First-Principles Simulations for the Effect of Zinc Ions on MnO2/water system

The aqueous zinc-ion battery (AZIB), with its excellent power density, high energy density, safety, and sustainability, is a potential energy storage device. Manganese dioxide (MnO2), as the cathode material of AZIB, has shown exceptional electrochemical performance due to its multiple valence states and outstanding ion storage performance. Nevertheless, the energy storage process of MnO2 remains controversial. In order to explain the energy storage process of MnO2 in AZIB, it is necessary to explore the effect of Zinc ions (Zn2+) on the structure of the MnO2/water interface at the atomic scale. The effect of Zn2+ on the structure of MnO2/water interface systems was examined in this work using molecular dynamics simulation. Two representative crystal phases of MnO2, including α-MnO2 and δ-MnO2, were considered. The results revealed that Zn2+ will affect the structural stability of the MnO2/water interface system, and the degree of structural deformation in MnO2 varies with its crystal phase. Moreover, Zn2+ for both the α-MnO2/water and δ-MnO2/water systems prefer to be stabilized at the interface near MnO2 and the water, forming a spinel-like product. These results offer a fresh understanding of the MnO2 energy storage process in AZIBs and can help in the development of high-performance cathode materials.


Introduction
The aqueous zinc-ion battery is viewed as a promising energy storage material with the potential to supersede conventional lithium-ion batteries due to its high power density and energy density, ability to accommodate large current charge-discharge, low cost, and environmental safety [1,2].MnO 2 as the cathode material of AZIBs exhibits outstanding electrochemical performance due to its variable valence states and ion storage capability [3].Current experimental research on the energy storage mechanism of Zn-MnO 2 batteries has found that the energy storage mechanism of MnO 2 is extremely complex.Taking α-MnO 2 as an example, five different energy storage processes have been reported, which include insertion/extraction of Zn 2+ [4,5], insertion/extraction of H + /Zn 2+ [6,7], chemical conversion reaction [8], dissolution-deposition mechanism [9], and the combination of insert and convert reaction [10], but a unified conclusion has not been reached yet.To understand the energy storage process of MnO 2 in AZIB, atomic-level theoretical research on the influence of Zn 2+ on the structure of the MnO 2 /water interface is required.Density functional theory (DFT), as a computational method, can simulate micro-physical and chemical phenomena at the molecular scale and theoretically elucidate the possibility of reactions and has been widely used in catalysis and energy storage fields.MnO 2 has many phase structures, of which α-MnO 2 and δ-MnO 2 are two representative structures.The α-MnO 2 has a typical 2×2 tunnel structure with large channels, and δ-MnO 2 features a typical layered structure with ample interlayer spacing.In this paper, we chose α-MnO 2 and δ-MnO 2 to examine the impact of Zn 2+ on the MnO 2 /water interface system by DFT calculation.

Method
All DFT calculations were performed using the MedeA-VASP software package [11,12].For the expansion of the electronic wave functions, projector-augmented wave pseudopotentials with a 500 V cutoff energy were used [13].The Perdew-Burke-Ernzerhof generalized gradient approximation was utilized in this study [14].The energy criteria were set to 1.0 × 10 -5 eV, and the force criteria were set to 0.01 eV•Å -1 .The lattice parameters of α-MnO 2 /water and δ-MnO 2 /water systems are shown in Table 1.The α-MnO 2 /water system consists of 16 Mn, 58 O, and 52 H.The δ-MnO 2 /water system consists of 18 Mn, 51 O, and 30 H. Individual simulations were conducted for each model using the canonical ensemble (NVT) and a Nosé thermostat.To expedite the interface development, the temperature was fixed at 330 K.The simulations for each model lasted for 30 ps with a time step of 0.5 fs.

Results and conclusions
To investigate the impact of Zn 2+ on the MnO 2 / water interfacial system, we conducted three dynamics simulations for each crystal phase: MnO 2 / water interface system without Zn 2+ , MnO 2 /water interface system with a Zn 2+ in MnO 2 , and MnO 2 /water interface system with two Zn 2+ in water.

α-MnO 2 /water interface system without Zn 2+
To investigate the impact of Zn 2+ on the α-MnO 2 /water interface system, we first examined the system in the absence of Zn 2+ .Figure 1 demonstrates that in the absence of Zn 2+ , the atoms of α-MnO 2 undergo small-range vibrations at their respective positions, and the crystal structure remains unchanged.These observations suggest that the α-MnO 2 /water system is stable without the addition of external Zn 2+ .

α-MnO 2 /water interface system with a Zn 2+ in MnO 2
To investigate the effect on the α-MnO 2 /water interface system when a Zn 2+ insert MnO 2 , we posited a Zn 2+ at the center of the α-MnO 2 tunnel structure and then simulated the molecular dynamics of this system.The simulation results as depicted in Figure 2, when a Zn 2+ insert MnO 2 , it is compounded with α-MnO 2 and formed a stable spinel-like ZnMn 2 O 4 , which is consistent with the Zn 2+ insertion/extraction mechanism reported in earlier studies [15,16].Interestingly, the Zn 2+ moved from the center of the α-MnO 2 tunnel to the interface between α-MnO 2 water, where it formed the spinel-like ZnMn 2 O 4 and vibrated near this position without leaving the tunnel structure.This observation indicates that the spinellike ZnMn 2 O 4 compound is stable under these simulated conditions.Moreover, we noticed some deformation in the crystal structure of α-MnO 2 , which could be attributed to the Mn ions in the MnO 6 octahedra changing from +4 to a larger +3 radius during the Zn 2+ insertion process, leading to the expansion of the α-MnO 2 unit cell.

α-MnO 2 /water interface system with two Zn 2+ in water.
To further investigate the effect of Zn 2+ on the α-MnO 2 /water interface system, we performed molecular dynamics simulations by placing two Zn 2+ in the water layer of the α-MnO 2 /water interface system.The changes in the system were then observed as depicted in Figure 3, and one of the Zn 2+ remained in the aqueous solution.The other Zn 2+ was inserted the tunnel of α-MnO 2 , and exhibited only small vibrations, indicating the structural stability of the product.Upon extending the simulation period along the z-axis, we observed that the Zn 2+ entered the upper tunnel of α-MnO 2 , forming a spinel structure similar to ZnMn 2 O 4 .We also noted some deformation in the crystal structure of α-MnO 2 , which was not significantly different from the case when only one Zn 2+ was added.3.2.δ-MnO 2 /water interface system 3.2.1.δ-MnO 2 /water interface system without Zn 2+ .
To further investigate the impact of Zn 2+ on the δ-MnO 2 /water interface system, we also examined the δ-MnO 2 /water interface system in the absence of Zn 2+ .It can be observed from Figure 4 that the crystal structure of δ-MnO 2 undergoes some degree of deformation even in the absence of Zn 2+ .The displacement of Mn ions is more pronounced, and the layered structure is disrupted, indicating that the δ-MnO 2 structure is inherently unstable and prone to changes.

δ-MnO 2 /water interface system with a Zn 2+ in MnO 2
We hypothesized that a Zn 2+ occupies the central position in the δ-MnO 2 tunnel structure, and we conducted molecular dynamics simulations to investigate the resulting system at the water interface.interface, where it reacts with δ-MnO 2 to form a spinel-like ZnMn 2 O 4 product.The product remains stationary within the δ-MnO 2 structure, indicating its equilibrium state and stability.However, the δ-MnO 2 layered structure undergoes significant deformation and loses its original layered structure, which may be due to its inherent instability.

δ-MnO
2 /water interface system with two Zn 2+ in water.
To further investigate the effect of Zn 2+ on the δ-MnO 2 /water interface system, we performed molecular dynamics simulations by placing two Zn 2+ in the water layer of the δ-MnO 2 /water interface system. Figure 6 demonstrates that the two Zn 2+ both move towards the interface between δ-MnO 2 and water.Upon adding periodicity in the x and z directions, we observed that the Zn 2+ remains at the δ-MnO 2 /water interface, creating a product similar to ZnMn 2 O 4 or ZnMn 3 O 7 •2H 2 O.We can also note substantial structural distortion of δ-MnO 2 , with the original layered structure almostly disappearing.

Conclusions
This article discusses the impact of Zn 2+ on the structure of MnO 2 /water interface systems by DFT calculation.The simulation results show that Zn 2+ will affect the structural stability of the MnO 2 /water interface system and result in structural deformation of MnO 2 .The degree of structural deformation in MnO 2 varies with its crystal phase.Specifically, the initial layered structure of δ-MnO 2 was almost destroyed, whereas the initial tunnel structure of α-MnO 2 was retained.It was also found that δ-MnO 2 can insert more Zn 2+ compared to α-MnO 2 , but with relatively poor stability, leading to significant lattice deformation during the insertion process.Furthermore, Zn 2+ for both the α-MnO 2 /water and δ-MnO 2 /water systems prefer to be stabilized at the interface near MnO 2 and the water, forming a spinellike product.Overall, these findings will contribute to the further enhancing the comprehension of the storage mechanism of MnO 2 and have significant implications for improving the performance of MnO 2 cathode materials.

Figure 1 .
Figure 1.Molecular dynamics simulation of the α-MnO 2 /water interface system without Zn 2+ .The initial and final structures are shown in the side view (a, b) and top view (c, d), respectively.In the visualization, the Mn, O, and H atoms are represented by purple, red, and white balls, respectively.

Figure 2 .
Figure 2. Molecular dynamics simulation of the α-MnO 2 /water interface system with a Zn 2+ .(a-c) Side views of the initial structure, final structure, and trajectory of Zn 2+ during the simulation, respectively.(d-f) Top views of the initial structure, final structure, and trajectory of Zn during the simulation without display of water, respectively.The purple, red, white, and blue balls symbolize Mn, O, H, and Zn atoms, respectively.

Figure 3 .
Figure 3. Molecular dynamics simulation of the α-MnO 2 /water interface system with two Zn.(a-b) The initial and final structure is shown in side view, respectively (c) The trajectory of Zn during the simulation.(d-e) The initial and final structures are shown in the top view, respectively.The purple, red, white, and blue balls symbolize Mn, O, H, and Zn atoms, respectively.

Figure 4 .
Figure 4. Molecular dynamics simulation of the δ-MnO 2 /water interface system without Zn.The simulation's initial and final structure are shown in side view (a, b) and top view (c, d), respectively.

Figure 5
shows that the Zn 2+ migrates from the interlayer of δ-MnO 2 to the vicinity of the δ-MnO 2 /water NESP-2023 Journal of Physics: Conference Series 2592 (2023) 012032

Figure 5 .
Figure 5. Molecular dynamics simulation of the δ-MnO 2 /water interface system with a Zn.(ac) Side views of the initial structure, final structure, and trajectory of Zn during the simulation, respectively.(d-f) Top views of the initial structure, final structure, and trajectory of Zn during the simulation, respectively.The purple, red, white, and blue balls symbolize Mn, O, H, and Zn atoms, respectively.

Figure 6 .
Figure 6.Molecular dynamics simulation of the δ-MnO 2 /water interface system with two Zn.(a-c) Side views of the initial structure, final structure, and trajectory of Zn during the simulation, respectively.(d-e) Top views of the initial structure and final structure during the simulation, respectively.The purple, red, white, and blue balls symbolize Mn, O, H, and Zn atoms, respectively.