Lithium iron oxide as alternative anode for li-ion batteries

Lithium iron oxide as alternative anode for li-ion batteries

International Journal of Inorganic Materials 2 (2000) 365–370 Lithium iron oxide as alternative anode for li-ion batteries Pier Paolo Prosini a , *, ...

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International Journal of Inorganic Materials 2 (2000) 365–370

Lithium iron oxide as alternative anode for li-ion batteries Pier Paolo Prosini a , *, Maria Carewska a , Stefano Loreti a , Carla Minarini b , Stefano Passerini a a

ENEA, C.R. Casaccia, Via Anguillarese 301, S. Maria di Galeria, Rome 00060, Italy b ENEA, C.R. Portici, Loc. Granatello, Portici, Napoli, Italy Accepted 20 March 2000

Abstract Lithium–iron oxide Li–Fe–O was synthesized by solid state reaction between Li 2 CO 3 and Fe 2 O 3 . The sample was characterized by X-ray powder diffraction. The XRD patterns showed well defined reflections corresponding to a-LiFeO 2 and the spinel LiFe 5 O 8 in a molar ratio of 9:1. The material was tested as alternative anode for lithium-ion batteries. It exhibited good cyclability delivering about 120 mAh / g after 500 deep charge / discharge cycles. Unlikely, the use of the material as intercalation anode in practical cells is hindered by the irreversible uptake of lithium that takes place during the first lithium insertion. X-ray diffraction pattern showed that during this step a reduction of the lithium iron oxide occurs leading to the formation of lithium oxide and iron metal.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. oxides; C. X-ray diffraction

1. Introduction The introduction of carbonaceous materials (graphite and cokes) as lithium intercalation anodes has allowed the wide commercialization of rechargeable lithium batteries usually called lithium-ion batteries. Nevertheless the use of these materials presents a major drawback, the safety. Carbonaceous materials intercalate lithium at potentials as low as the lithium deposition potential. A not appropriate charge of the battery can cause the deposition of very reactive metallic lithium, which might react with the solvent to generate flammable gas mixture. In extreme situations the over-lithiated carbonaceous anode can cause explosion with flame. Thus, a careful control of the charge procedure is needed to avoid major accidents. The research of new anode able to intercalate lithium at higher voltages has led to the identification of several materials. Cobalt nitrides [1], manganese nitrides [2] and transition metal oxides characterized by a low intercalation potential such as WO 2 and MoO 2 [3], Nb 2 O 5 [4], Li 4 Ti 5 O 12 , Li 4 Mn 5 O 12 , Li 12 Mn 4 O 9 [5], Li[Li 1 / 3 Ti 5 / 3 ]O 4 [6], TiO 2 [7], and CuCoS 3 O 8 [8], have received great *Corresponding author. Tel.: 139-6-3048-6768; fax: 139-6-30486357. E-mail address: [email protected] (P.P. Prosini).

attention. They present high reversibility for lithium insertion but their capacity is somewhat limited. Recently, Idota et al. [9,10], have shown that amorphous tin oxides have very high capacity (0.6 Ah / g) for lithium intercalation. Unlikely, such a performance is also accompanied by a large irreversible capacity in the first cycle. Dahn et al. [11] have shown that the initial irreversible capacity is related with the reaction of lithium and the oxide to form lithium oxide and metallic tin. Further lithium intercalation leads to the mostly reversible formation of a Li–Sn alloy [12,13]. Iron oxides have also been proposed as intercalation negative electrodes because of their availability and low price. Ohzuku et al. have electrochemically reduced Fe 2 O 3 . They found that two equivalents of lithium were inserted and proposed the formation of FeO and Li 2 O [14]. In a following work, Scrosati et al. prepared Li 6 FeO 3 by exhaustive electro-reduction of Fe 2 O 3 and demonstrated its use as alternative anode in lithium batteries [15]. Abraham et al. prepared Lix Fe 2 O 3 via a room temperature reaction involving the in situ generation of Li–naftalide in tetrahydrofurane [16]. More recently, sodium ferrite Na 2 O– 1.5Fe 2 O 3 electrode was tested as alternative negative electrode whit a reversible specific capacity of up to 0.36 Ah / g. The material showed an irreversible behavior on the first cycle where the cycle efficiency was about 51% [17]. The aim of this work is to characterize the host

1466-6049 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00028-3

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capability of lithium iron oxide synthesized by a solid-state reaction and to evaluate its use as alternative anode in lithium-ion batteries. The investigations have been focused on the structural modification taking place in the oxide during the first lithium insertion as well as on the lithium cycling (insertion–release) behavior.

2. Experimental Li–Fe–O was prepared by reacting 3.714 g of Li 2 CO 3 (Carlo Erba, Reagent Grade) with 11.577 g of Fe 2 O 3 (Aldrich, Reagent Grade) at 8508C for 10 h. Prior thermal treatment the powders were dried at 1208C (18 h) then mixed with 50 cc of acetone and grounded in an orbital mill for 30 min. Composite cathode tapes were made by roll milling a mixture of 60% active material with 30% of binder (Teflon, DuPont) and 10% of carbon (SuperP, MMM Carbon). Electrodes were punched in form of discs typically with a diameter of 8 mm. A typical electrode weighed 4.2 mg corresponding to an active material mass loading of 5 mg / cm 2 . The electrodes were assembled in a sealed cell formed by a polypropylene T-type pipe connector with three cylindrical stainless steel (SS316) current collectors. A lithium foil was used both as anode and reference electrode and a glass fiber was used as separator. The cell was filled with propylene carbonate / LiClO 4 1 M electrolyte solution. Cells were tested with constant charge and discharge currents, and were cycled between fixed

voltage limits. The cycling tests were carried out automatically by means of a battery cycler (Maccor 2000). Composite cathode preparation, cell assembly, test and storage were performed in a dry room (R.H.,0.2%). XRD patters of the powder was carried out by means of an X PERT-MPD diffractometer using Cu ka radiation. The measure was carried out in a theta-2theta configuration with a monochromator, programmable receiving slit and a Xe filled proportional detector. XRD patterns of pristine as well as electrochemically lithiated electrodes were obtained by means of an Italstructures diffractometer using a focused Co ka radiation and equipped with a PSD (INEL) curved detector.

3. Results and discussion The XRD spectrum of the material is illustrated in Fig. 1 upper curve. The comparison with published spectra of Li–Fe–O (JCPDS Data Base) reveals the presence of two phases, namely a-LiFeO 2 [18] and the spinel LiFe 5 O 8 [19], which spectra are also shown in Fig. 1. The spinel is most likely formed as a result of the partial loss of Li 2 O at high temperature [20]. The Rietveld method was used to simulate the spectrum and to evaluate the molar ratio of the two phases. The lower curve in Fig. 1 shows the difference between the experimental spectrum and the linear combination of the a-LiFeO 2 and LiFe 5 O 8 diffraction patterns. The two phases are in a molar ratio of 9:1,

Fig. 1. From the top: XRD spectra for the sample and simulated XRD spectra for a-LiFeO 2 and LiFe 5 O 8 . The lower curve is the difference between the experimental spectrum and the linear combination of the a-LiFeO 2 and LiFe 5 O 8 diffraction patterns.

P.P. Prosini et al. / International Journal of Inorganic Materials 2 (2000) 365 – 370

respectively. The extremely low value of the difference is a clear indication of the fit goodness. The cycling behavior of the Li–Fe–O electrode is shown in Fig. 2. The lithium insertion and release processes were driven at 0.6 mA / cm 2 corresponding to a specific current of 65 mA / g. The electrode showed an initial low value of the charge coefficient, i.e., the ratio between the charge released inserted (also shown in Fig. 2). This indicates that the lithium released during the initial cycles was substantially less than the amount inserted. However, after the 10th cycle the charge coefficient rose to values higher than 0.9. Finally, after the 20th cycle the lithium insertion / release process proceeds with full reversibility. A specific capacity of about 150 mAh / g was delivered from the electrode at the first cycle. The specific capacity decreases upon cycling, but after 500 cycles the electrode was still able to deliver about 60 mAh / g. The capacity delivered from the electrode depended on the discharge current used. Fig. 3 shows the capacity delivered by the same electrode at different current densities in reversible cycles, i.e., with a charge coefficient of about one. At 0.2 mA / cm 2 a reversible capacity of about 120 mAh / g was obtained. To fully understand the nature of the irreversible lithium uptake taking place in the initial cycles the measurement of the quasi-equilibrium electrode voltage as a function of the amount of lithium insertion was performed. The electro-

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chemical lithium insertion was driven at a very low current density (0.06 mA / cm 2 , 17 mA / g) in order to fully lithiate the electrode and to avoid large polarization and ohmic overvoltages. The results are shown in Fig. 4. After an initial steep drop the voltage curve shows a plateau at 0.9 V (versus Li) that extends for about 0.5 equivalents of lithium. Upon further lithium insertion the voltage slowly declined to reach a mostly flat region located around 0.8 V (versus Li). This region extends for about three equivalents of lithium. It is composed of several parts identified by relatively small voltage fluctuations that could be associated to different processes. Finally, a third region in which the voltage gradually declines down to 0.5 V (versus Li) is shown in the figure. This final region was previously associated to electrolyte decomposition [16,17]. The diffraction pattern of the fully lithiated Li–Fe–O electrode is illustrated in Fig. 5 (upper curve) and compared with the pattern of the pristine material. After the electrochemical lithium intercalation only a small fraction of the material is left in the original crystalline form. In addition, several new peaks are seen in the pattern of the lithiated Li–Fe–O compound. The new peaks are very broad thus indicating the formation of nano-crystalline phases. The peak assignment process is not completed yet, but two peaks ([12 and [20) certainly belong to metallic iron while peak [7 belongs to Li 2 O. Iron and lithium oxide are most likely the products of the irreversible process that takes place in the

Fig. 2. Behavior of a Li–Fe–O electrode upon cycling. The specific capacity (left) and the charge coefficient (right) versus cycle number are illustrated. The lithium insertion process was driven at 0.6 mA / cm 2 (65 mA / g). Reference and counter electrode: lithium. Electrolyte: 1 M LiClO 4 in propylene carbonate. T5208C.

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Fig. 3. Specific capacity delivered by a Li–Fe–O electrode as a function of the current density and cycle number. The current densities used were j50.2 mA / cm 2 (square), j50.3 mA / cm 2 (up triangle), j50.4 mA / cm 2 (circle), j50.5 mA / cm 2 (diamond), j50.6 mA / cm 2 (down triangle). Reference and counter electrode: lithium. Electrolyte: 1 M LiClO 4 in propylene carbonate. T5208C.

first cycle in the voltage range between 1 V (versus Li) and 0.8 V (versus Li). The modifications induced in the material by the lithium

insertion process are shown very clearly in the differential capacity plot reported in Fig. 6. The experiment was performed by cyclically driving the lithium insertion / re-

Fig. 4. Quasi-thermodynamic voltage versus composition behavior of a Li–Fe–O electrode. The lithium insertion process was driven at 0.06 mA / cm 2 (17 mA / g). Reference and counter electrode: lithium. Electrolyte: 1 M LiClO 4 in propylene carbonate. T5208C.

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Fig. 5. XRD patterns of pristine (lower curve) and fully lithiated (upper curve) Li–Fe–O based electrode.

Fig. 6. Differential capacity versus electrode voltage behavior of a Li–Fe–O electrode. The inset illustrates the behavior in following cycles. The lithium insertion process was driven at 0.06 mA / cm 2 (17 mA / g). Reference and counter electrode: lithium. Electrolyte: 1 M LiClO 4 in propylene carbonate. T5208C.

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lease process in galvanostatic conditions. During the first cycle the lithium insertion proceeds through different steps as indicated by the presence of two cathodic peaks. The features can be easily associated with the voltage plateaus seen in Fig. 4. In the following cycles only one cathodic peak (with a shoulder on the higher-voltage side) is seen. In the second cycle the peak moves toward higher voltages at about 1.02 V (versus Li). From the third cycle, the cathodic peak shifts back to lower voltages to finally stabilize (see inset) at about 0.7 V (versus Li). In addition, the peak broadens on cycling thus indicating a widening of the voltage change in the insertion plateau. These effects are certainly related to the severe modification of the crystalline structure of the material induced by the lithium insertion. The modifications take place mostly during the first intercalation cycle and, at a lower extent, in the following cycles. Summarizing, during the electrochemical reduction lithium can react with the oxide to form lithium oxide and iron metal as indicated by the following equation: LiFe x O y 1 (2y 2 1)Li 1 1 (2y 2 1)e 2 → yLi 2 O 1 xFe

(1)

The initial irreversible behavior and the capacity fading observed for the material have to be associated with this irreversible reduction. However, the disappearance of the first two reduction peaks and the appearance of a new peak located at higher potential, indicates that different irreversible processes take place upon lithium insertion in the Li–Fe–O material. The first irreversible process is related with the first voltage plateau in Fig. 4 and then with the first cathodic peak in the differential capacity versus voltage plot of Fig. 6. It involves the irreversible reduction of the Li–Fe–O material with one equivalent of lithium (1 mole of electrons). At lower voltage, a second irreversible lithium insertion process takes place. The extent of this process is about two equivalent of lithium. At the second discharge cycle a new lithium insertion process takes place. Surprisingly the voltage plateau is shifted at a higher voltage than in the pristine material. Upon cycling, the intercalation peck moves towards more negative potential and became broad thus indicating a non perfectly reversible lithium insertion process.

4. Conclusion A Li–Fe–O compound corresponding to a mixture of LiFeO 2 and LiFe 5 O 8 was synthesized by solid-state reaction of Li 2 CO 3 and Fe 2 O 3 at high temperature. The material was tested as intercalation anode for lithium batteries. Cycled at high current density it offered a specific capacity of about 150 mAh / g at the first cycle. The specific capacity decreases upon cycling, but after 500

cycles the electrode was still able to deliver about 60 mAh / g. At lower current densities the same electrode was able to delivered about 120 mAh / g. This reasonable value is susceptible of extensive improvement upon morphological, compositional and engineering optimization of the electrode. Unlikely, the lithium iron oxide characterized in this paper shares with the other anodic intercalation oxides a somewhat large initial irreversible capacity. The irreversibility is associated with the lithium uptake that results in major structural modifications. Only a fraction of the lithium inserted in the first cycles is reversibly released and inserted in following cycles and this leads to a low overall capacity of the battery.

Acknowledgements The authors would like to thank MICA (Ministero per l’Industria, il Commercio e l’Artigianato) for the financial support. Doctor F. Cardellini (ENEA) is kindly acknowledged for XRD measurements.

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