MnO 2 as an ionic conductor with randomly distributed nano-sized layers

Here we show that through a straightforward synthesis it is possible to create a bulk material, Na0.35MnO2, with isolated sheets. Due to such arrangement of the oxide layers, this ionic conductor was found to be genuinely pseudocapacitive, with charge storage not limited by diffusion of ions between stacked layers, resulting in capacitance values of 190 F g-1 under exceptionally high current rates of up to 200 A g-1.


Introduction
Manganese oxides have been widely investigated for supercapacitor applications. [1][2][3][4] Numerous different compounds based around MnO x , as well as variations in structure and morphology have been reported. [5][6][7][8][9][10] One of the more promising materials is cation pre-intercalated manganese oxides. Here the cation located in-between the MnO 2 layers is involved in the charge storage mechanism; thus enabling the electrolyte to maintain both a stable salt concentration and ionic conductivity during charge and discharge. 11 Limitations to intercalationbased charge storage from solid state diffusion result in the high diffusion barrier of cations. This reduces the power performance, although pseudocapacitance may be increased at very low currents or scan rates. [12][13][14][15] However, important inuences of dimension and crystalline size of cation-preintercalated MnO 2 nanosheets on the cation diffusion rate within 2D MnO 2 have not yet been fully explored.
Materials of reduced dimensionality have been under intense investigation since the discovery of Buckminsterfullerene (0D), 16 carbon nanotubes (1D) 17 and graphene (2D). 18 In particular, 2D materials are being critically examined for a whole host of applications in energy, materials, and engineering. [19][20][21][22][23] The challenge of fully utilising 2D materials is the ability to scale up the synthesis in concert with maintaining the remarkable material properties reported from studies of single isolated akes. [24][25][26] Isolated in suspension, 2D sheets, once sedimented, tend to stack, forming a 3D-ordered material with either ideal crystallographic interlayer registry or restricted to equidistant layers. 23,[27][28][29][30][31] This inevitably leads to the loss of the unique 2D properties, such as vastly accessible surface area. The challenge of making truly 2D materials is always accompanied by the difficulty of establishing their structure using direct, powder diffraction, methods since the commonly used diffraction methodologies are not adequately suited for materials with lower than perfectly 3D ordering. Powder diffraction data from the literature, when provided, indicate that the majority of reportedly 2D-material powders routinely have the diffraction pattern similar to their 3D counterparts albeit oen with a small and anisotropic crystallite size. 32,33 Here we show that a reliable structural model of a 2D-ordered compound, in this case Na 0.35 MnO 2 , can be obtained through powder diffraction data and the fundamental equation of Debye. As a consequence, through the interactive inuences of both dimension and crystalline size of the cation-pre-intercalated MnO 2 nanosheets, the capacitance was found not to be limited by solid-state diffusion.

Material preparation
The synthesis route for sodium pre-intercalated manganese oxide (Na x MnO 2 ) was devised where conditions would promote the formation of a disordered material. 3 mM MnSO 4 (aq.) (Hayashi Pure Chemical, Japan) and 2 mM KMnO 4 (aq.) (Shimakyu's Pure Chemical, Japan) were added together at 30 C, followed by the addition of 15 mM Na 2 SO 4 (Showa Chemical Co. Ltd, Japan). Aer mixing, the pH of the solution was adjusted to 12.3 via dropwise addition of 1 M NaOH (aq.) and was then transferred to a Teon-lined pressure vessel and heated within an oven (Memmert) to 75 C (AE0.5 C) for 12 h. Aer cooling, the precipitate was ltered and washed with deionised water. The mild temperatures and the inclusion of a molar excess of alkali metal cations with respect to manganese disrupt the formation of a highly crystalline material. By carrying out the synthesis under strict temperature control, we were able to obtain a single phase product. The stoichiometry of resultant material was determined as Na 0.35 MnO 2 by elemental analysis via inductively coupled plasma optical emission spectrometry, ICP-OES (see ESI †).

Electrode preparation
The coating slurry was prepared by mixing 70 wt% of Na 0.35 MnO 2 , 20 wt% of carbon black (Vulcan XC72, USA), and 10 wt% of poly(vinylidene diuoride) (PVdF) binder (Sigma-Aldrich), which was dispersed in N-methyl-2-pyrrolidone (NMP, Tedia Company Inc. USA) followed by sonication for 60 min. The slurry was coated on a graphite substrate (1 cm Â 1 cm) and dried in an oven for 12 h at 45 C to obtain the electrodes. The mass of Na 0.35 MnO 2 as the electroactive material in this study was xed to be 0.5 mg cm À2 , resulting in the total mass loading of 0.71 mg cm À2 .

Structural characterisation
X-ray powder diffraction data were collected in a capillary mode on PANalytical Empyrean using Mo Ka 1,2 radiation (see ESI † for more details). Raman spectra were recorded within an argon containing airtight cell using a Raman microscope (Renishaw in Via), with the He-Ne laser (632.8 nm) illumination focused through an inverted microscope (Leica), via a 50Â objective lens (Leica). In order to avoid local heating by the laser, the sample surface exposure was carefully controlled via the use of an appropriate lter. The acquisition time for each spectrum was 300 s. Scanning electron microscopic (SEM) images were collected with a eld-emission scanning electron microscope (FE-SEM, Hitachi SU8010). Transmission electron microscopic (TEM) and selected-area electron diffraction (SAED) images were recorded using a JEOL, JEM 3010 microscope with an accelerating voltage of 300 kV and a high-resolution transmission electron microscope (HRTEM, JEOL, 2100F) at 200 kV. Nitrogen adsorption and desorption isotherms were measured at 77.3 K by means of the Micromeritics ASAP 2020 surface area and porosity analyser.

Electrochemical characterisation
Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests of as-prepared samples were conducted on an electrochemical station (CHI 1128C, CH Instruments, USA). The specic capacitance of Na 0.35 MnO 2 was calculated from the discharge curves: where C S,Mn , m, DV, I, and Dt are the specic capacitance of Na 0.35 MnO 2 , mass loading of Na 0.35 MnO 2 on the substrate, potential window of discharge tests, applied current, and discharge time, respectively. 34

Results and discussion
Powder X-ray diffraction pattern of Na 0.35 MnO 2 , shown in Fig. 1 and ESI, † is characterised by a very low signal-to-noise ratio, indicating that the compound is largely disordered. The regions of the pattern representing coherent scattering are reminiscent of those previously calculated for a single layer of MnO 2 and contain broad asymmetric peaks. 35 The structure of the ordered regions has been determined using a newly developed renement method based on the fundamental scattering equation of Debye 36 and thus far employed to determine the shape and size of 3D nanoparticles 37 and of 2D graphene layers in partially disordered carbon blacks. 38 A single innite layer of the manganese oxide from the structure of hexagonal H-birnessite 39 was used as a starting model for the renement. The shape of each trial layer was assumed to be an elliptical cylinder, with the axes' lengths of the ellipse, lattice parameters, amplitudes of mean-square displacements of atoms and the directional angle of the cylindrical cut-out serving as variables in the renement.
The entire experimental powder diffraction prole of Na 0.35 MnO 2 ts exceptionally well (c 2 ¼ 1.6), see Fig. 1, by the pattern calculated from the fragment of a MnO 2 layer shown in Fig. 2(a), with Mn-O distances of 0.1863 nm and the lengths of the elliptical axes of 2 and 4 nm. A nano-sized region of an individual layer tting the whole experimental data proves that such region represents all the order within the powder. There is no crystallographic registry, or even stacking in any regular fashion, of the MnO 2 layers, in Fig. 2(b), and no ordered pattern in the positions of the sodium atoms.
The structure and domain size of Na 0.35 MnO 2 were observed by eld emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), see Fig. 3. The powder consists of particles 200-250 nm in length and 150-200 nm in width, from Fig. 3(a) and (b). Moreover, the high-magnication SEM image in Fig. 3(b) shows the aggregates of Na 0.35 MnO 2 primary particles without a clear inter-particle boundary. TEM images in Fig. 3(c)-(e) show that domains consist of poorly ordered and randomly stacked MnO 2 layers. The disperse rings in the selected area electron diffraction (SAED) pattern indicate the poor crystallinity of Na 0.35 MnO 2 as shown in Fig. 3(f). It is important to highlight that Na 0.35 MnO 2 is sensitive to the high energy electron beam and if viewed at higher magnications (>Â800k), it transforms to the hausmannite crystalline phase of Mn 3 O 4 (see ESI †). The lattice parameters are typical of Mn 3 O 4 , and the change in crystallinity is conrmed by dened rings within the SAED image and Raman spectroscopy. 40,41 The sheet length of the ordered Mn 3 O 4 layers formed under the electron beam are seen to be either in the 2 nm or 4-5 nm range. This indirect observation corroborates our structure model.
The Raman spectrum of Na 0.35 MnO 2 (Fig. 4) resembles some of the characteristics of the spectrum typically reported for Nabirnessite MnO 2 and all four tted peaks can be assigned to Mn-O vibrational modes (Table 1). 41 In particular, the Raman bands at 651 cm À1 , 577 cm À1 and 502 cm À1 can be assigned respectively to the symmetric stretching vibration of Mn-O band of MnO 6 groups, Mn-O stretching within the basal plane of the MnO 6 sheet and Mn-O stretching vibration of MnO 6 octahedra. 42,43 The lack of order in the positions of the sodium atoms is supported by the absence of bands associated with the stretching modes of NaO 6 octahedron and NaO 4 tetrahedron for Na-birnessite that are typically observed below 450 cm À1 . 43,44 The current response was found to be linearly proportional to the scan rate of CV when the scan rate is varied between 5 and 1000 mV s À1 (Fig. 5(a)), thereby illustrating that Na 0.35 MnO 2 is genuinely pseudocapacitive, where the charge storage mechanism is based on the surface redox processes (see ESI †).
The ideal capacitive performance is demonstrated by the near-rectangular CV curve measured at 100 mV s À1 (Fig. 5(b)), which is in contrast to chemically similar compounds that display distinct redox peaks in their CVs. 15,47 Moreover,   rectangular CV curves were measured from scan rates between 5 to 1000 mV s À1 (see ESI †). The pseudocapacitive nature of Na 0.35 MnO 2 can be further supported by a low specic surface area of 15 m 2 g À1 , using the Brunauer-Emmett-Teller (BET) method (see ESI †), with a total porosity of 0.0595 cm 3 g À1 . For bulk MnO 2 , pseudocapacitance is derived from the redox activity of manganese at the surface/near surface region of the active material 1 whereas the interior of MnO 2 crystals remains mostly inactive. 48 Typically, the charge storage mechanism in neutral pH electrolytes is based on the extraction of supercially adsorbed/absorbed Na + cations according to the reaction: (MnO 2 ) surface + Na + + e À 4 (MnO 2 À Na + ) surface Once within the aqueous electrolyte, we surmise that Na + and H 2 O are located randomly between the MnO 2 sheets, permitting redox processes for both the surface and interior of the sheets. The arrangement of the sheets allow facile exchange of Na + and H 2 O from and to the bulk electrolyte during charge and discharge. Na + has a dual role of allowing the formation of a disordered material during synthesis and being ready present to partake in the initial exchange of speciation at the interface during electrode polarisation.
To further explore the advanced pseudocapacitive characteristics of the Na 0.35 MnO 2 , galvanostatic charge-discharge measurements were carried out at various current densities and the results are shown in Fig. 5(c). The charge curves measured at all current densities are highly symmetrical to their corresponding discharge counterparts, revealing the ideal capacitive property of the Na 0.35 MnO 2 electrode. The specic capacitance, (C S,Mn ) of Na 0.35 MnO 2 obtained at 10 A g À1 , calculated from the discharge curves in Fig. 5(c) was 204 F g À1 (see Experimental). The iR drop (0.07 V) is low, even at the high current density of 100 A g À1 , indicating low internal resistance of the electrode  material. Accordingly, the C S,Mn values of Na 0.35 MnO 2 measured at 20, 50, 100, 150, and 200 A g À1 are equal to 199,195,193,192 and 192 F g À1 , respectively (see Fig. 5(d)). 94% capacitance is retained even when the current density has been increased by a factor of 20. Fig. 5(e) shows the cycle-life data of the Na 0.35 MnO 2 in which no signicant capacitance loss (<5%) was observed aer 5000 charge-discharge cycles. The stability of Na 0.35 MnO 2 is supported by the synchrotron X-ray absorption measurements whereby the X-ray absorption near-edge structure (XANES) spectra demonstrate similar features for as-prepared and cycled (100 and 1200 cycles) Na 0.35 MnO 2 . A multiple structure of pre-edge region at ca. 6541 eV can be observed for Na 0.35 MnO 2 (Fig. 5(f) and ESI †), which are attributed to the partially allowed transition of a 1s electron to an unoccupied 3d orbital. 49 The split in the pre-edge peaks was reported as the separation of degenerated 3d levels under the octahedral crystal eld, which are corresponding to 1s to 3d (t 2g ) and 1s to 3d (e g ) transitions, respectively. 49 The main absorption edge around 6560 eV is assigned to the purely dipole-allowed 1s to 4p transition. 49 According to the quasilinear relationship, the Mn oxidation state of the as-prepared sample is 3.57, indicating a minor sub-stoichiometric presence of oxygen within the material (i.e. Na 0. 35  The outstanding capacitive performance at high currents up to 200 A g À1 can be explained by the nanostructure and sheet dimensions of the synthesised Na 0.35 MnO 2 that allow ion transport throughout the material that is not limited by solid state diffusion, in contrast to crystalline Na 0.35 MnO 2 nanowires, where capacitance is shown to decrease signicantly with the scan rate of CV. 15

Conclusions
In summary, we present powders of 2D-ordered manganese dioxide that comprised of randomly distributed manganese oxide layers, size 2 Â 4 nm, and sodium atoms located amongst them, also without forming a regular pattern. The dimensionality of the ordered domain is established by Debye renement, an unrivalled tool for the task.
Due to such arrangement of the oxide layers, this ionic conductor was found to be genuinely pseudocapacitive, with charge storage not limited by diffusion of ions between stacked layers. Under substantial current rates, up to 200 A g À1 , a capacitance greater than 190 F g À1 was maintained. The material was incredibly stable, with no signicant capacitance loss (<5%) aer 5000 charge-discharge cycles.
The synthesised Na 0.35 MnO 2 exhibits fast ion mobility due to the formation of short interlayers (2 Â 4 nm) and signicantly capacitance was found to be not limited by the solid-state diffusion. The distinctive material property is its capacitance retention at exceptionally high current rates, thereby opening up a new approach of material design to take full advantage of surface redox sites to store and deliver charge.