LiFePO₄ nanoparticles growth with preferential (010) face modulated by Tween-80

Abstract

Small grain size combined with large I₍₀₂₀₎/I₍₁₁₁₎ ratio have been proven to be an effective strategy to improve the electrochemical properties of LiFePO₄ material due to increased Li-ion diffusion. In view of this, we use Tween-80 as the surfactant in hydrothermal synthesis to modulate both crystal size and orientation of LiFePO₄. It is indicated that the LiFePO₄ particles synthesized via the Tween-80 modified hydrothermal method exhibited a small mean diameter of 100 nm and a large I₍₀₂₀₎/I₍₁₁₁₎ ratio of 1.19. Whereas in the surfactant-free hydrothermal system, the obtained LiFePO₄ particles exhibit a relatively large mean diameter of 200 nm and a low I₍₀₂₀₎/I₍₁₁₁₎ ratio of 0.84. LiFePO₄ particles with small grain size and a large I₍₀₂₀₎/I₍₁₁₁₎ ratio can be easily brought into contact with electrolyte, facilitating electric and Li-ion diffusion. After being coated with conductive carbon, the synthesized LiFePO₄/C nanoparticles perform a high Li-ion diffusion coefficient of 1.79 × 10⁻¹³ cm² s⁻¹. It presents a large reversible capacity of 166.5 mA h g⁻¹ at 0.1 C and even a high rate capacity of 119.6 mA h g⁻¹ at 20 C.

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1. Introduction

As reversible electrochemical energy storage devices, rechargeable lithium ion batteries (LIBs) have long been considered as key enablers for large scale use of various attractive renewable energy resources in future low-carbon society.^1,2 Nowadays, LIBs are not only widely used in portable electronics, such as cell phones, cameras, toys and laptops, but have also been gradually adopted for increasingly large scale applications, such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and stationary electric power storage.³ The major challenge in adopting LIBs to satisfy the needs of modern society and emerging ecological concerns is unquestionably cathode materials where the Li-ions extraction/insertion process occurs.⁴ Fortunately, olivine-structured LiFePO₄ is seen as a promising cathode material for LIBs owing to its high theoretical capacity (170 mA h g⁻¹), adequate operating voltage (3.45 V vs. Li⁺/Li), long cycle life (>2000 cycles), superior safety, low cost and low toxicity, as well as abundant raw material availability and environmental benignity.^5–7 However, the inherent shortcomings of LiFePO₄, such as the material's low electrical conductivity and slow Li-ion diffusion, result in poor electrochemical performance and hinder its commercial applications.⁸ Reducing the grain size together with adjusting the crystal orientation have been proven to be an effective strategy to enhance the Li-ion diffusion of LiFePO₄ and make LiFePO₄ a technically feasible cathode material for high-power density LIBs.^9–11

In comparison with conventional solid-phase synthesis methods, the low temperature hydrothermal method, due to its fast reaction rate, mild synthesis conditions, is more suitable for controlling crystal size and achieving a high degree of crystallinity, purity, and narrow particle size distribution.^6,12–14 The hydrothermal method has been used for the synthesis of LiFePO₄ nanoparticles including nanorods,¹⁵ nanoplate,^6,16 nanowire,¹⁷ and nanosphere.¹⁸ The use of surfactants during hydrothermal synthesis is a promising route to control and modify the size and crystal orientation of LiFePO₄. For instance, Ferrari et al.¹⁹ utilized a polymeric surfactant (polyvinylpyrrolidone, PVP) in the hydrothermal reaction to synthesize platelet-like LiFePO₄ particles with thicknesses of approximately 0.5–1 μm and diameters of 2–7 μm. This method gave preferred crystal growth along the (010) face and yielded materials with discharge capabilities of 153 and 80 mA h g⁻¹ at rates of 0.1 C and 5 C, respectively. Pei et al.²⁰ used the surfactant of sodium dodecyl benzene sulfonate (SDBS) in the hydrothermal reaction to synthesize LiFePO₄ nanoplates (20 nm long and 50 nm wide) with larger surface areas along the (010) plane. These materials showed discharge capacities of 162.9 and 107.9 mA h g⁻¹ at rates of 0.1 C and 10 C, respectively.

Herein, we successfully prepared LiFePO₄ nanoparticles (rather than platelet-like LiFePO₄ particles) with the preferential (010) face using a non-ionic surfactant Tween-80 (i.e., polyoxyethylene sorbitan monooleate) as surfactant via hydrothermal synthesis. Characterization by XRD, FTIR, SEM and size distribution analysis all indicated that the use of Tween-80 can successfully reduce the grain size and modulate the crystal growth along the (010) facet of LiFePO₄ particles. To the best of our knowledge, no report has been found so far that using Tween-80 as the surfactant to hydrothermal synthesis of LiFePO₄ materials, since Tween-80 has already been used as an additive to synthesize nanoparticles of silver,²¹ nickel,²² copper,²³ gold,²⁴ and α-Fe₂O₃.²⁵ Additionally, we believe that this facie route shows potential for preparing other olive-structured LiMPO₄ (M = Mn, Co and Ni) nanoparticles with preferential growth along (010) face.

2. Experimental

2.1 Materials synthesis

The starting materials for the hydrothermal reaction were FeSO₄·7H₂O, LiOH·H₂O, H₃PO₄ and Tween-80. All of the materials were of analytical grade and were used without further purification. Deionized water was used throughout the process. A mixed solution (0.3 L) of LiOH and Tween-80 was loaded into the vessel, followed by 0.1 L of a H₃PO₄ solution. Finally, 0.2 L of a FeSO₄ solution was added dropwise into the stirring solution under a nitrogen atmosphere. The resultant Li : Fe : P molar ratio was 3 : 1 : 1, and the concentrations of Fe²⁺ and Tween-80 were 0.5 and 0.1 mol L⁻¹, respectively. After vigorous stirring, the mixture was transferred into a crystallizer. The prepared precursor mixture named Precursor-Tween. As a control, a precursor mixture in the absence of Tween-80 was also prepared, which was denoted as Precursor.

The precursor mixture was transferred into a 0.5 L stainless steel autoclave (Dalian Co., China, Model GCF-0.5L) under a nitrogen atmosphere where it underwent hydrothermal crystallization at 180 °C for 3 h. After the reaction, the resulting suspension was filtered and washed several times with deionized water and ethanol. The solids were then dried at 120 °C for 12 h under vacuum, after which the LiFePO₄ powder was mixed with 17 wt% glucose. In order to improve electronic conductivity, the material then underwent calcination at 750 °C for 6 h in a tubular furnace under a nitrogen atmosphere, yielding LiFePO₄ particles coated with carbon (LiFePO₄/C). The prepared LiFePO₄ and LiFePO₄/C samples were denoted as LFP-Tween and LFP/C-Tween. The LiFePO₄ and LiFePO₄/C samples prepared in the absence of Tween-80 were denoted as LFP and LFP/C for comparison (Scheme 1).

Scheme 1 Schematic illustration for the preparation of LiFePO₄/C.

2.2 Materials characterization

Phase identification was carried out by X-ray diffraction (XRD, Rigaku D/max 2500 V/PC) with a step of 0.017° in a 2θ range from 10° to 65° using Cu-Kα radiation. Particle morphologies were characterized using a Hitachi scanning electron microscopy (SEM, Hitachi Ltd., Japan, Model S-4800) with an X-ray energy dispersive spectroscope (EDS). Particle size distribution was measured with a laser granulometer (Malvern Instruments Ltd., UK, Model Masterzizer 2000), using water as the dispersing agent. The FTIR spectra of the powders were recorded on a Nicolet iZ10 spectrometer (Thermo scientific) from 4000 to 400 cm⁻¹ at room temperature.

2.3 Electrochemical measurements

The electrode was prepared by mixing LiFePO₄/C particles, carbon conductive additive (Super P-MMM carbon) and vinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1. All electrodes were cut into disks with a diameter of 1.3 cm and a thickness of 28 μm. The average mass loading of active material was calculated to be 2.2 mg cm⁻². These film-type LiFePO₄ electrodes assembled in coin cells were separated from the counter electrode (lithium metal) by a Celgard 2400 separator. The electrolyte was 1.0 M LiPF₆ in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (1 : 1 : 1, by volume). The assembly was operated in an argon-filled glove box (Mikrouna Co., Ltd., China, Model Super 1220/750). Cathode performance, evaluated in terms of charge–discharge curves and cycling capacities, was measured using an automatic charge–discharge instrument (Land Co., China, Model CT2001A) between the cut-off voltages of 2.2 V and 4.2 V. All of the electrochemical measurements were carried out at ambient temperature (25 ± 2 °C). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) curves were performed on electrochemical analyzer (ZAHN-ER ZENNIUM).

3. Results and discussion

3.1 Tween-80 affects the formation of Fe₃(PO₄)₂·8H₂O crystal before hydrothermal crystallization

The precursor precipitates, Li₃PO₄ and Fe₃(PO₄)₂·8H₂O, were prepared by mixing the raw solutions of LiOH, H₃PO₄, FeSO₄ and Tween-80. From the XRD characterization in Fig. 1 it can be seen that, without the addition of Tween-80, both reflection peaks of Li₃PO₄ and Fe₃(PO₄)₂·8H₂O are observed. When the surfactant Tween-80 was introduced, the peak intensity of Fe₃(PO₄)₂·8H₂O increased, implying that Tween-80 can facilitate the formation of Fe₃(PO₄)₂·8H₂O in the precursor mixture.

Fig. 1 XRD patterns of the precursor precipitates synthesized in the absence (Precursor) and presence of Tween-80 (Precursor-Tween).

FTIR analysis was carried out in order to understand how Tween-80 facilitates the formation of Fe₃(PO₄)₂·8H₂O crystal. The FTIR spectrum of neat Tween-80 is also given for comparison. As shown in Fig. 2, the sample Precursor shows typical peaks at 1054 cm⁻¹ and 837 cm⁻¹ corresponding to the P–O bond of PO₄³⁻.²⁶ For the Tween-80 modified precursor precipitates, the presence of the peaks of surfactant involving –H₂C–O–CH₂– (946 cm⁻¹), –CO–O–CH₂– (1100 cm⁻¹), –CH₂–CH₃ (2855 cm⁻¹ and 2900 cm⁻¹)^27,28 indicates that the functional groups of Tween-80 have been anchored onto the surface of the precursor precipitates. The absorption band due to C=O stretch at 1732 cm⁻¹, however, does not appear in Tween-80 modified precursor precipitates. At the same time, a broad band centered around 1640 cm⁻¹ assigned to the C–O–Fe bond formation is clearly observed.²⁵ This indicates that there might exist chemical interaction between Tween-80 and Fe₃(PO₄)₂·8H₂O precipitate via the oxygen of C=O group.²⁵ These Tween-80 capped Fe₃(PO₄)₂·8H₂O precipitates are covered or encapsulated by surfactant micelles, leading to a reduction of Fe₃(PO₄)₂·8H₂O precipitates in the precursor solution. Therefore, the dissolution reaction of the two precipitates needs to maintain equilibrium by encouraging the dissolution of the first precipitate Li₃PO₄ (with K_sp = 3.2 × 10⁻⁹) and accelerating the precipitation of Fe₃(PO₄)₂·8H₂O (with K_sp = 1.0 × 10⁻³⁶),²⁹ which is consonant with the XRD observation in Fig. 1.

Fig. 2 FTIR spectra of the precursor precipitate synthesized in the absence (Precursor) and in the presence of Tween-80 (Precursor-Tween), together with the FTIR spectrum of pure Tween-80.

To investigate the effect of the surfactant-modified precursor on the crystal growth of LiFePO₄, we treated the precursor mixtures in the autoclave with a fast rising temperature to 125 °C, 130 °C, 150 °C, respectively, and then quickly cooled down to room temperature. It is observed that for the Precursor with thermal treatment to 150 °C there were still small amounts of Fe₃(PO₄)₂·8H₂O in the particles. Whereas for the Precursors-Tween, perfect crystal LiFePO₄ particles (Fig. S1†) were formed. This suggested that the more Fe₃(PO₄)₂·8H₂O crystals appeared in the precursor mixture, the quicker the crystals of LiFePO₄ formed during the next hydrothermal crystallization process.

3.2 Tween-80 affects the crystal orientation of LiFePO₄ through hydrothermal crystallization

The prepared precursor mixtures were transferred into the 0.5 L stainless steel autoclave to synthesize the LiFePO₄ crystals via hydrothermal crystallization for 3 h at the temperatures of 150 °C, 160 °C, 170 °C and 180 °C, respectively. The XRD patterns of the obtained samples are shown in Fig. 3. It is clear that all of the observed peaks can be indexed to the orthorhombic olivine-type LiFePO₄ with a space group of Pnma (ICDD PDF no. 81-1173) and no impurity phase was detected. However, the relative intensities of the peaks involving (020) and (111) are clearly different between the LiFePO₄ particles synthesized with and without Tween-80. As shown in Fig. 3a, all of the XRD patterns of LiFePO₄ particles from Precursor are dominated by four lines including (101), (111), (020) and (311). Among which, line (311) exhibits the sharpest intensity, which is consisted with the previous reported studies, suggesting that the LiFePO₄ particles prepared without Tween-80 are randomly oriented. However, the sharply increased (020) diffraction line in the LiFePO₄ particles prepared from the Tween-80 modified precursor (Fig. 3b) indicated that the LiFePO₄ crystals preferentially grew along the (010) facet.

Fig. 3 XRD patterns of the LiFePO₄ prepared via hydrothermal crystallization from Precursor (a) and Precursor-Tween (b) for 3 h at various temperatures.

In order to confirm the LiFePO₄ crystals grow preferentially along the (010) facet, the peak intensity ratios of I₍₀₂₀₎/I₍₁₁₁₎ for the synthesized LiFePO₄ were calculated according to the XRD patterns (see Fig. 4). It is clear that the I₍₀₂₀₎/I₍₁₁₁₎ ratios for LiFePO₄ synthesized from Precursor at the temperatures of 150 °C, 160 °C, 170 °C and 180 °C are 0.75, 0.79, 0.80 and 0.84, respectively; whereas it is 0.90, 1.00, 1.10 and 1.19 for LiFePO₄ synthesized from Precursor-Tween. The increase in I₍₀₂₀₎/I₍₁₁₁₎ ratio demonstrated that the preferred orientation along (010) facet of LiFePO₄ was successfully achieved by using Tween-80 during hydrothermal synthesis.

Fig. 4 Peak intensity ratios of I₍₀₂₀₎/I₍₁₁₁₎ for LiFePO₄ prepared via hydrothermal crystallization from Precursor and Precursor-Tween for 3 h at various temperatures.

It is well known that the crystal growth is mainly controlled by nuclei surface energy. Nuclei surfaces with low surface energy have a faster growth rate along these directions.¹² During the LiFePO₄ nuclei growing process, the functional groups of surfactant Tween-80 may adhere to the outer layer of (010) surfaces of the newly created LiFePO₄ nuclei, making the Tween-80-terminated (010) surfaces more stable and further suppressing crystal growth along the (010) facet. It is reported that the Li-ions migrate along [010] direction,³⁰ so reducing the length in [010] direction and enlarging the surface area of (010) facet can accelerate the Li-ions diffusion. Therefore, with the assistance of Tween-80, the LiFePO₄ grows more slowly along the [010] direction, resulting in a large scale of (010) facet for the Li-ions diffusion, and hence enhancing the diffusive rate of Li-ions and improving the electrochemical performance of LiFePO₄ material. In addition, the I₍₀₂₀₎/I₍₁₁₁₎ ratio is also affected by and increases with the hydrothermal reaction temperature. LiFePO₄ particles synthesized from the Precursor-Tween via hydrothermal crystallization at 180 °C for 3 h showed the largest (010) facet, and were chosen as the subject for further investigation.

3.3 Effect of carbon coating on the structure and morphology of LiFePO₄/C

Due to the inherently low electronic conductivity, the obtained LiFePO₄ particles need to be coated with conductive carbon so as to make them suitable for cathode application. Fig. 5a shows the XRD pattern of the LiFePO₄/C samples synthesized from the Precursor and Precursor-Tween via hydrothermal crystallization at 180 °C for 3 h. It is clear that the XRD spectra of both of the LiFePO₄/C samples match well with the standard patterns of orthorhombic olivine LiFePO₄ (ICDD PDF no. 81-1173), indicating that the carbon coating has no impact on the phosphor-olivine's structure. However, the I₍₀₂₀₎/I₍₁₁₁₎ ratio in the LFP/C-Tween sample (1.35) become greater than that of the LFP/C sample (0.91), which could be attributed to the better crystallization of the coated amorphous carbon layer after the post-heat treatment.⁶

Fig. 5 (a) XRD patterns, (b) particle size distribution, (c and d) SEM images, and (e and f) HRTEM images of the LiFePO₄/C samples synthesized in the absence (LFP/C) and presence (LFP/C-Tween) of Tween-80. The inserted section in (e and f) is the TEM images of the sample LFP/C and LFP/C-Tween, respectively.

In addition, the particle size distribution of the obtained LiFePO₄/C samples was also investigated and the results were listed in Fig. 5b. It is clear that the LFP/C sample shows a broad particle size distribution with peak located at 1.26 μm. Whereas the LFP/C-Tween sample illustrates a relatively narrow size distribution with decreased peak value located at 0.83 μm. This reduction of particle size is also confirmed by SEM.

SEM and TEM analysis were carried out to reveal the morphology and structure of the LiFePO₄/C samples. From the SEM images in Fig. 5c and d, it can be seen that both of the LiFePO₄/C particles exhibit a rod-like structure, indicating the shape does not change after the addition of surfactant Tween-80. However, the LFP/C-Tween particles exhibit smaller size (∼100 nm) than that of LFP/C (∼200 nm), which can be attributed to the faster dissolving rate of Precursor-Tween and more rapid growth rate of LiFePO₄, as suggested by the XRD patterns given in Fig. S1.† Furthermore, the HRTEM images in Fig. 5e and f also confirmed the preferred orientation of LiFePO₄ crystal modified by surfactant Tween-80. For the sample LFP/C, the interplanar distance of 3.476 Å corresponds to the (111) plane of orthorhombic LiFePO₄. Whereas for the sample LFP/C-Tween, HRTEM image shows the lattice interplanar spacing of 3.478 Å and 3.003 Å assign to the (111) and (020) planes of LiFePO₄ respectively, which indicates that the largest exposed plane of the LiFePO₄/C is the (020) facet, giving the strongest (020) line in the XRD pattern (Fig. 5a). In addition, TEM images in the inserted section of Fig. 5e and f indicate the amorphous carbon layer. The conductive layer of LFP/C-Tween (1.8 nm) is thinner than that of LFP/C, which might ascribed to the smaller grain size of LFP/C-Tween particles.

Base on the above analysis, it can be concluded that using surfactant Tween-80, LiFePO₄/C material with enlarged (010) facet and smaller grain size can be synthesized hydrothermally, which can provide a more favourable pathway for Li-ions transport and larger contact area of electrode/electrolyte, resulting in the improved electrochemical performance.

3.4 Tween-80 improves the electrochemical performance of LiFePO₄/C

The electrochemical properties of the LFP/C sample and Tween-80 modified LFP/C-Tween sample as Li-ions battery cathode were evaluated and compared at different current rate from 0.1 C to 20 C (1 C = 170 mA h g⁻¹) in the voltage range of 2.2–4.2 V vs. Li/Li⁺. Fig. 6a shows the initial charge–discharge curves at 0.1 C. The long, flat voltage plateaus at 3.42 V were observed during both charge and discharge processes, indicating the typical two-phase reaction between FePO₄ and LiFePO₄.¹ The LFP/C sample shows initial specific discharge capacity of 157.7 mA h g⁻¹ at 0.1 C, and an enhanced value 166.5 mA h g⁻¹ was observed for the surfactant modified LiFePO₄/C sample. This improved discharge capacity can be attributed to the smaller particle size, especially the enlarged (010) facet of the surfactant modified LiFePO₄/C.

Fig. 6 Electrochemical properties of LiFePO₄/C samples: (a) charge and discharge profiles measured at 0.1 C, (b) discharge profiles of the LFP/C-Tween sample measured at various C-rates, (c) C-rate capabilities of LiFePO₄/C samples, (d) CV profiles of the LiFePO₄/C samples between 2.2 and 4.2 V.

As shown in Fig. 6b, the LFP/C-Tween materials still exhibited higher specific capacities of 129 mA h g⁻¹ and 119.6 mA h g⁻¹ as the current rate rose to 10 C and 20 C, and the discharge voltage plateau at 20 C is still higher than 3 V, implying low polarization.

The low polarization of Li/LiFePO₄ suggests small electrolyte resistance and interfacial resistances on both the anode and cathode, which is desirable for achieving high rate performance.¹ Fig. 6c compares the cycle performance of LiFePO₄/C particles at the current rates from 1 C to 20 C. Even at high rate of 20 C, the special capacity of LFP/C-Tween particles was still maintained at 93% after 50 cycles, which is 13% higher than that of the LFP/C particles. We compared our results with those reported by other research groups in Table 1. It is clear that the present LiFePO₄/C composite exhibits the highest rate capacity at 20 C. All of these improvements are attributed to the enlarged crystal orientation along the (010) facet and the reduced Li-ion transport length of LiFePO₄/C. In addition, the energy density of the cathode LiFePO₄/C (calculated by integrating the discharge curve after 50 cycle at 20 C) was 380 W h kg⁻¹. This value meets the target specific energy density for EV battery, indicating great potential for commercial applications.

Table 1 Comparison of high rate performance of LiFePO₄ published in recent years

Method	Structure	Capacity (mA h g^−1)		Reference
		10 C	20 C
Carbothermal method	LiFePO4/C	90	70	31 (2013)
Chemical vapor deposition	LiFePO4/graphic	100	80	32 (2013)
Mechanochemical activation	LiFePO4/C	111	92	33 (2009)
In situ polymerization restriction method	LiFePO4/C	115.6	84.5	34 (2013)
Carbothermal reduction route	Cl-doped LiFePO4/C	120	90	35 (2010)
Solid-state method	Li0.97Na0.03Fe0.97Ti0.03PO4/C	121.4	97.3	36 (2013)
Hydrothermal method	LiFePO4/C + graphic	124	112	37 (2011)
Tween-80 modified hydrothermal method	LiFePO4/C	129	119.6	This work

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Fig. 6d shows the first-round CVs profiles of LFP/C and LFP/C-Tween sample for an open circuit voltage (OCV) of 3.42 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹. The CV profiles show one distinct anodic peak (charge) and cathodic peak (discharge) corresponding to the charge–discharge reaction between LiFePO₄ and FePO₄. The LFP/C-Tween sample shows a higher peak current than that of LFP/C sample, indicating a better rate capability. In addition, the peak potential difference between the anodic and cathodic peaks is 0.19 V for LFP/C-Tween particles, whereas it is 0.30 V for LFP/C particles. A small peak potential difference in the CV curve suggests that the reversibility of the electrode reaction of LFP/C-Tween is markedly ameliorated by the preferential growth of the (010) plane and reduced grain size.

Fig. 7a shows the electrochemical impedance spectroscopy (EIS) of the synthesized LiFePO₄/C particles. It is clear that the impedance spectrum is composed of an intercept at high frequency, a depressed semicircle at medium frequency and a straight line in low frequency region. A simple equivalent circuit model in the inserted section of Fig. 7a is constructed to analyse the impedance spectra. The intercept impedance on the real resistance axis is attributed to the ohmic resistance (R_Ω), which represents the resistance between electrolyte and electrode. The semicircle at medium frequency relates to the charge-transfer (R_ct) in cathode–electrolyte interface while the straight line in low frequency region relates to Warburg impedance (Z_w) associated with Li-ion diffusion in LiFePO₄/C crystal. The numerical value of the diameter of the semicircle on the real axis is approximately equal to R_ct. The R_ct value for LFP/C-Tween cathode is about 37 Ω, much smaller than that of LFP/C cathodes (63 Ω). This indicated that the impedance of electrochemical reaction was mediated by using surfactant Tween-80 during the synthesis, which is consistent with their electrochemical properties.

Fig. 7 (a) The equivalent circuit and Nyquist plots of the LiFePO₄/C samples. Frequency range: 0.1 Hz–100 kHz; (b) the relationship of the resistance (Z′) with the inverse square root of the angular speed for the LiFePO₄/C samples.

In addition, the Li-ions diffusion coefficient of the LiFePO₄/C was also calculated based on the low frequency spots.³⁸ According to the slopes of the lines (Fig. 7b), the Li-ions diffusion coefficient in the LFP/C-Tween sample was calculated to be 1.79 × 10⁻¹³ cm² s⁻¹, which is higher than that of LFP/C (1.15 × 10⁻¹³ cm² s⁻¹). This enhanced Li-ions diffusion is ascribed to the enlarged (010) lattice plane and reduced particle size.

4. Conclusions

A facile Tween-80 modified hydrothermal method has been developed to modulate the size and crystal orientation of LiFePO₄ crystal to improve its electrochemical performance. The addition of Tween-80 not only facilitates the formation of intermediate Fe₃(PO₄)₂·8H₂O particles in the precursor mixture and accelerates the phase evolution and nucleation rate of LiFePO₄ crystals, but also adjusts the LiFePO₄ crystal preferentially growth along the (010) facet during hydrothermal crystallization process. The LiFePO₄/C particles synthesized by Tween-80 modified hydrothermal reaction exhibit small grain size (∼100 nm), high I₍₀₂₀₎/I₍₁₁₁₎ ratio (1.35), and thin carbon layer (1.8 nm), which leads to excellent discharge capabilities of 166.5 and 119.6 mA h g⁻¹ at the current rates of 0.1 C and 20 C, respectively. These results indicate that the Tween-80 modified hydrothermal method is an effective route to synthesize LiFePO₄ with excellent electrochemical performance.

Acknowledgements

This work is financially supported by the Special Funds for Major State Basic Research Program of China (2012CB720300), NSFC (21476158), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1161).

Notes and references

1. F. Yu, L. L. Zhang, Y. C. Li, Y. X. An, M. Y. Zhu and B. Dai, RSC Adv., 2014, 4, 54576–54602.
2. A. K. Padhi, K. Nanjundaswamy and J. B. D. Goodenough, J. Electrochem. Soc., 1997, 144, 1188–1194.
3. F. Yu, J. J. Zhang, Y. F. Yang and G. Z. Song, J. Power Sources, 2009, 189, 794–797.
4. F. Yu, L. L. Zhang, M. Y. Zhu, Y. X. An, L. L. Xia, X. G. Wang and B. Dai, Nano Energy, 2014, 3, 64–79.
5. F. Yu, J. J. Zhang, Y. F. Yang and G. Z. Song, Electrochim. Acta, 2009, 54, 7389–7395.
6. X. Q. Ou, H. C. Gu, Y. C. Wu, J. W. Lu and Y. J. Zheng, Electrochim. Acta, 2013, 96, 230–236.
7. M. S. Whittingham, Chem. Rev., 2004, 104, 4271–4301.
8. N. Ravet, M. Gauthier, K. Zaghib, J. B. Goodenough, A. Mauger, F. Gendron and C. M. Julien, Chem. Mater., 2007, 19, 2595–2602.
9. R. G. Mei, X. R. Song, Y. F. Yang, Z. G. An and J. J. Zhang, RSC Adv., 2014, 4, 5746–5752.
10. M. M. Chen, Q. Q. Ma, C. Y. Wang, X. Sun, L. Q. Wang and C. Zhang, J. Power Sources, 2014, 263, 268–275.
11. J. Su, B. Q. Wei, J. P. Rong, W. Y. Yin, Z. X. Ye, X. Q. Tian, L. Ren, M. H. Cao and C. W. Hu, J. Solid State Chem., 2011, 184, 2909–2919.
12. X. Q. Ou, L. Pan, H. C. Gu, Y. C. Wu and J. W. Lu, J. Mater. Chem., 2012, 22, 9064–9068.
13. G. C. Liang, L. Wang, X. Q. Ou, X. Zhao and S. Z. Xu, J. Power Sources, 2008, 184, 538–542.
14. Z. M. Cui, W. Y. Yuan and C. M. Li, J. Mater. Chem. A, 2013, 1, 12926–12931.
15. F. Teng, S. Santhanagopalan, R. Lemmens, X. Geng, P. Patel and D. D. Meng, Solid State Sci., 2010, 12, 952–955.
16. S. L. Yang, X. F. Zhou, J. G. Zhang and Z. P. Liu, J. Mater. Chem., 2010, 20, 8086–8091.
17. S. Lim, C. S. Yoon and J. Cho, Chem. Mater., 2008, 20, 4560–4564.
18. G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo and N. Penazzi, J. Power Sources, 2006, 160, 516–522.
19. S. Ferrari, R. L. Lavall, D. Capsoni, E. Quartarone, A. Magistris, P. Mustarelli and P. Canton, J. Phys. Chem. C, 2010, 114, 12598–12603.
20. B. Pei, H. X. Yao, W. X. Zhang and Z. H. Yang, J. Power Sources, 2012, 220, 317–323.
21. L. Kvitek, M. Vanickova, A. Panacek, J. Soukupova, M. Dittrich, E. Valentova, R. Prucek, M. Bancirova, D. Milde and R. Zboril, J. Phys. Chem. C, 2009, 113, 4296–4300.
22. A. L. Wang, H. B. Yin, M. Ren, H. H. Lu, J. J. Xue and T. S. Jiang, New J. Chem., 2010, 34, 708–713.
23. X. F. Zhang, H. B. Yin, X. N. Cheng, H. F. Hu, Q. Yu and A. L. Wang, Mater. Res. Bull., 2006, 41, 2041–2048.
24. T. Premkumar, D. Kim, K. Lee and K. E. Geckeler, Macromol. Rapid Commun., 2007, 28, 888–893.
25. Y. Khan, S. K. Durrani, M. Siddique and M. Mehmood, Mater. Lett., 2011, 65, 2224–2227.
26. A. Ait Salah, P. Jozwiak, K. Zaghib, J. Garbarczyk, F. Gendron, A. Mauger and C. M. Julien, Spectrochim. Acta, Part A, 2006, 65, 1007–1013.
27. Y. Khan, S. K. Durrani, M. Mehmood, J. Ahmad, M. R. Khan and S. Firdous, Appl. Surf. Sci., 2010, 257, 1756–1761.
28. A. Amani, P. York, H. De Waard and J. Anwar, Soft Matter, 2011, 7, 2900–2908.
29. M. H. Lee, J.-Y. Kim and H.-K. Song, Chem. Commun., 2010, 46, 6795–6797.
30. Y. Y. Li, F. E. Gabaly, T. R. Ferguson, R. B. Smith, N. C. Bartelt, J. D. Sugar, K. R. Fenton, D. A. Cogswell, A. L. D. Kilcoyne, T. Tyliszczak, M. Z. Bazant and W. C. Chueh, Nat. Mater., 2014, 13, 1149–1156.
31. Q. Wang, D. Wang and B. Wang, Ionics, 2013, 19, 245–252.
32. H. Bi, F. Q. Huang, Y. F. Tang, Z. Q. Liu, T. Q. Lin, J. Chen and W. Zhao, Electrochim. Acta, 2013, 88, 414–420.
33. D. Zhang, X. Yu, Y. F. Wang, R. Cai, Z. P. Shao, X. Z. Liao and Z. F. Ma, J. Electrochem. Soc., 2009, 156, A802–A808.
34. J. Chen, Y. C. Zou, F. Zhang, Y. C. Zhang, F. F. Guo and G. D. Li, J. Alloys Compd., 2013, 563, 264–268.
35. C. S. Sun, Y. Zhang, X. J. Zhang and Z. Zhou, J. Power Sources, 2010, 195, 3680–3683.
36. H. B. Shu, X. Y. Wang, W. C. Wen, Q. Q. Liang, X. K. Yang, Q. L. Wei, B. A. Hu, L. Liu, X. Liu, Y. F. Song, M. Zho, Y. S. Bai, L. L. Jiang, M. F. Chen, S. Y. Yang, J. L. Tan, Y. Q. Liao and H. M. Jiang, Electrochim. Acta, 2013, 89, 479–487.
37. X. F. Zhou, F. Wang, Y. Zhu and Z. P. Liu, J. Mater. Chem., 2011, 21, 3353–3358.
38. H. Liu, P. Zhang, G. C. Li, Q. Wu and Y. P. Wu, J. Solid State Electrochem., 2008, 12, 1011–1015.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14791j

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