English

• Supercapacitors are suitable for a broad range of grid energy storage and high-power demand facilities due to their high-power density and excellent cycling life [1-7]. As a promising alternative to commercial electric double layer (EDL) capacitance materials, pseudocapacitive materials function through rapid Faradaic reactions, which lead to capacitive electrochemical characteristics and battery-level capacity [8,9]. In particular, unlike layered battery-type materials with diffusion-controlled kinetics [10], the intercalation pseudocapacitance is without phase change and is not limited by solid-state diffusion, which are promising for developing energy storage devices with both high-power density and high energy density [11,12].

  Among intercalation pseudocapacitive materials, birnessite (M_xMnO₂·nH₂O, M = Li, Na, K, Mg and so forth) has attracted considerable attention due to its excellent performance and abundance in raw materials [13-16]. Compared to other layered materials, e.g., P2-Na_0.67MnO₂, the birnessite shows high-rate capability and stable cycling performance (Table S1 in Supporting information). The electrochemical behavior and charge storage mechanisms of birnessite in aqueous electrolytes have been widely investigated [13,17-20]. Their quasi-rectangular cyclic voltammetry (CV) curves and linear galvanostatic charge and discharge (GCD) profiles show typical capacitive behavior in aqueous electrolytes [13,17]. These electrochemical features display fingerprints similar to those of EDL models of carbon materials. The electrochemical quartz crystal microbalance (EQCM) is considered a powerful operando tool for sensitively monitoring the gravity changes during real-time electrochemical reactions [21]. Recently, Boyd et al. studied the charge storage mechanism of birnessite in aqueous electrolytes via EQCM and demonstrated that interlayer expansion upon oxidation is due to opposing fluxes of K⁺ and H₂O from the middle of the interlayer [19]. Other studies also pointed out that the (de)intercalation behavior of cations serves as the primary factor in the charge storage mechanism accompanied by an opposite flux of structural water [20,22]. The birnessite electrode shows the box-like CV shape in the aqueous electrolyte of 1 mol/L Na₂SO₄ in water (Figs. 1a and b), Nevertheless, aqueous electrolytes naturally suffer from a narrow electrochemical stability window [23], which limits the overall energy density.

  Figure 1

  

  Figure 1.  Comparison of charge storage of birnessite in aqueous and organic electrolytes. (a) CV curves of birnessite in the aqueous and organic electrolytes. Schematics of hydrated birnessite measures in the (b) aqueous electrode of 1 mol/L Na₂SO₄ in water and (c) organic electrode of 1 mol/L NaPF₆ in diglyme (denoted as DGDE).

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  Widening the application of layered birnessite to organic electrolytes is desirable for improved performance [23-26]. However, the CV curves of birnessite in an organic electrolyte show reversible broad redox peaks, which are quite different from the box-like shape in an aqueous electrolyte (Fig. 1c). It remains the question about the functions of confined water in birnessite when cycled in sodium-based organic electrolytes (Fig. 1c). Herein, we systematically investigate the electrochemical properties of birnessite (Na_0.4MnO₂·0.53H₂O) in sodium-based organic electrolytes. The Na_0.4MnO₂·0.53H₂O cathode delivers a high capacity of 185 mAh/g in a potential window of 1.5−4 V vs. Na⁺/Na and a high capacitive contribution of 85.6% at 1.0 mV/s in the organic electrolyte of 1 mol/L NaPF₆ in diethylene glycol dimethyl ether (DGDE). Ex situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) reveal a non-phase-change sodium-ion storage mechanism. Importantly, operando EQCM is used to quantitatively analyze the specific dynamic (dis)charging processes in three different organic electrolytes. This demonstrates the dominance of Na⁺ (totally desolvated) intercalation into Na_0.4MnO₂·0.53H₂O in organic electrolytes, while EDL surface adsorption contributes ~9.0% of the total capacity.

  Layered birnessite (Na_0.4MnO₂·0.53H₂O) was synthesized by a hydrothermal method (see Supporting information for more details) [27]. Birnessite is a two-dimensional layered structure composed of layers of edge-sharing manganese oxide octahedra, nanoconfined interlayer water and intercalated cations (inset of Fig. 2a) [28]. The XRD pattern (Fig. 2a) shows typical (00l) reflection peaks that correspond to the layered birnessite structure (JCPDS No. 43–1456). The diffraction peak at 12.48° is attributed to a layer d₀₀₁ of ~7.1 Å, which is suitable for ion diffusion [25]. Raman spectrum (Fig. 2b) shows a single peak at approximately 647 cm^–1, corresponding to the symmetric stretching vibration of Mn–O in [MnO₆] [29]. The FT-IR spectrum (Fig. S1 in Supporting information) shows that the peaks at ~3000−3600 cm^–1 correspond to –OH bond stretching, and those at ~1600 cm^–1 correspond to H–O–H bond bending [30], indicating the existence of interlayer water. The peak at ~500 cm^–1 represents the Mn–O stretching mode [25]. Detailed quantification of the water content was conducted by thermogravimetric analysis (TGA, Fig. 2c). An approximately 9.1% weight loss between 100 ℃ and 400 ℃ is considered the removal of structural water, corresponding to an amount of 0.53 H₂O per formula. Inductively coupled plasma-atomic emission spectrometry (ICP‒AES) analysis (Table S2 in Supporting information) indicates the molar ratio of Mn: Na is 1:0.4. Thus, the formula for the synthesized birnessite is Na_0.4MnO₂·0.53H₂O. The Brunauer‒Emmett‒Teller (BET) SSA is 59.7 m²/g (Fig. S2 in Supporting information). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figs. 2d−f) revealed lamellar morphologies. Fig. 2f shows the layered fringes of d₀₀₁ = 7.04 Å. The corresponding energy dispersive spectroscopy (EDS) elemental mappings (Fig. S3 in Supporting information) demonstrate the uniform distribution of all the elements in the synthesized birnessite sample.

  Figure 2

  

  Figure 2.  Characterization of Na_0.4MnO₂·0.53H₂O. (a) XRD pattern of Na_0.4MnO₂·0.53H₂O, inserted with a sketch of layered birnessite. (b) Raman spectrum of Na_0.4MnO₂·0.53H₂O, inserted with the vibration mode of Mn–O bonding. (c) TGA results of Na_0.4MnO₂·0.53H₂O. (d) SEM and (e, f) TEM images of Na_0.4MnO₂·0.53H₂O.

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  The electrochemical properties of Na_0.4MnO₂·0.53H₂O were investigated by half-cells with 1 mol/L NaPF₆ in DGDE as the electrolyte. Fig. 3a shows the transformed GCD profiles of Na_0.4MnO₂·0.53H₂O for the initial two cycles. The (dis)charging processes of birnessite are divided into three stages according to the slope: 1.5–2.5 V (Stage Ⅰ), 2.5–3.4 V (Stage Ⅱ), and 3.4–4 V vs. Na⁺/Na (Stage Ⅲ). Moreover, the GCD profiles at various specific currents ranging from 0.1 A/g to 15 A A/g are shown in Fig. 3b. The specific capacities are 185, 171, 138, and 101 mAh/g at 0.1, 0.2, 1, and 10 A/g (Fig. S4a in Supporting information), respectively, which are significantly higher than those of aqueous electrolytes (48.8 mAh/g at 0.5 A/g) [31]. The cycling stability is also evaluated at a specific current of 1 A/g, showing a capacity retention of 93% in organic electrolyte after 200 cycles (Fig. S4b in Supporting information). To analyze the reaction kinetics of Na_0.4MnO₂·0.53H₂O, the Na⁺ diffusion coefficient (D_Na⁺) was measured by the galvanostatic intermittent titration technique (GITT) (Fig. 3c). D_Na⁺ remains relatively high at approximately 4 × 10^–10 cm²/s, except for two minor drops at 2.5 and 3.5 V.

  Figure 3

  

  Figure 3.  (a) Charge and discharge curves of Na_0.4MnO₂·0.53H₂O. (b) Charge and discharge curves at different current densities of Na_0.4MnO₂·0.53H₂O. (c) GITT results and corresponding diffusion coefficients of Na_0.4MnO₂·0.53H₂O in the potential range of 1.5–4.0 V. (d) CV curves measured at 0.2 mV/s in different potential windows. (e) CV curves of Na_0.4MnO₂·0.53H₂O at different scan rates. (f) The calculated capacitive contribution of Na_0.4MnO₂·0.53H₂O at 1 mV/s.

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  Fig. 3d shows the CV curves of Na_0.4MnO₂·0.53H₂O in different potential windows. When the cutoff potential is above 2.5 V vs. Na⁺/Na, the quasi-rectangular CV characteristics are observed. It has been reported that birnessite displayed box-like shapes in 0–0.85 V vs. Ag/AgCl (refers to 2.93–3.78 V vs. Na⁺/Na and 0.22–1.07 V vs. standard hydrogen electrode (SHE)) in 0.5 mol/L K₂SO₄ aqueous electrolyte [19]. However, different from the CV shapes measured in aqueous electrolyte, the organic electrolyte enables further expanded the cutoff potential to 1.5 V vs. Na⁺/Na, and Na_0.4MnO₂·0.53H₂O display a couple of broad peaks at 2.39/2.58 V, attributed to Mn³⁺/Mn⁴⁺ redox reactions [32].

  The CV curves of Na_0.4MnO₂·0.53H₂O shows broad, reversible redox peaks with increasing sweep rates (Fig. 3e). According to the kinetics analysis of Na_0.4MnO₂·0.53H₂O, a detailed relationship between the peak current (i) and sweep rate (ν) was elucidated using Eq. 1 [33]:

  $ i=a v^b $

  (1)

  The b values (Fig. S5 in Supporting information) of the anodic and cathodic peaks are 0.975 and 0.976, respectively, indicating capacitive-dominated charge storage behavior. The charge storage contributions could be further quantitatively characterized by Eq. 2, where k₁ν is capacitive responses and k₂ν^1/2 is diffusion-controlled contributions [33]:

  $ i(V)=k_1 v+k_2 v^{1 / 2} $

  (2)

  The fitting result indicates a high capacitive contribution of 71.8% at 0.2 mV/s (Fig. S6 in Supporting information) and 85.6% at 1.0 mV/s (Fig. 3f). The charge dynamics of Na_0.4MnO₂·0.53H₂O is analyzed by Bode plots (Fig. S7 in Supporting information) [34,35]. The high phase angles over −60° at low frequence of 10 mHz indicate the capacitive dominated processes [34−36]. Based on the electrochemical behavior and kinetic analysis discussed above, it confirms that the charge storage dynamics of Na_0.4MnO₂·0.53H₂O include kinetically fast and reversible redox reactions, indicating the pseudocapacitive behavior.

  The detailed structural changes of Na_0.4MnO₂·0.53H₂O during (dis)charging were further explored. The GCD profiles and corresponding ex situ XRD patterns are shown in Fig. 4a. Only slight shifts of the (00l) diffraction peaks are observed, indicating a non-phase-change process during reversible Na⁺ (de)intercalation. Fig. 4b shows that the changes of d₀₀₁ are strongly related to the (de)intercalation amount of Na⁺ from Na_0.4MnO₂·0.53H₂O. Specifically, the d₀₀₁ increases from 7.07 Å to 7.09 Å at 4 V (Na_0.09MnO₂·0.53H₂O) during initial charging due to the enhanced repulsion between the layers caused by sodium ion extraction [25]. When Na⁺ fully inserts into the interlayers, the d₀₀₁ decreases to 6.95 Å at 1.5 V (Na_0.87MnO₂·0.53H₂O), yielding a total layer spacing change of 0.15 Å. In short, the intercalation of Na⁺ increases the attractive interactions between the layers, while the shielding effect provided by interlayer structural water relieves those interactions [25,37]. However, when the interlayer contains more sodium ions than in its equilibrium state (OCV = 3.0 V vs. Na⁺/Na), the amount of crystal water contained between the layers is insufficient to provide the shielding effect to relieve all the attraction caused by abundant Na⁺, resulting in significant layer distance changes from 1.5 V to 3 V. Overall, the 0.15 Å change in the total layer spacing indicates a quite reversible intercalation process and is consistent with previous reports of interlayer expansion of ~0.2 Å during discharge [38,39].

  Figure 4

  

  Figure 4.  (a) GCD curves of ex situ XRD and corresponding ex situ XRD patterns. (b) d₀₀₁ of the Na_0.4MnO₂·0.53H₂O electrode at different (dis)charging states calculated with ex situ XRD. (c) Ex situ Raman spectra of Na_0.4MnO₂·0.53H₂O. (d) Mn 2p and (e) O 1s XPS spectra of Na_0.4MnO₂·0.53H₂O at different (dis)charging states.

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  Ex situ Raman spectra (Fig. 4c) was applied to study the structural features during the electrochemical processes. The Raman spectrum measured from the electrode at the initial state is consistent with the result of the powder sample, showing a single ν₁ peak at ~640 cm^–1, corresponding to the symmetric stretching vibration of Mn–O bonds in [MnO₆] octahedra [29]. During the discharge process, the ν₁ peak shows a slight redshift, which is probably caused by the decrease in the average valence of Mn in that range. The increased proportion of Mn³⁺ in [MnO₆] octahedra leads to the elongation of Mn−O in the long axis of [MnO₆] octahedra and Jahn-Teller (J-T) distortions [40]. After discharge to 2.5 V, the ν₂ peak at ~575 cm^–1, related to the stretching vibration of Mn–O bonds in the basal plane of [MnO₆] sheets [29], appears. The I(ν₂)/I(ν₁) increases prominently from 2.5 V to 1.5 V, which is caused by the local lattice distortion of the [MnO₆] sheets [41]. Because Mn atoms are much heavier than O atoms, the stretching vibration of Mn–O should generally be considered to be related to oxygen atoms [42]. It is possible that the abundant Na⁺ between 2.5 V and 1.5 V has undesired interactions with lattice oxygen, thus causing local stress concentration in the plane and increased I(ν₂)/I(ν₁) [43].

  The changes in the valence and binding states of Na_0.4MnO₂·0.53H₂O were analyzed via XPS. The Mn 2p peaks are fitted with two differentiating peaks (Fig. 4d), which represent Mn³⁺ and Mn⁴⁺ [26]. The relative contents of Mn³⁺ and Mn⁴⁺ were calculated (Table S3 in Supporting information), and the average valence of Mn decreased with increasing content of Na⁺ in the layers. The changes in the valence of Mn³⁺/Mn⁴⁺ support pseudocapacitive Faradaic charge transfer, which is caused by intercalation in birnessite. In the O 1s spectra (Fig. 4e), lattice oxygen (O−Mn) and oxygen vacancies (O_vacancy) are detected at 529 and 531 eV, respectively [44]. The relative intensity of O_vacancy significantly increases at 1.5 and 2.5 V, indicating increased oxygen vacancies, which could be related to the abovementioned increase of I(ν₂)/I(ν₁) in ex situ Raman. Moreover, two minor peaks at 533 and 536 eV appear at 2.5 V and 1.5 V, which are attributed to C–O (O_solvent) and the Na auger peak (Na KLL) [45,46]. Since the only source of C–O in this testing system is the DGDE, the appearance of this peak during discharge may indicate the participation of the solvent. The increase in the Na auger peak and Na–F peak in the F 1s spectra (Fig. S8 in Supporting information) confirms the participation of sodium ions during the charge storage process [47].

  EQCM analysis was further performed to quantitatively investigate the charge storage mechanism. Fig. 5a shows that the Δmass periodically changes with respect to the potential during the measurement. Notably, the Δmass returns to its initial value after 2 cycles (Fig. S9 in Supporting information), indicating the reversibility of the mass change. The free water would undergo severe side reactions with PF₆⁻ in the organic electrolyte [48]. If any structural water is extracted from the interlayer, it is difficult to reversibly re-intercalate into the layers. Therefore, according to the operando EQCM results, it is concluded that structural water does not participate in ion/molar exchange in organic electrolytes, which is different from previous reports of birnessite measured in aqueous electrolytes [19]. The mass change (−δΔm/δ_t) vs. potential curve, referred to as "gravimetric CV" (gCV, Fig. 5b), has a tendency similar with the CV curve [19]. The result of mass change (Δm) vs. potential (V vs. Na⁺/Na) plots (Fig. 5c) further shows three stages of mass change, indicating that the electrochemical response of birnessite is mainly related to the charge carriers.

  Figure 5

  

  Figure 5.  (a) The change in mass and corresponding potential during measurement. (b) The gCV and CV curves of Na_0.4MnO₂·0.53H₂O at 5 mV/s. (c) CV and mass change vs. potential curves of the Na_0.4MnO₂·0.53H₂O electrode. The MCRs of Na_0.4MnO₂·0.53H₂O during the charge process in (d) 1 mol/L NaPF₆ in DGDE, (e) 1 mol/L NaClO₄ in DGDE and (f) 1 mol/L NaPF₆ in DME and (g-i) scheme of the electrode material in the respective electrolytes. (j) The intercalation contribution calculated in different electrolytes.

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  The mass to charge ratios (MCRs), corresponding to the molar mass change during the transfer of per electron, were further linearly fitted according to the mass change vs. charge number plots (Fig. 5d). The process is divided into three stages, in which the MCRs are 71.1, 42.3 and 27.3 g mol⁻¹ e⁻¹, which are lower than the MCR of 23 g mol e⁻¹ for net Na⁺ intercalation [19]. According to ex situ XRD results (Fig. 4b), the change of layer distance is only 0.15 Å, which is significantly lower than the reported results for solvent co-intercalation [49,50]. This result indicates that the co-intercalation reaction does not occur for large MCRs over 23 g mol⁻¹ e⁻¹. Expect for Na⁺ ions and confined water, the solvent molecules and anions might influence mass changes. To explore these insights, we measured the EQCM in two other electrolytes: 1 mol/L NaClO₄ in DGDE and 1 mol/L NaPF₆ in 1, 2-dimethoxyethane (DME), which replaced the anion and solvent, respectively. The Na_0.4MnO₂·0.53H₂O cathodes display overlapping CV curves for the three different electrolytes (Fig. S10 in Supporting information), indicating that the replacement of anions and solvents has no influence on their charge storage. However, their fitted MCRs change (Figs. 5e and f). There are slight changes in the MCR values when the anion is replaced (Fig. 5e). However, the MCR significantly changes when the solvent is switched from DGDE to DME (Fig. 5f). Therefore, as schematically shown in Figs. 5g-i, it indicates that the [Na-solvent]⁺ structure may have a significant influence on the charge storage mechanism of Na_0.4MnO₂·0.53H₂O.

  Based on the above discussion, we propose that the charge storage of Na_0.4MnO₂·0.53H₂O occurs in three stages, both of which include dominant Na⁺ intercalation and different degrees of solvated cation adsorption. The contributions of intercalated sodium ions and adsorbed solvated sodium ions in different electrolytes were calculated (Tables S4−S6 in Supporting information). Fig. 5j shows the calculated proportions of intercalated and adsorbed cations, which are similar for different electrolytes, suggesting the validity of the analytical model. During Stage Ⅰ, a significant amount of surface cation adsorption may originate from the positively charged bulk material, resulting in a high contribution of surface EDL adsorption of 18%. Ex situ O 1s XPS spectra (Fig. 4f) display a significantly increased signal from the O_solvent peaks, which confirms the surface EDL adsorption of solvated cations. At Stages Ⅱ and Ⅲ, the contributions of net Na⁺ intercalation are 95% and 98%, respectively, indicating that intercalation processes dominate the charge storage. Overall, the results of the EQCM indicate that the charge storage mechanism of birnessite is dominated by cation intercalation in organic electrolytes, showing that the specific adsorption of desolvated Na⁺ between the nanoconfined interlayer also results in a pseudocapacitance mechanism [19,51].

  The pseudocapacitive sodium-ion storage of birnessite in organic electrolyte is summarized in Fig. S12 (Supporting information). There are three stages dependent on the potential windows. Taking the discharging process as an example, in Stages Ⅲ and Ⅱ, corresponding to a discharge process from 4 V to 2.5 V vs. Na⁺/Na, the reaction is dominated by the intercalation of net Na⁺ and accompanied by a small amount of [Na-solvent]⁺ adsorption on the surface. In stage Ⅰ, Na⁺ intercalation dominates the charge contribution, but the contributions from adsorbed [Na-solvent]⁺ are largely increased. The adsorption of solvated cations is quantitively monitored by EQCM, indicating the changes of surface EDL structure during the electrochemical reactions. Previous studies on EDLs have been conducted mainly via MD simulations [52,53]. Our results suggest that the EQCM could be applied to investigate changes in interfaces by monitoring the migration of ions and solvents, providing a possible operando tool for assisting in research on interface chemistry. Moreover, the interlayer confined water is stable during the entire (dis)charge process, and solvent co-intercalation is not observed.

  In summary, we systematically investigate the charge storage mechanism of birnessite (Na_0.4MnO₂·0.53H₂O) in sodium-based organic electrolytes. Using operando EQCM, the stability of interlayer-confined water in organic electrolytes is demonstrated, and the deslovated Na⁺ ions intercalate into the water-confined interlayers of Na_0.4MnO₂·0.53H₂O in organic electrolytes, which is different from cation and H₂O co-intercalation in aqueous electrolyte. Additionally, surface EDL cation-dominated adsorption is detected by the EQCM, which contributes ~9.0% of the total capacity. According to ex situ XRD, Raman, XPS and kinetic analyses, deslovated Na⁺ ion intercalation is the non-phase-change and pseudocapacitive response process, delivering a high capacitive contribution of 85.6% at 1.0 mV/s. The Na_0.4MnO₂·0.53H₂O cathode delivers a high capacity of 185 mAh/g at 0.1 A/g and 101 mAh/g at 10 A/g, showing an ultrahigh rate capability. Combining the previous viewpoint of birnessite in aqueous electrolytes [19] and our results showing that hydrated interlayers allow desolvated Na⁺ pseudocapacitive intercalation in three organic electrolytes, we expand the perspective that the inner nanoconfined water environment is stable and importantly determines capacitive reactions. Using operando EQCM provides detailed information on the ion intercalation and surface EDL, while the latter raises extensive concerns and is worthy of further study.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Qinyao Jiang: Writing – original draft, Investigation, Data curation. Binhao Wang: Investigation, Data curation. Zerui Yan: Investigation, Data curation. Sicheng Fan: Investigation. Dafu Tang: Investigation. Biwei Xiao: Supervision, Funding acquisition. Qiulong Wei: Writing – review & editing, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22179113), the Guangdong High-Level Innovation Institute Project (No. 2021B0909050001), and the Fundamental Research Funds for the Central Universities (No. 20720230028).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110416.

  
  
• 1. [1]

     C. Zhao, Q. Wang, Z. Yao, et al., Science 370 (2020) 708–711. doi: 10.1126/science.aay9972

  2. [2]

     S.J. Shin, J.W. Gittins, C.J. Balhatchet, A. Walsh, A.C. Forse, Adv. Funct. Mater. (2023) 2308497. doi: 10.1002/adfm.202308497

  3. [3]

     J. Feng, Y. Wang, Y. Xu, et al., Adv. Mater. 33 (2021) 2100887.

  4. [4]

     Z. Xiong, P. Guo, Y. Yang, et al., Adv. Energy Mater. 12 (2022) 2103226.

  5. [5]

     H. Chen, K. Chen, J. Yang, et al., J. Am. Chem. Soc. 23 (2024) 15751–15760. doi: 10.1021/jacs.4c01395

  6. [6]

     J. Zhou, Q. Li, X. Hu, et al., Chin. Chem. Lett. 35 (2024) 109143.

  7. [7]

     Y. Wang, Y. Liu, X. Huang, et al., Chin. Chem. Lett. 35 (2024) 109301.

  8. [8]

     J. Ding, W. Hu, E. Paek, D. Mitlin, Chem. Rev. 14 (2018) 6457–6498. doi: 10.1021/acs.chemrev.8b00116

  9. [9]

     S. Fleischmann, J.B. Mitchell, R. Wang, et al., Chem. Rev 14 (2020) 6738–6782. doi: 10.1021/acs.chemrev.0c00170

  10. [10]

      G. Zhang, T. Xiong, L. Xia, et al., Batteries 12 (2022) 252. doi: 10.3390/batteries8120252

  11. [11]

      W. van den Bergh, M. Stefik, Adv. Funct. Mater. 31 (2022) 2204126.

  12. [12]

      C. Choi, D.S. Ashby, D.M. Butts, et al., Nat. Rev. Mater. 5 (2020) 5–19.

  13. [13]

      M. Mateos, N. Makivic, Y. -S. Kim, B. Limoges, V. Balland, Adv. Energy Mater. 23 (2020) 2000332.

  14. [14]

      W. Guo, C. Yu, S. Li, et al., Nano Energy 57 (2019) 459–472.

  15. [15]

      T. Wang, X. Zhu, S.V. Savilov, S.M. Aldoshin, H. Xia, Funct. Mater. Lett. 14 (2021) 2130013.

  16. [16]

      L. Yan, X. Li, H. Pan, ACS Appl. Mater. Interfaces 16 (2024) 26280–26287. doi: 10.1021/acsami.4c04256

  17. [17]

      I.I. Misnon, R.A. Aziz, N.K.M. Zain, et al., Mater. Res. Bull. 57 (2014) 221–230.

  18. [18]

      S. Boyd, R. Dhall, J.M. LeBeau, V. Augustyn, J. Mater. Chem. A 44 (2018) 22266–22276. doi: 10.1039/c8ta08367c

  19. [19]

      S. Boyd, K. Ganeshan, W. -Y. Tsai, et al., Nat. Mater. 20 (2021) 1689–1694. doi: 10.1038/s41563-021-01066-4

  20. [20]

      S. Kim, S. Lee, K.W. Nam, et al., Chem. Mater. 28 (2016) 5488–5494. doi: 10.1021/acs.chemmater.6b02083

  21. [21]

      E. Bendadesse, A.V. Morozov, A.M. Abakumov, et al., ACS Nano 16 (2022) 14907–14917. doi: 10.1021/acsnano.2c05784

  22. [22]

      S.L. Kuo, N.L. Wu, J. Electrochem. Soc. 153 (2006) A1317. doi: 10.1149/1.2197667

  23. [23]

      T. Lv, L. Suo, Curr. Opin. Electrochem. 29 (2021) 100818.

  24. [24]

      K.W. Nam, S. Kim, E. Yang, et al., Chem. Mater. 27 (2015) 3721–3725. doi: 10.1021/acs.chemmater.5b00869

  25. [25]

      Y. Jiang, S. Tan, Q. Wei, et al., J. Mater. Chem. A 6 (2018) 12259–12266. doi: 10.1039/c8ta02516a

  26. [26]

      Y. Zhao, Q. Fang, X. Zhu, et al., J. Mater. Chem. A 8 (2020) 8969–8978. doi: 10.1039/d0ta01480j

  27. [27]

      W. Gu, G. Lv, L. Liao, et al., J. Hazard. Mater. 338 (2017) 428–436.

  28. [28]

      S. Zhu, W. Huo, X. Liu, Y. Zhang, Nanoscale Adv. 2 (2020) 37–54. doi: 10.1039/c9na00547a

  29. [29]

      C. Julien, M. Massot, R. Baddour-Hadjean, et al., Solid State Ionics 159 (2003) 345–356.

  30. [30]

      M. Najdoski, V. Koleva, A. Samet, J. Phys. Chem. C 118 (2014) 9636–9646. doi: 10.1021/jp4127122

  31. [31]

      A.C. Alves, J.P. Correia, T.M. Silva, M.F. Montemor, Electrochim. Acta 454 (2023) 142418.

  32. [32]

      W. Zuo, X. Liu, J. Qiu, et al., Nat. Commun. 12 (2021) 4903.

  33. [33]

      T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Nat. Mater. 9 (2010) 146–151. doi: 10.1038/nmat2612

  34. [34]

      J. Jin, R. Wang, K. Yu, et al., Sep. Purif. Technol. 353 (2025) 128290.

  35. [35]

      R. Wang, J. He, C. Yan, et al., Adv. Mater. 36 (2024) 2402681. doi: 10.1002/adma.202402681

  36. [36]

      J. Ko, C. Lai, J. Long, et al., ACS Appl. Mater. Interfaces 12 (2020) 14071–14078. doi: 10.1021/acsami.0c02020

  37. [37]

      H. Yu, M. Aakyiir, S. Xu, J.D. Whittle, D. Losic, J. Ma, Today Energy 21 (2021) 100757.

  38. [38]

      X. Shan, F. Guo, D.S. Charles, et al., Nat. Commun. 10 (2019) 4975.

  39. [39]

      P. Xiong, R. Ma, N. Sakai, et al., ACS Appl. Mater. Interfaces 9 (2017) 6282–6291. doi: 10.1021/acsami.6b14612

  40. [40]

      H.Y. Asl, A. Manthiram, Science 369 (2020) 140–141. doi: 10.1126/science.abc5454

  41. [41]

      Y.K. Hsu, Y.C. Chen, Y.G. Lin, L.C. Chen, K.H. Chen, Chem. Commun. 47 (2011) 1252–1254.

  42. [42]

      Q. Zhang, M.D. Levi, Q. Dou, et al., Adv. Energy Mater. 9 (2019) 1802707.

  43. [43]

      L. Wang, Y. Lu, J. Liu, et al., Angew. Chem. Int. Ed. 52 (2013) 1964–1967. doi: 10.1002/anie.201206854

  44. [44]

      J. Dai, Y. Zhu, H.A. Tahini, et al., Nat. Commun. 11 (2020) 5657.

  45. [45]

      L. Gao, J. Chen, Q. Chen, X. Kong, Sci. Adv. 8 (2022) eabm4606.

  46. [46]

      X. Li, Z. Ao, J. Liu, et al., ACS Nano 10 (2016) 11532–11540. doi: 10.1021/acsnano.6b07522

  47. [47]

      C. Wu, Y. Yang, Y. Zhang, et al., Angew. Chem. Int. Ed. 63 (2024) e202406889.

  48. [48]

      L. Sheng, D. Zhu, K. Yang, et al., Nano Lett. 24 (2024) 533–540. doi: 10.1021/acs.nanolett.3c01682

  49. [49]

      H. Kim, J. Hong, G. Yoon, et al., Energy Environ. Sci. 8 (2015) 2963–2969.

  50. [50]

      F. Zhao, S. Zeng, L. Duan, et al., J. Phys. Chem. C 124 (2020) 28431–28436. doi: 10.1021/acs.jpcc.0c10237

  51. [51]

      Y. Ando, M. Okubo, A. Yamada, M. Otani, Adv. Funct. Mater. 30 (2020) 2000820.

  52. [52]

      Z. Wen, W. Fang, F. Wang, et al., Angew. Chem. Int. Ed. 63 (2024) e202314876.

  53. [53]

      H. He, J. Zhou, L. Yang, et al., J. Mater. Chem. A 12 (2024) 10279–10286. doi: 10.1039/d4ta00701h

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  Figure 1  Comparison of charge storage of birnessite in aqueous and organic electrolytes. (a) CV curves of birnessite in the aqueous and organic electrolytes. Schematics of hydrated birnessite measures in the (b) aqueous electrode of 1 mol/L Na₂SO₄ in water and (c) organic electrode of 1 mol/L NaPF₆ in diglyme (denoted as DGDE).

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  Figure 2  Characterization of Na_0.4MnO₂·0.53H₂O. (a) XRD pattern of Na_0.4MnO₂·0.53H₂O, inserted with a sketch of layered birnessite. (b) Raman spectrum of Na_0.4MnO₂·0.53H₂O, inserted with the vibration mode of Mn–O bonding. (c) TGA results of Na_0.4MnO₂·0.53H₂O. (d) SEM and (e, f) TEM images of Na_0.4MnO₂·0.53H₂O.

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  Figure 3  (a) Charge and discharge curves of Na_0.4MnO₂·0.53H₂O. (b) Charge and discharge curves at different current densities of Na_0.4MnO₂·0.53H₂O. (c) GITT results and corresponding diffusion coefficients of Na_0.4MnO₂·0.53H₂O in the potential range of 1.5–4.0 V. (d) CV curves measured at 0.2 mV/s in different potential windows. (e) CV curves of Na_0.4MnO₂·0.53H₂O at different scan rates. (f) The calculated capacitive contribution of Na_0.4MnO₂·0.53H₂O at 1 mV/s.

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  Figure 4  (a) GCD curves of ex situ XRD and corresponding ex situ XRD patterns. (b) d₀₀₁ of the Na_0.4MnO₂·0.53H₂O electrode at different (dis)charging states calculated with ex situ XRD. (c) Ex situ Raman spectra of Na_0.4MnO₂·0.53H₂O. (d) Mn 2p and (e) O 1s XPS spectra of Na_0.4MnO₂·0.53H₂O at different (dis)charging states.

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  Figure 5  (a) The change in mass and corresponding potential during measurement. (b) The gCV and CV curves of Na_0.4MnO₂·0.53H₂O at 5 mV/s. (c) CV and mass change vs. potential curves of the Na_0.4MnO₂·0.53H₂O electrode. The MCRs of Na_0.4MnO₂·0.53H₂O during the charge process in (d) 1 mol/L NaPF₆ in DGDE, (e) 1 mol/L NaClO₄ in DGDE and (f) 1 mol/L NaPF₆ in DME and (g-i) scheme of the electrode material in the respective electrolytes. (j) The intercalation contribution calculated in different electrolytes.

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