A New Design Strategy of Series of Tetrazole-Based High-Energy-Density Energy Storage Molecular Systems

Abstract

Innovative energy storage technologies in the energetic materials field represent a critical frontier in energy research. Consequently, we developed a performance regulation strategy based on tetrazolyl high-energy-density energy storage molecular systems and theoretically assessed their energetic properties and safety profiles. The findings reveal that substituent characteristics profoundly affect the performances of these energy storage molecular systems. The molecule systems ((1-amino-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate, ((1-azido-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate, ((1-nitro-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate, and especially ((1-azido-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate, exhibit exceptional performances, including high density, high heat of formation, high detonation velocity and pressure, zero oxygen balance, and low impact sensitivity, qualifying them as high-energy-density and low-sensitivity candidates. This work offers novel pathways for advancing energy storage technologies in energetic materials field.

1. Introduction

Faced with increasingly urgent environmental protection requirements and sustainable development goals, innovation in energy storage technology is particularly crucial and is one of the core directions of energy technology development [1,2,3,4,5]. Energetic materials have unique characteristics in this context. These energy storage media can quickly undergo chemical reactions under specific external stimuli, instantly releasing huge destructive energy. Therefore, they have been widely used in many fields such as national defense, mining, and so on [6,7,8,9].

Developing high-energy-density and low-sensitivity materials has become a core focus in this field [10,11,12,13,14,15]. However, the inherent contradiction between high energy output and low sensitivity makes the development of such new materials a key challenge. Given the continuous growth in demand for high-energy materials, the development of new candidate materials with excellent energy properties and high safety levels has become an urgent need. High-nitrogen heterocyclic structures and their derivatives are key components of energetic systems and a promising class of high-energy-density substances [16,17,18,19,20]. In addition, such compounds mainly release nitrogen gas during the reaction, without causing environmental pollution, which meets the needs of new energy storage technologies for green environmental protection and sustainable development [21,22,23,24,25,26].

High-nitrogen heterocyclic structures generally have low sensitivity and high thermal stability. Meanwhile, the higher nitrogen content typically results in greater heat generation, making it superior to ordinary energetic compounds. For example, Thoenen et al. recently investigated high-nitrogen materials derived from 5-nitromethyl-1H-tetrazole [27]. Therefore, by combining and designing multiple high-nitrogen heterocyclic structures, we expect to obtain compounds with both high energy density and excellent detonation performance.

Based on the above viewpoint, a new structural series of energetic compounds centered on nitrogen and containing multiple tetrazole rings is proposed. This work innovatively designed a series of novel tricyclic tetrazole-based energy storage molecules by introducing various types, quantities, and positions of energetic groups on the symmetrical skeleton of three tetrazole rings centered on nitrogen atoms, taking both detonation performance and stability into consideration. By utilizing density functional theory (DFT), the electronic configuration, energy properties, and stability of high-energy materials were systematically studied, contributing innovative solutions to the advancement of energy storage technology in the high-energy materials field.

2. Computational Methods

Based on the Gaussian 09 software package [28], the structures of all these energy storage molecular systems were fully optimized based on the DFT-B3LYP/6-311G** basis set [29,30]. Vibrational analysis shows no imaginary frequency, indicating that the obtained structures are stable conformations. On this basis, the heat of formation (HOF, Δ_fH_298K(s)) of these molecules was calculated through isodesmic reaction and atomization reaction pathways, and the corresponding equations are presented as follows [31]:

$$X-nR+nN{H}_3\to X+nN{H}_{2}R$$

(1)

$${\mathsf{\Delta }}_r{H}_{298K}=\Sigma {\mathsf{\Delta }}_f{H}_{298K}(P)-\Sigma {\mathsf{\Delta }}_f{H}_{298K}(R)$$

(2)

$${\mathsf{\Delta }}_r{H}_{298K}=\mathsf{\Delta }{E}_{298K}+\mathsf{\Delta }(PV)=\mathsf{\Delta }{E}_0+\mathsf{\Delta }{E}_{ZPE}+\mathsf{\Delta }{E}_T+\mathsf{\Delta }nRT$$

(3)

in which X is molecular ring skeleton, R is energetic groups, $\mathsf{\Delta }{E}_0$ is the difference in total energies between products and reactants, $\mathsf{\Delta }{E}_{ZPE}$ is the difference in zero-point energies between products and reactants, and $\mathsf{\Delta }{H}_T$ is the difference in thermal correction between products and reactants.

The molecular density is obtained by the following equation:

$$\rho =\alpha M/V_M+\beta (\nu {\sigma }_{\mathrm{tot}}^2)+\gamma$$

(4)

in which M is the molecular mass, V_M is the molecular van der Waals volume, ${\sigma }_{\mathrm{tot}}^2$ is the variance of the total electrostatic potential on the molecular surface, ν is the charge balance degree, and the coefficients α, β, and γ are 0.9183, 0.0028, and 0.0443, respectively.

The detonation performances of these energy storage molecular systems, including detonation velocity (D) and detonation pressure (P), were calculated by Kamlet–Jacobs (K–J) equations, and the expressions are presented as follows [32]:

$$D=(1.011+1.312\rho ){(N{\overline{M}}^{1/2}{Q}^{1/2})}^{1/2}$$

(5)

$$P=1.558{\rho }^{2}N{\overline{M}}^{1/2}{Q}^{1/2}$$

(6)

in which ρ is the density; N is the detonation gases moles per gram compound; $\overline{M}$ is the average gases molecular weight; and Q is the heat of detonation.

Impact sensitivity (h₅₀, cm) can be used to assess the safety performances of these energy storage molecular systems, and the calculation equation is as below [33]:

$$h_{50}=\alpha {\sigma }_{+}^2+\beta \nu +\gamma$$

(7)

in which ${\sigma }_{+}^2$ is the variance of the positive electrostatic potential on the molecular surface; ν is the charge balance degree; and the coefficients α, β, and γ are −0.0064, 241.42, and −3.43, respectively.

3. Results and Discussion

3.1. Molecular Structure

This work designed a series of novel tricyclic tetrazole-based energy storage molecules by introducing various types, quantities, and positions of energetic groups (-NH₂, -NO₂, -ONO₂, -N₃) on the symmetrical skeleton of three tetrazole rings centered on nitrogen atoms with the aim of regulating their structure and properties while balancing detonation performance and stability. According to the number of substituents, these energy storage molecules are divided into three series: A, B, C, and D (see Figure 1). Among them, molecule A1 is a benchmark molecule with hydrogen atoms (or no energetic substituents). All molecules have a symmetrical structure, with the core consisting of a central nitrogen atom connected to three tetrazole rings through three C-N bonds and modified with abundant energetic groups. Figure 2 shows the optimized structures of these energy storage molecular systems. It was found that, except for the benchmark molecule A1, the three tetrazole rings of these molecules are not coplanar. This is due to the steric hindrance effect, which causes the various components within the molecule to be as far apart as possible, thus achieving the lowest energy stable configuration of the system. Meanwhile, due to the electron repulsion effect, the substituents exhibit a certain degree of distortion, indicating the influence of different substituents on the overall configuration.

3.2. Frontier Molecular Orbitals

Frontier molecular orbitals, namely, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The energy levels and symmetry of these orbitals play a crucial role in the reaction process, such as the positions of electrophilic and nucleophilic reactions, cycloaddition reactions, electrocyclic reactions, and so on. Figure 3 shows the HOMO and LUMO of these high-energy-density nitrogen-rich energy storage molecular systems. Orbital energy level diagrams depict red areas as the positive phase and green areas as the negative. For molecule A1, its HOMO is mainly contributed by the whole molecular skeleton, while its LUMO is contributed by the tetrazole ring, indicating that the electronic excitation of A1 molecule mainly occurs from the HOMO of the whole molecule to the LUMO of the tetrazole ring. The HOMO of the B series molecules is mainly contributed by the whole molecular skeleton, while their LUMO is mainly contributed by all remaining parts except for the -NH₂ substituent and its connected tetrazole ring (such as molecules B1, B3, and B4), indicating that electronic excitation of the B series molecules mainly occurs from the HOMO of the whole molecule to the LUMO of the non-NH₂ substituent and its connected tetrazole ring. For C series molecules, the HOMO of all these molecules is mainly contributed by the whole molecular skeleton, while their LUMO is mainly contributed by all remaining parts except for the -N₃ and -ONO₂ substituents and the tetrazole ring they are connected to (such as molecules C3, B4, and C5), indicating that electronic excitation of C-series molecules mainly occurs from the HOMO of the whole molecule to the LUMO of the tetrazole ring and the LUMO of the nitro substituent. For D series molecules, their HOMO is mainly contributed by the whole molecular skeleton, while their LUMO is mainly contributed by all remaining parts except for the -ONO₂ substituent and the tetrazole ring they are connected with. This indicates that the electronic excitation of D series molecules mainly occurs from the HOMO of the whole molecule to the LUMO of the tetrazole ring and the LUMO of the nitro substituent. All or most of the HOMO and LUMO of these molecules involve molecular skeletons, indicating that these molecular skeletons are influenced by electron transference between their HOMO and LUMO.

The chemical reactivity of these energy storage molecules can be quantitatively assessed using the HOMO-LUMO energy gap (ΔE). Figure 4 displays the corresponding HOMO, LUMO, and ΔE values. Typically, a larger ΔE correlates with greater molecular stability, as it impedes electron transitions between these orbitals. The LUMO energy level position affects the ability of the molecule to accept electrons (electrophilicity). The LUMO energy levels of the D series molecules in the figure are significantly lower than those of other series molecules, indicating that they have the strongest electron accepting ability (strong electrophilicity), which is closely related to the strong electron withdrawing groups (nitro, nitrate ester groups) present in their structure. The LUMO of the A series molecules has the highest energy level and the weakest electrophilicity. The HOMO energy level reflects the ability of a molecule to provide electrons (nucleophilicity). From the figure, it can be seen that the HOMO energy levels of the C series molecules are relatively higher, indicating that they are more prone to losing electrons (with strong reducibility), which is related to the strong electron donating groups present in their structure. The relatively low HOMO energy levels in the D series molecules indicate a weak ability to provide electrons. As shown in Figure 4b, there are significant differences in ΔE values between different series molecules. The A series molecules have the largest ΔE values, indicating that their electronic excitation requires higher energy, the molecular ground state stability is the highest, and the chemical reaction activity is relatively lower. The B and C series molecules have a centered ΔE values and moderate stability. The D series molecules have the smallest ΔE values and are most prone to electronic transitions, indicating relatively poor ground state stability and the possible highest chemical reactivity. Overall, molecule A1 with amino substituents achieved good electronic balance, which endowed the molecule with high kinetic stability but low chemical reactivity. Molecules with coexisting push–pull electron effects in substituent combinations (such as the coexistence of amino and nitro groups) (B and C series) have relatively balanced chemical properties and certain reactivity. Molecules with strong electron withdrawing substituents (such as multiple nitro and nitrate ester groups) (D series) exhibit strong electrophilicity and are expected to have relatively poor stability and high sensitivity.

3.3. Vibrational Properties

Based on the same method and basis set, the vibrational frequencies of these energy storage molecular systems were calculated, and all molecules are the minimum points on the potential energy surface without imaginary frequencies. Through frequency analysis, the theoretically predicted infrared (IR) spectra are shown in Figure S1.

For tricyclic tetrazole-based energy storage molecules, the stretching vibrational absorption bands of the N-N and N=N bonds in the tetrazole ring skeleton are mainly located in 1100–1300 cm⁻¹, while the weak absorption peak at 500–700 cm⁻¹ corresponds to the planar bending vibration of the tetrazole ring. Different substituents of molecules exhibit characteristic absorption peaks. For example, the strongest absorption peak 2263 cm⁻¹ of molecule B1 corresponds to the stretching vibration of -N₃, the strongest absorption peak 797 cm⁻¹ of molecule B2 corresponds to the bending vibration of the nitro group in -ONO₂, the symmetric stretching vibration of the N-H bond on -NH₂ in molecule B3 is 3486 cm⁻¹, and the asymmetric stretching vibration is 3564 cm⁻¹ and 3573 cm⁻¹, respectively. The absorption peak at 1770 cm⁻¹ in molecule B4 is the asymmetric stretching vibration of -NO₂, the stretching vibration of -N₃ in molecule C1 is located at 2263 cm⁻¹, and the asymmetric stretching vibration of -NO₂ is located at 1761 cm⁻¹ and 1796 cm⁻¹. The rocking vibration and stretching vibration of -N₃ in molecule C3 are at 529 cm⁻¹ and 2262 cm⁻¹, respectively, and the rocking vibration of -NO₂ and the -NO₂ in -ONO₂ in molecule D2 are located at 165 cm⁻¹ and 107 cm⁻¹, respectively.

3.4. Energy Characteristics

Energetic properties, containing density (ρ), heat of formation (Δ_fH_298K(s)), detonation heat (Q), detonation velocity (D), detonation pressure (P), and oxygen balance (OB), represent key attributes of energetic molecular systems. Figure 5a demonstrates how substituent type, quantity, and location substantially influence molecular density. Generally, density increases with substituent count. The benchmark molecule A1 has the lowest ρ value of 1.75 g·cm⁻³ due to its lack of any substituents. For B series energy storage molecules with one fixed -NH₂ substituent, the ρ values of all molecules are no more than 1.90 g·cm⁻³ and even those of four molecules B1, B3, B4, and B5 are no more than 1.80 g·cm⁻³, which is owing to the presence of more -NH₂ and -N₃ in these molecules that contribute less to the ρ of them. For C series power molecules with one fixed -N₃ substituent, the ρ values of these molecules are significantly higher than those of the B series molecules, except for molecules C4 and C5, because these two molecules contain two -N₃ substituents that contribute less to the ρ. For molecule C1 containing only one -N₃ and two -NO₂, molecule C2 containing only one -N₃ and two -ONO₂, and molecule C3 containing only one -N₃ and one -NO₂ and one -ONO₂, their density values are close to 1.90 g·cm⁻³, which are 1.88 g·cm⁻³, 1.88 g·cm⁻³, and 1.89 g·cm⁻³, respectively, which is more than that of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) [34], indicating that -NO₂ and -ONO₂ contribute more to the ρ of energy storage molecules than that of -N₃. It can be clearly seen that the ρ of D series molecules is higher than 1.90 g·cm⁻³, with D1 being 1.92 g·cm⁻³ and D2 being 1.93 g·cm⁻³, which is more than that of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) [34]. This is because the substituents of the D series molecules are all composed of -NO₂ or -ONO₂, which contribute significantly to the ρ of energy storage molecules. Analysis of ρ reveals the substituent impact sequence is -ONO₂ > -NO₂ > -N₃ > -NH₂. Specifically, incorporating -ONO₂ or -NO₂ groups effectively enhances ρ values, whereas -N₃ and -NH₂ substitutions yield minimal density improvement. These findings provide critical guidance for designing high-energy-density compounds. Notably, molecules B2, B8, B9, C1, C2, C3, D1, and D2 surpass RDX in density, positioning them as promising candidates.

Figure 5b presents the Δ_fH_298K(s) for these energy storage molecules. All molecules exhibit substantially higher values than HMX, even exceeding 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) [34] despite substituent variations. This confirms their significant detonation capabilities. Additionally, Q values were assessed and are displayed in Figure 5c. It can be seen that except for molecules A1 and B3, all molecules have higher Q values than HMX, and even higher Q values than CL-20, with the highest Q value of 1912 cal·g⁻¹ for molecule C2. The benchmark molecule A1 has the lowest Q value of 1167 cal·g⁻¹ due to its lack of any power substituents, which is consistent with that of ρ. For B series energy storage molecules with one fixed -NH₂ substituent, the Q values of all molecules are no more than 1800 cal·g⁻¹, except for that of B2 and B9, with Q values of more than 1800 cal·g⁻¹, owing to the presence of more -ONO₂ or -NO₂ in these two molecules, which contribute more to the Q. For C series power molecules with one fixed -N₃ substituent, the Q values of these molecules are significantly higher than those of the B series molecules, except for molecule C4, because molecule C4 contains two -N₃ substituents that contribute less to the Q. For molecule C2, containing only one -N₃ and two -ONO₂, and molecule, C3 containing only one -N₃, one -NO₂, and one -ONO₂, their Q values are around 1900 cal·g⁻¹, which are 1912 cal·g⁻¹ and 1897 cal·g⁻¹, respectively, which are far higher than that of CL-20, indicating that -NO₂ and -ONO₂ contribute more to the Q of energy storage molecules than other substituents. It is obvious that molecule D1 has a relatively higher Q value of 1846 cal·g⁻¹, while molecule D2 has a relatively lower Q value of 1689 cal·g⁻¹, which is also higher than that of CL-20. Analysis of Q reveals that the substituent efficacy order is -ONO₂ > -NO₂ > -N₃ > -NH₂, aligning with the trends of ρ. Specifically, incorporating -ONO₂ or -NO₂ groups more effectively enhances Q values than -N₃ or -NH₂ substitutions in these molecules. For example, the order of Q values for molecules B3, B4, and B5 is B3 < B4 < B5, and that for molecules B1 and B3 is B1 > B3.

To further assess the detonation characteristics, D and p values were computed and presented in Figure 5d,e. These results demonstrate that substituent type, quantity, and location significantly influence their detonation parameters. Clearly, increased energetic substituent count enhances molecular detonation properties. This aligns with trends showing that D series molecules exceed C series in D and p values, which surpass B series molecules. Overall, these molecules demonstrate strong detonation performances (D > 8.5 km·s⁻¹, P > 30 GPa), except for the benchmark molecule A1, owing to absent energetic substituents. Specifically, for B series energy storage molecules with one fixed -NH₂ substituent, the D and p values of these nine molecules are more than 8.5 km·s⁻¹ and 30 GPa, respectively; the D values of four molecules B2, B7, B8, and B9 are more than 9.0 km·s⁻¹; and the p values of two molecules B2 and B9 are more than 40 GPa, owing to the presence of more -ONO₂ or -NO₂ in these molecules, which contribute more to the D and P. For the C series power molecules with one fixed -N₃ substituent, the D and p values of all these molecules are significantly higher than those of the B series molecules, except for molecules C4 and C5; this is due to their substituents containing relatively less -NO₂ or -ONO₂—i.e., more -N₃. For example, molecule C1 has one -N₃ and two -NO₂, C3 has one -N₃, one -NO₂, and one -ONO₂, and C4 has one -NO₂ and two -N₃; their D and p values follow the order of C4 < C1 < C3, indicating that the contribution order to the D and P of the energy storage molecules is -ONO₂ > -NO₂ > -N₃. For molecule C2, containing only one -N₃ and two -ONO₂, and molecule C3, containing only one -N₃, one -NO₂, and one -ONO₂, their D and p values are both over 9.6 km·s⁻¹ and 42 GPa, which are far higher than that of HMX and equivalent to that of CL-20, indicating that -NO₂ and -ONO₂ contribute more to the Q of energy storage molecules than other substituents. Moreover, molecule B1 has one -NH₂ and two -N₃, and B3 has one -N₃ and two -NH₂; their D and p values follow the order of B1 > B3, suggesting that the contribution order to the D and P of the energy storage molecules is -N₃ > -NH₂. It is obvious that molecule D1 has the highest D and p values (9.71 km·s⁻¹ and 42.09 GPa), and molecule D2 also has relatively higher D and p values (9.54 km·s⁻¹ and 43.49 GPa), which are equivalent to that of CL-20. Analysis establishes the substituent contribution order to D and P as -ONO₂ > -NO₂ > -N₃ > -NH₂, which agree well with that of ρ and Q. This correlation arises because D and P derive from ρ and Q. Specifically, -NO₂ or -ONO₂ incorporation yields greater D and P enhancement in these molecules than the -N₃ or -NH₂ groups.

Oxygen balance (OB) further serves as a key indicator of oxygen sufficiency during explosive decomposition to form carbon and hydrogen oxides. Typically, positive OB values correlate with enhanced energy properties. It can be seen in Figure 5g that all these molecules show negative OB values except for molecules D1 (4.3%) and D2 (8.24%). It is notable that molecule C2 exhibits the standard zero oxygen balance. On the whole, the OB values of all these energy storage molecules obey the order of D series > C series > B series > A series, suggesting that the order of contribution to OB values is -ONO₂ > -NO₂ > -N₃ > -NH₂, which is consistent with that of ρ, Q, D, and P. Namely, introducing -ONO₂ and -NO₂ is more beneficial for increasing the OB values of these molecules than other substituents, such as -N₃ and -NH₂. For example, molecule B5, containing one -ONO₂ and two -NH₂, has the highest OB value (−25.63%); molecule B4, containing one -NO₂ and two -NH₂, has the second highest OB value (−32.41%); molecule B1, containing one -NH₂ and two -N₃, has the third highest OB value (−35.2%); and molecule B3, containing one -N₃ and two -NH₂, has lowest OB value (−43.81%).

In summary, molecules B2, B8, B9, C1, C2, C3, D1, and D2 exceed HMX in D and P, positioning them as promising candidates of high-energy-density molecules.

3.5. Safety Evaluation

The molecular surface electrostatic potential (ESP) is of great significance in predicting the reactivity of molecules [35,36,37]. Figure 6 shows the ESP of these energy storage molecular systems.

It can be seen that the ESP values of these molecules span up to 37.65 kcal·mol⁻¹, indicating severe charge separation within the molecules. This is mainly due to the strong interaction between the inherent electron-rich property of the tetrazole ring and the extreme and opposite electronic effects of the four substituents. Specifically, -NO₂ and -ONO₂, as strong electron withdrawing groups, exhibit extremely negative ESP values in the rings they are connected to and their surrounding atoms, making them the strongest electron acceptor regions. As a strong electron donating group, -NH₂ exhibits a significant increase in electron density in the connected rings and adjacent regions, resulting in strong negative ESP values. However, the H atom on its N-H bond will exhibit significant positive ESP values due to the electron being pulled towards N atom, becoming a strong electrophilic point. Molecules containing -N₃ substituents with a moderate electron withdrawing induction effect exhibit moderate negative ESP values at the terminal N atom, while those in the vicinity of the N atom connected to the tetrazole ring are slightly positive, and the overall ESP performance is between -NH₂ and -NO₂, but with more electron withdrawing. In summary, these energy storage molecules exhibit highly non-uniform and polarized electrostatic potential distributions driven by the extreme electronic effects of the substituents. Amino groups provide strong electron donating and hydrogen bond donor sites, nitro and nitrate ester groups create strong electron withdrawing centers and super strong hydrogen bond acceptors and energy sources, and azide groups contribute reactivity and moderate polarity. This unique ESP distribution indicates their excellent multiple hydrogen bonding ability, abundant reaction sites, and enormous potential as high-energy-density compounds.

In addition, the surface area distributions within the different electrostatic potential ranges of these high-energy-density nitrogen-rich energy storage molecular systems were calculated. As a benchmark molecule, the surface area of the A1 molecule in different electrostatic potential ranges gradually decreases with the increase in electrostatic potential and is lower than that of any other substituted molecule in this electrostatic potential range, which is attributed to the intrinsic weak electron-rich property of its tetrazole ring. By comparing the surface area distribution of different molecules in different electrostatic potential ranges, it can be inferred that the contribution order of different substituents to negative ESP is -NH₂ > -N₃> -ONO₂ > -NO₂. This is because the nitrogen atom in -NH₂ exhibits sp³ hybridization, with a lone pair of electrons perpendicular to the molecular plane and completely exposed to the surface, enhancing the electron density of the connected tetrazole ring through electron pushing. The terminal N⁻ (sp hybridization) in -N₃ provides localized lone pair electrons, partially offsetting the contribution of lone pair electrons (still showing net negativity). The resonant structure of -ONO₂ causes oxygen lone pair electrons to delocalize, resulting in low surface exposure and weakening the electron density of connected atoms. The lone pair of -NO₂ electrons are completely delocalized to the N=O bond, with no effective negative charge region on the surface and almost no negative ESP contribution. Overall, the surface area distributions of all these molecules in different electrostatic potential ranges are relatively uniform.

Safety properties constitute critical considerations for energy storage molecular systems. Impact sensitivity (h₅₀, cm) quantifies these molecules’ safety behavior. Higher h₅₀ values indicate lower molecular sensitivity. Comparative h₅₀ data for these molecules and common energetic benchmarks appear in Figure 5f. It is found that substituent type, quantity, and location substantially influence h₅₀ values. Increased energetic group count typically reduces safety performance. Nevertheless, almost all molecules demonstrate sufficient h₅₀ values, validating our effective energy storage molecular design strategy. Specifically, the benchmark molecule A1 has a relatively higher h₅₀ value (30.35 cm) due to its lack of any power substituents. For the B series energy storage molecules with one fixed -NH₂ substituent, the h₅₀ values of all molecules are more than 26 cm, with even molecules B3 and B7 being over 40 cm, except for molecules B8 and B9, owing to their increase -NO₂ or -ONO₂, which contribute more to their D and P. Obviously, it can be seen that the h₅₀ values of the C series power molecules are slightly lower than those of the B series molecules, and those of the D series power molecules are slightly lower than those of the C series molecules. Specifically, the B series molecules exhibit h₅₀ values between 17.93 and 41.12 cm, while those of the C series molecules range from 15.22 to 37.43 cm, and the D series molecules range from 17.76 to 31.47 cm. This is due to the order of contribution to the molecules’ energetic performances (D, P) being -ONO₂ > -NO₂ > -N₃ > -NH₂, which is consistent with the inherent contradiction between the energy and sensitivity of energy storage molecules. That is, introducing substituents with a relatively lower energy contribution, such as -NH₂, is beneficial for increasing the h₅₀ value and enhancing the safety performance.

Collectively, molecules B2, C2, and D2 demonstrate superior energy metrics and enhanced safety profiles compared to HMX, establishing them as promising high-energy-density and low-sensitivity candidates.

4. Conclusions

In this work, we innovatively designed a series of novel tricyclic tetrazole-based energy storage molecules by introducing various types, quantities, and positions of energetic groups on the symmetrical skeleton of three tetrazole rings centered on nitrogen atoms, taking both detonation performance and stability into consideration. Our findings reveal that substituent characteristics profoundly affect comprehensive performance.

Investigations into frontier orbitals reveal that all molecules’ molecular skeletons involve both HOMO and LUMO, signifying skeleton reinforcement or diminishment resulting from electron transfer between these orbitals. Analysis of HOMO-LUMO energy gaps demonstrates that specific substituents increase molecules’ ΔE value, consequently enhancing its relative stability. ESP analysis indicates that these energy storage molecules exhibit highly non-uniform and polarized electrostatic potential distributions driven by the extreme electronic effects of substituents. Vibrational analysis gives the characteristic absorption peaks of the tetrazole ring and different substituents.

Analysis of energy characteristics, including ρ, Q, D, and P, reveals that the substituent impact sequence is -ONO₂ > -NO₂ > -N₃ > -NH₂. That is, incorporating -ONO₂ or -NO₂ effectively enhances energy levels, whereas -N₃ and -NH₂ substitutions yield minimal improvement. Discussions on safety performances suggest that the order of contribution to safety performances is the opposite to that of energy characteristics, which is consistent with the inherent contradiction between the energy and sensitivity of energy storage molecules. Taking both energy characteristics and safety performances into account, molecules ((1-amino-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate (B2), ((1-azido-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate (C2), and ((1-nitro-1H-tetrazol-5-yl)azanediyl)bis(1H-tetrazole-5,1-diyl) dinitrate (D2) possess higher density, higher heat of formation, higher detonation velocity and pressure, and lower impact sensitivity, demonstrating superior energy metrics and enhanced safety profiles compared to HMX, establishing them as promising high-energy-density and low-sensitivity candidates.

This work is expected to introduce innovative solutions to drive progress in energy storage technology for energetic materials.