Rare earth pillars for stable layered birnessite cathodes propelling aqueous zinc-ion batteries with ultra-long cyclability

Abstract

The weak structural stability, low intrinsic conductivity, and strong electrostatic interaction of cathode materials are still bottlenecks in aqueous zinc-ion batteries. Herein, a novel win–win strategy was proposed to fabricate a yttrium ion pre-intercalated birnessite-MnO₂ cathode material. Benefiting from the unique advantages of rare earth ions with large radii, it could serve as an interlayer pillar in the crystal lattice to stabilize the structure and enhance ionic conductivity. Furthermore, the high valence state of rare earth ions could significantly weaken the electrostatic interaction between zinc ions and host structures, thereby reducing charge transfer resistance and promoting ion transport. As a result, Y_0.04K_0.16Mn₂O₄·2.3H₂O exhibits an ultra-long cycling stability of 24 000 cycles at a high current density of 8 A g⁻¹, and the average capacity decay rate is only 0.002% per cycle. This work paves the way for the application of rare earth elements in energy storage.

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The 10th anniversary statement

Warmly celebrating the 10th anniversary of the establishment of Inorganic Chemistry Frontiers. I am Yaping Du, a professor at Nankai University. My major is rare earth functional materials and applications. Over the past decade, we have witnessed tremendous growth in Inorganic Chemistry Frontiers. This journal has not only served as an excellent platform to showcase important research work but also provided the impetus for our career development. It is truly gratifying to publish in this esteemed journal. I sincerely hope that the journal will continue to uphold high standards in the future and deliver more valuable content to academia and society.

Introduction

Aqueous zinc-ion batteries (AZIBs) have exhibited several unrivaled advantages, and have attracted the attention of researchers. Zinc metal has abundant reserves, suitable redox potential (−0.76 V vs. SHE), and high theoretical specific capacity (820 mA h g⁻¹).^1,2 Birnessite-MnO₂ is regarded as a highly promising cathode material for AZIBs owing to its low cost, high redox potential (1.3–1.4 V), and larger layered spacing (0.7 nm).³ However, its weak structural stability, low intrinsic conductivity, and strong electrostatic interaction with Zn²⁺ are still bottlenecks in the development of AZIBs.

Up to now, chemists have offered many viable solutions, mainly including conductive material composites, conductive self-supporting substrate,⁴ defect engineering, amorphous engineering,⁵ and ion pre-intercalation. Among them, ion pre-intercalation could play “pillar” and “electrostatic shielding” roles, which is an effective strategy to significantly improve the structural stability and conductivity of electrode materials, weaken electrostatic interactions, and accelerate electrochemical reaction kinetics.⁶ Compared to transition metal ions such as Fe²⁺ (76 pm), Mg²⁺ (72 pm), and Cu²⁺ (72 pm), rare earth ions have larger ionic radii (such as Y³⁺ (102 pm), La³⁺ (116 pm), and Ce³⁺ (114 pm)).⁷ This benefits their role as interlayer pillars in the crystal lattice to stabilize the structure and enhance ionic conductivity. Furthermore, the high valence states of rare earth ions will significantly weaken the electrostatic interactions between zinc ions and host structures, thereby reducing charge transfer resistance and promoting ion transport.⁶ Besides, rare earth elements are strategic elements known as industrial monosodium glutamate, and their little use can make a huge impact. They have been widely utilized in high-performance lasers,⁸ permanent magnets,⁹ and photocatalytic water oxidation.¹⁰ Nevertheless, rare earth element research in the field of energy storage is still in its early stages. More importantly, rare earth elements with large ionic radii and high electric charge have unparalleled potential in the modification of electrode materials. It should be mentioned that yttrium was the first rare earth element identified in the world, and its abundance in the Earth's crust is comparable to that of metallic zinc and its price is low.¹¹ Therefore, choosing yttrium ions as a pre-intercalation source can not only achieve the goal of improving the electrochemical performance of electrode materials, but also meet the requirements of economic sustainable development.

Herein, a novel win–win strategy was proposed to fabricate the yttrium ion pre-intercalated birnessite-MnO₂ cathode material. The rare earth pillars expand the lattice spacing of birnessite-MnO₂ and stabilize the layered structure. DFT calculations reveal that yttrium ion pre-intercalation enhances the conductivity of the electrode material and weakens the electrostatic interaction between zinc ions and the host structure. Additionally, the migration barrier of zinc ions in the host structure is significantly reduced from 1.84 to 0.32 eV, which is conducive to accelerating the reversible insertion/extraction process of Zn²⁺. As a result, Y_0.04K_0.16Mn₂O₄·2.3H₂O exhibits an ultra-long cycling stability of 24 000 cycles at a high current density of 8 A g⁻¹, and the average capacity decay rate is only 0.002% per cycle.

Results and discussion

The microstructure of Y_0.04K_0.16Mn₂O₄·2.3H₂O was analyzed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). It shows that the flower-like microsphere (with an average diameter of about 1 μm) is composed of nanosheets (Fig. 1a–c). Fig. 1d depicts the selected area electron diffraction (SAED) pattern of Y_0.04K_0.16Mn₂O₄·2.3H₂O, revealing that it is a polycrystalline material. The related diffraction rings correspond to the (2 0 0), (−1 1 2), and (3 1 0) crystal planes, respectively. The high-resolution TEM (HRTEM) image displays a lattice spacing of 0.252 nm (Fig. 1e), which can be attributed to the (2 0 0) crystal plane of birnessite-MnO₂ (PDF#80-1098). In comparison, the (200) crystal plane of K_0.24Mn₂O₄·1.9H₂O corresponds to a lattice spacing of 0.248 nm (Fig. S1c†), which indicates that yttrium ion pre-intercalation expands the interlayer spacing. Energy-dispersive X-ray spectroscopy mappings of Y_0.04K_0.16Mn₂O₄·2.3H₂O are shown in Fig. 1f, which exhibits a uniform distribution of Mn, O, and Y elements. Under the same reaction conditions, a series of Y_xK_yMn₂O₄·nH₂O with different Y/Mn atomic ratios was synthesized. The SEM and TEM images of other as-prepared samples except Y_0.04K_0.16Mn₂O₄·2.3H₂O are given in Fig. S1–S3.† They are all flower-like microspheres of nanosheets, which indicates that a small amount of yttrium ions will not significantly change the microscopic morphology of manganese dioxide. The as-prepared samples were quantitatively analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The results are shown in Fig. 1g and Table S1.† The atomic ratios of K/Mn in K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O are 0.12 and 0.08, respectively. Additionally, the atomic ratio of Y/Mn is 0.02 in Y_0.04K_0.16Mn₂O₄·2.3H₂O. The water content of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O was determined by the thermogravimetric (TG) test (Fig. 1h and i). In the temperature range below 100 °C, the main loss is that of physically adsorbed water, and the weight loss of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O is 6.43% and 6.71%, respectively. The weight loss of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O in the range of 100–450 °C is 15.56% and 18%, respectively, confirming the existence of crystal water. For comparison, the TG analysis results of other samples are shown in Fig. S4 and S5.† Based on the above detailed analysis, it can be determined that the molecular formulas of the as-prepared samples are K_0.24Mn₂O₄·1.9H₂O, Y_0.03K_0.19Mn₂O₄·2.2H₂O, Y_0.04K_0.16Mn₂O₄·2.3H₂O and Y_0.06K_0.15Mn₂O₄·2.4H₂O, respectively. Interestingly, as the content of yttrium element increases, the potassium content decreases and that of the water molecules increases. This shows that hydrated yttrium ions replace a part of the potassium ions between the layers, which can increase the interlayer spacing, and can also act as a “pillar” to enhance structural stability, thus enhancing the zinc ion storage capacity.^3,12

Fig. 1 Structural and morphological characterization of Y_0.04K_0.16Mn₂O₄·2.3H₂O: (a) SEM image. (b and c) TEM images. (d) SAED pattern. (e) HRTEM image. (f) Elemental mapping images. (g) ICP-OES results of the obtained samples. (h and i) TG curves of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O.

X-ray diffraction (XRD) was used to examine the crystal structure of the as-prepared samples (Fig. 2a). There are some obvious diffraction peaks at 12.2°, 25.0°, and 36.7°, which can be assigned to the (0 0 1), (0 0 2), and (−1 1 1) crystal planes of birnessite-MnO₂ (PDF#80-1098).¹³ Notably, no other impurity peaks appear in the XRD patterns. Importantly, the diffraction peak corresponding to the (0 0 1) crystal plane shifts to a small angle direction, indicating that the introduced yttrium ions can enhance interlayer spacing (Fig. 2b).¹⁴Fig. 2c shows the Raman spectra of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O. The peaks of K_0.24Mn₂O₄·1.9H₂O at 359 cm⁻¹ and 643 cm⁻¹ are attributed to the vibration of Mn–O. Compared with K_0.24Mn₂O₄·1.9H₂O, the Raman peaks of Y_0.04K_0.16Mn₂O₄·2.3H₂O are red-shifted to positions 336 cm⁻¹ and 635 cm⁻¹, respectively. In addition, Y_0.04K_0.16Mn₂O₄·2.3H₂O exhibits a significant Raman signal at 565 cm⁻¹. It indicates that the stretching vibration of the Mn–O bond is affected by yttrium ion intercalation.¹⁵ The X-ray photoelectron spectroscopy (XPS) testing was used to further explore the elemental valence state and chemical composition of Y_0.04K_0.16Mn₂O₄·2.3H₂O. In the high-resolution Y 3d spectrum, the binding energy of the Y 3d_3/2 peak located at 159.7 eV (Fig. 2d), which was situated between Y⁰ (158.8 eV) and Y³⁺ (160.5 eV) states, suggests the presence of ionic Y^δ+ (0 < δ < 3) in Y_0.04K_0.16Mn₂O₄·2.3H₂O.¹⁶ The Mn 2p XPS spectrum indicates the binding energy of Mn 2p_3/2 and Mn 2p_1/2 at 642.66 and 653.89 eV, respectively (Fig. 2e). The high-resolution XPS spectrum of O 1s is shown in Fig. 2f. It's fitting peaks can be decomposed into the Mn–O–Mn bond, Mn–O–H bond, and H–O–H bond, which are located at 530.09 eV, 531.56 eV, and 532.81 eV, respectively.¹⁷ The Mn–O–H bond and H–O–H bond prove the existence of crystal water in Y_0.04K_0.16Mn₂O₄·2.3H₂O once again, which is in good agreement with the analysis results of TG.¹⁸ The energy band structure, total density of states (TDOS) and partial density of states (PDOS) of Y_0.04K_0.16Mn₂O₄·2.3H₂O are shown in Fig. 2g–i. Compared with K_0.24Mn₂O₄·1.9H₂O (0.965 eV) (Fig. S6†), Y_0.04K_0.16Mn₂O₄·2.3H₂O exhibits a smaller energy band gap (0.719 eV), which indicates that the pre-intercalation of Y ions rearranges the electronic structure and improves the conductivity.

Fig. 2 (a and b) XRD patterns. (c) Raman spectra of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O. (d–f) High-resolution XPS spectra of Y 3d, Mn 2p, and O 1s in Y_0.04K_0.16Mn₂O₄·2.3H₂O. (g–i) The calculated band structure, TDOS patterns, and PDOS patterns of Y_0.04K_0.16Mn₂O₄·2.3H₂O.

Fig. 3a depicts the cyclic voltammetry (CV) curve of Y_0.04K_0.16Mn₂O₄·2.3H₂O throughout the initial four cycles at 0.1 mV s⁻¹. Two pairs of distinct redox peaks located at 1.39/1.59 V and 1.26/1.54 V correspond to the insertion/extraction of H⁺ and Zn²⁺, respectively.¹⁹ The perfect overlap of the redox peak position indicates that Y_0.04K_0.16Mn₂O₄·2.3H₂O has excellent structural stability. The discharge capacities of K_0.24Mn₂O₄·1.9H₂O at 0.1, 0.5, 1.0, 4.0, 8.0, and 10 A g⁻¹ are 253.4, 179.9, 132.1, 86.3, 70.7, and 66 mA h g⁻¹, respectively. In comparison, Y_0.04K_0.16Mn₂O₄·2.3H₂O could exhibit higher discharge capacities at the same current density, which are 292.8, 259.1, 222.5, 138.3, 113.6, and 106.8 mA h g⁻¹, respectively (Fig. 3b). Remarkably, Y_0.04K_0.16Mn₂O₄·2.3H₂O has a better rate performance, which indicates that the conductivity is enhanced by the intercalation of Y ions. The rate performance of Y_0.03K_0.19Mn₂O₄·2.2H₂O and Y_0.06K_0.15Mn₂O₄·2.4H₂O is given in Fig. S7 and S8.† Cycling performance tests of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O were conducted at a current density of 2.0 A g⁻¹ (Fig. 3c). The results show that the discharge capacity of K_0.24Mn₂O₄·1.9H₂O decays rapidly in the first 200 cycles. Nevertheless, the discharge capacity of Y_0.04K_0.16Mn₂O₄·2.3H₂O remains at a high level of 109.1 mA h g⁻¹ after 1000 cycles. This shows that yttrium ion intercalation significantly enhances the structural stability of the electrode material, thus making Y_0.04K_0.16Mn₂O₄·2.3H₂O have better cycling stability than K_0.24Mn₂O₄·1.9H₂O.²⁰ The cycling performance of Y_0.03K_0.19Mn₂O₄·2.2H₂O and Y_0.06K_0.15Mn₂O₄·2.4H₂O at 2.0 A g⁻¹ is given in Fig. S9 and S10.† Furthermore, an ultra-long cycling performance test of Y_0.04K_0.16Mn₂O₄·2.3H₂O was performed at a high current density of 8.0 A g⁻¹. As shown in Fig. 3d, it can exhibit an initial discharge capacity of 135.9 mA h g⁻¹. The average capacity decay rate is only 0.002% per cycle during the 24 000 cycles and the coulombic efficiency remained at ∼100%. The cycling performance is better than those of most reported AZIB cathode materials, as displayed in Fig. 3e and summarized in Table S2.† The Galvanostatic Charge/Discharge (GCD) curves of Y_0.04K_0.16Mn₂O₄·2.3H₂O show two well-maintained charge and discharge platforms and high specific capacities at different current densities, corresponding to the H⁺/Zn²⁺ insertion/extraction.²¹ It matched well with the CV test results. In addition, their average discharge voltage is about 1.35 V (Fig. 3f). Notably, Y_0.04K_0.16Mn₂O₄·2.3H₂O can exhibit an energy density of 395.3 W h kg⁻¹ at a power density of 135 W kg⁻¹, and can display a very impressive energy density (144.2 W h kg⁻¹) even at a power density of 13 500 W kg⁻¹, which is at a very high level among reported AZIB cathode materials (Fig. 3g and Table S3†). Such high specific energy benefits from high discharge capacity and discharge voltage. To verify the feasibility of the Zn||Y_0.04K_0.16Mn₂O₄·2.3H₂O cell as a power source, the two cells were integrated in series with an open circuit voltage of about 2.8 V, which is enough to light a blue light-emitting diode (Fig. 3h).

Fig. 3 (a) CV curve of Y_0.04K_0.16Mn₂O₄·2.3H₂O. (b) Rate performance of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O at various current densities. (c) Cycling performance of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O at 2 A g⁻¹. (d) Ultra-long cyclability of Y_0.04K_0.16Mn₂O₄·2.3H₂O at 8 A g⁻¹. (e) Comparison of cycling performance with those of other reported cathodes in AZIBs. (f) GCD curves of Y_0.04K_0.16Mn₂O₄·2.3H₂O. (g) Comparison with other aqueous AZIBs (Ragone plot). (h) Optical image of the blue light-emitting diodes powered by two Zn||Y_0.04K_0.16Mn₂O₄·2.3H₂O cells integrated in series.

The electrochemical reaction kinetics of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O were evaluated through a series of tests. Fig. 4a shows the CV test of Y_0.04K_0.16Mn₂O₄·2.3H₂O, which has two pairs of redox peaks. As the scan rate gradually increases, the shape of the peaks hardly changes, indicating its excellent structural stability.²² The relationship between the peak current (i_p) and scan rate (v) could be described according to the following two formulas:²³

i_p = av^b (1)

log(i_p) = b log(v) + log a (2)

Fig. 4 (a) CV profiles of Y_0.04K_0.16Mn₂O₄·2.3H₂O at different scan rates. (b) log i and log v plots at specific peak currents. (c) Capacitive contribution of Y_0.04K_0.16Mn₂O₄·2.3H₂O. (d) EIS results of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O. (e and f) Differential charge density of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O. (g and h) Illustration of the Zn²⁺ diffusion path in K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O. (i) The energy barriers of Zn²⁺ migrating in K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O.

Among them, a and b are two changeable constants, and the b value is connected to the charge storage way. When b = 0.5, the charge storage is affected by ion diffusion. When b = 1, it is controlled by capacitive behavior. The b values corresponding to the four obvious redox peaks are 0.62, 0.60, 0.77, and 0.54, as determined by fitting log(i_p) and log(v). The b value between 0.5 and 1 indicates that the charge storage mode of Y_0.04K_0.16Mn₂O₄·2.3H₂O is controlled jointly by ion diffusion and capacitive behavior (Fig. 4b).²⁴ The contribution of ion diffusion and capacitance behavior can be calculated using the following formula:

i_p = k₁v + k₂v^1/2 (3)

where the ratio of k₁v and i_p is the capacitance behavior contribution and the ratio of k₂v^1/2 and i_p corresponds to the ion diffusion contribution. As the scan rate increases from 0.1 to 0.5 mV s⁻¹, the capacitance contribution ratio improves from 43.89% to 63.4% (Fig. 4c). Furthermore, the electrochemical impedance spectroscopy (EIS) test results show that the charge transfer resistance (R_ct) value of Y_0.04K_0.16Mn₂O₄·2.3H₂O (153.3 Ohm) is smaller than that of K_0.24Mn₂O₄·1.9H₂O (385.8 Ohm), which indicates that yttrium ion intercalation can effectively accelerate charge transfer (Fig. 4d). Fig. 4e and f show the differential charge density of K_0.24Mn₂O₄·1.9H₂O and Y_0.04K_0.16Mn₂O₄·2.3H₂O respectively, where light blue and yellow shaded areas represent the charge depletion and accumulation regions. The results show that the interaction between zinc and O atoms in Y_0.04K_0.16Mn₂O₄·2.3H₂O is weaker than that in K_0.24Mn₂O₄·1.9H₂O, indicating that Y ion pre-intercalation could promote charge transfer. The diffusion energy barrier of Zn²⁺ in K_0.24Mn₂O₄·1.9H₂O (Fig. 4g) and Y_0.04K_0.16Mn₂O₄·2.3H₂O (Fig. 4h) was further calculated. As shown in Fig. 4i, the Zn²⁺ diffusion energy barrier of Y_0.04K_0.16Mn₂O₄·2.3H₂O (0.32 eV) is much lower than that of K_0.24Mn₂O₄·1.9H₂O (1.84 eV), which indicates that Y ion pre-intercalation could help accelerate the reaction kinetics.²⁵ In addition, the GITT test was carried out on the Y_0.04K_0.16Mn₂O₄·2.3H₂O material at a current density of 0.1 A g⁻¹ (Fig. S11†).

The electrochemical energy storage mechanism of Y_0.04K_0.16Mn₂O₄·2.3H₂O was explored using in situ XRD testing techniques. The GCD curve of Y_0.04K_0.16Mn₂O₄·2.3H₂O is shown in Fig. 5a, and the contour map of the phase evolution under various charge and discharge states is displayed in Fig. 5b. The diffraction peak at 26.5° remains constant during the charge and discharge processes, which may be attributed to the characteristic peaks of the carbon paper (current collector). As the discharge progresses, new diffraction peaks at 28.1° and 35.0° can be observed, and they progressively fade away over the ensuing charging process. These peaks can be attributed to the formation of Zn₄SO₄(OH)₆·xH₂O (ZSH).²⁶ The change in the intensity of the ZSH diffraction peak verifies that it is formed gradually throughout discharge and dissolved in a reversible way during charge. It is consistent with numerous recent reports that accumulating OH⁻ causes ZSH to form on the cathode surface, whereas H⁺ diffuses toward the active material.²⁷ The diffraction peak at 32.9° matches well with ZnSO₄·xH₂O, which can be explained by the consumption of water molecules during the formation of ZSH.²⁸ In addition, only the characteristic peak corresponding to the (1 1 0) crystal plane of Y_0.04K_0.16Mn₂O₄·2.3H₂O underwent a slight shift, which is related to the reversible intercalation/extraction of Zn²⁺ in the layered structure.²⁹ The valence state evolution of Zn, Mn and O elements under electrochemical charge and discharge conditions was further investigated using ex situ XPS technology. In the fully discharged state, there are two prominent peaks at 1045.77 and 1022.73 eV attributed to Zn 2p. When it is fully charged, the signal weakens significantly (Fig. 5c). It demonstrates the reversible intercalation/extraction of zinc ions during charge and discharge processes.²² The spin energy separation of Mn 3s increases from 4.95 eV at the initial state to 5.06 eV at the completely discharged state, possibly due to the intercalation of H⁺/Zn²⁺ leading to the decrease of high-valent Mn ions. At the fully charged state, the spin energy separation of Mn 3s is reduced to 4.29 eV, indicating that the extraction of H⁺/Zn²⁺ increases the valence state of Mn. Notably, the peak of Zn 3p appearing in the XPS spectrum of Mn 3s indicates the presence of ZSH (Fig. 5d). When fully discharged to 0.8 V, the enhancement of the peak at 532.2 eV in O 1s implies that abundant OH⁻ and water molecules lead to the formation of ZSH. The intensity of the peak at 529.6 eV representing the Mn–O bond changes drastically under different electrochemical states. Specifically, the Mn–O bond signal is very weak in the fully discharged state, which may be caused by the fact that a part of Y_0.04K_0.16Mn₂O₄·2.3H₂O is covered by ZSH.¹⁵ In the fully charged state, the peak signal of the Mn–O bond is significantly enhanced along with the reversible disappearance of ZSH (Fig. 5e). This demonstrates that Y_0.04K_0.16Mn₂O₄·2.3H₂O has an excellent structural stability. The analysis results of ex situ XPS are compatible with in situ XRD results. According to the results of the aforesaid investigation, Y_0.04K_0.16Mn₂O₄·2.3H₂O undergoes extremely reversible co-intercalation/extraction of H⁺/Zn²⁺ during the reaction process.

Fig. 5 (a) GCD curve of Y_0.04K_0.16Mn₂O₄·2.3H₂O. (b) In situ XRD contour map analysis in the 2nd cycle. (c–e) High resolution XPS spectra of Zn 2p, Mn 3s and O 1s at different states.

Conclusions

In summary, novel yttrium ion pre-intercalated birnessite-MnO₂ cathode materials were fabricated by a simple one-step hydrothermal method. Both experimental results and DFT calculations demonstrate that the yttrium ion pre-intercalation could play “pillar” and “electrostatic shielding” roles. It is an effective strategy to significantly improve the structural stability and conductivity of electrode materials, weaken electrostatic interactions, and accelerate electrochemical reaction kinetics. As a cathode material for AZIBs, Y_0.04K_0.16Mn₂O₄·2.3H₂O exhibits an ultra-long cycling stability of 24 000 cycles at 8 A g⁻¹, and the average capacity decay rate is only 0.002% per cycle, indicating that it has an excellent structural stability.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22269024 and 22371131), the Ph.D. Research Startup Foundation of Yan'an University (YDBK202022), and the Key Research and Development Program of Shaanxi Province (2024GX-YBXM-439). We also acknowledge the support from the National Science Fund for Distinguished Young Scholars (22425503), the 111 Project (B18030) from China, the Outstanding Youth Project of Tianjin Natural Science Foundation (20JCJQJC00130), and the Key Laboratory of Rare Earths, Chinese Academy of Sciences. We also thank “Yan-Tu-Hui Test Center” for in situ XRD analysis and tests.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02654c

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