Hydrothermal Synthesis of α-MnO 2 and β-MnO 2 Nanorods as High Capacity Cathode Materials for Sodium Ion Batteries

Two types of MnO2 polymorphs, α-MnO2 and β-MnO2 nanorods, have been synthesized by a hydrothermal method. Their crystallographic phases, morphologies, and crystal structures were characterized by XRD, FESEM and TEM analysis. Different exposed crystal planes have been identified by TEM. The electrochemical properties of α-MnO2 and β-MnO2 nanorods as cathode materials in Na-ion batteries were evaluated by galvanostatic charge/discharge testing. Both α-MnO2 and β-MnO2 nanorods achieved high initial sodium ion storage capacities of 278 mA h g−1 and 298 mA h g−1, respectively. β-MnO2 nanorods exhibited a better electrochemical performance such as good rate capability and cyclability than that of α-MnO2 nanorods, which could be ascribed to a more compact tunnel structure of β-MnO2 nanorods. Furthermore, the one-dimensional architecture of nanorods could also contribute to facile sodium ion diffusion in the charge and discharge process.


Density Function Theory (DFT) Calculation Method
DFT calculations were performed using the Vienna Ab initio Simulation Package [1][2][3][4] based on the Generalized Gradient Approximation (GGA) [5] with Hubbard U (U=5.2 eV for the transition metal Mn) [6,7] corrections (GGA+U). [8]The projector augmented wave potentials [9] with the cutoff energy of 450 eV applied.Relaxation simulations were performed for ionic positions, unit cell shape, and unit cell size.For Mn, the 3p, 3d, and 4s states were treated as valence states.For O, the 2s and 2p states were treated as valence states.The conjugate gradient scheme is used to optimize the atom coordinates until the force is less than 0.01 eV Å −1 .
To calculate the Na + ions binding energy, the energy of 1 × 1 × 1 unit cell of body-centered cubic Sodium crystal was calculated.Then the single Na atom energy (E Na ) was obtained by dividing the sodium number of the unit cell.The binding energies (E b ) were calculated by: Where, E facets absorbed Na + ions , E facets , and n are the energy of facets absorbed with Na + ions, the energy of the facets and the absorbed Na + ions number, respectively.
Here, we further applied the DFT calculations to analyse the sodium ions' interaction with different facets of the β-MnO 2 ({001} and {110}) and α-MnO 2 ({001} and {100}).The detailed calculation method and relaxed crystal structures were supplied in the last session of this response letter.
As shown in Figs.S9 a and b, after the lattice and atoms relaxation calculations on {001} facets of β-MnO 2 , all the Na + ions stabilize on the 8h Wyckoff position (P4 2 /mum mnm space group), that are above the bottleneck (window) of the [1 × 1] tunnel of β-MnO 2 .Na + ions are two-fold coordinated with the oxygen ligands of the MnO 6 octahedron with the 2.262 Å bond length as marked in the side view of the relaxed {001} facets of β-MnO 2 (Fig. S9b).While for the I4/m space group α-MnO 2 , after the lattice and atoms relaxation calculations (Figs.S9 c   and d), in each unit cell, two Na + ions stabilize on the 8g Wyckoff position, that is above the bottleneck (window) of the [1 × 1] tunnel of α-MnO 2 , that are two-fold coordinated with the oxygen ligands of the MnO 6 octahedron with the 2.245 Å bond length.One Na + ion localizes on the 2a Wyckoff position (the cavity centre of the [2 × 2] tunnel), eight-fold coordinated with the oxygen ligands of the MnO 6 octahedron with an average ~2.61Å bond length (form the cube shortened along c axis geometry) as marked in the side view of the relaxed {001} facets of α-MnO 2 (Fig. S9c).There is one more Na + ion occupying the 2b Wyckoff position, forming a four-fold coordination geometry with an average ~2.61Å bond length.VESTA. [11]e Na + ion binding energy on the relaxed {001} and {110} facets of β-MnO 2 are -0.709 and -0.976 eV, respectively, as shown in Table S1.They are smaller than the values of relaxed  VESTA. [11]ble S1.Binding energy (eV) of the Na + ions on the relaxed facets of α-MnO 2 and β-MnO 2 .

Figs.
Figs. S10 a and b show the lattice and atoms relaxed oxygen termination {110} facets of β-MnO 2 (oxygen termination {110} facets of β-MnO 2 have 131 meV lower free energy compared with the manganese terminated facets).All the Na + ions two-fold coordinate with the 4f Wyckoff position oxygen, forming the 2.192 Å bond length as shown in Fig. S10b.While for the oxygen terminated {100} facets of α-MnO 2 (Figs.S10 c and d, oxygen termination {100} facets of α-MnO 2 have 954 meV lower free energy compared with the manganese terminated facets), in each unit cell, one Na + ion four-fold coordinates with 8h Wyckoff position oxygens (with two 2.168 Å bond lengths and two 2.587 Å bond lengths), that is inserted within the half [1 × 1] tunnel.Another Na + ion inserts the half [2 × 2] tunnels, forming the V coordination geometry with 8h Wyckoff position oxygens (with four ~2.32 Å bond lengths and one 2.665 Å bond length) as shown in Figs.S10 c and d.

Fig. S10 .
Fig. S10. a and b.Top (a) and side (b) views of the relaxed {110} facets of β-MnO 2 absorbed with the Na + ions.c and d.Top (c) and side (d) views of the relaxed {100} α-MnO 2 absorbed with the Na + ions.Yellow, purple, and red spheres are Na, Mn, and O atoms, respectively.Red, green, and blue coloured arrows represent the a, b, and c axes, respectively.Plotted by the