First Principles Study of Oxygen Adsorption on Li-MO 2 (M = Mn, Ti and V) (110) Surface
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Journal of The Electrochemical
Society
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First Principles Study of Oxygen Adsorption on Li-MO2 (M = Mn, Ti and
V) (110) Surface
To cite this article: Khomotso P. Maenetja and Phuti E. Ngoepe 2021 J. Electrochem. Soc. 168 070556
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Journal of The Electrochemical Society, 2021 168 070556
First Principles Study of Oxygen Adsorption on Li-MO2 (M = Mn,
Ti and V) (110) Surface
Khomotso P. Maenetja and Phuti E. Ngoepez
Materials Modelling Centre, University of Limpopo, Sovenga 0727, South Africa
Metal-air batteries are attractive for any application where weight is a primary concern, such as in mobile devices. Since oxygen
doesn’t need to be stored in the battery, the cathode is much lighter than that of a lithium-ion battery, which gives lithium-air
batteries their high energy density. Density functional theory study (DFT) is employed in order to investigate the surfaces of,
β-MnO2, β-TiO2 and β-VO2 (β-MO2) which act as catalysts in metal-air batteries. Adsorption of oxygen at (110) Li-MO2 is
investigated, which is important in the discharging and charging of Li–air batteries. Oxygen adsorption on Li/MO2 was simulated
and we found that in all the metal oxides (MnO2, TiO2 and VO2) comprises most stable orientation is the dissociated composition
where there is an oxygen atom on the “bulk-like” positions on top of each of the M cations. The surface lithium peroxide for MO2
simulated produces clusters with oxygen - oxygen bond lengths that are comparable to the calculated bulk and monomer discharge
products reported in literature.
© 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access
article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/
1945-7111/ac1640]
Manuscript submitted May 10, 2021; revised manuscript received July 3, 2021. Published July 28, 2021.
A high energy demand as a result of a growth in living standards progress of manganese oxides with various crystal structures for
and population has stimulated the efforts to develop high energy Li–O2 battery application, and demonstrated design strategies of
density power sources. Even though there have been improvements Mn-based oxides cathodes and the effect of crystal structure of
in Li-ion battery technology, these have not kept pace with the MnO2 on the overall performance of Li–O2 batteries. Such review
development of portable devices, leaving a so-called “power gap” substantially affirmed that manganese oxides, as oxygen cathode
that is broadly anticipated to grow in coming years. As an catalysts, can effectively promote the reversible formation and
alternative, metal air batteries are gaining a lot attention due to their decomposition of Li2O2, and can effectively reduce the OER and
ability to deliver high theoretical specific energies, which are almost ORR overpotentials.
6–10 times Li-ion batteries.1–3 However, the fundamental challenges Transition metal oxides have been widely used in catalysing
that limit the use of Li/Na air battery technology is the ability to find ORR and OER, their activities remain to be encourage in some
a catalyst that will expedite the formation and decomposition of cases. There were some reported works using valence moderating
Li2O2 during charging and discharging cycle. strategy to intensify the OER catalytic activity. Things seem to work
Catalytic materials have been proposed and synthesized to well when some lower valence fraction was introduced into the
promote the oxygen reduction reactions and oxygen evolution pristine transition metal oxides.30,31
reactions ORR/OER process, which can be mainly classified into Catalytic studies in Li air batteries that have been conducted from
three groups: carbon-based materials4–6 noble metal/metal oxides,7–9 all types of catalysts (carbon based material, noble metals and metal
and transition metal oxides.10–12 Several studies have reported the oxide) both experimentally and computationally demonstrate how
use of noble metals and metal oxides as a catalyst for Li air they improve performance of Li air batteries, specifically the ORR/
batteries.13–15 Transition metal oxides such as Co, Mn oxides have OER.
also been considered as potential candidate electrocatalysts for bi- Although much progress has been made, there are still some
functional oxygen electrodes due to their high catalytic activity and challenges that need to be addressed prior implementation of
good corrosion stability for Li air batteries.16,17 Among transition positive-electrode catalysts in Li-O2 batteries. Fundamental research
metal oxides, manganese dioxides have been widely investigated as about the battery reaction mechanism is still needed, which could
the catalyst for the ORR/OER in non-aqueous lithium-oxygen speed up the oriented design and optimization of catalysts. Research
batteries,18–20 due to the low cost, environmental friendliness, and on ensuring that good conductivity and large exposure surface area
easy preparation.21–23 Charge voltages of these un-catalysed cells are of catalysts, the synthesis of nano-crystallised catalysts and porous
similar to those of the MnO2 catalysed cells while both of these electrode structures should be explored.32 However, such studies
exhibits higher charge voltages than the cobalt-catalysed cells.24 have not clearly quantified the energetics related to different
Song et al. demonstrated that the relatively large amount of Mn catalysts. In this paper all the energetics are clearly shown mainly
(III) exposed on the MnO2 surface can improve the catalytic activity focusing on the ORR which will also help predict the OER, as an
of MnO2, enabling homogeneous discharge product distribution extension of our previous work on MnO2 and compared with other
which will lead to a higher capacity, good rate performance, and metal oxides. Redox properties of the most stable surface (110)
lower overpotential.25 Hu et al. prepared a carbon- and binder-free β-MnO2, β-TiO2 and VO2 are investigated; we determine the most
MnO1.957/Ni foam electrode by annealing the MnO2/Ni foam stable composition with different amounts of oxygen on the surface
electrode at 350 °C for 60 h.26 Their results also demonstrated that and the type of configuration that is more favourable in terms of
more Mn(III) and oxygen vacancies in MnO2 can improve the orientation as well. We therefore adsorb with Li on a stoichiometric
catalytic activity. However, in these reported results, Mn(III) and surface, and determine the stable site on which Li adsorbs taking into
oxygen vacancy were generated by a calcination method,26,27 while account charge transfer between the adatom and surface M cation.
the high-temperature treatment will seriously lead to the surface We further discuss the thermodynamics of oxygen adsorption on
oxidation of the substrate28 and the collapse of MnO2−x lithiated surfaces which is basically what is happening during
nanosheets,26 which decrease the conductivity of substrate and the charging and discharging of Li-air batteries.
active surface areas of MnO2−x. Liu et al.29 provided an overview on
Methodology
Periodic density functional theory (DFT) calculations were
z
E-mail: phuti.ngoepe@ul.ac.za performed with the VASP code,33,34 within the generalized gradient
Journal of The Electrochemical Society, 2021 168 070556
Table I. 1: Lattice parameters of the MnO2, TiO2 and VO2 bulk We considered the adsorption of oxygen (two O atoms per
structure. surface cell, Γ = 2) on the Li/MO2 (110) surface. In order to
investigate the stability of Li–O–O–Li species, which are known to
3
Structure (Rutile) a (Ǻ) c (Ǻ) V (Ǻ ) be important in the Li–air battery, we also consider two Li atoms per
surface cell, assuming that they both occupy the most stable bbi
0MnO2 4.366 4.41038 2.961 2.88739 56.44 sites. It should be noted that since there are only two bbi sites at each
TiO2 4.627 4.95440 3.008 2.95938 64.40 surface in our simulation cell, this structure corresponds to full
VO2 4.617 4.55438 2.774 2.85739 59.13 coverage of the bbi sites at the surface, that is, a monolayer of Li
adatoms. Therefore, based on the results discussed above, all the M
(5-fold) surface cations can be expected to be reduced before oxygen
approximation (GGA) in the form of PBE exchange correlation adsorption, which has been confirmed using Bader analys.43
functional.35 The number of planewaves was determined by a cutoff We investigate a number of oxygen adsorption configurations, in
kinetic energy of 600 eV and the Brillouin zone sampling scheme of some cases keeping the molecular bond between the two oxygen
Monkhorst-Pack with 6 × 6 × 9 and 6 × 6 × 1 k-points mesh for the atoms, and in others assuming dissociative adsorption. We found
bulk and surface structures were used, respectively. For these four different stable configurations, which are shown in Fig. 2. Some
calculations we use Liechtenstein’s non-simplified rotationally of the configurations initially considered were found to be unstable
invariant Hubbard correction with effective Coulomb parameter set (in the sense of not being a minimum in the adsorption energy
U = 2.8 eV and exchange parameter J = 1.2 eV and U = 4.6 eV and landscape).
the exchange parameter J = 0.0 eV.36,37 The calculations for VO2 For example, although a configuration with one oxygen atom
were performed without the Hubbard correction and were not spin directly on top of each Li is a stationary point, it is in fact unstable
polarized. and relaxes (if the symmetry of the initial configuration is broken) to
The lattice parameters were in good agreement with the experi- a peroxo configuration bridging between two Li atoms, as in Fig. 2
mental with deviations of approximately +0.8% and −3.1% for a (iii). The oxygen adsorption energy in this Li-peroxo configuration is
and c, respectively, and of 1.6% in the cell volume for the MO2 −1.81 eV/O2, −2.23 eV/O2, and −4.03 eV/O2 for MnO2, TiO2 and
shown in Table I. VO2 respectively as shown in Table II and Fig. 2 iii. In this
When the bulk structure was allowed to relax fully and cleaved a configuration, the calculated O–O bond lengths, associated with
(110) surface which was allowed to converge as well and the surface MnO2 and TiO2 catalysts, are around 1.55 Å and are consistent with
energy was obtained using the expression those of the hexagonal bulk and monomer Li2O2 (1.55 Å).
Adsorption of oxygen, as atoms and molecules, on lithiated metal
Eslab − Ebulk oxide surfaces will obviously yield discharge products of varying
γ= [1]
2A stability which will be attested to by corresponding adsorption
energies.
where Eslab is the energy per slab unit cell, Ebulk is the energy of the A peroxo group bridging directly between the two previously
equivalent amount of bulk solid and A is the surface area. Lithium unsaturated Mn and Ti cations, as in Fig. 2. ii) (Eads = −2.01 eV/O2,
and oxygen adsorption and co-adsorption on a clean (110) surfaces is −4.35 eV/O2 for MnO2 and TiO2 respectively) is noted. It is further
performed in such a way that stoichiometry and symmetry are observed that the O–O separations are 1.76 and 1.78 Å for MnO2 and
preserved throughout the calculations Further details on the meth- TiO2 respectively, which are relatively larger than those of the Li2O2
odology can be found elsewhere.41,42 monomer and bulk hexagonal forms (1.55 Å). However, when
oxygens are located directly above the V cations a peroxo bridging
Results and Discussion group is not formed, on the contrary, a dissociated configuration is
Oxygen adsorption at the Li/MO2 (110) surface.—Examine the stable with the adsorption energy of −4.61 eV/O2 for VO2 and this is
catalytic influence of metal oxide (110) surfaces, on the formation of further confirmed by an extended the O–O bond length to 2.96 Å.
discharge products in the Li-air batteries. Some of the prominent The least stable which involves a peroxo perpendicular to the surface
products that have been reported widely in literature are depicted in and binding to Li and M produced (Eads = −1.27 eV/O2,
Fig. 1, that is, bulk Li2O2 hexagonal and P-6 symmetry and −0.79 eV/O2, −4.06 eV/O2 for MnO2, TiO2 and VO2 respectively).
monomers with O–O bond length of 1.55 Å (hexagonal and Generally, the O–O bond lengths of the products, shown in Fig. 2
monomer) whereas the P-6 symmetry has O–O bondlength of iv), are reduced relative to those of the bulk and monomer of Li2O2.
1.85 Å. The other unstable discharge product in Li-air battery is Furthermore, the O–O bond length on the MnO2 (1.34 Å) and VO2
LiO2 which is also shown in Fig. 1 (pnnm symmetry and its (1.36 Å) are closer to those of the LiO2 monomer which has O–O
monomer with bondlength of 2.45 and 1.38 Å respectively). bond length of 1.38 Å. However, the O–O bond length on the TiO2
Figure 1. Discharge products in Li-air battery; structure of Li2O2 (a–c), P-6, hexagonal (P63/mmc) and a monomer respectively; structure of LiO2 (d &e) Pnnm
and its monomer respectively showing the relaxed O–O distances.
Journal of The Electrochemical Society, 2021 168 070556 Figure 2 . Stable adsorption configurations for two oxygen atoms adsorbed on the (a) Li/MnO2 (b) Li/TiO2 and (c) Li/VO2 (110) surface. Table II. Adsorption energies for oxidation of the lithiated MO2 (MnO2, TiO2 and VO2). Configuration MnO2 Ads Energy (eV) TiO2 Ads Energy (eV) VO2 Ads Energy (eV) Dissociative −2.25 −4.35 −4.88 Peroxo on M −2.01 −4.35 −4.61 Peroxo on Li −1.81 −2.23 −4.03 Peroxo on M/Li −1.27 −0.79 −4.06 surface (1.43 Å), does not compare with that of any known discharge (Eads = −2.25 eV/O2, −4.35 eV/O2 and −4.88 eV/O2). The dis- products, but lies between that of monomers for Li2O2 and LiO2. sociative configuration is further confirmed by large O–O bond However, the most stable configuration found was the dis- lengths of approximately 2.96 Å in magnitude, for all metal oxides sociative adsorption where there is an oxygen atom on the in this study. It is observed that all the metal oxides seems to have “bulk-like” positions on top of each of the M cations, but with the same favourable configuration (dissociative) in terms of additional bonds formed with the Li adatoms, as in Fig. 2 i yielded thermodynamic stability.
Journal of The Electrochemical Society, 2021 168 070556
Figure 3. MnO2 surface (110) adsorption and lithium peroxide bulk
energetics. Figure 4. TiO2 surface (110) adsorption and lithium peroxide bulk
energetics.
Effect of MO2 in the cathode reaction in a Li–air battery.—
During the battery discharge process, molecular oxygen is reduced presence and in the absence of TiO2). When comparing the energy of
in the cathode, in the presence of Li cations and electrons, forming formation to the most stable configuration of the surface lithium
lithium peroxide (Li2O2) particles: oxide in Fig. 2b i and ii and Fig. 4 (dissociated and peroxo on Ti
configuration), the formation energy of the bulk is higher by 1.83 ev
O 2 + 2Li+ + 2e− → Li2 O 2 [2] Li−1. This implies that, for such configurations, the formation of
Li2O2 will not be expedited since clusters are too stable and would
The essential prerequisite for the successful operation of a recharge- stick to the TiO2 surface. It can therefore be surmised that the initial
able Li air battery is the formation of Li2O2 as a reaction product reduction of oxygen in the cathode occurs more favourably via the
during discharge and the decomposition of Li2O2 to Li and O2 peroxo on Li and peroxo on Ti/Li at the lithiated TiO2 surface,
during charging. However, one of the critical problems in the non- forming the structures described in Figs. 2b and 4, than via the
aqueous Li air batteries employing carbon-based oxygen electrode is formation of small unsupported Li2O2 clusters.
the very large polarization that occurs during the discharge/charge Lastly, on comparing the energy of formation of the bulk Li2O2,
process. The high cell polarization is mainly attributed to the low which is −2.52 eV/Li and shown in Figs. 1b and 5, and the energy of
catalytic activity of carbon and to the high activation energies formation of the surface lithium oxide at VO2 (110) figure 4.9
required for the formation of Li2O2 during discharge and the (dissociative, peroxo on V, peroxo on Li and peroxo on V/Li)], the
decomposition of the Li2O2 during charging. It has been confirmed energy of formation for the bulk is higher by approximately 2.4 eV
that the discharge/charge efficiencies can be improved by the Li−1 in all stable configurations for oxygen adsorption on lithiated
addition of catalytic materials to the carbon supported oxygen surfaces. This is almost twice the value of the formation energy of
electrodes. The catalysts included in the oxygen electrode such as the bulk Li2O2 implying that in all such configurations, formation of
MO2 can affect the discharge/charge potentials and determine the Li2O2 will not be encouraged since the clusters are too stable and
rechargeability of the cells.44 Hence, the energetics associated with would stick to the VO2 surface.
MnO2, TiO2 and VO2 as catalysts and how they impact on the Although several studies have alluded to the effectiveness of
formation of Li2O2 as discharge products will be discussed. transition metal oxide catalysts on the performance of Li-air
Firstly, a comparison of the formation energy of the hexagonal batteries, the related energetics have not been adequately quantified.
bulk Li2O2, which is −2.52 eV/Li,45 shown in Fig. 1b and Fig. 3 It has mainly been mentioned how the catalysts improve the
with the energy of formation of the surface lithium oxide at MnO2 formation and decomposition of Li2O2 during discharging and
(110) is insightful. The energy of formation for the bulk is lower by charging. The energetics calculated for configurations related to
1.25 eV Li−1 relative to the least stable configuration for oxygen the three chosen rutile type metal oxides, i.e. MnO2, TiO2 and VO2
adsorption on lithiated surfaces Fig. 2a iv (peroxo on Mn/Li) and as possible catalysts in the discharge process of Li-O2 batteries have
Fig. 3. Further comparisons of the bulk energy of formation to the clearly depicted, that the MnO2 could expedite nucleation and
most stable configuration of the surface lithium oxide Fig. 2a i growth of the Li2O2 discharge product, across all configurations. It
(dissociated configuration) and Fig. 3 shows that the former is lower has further been shown that TiO2 promotes nucleation for limited
by 0.27 ev Li−1. This agrees with the experimental observation that configurations, in particular peroxo on Li with O–O bondlength of
Li2O2 is the main product of the cathode reaction in Li–air batteries 1.55 Å and peroxo on Ti/Li with the O–O bondlength of 1.43 Å,
(both in the presence and in the absence of MnO2).46 The initial whereas the VO2 does not appear to play any significant role in this
reduction of oxygen in the cathode occurs more favourably via the process, for all configurations considered.
dissociative adsorption of the oxygen molecule at the lithiated MnO2 Indeed, in previous studies, manganese dioxides have been
surface, forming the structures described in Fig. 2a, than via the widely investigated as catalysts, among transition metal oxides, for
formation of small unsupported Li2O2 clusters. Hence when the the ORR/OER in non-aqueous lithium-oxygen batteries, due to the
Li2O2 particles grow in contact with such MnO2 surfaces, then low cost, environmental friendliness, and easy preparation.21–23 In
the barrier for the formation of the lithium peroxide particles at the particular, it was shown that nanostructured MnO2 in different
cathodes would be reduced. polymorphic states are able to catalyse the formation and decom-
We further consider the impact of TiO2 as a catalyst on the position of Li2O2 in the cathode, thus decreasing the overpotentials
growth of the Li2O2 product. The energy of formation for the bulk is required for the operation of the Li–air cell.47 In that case, it was the
lower by 0.29 eV Li−1 for peroxo on Li (Fig. 2b iii) and 1.73 eV surface rather than the bulk of the manganese oxide which controlled
Li−1 for the peroxo on Ti/Li (Fig. 2b iv), as also shown in Fig. 4. its function in the cathode, and therefore the lack of Li intercalation
This agrees with the experimental observation that Li2O2 is the in bulk β- MnO2 was not expected to limit its utilization. The current
main product of the cathode reaction in Li–air batteries (both in the study has provided an explanation, based on the energetics, as to
Journal of The Electrochemical Society, 2021 168 070556
Li2O2 Is energetically more favourable than the formation of gas-
phase lithium peroxide (Li2O2) monomers, but still more favourable
than the formation of Li2O2 bulk, which suggests that the presence
of β-VO2 in the cathode of a Li air battery increases the energy for
the initial reduction of oxygen which makes VO2 an undesirable
catalyst candidate.
Acknowledgments
National Research Foundation for financial assistance and South
African Research Chair Initiative of the Department of Science and
Technology. The calculations performed were carried out at the
Centre for High Performance Computing in Cape Town, South
Africa, some were performed at our local clusters at Materials
Modelling Centre, University of Limpopo, South Africa. Special
thanks to Prof Ricardo Grau Crespo for advises and input.
ORCID
Khomotso P. Maenetja https://orcid.org/0000-0002-3199-0946
Figure 5. VO2 surface (110) adsorption and lithium peroxide bulk ener-
getics. Phuti E. Ngoepe https://orcid.org/0000-0003-0523-5602
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