Comparative study of Li[Co1-zAlz]O2 prepared by solid-state and co-precipitation methods

Electrochimica Acta 54 (2009) 4655–4661 Contents lists available at ScienceDirect Electrochimica Acta journa l homepage: www.e lsev ier .com Compa by and co- Wenbin a Department o b School of Met a a r t i c l Article history: Received 24 D Received in re Accepted 24 M Available onlin Keywords: Aluminum sub Lithium-ion ba Aluminum sub materials Co-precipitatio Solid-state synthesis ere ith z -stat tent les w =0.95 s bet meas Litera e ra samp thermal stability of delithiated Li[Co1−zAlz]O2 in electrolyte. © 2009 Elsevier Ltd. All rights reserved. 1. Introdu The saf ued succe are presen where the can be ma often incor LiFePO4 an have small mon layer research fo itive electr only a sma through su Li[Ni1/2−zM The im ity of lay For exam Li[Ni1−zAlz material w ∗ Correspo E-mail ad 0013-4686/$ doi:10.1016/j ction ety of lithium-ion batteries is central to their contin- ss in the market place. LiCoO2-based Li-ion batteries tly dominant in cell-phone and computer applications thermal instability of delithiated LiCoO2 in electrolyte naged due to the small size of the cells. Larger cells porate less expensive and less reactivematerials such as d LiMn2O4 positive electrode materials. However these er volumetric energydensity thanLiCoO2 andother com- ed positive electrode materials. There has recently been cusing on improving the thermal stability of layeredpos- ode materials so they are more benign than LiMn2O4 at ll penalty in energy density. This has been accomplished bstitutions of Al in Li[Ni1/3Mn1/3Co1/3−zAlz]O2 [1] and n1/2−zAl2z]O2 [2]. pact of Al substitutions on the thermal stabil- ered lithium transition metal oxides is well-known. ple, Ohzuku’s group showed that Al-substitution in ]O2 reduced the reactivity of the charged electrode ith electrolyte [3]. Al substitutions have been used to nding author. Tel.: +1 902 494 2991; fax: +1 902 494 5191. dress: jeff.dahn@dal.ca (J.R. Dahn). make Li[Ni0.8Co0.15Al0.05]O2 (NCA) with improved thermal sta- bility compared to the parent material without Al. Ceder et al. have reported that Al-substituted LiCoO2 has a higher average potential versus Li than LiCoO2 according to the first principles calculation and that it has better thermal stability [4,5]. In spite of these well-known results, the mechanism for the thermal sta- bility improvement is not understood in detail. In addition, the impact of aluminum on the kinetics of the reaction between delithiated positive electrode materials and electrolyte at high temperature is not known. These reaction kinetics are required for models of the response of full Li-ion cells to abuse scenarios [6–8]. MacNeil et al. have made substantial studies of the kinetics of the reactions between delithiated LiCoO2 and electrolyte at ele- vated temperature [9,10]. It is our goal to build on this earlier research by studying the impact of Al on the kinetics of reaction between delithiated Li[Co1−zAlz]O2 and electrolyte. In such studies, Li[Co1−zAlz]O2 would serve as amodel systemandhopefully results learned could be applied to other Al-substituted layered transition metal oxides. As the first step, a careful characterization of synthe- sis, structure and electrochemical properties of Li[Co1−zAlz]O2 is required. Scattered results can be found in the literature already, and will be referred to as appropriate below, but none is com- plete to the level we would like. In this paper, the co-precipitation methodand the traditional solid-statemethodwereused toprepare Li[Co1−zAlz]O2. We document the structure and electrochemical – see front matter © 2009 Elsevier Ltd. All rights reserved. .electacta.2009.03.068 rative study of Li[Co1−zAlz]O2 prepared precipitation methods Luoa,b, J.R. Dahna,∗ f Physics and Atmospheric Science, Dalhousie University, Halifax B3H3J5, Canada allurgical Science and Engineering, Central South University, Changsha 410083, PR Chin e i n f o ecember 2008 vised form 22 March 2009 arch 2009 e 5 April 2009 stituted LiCoO2 tteries stituted positive electrode n a b s t r a c t Li[Co1−zAlz]O2 (0≤ z≤0.5) samples w lattice constants varied smoothly w state samples above z=0.2. The solid distribution when the aluminum con stoichiometric Lix[Co0.9Al0.1]O2 samp nominal compositions x= Li/(Co+Al) ples suggest the solid solution limit i Li[Co1−zAlz]O2 samples were used to be about −250±30 (mAh/g)/(z=1). Li[Mn2−yAly]O4 demonstrates the sam serve as baseline characterization of / locate /e lec tac ta solid-state prepared by co-precipitation and solid-state methods. The for the co-precipitated samples but deviated for the solid- e method may not produce materials with a uniform cation is large or when the duration of heating is too brief. Non- ere synthesized by the co-precipitation method at various , 1.0, 1.1, 1.2, 1.3. XRD patterns of the Lix[Co0.9Al0.1]O2 sam- ween Li/(Co+Al) = 1.1 and 1.2. Electrochemical studies of the ure the rate of capacity reduction with Al content, found to ture work on Li[Ni1/3Mn1/3Co1/3−zAlz]O2, Li[Ni1−zAlz]O2 and te of capacity reduction with Al/(Al +M) ratio. These studies les to be used to determine the impact of Al content on the 4656 W. Luo, J.R. Dahn / Electrochimica Acta 54 (2009) 4655–4661 performance of Li[Co1−zAlz]O2 prepared by twomethods and com- pare to literature results. 2. Experimental 2.1. Material preparation 2.1.1. Li[Co1−zAlz]O2 samples prepared by the co-precipitation method A LiOH·H2O (Sigma Aldrich, 98%) solution and a mixed solution of Co(NO3)2·6H2O (Sigma Aldrich, 98%) and Al(NO3)3·9H2O (Sigma Aldrich, 98%) were simultaneously added over the course of about 30min to a stirredflask using a two-channel peristaltic pump (Mas- terflex C/L pump, Barnant Co.). The concentrations of the solutions were adjusted to set the Al:(Al +Co) ratio, z. The precipitate was fil- tered out and washed with distilled water several times to remove any dissolved salts and dried at 80 ◦C overnight. The variation in structural properties and chemical composition of the precipitate with Al content, z, have been reported in another publication [11]. The dried precipitate was mixed with an appropriate amount of Li2CO3 (Alfa Aesar, 99%) and ground. The precursors were heated in air. Samples were heated at selected temperatures for selected times to be described below. Samples with 0≤ z≤0.5 in Li[Co1−zAlz]O2 were prepared. In addition, non-stoichiometric Lix[Co0.9Al0.1]O2 samples were synthesized at various nominal compositions x= Li/(Co+Al) = 0.95, 1.0, 1.1, 1.2, 1.3. 2.1.2. Li[Co1−zAlz]O2 samples prepared by the solid-state method The starting materials for synthesis were Li2CO3 (Alfa Aesar, 99%), Co3O4 (Alfa Aesar, 99.7%) and Al(OH)3 (Sigma Aldrich). Sto- ichiometric amounts of Co- and Al-containing starting materials along with an appropriate amount Li2CO3 were mixed and ground togetherusing anautomatic grinder (RetschRM-0). Finally the sam- ples were heated at a selected temperature for a selected time in air. 2.2. Material characterization 2.2.1. X-ray diffraction XRD patterns were collected with a Siemens D5000 diffrac- tometer equipped with a Cu target x-ray tube and a diffracted beammonochromator. Li[Co1−zAlz]O2 samplesweremeasuredover a scattering angle range between 10◦ and 90◦ using 0.05◦ steps and a 10 s counting time. 2.2.2. Electrochemical testing Coin cells (23mm diameter and 2.5mm thick) were used for testing the electrochemical performance of the samples. Positive electrodes were prepared by mixing the active material, Super S Carbon Black, and PVDF in a weight ratio of 90:5:5. An appropriate amount of NMP was added to the mixture to form a slurry, which was then thoroughly mixed. The slurry was coated on Al foil and dried overnight in an oven. The electrochemical cells used a single lithium metal foil as the counter electrode, Celgard 2320 microp- orous film as the separator and 1MLiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:2, v/v) as the electrolyte. Cells were assembled in an argon-filled glove box. The cells were removed from the glove box and connected to a computer-controlled charg- ing system (E-One/Moli Energy). The cells were initially charged and discharged at a C/20 rate between 3.3 and 4.3V versus Li metal for two cycles. After the first two cycles, the charge–discharge rate was increased to C/5. 2.2.3. SEM testing A Hitachi S4700 field-emission scanning electron microscope (SEM) was used to image the materials. Fig. 1. XRD pa he co- samples are in lumn top panels. tterns of LiCo1−zAlzO2 (0≤ z≤0.5) synthesized by both the solid-state method and t the left column and the patterns of the co-precipitation samples are in the right co precipitation method at 900 ◦C for 3h. The patterns of the solid-state . Three Bragg peaks are indexed based on the LiCoO2 structure in the W. Luo, J.R. Dahn / Electrochimica Acta 54 (2009) 4655–4661 4657 Fig. 2. SEM images of LiCo0.7Al0.3O2 synthesized by the solid-state method A, B and the co-precipitation method C, D. Fig. 3. XRD patterns of LiCo0.8Al0.2O2 synthesized by both the solid-state method and the co-precipitation method. The XRD pattern shown in the bottom panel is for a sample synthesized by the co-precipitation method and then heated at 750 ◦C for 3h. The remaining XRD patterns are for samples synthesized by the solid-state method at 750 ◦C with different heating times. 3. Results and discussion Fig. 1 s LiCo1−zAlzO state and co single phas broader Bra show the s gesting the panels may Bragg peak imageswer Fig. 2 sh LiCo0.7Al0.3 state samp Fig. 4. Lattice for samples pr and by the sol hows the XRD patterns (between 60◦ and 75◦) of 2 (0≤ z≤0.5) synthesizedat900 ◦C for3hbyboth solid- -precipitationmethods. All samples appear at first to be e, however the solid-state samples show substantially gg peaks for z>0.2. The right panels of Fig. 1 clearly mooth shift of the Bragg peak positions versus z, sug- broad Bragg peaks for the solid-state samples in the left be caused by non-uniform Al distribution. Increased width can also be caused by smaller grain size, so SEM eused to ensure that theparticle sizeswere comparable. ows scanning electron micrographs of two samples of O2. Fig. 2A and B show the morphology of the solid- le. Fig. 2C and D show the morphology of the sample constants, a [panel A] and c [panel B] of Li[Co1−zAlz]O2 versus z. Data epared by the co-precipitation method (750 ◦C, 20h and 900 ◦C, 3h) id-state method (900 ◦C, 3h) are included. 4658 W. Luo, J.R. Dahn / Electrochimica Acta 54 (2009) 4655–4661 Fig. 5. Lattice constants versus z for Li[Co1−zAlz]O2 samples prepared by the co- precipitation method (900 ◦C, 3h) compared to previous literature reports. —co- precipitation method (900 ◦C; 3h) are compared to the results from �—ref. [12], �—ref. [13], �—ref. [14], �—ref. [15], +—ref. [16], —ref. [17]. made by the co-precipitation route. The sample synthesized by the solid-state method shows crystallites with flat facets, having primary particle sizes around 2�m. The sample synthesized by co- precipitation route shows a similar morphology, but has primary particles that are around 1�m. The solid-state sample has larger particle size, but broader Bragg peaks, suggesting that non-uniform cation distr Fig. 3 sh both the so sample in b method at 7 ing panels method at are observe the solid-st Fig. 7. Lattice constants of Lix[Co0.9Al0.1]O2 (x=0.95, 1.0, 1.1, 1.2 and 1.3) synthesized by the co-precipitation method (900 ◦C, 3h). 16h. A pure phase of LiCo0.8Al0.2O2 is only obtained by heating 20h at 750 ◦C for the solid-state method. Based on the results in Fig. 3, it can be concluded that shorter heating times are required for samples prepared by the co-precipitation method, as expected. Fig. 4 shows the lattice constants (a and c) as a function of z in LiCo1−zAlzO2 for samples prepared by the solid-state (900 ◦C, 3h) and co-precipitation methods (both 900 ◦C, 3h and 750 ◦C, 20h). The lattice constantswere determined by least squares refinements meas e lat tice s bu ugge h a u , con 5 zAlz Fig. 6. XRD pa shown. The lef for all samples ibution is the cause as was suggested above. ows the XRD patterns of LiCo0.8Al0.2O2 synthesized by lid-state method and the co-precipitation method. The ottom panel was synthesized by the co-precipitation 50 ◦C for 3h. The sample is a single phase. The remain- in Fig. 3 show samples synthesized by the solid-state 750 ◦C for different times (3–28h). Some Li2CO3 peaks d in the XRD patterns of LiCo0.8Al0.2O2 synthesized by ate method and heated at 750 ◦C for 3h, 6h, 12h and to the ple. Th The lat sample again s als wit is large Fig. Li[Co1− tterns of Lix[Co0.9Al0.1]O2 (x=0.95, 1.0, 1.1, 1.2 and 1.3) synthesized by the co-precipitat t panels show the presence of Bragg peaks from Li2CO3 in the x=1.2 and 1.3 patterns. The r . ured positions of at least 10 Bragg peaks for each sam- tice constant a decreases and c increases as z increases. constants vary smoothly with z for the co-precipitated t deviate for the solid-state samples above z=0.15. This sts that the solid-statemethodmaynot producemateri- niform cation distribution when the aluminum content sistent with the results in Fig. 1. shows the variation of the lattice constants of ]O2 (co-precipitation method, 900 ◦C, 3h) compared to ion method (900 ◦C, 3h) Two regions of the diffraction patterns are ight panels show that the 018, 110 and 111 Bragg peaks are crystalline W. Luo, J.R. Dahn / Electrochimica Acta 54 (2009) 4655–4661 4659 Fig. 8. The potential (V) versus specific capacity (mAh/g) of Li/Li[Co1−zAlzO2]cells cycled between 3.3 and 4.3V at a rate of C/20. Samples prepared by the solid-state method (900 ◦C, 3h) are shown in the left columns and samples prepared by the co-precipitation method (900 ◦C, 3h) are shown in the right panels. literature results [12–17]. The literature results are for samples prepared by a variety of methods, including the sol–gel method, the emulsion drying method, the co-precipitation method, the citrate prec methods (th precipitationmethodand the citrate precursormethod) that ensure good Co-Al cation mixing in the precursor yield results in good agreement with our work. By contrast, the data set ( ) in Fig. 5 in p ds, w Fig. 9. The dif method (900 ◦ ursor method and the solid-state method. In general, e sol–gel method, the emulsion dryingmethod, the co- that is metho ferential capacity (dQ/dV) versus potential (V) of Li/Li[Co1−zAlz]O2 cells cycled between C, 3h) are shown in the left columns and samples prepared by the co-precipitation meth oor agreement is for samples prepared by solid-state hich may not lead to good cation mixing. 3.3 and 4.3V at a rate of C/20. Samples prepared by the solid-state od (900 ◦C, 3h) are shown in the right panels. 4660 W. Luo, J.R. Dahn / Electrochimica Acta 54 (2009) 4655–4661 Fig. 6 shows the XRD patterns of Lix[Co0.9Al0.1]O2 (x=0.95, 1.0, 1.1, 1.2, 1.3) synthesizedat900 ◦C for3hbyco-precipitationmethod. The value x in the figure shows the nominal Li/(Co+Al) molar ratio. The left panels in Fig. 6 show the portion of the XRD pattern where the Li2CO3 right panel resolved K� single phas 0.95≤ x≤1. region was Thus, the ad non-stoichi Fig. 7 sh molar ratio with x. This LixCoO2 sam Fig. 8 sh having Li[Co from solid- those from column. Th the increasi cannot be o ing. Therefo content of a Fig. 8 sh the smalles versible ca irreversible strongly for x-ray diffra the irrevers be more cha origin of th Fig. 9 s described b are describ are describe associated w ened and sh solid-state to z=0.25, samples is n agreement dQ/dV for t the second the reasons Fig. 10a (4.3–3.3V, for the soli samples (90 sample typ solid-state precipitated in the solid average am dashed line samples in samples sho as the sam als designer with the inc The imp ties of Li[N [20] and Li Li[Co1−zAlz] Panel a) First discharge capacities as a function of z in Li[Co1−zAlz]O2 (z=0, , 0.15, 0.2, 0.25,0.3, 0.4, and 0.5). Samples were prepared by co-precipitation state methods as indicated in the legend. The discharge capacities for sam- ared in thiswork:� solid-state (900 ◦C, 3h), co-precipitation (900 ◦C, 3h), o-precipitation (750 ◦C, 20h) are compared to previous literature work—� � ref. [17], + ref. [19]. Panel b) First discharge capacities versus z=Al/(Al +M) amples prepared in this work (same as Panel a) compared to different Al- ositive electrodematerials in the literature: LiNi1/3Mn1/3Co(1/3−z)AlzO2 ref. Ni1−zAlzO2 ref.[20]; LiMn2−yAlyO4 ref.[21]. s been renormalized and plotted as z= y/2 along the x-axis so lmaterials canbe comparedon the samegraph. The impact of tions on cell capacity for all these systems, both layered-type inel-type is basically identical. If each addedAl atom reduced acitybyone lithiumatom, then the slopeof the lines through data in Fig. 10a and b should be about −280 (mAh/g)/(z=1) approximately matches the data. This “universality” is pre- ly a result of the fact that Al3+ cannot be oxidized while er cations in the samples described in Fig. 10 undergo an e oxidation state change of +1 when one Li per metal is ed. clusions o1−zAlz]O2 samples were prepared by co-precipitation and tatemethods. Cationuniformityasmeasuredbysharpnessof eaks, smooth variation of lattice constantswith z and differ- capacity versus potential was better for the co-precipitated s. Lattice constant versus z behaviour for samples prepared co-precipitation routewas found toagreewellwith literature ses where uniform cation mixing was ensured. -stoichiometric Lix[Co0.9Al0.1]O2 samples with x=0.95, 1.0, and 1.3were prepared by the co-precipitationmethod. Sam- ith x<1.2 were found to be single phase. impurity appears for the x=1.2 and x=1.3 samples. The s show that the XRD peaks remain sharp with well- doublets in all cases. The results in Fig. 6 suggest that e samples can be prepared under these conditions for 1. Delmas et al. [18] showed that a similar single phase possible for LixCoO2 samples synthesized at 900 ◦C. dition of Al does not strongly impact the single phase ometric range. ows the lattice constants, a and c, plotted versus x, the Li/(Co+Al). The lattice constants basically do not vary is in agreement with the findings of Delmas et al. for ples [See Table 1 in reference [18]]. ows the first charge/discharge cycling curves for the cells 1−zAlz]O2 electrodes. The results from electrodesmade state samples (900 ◦C, 3h) are in the left column while co-precipitated samples (900 ◦C, 3h) are in the right e reversible capacity of Li[Co1−zAlz]O2 decreases with ng Al content. As is well-known [5] this is because Al3+ xidizedwhile Co3+ can be oxidized to Co4+ during charg- re, substitution of Al3+ for Co3+ in LiCoO2 decreases the ctive species. ows that the co-precipitated sample with z=0 has t charge–discharge polarization and the smallest irre- pacity of all the samples. Fig. 8 also shows that the capacity increases with z and that it increases more the samples prepared by co-precipitation. Based on the ction studies in Figs. 1 and 4, we believe the variation of ible capacity with z for the co-precipitated samples to racteristic of samples with uniform cation mixing. The is irreversible capacity needs to be understood. hows the differential capacity of the same samples y Fig. 8 plotted versus potential. The solid-state samples ed by the left column and the co-precipitated samples d by the right column. The sharp peak in dQ/dV at z=0, ith the insulator–metal transition [5] is rapidly broad- ifted to higher potential as z increases. However, in the samples, a small remnant peak exists, even all the way again suggesting that cation mixing in the solid-state ot complete. The average potential increases with z, in with previous reports [5]. One curious feature is that he first charge becomes quite different from dQ/dV for charge as z increases. Presumably this is associatedwith for the irreversible capacity and deserves more study. shows the variation of