Templating Nanostructured Aromatic Based Materials as Possible Anode Electrodes for Na-ion Batteries: A Computational DFT Approach

Na-ion batteries have garnered significant attention as a favorable substitute to the prevalent Li-ion batteries, which presently dominate the storing energy landscape. Sodium, being more abundant and cost-effective than lithium, presents a sustainable choice for storing energy on large scale, marking a important step towards a more sustainable environment [1]. Sodium-ion batteries present hope for a greener future. By using sodium ions rather of the more common lithium ions, these batteries are not only cheaper but also better for the environment [2].

A key advantage of sodium-ion batteries lies in their utilization of abundant materials. With sodium ranking as the sixth most abundant element on our planet, its general availability exceeds that of lithium. This intrinsic abundance implies that scaling up the production of sodium-ion batteries could be achieved more seamlessly, without facing the geopolitical challenges associated with lithium-ion battery production [3]. Moreover, the materials used in sodium-ion batteries are generally safer and less harmful than those used in lithium-ion batteries. LIBs often lean on resources like cobalt, which are frequently sourced under environmentally damaging and ethically concerning conditions [4]. Contrariwise, sodium-ion batteries can utilize materials like $\mathrm{Mg}$, which are more abundant and environmentally sustainable. This distinction positions sodium-ion batteries as potentially more cost-effective than their lithium-ion counterparts [5]. With the surge in battery demand, the limitations and increasing prices of lithium are becoming evident. In contrast, the vast availability and affordability of sodium present a more sustainable avenue for large-scale battery production [6]. Furthermore, sodium-ion batteries have the potential to offer higher energy densities and more extended lifetimes when compared to LIBs. However, making high-performance SIBs comes with its own set of challenges, such as dealing with the relatively larger size of $\mathrm{Na}$ ions and the absence of appropriate electrode materials [7].

Sodium-ion batteries (SIBs) function based on the same basic principle as other rechargeable batteries, involving the movement of ions between two electrodes through an electrolyte during charging and discharging cycles. In the case of SIBs, the charge carriers are sodium ions $(\mathrm{Na}+)$, in contrast to lithium ions used in lithium-ion batteries [8]. During the charging process, sodium ions are drawn from the cathode material, generally made of layered transition metal oxides or polyanionic compounds. These ions cross through the electrolyte, usually a salt dissolved in an organic solvent, and get stored in the anode material, generally composed of carbon. Meanwhile, electrons transfer through the external circuit to the anode, creating a voltage difference between the two electrodes [9]. When the battery discharges, the process inverts. The stored $\mathrm{Na}$ ions in the anode travel back to the cathode through the electrolyte, while electrons float in external circuit, generating an electric current to power devices. This cycle can be replicated multiple times, enabling the battery to be recharged and used again [10].

The performance SIBs relies on various factors, including the choice of electrode and electrolyte materials, the structure of the cell, and the operating conditions. One of the primary challenges in SIB development is the quest for materials that can present high capacity, stable cycling, and fast charge/discharge rates while being cost-effective and abundant. Some studies have also concentrated on transition metal oxides for various electrochemical aspects of energy generation and storage [11]. Continued research in this domain seeks to enhance the efficiency and reliability of SIBs for diverse energy storage applications. 2,4-Chloronitrotoluenes and pyrazine have appeared as promising anode materials for $\mathrm{Na}-\mathrm{ion}$ Batteries (SIBs) due to their strong conductivity and capacity. 2,4-Chloronitrotoluenes, a type of carbon with a special cylindrical structure, offer a considerable surface area and superior electrical conductivity [12]. On the other hand, pyrazine is an organic molecule with a flat structure that can host sodium ions between its layers [13]. This study employed density functional theory (DFT) to examine 2,4chloronitrotoluenes and pyrazine, providing valuable insights into their electrochemical properties as anode materials for SIBs. These understandings have played a pivotal role in driving the design and enhancement of these materials to boost battery performance. Key properties such as adsorption energies, HOMO energy, LUMO energy, and the energy difference between HOMO and LUMO orbitals were closely examined.

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Figure 1. New sodium-ion battery materials: 2,4-Chloronitrotoluene and pyrazine

3.1 Geometrically optimized structures of 2,4-Chloronitrotoluenes and pyrazine with $\mathrm{Na}$

This study involved an analysis of the interaction between sodium ions and 2,4-Chloronitrotoluenes and Pyrazine molecules. The investigation entailed an examination of four distinct optimized structures for each compound. Optimized structures were generated by systematically altering the positions of sodium ions within the molecular framework. The optimization was conducted using the 'optimization' job type, and DFT served as the ground-state method. The calculations were performed with default spin settings and a 6-31G basis set, providing insights into the most energetically favorable configurations of sodium ions within the molecules. The results obtained from this study have meaningful implications for the potential use of these molecules as electrode materials in SIBs.

In Figure 2, we showcase a visual representation of our optimized structures of 2,4-chloronitrotoluene with sodium ion configurations, providing insight into their spatial arrangement and geometry.

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Figure 2. Optimized structures of 2,4-Chloronitrotoluenes and pyrazine with sodium ions

3.2 Theoretical capacity and energy density

The Theoretical capacity ($Q_{\text {theoretical }}$) of the battery can be calculated using Faraday's law:

$\mathrm{Q}_{\text {theoretical }}=\mathrm{C}=\mathrm{nF} / \mathrm{M}$              (1)

The theoretical capacity, represented as $Q_{\text {theoretical, is }}$ measured in $\mathrm{Ah} / \mathrm{g}$ or $\mathrm{Ah} / \mathrm{cm}^3$. Here, 'n' represents the number of electrons participating in the redox reaction, ' $F$ ' is the Faraday constant $(96,485 \mathrm{C} / \mathrm{mol})$, and ' $\mathbf{M}$ ' represents the molar mass of the active material. Pyrazine, having a molecular weight of $80.10 \mathrm{~g} / \mathrm{mol}$, is capable of engaging in a sodium-ion battery configuration with a distinct charge transfer reaction.

$\mathrm{C}_4 \mathrm{H}_4 \mathrm{~N}_2+\mathrm{Na}^{+}+\mathrm{e}^{-} \rightarrow \mathrm{NaC}_4 \mathrm{H}_4 \mathrm{~N}_2$                  (2)

In this reaction, one electron is transferred per sodium ion, so $\mathrm{n}=1$. Plugging in the values, we get $\mathrm{Q}=1204 \mathrm{Ah} / \mathrm{g}$. The charge transfer mechanism for $2,4-\mathrm{CNT}$ in a sodium ion battery is:

$2,4-\mathrm{CN}+\mathrm{Na}^{+}+e^{-} \rightarrow \text { Sodium 2,4-CN }$                     (3)

In this reaction, one electron is transferred per sodium ion, so $\mathrm{n}=1$. The molar mass of $2,4-\mathrm{CNT}$ is $181.59 \mathrm{~g} / \mathrm{mol}$. Plugging in the values, we get: $\mathrm{Q}=(1\times 96,485 \mathrm{C} / \mathrm{mol}) /$ $181.59 \mathrm{~g} / \mathrm{mol}=531 \mathrm{Ah} / \mathrm{g}$.

In order to convert the units of theoretical Capacity from $\mathrm{Ah} / \mathrm{g}$ to $\mathrm{Wh} / \mathrm{kg}$ we need to multiply it with Vcell and divide it by 1000 . In order to calculate Vcell we use $\mathrm{V}_{\text {cell }}=\Delta \mathrm{G} / \mathrm{nF}$, Where $\mathrm{F}$ is Faradays constant $\mathrm{n}$ is charge on sodium Delta $\mathrm{G}$ is gibbs free energy. We used B3LYP 6-31+G basis for calculations. The values of $\mathrm{V}_{\text {cell }}$ of 2,4-Chloronitrotoluenes and pyrazine were calculated as 5.621 volts and 1.778 volts respectively and the energy densities are calculated as 2.991 $\mathrm{Wh} / \mathrm{kg}$ and $2.140 \mathrm{Wh} / \mathrm{kg}$ respectively.

3.3 Sodium Ion adsorption energies

The adsorption energy of sodium in 2,4-Chloronitrotoluenes and pyrazine plays a crucial role in determining capacity of Na-ion intercalation, stability and durability of the anode material [18]. A high adsorption energy of sodium in 2,4-Chloronitrotoluenes and pyrazine can result in strong binding between the sodium ions and the electrode, which can improve rate capability and cycling stability of the electrode material. However, if the adsorption energy is too high, the sodium ions may become strongly trapped in the electrode material, which can reduce the capacity and voltage of the battery. [18] In our study we calculated the adsorption energies of sodium on different transition structures of 2,4-Chloronitrotoluene and Pyrazine by selecting the job as frequency and the method as ground state, DFT, default spin and the Basis as $6-31 \mathrm{G}$ in computational engineering modeling package. The adsorption energies can be calculated as [19].

$\mathbf{E}_{\text {ad }}=E_{\text {total }}(o p t)-\left(E_{\text {mon } 1}(o p t)+E_{\text {mon } 2}(o p t)\right)$         (4)

Table 1. Adsorption energies of Na-ion on 2,4-CN

Table 2. Adsorption energies of Na-ion on Pyrazine

It can be seen from the above result that the adsorption energies of sodium in 2,4- Chloronitrotoluene varies between 21.621 eV to 30.068 eV the higher Ead value for CN4 is due to the presence of a new lone pair on the 2,4-Chloronitrotoluene molecule. When a sodium ion adsorbs onto the 2,4-Chloronitrotoluene molecule, it can interact with the electron density in the molecule, including any lone pairs. The adsorption energy of sodium in Pyrazine was measured as 16.5266 eV. With respect to adsorption energy pyrazine is favorable electrode material because its adsorption energy is not too high like 2,4-Chloronitrotoluene. Sodium ions may become strongly trapped in the 2,4- Chloronitrotoluene, which can reduce the capacity and voltage of the battery.

3.4 Bond lengths

The bond length between $\mathrm{Na}$ with 2,4-Chloronitrotoluene and pyrazine can have a significant effect on the working of a sodium-ion battery. In general, the shorter the length of bond between $\mathrm{Na}$ and anode material, the stronger the connection between them, which can improve the battery performance [20].

This happens due to sodium $(\mathrm{Na})$ creating a strong connection with the anode material, it stores and carries $\mathrm{Na}$ ions within the material, boosting the battery's capacity and long-term stability. On the flip side, if the bond between them is too tight, it can restrict the movement of $\mathrm{Na}$ ions, leading to slow ion diffusion and ultimately reducing the battery's performance [21]. In our research we calculated the bond length of different transition states of $\mathrm{Na}$ with 2,4Chloronitrotoluene and pyrazine. The results are expressed below.

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Figure 3. HOMO and LUMO orbitals of 2,4-Chloronitrotoluene with sodium ions

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Figure 4. HOMO and LUMO orbitals of pyrazine with sodium ions

As shown from the results that the $\mathbf{E}_{\text {HLG }}$ of 2,4-Chloronitrotoluene with sodium varies from $-1.215 \mathrm{eV}$ to $1.753 \mathrm{eV}$ and for pyrazine with sodium it varies from -2.759 $\mathrm{eV}$ to $-3.049 \mathrm{eV}$. So, 2,4-Chloronitrotoluene will be favorable because its $\mathbf{E}_{\text {HLG }}$ is not too high unlike pyrazine. In case of pyrazine when $\mathbf{E}_{\text {HLG }}$ is too high because of this the electrodes can lead to a higher risk of unwanted side reactions,  such as electrolyte decomposition, which can degrade the battery's performance over time.

Table 7. Specific capacitance, energy density and adsorption energy of anode material for sodium ion batteries

The authors also would like to extend their sincere appreciation to Researchers Supplorting Project number (RSPD2024R612), King Saud University, Riyadh, Saudi Arabia.