14-electron reduced Mo^IV₆-ε-Keggin polyoxometalates: highly stable and reversible electron/Li⁺ sponge materials

Abstract

First, 14-electron reduced Mo^IV₆-ε-Keggin polyoxometalates (POMs), namely14e-[Sb₂Mo^IV₆Mo^V₂Mo^VI₄O₃₂(OH)₂py₆]²⁻ (1), 14e-[(GeO)₂Mo^IV₆Mo^V₂Mo^VI₄O₃₄(OH)₂py₆]⁴⁻ (2), and 14e-[Sb₂ZnMo^IV₆Mo^V₂Mo^VI₄O₃₄py₇] (3) (py = pyridine), were prepared. X-ray structural analyses of 1–3 revealed two triangularly metal–metal Δ-bonded 6e-[Mo^IV₃O₄] incomplete cubane-type units and one 2e-[Mo^V₂O₄], which account for the hollow ε-Keggin structures free of central heteroatoms. Long-range π-stacking interactions and super electron-rich Keggin structures of 14e-1 and 14e-2 lead to their 260 times higher electrical conductivity than conventional fully oxidized Mo^VI-Keggin POMs. The hollow Mo^IV₆-ε-Keggin POM 1 exhibited structural integrity retained during 24-electron charging/discharging processes when being simultaneously monitored by ex situ XPS and IR. A dynamic study of Li-ion migration in 1 revealed its dominant capacitor-like Li⁺-storage mechanism and effective/fast absorption/desorption of Li⁺. Therefore, it exhibited a high discharge specific capacity (303 mA h g⁻¹, 50 mA g⁻¹), superior rate and cycling performance, especially at an extremely high current density (121.6 mA h g⁻¹, 100 cycles, 8.0 A g⁻¹), which remarkably enhances the performance of conventional fully oxidized Mo^VI-Keggin cathode materials, and provides a new option for using super electron-rich, hollow POMs to improve the electrochemical performance of POM electrode materials.

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Introduction

The global rush to search for advanced battery materials has led to increased interest in the study of polyoxometalates (POMs) as electrode materials for lithium batteries because of their diverse structures and potentially unique ability to accept/release multiple electrons while retaining structural integrity.^1–3 Recently, high-capacity tailored POMs and their derivative electrode materials have been reported.^4–8 Of the most essential but challenging issues are the number of redox electrons, structural integrity associated with the multielectron redox processes and electrical conductivity.^9–11 The former two are highly dependent on the number of incomplete cuboidal [Mo₃(μ₃-O)(μ₂-O)₃] functional units of POMs^11,12 since they are able to accept six electrons to form well-established incomplete cubane-type [Mo₃(μ₃-O)(μ₂-O)₃] units stabilized by strong triangular d²–d² metal–metal Δ-bonds.^11–15 Hence, the incomplete cuboidal [Mo₃O₄] units can serve as reversible 6e-redox functional groups via Mo–Mo Δ-bond breakage and re-formation while maintaining their structural integrity. Conventional Keggin-type species [X@M₁₂O₄₀]^q− (M = Mo, W) unexceptionally embedded with central atoms X, the earliest discovered and most extensively studied POMs,¹⁶ are of special importance in the search for advanced molecular cluster batteries since they are composed of four incomplete cuboidal [M₃(μ₃-O)(μ₂-O)₃] units. [PMo^VI₁₂O₄₀]³⁻ has been reported as a cathode material for lithium-ion batteries, which exhibits a specific capacity of 260 mA h g⁻¹ at a current density of 1 mA.¹⁴ The high specific capacity was ascribed to the reversible [Mo^VI₃O₄]₄ ↔ 24e-[Mo^IV₃O₄]₄ interconversion according to in operando Mo K-edge X-ray absorption fine structure measurements.¹⁴ However, the [PMo^VI₁₂O₄₀]³⁻ cathode material revealed remarkable capacity degradation after 10 cycles.¹⁴ Similarly remarkable capacity degradation has also been observed in other Keggin-POM cathode materials.^14,17–24 Subsequent first-principles molecular dynamics (FPMD) simulations showed that the highly symmetric 24e-reduced Keggin structure would dissociate when its symmetric constraints were removed, suggesting configurational change.¹¹ One of the important factors leading to this structural instability of the super-reduced [PMo^IV₁₂O₄₀]²⁷⁻ has been revealed by recent findings that both 6e- and 12e-reduced Mo^IV_3,6-Keggin POMs^12,25–29 are unexceptionally free of a central heteroatom because of the absence of additional coordinating ability of the μ₃-O atoms caused by the much weaker trans-Mo^IV-pyridine dative bonds and hence creation of stronger μ₃-O–Mo^IV₃ bonding. However, strong additional μ₃-O bonding ability associated with weak Mo^VI₃–μ₃-O bonds is important in order to stabilize central heteroatomic groups such as {PO₄}. Hence, one effective route to improve and enhance battery performance of POMs is to use empty Mo^IV_3n-Keggin species to maintain structural integrity during multielectron redox processes. Moreover, deeply reduced Mo^IV_3n-POMs with markedly smaller HOMO–LUMO gaps and a large number of metal–metal bonding electrons can effectively increase electrical conductivity compared to conventional fully oxidized POMs.³⁰ However, the synthetic and structural chemistry of Mo^IV₃-POMs has been a long-term uncharted research area, which has led to very limited availability of structural information about the multielectron-reduced states of POM electrode materials. In 2013 and 2018, synthetic strategies for the partial oxidation of [Mo^IV₃O₄(Hnta)₃]²⁻ (ref. 12) and the [M^IV₃O₂(O₂CCH₃)₆(H₂O)₃]²⁺ precursor¹⁵ (Hnta = nitrilotriacetic acid) were developed, leading to the rapid development of Mo^IV_3n-POM (n = 1–4) chemistry (Fig. S1†).^{12,15,25–29,31,32} Deeply reduced Mo^IV_3n-POMs^{12,14,15,25–29,31,32} featuring strong triangular metal–metal Δ-bonds are of special importance in respect of (i) a varied combination of 6e-Mo^IV₃ subunits can dramatically change the structures of POMs to form novel types of Mo^IV_3n-POMs;^{12,15,25–29,31,32} (ii) each [Mo^IV₃O₄] can act as a reversible 6e-redox functional unit via Δ-bond breakage and re-formation while maintaining structural integrity; (iii) inert terminal Mo^V,VI=O double bonds are replaced by active Mo^IV ←: L dative bonds (L = Lewis base ligand), which functionalize POMs with Mo^IV₃ Lewis acidic active sites to form Mo^IV₃⋯O_xM_y subnano Lewis catalysis fields (LCFs) with extensive Lewis acid–base synergistic effects;^25–29,32 (iv) a varied number of Δ-bonded Mo^IV₃ triangles remarkably change electronic structures and decrease HOMO–LUMO gaps to different extents to tune the photoelectric properties and enhance the electrical conductivity of POMs; (v) a large number of surface Mo^IV_3n bonding electrons can act as an “electron sponge/reservoir”^14,33 through the breakage and re-formation of Δ-bonds; (vi) peripheral modifiable pyridine ligands allow for the functionalization of Mo^IV₃-POMs via crystal engineering/ligand substitution/π⋯π interactions for varied purposes; (vii) physicochemical properties such as conductivity can be reinforced by efficient Mo^IV₃ → O_xM^VI_y electron transfer and the incorporation of heterometal addenda atoms; and (xiii) X-ray single-crystal structural characterization of mixed-valence Mo^IV,V,VI-POMs is critical for understanding the Mo^IV ↔ Mo^V ↔ Mo^VI structural evolution during charging/discharging processes of POM electrode materials.^{11,14,34–36} POM materials have shown excellent electrochemical performance as anode materials and have been widely reported.^3,37,38 However, the use of POMs as cathode materials has been less reported and their performance is generally poor^{14,17,18,20,21,24,39} (Table S8, ESI†) due to the poor electrical conductivity and structural reversibility of POMs in super-electron redox processes at higher voltage ranges (1.5–4.0 V, vs. Li/Li⁺). In this work, compared to conventional fully oxidized POMs, three empty electron-rich [Mo^IV₃O₄]₂-incorporated Keggin POMs, namely [Sb₂Mo^IV₆Mo^V₂Mo^VI₄O₃₂(OH)₂py₆]²⁻ (1), [(GeO)₂Mo^IV₆Mo^V₂Mo^VI₄O₃₄(OH)₂py₆]⁴⁻ (2) and [Sb₂ZnMo^IV₆Mo^V₂Mo^VI₄O₃₂(OH)₂py₇] (3), exhibit remarkably enhanced cycling life and battery performance, even at an extremely high current density, as cathode materials for lithium batteries.

Results and discussion

Synthesis and structure

The preparation of deeply reduced Mo^IV-POMs has been a long-term challenging issue because (i) discrete Mo^IV is unstable and easily oxidized to Mo^V,VI and (ii) in contrast to W^V in POMs,^40–43 Mo^V has a strong tendency to form metal–metal-bonded Mo^V₂ dimer and never disproportionates to Mo^IV and Mo^VI. In 2013, the first Mo^IV-POM [Mo^IV₆Mo^VI₇O₃₆py₆]⁴⁻ (9, Fig. S1†) was prepared.¹² In 2018, inspired by the established [Mo^IV₃(μ₃-O)₂] → [Mo^IV₃O₄] skeletal conversion,^34–36 the oxidation-controllable bioxo-capped precursor [Mo^IV₃O₂(O₂CCH₃)₆(H₂O)₃]²⁺ was utilized¹⁵ to synthesize a series of Mo^IV_3n-POMs (4–28, Fig. S1†), leading to the rapid development of Mo^IV-POM chemistry.^{12,15,25–29,31,32} As shown in Fig. 1, the empty Mo^IV₆-Keggin anions 1(2) were prepared from solvothermal oxidative aggregation of a mixture of [Mo^IV₃O₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄ and Sb₂O₃(GeO₂) in py/DMF/H₂O/2-pyCO₂H at different temperatures for 3 days. The dianionic 1 and tetraanionic 2 were charge balanced by 2{HNH(CH₃)₂}⁺ and 2{Hpy}⁺·2{HNH(CH₃)₂}⁺, respectively, wherein dimethylamine commonly came from the solvothermal decomposition of N,N-dimethylformamide (DMF). The electroneutral 3 resulting from the additional electrophilic addition of {Znpy}²⁺ was obtained from a similar preparative procedure to that of 1 but at an enhanced solvothermal temperature. POMs 1–3 have been characterized by IR (Fig. S11†), TGA (Fig. S7†) and XPS (Fig. S9†). As-prepared and dried samples of 1 show identical IR and XPS spectra (Fig. S11a and S9a–b†), indicative of their retention of structural integrity. The purity of 1–3 has been verified by PXRD (Fig. S8†) and elemental analyses (Experimental procedures, ESI†).

Fig. 1 Solvothermal syntheses and atomically labelled structures of the deeply reduced hollow Mo^IV₆-Keggin POMs 1–3 with hydrogen atoms omitted for clarity. Selected average bond lengths (Å) in 1: Mo^IV–Mo^IV (Mo^V–Mo^V/Mo^V⋯Mo^VI/Mo^VI⋯Mo^VI), 2.5073(7) ((2.596/3.352)/3.309); Mo^IV–N (Mo^V=O/Mo^VI=O), 2.233 (1.689(6)/1.699(4)); Mo^IV–μ₃-O (Mo^V,VI–μ₃-O), 2.036(2.260); Mo^IV–μ₂-O–Mo^IV (Mo^IV–μ₂-O–Mo^VI/Mo^IV–μ₃-O–Sb^III), 1.939/1.935/2.128(3). Sb^III–O, 1.965. Colour code: Mo^IV, pink; Mo^V, light green; Mo^VI, teal; Sb, pale blue; Ge, olive; Zn, turquoise; O, red; N, blue; and C, gray.

As shown in Fig. 1, 1 features a Keggin structure with crystallographic C_2v-symmetry. The electrophilic addition of two Sb^III atoms occurred as commonly encountered for the deeply reduced Mo^IV₃-Keggin adducts 8 and 9 ³² (Fig. S1†). Such addition has not been observed for the 6e- or 12e-reduced W^VI-incorporated Mo^IV₃-Keggin species 4–7 because of the W^VI-induced lower nucleophilicity.³² The existence of two 6e-reduced [Mo^IV₃O₄] units has been confirmed by the XPS spectrum (Fig. S9†) and the strong triangular Mo^IV–Mo^IV bonds (av. 2.5047(8) Å) (Fig. 1). The markedly shorter Mo^IV₃–μ₃-O₅ bonds (av. 2.030(4) Å) induced by the weak trans Mo^IV–N (2.225(3) Å) compared to Mo^V,VI–μ₃-O₈ ones (av. 2.256(3) Å) lead to the absence of additional coordinating ability of the former and hence the central atoms. The two much weaker Mo^VI₃–μ₃-O bonds resulting from the strong trans Mo^VI=O double bonds are expectedly stabilized by the bonded protons. As shown in Fig. 1 and Fig. S1,† differing from 6e-α-Keggin 4, 12e-β-Keggin 9 and 12-e-γ-Keggin 5–8, 14e-1 with its unusual further 2e-reduction at Mo(V)3–Mo(V)3′ confirmed by XPS (Fig. S9a†) and its short bond length of 2.5896(8) Å, has an ε-Keggin conformation induced by the Mo(V)3–Mo(V)3′ metal–metal bond (Table S2†). As displayed in Fig. 1, 2 has a similar ε-Keggin structure with two {GeO}²⁺ adducts instead of Sb^III. Differing from ε-Keggin 1 and 2, 3 bears one more electrophilic addition of {Znpy}²⁺ at three μ₂-O of the two bottom Mo^VMo^VI₂ triads, responsible for its electroneutrality. Fig. S3† shows edge-to-face π-stacking interactions between coordinating and lattice py rings in 1 and 2. They constitute 1D π⋯π interacted chains with dihedral angles/centroid distance/slipped distance of 74°/4.8/0.94 Å, in contrast to the face-to-face π⋯π interactions (7.5°/3.6 Å) between the two almost parallel coordinating py rings. Differing from 1 and 2, 3 has no lattice py molecules induced by electroneutrality and lacks effective intermolecular π⋯π stacking interactions.

Electronic structures and bonding

As shown in Fig. 2, first-principles density functional theory (DFT) calculations for the deeply reduced Keggin structures 1–3 and the conventional fully oxidized Keggin structures [PMo^VI₁₂O₄₀]³⁻ and [SiW^VI₁₂O₄₀]⁴⁻ have been carried out to understand their differences including energy levels and bonding evolution induced by increasing numbers of reduced electrons and heterometals. Detailed DFT calculations are provided in the ESI section.† The HOMO of 1/2 is dominated by d¹(Mo^V)–d¹(Mo^V) and Mo^V–μ₂O bonding orbitals in the Mo^V₂O₄ dimer, while that of 3 is mainly contributed by d²(Mo^IV)–d²(Mo^IV) and Mo^IV–μ_2,3O bonding interactions in the [Mo^IV₃O₄] incomplete cuboidal units caused by the electrophilic addition of {Znpy}²⁺. The next six frontier MOs (HOMO−1 to HOMO−6) of 1–3 are dominated by Mo^IV–Mo^IV and Mo^IV–μ_2,3-O bonding in the two [Mo^IV₃O₄] units (Fig. S25–S27†). They have markedly higher energy levels than the fully oxidized Mo^VI/W^VI-Keggin HOMO dominated by μ₂-O 2p orbitals (Fig. 2, Table S9 and Fig. S24†) and hence smaller HOMO–LUMO gaps, which, together with the 14 metal–metal bonding electrons, should be beneficial for their electrical conductivity as described later. This has been supported by UV-vis spectrophotometric measurements of 1/2/3 (Fig. S12†) that exhibit much smaller optical band gaps of 1.67/1.72/1.59 eV at 800 nm than those of conventional fully oxidized POMs such as H₃[P@W^VI₁₂O₄₀], H₆[P₂W^VI₁₈O₆₂] and H₃[P@Mo^VI₁₂O₄₀] (about 2.50 eV)^44,45 according to the Tauc plot function. When 14e-1 undergoes a further 10e reduction to produce 24e-1 (S10-1, ESI†), the higher energy level of the HOMO featuring d²–d² bonding interactions in the [Mo^IV₃O₄] units leads to a decreased HOMO–LUMO gap of 1.12 eV (Fig. S22†).

Fig. 2 A comparison of energy levels and HOMO–LUMO gaps of empty Mo^IV₃-POMs 1–3, and fully oxidized Keggin POMs [PMo^VI₁₂O₄₀]³⁻ and [SiW^VI₁₂O₄₀]⁴⁻.

Electrical conductivity

Electrical conductivity of electrode materials has a considerable impact on electrochemical performance.⁴⁶ It largely depends on extended interactions,^47,48 HOMO–LUMO gaps and availability of delocalized electrons.³⁰ As detailed above, long-range π-stacking interactions,^47,48 narrower HOMO–LUMO gap and metal–metal bonding electrons³⁰ can effectively improve electrical conductivity. Conventional fully oxidized POMs were known to suffer from poor electrical conductivity.^46,49–51 As shown in Table S7† and Fig. 3, the electrical conductivity of 14e-1 (4.61 × 10⁻⁹ S cm⁻¹) is more than 260 times that of fully oxidized TBA₃[P@Mo₁₂O₄₀] (TBA = n-Bu₄N⁺),⁵¹ which is largely responsible for the markedly enhanced battery performance described below. 14e-2 with almost the same long-range π⋯π interactions has a similar conductivity (2.72 × 10⁻⁹ S cm⁻¹) but 14e-3 lacking long-range π⋯π interactions exhibits significantly smaller conductivity (2.91 × 10⁻¹⁰ S cm⁻¹).

Fig. 3 I–V characteristics of Mo^IV₆-POM 1 (red), Mo^IV₆-POM 2 (blue) and Mo^IV₆-POM 3 (olive) at 30 °C, respectively.

Electrochemical properties

The reversible multielectron redox and Li⁺/electron storage behavior of Mo^IV₆-Keggin cathode materials 1–3 for lithium batteries was investigated. 1 exhibits reversible redox behavior over the voltage range of 1.5–4.0 V (vs. Li/Li⁺). The cyclic voltammetry (CV) curves for the first five and 10th cycles of the lithium half-cell are illustrated in Fig. 4a. In the first CV curve sweep, a weak cathodic peak at 2.08 V and four anodic peaks at 2.22, 2.97, 3.22 and 3.58 V (vs. Li/Li⁺) appeared. The irreversible weak cathodic peak at 2.08 V and anodic peak at 3.58 V disappeared in the following cycles, which may be ascribed to the irreversible intercalation and deintercalation of Li⁺, respectively. In the second sweep, it also showed four cathodic peaks, centered at 2.93, 2.74, 2.18 and 1.76 V (vs. Li/Li⁺), and four corresponding anodic peaks at 3.22, 2.97, 2.22 and 1.89 V, respectively. The former three pairs of redox peaks are similar to those of [PMo^VI₁₂O₄₀]³⁻ (2.85/3.2, 2.68/2.86 and 2.32/2.42 V, vs. Li/Li⁺). These redox peaks are associated with the reversible Mo^IV ↔ Mo^VI conversion according to the ex situ XPS results and related work reported previously.^11,14,52 The main anodic peak at 2.97/3.22 V and main cathodic peak at 2.74/2.93 V were obviously enhanced over 5 cycles, in accordance with the increasing charging/discharging specific capacity over several cycles (Fig. 4b). The intensified redox peaks should be associated with the deintercalation/intercalation behavior of Li⁺ and the “activating” process in the first few cycles.^52–54 In those cycles with the crystal → amorphous phase conversion, the deintercalation/intercalation behavior of Li⁺ is replaced by the absorption/desorption effects.^52–55 The CV of the 5th sweep nicely overlaps with those of the following sweeps, indicative of highly reversible redox behavior and the strong structural integrity during the charge–discharge process. Mo^IV₆-POMs 2 and 3 also exhibit similar reversible redox behaviors (Fig. S13†). Additionally, the small polarization between the reduction and oxidation processes forebodes excellent rate performance of the cathode material.

Fig. 4 (a) CV curves of Mo^IV₆-POM 1 at a scan rate of 0.5 mV s⁻¹ and (b) galvanostatic charge/discharge voltage profiles of the 1st to 5th cycles. Cycle performance with coulombic efficiency of Mo^IV₆-POMs 1–3 (c, 2.0 A g⁻¹) and Mo^IV₆-POM 1 (d, 8.0 A g⁻¹) over 100 and 350 cycles, respectively.

The galvanostatic charge/discharge profiles of 1 are shown in Fig. 4b. Two plateau-like potential regions, 3.2–2.75 and 1.76–1.5 V (vs. Li/Li⁺), were observed during the second discharging process, which is in good agreement with the CV results. The voltage gradually decreases from the initial open potential of about 2.89 V to 1.5 V (vs. Li/Li⁺) and exhibits a discharge capacity of 177 mA h g⁻¹. It presents a high discharge specific capacity of 303 mA h g⁻¹ in the 3rd cycle at a current density of 50 mA g⁻¹ (30 °C). It presents much better rate performance over the voltage range of 1.5–4.0 V (Fig. S14†) compared to not only conventional pure POM electrode materials but also POMs functionalized with nanocarbon materials such as rGO@PANI/PW₁₂ (285 mA h g⁻¹),²³ PANI/PMo₁₂ (210 mA h g⁻¹)²¹ and SiW₁₂/rGO (275 mA h g⁻¹)²² (Table S8†). As shown in Fig. 4c, Mo^IV₆-ε-Keggin cathode materials 1–3 each exhibit a high reversible discharge capacity of about 147, 142 and 117 mA h g⁻¹ after 100 cycles at a high current density of 2.0 A g⁻¹, respectively. In particular, 1 exhibits an amazing long-term cycling performance with reversible capacity of 122 and 101 mA h g⁻¹ after 100 and 350 cycles, respectively, even at a high current density of 8.0 A g⁻¹ (Fig. 3d). Notably, the capacity retention rate after 350 cycles achieved an impressive value of 106.9% compared to the first cycle. 2 and 3 exhibit similar cycling performances at extremely high current density (8.0 A g⁻¹), which present high discharge specific capacity values of 94 and 86 mA h g⁻¹ after 100 cycles (Fig. S15 and S16†), respectively. The better performance of the super electron-rich ε-Keggin POM 1 should be attributed to its higher electrical conductivity and structural stability. As indicated in Table S8,† the deeply reduced empty Mo^IV₆-Keggin cathode materials 1–3 exhibit significantly higher discharge specific capacity and longer cycling life than traditional fully oxidized Keggin POMs and even POMs functionalized with nanocarbon materials (Table S8†). The remarkably enhanced cycling performance, especially at extremely high current density (2 and 8 A g⁻¹), exhibited by deeply reduced empty Mo^IV₆-ε-Keggin POMs compared to conventional fully oxidized POMs is closely related to their unique structural integrity retained during the course of charging/discharging processes, which will be detailed below.

The poor electrochemical performance of fully oxidized POMs might be attributable to the unstable deeply reduced structures when discharged to 1.5 V. Both 6e- and 12e-reduced Mo^IV_3,6-Keggin POMs are unexceptionally free of central heteroatoms (As, Ge, P, Si etc.)^{12,15,25,26,28,29} because of the absence of additional coordinating ability of the μ₃-O atoms caused by the much weaker trans-Mo^IV-pyridine dative bonds (2.23 Å) and hence the stronger μ₃-O–Mo^IV₃ (2.04 Å) bonding, as detailed in Fig. 5. However, the strong additional μ₃-O bonding ability associated with the weak Mo^VI₃–μ₃-O bonds (2.45 Å) is essential to stabilize central heteroatomic groups such as {PO₄} (P=O, 1.52 Å) for fully oxidized Keggin POMs (Fig. 5a). In addition, hollow Mo^IV_3n-Keggin structures with almost intact terminal Mo^IV,V,VI ← py bonds (and hence similar trans-Mo^IV,V,VI–μ₃-O bonds) benefit in retaining structural stability during super-electron redox processes. Terminal Mo^IV,V,VI ← py dative bonds were observed to coexist in single Mo^IV,V,VI-POMs with similar Mo–N distances (2.18–2.32 Å),³² suggesting that the Mo ← py groups of 1 almost remain intact during the charging/discharging process and hence contribute to the structural stability in contrast to fully oxidized Keggin POMs with their remarkable structural change from doubly bonded Mo^V,VI=O (1.68 Å, Fig. 5) to dative bonded Mo^IV ← O²⁻ (2.14 Å)¹⁴ (and hence markedly different trans-Mo^IV,V,VI–μ₃-O bonds, Mo^V,VI–μ₃-O, 2.45 Å → Mo^IV–μ₃-O, 2.04 Å) during super-electron redox processes. Therefore, the much reduced structural change and the absence of endohedral heteroatoms in the deeply reduced hollow Mo^IV₆-Keggin POMs lead to advantageous electrochemical performance compared to fully oxidized Keggin POMs and even those functionalized by nanocarbon materials (Table S8†) especially at an extremely high current density.

Fig. 5 A structural comparison (Å) of the fully oxidized [Mo^VI₃O₄] unit in the conventional endohedral α-Keggin [PMo^VI₁₂O₄₀]³⁻ (a) and the 6e-reduced [Mo^IV₃O₄] unit in super-reduced hollow ε-Keggin 1 (b, Sb, C and H atoms are omitted for clarity).

{Mo^IV₃}₄ ⇌ {Mo^VI₃}₄ 24e-redox monitored by ex situ IR/XPS

The reversible Mo^IV₃ ⇌ Mo^VI₃ 6e-redox is characteristic of the interconversion between the terminal Mo^IV ←: O²⁻ dative bond and the Mo^VI=O double bonds^11,14 and hence can be monitored by both IR and XPS since the Mo^V/VI=O bond displays a characteristic stretching vibration between 939 and 955 cm⁻¹. Fig. 6 shows the ex situ XPS/IR diagrams measured in the course of discharging → charging → discharging processes at different voltages (Fig. S10 and Table S6†). As-prepared 1 shows a strong Mo^VI/V=O (939 cm⁻¹) IR band and Mo^IV/Mo^V/Mo^VI (3 : 1 : 2) XPS peaks. When discharged to 1.5 V, all Mo^VI atoms were reduced to Mo^IV as demonstrated by the appearance of Mo^IV XPS peaks only, which is in accordance with the disappearance of the Mo^VI/V=O peaks at about 946 cm⁻¹. The Mo^VI/V=O IR bands re-appeared when charged to 3.0 V concomitant with the appearance of the Mo^VI/Mo^V/Mo^IV XPS peaks. Once 1 was further charged to 4.0 V, all Mo^IV/V atoms were oxidized to Mo^VI as confirmed by the presence of Mo^VI XPS peaks only and stronger Mo^VI=O peaks. The XPS peaks of Mo^IV and Mo^V reappeared when discharged to 3.0 V with strong Mo^VI/V=O IR bands. The re-discharge to 1.5 V led to the disappearance of the Mo^VI/V=O peaks at 946 cm⁻¹ concomitant with the observation of Mo^IV-XPS peaks only. The ex situ IR/XPS results and the highly stable cycling performance of Mo^IV₆-Keggin POMs reveal that a highly reversible 24-electron redox reaction occurs during the charging–discharging process (1.5–4.0 V).

Fig. 6 Ex situ XPS/FT-IR spectra of the cathode materials 1 at different cutoff voltages.

Li-ion migration dynamics

The Mo^IV₆-POMs bear six Mo^IV ←: NC₅H₅ groups, which, in addition to taking part in long-range π-stacking interactions, also lead to the larger void spaces favoring Li⁺ migration. The total potential solvent-accessible void volume of Mo^IV₆-Keggin POMs 1–3 was calculated to be 1001.0, 924.6 and 57.2 ų (12.1%, 11.0% and 0.9% of the respective unit cell volume) using PLATON.⁵⁶ This has been supported by the fact that the consistent Li⁺ diffusion coefficient (D_Li) of 1 (10⁻¹² to 10⁻¹⁰ cm² s⁻¹, Fig. 7a), which is 10–100 times greater than that of classical intercalation compounds LiFePO₄/LiCoO₂ (10⁻¹⁴ to 10⁻¹¹ cm² s⁻¹).^57,58 Rate-sweep CV provides deeper insights into the charge storage kinetics on the basis of the following equation:⁵⁹

i = av^b

where i is the current density (A), a and b are the adjustable parameters, and v is the scan rate (V s⁻¹). The b value is the slope of the plot of log i vs. log v. The b values for redox peaks A, B, C and D are 0.74, 0.88, 0.84 and 0.79 for Mo^IV₆-POM 1, respectively (Fig. 5b and c), and log(i) − log(v) also exhibits similar linear relationships for Mo^IV₆-POM 2/3, which suggest that the charge storage of Mo^IV₃-POMs is affected by both capacitor-like and diffusion-controlled behaviors:⁵⁹

i(V) = k₁v + k₂v^1/2

where k₁v and k₂v^1/2 are the current response contributions of surface capacitive effects and diffusion-controlled behaviours, respectively. The contribution of the capacitive fraction is 66.3% at a sweep rate of 2 mV s⁻¹ and increases to 81.7% at 10 mV s⁻¹ for 1 (Fig. 7d and Fig. S17†). Mo^IV₃-POMs 2 and 3 exhibit similar behaviors (Fig. S19 and S21†), indicating that capacitor-like charge storage dominates for Mo^IV₃-POM electrode materials. The charge storage behavior dominated by capacitive effects, fast Li⁺ diffusion and unique structural stability is believed to account for their excellent cycling life and rate performance at high current density. A comparison of the super electron-rich empty Mo^IV₃-ε-Keggin cathode materials 1–3 and fully oxidized M^VI-Keggin species is given in Table S8.† The highly enhanced discharging specific capacity, superior rate capacity and cycling performance of the former can be ascribed to the significantly enhanced electrical conductivity induced by the super electron-rich empty ε-Keggin structure and π⋯π stacking interactions, void spaces induced by coordinating and lattice py groups, and the good structural integrity retained during the super redox process.

Fig. 7 (a) GITT curve of Mo^IV₆-POM 1. (b) Rate-scan CV curves of Mo^IV₆-POM 1 at various sweep rates. (c) Peak current I_p as a function of the square root of the scan rate v^0.5. (d) Normalized contribution ratio of capacitive effects and diffusion-controlled behaviors at different scan rates (2–10 mV s⁻¹) of Mo^IV₆-POM 1.

Conclusions

The deeply reduced 14e-Mo^IV₆-Keggin POMs 1–3 prepared exhibit expected hollow Keggin structures as a consequence of the extremely weak additional coordination ability of capping oxygen atoms bonded to the strongly metal–metal-bonded Mo^IV₃ triangles. The ex situ XPS/IR monitoring and highly enhanced cycling performance during the charging/discharging process (1.5–4.0 V) of 1–3 indicate that the deeply reduced hollow ε-Keggin structures have proven essential to maintain the structural integrity in the course of the super 24e-redox process. This, together with the markedly enhanced electrical conductivity resulting from long-range π⋯π stacking interactions, smaller HOMO–LUMO gaps, a super electron-rich structure, and fast Li⁺ migration kinetics, accounts for their high discharging specific capacity and superior rate/cycling performance especially at high current density. This work reveals the reasons for the poor performance of conventional fully oxidized POM materials as electrode materials and the super electron-rich Keggin POMs providing a new way of thinking to improve POM-based electrode materials.

Author contributions

The experiments and theoretical calculations were mainly performed by JZ, MC, FY and CMR. LX experimentally and theoretically guided and financially supported this work. The work was written by JZ and LX.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21873102).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2246017 (1), 2246005 (2) and 2245819 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01828h

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