Facile and economical synthesis for “plum pudding”-shaped porous LiFePO₄/carbon composites for lithium ion batteries

Abstract

A facile and economical template-free method has been developed to prepare “plum pudding”-shaped porous LiFePO₄/C electrode materials for lithium ion batteries, which are synthesized by a one-step, dry ball milling with inexpensive Fe³⁺ salt as the raw material assisted by carbothermal reduction. Compared with a sample by ball milling with water, dry ball milling was beneficial to homogeneous nucleation of LiFePO₄ in drying and subsequent thermal treatment processes. This material shows plentiful LiFePO₄ nanospheres (∼200 nm) uniformly lodged in the 3D porous carbon architecture as an interconnected conductive framework due to the dry ball milling process. The dry-milling sample possessed nanoscale, active electrode materials (an average size distribution of ∼200 nm) with increased crystallinity, high surface area (up to 140 m² g⁻¹) and enhanced electronic conductivity contribute to improve the rate capability of the battery. The capacity of this “plum pudding”-shaped porous LiFePO₄/C electrode materials achieved 157.4 mA h g⁻¹ (92.6% of theoretical capacity) at the 0.1 C discharge rate and the practical charge capacity, 154.4 mA h g⁻¹, has been achieved after 100 cycles.

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Introduction

LiFePO₄ has recently been attracting a great deal of attention as a promising substitute for LiCoO₂ due to its potential uses.^1–3 As a new cathode material for lithium rechargeable batteries, LiFePO₄ has many advantages over conventional cathode materials, such as high competitive theoretical capacity (170 mA h g⁻¹), stable voltage (3.4 V), low cost, environmentally benign, and safety.^4,5 Therefore it has been used in batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs).^6–9 However, the low intrinsic electrical conductivity and the sluggish Li⁺ diffusion across the LiFePO₄/FePO₄ interface have limited its application in high-power-density batteries .The poor high-rate performances of pure LiFePO₄ make it difficult to make full use of LiFePO₄ cathodes in lithium rechargeable batteries.^10–13 Thus far, many approaches have been adopted to improve the electrical conductivity and Li⁺ diffusion capability of LiFePO₄ by selective doping of supervalent cations in Fe sites,^6,10,11 reducing the particle-size to the sub-micron and even nano-scale level,^12,14 creating an ion-conducting surface phase (such as Li₄P₂O₇),¹² and coating the LiFePO₄ particles with electronically conductive agents such as carbon,^3,15–20 metals or metal oxides,^21–23 conductive polymer²⁴ and so on.

The electrochemical performance of electrode materials is closely related to its surface area, particle size and distribution, particulate morphology, carbon content, carbon morphology, phase purity, and so on.^25–27 Based on previous publications,^3,15–20 the 3D architecture has been considered an alternative optimum structure design for electrode materials. The benefits that may be realized for this structure are improvements in energy per unit area and high-rate discharge capabilities. Among the different 3D architectures for LiFePO₄ cathode materials, the 3D porous carbon framework architectures that combine host LiFePO₄ particles into a single incorporated entity for lithium ion storage can be effective. The use of 3D porous carbon framework has several advantages:^3,15–20 (1) the growth of LiFePO₄ particles can be significantly restricted on the nanoscale so as to shorten the diffusion route of lithium ions by the solid carbon architecture in synthesis and this 3D porous carbon framework also effectively prevent unfavorable aggregations of active material particles; (2) the strain on or from LiFePO₄/FePO₄ two-phase transformation also could be released by the rigid carbon skeleton during the lithium ion insertion/extraction; (3) an important feature of the carbon framework is to slow or prevent detrimental corrosion processes in lithium ion batteries; and (4) the excellent inter-particle electronic conductivity and continuous high surface area porous network allow an efficient transport route for electrolytes throughout the electrode, and hence may offer high energy and power capacities in lithium ion batteries. Among the various methods used to create this versatile porous carbon framework for lodging LiFePO₄ nanoparticles, template approaches have been employed.^{16–18,28–30} Doherty et al.¹⁷ reported the preparation of hierarchically porous monolithic LiFePO₄/C composite by a nanocasting templating technique. This composite monolith electrode attained capacities of 140 mA h g⁻¹ at a discharge rate of 0.1 C and 100 mA h g⁻¹ at 5 C. However, the preparation of meso/macroporous silica monolith and a chemical etch of the monolith after carbonizing is time consuming. They also used colloidal crystal as a template to produce hierarchically porous LiFePO₄.¹⁶ The authors succeeded in preparing electrode materials with the largest pores, around 100 nm in diameter, which showed better discharge capacities for LiFePO₄ of 160 mA h g⁻¹ at 0.1 C and 115 mA h g⁻¹ at a fast discharge rate of 5 C. However, this process is very complex, including the preparation of PMMA colloidal crystal template. Thus, template approaches using triblock copolymer,^16,18 citric acid,²⁸ and other hard or soft templates,^17,29,30 always are difficult to expand to large-scale commercial applications due to fatal disadvantages related to high cost and complicated synthetic procedures.^31,32

In this work, we developed a one-step template-free methodology for preparing “plum pudding”-shaped porous LiFePO₄/C composites, with LiFePO₄ nanospheres within a 3D porous carbon framework. This facile and economical method can allow mixing the starting ingredients (inexpensive Fe³⁺ salt) by dry ball milling followed by the carbothermal reduction reaction at 700 °C, and glucose was used as the carbon source to produce a 3D carbon framework full of macro/mesoporous pores. In order to create this versatile carbon framework, rather high carbon content is unavoidable; consequently, attempts are underway solve this problem.^17,18 The “plum pudding”-shaped porous LiFePO₄/C composites reveal considerably enhanced electronic conductivity and remarkably high surface area, which may provide materials with interesting properties for applications in lithium ion batteries.

Experimental

Material preparation

The “plum pudding”-shaped porous LiFePO₄/C composite was prepared via ball milling. The starting materials—LiAC·2H₂O, Fe(NO₃)₃·9H₂O, NH₄H₂PO₄ and glucose—were placed in a stainless steel cylindrical container with stainless steel balls. The precursors were mixed by ball milling at 400 rpm for 4 h and subsequently aged for 12 h in tanks. The mixture was initially dried in a vacuum oven at 80 °C, hand grinded with an agate pestle and mortar and then heated in nitrogen atmosphere at 700 °C for 10 h. All heating rates were 5 °C min⁻¹. The products were obtained after cooling at room temperature. The molar ratio of Li : Fe : P : C in the precursor was 1 : 1 : 1 : 2. Carbon converted from glucose acted as the reducing agent in the synthesis process and as the conducting agent in the resulting sample; the carbon content in the LFP-A is 31.2% (Fig. S1, ESI†). Two sample types, including dry ball milling and ball milling with water, were labeled LFP-A and LFP-B, respectively.

Structural and morphological characterization

The crystalline phases of LiFePO₄/C samples were identified through X-ray diffraction (XRD) under a Bruker D8 X-ray diffraction meter with monochromatic Cu-K radiation (λ = 0.154056 nm). The size and surface morphology of the samples were observed with a Hitachi S-4800 field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) images were captured on the JEM-2100 instrument microscopy at an acceleration voltage of 200 kV to investigate the diameter and distribution of LiFePO₄ particles within the carbon framework as well as characteristics of the carbon-coating layer. Thermogravimetric (TG) measurements of the products were monitored using a NETZSCH STA 409 PC from 30 to 900 °C under oxygen with a heating rate of 10 °C min⁻¹. The porous structures of the samples were measured by an N₂ adsorption isotherm using a Micromeritics ASAP 2010 at 77 K. The specific surface areas were calculated using the Brunauer–Emmet–Teller (BET) method. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas based on adsorption data in the partial pressure (P/P₀) range of 0.05–0.2. The pore volumes and pore size distributions derived from the desorption branches of isotherms were estimated based on the Barett–Joyner–Halenda (BJH) model; the total pore volumes (V_pore) were computed from the adsorbed amount at a relative pressure (P/P₀) of 0.992.

Electrochemical tests

Test cathodes were prepared by mixing with acetylene black and polyvinylidene fluoride (PVDF) with a weight ratio of 75 : 15 : 10 in N-methyl-2-pyrrolidone (NMP) solvent to produce the slurry. After mixing for 24 hours, the resultant viscous slurry was spread into the aluminum current collector and dried at 120 °C overnight under vacuum. After rolling, the obtained sheets were cut into circular strips of 16 mm in diameter, and about 3.0 ± 0.1 mg cm⁻² active materials were loaded on an Al foil. The strips were dried at 80 °C for 24 hours. Electrochemical measurements were conducted in Li test cells with lithium foil as counter and reference electrodes. All test cells contained 1.0 mol L⁻¹ LiPF₆/EC-DEC (1 : 1 vol%) as an electrolyte and were assembled in an argon-filled glove box. Galvanostatic charge/discharge reactions were performed in the voltage range of 2.5–4.2 V on the CT2001A LAND Battery Tester. The electrochemical storage capacities of samples were calculated from the mass of LiFePO₄ with the amount of carbon subtracted. The cyclic voltammetry (CV) of samples was performed with the scanning rate at 1 mV s⁻¹ on the CHI660C electrochemical workstation (Chen Hua Instruments, China). Electrochemical impedance spectroscopy (EIS) measurements using a Solartron 1260 frequency response analyser coupled to a Solartron 1287 potentiostat were obtained at frequencies between 100 kHz and 0.01 Hz. The amplitude of the sinusoidal potential signal was 10 mV.

Results and discussion

Crystalline structure analysis

Fig. 1 shows the XRD patterns of LiFePO₄/C composites with water or without mediums during ball milling followed by the carbothermal reduction reaction at 700 °C. The sharp diffraction peaks of the samples without any obvious impurity phase indicate well-crystallized LiFePO₄/C in an olivine type structure indicates successful synthesization. All diffraction peaks can be indexed on the basis of orthorhombic olivine-type structure (JCPDS no. 83-2092) with the Pnma space group. Moreover, widening of peaks occurs as dry milling was increased, suggesting a decrease in LiFePO₄ crystallite diameter, which is in agreement with the results of FE-SEM and TEM images shown in Fig. 2. The lattice constants of the as-synthesized samples are listed in Table 1, similar to the previous report.¹ In addition, the reduced lattice constants indicate that the crystalline growth of LiFePO₄ crystals may be restricted due to the hard carbon framework.²⁰

Fig. 1 XRD patterns of LiFePO₄/C composites.

Fig. 2 FE-SEM and TEM images of LiFePO₄/C composites: (a) and (c) LFP-A; (b) and (d) LFP-B; (e) and (f) TEM and HR-TEM images of LFP-A.

Table 1 Structural lattice parameters and cell volumes of LiFePO₄/C composites

Sample	a (Å)	b (Å)	c (Å)	V (Å^3)
Standard pattern	10.347	6.019	4.704	292.9
LFP-A	10.318	6.022	4.699	292.0
LFP-B	10.367	6.020	4.690	292.7

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Morphological analysis

Fig. 2(a) displays an FE-SEM image of the LiFePO₄/C composite heat treated at 700 °C for 10 h after dry milling. As illustrated in Fig. 2(a), many densely aggregated LiFePO₄ nanospheres cover the surface of the particles. These LiFePO₄ nanospheres show a uniform distribution of particle size with an average diameter of ∼150 nm. The elemental mappings of C, Fe, P, and O for LFP-A are shown in Fig. S2 (in ESI†), indicating a homogeneous distribution of the continuous guest carbon framework and host LiFePO₄ lodgers. In contrast, as shown in the SEM images (Fig. 2(b)), the LFP-B consisted of LiFePO₄ nanospheres of about 100–500 nm in diameter, with a larger particle size distribution and a serious agglomeration of LiFePO₄ particles, which is consistent with its TEM image (Fig. 2(d)). Furthermore, as seen in the TEM image (Fig. 2(c)), the LFP-A shows significant plum pudding shape consisting of numerous LiFePO₄ nanospheres of about 100–150 nm in diameter uniformly embedded in a carbon matrix. The smaller LiFePO₄ nanospheres inside of the samples, especially LFP-A, indicate that this carbon matrix has a significant effect on the crystal size of LiFePO₄ due to its additional role as a barrier layer to restrict LiFePO₄ crystal growth during thermal treatment. Next, the region marked with red and blue rectangles in Fig. 2(c) analyzed by HRTEM is shown in Fig. 2(e); interspaces of the larger LiFePO₄ nanospheres were filled with abundant smaller spheres in the continuous carbon matrix, which contributed to obtaining higher tap density of this cathode material to guarantee high-energy density in working electrodes.²⁰ From the deep observation marked with the red rectangle in HRTEM image in Fig. 2(f), it can be seen that the carbon layer closely coated on the surface of LiFePO₄ nanosphere of the LFP-A is about 4 nm thick. Meanwhile, a virtual fingerprint is shown in the core of the particle, indicating LiFePO₄ particles with a high degree of crystallization. This carbon structure in combination with the perfect carbon coating layer enhances greatly the electronic conductivity of LiFePO₄. It is believed that the small particle size of LiFePO₄ is useful for the intercalation/deintercalation process of lithium ions, that is, the aforementioned LiFePO₄ nanospheres. In addition, a large number of studies have shown that lithium ion diffusion kinetics are related to superior high-power electrochemical performance and properties. The ability of lithium ions to travel across the interface between LiFePO₄ and electrolyte phases is crucial for ultrafast diffusion, so this perfect carbon coating layer could accelerate the mobility of lithium ions. Therefore, this “plum pudding”-shaped porous LiFePO₄/C composite is very suitable as an electrode material to improve the electrochemical properties of lithium ion batteries.

Further details regarding porosities of this intriguing LiFePO₄/C composite material were obtained through nitrogen adsorption measurements. The nitrogen adsorption–desorption isotherms of both LFP-A and LFP-B composite material exhibited typical IV shapes with H3-type hysteresis loops in Fig. 3(a),³³ revealing the porous characteristic of them all with abundant macro/mesopores. In addition, the adsorption–desorption curves do not level off at relative pressures close to the saturation vapor pressure, suggesting the presence of slit-like pores that might be caused by LiFePO₄ nanoparticles. The Barrett–Joyner–Halenda (BJH) desorption pore-size distribution curves of the composite material calculated from the desorption branch were shown in Fig. 3(b), based on the Kelvin equation and corrected for multilayer adsorption. The pore sizes, BET surface areas and pore volumes of materials calculated from the N₂ adsorption–desorption data are summarized in Table 2. In contrast to previous LiFePO₄/C products, the BET surface area of these samples is obviously higher, up to more than 100 m² g⁻¹. This appreciably high BET surface area probably results from the porous carbon framework formed from the vigorous gas evolution produced from precursor decompositions (mainly nitrate, ammonium dihydrogen phosphate and organic compounds) during thermal treatment.^20,32,34,35 The LFP-A sample with the higher BET surface area may benefit from a relatively regular spherical type with uniform scale of LiFePO₄ nanospheres. In addition, the nanoscale size of the highly dispersed LiFePO₄ also results in increased surface area. Overall, the significant porosity of cathode materials can facilitate the access and accommodation of electrolytes and shortens the diffusion length of lithium ions to achieve high power density in electrode materials.

Fig. 3 Nitrogen adsorption–desorption isotherms (a) and pore size distribution curves (b) of LiFePO₄/C composites.

Table 2 Pore structure parameters of LiFePO₄/C composites

sample	Specific surface area (m^2 g^−1)	Pore volume(cm^3 g^−1)	Aperture (nm)
LFP-A	140.6	0.165	6.5
LFP-B	102.3	0.167	4.7

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As discussed above, the LiFePO₄/C composite shows that LiFePO₄ nanospheres are tightly lodged in the 3D porous carbon framework, visible in the schematic diagram in Fig. 4. The formation process of the “plum pudding”-shaped porous LiFePO₄/C composite (LFP-A) can be visualized based on the following description. The starting materials can be homogeneously mixed by ball milling, and sealed stainless steel tanks provide a relatively high temperature and pressure environment after ball milling, which is beneficial to the initial nucleation of spherical LiFePO₄ nanoparticles and formation of the carbon framework.^20,26 In addition, the dispersion medium in the ball-milling process has an effect on granule dispersion and aggregation state of the raw material. This effect leads to a mixed particle reunion state of the pulp after drying, finally influencing the mass transfer process of reactants in the sintering process. Water, of course, is a common polar solvent and possesses high boiling point. The higher the boiling point, the slower the drying rate. In the drying process after ball milling, the augmented and formed speed of particles will be slow, and the component segregation degree will increase. Therefore, the aggregation of particles and degree of segregation of composites are greater due to the slow drying rate of large amounts of water, thus leading to relatively low particle distribution. In addition, the sealed stainless steel tanks provide a relatively high temperature environment for ball milling. And ferric nitrate is a special iron source, as it has a lot of crystal water and low melting point (47.2 °C), so melts in the process of ball milling. The molten ferric nitrate and some water derived from the crystal water likely serve as a dispersant, which would enhance the effects of dry ball milling. Moreover, because of proportionately less water, the drying rate is accelerated, thus leading to better particle size distribution and more uniform structure from dry ball milling.

Fig. 4 Schematic diagram of the “plum pudding”-shaped porous LiFePO₄/C composite in 3D structure.

Electrochemical performance

The first and fifth cycles in cyclic voltammogram (CV) determination of the as-obtained LiFePO₄/C cell tested between 2.5 V and 4.2 V at a scanning rate of 0.1 mV s⁻¹ is shown in Fig. 5. All samples showed very high peak symmetry, indicating that all had good cycle reversibility, benefiting from the established 3D porous carbon fabric as a conductive network for ultrafast electron transfer and a capacious container for sufficient electrolyte penetration. The reduction and oxidation peaks in the first cycle of the LFP-A appear at 3.19 V and 3.68 V, indicating that Fe²⁺/Fe³⁺ redox pairs contribute to the gain and loss of electrons in the LiFePO₄ crystal structures of the sample during the lithium insertion and extraction process. The voltage difference between the oxidation and reduction peaks is 0.48 V. However, in the first cycle CV curve of the LFP-B, the reduction peak is observed at a potential of 3.15 V and the corresponding oxidation peak is observed at 3.77 V, resulting in a voltage difference of 0.62 V. The CV curve for the LFP-A with dry milling showed a better peak symmetry and narrower voltage separation. In addition, the reproducibility in CV measurements among samples is shown in Fig. 5. After four cycles, LFP-A shows much better reproducibility than LFP-B, proving the outstanding reversibility of LFP-A as cathode material. As discussed previously, the LFP-A prepared by dry milling exhibited weaker polarization and more lithium ions and electrons participating actively in redox reactions, which allowed reversible electrochemical reactions during extraction and insertion of lithium ions. Moreover, the LFP-A showed higher peak current density, visible in the SEM and TEM of LFP-A (Fig. 2). These nanoscale LiFePO₄ spheres can provide various advantages in terms of reducing the diffusion path of lithium ions and offering more active sites for electrochemical reactions.

Fig. 5 First and fifth cycles in cyclic voltammogram (CV) determination of LFP-A and LFP-B at scan rate 0.1 mV s⁻¹.

To further demonstrate the effects of ball milling in the kinetic process of the electrode materials, electrochemical impedance spectra (EIS) measurements were carried out in three two-electrode coin cells under the discharging platform condition. Fig. 6 shows EIS data of LiFePO₄/C samples with and without solvent. The corresponding equivalent circuit is proposed to fit the impedance plots as seen in the inset image of Fig. 6. R_s is caused by the liquid phase ohmic resistance when charges transfer from the counterelectrodes to the electrolyte, the constant phase element (CPE) represents the charge-transfer process at the electrolyte/electrode interface, R_ct results from the charge-transfer resistance, W₀ is caused by the nonhomogeneous diffusion impedance, and W₀ − T is the Warburg coefficient. The fitting parameters using Zview impedance analysis software 2.80 are shown in Table 3.

Fig. 6 Electrochemical impedance spectra measurements (EIS) of LFP-A and LFP-B (inset: equivalent circuit).

Table 3 Fitted results of electrochemical impedance spectra for LiFePO₄/C composites

Sample	Rs (Ω)	CPE (μF)	Rct (Ω)	W0 − T
LFP-A	25.8	24.0	483	0.35
LFP-B	19.7	10.5	550	0.32

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All EISs are comprised of an intercept at high frequency followed a depressed semicircle in the middle-high frequency region and a sloping line in the low frequency region. The high frequency intercept represents the ohmic resistance (R_s) at the real axis, while the semicircle middle-high frequency region basically deals with the complicated reactions occurring at the electrolyte–electrode interface, which mainly includes the charge-transfer resistance of electrons and lithium ions (R_ct) and corresponding capacities (CPE). The sloping line ascribes the Warburg impedance (W₀), associated with lithium ion diffusion through the LiFePO₄ electrode, as indicated in the inset of Fig. 6. In the middle-high frequency region, the LFP-A exhibited a smaller charge-transfer resistance while the LiFePO₄/C composite prepared from ball milling with water solvent had the largest value. The lower R_ct value of the LFP-A indicates a lower electrochemical polarization and the charge-transfer resistance may be the limiting factor for the electrochemistry results, which can be attributed to smaller particles. The smaller particle size is associated with the lower electronic and/or ionic resistance at the boundary of the crystallites within each polycrystalline particle of the active material, thus improving the reversible capacity of the LiFePO₄/C material. Furthermore, narrower particle size distribution and uniform dispersion of the LiFePO₄ nanospheres help to form the interconnected carbon framework, which significantly improve the conductivity of the LiFePO₄/C composite.

The first charge-discharge profiles of the LiFePO₄/C samples at the 0.1 C rate are shown in Fig. 7(a). For comparison, samples of LiFePO₄/C produced by ball milling with water solvent or solvent-free are also shown by dotted lines in the figure. All samples possess a flat plateau around 3.4 V, corresponding to the two-phase (LiFePO₄ ↔ FePO₄) transformation model. Near the starting and ending discharge, two slopes are observed, attributed to the small single-phase domain as reported by Yamada and co-workers. The LFP-A has a discharge capacity of 157 mA h g⁻¹, which corresponds to 92% of the theoretical capacity of LiFePO₄ (170 mA h g⁻¹), whereas sample LFP-B shows reduced capacities (140 mA h g⁻¹). Furthermore, the solvent-free LiFePO₄/C has a much smaller polarization loss and irreversible capacity. Taking into account the cyclability of the samples, Fig. 7(b) shows remarkable electrochemical cycling stability of LFP-A with less than 6% decay in discharge capacity (better than 94% retention) up to 100 cycles. The LFP-B electrode material had only 89.6% capacitance retention after 100 cycles. In addition, the Coulombic efficiency of the LFP-A is higher than LFP-B, which maintained above 99% for all the charge and discharge processes, confirming excellent reversibility. Next, the discharge capacities of the LiFePO₄/C composites prepared into electrodes are presented in Fig. 7(c and d); all samples were cycled 5 times at various discharge current rates of 0.1 C, 1 C, 2 C, 5 C, and 10 C. The capacity of the LiFePO₄/C composite prepared with water solvent decreased quickly with the increase in discharge rate: 144, 121, 100, 79, and 56 mA h g⁻¹ at the discharge current rate from low to high, respectively. The “plum pudding”-shaped porous LiFePO₄/C composite (LFP-A) by ball milling for 4 h without solvent showed higher capacities at all investigated discharge rates—157, 138, 122, 110, and 82 mA h g⁻¹ at 0.1, 1, 2, 5, and 10 C discharge rates, respectively. This excellent electrochemical performance is likely due to the structural features of the composite material, with plentiful LiFePO₄ nanospheres uniformly lodged in the 3D porous carbon architectures. The fast transport of electron and lithium ion between LiFePO₄ nanospheres and electrolyte enhances lithium ion diffusion in the LiFePO₄/C composite, which can be ascribed to the 3D porous and equally distributed nanoscale LiFePO₄. Moreover, the thin carbon layer coating and the interconnected conductive carbon framework are beneficial toward improving electron diffusion. Therefore, this “plum pudding“-shaped porous LiFePO₄/C composite (LFP-A) as cathode material can apply high energy and power densities in lithium ion batteries.

Fig. 7 Electrochemical performance of LiFePO₄/C composites: (a) first charge/discharge curves and (b) cycle performance and Coulombic efficiency curves of LFP-A and LFP-B at 0.1 C; rate capabilities of LEP-A (c) and LFP-B (d). Cells were charged to 4.2 V at 0.1 C and then discharged at various rates.

Conclusion

A facile and economical one-step method based on a ball milling approach has been established for synthesis of a “plum pudding”-shaped porous LiFePO₄/C composite. The presence of porosity carbon framework architecture for lodging LiFePO₄ nanospheres is an effective approach to improve performance of the “plum pudding”-shaped porous LiFePO₄/C electrode. This unique carbon structure could enhance electrical conductivity by increasing electron transport between LiFePO₄ nanospheres and provide abundant porosities, allowing the electrolyte to penetrate deep into the electrode material. SEM and TEM observations reveal that the LiFePO₄ nanospheres mediated by dry ball milling consist of nearly homogenous nanocrystals with a size of ∼150–200 nm, while the LiFePO₄ nanospheres by ball milling with water leads to large agglomerates. Galvanostatic testing showed that the “plum pudding”-shaped porous LiFePO₄/C composite exhibits a high electrochemical activity in terms of discharge capacity, cyclability, and rate capability, such as largest reversible capacity of 157.8 mA h g⁻¹ at 0.1 C, best rate capability of 84.7 mA h g⁻¹ at 10 C, and excellent cyclic stability, which retrieves nearly 98.1% of the starting capability in 100 cycles after discharging at 0.1 C. As such, cathodes fabricated from this composite are promising for high-power, electrical energy storage applications such as electric vehicles and power tools.

Acknowledgements

The authors appreciate financial support from the National Natural Science Foundation of China (51372115).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05342g

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