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Glucose-Assisted Synthesis of Highly Dispersed LiMnPO4 Nanoparticles at a Low Temperature for Lithium Ion Batteries
Electrochimica Acta 189 (2016) 205–214
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Glucose-Assisted Synthesis of Highly Dispersed LiMnPO4 Nanoparticles at a Low Temperature for Lithium Ion Batteries Zhengzheng Xiea , Kun Changa,* , Bao Lia , Hongwei Tanga , Xiaoning Fua , Zhaorong Changa,* , Xiao-Zi Yuanb , Haijiang Wangb a Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P.R. China b National Research Council of Canada, Vancouver, BC V6T 1W5, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 November 2015 Received in revised form 15 December 2015 Accepted 15 December 2015 Available online 18 December 2015
The cathode material of the LiMnPO4/C composite for lithium-ion batteries is successfully synthesized via a one-step glucose-assisted liquid-phase method in ethylene glycol (EG). The crystalline structure, morphology, micro-structure and particle size are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). XRD results show that the pure phase of LiMnPO4 with high crystallinity can directly be prepared in the liquid-phase assisted by glucose. SEM measurements confirm the uniform-sized nanorods of the LiMnPO4 morphology with a width of 20– 50 nm and a length of 50–80 nm. TEM characterization reveals that the surface of the obtained LiMnPO4 nanorods is coated with a homogeneous carbon layer after a short heat treatment at a high temperature in the presence of glucose. This can be explained by the fact that the glycol glucoside generated during the refluxing of EG with glucose can effectively inhibit the growth and agglomeration of particles. Results of electrochemical tests show that the prepared LiMnPO4/C nanorods exhibit not only a high initial discharge capacity of 155.3 mAh g1 but also a good cycling stability, which retains 94% of the initial capacity over 100 cycles at 0.05 C. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium manganese phosphate Cathode material Nanoparticles Glucose assisted Liquid phase method
1. Introduction Lithium ion batteries as an excellent performance rechargeable green power have been widely used in portable electronic products, communication tools and are being developed for electric vehicles power. Consequently, they are promoted to safety, environmental protection, low cost and high specific energy [1–3]. LiFePO4 with an olivine structure as a cathode material has become a research hot spot quickly after it was first reported in 1997 by Padhi et al [4], due to the characteristics of high specific capacity, good thermal stability, excellent cycling performance, abundant raw materials and environmental friendliness [5,6]. The issues related to low ion and electron conductivity of the LiFePO4 material have basically been solved by reducing particle size [7], doping metal ions [8] and coating carbon on the surface of the particles [9]. Presently, the capacity of commercialized LiFePO4 mainly used in electric vehicles and energy storage areas has reached
* Corresponding authors. Tel.: +86 373 3326335. E-mail addresses: [email protected] (K. Chang), [email protected] (Z. Chang). http://dx.doi.org/10.1016/j.electacta.2015.12.111 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
160 mAh g1, which is close to its theoretical capacity (170 mAh g1) [10–12]. However, the low specific energy (theoretical value is 578 Wh kg1) of LiFePO4 due to the low tap density (1.1–1.3 g cm3) and working voltage (3.4 V) limits the mileage of electric vehicles powered by Li-ion batteries [13–15]. It is known that LiMnPO4 has the same olivine structure and theoretical capacity as LiFePO4, however with a working voltage of 4.1 V (vs. Li+/Li) [16], which is within the electrochemical window of the existing electrolyte system [17,18]. As such, the theoretical specific energy of LiMnPO4 can reach 700 Wh kg1 because of the high working voltage, about 20% higher than LiFePO4 [17,19,20]. In addition, the outstanding characteristics of the LiMnPO4 such as abundant raw material resources, low cost, environmental friendliness, stable structure, good chemical compatibility and high safety endow it to be a quite promising cathode material for lithium ion batteries. Nevertheless, comparing with LiFePO4, the electronic conductivity and ion diffusion coefficient of LiMnPO4 are much lower [21], which leads the material prone to polarization with a poor rate performance. The band gap of LiMnPO4 calculated by Yamada et al. with the first principle is 2 eV, which means LiMnPO4 is almost an insulator [22]. Hence, it is very difficult to synthesize the LiMnPO4
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material with an excellent electrochemistry performance. It is also the main obstruction to realize the industrialization of the material so far. Researches have revealed that the nanocrystallization of material particles, doping of metal ions and carbon coating on the surface of nanoparticles are effective ways to improve the ion diffusion coefficient and electron conductivity [22–25]. However, the electrochemical performance of LiMnPO4 depends on the particle size and dispersion more heavily than that of LiFePO4 due to the insulation of the LiMnPO4 itself. Compared with ball-milling [11,12,26] and sol-gel [27],mainly based on a high temperature solid phase sintering, the liquid phase hydrothermal or solvent method [28] and polyol method [29] are more suitable for the preparation and regulation of nanoparticles. It is found that most nanoscaled LiMnPO4 particles synthesized by the hydrothermal or polyol methods are in one dimension only, whereas other dimensions are still in a micron scale. In addition, these micronano particles are easy to agglomerate. Dong et al. [30] synthesized the LiMnPO4 nanosheets with a thickness of 50 nm and a width of 2 mm via a hydrothermal route in a high-pressure container heated at 200 C for 12 h, and the nanosheets exhibit a discharge capacity of 107.5 mAh g1 at 0.05 C. Wang et al. [31] synthesized LiMnPO4 nanosheets with a thickness of 20–30 nm and a width of 3.5 um by a polyol method of refluxing 5 h at 100 C. The resulting material achieved a discharge capacity of 145 mAh g1 at 0.05 C. As these synthesized LiMnPO4 materials are not full-dimensional sized nanoparticles and easy to aggregate, the lithium ions diffusion and electrons conductivity are greatly restricted. In our previous work, dimethyl sulfoxide (DMSO) solution-phase method was employed to synthesized hollow spindle LiMnPO4 particles with a width of 200 nm and length of 500–700 nm, which exhibited the enhanced electrochemical performances [32]. To further improve the contact between LiMnPO4 materials and electrolyte, nanocrystalized synthesis of LiMnPO4 particles especially the full-dimensional size are expected to fulfill the uniformity of carbon layer coating and fast transport of lihthium ions, facilitating the enhancement of electrochemical performances of the electrodes. In this context, we adopted a unique approach in an attempt to synthesize uniform LiMnPO4 nanorods. First a certain amount of
glucose was added into the ethylene glycol (EG) solution during pretreatment refluxing, and LiOH, MnSO4 and H3PO4 were then added and refluxed at 140 C under ambient pressure. It is found that the obtained LiMnPO4 particles are uniform nanorods with a diameter of 50 nm and a length of 80 nm. Moreover, the prepared LiMnPO4 nanorods show a good dispersion and excellent electrochemical performance after being coated carbon. Interestingly, the color of the solution became light yellow when glucose was added into EG during refluxing. If glucose and LiOH were together added into EG during refluxing, the solution turned to sepia and viscous. For the above phenomenon, we analyzed the effect of intermediate possibly generated in the pretreatment refluxing on the particle size and dispersion of LiMnPO4. The research confirms that the product generated by EG and glucose after refluxing and the adding sequence of LiOH have a great effect on the size, dispersion and the electrochemical properties of the LiMnPO4 particles. This work provides a new method for the synthesis of LiMnPO4 nano-particles with a full-dimension and non-aggregation. To the best of our knowledge, there have been no relevant literature reported so far.
2. Experimental 2.1. Synthesis of LiMnPO4 and the LiMnPO4/C The preparation procedures are shown in Fig. 1 . Solution A was obtained by dissolving 0.06 mol LiOH into 10 mL deionized water. Meanwhile, solution B1 was obtained by the heating of the solution containing 30 ml EG and 3 g glucose. This was done at 140 C under the protection of N2 for 2 h. Next, Solution C was produced after solutions of A and B1 were together stirred uniformly. The D solution was received by dissolving 0.02 mol MnSO4 and 0.02 mol H3PO4 into 15 mL deionized water. Solution C were poured into D and stirred uniformly with 30 mL EG, the proportion of EG to H2O is 60: 25, and then refluxed at 140 C under the protection of N2 for 12 h. Sample 1 (marked as S1) was obtained after the precipitate was centrifuged at a rotating speed of 8000 r min1, washed with deionized water and dried for 12 h at 80 C.
Fig. 1. Schematic illustration of the synthesis process of the LiMnPO4/C cathode materials.
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The B2 solution was obtained after 30 mL EG was heated at 140 C with the protection of N2 for 2 h. Sample 2 (marked as S2) serves as a comparative sample with no glucose refluxing via the same method as S1. The B3 solution was obtained after that the 30 mL EG dissolving 3 g glucose and 0.06 mol LiOH was heated at 140 C under the protection of N2 for 2 h (because all the required LiOH has been added, solution B3 is solution C here). The S3 sample was obtained via the same method as S1. Carbon coating process was conducted through grinding the proposed samples with a certain amount of glucose, followed by calcination in a tube furnace under an atmosphere of 95% Ar, 5% H2 at 500 C for 5 h with a heating rate of 10 C min1.
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which maintains <0.5 ppm of O2 and <0.5 ppm of H2O. Lithium metal was used as the counter and reference electrodes. 1 M LiPF6 in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1 volume ratio) was used as the electrolyte, while Celgard 2400 (Celgard polypropylene) was employed as separator. The cells were kept stewing for 12 h and then tested using a Land CT2001A battery tester (Wuhan Jinnuo Electronics Co. Ltd. China) in a potential range of 2.44.8 V at room temperature and rates of 0.05C, 0.1C, 1C and 5C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were investigated on an electrochemical workstation (Shanghai Chenhua instrument company, China). The CV test scan rates were from 0.1 mVs1 to 0.5 mVs1 over a cell voltage from 2.4 V to 4.8 V. For EIS, the scan frequency ranged from 1 Hz to10 kHz.
2.2. Characterization 3. Results and discussion Phase analysis and cell parameter determination of all samples were performed by X-ray diffraction(XRD) using a D8 X diffract meter (Bruker, Germany) with Cu-Ka radiation and graphite monochromator. The working voltage was 35 KV and working current was 30 mA. The scanned data were collected in a 2u range from 1080 . The particle morphology of the synthesized materials was analyzed by scanning electron microscopy (SEM) (SEM-6701F, JEOL, Japan) and transmission electron microscopy (TEM) (JEOL JEM 2010, Japan). Solutions of B1, B2 and B3 were scanned with Hydrogen nuclear magnetic resonance spectroscopy (HNMR) (Avance, 400 MHz, Bruker, Switzerland). 2.3. Electrochemical performance tests The electrochemical performance of as-prepared samples S1, S2 and S3 were investigated at room temperature on a 2025 type coin cell. The electrode was prepared as follows: active material, PVDF (polyvinylidene fluoride) and acetylene black were mixed at a weight ratio of 70:10:20. After adding a certain amount of Nmethyl-2-pyrrolidone (AR, purchased from Sinopharm Chemical Reagent Co., Ltd.), the mixture was ground uniformly with a pulp refiner. The obtained slurry was then spread on a piece of aluminum foil with a diameter of 12 mm, and spreading area of 1.13 cm2. The cathode electrode with an active material loading of 0.8 mg was obtained after the coated foil was dried at 70 C for 5 h, pressed, and dried again at 120 C for 12 h under a vacuum (0.1 MPa) atmosphere. The coin cells (CR2025) were assembled in a Mbraun inert gas glove box (UNILAB-ECO (1800/780), Germany)
3.1. Structure and morphology characterization Fig. 2 shows the XRD patterns of as-prepared LiMnPO4 samples. All diffraction peaks could readily be indexed to orthorhombic olivine-type structure with a space group of Pnmb (JCPDS No. 33– 0803) without any detectable peaks from impurities. On the XRD patterns of the samples of S1/C, S2/C and S3/C, no diffraction peaks of elemental carbon appear, indicating that the carbon generated by glucose pyrolysis in the material may exist in an amorphous form [33]. It is also worthy of noting that the XRD peaks of the samples after coating C are not significantly enhanced compared with those without carbon-coating, which means that the samples synthesized in the liquid phase already have quite high degree of crystallinity. Based on the XRD spectra, the crystal cell parameters, crystal cell volume, and grain size of all samples are calculated and listed in Table 1. It can be seen that the crystal cell parameters and the cell volume of the three samples agree well with the literature data [34,35]. As Fig. 2b shows, the diffusion path of Li+ in LiMnPO4 is one dimensional channel along the b axis [36]. To further compare the crystal orientations of the two materials, Table 1 also lists the hkl plane distance and the full width at half maximum (FWHM) of the main crystal plane diffraction peaks. The FWHM of the main crystal plane diffraction peaks (111, 2 0 0 and 1 3 1) in S1 is less than that of S2 and S3. The FWHM of an XRD peak (i.e., hkl) gives a thickness of the crystallite in one particle direction. According to Scherrer equation, the grain size of the main crystal plane direction was
Fig. 2. XRD patterns of different samples: (a) S1 obtained with the addition of glucose into EG; S2 prepared without the addition of glucose; S3 synthesized with the addition of glucose and LiOH into EG; and S1/C, S2/C and S3/C obtained after a short high temperature treatment with certain amount of glucose. (b) the schematic diagram of Li+ diffusion channels in LiMnPO4.
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Table 1 Parameters obtained from XRD patterns for samples S1, S2 and S3. Cell parameters a/Å b/Å c/Å 6.100 10.460 4.744
Unit cell volume/Å3
hkl plane distance/Å
FWHM/degree
Grain size/Å
Diffraction peak intensity ratio (I200/I131)
S1
6.0982 10.4514 4.7449
302.4
6.0969 10.4612 4.7610
303.6
S3
6.108 10.4595 4.732
302.3
FWHM111 = 0.279 FWHM200 = 0.269 FWHM131 = 0.268 FWHM111 = 0.291 FWHM200 = 0.309 FWHM131 = 0.308 FWHM111 = 0.270 FWHM200 = 0.286 FWHM131 = 0.287
XS111 = 313 XS200 = 329 XS131 = 336 XS111 = 298 XS200 = 281 XS131 = 286 XS111 = 325 XS200 = 307 XS131 = 310
0.795
S2
d111 = 3.5287 d200 = 3.0503 d131 = 2.5523 d111 = 3.5294 d200 = 3.0509 d131 = 2.5523 d111 = 3.5292 d200 = 3.0530 d131 = 2.5523
Sample LiMnPO4 (#330803)
calculated. It is found that the grain size of S1 is greater than that of S2 and S3 in the crystal plane direction perpendicular to (111), (2 0 0) and (1 3 1), which indicates the crystals growth should be preferential in the vertical direction of the main crystal face. In particular, grain growth on the (2 0 0) crystal face for S1 is much bigger than that of other samples. This lattice orientation makes it easier for the deintercalation of lithium ions, which is beneficial to the performance improvement [6,31,37]. The peak intensity ratio of I200/I131can also be an indicator of crystalline grains growth and the I200/I131 values for S1 (with glucose assisted) is 0.795, which is greater than that of standard LiMnPO4, 0.78. This change indicates the S1 crystalline grains are orientated in the a–c plane [31,38,39]. Thus, the added glucose, as an additive agent in the EG/H2O system, facilitates the LiMnPO4 crystal growth in a special crystal face,
0.772
0.788
which is conducive to the deintercalation of lithium ions and electrochemical performances. This conclusion is further confirmed by the following electrochemical performance tests. Fig. 3 gives a general morphology view of different samples. While S1 and S2 samples, as shown in Fig. 3a and b,exhibitananorod structure of LiMnPO4, S3 sample in Fig. 3c displays an irregular granular aggregates. From the TEM images of Fig. 3d to f, it can be seen that the nanoparticle sizes of the samples decrease from S1 to S3. For S1, as shown in the insert distribution, the particle size are mostly in the range of 6090 nm as opposed to a particle size of 5080 nm for S2, shown in the insert of Fig. 3e. Fig. 3f indicates that S3 has 0.8 mm aggregates in size, which consists of many nanoparticles. However, among the three samples, the S1 sample shows the best dispersion. To enhance
Fig. 3. SEM images of (a) S1, (b) S2 and (c) S3, and TEM images of (d) S1, (e) S2 and (f) S3, the inserts show the particle size distribution. (g) (h) and (i) HRTEM images of S1, S2 and S3.
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the conductivity, carbon coating is usually employed on the surface modification of LiMnPO4 material. Generally, a good dispersion of nanoparticles is beneficial for a uniform carbon coating. HRTEM images of Fig. 3g to i demonstrate that all the synthesized samples show a clear and regular crystal lattice, indicating that the proposed synthesis in liquid phase facilitates the formation of LiMnPO4 with a good crystallinity and pure phase. The measured lattice values in Fig. 3g to i are 4.3, 2.3 and 4.3 Å, respectively, corresponding to the crystal planes of (0 11), (0 1 2) and (0 11). In all the three samples, the primary LiMnPO4 nanoparticles could be obtained. As shown in Fig. 3, it can be concluded that S1 with the addition of glucose shows the best particle uniformity and dispersion, whereas, without the addition of glucose, the particle
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uniformity and dispersion of the S2 decreases. Furthermore, with the addition of glucose and LiOH, serious agglomeration can be observed for the synthesized LiMnPO4 particles (S3). To further reveal the crystal structures of three samples after coating carbon, TEM and HRTEM characterizations were employed as shown in Fig. 4 . Fig. 4a, c and e give the general view of morphologies of LiMnPO4 coated carbon layer. It can be observed that all the samples keep the initial structure and morphologies of nanorod of as-prepared samples. Fig. 4b and d are the corresponding HRTEM images of S1 and S2, which obviously show amorphous carbon from glucose cracking coated on the surface of LiMnPO4 nanoparticles, with a carbon layer thickness of 34 nm. Due to the aggregation of nanoparticles, it is difficult to coat carbon layer
Fig. 4. The TEM and HRTEM images of S1/C(a and b), S2/C (c and d) and S3/C(e and f).
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uniformly on each particle surface of S3. As seen in the schematic illustration of Fig. 4f, the nano-sized LiMnPO4 particles are prone to aggregate into micro-sized particles. During the process of carbonization, the carbon layer can only be coated on the surface of large granular aggregates, leaving the inner nanoparticles uncoated or inefficiently coated, seriously restricting the conductivity improvement of the LiMnPO4 material. As such, to improve the conductivity of LiMnPO4, a full-dimensional nano-sized structure with a good dispersion of the material plays a key role in a uniform and effective carbon coating. 3.2. Electrochemical properties To increase the conductivity of electrode materials and lithium ions diffusion, carbon coating is usually employed before LIBs performance test. Fig. 5 shows the electrochemical properties of samples S1/C, S2/C and S3/C. As shown in Fig. 5a from the initial charge and discharge curves, all the LiMnPO4 samples exhibit a reversible plateau around 4.25 and 4.1 V vs. Li/Li+, which is typical for the Mn(II) $ Mn(III) redox with manganese phosphates. The first discharge capacities of S1, S2 and S3 are 155.3, 140.5 and 141.1 mAh g1 at 0.05 C, respectively. Among them, S1/C has the highest discharge capacity, indicating that the addition of glucose during the pretreatment refluxing helps improve the performance of LiMnPO4/C, as will be discussed later. Fig. 5b shows the charge and discharge curves of sample S1/C at different rates of 0.05 C, 0.1 C, 1 C and 5 C, which can be read their discharge capacity of
155.3, 140, 123.4 and 89.8 mAh g1, respectively. It also reflects that S1/C electrode shows a good rate capability. Cycle performance and capacity recovery of S1/C, S2/C and S3/C at different rates are shown in Fig. 5c. Clearly, for the first ten cycles at 0.05 C and 0.1 C, the capacity difference of S1/C and S2/C is not much. This difference increases as the rate increases to 1 C and 5 C, and reaches the maximum at 5 C. Also, the capacity of S1/C is better recovered than that of S2/C and S3/C, which indicates that the electrochemical properties of samples depend not only on the size of the particles, but also on the dispersion of particles. A good dispersion of particles is advantageous for an effective carbon coating on the surface of the electrode materials, which in turn increases the conductivity of the electrodes, accelerates the rate of lithium ion, and reduces the degree of polarization. Cycling behavior of three electrodes at 0.05 C rate can be seen in Fig. 5d. It can be seen that after 100 cycles, 94% of capacity retention rate of S1/C is higher than that of S2/C and S3/C with 90.5% and 87.1%, respectively. It is calculated that the first cyclic coulomb efficiency of sample S1/C is 90% at 0.05 C rate. After 100 cycles, it still can maintain over 99%, which also indicates the good cycling performance of proposed sample. Cyclic voltammetry curves and AC impedance of S1/C, S2/C and S3/C electrodes are shown in Fig. 6 . Fig. 6(a), (b), and (c) show, respectively, the cyclic voltammetry of S1/C, S2/C and S3/C after 5 cycles with a test voltage of 2.4 V-4.8 V and a scanning speed of 0.1 mV s1, 0.3 mV s1 and 0.5 mV s1. All the samples have obvious oxidation and reduction peaks, which correspond to the insertion-
Fig. 5. (a) initial charge and discharge curves of the three samples: S1/C, S2/C and S3/C at 0.05C. (b) charge and discharge curves of the S1/C sample at 0.05C, 0.1C, 1C and 5C. (c) rate performance of the acquired samples: S1/C, S2/C and S3/C at 0.05C, 0.1C, 1C and 5C. (d) cycling performance of the acquired samples at 0.05C.
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Fig. 6. CV profiles at different sweep rates of 0.1 mA s1, 0.3 mV s1 and 0.5 mV s1 and graphs of normalized peak current vs. square root of the scan rate for different samples: (a) S1/C, (b) S2/C and (c) S3/C. (d) the AC impedance fitting curves for the inset equivalent circuit of the samples S1/C, S2/C and S3/C.
extraction reactions of lithium ion in the respective charging and discharging processes. With the increase in scanning rate, the peak potentials and peak currents change regularly. Obviously, the peak current of sample S1/C is larger than that of S2/C and S3/C at respective scanning rates. This can be explained by the good dispersion and uniformity of the carbon layer on the surface of S1 particles, which are conductive to the transfer of Li+ between LiMnPO4/C and electrolyte, thus accelerating the reaction velocity. The ip vs. v1/2 graphs of every sample show a good liner relationship. The apparent diffusion coefficient values are calculated from CV using the Randles-Sevcik equation [40,41]: ip ¼ 2:69 105 ACD1=2 n3=2 v1=2 Where ip is the peak current, n is the number of electrons per molecule. c is the concentration of Li+ species. A is the contact area of the electrode. D is the lithium ion diffusion coefficient. v is the scan rate. According to the formula, the lithium ion diffusion coefficients of S1/C, S2/C and S3/C are calculated to be 1.083 1016 cm2 s1, 1.889 1016 cm2 s1 and 4.774 1017 cm2 s1 respectively. Which mean that the S1/C has better ability of ion transmission than others because of full dimensional nanostructure and good dispersion. As a result S1/C exhibited the best electrochemical performance. It can be seen in the graphs that the oxidation peak and reduction peak area ratio is close to 1, with a corresponding potential difference of 2.1 V, which indicates that the material has a good cyclic reversibility. This is consistent with the results of the previous test, suggesting that the addition of
glucose be beneficial to the dispersion of the particles in the process of refluxing and the improvement of the electrode material performance. To further analyze the electrochemical properties of the three samples, we have performed the AC impedance tests of the samples. Fig. 6d shows the impedance spectra and the equivalent circuit of the three samples. Rs represents the internal resistance of the test battery, R1 and CPE1 are associated with the resistance and constant phase element of SEI film, R2 and CPE2 are associated with the charge-transfer resistance and constant phase element of the electrode/electrolyte interface, W1 represents the Warburg resistance corresponding to the lithium-diffusion process. As shown in Fig. 6d, the high frequency semicircle is corresponding to the resistance R1 and CPE1 of SEI film, the semicircle in medium frequency region is assigned to the charge-transfer resistance R2 and CPE2 of electrode/electrolyte interface. The inclined line corresponds to the lithium-diffusion process within bulk of the electrode material. The kinetic differences of the three electrodes were further investigated by modeling electrochemical impendence spectra based on the modified Randles equivalent circuit. The fitted impedance parameters are listed in Table 2. The value of R1 and R2 of the S1/C are 16.36 and 71.79 V, respectively, which are significantly lower than those of other samples. This fact confirms that the full-dimensional LiMnPO4 nanoparticles coated carbon layer uniformly is beneficial for the lithium ions fast transport during the cycles and therefore can enhance the electrochemical performances.
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Table 2 Impedance parameters derived using an equivalent circuit model for S1/C, S2/C and S3/C. Fresh cells Electrode material
Rs(V)
R1(V)
R2(V)
CPE1 (106F)
CPE2 (106F)
S1/C S2/C S3/C
7.46 1.82 2.96
16.36 55.84 27.39
71.79 129.5 191.7
1.22 6.79 2.16
5.74 6.62 2.90
3.3. Effect of adding glucose As mentioned above, the addition of glucose to EG during pretreatment refluxing has a great influence on the size, dispersion and electrochemical properties of the sample particles. The synthesized S1 with the addition of glucose has full-dimensioned nanoparticles with a good dispersion. The good dispersion makes the surface of every single particle be uniformly coated with a layer of carbon generated by the glucose cracking at high temperature, which significantly increases the conductivity of the particles. The full-dimensioned nanoparticles can effectively shorten the path of Li+ transfer. As a matter of fact, it is the effective combination of the good dispersion and the full-dimensioned nanosize improves the conductivity and ionic diffusion rate of LiMnPO4, which in turn exhibits excellent electrochemical performances. During the synthesis process, we noticed that the obtained EG with glucose solution, EG without glucose, and EG with glucose and LiOH after the pretreatment refluxing have obviously different colors. As such, we analyzed the different reflux solution with H-NMR, as shown in Fig. 7 . As shown in the insert of Fig. 7, the B1 solution is bright yellow; B2 is colorless and transparent; and B3 is brown soluble colloidal liquid. To facilitate the comparison, EG with glucose before reflux is also chosen, together with B1, B2 and B3, for H-NMR scan. The corresponding H-NMR spectrum are II, III, I, and IV, respectively. Comparing these spectra, it can be found that spectrum II has more
peaks than spectrum I. Peaks at 3.1 ppm, 4.3 ppm, 4.6 ppm, 4.7 ppm, 6.3 ppm and 6.6 ppm are characteristic for glucose. Comparing spectrum III with II, the peaks appearing at 2.4 ppm and 2.6 ppm are different from the characteristic peaks of glucose, which indicates that different H atoms appear in solution B1. The original glucose characteristic peaks are weakened and the color of the solution becomes bright yellow. These phenomena indicate the chemical reaction has occurred in a portion of glucose added in EG, and generates new substances. The new substances could be glycol glucoside according to other reports [42]. From the spectrum IV for the brown and sticky solution B3, it can be observed that the characteristic peaks of glucose and glycol glucoside disappear. This may be due to the H atoms of carbonyl of glucose and glycol glucoside are replaced by Li+. The phenomena may be explained by the certain degree carbonization occurred in glucose in the presence of alkali, and the glucose polymerized to polysaccharide under the effect of hydroxyl, increasing the solution viscosity, and making the color of the solution brown. Fig. 8 shows the Schematic illustration for the formation of alkyl polyglucoside (APG) and its function on LiMnPO4 particles. During the pretreatment refluxing, APG is generated due to the addition of glucose into EG. With the presence of APG, a non-ionic surface active agent with excellent performance [43], dispersion in aqueous solution of EG is effective on the surface of LiMnPO4 crystal nucleus. The reason of LiMnPO4 shows a good uniformity and dispersion is that APG inhibits the growth and agglomeration of the particles. A good dispersion of the particles makes every particle be coated with a uniform layer of carbon in the subsequent carbon-coating process at high temperature. The full-dimensioned and nano-scaled particles effectively shorten the diffusion path of Li+, and the uniform carbon layer improves the electronic conductivity of particles. The synergistic effect of the two aspects gives the LiMnPO4/C material an excellent electrochemical performance. Although the LiMnPO4 prepared in the reflux process of EG without the addition of glucose has a structure of nanorod, the particles are prone to agglutinate, especially in the case with the addition of glucose and LiOH, the particles are more likely to
Fig. 7. H-NMR spectrum of different solutions: (I) the refluxing solution of EG (B2), (II) the glucose added EG before reflux, (III) the refluxing solution of EG with glucose (B1), and (IV) the refluxing solution of EG with glucose and LiOH (B3). The insert shows the photo-pictures of three solutions: B1, B2 and B3.
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Fig. 8. Schematic illustration of the APG formation and its function on LiMnPO4 particles.
agglutinate. The cracked carbon can only be coated on the surface of the aggregates in the subsequent high temperature carboncoating process, which leads to a significant extension of the diffusion path of Li+ in the aggregate particles. In addition, the uneven surface of aggregates makes the carbon layer uneven. These two factors lead to a decrease in the Li+ diffusion rate and the reduction of electrical conductivity of particles. 4. Conclusions The pure phase LiMnPO4 nanorods with a width of 20–50 nm and a length of 50–80 nm were successfully synthesized via a glucose-assisted method in EG. The synthesized LiMnPO4 nanorods have good uniformity and dispersion. The LiMnPO4/C composite with a layer of uniform carbon coat was obtained after a short carbon-coating treatment at high temperature. APG generated in the refluxing process effectively inhibits the growth and agglomeration of the particles. Electrochemical tests show that the synthesized LiMnPO4/C nanorods have excellent electrochemical properties. The discharge capacities of sampleS1/C at different rates of 0.05 C, 0.1 C, 1 C and 5 C reached 155.3 mAh g1, 140 mAh g1, 123.4 mAh g1and 89.8 mAh g1 respectively. After 50 cycles, a capacity of 147.5 mAh g1is still maintained with a retention rate of 95%.This proposed method has the advantages of simple process and low reflux temperature. In particular, it directly synthesizes the full-dimensioned and nanoscaled LiMnPO4 in the liquid phase with good uniformity and dispersion. Acknowledgements This work is financially supported by the Natural Science Foundation of China under approval No. 21071046, the Natural Science Foundation of China for Young under approval No. 21303042 and 21203056, and the Program for Innovative Research Team in University of Henan Province (No. 14IRTSTHN005).
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Glucose-Assisted Synthesis of Highly Dispersed LiMnPO4 Nanoparticles at a Low Temperature for Lithium Ion Batteries Zhengzheng Xiea , Kun Changa,* , Bao Lia , Hongwei Tanga , Xiaoning Fua , Zhaorong Changa,* , Xiao-Zi Yuanb , Haijiang Wangb a Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P.R. China b National Research Council of Canada, Vancouver, BC V6T 1W5, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 November 2015 Received in revised form 15 December 2015 Accepted 15 December 2015 Available online 18 December 2015
The cathode material of the LiMnPO4/C composite for lithium-ion batteries is successfully synthesized via a one-step glucose-assisted liquid-phase method in ethylene glycol (EG). The crystalline structure, morphology, micro-structure and particle size are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). XRD results show that the pure phase of LiMnPO4 with high crystallinity can directly be prepared in the liquid-phase assisted by glucose. SEM measurements confirm the uniform-sized nanorods of the LiMnPO4 morphology with a width of 20– 50 nm and a length of 50–80 nm. TEM characterization reveals that the surface of the obtained LiMnPO4 nanorods is coated with a homogeneous carbon layer after a short heat treatment at a high temperature in the presence of glucose. This can be explained by the fact that the glycol glucoside generated during the refluxing of EG with glucose can effectively inhibit the growth and agglomeration of particles. Results of electrochemical tests show that the prepared LiMnPO4/C nanorods exhibit not only a high initial discharge capacity of 155.3 mAh g1 but also a good cycling stability, which retains 94% of the initial capacity over 100 cycles at 0.05 C. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium manganese phosphate Cathode material Nanoparticles Glucose assisted Liquid phase method
1. Introduction Lithium ion batteries as an excellent performance rechargeable green power have been widely used in portable electronic products, communication tools and are being developed for electric vehicles power. Consequently, they are promoted to safety, environmental protection, low cost and high specific energy [1–3]. LiFePO4 with an olivine structure as a cathode material has become a research hot spot quickly after it was first reported in 1997 by Padhi et al [4], due to the characteristics of high specific capacity, good thermal stability, excellent cycling performance, abundant raw materials and environmental friendliness [5,6]. The issues related to low ion and electron conductivity of the LiFePO4 material have basically been solved by reducing particle size [7], doping metal ions [8] and coating carbon on the surface of the particles [9]. Presently, the capacity of commercialized LiFePO4 mainly used in electric vehicles and energy storage areas has reached
* Corresponding authors. Tel.: +86 373 3326335. E-mail addresses: [email protected] (K. Chang), [email protected] (Z. Chang). http://dx.doi.org/10.1016/j.electacta.2015.12.111 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
160 mAh g1, which is close to its theoretical capacity (170 mAh g1) [10–12]. However, the low specific energy (theoretical value is 578 Wh kg1) of LiFePO4 due to the low tap density (1.1–1.3 g cm3) and working voltage (3.4 V) limits the mileage of electric vehicles powered by Li-ion batteries [13–15]. It is known that LiMnPO4 has the same olivine structure and theoretical capacity as LiFePO4, however with a working voltage of 4.1 V (vs. Li+/Li) [16], which is within the electrochemical window of the existing electrolyte system [17,18]. As such, the theoretical specific energy of LiMnPO4 can reach 700 Wh kg1 because of the high working voltage, about 20% higher than LiFePO4 [17,19,20]. In addition, the outstanding characteristics of the LiMnPO4 such as abundant raw material resources, low cost, environmental friendliness, stable structure, good chemical compatibility and high safety endow it to be a quite promising cathode material for lithium ion batteries. Nevertheless, comparing with LiFePO4, the electronic conductivity and ion diffusion coefficient of LiMnPO4 are much lower [21], which leads the material prone to polarization with a poor rate performance. The band gap of LiMnPO4 calculated by Yamada et al. with the first principle is 2 eV, which means LiMnPO4 is almost an insulator [22]. Hence, it is very difficult to synthesize the LiMnPO4
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material with an excellent electrochemistry performance. It is also the main obstruction to realize the industrialization of the material so far. Researches have revealed that the nanocrystallization of material particles, doping of metal ions and carbon coating on the surface of nanoparticles are effective ways to improve the ion diffusion coefficient and electron conductivity [22–25]. However, the electrochemical performance of LiMnPO4 depends on the particle size and dispersion more heavily than that of LiFePO4 due to the insulation of the LiMnPO4 itself. Compared with ball-milling [11,12,26] and sol-gel [27],mainly based on a high temperature solid phase sintering, the liquid phase hydrothermal or solvent method [28] and polyol method [29] are more suitable for the preparation and regulation of nanoparticles. It is found that most nanoscaled LiMnPO4 particles synthesized by the hydrothermal or polyol methods are in one dimension only, whereas other dimensions are still in a micron scale. In addition, these micronano particles are easy to agglomerate. Dong et al. [30] synthesized the LiMnPO4 nanosheets with a thickness of 50 nm and a width of 2 mm via a hydrothermal route in a high-pressure container heated at 200 C for 12 h, and the nanosheets exhibit a discharge capacity of 107.5 mAh g1 at 0.05 C. Wang et al. [31] synthesized LiMnPO4 nanosheets with a thickness of 20–30 nm and a width of 3.5 um by a polyol method of refluxing 5 h at 100 C. The resulting material achieved a discharge capacity of 145 mAh g1 at 0.05 C. As these synthesized LiMnPO4 materials are not full-dimensional sized nanoparticles and easy to aggregate, the lithium ions diffusion and electrons conductivity are greatly restricted. In our previous work, dimethyl sulfoxide (DMSO) solution-phase method was employed to synthesized hollow spindle LiMnPO4 particles with a width of 200 nm and length of 500–700 nm, which exhibited the enhanced electrochemical performances [32]. To further improve the contact between LiMnPO4 materials and electrolyte, nanocrystalized synthesis of LiMnPO4 particles especially the full-dimensional size are expected to fulfill the uniformity of carbon layer coating and fast transport of lihthium ions, facilitating the enhancement of electrochemical performances of the electrodes. In this context, we adopted a unique approach in an attempt to synthesize uniform LiMnPO4 nanorods. First a certain amount of
glucose was added into the ethylene glycol (EG) solution during pretreatment refluxing, and LiOH, MnSO4 and H3PO4 were then added and refluxed at 140 C under ambient pressure. It is found that the obtained LiMnPO4 particles are uniform nanorods with a diameter of 50 nm and a length of 80 nm. Moreover, the prepared LiMnPO4 nanorods show a good dispersion and excellent electrochemical performance after being coated carbon. Interestingly, the color of the solution became light yellow when glucose was added into EG during refluxing. If glucose and LiOH were together added into EG during refluxing, the solution turned to sepia and viscous. For the above phenomenon, we analyzed the effect of intermediate possibly generated in the pretreatment refluxing on the particle size and dispersion of LiMnPO4. The research confirms that the product generated by EG and glucose after refluxing and the adding sequence of LiOH have a great effect on the size, dispersion and the electrochemical properties of the LiMnPO4 particles. This work provides a new method for the synthesis of LiMnPO4 nano-particles with a full-dimension and non-aggregation. To the best of our knowledge, there have been no relevant literature reported so far.
2. Experimental 2.1. Synthesis of LiMnPO4 and the LiMnPO4/C The preparation procedures are shown in Fig. 1 . Solution A was obtained by dissolving 0.06 mol LiOH into 10 mL deionized water. Meanwhile, solution B1 was obtained by the heating of the solution containing 30 ml EG and 3 g glucose. This was done at 140 C under the protection of N2 for 2 h. Next, Solution C was produced after solutions of A and B1 were together stirred uniformly. The D solution was received by dissolving 0.02 mol MnSO4 and 0.02 mol H3PO4 into 15 mL deionized water. Solution C were poured into D and stirred uniformly with 30 mL EG, the proportion of EG to H2O is 60: 25, and then refluxed at 140 C under the protection of N2 for 12 h. Sample 1 (marked as S1) was obtained after the precipitate was centrifuged at a rotating speed of 8000 r min1, washed with deionized water and dried for 12 h at 80 C.
Fig. 1. Schematic illustration of the synthesis process of the LiMnPO4/C cathode materials.
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The B2 solution was obtained after 30 mL EG was heated at 140 C with the protection of N2 for 2 h. Sample 2 (marked as S2) serves as a comparative sample with no glucose refluxing via the same method as S1. The B3 solution was obtained after that the 30 mL EG dissolving 3 g glucose and 0.06 mol LiOH was heated at 140 C under the protection of N2 for 2 h (because all the required LiOH has been added, solution B3 is solution C here). The S3 sample was obtained via the same method as S1. Carbon coating process was conducted through grinding the proposed samples with a certain amount of glucose, followed by calcination in a tube furnace under an atmosphere of 95% Ar, 5% H2 at 500 C for 5 h with a heating rate of 10 C min1.
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which maintains <0.5 ppm of O2 and <0.5 ppm of H2O. Lithium metal was used as the counter and reference electrodes. 1 M LiPF6 in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1 volume ratio) was used as the electrolyte, while Celgard 2400 (Celgard polypropylene) was employed as separator. The cells were kept stewing for 12 h and then tested using a Land CT2001A battery tester (Wuhan Jinnuo Electronics Co. Ltd. China) in a potential range of 2.44.8 V at room temperature and rates of 0.05C, 0.1C, 1C and 5C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were investigated on an electrochemical workstation (Shanghai Chenhua instrument company, China). The CV test scan rates were from 0.1 mVs1 to 0.5 mVs1 over a cell voltage from 2.4 V to 4.8 V. For EIS, the scan frequency ranged from 1 Hz to10 kHz.
2.2. Characterization 3. Results and discussion Phase analysis and cell parameter determination of all samples were performed by X-ray diffraction(XRD) using a D8 X diffract meter (Bruker, Germany) with Cu-Ka radiation and graphite monochromator. The working voltage was 35 KV and working current was 30 mA. The scanned data were collected in a 2u range from 1080 . The particle morphology of the synthesized materials was analyzed by scanning electron microscopy (SEM) (SEM-6701F, JEOL, Japan) and transmission electron microscopy (TEM) (JEOL JEM 2010, Japan). Solutions of B1, B2 and B3 were scanned with Hydrogen nuclear magnetic resonance spectroscopy (HNMR) (Avance, 400 MHz, Bruker, Switzerland). 2.3. Electrochemical performance tests The electrochemical performance of as-prepared samples S1, S2 and S3 were investigated at room temperature on a 2025 type coin cell. The electrode was prepared as follows: active material, PVDF (polyvinylidene fluoride) and acetylene black were mixed at a weight ratio of 70:10:20. After adding a certain amount of Nmethyl-2-pyrrolidone (AR, purchased from Sinopharm Chemical Reagent Co., Ltd.), the mixture was ground uniformly with a pulp refiner. The obtained slurry was then spread on a piece of aluminum foil with a diameter of 12 mm, and spreading area of 1.13 cm2. The cathode electrode with an active material loading of 0.8 mg was obtained after the coated foil was dried at 70 C for 5 h, pressed, and dried again at 120 C for 12 h under a vacuum (0.1 MPa) atmosphere. The coin cells (CR2025) were assembled in a Mbraun inert gas glove box (UNILAB-ECO (1800/780), Germany)
3.1. Structure and morphology characterization Fig. 2 shows the XRD patterns of as-prepared LiMnPO4 samples. All diffraction peaks could readily be indexed to orthorhombic olivine-type structure with a space group of Pnmb (JCPDS No. 33– 0803) without any detectable peaks from impurities. On the XRD patterns of the samples of S1/C, S2/C and S3/C, no diffraction peaks of elemental carbon appear, indicating that the carbon generated by glucose pyrolysis in the material may exist in an amorphous form [33]. It is also worthy of noting that the XRD peaks of the samples after coating C are not significantly enhanced compared with those without carbon-coating, which means that the samples synthesized in the liquid phase already have quite high degree of crystallinity. Based on the XRD spectra, the crystal cell parameters, crystal cell volume, and grain size of all samples are calculated and listed in Table 1. It can be seen that the crystal cell parameters and the cell volume of the three samples agree well with the literature data [34,35]. As Fig. 2b shows, the diffusion path of Li+ in LiMnPO4 is one dimensional channel along the b axis [36]. To further compare the crystal orientations of the two materials, Table 1 also lists the hkl plane distance and the full width at half maximum (FWHM) of the main crystal plane diffraction peaks. The FWHM of the main crystal plane diffraction peaks (111, 2 0 0 and 1 3 1) in S1 is less than that of S2 and S3. The FWHM of an XRD peak (i.e., hkl) gives a thickness of the crystallite in one particle direction. According to Scherrer equation, the grain size of the main crystal plane direction was
Fig. 2. XRD patterns of different samples: (a) S1 obtained with the addition of glucose into EG; S2 prepared without the addition of glucose; S3 synthesized with the addition of glucose and LiOH into EG; and S1/C, S2/C and S3/C obtained after a short high temperature treatment with certain amount of glucose. (b) the schematic diagram of Li+ diffusion channels in LiMnPO4.
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Table 1 Parameters obtained from XRD patterns for samples S1, S2 and S3. Cell parameters a/Å b/Å c/Å 6.100 10.460 4.744
Unit cell volume/Å3
hkl plane distance/Å
FWHM/degree
Grain size/Å
Diffraction peak intensity ratio (I200/I131)
S1
6.0982 10.4514 4.7449
302.4
6.0969 10.4612 4.7610
303.6
S3
6.108 10.4595 4.732
302.3
FWHM111 = 0.279 FWHM200 = 0.269 FWHM131 = 0.268 FWHM111 = 0.291 FWHM200 = 0.309 FWHM131 = 0.308 FWHM111 = 0.270 FWHM200 = 0.286 FWHM131 = 0.287
XS111 = 313 XS200 = 329 XS131 = 336 XS111 = 298 XS200 = 281 XS131 = 286 XS111 = 325 XS200 = 307 XS131 = 310
0.795
S2
d111 = 3.5287 d200 = 3.0503 d131 = 2.5523 d111 = 3.5294 d200 = 3.0509 d131 = 2.5523 d111 = 3.5292 d200 = 3.0530 d131 = 2.5523
Sample LiMnPO4 (#330803)
calculated. It is found that the grain size of S1 is greater than that of S2 and S3 in the crystal plane direction perpendicular to (111), (2 0 0) and (1 3 1), which indicates the crystals growth should be preferential in the vertical direction of the main crystal face. In particular, grain growth on the (2 0 0) crystal face for S1 is much bigger than that of other samples. This lattice orientation makes it easier for the deintercalation of lithium ions, which is beneficial to the performance improvement [6,31,37]. The peak intensity ratio of I200/I131can also be an indicator of crystalline grains growth and the I200/I131 values for S1 (with glucose assisted) is 0.795, which is greater than that of standard LiMnPO4, 0.78. This change indicates the S1 crystalline grains are orientated in the a–c plane [31,38,39]. Thus, the added glucose, as an additive agent in the EG/H2O system, facilitates the LiMnPO4 crystal growth in a special crystal face,
0.772
0.788
which is conducive to the deintercalation of lithium ions and electrochemical performances. This conclusion is further confirmed by the following electrochemical performance tests. Fig. 3 gives a general morphology view of different samples. While S1 and S2 samples, as shown in Fig. 3a and b,exhibitananorod structure of LiMnPO4, S3 sample in Fig. 3c displays an irregular granular aggregates. From the TEM images of Fig. 3d to f, it can be seen that the nanoparticle sizes of the samples decrease from S1 to S3. For S1, as shown in the insert distribution, the particle size are mostly in the range of 6090 nm as opposed to a particle size of 5080 nm for S2, shown in the insert of Fig. 3e. Fig. 3f indicates that S3 has 0.8 mm aggregates in size, which consists of many nanoparticles. However, among the three samples, the S1 sample shows the best dispersion. To enhance
Fig. 3. SEM images of (a) S1, (b) S2 and (c) S3, and TEM images of (d) S1, (e) S2 and (f) S3, the inserts show the particle size distribution. (g) (h) and (i) HRTEM images of S1, S2 and S3.
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the conductivity, carbon coating is usually employed on the surface modification of LiMnPO4 material. Generally, a good dispersion of nanoparticles is beneficial for a uniform carbon coating. HRTEM images of Fig. 3g to i demonstrate that all the synthesized samples show a clear and regular crystal lattice, indicating that the proposed synthesis in liquid phase facilitates the formation of LiMnPO4 with a good crystallinity and pure phase. The measured lattice values in Fig. 3g to i are 4.3, 2.3 and 4.3 Å, respectively, corresponding to the crystal planes of (0 11), (0 1 2) and (0 11). In all the three samples, the primary LiMnPO4 nanoparticles could be obtained. As shown in Fig. 3, it can be concluded that S1 with the addition of glucose shows the best particle uniformity and dispersion, whereas, without the addition of glucose, the particle
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uniformity and dispersion of the S2 decreases. Furthermore, with the addition of glucose and LiOH, serious agglomeration can be observed for the synthesized LiMnPO4 particles (S3). To further reveal the crystal structures of three samples after coating carbon, TEM and HRTEM characterizations were employed as shown in Fig. 4 . Fig. 4a, c and e give the general view of morphologies of LiMnPO4 coated carbon layer. It can be observed that all the samples keep the initial structure and morphologies of nanorod of as-prepared samples. Fig. 4b and d are the corresponding HRTEM images of S1 and S2, which obviously show amorphous carbon from glucose cracking coated on the surface of LiMnPO4 nanoparticles, with a carbon layer thickness of 34 nm. Due to the aggregation of nanoparticles, it is difficult to coat carbon layer
Fig. 4. The TEM and HRTEM images of S1/C(a and b), S2/C (c and d) and S3/C(e and f).
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uniformly on each particle surface of S3. As seen in the schematic illustration of Fig. 4f, the nano-sized LiMnPO4 particles are prone to aggregate into micro-sized particles. During the process of carbonization, the carbon layer can only be coated on the surface of large granular aggregates, leaving the inner nanoparticles uncoated or inefficiently coated, seriously restricting the conductivity improvement of the LiMnPO4 material. As such, to improve the conductivity of LiMnPO4, a full-dimensional nano-sized structure with a good dispersion of the material plays a key role in a uniform and effective carbon coating. 3.2. Electrochemical properties To increase the conductivity of electrode materials and lithium ions diffusion, carbon coating is usually employed before LIBs performance test. Fig. 5 shows the electrochemical properties of samples S1/C, S2/C and S3/C. As shown in Fig. 5a from the initial charge and discharge curves, all the LiMnPO4 samples exhibit a reversible plateau around 4.25 and 4.1 V vs. Li/Li+, which is typical for the Mn(II) $ Mn(III) redox with manganese phosphates. The first discharge capacities of S1, S2 and S3 are 155.3, 140.5 and 141.1 mAh g1 at 0.05 C, respectively. Among them, S1/C has the highest discharge capacity, indicating that the addition of glucose during the pretreatment refluxing helps improve the performance of LiMnPO4/C, as will be discussed later. Fig. 5b shows the charge and discharge curves of sample S1/C at different rates of 0.05 C, 0.1 C, 1 C and 5 C, which can be read their discharge capacity of
155.3, 140, 123.4 and 89.8 mAh g1, respectively. It also reflects that S1/C electrode shows a good rate capability. Cycle performance and capacity recovery of S1/C, S2/C and S3/C at different rates are shown in Fig. 5c. Clearly, for the first ten cycles at 0.05 C and 0.1 C, the capacity difference of S1/C and S2/C is not much. This difference increases as the rate increases to 1 C and 5 C, and reaches the maximum at 5 C. Also, the capacity of S1/C is better recovered than that of S2/C and S3/C, which indicates that the electrochemical properties of samples depend not only on the size of the particles, but also on the dispersion of particles. A good dispersion of particles is advantageous for an effective carbon coating on the surface of the electrode materials, which in turn increases the conductivity of the electrodes, accelerates the rate of lithium ion, and reduces the degree of polarization. Cycling behavior of three electrodes at 0.05 C rate can be seen in Fig. 5d. It can be seen that after 100 cycles, 94% of capacity retention rate of S1/C is higher than that of S2/C and S3/C with 90.5% and 87.1%, respectively. It is calculated that the first cyclic coulomb efficiency of sample S1/C is 90% at 0.05 C rate. After 100 cycles, it still can maintain over 99%, which also indicates the good cycling performance of proposed sample. Cyclic voltammetry curves and AC impedance of S1/C, S2/C and S3/C electrodes are shown in Fig. 6 . Fig. 6(a), (b), and (c) show, respectively, the cyclic voltammetry of S1/C, S2/C and S3/C after 5 cycles with a test voltage of 2.4 V-4.8 V and a scanning speed of 0.1 mV s1, 0.3 mV s1 and 0.5 mV s1. All the samples have obvious oxidation and reduction peaks, which correspond to the insertion-
Fig. 5. (a) initial charge and discharge curves of the three samples: S1/C, S2/C and S3/C at 0.05C. (b) charge and discharge curves of the S1/C sample at 0.05C, 0.1C, 1C and 5C. (c) rate performance of the acquired samples: S1/C, S2/C and S3/C at 0.05C, 0.1C, 1C and 5C. (d) cycling performance of the acquired samples at 0.05C.
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Fig. 6. CV profiles at different sweep rates of 0.1 mA s1, 0.3 mV s1 and 0.5 mV s1 and graphs of normalized peak current vs. square root of the scan rate for different samples: (a) S1/C, (b) S2/C and (c) S3/C. (d) the AC impedance fitting curves for the inset equivalent circuit of the samples S1/C, S2/C and S3/C.
extraction reactions of lithium ion in the respective charging and discharging processes. With the increase in scanning rate, the peak potentials and peak currents change regularly. Obviously, the peak current of sample S1/C is larger than that of S2/C and S3/C at respective scanning rates. This can be explained by the good dispersion and uniformity of the carbon layer on the surface of S1 particles, which are conductive to the transfer of Li+ between LiMnPO4/C and electrolyte, thus accelerating the reaction velocity. The ip vs. v1/2 graphs of every sample show a good liner relationship. The apparent diffusion coefficient values are calculated from CV using the Randles-Sevcik equation [40,41]: ip ¼ 2:69 105 ACD1=2 n3=2 v1=2 Where ip is the peak current, n is the number of electrons per molecule. c is the concentration of Li+ species. A is the contact area of the electrode. D is the lithium ion diffusion coefficient. v is the scan rate. According to the formula, the lithium ion diffusion coefficients of S1/C, S2/C and S3/C are calculated to be 1.083 1016 cm2 s1, 1.889 1016 cm2 s1 and 4.774 1017 cm2 s1 respectively. Which mean that the S1/C has better ability of ion transmission than others because of full dimensional nanostructure and good dispersion. As a result S1/C exhibited the best electrochemical performance. It can be seen in the graphs that the oxidation peak and reduction peak area ratio is close to 1, with a corresponding potential difference of 2.1 V, which indicates that the material has a good cyclic reversibility. This is consistent with the results of the previous test, suggesting that the addition of
glucose be beneficial to the dispersion of the particles in the process of refluxing and the improvement of the electrode material performance. To further analyze the electrochemical properties of the three samples, we have performed the AC impedance tests of the samples. Fig. 6d shows the impedance spectra and the equivalent circuit of the three samples. Rs represents the internal resistance of the test battery, R1 and CPE1 are associated with the resistance and constant phase element of SEI film, R2 and CPE2 are associated with the charge-transfer resistance and constant phase element of the electrode/electrolyte interface, W1 represents the Warburg resistance corresponding to the lithium-diffusion process. As shown in Fig. 6d, the high frequency semicircle is corresponding to the resistance R1 and CPE1 of SEI film, the semicircle in medium frequency region is assigned to the charge-transfer resistance R2 and CPE2 of electrode/electrolyte interface. The inclined line corresponds to the lithium-diffusion process within bulk of the electrode material. The kinetic differences of the three electrodes were further investigated by modeling electrochemical impendence spectra based on the modified Randles equivalent circuit. The fitted impedance parameters are listed in Table 2. The value of R1 and R2 of the S1/C are 16.36 and 71.79 V, respectively, which are significantly lower than those of other samples. This fact confirms that the full-dimensional LiMnPO4 nanoparticles coated carbon layer uniformly is beneficial for the lithium ions fast transport during the cycles and therefore can enhance the electrochemical performances.
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Table 2 Impedance parameters derived using an equivalent circuit model for S1/C, S2/C and S3/C. Fresh cells Electrode material
Rs(V)
R1(V)
R2(V)
CPE1 (106F)
CPE2 (106F)
S1/C S2/C S3/C
7.46 1.82 2.96
16.36 55.84 27.39
71.79 129.5 191.7
1.22 6.79 2.16
5.74 6.62 2.90
3.3. Effect of adding glucose As mentioned above, the addition of glucose to EG during pretreatment refluxing has a great influence on the size, dispersion and electrochemical properties of the sample particles. The synthesized S1 with the addition of glucose has full-dimensioned nanoparticles with a good dispersion. The good dispersion makes the surface of every single particle be uniformly coated with a layer of carbon generated by the glucose cracking at high temperature, which significantly increases the conductivity of the particles. The full-dimensioned nanoparticles can effectively shorten the path of Li+ transfer. As a matter of fact, it is the effective combination of the good dispersion and the full-dimensioned nanosize improves the conductivity and ionic diffusion rate of LiMnPO4, which in turn exhibits excellent electrochemical performances. During the synthesis process, we noticed that the obtained EG with glucose solution, EG without glucose, and EG with glucose and LiOH after the pretreatment refluxing have obviously different colors. As such, we analyzed the different reflux solution with H-NMR, as shown in Fig. 7 . As shown in the insert of Fig. 7, the B1 solution is bright yellow; B2 is colorless and transparent; and B3 is brown soluble colloidal liquid. To facilitate the comparison, EG with glucose before reflux is also chosen, together with B1, B2 and B3, for H-NMR scan. The corresponding H-NMR spectrum are II, III, I, and IV, respectively. Comparing these spectra, it can be found that spectrum II has more
peaks than spectrum I. Peaks at 3.1 ppm, 4.3 ppm, 4.6 ppm, 4.7 ppm, 6.3 ppm and 6.6 ppm are characteristic for glucose. Comparing spectrum III with II, the peaks appearing at 2.4 ppm and 2.6 ppm are different from the characteristic peaks of glucose, which indicates that different H atoms appear in solution B1. The original glucose characteristic peaks are weakened and the color of the solution becomes bright yellow. These phenomena indicate the chemical reaction has occurred in a portion of glucose added in EG, and generates new substances. The new substances could be glycol glucoside according to other reports [42]. From the spectrum IV for the brown and sticky solution B3, it can be observed that the characteristic peaks of glucose and glycol glucoside disappear. This may be due to the H atoms of carbonyl of glucose and glycol glucoside are replaced by Li+. The phenomena may be explained by the certain degree carbonization occurred in glucose in the presence of alkali, and the glucose polymerized to polysaccharide under the effect of hydroxyl, increasing the solution viscosity, and making the color of the solution brown. Fig. 8 shows the Schematic illustration for the formation of alkyl polyglucoside (APG) and its function on LiMnPO4 particles. During the pretreatment refluxing, APG is generated due to the addition of glucose into EG. With the presence of APG, a non-ionic surface active agent with excellent performance [43], dispersion in aqueous solution of EG is effective on the surface of LiMnPO4 crystal nucleus. The reason of LiMnPO4 shows a good uniformity and dispersion is that APG inhibits the growth and agglomeration of the particles. A good dispersion of the particles makes every particle be coated with a uniform layer of carbon in the subsequent carbon-coating process at high temperature. The full-dimensioned and nano-scaled particles effectively shorten the diffusion path of Li+, and the uniform carbon layer improves the electronic conductivity of particles. The synergistic effect of the two aspects gives the LiMnPO4/C material an excellent electrochemical performance. Although the LiMnPO4 prepared in the reflux process of EG without the addition of glucose has a structure of nanorod, the particles are prone to agglutinate, especially in the case with the addition of glucose and LiOH, the particles are more likely to
Fig. 7. H-NMR spectrum of different solutions: (I) the refluxing solution of EG (B2), (II) the glucose added EG before reflux, (III) the refluxing solution of EG with glucose (B1), and (IV) the refluxing solution of EG with glucose and LiOH (B3). The insert shows the photo-pictures of three solutions: B1, B2 and B3.
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Fig. 8. Schematic illustration of the APG formation and its function on LiMnPO4 particles.
agglutinate. The cracked carbon can only be coated on the surface of the aggregates in the subsequent high temperature carboncoating process, which leads to a significant extension of the diffusion path of Li+ in the aggregate particles. In addition, the uneven surface of aggregates makes the carbon layer uneven. These two factors lead to a decrease in the Li+ diffusion rate and the reduction of electrical conductivity of particles. 4. Conclusions The pure phase LiMnPO4 nanorods with a width of 20–50 nm and a length of 50–80 nm were successfully synthesized via a glucose-assisted method in EG. The synthesized LiMnPO4 nanorods have good uniformity and dispersion. The LiMnPO4/C composite with a layer of uniform carbon coat was obtained after a short carbon-coating treatment at high temperature. APG generated in the refluxing process effectively inhibits the growth and agglomeration of the particles. Electrochemical tests show that the synthesized LiMnPO4/C nanorods have excellent electrochemical properties. The discharge capacities of sampleS1/C at different rates of 0.05 C, 0.1 C, 1 C and 5 C reached 155.3 mAh g1, 140 mAh g1, 123.4 mAh g1and 89.8 mAh g1 respectively. After 50 cycles, a capacity of 147.5 mAh g1is still maintained with a retention rate of 95%.This proposed method has the advantages of simple process and low reflux temperature. In particular, it directly synthesizes the full-dimensioned and nanoscaled LiMnPO4 in the liquid phase with good uniformity and dispersion. Acknowledgements This work is financially supported by the Natural Science Foundation of China under approval No. 21071046, the Natural Science Foundation of China for Young under approval No. 21303042 and 21203056, and the Program for Innovative Research Team in University of Henan Province (No. 14IRTSTHN005).
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