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

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 Cp,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 ΔdHLFN = -206.537 ± 2.8 kJ mol-1, the standard enthalpy of formation ΔfH0 = -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 ΔfG0298 = -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], Li2n-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. Li3-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). 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).

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.

Figure 1

Scheme (a) and photo (b) of Netzsch APTAC chamber. 1—machined insulation; 2—sample bomb; 3—safety thermocouple; 4—bottom thermocouple; 5—magnetic stirring; 6—containment vessel; 7—machined insulation; 8—side bottom heater; 9—side thermocouple; 10—control thermocouple; 11—top heater; 12—tube heater.

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 Kα-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 2

XRD pattern of synthesized (a) Li3N at 170 °C for 5 h at N2 atmosphere (709 kPa) and (b) Fe2N at 530 °C for 5 h at NH3 atmosphere.

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.

Figure 3

XRD patterns of Li3FeN2 after heat treatment at 750, 800, and 850 °C for 10 h in N2 atmosphere. The lines in the bottom indicate the diffraction positions of the Li3FeN2 structure (PDF #01-080-0718).

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.

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 (ΔoxHLFN):Li and single nitrides were synthesized by reactions, as described in the Experimental section:6Li + N 2Fe + 2NH

For enthalpy calculation, we used thermodynamic cycle with the following reactions, as shown in Figure 4:Li Li 2Fe N where (aq) means “aqueous”. The standard enthalpy (ΔdHLFN) has been determined in the calorimeter. The received value was equal to −1972.96 ± 25 J g−1, as shown in Table 3.

Figure 4

Thermochemical cycle scheme. Dissolution enthalpy connection of Li3FeN2 with its formation enthalpies from single nitrides.

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 ΔoxHLFN is obtained by the next equation:Δ

The values of ΔdHLi3N, ΔdHFe2N, and ΔdHN2 were also measured by the calorimetry method. Measurement results are shown in Table 3. The value of ΔoxHLFN by Equation (8) is equal to −94.833 kJ mol−1. The negative value of ΔoxHLFN 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

Standard enthalpies for the calculation were taken from the handbooks [76,77], as shown in Table 4.

Table 4

Standard enthalpies of formation from elements (ΔfH0).

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 ΔfH0 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 where Cp—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:

Figure 5

Temperature dependences of the experimental, recalculated, and Neumann–Kopp rule heat capacities of Li3FeN2. The line for the Neuman–Kopp rule is given as an approximating allometric line.

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(Li and m(LFN) = m[1 − ω(Li

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

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 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. 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: where Cp—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

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

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: where S is entropy, ΔHk is enthalpy of the k-th phase transition, and Tk is temperature of the k-th phase transition (0 < Tk < 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: 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(Li

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]: where KH is Herz constant (KH = 20.5), M is molar mass, Cp,298 is isobaric heat capacity, and m is atoms per formula. According to Equation (21) and considering Cp,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):

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 + Li

To determine the Gibbs free energy of the reaction, it is required to subtract from values of the Gibbs energy for initial reagents of the reaction. The 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 Cp(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.