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. 2026 Oct 22. Online ahead of print. doi: 10.1039/d6ra08458c

First-principles investigation of penta-NiO2 monolayer for selective detection of thermal-runaway gases from lithium-ion batteries

Md Aminul Haque a, Methila Akter a, Abu Talha a, Md Alamgir Badsha b, Abdullah Al Roman a,✉
PMCID: PMC13689739  PMID: 42923743

Abstract

Lithium-ion battery thermal runaway releases hazardous gases that require rapid and reliable detection to ensure operational safety. In this work, first-principles density functional theory calculations are employed to systematically investigate the sensing performance of a two-dimensional penta-NiO2 monolayer toward representative thermal-runaway gases, including H2, CO, CO2, C2H4, CH4, and HF. The optimized penta-NiO2 monolayer exhibits favorable structural and dynamic stability and possesses a narrow semiconducting band gap of 0.621 eV. Gas adsorption substantially modifies the electronic properties of the monolayer, with the magnitude of the response strongly dependent on the adsorbed species. Among the investigated gases, HF exhibits the strongest interaction with penta-NiO2, characterized by an adsorption energy of −0.831 eV, pronounced charge redistribution, a large band-gap reduction to 0.153 eV, and a work-function change of 0.85 eV. The pronounced response to HF is associated with its strong interaction with the surface, promoted by the high electronegativity of fluorine, which leads to substantial interfacial charge redistribution and stronger perturbation of the electronic states. CO also produces a significant electronic response, reducing the band gap to 0.242 eV and inducing a work-function change of 0.31 eV. In contrast, H2, CH4, CO2, and C2H4 exhibit comparatively weak interactions and produce only modest modifications of the electronic structure. Electron-density difference, electron-localization-function, projected-density-of-states, and reduced-density-gradient analyses consistently reveal the distinct nature of the gas–surface interactions. The calculated temperature-dependent sensing response further demonstrates the strong selectivity of penta-NiO2 toward HF and CO. These results suggest that penta-NiO2 is a promising theoretical candidate for selective detection of HF and CO associated with lithium-ion battery thermal runaway.

Lithium-ion battery thermal runaway releases hazardous gases that require rapid and reliable detection to ensure operational safety.graphic file with name d6ra08458c-ga.webp

1. Introduction

Lithium-ion batteries (LIBs) have become central to modern energy-storage technologies because of their high energy density, long cycle life, and suitability for applications ranging from portable electronics and electric vehicles to large-scale energy-storage systems. Nevertheless, the increasing demand for high-energy-density LIBs has intensified concerns regarding their safety under thermal, electrical, and mechanical abuse.1,2 Thermal runaway (TR) is particularly hazardous because the accumulation of heat can accelerate a series of exothermic reactions within the cell, leading to rapid temperature and pressure escalation, cell venting, fire, and, in severe cases, explosion. The decomposition of electrolyte and electrode components during this process simultaneously produces a complex mixture of flammable and toxic gases, making gaseous evolution an important chemical signature of battery failure.3–6 Recent reviews and experimental studies have consequently identified gas evolution as an important target for early-warning strategies because gas generation can precede or accompany the rapid temperature rise associated with catastrophic failure.7 The composition of thermal-runaway gases depends on battery chemistry, state of charge, temperature, and failure conditions, but H2, CO, CO2, CH4, and C2H4 are repeatedly identified among the characteristic gaseous products of LIB abuse, while HF is of particular concern because of its toxicity and corrosive nature.5 Recent computational studies have therefore selected combinations of H2, CO, CO2, CH4, C2H4, C2H2, and HF as representative target molecules for evaluating candidate sensing materials. For example, Pd-doped HfSSe has been investigated for H2, CO, CO2, CH4, C2H2, and C2H4, whereas Pt-decorated AlN has been evaluated for H2, CO, CH4, C2H4, C2H2.8 Likewise, CuO/ZnO-modified GeC has been examined for H2, CO, CH4, C2H4, C2H2 demonstrating the increasing emphasis on multi-gas recognition rather than detection of a single decomposition product.9 These studies collectively indicate that the ability to distinguish different thermal-runaway gases is an important requirement for developing reliable battery-safety sensors. Conventional gas sensors based on metal oxides and related semiconductors have demonstrated useful responses toward hazardous gases, but their performance can be constrained by operating temperature, selectivity, response/recovery characteristics, and the availability of chemically active surface sites.10 The development of low-dimensional materials has therefore provided new opportunities for designing sensors with tunable electronic structures and high surface accessibility.11 In particular, atomically thin 2D materials offer a large fraction of exposed atoms for direct interaction with adsorbates, while their electronic properties can be strongly perturbed by molecular adsorption.12 The resulting changes in charge transfer, electronic density, band structure, density of states, and work function can serve as measurable signatures of gas recognition. A study of Rh-decorated PdS2 demonstrated this concept by combining adsorption energy, charge transfer, electron-density redistribution, band structure, DOS, work-function variation, and recovery behavior to evaluate thermal-runaway gas sensing.5 Similar multidimensional approaches have subsequently been applied to Cu-modified HfS2, TM-decorated HfS2, Au-doped CdS, Pd-doped HfSSe, and Cu-decorated h-BN.8,13–15

Recent studies demonstrate that modifying a 2D host with chemically active atoms or clusters can substantially enhance its interaction with thermal-runaway gases. Cu-decorated HfS2, for example, exhibits enhanced adsorption toward selected C2H4, CO, and H2 molecules, while Cu-embedded HfS2 shows stronger interactions with CH4 and CO2.13 Transition-metal decoration of HfS2 with Co, Rh, or Ir has likewise been explored to improve the adsorption and sensing of C2H4, CH4, and CO.14 More recently, Ag-, Cu-, and Pt-doped HfS2 have been investigated for CO, CO2, and C2H2, with Cu-HfS2 exhibiting particularly strong interaction with CO.16 Single-atom and cluster engineering have also been explored using h-BN, where transition-metal single atoms and Pd/Rh/Ru clusters can significantly modify the adsorption characteristics of thermal-runaway gases.17 These developments establish chemical functionalization as an effective route for enhancing gas–surface interactions; however, they also motivate the search for intrinsically active low-symmetry materials that may achieve useful gas discrimination without relying exclusively on external metal decoration or defect engineering. Pentagonal 2D materials provide a fundamentally different structural platform from conventional hexagonal lattices because their non-hexagonal bonding arrangement can generate low-symmetry atomic environments and distinctive electronic characteristics. Such structural asymmetry may influence the adsorption geometry, charge redistribution, and orbital interaction of gas molecules, potentially providing additional opportunities for selective molecular recognition.18,19 Our penta-NiO2 system is particularly interesting in this context because its Ni–O framework combines a pentagonal geometry with a semiconducting electronic structure, creating a potentially active surface for gas adsorption and electronic sensing. However, while recent research has extensively examined chemically modified HfS2, HfSSe, CdS, AlN, GeC, and h-BN for LIB thermal-runaway detection, systematic investigation of an intrinsically pentagonal NiO2 monolayer toward a broad set of characteristic thermal-runaway gases remains limited.13 This provides a clear motivation for assessing whether the unique structural and electronic characteristics of penta-NiO2 can generate sufficiently distinct adsorption-induced responses for thermal-runaway gas recognition.

In this work, first-principles density functional theory calculations are employed to systematically investigate the adsorption and sensing behavior of penta-NiO2 toward six representative LIB thermal-runaway gases, namely H2, CO, CO2, C2H4, CH4, and HF. Rather than evaluating sensing performance solely from adsorption energies, the present study establishes a comprehensive structure–interaction–response relationship by examining the energetically favorable adsorption configurations, adsorption energies, charge transfer, electron-density difference, electron-localization function, band structures, DOS/PDOS, work-function changes, reduced-density-gradient analysis, sensing response, and recovery characteristics. Such a combination of descriptors follows the increasingly comprehensive methodology adopted in recent first-principles studies of LIB thermal-runaway gas sensors. The objective is to determine how different gases perturb the electronic structure of penta-NiO2 and whether these changes produce distinguishable theoretical responses for gas recognition. Through this approach, the study aims to establish the fundamental sensing mechanism of penta-NiO2 and assess its potential as an alternative low-dimensional material for early-warning monitoring of LIB thermal runaway.

2. Computational details

First-principles calculations were carried out with the CASTEP module in Materials Studio. Exchange–correlation effects were described using the PBE functional within the GGA framework.20 For electron–ion interactions, ultrasoft pseudopotentials were used, while Grimme's DFT-D2 correction was considered for long-range van der Waals interactions. For the penta-NiO2 monolayer, a 2 × 2 × 1 supercell containing 24 atoms (8 Ni and 16 O) was used in all calculations. A vacuum spacing of 20 Å was maintained along the z-direction to prevent interactions between adjacent periodic structures. The plane-wave cutoff energy was set to 600 eV, and the Brillouin zone was sampled using a 3 × 3 × 1 Monkhorst–Pack k-point mesh, which was also employed for the electronic-property calculations. The pristine monolayer, isolated gas molecules, and adsorption configurations were geometrically optimized. The convergence criteria were set to 2 × 10−5 eV per atom for total energy and 0.03–0.05 eV Å−1 for maximum force. A stress tolerance of 0.1 GPa and a maximum atomic displacement of 0.002 Å were used as additional optimization criteria. Different adsorption sites and molecular orientations were considered to determine the favorable adsorption configurations. The Adsorption Locator module in Materials Studio was used for this analysis.21–23 Spin-polarized calculations were also performed for the pristine penta-NiO2 monolayer to examine its possible magnetic behavior. To quantify the interaction between the gas molecule and the penta-NiO2 surface, the adsorption energy (Eads) was determined according to the equation given below:24

Eads = Esheet+gas − Esheet − Egas 1

where, Esheet+gas, Esheet and Egas represent the total energy of the gas-adsorbed system, energy of the pristine penta-NiO2 and the energy of the isolated gas molecule respectively. The stability of the penta-NiO2 sheet was evaluated through its cohesive energy per atom (Ecoh) calculated from the equation below:25,26

2. 2

where, Esheet is the total energy of the penta-NiO2 sheet, ENi, and Eo are the energies of the single Ni and O atoms, respectively, and x, y, N denote the number of Ni atoms, O atoms and the total number of atoms in the sheet, respectively.

The formation energy of the penta-NiO2 monolayer was calculated to evaluate its energetic stability. Since the computational unit cell contains two Ni atoms and four O atoms, the formation energy per atom was calculated as:

2. 3

where Etotal is the total energy of the unit cell, and µNi and µO represent the chemical potentials of Ni and O, respectively. The factor of 6 corresponds to the total number of atoms in the unit cell.

To determine the sensor's recovery time (τ), the standard kinetic formulation rooted in transition state theory was employed:27

2. 4

where, v is attempting frequency 1012 Hz, k is the Boltzmann constant, and T is the temperature 298 K. The work function is calculated using the following formula:21,28

Φ = |V∞ − EF| 5

where, V∞ is the electric potential of a single electron in vacuum and can be neglected (V∞) V∞ ≈ 0.29

To determine the sensing response of the material, the following mathematical formulation was employed.

2. 6

Here, S represents the sensitivity of the material, ΔBG represents the change in band gap between the gas-adsorbed and pristine sheets. KB denotes the Boltzmann constant (8.62 × 10−5 eV K−1), and T refers to the absolute temperature.30

To map the weak intermolecular forces between the gaseous species and the sensing substrate, reduced density gradient (RDG) analyses were executed and subsequently visualized utilizing the Multiwfn and VMD software packages.28,31–34 The calculations performed in this study were used to analyze the adsorption characteristics and chemical interactions between the penta-NiO2 nanosheet and gases generated during lithium-ion battery thermal runaway.

3. Results and discussion

3.1. Geometry analysis

Fig. 1 illustrates the relaxed structure of the penta-NiO2 monolayer. The penta-NiO2 monolayer retains its characteristic pentagonal arrangement while adopting a buckled low-symmetry configuration. The resulting lattice constants are a = 4.47 Å and b = 4.44 Å. The Ni–O and O–O bond lengths are 1.8535 Å and 1.59 Å, respectively, with a Ni–O–Ni bond angle of 116.426°. These structural parameters are consistent with previously reported transition-metal oxide and pentagonal oxide materials, supporting the reliability of the optimized structure.18,35 The deviation of the Ni–O–Ni angle from ideal symmetric geometries reflects the intrinsic distortion associated with the pentagonal framework. Compared with the previously reported 1T-NiO2 monolayer, which adopts a trigonal structural arrangement,36 the present penta-NiO2 monolayer exhibits a distinct buckled pentagonal topology and local bonding environment. This structural difference establishes penta-NiO2 as a distinct structural configuration among the reported two-dimensional NiO2 systems. The optimized structural parameters of the penta-NiO2 monolayer are provided in Table 1. Upon interaction with gas molecules, only minor changes in the Ni–O–Ni bond angle and local bond lengths of the penta-NiO2 monolayer are observed, while the gas molecules remain nearly unchanged, indicating weak surface interaction, as summarized in Table 1.

Fig. 1. Optimized geometries of (a) penta-NiO2, (b) C2H4 + NiO2, (c) CH4 + NiO2, (d) CO2 + NiO2, (e) CO + NiO2, (f) H2 + NiO2, and (g) HF + NiO2.

Fig. 1

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Table 1. The minimum adsorption distance (dmin), Ni–O bond distance, Ni–O–Ni bond angle, recovery time (RT), and adsorption energy (Eads) of gas adsorbed penta-NiO2 monolayer.

Structures d min (Å) Bond distance (Å) Bond angle R T (s) E ads (eV)
NiO2 + C2H4 2.606 1.856 116.990 2.32 × 10−4 −0.494
NiO2 + CH4 2.533 1.866 116.374 2.73 × 10−10 −0.144
NiO2 + CO2 3.330 1.853 116.878 1.58 × 10−10 −0.130
NiO2 + CO 2.238 1.878 116.849 1.02 × 10−4 −0.473
NiO2 + H2 2.829 1.853 117.140 1.69 × 10−8 −0.25
NiO2 + HF 2.531 1.852 117.124 113.35 −0.831
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The calculated cohesive energy of the penta-NiO2 monolayer is −5.51 eV per atom, suggesting its energetic stability. Following the sign convention adopted in eqn (2), the negative value indicates that the total energy of the monolayer is lower than the sum of the energies of the corresponding isolated atoms. The obtained cohesive energy falls within the range of values previously reported for stable pentagonal and transition-metal oxide monolayers.35,37,38 The calculated formation energy of the penta-NiO2 monolayer is −0.937 eV per atom, as obtained using eqn (3). The negative value indicates favorable formation relative to the chosen reference states, supporting the energetic favorability of the monolayer. The stability mainly arises from the strong Ni–O bonding network in the pentagonal framework. Low-symmetry pentagonal structures also exhibit anisotropic charge distribution and tunable electronic properties, making them promising for nanoelectronic, catalytic, and gas-sensing applications.39–41

3.2. Phonon dispersion analysis

Fig. S1 (SI) presents the phonon dispersion of penta-NiO2 computed along the Γ–X–S–Y–Γ direction, which is used to verify the dynamical stability of the monolayer. The phonon spectrum of penta-NiO2 consists of three acoustic branches, namely out-of-plane (ZA), transverse (TA), and longitudinal (LA), along with several optical modes arising from Ni–O lattice vibrations No significant imaginary phonon modes are observed throughout the calculated phonon spectrum, supporting the dynamical stability of the penta-NiO2 monolayer.

A slight imaginary frequency appears in the flexural ZA branch close to the Γ point, corresponding to out-of-plane lattice vibrations. Such soft phonon modes are commonly reported in first-principles phonon calculations of two-dimensional materials and are typically attributed to numerical convergence limitations or the intrinsic flexural nature of atomically thin lattices rather than a true structural instability.42–44 This behavior of the ZA branch is not unique to penta-NiO2 and has also been found in graphene, transition-metal dichalcogenides, and various other two-dimensional materials.43–45

The optical branches reaching ∼700 cm−1 indicate strong Ni–O bonding and lattice stiffness. These phonon characteristics support the dynamical stability and theoretical viability of the penta-NiO2 monolayer, while its experimental synthesizability remains to be established experimentally.

3.3. Adsorption energy and recovery time analysis

The adsorption behavior of gas molecules on the penta-NiO2 surface was evaluated using the adsorption energy, which reflects the strength of gas–surface interactions and their influence on the sensing response.46 Accordingly, the adsorption energies of the selected target gases, namely H2, HF, CO2, C2H4, CH4, and CO, on the monolayer were systematically determined. The calculated adsorption energies and equilibrium separation distances for all adsorption configurations are given in Table 1.

The negative adsorption energies indicate that the gas molecules bind favorably to the NiO2 monolayer relative to the chosen reference states, but they do not by themselves establish thermodynamic spontaneity at finite temperature. Stronger adsorption is typically associated with shorter equilibrium separations, arising from enhanced electronic coupling and greater orbital overlap between the adsorbate and substrate. Overall, stronger adsorption is accompanied by shorter equilibrium distances, indicating a clear correlation between adsorption strength and the interfacial geometry.

The adsorption energies provide an indication of the strength of the gas–surface interactions; however, the nature of adsorption was determined by considering the adsorption energy together with the adsorption distance, charge transfer, ELF, PDOS, and structural deformation. Among the investigated gases, HF exhibits the strongest adsorption on the penta-NiO2 monolayer, with an adsorption energy of −0.831 eV and an equilibrium adsorption distance of 2.531 Å, indicating a chemisorption-dominated interaction. The strong HF interaction is consistent with the pronounced charge redistribution observed at the adsorption interface and the substantial modification of the electronic structure discussed below. The high electronegativity of fluorine provides a plausible origin for this enhanced interfacial interaction and charge redistribution. CO also shows relatively strong adsorption, suggesting significant electronic coupling with the surface and strong physisorption behavior. In contrast, C2H4 and H2 exhibit intermediate adsorption characteristics, while CH4 and CO2 display comparatively weak interactions. For these weaker adsorption systems, the relatively small interaction strength and limited interfacial perturbation are consistent with their smaller changes in the electronic properties of penta-NiO2. Overall, the adsorption characteristics reveal a gradual transition from chemisorption to weak physisorption with decreasing interaction strength, consistent with the observed adsorption energies and equilibrium distances. Nevertheless, the observed deviations indicate that adsorption strength is also influenced by electronic factors, including charge transfer and adsorbate–surface orbital hybridization.47 This combined analysis confirms that penta-NiO2 exhibits strong selectivity toward HF and CO, moderate sensitivity toward C2H4, while CH4 and CO2 interact only weakly with the surface, resulting in a clear distinction in the sensing response (Fig. 2).

Fig. 2. Adsorption energy of gas molecules on pristine NiO2.

Fig. 2

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The reversibility of a gas sensor is closely related to its recovery time, which reflects how readily the adsorbed gas leaves the surface and how quickly the sensor returns to its initial electronic configuration.48 In the recovery-time calculations, an attempt frequency of 1012 s−1 was adopted as a conventional order-of-magnitude value commonly used in theoretical gas-sensing studies to estimate thermally activated desorption. Although the actual attempt frequency may vary among different gas molecules and adsorption configurations due to differences in their vibrational characteristics, a common value was used for all systems to provide a consistent basis for comparing their relative recovery behavior. For the penta-NiO2 monolayer, the estimated recovery times vary markedly among the investigated gases. Consistent with transition-state theory, stronger surface binding leads to prolonged desorption times.49 The calculated recovery times are consistent with the adsorption characteristics discussed above. HF shows the slowest recovery because it binds to the penta-NiO2 surface through chemisorption, reflecting its comparatively strong adsorption. In contrast, CO and C2H4 exhibit very short recovery times, consistent with their relatively weaker adsorption and rapid desorption from the surface. H2, CH4, and CO2 exhibit extremely short recovery times, which can be attributed to their weak physisorptive interactions with the surface. Although rapid desorption can facilitate sensor recovery and improve reversibility, extremely short recovery times, particularly for CO and C2H4, may also result in limited residence time of the gas molecules on the surface and consequently reduce the duration of the measurable sensing response. Overall, stronger gas–surface interactions result in longer desorption times. Thus, the short recovery times of CO and C2H4 indicate favorable desorption kinetics, but their practical sensing performance should be evaluated by considering recovery time together with adsorption strength and the resulting sensing response.

3.4. Mulliken charge analysis

Mulliken population analysis was used to characterize bonding and charge redistribution in the pristine penta-NiO2 monolayer and its complexes with various gas molecules. Mulliken bond populations provide a qualitative measure of bond covalency where larger positive values indicate increased electron sharing (more covalent character), while values approaching zero indicate predominantly ionic interactions. Consistent with this criterion, Ni–O bonding in the pristine sheet is mainly ionic, with substantial charge transfer from Ni to O.50Table 2 summarizes the average mulliken charges for the pristine sheet and after adsorption of C2H4, CH4, CO2, H2, HF, and CO. The atomic charge distributions reveal the redistribution of electron density within the adsorbed molecules and at the adsorption interface. In hydrocarbons (CH4 and C2H4), carbon carries a negative charge, whereas hydrogen is positively charged, consistent with the higher electronegativity of carbon. In contrast, carbon carries a positive charge in CO and CO2, where the more electronegative oxygen atoms withdraw electron density from carbon. Hydrogen also exhibits different charge values depending on the adsorbed molecule. In the HF complex, hydrogen carries a positive charge, whereas fluorine carries a negative charge, consistent with their difference in electronegativity. The substrate atoms also exhibit variations in their Mulliken charges following gas adsorption. The Ni charge increases in the HF complex but decreases in the CO2 complex compared with the pristine monolayer. Similarly, the O charge becomes more negative in the HF complex than in the pristine sheet. These variations indicate changes in the local atomic charge distributions upon adsorption. The calculated charge transfer results indicate that the direction of electron transfer depends on the adsorbed gas. According to the summed Mulliken charges, CO2, CO, CH4, HF, and H2 act as electron acceptors, whereas C2H4 acts as a weak electron donor to the penta-NiO2 monolayer. Among the investigated gases, CO2 exhibits the strongest electron-accepting behavior, with a net charge of −0.81e, followed by HF and CO, with net charges of −0.10e and −0.12e, respectively. C2H4, in contrast, donates a small amount of electron density to the substrate, corresponding to a net charge of +0.078e. These results indicate that gas adsorption produces adsorbate-dependent charge redistribution, which can influence the electronic properties of penta-NiO2 and contribute to its gas-sensing response.

Table 2. Average Mulliken charge distribution of pristine NiO2 and its complexes.

Elements NiO2 C2H4 + NiO2 CH4 + NiO2 CO2 + NiO2 H2 + NiO2 HF + NiO2 CO + NiO2
Ni 0.86 0.865 0.858 0.852 0.863 0.895 0.871
O −0.86 −0.85 −0.852 −0.85 −0.855 −0.88 −0.855
C __ −0.555 −1.02 0.91 0.31
H __ 0.297 0.227 __ −0.035 0.47 __
F __ __ __ __ __ −0.57 __
O* __ __ __ −0.86 __ __ −0.43
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3.5. Electron density difference analysis

The distribution of electronic charge after gas adsorption is analyzed using EDD maps, as shown in Fig. 3. In the EDD plots, the green isosurfaces represent electron accumulation, indicating an increase in electron density, whereas the yellow isosurfaces represent electron depletion, indicating a decrease in electron density. These features illustrate the redistribution of electron density upon gas adsorption. For the pristine penta-NiO2 monolayer, charge accumulation is mainly localized around oxygen sites, whereas depletion occurs around nickel sites. This redistribution originates from the higher electronegativity of oxygen compared with nickel, which drives electron transfer from Ni to O and contributes to the ionic character of the Ni–O bonds. This charge distribution is in good agreement with the mulliken population analysis.

Fig. 3. Electron density difference (EDD) images of complex structures: (a) C2H4 + NiO2, (b) CH4 + NiO2, (c) CO2 + NiO2, (d) CO + NiO2, (e) H2 + NiO2, (f) HF + NiO2 respectively.

Fig. 3

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Each gas molecule induces a different degree of charge rearrangement on the penta-NiO2 surface, reflecting the variation in their adsorption characteristics. In the C2H4 and CH4 complexes, charge accumulation is primarily distributed around the carbon atoms and the Ni–C interface, whereas charge loss occurs mainly near the hydrogen atoms and the interacting Ni site, demonstrating electron donation from the hydrocarbons to the penta-NiO2 surface. In contrast, CO2 and CO exhibit electron depletion around the carbon atom, while electron accumulation is localized on the oxygen atom of the adsorbed molecule and within the Ni–molecule interfacial region, indicating adsorption-induced polarization accompanied by interfacial bonding. The H2 adsorbed system shows only slight electron redistribution, with weak accumulation around the molecule and limited depletion on the neighboring Ni atom, reflecting its weak interaction with the monolayer. Among all the investigated gases, HF induces the most pronounced redistribution of electron density, with accumulation around the F atom and depletion near the adsorption-site Ni atom. This is attributed to the high electronegativity of F and the strong polarity of the H–F bond, which promote charge transfer and stronger electronic coupling with the penta-NiO2 surface. Overall, the EDD results are fully consistent with the Mulliken population analysis, demonstrating that each adsorbed gas modifies the local charge distribution of the penta-NiO2 monolayer to a different extent, thereby influencing its electronic response and gas-sensing performance.

3.6. Electron localization function (ELF)

The Electron Localization Function (ELF) analysis was carried out to assess electron localization behavior and gain insight into the nature of chemical bonding in the studied structures.51,52 While charge-density-difference analysis provides a spatial picture of charge redistribution and quantitative charge analysis can estimate the amount of charge transferred, ELF offers complementary information about the spatial localization of electrons and the bonding character of the interface. As illustrated in the ELF map in Fig. 4, the green zones denote regions characterized by a high concentration of localized electrons.

Fig. 4. Electron localization function images of complex structures: (a) C2H4 + NiO2, (b) CH4 + NiO2, (c) CO2 + NiO2, (d) CO + NiO2, (e) H2 + NiO2, (f) HF + NiO2 respectively.

Fig. 4

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For the pristine penta-NiO2 monolayer, the localized electrons are predominantly concentrated around the O atoms owing to their higher electronegativity than Ni, whereas comparatively lower localization is observed around the Ni atoms. Upon gas adsorption, the ELF distributions reveal that C2H4, CH4, CO2, H2, and CO retain localized electron density within the individual species, with no continuous electron-localization region formed between the adsorbates and the penta-NiO2 surface. This behavior indicates negligible electron pairing across the interface and is characteristic of weak physical adsorption. In contrast, the HF adsorbed system exhibits a distinct electron-localization region connecting the F atom and the neighboring Ni atom, demonstrating electron pairing at the interface and a stronger chemical interaction with the penta-NiO2 monolayer.

3.7. Band gap analysis

The electronic band structures of pristine penta-NiO2 and its complexes are shown in Fig. 5. The obtained results reveal the changes in the electronic properties of the nanosheet after gas adsorption. The pristine penta-NiO2 monolayer exhibits semiconducting behavior with a band gap of 0.621 eV, indicating a narrow-gap system favorable for sensing applications. Spin-polarized calculations were also performed to examine the possible magnetic behavior of pristine penta-NiO2. The optimized structure converged to a zero total magnetic moment, indicating a nonmagnetic ground state within the present computational framework. For comparison, the previously reported 1T-NiO2 monolayer exhibits an indirect PBE band gap of 1.38 eV,36 which is considerably larger than the 0.621 eV band gap obtained for the present penta-NiO2 structure. This difference in band gap further highlights the influence of the distinct structural topology on the electronic properties of NiO2.

Fig. 5. Band structures of pristine and complex structures: (a) NiO2, (b) C2H4 + NiO2, (c) CH4 + NiO2, (d) CO2 + NiO2, (e) CO + NiO2, (f) H2 + NiO2, (g) HF + NiO2 respectively.

Fig. 5

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After gas adsorption, only minor band-gap reductions are observed for C2H4, CH4, CO2, and H2 with decreases of 2.09%, 4.51%, 1.45%, and 3.22%, respectively. These small variations indicate weak interaction with the surface and limited perturbation of the electronic structure. In contrast, HF and CO adsorption cause much larger reductions, lowering the band gap to 0.153 and 0.242 eV, which correspond to decreases of 75.36% and 61.03%, respectively. This pronounced narrowing indicates stronger orbital interaction and charge redistribution near the Fermi level. Overall, the results show that C2H4, CH4, CO2, and H2 induce only minor electronic changes, whereas HF and CO strongly modify the band structure and are therefore expected to produce a larger conductivity response. These findings suggest that penta-NiO2 is more sensitive to HF and CO, supporting its potential use in gas-sensing applications.

3.8. Density of states analysis

The electronic structure of the system was further examined through density of states (DOS) analysis. The presence or absence of states at the Fermi level (EF) determines whether the material exhibits metallic or semiconducting behavior.53 To further understand the electronic behavior of the penta-NiO2 monolayer upon gas adsorption, the projected density of states (PDOS) was evaluated, as displayed in Fig. 6. The calculated band structure of penta-NiO2 shows a semiconducting nature, with a band gap of 0.621 eV located at the Fermi level (EF = 0 eV). The states close to the valence-band maximum predominantly originate from Ni-3d orbitals, while O-2p states contribute over a broader energy range in both the valence and conduction bands. The noticeable overlap between Ni-3d and O-2p states in the valence region indicates strong Ni–O hybridization within the pristine structure. Adsorption of C2H4, CH4, CO2, and H2 introduces additional molecular states mainly within the valence-band region. For C2H4 and CH4, the dominant C-2p contributions are concentrated around −4.5 to −3.5 eV, accompanied by smaller contributions from H-1s states. These localized molecular states remain relatively distant from the band edges, which can be attributed to the weak interaction between the hydrocarbon molecules and the penta-NiO2 surface. In the case of CO2, the C-2p states are distributed over a wider energy range within the valence band and extend slightly into the conduction region. This broader contribution is associated with the multiple bonding characteristics of CO2 and the stronger polarization induced during adsorption. Meanwhile, the H-1s states associated with H2 remain mainly confined to lower energies. Consequently, the adsorption of C2H4, CH4, CO2, and H2 causes only minor modifications to the states near the band edges, which agrees well with the small variations observed in their calculated band gaps. Among the investigated gases, HF exhibits a distinct electronic response. The F-2p states appear mainly between −3.0 and −2.5 eV in the valence band and contribute noticeably to the DOS in this region. However, the states near the band edges remain dominated by Ni-3d orbitals. The strong influence of HF adsorption on the electronic structure can be attributed to the high electronegativity of fluorine, which promotes considerable charge redistribution between HF and the penta-NiO2 surface. Although the F-2p contribution is largely confined to the lower valence band, HF adsorption is associated with a substantial reduction in the calculated band gap, indicating that the interaction with the surface modifies the electronic structure more strongly than in the other adsorption systems. Overall, the PDOS results show that adsorption of C2H4, CH4, CO2, and H2 has minor influence on the electronic states around the gap region, whereas HF leads to a more noticeable change in the electronic structure, consistent with its stronger impact on the band gap.

Fig. 6. DOS spectra of pristine and gas adsorbed penta-NiO2 monolayer: (a) NiO2, (b) C2H4 + NiO2, (c) CH4 + NiO2, (d) CO2 + NiO2, (e) H2 + NiO2, (f) HF + NiO2, (g) CO + NiO2, respectively.

Fig. 6

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3.9. Work function analysis

The work function describes the energy difference needed for an electron to move from the Fermi level (EF) to the vacuum level.54 Work-function analysis was conducted to assess the changes in the surface electronic properties of penta-NiO2 induced by gas adsorption. Table 3 summarizes the work-function changes caused by gas adsorption, revealing molecule-dependent variations associated with charge transfer and interfacial dipole formation. The relatively small changes observed for H2, CH4, CO2, and C2H4 indicate weak perturbation of the penta-NiO2 electronic structure, whereas CO and HF adsorption produce a much stronger modulation of the work function, suggesting more pronounced electronic interactions at the adsorption interface. In particular, the significant response associated with HF adsorption implies substantial charge redistribution and a stronger influence on the surface electronic environment. Because the work function is highly sensitive to adsorption-induced electronic changes, its variation can serve as an effective signal for work-function-based sensing. The considerably larger response generated by HF and CO compared with the other investigated gases demonstrates the ability of penta-NiO2 to distinguish between different adsorbates through measurable changes in its surface electronic characteristics. These results highlight the potential of penta-NiO2 as a promising candidate for gas-sensing applications, especially for the detection of HF and CO molecules.

Table 3. Work-function and band gap of the studied structures.

Structures Work function Φ (eV) ΔΦ (eV) E g (eV)
Pristine NiO2 4.29 __ 0.621
C2H4 + NiO2 4.14 0.15 0.608
CH4 + NiO2 4.22 0.07 0.593
CO2 + NiO2 4.18 0.11 0.612
H2 + NiO2 4.26 0.03 0.601
HF + NiO2 5.14 0.85 0.153
CO + NiO2 4.60 0.31 0.242
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3.10. Sensitivity analysis

The sensitivity of the penta-NiO2 monolayer toward various gas molecules was assessed over the temperature range of 300–500 K, and the calculated results are listed in Table 4.

Table 4. Sensitivity of penta-NiO2 towards different gas molecules at different temperatures.

System Sensitivity_300 K (a.u.) Sensitivity_400 K (a.u.) Sensitivity_500 K (a.u.)
C2H4 + NiO2 0.286 0.207 0.163
CH4 + NiO2 0.718 0.501 0.384
CO2 + NiO2 0.190 0.139 0.110
H2 + NiO2 0.472 0.336 0.261
HF + NiO2 8.51 × 103 8.85 × 102 2.27 × 102
CO + NiO2 1.52 × 103 2.43 × 102 8.02 × 101
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At 300 K, the sensitivity of penta-NiO2 varies significantly depending on the adsorbed gas molecule. Among the investigated gases, HF exhibits the highest response with a sensitivity in the order of 103, followed closely by CO with a similar magnitude. The other investigations of gases show considerably weaker responses, with CH4 exhibiting the highest sensitivity among them, followed by H2, C2H4, and CO2. The relatively low sensitivity responses of these gases can be attributed to their weaker interactions with the penta-NiO2 surface, resulting in limited modulation of the electronic properties. A temperature increase between 300 and 400 K results in a noticeable decrease in sensitivity for all gas-adsorbed systems, whereas a further rise to 500 K causes a more gradual reduction. For example, the sensitivity of the HF adsorbed penta-NiO2 system decreases from about 103 at 300 K to about 102 at higher temperatures, while the CO adsorbed penta-NiO2 system shows a similar reduction from about 103 to around 101–102. The other gas systems also show a decrease in sensitivity, but the changes remain relatively small. The relative sensitivity ranking remains essentially unchanged across the temperature range and can be expressed as:HF > CO >> CH4 > H2 > C2H4 > CO2

These findings indicate that the penta-NiO2 monolayer exhibits substantially stronger sensing responses to HF and CO than to the other examined gases.

3.11. Optical properties analysis

The optical response of penta-NiO2 before and after gas adsorption is discussed in Fig. 7 based on the absorption coefficient, reflectivity, and optical conductivity spectra. The purpose of this analysis is to examine whether gas adsorption produces detectable changes in the optical response of penta-NiO2. Since adsorption can alter the electronic structure through charge transfer and gas–surface interactions, such changes may be reflected in the optical properties of the material.

Fig. 7. Optical properties of the studied structures: (a) absorption coefficient, (b) conductivity, (c) reflectivity, respectively.

Fig. 7

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The optical absorption spectra of pristine and gas-adsorbed penta-NiO2 monolayer are shown in Fig. 7a. All systems exhibit significant absorption in the ultraviolet region, with absorption coefficients reaching values between 104 and 105 cm−1, indicating the excellent UV absorption capability of the monolayer. Although gas adsorption does not significantly alter the overall absorption behavior, it modulates the absorption intensity to different extents, indicating that the optical response is sensitive to the presence of gas molecules. Among the investigated configurations, CO2 adsorbed penta-NiO2 shows the most pronounced enhancement, with a sharp peak above 5 × 104 cm−1 near 150 nm. By contrast, CH4 and H2 adsorbed systems exhibit comparatively stronger absorption in the near-UV and visible regions, whereas the remaining cases show only moderate deviation from the pristine spectrum. Notably, the absorption edge remains nearly unchanged upon adsorption, indicating that the gas molecules exert only a limited influence on the fundamental optical transition threshold. This behavior suggests that the adsorption-induced modulation mainly affects the transition intensity rather than causing a major perturbation of the electronic structure. Overall, the sensitivity of the absorption coefficient to different gas molecules highlights the potential of penta-NiO2 monolayers for optical gas-sensing applications.

The reflectivity response of the penta-NiO2 monolayer changes after gas adsorption, with the magnitude of the variation depending on the adsorbed species. As illustrated in Fig. 7c, the pristine monolayer exhibits a maximum reflectivity of approximately 8.8% in the visible region. Adsorption of CH4, C2H4, CO, CO2, and H2 results in only minor changes throughout the wavelength range investigated, indicating a limited influence on the optical response. In contrast, HF adsorption causes a pronounced increase in reflectivity beyond 1000 nm, producing the largest optical variation among all the considered gas molecules.

The optical conductivity spectra of the pristine and gas-adsorbed penta-NiO2 monolayers are presented in Fig. 7b. All systems exhibit high optical conductivity in the ultraviolet region, with a pronounced peak near 150 nm. Among the investigated gases, CO2 adsorption exhibits the highest conductivity at this wavelength, whereas the CH4, C2H4, CO, and H2 adsorbed systems show only slight changes relative to the pristine monolayer. Beyond 250 nm, the conductivity of the pristine, CH4, C2H4, CO2, and H2 adsorbed systems gradually increases, reaching a broad maximum around 410 nm, before decreasing at longer wavelengths. In contrast, the HF and CO adsorbed systems do not exhibit this broad maximum and instead maintain comparatively lower conductivity across the near-ultraviolet and visible regions, followed by a slight increase toward 600 nm. In summary, the optical spectra reveal that the response of the penta-NiO2 monolayer depends on the adsorbed gas molecule. Among the investigated gases, HF induces the most pronounced spectral changes, confirming its stronger interaction with the monolayer.

3.12. Reduced density gradient analysis

To gain a deeper understanding of the adsorption mechanism, (Reduced Density Gradient) RDG analysis was employed to visualize weak interactions and distinguish the different noncovalent features present in the adsorption systems.28,55 Compared with the Quantum Theory of Atoms in Molecules (QTAIM), RDG analysis provides a more intuitive way to visualize weak interactions, particularly in large or structurally complex molecular systems. RDG analysis is based on the electron density (ρ) and the second eigenvalue (λ2) of the electron-density Hessian matrix. The sign(λ2)ρ value is then used to qualitatively characterize different types of interactions. A negative sign(λ2)ρ value indicates an attractive interaction, while a positive value represents steric repulsion. Values close to zero are associated with weak interactions, including van der Waals and π–π interactions.28,56 RDG isosurfaces were used to visualize the interaction regions. The colors of the isosurfaces provide a qualitative representation of the interaction regions, with blue representing attractive regions, green indicating weak noncovalent interactions, and red identifying steric repulsion. The color scale is interpreted as a qualitative indicator rather than as independent evidence of a specific bonding character or adsorption mechanism. Fig. 8 depicts the RDG scatter plots together with the associated gradient isosurfaces for the gas-adsorbed complexes. These results were generated according to the equation given below:

3.12. 7

Here, ρ(r) refers to the electron density distribution, and ∇ρ(r) represents the corresponding density gradient. RDG analysis is obtained from these density-related quantities, while the sign of the second eigenvalue of the electron-density Hessian matrix, λ2(r), is used to distinguish different types of interactions. As shown in Fig. 8, the adsorption systems of CH4, C2H4, CO, CO2, and H2 are characterized mainly by green isosurfaces, while the corresponding scatter plots display green spikes around sign(λ2)ρ ≈ 0. These observations indicate that the interactions of these gases with the penta-NiO2 surface are predominantly weak and noncovalent in nature. In contrast, the HF-adsorbed system exhibits a distinct blue interaction region, together with a negative sign(λ2)ρ distribution, indicating a more pronounced attractive interaction with the penta-NiO2 surface. HF shows a comparatively stronger interaction with the monolayer, which is also reflected in its higher adsorption energy and is consistent with the corresponding charge-transfer results.

Fig. 8. Reduced density gradient (RDG) isosurfaces and corresponding sign(λ2)ρ scatter plots for (a) H2, (b) CH4, (c) C2H4, (d) CO2, (e) CO, and (f) HF adsorbed on the penta-NiO2 monolayer.

Fig. 8

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4. Conclusions

In summary, first-principles calculations were employed to investigate the structural stability and gas-sensing characteristics of a penta-NiO2 monolayer toward representative gases associated with lithium-ion battery thermal runaway. The optimized monolayer exhibits favorable structural and dynamical stability and a narrow semiconducting band gap of 0.621 eV. The adsorption calculations demonstrate a pronounced gas-dependent interaction strength, with HF showing the strongest adsorption energy of −0.831 eV, followed by C2H4, CO, H2, CH4, and CO2. The strong HF interaction is accompanied by substantial charge redistribution, a large reduction in the band gap, and a significant work-function change, indicating a pronounced electronic response. CO also produces considerable modulation of the electronic structure and work function while exhibiting a substantially shorter recovery time than HF. In contrast, H2, CH4, CO2, and C2H4 interact relatively weakly with the penta-NiO2 surface and induce only limited changes in its electronic properties. The Mulliken charge, electron-density-difference, ELF, PDOS, and RDG analyses consistently indicate that the interaction mechanism varies from predominantly weak noncovalent adsorption for most gases to a stronger chemisorption interaction for HF. The temperature-dependent calculations further show that the relative sensing response follows the order HF > CO > CH4 > H2 > C2H4> CO2 over the investigated temperature range.

Overall, the combined adsorption, electronic, charge-transfer, and optical analyses indicate that penta-NiO2 has strong theoretical potential for a differential response to HF and CO generated during lithium-ion battery thermal runaway. Nevertheless, experimental validation, substrate effects, gas concentration dependence, competing-gas effects, and device-level response should be investigated in future work before practical sensor applications can be established.

Author contributions

Md Aminul Haque: formal analysis, data curation, investigation, visualization, writing – original draft; Methila Akter: formal analysis, investigation, visualization, writing – original draft, Abu Talha: formal analysis, software, visualization, writing – original draft; Dr Md. Alamgir Badsha; software, visualization, writing – original draft, Abdullah Al Roman: conceptualization, methodology, project administration, supervision, writing – review and editing.

Conflicts of interest

The authors of this manuscript have no conflicts of interest to declare.

Supplementary Material

RA-OLF-D6RA08458C-s001

Acknowledgments

This research was made possible by the generous financial support of the Ministry of Science & Technology, Bangladesh, through the R&D project grant 2025–2026, for which the authors are deeply thankful.

Data availability

Research data will be made available upon reasonable request.

Supplementary information (SI): phonon of penta-NiO2. See DOI: https://doi.org/10.1039/d6ra08458c.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA08458C-s001
RA-OLF-D6RA08458C-s001.pdf (150.3KB, pdf)

Data Availability Statement

Research data will be made available upon reasonable request.

Supplementary information (SI): phonon of penta-NiO2. See DOI: https://doi.org/10.1039/d6ra08458c.

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