High-Voltage Polyanion Positive Electrode Materials

Abstract

High-voltage generation (over 4 V versus Li⁺/Li) of polyanion-positive electrode materials is usually achieved by Ni³⁺/Ni²⁺, Co³⁺/Co²⁺, or V⁴⁺/V³⁺ redox couples, all of which, however, encounter cost and toxicity issues. In this short review, our recent efforts to utilize alternative abundant and less toxic Fe³⁺/Fe²⁺ and Cr⁴⁺/Cr³⁺ redox couples are summarized. Most successful examples are alluaudite Na₂Fe₂(SO₄)₃ (3.8 V versus sodium and hence 4.1 V versus lithium) and β₁-Na₃Al₂(PO₄)₂F₃-type Na₃Cr₂(PO₄)₂F₃ (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing ΔG by edge-sharing Fe³⁺-Fe³⁺ Coulombic repulsion and the use of the 3d²/3d³ configuration of Cr⁴⁺/Cr³⁺ are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.

1. Introduction

Polyanion-positive electrode material for lithium batteries was identified by Delmas, Goodenough, and their co-workers for the NASICON M₂(XO₄)₃ framework in the 1980s [1,2,3]. Later on, Padhi, Nanjundaswamy, and Goodenough discovered a very promising positive electrode material, LiFePO₄ [4], which is now widely commercialized for stationary use or a power source for electric vehicles. A common advantage of polyanion-type electrodes is their long-term stability of operation due to the rigid structural framework. Additional advantages inherent to LiFePO₄ that have led to its commercial application are (i) lithium can be extracted at the first charge and functions as a charge carrier, moving back and forward upon charge/discharge, (ii) it can withstand self-decomposition to guarantee a high level of safety, and (iii) it has a suitable operating voltage of 3.4 V versus lithium, which is not so high that it decomposes electrolytes but not too low that energy density is sacrificed [5].

Toward higher voltages, Mn analogue LiMnPO₄ (4.1 V versus lithium) was investigated but its low electrochemical activity was not acceptable, and this negative feature is common for all Mn-based polyanion-positive electrode materials [4,6,7,8]. Vanadium-based compounds such as LiVPO₄F [9] operate well at a reasonable voltage range around 4.2 V, but they have been excluded as a commercial option due to the element’s toxicity and the volume change during Li⁺ de/intercalation [10,11]. For an even higher voltage, Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ redox couples show activity at >4.5 V [12,13,14], but their highly oxidizing nature induces several side reactions unless careful design is applied to both the electrolyte and electrode composite. For a sodium analogue, electrode operation at higher voltages is more important as the Na/Na⁺ potential is ca. 0.3 V higher than the Li/Li⁺ potential. Within the polyanionic materials, strategic design toward high-voltage operation is almost the same in the case of lithium, achieved by introducing V, Co, Ni as a redox center, as represented by Na₄Co₃(PO₄)₂P₂O₇ [15,16,17].

However, the use of V, Co, or Ni is a challenging option for battery engineers as they entail cost and toxicity issues. In particular, for a sodium battery system, a high-voltage system with more abundant and cheap elements would be ideal. In this short review article, after summarizing the influential factors dominating positive electrode voltage, our recent successful attempts to activate Fe³⁺/Fe²⁺ and Cr⁴⁺/Cr³⁺ redox couples, in Na₂Fe₂(SO₄)₃ (3.8 V versus sodium and hence 4.1 V versus lithium) [18] and β₁-Na₃Al₂(PO₄)₂F₃-type Na₃Cr₂(PO₄)₂F₃ (4.7 V versus sodium and hence 5.0 V versus lithium) [19], will be demonstrated.

2. Toward Higher Voltage

2.1. Inductive Effect in Polyanionic Compounds

The voltage trend of polyanion-based positive electrode materials roughly follows the formal charges of the central atoms in the polyanions, consisting of the idea of the inductive effect [20]. The presence of strong X-O covalency stabilizes the antibonding M³⁺/M²⁺ state through an M-O-X inductive effect to generate an appropriately high voltage. 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 M³⁺/M²⁺ redox energy to useful levels.

2.2. Thermodynamic Modification

In essence, the voltage is defined as the difference between the lithium chemical potential in the cathode and in the anode, leading to a simple thermodynamic definition, ignoring PV and TS terms: (P = pressure, V = volume, T = temperature, and S = entropy), E = (G_Li + G_charged − G_discharged)/nF. where G_Li, G_charged, and G_discharged, are the Gibbs free energies of lithium metal, charged cathode, and discharged cathode, respectively; n is the number of electrons in the redox reaction, and F is the Faraday constant. The overall thermodynamic scheme for voltage generation is summarized in Figure 1 based on the Born–Haber cycle [21].

2.3. Choice of Transition Metals

Figure 2 shows a schematic derivation of the operation voltages and d-band positions of 3d transition metal phosphates in sodium ion batteries. [19] In general, a transition metal ion Mⁿ⁺ with a higher atomic number has a deeper valence level owing to a larger effective nuclear charge, resulting in a higher M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox potential. Naturally, phosphates with end representatives of the 3d series, such as Co²⁺ and Ni²⁺, typically Na₂CoPO₄F and Na₄M₃(PO₄)₂P₂O₇ (M = Co²⁺ and Ni²⁺), have been reported as high-voltage (4.3 V, 4.4 V, 4.8 V, respectively) cathode materials [16,17,22,23,24]. However, end representatives of the 3d series suffer from energy-level increments either by spin exchange penalty or crystal field splitting. On the other hand, the 3d³ electron of Cr³⁺ in the t_2g orbital is free from both spin exchange and crystal field splitting, which can be compensated for the smaller nuclear charge. Indeed, Cr⁴⁺/Cr³⁺ redox couples in phosphates generate >4.5 V vs. Na/Na⁺ (as presented below) [19], comparable to Co- or Ni-based phosphates.

3. High-Voltage System with Fe³⁺/Fe²⁺ and Cr⁴⁺/Cr³⁺

3.1. Pyrophosphates

Of particular interest is the triplite phase of LiFeSO4F [25] and metal-doped Li2FeP2O7 [26,27] possess edge-sharing FeO₆ octahedra to minimize the Fe-Fe distance, as distinguished from other, lower-voltage Fe-based polyanion electrodes with corner-sharing octahedra. 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.

The cell voltage for these two materials can reach as high as 3.9 V (vs. 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 solid. As shown in Figure 3, the potential tunability for the Fe³⁺/Fe²⁺ redox couple at the unusually high-voltage region of 3.5–3.9 V vs. lithium is similar for any metal M doping in the Li₂M_xFe_1-xP₂O₇ system [27]. The phenomena include two aspects: (1) two redox reactions at different potentials are stabilized with the doping of foreign metal M, (2) with more dopant, both of the redox reactions upshift to higher potential, and one even approaches 4 V. Substitution of M into Fe sites may suppress the migration of Fe from the FeO₅ site upon charging, and the two original, distinct Fe sites become robust to stabilize the edge-sharing geometry of FeO₅ and FeO₆ polyhedrals with large Fe³⁺-Fe³⁺ coulombic repulsion energy, leading to the two distinct redox reactions with inherently high potentials. The change in the relative energy of the intermediate compounds, which is induced by the unfavorable V_Li^’-M²⁺(M_Fe^×) and/or Li_Li^×-Fe³⁺(Fe_Fe•) interaction in the doping case, may be a reason for the further potential upshifting. The classic inductive effect cannot explain the redox potential upshifting phenomenon in this case.

3.2. Alluaudites

Compaction of the MO₆ dimer can be more pronounced in an alluaudite framework, where two edge-shared MO₆ octahedra are bridged by a small XO₄ tetrahedron and the M-M distance becomes much shorter (Figure 4). During the search along the Na₂SO₄-FeSO₄ tie line, we discovered the first sulfate compound with an alluaudite-type framework [18]. Deviating sharply from most of the A_xM₂(XO₄)₃-type compounds adopting the NASICON-related structures, Na₂Fe₂(SO₄)₃ does not contain the lantern units [M₂(XO₄)₃]. It would be convenient to denote AA’BM₂(XO₄)₃ as general alluaudite-type compounds, where A = partially occupied Na(2), A’ = partially occupied Na(3), B = Na(1), M = Fe²⁺, and X = S in the present case.

The Na₂Fe₂(SO₄)₃ offers an average potential of 3.8 V (vs. Na/Na⁺), with smooth, sloping charge–discharge profiles over a narrow voltage range of 3.3–4.3 V, which is the highest Fe³⁺/Fe²⁺ redox potential obtained in any material environment (Figure 5) [18]. The abnormally high voltage can be explained by the thermodynamic definition of voltage explained in Section 2.2; the edge-sharing geometry of the Fe octahedra in Na₂Fe₂(SO₄)₃ will raise G_charged due to the strong Fe³⁺–Fe³⁺ repulsion, leading to high E. Additionally, it offers excellent rate kinetics and cycling stability without requiring any additional cathode optimization. It forms an open framework host for the efficient (de)intercalation of Na ions with very low activation energy.

An remarkable feature is that, now, the most commonly accessible redox Fe³⁺/Fe²⁺ can, in principle, generate the high voltage of 3.8 V vs. sodium (and hence 4.1 V vs. lithium). However, the hygroscopicity of the sulphate compounds must be carefully managed.

3.3. β₁-Na₃Al₂(PO₄)₂F₃-Type Fluoride Phosphates

Considering the d-level considerations in Section 2.3, an extremely high operating potential of 4.7 V vs. Na/Na⁺ was identified in Na_3–xCr₂(PO₄)₂F₃ (0 < x < 1) on account of the Cr⁴⁺/Cr³⁺ (3d²/3d³) redox couple, providing a promising design strategy for a high-voltage positive electrode material [19]. Whilst further Na⁺ extraction (x > 1) to form NaCr₂(PO₄)₂F₃ above 5.0 V vs. Na/Na⁺ remains elusive, optimization of the durable cell components for high-voltage operation may enable more activity and greater reversibility. Overall, utilizing low-cost Cr⁴⁺/Cr³⁺ (3d²/3d³), instead of Co³⁺/Co²⁺ (3d⁶/3d⁷) or Ni³⁺/Ni²⁺ (3d⁷/3d⁸) as in previously reported polyanion compounds, is worthwhile for the realization of batteries with higher energy density.

4. Summary and Perspective

Initiated by Delmas, Goodenough, and co-workers in the 1980s, polyanion-type positive electrode materials now represent a large group of materials for reversible Li⁺, Na⁺, and K⁺ insertion. With a suitable combination of transition metal and framework structure, the operating voltage can be tuned, leading sometimes to a suitable high-voltage range for practical application. Although LiFePO₄ is the only compound that has been widely applied for commercial use to date, continuous exploration is ongoing in the community toward better batteries with lower cost, high voltage, high safety, and a long calendar life. In addition to the widely examined redox couple based on Fe³⁺/Fe²⁺, Cr⁴⁺/Cr³⁺ could be an inexpensive yet higher-voltage option for future material development in polyanion compounds.