1 Introduction

The design and synthesis of transition metal compounds with targeted properties and enhanced performances is important to develop the present and future technologies [1]. For fast energy storage applications, novel electrode materials are urgent to enhance both their capacities and charge storage kinetics [2, 3]. Recently, two novel complex niobium tungsten oxides (Nb16W5O55 and Nb18W16O93) were synthesized to effectively use superstructure motifs to provide stable host structures for lithium intercalation with facile and defect-tolerant lithium diffusion and multi-electron redox [4]. At a current density of 20 C, the Nb18W16O93 material shows a capacity of ~ 150 mAh g−1. Even at a current density of 60 C (8.9 A g−1) and 100 C (14.9 A g−1), the capacities are 105 and 70 mAh g−1, respectively. For fast lithium-ion energy storage, nanostructured materials, e.g., TiO2, Li4Ti5O12, and Nb2O5, were proved to overcome poor ionic diffusion and electronic properties [5,6,7]. Among these materials, orthorhombic Nb2O5 (T-Nb2O5) was found to show the anomalously fast energy storage behavior (60 C rate) [5, 6], owing to its specific crystal structure. In the T-Nb2O5 crystal, loosely packed oxygen atomic layers and densely packed Nb–O atomic layers form a quasi‐2D network for Li‐ion incorporation, which allows direct Li-ion transport between bridging sites with very low steric hindrance [5]. Therefore, finding and designing electrode materials with specific atomic arrangements are still needed to obtain enhanced electrochemical properties [8, 9].

Niobium‐based oxides, i.e., Nb2O5, TiNbxO2+2.5x compounds, M–Nb–O (M = Cr, Ga, Fe, Zr, Mg, etc.) family, etc., showed fast charge storage performances because of their unique quasi‐2D networks for Li‐ion incorporation and both open and stable Wadsley–Roth shear crystal structures [10]. The finding and unraveling of new M–Nb–O materials systems are of great interest. Li–La–Nb–O and La–Nb–O compounds were served as solid-state electrolytes for the lithium-ion battery and fuel cell, which may show a fast Li-ion migration behavior. La–Nb–O compounds mainly contain La3NbO7, LaNbO4, LaNb3O9, LaNb5O14, La2Nb12O33, etc. [11, 12]. Most of niobium oxides consist of octahedral NbO6 structures with different degree distortions, while the tetrahedral NbO4 structure only exists in the LaNbO4 compound [13,14,15]. However, among niobium oxide compounds, the NbO4 tetrahedron is rarely found, because the large Nb5+ atom cannot fit into the oxygen-anion tetrahedron. Electronegativity is a useful guideline tool to design targeted electrode materials [9]. With applying electronegativity of metal cations, the change of redox potential and the selectivity of targeted doping element can be achieved in the design of electrode materials [16,17,18]. Ionic electronegativity values of Nb5+ and La3+ are 1.862 and 1.327, respectively. In different La–Nb–O compounds, La3+ can decrease the orbital overlap by easing some charge densities of Nb–O bonds, leading to easy enable redox reaction of Nb5+/Nb4+ and lithium-ion diffusion. With high electronegativity, tetrahedral NbO43− species often do not exist in aqueous solution [19], except with the help of La3+.

In this work, we synthesized orthorhombic pyrochlore LaNb5O14, perovskite LaNb3O9, and monoclinic LaNbO4 compounds. In LaNb5O14, each Nb atom is surrounded by six or seven oxygen atoms to form a NbO6 octahedron or tilted pentagonal bipyramids, while in LaNb3O9, each Nb atom forms a NbO6 octahedron and 2/3 of La cation sites are vacant (Fig. 1). In the LaNbO4 compound, a La cation is eightfold coordination and a Nb cation and 4 O anions form an NbO4 tetrahedron. Served as electrode materials for lithium-ion batteries, the electrochemical performances of these La–Nb–O compounds were systematically studied.

Fig. 1
figure 1

Crystal structures of a LaNb3O9, b LaNb5O14, and c, d monoclinic LaNbO4. LaNb3O9 includes octahedral NbO6 and LaO12 structures. LaNb5O14 includes octahedral NbO6, pentahedral NbO7 and LaO12 structures. LaNbO4 includes tetrahedrally coordinated NbO4 and hexahedral LaO8 units

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2 Experimental

2.1 Material synthesis

La–Nb–O samples were synthesized with a one-step solid-state reaction. The stoichiometric amounts of Nb2O5 and La2O3 were weighed (Table 1) and milled in a mortar for 1 h. Then, the mixed powder was pressed into pellets under a pressure of 30 MPa and calcined in air at 1080 °C for 10 h. After that, the pellets were crushed and milled for further usage.

Table 1 Experimental conditions of the as-synthesized samples and discharge capacitor
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2.2 Material characterization

The phase, image, and element mappings of La–Nb–O samples were analyzed by X-ray diffraction (XRD) with a D8 Focus X-ray diffractometer (Bruker) and field-emission scanning electron microscope (SEM, Hitachi S-4800). Raman spectra were measured in a Jobin–Yvon Horiba T64000 Raman triple grating spectrometer (Horiba Ltd., France) with a green line of Ar+ laser (514.5 nm radiation).

2.3 Electrode preparation

The La–Nb–O sample was mixed with carbon black and polyvinylidene fluoride (PVDF) with a mass ratio of 70:15:15 to form slurry and coated on an Al foil, which was served as the cathode. The assembled half-cells used lithium metal as the anode. The cathode and anode were assembled into a CR2032 coin cell in an Ar-filled glovebox. The separator is a polypropylene film (Celgard 2400) and the electrolyte is 1 mol L−1 LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol%).

2.4 Electrochemical measurement

A charge–discharge cycling test of the assembled half-cell was performed on a LAND CT2001A system in the voltage range of 1.0–3.0 V (vs. Li+/Li) at different current densities. Cyclic voltammogram (CV) curves were measured at an electrochemical workstation (CHI 660E).

3 Results and discussion

Different La–Nb–O compounds were synthesized by solid-state reaction. Figure 2 show XRD patterns of as-synthesized La–Nb–O compounds. When the atom ratios of Nb:La vary from 7:1 to 4:1, the main phase of samples is orthorhombic pyrochlore LaNb5O14 (JCPDS No. 76-263) with the trace phase of monoclinic Nb2O5 (JCPDS No. 72-1121). Orthorhombic pyrochlore LaNb5O14 is constituted by corner and edge-sharing NbO6 and NbO7 polyhedra, which also includes loosely packed La–O atomic layers and densely packed Nb–O atomic layers, forming a quasi‐2D network for Li‐ion insertion (Fig. 1b). When the atom ratios of Nb:La are between 3:1 and 2:1, the main phase of samples is orthorhombic perovskite LaNb3O9 (JCPDS No. 72-2080) with the trace phase of monoclinic LaNbO4. Pure phase monoclinic LaNbO4 (JCPDS No. 22-1125) was obtained at the atom ratio 1:1 of Nb:La. Monoclinic LaNbO4 includes isolated NbO4 tetrahedra which are interlinked by eightfold coordination LaO8 (Fig. 1c, d). SEM shows that the as-obtained samples have large particles with several micrometer and uniform distribution of La, Nb, and O elements (Fig. 3).

Fig. 2
figure 2

a, b XRD patterns of La–Nb–O compounds with different atom ratios of Nb:La (JCPDS No. 76-263 for LaNb5O14, JCPDS No. 72-1121 for monoclinic Nb2O5, JCPDS No. 72-2080 for LaNb3O9, and JCPDS No. 22-1125 for monoclinic LaNbO4)

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Fig. 3
figure 3

SEM images and element mapping of a 1# sample (the atom ratio of Nb:La is 7:1), b 5# sample (the atom ratio of Nb:La is 3:1), and c 7# sample (the atom ratio of Nb:La is 1:1)

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Raman spectroscopy is a powerful tool for probing chemical and structural properties of La–Nb–O materials. As shown in Fig. 4, the band in the range of 450–850 cm−1 can be assigned to the Nb–O-stretching modes. The O–Nb–O-bending modes appear below 450 cm−1, which are strongly interacted with the O–La–O-bending and La–O-stretching modes [19, 20]. The 670 cm−1 band originates from the symmetric stretching mode of NbO6 octahedra (Fig. 4d). Both LaNb5O14 and LaNb3O9 compounds include the NbO6 octahedral structure with different degree distortions. Therefore, LaNb5O14 and LaNb3O9 compounds show similar Raman bands. However, only the NbO4 tetrahedral structure exists in the LaNbO4 compound. The Raman bands centered at 810 and 320 cm−1 are due to the Nb–O symmetric modes of NbO4 tetrahedral structure.

Fig. 4
figure 4

a Raman spectra of La–Nb–O compounds with different atom ratios of Nb:La; b–d Enlarged Raman spectra of La–Nb–O compounds with different atom ratios of Nb:La

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Electrochemical performances of La–Nb–O compounds were further studied as electrode materials for lithium-ion batteries. CV was used to demonstrate their electrochemical reaction mechanisms. Figure 5 shows CV curves of La-Nb–O compounds with different atom ratios of Nb:La at scan rate of 0.5 mV s−1 and potential range of 1.0–3.0 V. These La–Nb–O electrodes have high working voltages of > 1.0 V, which can suppress the formation of the solid electrolyte interface film and lithium dendrites, ensuring the safety of working batteries [10]. When the atom ratios of Nb:La were changed from 7:1 to 5:1, one couple of main redox peaks centered at 1.59 and 1.77 V and weak redox peaks of 2.05/1.90 V were appeared. The redox peaks are assigned to the redox reaction of Nb5+/Nb4+ couple in different chemical environments of NbO6 and NbO7 [21]. Beginning from 4:1 of the Nb:La atom ratio, positions of main redox peaks were changed to 1.16 and 1.29 V, while the intensity of redox peaks of 1.59 and 1.77 V decreased. Only NbO6 and NbO4 exist in LaNb3O9 and LaNbO4, respectively. The above results show that rare earth La3+ can change the thermodynamic potential of Nb5+/Nb4+ redox reaction by changing the crystal structure and Nb coordination condition.

Fig. 5
figure 5

CV curves of La–Nb–O compounds with different atom ratios of Nb:La at a scan rate of 0.5 mV/s and a potential range of 1–3 V

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Furthermore, we studied the charge–discharge performances of these La–Nb–O compounds. The first discharge capacities at the current density of 100 mA g−1 are 175, 160, 116, 112, 138, 122, and 60 mAh g−1 for different samples (Table 1, Fig. 6 and 7). The La–Nb–O compound with the Nb:La atom ratio of 7:1 shows the highest capacity among 1–7# La–Nb–O compounds. The capacity contribution originates from Nb5+/Nb4+ redox reaction coupled with lithium-ion intercalation. Thus, LaNb5O14 with a quasi‐2D network structure can favor the insertion of Li‐ion (Fig. 1b) to obtain a high capacity. The rate capability was tested at various current densities from 0.1 to 2 A g−1 (Fig. 6a). The La–Nb–O compound with the Nb:La atom ratio of 2:1 exhibits a higher capacity especially at a discharge rate of 2 A g−1 than that of 1–5# and 7# La–Nb–O compounds. The NbO6 framework structure and many La vacant sites in LaNb3O9 provide a stable host for fast lithium intercalation. Owing to high electronegativity of tetrahedral NbO43− species in LaNbO4, this compound only displays the smallest capacity and bad rate performance than that of 1–6# La–Nb–O compounds [19, 22]. Electrochemical impedance spectra are used to analyze the resistance during electrochemical reaction. Figure 6d shows Nyquist plots of as-synthesized La–Nb–O compounds. The calculated charge transfer resistances of most La–Nb–O compounds have small values of < 50 Ω, which indicate that La–Nb–O electrodes have low resistances.

Fig. 6
figure 6

Electrochemical performances of La–Nb–O compounds with different atom ratios of Nb:La. a Rate performance of 1#-7# La–Nb–O compounds; b, c charge–discharge curves of 1# and 7# samples, respectively; d–f EIS curves, b values, and lg (current) vs. lg (scan rate) of 1#–7# La–Nb–O compounds

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Fig. 7
figure 7

Charge–discharge curves of La–Nb–O compounds with different Nb:La atom ratios

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To study the fast rate performance, we studied the charge storage kinetics of La–Nb–O compounds using CV curves at different scan rates. The relationship of the peak current and scan rate satisfies the following equation [23, 24]:

$$i = {a}\nu^{b} ,$$
(1)

where b = 0.5 indicates that the electrochemical reaction is controlled by a semi-infinite diffusion process, whilst b = 1 indicates a capacitive behavior. The b values for the La–Nb–O compounds are between 0.6 and 0.8, indicating a hybrid diffusion and capacitive processes (Fig. 6e, f) [25, 26]. The above results show that La–Nb–O compounds with specific crystal structures can provide the lithium diffusion pathway for fast lithium intercalation reaction. The design of novel complex compounds with targeted performance needs the control of the crystal structure and ionic electronegativity of component elements [27].

To study the energy storage mechanism of these electrode materials, XRD and Raman analyses were used to measure the electrode after electrochemical reaction (Fig. 8). LaNb5O14, LaNb3O9, and LaNbO4 phases are not changed after lithium insertion reaction (Fig. 8a). The existence of the peak at 805 cm−1 indicates the retaining of the NbO4 structure in the LaNbO4 compound. Raman peaks of La–Nb–O compounds with Nb:La atomic ratios of 7:1 and 3:1 are at 649 and 647 cm−1, which are consistent with Nb–O-stretching vibration of NbO6 or NbO7 structures [19, 20]. The results show that the La–Nb–O compound is an intercalation-type material for lithium storage.

Fig. 8
figure 8

a XRD patterns and b Raman spectra of La–Nb–O compounds after lithium-ion insertion with different atom ratios of Nb:La

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4 Conclusion

In conclusion, orthorhombic pyrochlore LaNb5O14, perovskite LaNb3O9, and monoclinic LaNbO4 compounds were obtained among La–Nb–O compounds. Pyrochlore LaNb5O14 showed the highest discharge capacities of 175 mAh g−1 at the current density of 100 mA g−1, when used as cathode materials for fast lithium-ion energy storage. Perovskite LaNb3O9 displayed the highest discharge capacities of 51 mAh g−1 at current density of 2 A g−1. In both LaNb5O14 and LaNb3O9, La cation is 12-fold coordination and Nb cations form a sixfold and sevenfold coordination polyhedra, which construct the quasi-2D lithium-ion diffusion network. LaNbO4 with a NbO4 tetrahedral structure showed bad electrochemical performances. The results showed that the enhanced performances of cathode materials originated from appropriate three-dimensional oxide crystal structures, which might be identified and selected through ionic electronegativity scale.