Sucrose-aided combustion synthesis of nanosized LiMn1.99−yLiyM0.01O4 (M = Al3+, Ni2+, Cr3+, Co3+, y = 0.01 and 0.06) spinels: Characterization and electrochemical behavior at 25 and at 55 °C in rechargeable lithium cells
Introduction
Several manganese oxides have been considered as electrode materials for various types of batteries, including rechargeable lithium ion batteries [1], [2]. Among them, lithium manganese oxide spinel LiMn2O4 is particularly attractive as a promising positive electrode active material, hereafter named cathode material, for the new generations of Li-ion batteries. It is a mixed ionic–electron conductor that intercalates reversibly Li+ ions at about 4 V [3]. The LiMn2O4 itself is cheap, non-toxic and can be easily prepared. However it shows a capacity fade on cycling at room temperature, and a more severe one at high temperature, ≈50 °C [4], [5], [6], [7], [8], [9], [10]. Causes of the capacity fade are so far complex and several reasons have been given to account for it, among them: (i) dissolution of Mn2+ formed by the disproportionation reaction 2Mn(solid)3+ → Mn(solid)4+ + Mn(solution)2+ due to the acid attack of the HF generated by hydrolysis of the LiPF6 of the electrolyte [1], [5], [6], [7], [8]; (ii) the onset of the Jahn–Teller effect at the end of the discharge on the surface of the particles [8]; (iii) mechanical instability generated by structural changes occurring during Li+-de/insertion in the charge/discharge steps [9], [10]. It is possible that the three phenomena contribute simultaneously to the capacity fade of the LiMn2O4 electrode. It has been shown that doping with several foreign cations such as Li+, Ni2+, Al3+, Cr3+, Co3+, etc. improves the cycling performances of the LiMn2O4-based electrodes [1], [10], [11], [12], [13], [14], [15]. In fact, substitution for Mn3+ by other metals decreases the dissolution of the LiMn2O4-based spinels [7], [16], suppresses the Jahn–Teller effect [17], [18], [19], [20] and reduces the volume difference between the charged/discharged structures of the doped spinel [9], [10], [12]. Recently, it has been demonstrated that double doping, particularly when Li+ is one of the doping cations, improves the cycling performance of the LiMn2O4-based cathodes [10], [21].
For practical applications, high rate capability is one of the most important characteristics of electrode materials. It has been shown that nanostructured electrodes are able to drain high capacity at high currents, because diffusion path of Li+ in the solid is significantly smaller than in electrodes of the same materials but with higher particle size [22], [23], [24], [25], [26], [27]. We think that it is plausible to assume that to develop doubly doped spinels with nanometer size would be helpful in improving both the rate capability and the cyclability of the cathodes.
Aimed to obtain LiMn2O4-based cathodes for Li-ion batteries with high electrochemical performances both at 25 and 55 °C, we have synthesized nanosized doubly doped LiMn1.99−yLiyM0.01O4 spinels with two different Li-excess content, y = 0.01 and 0.06. Several Mn+-dopant cations (M = Al3+, Ni2+, Cr3+, Co3+) has been tested out. The samples have been obtained by an original, competitive, cheap and straightforward sucrose-aided combustion method that affords homogeneous and single phase compounds. The samples obtained have been characterized by X-ray powder diffraction (XRD), differential and thermogravimetric analysis (DTA/TG) and transmission electron microscopy (TEM). The influence of the Li-excess, the Mn+-dopant cation and the amount of fuel used in the combustion synthesis on the electrochemical behavior of the spinels in a lithium cell at room and at elevated temperature (55 °C) has been studied.
Section snippets
Experimental
The LiMn1.99−yLiyM0.01O4 (M = Al3+, Ni2+, Cr3+, Co3+, y = 0.01 and 0.06) spinels have been synthesized by the sucrose-aided combustion method [22], from reagent grade Li2CO3, Ni(NO3)2·6H2O, Al(NO3)3·9H2O, Co(NO3)2·6H2O and Cr(NO3)3·9H2O. Mn metal dissolved in 0.25 M HNO3 was used as manganese source. Preset amounts of dried Li2CO3 were dissolved in a small amount of diluted HNO3 to avoid vigorous evolution of CO2. A stoichiometric amount of the previously dissolved Mn was added to the Li-containing
Structural, thermal and morphological characterization
The XRD patterns obtained for the “as prepared” samples are practically identical. They show broad diffraction lines, which can be fully indexed in the Fd3m space group. As an example, patterns recorded for the LiMn1.99−yLiyM0.01O4 (M = Ni2+, Al3+; y = 0.01 and 0.06) spinels are presented in Fig. 1. Values of lattice parameter, ac, determined for these samples are summarized in Table 1. Lattice parameters determined for the Cr3+ and Co3+-doped spinels are slightly larger than those determined for
Conclusions
The sucrose aided combustion synthesis is an attractive, powerful and straightforward method for the synthesis of nanosized LiMn2O4-based spinels with complex composition, as the LiMn1.99−yLiyM0.01O4 (M = Al, Ni, Cr, Co, y = 0.01, 0.06) compounds synthesized in this work. Our results point out that the amount of fuel (sucrose) in the reaction mixture controls the particle size and the composition of the “as prepared” spinels. Further thermal treatment at 700 °C of the “as prepared” samples yields
Acknowledgements
Financial support through project MAT 2008-03182 (MICINN), and the joint project CSIC-Bulgarian Academy of Sciences no. 2007BG0018 is gratefully recognized.
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Enhanced cycle and rate performances of Li(Li<inf>0.05</inf>Al<inf>0.05</inf>Mn<inf>1.90</inf>)O<inf>4</inf> cathode material prepared via a solution combustion method for lithium-ion batteries
2017, Solid State IonicsCitation Excerpt :It is reported that Li-rich spinel with Al doping can notably improve the cycling stability and elevated-temperature cyclic performance of LiMn2O4 [20]. Recently, some groups focus on the preparation and electrochemical performances by adopting the (Li, Al)-co-doping via the different synthesis methods, resulting in making a significant progress and obtaining the superior electrochemical properties [20–23]. Prabu et al. [20] prepared the Li(Li0.1Al0.1Mn1.8)O4 at different temperatures by polymer precursor method using polyvinyl pyrrolidone (PVP) and PVP acted as a capping agent, and the phases prepared at 650 °C showed a reversible capacity of 114 mAh g− 1 with a 92% capacity retention of it after 50 cycles at 55 °C and 0.5C.