Materials and Product EngineeringInfluence of synthesis parameters on the properties of LiFePO4/C cathode material☆
Graphical abstract
The choice of dispersive agent in ball-milling is critical in determining the existent state of amorphous carbon in final product LiFePO4/C. The dispersion state of ball-milled Fe2O3, NH4H2PO4, Li2CO3 and glucose in acetone is greatly different from those in ethanol and water for their different molecular polarities. Meanwhile, acetone has a much lower boiling point of 56.2 °C than ethanol and water, which ensures a faster drying of the ball-milled pulp to alleviate severe segregation of ingredients. An ideal and more even distribution of amorphous carbon in LiFePO4/C synthesized using acetone in place of ethanol as the dispersive agent in ball-milling leads to a superior electrochemical performance.
Introduction
The issues of energy crisis and pollution are formidable to mankind and a sustainable development needs green energy supply. The burning of fossil fuels for heat and electricity generation has long seriously contributed to the rise in CO2 concentration in atmosphere, resulting in drastic climate changes worldwide. At present, electric vehicles are regarded as the solution to CO2 emission reduction through higher energy efficiency by making use of regenerative braking [1]. However, this reduction is limited by the present electrical energy supply nearly 70% of which is generated by burning fossil fuels [2]. Thus, the adoption of electric vehicles in cities to a certain degree can only transfer urban pollution to places where electricity is generated [3]. The introduction of a green grid is the ultimate solution, but the solar and wind power is unstable due to weather changes and as a result causes fluctuations on the grid. Therefore, the increase in percentage of renewable power on the grid depends on the successful large-scale stationary storage of electrical energy [4].
The lithium ion cell outperforms other battery systems, such as lead–acid, Ni–Cd and Ni–MH, in many aspects, for example, cell voltage, gravimetric and volumetric energy density/power, cycle life and so on [5]. Among the cathode materials for lithium ion cells, olivine-structured LiFePO4 holds the desirable merits of abundant raw materials, non-toxicity, high thermal stability, suitable voltage of 3.45 V (vs Li+/Li) and theoretical capacity of 170 mA·h·g− 1 [6], [7], [8]. It meets both demands of high energy density and environmental friendliness and is an adequate cathode for power battery and stationary storage of electrical energy generated by renewable power [9]. The worst drawback of the cathode is its intrinsic low electronic conductivity and a viable solution is the application of LiFePO4/C composite [10].
The adoption of Fe2 + source FeC2O4·2H2O for LiFePO4/C synthesis via solid state reaction possesses advantageous attributes of simple procedure and product with good electrochemical performance, which is in particular suitable for mass production of the cathode material [11]. But FeC2O4·2H2O, along with FePO4·2H2O and Fe3(PO4)2·8H2O, contains crystal water, which tends to be lost and brings about the need of chemical analysis for stoichiometric use in LiFePO4/C preparation. In addition, the oxidation of Fe2 + in FeC2O4·2H2O and Fe3(PO4)2·8H2O results in the same issue. Fe2O3 is chemically stable, consists of no crystallized water and is an ideal iron source for producing LiFePO4/C. The ferric iron in Fe2O3 must be reduced in LiFePO4 formation, which is generally realized by use of carbon or carbon-containing reductants [12]. In the study, Fe2O3, NH4H2PO4, Li2CO3 and glucose (C6H12O6·H2O) are applied to LiFePO4/C synthesis and effects of synthesis parameters on the properties of LiFePO4/C are investigated. The interesting results of low sintering temperature favoring carbon maintenance in LiFePO4/C and ball-milling dispersive agent affecting the properties of LiFePO4/C are for the first time reported by the work.
Section snippets
Experimental
Stoichiometric Fe2O3, NH4H2PO4 and Li2CO3 and certain amount of glucose were mixed and ball-milled for 4 h in a dispersive agent to ensure homogenous mixing. The pulp was dried at 80 °C overnight to vaporize volatile components. The dried mixture was pressed in a crucible and sintered for 15 h in an argon atmosphere. Changes in sintering temperature, carbon content and dispersive agent were carried out for LiFePO4/C synthesis. Carbon content of LiFePO4/C was determined by dissolving LiFePO4/C in
Results and Discussion
Fig. 1 indicates that LiFePO4 starts crystallizing even below 300 °C. Fe2O3, NH4H2PO4, Li2CO3 and glucose can be completely converted into LiFePO4/C at 500 °C below which the main impurities in the sintered products are Fe2O3, Li3PO4 and Li3Fe2(PO4)3. In particular at 300 °C, Li3Fe2(PO4)3 is detected, which is similar to the work by Ravet and co-workers. in which FePO4·2H2O instead of Fe2O3 and NH4H2PO4 was used as the Fe and PO4 sources [13]. For convenience, the samples obtained at 500 °C, 600 °C,
Conclusions
From above, it can be concluded that sintering temperature and carbon content must be carefully optimized for synthesizing LiFePO4/C with superior electrochemical performance and tap density. The increase in sintering temperature leads to a higher degree of crystallinity and bigger crystallite/particle size for LiFePO4, more Fe2P formation and lower carbon content in LiFePO4/C. 700 °C is the optimum sintering temperature. Higher carbon content in the range of 4.48%–11.03% leads to better rate
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