Abstract
The xLiMnPO4·yLi3V2(PO4)3/C (x/y = 1 : 0, 12 : 1, 8 : 1, 6 : 1, 4 : 1, 0 : 1) composite cathode materials are synthesized using tributyl phosphate as a novel organic phosphor source via a solid-state reaction process. All obtained xLiMnPO4·yLi3V2(PO4)3/C composites present similar particles morphology with an average size of ca. 100 nm and low extent agglomeration. The electrochemical performance of pristine LiMnPO4/C can be effectively improved by adding small amounts of Li3V2(PO4)3 additives. The 4LiMnPO4·Li3V2(PO4)3/C has a high discharge capacity of 143 mAh g−1 at 0.1 C and keeps its 94% at the end of 100 cycles.
1. Introduction
Recently, 4 V olivine-structured LiMnPO4 and monoclinic Li3V2(PO4)3 attract much attention as polyanionic cathode materials for lithium ion batteries because of their superior thermal and cycling stabilities [1–4]. LiMnPO4, owing to the outstanding advantages of low cost and nontoxicity, exhibits more attraction than Li3V2(PO4)3 for use in power batteries and energy storage systems. However, the one-dimensional Li+ transport pathway in LiMnPO4 crystal restricts its electrochemical performance at high charge-discharge currents [5]. By contrast, the open three-dimensional pathway in Li3V2(PO4)3 crystal enables fast Li+ migration, resulting in the high rate capability [6]. However, the relatively high cost of raw materials for Li3V2(PO4)3 hinders its large amount of industrial production.
More recently, several reports have proved that the electrochemical activity of bulk LiMnPO4 can be effectively enhanced by incorporating with a small amount of Li3V2(PO4)3 additives [7–9]. On one hand, the structure modification of V doping at Mn sites can increase the electronic and ionic transport speed of LiMnPO4 phase [10–12]. On the other hand, the homogeneous dispersion of Li3V2(PO4)3 crystallites in LiMnPO4 body reduces Li-diffusion distance in the LiMnPO4 [13]. Li et al. [14] prepared ()LiMnPO4·xLi3V2(PO4)3/C composites through a sol-gel route, and 0.5LiMnPO4·0.5Li3V2(PO4)3/C showed the highest discharge capacity of 135 mAh g−1 at 0.1 C. Zhang et al. [15] synthesized LiMnPO4·Li3V2(PO4)3/C using rod-like MnV2O6·4H2O as precursor, which provided a reversible capacity of 126 mAh g−1 at 0.1 C. Chen et al. [16] reported the synthesis of 0.95LiMn0.95Fe0.05PO4·Li3V2(PO4)3 composite, which exhibited capacity of 176 mAh g−1 at 0.05 C and retained 112 mAh g−1 after 50 cycles. Qin et al. [17] synthesized the LiMnPO4-Li3V2(PO4)3/C composite materials at the sintering temperature of 600°C. The 0.6LiMnPO4·0.4Li3V2(PO4)3 exhibits the highest discharge capacity of 126 mAh g−1 at 0.1 C, against 76 mAh g−1 for the pristine LiMnPO4/C. Similarly, the synergistic effect of LiFePO4 and Li3V2(PO4)3 on physical and electrochemical behaviors is extensively investigated in recent studies [18–24].
It is noteworthy that the particle size, carbon layer, and phase distribution strongly affect the electrochemical activity of LiMnPO4-Li3V2(PO4)3 composite. In this paper, we use tributyl phosphate as a novel organic phosphor source to synthesize a series of xLiMnPO4·yLi3V2(PO4)3/C (abbreviated as xLMP·yLVP/C) composites by ball-milling followed by sintering process. The physical and electrochemical properties of LiMnPO4-based composites with different Li3V2(PO4)3 amounts are studied in detail.
2. Experimental
Stoichiometric Li2CO3, Mn(CH3COO)2·4H2O, V2O5, (C4H9O)3PO, and glycine were used as raw materials. Glycine acted as carbon source and the carbon amount of final products was controlled about 5 wt%. Appropriate quantities of reactants were ball-milled in ethanol for 6 h at 350 rpm. The obtained mixture was dried at 70°C and then heated at 700°C under Ar atmosphere for 10 h to yield the xLMP·yLVP/C (x/y = 1 : 0, 12 : 1, 8 : 1, 6 : 1, 4 : 1, 0 : 1) cathode materials.
X-ray diffraction study was performed on X’pert Pro diffractometer equipped with Cu Kα radiation at 30 mA and 40 kV. Field-emission scanning electron microscope (FE-SEM, HITACHI-S4800) and high-resolution transmission electron microscope (HRTEM, JEOL-2100F) were used to investigate the morphology, crystal structure, and elemental distribution of the samples. The residual carbon amount was evaluated by PE 2400 elemental analyzer.
The electrochemical properties were studied by charge-discharge cycling using CR2016 coin-type cells. The cathode materials comprised xLMP·yLVP/C, acetylene black, and poly(vinylidene fluoride) with a weight ratio of 8 : 1 : 1. Lithium-foil is used as anode, Entek ET20-26 microporous membrane as separator, and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (1 : 1, volume) as electrolyte. The assembled cells were cycled on a LANHE CT2001 battery testing system with a constant current-constant voltage mode between 2.0 and 4.5 V at 25°C. Cyclic voltammetry (CV) was conducted on a Chenhua CHI650D electrochemical workstation at a sweep rate of 0.1 mV s−1.
3. Results and Discussion
Figure 1 shows the XRD patterns of the as-prepared composites. The diffraction peaks in the sample with x/y = 1 : 0 can be assigned to olivine LiMnPO4 (space group Pnmb, JCPDS 74-0375) [1]. As x/y = 0 : 1, all peaks can be indexed to monoclinic Li3V2(PO4)3 (space group P21/n, JCPDS 47-0107) [22]. When x/y = 12 : 1, 8 : 1, 6 : 1, and 4 : 1, the diffraction peaks of xLMP·yLVP/C consist of LiMnPO4 and Li3V2(PO4)3 phases without any crystalline impurity. The diffraction peaks of Li3V2(PO4)3 become stronger with increasing Li3V2(PO4)3 amount. No detectable reflections for graphitic carbon are observed, which suggests that carbon in all composites is amorphous.
Figure 2 compares the powder morphologies of LMP/C, 8LMP·LVP/C, 4LMP·LVP/C, and LVP/C samples. All samples exhibit narrow particle size distribution with an average size of ca. 100 nm. Tributyl phosphate is a liquid organic molecule, compared with solid NH4H2PO4, which can favorably disperse in the precursors during ball-milling process. The decomposition of tributyl phosphate releases gas and forms some in situ carbon layer on the surface of xLMP·yLVP nuclei during the heating step. This effectively prevents the aggregation of small primary particles and further growth.
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TEM image of 4LMP·LVP/C in Figure 3(a) displays that the primary 4LMP·LVP nanoparticles are connected by amorphous carbon. The corresponding elemental mappings for Mn, V, and P illustrate that the LiMnPO4 particle contains Li3V2(PO4)3 phase and the Li3V2(PO4)3 particle contains LiMnPO4 phase (Figures 3(b)–3(d)). The atomic ratio of Mn/V/P presented in EDS spectrum basically accords with the theoretical value of 4LMP·LVP/C (Figure 3(e)). HRTEM image of 4LMP·LVP/C in Figure 3(f) shows that the particle surface is fully coated with amorphous carbon layer. The -spacing values of 0.524 and 0.352 nm correspond to the (020) and (111) crystalline planes of LiMnPO4, respectively, whereas the -spacing value of 0.431 nm is attributed to the (020) plane of Li3V2(PO4)3. The results demonstrate that both LiMnPO4 and Li3V2(PO4)3 nanocrystals coexist in the 4LMP·LVP/C particles.
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Figure 4 displays the typical charge-discharge and cycle life profiles of the xLMP·yLVP/C samples at 0.1 C. As shown in Figure 4(a), the LMP/C gives one reversible voltage plateau nearby 4.1 V associating with the phase transition of LiMnPO4↔MnPO4, while the LVP/C presents three reversible voltage plateaus nearby 3.6 V, 3.7 V, and 4.1 V corresponding to the sequential phase transitions of Li3V2(PO4)3↔Li2.5V2(PO4)3↔Li2V2(PO4)3↔LiV2(PO4)3, respectively [9]. As to the xLMP·yLVP/C composites, the distinct voltage plateaus of both LiMnPO4 and Li3V2(PO4)3 are observed. Furthermore, the electrochemical polarization of LiMnPO4 is effectively alleviated by Li3V2(PO4)3 additive, especially in the 4LMP·LVP/C composite. The discharge capacities of xLMP·yLVP/C composites with various x/y of 12 : 1, 8 : 1, 6 : 1, and 4 : 1 are 115, 124, 131, and 143 mAh g−1, respectively, much higher than that of LMP/C (102 mAh g−1). Figure 4(b) describes the cycle life profiles of the composite samples. After 100 cycles at 0.1 C, the capacity retention for LMP/C, 12LMP·LVP/C, 8LMP·LVP/C, 6LMP·LVP/C, and 4LMP·LVP/C is 98%, 96%, 95%, 95%, and 94%, demonstrating good cycling stability. By contrast, the LVP/C provides relatively lower capacity retention of 90% than the LMP-based composites, in spite of delivering the largest initial discharge capacity of 155 mAh g−1. Here, the capacity fade of xLMP·yLVP/C can be ascribed to the irreversible structural changes of Li3V2(PO4)3 when cycling a part of the third Li+ beyond 4.3 V [17].
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Figure 5 exhibits the discharge rate profiles of LMP/C and 4LMP·LVP/C samples ranging from 0.1 C to 2 C. Obviously, the discharge capacities of 4LMP·LVP/C remarkably surpass that of LMP/C at various rates. Moreover, the 4LMP·LVP/C can retain much higher discharge voltage plateau than LMP/C at high rates, such as at 1 C and 2 C. According to the HRTEM and EDS analysis in Figure 3, the LiMnPO4 primary particle is further divided by a small amount of Li3V2(PO4)3 nanocrystals, which shortens Li-diffusion distance in LiMnPO4 body resulting in the superior rate capability.
Figure 6 shows the typical cyclic voltammetry profiles of the xLMP·yLVP/C electrodes between 3.0 and 4.5 V. One redox couple peak at 3.92/4.33 V for LMP/C belongs to the reaction of Mn2+↔Mn3+. Meanwhile, three redox couple peaks at 3.56/3.61, 3.64/3.69, and 4.02/4.11 V for LVP/C correspond to the reaction of V3+↔V4+. For the 12LMP·LVP/C, 8LMP·LVP/C, 6LMP·LVP/C, and 4LMP·LVP/C samples, the redox couple peaks of LiMnPO4 and Li3V2(PO4)3 are both detected. More importantly, the interval between the Mn2+/Mn3+ redox potential peaks decreases from 0.41 V of LMP/C to 0.33 V of all LMP-LVP/C composites. The CV results coincide with the charge-discharge profiles, which reveals that the electrochemical activity and reversibility of LiMnPO4 are improved by incorporating with Li3V2(PO4)3.
4. Conclusions
In summary, the xLMP·yLVP/C nanoparticles are synthesized via a facile solid-state method using tributyl phosphate as phosphor source and glycine as carbon source. The use of tributyl phosphate is favorable for the formation of granular morphology with small particle size. XRD, HRTEM, and EDS mapping results indicate that the composites consist of LiMnPO4 and Li3V2(PO4)3 phases, and the Li3V2(PO4)3 nanocrystals disperse in the LiMnPO4 body particle. Among the LMP-LVP/C composites, the 4LMP·LVP/C delivers the largest discharge capacity of 143 mAh g−1 at 0.1 C along with superior rate capability and satisfactory cycle life. The LMP-LVP/C composite is promising as high-performance cathode material for rechargeable lithium batteries.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (no. 21401061), Natural Science Foundation of Anhui Province, China (no. 1308085QB41), Provincial Natural Science Research Foundation of Anhui Universities, China (nos. KJ2014A224 and KJ2015A332), and the Key Project of Anhui Universities Support Program for Outstanding Youth, China (no. gxyqZD2016111).