Heat-rate-controlled hydrothermal crystallization of high-performance LiMn0.7Fe0.3PO4 cathode material for lithium-ion batteries

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Heat-rate-controlled hydrothermal crystallization of high-performance LiMn0.7Fe0.3PO4 cathode material for lithium-ion batteries Yijun Songa, Yuanyuan Liua,∗, Xiuqin Oub a b

College of Chemistry and Chemical Engineering, Yantai University, Yantai, 264005, PR China Institute of Power Source and Ecomaterials Science, Hebei University of Technology, Tianjin, 300130, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium-ion batteries LiMn0.7Fe0.3PO4 Hydrothermal Supersaturation degree Heating rate

High supersaturation degree can endow hydrothermal products with small size, high crystallinity, designed morphology and orientation. However, adjusting the hydrothermal supersaturation level in organic-free systems is still a great challenge. In this work, high supersaturation degree is feasibly achieved via a green and efficient strategy just by elevating the hydrothermal heating rate, where cubic-like LiMn0.7Fe0.3PO4 cathode material with improved electrochemical kinetics is obtained. Detailed studies reveal that the elevated heating rate could improve the critical dissolved temperature of the precursor and lead to shortened time window for nucleation, and finally give rise to high supersaturation level and fast crystallization kinetics. By this heating-rate-controlled hydrothermal strategy, cuboid-like LiMn0.7Fe0.3PO4 particles with average length of 373 nm are successfully synthesized at the optimal heating rate of 5 °C/min. After coated with conductive carbon, the LiMn0.7Fe0.3PO4/C electrode possesses enhanced conductivity for both electron and Li+, which delivers superior electrochemical properties of 166.5 mAh·g−1 and 87.3 mAh·g−1 at the rates of 0.1 C and 10 C, respectively. We believe this work provides new perspectives for the synthesis of properties-controlled nanocrystals in the hydrothermal system.

1. Introduction Driven by the booming development of electronic industry, it is urgent to develop advanced cathode materials for lithium-ion batteries (LIBs) with long lifespan, superior safety, and high energy density [1]. Nowadays, olivine-typed LiMPO4 (M = Fe, Mn) are widely regarded as the potential cathode materials for LIBs due to their safety, nontoxicity and stability [2]. In particular, LiFePO4 material has already been commercialized, but its low potential (∼3.45 V vs. Li/Li+) leading to low energy density (586 Wh/kg) impedes its further extensive applications [3]. Another important material is LiMnPO4, its high potential of ∼4.10 V vs. Li/Li+ can provide nearly 20% energy density (697 Wh/ kg) more than LiFePO4. Nevertheless, the Mn2+ dissolution (JahnTeller effect) and worse electrical conductivity (10−10 S/cm) made LiMnPO4 suffered from severe capacity fading [4]. As the bridge that links the excellent performance of LiFePO4 and high potential of LiMnPO4, mixed-cation LiMnxFe1-xPO4 (0 < x < 1) materials meet the requirement in energy storage systems, which could efficiently extend the electrochemical window for high energy density [5,6]. Considering the improved electrochemical properties and relatively high potential, LiMnxFe1-xPO4 with x = 0.7 is accepted as the optimum choice for 4 V vs. (Li+/Li) cathode material and has attracted

∗

increasing attentions in recent years [7–11]. It has already demonstrated that small crystallite size [12], well-ordered crystal structure [13], particular distribution of Mn/Fe atom [6] and appropriate carbon coating [14,15] could afford LiMn0.7Fe0.3PO4 fast electrochemical kinetics. Therefore, purposeful controlling the crystal growth condition to produce LiMn0.7Fe0.3PO4 with designed qualities is the key to promoting its application. Compared with solid-state method, hydrothermal synthesis is a more feasible strategy to control the crystal growth condition since it takes place at the atomic level [16,17]. It is known that the hydrothermal supersaturation level is the driving force for the crystal nucleation and growth [18]. High supersaturation could provide fast nucleation rate and large nucleation sites, which is helpful to produce crystal with high crystallinity, small size, and well-defined morphology. In an effort to increase the supersaturation level, a number of strategies were reported such as elevating the temperature [19], adding organic matter [20], intensifying stir [21] and adopting microwave [22] or supercritical hydrothermal [18], etc. Although nanostructured LiMn0.7Fe0.3PO4 such as nanosheets [15], nanorods [20], and nanoplates [8] with excellent performance have been successfully synthesized, the above-mentioned strategies suffer from environmental pollution, complicated procedure, and high investment. It is still a

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.ceramint.2019.10.250 Received 13 August 2019; Received in revised form 8 October 2019; Accepted 26 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Yijun Song, Yuanyuan Liu and Xiuqin Ou, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.250

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calcined at 550 °C for 4 h in N2. The obtained LiMn0.7Fe0.3PO4/C particles were denoted as LMFP/C-3 °C/min, LMFP/C-5 °C/min, and LMFP/C-6.5 °C/min, respectively. To investigate the phase evolution during the LiMn0.7Fe0.3PO4 hydrothermal process, the intermediates within the temperature-raisingperiod were collected at 40, 60, 80, 120, 130, 135, 140 and 160 °C (heating rate of 3 °C/min), respectively. The colours of the collected slurries change from light green at 40 °C to deep green at 130 °C, and finally to dark yellow at 160 °C owing to their different composition. The slurries were filtered and dried, and then the obtained dry samples were respectively named as P-40 °C, P-60 °C, P-80 °C, P-120 °C, P130 °C, P-135 °C, P-140 °C, and P-160 °C.

challenge to obtain properties-controlled LiMn0.7Fe0.3PO4 at low cost in organic-free hydrothermal system. Previous studies on hydrothermal recrystallization reveal that high supersaturation could be easily achieved via a process control method just by elevating the heating rate [23]. The elevated heating rate would shorten the dissolved period of the precursor. According to LaMer mechanism [24–26], the shortened dissolved period can reduce the time window for nucleation, and thereby give rise to high supersaturation level. Furthermore, the heat and mass transfer would be enhanced and the time of secondary nucleation would also be minimized. By this heat-rate-controlled method, nanostructured CoFe2O4 [27], GdPO4 [28], and SrTiO3 [29] with well-defined shapes of square, wire, and sphere have been successfully synthesized. Recently, our group also adopted this strategy to synthesize LiFePO4, where flake-like LiFePO4 nanoparticles with excellent electrochemical performance were obtained at the optimal heating rate of 3 °C/min [30]. Our study showed that the slow heating rate produces LiFePO4 with big size and serious Li/Fe dislocation, while over rapid heating rate causes LiFePO4 less orientation. However, when introducing Mn2+ in LiFePO4, how will the heating rate alter the dissolved behavior of the precursor and consequently affect the properties of LiMn0.7Fe0.3PO4? If the dissolved temperature goes down, it is necessary to raise the heating rate. But is there an upper limit? Where is the limit? These questions should be explored so as to produce properties-controlled LiMn0.7Fe0.3PO4 with enhanced electrochemical kinetics. Herein, we attempted to prepare high-performance LiMn0.7Fe0.3PO4 via a heat-rate-controlled hydrothermal method for the first time. The effects of heating rate on the crystallinity, orientation, morphology, particle size and electrochemical performance of LiMn0.7Fe0.3PO4 have been investigated. Detailed studies reveal that the hydrothermal formative temperature of LiMn0.7Fe0.3PO4 is 120 °C, which is 10 °C lower than that of LiFePO4. Therefore, an elevated heating rate is necessary to decrease the dislocation and thus enhance the electrochemical performance of LiMn0.7Fe0.3PO4. However, over high heating rate brings out LiMn0.7Fe0.3PO4 less orientation, so the hydrothermal heating rate should be cautiously controlled. At the optimum rate of 5 °C/min, cuboid-like LiMn0.7Fe0.3PO4 nanoparticles with high crystallinity and small size are successfully synthesized. Considering the inherently low electronic conductivity, the as-synthesized LiMn0.7Fe0.3PO4 samples were then coated with conductive carbon so as to improve their rate capabilities. The carbon coated LiMn0.7Fe0.3PO4/C material shows superior rate capabilities and excellent capacity retention, which is necessary for the commercial application of LIBs.

2.2. Materials characterization The X-ray diffractometer (XRD, Rigaku SmartLab III) with Cu-Kα radiation (λ = 1.5406 Å) was used to characterize the crystalline structure of samples between 2θ = 5°–80° at 1°/min. A thermal field emission scanning electron microscopy (FESEM, JOEL JSM-7900 F) was used to observe the particle size, morphology, and microstructure of the samples. The nitrogen adsorption/desorption isotherms were measured at 77 K after degassing at 150 °C for 4 h (Micromeritics ASAP 2020). The ratio of Mn/Fe in the samples was analyzed by an X-ray fluorescence analyzer (XRF, Rigaku Supermini 200). The thermal gravimetric analysis (TGA) on the carbon-coated samples was carried out in air from room temperature to 800 °C at a heating rate of 5 °C/min (NetzschSTA 449C). Transmission electron microscopy (TEM) was conducted (Talos F2100×, Thermo Scientific) to observe the morphology and carbon layer of the samples. The four-point probe technology (San Feng SB 120) was performed to measure the electronic conductivity. The LiMn0.7Fe0.3PO4/C particles were compacted to be a column with diameter of 10 mm and thickness of 1.2 mm. The electronic conductivity is measured three times along the column at 25 °C, and then average the results. 2.3. Electrochemical performance The active materials, ethylene black, and polyvinylidene fluoride (PVDF) were mixed in N-methyl-2-pyrrolidinone (NMP) with the mass ratio of 8:1:1. The mixed slurry was casted onto an Al foil and dried at 120 °C under vacuum for 12 h. The mass loading of the electrode is ∼2.2 mg/cm2. The electrochemical properties were evaluated by a CR2032 coin cell which assembled in a glove box (Mikrouna Co., Ltd., China, Model Super 1220/750) using lithium metal as the anode and a Celgard 2400 separator. The electrolyte was 1.0 M LiPF6 in a solvent of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (volume ratio = 1:1:1). The galvanostatic charge/discharge (1C = 170 mAh·g−1) (Instrument model CT2001A, Land Co., China), cyclic voltammogram (CV) tests (Electrochemical workstation model CHI 660 E) and electrochemical impedance spectroscopy (EIS) measurements for the materials were carried out at 25 ± 2 °C.

2. Experimental section 2.1. Materials synthesis LiMn0.7Fe0.3PO4 was prepared by hydrothermal method using the raw materials of FeSO4·7H2O, MnSO4·H2O, H3PO4, and LiOH·H2O (Tianjin Guangfu Fine Chemical Research Institute, analytically pure). The raw materials were firstly dissolved in deionized water. The mixing process of the solution is dropping H3PO4 solution into the LiOH solution, and then dropping the premixing solution of MnSO4 and FeSO4 under N2 atmosphere. The mixture is greenish with final molar ratio of (Fe + Mn):P:Li = 1:1:3. Subsequently, the formed slurry was transferred into a 5 L stainless steel autoclave (Weihai Co., China, Model WHF-5 L) and heated with a temperature controller and an appropriate data acquisition system. The reactor was heated from 25 °C to 160 °C at different heating rates of 3 °C/min, 5 °C/min, and 6.5 °C/min, respectively. After reaction at 160 °C for 3 h, the produced deep yellow precipitates were filtered, washed and dried at 120 °C for 12 h, then calcined at 700 °C for 1 h under N2 aiming to enhance the crystallinity. The obtained samples were named as LMFP-3 °C/min, LMFP-5 °C/min, and LMFP-6.5 °C/min, respectively. To improve the electronic conductivity, the dried samples were mixed with glucose (20 wt %), where after

3. Results and discussion 3.1. Temperature curve during LiMn0.7Fe0.3PO4 hydrothermal process To figure out the temperature curve during LiMn0.7Fe0.3PO4 hydrothermal process, temperature spots within the temperature-raisingperiod were recorded from 100 °C to 160 °C and the results are shown in Fig. 1. Herein, the heating rate of 3 °C/min is chosen not only because the slow heating process is convenient for sample collection, but also it is easy to contrast the differences between LiFePO4 and LiMn0.7Fe0.3PO4. From Fig. 1 it can be seen that the precursor dissolved temperature for LiMn0.7Fe0.3PO4 hydrothermal system is 110 °C–127 °C, which is lower than that of LiFePO4 (131 °C–140 °C as described in our previous study [30]). To understand this better, the phase evolution of 2

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are the main intermediates since these hydrated phosphates form easily by spontaneous gelification of Fe2+, Mn2+ and PO43+. As temperature rises to 80 °C, the peaks of Mn3(PO4)2·7H2O and Fe3(PO4)2·8H2O become sharp while the peaks corresponding to Li3PO4 are weakened, implying large amount of Mn3(PO4)2·7H2O and Fe3(PO4)2·8H2O crystals are formed. The formative process of the intermediates could be explained by means of the solubility product principle. As we know, Li3PO4 is the first precipitation in the precursor mixing process. Although Li3PO4 has low Ksp of 3.2 × 10−9 (25 °C), there exists a small amount of free Li+ and PO43− in the solution. When introducing the aqueous solution of Fe2+/Mn2+, Fe2+ and Mn2+ simultaneously combine with free PO43− and rapidly generate Fe3(PO4)2·8H2O and Mn3(PO4)2·7H2O since Fe3(PO4)2·8H2O (Ksp = 1.0 × 10−36, 25 °C) and Mn3(PO4)2·7H2O (Ksp = 6.13 × 10−32, 25 °C) have lower solubility than Li3PO4. The consumed free PO43− ions facilitate the dissolution of Li3PO4, resulting in the reprecipitation of Fe3(PO4)2·8H2O and Mn3(PO4)2·7H2O again. Therefore, a large number of Fe3(PO4)2·8H2O and Mn3(PO4)2·7H2O appear in the intermediates at a low temperature of 40 °C, and their amount increase gradually as the temperature goes up to 80 °C, which is similar to the hydrothermal process of LiFePO4 reported by our group [16]. Furthermore, the peak at 2θ = 8.2° attributed to Mn3(PO4)2·7H2O shifts left compared with that of P-60 °C. According to the Bragg formula (2dsinθ = nλ), the decreased θ will cause expansive interplanar spacing (d) [16] and unstable crystal structure, resulting in the eventual dissolution of Mn3(PO4)2·7H2O. Therefore, the peaks belonging to Mn3(PO4)2·7H2O diminish obviously as temperature up to 120 °C. Meanwhile, these free Mn2+ dissolved from Mn3(PO4)2·7H2O combining with Fe2+ and PO43- generate a series of M2(PO4)3∙xH2O (M = Fe2+/Mn2+) precipitations rapidly. It is interesting to note that relatively weak peaks assigned to LiMnPO4 appear at a temperature as low as 120 °C, which could be ascribed to the partial substitution of Fe by Mn. As temperature rises to 130 °C, the intermediates dissolve rapidly and the system reaches oversaturation, leading to a fast formative rate of LiMn0.7Fe0.3PO4 nuclei. After heated above 140 °C, all of the peaks in Fig. 2 are attributed to LiMn0.7Fe0.3PO4 and split better (2θ = 55.5° and 61.6°), indicating pure phase LiMn0.7Fe0.3PO4 with well-developed crystallinity is obtained. Based on the above XRD analysis, it is clear that there exists a process of precursor dissolution to the recrystallization of LiMn0.7Fe0.3PO4, which is in good agreement with the SEM results with regard to the morphology change of the intermediate particles as temperature in Fig. S1. Therefore, we infer that the hydrothermal synthesis of LiMn0.7Fe0.3PO4 should be described as a three-step dissolution-precipitation process including precursor dissolution, nucleation, and crystal growth. Although the hydrothermal formation mechanism of LiMn0.7Fe0.3PO4 is the same as LiFePO4, it is worth emphasizing that the generation of LiMn0.7Fe0.3PO4 tends to happen at 120 °C, which is 10 °C lower than that of LiFePO4. Such a low reactive temperature would cause the dislocation of Li+ by Fe2+ or Mn2+ [6], and thus resulting in weak electrochemical performance, especially the cycle performance of LiMn0.7Fe0.3PO4. Therefore, in an effect to obtain LiMn0.7Fe0.3PO4 with excellent electrochemical properties, an elevated heating rate should be necessary.

Fig. 1. Temperature curve during LiMn0·7Fe0·3PO4 hydrothermal process recorded from 100 °C to 160 °C at the heating rate of 3 °C/min.

the precursor with temperature is analyzed in the following section.

3.2. Hydrothermal formation mechanism of LiMn0.7Fe0.3PO4 The XRD patterns of the intermediates collected at different temperatures are shown in Fig. 2. In the XRD results of P-40 °C and P-60 °C, it could be observed that Li3PO4, Fe3(PO4)2·8H2O and Mn3(PO4)2·7H2O

3.3. Effect of heating rate on LiMn0.7Fe0.3PO4 crystal To understand the elevated heating rate on the properties of LiMn0.7Fe0.3PO4, the precursor was treated from 25 °C to 160 °C at different heating rates of 3 °C/min, 5 °C/min, and 6.5 °C/min, and then reacted at 160 °C for 3 h. The XRD patterns of these synthesized samples are shown in Fig. 3a. It can be seen that all of the XRD patterns match well with the orthorhombic olivine structure of LiMnPO4 (JCPDS No.77–0178) and no impurities detected, implying the as-synthesized LiMn0.7Fe0.3PO4 possess a high crystallinity and perfect crystalline structure [10]. The element analysis in Table 1 reveals that the concentrations of Mn and Fe for all samples are in accord with the

Fig. 2. XRD patterns of the intermediates collected at the temperatures of 40 (P40), 60 (P-60), 80 (P-80), 120 (P-120), 130 (P-130), 135 (P-135), 140 (P-140) and 160 °C (P-160) during hydrothermal temperature-rising period at the heating rate of 3°/min. 3

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Fig. 3. XRD (a) and the Rietveld refinement patterns (b) of the LiMn0·7Fe0·3PO4/C samples hydrothermally synthesized at 160 °C for 3 h at the heating rates of 3 °C/ min, 5 °C/min and 6.5 °C/min, respectively.

stoichiometric relationships of phosphates, suggesting the different hydrothermal heating rates have no influence on the phase composition of LiMn0.7Fe0.3PO4. However, the XRD patterns show a delicate difference at the relative intensity of the (020) plane, which exposes the crystal growth direction as discussed in our previous work [19,30]. The compared results of these selected peaks in Table 1 indicate that the intensity ratio increases slightly with the elevated heating rate, inferring more crystal faces are able to expose at a higher heating rate. Furthermore, the mean crystal size estimated by the Scherrer equation (D = 0.9λ/βcosθ) [31] displays a descending trend as the heating rate rising from 3 °C/min to 6.5 °C/min, which might be ascribed to their enhanced supersaturation and fasted nucleation rate. It is known that LiMn0.7Fe0.3PO4 with small crystal size is helpful to improve the discharge capacity, power capability, and cycling stability. In order to further explore the structural information of LiMn0.7Fe0.3PO4 affected by the elevated heating rate, Rietveld refinement was conducted using EXPGUI software. The XRD patterns in Fig. 3b show good consistency between the experimental and calculated ones, and the factors of Rwp and Rp for all samples are below 5% in Table 2, manifesting the Rietveld results are reliable and acceptable. The estimated lattice parameters in Table 2 indicated that both a and c axes shrink while the b axis stretches gradually when elevating the heating rate, which finally leading to smaller lattice volume since the change of b axis has little contribution to the volume. The decreased lattice volume means there is a reduced average Mn/Fe–O bond length at a higher heating rate. As well known, short bond length could reduce

Table 2 The Rietveld refinement for the LiMn0·7Fe0·3PO4/C samples. Samples

LMFP/C-3°C/ min LMFP/C-5°C/ min LMFP/C6.5 °C/min

Lattice parameter (Unit: Å)

Reliability factors

a

b

c

V

Rwp (%)

Rp (%)

χ2

10.4306

6.0481

4.7249

298.07

2.21

1.69

1.626

10.3907

6.0577

4.7128

296.64

2.19

1.70

1.734

10.3723

6.0680

4.7105

296.47

2.20

1.71

1.741

the ionicity of Mn–O bond in LiMn0.7Fe0.3PO4, which can accelerate the electron polaron hopping between adjacent cationic centers and thus improve the electrochemical performance of the electrodes. Another noteworthy phenomenon is the slightly stretched b axis as the elevated heating rate, which is consistent with the XRD analysis of the (020) plane in Fig. 3a. A large parameter of b impedes ion transport since Li+ diffuses along the b-axis in LiMn0.7Fe0.3PO4 (one-dimensional channel as shown in Fig. S2). Clearly, the heating rate should be cautiously controlled. Furthermore, the differences of lattice parameters between LMFP/C-5°C/min and LMFP/C-6.5 °C/min are negligible in Table 2, suggesting that over quick heating rate has limited influence on the crystal structure of LiMn0.7Fe0.3PO4, which might be the upper limit of the heating rate.

Table 1 The results of intensity ratio, XRF, crystal size and BET for the LiMn0·7Fe0·3PO4/C samples. Samples

LMFP/C-3°C/min LMFP/C-5°C/min LMFP/C-6.5 °C/min

Intensity ratio I(020)/I(111)

I(020)/I(121)

I(020)/I(131)

0.269 0.305 0.313

0.252 0.293 0.307

0.209 0.248 0.259

4

Mn/Fe ratio

Crystal size (nm)

Specific surface area (m2/g)

70.42/29.58 70.37/29.63 70.35/29.65

77.26 70.48 67.79

9.93 14.92 13.91

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Fig. 4. Temperature curves recorded during hydrothermal heating process from 100 °C to 160 °C (a), FE-SEM images (b–d) and TEM images (inset) for the LiMn0·7Fe0·3PO4/C samples synthesized at 160 °C for 3 h by the hydrothermal heating rates of 3 °C/min, 5 °C/min and 6.5 °C/min, respectively.

5 °C/min has relatively larger specific surface areas than other samples (Table 1) owing to its small particle size and good dispersion. Based on the above XRD and SEM analysis, it can be found that the effect of the heating rate on LiMn0.7Fe0.3PO4 crystal is multifaceted. With the increase in heating rate, the crystal granularity is reduced and the degree of crystallinity is enhanced, which is benefit for the electrochemical kinetics of LiMn0.7Fe0.3PO4. While ultra-fast heating rate induces LiMn0.7Fe0.3PO4 less orientation, which leads the b-axis slightly lengthen and consequently impedes Li+ diffusion. Therefore, to get LiMn0.7Fe0.3PO4 material with excellent electrochemical performance, the hydrothermal heating rate should be cautiously controlled.

The temperature curves at various hydrothermal heating rates were monitored from 100 °C to 160 °C, and the results are shown in Fig. 4a. It can be seen that the dissolved temperature rises from 110 °C-127 °C to 115 °C-135 °C as the heating rate increasing from 3 °C/min to 5 °C/min, and which is even higher at 6.5 °C/min (no endothermic peak within the curve). It is noted that this variation has an impact on the morphology and size of LiMn0.7Fe0.3PO4/C as shown in Fig. 4b–d. The sample LiMn0.7Fe0.3PO4/C synthesized at 3 °C/min exhibits approximately plate-like morphology with an average length of 454 nm. Whereas LiMn0.7Fe0.3PO4/C prepared at 5 °C/min shows a cuboid-like shape with obviously reduced size of 373 nm (length). Their apparently different temperature curves could account for this. The low dissolved temperature and longish dissolved span at 3 °C/min cause a slow crystallization process, leading to the generation of large particles. While the elevated heating rate of 5 °C/min improves the critical dissolved temperature, which subsequently brings about high supersaturation and fast crystallization kinetics, resulting in the formation of LiMn0.7Fe0.3PO4 particles with small size and narrow distribution (Fig. 4c). It is known that small particle exhibits a variety of fascinating and admirable properties for LIBs, such as large surface area, low diffusion distance, high conductivity for both electron and Li+. Besides, the nanocrystals in Fig. 4d are observed to agglomerate together, which might because fine crystalline with larger surface energy are easier to adsorb together. Some hollow structures as described in our previous work are also detected on the surface of LiMn0.7Fe0.3PO4/C particles in Fig. 4c and d, which is the result of accelerated nucleation rate and benefit for Li+ diffusion [16]. The LiMn0.7Fe0.3PO4/C synthesized at

3.4. Electrochemical characterization The as-prepared LiMn0.7Fe0.3PO4 samples were coated with conductive carbon using glucose as the carbon source. The small amounts of carbon in Table 3 (calculated by TGA analysis in Fig. S3 [32,33]) and an appropriate carbon layer of ∼3 nm (TEM in Fig. S4) improve the electronic conductivities significantly [34] (Table 3). The first chargedischarge curves of these carbon coated LiMn0.7Fe0.3PO4/C electrodes at 25 °C within 2.5–4.5 V (vs. Li/Li+) at 0.1 C are shown in Fig. 5a. It can be seen that all of the samples possess two characteristic potential plateaus at 4.10 V and 3.45 V (vs. Li/Li+), corresponding to the redox couples of Mn3+/Mn2+ and Fe3+/Fe2+. The LiMn0.7Fe0.3PO4/C sample prepared at 3 °C/min shows a discharge capacity of 128.3 mAh·g−1 at 0.1 C. As the heating rate increasing to 5 °C/min, the discharge capacity improves to 166.5 mAh·g−1 at 0.1 C. The LMFP/C-5 °C/min sample also

Table 3 Carbon contents, Electronic conduction, Resistance and DLi of the LiMn0·7Fe0·3PO4/C samples. Samples

Carbon contents (wt%)

Electronic conduction (S/cm)

Rs (ohm)

Rct (ohm)

DLi (cm/S)

LMFP/C-3°C/min LMFP/C-5°C/min LMFP/C-6.5 °C/min

3.32 3.57 3.61

2.62 × 10−2 2.75 × 10−2 2.69 × 10−2

1.47 1.73 1.80

219.71 84.18 120.53

9.98 × 10−15 3.02 × 10−14 1.21 × 10−14

5

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Fig. 5. The charge-discharge curves at 0.1 C (a), the discharge capabilities at various rates (b), and the cyclic performances at 5 C (c) for the LiMn0·7Fe0·3PO4/C samples prepared at the hydrothermal heating rates of 3 °C/min, 5 °C/min and 6.5 °C/min, respectively.

exhibits better capacity retention at various current rates (Fig. 5b) and excellent discharge capacity of 87.3 mAh·g−1 even at the high rate of 10 C, which is large than the uncoated LiMn0.7Fe0.3PO4 in Fig. S5 and necessary for the commercial use of LIBs, especially for the power battery. To further demonstrate the improved performance, long-cycling test is carried out at the rate of 5 C as shown in Fig. 5c. The LiMn0.7Fe0.3PO4/C sample prepared at 5 °C/min can display excellent capacity retention after 600 cycles and maintain superior columbic efficiency of approximately 99–100% during the long cycling process. The high discharge capacity and excellent cycling stability of LMFPC5°C/min could be attributed to its decreased lattice volume, smaller particle size, and relatively larger surface area than other samples, which can greatly promote the penetration of liquid electrolyte while facilitating the Li+ diffusion and electron transport. In addition, the electrochemical properties of LMFP/C-6.5 °C/min (152.9 mAh·g−1 at 0.1 C) are slightly lower than that of LMFPC-5 °C/min, which could be attributed to its sluggish reaction kinetics resulting from its stretched b axis and agglomerated structure. The LiMn0.7Fe0.3PO4/C electrodes are further assessed by cyclic voltammetry (CV) in order to understand the Li+ diffusion behavior. Fig. 6 presents the CV curves of these LiMn0.7Fe0.3PO4/C electrodes at a scan rate of 0.1 mV/s. It can be seen that all of the CV curves have two pairs of redox peaks, corresponding to the redox couples of Mn3+/ Mn2+ and Fe3+/Fe2+ [34–36]. For the LiMn0.7Fe0.3PO4/C sample prepared at 3 °C/min, the potential difference in the redox peaks of Fe3+/Fe2+ and Mn3+/Mn2+ are 0.25 V and 0.47 V (Table 4), respectively. While for the sample prepared at 5 °C/min, these potential differences decrease to 0.33 V and 0.16 V, respectively. And at the same time, relatively sharper peaks and larger peak current are observed, evidencing the beneficial effect of elevated heating rate on the redox activity and Li-diffusion kinetics during the charge/discharge process. The potential difference in the redox peaks for Fe3+/Fe2+ is smaller than that of Mn3+/Mn2+ since the latter reaction is kinetically more difficult. As the heating rate increases to 6.5 °C/min, the potential differences become slightly larger, suggesting that over quick heating rate has a detrimental effect on the reversibility, which might be attributed

Fig. 6. The first cycle cyclic voltammogram of LiMn0·7Fe0·3PO4/C samples prepared at the hydrothermal heating rates of 3 °C/min, 5 °C/min and 6.5 °C/ min, respectively. Scan rate: 0.1 mV/s. Potential range: 2.0–4.5 V vs. Li+/Li. Table 4 CV results of the LiMn0·7Fe0·3PO4/C samples. Samples

LMFP/C-3°C/min LMFP/C-5°C/min LMFP/C-6.5 °C/min

Potential values (V) EpA

EpB

EpC

EpD

△EAB

△ECD

3.67 3.61 3.63

3.42 3.45 3.44

4.28 4.22 4.24

3.81 3.89 3.87

0.25 0.16 0.19

0.47 0.33 0.37

to its stretched b axis and agglomerated structure. In contrast, the LiMn0.7Fe0.3PO4/C prepared at 5 °C/min exhibits the smallest potential differences between redox peaks and the highest peak current originating from its smallest size and relatively large surface area. 6

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Fig. 7. The Nyquist plots (a) and relationship between -Z″ and ω−1/2 (b) of LiMn0·7Fe0·3PO4/C samples prepared at hydrothermal heating rates of 3 °C/min, 5 °C/min and 6.5 °C/min, respectively.

enhanced, however, over heating rate will bring out LiMn0.7Fe0.3PO4 less orientation. Considering its multifaceted effects, the hydrothermal heating rate should be cautiously controlled in order to get LiMn0.7Fe0.3PO4 with excellent electrochemical performance. The LiMn0.7Fe0.3PO4/C electrode prepared at the optimal heating rate of 5 °C/min presents enhanced conductivities for both electron and Li+, delivering superior electrochemical performances of 166.5 mAh·g−1 and 87.3 mAh·g−1 at 0.1 C and 10 C, respectively. We believe that this work not only enlightens a way for the fabrication of electrode materials in advanced batteries, but also provides new perspectives for the properties controlling of the product during the hydrothermal crystallization process.

The electrode kinetics was further studied by electrochemical impedance spectroscopy (EIS) and the results are shown in Fig. 7a. It can be seen that all of the curves present a distorted frequency semicircle in the high-frequency region followed by an oblique line in the low-frequency region. The semicircle is associated with the charge transfer resistance (Rct) at the interface of electrode and electrolyte, and the oblique line is referred to the Li+ diffusion in the bulk of the electrode. The values of Rct fitted with ZView 2 software are shown in Table 3. The obviously decreased Rct of LiMn0.7Fe0.3PO4/C prepared at 5 °C/min as compared with other samples suggesting good charge transfer kinetics. DLi+ = R2T2/2A2n4F4C2σ2 Z' = Re + Rct + σω

−1/2

(1) (2)

Declaration of competing interest

The Li+ diffusion coefficients DLi+ can be calculated from the slope of the oblique line in the low-frequency region by eqns. (1) and (2), where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidation, F is the Faraday constant, C is the concentration of Li+ (0.00228 moL/cm3 [37]), σ is the Warburg factor associated with Z′ and ω. The linear relationship between Z′ and ω−1/2 is shown in Fig. 7b and the calculated DLi+ are listed in Table 3. In contrast, the sample LiMn0.7Fe0.3PO4/C prepared at 5 °C/min exhibits the lowest Rct and largest DLi+, which could provide enhanced charge transfer kinetics and improved transmission speed of Li+ during the insertion/extraction process. As a result, the LMFP/C-5 °C/min sample exhibits superior rate capability and excellent cycling stability when acting as cathode materials for LIBs.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.250. References

4. Conclusions The phosphates of Li3PO4, Fe3(PO4)2·8H2O, Mn3(PO4)2·7H2O and (Mn, Fe)3(PO4)2·4H2O are the main intermediates during LiMn0.7Fe0.3PO4 hydrothermal process. The Li3PO4 and Fe3(PO4)2·8H2O crystal are the main intermediates when the temperature below 40 °C, and then Mn3(PO4)2·7H2O and (Mn, Fe)3(PO4)2·4H2O intermediates generate at 60–80 °C. Finally, the LiMn0.7Fe0.3PO4 nuclei appear at a low temperature of 120 °C till all of the intermediates transform into LiMn0.7Fe0.3PO4 crystal at 160 °C. The morphology, orientation, size, and electrochemical performances of LiMn0.7Fe0.3PO4 are highly depended on the hydrothermal heating rate since it affects the precursor's dissolved behavior. The low heating rate prolongs the dissolving span and leads to a slow crystallization process, resulting in the generation of large LiMn0.7Fe0.3PO4 particles. With the increase in heating rate, the crystal granularity is reduced and the degree of crystallinity is

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