Synthesis Method and Thermodynamic Characteristics of Anode Material Li₃FeN₂ for Application in Lithium-Ion Batteries

Abstract

Li₃FeN₂ material was synthesized by the two-step solid-state method from Li₃N (adiabatic camera) and FeN₂ (tube furnace) powders. Phase investigation of Li₃N, FeN₂, and Li₃FeN₂ was carried out. The discharge capacity of Li₃FeN₂ is 343 mAh g⁻¹, which is about 44.7% of the theoretic capacity. The ternary nitride Li₃FeN₂ molar heat capacity is calculated using the formula C_p,m = 77.831 + 0.130 × T − 6289 × T⁻², (T is absolute temperature, temperature range is 298–900 K, pressure is constant). The thermodynamic characteristics of Li₃FeN₂ have the following values: entropy S⁰₂₉₈ = 116.2 J mol⁻¹ K⁻¹, molar enthalpy of dissolution Δ_dH_LFN = −206.537 ± 2.8 kJ mol⁻¹, the standard enthalpy of formation Δ_fH⁰ = −291.331 ± 5.7 kJ mol⁻¹, entropy S⁰₂₉₈ = 113.2 J mol⁻¹ K⁻¹ (Neumann–Kopp rule) and 116.2 J mol⁻¹ K⁻¹ (W. Herz rule), the standard Gibbs free energy of formation Δ_fG⁰₂₉₈ = −276.7 kJ mol⁻¹.

1. Introduction

In the world of technological development, energy sources are being severely depleted. In this regard, the issues related to creating new energy sources, in particular renewable energy sources, are being considered.

Secondary batteries, such as lithium-ion, lithium sulfur, and hydrogen batteries, are attracting increased attention for their development and production. Probably, one of the prospective renewable sources of energy is the lithium-ion battery (LIB) as an energy source for many applications, such as electric cars and buses, laptops, mobile phones, etc. LIBs solve the problems of high energy requirements (energy and power density, cycle life), environmental efforts, and relatively low cost.

A lot of efforts were directed to the development of more advanced batteries. For example, different approaches for LIB’s development were used, such as nanostructured materials [1,2,3,4,5,6,7,8,9,10,11,12,13], the growth of the capacity and voltage of cathode materials [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30], hollow and porous and structures [13,31,32,33,34,35,36,37,38,39,40,41,42,43,44], safety issues, including separator and liquid electrolyte studies [45,46,47,48,49,50,51,52,53,54,55,56], etc. As a prospective current source for electric vehicles (EV), LIBs have proven their market position. To receive high-performance lithium-ion batteries, it is required to improve the specific capacity of active (electrode) materials.

Thus, a lot of efforts were focused on the fabrication of anode materials with high theoretical specific capacity. For example, silicon has attracted the attention of the LIBs industry as an anode material with ultrahigh specific capacity (4212 mAhg⁻¹), although the large volume expansion of silicon during the charge/discharge process (300%) leads to a capacity decrease and reduced cycle life [57].

Another popular anode material with high performance is Li₄Ti₅O₁₂. This anode material attracted attention due to its low manufacturing cost, high safety, and environmental friendliness [58,59]. However, Li₄Ti₅O₁₂ has poor electrical conductivity of 10⁻⁸–10^˗13 S cm⁻¹, a low lithium diffusion coefficient (10⁻⁹–10⁻¹⁶ cm⁻² s⁻¹), and a low theoretical capacity of 175 mAh g⁻¹ [60,61,62,63].

Previous works shows good electrochemical properties of Li₃N-type anodes, e.g., Li₂Na₄N₂ and Li₄Na₂N₂ phases [64], LiBeN [65], Li₃N-Mg₃N₂ [66], Li_2n-1MN [67], and Li₃FeN₂ [68,69]. Thus, as Li₃FeN₂ materials have transition metal, it could not be used as solid electrolyte because transition metals might produce conduction electrons, which is unacceptable for a solid electrolyte of lithium battery. However, this quality is advantageous for using this material as an electrode. Li_3-xFeN₂ (0 < x < 1) has a high capacity of 260 mAh g⁻¹ [67]. In addition, the charge–discharge potentials between 0 and 2 V (vs. Li) were very flat for x = 0.1–0.7.

Li₃FeN₂ was first synthesized by Frankenburger et al. by the reaction of lithium nitride (Li₃N) with elemental Fe in N₂ atmosphere [70]. After decades, Fromont investigated the reaction of Li₃N with iron using thermogravimetry [71]. These studies show that Li₃FeN₂ was indexed by an orthorhombic cell with lattice parameters a = 9.65 Å, b = 8.66 Å, and c = 8.38 Å. Emery et al. [70] show the solid-state synthesis of Li₃N with Fe powder in atmosphere, which shows a cationic mixing in Li₃FeN₂ compound.

Li₃FeN₂ is a prospective material for hydrogen storage because of its hydrogen uptake capacity of 2.7 wt %, of which about 1.5 wt % was reversible [69,72,73].

In this article, the two-step synthesis and properties of promising anode material Li₃FeN₂ are shown. Firstly, Li₃N synthesis was obtained in an adiabatic chamber. Then, mixed with iron nanopowder, Li₃FeN₂ was obtained at a tube furnace. Two-step synthesis was chosen for the synthesis of high-purity complex nitride Li₃FeN₂.

2. Materials and Methods

A 16 mm diameter and 0.6 mm lithium plate sliced and polished in an argon glovebox, iron nanopowder, nitrogen, and ammonia (NH₃) were used as starting components for Li₃N, Fe₂N, and Li₃FeN₂ synthesis (

Table 1. Summary of chemicals descriptions.

Name	Formula	Source	Purity, %
Iron nanopowder	Fe	Changsha Easchem Co., Ltd. (Changsha, China)	99.9
Lithium	Li	Xiamen Tmax Battery Equipments Ltd. (Xiamen, China)	99.9
Nitrogen	N2	Qingdao Guida Special Gas Co., Ltd. (Qingdao, China)	99.9–99.999
Ammonia	NH3	Wuhan Newradar Trade Company Ltd. (Wuhan, China)	99.9
Lithium nitride	Li3N	Prepared here	98.9 1
Iron nitride	Fe2N	Prepared here	98.4 1
Lithium iron nitride	Li3FeN2	Prepared here	99.1 1

¹ Purity according to XRD analysis.

). The purity of materials shown in Table 1 is according to suppliers’ data. Lithium sliced plates were put into a titanic autoclave nitrogen-filled bomb of Netzsch APTAC 264 (Selb, Germany), as shown in Figure 1. The Li₃N synthesis parameters are next: the temperature is 170 °C, heat rate is 2 °C/min, synthesis time is 6 h, and nitrogen pressure is ≈709.3 kPa (7 atm).

Iron nanopowder and nitrogen were used as a source for Fe₂N. Ceramic crucible with initial powder was put into the tube furnace (BTF−1700C, (Hefei, China). The tube has been purged by ammonia (NH₃) for 30 min before synthesis. Synthesis was carried out in NH₃ atmosphere at 530 °C for 6 h with a heat rate of 8 °C/min. Mechanically mixed and powder was hot pressed for 2 h at 1100 °C. The received hot-pressed sample was heated in N₂ atmosphere for 10 h at 700 °C (heat rate was 5 °C/min). After heat treatment, the sample was mechanically ground into ivory-colored powder.

XRD analysis was held with a Bruker D8 Advance (Karlsruhe, Germany) with a step of 0.02°. Structural parameters were refined by the Rietveld method using TOPAS5 software.

X-ray diffraction analysis (XRD) was used as the structure analysis method for the synthesized nitrides powders investigated. XRD analysis was performed with a Bruker D8 ADVANCE diffractometer with a vertical goniometer and Cu K_α-radiation. The diffraction angles (2θ) are 5–100°, 10–80°, and 5–120° for Li₃N, Fe₂N, and Li₃FeN₂, respectively.

Calorimetric measurements were performed using a TAM IV Microcalorimeter (Shanghai, China) at 298 K with the cell volume of 20 mL. Aqueous solution of 1 mol dm⁻³ HCl was used for the calorimetric cell ampoule. The ampoule was broken when thermal equilibrium was established, and nitride powder began to dissolve in HCl solution. Thermo-EMF vs. time was registered during the dissolution process providing the heat dissolution curve. Integration of this curve gave the value of dissolution enthalpy.

3. Results

Figure 2 shows the XRD pattern of synthesized Li₃N (a) and Fe₂N (b) powders. All peaks are in good correlation with database one. Li₃N has a P6/mmm space group with lattice parameters a = 3.6711 Å, b = 3.6711 Å, and c = 3.8770 Å, which are in good correlation with [74] and PDF #30-0759. Fe₂N reflection peaks also are in good correlation with [75] and PDF #50-0978. The space group of Fe₂N is P312 with lattice parameters a = 4.7912 Å, b = 4.7912 Å, and c = 4.416 Å.

Figure 3 shows the XRD pattern of Li₃N, Fe₂N, and Li₃FeN₂ after heat treatment in an Netzsch APTAK chamber, tube furnace with ammonia atmosphere, and tube furnace with nitrogen atmosphere, respectively. Lattice parameters, a and c, calculated by the Rietveld method for Li₃FeN₂ are a = 4.872 Å, b = 9.677 Å, and c = 4.792 Å, respectively, in the Ibam space group. XRD patterns of Li₃FeN₂ synthesized at different temperatures are shown in Figure 3. The sample synthesized at 850 °C shows a high purity of 97.2% with Li₂O impurity. Other samples include such impurities as Li₂O (PDF #01-076-9237), Li₅FeO₄ (PDF #01-075-1253), and LiFeO₂ (PDF #74-2284). Samples synthesized at 850 °C have only Li₂O impurity; thus, further investigation of the compounds were conducted with materials synthesized at 850 °C.

The structure refinement defined that Li⁺ is in 4b and 8g, Fe⁺³ is in 4a, and N⁻³ is in 8j sites. All calculations were carried out with using TOPAS 4 software by Bruker. The final structure parameters (including site occupancy) are listed in

Table 2. Structure characteristics of Li3FeN2.

Atom/Void	Site	g	x	y	z
Li1	8g	0.91	0.0	0.25745	0.25
Li2	4b	1	0.0	0.5	0.25
Fe	4a	1	0.0	0.0	0.25
N	8j	0.98	0.219979	0.113757	0.5

.

4. Discussion

4.1. The Standard Enthalpy of Formation

The formation enthalpy of Li₃FeN₂ (LFN) compound from single nitrides Li₃N and Fe₂N is calculated using the following equation (Δ_oxH_LFN):

Li₃N + 0.5Fe₂N + 0.25N₂ → Li₃FeN₂,

(1)

and single nitrides were synthesized by reactions, as described in the Experimental section:

6Li + N₂ → 2Li₃N,

(2)

2Fe + 2NH₃ → 2Fe₂N + 3H₂.

(3)

For enthalpy calculation, we used thermodynamic cycle with the following reactions, as shown in Figure 4:

Li₃FeN₂ + 6HCl_(aq) → 3LiCl_(aq) + FeCl_3(aq) + N₂↑+ 3H₂,

(4)

Li₃N + 4HCl_(aq) → 3LiCl + NH₄Cl,

(5)

2Fe₂N + 8HCl_(aq) → 4FeCl₂ + 2NH₃ + H₂,

(6)

N₂ + 8HCl_(aq) → 2NH₄Cl + 3Cl₂,

(7)

where (aq) means “aqueous”. The standard enthalpy (Δ_dH_LFN) has been determined in the calorimeter. The received value was equal to −1972.96 ± 25 J g⁻¹, as shown in

Table 3. Values of specific and molar enthalpies of dissolution (298 K, p = 101 kPa, 1 mol dm⁻³ HCl).

Compound	Specific Enthalpy, J g−1	Molar Mass, g mol−1	Molar Enthalpy of Dissolution, kJ mol−1	Ref.
Li3N	−3163.853 ± 30	34.83	−110.197 ± 1.7	this work
Fe2N	−13.79 ± 1.5	125.701	−1.734 ± 0.04	this work
N2	−71.716 ± 10	28.014	−2.56 ± 0.12	this work
Li3FeN2	−1972.96 ± 25	104.684	−206.537 ± 2.8	this work
Li3Na3N2	−2285.96 ± 13.4	117.807	−269.3018	[66]

.

The resulting value of Δ_oxH_LFN is obtained by the next equation:

Δ_oxH_LFN = Δ_dH_Li3N + 0.5Δ_dH_Fe2N + 0.25Δ_dH_N2 − Δ_dH_LFN.

(8)

The values of Δ_dH_Li3N, Δ_dH_Fe2N, and Δ_dH_N2 were also measured by the calorimetry method. Measurement results are shown in Table 3. The value of Δ_oxH_LFN by Equation (8) is equal to −94.833 kJ mol⁻¹. The negative value of Δ_oxH_LFN defines Li₃FeN₂ as a stable phase. In addition, it is it is energetically favorable to synthesize LFN from single nitrides.

At last, the enthalpy of formation of Li₃FeN₂ from elements can now be calculated using the following equation:

Δ_fH_LFN = Δ_fH_Li3N + 0.5Δ_fH_Fe2N + 0.25Δ_fH_N2 + Δ_oxH_LFN.

(9)

Standard enthalpies for the calculation were taken from the handbooks [76,77], as shown in

Table 4. Standard enthalpies of formation from elements (Δ_fH⁰).

Compound	ΔfH0298.15, kJ mol−1	Reference
Li3N(cryst)	−196.78 ± 0.3	[76]
Fe2N(cryst)	−3.77 ± 0.1	[76]
N2(gas)	8.67 ± 0.1	[77]
Li3FeN2(cryst)	−291.331 ± 5.7	this work
LiCaN(cryst)	−216.8 ± 10.8	[78]
Li3BN2(cryst)	−534.5 ± 16.7	[79]
Li3AlN2(cryst)	−567.8 ± 12.4	[79]
LiMoN2(cryst)	−386.0 ± 6.4	[80]
Li7MnN4	−661	[81]

The subscripts (cryst) and (gas) mean “crystalline” and “gaseous”, correspondingly.

.

The calculated value of the enthalpy of Li₃FeN₂ formation by Equation (9) is −291.331 ± 5.7 kJ mol⁻¹, Table 4. The enthalpy of formation Δ_fH⁰ for Li₃FeN₂ has the same order as for similar compounds, namely lithium metal nitrides (Table 4). That fact indirectly confirms the correctness of measurements. The value of formation enthalpy, calculated by Equation (9), can be used in thermodynamic estimation and the modeling of Li₃FeN₂ reactivity.

4.2. The Isobaric Heat Capacity

The temperature dependence of the isobaric heat capacity of the Li₃FeN₂ is shown in Figure 5. According to XRD data (Figure 3), the obtained powder material contains a certain amount of lithium oxide Li₂O. This impurity quantity must be taken in consideration for valuation of the heat capacity of the Li₃FeN₂. This impurity could appear during the synthesis process or contact with oxygen in air atmosphere. XRD quantitative methods have limitations, but the heat capacity of a two-phase system must be recalculated by additive consideration:

mC_p = m(LFN)C_p(LFN) + m(Li₂O)C_p(Li₂O),

(10)

where C_p—a specific heat capacity (pressure is constant), and m—a mass. The sample weight consists of synthesized compound (Li₃FeN₂) and impurity (Li₂O). So, the heat capacity of Li₃FeN₂) is expressed from Equation (10) as:

$${\mathrm{C}}_{\mathrm{p}}\left(\mathrm{LFN}\right)=\frac{{\mathrm{mC}}_{\mathrm{p}}-\mathrm{m}\left({\mathrm{Li}}_2\mathrm{O}\right){\mathrm{C}}_{\mathrm{p}}\left({\mathrm{Li}}_2\mathrm{O}\right)}{\mathrm{m}\left(\mathrm{LFN}\right)}.$$

(11)

The weight of the included compounds can be found from the total mass of the sample, which are calculated through the weight fraction of lithium oxide, ω(Li₂O):

m(Li₂O) = mω(Li₂O)

(12)

and

m(LFN) = m[1 − ω(Li₂O)].

(13)

According to Equations (12) and (13), Equation (11) can be written as follows:

$${\mathrm{C}}_{\mathrm{p}}\left(\mathrm{LFN}\right)=\frac{{\mathrm{C}}_{\mathrm{p}}-{\mathrm{C}}_{\mathrm{p}}\left({\mathrm{Li}}_2\mathrm{O}\right)\mathsf{\omega }\left({\mathrm{Li}}_2\mathrm{O}\right)}{1-\mathsf{\omega }\left({\mathrm{Li}}_2\mathrm{O}\right)}.$$

(14)

Thereby, the heat capacity of LFN can be calculated from the experimental data and heat capacity of lithium oxide impurity. For Equation (14), it is required to know the dependence of the specific heat capacity of the lithium oxide from temperature. For this, tabulated data for the lithium oxide heat capacity [77] were used. For the temperature range of 300–900 K, the commonly used polynomial formula for the heat capacity is as follows:

C_p = a + bT − cT⁻²

(15)

where a, b, and c are empirical coefficients; T is the absolute temperature. The received coefficients for lithium oxide are a = 76.666 J mol⁻¹ K⁻¹, b = –13.63·10⁻³ J mol⁻¹ K⁻², and c = –18.624·10⁵ J mol⁻¹ K. The heat capacity of Li₃FeN₂ for the 300–900 K temperature range was recalculated using Equations (15) and (16) considering Li₂O’s impurity presence. According to XRD data (Figure 3), Li₃FeN₂ contains about 2.8 ± 0.04 wt % Li₂O. The experimental and recalculated LFN heat capacity is shown in Figure 5 and

Table 5. The temperature dependence of the experimental (exp.), recalculated by Equation (14) (rec.), and calculated by the Neumann–Kopp (N-K) rule (Equation (17)) heat capacities (Cp) of Li₃FeN₂(s).

T, K	Cp(exp.), J K−1 mol−1	Cp(rec.), J K−1 mol−1	Cp(N-K), J K−1 mol−1
300	126.9	124.1	117.8
400	134.1	132.6	130.7
500	146.3	144.3	141.9
600	160.5	158.3	152.8
700	173.8	171.9	163.4
800	183.3	180.7	173.6
900	186.1	178.8	183.5

. Empirical values for heat capacity were calculated by the Neumann–Kopp rule. This rule prescribes calculating the molar heat capacity of a complex compound from the heat capacities of constituent elements by adding them in with the corresponding compound stoichiometry. However, this calculation method gives good results for room temperatures and rough results for high temperatures. For more accurate results, binary compounds were used instead of single elements:

$${\mathrm{C}}_{\mathrm{p}}\left(\mathrm{CN}\right)={\displaystyle \sum }\mathrm{n}\left(\mathrm{BN}\right){\mathrm{C}}_{\mathrm{p}}\left(\mathrm{BN}\right)$$

(16)

where C_p—molar heat capacity, n—a stoichiometric coefficient, and CN and BN are complex and binary nitrides, correspondingly. For LFN, Equation (16) can be written as (according to Equation (1)):

C_p(LFN) = C_p (Li₃N) + 0.5C_p (Fe₂N) + 0.25C_p (N₂).

(17)

The dependence of the heat capacity by temperature calculated from Equation (17) using tabular data [77] is shown in Figure 5 and Table 5.

The temperature dependence of the heat capacity calculated by the Neumann–Kopp rule is in good correlation with the recalculated heat capacity (considering Li₂O impurity amount). However, XRD quantitative analysis gives rough results for the small presence of compounds in the material. For other quantitative methods, the amount of impurities can be measured more accurately: for example, thermogravimetry or volumetric methods.

4.3. Entropy

Entropy is another thermodynamic function that should be calculated. The Third Law of thermodynamics states, “The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).”. Thus, the entropy absolute value can be valued by the equation:

$$\mathrm{S}\left(\mathrm{T}\right)=\underset{0}{\overset{{\mathrm{T}}_1}{{\displaystyle \int }}}\frac{{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}\right)}{\mathrm{T}}\mathrm{dT}+\frac{\Delta {\mathrm{H}}_1}{{\mathrm{T}}_1}+\underset{{\mathrm{T}}_1}{\overset{{\mathrm{T}}_2}{{\displaystyle \int }}}\frac{{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}\right)}{\mathrm{T}}\mathrm{dT}+\frac{\Delta {\mathrm{H}}_2}{{\mathrm{T}}_2}+\cdots +\underset{{\mathrm{T}}_{\mathrm{k}}}{\overset{\mathrm{T}}{{\displaystyle \int }}}\frac{{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}\right)}{\mathrm{T}}\mathrm{dT}$$

(18)

where S is entropy, ΔH_k is enthalpy of the k-th phase transition, and T_k is temperature of the k-th phase transition (0 < T_k < T). Since the entropy can be calculated by the Neumann–Kopp rule, if there is no phase transition until the calculation temperature, entropy can be also calculated by the Neumann–Kopp rule:

$$\mathrm{S}\left(\mathrm{T}\right)=\underset{0}{\overset{\mathrm{T}}{{\displaystyle \int }}}\frac{{\displaystyle \sum }{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}, \mathrm{BN}\right)}{\mathrm{T}}\mathrm{dT}={\displaystyle \sum }\underset{0}{\overset{\mathrm{T}}{{\displaystyle \int }}}\frac{{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}, \mathrm{BN}\right)}{\mathrm{T}}\mathrm{dT}={\displaystyle \sum }\mathrm{S}\left(\mathrm{T},\mathrm{BN}\right),$$

(19)

where BN is the binary nitride compound (see Equation (16)). According to Equations (16) and (17), Equation (19) can be written in the following way:

S (LFN) = S(Li₃N) + 0.5S(Fe₂N).

(20)

The entropy of Li₃FeN₂ at room temperature is 113.2 J mol⁻¹ K⁻¹ according to Equation (20) and tabular data [82]. The additive rule for entropy calculation is suitable if the sum of the molar volumes of binary compounds differs a bit from the molar volume of the complex compound [83]. Thus, the molar volume for Li₃N is 27.2 cm³ mol⁻¹ (ρ = 1.28 g cm⁻³ [83]), for Fe₂N is 19.8 cm³ mol⁻¹ (ρ = 6.35 g cm⁻³ [83]), and for Li₃FeN₂ is 33.9 cm³ mol⁻¹ (ρ = 3.09 g cm⁻³ [84]). The sum of the molar volumes of binary nitrides with their corresponding coefficients is 37.1 cm³ mol⁻¹ and differs about 9% from the LFN molar volume, which allows usage of an additive scheme.

In addition, the LFN entropy can be calculated by the W. Herz rule [85]:

$${\mathrm{S}}_{298}^0={\mathrm{K}}_{\mathrm{H}}{\left(\mathrm{M}/{\mathrm{C}}_{\mathrm{p},298}\right)}^{1/3}\mathrm{m},$$

(21)

where K_H is Herz constant (K_H = 20.5), M is molar mass, C_p,298 is isobaric heat capacity, and m is atoms per formula. According to Equation (21) and considering C_p,298 from Table 5, the LFN entropy is 116.2 J mol⁻¹ K⁻¹. Thus, the LFN entropy calculated by the Herz rule is in good correlation with the Neumann–Kopp rule result.

4.4. The Standard Gibbs Free Energy

The enthalpy of formation and entropy calculated above allows evaluating the standard Gibbs free energy of Li₃FeN₂ formation (at T = 298 K):

$${\Delta }_{\mathrm{f}}{\mathrm{G}}_{298}^0={\Delta }_{\mathrm{f}}{\mathrm{H}}_{298}^0-298{\Delta }_{\mathrm{f}}{\mathrm{S}}_{298}^0.$$

(22)

The resulting value of the Gibbs free energy for Li₃FeN₂ at room temperature is −276.7 kJ mol⁻¹.

The next reaction is suggested for the determination of stability against metallic lithium with subsequent calculation of the Gibbs free energy at room temperature:

3Li + Li₃FeN₂ = 2Li₃N + Fe.

(23)

To determine the Gibbs free energy of the reaction, it is required to subtract from ${\Delta }_{\mathrm{f}}{\mathrm{G}}_{298}^0$ values of the Gibbs energy for initial reagents of the reaction. The ${\Delta }_{\mathrm{f}}{\mathrm{G}}_{298}^0$ for single elements is equal to zero, and for Li₃N, it is −128.6 kJ mol⁻¹ [82]. The Li₃FeN₂ Gibbs free energy has been calculated above. Thus, the Gibbs free energy for reaction (23) is 19.5 kJ mol⁻¹, and this reaction is thermodynamically impossible. Finally, Li₃FeN₂ is stable against metallic lithium at room temperature.

5. Conclusions

The thermodynamic characteristics were determined for Li₃FeN₂ anode material for a lithium-ion battery. The two-step synthesis method allowed producing a highly pure compound with less than 3 wt % of Li₂O impurity according to XRD data. The enthalpy of Li₃FeN₂ formation from binary nitrides was determined according to the measured enthalpy of dissolution of reagents and product of Li₃FeN₂ formation reaction. The obtained value is equal to −206.5 ± 2.8 kJ mol⁻¹. The Li₃FeN₂ standard enthalpy of formation from single elements is equal to −291.3 ± 5.7 kJ mol⁻¹. This value can be used in further thermodynamic modeling and determinations.

The heat capacity value was recalculated considering the presence of Li₂O impurity. The temperature dependence of the heat capacity is in good correlation with calculation by the Neumann–Kopp rule. Finally, the heat capacity can be described by formula C_p(T) = 78.997 + 0.132 × T + 4.654·10⁵ × T⁻², where T is absolute temperature. The LFN entropy is equal to 113.2 J mol⁻¹ K⁻¹, and the Gibbs free energy of Li₃FeN₂ formation is −276.7 kJ mol⁻¹. The calculations confirm that the Li₃FeN₂ material is stable against metallic lithium. All thermodynamic values and functions can be used for modeling and further calculations.