Iron phosphide (FeP) synthesis, and full cell lithium-ion battery study with a [Li(NiMnCo)O₂] cathode

Abstract

A scalable two-step strategy is utilized to synthesize iron phosphide (FeP) from iron oxide (Fe₂O₃). The as-obtained iron phosphide is tested as a negative electrode for lithium-ion batteries both in half cell and full cell configurations. The electrochemical performance vs. lithium exhibits a capacity retention of 53.7% of the second discharge capacity (622 mA h g⁻¹) after 50 cycles using a CMC binder, utilizing both the insertion and conversion reactions. Potential limitation after the first discharge to 0.25 V exhibits a capacity of 227 mA h g⁻¹ after 50 cycles, utilizing only an insertion plateau. The electrochemical performance of iron phosphide in the full cell configuration with a Li(NiMnCo)O₂ cathode exhibits promising lithium storage performance.

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Lithium-ion batteries (LIBs) play an important role in consumer electronic devices and are beginning to gain enormous interest in electric vehicles and renewable energy storage.¹ However, the performance of these devices in turn depends on the materials of construction. Moreover, lithium dendrite formation at high current rates and at low temperatures on the graphitic anode imposes a safety hazard.² This safety concern along with the demands for high energy density batteries stimulated researchers to find an alternative to graphite negative electrodes (theoretical capacity ∼ 372 mA h g⁻¹) used in today's lithium-ion battery technology. Though alloying anodes such as Si and Sn have higher capacities than graphite (Si ∼ 4200 mA h g⁻¹, Sn ∼ 994 A h g⁻¹), they experience huge volume changes (∼300%). These volume changes have restricted their commercial use by limiting the cycle life.³

After the discovery of conversion reaction (eqn (1)), materials which do not undergo classical intercalation mechanism are considered as potential negative electrodes for LIBs.⁴ The electrode reaction involves two or even more e⁻ transfer per 3d-metal as compared to only one e⁻ in commercial graphite electrodes.

where M = transition metal, X = anion, and n = formal oxidation state of X.

Transition-metal phosphides offer great advantage in LIBs compared to transition metal oxides and sulphides because of their high gravimetric storage capacities and relatively low potential conversion reaction plateaus.⁵ It has been observed that transition metal phosphides are associated with low polarization (ΔV ∼ 0.4 V) values compared to other binary compounds due to low polarity of metal–phosphorus bond.⁶ Transition metal phosphides based on Fe, Ni and Cu have been reported as lithium-ion battery anodes with high theoretical capacities.^7–9 Iron phosphides have high theoretical capacities and low reaction potential compared to other metal phosphides (FeP ∼ 926 mA h g⁻¹, FeP₂ ∼ 1365 mA h g⁻¹, FeP₄ ∼ 1789 mA h g⁻¹ [calculated based on Faraday's law]). However, poor cyclic stability of FeP due to large volume change during lithium charge–discharge processes and difficulties in their synthesis limited their application.^7,10–13 Metal phosphides synthesis often involves the utilization of toxic chemicals such as trioctyl phosphine (TOP) or high temperature reaction between elemental compounds (Fe and P) in sealed tubes at elevated temperatures.^7,14,15 The use of inexpensive and non-toxic raw materials to synthesize FeP with high purity was not explored.

In this report, we have constructed a strategy to synthesize iron phosphide (FeP) that avoids the use of TOP or elemental phosphorous. The resultant iron phophide (FeP) powder is studied for electrochemical performance as anode material for LiBs with different potential limitations. A full cell with composition of Li(NiMnCo)O₂ as cathode and FeP as anode is also fabricated. The full cell performance is also studied with different potential limitations with 40.4% capacity retention without potential limitation and 44.1% capacity retention with potential limitation on anode side. To the best of our knowledge, no significant effort was made to study full cell performance of FeP.

Experimental section

Synthesis of FeP

Fe₂O₃ was synthesized according to our previous work.¹⁶ Ammonium dihydrogen phosphate (NH₄H₂PO₄, Merck) and iron oxide (Fe₂O₃) were grinded in a mortar and pestle with the addition of polyethylene glycol-400 (PEG-400) for homogeneous mixing for 1 h. The resultant mixture was dried in an oven at 60 °C for 48 h. The dried sample was washed with deionised water and ethanol repeatedly, and dried in oven at 60 °C for 12 h. As-obtained mixture was heated in a tubular furnace with Ar/H₂ (90 : 10), or pure H₂ at 800 °C for 10 h to obtain FeP powder.

Materials characterization

The X-ray diffraction (XRD) analysis was carried out by Philips X'pert X-ray diffractometer with a Cu-Kα radiation at 40 KV and 20 mA. Morphological study was performed by Field Emission Scanning Electron Microscope (FE-SEM) [Carl Zeiss, Ultra 55] and Field Emission Transmission Electron Microscope (FE-TEM) using JEOL, 2100F.

Electrochemical testing

Negative electrodes were prepared by mixing as prepared FeP with super C-65 (Timcal, Switzerland) and sodium salt of carboxy methyl cellulose (CMC) (Sigma Aldrich) as binder in the weight ratio of 60 : 20 : 20. The slurry was made with de-ionised water as solvent and tape cast on Cu foil. Positive electrodes were prepared by mixing commercial Li(NiMNCo)O₂ (Gelon, China) with super C-65 and PVDF binder in the weight ratio of 88 : 8 : 4. The foils were dried at 80 °C for 10 h in oven. The dried films were cut into circular discs and used as working electrodes. The mass loading of FeP is 1 mg cm⁻² and NMC is 8 mg cm⁻². Argon filled glove box (Mbraun Labstar, Germany) was used to fabricate the half cells (CR 2032 coin cell) with O₂ and H₂O levels of ∼1 ppm. Lithium metal foil was used as reference and counter electrode, borosilicate glass fiber (Whatman, GF/D) as separator, and 1 M LiPF₆ dissolved in EC/DMC (1 : 1 wt) (LP-30, Merck) was used as electrolyte for half-cell fabrication. Galvanostatic charge/discharge cycling performance was carried out by Arbin instruments (BT-2000, USA) in the voltage range of 2.2–0.01 V vs. Li⁺/Li at 20 °C. Full cell was constructed with FeP as negative electrode and Li(NiMnCo)O₂ as positive electrode in CR 2016 coin cell using LP-30 electrolyte with 2 vol% vinylene carbonate (VC) as additive in the potential window of 1–4 V vs. Li/Li⁺.

Results and discussion

The synthesis strategy employed in this work is described in Fig. 1 along with cell configurations used in electrochemical studies.

Fig. 1 Schematic representation of two-step synthesis of FeP from Fe₂O₃ and the corresponding cell configurations.

Initially, Fe₂O₃ porous structures are synthesized using a simple polyol method.¹⁶ Fig. 2a shows the XRD pattern of the as synthesized Fe₂O₃, which represents the formation of rhombohedral α-Fe₂O₃ (JCPDS card no. 89-8104). The calcination (500 °C) of Fe–ethylene glycol complex formed in the polyol process results in the formation of porous Fe₂O₃ by the removal of ethylene glycol units. FE-SEM images shown in Fig. 2b depicts the porous nature of Fe₂O₃ synthesized using polyol process. The effect of temperature on the porous nature of Fe₂O₃ is reported in our previous work.¹⁶ The as synthesized Fe₂O₃ is grinded with ammonium di-hydrogen phosphate (NH₄H₂PO₄) with poly ethylene glycol to get homogeneous mixture. The resultant mixture is dried and heated in hydrogen atmosphere to obtain FeP powder. Fig. 2c shows the XRD pattern of the mixture heated at 800 °C in Ar/H₂ (90/10) atmosphere. The results suggest that the formation of FeP is not completed under these conditions and an intermediate phase of Fe₂P₂O₇ is formed.¹⁷ The formation of pyrophosphate is confirmed by the XRD analysis taken after 10 h of heating in Ar/H₂ at 800 °C.

Fig. 2 (a) XRD pattern and (b) FE-SEM images of Fe₂O₃ synthesized by polyol process, (c) XRD pattern heated in different atmospheres, (d) SEM, (e) TEM (f) HR-TEM and (g) SAED pattern of FeP synthesized from Fe₂O₃.

Later, the complete formation of FeP is achieved by heating in reducing atmosphere (100% H₂) at 800 °C. Similar formation of metal phosphides are observed using hydrogen plasma.¹⁸ Fig. 2c shows the XRD pattern of the samples heated in H₂ atmosphere which matches with the orthorhombic phase of FeP. The formation of pyrophosphate in less reducing atmosphere confirms that the process follows via the intermediate pyrophosphate step before converting to FeP. FEG-SEM images of the samples after complete formation of FeP show micron sized spheroidal particles (Fig. 2d). TEM image reveals the connection between two big particles, and high magnification image reveals the crystalline nature of FeP. Selected area diffraction (SAED) pattern shows the diffractions from (101) and (102) crystal planes of FeP (Fig. 2e–g). We believe that the formation of intermediate pyrophosphate phases before conversion to FeP has resulted in the complete morphological changes observed in the final products. From the morphological observations, the initial morphology of Fe₂O₃ has no effect on the final morphology of FeP. The main advantage of this preparation method compared to existing methods is utilizing an inexpensive precursor and avoiding the use of trioctyl phosphene (TOP) which produces toxic phosphene (PH₃) gas during the reaction as well as avoiding the use of elemental phosphorous (P) for the synthesis of FeP.

The concern for exploiting transition binary compounds in future applications can be addressed by controlling the kinetics of lithium insertion and lowering the charge/discharge polarization voltage, which may be responsible for the poor performance of conversion based anodes. The low polarization value of FeP might be a viable option for exploiting in future applications. The electrochemical reaction of FeP with Li undergoes conversion reaction like any other transition metal oxide materials. However, the conversion reaction of FeP with lithium occurs at relatively lower potential compared to other transition binary compounds. Fig. 3a represents charge/discharge profiles of as synthesized FeP against metallic lithium at 100 mA g⁻¹. The flat plateau at ∼0.1 V represents the two phase reaction of FeP with lithium, where it get converted to Fe nanoparticles and Li₃P. Tarascon et al., witnessed that the observed low potential plateau for FeP conversion during the first discharge is linked to kinetics.⁷

Fig. 3 (a and c) Charge–discharge profiles and (b and d) cycling performance of FeP with different potential limitations, and (e and f) differential capacity plots of second cycle with and without potential limitation, and (g) discharge profiles and (h) power capability of FeP at various current rates.

We have observed a potential plateau at ∼0.9–0.7 V in the first discharge only which may signify the small insertion of lithium into FeP crystal structure.⁷ The large insertion plateau at ∼0.9 V can also be mistaken as oxide impurity plateau (Fe₂O₃ conversion reaction). However, the first charge plateau is observed from ∼1–1.25 V, which is solely associated with the oxidation of Fe to FeP (Fe oxidation to Fe₂O₃ occurs from ∼1.5–2 V).¹⁶ The first discharge capacity is observed to be ∼988 mA h g⁻¹ with reversible charge capacity of ∼575 mA h g⁻¹ (Fig. 3a). The cyclic performance with PVDF binder shows rapid capacity decay, whereas a better performance is observed with CMC binder. After 50 charge/discharge cycles, a capacity of 334 mA h g⁻¹ is retained using CMC binder, which is 53.7% of second discharge capacity (Fig. 3b). The capacity degradation can be the direct result of employing micron size particles of FeP, which upon cycling lead to pulverization due to conversion reaction with Li (as observed in the morphological analysis after cycling in full cell). The reversible insertion of Li at higher potential from second discharge onwards can be visualized by limiting the potential.⁷ Fig. 3c shows the charge/discharge profiles of the electrode in which the potential is limited to 0.25 V during discharge after 1^st cycle. The second discharge capacity by limiting the potential to 0.25 V is 375 mA h g⁻¹, and a capacity of 227 mA h g⁻¹ is retained after 50 cycles using CMC binder. Unlike other transition binary compounds, FeP follows two-step insertion/conversion reaction from second cycle onwards. Differential capacity (dQ/dV) plots of the second cycle shows that the second discharge profile consists of two distinct peaks at ∼0.5 V and 0.2 V which involves the insertion of lithium at higher potential and subsequent conversion to Fe and Li₃P (represented as process A′ and B′ in Fig. 3e).^7,19 The corresponding peaks in charging are marked as A and B in Fig. 3e. The process B and B′ corresponds to the reversible insertion of Li. The differential plots of second cycle by limiting only the discharge potential consists of the B–B′ process (Fig. 3f). The observed cyclic stability by limiting the discharge potential to 0.25 V from second cycle onwards is due to the insertion of Li into FeP (process B–B′) as observed by S. Boyanov et al.^7,19 Though the gravimetric capacities obtained with FeP are less than graphite anodes, the volumetric capacity of FeP is 1.7 times (1377 A h L⁻¹) higher than that of graphite (840 A h L⁻¹). Table 1 shows the comparison of electrochemical performance between various FeP_y based systems in the literature and the FeP synthesized in present work.

Table 1 Comparison between electrochemical performance of FeP synthesized in this work with the literature

Synthesis method	Phase	Capacity	Ref.
Elements heated in sealed tubes	FeP	310 mA h g^−1, 50 cycles (potential limit)	7
Fe(N(SiMe3)2)3 + Ph3 in THF	FeP2 (amorphous)	906 mA h g^−1, 10 cycles (no rate capability)	10
Electrodeposition	FePy (y = 0.17, 0.32, 0.55) amorphous	FeP0.55 200 mA h g^−1, 20 cycles	11
		FeP0.32 150 mA h g^−1, 20 cycles
		FeP0.17 50 mA h g^−1, 20 cycles (no rate capacibility)
Mechanochemical method using Fe and red P	FeP, FeP2, FeP4 (mix phase)	520 mA h g^−1, 20 cycles 0 mA h g^−1@0.3 mA cm^−2	12
Ferrocene + PPh3 sealed tube	Nanorod FeP@C	480 mA h g^−1, 200 cycles	13
		30 mA h g^−1@600 mA g^−1
Fe2O3 + NH4H2PO4	FeP	334 mA h g^−1, 50 cycles (full potential window)	Present work
		227 mA h g^−1, 50 cycles (potential limit) 300 mA h g^−1@1 A g^−1

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The power capability of FeP anode is tested by applying various current rates ranging from 100 mA g⁻¹ to 1 A g⁻¹ as shown in Fig. 3g. The discharge capacity of FeP is 618 mA h g⁻¹, 495 mA h g⁻¹, 400 mA h g⁻¹ and 314 mA h g⁻¹ at 100 mA g⁻¹, 200 mA g⁻¹, 500 mA g⁻¹ and 1 A g⁻¹, respectively. After continuous charge–discharge cycles at various current rates, a capacity of 345 mA h g⁻¹ is retained at the end of 50^th cycle at 100 mA g⁻¹ (Fig. 3h).

While there are several reports on stable electrochemical performance in half cell configuration,²⁰ there are degradation mechanisms whose effect is only observed in full cell configuration, where lithium source is limited. Hence, getting stable electrochemical performance in full cell configuration is the real challenge. Therefore, we decided to perform a full cell study with FeP as negative electrode and commercial Li(NiMnCo)O₂ (NMC) as positive electrode to study the feasibility of FeP as prospective anode material for LIBs. The active materials loading on cathode side is maintained high in order to account for loss of lithium towards SEI formation in the first charge process (FeP theoretical capacity ∼ 926 mA h g⁻¹). Fig. 4a illustrates the charge–discharge profiles of FeP–NMC full cell for three cycles. The potential window of full cell (4–1 V) is based on the charge/discharge profiles of FeP and NMC in half-cell configurations. The initial charge process contains one small plateau from 3.2–3.4 V and a large plateau from 4–4.3 V, which arises from insertion and conversion reactions of FeP, respectively (Fig. 4a). The initial discharge process contains a single plateau from 3.3–2.4 V, which represents formation of FeP and Li intercalation into NMC (Fig. 4a). Subsequent charge–discharge profiles after the first cycle shows good reversibility of the system at C/5 current rate. Fig. 4b shows the normalized capacity of the cell to second discharge capacity with 41% retention after 50 cycles (300 mA h g⁻¹ based on FeP mass loading), utilizing both insertion and conversion reactions of FeP. The potential limitation after the first cycle in half cell configuration is resulted in stable performance with the insertion of Li as the only process. Similarly, in full cell, potential is limited on anode side after first cycle to utilize only the insertion of Li into FeP. The charge–discharge profiles show potential limitation from the second charge process onwards (Fig. 4c).

Fig. 4 (a and c) Charge–discharge profiles and (b and d) cycling performance of FeP as negative electrode in conjunction with NMC positive electrode [at C/5 current rate based anode loading capacity], and SEM images of (e) cross-sectional view of cycled FeP–NMC full cell, (f) FeP and (g) NMC after cycling in full cell configuration.

The capacity retention with potential limitation was 53% after 50 cycles (200 mA h g⁻¹ based on FeP mass loading), normalized to second discharge capacity. Fig. 4e shows the SEM image of cross-sectional view of cycled FeP–NMC electrode. Morphological analysis of individual electrodes after cycling shows aggregated particles of FeP covered with electrolyte decomposition products (Fig. 4f). As mentioned earlier the pulverization of the active FeP materials is due to the large particle size of FeP. In contrast, NMC cathode shows no significant decomposition products on the particles surface (Fig. 4g). Further optimization to reduce the particle size of FeP and to increase the capacity retention is currently in progress.

Conclusions

In summary, we have utilized a two-step approach for the synthesis of FeP without using toxic chemicals such as TOP or elemental phosphorous. The conversion reaction of FeP in the full potential window has shown capacity degradation, but restricting the conversion reaction from the second discharge yields a capacity of 195 mA h g⁻¹ after 100 cycles from the insertion of Li at 0.5 V. Similarly, full cell performance with NMC utilizing full conversion reaction of FeP has shown 40.4% capacity retention after 50 cycles and 53% capacity retention with restricted potential window after 50 cycles. We believe by combining iron based phosphides (FeP) with suitable high voltage cathodes would result in safer, high energy density lithium-ion batteries.

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