Substituted cobalt oxides, LiM0.05Co0.95O2 (M 5 Mg21, Al31, and Ti41), have been synthesized using solid-state technique and

their performance in a 2032-type lithium rechargeable coin cell is reported. The synthesized powders were structurally analyzed

using X-ray diffraction ~XRD! and the surface morphology evaluated with scanning electron microscopy. XRD patterns indicate

that single-phase materials were formed involving Al-doped LiCoO2 . Electrochemical studies were carried out in the voltage

range 3.5-4.5 V ~vs. Li metal! using 1 M LiPF6 in ethylene carbonate/dimethyl carbonate as electrolyte. The doping involving 5%

Mg resulted in a charge/discharge capacity of ;160 mAh/g at C/5 rate and remained stable even after 50 cycles. To the best of our

knowledge, this is the first time that such high stable capacities have been obtained involving doped LiCoO2 when cycled up to

4.5 V

Gopukumar, S.

Yamaki, J.I.

Yoshio, M.

Zou, M.

IR@CECRI

Electrochemical and Solid-State Letters, 7 ~7! A176-A179 ~2004!0013-4651/2004/7~7!/A176/4/$7.00 © The Electrochemical Society, Inc.A176Synthesis and Electrochemical Performance of High VoltageCycling LiM0.05Co0.95O2 as Cathode Materialfor Lithium Rechargeable CellsMeijing Zou,a Masaki Yoshio,a,* S. Gopukumar,b,c,z and Jun-ichi Yamakib,*aDepartment of Applied Chemistry, Saga University, Saga 840, JapanbInstitute of Advanced Materials Chemistry and Engineering, Kyushu University. Kasuga 816-8580, JapanSubstituted cobalt oxides, LiM0.05Co0.95O2 (M 5 Mg21, Al31, and Ti41), have been synthesized using solid-state technique andtheir performance in a 2032-type lithium rechargeable coin cell is reported. The synthesized powders were structurally analyzedusing X-ray diffraction ~XRD! and the surface morphology evaluated with scanning electron microscopy. XRD patterns indicatethat single-phase materials were formed involving Al-doped LiCoO2 . Electrochemical studies were carried out in the voltagerange 3.5-4.5 V ~vs. Li metal! using 1 M LiPF6 in ethylene carbonate/dimethyl carbonate as electrolyte. The doping involving 5%Mg resulted in a charge/discharge capacity of ;160 mAh/g at C/5 rate and remained stable even after 50 cycles. To the best of ourknowledge, this is the first time that such high stable capacities have been obtained involving doped LiCoO2 when cycled up to4.5 V.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1738423# All rights reserved.Manuscript submitted July 23, 2003; revised manuscript received November 6, 2003. Available electronically April 26, 2004.Lithium-ion batteries are emerging as a major source of power inview of their varied applications ranging from cell phones to electricvehicles as also in the medical field. The system usually involves theuse of lithiated transition metal oxides, namely, LiCoO2 , LiNiO2 ,and LiMn2O4 as cathode material and also as lithium source. How-ever, among these materials, lithium cobalt oxide (LiCoO2) is themost widely used cathode material in the majority of commerciallyavailable lithium-ion batteries owing to its ease of synthesis andhigh reversibility.1 Despite the advantages of LiCoO2 , the maxi-mum attainable practical capacity is only around 137 mAh/g in thevoltage range 3-4.25 V even though the theoretical capacity isaround 273.8 mAh/g. Therefore, to obtain higher capacities onemust charge the cells to high voltages ~4.5 V!.2 However, such at-tempts have failed to produce stable cycling LiCoO2 due to struc-tural changes taking place.3 Attempts to improve the cyclability be-yond 4.25 V by doping have been reported by some authors.4,5However, such doping resulted in only inferior capacities. Theoret-ical studies6,7 suggest that doping of LiCoO2 with a transition metalion leads to an increase in capacity whereas nontransitional metalion doping leads to an increase in voltage at the expense of capacity.To overcome the cycling problems at high voltages, recently, Choet al.8-10 demonstrated that the coating of LiCoO2 with ceramic ma-terials like Al2O3 , ZrO2 , etc., delivers stable capacities even whencycled up to 4.5 V and was also confirmed by Chen and Dahn.2However, apart from these studies to the best of our knowledge thereis no report on the stable cycling of doped LiCoO2 at high voltages~4.5 V!. Hence, we thought it timely to synthesize carefully dopedLiCoO2 and present in this communication the electrochemical per-formance of the synthesized materials in a lithium rechargeable cell.Moreover, the present method of synthesis is more commerciallyacceptable considering the economy and ease of the bulk productionprocess involved.ExperimentalLiM0.05Co0.95O2 (M 5 Mg, Al, Ti) has been prepared usingCo3O4 ~.99.9% pure!, Li2CO3 and either Mg(NO3)2 , Al(NO3)3 ,or TiO2 as precursor materials. Stoichiometric amounts of the men-tioned materials were thoroughly mixed and melted in an argonatmosphere at 550°C for 6 h and then cooled. The synthesized pow-der is ground and mixed well before finally subjecting to further* Electrochemical Society Active Member.c Permanent address: Central Electrochemical Research Institute ~CSIR!,Karaikudi 630 006, India.z E-mail: deepika 41@rediffmail.com–annealing at high temperature of 850°C for 10 h and then cooled,mixed, and repeated annealing for a further 10 h. The mixture wascooled, mixed thoroughly, and was subjected to physical and elec-trochemical characterization.The prepared powders were characterized for their phase purityand structural features using powder X-ray diffraction ~XRD, Rint1000, Rigaku! using Cu Ka radiation. The particle morphologies ofthe powders were recorded using a scanning electron microscope~JSM-5300E, Japan Electron Ltd., Japan!.Electrochemical characterizations were performed in a 2032 coincell-type two-electrode assembly. Cathode mix was prepared using85% of the active material mixed with 15% Teflonized carbon. Theprepared mix was coated onto an aluminum foil using doctor bladetechnique and dried in an oven for 6-12 h at 120°C. The dried sheetof the cathode material was then roll pressed for increased adher-ence of the cathode mixture on the aluminum foil current collector.Circular disks of the cathodes were then punched and were used forfabricating the coin cell. The coin-type cells were assembled in anargon-filled glove box with the prepared circular disks ofLiM0.05Co0.95O2 as cathode, lithium ~Cyprus Foote Mineral Co.!metal anode, and with Celgard 3401 as separator in a nonaqueouselectrolyte consisting of 1 M LiPF6 in 1:2 by volume ethylene car-bonate ~EC!/dimethyl carbonate ~DMC! ~Ube Chemical, Japan!. Thecharge/discharge galvanostatic cycling of the fabricated cells wereperformed at C/5 rate in the voltage range 3.5-4.5 V using an auto-matic battery tester at room temperature.Results and DiscussionThe XRD patterns of the synthesized metal-doped LiCoO2 arepresented in Fig. 1. It is clearly observed that all the finger printpeaks, viz., 003, 101, 006, 102, 104, 108, and 110 are identifiablethereby suggesting the existence of a-NaFeO2 structure. The hex-agonal lattice parameters of the synthesized powders were evaluatedusing a least square fit and the values of a and c ~60.002! aresummarized in Table I along with the c/a ratio, an indicator ofmetal-metal layering distance. As can be observed from Table I thevalues of a and c are highest in Mg doping as compared to pristineLiCoO2 or with Al or Ti doping with a c/a ratio of 4.995 therebysuggesting that all the investigated doped LiCoO2 materials arelamellar, but the magnitude of a and c depends on the cationspresent in the slab ~to compensate for the charge of the substituent!and therefore should exhibit excellent electrochemical behavior.Further, it can be seen from Fig. 1 the evolution of secondary phasesmarked ‘‘X’’ in the Ti-doped sample and may be assigned to theElectrochemical and Solid-State Letters, 7 ~7! A176-A179 ~2004! A177emergence of spinel phases.11 Our XRD results are in confirmationto earlier reports available for the synthesis of metal ion ~Mg, Al, Ti!doped LiCoO2 .11-14Having confirmed the formation of single-phase metal ion-dopedmaterials, we examined the surface morphology of the synthesizedmaterials using scanning electron microscopy ~SEM!. Figure 2 de-picts the SEM photographs of the synthesized metal-doped powders.All the synthesized powders show particles of submicrometer sizeand therefore should aid the diffusion of lithium ions. Further, theparticles exhibit a uniform distribution and are smaller than the par-ent LiCoO2 .15To confirm the electrochemical performance, we cycled the fab-ricated 2032 lithium coin cell galvanostatically from 3.5 to 4.5 V.The cells exhibited an open-circuit voltage of around 2.9 V ~vs. Limetal!. The first charge and discharge pattern for the cells incorpo-rating the synthesized materials are shown in Fig. 3. It is observed inthe Al doping ~Fig. 3a! that the first charge is as high as 190 mAh/g,but the irreversible capacity is around 40 mAh/g in the first cycle.Moreover, phase change around 120-125 mAh/g is absent. However,interesting results are observed looking at the first cycle of the Mgdoping ~Fig. 3b!. The first charge capacity is around 185 mAh/g butmore than 90% of the capacity is retained in the first discharge or theirreversible capacity is only about 15-17 mAh/g. Moreover, Mg-doping also does not exhibit any phase transition such as in Al-doping at around 0.5 lithium extraction. Further, a different pictureis seen for the first charge/discharge cycling of Ti-doped LiCoO2~Fig. 3c!. First, the hump near the capacity of 120 mAh/g is ob-served even though the first charge and discharge capacities are 182and 162 mAh/g, respectively. Having been encouraged by the pre-liminary performance of the synthesized powders, we were inter-ested in carrying out the galvanostatic cycling studies up to 50cycles so as to ascertain the stability of the initially observed highcapacities.Figure 4a-c presents the cycling behavior of the synthesizeddoped cathode materials in a lithium rechargeable 2032-type coincell in the voltage range 3.5-4.5 V at the C/5 rate. It is seen from thefigures that the cells employing Al doping ~Fig. 4a! shows an initialcapacity of around 160 mAh/g but fades slowly on cycling andFigure 1. XRD patterns of LiM0.05Co0.95O2 M 5 (a) Al, ~b! Mg, or ~c! Ti.Table I. XRD lattice constants a , c and cÕa ratio for differentsynthesized doped LiCoO2 .Material a ~Å! c ~Å! c/aLiAl0.05Co0.95O2 2.813 14.066 5.000LiMg0.05Co0.95O2 2.818 14.075 4.995LiTi Co O 2.812 14.015 4.9840.05 0.95 2retains a capacity of approximately 140 mAh/g. Such fading hasalso been observed by other authors4,12 and is as expectedtheoretically.7 Excellent cyclability of Mg-doped material ~Fig. 4b!is exhibited over the investigated 50 cycles, i.e., there only being adifference of ;8 mAh/g from the first to the 50th discharge capaci-ties. This result looks interesting as to our knowledge this is the firsttime such stable high voltage capacities have been obtained eventhough some authors suggest improved performance up to 4.25 Vrange involving Mg-doped LiCoO2 .11 The enhanced performance ofMg doping cycling up to 4.25 V has been ascribed to enhancedelectronic conductivity. Finally, the cyclability of Ti-doped LiCoO2~Fig. 4c! even though it looks attractive in the initial cycles, fades toFigure 2. SEM photographs of LiM0.05Co0.95O2 .Electrochemical and Solid-State Letters, 7 ~7! A176-A179 ~2004!A178a stable value of around 143 mAh/g. The drop in capacity can beascribed due to the phase changes encountered around a capacity of125 mAh/g and also due to inactive Ti present. Thus, we can say thatdoping with divalent magnesium may deliver high capacities andalso produce good high voltage cycling cathode materials. A com-parison of our capacities with that obtained by coating8-10 indicatesthat our results are superior in view of the high capacities obtainedin a narrow voltage range of 3.5-4.5 V in contrast to values of ;165mAh/g reported by Cho et al.,8 in the voltage range 2.75-4.4 V.ConclusionWe may therefore conclude that metal-doped LiCoO2 materialssynthesized by the present procedure results in enhanced electro-chemical activity. It has been demonstrated for the first time that theFigure 3. First charge/discharge patterns of LiM0.05Co0.95O2 M 5 (a) Al,~b! Mg, or ~c! Ti.material, LiMg0.05Co0.95O2 , is a promising high voltage cycling ~4.5V! cathode material delivering capacities of 160 mAh/g at C/5 ratefor use in lithium rechargeable cells.AcknowledgmentOne author ~S.G.K.! thanks CECRI, Karaikudi ~CSIR, NewDelhi! for grant of leave and to Kyushu University for offering avisiting professorship.Saga University assisted in meeting the publication costs of this article.References1. C. Julien and S. Gastro-Garcia, J. Power Sources, 97Õ98, 290 ~2001!.2. Z. Chen and J. R. Dahn, Electrochem. Solid-State Lett., 5, A213 ~2002!.3. G. G. Amatucci, J. M. Tarascon, and L. C. Klein, Solid State Ionics, 83, 167 ~1996!.Figure 4. Galvanostatic cycling at C/5 rate in the voltage range 3.5 to 4.5 Vof LiM0.05Co0.95O2 M 5 (a) Al, ~b! Mg, or ~c! Ti.Electrochemical and Solid-State Letters, 7 ~7! A176-A179 ~2004! A1794. Y. Jang, B. Haung, D. R. Sadoway, G. Ceder, Y. M. Chiang, H. Liu, and H. Tamura,J. Electrochem. Soc., 146, 862 ~1999!.5. H. Tukamoto and A. R. West, J. Electrochem. Soc., 144, 3164 ~1997!.6. G. Ceder, Y. M. Chiang, D. R. Sadoway, M. K. Ayginol, Y. I. Jang, and B. Huang,Nature (London), 392, 894 ~1998!.7. S. Venkatraman, V. Subramanian, S. Gopukumar, N. G. Renganathan, and N. Mu-niyandi, Electrochem. Commun., 2, 18 ~2000!.8. J. Cho, C. Kim, and S. I. Yoo, Electrochem. Solid-State Lett., 3, 362 ~2000!.9. J. Cho, Y. J. Kim, and B. Park, Chem. Mater., 12, 3788 ~2000!.10. J. Cho, Y. T. Kim, and B. Park, Angew. Chem., Int. Ed. Engl., 40, 3367 ~2001!.11. S. Gopukumar, Y. Jeong, and K. B. Kim, Solid State Ionics, 159, 223 ~2003!.12. S. Levasseur, M. Mene´trier, and C. Delmas, J. Power Sources, 112, 419 ~2002!.13. G. A. Nazri, A. Rougier, and K. F. Mia, Mater. Res. Soc. Symp. Proc., 453, 63~1997!.14. S. Levasseur, M. Mene´trier, and C. Delmas, Chem. Mater., 14, 3584 ~2002!.15. M. Zou, M. Yoshio, S. Gopukumar, and J. Yamaki, Chem. Mater., 15, 4699 ~2003!.

Synthesis and Electrochemical Performance of High Voltage Cycling LiM0.05Co0.95O2 as Cathode Material for Lithium Rechargeable Cells

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