Introduction

Lithium-ion batteries (LIBs) have been widely used as renewable energy storage devices for electronic products and plug-in hybrid and pure electric vehicles due to their high-power density and stable cycling performance [1,2,3]. The layered α-LiFeO2 is considered one of the potential candidates as cathode material for LIBs due to its high theoretical capacity (282 mAh/g), environmental friendliness, and low cost [4, 5]. However, there are still some challenges remained for α-LiFeO2 cathodes, such as inherent sluggish kinetic and poor electronic conductivity, which severely disrupts the high-rate and cycling performance of batteries [6].

Generally, the nanostructured morphology such as nanowires, nanoparticles, and nanorods can enhance their material electrochemical properties [7,8,9]. Among different morphologies, one-dimensional (1D) nanorods have attracted numerous attentions due to their 1D transport pathway which enables good electron transport and fast lithium-ion insertion/removal [10]. However, there are still huge challenges to directly prepare α-LiFeO2 nanorods via conventional strategies where cubic crystals are usually obtained [11]. According to previous reports, α-FeOOH nanorods have been used as a self-template to prepare 1D iron-based materials, such as Fe2O3 and Fe3O4 [12, 13]. Inspired by these, in this work, we firstly synthesize α-FeOOH nanorods by hydrothermal method; then, by solid-state reaction method, α-LiFeO2 nanorods are obtained (Scheme S1). The excellent electrochemical performance of the as-prepared α-LiFeO2 nanorods is evaluated in LIBs, and the structure-property relationship is explored based on physical characterization results.

Experimental

Material synthesis

α-FeOOH nanorods were synthesized through a hydrothermal process. Firstly, 2.59 g Fe(NO)3·9H2O and 1.01 g KOH were dissolved in 80 ml distilled water, and 1 ml of H2O2 was added to remove the Fe2+ in the solution. After thoroughly stirred for 1 h, the solution was transferred into a 100-ml Teflon-lined stainless steel autoclave and left at 100 °C for 6 h. α-LiFeO2 nanorods were prepared via a low-temperature solid-state reaction method from the as-prepared α-FeOOH rods. Typically, the as-prepared α-FeOOH rods were mixed with LiOH·H2O and LiNO3 (Li+/Fe3+ = 4) by grinding and then dried at room temperature. After that, the powder was sintered at 400 °C for 6 h in muffle to obtain α-LiFeO2 nanorods.

Material characterization

XRD analysis, material morphologies, and electronic state of each element of the sample were characterized by X-ray diffraction (XRD, Bruker DX-1000, CuKα radiation), scanning electron microscopy (FESEM, JEOL, JSM-6360LV), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-3010), and X-ray photoelectron spectroscope (XPS, ESCALAB 250XI), respectively.

Electrochemical measurements

For battery test, the working electrode was prepared from 85% as-prepared α-LiFeO2 rods, 10% acetylene black, and 5% polyvinylidene fluoride. The lithium metal was used as counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and ethylmethyl carbonate (EC:DMC:EMC, 1:1:1 by volume). The batteries were assembled in an argon-filled glove box and tested on Land cell Systems (LAND CT2001A) in the voltage range of 1.5–4.8 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis were conducted with a CS-350 electrochemical workstation.

Results and discussion

XRD pattern of α-FeOOH nanorods is shown in Fig. S1. All peaks are indexed to α-FeOOH (Space group pbnm, JCPDS No. 29-0713) [14]. XRD pattern of α-LiFeO2 nanorods (Fig. 1a) shows all the diffraction peaks (111), (200), (220), (311), and (222) are well regarded to the layered structure of α-LiFeO2 (JCPDS No. 74-2284) with lattice constants a = 4.158 [14, 15]. No diffraction peak from α-FeOOH is observed, indicating the α-FeOOH nanorods are fully converted into α-LiFeO2 in the low-temperature solid-state reaction.

Fig. 1
figure 1

a XRD pattern of the as-prepared α-LiFeO2.b SEM image of α-FeOOH. c SEM, d TEM, and e HRTEM images with corresponding f SAED pattern of the as-prepared α-LiFeO2

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The SEM image (Fig. 1b) shows the width and length of the α-FeOOH nanorods are approximately 20 nm and 0.75 μm, respectively. Figure 1c shows the morphology of the obtained α-LiFeO2, which is mainly composed of nanorods, suggesting that the rod-like morphology of the α-FeOOH nanorods is remained after the solid-state reaction.

TEM image (Fig. 1d) further confirms the diameter of α-LiFeO2 nanorods is around 20 nm. The HRTEM image in Fig. 1e shows the lattice fringe spacing of 0.24 nm is consistent with the (111) crystal planes of α-LiFeO2. The corresponding selected area electron diffraction (SAED) pattern presented in Fig. 1e shows the diffraction rings from inside to outside as (111), (200), and (220), which are in good agreement with the XRD results.

The XPS survey of the α-LiFeO2 nanorods (Fig. 2a) confirms that the sample comprises Li, Fe, and O. The spectrum of Fe2p is depicted in Fig. 2b. The Fe2p spectrum exhibits two peaks at 711.15 eV and 724.89 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 spin orbit peaks of α-LiFeO2, respectively. Besides, both Fe 2p3/2 and Fe 2p1/2 main peaks show satellite peaks on their higher binding energy sides, thus confirming the Fe (III) oxidation state [16].

Fig. 2
figure 2

a XPS survey and b Fe2p spectrum of α-LiFeO2 nanorods

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CV, EIS, and charge-discharge curves of the α-LiFeO2 nanorods are depicted in Fig. 3 together with its cycling performance. Figure 3a shows CV curves in the voltage range of 1.5–4.8 V. The oxidation peaks appeared at 2.74 and 4.75 V are attributed to the charge process, and cathodic peaks at 1.65 V are associated with lithium insertion in the α-LiFeO2 nanorods [17]. Figure 3b displays Nyquist plots of the α-LiFeO2 electrode before and after 50 cycles of cycling at 0.1 C. Both figures contain a semicircle and a diagonal line representing the charge transfer resistance (Rct) and the Warburg impedance (ωo), respectively [18, 19].

Fig. 3
figure 3

a CV curves of the first three cycles, b Nyquist curves (the inset shows the equivalent circuit), c liner fitting curves of Ζ′ vs ω−1/2, and d cycling performance of α-LiFeO2 nanorods at 0.1, 0.2, and 0.5 C. e Specific charge-discharge curves at 0.1 C of α-LiFeO2 nanorods

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Table 1 lists the fitting value of charge transfer resistance (Rct) of α-LiFeO2 nanorods in batteries. The charge transfer resistance is 151.5 and 183 Ω before and after 50 cycles, respectively. The small potential difference (ΔE = 1.095 V) and impedance change value (ΔRct = 31.5 Ω) indicate that the rod-like α-LiFeO2 has a good electronic conductivity [20,21,22]. The lithium-ion diffusion coefficient can be calculated using following equations [23]

$$ {D}_{{\mathrm{Li}}^{+}}=\frac{R^2{T}^2}{2{A}^2{n}^4{F}^4{C}^2{\sigma}^2} $$
(1)

where DLi+ is the lithium-ion diffusion coefficient, A is surface area of the electrode, R is gas constant, n is number of electrons, T is absolute temperature, F is the Faraday constant, C is the Li-ion concentration in the electrode, and σ is the Warburg factor shown in Fig. 3c. The DLi+ value of the α-LiFeO2 electrode is also listed in Table 1. The decrease of DLi+ value after the 50 cycles is small, suggesting that one-dimensional ion transport channel provided by the nanorod structure improves the Li-ion diffusion rate and enables a better rate capability of α-LiFeO2.

Table 1 Fitting values of charge transfer resistance (Rct) and lithium-ion diffusion coefficient of the α-LiFeO2 electrode
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Figure 3 d and e show the cycling performance of α-LiFeO2 nanorod electrodes at 0.1, 0.2, and 0.5 C. The cycling performance of α-LiFeO2 is tested at 0.1, 0.2, and 0.5 C within the voltage range of 1.5–4.8 V (Fig. 3d). The initial discharge capacities of α-LiFeO2 are 165.85, 150.84, and 147.87 mAh/g, and these drop to 135.42, 105.23, and 100.88 mAh/g after 50 cycles, respectively. Compared with α-LiFeO2 bulk samples reported in literature [17, 24], the α-LiFeO2 nanorod electrode here delivers a better cycling performance at 0.2 and 0.5 C, which can be ascribed to the nanorod morphology which is beneficial for electron and lithium-ion transportation [24, 25]. The charge/discharge profiles (Fig. 3e) remain nearly the same after 50 cycles, suggesting a very small polarization during cycling process.

Conclusion

We successfully prepared α-LiFeO2 nanorods via a novel hydrothermal-assisted solid-state method by using α-FeOOH nanorods as the self-sacrifice template. The α-LiFeO2 nanorod can provide one-dimensional path facilitating the transportation of lithium-ions. Moreover, the good electron transport property of one-dimensional nanorods reduces the charge transfer impedance and thus also improves the electrode cycling performance. As a result, the α-LiFeO2 nanorod electrode presents a stable specific capacity of 139.8 mAh/g−1 at 0.1 C after 50 cycles, indicating the enhancement effect of the nanorod morphology for electrochemical applications.