Mesoporous CoFe₂O₄ octahedra with high-capacity and long-life lithium storage properties

Abstract

In this work, we present the synthesis of mesoporous CoFe₂O₄ octahedra via the sol–gel method and their application as anode materials for lithium-ion batteries. The samples are characterized with XRD, SEM, and BET techniques. Based on the combined advantages of porous features, anisotropic octahedral structural stability, and multicomponent effect, CoFe₂O₄ achieves good electrochemical lithium storage, especially with a high capacity (992 mA h g⁻¹ at 0.1 A g⁻¹ after 200 cycles) and ultralong cyclic life (3000 cycles at 5 A g⁻¹). Moreover, the structural stability of CoFe₂O₄ octahedra is probed by decomposing the electrodes after they have been cycled.

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Introduction

Transition metal oxides have intrigued extensive research interest for their applications as anode materials for lithium-ion batteries (LIBs) due to their high theoretical capacity, and low cost.^1–4 Metal oxides achieve lithium storage via a conversion reaction, which is associated with terrible volume variation and results in rapid capacity fading. Besides, the intrinsic poor ionic/electric conductivity of metal oxides will cause ineffective charge transfer and sluggish reaction kinetics. Recently, mixed transition metal oxides (MTMOs) have received growing attention because of their adjustable capacity and working voltage from the broad selectivity of compositions.^5–8 CoFe₂O₄, a ternary transition metal oxide with a spinel-structure, has been focused on as a promising candidate for LIB anodes.^9–14 However, CoFe₂O₄ also suffers from terrible capacity fading and poor rate performance. To solve these obstacles, CoFe₂O₄ has been combined with carbon materials and delivered enhanced electrochemical performance.^12,14 For instance, Sener and co-workers have deposited CoFe₂O₄ nanoparticles on Vulcan XC-72 and obtained high capacity.¹⁴ Another effective solution is nanoengineering. Film,¹³ nanorod,⁹ and nanotube¹⁰ structures have been developed to boost the electrochemical performance by shortening the Li⁺ diffusion path. Previous reports demonstrated that hollow¹¹ and porous¹² nanostructures can supply additional void space to accommodate the volume variation and obtain prolonged cyclic life. Despite these developments, the cyclic lives of CoFe₂O₄ based anodes have rarely been extended over 300 cycles and high-rate tests over 1 A g⁻¹ have rarely been reported. Therefore, the cyclic life and high-rate performance of CoFe₂O₄ anodes need further improvement to satisfy the demands for actual application.

Herein, we prepare mesoporous CoFe₂O₄ octahedra using a simple sol–gel method as LIBs anode materials for the first time. According to the previous reports, anisotropy octahedral structures possess structural advantages for lithium storages.^15,16 The mesoporous features can shorten the transport path of Li⁺/electron diffusion, buffer the volume changes, and increase the contact surface area between electrolyte and active materials. Based on the above combined benefits, the present sample exhibits good electrochemical properties, especially cyclic stability and ultralong lifespan at high current density.

Experimental

All reagents are of analytical grade and used without further purification. The typical sol–gel synthesis process is as follows: CoCl₂·6H₂O (0.02 mol), FeCl₃·6H₂O (0.01 mol), and 1.42 g of triblock copolymer pluronic F127 (HO(CH₂CH₂O)₁₀₆(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₁₀₆H) are dissolved in 30 mL ethanol under ultrasonic. The mixture is refluxed at 90 °C for 6 h. The resultant sol is dried at room temperature for 3–5 days till the gel is obtained. Then the resultant gel is calcinated at 500 °C in air for 2 h in a tube furnace to prepare the final products.

The CoFe₂O₄ is characterized with scanning electron microscope (SEM, JEOL JSM-7500F), and powder X-ray diffraction (XRD, Philips X'-pert X-ray diffractometer). N₂ adsorption/desorption isotherms are performed at 77 K on Micromeritics Co. Ltd, Tristar. The total specific surface area is calculated with the multipoint Brunauer–Emmett–Teller (BET) method.

CR2016-type coin cell is used to carry out the electrochemical experiments. The mixture of FeCo₂O₄, acetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 7 : 2 : 1 is compressed onto a copper foil and then vacuum dried at 120 °C for 24 h to fabricate the working electrode. The negative electrode is metallic lithium sheet. 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1 : 1 : 1 in volume) is used as electrolyte. The separator is Celgard 2300 microporous film. The cells are assembled in a glove box filled with high-purity argon. Charge–discharge tests are conducted between 3 and 0.01 V on a LAND CT2001A Battery Cycler. Cyclic voltammetry tests are conducted on a CHI760D (CH Instruments, Shanghai, China) using a coin cell at a scan rate of 0.05 mV s⁻¹. Electrochemical impedance spectroscopy is obtained over the frequency from 0.1 Hz to 100 kHz.

Results and discussion

The XRD pattern of CoFe₂O₄ octahedra is shown in Fig. 1a. All the powder XRD data can be well assigned to the cubic spinel structure CoFe₂O₄, which is consistent with standard cards (JCPDS 22-1086). No diffraction peaks for impurity phases can be detected, signifying high purity of the present sample. And the sharp diffraction peaks indicate its highly crystalline state. Fig. 1b displays the SEM image of CoFe₂O₄ octahedra, in which octahedral-like CoFe₂O₄ with sharp corners and edges are observed in high yield. The edge lengths range from ∼400 nm to 1 μm. The CoFe₂O₄ octahedra are also characterized with TEM. Fig. 1c displays the TEM image of the complete octahedral particles. The octahedra are too large and the electron cannot transmit it. The porous structures cannot be observed clearly. However, many bright contrasts can be detected on the edge of octahedra, suggesting the existence of pores. To reveal the porous feature more clearly, the TEM image is obtained from the broken particle and shown in Fig. 1d. One can see that, the CoFe₂O₄ is composed of compacted small particles. As shown in the inset, abundant bright contrast can be clearly observed between the adjacent particles, revealing the existence of pores. It is believed that, the anisotropy advantages endowed with octahedral structure can effectively boost the lithium storage properties of spinel metal oxide anode, such as improved durability and high reversible capacity.^15,16 The porous structures can shorten the diffusion path of Li⁺, increase the surface area of reaction, and accommodate the volume changes. Therefore, good electrochemical lithium storages can be expected from the octahedral CoFe₂O₄.

Fig. 1 (a) XRD, (b) SEM image, and (c and d) TEM images of CoFe₂O₄ octahedra. The inset in (d) is the magnified TEM image.

BET gas-sorption isotherms of CoFe₂O₄ octahedra are also obtained to further confirm the porous structure. Fig. 2a shows the N₂ sorption/desorption isotherms of CoFe₂O₄, in which the typical type IV curve can be detected, confirming its mesoporous nature.¹⁷ Calculated from that, CoFe₂O₄ exhibits the high BET surface of 46.7 m² g⁻¹ and pore volume of 0.2 cm³ g⁻¹. The corresponding Barrett–Joyner–Halenda (BJH) pore size distribution of CoFe₂O₄ octahedra is shown in Fig. 2b. Observed from it, CoFe₂O₄ shows a wide range in the pore size distribution, including four narrow peaks positioned at 3.6, 4.4, 5.7, and 7.6 nm, in addition to a wide peak centered at 13.2 nm.

Fig. 2 (a) The N₂ adsorption/desorption isotherms and (b) the corresponding pore size distribution of CoFe₂O₄ octahedra.

Inspired by the remarkable structural advantages, the electrochemical performance of CoFe₂O₄ octahedra is measured as anode materials for LIBs. Fig. 3a shows the 1st, 2nd, 5th, 10th, and 100th charge/discharge voltage profiles of CoFe₂O₄ at current density of 0.1 A g⁻¹. A main voltage plateau at ∼0.84 V can be detected at the first discharge sweep, which corresponds to the lithium intercalating into FeCo₂O₄ crystals. Further reaction with lithium results in a sloping voltage to the cutoff voltage of 0.01 V, at which the metallic Fe and Co nanoparticles are formed and dispersed in Li₂O matrix, and the electrolyte is decomposed to form solid electrolyte interphase (SEI) film and Li₂O.¹⁸ The main plateau (0.84 V) shifts higher to ∼1.2 V during the 2nd discharge cycle, indicating that the electrode materials occurs irreversible structural evolution during the 1st discharge process.¹² These voltage profiles are consistent with the previous reports.^10–12 Interestingly, the curves obtained at the 5th, 10th, and 100th cycles overlap very well, suggesting that the capacity can quickly stabilize at the 5th cycle after the initial testing cycles, which reveals the good reversibility of FeCo₂O₄ electrode.

Fig. 3 (a) The 1st, 2nd, 5th, 10th, and 100th galvanostatic charge/discharge profiles of CoFe₂O₄ octahedra at current density of 0.1 A g⁻¹. (b) The initial five CV curves of CoFe₂O₄ electrode at a scanning rate of 0.05 mV s⁻¹. (c) Cycling performance of CoFe₂O₄ electrode at a current density of 0.1 A g⁻¹. (d) Rate capability of CoFe₂O₄ octahedra. (e) Cyclic performance of CoFe₂O₄ at high current density of 5 A g⁻¹ up to ultralong life of 3000 cycles. The inset shows the cyclic performance of the initial 60 cycles. (f) EIS spectra of CoFe₂O₄ electrode after 2 cycles and 30 cycles.

The initial five CV curves of FeCo₂O₄ electrode are also collected. In Fig. 3b, a sharp peak at 0.58 V and a shoulder peak at 0.96 V can be detected at the 1st discharge scan, which should be ascribed to the conversion reaction of Fe³⁺ and Co²⁺ to metallic Fe, and Co, as well as the formation of Li₂O and SEI layer.^11,12 The broad peak of 1.76 V located at the following charge scan corresponds to the oxidation of metal particles to their oxidation state. After the 1st cycle, the density of cathodic peak decreases with the increasing cycles, accompanied by the positive shift of peak position. The similar phenomena can also be observed at the charge curves. These results are in accordance with the previous result.^11,12 Noting that, the 4th and 5th CV curves are almost overlapped, indicating that the capacity keeps stable after the initial 5 testing cycles. This is consistent with the result of Fig. 3a.

The cyclic performance of CoFe₂O₄ octahedra at low current density of 0.1 A g⁻¹ is shown in Fig. 3c. Obviously, CoFe₂O₄ exhibits high reversible capacity and excellent cyclic stability all through the test of 200 cycles. During the first cycle, CoFe₂O₄ octahedra anode delivers the high initial discharge and charge capacities of 1680 and 1270 mA h g⁻¹, respectively, corresponding to a coulombic efficiency (CE) of 75.6%. The irreversible capacity loss can be assigned to the formation of SEI film on the electrode surface. At the 2nd cycle, the CE increases sharply to 92.7% and keeps stable at ∼100% after the 5th cycle, revealing the highly reversibility of CoFe₂O₄ electrode. The discharge capacity decreases to 1288 and 1125 mA h g⁻¹ at the 2nd and 3rd cycles, respectively. After that, the reversible capacity keeps stable at ∼970 ± 20 mA h g⁻¹ throughout the test. Noting that, these values are higher than the theoretical capacity of CoFe₂O₄ (916 mA h g⁻¹), which probably should be attributed to the reversible formation of gel-like polymer film and the interfacial lithium storage onto the high surface endowed with porous feature.^19,20 Importantly, a high reversible capacity of 992 mA h g⁻¹ can be retained after a long test of 200 cycles. In comparison with the value at the 4th cycle (993 mA h g⁻¹), CoFe₂O₄ octahedra electrode almost shows no capacity fade after 200 cycles, delivering amazingly high capacity retention. It should be assigned to the superior structural stability of anisotropy octahedra and the porous features that can effectively release the mechanical strain associated with the repeated volume variation.

The rate capability of CoFe₂O₄ octahedra ranging from 0.1 to 5 A g⁻¹ is shown in Fig. 3d. As expected, the capacity decays when the current density increases. CoFe₂O₄ octahedra exhibit reversible capacity of 845, 715, 618, 519, 366 mA h g⁻¹ in the 3rd cycle when the current density is 0.2, 0.5, 1, 2 and 5 A g⁻¹, respectively. When the rate returns back to the low current density of 0.1 A g⁻¹, a high capacity of 827 mA h g⁻¹ can be recovered, showing the high reversibility of CoFe₂O₄ electrode. After the rate test, the same cell is further tested at a high current density of 5 A g⁻¹. In Fig. 3e, excellent cyclic stability can be obtained during this process. After the initial training process of ∼100 cycles, the capacity increases and maintains at 380 ± 15 mA h g⁻¹ all through the 3000 cycles, and a capacity of 384 mA h g⁻¹ can be retained after ultralong cyclic test of 3000 cycles. The capacity of 384 mA h g⁻¹ is higher than the value of 366 mA h g⁻¹ obtained at the rate test, further confirming the superior capacity retention and the excellent structural stability of CoFe₂O₄ electrode. It is worth noting that, it is the first time to extend the cyclic lifespan to 3000 cycles so far, which totally fulfills the practical demands for LIBs. Moreover, the reversible capacities during the whole cyclic process are comparable to the theoretical value of graphite, but charging the cell only needs an amazing short time of 5 min.

Fig. 3f illustrates the EIS spectra of CoFe₂O₄ octahedra after charge/discharge test of 2 cycles and 30 cycles, respectively, which is used as an effective technique to reveal the electrode interfacial kinetics. Clearly, both of the curves possess the depressed semicircle, which is related to the charge transfer resistances R_ct.^9,16,19 The charge transfer resistance of CoFe₂O₄ electrode obtained after 2 cycles is 48 Ω. After 30 cycles, the resistance slightly decreases to 45 Ω, which may be ascribed to the reconstruction of mesoporous octahedra during the reversible lithiation/delithiation.²¹ It is worth noting that, these values are smaller than that of 60 Ω obtained on the reported CoFe₂O₄ nanorods.⁹ The low charge transfer resistance of CoFe₂O₄ octahedra should be attributed to its specific porous structures, which supply pores for electrolyte penetration and shorten the Li⁺ diffusion path, thus decreasing the charge transfer resistance. This should be responsible for the excellent high-rate performance of CoFe₂O₄ octahedra.

To highlight the structural advantages of the present mesoporous octahedra, the lithium storage performance comparison between the present sample and the recently reported CoFe₂O₄ electrodes has been performed. In Table 1, our sample delivers higher capacity, better stability, and longer cyclic life than most of the reported CoFe₂O₄ electrode at low current density.^10,12,14 The hollow CoFe₂O₄ nanospheres presented by Wang et al. showed higher initial capacity of 2264 mA h g⁻¹ at a rate of 0.1C.¹¹ Unfortunately, its cyclic test only lasted 50 cycles. As for the high-rate performance, Chu et al. have synthesized CoFe₂O₄ film and it was tested at high rate of 1C, but the test only lasted 20 cycles.¹³ Wang and co-workers have prepared CoFe₂O₄ nanorods and demonstrated the good high-rate performance of over 300 cycles.⁹ Our groups recently developed CoFe₂O₄ nanotubes for LIBs anode, which exhibited 830 mA h g⁻¹ after 200 cycles at a high rate of 1 A g⁻¹.¹⁰ However, the cyclic lives of nanorods and nanotubes still cannot fulfil the actual demands (500–1000 cycles). Compared with these samples, our present CoFe₂O₄ octahedra achieve ultralong cycling life of over 3000 cycles at a higher rate of 5 A g⁻¹. Moreover, the reversible capacity (380 ± 15 mA h g⁻¹) obtained at such high-rate is comparable to the theoretical capacity of currently commercial carbon anodes.

Table 1 Comparison of the lithium storage performance between the CoFe₂O₄ octahedra and the recently reported CoFe₂O₄ based electrodes

Samples	Current rate (A g^−1)	Initial capacity (mA h g^−1)	Capacity retention (mA h g^−1)/(cycles)	Ref.
CoFe2O4 octahedra	0.1	1680	992/200	Present
	5		384/3000
CoFe2O4 nanorods	0.15	1694		9/2014
	1		800/300
CoFe2O4 nanotubes	0.1	1036	988/100	10/2015
	1		830/200
CoFe2O4 nanospheres	0.1C	2264	1185/50	11/2012
Nanospheres/CNT	0.2	1517	1045/100	12/2013
Thin films	1C	1280	610/20	13/2004
CoFe2O4–Vulcan	0.1C	863	Fade 5%/20	14/2015

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To reveal the root of the superior cycling stability of the CoFe₂O₄ octahedra, SEM and TEM images of active materials are obtained from the cells after 100 cycles at 0.1 A g⁻¹. As shown in the SEM image of Fig. 4a, most of the CoFe₂O₄ particles maintain the intact octahedral morphology, except that the surface of few particles has broken. It supplies additional surface for electrolyte penetration, which could explain the decrease in charge transfer resistance after 30 cycles.²¹ And the particle size shows no detectable change, suggesting that the loss of active materials is negligible. Therefore, no capacity fade can be found after 100 cycles. Fig. 4b displays the corresponding TEM image, in which the morphology of CoFe₂O₄ particles shows no detectable changes. However, in comparison with CoFe₂O₄ particles before electrochemical tested (Fig. 1c), the edge of these particles is no longer as sharp as it used to be. This should be assigned to the cycled electrochemical lithiation/delithiation. All the SEM and TEM results show the excellent structural stability of octahedral particles during repeated electrochemical cycles, which should be responsible for the excellent cycling stability and good capacity retention of the CoFe₂O₄ electrode.

Fig. 4 (a) SEM image and (b) TEM image of CoFe₂O₄ octahedra after 100 cycles at 0.1 A g⁻¹.

Conclusions

In conclusion, mesoporous CoFe₂O₄ octahedra have been synthesized through a sol–gel method followed by thermal treatment for the first time. The results demonstrate that the CoFe₂O₄ octahedra exhibit remarkably high and stable lithium storages as well as ultralong cycling life when tested as anode materials for LIBs. The CoFe₂O₄ electrode can deliver a high reversible capacity of 992 mA h g⁻¹ after 200 cycles at a current density of 0.1 A g⁻¹. It also shows the longest cyclic life of over 3000 cycles among the reported results of CoFe₂O₄-based electrode. The combined advantages of good structure durability, porous features, and multicomponent effect should be responsible for its excellent electrochemical lithium storage properties. These results indicate that the present CoFe₂O₄ octahedra would be a promising anode material for high-performance LIBs.

Acknowledgements

We gratefully acknowledge for the National Natural Science Foundation of China (21003079) and Shandong Provincial Natural Science Foundation, China (ZR2014JL015, ZR2014EMM004).

Notes and references

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