Why Fe in rechargeable batteries?

Building batteries is a multifaceted engineering riddle that attempts to garner maximum energy/power density without compromising cost and safety. As a lightweight center of reversible redox reaction in the cathode material, 3d transition metals (Mn, Fe, Co, Ni) offer higher electronegativity using electrons that bind more strongly to the nuclear charge, whereby they are preferable because they can generate higher voltage to gain energy density. Mn, Co, and Ni have been the dominant elements in cathode materials in the form of Li_xMO₂ (M = Co, Ni) and Li_xMn₂O₄ for commercial batteries until recently.^1,2 However, keeping an eye on natural abundance (see Table I) and material cost, Fe is gaining ground. Being the fourth most abundant element widely distributed in the earth’s crust, Fe will have less impact on the environment and low toxicity. Such attractive features inherent to Fe are particularly important for large-scale applications, particularly the strong social demand for vehicle and grid applications. This has led materials scientists to devote tremendous efforts to utilize the reversible redox reaction of Fe in the solid matrix form.

Table I.

Elements with a large Clarke number.

Based on information from Reference 42.

Li-battery cathode: Road to olivine LiFePO₄

The most efficient chemical composition of the cathode material can be denoted as Li_xM^4+/3+O₂ (0 < x < 1), where all of the lithium ions and electrons can contribute to a one molar electrode reaction in the condensed solid matrix through various close-packed oxygen arrays. In practical use, however, the lithium composition is limited to ca. 0.5 < x < 1 to avoid structural instability and safety risks due to oxygen evolution.³ A simple expansion of this design to Li_xFeO₂ has led to unimpressive battery performance, regardless of its polymorphic variations. In the ceramics literature, it is common to characterize structural stability on the basis of size and charge effects. The Pauling rules, useful in predicting the structure of many simple oxides, are examples of this. In ABO₂-type compounds (A, B = cations), the α-NaFeO₂ structure (isostructure with LiCoO₂ and LiNiO₂) is stabilized as the size difference between cations is increased and the charge on the B cation is reduced. Unfortunately, Li_xFeO₂ is unstable because the ratio of ionic radii r _Fe3+/r _Li+ = 0.88 (0.77–0.78 for LiCoO₂ and LiNiO₂) does not satisfy the semiempirical tolerance criterion, r _B/r _A < 0.86, for the layer structure.⁴

In order to overcome these difficulties, a series of compounds, including large polyanions (XO₄)^y– (X = S, P, As, Mo, W, y = 2 or 3), were explored, and the use of (PO₄)^3– and (SO₄)^2– has been shown to stabilize the structure and lower the Fe³⁺/Fe²⁺ redox energy to useful levels. The presence of strong X–O covalency stabilizes the antibonding Fe³⁺/Fe²⁺ state through an Fe-O-X inductive effect to generate an appropriately high voltage around 2.8–3.6 V versus lithium.⁵

Based on schematic energy diagrams for some phosphates and sulfides (see Reference 3), LiFePO₄ satisfied almost all criteria for use in a practical Li-ion battery:^3,6 (1) ablility to function as a lithium source upon oxidation from Fe²⁺ to Fe³⁺, (2) high operation voltage at 3.45 V, (3) full utilization of a large theoretical capacity of 170 mAh/g, (4) reasonable true density of 3.6 g/cc with a hexagonal oxygen close-packed array, (5) volume shrinkage (ca. 7%) upon charge process, compensating for the simultaneous expansion of the carbon-based negative electrode in a cell, (6) mixed polaronic/ionic conduction by its crystallographic nature, and, most importantly, and (7) extremely safe, with thermal stability over 700°C. The term “utilization” in (3) is very important, as the practical capacity in a battery does not show any correlation to the theoretical capacity, and low utilization means a larger number of “dead” atoms in the compound, as illustrated in gray areas in Figure 2. Two major drawbacks of LiFePO₄ were recognized as easily blocked one-dimensional lithium diffusion^7–10 and low electronic conductivity in slow hopping sequence,¹¹ but these were successfully overcome, as shown in the ideal charge-discharge curves realized by particle downsizing and carbon coating (Figure 3).^3,11–13 After 2006, high-rate Li-ion batteries with a LiFePO₄ cathode became commercially available and are widely used for power tools and larger scale applications.

Figure 2.

Capacity utilization (practical capacity versus theoretical ones) of various iron-based cathode materials for lithium-ion batteries. For comparison, those for LiCoO₂ and LiMn₂O₄ are also included. The practical capacity in a battery does not show any correlation to the theoretical capacity.

Figure 3.

(a) Crystal structure of LiFePO₄ with a visualized one-dimensional lithium diffusion path in green. The brown octahedra represent FeO₆, and the purple tetrahedra represent PO₄ units.⁹ (b) Typical charge-discharge (voltage versus stored charge) curves of LiFePO₄ (first three cycles). Very flat voltage profile at 3.45 V with large capacity close to the theoretical value (170 mAh/g) is suitable as a cathode material. PC/DMC (propylene carbonate/dimethyl carbonate) 1 M LiPF₆ is the electrolyte composition; 1 mol/l of LiPF₆ is dissolved into a 1:1 solution of PC to DMC.³

Where is the ultimate limit of Fe redox function?

Inspired by the success of LiFePO₄, several compounds were explored to further utilize the redox reaction of Fe more efficiently toward higher energy per weight/volume. Major directions were (1) increasing the number of redox electrons per transition metal, (2) minimizing the weight and volume of the host framework, and (3) raising the redox potential.

Lightening and compaction of host framework

In comparison to the oxide cathodes, the polyanionic framework compounds invariably suffer from lower theoretical capacity owing to the weight and volume penalty of constituent polyanion units. The metal borate (LiMBO₃) family forms a special class of polyanionic cathodes with the lowest-weight polyanions (BO₃)³⁻ with a molecular weight of 58.8, which is significantly lighter than the larger tetrahedral (PO₄)³⁻ unit with a molecular weight of 95.0. A large theoretical capacity of 220 mAh/g is calculated based on the LiFeBO₃ chemical formula, which is much larger than the 170 mAh/g for LiFePO₄.Furthermore, the true density of LiFeBO₃ is 3.46 g/cm³, which is close to that of LiFePO₄ and suitable for maximizing the volume energy density due to the minimum volume occupation of the planar triangle BO₃ oxyanion unit.^20,21 Carefully prepared Li_xFeBO₃ shows room temperature activity as a cathode with a capacity of >190 mAh/g at around 3 V versus Li (Figure 4).^20,21 A significant positive impact is realized by avoiding surface poisoning due to contact with the ambient atmosphere. However, the voltage window of 1.5–4.5 V to cover large polarization is too large for practical applications. Moisture sensitivity is another important factor that needs to be investigated before its application in commercial products can be considered. Once these problems are solved, borates, rather than heavier and larger phosphates, could be a new avenue for the exploration of novel electrode materials.

Figure 4.

Experimental and calculated open-circuit voltage of Li_xFeBO₃; average open-circuit voltages were calculated between the lowest energy intermediate phases demonstrated by the red line. Reprinted with permission from Reference 20. © 2010 Wiley.

Raising the Fe³⁺/Fe²⁺ redox potential

The cell voltages with M = Fe show the trend LiFeBO₃ < Li₂FeSiO₄ < LiFePO₄F < LiFePO₄ < Li₂FeP₂O₇ < LiFeSO₄F.^22,23 This trend (see Figure 5) roughly follows the formal charges of the central atoms in the polyanions, which is consistent with the idea of the inductive effect. More essentially, the voltage is defined as the difference between the lithium chemical potential in the cathode and in the anode, leading to the simple thermodynamic definition ignoring PV and TS terms (P = pressure, V = volume, T = temperature, and S = entropy),

(3)$$E = ({G_{Li}} + {G_{charged}} - {G_{discharged}})/nF.$$

Figure 5.

A historical diagram comparing the potential of Fe³⁺/Fe²⁺ redox couple versus lithium of various Fe-based cathode materials. “Phen” is the abbreviation of phenanthroline. Reprinted with permission from Reference 27. © 2012 Wiley.

where G _Li, G _charged, and G _discharged are the Gibbs free energies of Li metal, charged cathode, and discharged cathode, respectively, n is the number of electrons to the redox reaction, and F is the Faraday constant. A shorter Fe³⁺–Fe³⁺ distance in the charged state is effective for enlarging G _charged, and hence the operating voltage E, while the influence of the discharged state G _discharged with smaller charge Fe²⁺ can be subordinated in energetics.²⁴ Of particular interest is the triplite phase of LiFeSO₄F and metal doped Li₂FeP₂O₇, which possess edge sharing FeO₆ octahedra to minimize the Fe-Fe distance, as distinguished from other lower voltage Fe-based polyanion cathodes with corner-sharing octahedra. The cell voltage for these two materials can reach as high as 3.9 V (versus Li), which is higher than the value of 3.8 V calculated from the standard redox potentials. The latter has been suggested to be the highest achievable voltage for a Li-ion battery utilizing the Fe³⁺/Fe²⁺ redox couple in a solid.^25,26

A historical overview of the Fe³⁺/Fe²⁺ redox potential is summarized in Figure 5, in which an inspiring feature is that now the most commonly accessible redox Fe³⁺/Fe²⁺ can, in principle, generate a high voltage value of 3.9 V versus lithium (and hence 3.6 V versus sodium).²⁷ In spite of the larger G _charged, this can be realized without sacrificing the thermal stability (safety margin)²⁸ and is suitable to maximize energy density under the constraint of the electrolyte stability window.

Li versus Na: Similarities and differences

There has been a resurgence of interest recently in sodium (Na)-ion batteries (see the Kubota et al. article in this issue). Enlightened by the rich experience with compound structures in Li-ion batteries, new cathode materials based on layered oxides and polyanions have been continuously discovered. The main factors affecting the applicability of a cathode such as voltage, conductivity (electronic and ionic), and stability are being addressed. One notes that all of these factors are determined by the phase behavior of the material in different states of charge. A Na⁺ ion is much bigger than a Li⁺ ion (102 pm versus 76 pm), and intercalation of Na⁺ into typical hosts for Li⁺ ions may result in different phase behavior. Therefore, elucidation of the phase diagram will provide an essential basis for understanding the material performance.

As a typical example, Na_xFePO₄ and Li_xFePO₄ show completely different phase diagrams.²⁹ The volume expansion of FePO₄ upon sodiation (sodium intercalation) to NaFePO₄ is large, about 17% (Figure 6). The existence of an intermediate phase at x = 2/3 in the Na case, which is very stable up to 500°C, is somehow anticipated to relax the large lattice strain between the x = 0 and x = 1 phases. However, even with the existence of an intermediate phase at Na_2/3FePO₄, one notices that the expansion from FePO₄ to Na_2/3FePO₄ is still quite large (12.8%), nearly two times that for Li_xFePO₄ to change from x = 0 to 1 (Figure 6), which may disturb the sustainment of a coherent interface for smooth electrode reaction. Difficulties in direct synthesis, sluggish electrode kinetics, and low voltage around 2.7 V versus Na³⁰ are three major obstacles hindering NaFePO₄ from practical application.

Figure 6.

Schematic diagram comparing the volume expansion of FePO₄ upon lithium and sodium insertion. For Li_xFePO₄, no intermediate structure between 0 < x < 1 is found. For Na_xFePO₄, an intermediate phase at x = 2/3 exists to “buffer” the large volume expansion. The difference of ionic radii between Li⁺ and Na⁺ causes a significant difference in the phase diagram. Reprinted with permission from Reference 29. © 2013 American Chemical Society.

Toward Na-ion battery cathode: The present status

In spite of the great success with olivine LiFePO₄, the sodium analogue with the same structure has turned out to be unsuitable for an electrode. Although the switch to Na from Li is the conceptually obvious way to follow, simple extension by just replacing lithium with sodium does not always lead to similar performances, and in many cases ends in failure. Therefore, strategies should be directed toward specific exploration of new materials suitable for the sodium system.

Renewed interest in Na-ion chemistry has seen extensive research on various P2- and O3-type oxide layered compounds (see the Kubota et al. article in this issue) and their solid solutions, mostly based on expensive transition metals such as Co and Ni. Large-scale sodium batteries will be commercially viable with earth-abundant transition metals such as Fe. Different from the lithium case, Na_xFeO₂ satisfies the tolerance criterion, r _B/r _A < 0.86, thanks to the larger ionic radius of Na⁺ (r _Na+ = 102 pm, r _Li+ = 76 pm). To date, O3-type NaFeO₂^31,32 and P2-type Na_x[Fe_1/2Mn_1/2]O₂³³ have been reported, both suffering from low operating potential even by using a Fe⁴⁺/Fe³⁺ redox couple, and the stable reversible capacity is limited. Using the inductive effect in polyanion framework systems, the Fe³⁺/Fe²⁺ redox potential can be enhanced with full utilization of a one-electron reaction. In this pursuit, many ∼3 V Fe-based phosphate PO₄^3– insertion compounds other than the olivine phase have been reported. These include Na₂FePO₄F (3.06 V),¹⁵ Na₂FeP₂O₇ (2.9 V),³⁴ and Na₄Fe₃(PO₄)₂(P₂O₇) (3.2 V).³⁵ Among these, Na₂FeP₂O₇ (Figure 7) shows high current-rate performance and extreme thermal stability. The Sumitomo electric company chose Na₂FeP₂O₇ in parallel with NaCrO₂ as cathode materials for their prototype Na-ion battery pack using a molten-salt electrolyte operating around 60–90°C.^36,37 However, iron-based high-voltage materials of >3.5 V versus Na have yet to be reported and warrant exploration in the future.

Figure 7.

Charge-discharge curves of Na₂FeP₂O₇. (Inset) The dQ/dV curve (derivative of capacity by voltage) of charge-discharge profile showing several distinct peaks. The peak ∼3 V has three distinct steps marked as 1, 2, and 3. Applied current is represented as C/20, meaning full theoretical capacity is reached in 20 hours.³⁴

Summary

At the present stage, Na-ion batteries are not a strong competitor for Li-ion chemistry by only considering sodium’s greater abundance compared to the well-established state-of-the-art Li-ion power packs. However, it has rapidly attracted the attention of many materials scientists. Both weak Lewis acidity and the large polarizability of Na-ion allow fast charge transfer reactions at electrode/electrolyte interfaces and in bulk diffusion.^38–40

Furthermore, unlike the incremental progress of the mature Li-ion systems, researchers of the immature Na-ion systems based on Na-ion intercalation are likely to discover new electrodes with ordered functional structures due to the difference in the ionic radius of Na and 3d transition metals, which may enable Na-ion batteries to surpass Li-ion batteries. The abundance of Na should not be compensated for by combination with rare transition metals, thereby Fe is definitely the best to functionalize. Maximizing the redox function of Fe has been intensively studied in the lithium system, from which we can expect to realize >3.6 V operation in the Na system.

However, the present status is still far from this technical target. Operating voltage and reversible capacity of various known iron-based cathodes for Na-“ion” (cathode functions as the whole sodium source) battery systems are summarized in Figure 8. Further effort to reach a primary target of >3.6 V operation and capacity around 125 mAh/g can lead to an energy density >450 Wh/kg, which is comparable to those of LiMn₂O₄ (430 Wh/kg) and LiFePO₄ (500 Wh/kg) in lithium systems.⁴¹ Exploration for emerging Na-ion battery electrodes is still at an early stage, and further contributions from materials scientists are welcome.

Figure 8.

Overall comparison of the Fe-based cathode materials that can function as sodium sources in a Na-ion battery system. Polyanionic cathode materials are shown as green boxes and simple oxides/fluorides as blue. The yellow band indicates the voltage region, which ensures compatibility with Li-ion batteries. A reasonable target region is illustrated as a dashed red circle. (*The capacity of P2-Na_2/3[Fe_1/2Mn_1/2]O₂ is assumed by the inherent amount of sodium, including the Mn⁴⁺/Mn³⁺ redox reaction, as separately denoted with the dashed pale-blue box. Dashed pale green for NaFePO₄ indicates it cannot be directly synthesized and has sluggish kinetics in electrode reaction.)