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Combustion-derived nanocrystalline LiMn2O4 as a promising cathode material for lithium-ion batteries
Journal of Power Sources 275 (2015) 38e44
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Combustion-derived nanocrystalline LiMn2O4 as a promising cathode material for lithium-ion batteries Xuefeng Gao a, Yujing Sha a, Qian Lin a, Rui Cai a, Moses O. Tade b, Zongping Shao a, b, * a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People's Republic of China b Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia
h i g h l i g h t s Highly crystalline LiMn2O4 nanostructures were fabricated at low temperature. Dense nature of sample effectively minimized Mn2þ dissolution during process. Cycled at 5.0C for 100 cycles, the retention of charge/discharge capacity reached 93.5%. Favorable performance was still achieved when operated under improved temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2014 Received in revised form 12 October 2014 Accepted 16 October 2014 Available online 25 October 2014
In this study, nanocrystalline LiMn2O4 was synthesized by a simple combustion method and investigated for its utility as the positive electrode of a lithium-ion battery. X-Ray Diffraction characterization demonstrated that a basic crystallized spinel phase was already formed in the primary product from the direct combustion process, while pure phase LiMn2O4 was obtained after further calcination in air at relatively low temperature of 600 C. Characterization by SEM and HR-TEM as well as BET analysis showed that the LiMn2O4 compound had a primary particle size of 40e80 nm and that those particles were partially sintered to form 0.2e0.8 mm aggregates with few mesopores. The exposed surface area of the aggregates was low and mainly formed by the outer surfaces of the constituent particles, which is beneficial to reducing the interfacial area between the liquid electrolyte and LiMn2O4, thereby effectively mediating the Mn dissolution problem. As a result, the as-prepared LiMn2O4 showed a favorable capacity of 114 mAh g1 at a current rate of 0.2C and still retained a capacity of 84 mAh g1 at 5C. After 100 continuous cycles at 0.1C, a capacity of 108 mAh g1 was still maintained, compared to 120 mAh g1 at the first cycle. The results demonstrated that combustion synthesis-derived LiMn2O4 is a promising cathode material for lithium ion batteries (LIBs). © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Cathode LiMn2O4 Combustion synthesis Nanocrystalline
1. Introduction A battery is a device that converts chemical energy, which is stored inside of its electrodes, into electric power by means of electrochemical redox reactions. Among the various types of batteries, lithium-ion batteries (LIBs) are currently the most popular secondary batteries available because of their high voltage, light weight, high energy density, no memory effect, and environmental
* Corresponding author. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People's Republic of China. Tel.: þ86 25 83172256; fax: þ86 25 83172242. E-mail address: [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2014.10.099 0378-7753/© 2014 Elsevier B.V. All rights reserved.
friendliness. A conventional battery must have three main components: cathode, electrolyte and anode. In many cases, the electrode materials largely determine the potential window, energy density, rate performance, and cycling stability of a battery. The state-of-the-art cathode used in commercial LIBs for portable applications is layer-structured LiCoO2 oxide, which has a theoretical capacity of approximately 274 mAh g1 [1e3], but typically can only deliver a reversible capacity of ~140 mAh g1 [4]. Additionally, the toxicity and relatively high price of cobalt, poor cycling performance, and unfavorable thermal stability are substantial drawbacks of LiCoO2, and as such there have been considerable efforts over the past decade directed toward searching for alternative highperformance cathode materials for high power applications in particular.
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Spinel-type LiMn2O4 oxide, which has three-dimensional lithium diffusion paths, represents one of the most promising alternative cathode materials for next-generation LIBs. It is an outstanding material in several respects, in that it has a high discharge voltage plateau of approximately 4.0 V; it makes use of the natural abundance of Mn and is economically feasible; and it is also safer to use, as it is environmentally benign with low toxicity [5]. In spinel-type LiMn2O4 oxide, Mn ions are coordinated to O in an octahedral arrangement and form a cubic close-packed (CCP) structure, and the MnO6 octahedra share edges to form a threedimensional host for the Li guest ions [6]. Typically, almost all of the lithium ions in the tetrahedral sites of spinel LiMn2O4 can be extracted, resulting in a theoretical reversible capacity of approximately 148 mAh g1, which is comparable to the reversible capacity of state-of-the-art LiCoOx spinel-type cathodes [7]. This suggests that LiMn2O4 may be an ideal cathode substitute for LiCoOx in LIBs. In particular, it is generally accepted that LiMn2O4 is more suitable than LiCoOx for power battery applications [8]. Due to its relatively low lithium-ion diffusion coefficient (109e1011 cm2 s1) [9], poor electrode performance and in particular poor rate capacity has often been observed for coarsetype LiMn2O4. The decrease of particle size and the creation of nanopores inside the electrode material, which increased the specific surface area of the material, proved to be effective approaches to increase the electrocatalytic activity of the LiMn2O4 electrode [10e12]. For example, porous LiMn2O4 synthesized by a hydrothermal method showed a promising capacity of 128 mAh g1 at a current density of 0.2C rate and a favorable rate capability, with 83 mAh g1 retained at a current density of 20C [13]. However, one main problem that limits the practical application of nano-structured LiMn2O4 electrodes is the serious capacity fading that the material exhibits with galvanostatic chargeedischarge cycling, which can be partially attributed to the slow dissolution of Mn into the liquid electrolyte through the disproportionation of trivalent Mn (2Mn3þ(solid) / Mn4þ(solid) þ Mn2þ(solution)) [14]. The dissolution of divalent Mn brought about by the attack of HF, which is formed by LiPF6 and residual H2O in the electrolyte, not only causes a loss of active cathode material but also affects the anode and the electrolyte [15]. Currently, the most common way of increasing cathode performance while simultaneously maintaining good cycling stability is through a doping strategy or by coating the surface with a protective layer [16e22]. Our hypothesis is that if the lithium diffusion distance could be effectively decreased while the specific surface area is maintained at a low value, even without doping or surface coating, simultaneous high electrode performance and favorable cycling stability may still be achieved for pristine LiMn2O4. It follows from this rationale that the formation of micropores and mesopores should be avoided as the great specific surface of electrode exposed in electrolyte will greatly increase the risk of Mn dissolution, leading to a poor capacity retention. In this study, we report the straightforward synthesis of ultrafine LiMn2O4 powder by a cellulose-assisted combustion method that is nearly free of micropores and mesopores and can be employed as a highly electrochemically active cathode material in LIBs. Activated natural cotton was used as a porous support for the infiltration of lithium and manganese nitrates and glycine fuel, and also provided diffusion channels for maintaining a high oxidant atmosphere during the combustion process. Furthermore, the cotton support served to suppress the grain growth of nanocrystalline products on the cathode. The resultant material exhibited a good capacity, rate capacity and cycling stability. The reported material represents a new pathway forward in the development of high-performance LiMn2O4 cathodes for use in next-generation LIBs.
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2. Experimental 2.1. Powder synthesis The LiMn2O4 used in this work was prepared by a celluloseassisted combustion method [23]. De-seeded natural cotton of medical use was first immersed in concentrated nitric acid (67%) to allow the nitration of the surface groups of the cotton and the dissolution of some inorganic minerals. The treated cotton was then washed with de-ionized water to eliminate any residual free nitric acid, the treated cotton was dried at 120 C in an electric oven. In the second step, appropriate amounts of LiNO3 and Mn(NO3)2 were impregnated into the cotton at a molar ratio of 1.06:2 (6% extra Li source was used in consideration of Li evaporation during high temperature calcination), to serve as the raw materials for the LiMn2O4 phase formation. Then, a quantity of glycine that was three times the combined molar amount of LiNO3 and Mn(NO3)2 was added to the solution as the combustion improver. Subsequentially, the as-prepared solution was absorbed by proper amount of activated cellulose. The impregnated cellulose was then dried at 80 C and placed into an electric oven at 250 C. Sudden self-ignition was triggered [24] and resulted in a powder that was gray in color, which was further calcined at 600 C for 5 h to achieve fine crystallization of spinel LiMn2O4 and burn out any organic residuals, mainly leftover from the cellulose. 2.2. Characterization 2.2.1. Basic characterization X-ray diffraction (XRD) measurements were conducted to obtain structural information for the as-prepared LiMn2O4 and were carried out on a Bruker D8 advance diffractometer with filtered Cu Ka1 radiation (l ¼ 0.15406 nm) in the 2q range of 10e90 . Environmental scanning electron microscopy (HITACHI S4800) and transmission electron microscopy (JEOL JEM-2100) were performed to gain information about the morphology of the synthesized particulates. The surface area and corresponding pore size distribution were characterized by N2 adsorption-desorption testing at 77 K using a BELSORP II instrument (Japan) after sample pretreatment at 200 C for 6 h under vacuum. 2.3. Electrochemical testing LiMn2O4 was used to prepare the working electrode slurry by combining it with Super P and PVDF, added as conductor and binder, respectively (75:15:10 in weight). Analytical-grade NMP was used as the solvent. After all components were homogeneously dispersed in NMP, the slurry was subsequently coated onto an Al foil current collector by the blading method. Disk-shaped electrodes with a diameter of 14 mm were tailored from the coated foil and dried at 100 C in a vacuum for 12 h. Button cells (CR 2016) were then assembled with the as-prepared electrode as the working electrode, lithium foil as counter electrode, and a microporous polypropylene film (Celgard 2400) as the separator. The electrolyte used was 1 M LiPF6 dissolved in a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) (1:1 in volume, Shenzhen Capchem Technology Co., LTD., China). Chargedischarge curves were recorded on NEWARE BTS-5V 50 mA computer-controlled battery test station with a voltage range of 4.3e3.0 V. Corresponding rate performance and stability measurements were conducted using the same test station applying different current densities of 0.2, 0.5, 1, 2, 5, and 0.1C, respectively (1C ¼ 148 mA g1). Cyclic voltammetry tests were performed using a Princeton Applied Research PARSTAT 2273 advanced electrochemical system at a scanning rate of 0.2 mV s1 and a voltage range of 3.5e4.5 V.
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3. Results and discussion Although many techniques have been developed for the synthesis of LiMn2O4, most of them require multi-step procedures, making them difficult for scale up for scale-up and economically unattractive. In addition, the samples prepared by these methods, which exhibited high electrochemical performance, usually possessed high specific surface area [25e27]. The large interfacial area between the electrode and liquid electrolyte for these electrodes has sparked concerns about the dissolution of Mn2þ. Cellulose-based combustion synthesis was found to be a universal technique for the synthesis of ultrafine powders, and has been successfully applied in the preparation of a wide range of materials with synthesis scale-up capability [28e30]. The synthesis of LiMn2O4 by the cellulose-assisted combustion method is fairly simple in procedure. After impregnating the activated cotton fibers with LiNO3, Mn(NO3)2 and glycine solutions in the proper ratios and then drying the mixture at 80 C in an electrical oven, dry cellulose containing an uniform mixture of LiNO3, Mn(NO3)2 and glycine was obtained. As referred to what we previously reported, raising the oven temperature to 250 C at this point initialized a two-stage combustion. A first sudden flame combustion, which lasted for approximately 15 s and was accompanied by the release of a large amount of heat and gas, occurred, followed by a secondary, more gentle combustion that resulted in a flameless and which lasted for a couple of minutes. Fig. 1 shows the XRD patterns of the primary LiMn2O4 product obtained from the direct combustion of the solid precursor and also of the powder calcined at 600 C in air for 5 h. Basic diffraction peaks for a spinel-type phase were already present in the XRD patterns of the product obtained directly from the auto-combustion process. The high peak intensity suggests that the crystalline phase was already well developed, while the broad peak width is a sign that the product is composed of very small crystallites. During the direct combustion synthesis, the first auto-combustion process was due to the reaction between the metal nitrates (LiNO3, Mn(NO3)2) and glycine, which was very vigorous, with the instantaneous reaction temperature reaching over 1000 C [31]. Referring to the previous report published by Chick L. A. et al. [32], the ideal reaction between metal nitrates and glycine during combustion can be described as follows:
9MnðNO3 Þ2 þ 8NH2 CH2 COOH/9MnO2 þ 13N2 [ þ 16CO2 [ þ 20H2 O (1) 2LiNO3 þ 2NH2 CH2 COOH/Li2 O þ 2N2 [ þ 2CO2 [ þ 5H2 O (2)
It was worth that during the combustion process, reddish brown gas was released accompanied with the reaction, indicating a generation of N2O. Thus, the real reaction during combustion may be somewhat different form reactions 1 and 2. Such an exothermic reaction provided a huge amount of energy that promoted the formation of LiMn2O4 phase in a very short time (4MnO2 þ Li2 O/2LiMn2 O4 þ 0:5O2 [) [12,33]. The porous morphology of the surrounding cellulose provided a supply of free oxygen that served to maintain a highly oxidizing atmosphere critical to the formation of the LiMn2O4 spinel and the prevention of Mn reduction. Meanwhile, due to the short time of reaction as well as the blocking effect of the cellulose fibers, which acted as a micro reactor during synthesis, the sintering of particles was effectively suppressed. During the second combustion process, the sintering of the product particles was also limited because of the low temperature generated in this stage, in which the cellulose framework was largely burned out. As a result, for the as-obtained product, the basic phase of the material was developed and a small crystalline size was preserved. The product of the direct combustion was gray in color, implying the presence of a considerable amount of organic/ carbon residual. Thus, it was likely that the secondary modest combustion process was insufficient to consume all of the cotton fibers in the material. However, after subjecting to calcination in air for 5 h at 600 C, the sample turned black, implying the successful elimination of the organic residual. Sharp X-ray diffraction peaks were observed for the calcined sample, suggesting the formation of a well-crystallized product with an increased crystallite size. The broad peaks indicate that the powder was composed of ultrafine crystals. The crystalline size was quantified based on the half width of the diffraction peaks using the Scherrer equation, where D ¼ Kl/ Bcosq. The peaks arising from the (111), (311), (400) and (440) diffraction planes were selected for this analysis, and the average crystalline size based on above four diffraction peaks was found to be ~45.0 nm. Shown in Fig. 1b is the Rietveld refinement of the XRD patterns for the sample after calcination at 600 C, based on a cubic spineltype lattice structure with space group Fd3 m (JCDPS: 35-0782). Good fitting with low convergence reliability factors (Rp ¼ 2.81%, Rwp ¼ 3.61%, c2 ¼ 1.461) suggests the successful formation of wellcrystallized phase-pure LiMn2O4 spinel after calcination of the solid combustion synthesis precursor at relatively low temperature (600 C for 5 h). The derived lattice parameter was 8.21(3) Å, which matched soundly with literature results (8.24(7) Å). The small difference between the two values may suggest a slight deviation in the oxidation state and lithium content of the current sample from that presented in the literature. SEM images of the as-synthesized LiMn2O4 powder (600 C calcined) are shown in Fig. 2. The particulates had a semi-spherical
Fig. 1. (a) XRD patterns of the primary LiMn2O4 particles formed directly from combustion and of the product upon further calcination of the powder at 600 C; (b) Rietveld refinement of an in-situ XRD pattern of the LiMn2O4 powder after calcination at 600 C.
X. Gao et al. / Journal of Power Sources 275 (2015) 38e44
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Fig. 2. SEM images of the as-synthesized LiMn2O4 at different magnifications.
morphology and a particle size of 200e800 nm. This observation agrees well with the fact that materials with a cubic lattice structure should grow isotropically in all three directions, and thus particles of a more symmetrical shape such as spheres or cubes are easier to form. It further indicates that the combustion synthesis resulted in an ultrafine particle size. As mentioned previously, the uniformity and dimensions of the particles can be explained by the homogeneous mixing of the raw materials in the solution stage and the low calcination temperature, as well as the blocking effect of the cellulose on the contact of the particles during synthesis. The surface of the ultrafine particles, is smooth and dense, implying the dense nature of the as-prepared nanoparticles. In other words, the overall specific surface area of the product is derived mainly from the external surface area of the nanoparticles. Additionally, those ultrafine particles were found to have partially sintered to form larger aggregates. It is well known that current collection for nanoparticles is somewhat difficult. The formation of macroporous and micrometer-sized aggregates is actually beneficial to the application of LiMn2O4 as an electrode material because of the improvement in current collection efficiency. The nanosized primary particles provide a short lithium diffusion distance, while the macropores between nanoparticles allow for easy penetration by the liquid electrolyte; on the other hand, the formation of micrometer-sized aggregates allows for more facile and efficient current collection. Thus, improved electrode performance is expected when using this material. The electrochemical performance of the as-prepared LiMn2O4 formed from combustion synthesis will be discussed in detail later. The particulate morphology of the as-synthesized LiMn2O4 sample was further investigated by TEM and HR-TEM. As shown in Fig. 3a and b, the primary LiMn2O4 particles were spherical in shape and had particle sizes in the range of 40e80 nm, matching well with the diameter calculated from X-ray diffraction peaks (~45.0 nm). Those primary particles were further partially fused to form larger aggregates, corresponding to the particles as observed by SEM in Fig. 2. Within the larger aggregates, meso-to-macro pores with diameters in the range of 60e150 nm, as highlighted in the same image, were also formed from the packing of the nanoparticles. HR-TEM images of LiMn2O4 are presented in Fig. 3c. Clear lattice fringes with a spacing of 0.47 nm were observed, matching well with the distance between the (111) diffraction planes of a typical spinel-type LiMn2O4 crystal. To obtain more detailed information about the local crystal structure, selected area electron diffraction (SAED) patterns were analyzed. It is known that an accurate SAED pattern is acquired when the rays of the electron beam are perpendicular to the direction of the plane forming the diffraction pattern. Fig. 3d presents the SAED pattern corresponding to the HR-TEM images shown in Fig. 3c; it is composed of wellorganized dots in which the (111) diffraction plane could be identified. The (200) diffraction plane was also calculated along the
specific zone axis direction of [011]. These findings further supported the successful formation of the crystallized spinel LiMn2O4 phase derived from the combustion synthesis. The specific surface area and pore structure information of the as-synthesized LiMn2O4 (calcined at 600 C) was investigated by nitrogen adsorption-desorption analysis. Shown in Fig. 4 are the nitrogen adsorption-desorption curves measured at the boiling temperature of liquid nitrogen, as well as the corresponding pore size distribution curves. The nitrogen adsorption-desorption curves can be categorized as type II curves according to the IUPAC definition. Almost no hysteresis loop within the relative partial pressure (P/Po) range of 0.1e0.9 was observed in the curves. This implies that few mesopores present within the as-synthesized LiMn2O4 material. This conclusion was more clearly demonstrated from the pore size distribution curves of the uncalcined sample. According to BrunauereEmmetteTeller (BET) analysis, the combustion-derived LiMn2O4 had a specific surface area of only 3.74 m2 g1, which was relatively low compared with the porous LiMn2O4 reported in the literature (19.2 m2 g1) [34]. Generally speaking, the increase in specific surface area will increase the contact area between the electrode and liquid electrolyte; however, the possibility of Mn2þ dissolution into the electrolyte will also increase accompanied with large surface exposed in electrolyte. In comparison, the low specific area of 3.74 m2 g1 for as-prepared LiMn2O4 greatly contributes to the enhanced cycling stability of electrode. Assuming that the LiMn2O4 particles were spherical in shape and densely packed, the average diameter of the particles could be calculated based on the specific surface area of the LiMn2O4 powder and was determined to be approximately 0.15 mm. This roughly agrees with the minimum particle size observed by SEM analysis (Fig. 2). To investigate the electrochemical performance of the assynthesized LiMn2O4 as the positive electrode of a LIB, a half-cell was constructed with LiMn2O4 as the working electrode and metallic lithium as the counter and reference electrodes. Shown in Fig. 5 are the first galvanostatic chargeedischarge curves for the electrode at a current rate of 0.1C (a rate of 1C corresponds to a full charge/discharge of the theoretical capacity, i.e., 148 mA g1, in 1 h) within a potential range of 3.0e4.3 V. The charge and discharge curves demonstrated two consecutive slowly increasing (for charge) or decreasing (for discharge) plateaus, one at 3.98/4.2 V and the other at 4.12/3.93 V, respectively. The observation of two pairs of voltage platforms on the galvanostatic chargeedischarge curves is common for LiMn2O4-based electrodes and is believed to be associated with the two-step phase transformations of LiMn2O4/ Li0.5Mn2O4 and Li0.5Mn2O4/l-MnO2, correspondingly [12]. During the charge process, the extraction of half of the Liþ ions from the tetrahedral sites in LiMn2O4 through LieLi interaction occurred, leading to the formation of Li0.5Mn2O4 intermediate, which is associated with the first charge plateau located at approximately
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Fig. 3. (a, b) Typical TEM images, (c) HR-TEM image, and (d) SAED patterns taken along the [011] zone axes of the as-synthesized LiMn2O4 sample.
3.98 V. Then, a further extraction of the lithium from the tetrahedral sites occurred without the aid of any LieLi interaction, giving rise to the formation of a product with the final composition of lMnO2, which can be associated with the presence of the second charge plateau at approximately 4.2 V in the galvanostatic charge curve [35]. The initial discharge capacity was 120.5 mAh g1 at 0.1C,
which is approximately 81.4% of the theoretical capacity of LiMn2O4 (148 mAh g1), indicating that the as-prepared LiMn2O4 electrode made from the combustion synthesized material was discharged to the state of Li0.81Mn2O4 on average. In other words, it suggests that most parts of the electrode materials were electrochemically active for the lithium insertion reaction.
Fig. 4. Adsorption/desorption curves of as-prepared LiMn2O4; the inset image is the corresponding BJH picture.
Fig. 5. The first galvanostatic chargeedischarge curves of the as-synthesized LiMn2O4 (calcined at 600 C for 5 h) obtained at a 0.1C current rate.
X. Gao et al. / Journal of Power Sources 275 (2015) 38e44
The electrochemical behavior of the combustion-synthesized LiMn2O4 was investigated by cyclic voltammetry (CV). Shown in Fig. 6 are the first, second and 10th CV curves of the oxide scanned between the voltages of 3.5 and 4.5 V, taken at room temperature and at a scanning rate of 0.2 mV s1. All three CV curves were similar to each other in shape and were well-overlapped with slight increases of overpotential, suggesting good reversibility of electrochemical lithium intercalation/de-intercalation of the asprepared LiMn2O4. Furthermore, the observed peaks are relatively sharp, suggesting the high crystallinity of the LiMn2O4 in the electrode and good kinetics for lithium ion transport within its bulk. In connection with the platforms in galvanostatic charge/ discharge curves, the redox peaks in first CV curve, which located at 4.05 V/4.06 V and 4.19 V/3.93 V (Liþ/Li), indicated the extraction/ insertion of Liþ ions from/into half of the tetrahedral sites with LieLi interaction and from/into the other half of the tetrahedral sites without any LieLi interaction, respectively. The rate performance of the as-synthesized LiMn2O4 electrode was also investigated by varying the charge/discharge rate from 0.2C to a maximum of 5.0C. As shown in Fig. 7, excellent rate capacities were demonstrated in this test. Capacities of 114, 105, 100 and 96 mAh g1 were reached at 0.2, 0.5, 1.0 and 2.0C rates, respectively. Even with the further increase of charge/discharge rate to 5.0C, a reversible capacity of 84 mAh g1 was still retained. For comparison, similar capacity at low current density (110 mAh g1, 0.5C) was observed for porous LiMn2O4 microcubes, while the capacity at 5.0C was only 68 mAh g1 [36]. Compared with the Li-rich mesporous LiMn2O4 nanospheres reported by J. M. Kim et al. [25], cellulose assisted-combustion was more costeffective and simple than reported ultrasound-assisted refluxes; Meanwhile, rate capacitance at high rates (96 mAh g1 at 2C) was much higher than that of reported LiMn2O4 (80 mAh g1 at 2C). This observed capacity was also slightly higher than that of mesoporous LiMn2O4 synthesized by the template method (80 mAh g1 at 5.0C), providing further evidence for the good performance of LiMn2O4 synthesized by cellulose-assisted combustion combustion. The better rate capacity of the LiMn2O4 structure presented here suggests that lithium insertion occurs easier in this structure, likely due to the short lithium diffusion distance and the improved access of the liquid electrolyte provided by the macropores in the electrode material as demonstrated in Fig. 3. As mentioned, with the decrease of particle size and the concomitant increase in the specific surface area of the LiMn2O4
Fig. 6. Cyclic voltammograms of the as-prepared LiMn2O4 at a scan rate of 0.2 mV s1 after performing cycling tests at 0.1C over different time intervals.
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Fig. 7. Cycling performance of the as-prepared LiMn2O4 electrode at different charge/ discharge rates of 0.2C, 0.5C, 1C, 2C and 5C at room temperature.
electrode, cycling stability becomes a point of concern because of increased Mn dissolution into the liquid electrolyte. Fig. 8 shows the dependence of the capacity of LiMn2O4 electrode on cycling times; the electrode was cycled at 0.1C rate (at room temperature) (Fig. 8a), and also cycled at 0.5C at either room temperature or 55 C (Fig. 8b). Acceptable stability of the capacity was achieved under these conditions. For example, a capacity of 108 mAh g1 was still retained after 100 cycles at a 0.1C rate at room temperature; a comparison of this capacity to that measured during the first cycle, at 120 mAh g1, indicates a capacity retention of 90.1%. At a faster rate of 5.0C, a discharge capacity of 87 mAh g1 was still achieved after 100 galvanostatic charge/discharge cycles in comparison to a capacity of 93 mAh g1 upon first discharge, suggesting a capacity retention as high as 93.5%. The specific capacity and capacity retention at different rates were much higher than those reported unidimensional rod and quadrate lamina LiMn2O4 [37], which were synthesized by complex hydrothermal reactions and exhibited only ~84% capacity retention after cycling at 0.1C for 50 times (25 C). There are several causes for the slight capacity fade of LiMn2O4 electrodes [38], but it is generally believed that Mn dissolution is the most problematic source. When employed in practical LIBs, these electrodes may be operated at elevated temperatures, at which the Mn dissolution problem becomes more severe [39]. As shown in Fig. 8, an initial discharge capacity of 92 mAh g1 was achieved when the temperature of the system was 55 C. It is generally accepted that the lithium diffusion rate inside an oxide is improved with an increase in temperature, and thus lithium diffusion inside the bulk oxide should cause less polarization resistance to the lithium charge/discharge processes. Very interestingly, comparable capacities were observed for LiMn2O4 at room temperature and 55 C at the high capacity charge/discharge rate of 5.0C. This suggests that the lithium-ion diffusion occurring inside the oxide bulk was not the main source of the large polarization resistance of LiMn2O4 at high charge/discharge rates. Alternatively, this resistance may be explained by the nanocrystalline size of the combustion-synthesized LiMn2O4, which provided short lithium diffusion distance. After continuously cycling the cell 100 times, a capacity of 84 mAh g1 was still maintained, suggesting a capacity retention of 89.1%. This suggests that even when used at high temperature, the combustion-synthesized LiMn2O4 still exhibited a favorable cycling stability. As we know, the dissolution of Mn from LiMn2O4 is directly related to the specific surface area of the electrode [40]. Although the combustion-synthesized LiMn2O4 sample
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Fig. 8. Cycle performances at (a) 0.1C and at (b) 0.5C (tested at room temperature and 55 C, respectively) for 100 cycles.
was composed of nanocrystalline particles, which ensured a short diffusion distance for lithium ion and thus a high electrode performance, the dense nature of the nanocrystallites minimized the specific surface area of the electrodes. As a result, the dissolution of Mn2þ from LiMn2O4 into the liquid electrolyte was not significant, even at elevated temperatures. A further modification of the electrode surface, such as adding a protective layer such as TiO2 by atom layer deposition or another advanced technique, may further improve the cycling stability of the electrode material and improve its potential for practical application in LIBs. 4. Conclusions Highly crystalline LiMn2O4 nanostructures were successfully synthesized by a straightforward cellulose-assisted combustion method at the relatively low temperature of 600 C. The primary particles that were produced were only 40e80 nm in size with an average surface area of 3.74 m2. The nano-sized particles ensured short diffusion paths for lithium ions and electrons when applied as a lithium-ion cathode, while the dense nature of the sample minimized the surface area of the electrode exposed to the electrolyte and thus decreased Mn2þ dissolution during the process. Even when cycled at 5.0C for 100 cycles, the retention of charge/discharge capacity was 93.5%. When operated under harsher conditions, favorable performance was still achieved. Furthermore, when incorporated into an LIB cell, a capacity of 84 mAh g1 was maintained after 100 cycles at 5.0C with a capacity retention of 89.1%. In conclusion, as-prepared LiMn2O4 nanostructures derived from a cellulose-assisted combustion method exhibited excellent performances even under harsh conditions, and the successful application of the combustion method in the preparation of LiMn2O4 also shed light on the application of LiMn2O4 in electrochemical conversion and storage. As a universal synthesis method, cellulose-assisted combustion may also find application in the preparation of other nano-structured materials for functional applications. Acknowledgments This work was partially supported by the “Key Projects in Nature Science Foundation of Jiangsu Province” under contract no. BK2011030, “National Science Foundation for Distinguished Young Scholars of China” under contract no. 51025209, “National Nature Science Foundation of China” under contract no. 201103089, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Y. Shao-Horn, L. Croguennec, C. Delmas, E.C. Nelson, M.A. O'Keefe, Nat. Mater. 2 (2003) 464e467.
ron, R. Dedryve re, H. Martinez, M. Me ne trier, C. Denage, C. Delmas, [2] L. Dahe D. Gonbeau, Chem. Mater. 20 (2008) 583e590. [3] S. Jeong, S. Park, J. Cho, Adv. Energy Mater. 1 (2011) 368e372. [4] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Adv. Mater. 21 (2009) 4593e4607. [5] S. Lee, Y. Cho, H.K. Song, K.T. Lee, J. Cho, Angew. Chem. Int. Ed. 51 (2012) 8748e8752. [6] J.M. Tarascon, E. Wang, F.K. Shokoohi, J. Electrochem. Soc. 138 (1991) 2859e2864. [7] D.K. Kim, P. Muralidharan, H.W. Lee, R. Ruffo, Y. Yang, C.K. Chan, H.L. Peng, R.A. Huggins, Y. Cui, Nano Lett. 8 (2008) 3948e3952. [8] F.Y. Cheng, H.B. Wang, Z.Q. Zhu, Y. Wang, T.R. Zhang, Z.L. Tao, J. Chen, Energy Environ. Sci. 4 (2011) 3668e3675. [9] Z. Quan, S. Ohguchi, M. Kawase, H. Tanimura, N. Sonoyama, J. Power Sources 244 (2013) 375e381. [10] B.J. Liddle, S.M. Collins, B.M. Bartlett, Energy Environ. Sci. 3 (2011) 1339e1346. s-Solís, M.R. Palacín, J. Oro -Sole , A. Fuertes, G. Marb [11] J. Cabana, T. Valde an, A.B. Fuertes, J. Power Sources 166 (2007) 492e498. [12] D. Tonti, M.J. Torralvo, E. Enciso, I. Sobrados, J. Sanz, Chem. Mater. 20 (2008) 4783e4790. [13] Y.Z. Wang, X. Shao, H.Y. Xu, M. Xie, S.X. Deng, H. Wang, J.B. Liu, H. Yan, J. Power Sources 226 (2013) 140e148. [14] K.R. Ragavendran, L. Lu, K. Barner, A.K. Arof, J. Phys. Chem. C 117 (2013) 23547e23557. [15] Y. Yang, C. Xie, R. Ruffo, H. Peng, D.K. Kim, Y. Cui, Nano Lett. 9 (2009) 4109e4114. [16] H.M. He, J.J. Li, Y. Cai, Y.W. Wang, J.R. Ying, C.Y. Jiang, C.R. Wan, J. Power Sources 15 (2005) 216e222. [17] S.H. Ye, J.K. Bo, C.Z. Li, J.S. Cao, Q.L. Sun, Y.L. Wang, Electrochim. Acta. 55 (2010) 2972e2977. [18] C.Y. Wang, S.G. Lu, S.R. Kan, J. Pang, W.R. Jin, X.J. Zhang, J. Power Sources 189 (2009) 607e610. [19] D. Guan, J.A. Jeevarajan, Y. Wang, Nanoscale 3 (2011) 1465e1469. [20] J. Zhao, Y. Wang, J. Phys. Chem. C 116 (2012) 11867e11876. [21] S.C. Park, Y.M. Kim, Y.M. Kang, K.T. Kim, P.S. Lee, J.Y. Lee, J. Power Sources 103 (2001) 86e92. [22] D.H. Hu, S.X. Zhao, Y.F. Deng, C.W. Nan, J. Mater. Chem. A 1 (2013) 14729e14735. [23] Y.J. Sha, T. Yuan, B.T. Zhao, R. Cai, H.T. Wang, Z.P. Shao, J. Power Sources 231 (2013) 177e185. [24] T. Yuan, K. Wang, R. Cai, R. Ran, Z.P. Shao, J. Alloys Compd. 477 (2009) 665e672. [25] J.M. Kim, G. Lee, B.H. Kim, Y.S. Huh, G.W. Lee, H.J. Kim, Ultrason. Sonochem. 19 (2012) 627e631. [26] B.J. Li, H.Q. Cao, J. Shao, G.Q. Li, M.Z. Qu, G. Yin, Inorg. Chem. 50 (2011) 1628e1632. [27] J.Y. Luo, H.M. Xiong, Y.Y. Xia, J. Phys. Chem. C 112 (2008) 12051e12057. [28] W. Zhou, Z.P. Shao, R. Ran, H.X. Gu, W.Q. Wan, N.P. Xu, J. Am. Cera. Soc. 91 (2008) 1156e1162. [29] A.N. Shigapov, G.W. Graham, R.W. MaCabe, H.K. Plummer Jr., Appl. Catal. A Gen. 210 (2001) 287e300. [30] H. Birol, C.R. Rambo, M. Guiotoku, D. Hotza, Rsc Adv. 3 (2013) 2873e2884. [31] T. Mokkelbost, I. Kaus, T. Grande, M.A. Einarsrud, Chem. Mater. 16 (2004) 5489e5494. [32] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.F. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6e12. [33] S. Komaba, N. Yabuuchi, S. Ikemoto, J. Solid State Chem. 183 (2010) 234e241. [34] Q.T. Qu, L.J. Fu, X.Y. Zhan, D. Samuelis, J. Maier, L. Li, S. Tian, Z.H. Li, Y.P. Wu, Energy Environ. Sci. 4 (2011) 3985e3990. [35] M. Kitta, T. Akita, M. Kohyama, J. Power Sources 232 (2013) 7e11. [36] Y.M. Wu, Z.H. Wen, H.B. Feng, J.H. Li, Small 8 (2012) 858e862. [37] X.D. Xiang, Z. Fu, W.S. Li, J. Solid State Electrochem. 17 (2013) 1201e1206. [38] T.F. Yi, Y.R. Zhu, X.D. Zhu, J. Shu, C.B. Yue, A.N. Zhou, Ionics 15 (2009) 779e784. [39] S.C. Park, Y.M. Kim, S.C. Han, S. Ahn, C.H. Ku, J.Y. Lee, J. Power Sources 107 (2002) 42e47. [40] W. Choi, A. Manthiram, J. Electrochem. Soc. 153 (2006) A1760eA1764.
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Combustion-derived nanocrystalline LiMn2O4 as a promising cathode material for lithium-ion batteries Xuefeng Gao a, Yujing Sha a, Qian Lin a, Rui Cai a, Moses O. Tade b, Zongping Shao a, b, * a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People's Republic of China b Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia
h i g h l i g h t s Highly crystalline LiMn2O4 nanostructures were fabricated at low temperature. Dense nature of sample effectively minimized Mn2þ dissolution during process. Cycled at 5.0C for 100 cycles, the retention of charge/discharge capacity reached 93.5%. Favorable performance was still achieved when operated under improved temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2014 Received in revised form 12 October 2014 Accepted 16 October 2014 Available online 25 October 2014
In this study, nanocrystalline LiMn2O4 was synthesized by a simple combustion method and investigated for its utility as the positive electrode of a lithium-ion battery. X-Ray Diffraction characterization demonstrated that a basic crystallized spinel phase was already formed in the primary product from the direct combustion process, while pure phase LiMn2O4 was obtained after further calcination in air at relatively low temperature of 600 C. Characterization by SEM and HR-TEM as well as BET analysis showed that the LiMn2O4 compound had a primary particle size of 40e80 nm and that those particles were partially sintered to form 0.2e0.8 mm aggregates with few mesopores. The exposed surface area of the aggregates was low and mainly formed by the outer surfaces of the constituent particles, which is beneficial to reducing the interfacial area between the liquid electrolyte and LiMn2O4, thereby effectively mediating the Mn dissolution problem. As a result, the as-prepared LiMn2O4 showed a favorable capacity of 114 mAh g1 at a current rate of 0.2C and still retained a capacity of 84 mAh g1 at 5C. After 100 continuous cycles at 0.1C, a capacity of 108 mAh g1 was still maintained, compared to 120 mAh g1 at the first cycle. The results demonstrated that combustion synthesis-derived LiMn2O4 is a promising cathode material for lithium ion batteries (LIBs). © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Cathode LiMn2O4 Combustion synthesis Nanocrystalline
1. Introduction A battery is a device that converts chemical energy, which is stored inside of its electrodes, into electric power by means of electrochemical redox reactions. Among the various types of batteries, lithium-ion batteries (LIBs) are currently the most popular secondary batteries available because of their high voltage, light weight, high energy density, no memory effect, and environmental
* Corresponding author. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, Number 5 Xin Mofan Road, Nanjing, Jiangsu 210009, People's Republic of China. Tel.: þ86 25 83172256; fax: þ86 25 83172242. E-mail address: [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2014.10.099 0378-7753/© 2014 Elsevier B.V. All rights reserved.
friendliness. A conventional battery must have three main components: cathode, electrolyte and anode. In many cases, the electrode materials largely determine the potential window, energy density, rate performance, and cycling stability of a battery. The state-of-the-art cathode used in commercial LIBs for portable applications is layer-structured LiCoO2 oxide, which has a theoretical capacity of approximately 274 mAh g1 [1e3], but typically can only deliver a reversible capacity of ~140 mAh g1 [4]. Additionally, the toxicity and relatively high price of cobalt, poor cycling performance, and unfavorable thermal stability are substantial drawbacks of LiCoO2, and as such there have been considerable efforts over the past decade directed toward searching for alternative highperformance cathode materials for high power applications in particular.
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Spinel-type LiMn2O4 oxide, which has three-dimensional lithium diffusion paths, represents one of the most promising alternative cathode materials for next-generation LIBs. It is an outstanding material in several respects, in that it has a high discharge voltage plateau of approximately 4.0 V; it makes use of the natural abundance of Mn and is economically feasible; and it is also safer to use, as it is environmentally benign with low toxicity [5]. In spinel-type LiMn2O4 oxide, Mn ions are coordinated to O in an octahedral arrangement and form a cubic close-packed (CCP) structure, and the MnO6 octahedra share edges to form a threedimensional host for the Li guest ions [6]. Typically, almost all of the lithium ions in the tetrahedral sites of spinel LiMn2O4 can be extracted, resulting in a theoretical reversible capacity of approximately 148 mAh g1, which is comparable to the reversible capacity of state-of-the-art LiCoOx spinel-type cathodes [7]. This suggests that LiMn2O4 may be an ideal cathode substitute for LiCoOx in LIBs. In particular, it is generally accepted that LiMn2O4 is more suitable than LiCoOx for power battery applications [8]. Due to its relatively low lithium-ion diffusion coefficient (109e1011 cm2 s1) [9], poor electrode performance and in particular poor rate capacity has often been observed for coarsetype LiMn2O4. The decrease of particle size and the creation of nanopores inside the electrode material, which increased the specific surface area of the material, proved to be effective approaches to increase the electrocatalytic activity of the LiMn2O4 electrode [10e12]. For example, porous LiMn2O4 synthesized by a hydrothermal method showed a promising capacity of 128 mAh g1 at a current density of 0.2C rate and a favorable rate capability, with 83 mAh g1 retained at a current density of 20C [13]. However, one main problem that limits the practical application of nano-structured LiMn2O4 electrodes is the serious capacity fading that the material exhibits with galvanostatic chargeedischarge cycling, which can be partially attributed to the slow dissolution of Mn into the liquid electrolyte through the disproportionation of trivalent Mn (2Mn3þ(solid) / Mn4þ(solid) þ Mn2þ(solution)) [14]. The dissolution of divalent Mn brought about by the attack of HF, which is formed by LiPF6 and residual H2O in the electrolyte, not only causes a loss of active cathode material but also affects the anode and the electrolyte [15]. Currently, the most common way of increasing cathode performance while simultaneously maintaining good cycling stability is through a doping strategy or by coating the surface with a protective layer [16e22]. Our hypothesis is that if the lithium diffusion distance could be effectively decreased while the specific surface area is maintained at a low value, even without doping or surface coating, simultaneous high electrode performance and favorable cycling stability may still be achieved for pristine LiMn2O4. It follows from this rationale that the formation of micropores and mesopores should be avoided as the great specific surface of electrode exposed in electrolyte will greatly increase the risk of Mn dissolution, leading to a poor capacity retention. In this study, we report the straightforward synthesis of ultrafine LiMn2O4 powder by a cellulose-assisted combustion method that is nearly free of micropores and mesopores and can be employed as a highly electrochemically active cathode material in LIBs. Activated natural cotton was used as a porous support for the infiltration of lithium and manganese nitrates and glycine fuel, and also provided diffusion channels for maintaining a high oxidant atmosphere during the combustion process. Furthermore, the cotton support served to suppress the grain growth of nanocrystalline products on the cathode. The resultant material exhibited a good capacity, rate capacity and cycling stability. The reported material represents a new pathway forward in the development of high-performance LiMn2O4 cathodes for use in next-generation LIBs.
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2. Experimental 2.1. Powder synthesis The LiMn2O4 used in this work was prepared by a celluloseassisted combustion method [23]. De-seeded natural cotton of medical use was first immersed in concentrated nitric acid (67%) to allow the nitration of the surface groups of the cotton and the dissolution of some inorganic minerals. The treated cotton was then washed with de-ionized water to eliminate any residual free nitric acid, the treated cotton was dried at 120 C in an electric oven. In the second step, appropriate amounts of LiNO3 and Mn(NO3)2 were impregnated into the cotton at a molar ratio of 1.06:2 (6% extra Li source was used in consideration of Li evaporation during high temperature calcination), to serve as the raw materials for the LiMn2O4 phase formation. Then, a quantity of glycine that was three times the combined molar amount of LiNO3 and Mn(NO3)2 was added to the solution as the combustion improver. Subsequentially, the as-prepared solution was absorbed by proper amount of activated cellulose. The impregnated cellulose was then dried at 80 C and placed into an electric oven at 250 C. Sudden self-ignition was triggered [24] and resulted in a powder that was gray in color, which was further calcined at 600 C for 5 h to achieve fine crystallization of spinel LiMn2O4 and burn out any organic residuals, mainly leftover from the cellulose. 2.2. Characterization 2.2.1. Basic characterization X-ray diffraction (XRD) measurements were conducted to obtain structural information for the as-prepared LiMn2O4 and were carried out on a Bruker D8 advance diffractometer with filtered Cu Ka1 radiation (l ¼ 0.15406 nm) in the 2q range of 10e90 . Environmental scanning electron microscopy (HITACHI S4800) and transmission electron microscopy (JEOL JEM-2100) were performed to gain information about the morphology of the synthesized particulates. The surface area and corresponding pore size distribution were characterized by N2 adsorption-desorption testing at 77 K using a BELSORP II instrument (Japan) after sample pretreatment at 200 C for 6 h under vacuum. 2.3. Electrochemical testing LiMn2O4 was used to prepare the working electrode slurry by combining it with Super P and PVDF, added as conductor and binder, respectively (75:15:10 in weight). Analytical-grade NMP was used as the solvent. After all components were homogeneously dispersed in NMP, the slurry was subsequently coated onto an Al foil current collector by the blading method. Disk-shaped electrodes with a diameter of 14 mm were tailored from the coated foil and dried at 100 C in a vacuum for 12 h. Button cells (CR 2016) were then assembled with the as-prepared electrode as the working electrode, lithium foil as counter electrode, and a microporous polypropylene film (Celgard 2400) as the separator. The electrolyte used was 1 M LiPF6 dissolved in a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) (1:1 in volume, Shenzhen Capchem Technology Co., LTD., China). Chargedischarge curves were recorded on NEWARE BTS-5V 50 mA computer-controlled battery test station with a voltage range of 4.3e3.0 V. Corresponding rate performance and stability measurements were conducted using the same test station applying different current densities of 0.2, 0.5, 1, 2, 5, and 0.1C, respectively (1C ¼ 148 mA g1). Cyclic voltammetry tests were performed using a Princeton Applied Research PARSTAT 2273 advanced electrochemical system at a scanning rate of 0.2 mV s1 and a voltage range of 3.5e4.5 V.
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3. Results and discussion Although many techniques have been developed for the synthesis of LiMn2O4, most of them require multi-step procedures, making them difficult for scale up for scale-up and economically unattractive. In addition, the samples prepared by these methods, which exhibited high electrochemical performance, usually possessed high specific surface area [25e27]. The large interfacial area between the electrode and liquid electrolyte for these electrodes has sparked concerns about the dissolution of Mn2þ. Cellulose-based combustion synthesis was found to be a universal technique for the synthesis of ultrafine powders, and has been successfully applied in the preparation of a wide range of materials with synthesis scale-up capability [28e30]. The synthesis of LiMn2O4 by the cellulose-assisted combustion method is fairly simple in procedure. After impregnating the activated cotton fibers with LiNO3, Mn(NO3)2 and glycine solutions in the proper ratios and then drying the mixture at 80 C in an electrical oven, dry cellulose containing an uniform mixture of LiNO3, Mn(NO3)2 and glycine was obtained. As referred to what we previously reported, raising the oven temperature to 250 C at this point initialized a two-stage combustion. A first sudden flame combustion, which lasted for approximately 15 s and was accompanied by the release of a large amount of heat and gas, occurred, followed by a secondary, more gentle combustion that resulted in a flameless and which lasted for a couple of minutes. Fig. 1 shows the XRD patterns of the primary LiMn2O4 product obtained from the direct combustion of the solid precursor and also of the powder calcined at 600 C in air for 5 h. Basic diffraction peaks for a spinel-type phase were already present in the XRD patterns of the product obtained directly from the auto-combustion process. The high peak intensity suggests that the crystalline phase was already well developed, while the broad peak width is a sign that the product is composed of very small crystallites. During the direct combustion synthesis, the first auto-combustion process was due to the reaction between the metal nitrates (LiNO3, Mn(NO3)2) and glycine, which was very vigorous, with the instantaneous reaction temperature reaching over 1000 C [31]. Referring to the previous report published by Chick L. A. et al. [32], the ideal reaction between metal nitrates and glycine during combustion can be described as follows:
9MnðNO3 Þ2 þ 8NH2 CH2 COOH/9MnO2 þ 13N2 [ þ 16CO2 [ þ 20H2 O (1) 2LiNO3 þ 2NH2 CH2 COOH/Li2 O þ 2N2 [ þ 2CO2 [ þ 5H2 O (2)
It was worth that during the combustion process, reddish brown gas was released accompanied with the reaction, indicating a generation of N2O. Thus, the real reaction during combustion may be somewhat different form reactions 1 and 2. Such an exothermic reaction provided a huge amount of energy that promoted the formation of LiMn2O4 phase in a very short time (4MnO2 þ Li2 O/2LiMn2 O4 þ 0:5O2 [) [12,33]. The porous morphology of the surrounding cellulose provided a supply of free oxygen that served to maintain a highly oxidizing atmosphere critical to the formation of the LiMn2O4 spinel and the prevention of Mn reduction. Meanwhile, due to the short time of reaction as well as the blocking effect of the cellulose fibers, which acted as a micro reactor during synthesis, the sintering of particles was effectively suppressed. During the second combustion process, the sintering of the product particles was also limited because of the low temperature generated in this stage, in which the cellulose framework was largely burned out. As a result, for the as-obtained product, the basic phase of the material was developed and a small crystalline size was preserved. The product of the direct combustion was gray in color, implying the presence of a considerable amount of organic/ carbon residual. Thus, it was likely that the secondary modest combustion process was insufficient to consume all of the cotton fibers in the material. However, after subjecting to calcination in air for 5 h at 600 C, the sample turned black, implying the successful elimination of the organic residual. Sharp X-ray diffraction peaks were observed for the calcined sample, suggesting the formation of a well-crystallized product with an increased crystallite size. The broad peaks indicate that the powder was composed of ultrafine crystals. The crystalline size was quantified based on the half width of the diffraction peaks using the Scherrer equation, where D ¼ Kl/ Bcosq. The peaks arising from the (111), (311), (400) and (440) diffraction planes were selected for this analysis, and the average crystalline size based on above four diffraction peaks was found to be ~45.0 nm. Shown in Fig. 1b is the Rietveld refinement of the XRD patterns for the sample after calcination at 600 C, based on a cubic spineltype lattice structure with space group Fd3 m (JCDPS: 35-0782). Good fitting with low convergence reliability factors (Rp ¼ 2.81%, Rwp ¼ 3.61%, c2 ¼ 1.461) suggests the successful formation of wellcrystallized phase-pure LiMn2O4 spinel after calcination of the solid combustion synthesis precursor at relatively low temperature (600 C for 5 h). The derived lattice parameter was 8.21(3) Å, which matched soundly with literature results (8.24(7) Å). The small difference between the two values may suggest a slight deviation in the oxidation state and lithium content of the current sample from that presented in the literature. SEM images of the as-synthesized LiMn2O4 powder (600 C calcined) are shown in Fig. 2. The particulates had a semi-spherical
Fig. 1. (a) XRD patterns of the primary LiMn2O4 particles formed directly from combustion and of the product upon further calcination of the powder at 600 C; (b) Rietveld refinement of an in-situ XRD pattern of the LiMn2O4 powder after calcination at 600 C.
X. Gao et al. / Journal of Power Sources 275 (2015) 38e44
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Fig. 2. SEM images of the as-synthesized LiMn2O4 at different magnifications.
morphology and a particle size of 200e800 nm. This observation agrees well with the fact that materials with a cubic lattice structure should grow isotropically in all three directions, and thus particles of a more symmetrical shape such as spheres or cubes are easier to form. It further indicates that the combustion synthesis resulted in an ultrafine particle size. As mentioned previously, the uniformity and dimensions of the particles can be explained by the homogeneous mixing of the raw materials in the solution stage and the low calcination temperature, as well as the blocking effect of the cellulose on the contact of the particles during synthesis. The surface of the ultrafine particles, is smooth and dense, implying the dense nature of the as-prepared nanoparticles. In other words, the overall specific surface area of the product is derived mainly from the external surface area of the nanoparticles. Additionally, those ultrafine particles were found to have partially sintered to form larger aggregates. It is well known that current collection for nanoparticles is somewhat difficult. The formation of macroporous and micrometer-sized aggregates is actually beneficial to the application of LiMn2O4 as an electrode material because of the improvement in current collection efficiency. The nanosized primary particles provide a short lithium diffusion distance, while the macropores between nanoparticles allow for easy penetration by the liquid electrolyte; on the other hand, the formation of micrometer-sized aggregates allows for more facile and efficient current collection. Thus, improved electrode performance is expected when using this material. The electrochemical performance of the as-prepared LiMn2O4 formed from combustion synthesis will be discussed in detail later. The particulate morphology of the as-synthesized LiMn2O4 sample was further investigated by TEM and HR-TEM. As shown in Fig. 3a and b, the primary LiMn2O4 particles were spherical in shape and had particle sizes in the range of 40e80 nm, matching well with the diameter calculated from X-ray diffraction peaks (~45.0 nm). Those primary particles were further partially fused to form larger aggregates, corresponding to the particles as observed by SEM in Fig. 2. Within the larger aggregates, meso-to-macro pores with diameters in the range of 60e150 nm, as highlighted in the same image, were also formed from the packing of the nanoparticles. HR-TEM images of LiMn2O4 are presented in Fig. 3c. Clear lattice fringes with a spacing of 0.47 nm were observed, matching well with the distance between the (111) diffraction planes of a typical spinel-type LiMn2O4 crystal. To obtain more detailed information about the local crystal structure, selected area electron diffraction (SAED) patterns were analyzed. It is known that an accurate SAED pattern is acquired when the rays of the electron beam are perpendicular to the direction of the plane forming the diffraction pattern. Fig. 3d presents the SAED pattern corresponding to the HR-TEM images shown in Fig. 3c; it is composed of wellorganized dots in which the (111) diffraction plane could be identified. The (200) diffraction plane was also calculated along the
specific zone axis direction of [011]. These findings further supported the successful formation of the crystallized spinel LiMn2O4 phase derived from the combustion synthesis. The specific surface area and pore structure information of the as-synthesized LiMn2O4 (calcined at 600 C) was investigated by nitrogen adsorption-desorption analysis. Shown in Fig. 4 are the nitrogen adsorption-desorption curves measured at the boiling temperature of liquid nitrogen, as well as the corresponding pore size distribution curves. The nitrogen adsorption-desorption curves can be categorized as type II curves according to the IUPAC definition. Almost no hysteresis loop within the relative partial pressure (P/Po) range of 0.1e0.9 was observed in the curves. This implies that few mesopores present within the as-synthesized LiMn2O4 material. This conclusion was more clearly demonstrated from the pore size distribution curves of the uncalcined sample. According to BrunauereEmmetteTeller (BET) analysis, the combustion-derived LiMn2O4 had a specific surface area of only 3.74 m2 g1, which was relatively low compared with the porous LiMn2O4 reported in the literature (19.2 m2 g1) [34]. Generally speaking, the increase in specific surface area will increase the contact area between the electrode and liquid electrolyte; however, the possibility of Mn2þ dissolution into the electrolyte will also increase accompanied with large surface exposed in electrolyte. In comparison, the low specific area of 3.74 m2 g1 for as-prepared LiMn2O4 greatly contributes to the enhanced cycling stability of electrode. Assuming that the LiMn2O4 particles were spherical in shape and densely packed, the average diameter of the particles could be calculated based on the specific surface area of the LiMn2O4 powder and was determined to be approximately 0.15 mm. This roughly agrees with the minimum particle size observed by SEM analysis (Fig. 2). To investigate the electrochemical performance of the assynthesized LiMn2O4 as the positive electrode of a LIB, a half-cell was constructed with LiMn2O4 as the working electrode and metallic lithium as the counter and reference electrodes. Shown in Fig. 5 are the first galvanostatic chargeedischarge curves for the electrode at a current rate of 0.1C (a rate of 1C corresponds to a full charge/discharge of the theoretical capacity, i.e., 148 mA g1, in 1 h) within a potential range of 3.0e4.3 V. The charge and discharge curves demonstrated two consecutive slowly increasing (for charge) or decreasing (for discharge) plateaus, one at 3.98/4.2 V and the other at 4.12/3.93 V, respectively. The observation of two pairs of voltage platforms on the galvanostatic chargeedischarge curves is common for LiMn2O4-based electrodes and is believed to be associated with the two-step phase transformations of LiMn2O4/ Li0.5Mn2O4 and Li0.5Mn2O4/l-MnO2, correspondingly [12]. During the charge process, the extraction of half of the Liþ ions from the tetrahedral sites in LiMn2O4 through LieLi interaction occurred, leading to the formation of Li0.5Mn2O4 intermediate, which is associated with the first charge plateau located at approximately
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Fig. 3. (a, b) Typical TEM images, (c) HR-TEM image, and (d) SAED patterns taken along the [011] zone axes of the as-synthesized LiMn2O4 sample.
3.98 V. Then, a further extraction of the lithium from the tetrahedral sites occurred without the aid of any LieLi interaction, giving rise to the formation of a product with the final composition of lMnO2, which can be associated with the presence of the second charge plateau at approximately 4.2 V in the galvanostatic charge curve [35]. The initial discharge capacity was 120.5 mAh g1 at 0.1C,
which is approximately 81.4% of the theoretical capacity of LiMn2O4 (148 mAh g1), indicating that the as-prepared LiMn2O4 electrode made from the combustion synthesized material was discharged to the state of Li0.81Mn2O4 on average. In other words, it suggests that most parts of the electrode materials were electrochemically active for the lithium insertion reaction.
Fig. 4. Adsorption/desorption curves of as-prepared LiMn2O4; the inset image is the corresponding BJH picture.
Fig. 5. The first galvanostatic chargeedischarge curves of the as-synthesized LiMn2O4 (calcined at 600 C for 5 h) obtained at a 0.1C current rate.
X. Gao et al. / Journal of Power Sources 275 (2015) 38e44
The electrochemical behavior of the combustion-synthesized LiMn2O4 was investigated by cyclic voltammetry (CV). Shown in Fig. 6 are the first, second and 10th CV curves of the oxide scanned between the voltages of 3.5 and 4.5 V, taken at room temperature and at a scanning rate of 0.2 mV s1. All three CV curves were similar to each other in shape and were well-overlapped with slight increases of overpotential, suggesting good reversibility of electrochemical lithium intercalation/de-intercalation of the asprepared LiMn2O4. Furthermore, the observed peaks are relatively sharp, suggesting the high crystallinity of the LiMn2O4 in the electrode and good kinetics for lithium ion transport within its bulk. In connection with the platforms in galvanostatic charge/ discharge curves, the redox peaks in first CV curve, which located at 4.05 V/4.06 V and 4.19 V/3.93 V (Liþ/Li), indicated the extraction/ insertion of Liþ ions from/into half of the tetrahedral sites with LieLi interaction and from/into the other half of the tetrahedral sites without any LieLi interaction, respectively. The rate performance of the as-synthesized LiMn2O4 electrode was also investigated by varying the charge/discharge rate from 0.2C to a maximum of 5.0C. As shown in Fig. 7, excellent rate capacities were demonstrated in this test. Capacities of 114, 105, 100 and 96 mAh g1 were reached at 0.2, 0.5, 1.0 and 2.0C rates, respectively. Even with the further increase of charge/discharge rate to 5.0C, a reversible capacity of 84 mAh g1 was still retained. For comparison, similar capacity at low current density (110 mAh g1, 0.5C) was observed for porous LiMn2O4 microcubes, while the capacity at 5.0C was only 68 mAh g1 [36]. Compared with the Li-rich mesporous LiMn2O4 nanospheres reported by J. M. Kim et al. [25], cellulose assisted-combustion was more costeffective and simple than reported ultrasound-assisted refluxes; Meanwhile, rate capacitance at high rates (96 mAh g1 at 2C) was much higher than that of reported LiMn2O4 (80 mAh g1 at 2C). This observed capacity was also slightly higher than that of mesoporous LiMn2O4 synthesized by the template method (80 mAh g1 at 5.0C), providing further evidence for the good performance of LiMn2O4 synthesized by cellulose-assisted combustion combustion. The better rate capacity of the LiMn2O4 structure presented here suggests that lithium insertion occurs easier in this structure, likely due to the short lithium diffusion distance and the improved access of the liquid electrolyte provided by the macropores in the electrode material as demonstrated in Fig. 3. As mentioned, with the decrease of particle size and the concomitant increase in the specific surface area of the LiMn2O4
Fig. 6. Cyclic voltammograms of the as-prepared LiMn2O4 at a scan rate of 0.2 mV s1 after performing cycling tests at 0.1C over different time intervals.
43
Fig. 7. Cycling performance of the as-prepared LiMn2O4 electrode at different charge/ discharge rates of 0.2C, 0.5C, 1C, 2C and 5C at room temperature.
electrode, cycling stability becomes a point of concern because of increased Mn dissolution into the liquid electrolyte. Fig. 8 shows the dependence of the capacity of LiMn2O4 electrode on cycling times; the electrode was cycled at 0.1C rate (at room temperature) (Fig. 8a), and also cycled at 0.5C at either room temperature or 55 C (Fig. 8b). Acceptable stability of the capacity was achieved under these conditions. For example, a capacity of 108 mAh g1 was still retained after 100 cycles at a 0.1C rate at room temperature; a comparison of this capacity to that measured during the first cycle, at 120 mAh g1, indicates a capacity retention of 90.1%. At a faster rate of 5.0C, a discharge capacity of 87 mAh g1 was still achieved after 100 galvanostatic charge/discharge cycles in comparison to a capacity of 93 mAh g1 upon first discharge, suggesting a capacity retention as high as 93.5%. The specific capacity and capacity retention at different rates were much higher than those reported unidimensional rod and quadrate lamina LiMn2O4 [37], which were synthesized by complex hydrothermal reactions and exhibited only ~84% capacity retention after cycling at 0.1C for 50 times (25 C). There are several causes for the slight capacity fade of LiMn2O4 electrodes [38], but it is generally believed that Mn dissolution is the most problematic source. When employed in practical LIBs, these electrodes may be operated at elevated temperatures, at which the Mn dissolution problem becomes more severe [39]. As shown in Fig. 8, an initial discharge capacity of 92 mAh g1 was achieved when the temperature of the system was 55 C. It is generally accepted that the lithium diffusion rate inside an oxide is improved with an increase in temperature, and thus lithium diffusion inside the bulk oxide should cause less polarization resistance to the lithium charge/discharge processes. Very interestingly, comparable capacities were observed for LiMn2O4 at room temperature and 55 C at the high capacity charge/discharge rate of 5.0C. This suggests that the lithium-ion diffusion occurring inside the oxide bulk was not the main source of the large polarization resistance of LiMn2O4 at high charge/discharge rates. Alternatively, this resistance may be explained by the nanocrystalline size of the combustion-synthesized LiMn2O4, which provided short lithium diffusion distance. After continuously cycling the cell 100 times, a capacity of 84 mAh g1 was still maintained, suggesting a capacity retention of 89.1%. This suggests that even when used at high temperature, the combustion-synthesized LiMn2O4 still exhibited a favorable cycling stability. As we know, the dissolution of Mn from LiMn2O4 is directly related to the specific surface area of the electrode [40]. Although the combustion-synthesized LiMn2O4 sample
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Fig. 8. Cycle performances at (a) 0.1C and at (b) 0.5C (tested at room temperature and 55 C, respectively) for 100 cycles.
was composed of nanocrystalline particles, which ensured a short diffusion distance for lithium ion and thus a high electrode performance, the dense nature of the nanocrystallites minimized the specific surface area of the electrodes. As a result, the dissolution of Mn2þ from LiMn2O4 into the liquid electrolyte was not significant, even at elevated temperatures. A further modification of the electrode surface, such as adding a protective layer such as TiO2 by atom layer deposition or another advanced technique, may further improve the cycling stability of the electrode material and improve its potential for practical application in LIBs. 4. Conclusions Highly crystalline LiMn2O4 nanostructures were successfully synthesized by a straightforward cellulose-assisted combustion method at the relatively low temperature of 600 C. The primary particles that were produced were only 40e80 nm in size with an average surface area of 3.74 m2. The nano-sized particles ensured short diffusion paths for lithium ions and electrons when applied as a lithium-ion cathode, while the dense nature of the sample minimized the surface area of the electrode exposed to the electrolyte and thus decreased Mn2þ dissolution during the process. Even when cycled at 5.0C for 100 cycles, the retention of charge/discharge capacity was 93.5%. When operated under harsher conditions, favorable performance was still achieved. Furthermore, when incorporated into an LIB cell, a capacity of 84 mAh g1 was maintained after 100 cycles at 5.0C with a capacity retention of 89.1%. In conclusion, as-prepared LiMn2O4 nanostructures derived from a cellulose-assisted combustion method exhibited excellent performances even under harsh conditions, and the successful application of the combustion method in the preparation of LiMn2O4 also shed light on the application of LiMn2O4 in electrochemical conversion and storage. As a universal synthesis method, cellulose-assisted combustion may also find application in the preparation of other nano-structured materials for functional applications. Acknowledgments This work was partially supported by the “Key Projects in Nature Science Foundation of Jiangsu Province” under contract no. BK2011030, “National Science Foundation for Distinguished Young Scholars of China” under contract no. 51025209, “National Nature Science Foundation of China” under contract no. 201103089, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Y. Shao-Horn, L. Croguennec, C. Delmas, E.C. Nelson, M.A. O'Keefe, Nat. Mater. 2 (2003) 464e467.
ron, R. Dedryve re, H. Martinez, M. Me ne trier, C. Denage, C. Delmas, [2] L. Dahe D. Gonbeau, Chem. Mater. 20 (2008) 583e590. [3] S. Jeong, S. Park, J. Cho, Adv. Energy Mater. 1 (2011) 368e372. [4] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Adv. Mater. 21 (2009) 4593e4607. [5] S. Lee, Y. Cho, H.K. Song, K.T. Lee, J. Cho, Angew. Chem. Int. Ed. 51 (2012) 8748e8752. [6] J.M. Tarascon, E. Wang, F.K. Shokoohi, J. Electrochem. Soc. 138 (1991) 2859e2864. [7] D.K. Kim, P. Muralidharan, H.W. Lee, R. Ruffo, Y. Yang, C.K. Chan, H.L. Peng, R.A. Huggins, Y. Cui, Nano Lett. 8 (2008) 3948e3952. [8] F.Y. Cheng, H.B. Wang, Z.Q. Zhu, Y. Wang, T.R. Zhang, Z.L. Tao, J. Chen, Energy Environ. Sci. 4 (2011) 3668e3675. [9] Z. Quan, S. Ohguchi, M. Kawase, H. Tanimura, N. Sonoyama, J. Power Sources 244 (2013) 375e381. [10] B.J. Liddle, S.M. Collins, B.M. Bartlett, Energy Environ. Sci. 3 (2011) 1339e1346. s-Solís, M.R. Palacín, J. Oro -Sole , A. Fuertes, G. Marb [11] J. Cabana, T. Valde an, A.B. Fuertes, J. Power Sources 166 (2007) 492e498. [12] D. Tonti, M.J. Torralvo, E. Enciso, I. Sobrados, J. Sanz, Chem. Mater. 20 (2008) 4783e4790. [13] Y.Z. Wang, X. Shao, H.Y. Xu, M. Xie, S.X. Deng, H. Wang, J.B. Liu, H. Yan, J. Power Sources 226 (2013) 140e148. [14] K.R. Ragavendran, L. Lu, K. Barner, A.K. Arof, J. Phys. Chem. C 117 (2013) 23547e23557. [15] Y. Yang, C. Xie, R. Ruffo, H. Peng, D.K. Kim, Y. Cui, Nano Lett. 9 (2009) 4109e4114. [16] H.M. He, J.J. Li, Y. Cai, Y.W. Wang, J.R. Ying, C.Y. Jiang, C.R. Wan, J. Power Sources 15 (2005) 216e222. [17] S.H. Ye, J.K. Bo, C.Z. Li, J.S. Cao, Q.L. Sun, Y.L. Wang, Electrochim. Acta. 55 (2010) 2972e2977. [18] C.Y. Wang, S.G. Lu, S.R. Kan, J. Pang, W.R. Jin, X.J. Zhang, J. Power Sources 189 (2009) 607e610. [19] D. Guan, J.A. Jeevarajan, Y. Wang, Nanoscale 3 (2011) 1465e1469. [20] J. Zhao, Y. Wang, J. Phys. Chem. C 116 (2012) 11867e11876. [21] S.C. Park, Y.M. Kim, Y.M. Kang, K.T. Kim, P.S. Lee, J.Y. Lee, J. Power Sources 103 (2001) 86e92. [22] D.H. Hu, S.X. Zhao, Y.F. Deng, C.W. Nan, J. Mater. Chem. A 1 (2013) 14729e14735. [23] Y.J. Sha, T. Yuan, B.T. Zhao, R. Cai, H.T. Wang, Z.P. Shao, J. Power Sources 231 (2013) 177e185. [24] T. Yuan, K. Wang, R. Cai, R. Ran, Z.P. Shao, J. Alloys Compd. 477 (2009) 665e672. [25] J.M. Kim, G. Lee, B.H. Kim, Y.S. Huh, G.W. Lee, H.J. Kim, Ultrason. Sonochem. 19 (2012) 627e631. [26] B.J. Li, H.Q. Cao, J. Shao, G.Q. Li, M.Z. Qu, G. Yin, Inorg. Chem. 50 (2011) 1628e1632. [27] J.Y. Luo, H.M. Xiong, Y.Y. Xia, J. Phys. Chem. C 112 (2008) 12051e12057. [28] W. Zhou, Z.P. Shao, R. Ran, H.X. Gu, W.Q. Wan, N.P. Xu, J. Am. Cera. Soc. 91 (2008) 1156e1162. [29] A.N. Shigapov, G.W. Graham, R.W. MaCabe, H.K. Plummer Jr., Appl. Catal. A Gen. 210 (2001) 287e300. [30] H. Birol, C.R. Rambo, M. Guiotoku, D. Hotza, Rsc Adv. 3 (2013) 2873e2884. [31] T. Mokkelbost, I. Kaus, T. Grande, M.A. Einarsrud, Chem. Mater. 16 (2004) 5489e5494. [32] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.F. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6e12. [33] S. Komaba, N. Yabuuchi, S. Ikemoto, J. Solid State Chem. 183 (2010) 234e241. [34] Q.T. Qu, L.J. Fu, X.Y. Zhan, D. Samuelis, J. Maier, L. Li, S. Tian, Z.H. Li, Y.P. Wu, Energy Environ. Sci. 4 (2011) 3985e3990. [35] M. Kitta, T. Akita, M. Kohyama, J. Power Sources 232 (2013) 7e11. [36] Y.M. Wu, Z.H. Wen, H.B. Feng, J.H. Li, Small 8 (2012) 858e862. [37] X.D. Xiang, Z. Fu, W.S. Li, J. Solid State Electrochem. 17 (2013) 1201e1206. [38] T.F. Yi, Y.R. Zhu, X.D. Zhu, J. Shu, C.B. Yue, A.N. Zhou, Ionics 15 (2009) 779e784. [39] S.C. Park, Y.M. Kim, S.C. Han, S. Ahn, C.H. Ku, J.Y. Lee, J. Power Sources 107 (2002) 42e47. [40] W. Choi, A. Manthiram, J. Electrochem. Soc. 153 (2006) A1760eA1764.