Journal of Power Sources 625 (2025) 235658
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Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Elucidating the diffusion and interaction mechanisms of Zn2+ and H+ in
MnO2 for aqueous zinc-ion batteries: A DFT study
Yichun Zheng , Diancheng Chen , Yuling Zhao , Hongfei Bao , Yang Sun *
School of Materials, Sun Yat-sen -University, Shenzhen, 518107, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
• Investigated multiple MnO2 phases to
insertion
understand
Zn2+/H+
mechanisms.
• H+ ions face significant diffusion hin­
drance due to oxygen interactions.
• H+ insertion partially obstructs Zn2+
migration.
• Established a connection between Zn2+/
H+ insertion and MnO2 dissolution.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Manganese dioxide
Aqueous zinc-ion batteries
Density functional theory calculations
Cathode materials
Reaction mechanism
MnO2 is a promising cathode material for aqueous zinc-ion batteries (AZIBs) with potential applications in largescale energy storage systems. However, its practical use is hindered by poor cyclic stability and slow diffusion
kinetics. Despite considerable efforts, success has been limited due to an unclear understanding of the intrinsic
properties of Zn2⁺ and proton (H⁺) insertion into MnO2. This study investigates the predominant phases of
MnO2—α, β, δ, and γ—and elucidates their reaction mechanisms using first-principles computational methods
based on density functional theory (DFT). Our research reveals that H⁺ ions exhibit a higher initial insertion
voltage compared to Zn2⁺ ions, but their diffusion in MnO2 is significantly impeded by strong interactions with
oxygen. Additionally, H⁺ insertion partially obstructs Zn2⁺ ion migration. Furthermore, the insertion of Zn2⁺/H⁺
and the adsorption of H⁺ on the MnO2 surface are intricately linked to the dissolution process of manganese. This
work significantly enhances our fundamental understanding of MnO2 as a cathode material for AZIBs.
1. Introduction
The contemporary global energy landscape’s reliance on fossil fuels
presents significant environmental challenges and risks of resource
depletion [1]. To address these issues, secondary batteries have emerged
as efficient alternatives for energy conversion, with lithium-ion batteries
being a prominent example. However, the development of lithium-ion
batteries faces limitations related to lithium availability, safety, and
* Corresponding author.
E-mail address: sunyang5@mail.sysu.edu.cn (Y. Sun).
https://doi.org/10.1016/j.jpowsour.2024.235658
Received 13 August 2024; Received in revised form 10 October 2024; Accepted 18 October 2024
Available online 23 October 2024
0378-7753/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
environmental concerns [2,3]. In response to evolving energy demands
and sustainability imperatives, research efforts are now shifting towards
the next generation of secondary batteries. Aqueous batteries, known for
their low cost, high safety, and environmental compatibility, are gaining
increasing recognition [4,5]. Among these, metal zinc has been identi­
fied as an ideal anode material for aqueous batteries due to its high
theoretical capacity and low electrochemical potential [6,7].
Manganese dioxide (MnO2), extensively studied as a cathode mate­
rial for aqueous zinc-ion batteries, offers substantial theoretical capacity
and operating voltage. Despite this potential, MnO2 faces significant
challenges, including poor cyclic stability and sluggish ion diffusion
dynamics [8–10]. MnO2’s complexity is highlighted by its diverse
oxidation states and structural variations, which include various crys­
talline forms (α-, β-, γ-, δ-, ε-forms) and amorphous types. During elec­
trode reactions, MnO2 may undergo irreversible phase transitions,
leading to volume changes and structural collapse, resulting in notable
capacity loss [11,12]. The Coulombic interactions between Zn2+ and H+
ions contribute to slow reaction and diffusion kinetics, further compli­
cating the enhancement of MnO2’s electrochemical properties. Addi­
tionally, stress from ion insertion exacerbates the collapse of the crystal
structure [13,14]. To address these issues, strategies such as composite
modification [15,16], defect engineering [17], and the introduction of
heterogeneous atoms [18,19] have been employed to overcome these
limitations. While these methods have led to some improvements, the
resulting products still fall short of meeting practical production de­
mands. This highlights the urgent need to observe the insertion process
and gain a comprehensive understanding of the MnO2 cathode’s reac­
tion mechanisms.
The proposed mechanisms for MnO2 cathode reactions in aqueous
zinc-manganese batteries (AZMBs) include Zn2+/H+ insertion/extrac­
tion, H+-induced conversion reactions, and dissolution-deposition
mechanisms [20–22]. Initial research focused on the electrochemical
insertion and extraction of Zn2+, resembling processes in lithium-ion
batteries [23,24]. Another typical mechanism involves the insertion of
H+ and the co-insertion of Zn2+ and H+ ions [25–27]. Additionally,
during discharge, manganese is reduced to Mn2+ ions, which are sub­
sequently released into the electrolyte. During the charging cycle, these
Mn2+ ions contribute to the regeneration of MnO2 through the
dissolution-deposition mechanism. Studies have shown that this dis­
solution/deposition process often occurs simultaneously with chemical
conversion and co-insertion mechanisms, resulting in the formation of
multiple reaction intermediates [28–31]. Despite these proposed
mechanisms, the structural transformations of MnO2 upon Zn2+/H+
insertion remain inadequately explored, and the role of H+ in MnO2 is
still debated [32]. Therefore, a deeper investigation into the ion inser­
tion behavior and mechanisms within MnO2 is crucial to develop stra­
tegies for enhancing its performance as a cathode material.
Density Functional Theory (DFT) calculations offer valuable insights
into the structural properties and reaction mechanisms at the atomic
level. In this study, we used DFT to investigate the intrinsic character­
istics of four widely reported MnO2 phases: α-, β-, δ-, and γ-. Our results
show that H+ has a lower insertion formation energy compared to Zn2+
in MnO2, but a higher diffusion energy barrier, suggesting a greater
tendency to bond with oxygen rather than undergo reversible insertion.
In contrast, Zn2+ demonstrates superior migratory capabilities within
MnO2 due to lower diffusion barriers, although its diffusion is somewhat
impeded by the preferential insertion of H+. Additionally, ion insertion,
combined with the decomposition and adsorption of water molecules
from the electrolyte on the cathode surface, exacerbates the dissolution
of MnO2. These findings provide deeper insights into the reaction and
degradation mechanisms of manganese-based cathodes in AZMBs.
Table 1
Calculated and experimental lattice parameters of hollandite (α-MnO2), pyro­
lusite (β-MnO2), birnessite (δ-MnO2), and nsutite (γ-MnO2). Experimental lattice
parameters are adapted from Refs. [43–47].
Crystal Structure
α-MnO2
β-MnO2
δ-MnO2
γ-MnO2
Calculations
Experiment
a (Å)
b (Å)
c (Å)
9.88
9.73
4.46
4.40
2.91
2.87
13.94
13.71
9.88
9.73
4.46
4.40
2.91
2.87
2.90
2.87
2.85
2.85
2.87
2.87
–
–
4.46
4.46
(PBE)
(PBEsol)
(PBE)
(PBEsol)
(PBE)
(PBEsol)
(PBE)
(PBEsol)
a (Å)
b (Å)
c (Å)
9.80
9.80
2.85
4.39
4.39
2.87
2.85
2.85
–
13.70
2.86
4.46
*The c lattice constant (interlayer spacing) in δ-MnO2 is highly uncertain due to
the presence of intercalated cations or water in all reported samples of this
phase.
augmented wave (PAW) method within the framework of DFT [33–35].
We employed the generalized gradient approximation (GGA) for the
exchange-correlation energy functional. To determine the most effective
exchange-correlation functional, we computed the equilibrium struc­
tures and energies of the bulk phases of α-, β-, δ-, and γ-MnO2 using both
the Perdew-Burke-Ernzerhof (PBE) and PBEsol GGA functionals
[36–38]. To improve the accuracy of van der Waals interactions, we
used the DFT-D3 method with Becke-Jonson damping [39]. Our
computational approach included a plane-wave kinetic energy cutoff of
550 eV for the electronic wave function expansion. We ensured that total
energies converged within 10− 4 eV per formula unit, and that the final
forces on all ions were below 0.01 eV Å− 1. Given the importance of the
magnetic structure of the Mn sublattice in determining the stability of
MnO2 polycrystals, all magnetic configurations were initialized using
the ground state antiferromagnetic (AFM) sequence. To account for the
strong on-site Coulomb interactions among the d-orbital electrons of the
transition metals, the GGA + U approach was applied with an effective U
parameter of 3.9 eV for manganese [40]. Symmetry-breaking re­
laxations were allowed by not imposing symmetry constraints. The
surface model included a 15 Å vacuum layer to prevent interactions
between adjacent images. To determine the energy pathways and bar­
riers for Zn2+/H+ in MnO2, we utilized the climbing image nudged
elastic band (CI-NEB) method [41]. For analyzing chemical bonds be­
tween O and Zn/H/Mn, a projected crystal orbital Hamilton population
(COHP) analysis was performed using the LOBSTER package [42].
3. Results and discussion
3.1. Geometrical structures of unmodified MnO2
MnO2 is composed of MnO6 octahedra as its fundamental structural
unit, which assemble into various configurations through the sharing of
angles and edges, resulting in diverse MnO2 crystal structures. The most
widely reported cathode materials in aqueous zinc-ion batteries (AZIBs)
are tunnel-type α-MnO₂, β-MnO₂, γ-MnO₂, and layered structure δ-MnO₂.
Consistent with prior research, our findings indicate that the structure
with the lowest energy exhibits antiferromagnetic behavior [40].
Table 1 presents the relaxed lattice constants of different MnO2, ob­
tained through structural optimization, along with experimental data for
comparison. It is shown that the structural optimization with the PBEsol
functional yielded results more closely aligned with experimental values
in terms of lattice parameters. Consequently, the PBEsol functional was
selected for optimizing the crystal structure in subsequent calculations.
Fig. 1 illustrates the optimized geometric structures of MnO2 poly­
morphs. α-MnO2 features a 2 × 2 tunnel structure and belongs to the
body-centered tetragonal system with a space group of I4/m. Its wide
tunnel structure (~4.6 × 4.6 Å) facilitates the insertion of Zn2+ ions
[43]. β-MnO2, with a narrower 1 × 1 tunnel structure (~2.3 × 2.3 Å),
2. Computational methods
All calculations in this study were conducted using the Vienna Ab
initio Simulation Package (VASP) and employed the projected
2
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
Fig. 1. Common polymorphs of MnO2. The blue/green spheres and surrounding octahedra denote spin-up/down Mn atoms and MnO6 octahedra [48]. (For inter­
pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
theoretically restricts Zn2+ insertion. It crystallizes in the P42/mnm
space group, typical of a tetragonal rutile-type structure known for its
stability and distinctive properties [44]. The γ-MnO2 polymorph has an
orthorhombic structure with a space group Pnma and a more complex
tunnel arrangement. It consists of alternating 1 × 1 tunnels (~2.3 × 2.3
Å, pyrolusite) and 1 × 2 tunnels (~2.3 × 4.6 Å, ramsdellite), which
influence its electrochemical behavior, making it suitable for AZMB
applications [45]. δ-MnO2 possesses a two-dimensional layered struc­
ture with P2/m space group symmetry. Its large interlayer spacing
promotes faster diffusion and storage of Zn2+ ions, enhancing its effec­
tiveness as an electrode material [46,47].
In terms of electronic structure, all MnO2 phases are semiconductors,
though their electronic properties, including band gaps, vary due to
differences in their crystal structures. These structural variations
strongly influence the material’s conductivity and overall electro­
chemical performance. As shown in Fig. S1, the calculated band gaps for
pure α-, β-, δ-, and γ- MnO2 are 1.31, 0.25, 1.86, and 0.64 eV, respec­
tively. These values generally align with experimental data [40], with
minor discrepancies likely due to defects and impurities in the samples.
2 tunnel facilitates Zn2+ insertion more effectively than the 1 × 1 tun­
nels, with the optimal site being the center of the triangular prism. For
layered δ-MnO2, the octahedral position is the most favorable insertion
site. The average Zn-O bond lengths in α-MnO2, β-MnO2, γ-MnO2, and
δ-MnO2 are 1.935 Å, 2.027 Å, 1.953 Å, and 2.139 Å, respectively. In
tunnel-structured forms such as α-, β-, and γ-MnO2, shorter Zn-O bond
lengths generally correspond to more negative formation energies,
reflecting stronger Zn-O bonds and more stable electrochemical con­
figurations. Despite having the longest bond length (2.139 Å), δ-MnO2
shows the most negative formation energy (− 4.18 eV), likely due to its
unique layered structure, which enhances stability through additional
interlayer interactions and increased space for ion accommodation.
Based on the most stable Zn2+ locations, we optimized over 50 insertion
structures across the four MnO2 polymorphs, systematically simulating
the step-by-step insertion of Zn2⁺ ions at various concentrations.
The charge-discharge processes of the Zn2+ insertion/extraction
mechanism are represented by the half-cell reaction:
MnO2 + nZn2+ + 2ne− ↔ Znn MnO2
(1)
Here, n denotes the number of Zn2+ inserted into the MnO2 cathode. To
develop a concentration-dependent spectrum of the theoretical elec­
trode voltage, we calculated the energy of all intermediate phases at
various concentrations of n. For each value of n, the formation energy
was calculated using the following formula below, which is based on the
ground state energy E[Znn MnO2 ]:
3.2. Zn2+ insertion/extraction mechanism
In neutral and weakly acidic AZMBs, it is widely accepted that Zn2+
ions are inserted into the MnO2 crystal structure during discharge,
potentially causing phase changes. This electrochemical mechanism is
similar to that observed in “rocking chair batteries” [49,50]. However,
current research does not clearly elucidate the behavior of the four
different types of MnO2 during charge and discharge. To deepen our
understanding of the Zn2+ insertion mechanism, it is essential to analyze
these differences at the molecular scale.
First, we calculated the formation energy (Ef) of Zn2+ insertion,
revealing that at low concentrations, Zn2+ insertion into MnO2 is ener­
getically favorable (see Table S1 in the Supplemental Material). We
systematically evaluated multiple cation insertion sites and identified
the optimal Zn2+ insertion positions for each of the four MnO2 structures
(Figs. S2 and S3). In α-MnO2, the interaction between Zn2+ and sur­
rounding O2− ions pulls Zn2+ toward the tunnel corners. This wide
tunnel structure accommodates eight potential insertion sites, indicating
a high theoretical Zn2+ insertion capacity. In contrast, the optimal
insertion site in β-MnO2 is at the tunnel center. In γ-MnO2, the larger 1 ×
Ef [Znn MnO2 ] = E[Znn MnO2 ] −
] nmax − n
n [
E Znnmax MnO2 −
E[MnO2 ]
nmax
nmax
2+
(2)
The total energy of the MnO2 cathode with Zn inserted at con­
centration n and the maximum concentration is represented by
]
[
E[Znn MnO2 ] and E Znnmax MnO2 , respectively. E[MnO2 ] denotes the total
energy of the pristine MnO2 cathode. Based on equation (2), we calcu­
lated the formation energies of ZnnMnO2 for inserted configurations at
different concentration, as illustrated in Fig. 2. Furthermore, we depic­
ted the formation energy convex hull, which is the connecting line be­
tween the lowest energy ordered phases. Structures located on this
convex hull indicate thermodynamic stability and resistance to decom­
position into substitutional configurations, whereas structures above the
convex hull are in a metastable state and are more prone to
3
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
Fig. 2. Calculated formation energies and open-circuit voltage of MnO2 for Zn2+ insertion. (a–d) Formation energies for α-ZnnMnO2, β-ZnnMnO2, δ-ZnnMnO2, and
γ-ZnnMnO2 systems with respect to the pristine MnO2 and bulk Zn. (e–h) Calculated open-circuit voltage as a function of Zn concentration in the MnO2.
4
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Journal of Power Sources 625 (2025) 235658
Fig. 3. The diffusion path and energy barrier of Zn2+ ions in (a) α-MnO2; (b) β-MnO2; (c) δ-MnO2; and (d) γ-MnO2.
decomposition into other compounds. This methodology is extensively
applied in the study of metal ion battery electrodes [51].
In Fig. 2a, our observations for α-MnO2 show that the ZnnMnO2
compounds with n = 0.25, 0.75, and 0.875 lie on the convex hull,
indicating their thermodynamic stability. In contrast, compounds with n
= 0.125, 0.375, 0.5, and 0.625 are positioned above the convex hull,
signifying energy instability. For β-MnO2 (Fig. 2b), the ZnnMnO2 com­
pounds with n = 0.375, 0.5, and 0.75 are on the convex hull, suggesting
they are energetically stable. In the case of δ-MnO2 (Fig. 2c), ZnnMnO2
compounds with n = 0.25, 0.5, 0.75, 1.0, and 1.125 are located on the
convex hull, demonstrating stability. For γ-MnO2 (Fig. 2d), the ZnnMnO2
structures with n = 0.167, 0.333, 0.5, and 0.667 also lie on the convex
hull, indicating stability. Some structures exhibit thermodynamic sta­
bility at higher cation insertion concentrations, while those with lower
concentrations are unstable. This suggests that, in some cases, the lattice
adapts more effectively to increased cation insertion. Moreover, the
insertion of cations affects the overall charge balance and electronic
structure of the material. We hypothesize that at higher insertion con­
centrations, enhanced charge compensation mechanisms, such as elec­
tron density redistribution or the formation of new bonds, contribute to
maintaining structural stability.
Open-circuit voltage (OCV) is a crucial parameter for evaluating
battery performance, and increasing the voltage of AZIBs can enhance
the energy density of the battery. The OCV is determined by calculating
the change in total energy before and after the insertion of Zn2+ ions.
Considering that the contributions of volume and entropy to the chargedischarge process are negligible [52,53], their effects have been
excluded from this analysis. The OCV calculation is conducted within
the bounds of the theoretical maximum concentration for Zn2+ inser­
tion, as follows [54]:
]
[
]
[
E Znn2 MnO2 − E Znn1 MnO2 − (n2 − n1 )E[Zn]
(3)
(n2 − n1 )2e
[
]
[
]
In this context, E Znn2 MnO2 and E Znn1 MnO2 represent the total
energies of Znn MnO2 at two adjacent concentrations n2 and n1 of stable
structures, respectively. E[Zn] signifies the energy of the bulk metal.
Based on these stable intermediates and Equation (3), we analyzed the
concentration-dependent curve of the electrode voltage for the MnO2
cathode with inserted Zn2+ ions.
Fig. 2e–h illustrate the concentration-dependent curves of electrode
voltage for Zn atoms in MnO2 cathodes, plotted against the concentra­
tion n. The open-circuit voltage (OCV) of all Zn2+-inserted MnO2 cath­
odes consistently decreases with increasing Zn2+ concentration.
Specifically, for α-MnO2, the Zn2+ insertion voltages at different stages
are 0.98 V, 0.56 V, and 0.13 V, respectively. In the case of β-MnO2, the
voltages are 0.98 V, 0.97 V, and 0.073 V, respectively. Zn2+ insertion in
δ-MnO2 is characterized by five distinct voltage plateaus at 1.18 V, 1.16
V, 0.42 V, 0.37 V, and 0.047 V. For γ-MnO2, the insertion process yields
voltage plateaus at 0.78 V, 0.75 V, 0.67 V, and 0.34 V. Reported
experimental data shows that the initial potential for zinc ion insertion
typically ranges between 1.0 and 1.14 V [23], aligning closely with our
theoretical values. Some studies have successfully increased the
discharge voltage to approximately 1.2 V by optimizing the defect
structure and electrolyte. However, challenges such as irreversible phase
transitions continue to undermine the stability of subsequent discharge
processes [24,49,50]. The phase transformation of MnO2, induced by
the rising concentration of Zn2+ ions, plays a pivotal role in the reduc­
tion of electrode capacity. This phenomenon will be analyzed in detail
later in the article.
The rapid charge-discharge capacity is also a critical factor in
assessing electrode material performance. Considering that the rate
capability of AZMBs is intimately linked to the diffusion efficiency of
OCV = −
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Journal of Power Sources 625 (2025) 235658
Fig. 4. Calculated formation energies and open-circuit voltage of MnO2 for H+ insertion. (a–d) Formation energies for α-HnMnO2, β-HnMnO2, δ-HnMnO2, and
γ-HnMnO2 systems with respect to the pristine MnO2 and hydrogen molecule. (e–h) Calculated open-circuit voltage as a function of H concentration in the MnO2.
6
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Journal of Power Sources 625 (2025) 235658
Fig. 5. The diffusion path and energy barrier of H+ in (a) α-MnO2; (b) β-MnO2; (c) δ-MnO2; and (d) γ-MnO2.
Zn2+, we examined the diffusion paths and energy barriers for Zn2+ in
MnO2. Our findings, presented in Fig. 3, indicate that the highest
diffusion energy barrier for Zn2+ is 0.69 eV in β-MnO2. This is primarily
due to the restrictive nature of the 1 × 1 tunnel structure in β-MnO2,
which impedes Zn2+ diffusion. Conversely, the interlayer structure of
δ-MnO2 provides ample space for Zn2+ movement, resulting in a lower
diffusion energy barrier for Zn2+ ions.
(Figs. S4 and S5). In these cases, protons bind to oxygen atoms within
MnO2, causing slight local structural distortions.
Building on these identified stable positions, we further optimized
various insertion structures across different MnO2 polymorphs to
simulate the gradual insertion of H+ ions. As shown in Fig. 4a–d, we
calculated the formation energy for various HnMnO2 configurations and
plotted a convex hull diagram using equation (4). Based on the convex
hull, we calculated the discharge voltage for H+ insertion.
3.3. H+ insertion/extraction mechanism
Ef [Hn MnO2 ] = E[Hn MnO2 ] −
Several studies have demonstrated that MnO2 exhibits enhanced
electrochemical properties in aqueous electrolytes compared to nonaqueous electrolytes, a phenomenon attributed to electrochemical
mechanisms involving H+ ions [25,55]. Recent developments have
shown that H+ insertion/extraction also plays a role in aqueous zinc
batteries, marking a departure from the traditional Zn2+ ion-based en­
ergy storage mechanisms. Given their minimal mass and smallest ionic
radius, the incorporation of H+ ions tends to enhance the capacity of
aqueous batteries significantly. In some cases, the probability of Zn2+
intercalating into the MnO2 lattice is very low, with reversible H+
intercalation predominantly governing the electrochemical process [49,
56].
To elucidate the electrochemical mechanism of proton participation,
we conducted a computational analysis of H+ insertion behaviors using
DFT. Specifically, we calculated the formation energy of H+ insertion
into MnO2. As shown in Table S2, our findings indicate that H+ insertion
is energetically favorable and occurs more readily compared to Zn2+
insertion. In α-MnO2, the formation energy for cation insertion into 2 ×
2 tunnels is lower than that for 1 × 1 tunnels, due to the larger tunnel
size—a trend also observed in γ-MnO2. We investigated various insertion
sites and confirmed the optimal H+ insertion positions for each structure
OCV = −
] nmax − n
n [
E Hnmax MnO2 −
E[MnO2 ]
nmax
nmax
]
[
]
[
1
E Hn2 MnO2 − E Hn1 MnO2 − (n2 − n1 ) E[H2 ]
2
(n2 − n1 )e
(4)
(5)
[
]
Where E[Hn MnO2 ] and E Hnmax MnO2 denote the total energy of the
MnO2 cathode inserted with the concentration n and the maximum
concentration of protons nmax , respectively; E[MnO2 ] is the total energy
]
[
]
[
of the pristine MnO2 cathode; E Hn2 MnO2 and E Hn1 MnO2 represent
the total energies of Hn MnO2 at two adjacent concentrations n2 and n1 of
stable structures, respectively; E[H2 ] signifies the energy of a hydrogen
molecule.
Fig. 4e–h shows the variation in electrode voltage with concentration
n for H+ in MnO2. The OCV values for the H+ intercalation in α-MnO2,
β-MnO2, δ-MnO2, and γ-MnO2 systems range from 1.64 to 0.29 V, 1.62 to
0.28 V, 1.60 to 0.34 V, and 1.54 to 0.92 V, respectively. Since an ideal
cathode material is characterized by a high electrode voltage, our cal­
culations indicate that the discharge mechanism involving H+ insertion
outperforms that of Zn2+, particularly in the initial stages. Experimental
measurements of proton insertion potential, which range from 1.4 to 1.5
7
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
Fig. 6. Crystal orbital Hamiltonian population (COHP) of (a–d) H-O for α-H0.125MnO2, β-H0.125MnO2, δ-H0.125MnO2 and γ-H0.125MnO2; (e–h) Zn-O for
α-Zn0.125MnO2, β-Zn0.125MnO2, δ-Zn0.125MnO2 and γ-Zn0.125MnO2.
V [26,55], align with the initial theoretical values. The slight discrep­
ancy, with theoretical values being marginally higher, is likely due to
internal resistance and battery polarization, which cause some voltage
loss during measurement.
To understand the diffusion mechanism of H+ ions in MnO2, refer to
Fig. 5 for the ion diffusion pathway and the corresponding energy bar­
rier. Contrary to our initial expectations, our findings illustrate that
across four different MnO2 structures, H+ exhibits a higher diffusion
barrier than Zn2+, suggesting that H+ diffusion is more challenging. This
unexpected finding is intriguing given the smaller ionic radius of protons
and their reduced electrostatic repulsion with Mn. Therefore, we hy­
pothesize that the diffusion barrier for H+ in MnO2 is primarily influ­
enced by the interaction between H+ and the O atoms of MnO2.
To assess the stability of chemical bonds within inserted systems,
COHP calculations were conducted and analyzed. The resulting plots, as
illustrated in Fig. 6, display the COHP values for H-O and Zn-O bonds
across α-MnO2, β-MnO2, δ-MnO2, and γ-MnO2 systems, with lower in­
tegral values indicating increased bond stability. Notably, the diffusion
barriers for H+ in α-MnO2, δ-MnO2, and γ-MnO2 systems with stronger
H-O bond strengths are higher compared to β-MnO2, where the H-O
bond strength is weaker. Furthermore, the stronger H-O bond compared
to the Zn-O bond implies a more pronounced interaction between H+
and O2− , thereby impeding H+ diffusion and resulting in a higher
diffusion barrier for H+ relative to Zn2+. Besides, the stronger H-O bond
implies a greater propensity for H+ to engage in a transformation re­
action with O in MnO2, rather than migrating within the MnO2 lattice.
The bond lengths of H-O and Zn-O for the four crystal structures have
been calculated (Table S4), providing a clearer understanding of the
COHP analysis. Our findings are consistent with numerous experimental
observations. Several studies have confirmed that H+ in the electrolyte
can directly participate in the electrochemical reaction of the MnO2
cathode, leading to the conversion of MnO2 into products such as
MnOOH, Mn2O3, and Mn3O4 [55–58].
Given that H+ ions exhibit higher diffusion barriers than Zn2+, this
could theoretically reduce rate capability when H⁺ is involved in the
charge transport mechanism, particularly during rapid charging and
discharging. However, modifications in the insertion behavior and
diffusion pathway of H+ can reduce these barriers and enhance H+
transport. A novel H+ diffusion pathway (Fig. S6) in α-MnO2 was
revealed, induced by crystal structure distortion due to increased H+
insertion concentration. In the transition state (TS) configuration, the
H+ ion is position on an O ion that situated at the corner of the 2 × 2
tunnel, representing a metastable state. This state arises from the
tetragonal-orthorhombic (TO) distortion of the α-MnO2 lattice. Our
findings demonstrate a significant reduction in the energy barrier (0.76
eV) associated with this mechanism, resulting in enhanced H+ diffusion
(Fig. S7). Similar experimental results have been reported, showing that
activation of the Grotthuss mechanism or a reduction in the diffusion
path improves H+ insertion and diffusion in MnO2, subsequently
increasing the battery’s specific capacity [49,59,60].
On the other hand, the diffusion barrier also affects the structural
stability of the cathode during long-term cycling. While a higher diffu­
sion barrier may limit rate performance, it could also extend the battery
lifespan by reducing mechanical stress and degradation in the MnO2
structure. For Zn2+, the lower diffusion barrier facilitates its insertion
and extraction but can also lead to irreversible phase transitions and
capacity fading [23]. Therefore, the relationship between the diffusion
barriers and pathways of Zn2+ and H+ in MnO2, and their impact on
battery performance, is complex and multifaceted. Integrating these
theoretical insights with empirical battery testing is crucial for
advancing battery technology. Strategic modifications to lower the
diffusion barrier could improve rate performance, while maintaining a
certain barrier level may help enhance durability.
3.4. Zn2+ and H+ co-insertion/extraction mechanism
Numerous studies have demonstrated that both Zn2+ and H+ are
inserted in MnO2 during cathode discharge [61,62]. Therefore, studying
the co-insertion behavior of Zn2+ and H+ is crucial for advancing our
understanding of electrode reaction mechanisms in manganese-based
8
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
Fig. 7. (a–b) Calculated single and co-insertion energies for Zn2+ and H+ ions in MnO2; (c–h) Lowest energy configurations of MnO2 for single and co-insertion of
Zn2+ and H+ ions at various concentrations.
materials.
It is widely recognized that the crystal stability of electrode materials
plays a pivotal role in the performance and cycle life of rechargeable
batteries [63]. Accordingly, the crystal structure of MnO2 was thor­
oughly examined during the insertion of Zn2+ and H+ (Fig. S8). For
α-MnO2, the insertion of Zn2+ and H+ induces significant lattice
distortion, with the a-axis increasing by 17.57 % and the b-axis
decreasing by 6.47 % when the Zn2+ concentration reaches 0.85. At an
H+ concentration of 1.5, the a-axis expands by 22.27 % while the b-axis
contracts by 10.68 %, resulting in a noticeable change in the a/b ratio.
This substantial deformation could irreversibly transform α-MnO2 from
a tetragonal to an orthorhombic system, compromising its ability to
store cations [64]. In β-MnO2 and γ-MnO2, Zn2+/H+ insertion leads to an
increase in both the a-axis and b-axis. Specifically, at Zn2+ concentra­
tions of 0.75 and 0.67, the a and b axes expand by over 20 % and 10 %,
respectively, eventually destroying the tunnel structure. In δ-MnO2,
electrostatic repulsion between inserted Zn2+ and H+ ions causes
expansion of the a-axis and b-axis, with a maximum expansion rate of
13.94 %. Despite this, δ-MnO2 retains its layered structure throughout
the insertion process, as confirmed by computational simulations.
Thermodynamic analyses reveal that H+ insertion generally occurs
before Zn2+ insertion (Fig. 7). At low ion concentrations, Zn2+ and H+
ions tend to selectively occupy different tunnels or layers. Energy ana­
lyses suggest that at lower concentrations, Zn2+ and H+ facilitate each
other’s insertion. This indicates that H+ insertion into the cathode ex­
pands the lattice, thereby aiding Zn2+ insertion. The enhanced interac­
tion within the lattice at lower concentrations results in a shift in the
voltage plateau toward higher values. δ-MnO2 differs from the behavior
seen in other polymorphs, likely due to its distinct layered structure,
which more easily accommodates ions. As a result, the lattice expansion
typically associated with co-insertion is less pronounced, masking the
facilitative interaction between Zn2+ and H+ ions. At higher concen­
trations, Zn2+ and H+ ions tend to occupy the same tunnels or layers,
leading to electrostatic repulsion between them. Consequently, this
mutual occupation hinders ion insertion, as seen in β-MnO2, where the
small tunnel size makes Zn2+ insertion thermodynamically unfavorable
when many protons are already inserted. The simultaneous presence of
numerous Zn2+ and H+ ions not only destabilizes the cathode structure
but also causes an unfavorable shift in the voltage plateau, reducing
battery efficiency and capacity.
The observed electrostatic repulsion at high ion concentrations
highlights the importance of developing hybrid cathode materials.
Introducing dopants or secondary materials could buffer the interactions
between Zn2+ and H+, allowing for smoother diffusion and improved
electrochemical performance. Furthermore, electrolytes designed to
selectively enhance the mobility of one ion type could optimize the
insertion process and reduce competitive interactions.
As shown in Fig. S9 and Fig. S10, the diffusion barrier for Zn2+ and
+
H after their co-insertion is higher than that for the individual insertion
of either ion. This increase can be attributed to two main factors. First,
the synergistic interaction between Zn2+ and H+ strengthens their bond
with O2− , enhancing their overall interaction with O2− and thereby
raising the diffusion barrier. Second, the co-insertion of Zn2+ and H+
causes greater lattice distortion compared to the insertion of either ion
alone. This distortion is primarily due to the increased electrostatic
repulsion exerted on Mn by the combined presence of Zn2+ and H+,
which hinders their diffusion within the lattice.
The co-insertion effect in MnO2 leads to lattice deformation and
higher diffusion barriers, highlighting the importance of microstructural
design to facilitate ion diffusion. Developing cathode materials with
9
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
Fig. 8. Crystal orbital Hamiltonian population (COHP) of (a) Mn-O for pristine α-MnO2 surface and α-MnO2 surface after H+ adsorption; (b) Mn-O for pristine
β-MnO2 surface and β-MnO2 surface after H+ adsorption; (c) Mn-O for pristine δ-MnO2 surface and δ-MnO2 surface after H+ adsorption; (d) Mn-O for pristine γ-MnO2
surface and γ-MnO2 surface after H+ adsorption.
specific pore sizes or channel orientations can guide ions along preferred
diffusion pathways, reducing conflicts in densely packed ionic envi­
ronments. Additionally, pre-intercalating metal ions or water molecules
as structural pillars can enhance material stability and mitigate the
adverse effects of ion co-insertion [65,66].
However, some studies have shown that the ion insertion/extraction
process is often accompanied by the dissolution of the cathode. Addi­
tionally, due to the redox reaction of Mn (II)/(IV), the production of
Mn2+ ions contributes to the overall capacity [67–69].
To examine the dissolution of MnO2 during the insertion/extraction
process, we conducted COHP calculations to assess the strength of Mn-O
bonds in pristine MnO2, as well as in MnO2 with inserted Zn2+ and H+.
The integrated COHP (ICOHP) values (illustrated in Fig. S11) indicate a
decrease in the strength of Mn-O bonds in MnO2 upon the insertion of
Zn2+ or H+. This reduction signifies a weakened interaction between Mn
and O atoms, suggesting that Mn is more likely to dissociate from MnO2,
leading to the formation of Mn vacancies following the insertion of Zn2+
and H+. Table S3 shows the amount of charge transfer of four crystal
MnO2 in detail, revealing a slight decrease in the average Bader net
atomic charge of the Mn atoms after H+/Zn2+ insertion. These results
demonstrate that ion insertion leads to a redistribution of charge, ulti­
mately reducing the valence state of the Mn ions. To elucidate the
relationship between Mn valence states and the Jahn-Teller effect, the
projected density of states (PDOS) of Mn was analyzed (Fig. S12).
The 3d)
(
electron orbitals of Mn are categorized into two groups: eg dz2 , dx2 − y2
(
)
and t2g dxz , dyx , dxy . In pristine MnO2, the t2g orbitals of Mn4+ are fully
occupied, while the eg orbitals remain unoccupied. The Jahn-Teller ef­
fect typically arises with Mn3+ ions, causing the eg orbitals to split into
unoccupied dx2 − y2 and occupied dz2 orbitals near the Fermi level. Our
results show that upon Zn2+ or H+ insertion, the high-energy eg orbitals
split into two distinct groups, one positioned below and the other above
the Fermi level, indicating the activation of Jahn-Teller distortions in
Mn3+ ions. Further analysis of the MnO2 structures after the insertion of
Zn2+ or H+ ions shows that α-MnO2 and δ-MnO2, due to their wider
tunnels and layered structures, are better able to stabilize the local
environment of Mn ions, reducing the likelihood of distortion. In
contrast, β-MnO2 and γ-MnO2, with their more compact and inter­
connected frameworks, are more prone to distortion. The observation
rationalizes the co-occurrence of Zn2+/H+ insertion and Mn dissolution.
MnO2 enhances capacity through the solution-deposition process,
where Zn2⁺/H⁺ insertion promotes Mn2⁺ formation, driving the discharge
reaction. However, repeated deposition and dissolution cycles result in
material loss and structural changes within the electrode, compromising
its mechanical stability and reducing overall capacity. Therefore, it is
essential to manage these processes to improve the longevity and per­
formance of MnO2-based electrodes in electrochemical applications. We
10
Y. Zheng et al.
Journal of Power Sources 625 (2025) 235658
propose that controlling charging rates and avoiding overcharging are
crucial to minimizing Mn dissolution and structural degradation. Future
research should explore strategies such as electrolyte mod­
ification—possibly through additives that inhibit Mn dissolution—or the
development of more stable MnO2 structures to mitigate the dissolutiondeposition cycle during extended cycling.
Notably, the use of aqueous electrolytes makes the presence of water
unavoidable in the experiment. Experimental evidence suggests that
H2O can co-insert with Zn2+ and H+ in MnO2, particularly in the layered
δ-MnO2 structure [60,70]. The presence of water expands the interlayer
spacing, facilitating the insertion of Zn2+ and H+ and thereby increasing
the ion storage capacity. Additionally, water molecules act as lubricants
within the MnO2 lattice, enhancing ion transport. However, the
co-insertion of H2O is less common in structures with narrower chan­
nels, such as β-MnO2.
Besides ion insertion, the Mn2+/MnO2 reaction also involves H2O
molecules, and interfacial water plays a crucial role in determining
dissolution [71]. Based on prior reports, the optimal surfaces for each
structure—specifically, the (110) facet of α-MnO2, the (110) facet of
β-MnO2, the (001) facet of δ-MnO2, and the (131) facet of γ-MnO2—were
identified and employed for further analysis (refer to Fig. S13) [72–74].
On the (110) surface of α-MnO2, H2O molecule tends to undergo dehy­
drogenation. The dehydrogenation energy (Edehydro) of a H2O molecule
on this surface was calculated to be − 3.734 eV. This finding was
corroborated by Ab Initio Molecular Dynamics (AIMD) simulations
(Fig. S14). Similar behaviors were observed on the (110) surface of
β-MnO2, the (001) surface of δ-MnO2, and the (131) surface of γ-MnO2,
with dehydrogenation energies of − 4.232 eV, − 0.431 eV, and − 4.018
eV, respectively.
Furthermore, the surfaces of MnO2 across all four crystal structures
showed a tendency to directly adsorb H+ ions from the solution
(Fig. S15). To evaluate the stability of Mn-O bonds on the pristine MnO2
surface and after H+ adsorption, COHP calculations were conducted.
Results indicate that the strength of Mn-O bonds on the MnO2 surface
decreases following H+ adsorption, leading to a weakened interaction
between Mn and O atoms (refer to Fig. 8).
This weakening suggests an increased propensity for Mn to dissolve
and detach from MnO2 after H+ adsorption. As illustrated in Fig. S16, the
surface of MnO2 adsorbed by H+ exhibits lower Mn vacancy formation
energy, further supporting these findings. Notably, upon hydrogen
adsorption, Mn migration was observed on the (001) surface of δ-MnO2,
leading to the formation of a vacancy at the original Mn site. Our find­
ings underscore the significant role of H+ adsorption in promoting Mn
dissolution on the surface, emphasizing that the electrochemical reac­
tion dynamics are strongly influenced by the solution’s pH.
and dehydrogenation on the surface of MnO2, enhancing H+
adsorption and facilitating Mn dissolution.
Overall, this work enhances our understanding of the MnO2 elec­
trode reaction mechanism and provides valuable insights for developing
AZIBS with improved cycle stability.
CRediT authorship contribution statement
Yichun Zheng: Writing – original draft, Investigation, Formal
analysis, Data curation. Diancheng Chen: Writing – original draft, Data
curation. Yuling Zhao: Writing – review & editing. Hongfei Bao:
Writing – review & editing. Yang Sun: Writing – review & editing, Su­
pervision, Project administration, Methodology, Conceptualization.
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.
Acknowledgements
This work was supported by National Key R&D Program of China
(2022YFB2404700), the Natural Science Foundation of China
(22109186), Guangdong Innovative and Entrepreneurial Research
Team Program (2021ZT09L227). Y.S. was supported by the Funda­
mental Research Funds for the Central Universities, Sun Yat-sen Uni­
versity (22hytd01).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jpowsour.2024.235658.
Data availability
Data will be made available on request.
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