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Single-crystalline LiMn2O4 nanorods as cathode material with enhanced performance for Li-ion battery synthesized via template-engaged reaction
Solid State Ionics 239 (2013) 8–14
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Single-crystalline LiMn2O4 nanorods as cathode material with enhanced performance for Li-ion battery synthesized via template-engaged reaction Dan Zhan, Qinggang Zhang, Xiaohong Hu ⁎, Guozhu Zhu, Tianyou Peng ⁎ College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
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Article history: Received 29 September 2012 Received in revised form 4 March 2013 Accepted 8 March 2013 Available online 9 April 2013 Keywords: LiMn2O4 Single-crystalline nanorod Transformation Electrochemical property
a b s t r a c t Single-crystalline LiMn2O4 nanorods with a diameter of ~ 100 nm were synthesized via a template-engaged reaction by using tetragonal β-MnO2 nanorods as starting material. The investigations on the structures and morphologies of both the precursor and the final product reveal that a minimal structure reconstruction can be responsible for the chemical transformation from tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods. The obtained LiMn2O4 nanorods as cathode material for Li-ion battery exhibit superior high-rate capability and good cycling stability in a potential range of 3.5–4.3 V vs. Li+/Li, which can deliver an initial discharge capacity of 125 mAh g−1 (> 84% of the theoretical capacity of LiMn2O4) at a current rate of 0.5 C, and about 75% of its initial capacity can be remained after 500 charge–discharge cycles at a current rate of 3 C. Importantly, the rod-like nanostructure and single-crystalline nature are also well preserved after prolonging the charge/discharge cycling time at a relatively high current rate, indicating good structural stability of the single-crystalline nanorods during the lithium intercalation/deintercalation processes, and such high-rate capacity and cycling performance can be ascribed to the favorable morphology and the high crystallinity of the obtained LiMn2O4 nanorods. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable Li-ion battery, as one of the most important energy storage device, has attracted much interest owing to its high density, light-weight, and long cycle life. Some have been widely used in electronic devices, portable power tools, hybrid electric vehicles (HEVs) and many power supplies. Among those cathode materials for Li-ion battery reported, spinel LiMn2O4 is considered as one of the most promising candidates due to its advantages such as low-cost, environmental friendliness and high abundance [1]. However, the application of LiMn2O4 is confined by the capacity decay and cycling instability caused by manganese dissolution, oxygen vacancy, and Jahn–Teller effect [2–4]. Therefore, various approaches have been employed to solve the above problem. For example, coating and doping techniques have been applied to alleviate the dissolution of manganese [5,6]. Moreover, it has been reported that the particle size, morphology and crystallinity of materials strongly affect the discharge capacity, cycle life and rate performance of the Li-ion battery [7–12]. Therefore, LiMn2O4 with various nanostructured morphologies, such as nanoparticles [8], nanowires [9], nanorods [10], nanotubes [11], and even nanochain materials [12] have been prepared and used to improve the electrochemical performances of LiMn2O4 due to the short diffusion distances for Li + within the particles and the large surface-to-volume ratio, which allows for a large electrode–electrolyte contact interface [7]. ⁎ Corresponding authors. Tel./fax: +86 27 6875 2237. E-mail addresses: [email protected] (X. Hu), [email protected] (T. Peng). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.03.015
Among the various morphologies mentioned above, one-dimensional (1 D) nanowires or nanorods not only have large surface-to-volume ratio, but also provide efficient electron transport pathway. In addition, the nonwoven fabric morphology can suppress the aggregation and grain growth at an elevated temperature, and the potential barrier can be ignored to decrease the electronic resistance among the nanograins, which has been confirmed in dye-sensitized solar cells [13] and supercapacitors [14]. Those nanorod- or nanowire-like morphologies of LiMn2O4 seem also suitable for the application in Li-ion battery. However, nanorods with cubic spinel structure such as LiMn2O4 are difficult to synthesize because the cubic crystals usually cannot grow in a one-dimensional direction. Since β-MnO2 has been reported to have a particular 1D structure with diameters of 40–100 nm [15], it has been considered as a possible precursor to produce LiMn2O4 nanorods. Nevertheless, the low crystallinity of the β-MnO2 precursor often affects the grain size distribution of the lithiated phase, and it is also a formidable challenge to maintain the nanorod morphology during the following phase and chemical transformations [10,16,17]. Luo et al. [10] have synthesized LiMn2O4 nanorods with an initial discharge capacity of 100 mAh g−1 at a current rate of 1 C through a complex refluxing process, which involves poisonous acetonitrile and calcination process by using α-MnO2 nanorods as precursor. Moreover, β-MnO2 nanorods were also used as precursor to synthesize LiMn2O4 nanorods, which offer preferable electrochemical property such as more than 85% capacity retention after 100 cycles at a current rate of 1 C at room temperature [16,17]. Although the above LiMn2O4 nanorods were synthesized by regulating the nanorod morphology of the α-MnO2 or β-MnO2 precursor,
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at 80 °C for 24 h in air to obtain the β-MnO2 precursor. Secondly, the above β-MnO2 and LiOH·H2O were mixed at 2:1 molar ratio and dispersed into 5 ml methanol to form a thick slurry, which was further ground for several hours, and then dried at room temperature. The obtained powder was calcined in tube furnace at 700 °C for 10 h in air to obtain spinel LiMn2O4 nanorods (marked as LiMn2O4-NRS). As a contrast, LiMn2O4 particles were also prepared by a traditional solid state reaction method (marked as LiMn2O4-SSR) from commercially available β-MnO2 without any further treatment and LiOH·H2O mixed at the same molar ratio of 2:1. For all calcinations in tube furnace, samples were heated from room temperature at a heating rate of 10 °C min −1 and cooled naturally to room temperature. All reagents used were of analytical grade without further treatment. 2.2. Material characterization Fig. 1. XRD patterns of the obtained β-MnO2, LiMn2O4-NRS and LiMn2O4-SSR.
little information about the transformation from the tetragonal MnO2 to cubic spinel LiMn2O4 without morphology transition at an elevated temperature. Herein, single-crystalline LiMn2O4 nanorods with cubic spinel structure were synthesized based on a top chemical method as the above mentioned route by using tetragonal β-MnO2 nanorods as precursor, and the phase and composition transformation process in the template-engaged reaction from tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods was explored in detail. The electrochemical performance of the as-synthesized single-crystalline LiMn2O4 nanorods was studied by galvanostatic charge/discharge cycling. Moreover, the structure and morphology of the LiMn2O4 nanorods after prolonged electrochemical cycling time were also investigated, which confirmed the stability of the single-crystalline LiMn2O4 nanorods as cathode materials for rechargeable Li-ion battery, and it was found that the singlecrystalline LiMn2O4 nanorods showed much better electrochemical cycle performance as compared to the LiMn2O4 particles derived from a commercial β-MnO2. 2. Experimental 2.1. Material synthesis The synthesis of LiMn2O4 nanorods is carried out through a twostep procedure. Firstly, the tetragonal β-MnO2 nanorods were prepared by a hydrothermal process similar to the previous report [15]. Typically, 10 mmol MnSO4 and 10 mmol (NH4)2S2O8 were dissolved in 140 ml of distilled water to form a homogeneous solution under magnetically stirring at room temperature. The mixed solution was transferred to a 200 ml Teflon-lined stainless autoclave and heated at 120 °C for 12 h. The resultant precipitate was washed with deionized water and ethanol in succession to remove the sulfate ions and other remnants. The obtained black powder was subsequently dried
X-ray diffraction (XRD) measurements of β-MnO2 and LiMn2O4 were carried out by Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The morphologies were studied with a JSM 6700F field emission scanning electron microscope (FESEM) and a JEOL JEM-2100 high resolution transmission electron microscope (HRTEM). The BET surface areas were analyzed by nitrogen adsorption at 77 K with a Micromeritics ASAP2020 nitrogen adsorption apparatus. All samples were degassed at 120 °C for 5 h prior to the nitrogen adsorption apparatus. The tap density of the powders was characterized as follows. 2 g powder was placed in a 5 ml glass measuring cylinder, and then the cylinder was lifted at a height of 10 cm and dropped onto a 2 cm thick ebonite board for about 200 times. The tap density was calculated according to the mass and the final volume of the powder. The electrochemical measurement was performed by using CR2016 coin-type cells assembled in an argon-filled glove box (Mikrouna Super 1220/750) with lithium metal as the negative electrode. The cathode was fabricated by roll-pressing a mixed paste of the active material, carbon black and PTFE with a weight ratio of 80:15:5 into ca. 0.1 mm thick film and then pressing the electrode film onto a stainless mesh. The electrolyte was 1.0 M LiPF6 solved in EC/DMC (1:1 in volume). A Celgard 2300 microporous membrane was used as the separator. The charge–discharge cycles were performed at different C rate (for convenience, 1 C was defined as 148 mAh g−1) between 3.5 V and 4.3 V vs. Li+/Li at room temperature on a battery tester (LAND, CT2001A, China). 3. Results and discussion 3.1. Structure and morphology analyses of β-MnO2 and LiMn2O4 nanorods Fig. 1 shows the XRD patterns of the obtained β-MnO2, LiMn2O4-NRS and LiMn2O4-SSR. As can be seen, the precursor β-MnO2 derived from the hydrothermal process can be ascribed to tetragonal β-MnO2 (JCPDS no. 24-0735) with P42/mnm space group. The sharp and narrow
Fig. 2. SEM (a) and TEM (b) images of the β-MnO2 nanorods derived from a hydrothermal process, the inset in b is a high-magnification TEM image of one nanorod.
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Fig. 3. SEM images of the LiMn2O4-NRS (a) and the LiMn2O4-SSR (b), TEM images of LiMn2O4 nanorods at hlow (c) i and a high (inset in c) magnification, and HRTEM (d) image taken from the circled area in the section of (c), the inset in d is the corresponding FFT pattern recorded with 110 zone axis.
diffraction peaks with no additional impurity phase indicate that the obtained β-MnO2 has high crystallinity. And the LiMn2O4-NRS obtained by using the above β-MnO2 as precursor templates shows a XRD pattern of spinel LiMn2O4 (JCPDS no. 35-0782) with Fd3m space group, and no diffraction peaks of β-MnO2 can be detected, indicating the LiMn2O4-NRS exhibits a relatively high crystallinity and the β-MnO2 is fully transformed into LiMn2O4. The LiMn2O4-SSR derived from the solid state reaction process also shows a XRD pattern similar to the LiMn2O4-NRS. Moreover, it is observed that the full width at half maximum (FWHM) of the diffraction peaks for LiMn2O4-NRS is a little bigger than that for LiMn2O4-SSR, indicating the smaller gain size of LiMn2O4-NRS than that of LiMn2O4-SSR. SEM image (Fig. 2a) shows that the β-MnO2 consists of uniform nanorods with an average diameter of 50 nm and several hundred nanometer lengths. The rod-like nanostructure of the β-MnO2 can be further validated from the TEM images as shown in Fig. 2b. No any other particles can be found from the TEM images, confirming the high yield of β-MnO2 nanorods in the present hydrothermal process. From the TEM images, it can be observed that the nanorod has a diameter of about 50 nm and a length of hundred nanometers, which agrees with that observed from the SEM image. To further confirm whether the nanorod-like morphology can still retain after the high temperature treatment of the solid-state reaction, the SEM and TEM observations were shown in Fig. 3. Obviously, the LiMn2O4-NRS synthesized at 700 °C (Fig. 3a) also consists mainly of nanorods, which appear to have a larger average diameter of 100 nm and a shorter length. However, the LiMn2O4-SSR derived from the commercial β-MnO2 only consists of some irregular, aggregated particles as observed in Fig. 3b, which is consistent with that observed from the XRD pattern in the Fig. 1. More structure information about LiMn2O4 nanorods can be obtained from the TEM and HRTEM images in Fig. 3c and d. The LiMn2O4 nanorods have an average diameter of 100 nm, which is in accordance with the observation from the SEM image. The clear lattice fringes and the well-defined spots in FFT pattern reveal the single-crystal characteristics and high crystallinity of the LiMn2O4 nanorods. As depicted in Fig. 3d, (002) lattice fringes are parallel to the rod axis with d-spacing of about 0.40 nm. The
(111) lattice fringes can also be clearly observed, which corresponds to the strongest peak in the XRD pattern (Fig. 1). The above results imply that the nanorods can grow preferentially along the [110] direction of LiMn2O4. 3.2. Transformation of tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods To understand the transformation process of the tetragonal β-MnO2 nanorods to the cubic LiMn2O4 nanorods, XRD patterns and SEM images of the products derived from the reaction of β-MnO2 nanorods and LiOH·H2O were obtained at different reaction times and temperatures. As can be seen from Fig. 4, the [Mn2O4] frameworks (corresponding to the strong (111) diffraction peak) are formed after only 2 h of the reaction at 700 °C, and at least 8 h is needed to complete the phase transformation from tetragonal β-MnO2 to cubic LiMn2O4. The product exhibits
Fig. 4. XRD patterns of the β-MnO2 precursor and the product derived from the solidstate reaction with LiOH·H2O at 700 °C for different reaction times.
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Fig. 5. SEM images of the products derived from the solid-state reaction of β-MnO2 nanorods and LiOH·H2O at (a) 500 °C; (b) 550 °C; (c) 600 °C; (d) 650 °C; (e) 700 °C for 2 h; and (f) 700 °C for 10 h.
a relatively high crystallinity when the reaction time increased to 10 h. The SEM images (Fig. 5a–e) of the products derived from different temperatures for 2 h exhibit that the nanorod-like morphology can be maintained during the solid state reaction. Moreover, the SEM image (Fig. 5f) shows that the cubic LiMn2O4 derived from 10 h at 700 °C still retain the nanorod-like morphology. It also can be observed that the dimension involving a diameter of about 100 nm and a length of several hundred nanometers of the nanorods does not change evidently with the increasing heat temperature. Considering the above observation, we think that converting nanostructures of MnO2 to LiMn2O4 is based on a lithiation reaction [18]. During the transformation process, the Li+ ions can enter the square channels (2.3 Å × 2.3 Å [19]) of the tetragonal β-MnO2 with lattice constant of a = b = 4.399 Å and c = 2.874 Å as shown in Fig. 6. In response to the increasing electrostatic interaction between lithium and manganese ions, the packing way of anions has to convert from hexagonal close-packing to cubic closepacking. This process is accompanied by a cooperative displacement of one-half of the manganese ions into neighboring interstitial octahedral sites which transforms the rutile structure of β-MnO2 to the [Mn2]O4 framework of LiMn2O4 (a = 8.247 Å). Meanwhile, the reduction of some Mn 4 + to Mn 3 + leads to lattice expansion owing to the larger radius of Mn 3 + than Mn 4 +. The tetragonal-to-cubic transformation is similar to the model reported by David et al. [20], which requires a cooperation jump to
a nearest-neighbor site of half the cation array followed by an adjustment of the Mn–O distances to convert the tetragonal anion packing of rutile to the cubic close packing of anions in spinel structure. Considering the high mobility of Li + ions under a high-temperature condition, the transformation involves almost no diffusion barrier. Namely, the whole conversion might involve a minimal reorganization of structure in the parent solid, and leads to the formation of more stable spinel [Mn2O4] framework with a higher symmetry than that of the original template. Importantly, both nanorod morphology and high crystallinity of the precursor could be maintained with high fidelity in the final product, which is confirmed by Fig. 5. Some researchers take it as a self-sacrificial template or templateengaged mechanism [11]. The templates are engaged in chemical reactions as precursor and could be completely converted into the target product without additional post processing. In the present case, the β-MnO2 nanorods can act as self-sacrificial template to produce LiMn2O4 nanorods although further work is needed to clarify the relation between the morphology and the phase restructuring during the lithiation reaction. 3.3. Electrochemical performance Fig. 7 shows the initial charge–discharge curves of the LiMn2O4-NRS derived from different processes at a current density of 0.5 C
Fig. 6. Schematic illustration of the lithium intercalation process during the structure transformation from the tetragonal (the left) to the cubic (the right) product. For comparison, MnO6 octahedra for simplified crystal structure are only displayed in blue.
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Fig. 7. Initial charge–discharge curves of LiMn2O4-NRS and LiMn2O4-SSR at a current rate of 0.5C between 3.5 and 4.3 V.
Fig. 9. Discharge capacity retention as function of cycle numbers for LiMn2O4-NRS and LiMn2O4-SSR at current density of 3C.
(74 mA g ). As can be seen, two distinct plateaus at about 3.95 V and 4.15 V can be observed, indicating that the products undergo two distinct reversible oxidation and reduction processes, which are ascribed to the two-phase transformation of λ-MnO2/Li0.5Mn2O4 and Li0.5Mn2O4/LiMn2O4, respectively [21]. The first discharge capacity of the LiMn2O4-NRS as cathode material is about 125 mAh g−1, higher than that (104 mAh g−1) of the LiMn2O4-SSR derived from the commercial MnO2. This better discharge capacity can be attributed to the nanorod morphology, which is more convenient for the inserting and deinserting of Li+ ions because of the shorter diffusion distance and the larger surface area of the nanorods [22]. Although the first coulombic efficiency of the LiMn2O4-NRS is only about 90%, caused by the solid electrolyte interphase (SEI) formation and other Li-consuming surface reactions during the first charge/discharge cycling [23], the subsequent coulombic efficiency can reach nearly 100%. Compared with the LiMn2O4-SSR, the present single-crystalline LiMn2O4-NRS has a lower tap density (0.8 g cm−3 vs. 1.3 g cm−3), which may cause a lower volume energy density. However, the present single-crystalline LiMn2O4-NRS has the advantage in terms of weight energy density. To further show the advantage of the present LiMn2O4-NRS, a comparison of rate capability between nanorods and the bulk particles of LiMn2O4 was performed. Fig. 8 shows the galvanostatic cycling of the products at current rates ranging from 0.5 to 5 C. The discharge capacities of the two products decrease gradually with enhancing the current density, indicating the diffusion-controlled kinetics process for the electrode reactions of LiMn2O4 [24]. Moreover, LiMn2O4-NRS can deliver much higher capacity at a high
current density than that of the LiMn2O4-SSR, e.g. 78 mAh g − 1 and 6 mAh g − 1 at 5 C can be obtained for the nanorods and the aggregated particles, which correspond to 65% and 6% of their capacities at 0.5 C, respectively. Obviously, the rate capability of LiMn2O4-NRS is much better than that of the aggregated particles. According to the BET surface measurement, the LiMn2O4-NRS has a surface area of 19.1 m 2 g − 1, which is higher than that (9.2 m 2 g − 1) of the LiMn2O4-SSR. A higher surface area of the LiMn2O4-NRS can provide more contact area with the electrolyte and short diffusion length for the Li+ deintercalation/intercalation processes, and then resulting in an excellent rate capability. Fig. 9 shows the cycling performance of the LiMn2O4-NRS and the LiMn2O4-SSR at a high current rate of 3 C during 500 charge– discharge cycles. The obvious capacity decay can be seen especially in the first 200 cycles for the LiMn2O4-SSR, while the LiMn2O4-NRS cathode exhibits relatively lower capacity fading. As known, the capacity decay of spinel cathode for lithium ion battery is mainly due to Mn dissolution into the electrolyte during the charge–discharge cycles, oxygen defect in the spinel structure and Jahn–Teller effect of Mn 3 + in the cathode materials [2–4]. For the present singlecrystalline LiMn2O4-NRS, the high electrode/electrolyte contact area resulting from the high surface area (19.1 m 2 g − 1) leads to high capacity decay due to the Mn dissolution. On the other hand, the high crystallinity can improve the cycling stability on account of that the stable crystallographic structure can hinder the Mn dissolution [25]. Moreover, the LiMn2O4-NRS has a little higher slope (discharge capacity vs. cycle number) than the LiMn2O4-SSR in the following 200 to 500 cycles. Even so, about 73% of initial
Fig. 8. Discharge capacity of LiMn2O4-NRS and LiMn2O4-SSR under different current rates (0.5–5 C).
Fig. 10. Charge/discharge voltage profiles at 1C rate between 3.5 and 4.3 V for the first cycle, the 50th cycle and the 100th cycle.
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capacity for the single-crystalline LiMn2O4-NRS, on the whole, can still be retained after 500 charge–discharge cycles, while only 53% of initial capacity remained for the LiMn2O4-SSR, revealing that the LiMn2O4-NRS can deliver better cycling stability at a high rate than the LiMn2O4-SSR. 3.4. Structural stability of LiMn2O4 nanorods after the electrochemically cycling As mentioned above, the crystallographic structure may be the leading factor which slows down the capacity decay during the first 200 charge–discharge cycles. To further investigate whether the nanorods goes through structure collapse after the charge–discharge cycling, galvanostatic cycling was carried out and the voltage profiles were compared after different cycles at 1 C rate. As can be seen in Fig. 10, two successive well-defined discharge voltage plateaus for LiMn2O4-NRS can be clearly observed, indicating that the lithium ions are extracted and inserted from/into the spinel phase by a two-step process: the extraction/insertion of Li+ ions from/into one half of the tetrahedral sites with Li–Li interaction at 4.05/3.94 V (vs. Li/Li+) and that from/into the other half of the tetrahedral sites without Li–Li interaction at 4.17/ 4.08 V (vs. Li/Li+) [11]. Meanwhile, no obvious difference can be
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observed between the voltage profiles after 50 cycles and 100 cycles. As a characteristic of the well-defined spinel LiMn2O4, the two distinct charge/discharge plateaus suggest that the LiMn2O4-NRS can remain superior Li+ ion intercalation/decalation reversibility and high crystallinity. It could be inferred that the LiMn2O4-NRS can hold the prolonged electrochemical cycles without apparent structure deformation or collapse. Fig. 11 shows the characterization results of the LiMn2O4-NRS after prolonging the charge/discharge cycles at 1 C rate. As can be seen from Fig. 11a, the XRD patterns indicate that the cycled LiMn2O4-NRS can still preserve the spinel structure with good crystallinity even after 100 charge–discharge cycles, while the crystallinity becomes weak after 200 cycles. It was found that most of the cycled LiMn2O4-NRS can sustain the rod-like morphology even after 200 charge/discharge cycles, as observed from TEM image in Fig. 11b–e (the blurry and irregular substance was considered as carbon black and PTFE, which was added during the preparation of electrode [26]). Moreover, both of the SAED patterns in the inset of Fig. 11c and e also revealed that the cycled LiMn2O4-NRS is still single-crystalline. The above results confirmed the structure and morphology stability of LiMn2O4-NRS even after prolonged electrochemical cycles at a relatively high current rate, which is very important for a cathode material with enhanced performance such as
Fig. 11. XRD pattern (a) of LiMn2O4 nanorods after 100 and 200 charge/discharge cycles, TEM images (b, c, d, e) and the corresponding SAED pattern (the inset of c and e, respectively) of the LiMn2O4 nanorods after 100 charge/discharge cycles and 200 charge/discharge cycles at 1 C rate between 3.5 and 4.3 V, the SAED pattern was recorded with the zone axis 112 and 111, respectively. b and c correspond to that after 100 cycles and d and e correspond to that after 200 cycles.
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high capacity, high rate capability and good cycling performance for Li-ion battery at high power application. Based on the above results and discussion, the excellent electrochemical performance such as high capacity, high rate capability and good cycling stability of LiMn2O4 nanorods can be mainly ascribed to the following reasons. Firstly, the single-crystallinity of LiMn2O4 nanorods has some superiority such as well-defined geometry and good crystallization, which can improve the stability of the crystallographic structure during the electrochemical cycling process and the sequential atom arrangement also facilitates the fast Li + ion diffusion in the material. Secondly, the 1D nanostructure could provide a short diffusion pathway for Li + ions and electronics, and then facilitate the Li + deintercalation/intercalation processes. Moreover, the nanorod-like structure can also increase the electrode/electrolyte contact surface, and then enhance the utilization efficiency of the material. As a result, the as-synthesized LiMn2O4 nanorods exhibit outstanding charge/discharge performance as cathode material for Li-ion battery.
4. Conclusions In conclusion, we have developed an effective route to synthesize single-crystalline LiMn2O4 nanorods with cubic spinel structure by using tetragonal β-MnO2 nanorods as template, in which both morphology and single-crystal characteristics of the precursor can be retained after a high-temperature solid-state reaction with LiOH. Compared to the aggregated LiMn2O4 derived from commercially available β-MnO2, the as-synthesized single-crystalline LiMn2O4 nanorods exhibit superior electrochemical performance such as high-rate capability and cycling stability due to its single-crystalline nanorod structures, 1D crystal nature and the high crystallinity. The present findings suggest that the single-crystalline LiMn2O4 nanorods could be a promising cathode material for high-power lithium ion battery in view of the remarkably improved performance and moderate preparation method.
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Single-crystalline LiMn2O4 nanorods as cathode material with enhanced performance for Li-ion battery synthesized via template-engaged reaction Dan Zhan, Qinggang Zhang, Xiaohong Hu ⁎, Guozhu Zhu, Tianyou Peng ⁎ College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
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Article history: Received 29 September 2012 Received in revised form 4 March 2013 Accepted 8 March 2013 Available online 9 April 2013 Keywords: LiMn2O4 Single-crystalline nanorod Transformation Electrochemical property
a b s t r a c t Single-crystalline LiMn2O4 nanorods with a diameter of ~ 100 nm were synthesized via a template-engaged reaction by using tetragonal β-MnO2 nanorods as starting material. The investigations on the structures and morphologies of both the precursor and the final product reveal that a minimal structure reconstruction can be responsible for the chemical transformation from tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods. The obtained LiMn2O4 nanorods as cathode material for Li-ion battery exhibit superior high-rate capability and good cycling stability in a potential range of 3.5–4.3 V vs. Li+/Li, which can deliver an initial discharge capacity of 125 mAh g−1 (> 84% of the theoretical capacity of LiMn2O4) at a current rate of 0.5 C, and about 75% of its initial capacity can be remained after 500 charge–discharge cycles at a current rate of 3 C. Importantly, the rod-like nanostructure and single-crystalline nature are also well preserved after prolonging the charge/discharge cycling time at a relatively high current rate, indicating good structural stability of the single-crystalline nanorods during the lithium intercalation/deintercalation processes, and such high-rate capacity and cycling performance can be ascribed to the favorable morphology and the high crystallinity of the obtained LiMn2O4 nanorods. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable Li-ion battery, as one of the most important energy storage device, has attracted much interest owing to its high density, light-weight, and long cycle life. Some have been widely used in electronic devices, portable power tools, hybrid electric vehicles (HEVs) and many power supplies. Among those cathode materials for Li-ion battery reported, spinel LiMn2O4 is considered as one of the most promising candidates due to its advantages such as low-cost, environmental friendliness and high abundance [1]. However, the application of LiMn2O4 is confined by the capacity decay and cycling instability caused by manganese dissolution, oxygen vacancy, and Jahn–Teller effect [2–4]. Therefore, various approaches have been employed to solve the above problem. For example, coating and doping techniques have been applied to alleviate the dissolution of manganese [5,6]. Moreover, it has been reported that the particle size, morphology and crystallinity of materials strongly affect the discharge capacity, cycle life and rate performance of the Li-ion battery [7–12]. Therefore, LiMn2O4 with various nanostructured morphologies, such as nanoparticles [8], nanowires [9], nanorods [10], nanotubes [11], and even nanochain materials [12] have been prepared and used to improve the electrochemical performances of LiMn2O4 due to the short diffusion distances for Li + within the particles and the large surface-to-volume ratio, which allows for a large electrode–electrolyte contact interface [7]. ⁎ Corresponding authors. Tel./fax: +86 27 6875 2237. E-mail addresses: [email protected] (X. Hu), [email protected] (T. Peng). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.03.015
Among the various morphologies mentioned above, one-dimensional (1 D) nanowires or nanorods not only have large surface-to-volume ratio, but also provide efficient electron transport pathway. In addition, the nonwoven fabric morphology can suppress the aggregation and grain growth at an elevated temperature, and the potential barrier can be ignored to decrease the electronic resistance among the nanograins, which has been confirmed in dye-sensitized solar cells [13] and supercapacitors [14]. Those nanorod- or nanowire-like morphologies of LiMn2O4 seem also suitable for the application in Li-ion battery. However, nanorods with cubic spinel structure such as LiMn2O4 are difficult to synthesize because the cubic crystals usually cannot grow in a one-dimensional direction. Since β-MnO2 has been reported to have a particular 1D structure with diameters of 40–100 nm [15], it has been considered as a possible precursor to produce LiMn2O4 nanorods. Nevertheless, the low crystallinity of the β-MnO2 precursor often affects the grain size distribution of the lithiated phase, and it is also a formidable challenge to maintain the nanorod morphology during the following phase and chemical transformations [10,16,17]. Luo et al. [10] have synthesized LiMn2O4 nanorods with an initial discharge capacity of 100 mAh g−1 at a current rate of 1 C through a complex refluxing process, which involves poisonous acetonitrile and calcination process by using α-MnO2 nanorods as precursor. Moreover, β-MnO2 nanorods were also used as precursor to synthesize LiMn2O4 nanorods, which offer preferable electrochemical property such as more than 85% capacity retention after 100 cycles at a current rate of 1 C at room temperature [16,17]. Although the above LiMn2O4 nanorods were synthesized by regulating the nanorod morphology of the α-MnO2 or β-MnO2 precursor,
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at 80 °C for 24 h in air to obtain the β-MnO2 precursor. Secondly, the above β-MnO2 and LiOH·H2O were mixed at 2:1 molar ratio and dispersed into 5 ml methanol to form a thick slurry, which was further ground for several hours, and then dried at room temperature. The obtained powder was calcined in tube furnace at 700 °C for 10 h in air to obtain spinel LiMn2O4 nanorods (marked as LiMn2O4-NRS). As a contrast, LiMn2O4 particles were also prepared by a traditional solid state reaction method (marked as LiMn2O4-SSR) from commercially available β-MnO2 without any further treatment and LiOH·H2O mixed at the same molar ratio of 2:1. For all calcinations in tube furnace, samples were heated from room temperature at a heating rate of 10 °C min −1 and cooled naturally to room temperature. All reagents used were of analytical grade without further treatment. 2.2. Material characterization Fig. 1. XRD patterns of the obtained β-MnO2, LiMn2O4-NRS and LiMn2O4-SSR.
little information about the transformation from the tetragonal MnO2 to cubic spinel LiMn2O4 without morphology transition at an elevated temperature. Herein, single-crystalline LiMn2O4 nanorods with cubic spinel structure were synthesized based on a top chemical method as the above mentioned route by using tetragonal β-MnO2 nanorods as precursor, and the phase and composition transformation process in the template-engaged reaction from tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods was explored in detail. The electrochemical performance of the as-synthesized single-crystalline LiMn2O4 nanorods was studied by galvanostatic charge/discharge cycling. Moreover, the structure and morphology of the LiMn2O4 nanorods after prolonged electrochemical cycling time were also investigated, which confirmed the stability of the single-crystalline LiMn2O4 nanorods as cathode materials for rechargeable Li-ion battery, and it was found that the singlecrystalline LiMn2O4 nanorods showed much better electrochemical cycle performance as compared to the LiMn2O4 particles derived from a commercial β-MnO2. 2. Experimental 2.1. Material synthesis The synthesis of LiMn2O4 nanorods is carried out through a twostep procedure. Firstly, the tetragonal β-MnO2 nanorods were prepared by a hydrothermal process similar to the previous report [15]. Typically, 10 mmol MnSO4 and 10 mmol (NH4)2S2O8 were dissolved in 140 ml of distilled water to form a homogeneous solution under magnetically stirring at room temperature. The mixed solution was transferred to a 200 ml Teflon-lined stainless autoclave and heated at 120 °C for 12 h. The resultant precipitate was washed with deionized water and ethanol in succession to remove the sulfate ions and other remnants. The obtained black powder was subsequently dried
X-ray diffraction (XRD) measurements of β-MnO2 and LiMn2O4 were carried out by Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The morphologies were studied with a JSM 6700F field emission scanning electron microscope (FESEM) and a JEOL JEM-2100 high resolution transmission electron microscope (HRTEM). The BET surface areas were analyzed by nitrogen adsorption at 77 K with a Micromeritics ASAP2020 nitrogen adsorption apparatus. All samples were degassed at 120 °C for 5 h prior to the nitrogen adsorption apparatus. The tap density of the powders was characterized as follows. 2 g powder was placed in a 5 ml glass measuring cylinder, and then the cylinder was lifted at a height of 10 cm and dropped onto a 2 cm thick ebonite board for about 200 times. The tap density was calculated according to the mass and the final volume of the powder. The electrochemical measurement was performed by using CR2016 coin-type cells assembled in an argon-filled glove box (Mikrouna Super 1220/750) with lithium metal as the negative electrode. The cathode was fabricated by roll-pressing a mixed paste of the active material, carbon black and PTFE with a weight ratio of 80:15:5 into ca. 0.1 mm thick film and then pressing the electrode film onto a stainless mesh. The electrolyte was 1.0 M LiPF6 solved in EC/DMC (1:1 in volume). A Celgard 2300 microporous membrane was used as the separator. The charge–discharge cycles were performed at different C rate (for convenience, 1 C was defined as 148 mAh g−1) between 3.5 V and 4.3 V vs. Li+/Li at room temperature on a battery tester (LAND, CT2001A, China). 3. Results and discussion 3.1. Structure and morphology analyses of β-MnO2 and LiMn2O4 nanorods Fig. 1 shows the XRD patterns of the obtained β-MnO2, LiMn2O4-NRS and LiMn2O4-SSR. As can be seen, the precursor β-MnO2 derived from the hydrothermal process can be ascribed to tetragonal β-MnO2 (JCPDS no. 24-0735) with P42/mnm space group. The sharp and narrow
Fig. 2. SEM (a) and TEM (b) images of the β-MnO2 nanorods derived from a hydrothermal process, the inset in b is a high-magnification TEM image of one nanorod.
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Fig. 3. SEM images of the LiMn2O4-NRS (a) and the LiMn2O4-SSR (b), TEM images of LiMn2O4 nanorods at hlow (c) i and a high (inset in c) magnification, and HRTEM (d) image taken from the circled area in the section of (c), the inset in d is the corresponding FFT pattern recorded with 110 zone axis.
diffraction peaks with no additional impurity phase indicate that the obtained β-MnO2 has high crystallinity. And the LiMn2O4-NRS obtained by using the above β-MnO2 as precursor templates shows a XRD pattern of spinel LiMn2O4 (JCPDS no. 35-0782) with Fd3m space group, and no diffraction peaks of β-MnO2 can be detected, indicating the LiMn2O4-NRS exhibits a relatively high crystallinity and the β-MnO2 is fully transformed into LiMn2O4. The LiMn2O4-SSR derived from the solid state reaction process also shows a XRD pattern similar to the LiMn2O4-NRS. Moreover, it is observed that the full width at half maximum (FWHM) of the diffraction peaks for LiMn2O4-NRS is a little bigger than that for LiMn2O4-SSR, indicating the smaller gain size of LiMn2O4-NRS than that of LiMn2O4-SSR. SEM image (Fig. 2a) shows that the β-MnO2 consists of uniform nanorods with an average diameter of 50 nm and several hundred nanometer lengths. The rod-like nanostructure of the β-MnO2 can be further validated from the TEM images as shown in Fig. 2b. No any other particles can be found from the TEM images, confirming the high yield of β-MnO2 nanorods in the present hydrothermal process. From the TEM images, it can be observed that the nanorod has a diameter of about 50 nm and a length of hundred nanometers, which agrees with that observed from the SEM image. To further confirm whether the nanorod-like morphology can still retain after the high temperature treatment of the solid-state reaction, the SEM and TEM observations were shown in Fig. 3. Obviously, the LiMn2O4-NRS synthesized at 700 °C (Fig. 3a) also consists mainly of nanorods, which appear to have a larger average diameter of 100 nm and a shorter length. However, the LiMn2O4-SSR derived from the commercial β-MnO2 only consists of some irregular, aggregated particles as observed in Fig. 3b, which is consistent with that observed from the XRD pattern in the Fig. 1. More structure information about LiMn2O4 nanorods can be obtained from the TEM and HRTEM images in Fig. 3c and d. The LiMn2O4 nanorods have an average diameter of 100 nm, which is in accordance with the observation from the SEM image. The clear lattice fringes and the well-defined spots in FFT pattern reveal the single-crystal characteristics and high crystallinity of the LiMn2O4 nanorods. As depicted in Fig. 3d, (002) lattice fringes are parallel to the rod axis with d-spacing of about 0.40 nm. The
(111) lattice fringes can also be clearly observed, which corresponds to the strongest peak in the XRD pattern (Fig. 1). The above results imply that the nanorods can grow preferentially along the [110] direction of LiMn2O4. 3.2. Transformation of tetragonal β-MnO2 nanorods to cubic LiMn2O4 nanorods To understand the transformation process of the tetragonal β-MnO2 nanorods to the cubic LiMn2O4 nanorods, XRD patterns and SEM images of the products derived from the reaction of β-MnO2 nanorods and LiOH·H2O were obtained at different reaction times and temperatures. As can be seen from Fig. 4, the [Mn2O4] frameworks (corresponding to the strong (111) diffraction peak) are formed after only 2 h of the reaction at 700 °C, and at least 8 h is needed to complete the phase transformation from tetragonal β-MnO2 to cubic LiMn2O4. The product exhibits
Fig. 4. XRD patterns of the β-MnO2 precursor and the product derived from the solidstate reaction with LiOH·H2O at 700 °C for different reaction times.
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Fig. 5. SEM images of the products derived from the solid-state reaction of β-MnO2 nanorods and LiOH·H2O at (a) 500 °C; (b) 550 °C; (c) 600 °C; (d) 650 °C; (e) 700 °C for 2 h; and (f) 700 °C for 10 h.
a relatively high crystallinity when the reaction time increased to 10 h. The SEM images (Fig. 5a–e) of the products derived from different temperatures for 2 h exhibit that the nanorod-like morphology can be maintained during the solid state reaction. Moreover, the SEM image (Fig. 5f) shows that the cubic LiMn2O4 derived from 10 h at 700 °C still retain the nanorod-like morphology. It also can be observed that the dimension involving a diameter of about 100 nm and a length of several hundred nanometers of the nanorods does not change evidently with the increasing heat temperature. Considering the above observation, we think that converting nanostructures of MnO2 to LiMn2O4 is based on a lithiation reaction [18]. During the transformation process, the Li+ ions can enter the square channels (2.3 Å × 2.3 Å [19]) of the tetragonal β-MnO2 with lattice constant of a = b = 4.399 Å and c = 2.874 Å as shown in Fig. 6. In response to the increasing electrostatic interaction between lithium and manganese ions, the packing way of anions has to convert from hexagonal close-packing to cubic closepacking. This process is accompanied by a cooperative displacement of one-half of the manganese ions into neighboring interstitial octahedral sites which transforms the rutile structure of β-MnO2 to the [Mn2]O4 framework of LiMn2O4 (a = 8.247 Å). Meanwhile, the reduction of some Mn 4 + to Mn 3 + leads to lattice expansion owing to the larger radius of Mn 3 + than Mn 4 +. The tetragonal-to-cubic transformation is similar to the model reported by David et al. [20], which requires a cooperation jump to
a nearest-neighbor site of half the cation array followed by an adjustment of the Mn–O distances to convert the tetragonal anion packing of rutile to the cubic close packing of anions in spinel structure. Considering the high mobility of Li + ions under a high-temperature condition, the transformation involves almost no diffusion barrier. Namely, the whole conversion might involve a minimal reorganization of structure in the parent solid, and leads to the formation of more stable spinel [Mn2O4] framework with a higher symmetry than that of the original template. Importantly, both nanorod morphology and high crystallinity of the precursor could be maintained with high fidelity in the final product, which is confirmed by Fig. 5. Some researchers take it as a self-sacrificial template or templateengaged mechanism [11]. The templates are engaged in chemical reactions as precursor and could be completely converted into the target product without additional post processing. In the present case, the β-MnO2 nanorods can act as self-sacrificial template to produce LiMn2O4 nanorods although further work is needed to clarify the relation between the morphology and the phase restructuring during the lithiation reaction. 3.3. Electrochemical performance Fig. 7 shows the initial charge–discharge curves of the LiMn2O4-NRS derived from different processes at a current density of 0.5 C
Fig. 6. Schematic illustration of the lithium intercalation process during the structure transformation from the tetragonal (the left) to the cubic (the right) product. For comparison, MnO6 octahedra for simplified crystal structure are only displayed in blue.
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Fig. 7. Initial charge–discharge curves of LiMn2O4-NRS and LiMn2O4-SSR at a current rate of 0.5C between 3.5 and 4.3 V.
Fig. 9. Discharge capacity retention as function of cycle numbers for LiMn2O4-NRS and LiMn2O4-SSR at current density of 3C.
(74 mA g ). As can be seen, two distinct plateaus at about 3.95 V and 4.15 V can be observed, indicating that the products undergo two distinct reversible oxidation and reduction processes, which are ascribed to the two-phase transformation of λ-MnO2/Li0.5Mn2O4 and Li0.5Mn2O4/LiMn2O4, respectively [21]. The first discharge capacity of the LiMn2O4-NRS as cathode material is about 125 mAh g−1, higher than that (104 mAh g−1) of the LiMn2O4-SSR derived from the commercial MnO2. This better discharge capacity can be attributed to the nanorod morphology, which is more convenient for the inserting and deinserting of Li+ ions because of the shorter diffusion distance and the larger surface area of the nanorods [22]. Although the first coulombic efficiency of the LiMn2O4-NRS is only about 90%, caused by the solid electrolyte interphase (SEI) formation and other Li-consuming surface reactions during the first charge/discharge cycling [23], the subsequent coulombic efficiency can reach nearly 100%. Compared with the LiMn2O4-SSR, the present single-crystalline LiMn2O4-NRS has a lower tap density (0.8 g cm−3 vs. 1.3 g cm−3), which may cause a lower volume energy density. However, the present single-crystalline LiMn2O4-NRS has the advantage in terms of weight energy density. To further show the advantage of the present LiMn2O4-NRS, a comparison of rate capability between nanorods and the bulk particles of LiMn2O4 was performed. Fig. 8 shows the galvanostatic cycling of the products at current rates ranging from 0.5 to 5 C. The discharge capacities of the two products decrease gradually with enhancing the current density, indicating the diffusion-controlled kinetics process for the electrode reactions of LiMn2O4 [24]. Moreover, LiMn2O4-NRS can deliver much higher capacity at a high
current density than that of the LiMn2O4-SSR, e.g. 78 mAh g − 1 and 6 mAh g − 1 at 5 C can be obtained for the nanorods and the aggregated particles, which correspond to 65% and 6% of their capacities at 0.5 C, respectively. Obviously, the rate capability of LiMn2O4-NRS is much better than that of the aggregated particles. According to the BET surface measurement, the LiMn2O4-NRS has a surface area of 19.1 m 2 g − 1, which is higher than that (9.2 m 2 g − 1) of the LiMn2O4-SSR. A higher surface area of the LiMn2O4-NRS can provide more contact area with the electrolyte and short diffusion length for the Li+ deintercalation/intercalation processes, and then resulting in an excellent rate capability. Fig. 9 shows the cycling performance of the LiMn2O4-NRS and the LiMn2O4-SSR at a high current rate of 3 C during 500 charge– discharge cycles. The obvious capacity decay can be seen especially in the first 200 cycles for the LiMn2O4-SSR, while the LiMn2O4-NRS cathode exhibits relatively lower capacity fading. As known, the capacity decay of spinel cathode for lithium ion battery is mainly due to Mn dissolution into the electrolyte during the charge–discharge cycles, oxygen defect in the spinel structure and Jahn–Teller effect of Mn 3 + in the cathode materials [2–4]. For the present singlecrystalline LiMn2O4-NRS, the high electrode/electrolyte contact area resulting from the high surface area (19.1 m 2 g − 1) leads to high capacity decay due to the Mn dissolution. On the other hand, the high crystallinity can improve the cycling stability on account of that the stable crystallographic structure can hinder the Mn dissolution [25]. Moreover, the LiMn2O4-NRS has a little higher slope (discharge capacity vs. cycle number) than the LiMn2O4-SSR in the following 200 to 500 cycles. Even so, about 73% of initial
Fig. 8. Discharge capacity of LiMn2O4-NRS and LiMn2O4-SSR under different current rates (0.5–5 C).
Fig. 10. Charge/discharge voltage profiles at 1C rate between 3.5 and 4.3 V for the first cycle, the 50th cycle and the 100th cycle.
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capacity for the single-crystalline LiMn2O4-NRS, on the whole, can still be retained after 500 charge–discharge cycles, while only 53% of initial capacity remained for the LiMn2O4-SSR, revealing that the LiMn2O4-NRS can deliver better cycling stability at a high rate than the LiMn2O4-SSR. 3.4. Structural stability of LiMn2O4 nanorods after the electrochemically cycling As mentioned above, the crystallographic structure may be the leading factor which slows down the capacity decay during the first 200 charge–discharge cycles. To further investigate whether the nanorods goes through structure collapse after the charge–discharge cycling, galvanostatic cycling was carried out and the voltage profiles were compared after different cycles at 1 C rate. As can be seen in Fig. 10, two successive well-defined discharge voltage plateaus for LiMn2O4-NRS can be clearly observed, indicating that the lithium ions are extracted and inserted from/into the spinel phase by a two-step process: the extraction/insertion of Li+ ions from/into one half of the tetrahedral sites with Li–Li interaction at 4.05/3.94 V (vs. Li/Li+) and that from/into the other half of the tetrahedral sites without Li–Li interaction at 4.17/ 4.08 V (vs. Li/Li+) [11]. Meanwhile, no obvious difference can be
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observed between the voltage profiles after 50 cycles and 100 cycles. As a characteristic of the well-defined spinel LiMn2O4, the two distinct charge/discharge plateaus suggest that the LiMn2O4-NRS can remain superior Li+ ion intercalation/decalation reversibility and high crystallinity. It could be inferred that the LiMn2O4-NRS can hold the prolonged electrochemical cycles without apparent structure deformation or collapse. Fig. 11 shows the characterization results of the LiMn2O4-NRS after prolonging the charge/discharge cycles at 1 C rate. As can be seen from Fig. 11a, the XRD patterns indicate that the cycled LiMn2O4-NRS can still preserve the spinel structure with good crystallinity even after 100 charge–discharge cycles, while the crystallinity becomes weak after 200 cycles. It was found that most of the cycled LiMn2O4-NRS can sustain the rod-like morphology even after 200 charge/discharge cycles, as observed from TEM image in Fig. 11b–e (the blurry and irregular substance was considered as carbon black and PTFE, which was added during the preparation of electrode [26]). Moreover, both of the SAED patterns in the inset of Fig. 11c and e also revealed that the cycled LiMn2O4-NRS is still single-crystalline. The above results confirmed the structure and morphology stability of LiMn2O4-NRS even after prolonged electrochemical cycles at a relatively high current rate, which is very important for a cathode material with enhanced performance such as
Fig. 11. XRD pattern (a) of LiMn2O4 nanorods after 100 and 200 charge/discharge cycles, TEM images (b, c, d, e) and the corresponding SAED pattern (the inset of c and e, respectively) of the LiMn2O4 nanorods after 100 charge/discharge cycles and 200 charge/discharge cycles at 1 C rate between 3.5 and 4.3 V, the SAED pattern was recorded with the zone axis 112 and 111, respectively. b and c correspond to that after 100 cycles and d and e correspond to that after 200 cycles.
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high capacity, high rate capability and good cycling performance for Li-ion battery at high power application. Based on the above results and discussion, the excellent electrochemical performance such as high capacity, high rate capability and good cycling stability of LiMn2O4 nanorods can be mainly ascribed to the following reasons. Firstly, the single-crystallinity of LiMn2O4 nanorods has some superiority such as well-defined geometry and good crystallization, which can improve the stability of the crystallographic structure during the electrochemical cycling process and the sequential atom arrangement also facilitates the fast Li + ion diffusion in the material. Secondly, the 1D nanostructure could provide a short diffusion pathway for Li + ions and electronics, and then facilitate the Li + deintercalation/intercalation processes. Moreover, the nanorod-like structure can also increase the electrode/electrolyte contact surface, and then enhance the utilization efficiency of the material. As a result, the as-synthesized LiMn2O4 nanorods exhibit outstanding charge/discharge performance as cathode material for Li-ion battery.
4. Conclusions In conclusion, we have developed an effective route to synthesize single-crystalline LiMn2O4 nanorods with cubic spinel structure by using tetragonal β-MnO2 nanorods as template, in which both morphology and single-crystal characteristics of the precursor can be retained after a high-temperature solid-state reaction with LiOH. Compared to the aggregated LiMn2O4 derived from commercially available β-MnO2, the as-synthesized single-crystalline LiMn2O4 nanorods exhibit superior electrochemical performance such as high-rate capability and cycling stability due to its single-crystalline nanorod structures, 1D crystal nature and the high crystallinity. The present findings suggest that the single-crystalline LiMn2O4 nanorods could be a promising cathode material for high-power lithium ion battery in view of the remarkably improved performance and moderate preparation method.
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