Mild hydrothermal synthesis and crystal morphology control of LiFePO by lithium nitrate
Graphical abstract
Introduction
Lithium-ion batteries are a type of energy storage systems highly needed for portable and mobile applications, as well as in transportation systems [1]. The batteries are composed by the anode and cathode electrodes and the separator/electrolyte [2]. In relation to the anode, different materials have been used, such as carbonaceous materials [3], TiO [4], [5] and NiO [6]. The most used active materials for the cathode are LiCoO [7], lithium metal polyoxyanion LiVPO [8] and olivine structures such as LiMPO (M Mn, Fe, Ni, Co, and combinations of them) [9]. Almost 20 years after the first scientific report about the electrochemical activity of synthetic olivines [10], a number of approaches for the preparation of lithium iron phosphate, LiFePO have been described. LiFePO is often used as active material for cathode electrode due to its excellent advantages, such as low cost, thermal stability, high safety, long cycle life and excellent theoretical capacity (170 mAh g). It also shows some disadvantages with respect to other active material for cathodes, including low electrical conductivity, slow kinetics and low power weight density [11], [12], [13]. The preparation of LiFePO is based on two different synthesis, solid state methods and solution methods [14]. In the solid-state methods, the most common ones are solid state synthesis, mechanochemical activation, carbothermal reduction and microwave heating and in relation to solution methods, the most used are hydrothermal synthesis, sol–gel synthesis, spray pyrolysis and co-precipitation [14]. Recently, LiFePO particles have been synthetized by hydrothermal synthesis by near- and super-critical water through a batch reactor system [15].
Initially the synthesis of LiFePO was limited to high temperature solid-state reaction [10]. Later the mild hydrothermal method was gradually developed [16], [17], [18] and reached a quality that allowed the commercialization of LiFePO as cathode material for lithium batteries (Candiac, Canada) [19]. Typically, the hydrothermal method employs water solutions that contain precursors such as lithium hydroxide (LiOH), iron (II) sulfate (FeSO) and orthophosphoric acid (HPO) mixed in a ratio 3:1:1. The lithium precursors typically used for the synthesis of LiFePO by hydrothermal synthesis are: Lithium carbonate (LiCO) [20], Lithium dihydrogen phosphate (LiHPO) [21], Lithium iodide (LiI) [22], Lithium acetate (LiOOCCH) [23] and Lithium citrate (LiCHO) [24]. More than 80% of LiFePO synthesis reported in the literature uses LiOH as lithium precursor, which is approximately 15% more expensive (per mole of lithium content) when compared with LiCO [25]. Furthermore, LiOH is the slightly more expensive lithium precursor [25]. The temperature for synthesis is usually 180 or higher and the time for crystallization is typically a few hours. [19] Our research in the available literature found that LiNO is used for solid state reactions as low-cost source of Li for synthesis of LiFePO [26] but no records about mild hydrothermal synthesis were found. Additionally, a number of works showed the potential of crystal morphology control for tuned performance of LiFePO. As a result iron olivines in form of nanocubes [27], hollow spheres [28], dumbbell-like [29], nanorods [30], microspheres [31], three dimensional sandwich composites [32] and spindle-like particles [33] have been reported.
The goal of this work is the hydrothermal synthesis of LiFePO using LiNO as a source of lithium and the control of the resulting morphology. It is the first time that LiNO is introduced as a precursor for the preparation of phase pure LiFePO which may become an alternative to the known synthesis procedures for cathode materials with olivine structure, considering this precursor is readily available and cheap. Additionally, we show new types of crystal morphologies of LiFePO that can be applied as Li-ion cathode.
Section snippets
LiFePO synthesis and characterization
The hydrothermal synthesis was performed by mixing four individual solutions through of molar fraction of 3:1:1 (Li:Fe:P): (1) 1.03 g of LiNO or LiOH (98% Aldrich) dissolved in 5 g of HO with (0.6 or 1.3 g/L of sucrose (sugar)) or without sucrose, (2) 0.57 g of NaOH (97% Aldrich) dissolved in 6.76 g of HO for adjusting the final pH at 7, (3) 0.29 ml of HPO (85% Aldrich) dissolved in 5 g of HO and (4) 1.39 g FeSO7HO (99% Panreac) dissolved in 10 g of HO. The addition of each individual
X-ray diffraction studies
Fig. 1 shows powder XRD patterns of samples prepared at conditions that are described in Table 1. All XRD patterns were refined by the method of Rietveld which gave qualitative and quantitative information about each phase. Although synchrotron radiation study showed that a mixture of amorphous phase and iron olivine can be obtained at 105 [35] in the literature, temperatures below 175 are generally considered unfavorable for the hydrothermal synthesis of LiFePO [16]. Unlike this
Conclusions
In this work we have introduced for the first time LiNO as an efficient source of lithium for mild hydrothermal synthesis of LiFePO. By the analysis of the influence of the temperature, time, source of lithium and addition of sucrose our synthetic procedure offer an alternative approach towards preparation of important cathode material in battery industry. Furthermore, a new morphological type of LiFePO is revealed as well as its interesting electrochemical results that can be used for the
Acknowledgments
This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the projects PTDC/CTM-ENE/5387/2014 and UID/CTM/50025/2013 and grants SFRH/BPD/112547/2015 (C.M.C.) and IF/01516/2013 (S.F.). The authors thank financial support from the Basque Government Industry Department under the ELKARTEK Program.
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