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Electrochemical properties of nano- and micro-sized LiNi0.5Mn1.5O4 synthesized via thermal decomposition of a ternary eutectic Li–Ni–Mn acetate
Electrochimica Acta 55 (2010) 832–837
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of nano- and micro-sized LiNi0.5 Mn1.5 O4 synthesized via thermal decomposition of a ternary eutectic Li–Ni–Mn acetate X. Fang, Y. Lu, N. Ding, X.Y. Feng, C. Liu, C.H. Chen ∗ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 4 July 2009 Received in revised form 15 September 2009 Accepted 15 September 2009 Available online 22 September 2009 Keywords: Lithium nickel manganese oxide Eutectic Low temperature performance Capacity retention Lithium battery
a b s t r a c t Nano- and micro-sized LiNi0.5 Mn1.5 O4 particles are prepared via the thermal decomposition of a ternary eutectic Li–Ni–Mn acetate. Lithium acetate, nickel acetate and manganese acetate can form a ternary eutectic Li–Ni–Mn acetate below 80 ◦ C. After further calcination, nano-sized LiNi0.5 Mn1.5 O4 particles can be obtained at an extremely low temperature (500 ◦ C). When the sintering temperature goes above 700 ◦ C, the particle size increases, and at 900 ◦ C micro-sized LiNi0.5 Mn1.5 O4 particles (with a diameter of about 4 m) are obtained. Electrochemical tests show that the micro-sized LiNi0.5 Mn1.5 O4 powders (sintered at 900 ◦ C) exhibit the best capacity retention at 25 ◦ C, and after 100 cycles, 97% of initial discharge capacity can still be reached. Nano-sized LiNi0.5 Mn1.5 O4 powders (sintered at 700 ◦ C) perform the best at low temperatures; when cycled at −10 ◦ C and charged and discharged at a rate of 1 C, nano-sized LiNi0.5 Mn1.5 O4 powders can deliver a capacity as high as 110 mAh g−1 . © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries are one of the most successful power sources and have dominated the portable electronic device market for the past two decades. Nonetheless, to keep up with the fast development of the laptop central processing unit and the wide application of 3G techniques to cell phones, people have continued to search for new electrode materials with higher capacities and more power. Compared to traditional cathode materials, such as LiCoO2 (3.9 V), LiMn2 O4 (4.1 V) and LiFePO4 (3.5 V), spinel LiNi0.5 Mn1.5 O4 has a higher voltage (4.7 V) [1]. In 1996, LiNi0.5 Mn1.5 O4 was first reported to be a 3 V cathode material by Amine et al. [2]. Later, Dahn and coworkers discovered the 4.7 V voltage plateau of LiNi0.5 Mn1.5 O4 [3]. The theoretical capacity of LiNi0.5 Mn1.5 O4 is 146.7 mAh g−1 ; due to its high working potential, the energy density of LiNi0.5 Mn1.5 O4 is 20% higher than that of LiCoO2 . Thus, LiNi0.5 Mn1.5 O4 is seen as a potential cathode material for use in electric vehicles and energy storage systems in the future. A variety of synthetic methods for the preparation of LiNi0.5 Mn1.5 O4 have been reported; these include solid state reaction [4], sol–gel [5], co-precipitation [6,7], spray pyrolysis [8,9], electrophoretic deposition [10] and pulsed laser deposition [11]. Kim et al. [12] prepared the well-defined octahedral LiNi0.5 Mn1.5 O4 by the molten salt method starting with the mixture of LiCl
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.09.046
and Li/Ni/Mn hydroxides; the synthesized LiNi0.5 Mn1.5 O4 powders show excellent cycling performance. In this study, we chose to use acetates as starting materials; these can form a ternary eutectic Li–Ni–Mn acetate below 80 ◦ C. Though acetates have been widely used in the literature, most studies used a wet-chemical route assisted by organic materials, such as acrylic acid [13], citric acid [14], poly(ethylene glycol) [15] and poly(methyl methacrylate) [16]. There are also some reports of studies using the so-called “sol–gel” method. This method involves first dissolving the acetates in water and then evaporating the water to obtain a “gel” [17,18]. We believe that the above method should produce the same ternary Li–Ni–Mn acetate eutectic instead of the real gel. It should be mentioned that prior to our work, Lafont et al. [19] observed this eutectic phenomenon of Li–Ni–Mn acetate, which they called “green slurry,” but no extensive investigation was conducted. In Lafont’s work, the capacity at 2 C was observed to be only about 60 mAh g−1 . Our work shows that by simply optimizing the sintering temperature of the ternary eutectic Li–Ni–Mn acetate, we can obtain nano- and microsized LiNi0.5 Mn1.5 O4 particles with a capacity of about 100 mAh g−1 at 8 C. 2. Experimental procedures We mixed 2.488 g (10 mmol) nickel acetate (Ni(Ac)2 ·4H2 O), 7.352 g (30 mmol) manganese acetate (Mn(Ac)2 ·4H2 O) and 2.146 g (21 mmol) lithium acetate (LiAc·2H2 O) and milled the mixture by hand in a mortar. Then, the mixture was calcined at 300 ◦ C for 5 h. After milling by hand again, the powders were sintered in air at 300,
X. Fang et al. / Electrochimica Acta 55 (2010) 832–837
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Fig. 1. Images of the formation of a ternary eutectic Li–Ni–Mn acetate: (a) mechanical mixture of Li–Ni–Mn acetates; (b) Mn acetate (bottom portion) and Ni acetate (top portion); (c) the mechanical mixture (left) and Mn–Ni acetates without mixing at 80 ◦ C; (d) lying bottle of Li–Ni–Mn acetate-mixture at 80 ◦ C.
400, 500, 600, 700, 800, 900 and 1000 ◦ C for 10 h (with a heating rate of 3 ◦ C min−1 ) and allowed to cool naturally. The crystalline structures of the powders were characterized by X-ray diffraction (XRD, Philips X’Pert Pro Super, Cu K␣ radiation) with 2 in the range from 10◦ to 80◦ . The morphology and composition of the powders were determined by scanning electronic microscopy (SEM, JEOL-6970). The electrochemical characteristics of the products were evaluated with coin cells (CR2032 size) of Li/1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC, with a weight ratio of 1:1)/LiNi0.5 Mn1.5 O4 assembled in an argon-filled glove box (MBraun Labmaster 130). The positive electrode laminate was composed of LiNi0.5 Mn1.5 O4 (84 wt.%), acetylene black (8 wt.%) and poly(vinylidene fluoride) (PVDF, 8 wt.%). The cells were tested on a multi-channel battery test system (Shenzhen Neware Co. Ltd.) between 2.8 and 5.1 V (vs. Li+ /Li).
can be attributed to the formation of a ternary eutectic system, which guarantees the mixing of Li, Ni and Mn atoms at the atomic level. The TG-DTA curves of the Li–Ni–Mn acetate-mixture are shown in Fig. 2. There are several thermal steps: (i) the endothermic peak at 54 ◦ C is a result of the formation of a ternary eutectic solution; (ii) the endothermic peak at 118 ◦ C results from the loss of crystallization water, accompanied by a weight loss of 26% (the theoretical weight loss is 27.7%); (iii) the exothermic peak at 345 ◦ C is related to the decomposition of acetate, with a 41.4% weigh loss (the theoretical weight loss is 37.2%); (iv) the weight is nearly constant when the temperature is above 370 ◦ C; (v) the small endothermic reac-
3. Results and discussion 3.1. Ternary eutectic Li–Ni–Mn acetate The melting points of LiAc·2H2 O and MnAc2 ·4H2 O are 70 and 80 ◦ C, respectively. Unlike lithium and manganese acetates, NiAc2 ·4H2 O can be directly decomposed, rather than being melted, with an increase in temperature. In our experiment, we observed that a mixture of LiAc·2H2 O, MnAc2 ·4H2 O and NiAc2 ·4H2 O can form a ternary eutectic solution of Li–Ni–Mn acetate at 80 ◦ C (Fig. 1a, c and d). On the other hand, when MnAc2 ·4H2 O and NiAc2 ·4H2 O are put in a bottle one after another without mixing, only MnAc2 ·4H2 O melts at an elevated temperature while NiAc2 ·4H2 O remains a solid (Fig. 1b and c). Thus, the melting of the Li–Ni–Mn acetate-mixture
Fig. 2. TG-DTA curves of the ternary eutectic Li–Ni–Mn acetate.
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tion at 750 ◦ C is a result of the emission of oxygen in LiNi0.5 Mn1.5 O4 (LiNi0.5 Mn1.5 O4−ı ) [20]. 3.2. Structure and morphology The XRD patterns of the samples sintered at different temperatures (from 200 to 1000 ◦ C) are shown in Fig. 3. Due to the similar diffraction patterns of the spinel and layered structures, it is difficult to differentiate these structures for the samples sintered at low temperatures (below 500 ◦ C). We believe that the samples sintered at high temperatures (beyond 500 ◦ C) are of a pure spinel structure. This conclusion is supported by the electrochemical test, as shown below. The peak width at half-height decreases with an increase in the sintering temperature, which indicates improved crystallinity. The SEM images of the as-synthesized samples are shown in Fig. 4. When the temperature is below 700 ◦ C, nanoparticles are obtained. When the temperature increases to 800 ◦ C, the particles grow; at 900 ◦ C, the particle size is about 4 m. When the temperature increases to 1000 ◦ C, the particle size further increases to about 10 m. There are some small particles on the surface of the sample sintered at 1000 ◦ C; these particles may have arisen
Fig. 3. XRD patterns of the samples sintered at different temperatures (from 200 to 1000 ◦ C).
Fig. 4. SEM images of the as-synthesized samples.
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Fig. 5. Galvanostatic charge–discharge curves (2nd cycle) of the as-synthesized samples (cycled at 25 ◦ C, charged and discharged at a rate of 1/3 C).
from the decomposition of LiNi0.5 Mn1.5 O4 at high temperature [21]. 3.3. Electrochemical performance The galvanostatic charge–discharge curves (2nd cycle) of the as-synthesized samples are shown in Fig. 5. The sample sintered at 300 ◦ C has a typical voltage profile of a layered-structure cathode (e.g. LiMnO2 ) with a voltage plateau at around 4.0 V [22,23]. This result indicates that Mnn+ ions are only oxidized to trivalence (LiMnO2 ) at 300 ◦ C. When the sintering temperature increases to 400 ◦ C, a voltage plateau at 4.7 V appears, indicating the formation of LiNi0.5 Mn1.5 O4 . At around 3 V another voltage plateau appears that might be due to the transition from LiMn2 O4 to Li2 Mn2 O4 [24,25]. The powder sintered at 400 ◦ C may still have some trivalent Mn ions. It should be mentioned that in LiNi0.5 Mn1.5 O4 , all Mn ions should be tetravalent. When the sintering temperature increases to 500 ◦ C, the plateau at 3 V almost disappears, indicating that the impurity in LiMn2 O4 has been converted into LiNi0.5 Mn1.5 O4 . A short voltage plateau at around 4.1 V is still observed, however. This plateau may be attributed to the transition from Mn3+ to Mn4+ in LiNi0.5 Mn1.5 O4 [26]; thus, the real composition should be written as LiNi0.5 Mn1.5 O4−ı . The plateau at 3 V completely disappears in the sample sintered at 600 ◦ C and due to the oxidation of Mn3+ , the plateau at 4.1 V shortens. The sample sintered at 700 ◦ C can deliver a capacity of as high as 127 mAh g−1 . When the sintering temperature is above 700 ◦ C, the particle size increases significantly to a submicron-size (as shown in Fig. 4). The sample sintered at 800 ◦ C can only deliver a capacity of 121 mAh g−1 . This result has been repeated by sintering the sample again. The capacity reduction at 800 ◦ C is likely due to the growth of particles, which increases the length of the lithium diffusion path. It is also observed that the plateau at 4.1 V lengthens, due to the reduction of some Mn4+ ions to Mn3+ ions at high temperatures [21]. During the reduction
of Mn4+ ions, the concentration of oxygen vacancy also increases, leading to an increase in electronic conductivity. Nevertheless, the increase of electronic conductivity cannot completely compensate for the adverse effect of increasing the length of the diffusion path, which reduces capacity. As the temperature further increases to 900 ◦ C, the as-synthesized sample delivers the highest capacity of 129.2 mAh g−1 , yet 16.6% capacity comes from the 4.1 V plateau. As a result, the overall output energy is still lower than that from the sample sintered at 700 ◦ C, for which only 7.2% capacity comes from the 4.1 V plateau. The sample sintered at 1000 ◦ C exhibits a similar voltage profile as that sintered at 900 ◦ C, with a capacity of 125 mAh g−1 . Fig. 6 shows the cycling performance of the nano- and microLiNi0.5 Mn1.5 O4 sintered at different temperatures, charged and discharged at a rate of 1 C, cycled between 2.8 and 5.1 V at room
Fig. 6. Cycling performance of nano- and micro-LiNi0.5 Mn1.5 O4 (at 1 C).
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Fig. 7. Rate performance of nano-700 and micro-900 at 25 ◦ C. Fig. 8. Galvanostatic charge–discharge curves (2nd cycle) of nano-700 and micro900 (cycled at −10 ◦ C, charged and discharged at a rate of 1/3 C).
temperature. Obviously, the sample sintered at 900 ◦ C (micro-900) exhibits the highest capacity and the best capacity retention during cycling; after 100 cycles, 97% of initial discharge capacity can still be reached, with a capacity loss of less than 0.04 mAh g−1 per cycle. Although the sample sintered at 700 ◦ C (nano-700) also delivers a high capacity, the capacity retention is not as good as that of micro-900. The large surface area of the nano-700 accelerates the dissolution of Ni and Mn ions, which results in the loss of capacity of LiNi0.5 Mn1.5 O4 during cycling [27,28]. The ability to retain capacity during cycling is: micro-900 ≈ submicro-800 > nano-700 > nano600. As nano-700 and micro-900 exhibit higher capacities and relatively better capacity retention, their rate capability was further investigated. The rate performance at 25 ◦ C of nano-700 and micro900 (charged at a rate of 1 C) is shown in Fig. 7. Both nano- and micro-sized LiNi0.5 Mn1.5 O4 particles exhibit an excellent rate capability, with a capacity of about 100 mAh g−1 at 8 C. At the low rate, micro-900 displays a better rate capability, though when the rate increases to 6 C, nano-700 is better. At the low rate, the rate-limiting factor for the rate performance is the electronic conductivity, which can be enhanced by the formation of oxygen vacancy at the higher sintering temperature; thus, micro-900 is better under this condition; at the high rate, the limiting factor is the lithium diffusion in LiNi0.5 Mn1.5 O4 , and thus nano-700, which has a shorter diffusion path, is better. Again, for long-time cycling, micro-900 has a better capacity retention than nano-700. As a high energy density cathode material, LiNi0.5 Mn1.5 O4 is seen as a potential cathode material for electric vehicles and hybrid electric vehicles in the future. Thus, it is necessary to test its performance at low temperatures. We investigated the performance of nano-700 and micro-900 at −10 ◦ C. Fig. 8 shows the galvanostatic charge–discharge curves (2nd cycle) of nano-700 and micro-900. The voltage plateau still remains at about 4.6 V, without a significant drop like that found in LiMn2 O4 [29]. Fig. 9 shows the rate performance of the two samples cycled at −10 ◦ C, charged at a rate of 1 C (except for the first 10 cycles, during which the cells were charged and discharged at a rate of 1/3 C). Nano-700 was found to have a higher capacity, and at 1 C the capacity was still about 110 mAh g−1 . The capacity retention of nano-sized LiNi0.5 Mn1.5 O4 at low temperatures is much better than those traditional cathode materials such as LiMn2 O4 [29], LiCoO2 [30], LiFePO4 [31,32], and even V2 O5 nanofibers which has been reported to be fast Li-ion conductor [33,34]. For micro-900, the capacity was found to be only about 70 mAh g−1 at 1 C. Thus, we believe that nano-sized LiNi0.5 Mn1.5 O4 perform better at low temperatures.
Fig. 9. Rate performance of nano-700 and micro-900 at −10 ◦ C.
4. Conclusions Nano- and micro-sized LiNi0.5 Mn1.5 O4 particles are successfully synthesized via the thermal decomposition of a ternary eutectic Li–Ni–Mn acetate. When the sintering temperature is below 800 ◦ C, nano-sized LiNi0.5 Mn1.5 O4 powders are obtained. When the sintering temperature further increases, the particles grow, and finally, micro-sized LiNi0.5 Mn1.5 O4 powders are obtained. Electrochemical tests at room temperature show that micro-sized LiNi0.5 Mn1.5 O4 powders sintered at 900 ◦ C have the best capacity retention when cycled at 25 ◦ C; after 100 cycles, 97% of initial discharge capacity can still be reached. Nevertheless, nano-sized LiNi0.5 Mn1.5 O4 powders sintered at 700 ◦ C exhibit a higher capacity at low temperatures, and at a rate of 1 C it can still deliver a capacity of 110 mAh g−1 at −10 ◦ C. Both nano- and micro-sized LiNi0.5 Mn1.5 O4 particles exhibit excellent rate capabilities, with a capacity of about 100 mAh g−1 at 8 C (at room temperature).
Acknowledgements This study was supported by National Science Foundation of China (grant no. 20971117), the Education Department of Anhui Province (grant no. KJ2009A142) and the Solar Energy Operation Plan of Academia Sinica.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical properties of nano- and micro-sized LiNi0.5 Mn1.5 O4 synthesized via thermal decomposition of a ternary eutectic Li–Ni–Mn acetate X. Fang, Y. Lu, N. Ding, X.Y. Feng, C. Liu, C.H. Chen ∗ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 4 July 2009 Received in revised form 15 September 2009 Accepted 15 September 2009 Available online 22 September 2009 Keywords: Lithium nickel manganese oxide Eutectic Low temperature performance Capacity retention Lithium battery
a b s t r a c t Nano- and micro-sized LiNi0.5 Mn1.5 O4 particles are prepared via the thermal decomposition of a ternary eutectic Li–Ni–Mn acetate. Lithium acetate, nickel acetate and manganese acetate can form a ternary eutectic Li–Ni–Mn acetate below 80 ◦ C. After further calcination, nano-sized LiNi0.5 Mn1.5 O4 particles can be obtained at an extremely low temperature (500 ◦ C). When the sintering temperature goes above 700 ◦ C, the particle size increases, and at 900 ◦ C micro-sized LiNi0.5 Mn1.5 O4 particles (with a diameter of about 4 m) are obtained. Electrochemical tests show that the micro-sized LiNi0.5 Mn1.5 O4 powders (sintered at 900 ◦ C) exhibit the best capacity retention at 25 ◦ C, and after 100 cycles, 97% of initial discharge capacity can still be reached. Nano-sized LiNi0.5 Mn1.5 O4 powders (sintered at 700 ◦ C) perform the best at low temperatures; when cycled at −10 ◦ C and charged and discharged at a rate of 1 C, nano-sized LiNi0.5 Mn1.5 O4 powders can deliver a capacity as high as 110 mAh g−1 . © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries are one of the most successful power sources and have dominated the portable electronic device market for the past two decades. Nonetheless, to keep up with the fast development of the laptop central processing unit and the wide application of 3G techniques to cell phones, people have continued to search for new electrode materials with higher capacities and more power. Compared to traditional cathode materials, such as LiCoO2 (3.9 V), LiMn2 O4 (4.1 V) and LiFePO4 (3.5 V), spinel LiNi0.5 Mn1.5 O4 has a higher voltage (4.7 V) [1]. In 1996, LiNi0.5 Mn1.5 O4 was first reported to be a 3 V cathode material by Amine et al. [2]. Later, Dahn and coworkers discovered the 4.7 V voltage plateau of LiNi0.5 Mn1.5 O4 [3]. The theoretical capacity of LiNi0.5 Mn1.5 O4 is 146.7 mAh g−1 ; due to its high working potential, the energy density of LiNi0.5 Mn1.5 O4 is 20% higher than that of LiCoO2 . Thus, LiNi0.5 Mn1.5 O4 is seen as a potential cathode material for use in electric vehicles and energy storage systems in the future. A variety of synthetic methods for the preparation of LiNi0.5 Mn1.5 O4 have been reported; these include solid state reaction [4], sol–gel [5], co-precipitation [6,7], spray pyrolysis [8,9], electrophoretic deposition [10] and pulsed laser deposition [11]. Kim et al. [12] prepared the well-defined octahedral LiNi0.5 Mn1.5 O4 by the molten salt method starting with the mixture of LiCl
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.09.046
and Li/Ni/Mn hydroxides; the synthesized LiNi0.5 Mn1.5 O4 powders show excellent cycling performance. In this study, we chose to use acetates as starting materials; these can form a ternary eutectic Li–Ni–Mn acetate below 80 ◦ C. Though acetates have been widely used in the literature, most studies used a wet-chemical route assisted by organic materials, such as acrylic acid [13], citric acid [14], poly(ethylene glycol) [15] and poly(methyl methacrylate) [16]. There are also some reports of studies using the so-called “sol–gel” method. This method involves first dissolving the acetates in water and then evaporating the water to obtain a “gel” [17,18]. We believe that the above method should produce the same ternary Li–Ni–Mn acetate eutectic instead of the real gel. It should be mentioned that prior to our work, Lafont et al. [19] observed this eutectic phenomenon of Li–Ni–Mn acetate, which they called “green slurry,” but no extensive investigation was conducted. In Lafont’s work, the capacity at 2 C was observed to be only about 60 mAh g−1 . Our work shows that by simply optimizing the sintering temperature of the ternary eutectic Li–Ni–Mn acetate, we can obtain nano- and microsized LiNi0.5 Mn1.5 O4 particles with a capacity of about 100 mAh g−1 at 8 C. 2. Experimental procedures We mixed 2.488 g (10 mmol) nickel acetate (Ni(Ac)2 ·4H2 O), 7.352 g (30 mmol) manganese acetate (Mn(Ac)2 ·4H2 O) and 2.146 g (21 mmol) lithium acetate (LiAc·2H2 O) and milled the mixture by hand in a mortar. Then, the mixture was calcined at 300 ◦ C for 5 h. After milling by hand again, the powders were sintered in air at 300,
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Fig. 1. Images of the formation of a ternary eutectic Li–Ni–Mn acetate: (a) mechanical mixture of Li–Ni–Mn acetates; (b) Mn acetate (bottom portion) and Ni acetate (top portion); (c) the mechanical mixture (left) and Mn–Ni acetates without mixing at 80 ◦ C; (d) lying bottle of Li–Ni–Mn acetate-mixture at 80 ◦ C.
400, 500, 600, 700, 800, 900 and 1000 ◦ C for 10 h (with a heating rate of 3 ◦ C min−1 ) and allowed to cool naturally. The crystalline structures of the powders were characterized by X-ray diffraction (XRD, Philips X’Pert Pro Super, Cu K␣ radiation) with 2 in the range from 10◦ to 80◦ . The morphology and composition of the powders were determined by scanning electronic microscopy (SEM, JEOL-6970). The electrochemical characteristics of the products were evaluated with coin cells (CR2032 size) of Li/1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC, with a weight ratio of 1:1)/LiNi0.5 Mn1.5 O4 assembled in an argon-filled glove box (MBraun Labmaster 130). The positive electrode laminate was composed of LiNi0.5 Mn1.5 O4 (84 wt.%), acetylene black (8 wt.%) and poly(vinylidene fluoride) (PVDF, 8 wt.%). The cells were tested on a multi-channel battery test system (Shenzhen Neware Co. Ltd.) between 2.8 and 5.1 V (vs. Li+ /Li).
can be attributed to the formation of a ternary eutectic system, which guarantees the mixing of Li, Ni and Mn atoms at the atomic level. The TG-DTA curves of the Li–Ni–Mn acetate-mixture are shown in Fig. 2. There are several thermal steps: (i) the endothermic peak at 54 ◦ C is a result of the formation of a ternary eutectic solution; (ii) the endothermic peak at 118 ◦ C results from the loss of crystallization water, accompanied by a weight loss of 26% (the theoretical weight loss is 27.7%); (iii) the exothermic peak at 345 ◦ C is related to the decomposition of acetate, with a 41.4% weigh loss (the theoretical weight loss is 37.2%); (iv) the weight is nearly constant when the temperature is above 370 ◦ C; (v) the small endothermic reac-
3. Results and discussion 3.1. Ternary eutectic Li–Ni–Mn acetate The melting points of LiAc·2H2 O and MnAc2 ·4H2 O are 70 and 80 ◦ C, respectively. Unlike lithium and manganese acetates, NiAc2 ·4H2 O can be directly decomposed, rather than being melted, with an increase in temperature. In our experiment, we observed that a mixture of LiAc·2H2 O, MnAc2 ·4H2 O and NiAc2 ·4H2 O can form a ternary eutectic solution of Li–Ni–Mn acetate at 80 ◦ C (Fig. 1a, c and d). On the other hand, when MnAc2 ·4H2 O and NiAc2 ·4H2 O are put in a bottle one after another without mixing, only MnAc2 ·4H2 O melts at an elevated temperature while NiAc2 ·4H2 O remains a solid (Fig. 1b and c). Thus, the melting of the Li–Ni–Mn acetate-mixture
Fig. 2. TG-DTA curves of the ternary eutectic Li–Ni–Mn acetate.
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tion at 750 ◦ C is a result of the emission of oxygen in LiNi0.5 Mn1.5 O4 (LiNi0.5 Mn1.5 O4−ı ) [20]. 3.2. Structure and morphology The XRD patterns of the samples sintered at different temperatures (from 200 to 1000 ◦ C) are shown in Fig. 3. Due to the similar diffraction patterns of the spinel and layered structures, it is difficult to differentiate these structures for the samples sintered at low temperatures (below 500 ◦ C). We believe that the samples sintered at high temperatures (beyond 500 ◦ C) are of a pure spinel structure. This conclusion is supported by the electrochemical test, as shown below. The peak width at half-height decreases with an increase in the sintering temperature, which indicates improved crystallinity. The SEM images of the as-synthesized samples are shown in Fig. 4. When the temperature is below 700 ◦ C, nanoparticles are obtained. When the temperature increases to 800 ◦ C, the particles grow; at 900 ◦ C, the particle size is about 4 m. When the temperature increases to 1000 ◦ C, the particle size further increases to about 10 m. There are some small particles on the surface of the sample sintered at 1000 ◦ C; these particles may have arisen
Fig. 3. XRD patterns of the samples sintered at different temperatures (from 200 to 1000 ◦ C).
Fig. 4. SEM images of the as-synthesized samples.
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Fig. 5. Galvanostatic charge–discharge curves (2nd cycle) of the as-synthesized samples (cycled at 25 ◦ C, charged and discharged at a rate of 1/3 C).
from the decomposition of LiNi0.5 Mn1.5 O4 at high temperature [21]. 3.3. Electrochemical performance The galvanostatic charge–discharge curves (2nd cycle) of the as-synthesized samples are shown in Fig. 5. The sample sintered at 300 ◦ C has a typical voltage profile of a layered-structure cathode (e.g. LiMnO2 ) with a voltage plateau at around 4.0 V [22,23]. This result indicates that Mnn+ ions are only oxidized to trivalence (LiMnO2 ) at 300 ◦ C. When the sintering temperature increases to 400 ◦ C, a voltage plateau at 4.7 V appears, indicating the formation of LiNi0.5 Mn1.5 O4 . At around 3 V another voltage plateau appears that might be due to the transition from LiMn2 O4 to Li2 Mn2 O4 [24,25]. The powder sintered at 400 ◦ C may still have some trivalent Mn ions. It should be mentioned that in LiNi0.5 Mn1.5 O4 , all Mn ions should be tetravalent. When the sintering temperature increases to 500 ◦ C, the plateau at 3 V almost disappears, indicating that the impurity in LiMn2 O4 has been converted into LiNi0.5 Mn1.5 O4 . A short voltage plateau at around 4.1 V is still observed, however. This plateau may be attributed to the transition from Mn3+ to Mn4+ in LiNi0.5 Mn1.5 O4 [26]; thus, the real composition should be written as LiNi0.5 Mn1.5 O4−ı . The plateau at 3 V completely disappears in the sample sintered at 600 ◦ C and due to the oxidation of Mn3+ , the plateau at 4.1 V shortens. The sample sintered at 700 ◦ C can deliver a capacity of as high as 127 mAh g−1 . When the sintering temperature is above 700 ◦ C, the particle size increases significantly to a submicron-size (as shown in Fig. 4). The sample sintered at 800 ◦ C can only deliver a capacity of 121 mAh g−1 . This result has been repeated by sintering the sample again. The capacity reduction at 800 ◦ C is likely due to the growth of particles, which increases the length of the lithium diffusion path. It is also observed that the plateau at 4.1 V lengthens, due to the reduction of some Mn4+ ions to Mn3+ ions at high temperatures [21]. During the reduction
of Mn4+ ions, the concentration of oxygen vacancy also increases, leading to an increase in electronic conductivity. Nevertheless, the increase of electronic conductivity cannot completely compensate for the adverse effect of increasing the length of the diffusion path, which reduces capacity. As the temperature further increases to 900 ◦ C, the as-synthesized sample delivers the highest capacity of 129.2 mAh g−1 , yet 16.6% capacity comes from the 4.1 V plateau. As a result, the overall output energy is still lower than that from the sample sintered at 700 ◦ C, for which only 7.2% capacity comes from the 4.1 V plateau. The sample sintered at 1000 ◦ C exhibits a similar voltage profile as that sintered at 900 ◦ C, with a capacity of 125 mAh g−1 . Fig. 6 shows the cycling performance of the nano- and microLiNi0.5 Mn1.5 O4 sintered at different temperatures, charged and discharged at a rate of 1 C, cycled between 2.8 and 5.1 V at room
Fig. 6. Cycling performance of nano- and micro-LiNi0.5 Mn1.5 O4 (at 1 C).
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Fig. 7. Rate performance of nano-700 and micro-900 at 25 ◦ C. Fig. 8. Galvanostatic charge–discharge curves (2nd cycle) of nano-700 and micro900 (cycled at −10 ◦ C, charged and discharged at a rate of 1/3 C).
temperature. Obviously, the sample sintered at 900 ◦ C (micro-900) exhibits the highest capacity and the best capacity retention during cycling; after 100 cycles, 97% of initial discharge capacity can still be reached, with a capacity loss of less than 0.04 mAh g−1 per cycle. Although the sample sintered at 700 ◦ C (nano-700) also delivers a high capacity, the capacity retention is not as good as that of micro-900. The large surface area of the nano-700 accelerates the dissolution of Ni and Mn ions, which results in the loss of capacity of LiNi0.5 Mn1.5 O4 during cycling [27,28]. The ability to retain capacity during cycling is: micro-900 ≈ submicro-800 > nano-700 > nano600. As nano-700 and micro-900 exhibit higher capacities and relatively better capacity retention, their rate capability was further investigated. The rate performance at 25 ◦ C of nano-700 and micro900 (charged at a rate of 1 C) is shown in Fig. 7. Both nano- and micro-sized LiNi0.5 Mn1.5 O4 particles exhibit an excellent rate capability, with a capacity of about 100 mAh g−1 at 8 C. At the low rate, micro-900 displays a better rate capability, though when the rate increases to 6 C, nano-700 is better. At the low rate, the rate-limiting factor for the rate performance is the electronic conductivity, which can be enhanced by the formation of oxygen vacancy at the higher sintering temperature; thus, micro-900 is better under this condition; at the high rate, the limiting factor is the lithium diffusion in LiNi0.5 Mn1.5 O4 , and thus nano-700, which has a shorter diffusion path, is better. Again, for long-time cycling, micro-900 has a better capacity retention than nano-700. As a high energy density cathode material, LiNi0.5 Mn1.5 O4 is seen as a potential cathode material for electric vehicles and hybrid electric vehicles in the future. Thus, it is necessary to test its performance at low temperatures. We investigated the performance of nano-700 and micro-900 at −10 ◦ C. Fig. 8 shows the galvanostatic charge–discharge curves (2nd cycle) of nano-700 and micro-900. The voltage plateau still remains at about 4.6 V, without a significant drop like that found in LiMn2 O4 [29]. Fig. 9 shows the rate performance of the two samples cycled at −10 ◦ C, charged at a rate of 1 C (except for the first 10 cycles, during which the cells were charged and discharged at a rate of 1/3 C). Nano-700 was found to have a higher capacity, and at 1 C the capacity was still about 110 mAh g−1 . The capacity retention of nano-sized LiNi0.5 Mn1.5 O4 at low temperatures is much better than those traditional cathode materials such as LiMn2 O4 [29], LiCoO2 [30], LiFePO4 [31,32], and even V2 O5 nanofibers which has been reported to be fast Li-ion conductor [33,34]. For micro-900, the capacity was found to be only about 70 mAh g−1 at 1 C. Thus, we believe that nano-sized LiNi0.5 Mn1.5 O4 perform better at low temperatures.
Fig. 9. Rate performance of nano-700 and micro-900 at −10 ◦ C.
4. Conclusions Nano- and micro-sized LiNi0.5 Mn1.5 O4 particles are successfully synthesized via the thermal decomposition of a ternary eutectic Li–Ni–Mn acetate. When the sintering temperature is below 800 ◦ C, nano-sized LiNi0.5 Mn1.5 O4 powders are obtained. When the sintering temperature further increases, the particles grow, and finally, micro-sized LiNi0.5 Mn1.5 O4 powders are obtained. Electrochemical tests at room temperature show that micro-sized LiNi0.5 Mn1.5 O4 powders sintered at 900 ◦ C have the best capacity retention when cycled at 25 ◦ C; after 100 cycles, 97% of initial discharge capacity can still be reached. Nevertheless, nano-sized LiNi0.5 Mn1.5 O4 powders sintered at 700 ◦ C exhibit a higher capacity at low temperatures, and at a rate of 1 C it can still deliver a capacity of 110 mAh g−1 at −10 ◦ C. Both nano- and micro-sized LiNi0.5 Mn1.5 O4 particles exhibit excellent rate capabilities, with a capacity of about 100 mAh g−1 at 8 C (at room temperature).
Acknowledgements This study was supported by National Science Foundation of China (grant no. 20971117), the Education Department of Anhui Province (grant no. KJ2009A142) and the Solar Energy Operation Plan of Academia Sinica.
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