LiMn2O4尖晶石纳米棒作为锂离子电池阴极材料Spinel LiMn2O4 Nanorods as Lithium Ion Battery Cathodes

Spinel LiMn2O4Nanorods as Lithium Ion

Battery Cathodes

Do Kyung Kim,?P.Muralidharan,?Hyun-Wook Lee,?Riccardo Ruffo,?

Yuan Yang,§Candace K.Chan,|Hailin Peng,§Robert A.Huggins,§and Yi Cui*,§

Department of Materials Science and Engineering,Korea Ad V anced Institute of

Science and Technology(KAIST),Daejeon305-701,Korea,Dipartimento di Scienza

dei Materiali,Uni V ersita`degli Studi di Milano-Bicocca,V ia Cozzi53,

20135Milan,Italy,and Department of Materials Science and Engineering,

Department of Chemistry,Stanford Uni V ersity,Stanford,California94305

Received August10,2008

ABSTRACT

Spinel LiMn2O4is a low-cost,environmentally friendly,and highly abundant material for Li-ion battery cathodes.Here,we report the hydrothermal synthesis of single-crystalline -MnO2nanorods and their chemical conversion into free-standing single-crystalline LiMn2O4nanorods using a simple solid-state reaction.The LiMn2O4nanorods have an average diameter of130nm and length of1.2μm.Galvanostatic battery testing showed that LiMn2O4nanorods have a high charge storage capacity at high power rates compared with commercially available powders.More than85%of the initial charge storage capacity was maintained for over100cycles.The structural transformation studies showed that the Li ions intercalated into the cubic phase of the LiMn2O4with a small change of lattice parameter,followed by the coexistence of two nearly identical cubic phases in the potential range of3.5to4.3V.

Lithium ion batteries with high energy and power density are important for consumer electronic devices,portable power tools,and vehicle electri?cation.1-4Li x CoO2is a commonly used cathode material in commercial lithium ion batteries and has a charge capacity of140mAh/g with a practical value of x from0.5to1.However,the high cost,toxicity, and limited abundance of cobalt have been recognized to be disadvantageous.As a result,alternative cathode materials have attracted much interest.One promising candidate is spinel LiMn2O4,which has a charge storage capacity of148 mAh/g.5-9Spinel LiMn2O4has the advantages of low-cost, environmental friendliness,and high abundance. Nanostructuring battery electrode materials have been shown to enhance power performance due to the large surface-to-volumeratiothatallowsforalargeelectrode-electrolyte contact area.10-16Nanowires or nanorods are particularly attractive.Recently we have demonstrated examples of using Si and Ge nanowires as ultrahigh capacity anode materi-als.17,18The nanowire or nanorod morphology not only has a large surface-to-volume ratio but also provides ef?cient one-dimensional electron transport pathways and facile strain relaxation during battery charge and discharge.

A wide variety of synthetic approaches have been developed for the synthesis of LiMn2O4nanoparticles, including combustion,19,20sol-gel,21solution-phase,22and templating11methods.Aggregated LiMn2O4nanorods have also been produced as cathodes.23A facile method is to chemically convert -MnO2nanorods into LiMn2O4.Here we report the hydrothermal synthesis of single-crystalline -MnO2nanorods and their chemical conversion into free-standing single-crystalline LiMn2O4nanorods in a simple solid-state reaction.Battery testing showed that LiMn2O4 nanorods have a high charge storage capacity at high power operation,which is signi?cantly better than the commercially available powders with particle sizes around10μm.More than85%of the initial charge storage capacity is maintained for over100cycles.The structural transformation studies showed that the Li ions intercalated into the cubic phase of the LiMn2O4with a small change of lattice parameter, followed by the coexistence of two nearly identical cubic phases in the potential range of3.5to4.3V. Experiments.Synthesis of MnO2and LiMn2O4Nano-rods.Analytical grade Mn(CH3COO)2·4H2O,Na2S2O8(99.99% Aldrich),and deionized water were used to prepare -MnO2 nanorods by hydrothermal reaction as reported elsewhere.24 All chemicals were used without further puri?cation.In a typical synthesis,Mn(CH3COO)2·4H2O and Na2S2O8were dissolved at room temperature with a molar ratio of1:1in 80mL of distilled water by magnetic stirring to form a

*To whom correspondence should be addressed.E-mail:yicui@ https://www.sodocs.net/doc/9c743838.html,.

?Korea Advanced Institute of Science and Technology(KAIST).

?Universita`degli Studi di Milano-Bicocca.

§Department of Materials Science and Engineering,Stanford University.

|Department of Chemistry,Stanford University.

NANO LETTERS

2008 Vol.8,No.11 3948-3952

homogeneous clear solution.The mixed solution was trans-ferred to a 100mL Te?on-lined stainless steel autoclave and heated at 120°C for 12h in a preheated electric oven for the hydrothermal reaction.After the reaction,the ?nal precipitated products were washed sequentially with deion-ized water and ethanol to remove the sulfate ions and other remnants by ?ltration.The obtained powder was subse-quently dried at 100°C for 12h in air.

A typical synthesis of LiMn 2O 4nanorods was as follows:0.00143moles of LiOH ·H 2O and 0.0028moles of the as-synthesized -MnO 2nanorods were dispersed into 2mL high purity ethanol to form a thick slurry,ground to form a ?ne mixture for several hours,and dried at room temperature.The above process was repeated two to three times to produce a well-mixed powder.The powder was then calcined at 650,700,and 750°C in air for 10h.

The synthesized -MnO 2and LiMn 2O 4nanostructures were characterized using an X-ray diffractometer (XRD,Rigaku,D/MAX-IIIC X-ray diffractometer,Tokyo,Japan)with Cu K R radiation (λ)0.15406nm at 40kV and 40mA).The size and shape of the nanostructures were observed on a ?eld emission scanning electron microscope (FE-SEM Philips XL30FEG,Eindhoven,Netherland),and a high-resolution transmission electron microscope (HR-TEM,JEM 3010,JEOL,Tokyo,Japan).

Electrochemical In W estigation.The electrodes for elec-trochemical studies were prepared by making a slurry of 85wt %active material of LiMn 2O 4,10wt %conducting carbon black,and 5wt %polyvinylidene ?uoride (PVDF)binder in N -methyl-2-pyrrolidone (NMP)as the solvent.The slurry was applied using a doctor-blade onto an etched aluminum foil current collector and dried at 100°C for 12h in an oven.The coated cathode foil was then pressed to form a uniform layer and cut into a square sheet.

The electrochemical performance of the LiMn 2O 4was investigated inside a coffee bag (pouch)cell assembled in an argon-?lled glovebox (oxygen and water contents below 2and 0.1ppm,respectively).Lithium metal foil (Alfa Aesar)was used as the anode.Typical cathode loading was 1.5mg/cm 2.A 1M solution of LiPF 6in ethylene carbonate/diethyl carbonate (EC/DEC,1:1v/v)(Ferro Corporation)was used as the electrolyte with a Celgard 2321triple-layer polypropylene-based membrane as the separator.The charge -discharge cycles were preformed at different C rates between 3.5-4.3V at room temperature using Bio-Logic VMP3and Maccor 4300battery testers.Electrochemical potential spectroscopy was also used to investigate the structural changes in the nanorods.The potential was swept at steps of 3mV from 3.5to 4.3V and vice versa using a cut off current of 8mA/g.

Result and Discussion.The XRD pattern of the hydro-thermal synthesized -MnO 2corresponded to JCPDS data No.24-0735having tetragonal symmetry with P 42/mnm space group.No additional impurity peaks were detected (Figure 1a).SEM images (Figure 1b)showed that the particles consisted of nanorods with an average diameter of 90nm and an average length of 1.5μm.

The LiMn 2O 4XRD diffratogram showed features of the spinel structure with Fd 3m space group (JCPS card No.35-0782),with no peaks of the -MnO 2phase detected (Figure 2a).Thus the reaction between -MnO 2and LiOH at 700°C produced the pure LiMn 2O 4phase.To con?rm whether the nanorod morphology still remained after the high temperature solid-state reaction,we performed SEM analysis.Figure 2b shows that the LiMn 2O 4phase also consisted mainly of nanorods which appear to have a larger average diameter of 130nm but a shorter average length of 1.2μm than the starting -MnO 2nanorods.TEM images and diffraction patterns (Figure 2c)show that nanorods are single crystalline and grow along the <110>crystallographic direction.

The electrochemical results are reported in terms of voltage versus lithium concentration (x in Li x Mn 2O 4)at constant current (Figure 3a)and voltage versus the dif-ferential capacity (d Q /d V )in potentiostatic conditions (Figure 3b)for the ?rst charge and discharge process,as well as cycling properties (Figure 3c,d).During the ?rst charge,at least three electrochemical processes can be observed in the increase of potential (Figure 3a).First,only a small number of charges are stored between the open circuit voltage (around 2.9V)to 3.8V,which corresponds to removing the lithium excess from Li 1+x Mn 2O 4-δ(around 3.0V)and removing oxygen vacancies in the lattice (between 3.2and 3.7V).25Second,at a potential higher than 3.9V,the material shows the well-known behavior of lithium deintercalation from the LiMn 2O 4cubic spinel phase to the same phase of Li 1-x Mn 2O 4(between 3.9and 4.1V,delithiation,Figure 3a).Third,a mixture of two cubic phases (plateau around 4.15V in Figure 3a)is formed until a second single-phase domain of Li 0.2Mn 2O 4is reached.The high potential behavior (from 3.9to 4.2V)is reversible and two domains (single and two phases)are detected also during the reductive lithiation

Figure 1.(a)XRD pattern and (b)SEM images of -MnO 2as obtained from hydrothermal reaction.

(Figure 3b).Moreover very good energy ef?ciency is observed.In fact,the potential drop between the charge and the discharge process is only 40mV at the rate of 28.6mA/

g,indicating the fast kinetics of the system.Since we are interested in evaluating the material as the cathode in the Li-ion battery,we used a discharge cutoff value of 3.5V,which is similar to the requirement for commercial applica-tion.At the end of the ?rst discharge the charge excess is lost (Coulombic ef?ciency of 90%).We have to consider,however,that usually a battery pack is assembled with a certain amount of extra capacity at the cathode to compensate irreversible anode reactions taking place during the ?rst cycle (solid electrolyte interphase (SEI)formation,surface oxide reduction,etc.).In this case,it is an advantage to use the charge excess instead of loading more material at the cathode.The voltage versus d Q /d V curve clearly points out the different nature of the two processes at high potential (Figure 3b).The former one,at potential ranging from 3.9to 4.1V,displays a bell shape peak at 4.05V which has full width at half-maximum (FWHM)of 55mV while the latter process shows a spike peak at 4.15V with just 5mV in FWHM.The sharp peak observed in this latter peak (Figure 3b)corresponds to a ?at plateau in Figure 3a,a clear indication of the coexistence of two phases.The difference between the two processes is also evident during the discharge scan in which they occur at potentials of 4.10and 3.95V with FWHM of 10and 50mV,respectively.

The discharge speci?c capacity of the nanorods as a function of cycle number has been compared with the result obtained using an electrode obtained from com-mercial powders (LiMn 2O 4electrochemical grade,Sigma Aldrich)using the same preparation route (Figure 3c),that is,the commercial electrode is a mixture of active material,carbon black,and binder in the weight ratio of 85:10:5.The charge capacity is measured with the power rate from 0.1C (14.8mA/g)to 1C (148mA/g).We have observed that the nanorod morphology has a much higher charge capacity than the commercial powders at higher power rates.At the lowest current (from cycle 1to 5)both the samples have a speci?c capacity around 110mAh/g but increasing the rate to 0.2C (from cycle 6to 10)leads a large difference in performance:the speci?c capacity of the nanorod electrode remains almost constant between 110and 105mAh/g while that of com-mercial powder decreases to 70mAh/g.In most of the literature a larger amount of conductive agent (up to 20%)is added to the active LiMn 2O 4material and the speci?c discharge capacity obtained at moderate rates such as 0.2C is comparable to that of our nanorods.However,we point out that in our nanorod case,reducing the amount of carbon black (10wt %)and increasing the active material ratio means that more charges can be stored in the electrode regardless of the obtained LiMn 2O 4speci?c capacity.This advantage results from the one-dimensional electron transport and large surface area of the nanorods.The difference of speci?c charge capacity between the nanorods and com-mercial powders becomes larger with the further rate increase (Figure 3c).At the highest current (148mA/g),the nano-structured electrode can deliver a speci?c charge capacity (100mAh/g)twice of the commercial powders (50mAh/g).These data indicate that the large surface-to-volume ratio of the nanorods enhances greatly the kinetics of the LiMn 2O

4

Figure 2.(a)XRD pattern,(b)SEM images,(c)low-and high-resolution TEM of LiMn 2O 4nanorods as obtained from solid-state reaction between -MnO2nanorods and LiOH ·H 2

O.

Figure 3.(a)Galvanostatic (0.1C)?rst charge/discharge curve,(b)potentiostatic differential capacity vs voltage (dO/dV),(c)discharge speci?c capacity curve vs number of cycles for nanorods (black dots)and commercial powder electrode (white squares)at different power rates,and (d)charge (white)and discharge (red)speci?c capacity curve vs number of cycles for nanorod electrode at 1C rate.

electrodes.To evaluate the cyclability of the nanorod electrode at a high rate,we have performed 100cycles at 1C.The sample shows very good capacity retention.After 50and 100cycles,the capacity retention is 95and 85%,respectively (Figure 3d).The average Coulombic ef?ciency is 99.7%.Therefore the LiMn 2O 4nanorods can supply good capacity at high rates with high reversibility.

To better understand the correlation between the structure and the electrochemical behavior of the nanorods,we have exploited XRD and TEM on samples with different lithium amounts.Two electrochemical cells were stopped during the ?rst delithiation charge step (rate 14.8mA/g,0.1C)to obtain compositions of Li 0.8Mn 2O 4and Li 0.4Mn 2O 4,respectively.It was observed (Figure 4a)that the Li 0.8Mn 2O 4sample showed an XRD diffraction pattern corresponding to a single crystalline phase having cubic cell parameter of 8.227?,which is lower than the pristine LiMn 2O 4(8.243?).The decrease of cell parameter with delithiation has already been observed in the literature for cubic spinel.26The observed shrinkage of the cell parameter continues until the composi-tion reaches a critical value where a structural change takes place and the potential enters into the two phases domain at ～4.15V.In fact,the diffraction pattern of Li 0.4Mn 2O 4sample appears to be lower in intensity and each re?ection clearly splits into two peaks due to the presence of two different cubic phases in this composition domain.Two cubic cell

parameters were calculated as 8.24and 8.17?,respectively;therefore the sample should be a mixture of a Li x Mn 2O 4lithium rich and a Li y Mn 2O 4lithium poor compound where x is close to 1.0and y might be close to 0.2.We also used TEM to compare nanorods with composition of Li 0.8Mn 2O 4(Figure 4b)and Li 0.4Mn 2O 4(Figure 4c).In both samples,TEM images and electron diffraction showed that nanorods remained single crystalline.However,due to the small difference in cell parameter,the coexistence of the two-phase domains in the Li 0.4Mn 2O 4nanorods could not be resolved.The small difference in the two phases should facilitate the fast kinetics of battery charging discharging,consistent with our electrochemical data.

Conclusion.LiMn 2O 4nanorods with cubic spinel structure have been obtained with facile,low-cost and scalable hydrothermal and solid-state reaction methods.The nanorod morphology appears to be a very important step in improving the kinetic properties of the material and the nanorods are able to deliver 100mAh/g at a high current density of 148mA/g with high reversibility and good capacity retention after 100cycles.During the charge step two fundamental pro-cesses have been detected and investigated:the delithiation proceeds in a single cubic phase with decreasing of the cubic cell parameter,followed by the coexistence of two cubic phases with similar cell parameters.

Acknowledgment.The work is supported by the Global Climate and Energy Project at Stanford and King Abdullah University of Science and Technology.C.K.C.acknowledges support from a National Science Foundation graduate fel-lowship and Stanford Graduate Fellowship.D.K.K.would like to thank the SBS Foundation and Korea Research Foundation (KRF-2005-005-JO9701)for supporting his sabbatical leave.References

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