Regulated electrochemical performance of manganese oxide cathode for potassium-ion batteries: A combined experimental and ﬁrst-principles density functional theory (DFT) investigation

calculations predict a diffusion barrier of 0.31 eV for K + through the 1D tunnel of a -MnO 2 electrode, which is low enough to promote faster electrochemical kinetics. The nanorod structure of a -MnO 2 facilitates electron conductive connection and provides a strong electrode–electrolyte interface for the cathode, resulting in a very consistent and prevalent execution cathode material for KIBs.


Introduction
Rechargeable batteries as reasonable and practical energy storage systems that can integrate the major energy sources, such as wind and solar, into the electric grids are critical in today's energy-based society [1][2][3].Among the various rechargeable ion batteries , Na [7][8][9][10][11][12][13][14], Al [15][16][17], Mg [18][19][20][21][22] etc.), research groups have opted to move further with the investigation of potassium-ion batteries (KIBs) [23][24][25][26][27][28][29].Potassium is the second most abundant base metal after sodium as well [30].Furthermore, the KIB advancement can be achieved from the smallest potential for the K + /K redox pair in various common organic electrolytes used for ion battery applications [31][32][33][34][35][36][37][38].The practical as well as theoretical computations have revealed that the K + /K (-2.88 V) redox couple can show the least reduction potential in ordinary solvents such as propylene carbonate (PC) [39,40].This might guide towards a much broader voltage window, which would result in a high energy density for KIBs, as projected.Another advantage of KIBs is the fundamentally weaker Lewis acidity of K ion, resulting in small solvated ions.Furthermore, because K ions have a low desolvation energy, they may diffuse more quickly through the electrolyte/electrode interface in a coordinated pathway.Another advantageous feature is that potassium does not associate with any alloy formation it in contact with aluminum at relatively lower voltages, which corresponds to reduced cell production cost by replacing copper with an aluminum anode.
However, because this advancement is still in the planning stages, great foresight and planning advances are required to solve the existing issues.During the charge/discharge process, for example, the continuous insertion and extraction of K + with a greater radius of 0.138 nm will simply disrupt the structure of used cathode materials, achieving low capacity, reduced rate execution, poor stability, and even inactivity in electrochemical performance.Another disadvantage is that electrode materials which are relatively heavier are responsible for achieving low-energy density performance as well.Along these points, there is a crucial need to recognize first structural and electrochemical properties of various electrodes investigated for KIBs, and then develop electrochemically advantageous both cathode and anode materials, to manufacture full cell KIBs.There are reports on the KIBs, nevertheless, the total research articles in this field are still limited.
Carbonaceous materials [41], elementary substance materials (P [42], Sn [43], Sb [44]and Bi [45]), alloys (K-Na [46] and Sn 4 P 3 [47]), organics [48], transition metal (TM) carbides [49], oxides [50] and sulfides [51] have all been confirmed as potential anodes for KIBs.Until now, only a few materials (for example, organics, polyanionic combinations, and Prussian blue analogues) have been studied as cathodes for KIBs, confirming the feasibility of reversible K + deintercalation/intercalation with sufficient mechanism studies.Furthermore, the development of TM oxides with a high reversible capacity and stable electrochemical mechanism are effectively emphasized as cathode materials for Li/Na-ion batteries, which also add new members to electrode category for KIBs.
Mn-based materials are supposed to be promising cathodes for rechargeable batteries due to their inexpensive, abundance and non-toxic qualities [52].Because of their potential usage as cathode materials for Li/Na-ion batteries, Mn-based compounds have been extensively investigated [53].In addition, Mn-containing electrode materials for K-ion insertion/extraction have gained a lot of interest [54].Manganese dioxide (MnO 2 ), one of the Mncontaining oxides, has been extensively considered as one of a potential electrode materials because to its conventional benefits of high theoretical capacity, low cost and earth-abundance [55].The crystal structure of MnO 2 can be broadly classified into three types based on the interlinking configurations of the octahedral MnO 6 building blocks: one-dimensional (1D) tunnel structures (a-MnO 2 , b-MnO 2 , and c-MnO 2 ), two-dimensional (2D) layer structures (d-MnO 2 ), and three-dimensional (3D) mesh structures (k-MnO 2 ) [56].The morphology, crystal structure, size, and surface area of those MnO 2 crystal structures directly influence the electrochemical properties [57].Among the several structural polymorphs of MnO 2 , the a-MnO 2 phase has structural advancement with a greater interlayer dissipation, which is important for cation diffusion.a-MnO 2 structure provides paths for particle intercalation and deintercalation because to its open structure.In the present study, we suggest a hydrothermal approach for preparing homogenous a-MnO 2 nanorods for high-performance KIBs.The nanorod structure creates a web network, which enhances conductivity and allows volume expansion in the active material as well.We show that KIB with a voltage window of 1-4 V have higher capacity and rate capability with near to 100 % Coulombic efficiency, which is much better than the KIB with a voltage window of 1.5-4 V. Electrochemical tests with a voltage window of 1-4 V reveal that MnO 2 has an extremely high K-storage capacity (primary discharge capacity of 142 mAh/g), a high rate capability (C/20 to 5C), and moderate cycling stability (up to 100 cycles).Because it combines high capacity and stability with the help of structural advancement, this electrode material is a superior cathode material for next-generation KIBs.

a-MnO 2 preparation
In alkaline conditions, the oxidation of a manganese (II) acetate (98 %, Sigma-Aldrich) with ammonium persulfate (98 %, Sigma-Aldrich) results in MnO 2 preparation as illustrated in previously reported article [58] (Scheme 1).First and foremost, the desired sodium hydroxide (anhydrous, !98 %, Sigma-Aldrich) (100 ml) aqueous solution was prepared and divided into two parts: one for manganese acetate and the other for ammonium persulfate precursors.Then, the ammonium persulfate-containing mixture was progressively filled with the manganese-associated precursor to prepare a complex solution.This complex was placed in a Teflon lined hydrothermal autoclave and heated at 180 °C for 12 h.The resulting dispersion was washed with ethanol and water before drying for 12 h at 60 °C.
Two synthetic reactions are associated in our manganese dioxide preparation approach.Manganese hydroxide complex is acquired by the association of manganese acetate with sodium hydroxide the in the first step: In association earlier condition, the next step includes an oxidoreduction reaction between manganese hydroxide and persulfate anions:

Characterizations
The Powder XRD analysis was carried out using a Bruker D8 Advance diffractometer (CuKa = 1.54056 nm), which confirmed the formation of high-quality powders of a-MnO 2 .The Raman spectrum for the material was obtained using the Horiba LabRAM HR spectrometer, and the contrasting spectrum was recorded between 100 and 1000 cm À1 .The X-ray photoelectron spectroscopy (XPS) (ESCALAB 250 (ThermoElectron)) was examined using a monochromatic Al Ka radiation, hv = 1486.6eV.The morphologies were studied using field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) captured by using Hitachi S-4800 and FEI TECNAI G2 20 Twin instruments, respectively.
The details coin cell fabrication with electrochemical parameters and DFT Computation are illustrated in Supplementary Information, Section 1 and 2. The tetragonal a-MnO 2 structure with edge and corner-sharing MnO 6 units, as shown in the inset of Fig. 1b, provides adequate space for efficient K + ions diffusion, which is necessary for excellent electrochemical performance.Among all the MnO 2 polymorphs, this a-MnO 2 tunnel structure is particularly interesting due to the typical 1D structure allowing ions for (de)intercalation in conjunction with the change of manganese oxidation states during redox reactions.The crystallite size is also supported from the structural analysis [59,60].

Structural and morphological analysis
Meanwhile, the development of as-prepared a-MnO 2 was also analyzed by using the Raman spectrum for Fig. S1.It shows clear peaks detected at 179, 581, and 640 cm À1 allocated for assynthesized material [61].The intense peaks at 581 and 640 cm À1 are ascribed to the symmetric vibrations of the Mn-O and the modes that follow the interstitial space associated tetragonal a-MnO 2 structure.Furthermore, the peak at 179 cm À1 is due  to the MnO 6 octahedra related translational vibration for the ions into the tunnel structure [62].
However, XPS spectrum was engaged to examine the oxidation states of prepared a-MnO 2 (Fig. 2a).The NaOH precursor utilized in the synthesis of MnO 2 is mainly responsible for the existence of small Na KL1 and Na1 s peaks.The survey spectrum is consistent with earlier investigations [63][64][65][66][67][68][69].Fig. 2b, c shows the standard core-level XPS spectra of associated Mn 2p and O1s.As shown in Fig. 2b, the Mn 2p spectrum is separated into doublet pointing at 642.5 and 654.2 eV, which are associated with 2p3/2 and 2p1/2, respectively.The binding difference between the abovementioned doublet of Mn 2p is about 11.7 eV, implying the formation of Mn 4+ .Overall, the O1 s core-level spectrum is presented in Fig. 2c, which depicts two distinct surface oxygen parts, one at 530 eV which corresponds to lattice oxygen and the other at 531.3 eV which associates with loosely coordinated or defected oxygen [70].FESEM and HRTEM were used to study a comprehensive morphological and fundamental assessment of the a-MnO 2 nanorods.
The FESEM photos of coordinated a-MnO 2 nanorods with homogeneous size are clearly demonstrated in Fig. 3a, b.The TEM image in Fig. 3c, d shows that the as-organized a-MnO 2 nanorods associate with a uniform diameter between 10 and 50 nm.This morphology further promotes electrical conductivity and more charge transport channels by virtue of the higher surface area, achieving a good electrode/electrolyte interface and small ion diffusion length [ [71][72][73].The electron beam of the SEM was scanned across the sample along a predetermined line to detect X-rays at certain points along that line.An examination of the X-ray energy spectrum at each point show graph of the relative elemental concentration of each element as a function of position along the line.The data is repre-sented qualitatively by the line scan profile that depicts the increase or decrease of all elemental elements present at all places along the line (Fig. 3e).The Mn and O signals exhibit a steady intensity pattern over the length of the nanorod, indicating a homogenous distribution of the associated components; nonetheless, the presence of Mn and O in a 1:2 ratio suggests the presence of pure MnO 2 .K + /K during the 1st discharge, out of which only 0.13 potassium can be extracted on the run of subsequent charge process, causing a moderate reversible capacity of 40 mAh/g.We changed the discharge voltage limit from 1.5 to 1 V and evaluated the electrochemical properties of a-MnO 2 at 1-4 V since it showed greater intercalation in this window (Fig. 4c).As a result, there is 0.46 potassiation, leading to the formation of K 0.46 MnO 2 .During the primary charge, an irreversibility of 0.2 K is predicted at the final voltage of 4 V.In present case, the a-MnO 2 electrode reaches a high capacity of 142 mAh/g for the primary discharge process, with a charge capacity of 81 mAh/g and Coulombic efficiency of 57 %, as illustrated in Fig. 4d.In the two cells, the associated charge and discharge characteristics are almost identical.Even in both cases, an irreversible limit is seen at the first cycle, which is determined to be due to significant electrolyte decomposition to form solid elec- trolyte interphase (SEI) [ [74][75][76][77].The associated insertion/extraction reaction mechanism during the subsequent discharge/charge process for the MnO 2 electrode is described by the following equation [78,79]:

Electrochemical performance
In addition, a reasonable rate capability for KIBs was examined for the a-MnO 2 electrodes.Fig. 5 shows the rate performance of the a-MnO 2 electrode at various current densities ranging from C/20 to 5C.At 1.5-4 V, the a-MnO 2 electrode yields moderate reversible capacities up to C, but not able to achieve any capacity above C to 5C (Fig. 5a, b).A limited capacity is achieved when the current rate is again slowed down to C/20.Despite this, a-MnO 2 electrode executes better rate capability studies up to 5C in 1-4 V voltage range, addressing remarkable stability under all C-rates engaged in discharge/charge studies (Fig. 5c, d).K + have a lower Lewis acidity than Li + , resulting in less correspondence with the Lewis-base in the electrolyte, leading in fast rate execution [80].The open growth of a-MnO 2 would be advantageous for quick transport and diffusion of K + as well.Actually, the excellent rate performance appears to be attributable to MnO 0 2 s unique nanorod-like structure, which is responsible for numerous electrochemical sites and the reduction of the K ion diffusion route while also limiting volume change.
Because the polarization effect of K metal is more at high rates, the voltage loss under a high current at the electrode is negligible.Surprisingly, when current density increases, the discharge capacities decrease in case of two electrodes, owing to the sluggish rate of K + diffusion in the presence of high current density [81,82].Their exceptional rate presentations without sacrificing high working voltage can be a valuable addition to any practical energy storage application.
Furthermore, Fig. 6 depicts the cycling of a-MnO 2 electrodes at C rate under predetermined voltage windows up to 100 cycles.The electrode at 1.5-4 V window reveals superior cycling dependability (Fig. 6a) and stable Coulombic efficiency than the 1-4 V window (Fig. 6c).Surprisingly, in the 1-4 V range, the capacity rapidly drops as far as feasible to zero.After the initial cycle, the Coulombic efficiency increases with cycle numbers and reaches 100 % after a few cycles as seen in both functional voltage windows for MnO 2 electrode.Significant difference has been found in terms of capacity retention between the electrodes.However, their charge/discharge profiles are similar in both cases (Fig. 6b and d).It implies that changing the voltage window might have an influence on a-MnO 0 2 s electrochemical properties.Fig. S2 shows the electrochemical impedance spectroscopy (EIS) evolutions of the cells before and after stability with equivalent circuit diagram.While   the charge transfer resistance (R CT ) is depicted in the high frequency domain as a semicircular arc, the internal resistance (Rs) corresponds to an intersection point in the X-axis [83,84].The double layer component inside pores and diffusion in the MnO 2 structure are coupled by the constant phase element (CPE) and Warburg resistance (W).The R L component is linked to leakage resistance during electrochemical activities.It is evident that the Nyquist plot after stability test did not show any observable new semicircles, despite the semicircle's considerable increase with cycling.This reveals that cells cycled at high current undergo a significant rise in charge-transfer resistance and that the SEI layer formation is constrained as well [85].

Theoretical insights
The electronic structure and diffusion barrier of K + in the a- ; where E KMnO 2 ð Þis the energy of the K-intercalated a-MnO 2 , E MnO 2 ð Þis the energy of the pristine a-MnO 2 , E(K) is the energy K atom, and N electrons are the number of electrons transferred with the cation.Similar voltages have been reported for lithium (3.36 V) [86] and sodium (3.23-3.34V) [87] intercalation in a-MnO 2 .As shown in Fig. 7c, d, compared to the pristine material, the Fermi level K-intercalated a-MnO 2 is shifted to slightly above the edge conduction band (CB) edge, owing to the addition of an extra electron from the K + ion.An analysis of the projected density of states reveals that the valence band edge of both the pristine and K-incorporated a-MnO 2 is composed mainly of O-p states, whereas the conduction band edge is dominated by Mn-d states.Overall, the bandgap remained is not significantly affected by the incorporation of K + ion, predicted at 2.42 eV for the pristine a-MnO 2 and 2.30 eV for the K-a-MnO 2 .This results are consistent with a previous theoretical investigation of Na incorporation in MnO 2 [87].The reduction of the bandgap coupled with the shift of the Fermi level above the edge conduction band edge with K incorporation suggests improvement in the electrical conductivity of the K-a-MnO 2 compared to the pristine a-MnO 2 .The diffusion pathway for K in a-MnO 2 and the corresponding calculated energy profile are shown in Fig. 7e, f.As shown in Fig. 7e, the K diffusion path in a-MnO 2 is investigated along the 1D tunnel within the 1 Â 1 Â 2 supercell with the K atom at the hollow site.
The energy barrier for the migration of K + in the a-MnO 2 material is calculated at 0.31 eV, which is low enough to promote faster electrochemical kinetics for practical K-MnO 2 battery applications.The calculated migration energy barrier of K (0.31 eV), compared well with previous theoretical predictions for both Li ions and Na ions which are less than 0.3 eV [86].The energy barrier for the migration of Zn 2+ in the a-MnO 2 structure was predicted to be 0.497 eV [88], whereas Ca through a-MnO 2 is predicted to encounter an energy barrier of 0.19 eV [89].

Conclusions
In conclusion, the nanorod morphology of a-MnO 2 is effectively structured using hydrothermal method followed by an annealing procedure.A good capacity (142 mAh/g as first discharge capacity), an excellent rate property (up to 5C), superb cycling execution (over 100 cycles) at room temperature are achieved for the a-
Fig. 4 demonstrates the charge/discharge profiles and cycle execution of a-MnO 2 electrode (active electrode mass loading of 2-6 mg/cm 2 ) at different voltage windows in K cells linked to KPF 6 in ethylene carbonate (EC): Diethyl carbonate (DEC) electrolyte at current rate of C/20.As shown in Fig. 4a, 0.18 potassium can be inserted into a-MnO 2 by utilizing a C/20 rate up to 1.5 V against

Fig. 3 .
Fig. 3. (a, b) FESEM images and (c, d) HRTEM images of a-MnO 2 .(e) EDS line scanning for the a-MnO 2 ; the yellow line refers to manganese (Mn), and the cyan line is assigned for oxygen (O).

Fig. 4 .
Fig. 4. Electrochemical charge/discharge curves per K + ion and cyclic voltage profiles in half potassium cells for a-MnO 2 in 1 M KPF 6 in EC:DEC at current rate of C/20 with a voltage window of (a and b) 1.5-4 V, and (c and d) 1-4 V.

Fig. 5 .
Fig. 5. Rate capability studies and charge-discharge curves a-MnO 2 at different C-rates between C/20 and 5C with a voltage window of (a and b) 1.5-4 V, and (c and d) 1-4 V.

MnO 2
material are important factors that dictate the electrochemical performance.Considering these properties are not easy to determine experimentally, first-principles density functional theory (DFT) calculations were employed.The optimized structures of the pristine and K-intercalated a-MnO 2 with the corresponding charge density isosurface are shown in Fig. 7a, b.The lattice parameters are predicted at a = b = 9.763 Å, c = 2.872 Å (volume = 273.75Å 3 ) for the pristine a-MnO 2 and at a = b = 9.71 9 Å and c = 2.850 Å (volume = 269.21Å 3 ) for the K-intercalated a-MnO 2 .This indicates that the incorporation of potassium contracts the unit cell volume of a-MnO 2 by 1.67 %.Bader population analyses reveal an increase in the atomic charge of the O atoms near to the Na ion in the one-dimensional tunnel, with the 4 nearest ones gaining extra charges of $ 0.13 e À (as indicated on Fig. 7a, b).The change of atomic charges before and after the insertion of K + can be attributed to the release on of one electron which redistributes to the nearest O atoms, increasing their negative charges.The reaction of K with a-MnO 2 (K + MnO 2 ?KMnO 2 )

Fig. 6 .
Fig. 6.The cycling performance and charge-discharge profiles of a-MnO 2 at C rate with a voltage window of (a and b) 1.5-4 V, and (c and d) 1-4 V.

MnO 2
cathode by optimizing the voltage window.The fabricated K-ion battery exhibits such electrochemical performance at high voltage window of 1-4 V in comparison with literature [90,91].It is well acknowledged that remarkable improvement comes from the structural frameworks.The homogeneous structure provides more open framework associated with electrolyte-electrode interface, ensuring host ion insertion/extraction, and safeguarding the cathode from electrolyte interactions at high voltage.The redox reactions are aided by the a-MnO 2 , which allows for quick charge movement in the structure as supported by DFT calculations.As a result, this research presents the strategy to fabricate future cathode materials with suitable proposal for exhibiting an excellent electrochemical behavior and extended cycle reliability in the framework of potassium ion batteries.CRediT authorship contribution statement Bidhan Pandit: Conceptualization, Data curation, Formal analysis, Writing -original draft, Writing -review & editing, Investigation, Methodology, Resources, Software, Validation, Visualization, Writing -review & editing, Supervision.Sachin R. Rondiya: Data curation, Software, Investigation, Methodology, Resources, Validation, Visualization, Writing -review & editing.Shoyebmohamad F. Shaikh: Data curation, Methodology, Investigation.Mohd Ubaidullah: Data curation, Methodology, Investigation.Ricardo

Fig. 7 .
Fig. 7. Charge density distribution for (a) pristine and (b) K-intercalated a-MnO 2 and the corresponding partial density of states (c) and (d), respectively.(e) K ion diffusion pathway and (f) Energy profile of K ion diffusion path through a-MnO 2 1 Â 1 Â 2 supercell.