Trace Fe³⁺ mediated synthesis of LiFePO₄ micro/nanostructures towards improved electrochemical performance for lithium-ion batteries

Abstract

Various LiFePO₄ micro/nanostructures have been solvothermally synthesized using FeSO₄ and ethylene glycol (EG) as the reactant and reaction medium, respectively. The LiFePO₄ micro/nanostructures including nanoflakes, stacked microsheets, micro-dumbbells and micro-spindles have been selectively fabricated and tuned via adjusting trace Fe³⁺ obtained from oxidation of the reactant. The content of mediated-Fe³⁺ in the EG system plays an important role in the formation of the micro/nanostructures as well as the change in the pH value. In this work, the content of mediated-Fe³⁺ in the precursor solution as a problem worthy of attention was put forward and the evolution process of the LiFePO₄ micro/nanostructures has been extensively studied. Among these micro/nanostructures of LiFePO₄, the LiFePO₄ micro-dumbbells as a cathode material for lithium-ion batteries showed the most excellent electrochemical performance with a discharge capacity of 117 mA h g⁻¹ at a high rate of 10C (1C = 169 mA h g⁻¹), which demonstrates that the exposed crystal plane and morphology of LiFePO₄ play critical roles in the electrochemical performance. This work not only provides deeper knowledge into the formation mechanism of LiFePO₄ microstructures, but also paves a facile way to prepare scalable LiFePO₄ with a high rate performance and high tap density.

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1. Introduction

LiFePO₄ (LFP) has attracted much attention as a cathode material as it is less toxic, low cost, environmentally friendly and has superior safety and high stability.^1–6 However, the inferior high rate capability of LiFePO₄ material has drawbacks for its applications due to its low electronic conductivity and poor Li-ion diffusion. Compared to electronic conductivity, ionic conductivity or lithium-ion diffusion plays a major role in determining electrochemical reaction kinetics.⁴ Reduction of the particle size is an effective way to enhance ionic conductivity due to its role in shortening the Li⁺ migration pathway.⁷ However, it is not fully applicable to LFP. Considering Li-ions preferably move along the [010] direction in olive-structured LiFePO₄,⁸ LiFePO₄ nanoplates with preferential growth of the (010) lattice plane not only reduce the Li-ion transport length, but also enlarge the (010) surface, resulting in fast Li-ion diffusion. Obviously, nanoscale materials, especially with a plate structure, have attracted extensive interest because of their unique orientation and outstanding electrochemical performance.^9,10 Nevertheless, compared with micro-structured LiFePO₄, nanoscale LiFePO₄ materials tend to agglomerate irregularly due to their high interfacial energy, which severely affects the voltage plateau and cycling stability.^11,12 Furthermore, nanoscale LiFePO₄ materials with a higher surface area manifold the adverse electrode/electrolyte reactions, resulting in worse cycling performance.¹³ In addition, increasing the surface area will lead to the need for larger mass fractions of carbon to coat the particles, thereby resulting in a decrease in tap density.¹⁴ Low tap density seriously impacts the volumetric energy density of nanoscale LiFePO₄ materials.^15–17 Generally, the tap density for micro-sized LiFePO₄ is more than 1.0 g cm⁻³ compared with that of nanoscale LiFePO₄ materials (less than 1.0 g cm⁻³).¹⁶ Therefore, the drawbacks of nanoscale materials lead scientists towards special morphological LiFePO₄ with micro/nanostructures owing to the urgent requirements for energy and power demands. Recently, LiFePO₄ micro/nanostructures such as micro-spheres,¹⁸ micro-cages,¹³ hierarchical^16,19 and dumbbell-like structures^19–21 have already been synthesized. For example, LiFePO₄ with hierarchical microstructures was synthesized using Fe³⁺ salt as the starting material and poly(vinyl pyrrolidone) (PVP) as the surfactant in a benzyl alcohol system.¹⁹ A similar LiFePO₄ structure was synthesized using sodium dodecyl benzene sulphonic acid (SDBS) as the soft template in a mixed solvent (containing water) for 1 h of intensive pre-stirring to control particle morphology.²⁰ LiFePO₄ microspheres with an open three-dimensional (3D) porous microstructure were reported through a solvothermal approach using Fe³⁺ salt as the starting material.²¹ It has been found that most of the micro/nanostructures of LiFePO₄ are synthesized from Fe³⁺ contained in the starting reactant. However, these methods or techniques involve the use of excess reductant or surfactants.

Herein, a template-free solvothermal process has been developed to fabricate dumbbell-shaped LiFePO₄ micro/nanostructures using FeSO₄ as starting materials and ethylene glycol (EG) as solvents. In the present study, trace Fe³⁺ obtained from the oxidation of FeSO₄·7H₂O in air plays a crucial role in the formation of LiFePO₄ with different micro/nanostructures. The growth mechanism of micro/nanostructures of LiFePO₄ has been extensively studied.

2. Experimental

2.1 Reagents

Iron(II) sulfate heptahydrate (FeSO₄·7H₂O, >99 wt%), lithium hydroxide monohydrate (LiOH·H₂O, >95 wt%), ethylene glycol (EG, >99.8 wt%), phosphoric acid (H₃PO₄, >85 wt%) and ethanol were purchased from Shanghai Chemical Ltd. All chemicals are of analytical grade and were used without further purification.

2.1.1 Synthesis of LiFePO₄ micro/nanostructures. LiFePO₄ micro/nanostructures with different morphologies were prepared by a facile solvothermal process. In a typical protocol for micro-dumbbell LiFePO₄ micro/nanostructures, 5 mmol FeSO₄·7H₂O was added into 20 ml ethylene glycol (EG) to form a clear solution by supersonic treatment for 0.5 h. The solution of FeSO₄ in ethylene glycol was aged with magnetic stirring for 15 h in air. During the process, the color of the solution turned from light green to orange. Then, 6 mmol H₃PO₄ was added dropwise into the previous solution. Subsequently, a 20 ml LiOH (13 mmol) EG solution was added into the mixture solution, quickly resulting in the formation of a dark green suspension. After stirring, the mixtures were transferred into a 60 ml stainless steel autoclave, sealed and kept at 180 °C for 10 h. The obtained precipitate was collected by centrifugation, washed by water and ethanol several times, then dried in a vacuum at 60 °C for 10 h. Nanoflakes, stacked microsheets and micro-spindles of LiFePO₄ were prepared via the same procedure except that the FeSO₄–ethylene glycol (EG) solution was aged at room temperature for 0 h, 10 h and 24 h, respectively. Nanoflakes, stacked microsheets, micro-dumbbells and micro-spindles are denoted as LFP-1, LFP-2, LFP-3 and LFP-4, respectively. In order to enhance the electrochemical performance, a LiFePO₄ powder was heated at 550 °C for 10 hours in a tubular furnace with C₂H₂–Ar (10 : 90, volume ratio) atmosphere. The sintered LiFePO₄ products obtained from the four samples (LFP-1, LFP-2, LFP-3, and LFP-4) are denoted as LFP-1A, LFP-2A, LFP-3A, and LFP-4A respectively. Moreover, a control experiment was carried out to emphasize the role of the content of mediated-Fe³⁺. The method was basically the same except for the atmosphere of the pre-stirred solution. The control experiment in the FeSO₄–EG solution was stirred for 24 h under inert conditions, then the aforementioned steps were repeated. The obtained precipitate was collected and is denoted LFP-5.

2.1.2 Characterizations. The LiFePO₄ samples were characterized using XRD (Philips X′ Pert Super diffractometer, Cu Kα, λ = 1.54178 Å), scanning electron microscopy (SEM, JSM-6700F), transmission electron microscopy (TEM, H7650, HRTEM, JEOL-2010), and Raman spectroscopy (Lab-RAM HR UV/Vis/NIR) with Raman shifts from 400 to 2000 cm⁻¹. UV-Vis absorption spectra were recorded with a Shimadzu UV-2450 UV-Vis spectrometer. The weight percentage of carbon was characterized by elemental analysis (EA, Elementarvario EL cube, Thermal Conductivity Detector) in a pure oxygen atmosphere. X-Ray photoelectron spectroscopy (XPS) measurements were recorded on a GESCALAB KII-ray photoelectron spectrometer.

2.1.3 Electrochemical measurements. Electrochemical experiments were tested using CR2016 coin cells. The electrodes were prepared by mixing the obtained materials, super P and PVDF binder dispersed in N-methylpyrrolidone (NMP), at a weight ratio of 70 : 20 : 10. Firstly, the active material and super P were mixed thoroughly, then the binder was added and the mass was mixed again. The obtained slurry was coated on Al foil and dried at 110 °C for 10 h in a vacuum. The loading of active materials was 1.5–2.0 mg cm⁻². Li metal acted as a counter electrode. The electrolyte used for testing was 1 M LiPF₆ in 1 : 1 EC/DEC and the separator was Celgard 2400. The cells were assembled in an argon-filled glove with the density of water and O₂ all below 1 ppm and then aged for 12 h before testing to guarantee full access of the electrolyte with the electrode. Galvanostatic discharge–charge experiments were conducted on a battery-testing system (Land-CT2001A) in the potential range of 2–4.3 V (versus Li/Li⁺) at designated C-rates at room temperature (1C = 169 mA g⁻¹). Cyclic voltammetry (CV) was performed using a CHI660D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (AUT302N, Metrohm, Switzerland) with an AC amplitude of 5 mV in the frequency range of 10 kHz to 1 Hz.

3. Results and discussion

3.1 Characterization of LiFePO₄ micro/nanostructures

Typical XRD patterns for the as-obtained product synthesized by a solvothermal route are presented in Fig. 1. All the peaks of Fig. 1 can be indexed as the orthorhombic phase of LiFePO₄ with an ordered olivine structure and a Pnma space group (JCPDS Card no. 81-1173). As shown in Fig. 1, the peak area intensity ratios of I₍₀₂₀₎/I₍₂₀₀₎ for different samples are 4, 5.08, 3.72, and 1.90, respectively. In particular, it is clearly observed that the peak intensity of (200) increases dramatically with a prolonged pre-stirring time of 24 h, demonstrating the preferential growth of LiFePO₄ along different crystal planes. To further confirm the sample structures, the as-prepared samples have been investigated by scanning electron microscopy (SEM) and transmission electron microscopy. Fig. 2(a and b) shows that the obtained LiFePO₄ sample (LFP-1) consists of uniform nanoflakes with an average size of 150 nm when the FeSO₄ solution was aged at room temperature for 0 h. In the present study, we found that the LiFePO₄ nanoflakes tend to obviously stack when the FeSO₄ solution was aged at room temperature. As shown in Fig. 2(c and d), LiFePO₄ stacked microsheets (LFP-2) that are 2 μm in width, 5 μm in length and 30 nm in thickness are achieved when the aged time is 10 h. When the stirring time was prolonged to 24 h, well-defined micro-spindles of LiFePO₄ (LFP-4) are observed, as shown in Fig. 2e. It is worth mentioning that the micro-spindles that are 2 μm in width and 5 μm in length can be seen and both ends of the micro-spindles are roughly structured. With the prolonged aging time of the FeSO₄ solution, a LiFePO₄ microparticle was assembled by nanostructures with different morphologies.

Fig. 1 XRD patterns of the samples: (a) LiFePO₄ nanoflakes (LFP-1), (b) LiFePO₄ stacked microsheets (LFP-2), (c) LiFePO₄ micro-dumbbells (LFP-3) and (d) LiFePO₄ micro-spindles (LFP-4).

Fig. 2 SEM images and corresponding magnified images of LiFePO₄ with variations in the aging time: (a and b) 0 h, (c and d) 10 h, and (e and f) 24 h.

On the basis of the XRD patterns and SEM images, high-resolution transmission electron microscopy (HRTEM) has been undertaken to confirm the exposed plane of LiFePO₄. Fig. 3 shows transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SEAD) patterns of the three LiFePO₄ samples. The HRTEM images derived from the enlarged image of the marked circular region and the corresponding SAED pattern reveal the crystal orientation of the samples. All samples except LFP-4 have predominantly (010) faces exposed, according to their SEAD patterns (inset). It is demonstrated that the large facet of the LFP-1 crystal corresponds to the ac-plane. The exposed (010) face is perpendicular to the (001) and (100) face. The lattice distances of the HRTEM images are 1.02 nm and 0.46 nm, respectively, which correspond to the (001) and (100) crystal plane, as shown in Fig. 3b. Similar results can be easily obtained from the LFP-2 sample. The HRTEM images reveal clear lattice fringes, which is in good accordance with the d-spacing obtained from the XRD patterns. The majority of the exposed (010) plane still remains from the SAED pattern of LFP-2 demonstrating that the LFP-2 sample agglomerated along the (001) and (100) planes to form a stacked microsheet morphology via interconnection of the (010) facets. The results are in line with previous reports, which demonstrated that a hierarchical structure consists of exposed nanosheet (010) faces.^21,22 The morphology of the as-prepared sample becomes micro-spindles when the aging time reached 24 h, as shown in Fig. 3(e and f). The related SAED pattern in Fig. 3f shows that the domain of the (200) facet is exposed at the end of the micro-spindles. The SAED pattern and HRTEM image of a side view of the micro-spindles are shown in Fig. S1 (in the ESI†). The LFP-4 product grows along the [010] direction and the (010) face is no longer the main exposed face in this structure. The intensity enhancement of the (200) peak of the XRD pattern is attributed to the (010) crystal plane piling together to form the (200) crystal plane.

Fig. 3 TEM images of the samples: (a) LFP-1, (c) LFP-2, and (e) LFP-4. Insets of (a, c and e): magnified TEM images of the corresponding samples. HRTEM images of the samples: (b) LFP-1, (d) LFP-2, and (f) LFP-4. Insets of (b, d and f): selected area electron diffraction patterns of the corresponding samples.

Various micro/nanostructures of LiFePO₄ can be selectively synthesized by tuning the aging time of the FeSO₄ solution. In particular, when the aging time was prolonged to 15 h, the dumbbell-shaped products that were 4 μm in width and 6 μm in length have been clearly observed, as shown in Fig. 4a. Compared with the micro/nanostructures of LiFePO₄ mentioned above, the micro-dumbbells of LiFePO₄ have a unique microstructure consisting of primary nanoflakes. The micro-dumbbells of LiFePO₄ are composed of many small nanoflakes (Fig. 4b), which produce several interstices by this disordered stacking. Disordered stacking generates interstices between primary nanoflakes, which is beneficial for electrolyte penetration and providing a greater interface area between the electrode material and the electrolyte.¹⁸ There is a general tendency that the large microsheets (LFP-2) become small pieces (LFP-3) due to a prolonged pre-stirring time, which is shown in the SEM images. The SAED pattern of the micro-dumbbells of LiFePO₄ proved that the (010) crystal plane could still be detected, implying these small pieces still maintain the same orientation.

Fig. 4 (a and b) SEM image and magnified image of LiFePO₄ with micro-dumbbell structures (aging time of 15 h). (c and d) TEM image and HTEM image of the LiFePO₄ micro-dumbbells (inset of (d): selected area electron diffraction pattern of the corresponding sample). (e) HADDF-STEM image and the elemental mapping images of the LiFePO₄ micro-dumbbell and (f) the corresponding EDX spectrum of the products (the inset shows the compositions of Fe and P).

Elemental mapping by energy dispersive X-ray spectroscopy (EDS) of the LiFePO₄ micro-dumbbells is shown in Fig. 4e. The images of the elements Fe and P closely match with the corresponding HADDF-STEM image (Fig. 4e) indicating a homogenous chemical composition. The energy dispersive X-ray (EDX) spectrum (Fig. 4f) reveals that the LiFePO₄ micro-dumbbells contain P, Fe and O elements. The atomic ratio of Fe/P in the LiFePO₄ micro-dumbbells is 23 : 26, approximately 1 : 1, which matches with the chemical composition of LiFePO₄.

3.2 Growth mechanism of the LiFePO₄ micro/nanostructures

As is well known, Fe²⁺ will gradually be oxidized to Fe³⁺ in solution, which generally results in a pH decrease. From Table 1, we can see that the pH value for the FeSO₄ solution can be adjusted, ranging from 3.45 to 2.41, when the aging time at room temperature is varied (0–24 h). The pH value of the solution will be different when the FeSO₄ solution is aged at room temperature for various times. In addition, with a prolonged aging time, the pH value of the aged FeSO₄ solution decreases gradually. All of this can be illustrated by the following reactions:

4Fe²⁺ + O₂ + 4H⁺ ⇔ 4Fe³⁺ + 2H₂O (1)

Fe³⁺ + 3OH⁻ ⇔ 4Fe(OH)₃ (2)

Table 1 The pH of the solution changes during the pre-stirring process

	pH value (aged FeSO4 solution)	pH value (solution before the solvothermal reaction)
LFP-1	3.45	5.83
LFP-2	3.01	5.25
LFP-3	2.87	4.9
LFP-4	2.41	4.3

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In order to determine the concentration of Fe³⁺ formed in the initial stage, ultraviolet spectrophotometry analysis has been carried out and shown in Fig. 5a. The concentration of ascorbic acid can be determined by the absorbance peak located at 265 nm in neutral media. The concentration of Fe³⁺ can be calculated according to the oxidation reaction of ascorbic acid with Fe³⁺. The absorption intensity for the peak located at 265 nm is inversely proportional to the concentration of Fe³⁺. The calculated ion mole ratios of Fe³⁺/Fe_total are 1.39%, 1.85%, 2.32% and 3.28% when the solution was aged at room temperature for 0 h, 10 h, 15 h, and 24 h, respectively.

Fig. 5 (a) UV-Vis absorption of the FeSO₄ solution aged at room temperature for different lengths of time with the same amount of ascorbic acid. (b) Fe2p spectra of the four LiFePO₄ samples (LFP-1, LFP-2, LFP-3, and LFP-4).

Moreover, X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemical state of the elements in the samples. The high-resolution Fe2p spectra for the four LiFePO₄ samples are presented in Fig. 5b. The binding energies of 709 and 723 eV can be ascribed to Fe2p3/2 and Fe2p1/2, respectively.

In addition, a satellite peak (715 eV) provided good evidence for the oxidation states of Fe²⁺ ions in the LiFePO₄ samples.²³ The results of XPS further confirmed the purity of the LiFePO₄ samples, indicating no existence of pre-formed Fe³⁺ from the first stage in the final product. In the current protocol, pre-formed Fe³⁺ was probably transferred to Fe²⁺ by EG.^13,18,24 In order to confirm that the content of mediated-Fe³⁺ in the EG system is a unique factor which effects micro/nanostructured LiFePO₄, rather than another function of EG, an experimental control was employed where the FeSO₄–EG solution was stirred under inert conditions. The LFP-5 sample showed similar morphology compared with LFP-1, which supported the role of the content of mediated-Fe³⁺ in the formation of micro/nanostructures. Related equipment and SEM images are shown in Fig. S2 (in the ESI†).

In the ethylene glycol solution, LiFePO₄ nanoflakes with a main exposed (100) face were gradually transferred to spindle plates with a predominantly exposed (010) face as the pH value of solution increased from 2.56 to 5.80.²⁵ A previous study revealed that the pH value of the reaction solution plays a crucial role in the formation of LiFePO₄ nanostructures with different crystal orientations.² Based on the above analysis, pre-formed trace Fe³⁺ plays a crucial role in the formation of LiFePO₄ micro/nanostructures. In addition, the pH value of the solution was mediated via oxidation of Fe²⁺ and reduction of Fe³⁺. The formation of LiFePO₄ micro/nanostructures is sensitive to the pH value of the reaction system.²⁶ In this work, the formation of LiFePO₄ with different micro/nanostructures can be realized by adjusting the amount of Fe³⁺.

3.3 Carbon coating and electrochemical measurements

In order to enhance the electrochemical performance, all the samples (LFP-1, LFP-2, LFP-3, and LFP-4) were heated at 550 °C for 10 h in a tubular furnace with C₂H₂–Ar (10 : 90, volume ratio) atmosphere. The XRD patterns of LiFePO₄ with further carbon coating are shown in Fig. S3 (in the ESI†). The high magnification SEM images of the carbon coated LiFePO₄ samples are shown in Fig. S4 (in the ESI†). As shown in Fig. S4,† LiFePO₄ micro/nanostructures are also uniform, even though further calcination also maintains the original shape. Fig. 6(a–d) shows Raman spectra of the four samples (LFP-1A, LFP-2A, LFP-3A, and LFP-4A). The two pronounced peaks at 1349 and 1598 cm⁻¹ correspond to the D and G bands of carbon. The D-band peak at 1349 cm⁻¹ is ascribed to disordered carbon, edge defects, vacancies defects and topological defects, while the G-band peak at 1598 cm⁻¹ corresponds to the tangential stretching (E_2g) mode of graphite.^27,28 In addition, there is a relatively weak peak at 950 cm⁻¹, which can be indexed to the symmetric PO₄ stretching vibration of LiFePO₄.²⁹ Elemental analysis on an Elementar vario EL cube confirms that the carbon content of the LiFePO₄ samples (LFP-1A, LFP-2A, LFP-3A, and LFP-4A) is 2.52%, 2.21%, 2.15%, and 1.98%, respectively. The tap density of the four samples (LFP-1A, LFP-2A, LFP-3A, and LFP-4A) is 0.93 g cm⁻³, 1.21 g cm⁻³, 1.33 g cm⁻³, and 1.45 g cm⁻³, respectively. The LFP-3A sample shows a relatively higher tap density (1.33 g cm⁻³). It is noteworthy that LiFePO₄ nanoflakes (LFP-1A) have a lower tap density (0.93 g cm⁻³). Hierarchical structured LiFePO₄ has a higher tap density (1.4 g cm⁻³) compared to nanoscale particles (less than 1.0 g cm⁻³).¹⁶ This experiment demonstrates that trace Fe³⁺ facilitates the formation of self-assembled micro-sized LiFePO₄ with a high tap density, and this synthetic strategy is also beneficial for the enhancement of batteries’ volumetric energy density.⁷ Fig. 7a shows the typical discharge profiles of LFP-3A. When the current rate increases, the discharge current rate increases and the discharge plateaus shift toward slightly lower potentials,³⁰ which indicate a slight polarization tendency. As the current densities increase from 0.5C to 1C, 2C, 5C, 8C, and 10C, the reversible capacities decrease from 160.2 mA h g⁻¹ to 148.8 mA h g⁻¹, 137.9 mA h g⁻¹, 117.6 mA h g⁻¹, and 105.6 mA h g⁻¹, respectively. Fig. 7b presents a comparison of the galvanostatic discharge profiles of LFP-1A, LFP-2A, LFP-3A and LFP-4A at different rates in the voltage window of 2.0–4.3 V. The electrochemical performances of LFP-1A and LFP-3A present a similar rate performance. As is well known, the crystal structure of olivine LiFePO₄ allows for a preferential migration of Li ions along the [010]_Pnma channel. The LFP-1A sample has benefited from the nanoflakes which have a faster lithium-ion diffusion due to shortening of the Li⁺ migration pathway. Nonetheless, tap density is an important factor that needs to be considered for fabricating real batteries.^16,17 LFP-3A, with a higher tap density, takes advantage of the nanoflakes with faster lithium-ion diffusion and shows good high-rate performance. It can be seen that both LFP-2A and LFP-3A show high specific capacities at lower rates, such as 160 mA h g⁻¹ at 0.5C. In particular, the capacity of LFP-3A was also maintained at 105 mA h g⁻¹ at a rate as high as 10C compared to LFP-2A. Among these samples, LFP-4 shows the lowest specific capacity of 140 mA h g⁻¹ at 0.5C. This is because the main exposed (100) face of LFP-4 leads to less diffusion of the lithium-ion. Fig. 7c exhibits cyclic voltammetry (CV) curves of LFP-3A with various potential scan rates from 0.1 mV s⁻¹ to 2 mV s⁻¹. All the CV curves are characterized by two obvious redox peaks in the range of 3.30–3.60 V, which should be attributed to the Fe²⁺/Fe³⁺ redox couple reaction.³¹ At a low scan rate of 0.1 mV s⁻¹, the reduction and oxidation peaks appear at 3.372 and 3.517 V, respectively. The potential difference between the cathodic and anodic peaks is 145 mV, which has no obvious voltage separation and indicates good reversibility of the Li⁺ ion extraction/insertion reactions. Even at a high scan rate of 2 mV s⁻¹, the CV profile is also well-defined with no significant distortion. The results reflect good kinetic properties. Thus, determination of the diffusion coefficient of lithium ions is an effective way to understand the four samples. The peak current I_p (based on the anodic peak) would be proportional to the square root of the scan rate (ν) in Fig. 7d. For semi-infinite and finite diffusion systems, a linear relation can be used to calculate the Li⁺ diffusion coefficient (D_Li⁺) according to the Randles–Sevcik equation:³²

I_p = (2.69 × 10⁵)n^3/2αD_(Li)^1/2c(Li)*ν^1/2

where I_p is the peak current (A), n is the number of electrons for the reaction (for Fe²⁺/Fe³⁺, it is 1), α is the active surface area of the electrode (cm²), c(Li)* is the shuttle concentration (mol cm⁻³), D_(Li) is the diffusion coefficient of Li in the electrode (cm² s⁻¹) and ν is the scanning rate (V s⁻¹). According to the slope of the linear fit, the Li ion diffusion coefficients of LFP-1A, LFP-2A, LFP-3A, and LFP-4A are calculated to be 2.707 × 10⁻¹⁰, 1.348 × 10⁻¹⁰, 3.448 × 10⁻¹⁰ and 4.483 × 10⁻¹¹ cm² s⁻¹, respectively. It was found that the Li ion diffusion coefficient for LFP-3A is much higher than the ones obtained by other groups.^33–35 Such a fast ionic conductivity characteristic proved that LFP-3A has better reaction kinetics with an excellent reversibility of the lithium intercalation/deintercalation processes. Fig. 7e presents the cycling performance of LFP-3A, which can deliver an initial discharge capacity of 109.2 mA h g⁻¹ at a high current rate of 10C and, after 600 cycles, a capacity of 104.3 mA h g⁻¹ can still be maintained. In addition, the coulombic efficiency was always maintained around 100%, indicating the good reversibility and rate performance of lithium intercalation and deintercalation in this electrode material. To determine the origin of the improvement in the rate performance, an AC impedance measurement was carried out. Electrochemical impedance spectroscopy (EIS) of the LiFePO₄ materials was performed in the fresh state and are presented as Nyquist plots (Fig. 8a). Each plot consists of a semicircle in the high frequency region which can be related to the charge transfer process and a sloped line in the low frequency region attributed to the mass transfer of the lithium ion.^36,37 The equivalent circuits for the impedance data are shown in Fig. 8b. According to the fitting results, the high frequency semicircle corresponds to the electrolyte resistance (R_s), while the semicircle at high-middle frequencies can be assigned to the charge transfer resistance (R_p) of the LiFePO₄ materials. The sloped line at the low frequency region can be related to Warburg impedance (Z_w),³⁷ which is attributed to the solid state diffusion of Li⁺ in the LiFePO₄ electrode. The R_s values of the LiFePO₄ electrode are similar and as low as ∼1 Ω, demonstrating that the LiFePO₄ electrodes in the fresh state exhibit similar electrolyte resistance and high Li⁺ transport.³⁸ However, R_p values are significantly different among these four samples. LiFePO₄ micro-dumbbells and nanoflakes possess an obviously lower surface charge transfer (116 Ω) resistance compared to LiFePO₄ stacked microsheets and micro-spindles (149 Ω and 195 Ω, respectively), indicating a much faster electron and Li-ion diffusion on the interface of the LiFePO₄ materials.

Fig. 6 Raman spectra of LiFePO₄ (a) LFP-1A (b) LFP-2A (c) LFP-3A and (d) LFP-4A.

Fig. 7 (a) Discharge curves of LFP-3A at different current rates in the voltage window of 2.0–4.3 V; (b) comparison of the galvanostatic discharge profiles of LFP-1A, LFP-2A, LFP-3A and LFP-4A at different rates in the voltage window of 2.0–4.3 V; (c) CV curves of the LFP-3A sample tested at various scan rates. (d) The linear fitting of the peak current versus square root of the scan rate. (e) Long-term cycling performance of LFP-3A evaluated at a 10C rate for 600 cycles.

Fig. 8 (a) Nyquist plots obtained for the LiFePO₄ electrodes in the fresh state and (b) equivalent circuits used to model the impedance spectra.

In a word, the as-prepared LiFePO₄ micro-dumbbells show a high tap density and a most excellent electrochemical performance, which can be attributed to their unique micro/nanostructure. The primary nanoflakes provide increasingly effective reaction areas and short Li⁺ diffusion distances due to the exposed (010) plane making the transportation of lithium ions easier during the charge and discharge process.^1,9 A micro-sized structure will assure a high tap density and cycling stability.¹³ Fabricating exposed special plane micro/nanostructured LiFePO₄ is a feasible approach because it combines the merit of microstructures and the advantage of the small plate.

4. Conclusions

In summary, a facile and template-free solvothermal process has been adopted to synthesize various LiFePO₄ micro/nanostructures using FeSO₄ as a reactant. The LiFePO₄ micro/nanostructures including nanoflakes, stacked microsheets, micro-dumbbells and micro-spindles have been prepared selectively by adjusting the aging time of the FeSO₄ solution at room temperature. During the aging process for FeSO₄, trace Fe²⁺ should be partially oxidized into Fe³⁺, which results in the changing of the pH value of the reaction system. The pH value, one of the most important parameters, can be adjusted by controlling the formation of trace Fe³⁺. Moreover, it plays an important role in the formation of LiFePO₄ micro/nanostructures. Among these LiFePO₄ micro/nanostructures, the LiFePO₄ micro-dumbbells show a high tap density and a most excellent electrochemical performance with a discharge capacity of 110 mA h g⁻¹ at a high rate of 10C (1C = 169 mA h g⁻¹). This work not only provides deeper knowledge into the formation mechanism of LiFePO₄micro/nanostructures, but also paves a facile way to prepare LiFePO₄ that is scalable, has a high rate performance and high tap density and may even extend to the preparation to other phosphates.

Acknowledgements

This work was financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Natural Science Fund of China (No. 21471142, 2120115 8) and the authors thank Dr Wanqun Zhang (Chemistry Experiment Teaching Center, USTC) for the XRD analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Auxiliary analysis such as SEM images, TEM images, SAED pattern, and HRTEM images. See DOI: 10.1039/c5ra22373c

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