Nickel-doped Li₂MoO₄ as a high-performance anode material for rechargeable lithium-ion batteries

Abstract

Transition metal oxides are promising anode materials for rechargeable lithium-ion batteries (LIBs) because of their high theoretical specific capacity. Li₂MoO₄ (LMO) has a high specific capacity due to its variable oxidation state and alloying reaction, but the low conductivity and large volume expansion significantly impede its practical applications. To overcome these challenges, here we propose a nickel-doping strategy to prepare Li₂Ni_xMo_1−xO₄ (LNMO) as an anode of LIBs by doping Li₂MoO₄ with Ni²⁺ ions using a sol–gel process. The Ni-doping can not only help minimize the path length for ion and electron transport, thus enhancing electron transmission or conductivity, but also offers a mechanical cushion against volume expansion and contraction amid regular insertion and removal of Li⁺. As a result, the as-developed LNMO anode exhibits a higher reversible charge/discharge specific capacity than LMO, in addition to excellent cycling stability. The Li₂Ni_0.05Mo_0.95O₄ anode exhibits a stable lithium storage capacity of 487.2 mA h g⁻¹, much higher than the 210.9 mA h g⁻¹ for the LMO anode after 100 charge/discharge cycles at a current density of 100 mA g⁻¹. This work offers a facile yet effective approach to creating high-performance Li₂MoO₄ anode materials, thus promoting their real-world application in rechargeable LIBs.

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Introduction

The energy and environmental challenges created by the ongoing consumption of old petrochemical materials are becoming increasingly visible, and hence there is a pressing need for developing energy storage systems with high energy density, safety, and efficiency.^1–4 As one of the most widely used energy storage systems, lithium-ion batteries (LIBs) feature high energy density, low self-discharge, high operating voltage, and long service life, and are thus being extensively employed in portable electronic gadgets, new energy vehicles, and energy storage devices.^5–7 Currently, graphite is the most frequently used negative electrode material for LIBs in the practical battery industry because of its high theoretical capacity that reaches as high as 372 mA h g⁻¹, assuming a low voltage plateau. Unfortunately, the potential of lithium embedded in graphite is very low, close to the potential of lithium plating, which can easily lead to a short-circuit of the battery due to the formation of lithium dendrites.⁸ Therefore, there is a pressing need for high-performance LIB anode candidates that enable high charge/discharge efficiency, high energy density, long cycle life and safety.⁹ However, to date it has remained highly challenging to develop high-capacity LIB anodes as an alternative to the graphite anode.^10–14

Metal molybdates represent one of the most promising anode materials for LIBs because of their high specific capacity as a result of the fast shift between the two molybdenum oxidation states of Mo⁶⁺/Mo⁴⁺ during the oxidation/reduction reaction in the discharge/charge process besides the low cost.^15–18 Mo-based anode materials feature environmental friendliness, low cost, abundant resources, and good safety.^19,20 However, due to their relatively low electronic conductivity and large volume changes, they often fail to maintain their initial high capacity during charging and discharging.^21,22 Meanwhile, Li/Mo (LMO) anode materials normally experience significant volume expansion, namely structural collapse and deformation due to the Jahn–Teller phenomenon. Meanwhile, the formation of an unstable solid electrolyte interface (SEI) can lead to significant volumetric fluctuations in battery materials during the lithiation/delithiation process.^23–25 Hence, many strategies, including particle size reduction,²⁶ preparation of nanopowders,^27–29 carbon coating,^30,31 and doping with metal ions,^32,33 have been developed to address these issues.

To address the relatively low electronic conductivity and large volume change issues of LMO, the construction of nanoscale LMO materials has recently emerged as an effective strategy. Indeed, Ni²⁺ is considered to be one of the most suitable dopants for the construction of nanoscale lithium molybdate materials. Because of the similar ionic radii of Ni²⁺ (ionic radius 0.69 Å) and Mo⁶⁺ (ionic radius 0.59 Å), the Ni²⁺-doping will not change the crystal parameters of LMO.

To prepare high-performance LMO anode materials, we, here, propose a Ni-doping strategy to design Li₂Ni_xMo_1−xO₄ (LNMO) as anode materials for LIBs via a facile sol–gel strategy.^34,35 Due to Ni doping, the path length of ion and electron transport can be appropriately expanded, thus enhancing electron transmission, and a mechanical cushion effect is provided against volume changes; as a result, the as-designed LNMO anode shows a superior reversible charge/discharge specific capacity to LMO, and excellent cycling stability.^36,37 It also exhibits a stable lithium storage capacity of 487.2 mA h g⁻¹, an over 200% increase as compared to LMO after 100 charge/discharge cycles at a current density of 100 mA g⁻¹. This work offers a facile and promising approach for the development of high-performance LMO anode materials, which hold great potential as commercially viable anode materials for rechargeable LIBs.

Experimental

Li₂Ni_xMo_1−xO₄ materials were fabricated by a straightforward, versatile sol–gel method. (NH₄)₆Mo₇O₂₄·4H₂O, CH₃COOLi, NiSO₄·6H₂O, and 0.2 g of H₂C₂O₄ were dispersed in 40 mL of deionized water as chelating agents by stirring. The obtained homogeneous solution was heated to 80 °C and stirred continuously until the solvent was removed to form the viscous precursor. Then, the precursor was dried at 80 °C for 14 h in an oven. The resultant powder was calcined at 500 °C for 10 h, and the nickel-doped lithium molybdate material (Li₂Ni_xMo_1−xO₄, x = 0.03, 0.05, 0.1) was obtained after cooling to room temperature.

Material properties

X-ray powder diffraction (XRD) was performed using a (Rigaku Shrewd Lab) X-ray beam diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) was conducted on a field emission filtered electron magnifier (Quanta-250). High resolution transmission electron microscopy (HRTEM; 2010F, JEOL) was carried out at two hundred kilovolts. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Escalab 250Xi.

Electrochemical measurements

According to the mass ratio (active substance : conductive agent super P : adhesive polyvinylidene fluoride = 8 : 1 : 1), the prepared mixed slurry was uniformly coated on copper foil to make the negative electrode sheet, and the negative electrode sheet was obtained by vacuum drying at 80 °C for 10 h. Typically, the mass loading was 20–25 mg, and a half-cell (CR2016) was used for electrochemical performance testing. Lithium wafers were used as counter electrodes, a solution of LiPF₆ and EC : DMC with a volume ratio of 1 : 1 was used as an electrolyte, and a polypropylene porous membrane (Celgard-2400) was used as the diaphragm. The CR2016 coin cell batteries were assembled in an Ar-filled glovebox that was operated under a vacuum with an H₂O/O₂ concentration of less than 0.1 ppm. Discharge/charge tests were conducted on a battery system (Land, CT2001A, China). Constant current charge/discharge testing and multiplier performance testing were performed on a Blue Power system (Land CT2001A). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed on a CHI660E electrochemical workstation.

Results and discussion

Fig. 1a shows the sol–gel preparation procedure of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1). Fig. 1b shows the XRD patterns of the pure LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1). As can be seen, no additional peaks can be found in the patterns, and the diffraction peaks of all samples align primarily with the standard card of Li₂MoO₄ (PDF serial number 12-0763), space group P3₂. The diffraction peaks including (012), (211), (220), (122), and (231) are attributed to the tripartite crystal system. Furthermore, the XRD spectra of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) do not display any XRD peaks associated with the newly formed stage containing nickel particles. Thus, nickel doping has a slight effect on the structure of Li₂MoO₄. After introducing nickel into the LMO structure, certain positions of the molybdenum ions are filled with nickel ions, reducing the oxidation state of adjacent molybdenum atoms from +6 to +4 valence. The nickel doping and change in molybdenum oxidation state leads to the creation of more free electrons throughout the crystal structure, bringing about an increase in conductivity, which is conducive to improving the electrochemical performance of LMO as an anode electrode in LIBs. The primary XRD peaks of the nickel-doped samples exhibited a shift in Fig. 1c. According to the Bragg equation 2d sin θ = nλ, Ni²⁺ (ionic radius 0.69 Å) has successfully replaced Mo⁶⁺ (ionic radius 0.59 Å). The interlayer spacing increases along the c-axis and the diffraction peaks become stronger, implying that Li₂MoO₄ lattices are doped with Ni²⁺. The crystal structure of the nickel-doped LMO is slightly deformed with increasing content of nickel ions. This indicates that nickel has been successfully and uniformly doped into the LMO.

Fig. 1 (a) Schematic of the fabrication process of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples, (b) XRD patterns of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples, (c) details of locally amplified peaks of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples, (d) Rietveld refinement results of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples, and (e) crystal structure of Li₂Ni_0.05Mo_0.95O₄.

The XRD data of Li₂Ni_0.05Mo_0.95O₄ were refined by GSAS software using the Rietveld method.³⁸ The detailed refinement results are shown in Fig. 1d. The R_p, R_wp and χ² values are 0.0461, 0.0318, and 0.0178, respectively. The black cross symbols represent the unprocessed data obtained from XRD measurements, and the red solid line represents the XRD pattern that has been fitted. The colorful straight lines illustrate the disparities between the refined and experimental data. The fuchsia color indicates data reflection on the location. The measured XRD data matches well with the fitted XRD data, indicating that the results are reliable.

For Li₂Ni_0.05Mo_0.95O₄, the results obtained after structural analysis of the material using VESTA³⁹ software are shown in Fig. 1e. The results show that the Ni atoms successfully take the place of Mo atoms and coexist with Mo atoms. The structural analysis shows that the nickel atoms are successfully doped. The lattice parameters and R-factor data for Li₂Ni_0.05Mo_0.95O₄ are listed in Table 1.

Table 1 Refined lattice parameters along with R-factors

Li2Ni0.05Mo0.95O4 lattice parameters
Space group	a (Å)	b (Å)	C (Å)	α (°)	β (°)	γ (°)	R p	R w	χ ^2
P32	14.2892 (16)	14.2892 (16)	9.5630 (14)	90.0	90.0	120.0	2.81	3.57	1.907

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Fig. 2 shows the SEM images of pure LMO and Li₂Ni_0.05Mo_0.95O₄ samples. In Fig. 2a and b, the particle surface of pure LMO is much rougher than that of Li₂Ni_0.05Mo_0.95O₄, and the particle size is irregular and the structure compact. As depicted in Fig. 2c and d, the morphology of Li₂Ni_0.05Mo_0.95O₄ is more uniform, with a more regular shape and size, compared with the pure LMO sample. The results show that the Li₂Ni_0.05Mo_0.95O₄ material features great structural stability.

Fig. 2 SEM images of (a and b) LMO and (c and d) Li₂Ni_0.05Mo_0.95O₄ at different magnifications.

Both samples have irregular shapes and similar morphologies (Fig. 3a and c). This indicates that nickel doping does not change the structure of LMO. Fig. 3d shows the electron diffraction pattern of Li₂Ni_0.05Mo_0.95O₄ for 241 facets. Fig. 3e shows the lattice stripes of LMO (232) and LMO (400) with distinct lattice stripes and lattice spacings of 0.242 nm and 0.210 nm, respectively. Fig. 3f and g present diffraction points of LMO (232) and (422) with their corresponding crystalline surfaces and lattice stripes. Fig. 3h shows the analysis of the lattice stripes of Li₂Ni_0.05Mo_0.95O₄, and in it the lattice stripes of LMO (152) and LMO (422) are evident with the lattice spacings of 0.205 nm and 0.211 nm, respectively, with diffraction points and their corresponding crystalline surfaces and lattice stripes (Fig. 3i and j). The lattice stripe d value of the (422) crystalline surface is also slightly larger for the comparison of LMO and Li₂Ni_0.05Mo_0.95O₄, consistent with the XRD shift results. This is in good agreement with the XRD results. Fig. 3k shows an image of the energy dispersive X-ray (EDX) mapping of Li₂Ni_0.05Mo_0.95O₄ showing the presence of Mo, Ni, and O. The elements of Mo, Ni, and O are uniformly dispersed in Li₂Ni_0.05Mo_0.95O₄. These results strong verify that nickel has been successfully doped into the chemical structure of LMO.

Fig. 3 (a) Transmission electron microscope (TEM) images and (b) electron diffraction pattern of LMO, (c) TEM image and (d) electron diffraction pattern of Li₂Ni_0.05Mo_0.95O₄, (e) high-resolution TEM image of LMO and (f and g) lattice diagrams of its (232) and (422) crystalline surfaces, (h) high-resolution TEM image of Li₂Ni_0.05Mo_0.95O₄ and (i and j) lattice diagrams of its (152) and (422) crystalline planes, and (k) elemental maps of Li₂Ni_0.05Mo_0.95O₄.

The elemental compositions of LMO and Li₂Ni_0.05Mo_0.95O₄ were measured using X-ray photoelectron spectroscopy (XPS). Fig. 4a shows the full spectrum of LMO, and Fig. 4b gives its C 1s spectrum with three deconvolution peaks centred at 284.8, 286.88 and 288.28 eV. Fig. 4c shows the Mo 3d spectrum with Mo 3d_5/2 and Mo 3d_3/2 peaks located at 235.78 and 232.68 eV. Fig. 4d–f show that Li₂Ni_0.05Mo_0.95O₄ consists of Li, Mo, Ni, O, and C elements, which is consistent with the mapping test. Fig. 4d shows the full spectrum of Li₂Ni_0.05Mo_0.95O₄, and Fig. 4e shows its high-resolution C 1s spectrum with three deconvolution peaks at 284.8, 286.15, and 287.65 eV. Fig. 4f shows the Mo 3d spectra of Li₂Ni_0.05Mo_0.95O₄. The Mo 3d binding energies of Li₂Ni_0.05Mo_0.95O₄ and LMO are around 232.1 eV and 235.3 eV, respectively, which are consistent with the binding energy (BE) of pure Li₂MoO₄.⁴⁰

Fig. 4 (a) Full-scan, and (b) high-resolution C 1s and (c) Mo 3d of Li₂MoO₄, (d) full-scan, and (e) high-resolution C 1s and (f) Mo 3d of Li₂Ni_0.05Mo_0.95O₄, (g) Li₂Ni_0.05Mo_0.95O₄ high-resolution Ni 2p in situ XPS, (h) Li₂Ni_0.05Mo_0.95O₄ high-resolution Mo 3d in situ XPS.

Fig. 4g shows the in situ Ni 2p spectra with peaks at Ni 2p_1/2 (872.88 eV) and Ni 2p_3/2 (855.18 eV) before light illumination, and Ni 2p_1/2 (855.45 eV) and Ni 2p_3/2 (871.35 eV) after light illumination.⁴¹Fig. 4h shows the in situ Mo 3d spectra with peaks of Mo 3d_3/2 (235.78 eV) and Mo 3d_5/2 (232.68 eV) before light illumination, and Mo 3d_3/2 (234.48 eV) and Mo 3d_5/2 (231.28 eV) after light illumination. The binding energy peaks of both Mo 3d and Ni 2p ions are shifted slightly to smaller BE positions after illumination. This is because during the reduction reaction, the discharge process of Li₂Ni_0.05Mo_0.95O₄, some of both Mo and Ni ions are reduced, leading to reduced binding energies, and the chemical valence states are lowered. As a result, more lithium ions undergo charge-site complementation, which is conducive to the embedding of more lithium ions and improves the specific capacity of Li₂Ni_0.05Mo_0.95O₄. This means that some Mo⁶⁺ and Ni²⁺ ions in Li₂Ni_0.05Mo_0.95O₄ are reduced to Mo⁴⁺ and Ni⁰⁺, resulting in a new charge equilibrium. With the increase of Ni²⁺ content, some Ni²⁺ ions can move to the Li⁺ sites, resulting in the formation of a NiO-like protective layer, which leads to the formation of a small amount of NiO protective layer and facilitates the Li⁺ transfer. The NiO-like transition phase acts as a column support layer and helps to inhibit the formation of electrolyte–cathode interface side-effect (SEI) membrane structures, while Mo ions migrate from the transition metal plate to the Li plate, forming a cationic mixed structure near the surface associated with the MoO₃ fraction.⁴² Overall, the above XPS and mapping results further confirm that nickel has been doped into the chemical structure of LMO successfully.

Electrochemical properties

Li₂MoO₄ and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) anodes were studied by using cyclic voltammetry (CV). Fig. 5 shows the initial cyclic voltammetry (CV) curves of Li₂MoO₄ and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) materials. The voltage range is 0.01–3.0 V and the scan rate is 0.1 mV s⁻¹. For Li₂MoO₄ and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) electrodes, there are two reversible redox peaks observed at 1.2 and 1.6 V. The similarity of these redox peaks indicates that the changed materials possess better structural stability. In the cathodic scanning curves, the reduction of metal ions (Li¹⁺ → Li⁰, Ni²⁺ → Ni⁰⁺, and Mo⁶⁺ → Mo⁰⁺) is assigned to the alloying process of nickel, and in the oxygen scanning curves, the oxidation peaks can be attributed to the de-alloying process of the metal (Li⁰ → Li¹⁺, Ni⁰⁺ → Ni²⁺, and Mo⁰⁺ → Mo⁶⁺). The lithium storage mechanism of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) materials can be elaborated as follows. Accompanied by the decomposition and reduction of Li₂Ni_xMo_1−xO₄, the metallic elements Li, Ni, and Mo are formed. Simultaneously, nickel undergoes a chemical reaction with lithium ions, resulting in the formation of an alloy. Additionally, the metallic components of molybdenum and nickel experience additional oxidation, leading to the creation of Li₂O, NiO, and MoO₃, respectively. The lithiation delithiation process of Li₂MoO₄ can be summarized as follows.^43,44

Li₂MoO₄ + xLi⁺ + xe⁻ → Li_2+xMoO₄ (0 < x < 1) (1)

Li_2+xMoO₄ + (6 − x)Li⁺ + (6 − x)e⁻ → 4Li₂O + Mo (2)

Mo + zLi₂O ⇌ Li_yMo^4.3+O_z + 4.3Li⁺ + 4.3e⁻ (3)

Fig. 5 The initial CV curves of LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) at a scan rate of 0.1 mV s⁻¹ between 0.01 and 3.0 V.

The rate performance is an important factor affecting the practical application of batteries. Fig. 6a shows the performance plots of LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, and 0.1) at 0.01–3.0 V, 60 cycles, and different current densities of 100 mA g⁻¹, 200 mA g⁻¹, 300 mA g⁻¹, 500 mA g⁻¹, 1 A g⁻¹, and 100 mA g⁻¹. At an initial current density of 100 mA g⁻¹, the specific capacities of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) in the first discharge are 764.9, 1199.6 and 718.1 mA h g⁻¹, which are higher than that of Li₂MoO₄ (663.2 mA h g⁻¹). When the current density returns to 100 mA h g⁻¹, the release limits of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) cathodes are reestablished to 484.5, 686.6, and 566.8 mA h g⁻¹, which are higher than that of Li₂MoO₄ (365 mA h g⁻¹). The retention rates of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) reach 69.73%, 74.32%, and 79.9%, respectively, which are higher than the 60.68% of Li₂MoO₄. The fast lowering in the particular limit of all examples is mainly due to the arrangement of the SEI film and the irreversible underlying change of the cathode material. Nevertheless, the samples exhibit a gradual increase in specific capacity as the current density declined. This can be attributed to the formation of a gel-like film on the surface of the conductor during activation. This film aids in the storage of metallic elements within the interface. Ultimately, the precise volume of all samples remains unchanged after a certain increase. Indeed, this is a prevalent occurrence observed in transition metal oxide materials.

Fig. 6 (a) Rate performances of the samples obtained at different current densities, (b) cycling performances of the prepared samples at 0.1 A g⁻¹ current density; constant current charge/discharge curves for the (c) 1st cycle and (d) 10th cycle, and (e) specific capacity plot of Li₂Ni_0.05Mo_0.95O₄versus previously reported molybdenum-based and nickel-doped materials for multiplicative performance at different current densities.

Fig. 6b illustrates the continuous cycling process at a rate of 100 mA g⁻¹, with charging and discharging occurring within a voltage range of 0.01 to 3.0 volts. It is obvious that the Li₂Ni_0.05Mo_0.95O₄ electrode shows better cycling performance. The discharge capacities of Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) electrodes are much higher than that of Li₂MoO₄ (169.2 mA h g⁻¹) over lengthy cycling. Specifically, the discharge capacities are 176.7, 482.7, and 324.9 mA h g⁻¹ for Li₂Ni_xMo_1−xO₄ with the x values of 0.03, 0.05, and 0.1, respectively. In particular, Li₂Ni_0.05Mo_0.95O₄ exhibits a higher multiplicative capacity compared with the LMO electrode due to the small amount of nickel doping only producing some nanoscale NiO structures within the surface, which can enhance the ionic conductivity and facilitate the migration of lithium ions into and out of the electrode. Excessive nickel significantly enhances the level of ion mixture, leading to impeded migration of Li⁺ ions and degradation of chemical performance.

Fig. 6c and d show the discharge–charge curves of LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) after the 1st and 10th cycles at a current density of 100 mA g⁻¹. The first discharge specific capacity and charge specific capacity of LMO, Li₂Ni_0.03Mo_0.97O₄, Li₂Ni_0.05Mo_0.95O₄, and Li₂Ni_0.1Mo_0.9O₄ are 673.4/1148.3, 642/1015.8, 891.4/1347, and 713.6/1045.5 mA h g⁻¹, respectively. The tenth cycle discharge specific capacity and charge specific capacity of LMO and Li₂Ni_xMo_1–xO₄ (x = 0.03, 0.05, 0.1) are 470/486.3, 683.3/694.7, 844.2/851.6, and 734.1/743.7 mA h g⁻¹, respectively. In the first charge/discharge curves, one can calculate that their first cycle coulombic efficiencies respectively are 58.64%, 63.20%, 66.15%, and 68.25%. Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) materials exhibit higher capacity than LMO, suggesting that nickel doping and the good crystal structure stability synergistically improve the electrochemical performance of LMO, which can accommodate lithium ions during discharge and release lithium ions during charging for repetitive high cycling.

Fig. 6e compares the specific capacity of Li₂Ni_0.05Mo_0.95O₄ with previously reported molybdenum-based^{27,37,45–48} and nickel-doped^49,50 materials in terms of multiplicative performance. At identical current densities, with increasing current density, in the range of 5–10 cycles, the specific capacity of Li₂Ni_0.05Mo_0.95O₄ is higher than that of other Mo-based or Ni-doped battery anode materials, despite a slight decrease in specific capacity. The higher specific capacity of Li₂Ni_0.05Mo_0.95O₄ indicates that the addition of nickel significantly enhances the capacity of LMO. Table 2 summarizes the detailed comparison of specific properties of the as-prepared Li₂Ni_0.05Mo_0.95O₄ with previous materials.

Table 2 Comparison of the multiplicative properties of Li₂Ni_0.05Mo_0.95O₄ with those of previously reported Mo-based and Ni-doped materials

Materials		Current density (A g^−1)	Specific volume (mA h g^−1)	Cycle number	References
Mo-based	Li2Ni0.05Mo0.95O4	200	749.2	20	This work
	Li2Ni0.05Mo0.95O4	500	585	40	This work
	α-ZnMoO4	200	400	20	41
	Cu@MoO2@C	200	700	30	42
	LiMoO2@C	200	580	30	43
	NiMoO4	500	493	30	25
	Na2MoO4–41.4%-TiO2	200	201	20	44
	ZnMoO4	200	230	10	33
Ni-doped	Ni-LVO	200	598	15	45
	TiO2–Ni-400	500	200	30	46

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The charge transfer resistance of the LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) materials was further investigated using the EIS technique. As shown in Fig. 7, all impedance curves show compression semicircles from high to medium, and the fitting results are shown in Table 3. Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) has different semicircle diameters for different x values. The smaller the R_ct value, the faster the ion transfer kinetics and the better the electrochemical performance. Obviously, the introduction of Ni²⁺ affects the conductivity of LMO. The Li₂Ni_0.05Mo_0.95O₄ sample has the smallest R_ct value, which is attributed to the appropriate amount of Ni²⁺ dopant enhancing the electronic conductivity and ionic diffusion coefficient. The EIS fitting results are also in line with the characterization results, demonstrating the advantages of nickel doping in terms of fast lithium transfer kinetics. The lithium-ion diffusion coefficients (D_Li⁺) in Fig. 7a and b were calculated based on the following two equations.^51,52

Z_re = R_s + R_ct + σω^−1/2 (4)

D_Li⁺ = 0.5R²T²/A²n⁴F⁴C²σ² (5)

Fig. 7 (a) Nyquist plots for LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples (inset: equivalent circuit used for curve fitting), and (b) the relationship between Z′ and ω^−1/2 for LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples.

Table 3 Calculated resistance of prepared LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) samples

Sample	R S	R ct	δ (Ω cm^2 s^−1/2)	D Li^+/cm^2 s^−1
Li2MoO4	11.857	2.4124	10.133	1.259 × 10^−14
Li2MoO4-0.03Ni	3.7419	1.2640	8.296	1.878 × 10^−14
Li2MoO4-0.05Ni	1.4143	0.4308	6.362	3.194 × 10^−14
Li2MoO4-0.1Ni	3.1288	1.5606	7.154	2.526 × 10^−14

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R represents the gas constant of 8.314 J mol⁻¹ K⁻¹. T represents the absolute temperature of 298.15 K. A represents the surface area of the cathode electrode (0.785 cm⁻²). n represents the quantity of electrons exchanged in the redox pair during the half-reaction, whereas F denotes the Faraday constant of 96 485 C mol⁻¹. The concentration of lithium ions in the solid is 5.86 × 10⁻³ mol cm⁻³, denoted as C. The fitting results are shown in Table 3. The R_s and R_ct values of LMO and Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) are close, and the higher R_ct of LMO may be attributed to its larger primary grain size. According to the derivation of eqn (4) and (5), the lithium ion diffusion coefficient of Li₂Ni_0.05Mo_0.95O₄ is 3.194 × 10⁻¹⁴ cm² s⁻¹, which is about 2.5 times higher than the 1.259 × 10⁻¹⁴ cm² s⁻¹ of LMO due to the lower Li/Ni exchange ratio in the lattice structure. This may be one of the reasons for the better electrochemical performance of Li₂Ni_0.05Mo_0.95O₄ in comparison to LMO. The improved electrochemical performance of the macroporous cathode can be attributed to its higher Li-ion diffusion coefficient and its shorter Li-ion transfer channel. The results align with the outcomes of the charging and discharging processes.

Conclusion

In this work, we have proposed a Ni-doping sol–gel strategy to synthesize Li₂Ni_xMo_1−xO₄ (x = 0.03, 0.05, 0.1) as anode materials for LIBs. Compared with other materials, Li₂Ni_0.05Mo_0.95O₄ exhibits superior electrochemical properties, such as a higher reversible charge/discharge specific capacity and excellent cycling stability compared to LMO. It also features a stable lithium storage capacity of 487.2 mA h g⁻¹, with an over 200% increase compared with LMO after 100 charge/discharge cycles at a current density of 100 mA g⁻¹. Obviously, the sol–gel method enhances the electrochemical properties of molybdates, and the metal molybdates are expected to be used as commercially viable anode materials for lithium batteries.

Data availability

All the data supporting this article have already been included in this article and no new data were generated or analysed as part of this article.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52171212, 51972151) and the Australian Research Council (Grant No. DP240102628, DP240102728).

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