Article pubs.acs.org/cm
Synthesis of Nanometric LiMnPO4 via a Two-Step Technique Maja Pivko,†,‡ Marjan Bele,†,‡ Elena Tchernychova,†,‡ Nataša Zabukovec Logar,†,‡ Robert Dominko,†,* and Miran Gaberscek†,‡,§ †
National Institute of Chemistry, Hajdrihova 19, SI-1000, Ljubljana, Slovenia CO-NOT, Hajdrihova 19, SI-1000, Ljubljana, Slovenia § Faculty of Chemistry and Chemical Technology, Universtiy of Ljubljana, Askerceva 6, SI-1000 Ljubljana ‡
ABSTRACT: A two-step procedure for preparation of LiMnPO4 with small particle size (15−20 nm) and embedded in a carbon matrix is presented. The crucial point that prevents excessive particle growth is the avoidance of lithium in the first firing step, so that small Mn2P2O7 particles embedded in carbon are obtained. Because of the carbon matrix, the Mn2P2O7 particles also cannot grow in the second step, which involves lithiation and heating to 700 °C in argon. The prepared LiMnPO4 shows a high theoretical capacity (up to 95% of the theoretical value) and a stable cycling (>130 mAh/ g, even after 100 cycles at 55 °C and a rate of C/20). At room temperature and using the CC−CV mode, the performance is comparable to the best result shown in the literature so far. Finally, the performance of LiMnPO4 is briefly compared with that of LiFePO4. KEYWORDS: two step synthesis, Mn2P2O7, LiMnPO4, kinetics, Li-ion batteries
1. INTRODUCTION During the past decade, polyoxyanion cathode materials have been widely studied as cathode materials for Li-ion batteries.1−9 In particular, LiFePO4, which is the most researched among these compounds, possesses inherent chemical stability and thus improved safety, compared to the classical transition-metal oxide cathode materials. Since 1997, intense research activities have pushed the performance of LiFePO4 from a barely electrochemically active material to a material that can now deliver full capacity at very high cycling rates. This was possible through the use of a combined approach of particle size reduction and proper spatial arrangements of individual phases (active material, carbon, binder, pores, etc.) that constitute a typical cathode. Given the great success of the LiFePO4 and the knowledge gained thereof, it is hard to understand why the development of the manganese analogue has lagged far behind. Namely, an obvious advantage of LiMnPO4 is its redox potential (4.1 V vs Li/Li+), which is 0.7 V higher than that of LiFePO4 but still within the typical electrolyte stability window. However, the kinetics of LiMnPO4 are unusually sluggish, possibly due to the intrinsically low ionic and electronic conductivity and the interfacial strain between the lithiated and delithiated phase.10−12 These limitations could be overcome by the use of very small particles (probably on the order of 10 nm) embedded in a conductive (electronically and ionically) matrix. At the least, such recipes have worked for LiFePO4, but also for silicates13−15 or transition-metal titanium oxides.16 This way, the supply of both charge carriers to the active surface area is maximal, while the solid-state diffusion paths are significantly reduced. © 2012 American Chemical Society
To achieve the desired composite architecture, one needs to focus on the material synthesis since the particle size, the degree of particle agglomeration as well as the local distribution of conductive “phases” (native carbon, pores etc.) are difficult to regulate once the composite materials have been synthesized. Several different types of synthesis of LiMnPO4 have been proposed up to date. Among the most promising seem to be the direct precipitation of LiMnPO4,17,18 the sol−gel synthesis,19,20 the polyol synthesis,21,22 ceramic synthesis,23,24 the ionothermal synthesis,25 the spray pyrolysis followed by wetball milling26 and the ultrasonic spray pyrolysis followed by ball milling.12 Whereas the low-temperature treatments typically lead to small and uniform active,17,21,24 any heating step to higher temperatures usually leads to uncontrolled particle agglomeration and/or particle growth. Both latter phenomena have a negative effect on the reversible capacity and the rate capability of LiMnPO4. Note, however, that, in cases involving native carbon, a treatment up to 700 °C is needed in order to reach sufficient carbon conductivity. With regard to the sol−gel approach, simultaneous synthesis and carbon coating formation typically leads to even lower electrochemical performance of LiMnPO4.20 On the other hand, it is known from ceramic processing27 that light elements, such as lithium, increase the particle growth rate at increased temperatures, because of the increased diffusion rate of the light element. In this paper, we present a novel approach toward a Received: October 14, 2011 Revised: February 7, 2012 Published: February 7, 2012 1041
dx.doi.org/10.1021/cm203095d | Chem. Mater. 2012, 24, 1041−1047
Chemistry of Materials
Article
Figure 1. Schematic presentation of the synthesis procedure. microscopy (TEM/STEM) system that was equipped with an energy-dispersive X-ray (SiLi) detector. The TGA−DTA measurements were performed in air flow at a heating rate of 5 °C min−1 with a simultaneous thermal analysis system (Model STA 409, Netzsch, Selb, Germany). Preparation of the Electrodes and Electrochemical Tests. Electrode composites of active material, (carbon black, CB) and polyvinyldene fluoride (PVdF) in the ratio of 8:1:1 were prepared by ball milling the composite mixture at 300 rpm for 30 min. The obtained slurry was cast onto a circular aluminum foil with a diameter of 16 mm (2 cm2). Before use, the electrodes with a loading of 2−3 mg were dried under vacuum at 90 °C for at least 12 h. The electrodes were then transferred and kept in a glovebox (
