Properties of lithium iron phosphate prepared by biomass-derived carbon coating for flexible lithium ion batteries
Graphical abstract
Introduction
Lithium batteries are considered as an attractive power source for various applications, such as cellular phones, notebook computers, electric vehicles (EVs), and energy storage systems (ESSs) [1]. Currently, lithium metal oxides (LiMO2, M = Co, Mn, Ni, Al) are the most commonly used as cathode material in lithium batteries. However, they are relatively expensive and have a lower stability of crystal structure. Much research has been devoted to finding low cost, effective replacements for LiMO2, especially for batteries destined for transportation applications. Lithium iron phosphate has been recognized as a good alternative cathode material for replacing LiMO2 since Padhi et al. first reported the redox reaction of lithium iron phosphate (LiFePO4) in 1997 [2]. LiFePO4 is a less expensive, abundant, environmentally friendly, and stable material with a high theoretical capacity of 170 mAh g−1. It provides an operation potential of about 3.4 V versus Li+/Li and exhibits stable cell operation by providing a large over-voltage margin for decomposition of liquid electrolyte [[2], [3], [4]]. Optimized preparation processes facilitated to achieve the desired particles size and morphology are gaining importance to improve the properties of LiFePO4. The rate capability limitation of LiFePO4 has come from the poor electronic conductivity and the slow lithium ion transfer across the phase boundary of LiFePO4/FePO4 [[2], [3], [4], [5], [6]]. Various approaches for enhancing the electronic conductivity of LiFePO4 have been attempted, such as coating the particles with carbon. In 1999, Ravet et al. reported that LiFePO4 had a capacity of 160 mAh g−1 at 1 C-rate at 80 °C when coated with carbon [7]. Carbon coating is usually formed by carbonization of organic/polymeric compounds during the synthesis of LiFePO4 [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] or by mixing with carbon materials [[21], [22], [23], [24], [25], [26]] and biomass materials [[27], [28], [29], [30], [31]]. Recent studies on the effect of the surface carbon structure on the electrochemical performance of LiFePO4 revealed that sp2-coordinated carbon resulted in better conductivity than sp3-coordinated carbon and in enhanced electrochemical performance [8,10,20]. Moreover, the carbon composite LiFePO4 electrode of high conductivity applied into flexible lithium ion batteries [[32], [33], [34]].
We previously reported the synthesis of LiFePO4 by a modified mechanical activation method [24,25]. The modification in the solid-state synthesis process allowed achieving phase-pure, small crystallites of the electrode material with a uniform carbon coating, which could obviously improve the electrochemical performance. Although methods of improving the electrochemical properties LiFePO4 have been studied by many research groups, the effects of biomass-derived carbon coating on the physical and electrochemical performance of LiFePO4 have not been defined, especially for application of the coated material in polymer batteries. Orange peel biomass is used as a carbon source for coating the surface of LiFePO4. The focus of the present study is to compare the physical and electrochemical performance of flexible polymer batteries employing pure LiFePO4 with that based on biomass-derived carbon-coated LiFePO4 prepared by a modified mechanical activation process.
Section snippets
Experimental
Pure LiFePO4 (LFP) and carbon-coated LiFePO4 (C-LFP) were synthesized by a modified mechanical activation method from the precursors (Li2CO3, FeC2O4·2H2O, and NH4H2PO4, 99% purity from Aldrich) in stoichiometric quantities. The orange peel precursor was immersed in 100 mL of 7% KOH solution at room temperature for 24 h and then dried at 80 °C for carbon coating on the surface LiFePO4, as shown in Fig. 1. The KOH was used as activating agent. Pure LiFePO4 was synthesized without orange peel
Results and discussion
The chemical composition ratio of the sample matched the theoretical molar ratio of Li:Fe:P (1.01:0.99:1.00, within the error range of ICP), and elemental analysis revealed that the C-LFP composite contained 6.0 wt% carbon. The XRD spectra of the synthesized samples are shown in Fig. 2. The structures of the two different samples were identified as belonging to the Pnma space group with the orthorhombic olivine structure. The diffraction patterns obtained for the two samples are in close
Conclusion
Olivine-LFP materials (pure LFP and C-LFP) were synthesized by a modified mechanical activation process followed by firing, and orange peel was used as the carbon precursor in the case of C-LFP. The effects of coating LFP with biomass-derived carbon on the physical and electrochemical performance of the resulting composite were investigated by X-ray diffraction, FT-IR, TGA, SEM, TEM, and galvanostatic charge-discharge analyses. The initial addition of dried orange peel, along with the other
Acknowledgements
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017M1A2A2087577 and 2018R1A4A1024691). This research was partially supported by the Cheongju University Research Scholarship Grants in 2017.
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