LiMn2O4 Nanotube as Cathode Material of Second-Level Charge

Because of the nanostructure and preferred orientation, it shows a superfast second-level charge capability as a cathode for aqueous rechargeable lith...
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LiMn2O4 Nanotube as Cathode Material of Second-Level Charge Capability for Aqueous Rechargeable Batteries Wei Tang,†,‡ Yuyang Hou,† Faxing Wang,† Lili Liu,† Yuping Wu,*,† and Kai Zhu‡ †

New Energy and Materials Laboratory (NEML), Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China ‡ Shanghai Institute of Space Power-Sources (SISP), Shanghai Academy of Spaceflight Technology, Shanghai 200233, China S Supporting Information *

ABSTRACT: LiMn2O4 nanotube with a preferred orientation of (400) planes is prepared by using multiwall carbon nanotubes as a sacrificial template. Because of the nanostructure and preferred orientation, it shows a superfast second-level charge capability as a cathode for aqueous rechargeable lithium battery. At the charging rate of 600C (6 s), 53.9% capacity could be obtained. Its reversible capacity can be 110 mAh/g, and it also presents excellent cycling behavior due to the porous tube structure to buffer the strain and stress from Jahn−Teller effects. KEYWORDS: Aqueous rechargeable lithium battery, second-level charge, LiMn2O4 nanotube, (400) planes, cathode

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electrode materials of solar cells and lithium ion batteries were reported by controlling the preparation conditions such as additives and surroundings, and their performance has greatly improved.16−20 According to the above findings, in this Letter we designed a LiMn2O4 nanotube with exposed (400) planes by using carbon nanotubes (CNTs) as a sacrificed template. It shows that the second-level charge performance and even the amount of conductive agent is in the range for normal practical application. Its charge capacity can be 59.3 mAh/g (54% of the capacity) at the charge rate of 600C (6 s) as a cathode material for aqueous rechargeable lithium batteries (ARLBs) in 0.5 M Li2SO4 aqueous solution. The synthesis of the LiMn2O4 nanotube (see Experimental details in Supporting Information) is schematically presented in Figure 1. Multiwall carbon nanotubes (MWCNTs) were dispersed in concentrated nitric acid to remove some impurities and to endow the surface with hydrophilic groups such as −OH and −COOH. Next, the acid-treated MWCNTs were immersed into an aqueous solution of KMnO4 at 85 °C for 1 h and coated with MnO2. Finally, the MnO2-coated MWCNTs were mixed with LiOH, heat treated at 700 °C to remove the MWCNTs, and them the targeted LiMn2O4 nanotubes were achieved.17 Scanning electron micrograph (SEM) and transmission electron micrograph (TEM) of the MnO2-coated MWCNTs are shown in Figure 2a,b respectively. It is clear that the MWCNTs were fully and uniformly coated with MnO2, and the MnO2 coating exists like some needles or rods on the surface of the MWCNTs of some orientation. SEM and TEM micro-

ecently the energy shortage and environmental pollution became serious due to the economic development and the increasing population. As a result, renewable energies such as solar energy and wind energy have become one hot focus.1 However, supply as well as the demand for these renewable energy sources fluctuates with time and season of the year. As a result, their stability of power supply from these energy sources should be adjusted by energy storage systems with a fast response for the frequent supply demanding changes.1,2 So far, the present power sources, including lithium ion batteries, are encountering different kinds of challenges or problems. Consequently, safe and reliable energy storage systems with high rate capability are urgently needed to utilize these renewable energies including their connection to power grids and use in hybrid Electric Vehicles (EVs).1−13 Recently, high rate performance of cathode material (LiFePO4) up to 400 C (full discharge in 9 s) was reported by adding 65% conductive agent in the electrode. It is a good suggestion of the possibility of realizing superfast charge performance though the conductive agent is higher than that for practical application.14 To our best knowledge, how to prepare electrode material for batteries with capability to charge up to 400C or even higher without adding too much conductive agent has never been tried and it is a serious challenge according to Goodenough et al.15 In our latest work, we replace the organic electrolyte of lower ionic conductivity with the aqueous electrolyte of higher ionic conductivity; the nanocathode can charge at the rate of 90C in about 40 s.12,13 This suggests that second-level charge performance for rechargeable batteries is possible to achieve.14 It is known that from the nature there are a lot of materials rich in facets. Each different facet has different numbers of atoms and effects. According to these natural findings, recently nanomaterials with tailored crystal orientation or facet for © 2013 American Chemical Society

Received: January 16, 2013 Revised: March 16, 2013 Published: March 28, 2013 2036

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Figure 3. Characterization of the prepared LiMn2O4 nanotube showing the X-ray diffraction pattern.

Figure 1. Schematic presentation of the material synthesis. (a) Acid treatment: MWCNTs were dispersed in 6 M HNO3 for 2 h with sonication to remove the impurities and endow the surface with hydrophilic groups such as −OH and −COOH. (b) Coating MnO2 on MWCNTs: the acid treated MWCNTs were immersed into an aqueous solution of 10 mM KMnO4 and 20 mM H2SO4 at 85 °C for 1 h. (c) Heat-treating to prepare LiMn2O4 nanotube: the MnO2-coated MWCNTs were mixed with LiOH with molar ratio of Li/Mn = 0.5 and then heat treated at 700 °C for 8 h in the air atmosphere.

very well with the standard pattern except the (400) planes. In the standard LiMn2O4, the intensity of (111) planes are much stronger than that of (400) ones. In our prepared LiMn2O4 nanotubes, their intensities are almost the same. This suggests that the LiMn2O4 has a crystal orientation. Such crystal orientation will play an important role during the lithium ion intercalation/deintercalation reaction.18 By the way, there is some minor impure phase, which is indicated as Mn2O3. This is maybe caused by the gas-evolution of the MWCNTs during the heat-treatment, which took away some lithium and caused not enough lithium to react with the Mn oxides. The electrochemical behavior of the prepared LiMn2O4 nanotube in 0.5 M Li2SO4 aqueous electrolyte is shown in Figure 4. It can be seen that there are also two redox couples at the scan rate of 1 mV/s, which are situated at 0.77/0.73 V and 0.916/0.85 V (vs saturated calomel electrode (SCE)), respectively. They are consistent with the intercalation and deintercalation of Li+ ions into/from the host spinel structure in organic and aqueous electrolytes.14,19 With the increase of the scan rate, the peak separation increases due to the polarization. However, the peaks retain the well-defined shape even when the scan rate increases to 100 mV/s. This is much better than the spinel LiMn2O4 from traditional solid-state reactions and the other nanomaterials.13 According to such a superior cyclic voltammogram (CV) response, a superfast or second-level charge performance will be possible. At the current density of 500 mA/g (about 4.5 C), the reversible capacity of the LiMn2O4 nanotube is 110 mAh/g, which is competitive with that in the organic electrolytes. When the charging and discharging current density increases, the LiMn2O4 displays perfect capacity retention with a little capacity decrease. The main reason is that the overpotential from the electrochemical reactions and the internal resistance (Figure S1 in the Supporting Information) is very small. The nanotube LiMn2O4 shows a discharge capacity of 102 mAh/g at the charge and discharge current densities of 1000 mA/g (9.1 C), 100 mAh/g at those of 5000 mA/g (45.5 C), and 99 mAh/ g even at those of 10 000 mA/g (91 C) between 0 and 1.05 V. The capacity at 91 C is 91% of that at 4.5 C. The flat plateaus in the charge and discharge curves are well-defined at all current densities and the voltage decrease is very slight at very high current densities. Also, the charging and discharging plots almost overlap with each other at the current densities of 5000 and 10000 mA/g, suggesting that this nanotube material has superfast electrochemical kinetics. To our best knowledge, this is very rare for electrodes of batteries.14,19 In addition, as shown in Figure 3c when the charging potential is increased to 1.4 V, 97.3 mAh/g (88.5% of the capacity), 80.2 mAh/g (72.9% of the capacity), and 59.3 mAh/

Figure 2. Characterization. (a) SEM and (b) TEM micrographs of MnO2 precursor coating on MWCNTs; (c) SEM and (d) TEM micrographs of the prepared LiMn2O4 nanotubes.

graphs of the prepared LiMn2O4 nanotubes (Figure 2c,d, respectively) show that the synthesized LiMn2O4 material consists of uniform nanotubes without other morphologies in the sample. The nanotube has a length over 2 μm and a tetragonal open end with an edge length of about 80−100 nm (shown in the inset of Figure 2d). Evidently, the thickness is less than that of the MnO2 coating, which is about 10 nm. The main reason is that the MnO2 coating is not dense and it condenses during the heat treatment. It is known that MWCNTs consist of the graphene sheet rolled into concentric cylinders. The graphene plane can coordinate with Mn atoms due to the π−3d orbit interaction. By the way, it does not repulse the oxygen due to the existence of −OH and −COOH groups from the acid treatment. As a result, the orientated MnO2 was deposited on the MWCNTs by using the surface graphene sheets as the templates. Consequently, the as-prepared LiMn2O4 also keeps retention of a typical crystal orientation. According to the XRD patterns of the as-prepared LiMn2O4 shown in Figure 3, most peaks fit 2037

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Figure 4. Electrochemical characterization of the prepared LiMn2O4 nanotube. (a) CV at different scan rate measured using Ni as the counter electrode and SCE as the reference electrode, (b,c) charging and discharging curves measured using Ni as the counter electrode and SCE as the reference electrode, and (d) cycling behavior of the prepared LiMn2O4 nanotube tested by 2-electrode cells by using active carbon as the counter electrode. All electrochemical measurements were carried out at room temperature in 0.5 M Li2SO4 aqueous solution, and the capacity is calculated based on the weight of active LiMn2O4.

resistance is very small. (4) In the aqueous electrolyte, there is no need for solid electrolyte interface (SEI) film or layer, which will contribute to some internal resistance, since both Mn3+ and Mn4+ are stable,1,3,4 which is quite different from that in the organic electrolyte. In the case of the latter, an SEI film is necessary to reduce or inhibit the rapid dissolution of both Mn3+ and Mn4+.1,23c If this kind of material is used for power system of electric vehicles, it means that the battery can be charged very quickly and the charging time can be markedly less than the time required for filling gasoline, usually about one minute. The response to the supply and demand of grids will be also very rapid. It provides great stimulus to the development of electric vehicles and smart grids.6d,10e The cycling behavior of the LiMn2O4 nanotube as the cathode material for ARLBs is shown in Figure 4d, which was obtained by using a two-electrode cell consisting of the nanoLiMn2O4 working electrode and active carbon counter electrodes at the current density of 500 mA/g (about 4.5C) between 0 and 1.8 V. The weight ratio of these two electrodes is fixed at 1:3.2 (LiMn2O4/activated carbon). Our former results showed that the activated carbon absorption and desorption of alkali ions with good reversibility and the kinetics is very fast.22 This kind of testing will not present any adverse influence on the electrochemical performance of the working electrode and the measured capacity since the activated carbon is very stable during cycling.5,6d,12 The nano-LiMn2O4 electrode achieves excellent capacity retention with no definite capacity loss after 1200 cycles. Actually, during the initial 200 cycles, the

g (53.9% of the capacity) could be obtained even at charging rates of 120C (30 s), 300C (12 s), and 600C (6 s), respectively. That means that the charge process of this system can be finished within several seconds as shown in the inset of Figure 3c by zooming out the x-axis range from 0 to 0.5 min. This superfast second-level charge capability can be ascribed to the following reasons: (1) The unique nano characteristics (nanotube) of the prepared LiMn2O4 materials. The thickness of the wall is very thin, only about 10 nm. It is well-known that nanostructure makes Li+ ions deintercalate and intercalate very easily because of very short diffusion distance.21 (2) The crystal orientation of the LiMn2O4 nanotube. Since the (400) planes of the prepared LiMn2O4 nanotube present very strong intensity, it means that there are more (001) or (010) planes on the edges of these planes or vertical to (400) planes. The 8a sites for lithium intercalation and deintercalation are situated at the (001) or (010) planes. This suggests that more Li sites are exposed to the aqueous electrolyte due to the preferred growth of (400) planes. The intercalation and deintercalation of lithium ions becomes much easier.20b−d Of course, further studies are needed for the action of the oriented (400) planes.16 (3) The ionic conductivity of the aqueous electrolyte is about 2 orders of magnitude higher than that of the organic electrolytes.1,3,4,7 This is also one reason that LiMn2O4 presents better rate performance than that in the organic electrolytes. The Nyquist plot (Supporting Information Figure S1) taken at different charging stages in three-electrode cells agrees with the second-level charge performance very well since the internal 2038

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capacity of the electrode material tends to increase, which is probably caused by the immersion of the aqueous electrolyte into the inner of the nanotubes. Another reason is perhaps that the unreacted Mn2O3 reacts with lithium to produce LiMn2O4 during the charge and discharge process. This excellent cycling behavior is similar to our former results reported, which demonstrated retention of 93% capacity after 10 000 full cycles.13 It is known that during the charge and the discharge processes for LiMn2O4 that there is a Jahn−Teller effect, which can be seen clearly from the two plateaus during the charge and the discharge processes. This will usually lead to strain and stress and the resulting poor cycling behavior.23 In the case of our prepared LiMn2O4 nanotube, the strain and stress from the Jahn−Teller effects can be buffered by the porous nanotube structure and good cycling behavior is ensured. Another reason is that the amount of protons is very small, about 10−7 M (10−4 ppm), in the neutral aqueous electrolyte.13 As a result, the possible dissolution of Mn element is much smaller than that in the organic electrolytes. In the organic electrolytes, the amount of HF will be at least 10 ppm. If there is some amount of water, the content will be larger. This HF acid will lead to capacity fading due to the destroying of the spinel structure by dissolving Mn element.23 In the case of the stability of Mn3+ and Mn4+, they are very stable in the aqueous electrolyte, which is quite different from its unstable behavior in the organic electrolytes, and the so-called SEI film or layer for the organic electrolyte is not needed to ensure excellent cycling.23 In terms of cycling, it can compete with the well studied LiFePO4,14,19,24 and thia also means that the ARLB is of great promise for electric vehicle and smart grids. However, before ARLB can be really applied as the main power sources of electric vehicle, its energy density still needs some further improvement.25 In conclusion, LiMn2O4 nanotube with an exposed (400) planes is successfully prepared through a two-step method by using MWCNTs as a sacrificial template. It delivers a high capacity of 110 mAh/g at 4.5C and presents very good capacity retention at superfast second-level charge and discharge rates. Even at the charging rate of 600C (6 s), 53.9% of the capacity could be obtained. Its cycling behavior is very good. It can meet the stringent requirements for high-power applications such as hybrid EVs, EVs, and smart grids since its charging rate seems faster than filling gasoline.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and Nyquist plot of the LiMn2O4 nanotube in 0.5 M Li2SO4 aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-21-55664223. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from MOST (2010DFA61770), STCSM (12JC1401200), and NSFC (21073046) is greatly appreciated. 2039

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