f shows that the rate performance of the KTMO-F7 electrode exhibits discharge capacities of 132.4 mA h g⁻¹, 110 mA h g⁻¹, 99.2 mA h g⁻¹, 90.5 mA h g⁻¹, 78.3 mA h g⁻¹, 61.3 mA h g⁻¹ and 51.9 mA h g⁻¹ at 0.1, 0.2, 0.3, 0.5, 1, 2, and 3 C, respectively, and when the rate reverts back to 0.1 C, the discharge capacity can be satisfactorily restored. To more comprehensively evaluate the performance advantages of KTMO-F7, this work compared its electrochemical performance with that of reported layered cathode materials for PIBs, and the relevant data are listed in Table S4.† The comparison reveals that the F-doped KTMO-F7 material has a significant advantage in capacity, which is remarkably higher than that of most materials. Meanwhile, such a high-capacity retention rate also proves that the KTMO-F7 material has great development potential. To further confirm the high-rate performance of KTMO-F7, the K⁺ diffusion coefficient (D_K⁺) of the cathode was evaluated by CV measurement (Fig. 3g, h, Fig. S5, and Table S5, ESI†). The calculated D_K⁺ values for the cathodic and anodic peaks are approximately 9.85 × 10⁻¹¹ and 1.9 × 10⁻¹¹ cm² s⁻¹, respectively. In contrast, KTMO-F7 shows a smaller overpotential and a larger K⁺ diffusion coefficient, confirming its superior reaction kinetics and rate capability. This is nearly consistent with the values obtained from the GITT measurement of the low-voltage phase, and the calculation details are provided in the ESI (Fig. S6, S7 and Table S6†).

To investigate more deeply the surface characteristics and internal kinetic mechanisms of the KTMO-F7 material during the reaction process, we pre-cycled this material and conducted in situ electrochemical impedance spectroscopy (EIS) tests during the second charge–discharge cycle. Fig. 4a shows the pre-selected voltage for the in situ EIS test, while Fig. 4b and c are the Nyquist plots corresponding to the charging and discharging stages, respectively. As shown in Fig. 4b, the charge transfer resistance during the charging process exhibits a slowly increasing trend. Notably, compared with KTMO, the KTMO-F7 sample does not experience a sudden jump in charge transfer resistance at 3.6 V and beyond. Moreover, until the charging reaches 4.0 V, the resistance value of the KTMO-F7 sample is consistently significantly lower than that of KTMO, which may imply that the side reactions of KTMO-F7 in the high-voltage region have been effectively suppressed. In addition, during the discharging process (Fig. 4c), KTMO-F7 and KTMO show certain similarities, with both presenting a trend of first decreasing, then increasing, and subsequently decreasing again. This may be due to the comprehensive effects of the Mn³⁺/Mn⁴⁺ redox reaction occurring during the discharging process and the side reactions between the material surface and the electrolyte on the electrode material structure, ion transport channels, and the electrode/electrolyte interface at different stages.^35,36

Fig. 4 In situ EIS test of the KTMO-F7 sample. (a) The curve of voltage variation with time during the second cycle process, and (b) the corresponding Nyquist plots during the charging state, and (c) during the discharging state. (d) The corresponding equivalent circuit diagram and Nyquist plots for (e) KTMO-F5, (f) KTMO-F7, and (g) KTMO-F10 in the initial state and after 500 cycles.

Considering that materials need to undergo a large number of cycles in practical application scenarios, the various physical and chemical changes they suffer will be more pronounced and complex. Based on this, we compared and analyzed the impedance of four materials before and after 500 cycles to evaluate their performance degradation under conditions of long-term use (Fig. S8†). As shown in Fig. 4e–g and Fig. S9 (ESI),† both before and after cycling, KTMO-F7 exhibits the smallest charge transfer resistance, which proves that an appropriate amount of F doping can effectively reduce the charge transfer resistance and improve the electronic conductivity of the material. Subsequently, the equivalent circuit diagram shown in Fig. 4d was used for fitting, and the specific results are detailed in Table S7.†

To elucidate the reaction mechanism and changes in the crystal structure of this as-prepared material during the K⁺ (de)intercalation process, ex situ XRD patterns were performed at 0.1 C within the voltage range of 1.5–4.0 V, with the findings shown in Fig. 5. The (002) and (004) peaks of the P2 phase primarily shift to a lower angle (Fig. 5b and c), indicating that the c-axis expands due to the increased repulsion force between adjacent oxygen layers when K⁺ is extracted. It should be emphasized that the XRD spectra of KTMO-F7 exhibit an opposing variation trend during discharging. As shown in Fig. 5b and c, the characteristic peak (002) corresponding to the P2 phase remains steady and resumes its original state after a full cycle. Furthermore, no new peaks, such as those for OP4 or O2 phases, emerge or disappear when in the deep depotassiation state (4.0 V), which differs from the previously reported phase transitions of P2-type materials during charge and discharge. This phenomenon is consistent with the previous in situ EIS results, demonstrating that the reaction in the high-voltage region was suppressed to a certain extent. Additionally, the refinement outcomes demonstrate slight fluctuations in the lattice parameter (c) during cycling, with the utmost variation reaching merely 2.2% (Fig. 5d). This micro-strain structural evolution accentuates the stabilizing effect of F substituting for O sites, providing a structural perspective for electrochemical stability.

Fig. 5 (a) Ex situ XRD patterns and corresponding charge and discharge curves of KTMO-F7; (b) and (c) the corresponding mountain profiles at various angles, and (d) the calculated c-axis evolution for KTMO-F7.

Conclusions

In summary, we have put forward a layered KTMO-F7 cathode that possesses outstanding structural stability and rate capability by means of a design strategy involving F substitution for O sites. Specifically, due to the greater electronegativity of F, the TM–F bond tends to have a higher binding energy. Simultaneously, the ratio of Mn³⁺/Mn⁴⁺ is regulated to suppress the Jahn–Teller effect of Mn³⁺, and the harmful structural transformation is significantly alleviated, resulting in superior cycling stability. The introduction of F ions within the O slabs broadens the interlay spacing of the K layer and allows for fast K⁺ diffusion kinetics. Consequently, the KTMO-F7 cathode shows remarkable rate performance (delivering 51.9 mA h g⁻¹ at 3 C) and durable cycling stability (with 76.8% capacity retention after 100 cycles at 1 C), outperforming the undoped KTMO cathode material. In conjunction with Raman spectroscopy analysis, we illustrate that F substitutes for O, forming a stronger covalent bond with TM. This substitution effectively fortifies the crystal structure, as evidenced by our findings. Therefore, the anion F substitution at the O site strategy is a reliable method for the systematic design of cathode materials, and its effectiveness fully ensures a favorable ion diffusion rate and reliable structural stability.

Author contributions

Yurong Wu: writing – original draft, validation, formal analysis, and data curation; Ziyun Zhang: data curation, investigation, and methodology; Jiangshan Huo: visualization and methodology; Runguo Zheng: investigation and formal analysis; Zhishuang Song: investigation and formal analysis; Zhiyuan Wang: conceptualization, funding acquisition, methodology, supervision, and writing – review & editing; Yanguo Liu: investigation and supervision; and Dan Wang: investigation and formal analysis.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 52071073, 52177208, and 52171202), the Hebei Province “333 Talent Project” (No. C20221012), the Science and Technology Project of the Hebei Education Department (BJK2023005), the Fundamental Research Funds for the Central Universities (2024GFZD002), and the Natural Science Foundation of Hebei Province (E2024501015).

References

1. H. Hui, X. Ma, X. Zhao, R. Zheng, Z. Wang, Z. Song, H. Sun, Y. Liu and D. Wang, Enhancing the potassium-ion storage performance of potassium manganese oxide cathode by single-element doping strategies, J. Power Sources, 2025, 626, 235780.
2. Z. Zhang, H. Hui, Q. Chen, Z. Wang, R. Zheng, H. Sun and Y. Liu, Component design and regulation to stabilize P3-type Mn-based layered cathodes for potassium-ion batteries, Ceram. Int., 2024, 50, 5974–5981.
3. X. Zhang, Z. Wei, K. N. Dinh, N. Chen, G. Chen, F. Du and Q. Yan, Layered Oxide Cathode for Potassium-Ion Battery: Recent Progress and Prospective, Small, 2020, 16, 2002700.
4. K. Sun, S. H. Luo, G. D. Hao, S. Guo, L. X. Qian, S. X. Yan and Q. Wang, Review on Layered Manganese–Based Metal Oxides Cathode Materials for Potassium–Ion Batteries: From Preparation to Modification, Chem. Rec., 2024, 24, e202300327.
5. L. Duan, C. Shao, J. Liao, L. Song, Y. Zhang, R. Li, S. Guo, X. Zhou and H. Zhou, A P2/P3 Biphasic Layered Oxide Composite as a High-Energy and Long-Cycle-Life Cathode for Potassium-Ion Batteries, Angew. Chem., Int. Ed., 2024, 63, e202400868.
6. H. Kim, J. C. Kim, S. H. Bo, T. Shi, D. H. Kwon and G. Ceder, K–Ion Batteries Based on a P2−Type K_0.6CoO₂ Cathode, Adv. Mater., 2017, 7, 201700098.
7. H. Kim, D. H. Seo, J. C. Kim, S. H. Bo, L. Liu, T. Shi and G. Ceder, Investigation of Potassium Storage in Layered P3−Type K_0.5MnO₂ Cathode, Adv. Mater., 2017, 29, 1702480.
8. S. H. Guo, P. Liu, H. J. Yu, Y. B. Zhu, M. W. Chen, M. Ishida and H. S. Zhou, A Layered P2− and O3−Type Composite as a High–Energy Cathode for Rechargeable Sodium–Ion Batteries, Angew. Chem., Int. Ed., 2015, 54, 5894–5899.
9. J. Zhou, Y. Wang, J. Wang, Y. Liu, Y. Li, L. Cheng, Y. Ding, S. Dong, Q. Zhu, M. Tang, Y. Wang, Y. Bi, R. Sun, Z. Wang and H. Wang, Low-temperature and high-rate sodium metal batteries enabled by electrolyte chemistry, Energy Storage Mater., 2022, 50, 47–54.
10. K. Lei, Z. Zhu, Z. Yin, P. Yan, F. Li and J. Chen, Dual Interphase Layers In Situ Formed on a Manganese-Based Oxide Cathode Enable Stable Potassium Storage, Chem, 2019, 5, 3220–3231.
11. K. Zhang, D. Kim, Z. Hu, M. Park, G. Noh, Y. Yang, J. Zhang and V. W. Lau, Manganese based layered oxides with modulated electronic and thermodynamic properties for sodium ion batteries, Nat. Commun., 2019, 10, 5203.
12. P.-F. Wang, T. Jin, J. Zhang, Q.-C. Wang, X. Ji, C. Cui, N. Piao, S. Liu, J. Xu, X.-Q. Yang and C. Wang, Elucidation of the Jahn-Teller effect in a pair of sodium isomer, Nano Energy, 2020, 77, 105167.
13. J. J. Liang, C. H. Lin, X. C. Meng, M. Liang, J. Lai, X. H. Zheng, Q. Z. Huang, L. Y. Liu and Z. C. Shi, P3-Type K_0.45Co_1/12Mg_1/12Mn_5/6O₂ as a superior cathode material for potassium-ion batteries with high structural reversibility ensured by Co-Mg Co-substitution, J. Mater. Chem. A, 2021, 9, 17261–17269.
14. L. Y. Liu, J. J. Liang, W. L. Wang, C. Han, Q. B. Xia, X. Ke, J. Liu, Q. F. Gu, Z. C. Shi, S. L. Chou, S. X. Dou and W. J. Li, A P3-Type K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ Cathode Material for Potassium-Ion Batteries with High Structural Reversibility Secured by the Mg-Ni Pinning Effect, ACS Appl. Mater. Interfaces, 2021, 13, 28369–28377.
15. Y. S. Zheng, J. F. Li, S. P. Ji, K. S. Hui, S. Wang, H. F. Xu, K. X. Wang, D. A. Dinh, C. Y. Zha, Z. P. Shao and K. N. Hui, Zinc–Doping Strategy on P2−Type Mn–Based Layered Oxide Cathode for High–Performance Potassium–ion Batteries, Small, 2023, 19, 2302160.
16. R. Huang, Q. Xue, J. Lin, X. Zhang, J. Zhou, F. Wu, L. Li and R. Chen, Layered K_0.54Mn_0.78Mg_0.22O₂ as a high-performance cathode material for potassium-ion batteries, Nano Res., 2022, 15, 3143–3149.
17. Z. Xiao, F. Xia, L. Xu, X. Wang, J. Meng, H. Wang, X. Zhang, L. Geng, J. Wu and L. Mai, Suppressing the Jahn-Teller Effect in Mn-Based Layered Oxide Cathode toward Long-Life Potassium-Ion Batteries, Adv. Funct. Mater., 2022, 32, 2108244.
18. Q. Zhang, C. Didier, W. K. Pang, Y. J. Liu, Z. J. Wang, S. Li, V. K. Peterson, J. F. Mao and Z. P. Guo, Structural Insight into Layer Gliding and Lattice Distortion in Layered Manganese Oxide Electrodes for Potassium–Ion Batteries, Adv. Energy Mater., 2019, 9, 1900568.
19. Y.-S. Xu, Q.-H. Zhang, D. Wang, J.-C. Gao, X.-S. Tao, Y. Liu, Y.-G. Sun, L. Gu, B.-B. Chang, C.-T. Liu, S.-Q. Shi and A.-M. Cao, Enabling reversible phase transition on K_5/9Mn_7/9Ti_2/9O₂ for high-performance potassium-ion batteries cathodes, Energy Storage Mater., 2020, 31, 20–26.
20. M. K. Cho, J. H. Jo, J. U. Choi and S.-T. Myung, Cycling Stability of Layered Potassium Manganese Oxide in Nonaqueous Potassium Cells, ACS Appl. Mater. Interfaces, 2019, 11, 27770–27779.
21. Y. You, S. Xin, H. Y. Asl, W. Li, P.-F. Wang, Y.-G. Guo and A. Manthiram, Insights into the Improved High-Voltage Performance of Li-Incorporated Layered Oxide Cathodes for Sodium-Ion Batteries, Chem, 2018, 4, 2124–2139.
22. Y. Shen, C. Cheng, X. Xia, L. Wang, X. Zhou, P. Zeng, J. Zeng and L. Zhang, Achieving structurally stable O3-type layered oxide cathodes through site-specific cation-anion co-substitution for sodium-ion batteries, J. Energy Chem., 2024, 93, 411–418.
23. J. Mu, T. Cai, W. Dong, C. Zhou, Z. Han and F. Huang, Biphasic high-entropy layered oxide as a stable and high-rate cathode for sodium-ion batteries, Chem. Eng. J., 2023, 471, 144403.
24. F. Ding, H. Wang, Q. Zhang, L. Zheng, H. Guo, P. Yu, N. Zhang, Q. Guo, F. Xie, R. Dang, X. Rong, Y. Lu, R. Xiao, L. Chen and Y.-S. Hu, Tailoring Electronic Structure to Achieve Maximum Utilization of Transition Metal Redox for High-Entropy Na Layered Oxide Cathodes, J. Am. Chem. Soc., 2023, 145, 13592–13602.
25. C. Zhou, L. Yang, C. Zhou, J. Liu, R. Hu, J. Liu and M. Zhu, Fluorine-substituted O3-type NaNi_0.4Mn_0.25Ti_0.3Co_0.05O_2(-xF_x cathode with improved rate capability and cyclic stability for sodium-ion storage at high voltage, J. Energy Chem., 2021, 60, 341–350.
26. K. Liu, S. S. Tan, J. Moon, C. J. Jafta, C. Li, T. Kobayashi, H. Lyu, C. A. Bridges, S. Men, W. Guo, Y. F. Sun and J. L. Zhang, Insights into the Enhanced Cycle and Rate Performances of the F–Substituted P2−Type Oxide Cathodes for Sodium–Ion Batteries, Adv. Energy Mater., 2020, 10, 2000135.
27. M. S. Chae, H. J. Kim, J. Lyoo, R. Attias, Y. Gofer, S. E. Hong and D. Aurbach, Anomalous Sodium Storage Behavior in Al/F Dual–Doped P2−Type Sodium Manganese Oxide Cathode for Sodium–Ion Batteries, Adv. Energy Mater., 2020, 10, 2002205.
28. H. Chen, Z. Wu, Y. Zhong, T. Chen, X. Liu, J. Qu, W. Xiang, J. Li, X. Chen, X. Guo and B. Zhong, Boosting the reactivity of Ni²⁺/Ni³⁺ redox couple via fluorine doping of high performance Na_0.6Mn_0.95Ni_0.05O_2(-xF_x cathode, Electrochim. Acta, 2019, 308, 64–73.
29. Y. Y.-S. Xu, M.-Y. Qi, Q.-H. Zhang, F.-Q. Meng, Y.-N. Zhou, S.-J. Guo, Y.-G. Sun, L. Gu, B.-B. Chang, C.-T. Liu, A.-M. Cao and L.-J. Wan, Anion Doping for Layered Oxides with a Solid-Solution Reaction for Potassium-Ion Battery Cathodes, ACS Appl. Mater. Interfaces, 2022, 14, 13379–13387.
30. C. L. Liu, S. H. Luo, H. B. Huang, Y. C. Zhai and Z. W. Wang, Layered potassium-deficient P2- and P3-type cathode materials K_xMnO₂ for K-ion batteries, Chem. Eng. J., 2019, 356, 53–59.
31. C. C. Wang, L. J. Liu, S. Zhao, Y. C. Liu, Y. B. Yang, H. J. Yu, S. Lee, G. H. Lee, Y. M. Kang, R. Liu and F. J. Li, Tuning local chemistry of P2 layered-oxide cathode for high energy and long cycles of sodium-ion battery, Nat. Commun., 2021, 12, 2256.
32. Z. Chen, M. Yang, G. Chen, G. Tang, Z. Huang, M. Chu, R. Qi, S. Li, R. Wang, C. Wang, T. Zhang, J. Zhai, W. Zhao, J. Zhang, J. Chen, L. He, J. Xu, W. Yin, J. Wang and Y. Xiao, Triggering anionic redox activity in Fe/Mn-based layered oxide for high-performance sodium-ion batteries, Nano Energy, 2022, 94, 106958.
33. W. P. Kang, P. Ma, Z. N. Liu, Y. Y. Wang, X. T. Wang, H. Chen, T. L. He, W. C. Luo and D. F. Sun, Tunable Electrochemical Activity of P2-Na_0.6Mn_0.7Ni_0.3O_2(-xF_x Microspheres as High-Rate Cathodes for High-Performance Sodium Ion Batteries, ACS Appl. Mater. Interfaces, 2021, 13, 15333–15343.
34. D. Wang, X. Feng, H. He, Z. Wang, R. Zheng, H. Sun, Y. Liu and C. Liu, Amorphous FeCoNiBO_x nanosheets as highly active and durable electrocatalysts for oxygen evolution reaction in alkaline electrolyte, New J. Chem., 2022, 46, 22989–22993.
35. S. J. Kim, J. H. Hong and Y. C. Kang, Spray-assisted synthesis of layered P2-type Na_0.67Mn_0.67Cu_0.33O₂ powders and their superior electrochemical properties for Na-ion battery cathode, Appl. Surf. Sci., 2023, 611, 155673.
36. K. Lee, S. Shin, T. Degen, W. Lee and Y. S. Yoon, In situ analysis of SnO₂/Fe₂O₃/RGO to unravel the structural collapse mechanism and enhanced electrical conductivity for lithium-ion batteries, Nano Energy, 2017, 32, 397–407.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00385g

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