Solid State Ionics 249–250 (2013) 105–111

Contents lists available at ScienceDirect

Solid State Ionics

j ourna l homepage: www.e lsev ie r .com/ locate /ss i

High capacity Li[Ni0.8Co0.1Mn0.1]O2 synthesized by sol–gel andco-precipitation methods as cathode materials for lithium-ion batteries

Huaquan Lu a,b, Haitao Zhou b, Ann Mari Svensson b, Anita Fossdal c, Edel Sheridan c,Shigang Lu a, Fride Vullum-Bruer b,⁎a R&D Center for Vehicle Battery and Energy Storage, General Research Institute for Non-ferrous Metals, 100088 Beijing, Chinab Department of Material Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norwayc SINTEF Materials and Chemistry, 7465 Trondheim, Norway

⁎ Corresponding author at: Department of Materials SAlfred Getzv.2, 7491 Trondheim, Norway. Tel.: +47 735 9

E-mail address: fride.vullum-bruer@material.ntnu.no

0167-2738/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.ssi.2013.07.023

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 December 2012Received in revised form 5 July 2013Accepted 31 July 2013Available online 2 September 2013

Keywords:Lithium-ion batteryCathode materialSol–gelCo-precipitationLayered structure

Layered Li[Ni0.8Co0.1Mn0.1]O2 cathode materials have been prepared by sol–gel and co-precipitation methods.The structural, morphological and electrochemical properties of the materials were compared. The XRD patternsshow that both the sol–gel and the co-precipitation method formed single phase materials with good layeredcharacteristics. Rietveld refinement reveals some differences in cation disorder between the two materials,where the sample synthesized by the sol–gel method shows lower Li/Ni cation disorder. SEM and BET measure-ments show that the sol–gel sample consists of relatively less aggregated particles giving larger BET surface areacompared to the co-precipitation sample. Electrochemical tests indicate that thematerial prepared by the sol–gelmethod has slightly better electrochemical properties, with an initial discharge capacity of 200 mAh·g−1 andcapacity retention of 82.2% after 50 cycles at a cycling rate of 0.5 C, as well as better capability at 5 C. The im-proved performances of the sol–gel synthesized material may be attributed to the low Li/Ni disorder combinedwith high surface area, the latter increasing the interfacial contact area between the electrolyte and the activematerial. Effects of calcination conditions on the structure and electrochemical performance of the materialswere also investigated. The electrochemical performance was improved by either increasing the O2 concentra-tion in the calcination atmosphere, or by increasing the flow rate of air, showing the potential of developinglow-cost synthesis routes for high-quality cathode materials.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Increasing the capacity of cathodematerials is one of themost effec-tiveways to improve the energy density of lithium-ion batteries tomeetthe requirements for higher energy applications. Recently, the layeredstructure series material LiNi1 − x − yCoxMnyO2 (NCM) has receivedincreased attention [1–3], because they integrate the high capacity ofLiNiO2, good cycle performance of LiCoO2 and low price of LiMnO2.Among various LiNi1 − x − yCoxMnyO2 (NCM) materials, the composi-tion LiNi1/3Co1/3Mn1/3O2 is one of the most extensively studied mate-rials [3–10], which could deliver a capacity of about 160 mAh·g−1

when charged to 4.3 V. LiNi1/3Co1/3Mn1/3O2 is now being used inmany commercial batteries to partly substitute for LiCoO2, butLiNi1/3Co1/3Mn1/3O2 does not offer many advantages when used inelectric vehicle batteries compared to LiFePO4 and LiMn2O4, especiallyconsidering the cost and safety. It is well known that the capacity ofthe layered series materials LiNi1 − x − yCoxMnyO2 (NCM) is stronglydependent on the nickel content since Ni is the main active redox

cience and Engineering, NTNU,3976; fax: +47 735 50203.(F. Vullum-Bruer).

ghts reserved.

species. Thus, increasing the nickel content in NCM materials is aneffective way to increase their charge–discharge capacity [11–13].High nickel content NCM materials, such as Li[Ni0.8Co0.1Mn0.1]O2, arevery attractive cathode materials for lithium-ion batteries in electricand hybrid vehicle applications because of their relatively low costand high reversible capacity of approximately 200 mAh·g−1.

The electrochemical performance of cathodes, such as cyclability andrate capacity, is very dependent on the synthesis method which can beused to control crystallinity, phase purity, particle morphology, particlesize, and cation disorder in the structure of the cathode materials. Forsynthesis of low nickel content NCM materials like LiNi1/3Co1/3Mn1/3O2, a variety ofmethods, such as co-precipitation [3–5], sol–gel synthe-sis [6,7], solution combustion [8], and spray pyrolysis [9,10], have beenreported. However, the high nickel content NCMmaterials are more dif-ficult to synthesize in consistent quality due to thedifficulty in complete-ly oxidizing the Ni2+ to Ni3+, even in pure O2 atmosphere. Incompleteoxidation will eventually lead to impurities, large cation disorder andlithium deficiency [14–17], which will strongly affect the structural andelectrochemical properties of the materials. Furthermore, high nickelcathode materials are not quite stable when exposed to air for a longtime, because they can react with CO2 or H2O in air to form Li2CO3 orLiOH [18,19]. This process can be accelerated as the temperature is

106 H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

elevated [20], which implies thatwhen calcined in air, there is also a pos-sibility of forming some Li2CO3 or LiOH impurities. In order to obtain highnickel cathode materials with better electrochemical performance, pureoxygen atmosphere rather than air has often been used in previousworks [17,21,22]. However, using pure oxygen as calcination atmo-sphere is challenging and costly. Understanding how the calcination at-mosphere affects the structure and electrochemical performance of theLi[Ni0.8Co0.1Mn0.1]O2 materials, and investigating the possibility of syn-thesizing high quality Li[Ni0.8Co0.1Mn0.1]O2 in air atmosphere is veryimportant for the further commercialization of this material.

Li[Ni0.8Co0.1Mn0.1] materials had been reported in several workswith discharge capacities of around 200 mAh·g−1 [11,18,21–23]. Thetypical Ni2+ content on Li sites is around 4% [11,24,25]. At low C-rates(i.e. 0.1–0.2 C), capacity retention of 94% for 50 cycles was demonstrat-ed [11,24], while at higher C-rate (i.e. 0.5 C), capacity retention of 83.5%after 70 cycles was exhibited [25]. All of the above Li[Ni0.8Co0.1Mn0.1]materials are fabricated via co-precipitation routes. It is seldom report-ed to be prepared by other methods like sol–gel method. Sol–gel is acommonly used method to synthesize material with high homogeneityand good stoichiometry, although the gel drying stepsmight complicatethe scale-up to industry production.

In this work, we report on the synthesis of Li[Ni0.8Co0.1Mn0.1]O2

cathode materials by sol–gel (SG) and co–precipitation (CP) methodsand also material calcination in different atmosphere conditions (airor oxygen and flow rate). The effects of different preparation methodsand calcination conditions on the structure, morphology and electro-chemical performance of Li[Ni0.8Co0.1Mn0.1]O2 cathode materials areinvestigated, with the aim of guiding the development of suitable syn-thesis routes.

2. Experimental

2.1. Material synthesis

2.1.1. Sol–gel method (SG)Amixture of LiNO3 (98%, Sigma-Aldrich), Ni(NO3)2·6H2O (98%, Sigma-

Aldrich), Co(NO3)2·6H2O (98%, Sigma-Aldrich) andMn(NO3)2·4H2O (97%,Sigma-Aldrich) with a molar ratio of Li:Ni:Co:Mn = 1.05:0.8:0.1:0.1was dissolved in distilled water. Citric acid (99%, Sigma-Aldrich) wasalso dissolved in distilled water in a separate container (the molarratio of metal ions:citric acid = 1:1). The two solutions were mixedtogether, and the pH of the solution was adjusted to 7.0 by addingNH3·H2O (ACS Reagent, Sigma-Aldrich). The mixed solution was evap-orated at 80 °C under vigorous stirring until a viscous gel was formed.The gel was dried and preliminarily annealed at 480 °C for 5 h in airto decompose the organic components. Finally, Li[Ni0.8Co0.1Mn0.1]O2

samples were obtained by calcination of the decomposed mixture at750 °C for 15 h in different calcination atmospheres listed in Table 1.

2.1.2. Co-precipitation method (CP)Metal nitrates (molar ratio of Ni:Co:Mn = 0.8:0.1:0.1) were

dissolved in distilled water to form an aqueous solution with a con-centration of 2.0 mol·L−1. Stoichiometric Na2CO3 (99%, Sigma-Aldrich) was also dissolved in distilled water to get a 2 mol·L−1 so-lution. The two solutions were added slowly into a reactor, in whichdistilled water was under vigorous stirring. At the same time, the

Table 1Samples synthesized by sol–gel and prepared in different calcination atmospheres.

Sample ID Calcination atmosphere Flow rate (L·h−1)

(a) Air 3.5(b) 50% O2 + 50% N2 3.5(c) 75% O2 + 25% N2 3.5(d) 100% O2 3.5(e) Air 10

desired amount of NH3·H2O solutionwas fed into the reactor. Duringthe reaction process, the pH, temperature and stirring speed werecontrolled to 7.5, 50 °C and 650 rpm, respectively. After aging for12 h, the precipitated Ni0.8Co0.1Mn0.1CO3 was filtered, washed, anddried in a vacuum oven for 12 h at 80 °C. The prepared carbonatewas mixed with LiNO3 (5% excess) in an agate mortar and prelimi-narily annealed at 480 °C for 5 h in air to decompose the carbonateand nitrate. Finally, the Li[Ni0.8Co0.1Mn0.1]O2 sample was obtainedby calcination of the decomposed mixture at 750 °C for 15 h in O2

at a flow rate of 3.5 L·h−1.

2.2. Material characterization

Powder X-ray diffraction (XRD) was performed on a Bruker AXSD8 Focus diffractometer with a LynxEye PSD using Cu Kα radiation(λ = 0.15406 nm) using a 0.2 mm slit. The diffraction data wasrecorded in the 2θ range of 10–100° with a step size of 0.02° and acount time of 3 s per step. Rietveld refinements were performed onthe collected XRD data using TOPAS R (Bruker AXS) version 2.1.The morphology and the particle size of the sample were observedusing a field emission scanning electron microscope (FESEM, ZeissUltra 55). The specific surface areas of the materials were measuredby the BET method on a Micromeritics Tristar 3000 analyzer. The Li,Mn, Ni, and Co molar ratios in the synthesized samples were deter-minedwith high resolution inductively coupled plasmamass spectrom-etry (ICP-MS) on Thermo Scientific ELEMENT 2.

Electrochemical cycling of the synthesized materials was per-formed in coin cells (CR2016) at room temperature. The cathodewas prepared by tape casting a mixed slurry onto aluminum foil(battery grade) by using an RK K-control coater (UK). The slurrywas composed of 85 wt.% active cathode material, 8 wt.% Super-Pcarbon black, and 7 wt.% polyvinylidene difluoride (PVDF, Kynar,reagent grade) binder dissolved in N-methyl l-2-pyrrolidene (NMP,Sigma-Aldrich, N99%). After drying the tape casted cathodes weredried overnight at 120 °C in a vacuum chamber, and the CR2016coin cells were assembled in an MBraun argon filled glove box. 1 MLiPF6 (Aldrich, ≥99.99%) in 1:1 ethylene carbonate (EC, Sigma, 99%)/diethyl carbonate (DEC, Aldrich,≥99%) was used as electrolyte, lithiumfoil as anode and Celgard 2400 membrane as separator. The cathodeelectrode loading was about 5–7 mg/cm2. Three electrode cells(Hohsen, Japan) were assembled for cyclic voltammogram (CV), usinglithium foil as the reference electrode. After assembly, the cells wereallowed to rest for 10 h before electrochemical characterization usinga Maccor 4200 battery test system (USA). For the rate capability test,the cells were charged to 4.3 V with a current density of 0.1, 0.2, 0.5,1.0, 2.0, and 5.0 C, then kept at 4.3 V until the current density wasbelow 0.1 C (18 mA⋅g−1), followed by discharging at the same rateas the charging rate. Long term cycling was performed at 0.5 C for50 cycles. Cyclic voltammograms of the electrodes were obtained inthe three electrode test cell with a VMP3 electrochemical workstation(Bio-Logic, France) in the potential range of 2.5–4.5 V at a scanningrate of 0.1 mV·s−1.

3. Results and discussion

3.1. Comparison of co-precipitation and sol–gel synthesis routes

The compositions of the two samples synthesized via sol–gel and co-precipitation routes based on ICP-MS are Li1.013Ni0.796Co0.101Mn0.103O2

and Li1.037Ni0.798 Co0.101Mn0.100O2, respectively, which are very closeto the targeted stoichiometry of Li[Ni0.8Co0.1Mn0.1]O2. Excess lithiumcontent is apparently from the excessive lithiumused for compensatingthe loss at high temperature.

Fig. 1 displays the Rietveld refinement results of X-ray diffractionpatterns (XRD) of the Li[Ni0.8Co0.1Mn0.1]O2 powders synthesized viasol–gel and co-precipitation routes. All the diffraction peaks can be

Fig. 1. Comparison of the experimental and calculated XRD patterns obtained by Rietveldrefinement for the Li[Ni0.8Co0.1Mn0.1]O2 powders synthesized via sol–gel (a) and co-precipitation route (b).

107H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

indexed on the basis of a hexagonal structure of α-NaFeO2-type (spacegroup R-3m) [11], and no impurity phase is detected in the patterns. Inthe layered structure, the oxygen sub-lattice forms an fcc lattice with ac-direction distortion, which results in a clear splitting of the (006)/(102) and the (108)/(110) peaks. A good resolution of the (006)/(102) and the (108)/(110) reflection pairs is typical of an ideal layeredstructure [26]. It is also found that the co-precipitation route producessamples with a clear splitting of the (006)/(102) and (108)/(110)peaks, indicating a preferred layered structure. Here, Rietveld refine-ments were carried out for the samples by using the space group ofR-3m and assuming that Ni2+ located at the 3b sites can be exchangedwith Li+ occupied at the 3a site due to the similarity of their ionic radii(Li+: 0.69 Å; Ni2+: 0.76 Å). Fig. 1 shows good agreement betweenthe experimental and calculated XRD patterns for both samples. TheRietveld refinement results and reliability factors are summarized inTable 2. From these refinements the amount of Ni2+ that replaces Li+

in the Li+-layers can be calculated, and this is found to be 2.44% and4.02% for the sol–gel and the co-precipitation routes, respectively.

Table 2Rietveld refinements of XRD patterns from Li[Ni0.8Co0.1Mn0.1]O2 synthesized by sol–gel and co

Sample a (Å) c (Å) Cell volume (Å3

Sol–gel 2.8725(0) 14.1990(3) 101.46(3)Co-precipitation 2.8707(0) 14.1982(7) 101.33(1)

Fig. 2 shows the FESEM images of Li[Ni0.8Co0.1Mn0.1]O2 powderssynthesized by the two different methods. The powders of both thesamples consist of particles with a diameter of 200–400 nm. It seemsthat the co-precipitation sample has smaller primary particle size butis in a serious aggregation. The aggregation makes it more difficult tobreak up during preparation of the electrode, whichwould give a small-er total surface area and hence less active surface area in contact withthe electrolyte compared to the more porous sol–gel synthesized sam-ple. The results from the BET surface measurements support the obser-vations from the FESEM images and clearly show that the surface area ofthe sample synthesized by the sol–gel method (1.18 m2·g−1) is muchlarger than the sample synthesized by the co-precipitation method(0.293 m2·g−1). The larger BET surface area of the sol–gel sample canbe explained by the large quantities of gas released during the thermaldecomposition of the gel, which would produce a more porous sampleand prevent hard agglomerates from forming.

Fig. 3 shows the charge/discharge curves for the initial cycle be-tween 2.5 and 4.3 V at room temperature at a current rate of 0.1 C(18 mA·g−1). The initial discharge capacity of the sample synthe-sized by the co-precipitation method is 195.7 mAh·g−1 with acolumbic efficiency of about 88.5%. The result is comparable to thatof the Li[Ni0.8Co0.1Mn0.1]O2 reported in literature [11,23,24], whichwas also prepared by co-precipitation method. The sample synthe-sized by the sol–gel method delivered an initial discharge capacityof 200.1 mAh·g−1 with a columbic efficiency of about 91.4%.

The capability at different discharge current rates of the Li[Ni0.8Co0.1Mn0.1]O2 samples is shown in Fig. 4. From 0.1 C to 2 C,the twomaterials show similar electrochemical performances. However,when the discharge current rate is increased to 5 C, the sample synthe-sized by the sol–gelmethod showsmore stable andhigher capacity com-pared to the sample synthesized by the co-precipitation method. Forinstance, the discharge capacity of the sol–gel sample is about 162 and140 mAh·g−1 at 1 C and 5 C, respectively. When the discharge currentrate goes back to 0.1 C, the discharge capacity of the two samples didnot showmuchdifference, and their capacities returned to approximate-ly 175 mAh·g−1. The difference in the discharge capacity between thesol–gel and the co-precipitated samples at high C-rates can possibly beexplained by the faster charge transfer in the sol–gel synthesized sampledue to higher surface area and higher porosity.

Fig. 5 shows the cycling performance of Li[Ni0.8Co0.1Mn0.1]O2 syn-thesized by the sol–gel and the co-precipitation methods cycled at acurrent rate of 0.5 C between 2.5 and 4.3 V. The samples synthesizedby the sol–gel method show capacity retention of 89.1%, 75.8% and65.4% for 50, 100 and 150 cycles, respectively. The sample synthesizedby co-precipitation shows lower capacity retention of 85.2%, 73.8%and 58.2% during the same cycling period. From the results of initialcharge–discharge, rate capability and cycling performance, we foundthat the sample synthesized by the sol–gel method displayed slightlyhigher initial discharge capacity, improved capacity retention and betterrate capability at high discharge rates compared to the sample synthe-sized by the co-precipitation method.

The better electrochemical performance of the sol–gel synthesizedmaterial might be attributed to the lower Li/Ni disorder in addition tothe higher surface area, the latter providing better contact between theelectrolyte and the active material. The fact that the co-precipitatedsample has a higher Li/Ni disorder leads to lower Li diffusivity, whichwill drive down its capacity at high rates.

Fig. 6 shows the first cycle CV curves of Li[Ni0.8Co0.1Mn0.1]O2 synthe-sized via the sol–gel and the co-precipitation routes in the potential

-precipitation methods.

) Ni2+ in Li+ layer (%) Crystal size (nm) Rwp (%)

2.44 98.9 1.514.02 102.1 1.35

Fig. 2. FESEM images of the Li[Ni0.8Co0.1Mn0.1]O2 powders synthesized via sol–gel (SG) and co-precipitation (CP) route.

108 H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

range of 2.5–4.5 V at a scanning rate of 0.1 mV·s−1. During charge–discharge, layered cathode materials with high nickel content suchas LiNiO2 and LiNi0.8Co0.15Al0.05O2, often exhibit four different phases(one monoclinic phase, M, and three hexagonal phases, H1, H2, andH3) [27–31]. As shown in Fig. 6, the curves of the two samples bothshow three pairs of redox peaks. The first oxidation peak corre-sponds to the material transformation from the pristine hexagonalphase (H1) to a new monoclinic phase (M), the second one corre-sponds to the transformation between the monoclinic phase and asecond hexagonal phase (H2), and the last one corresponds to H2transformation to a third hexagonal phase (H3). Because of thecomplicated phase transformations during charge and discharge,the material often shows unstable cycle performance. Furthermore,the high concentration of unstable Ni4+ in the delithiated state (H3phase) can be easily reduced to the divalent and insulating NiOphase at the cathode surface [32,33], which would also result in

Fig. 3. The initial charge and discharge curves of Li[Ni0.8Co0.1Mn0.1]O2 synthesized via sol–gel (SG) and co-precipitation (CP) route between 2.5 and 4.3 V at a current rate of 0.1 C.

capacity fading during cycling. It helps to understand why the mate-rial is often facing rapid capacity loss during cycling as shown inFig. 5.

3.2. The effects of different calcination atmospheres on the performance of Li[Ni0.8Co0.1Mn0.1]O2 synthesized by sol–gel method

In the following section, the potential for replacing pure O2 with airfor the calcination step is investigated. The Li[Ni0.8Co0.1Mn0.1]O2 cathodematerials were synthesized by the sol–gelmethod and calcined in differ-ent atmospheres as listed in Table 1.

XRD recorded for the Li[Ni0.8Co0.1Mn0.1]O2 powders is given in Fig. 7.For the sample synthesized in air (sample a), some impurity peaks(marked by *) appeared in the XRD pattern. These impurity peaks canbe attributed to Li2CO3 (JCPDS: 83-1454), related to the formation of aLi2CO3 phase through the reaction with CO2 in the air. For the other

Fig. 4. The rate capability of Li[Ni0.8Co0.1Mn0.1]O2 synthesized via sol–gel (SG) andco-precipitation (CP) route.

Fig. 5. The cycling performance of Li[Ni0.8Co0.1Mn0.1]O2 synthesized via sol–gel (SG) andco-precipitation (CP) route cycled at a current rate of 0.5 C between 2.5 and 4.3 V.

Fig. 7. X-ray diffraction patterns of Li[Ni0.8Co0.1Mn0.1]O2 powder synthesized in differentcalcination atmospheres (3.5 L·h−1): (a) air; (b) 50% O2 + 50% N2; (c) 75% O2 + 25%N2; and (d) 100% O2.

109H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

samples (samples b, c and d), calcined in themixed gas of N2/O2 or pureO2, no obvious Li2CO3 impurity peaks were observed. The fundamentalpeaks of all the samples can be indexed to theα-NaFeO2-type structurewith an R-3m space group. All the XRD patterns show a clear splittingbetween the (006)/(102) and (108)/(110) peaks which confirms thatthese samples have good layered characteristics.

The results discussed above indicated that when calcined in air ata flow rate of 3.5 L·h−1, the desired O2 concentration could not besupplied. However, by increasing the flow rate from 3.5 L·h−1 to10 L·h−1 the impurity peaks of the Li2CO3 phase disappeared, asshown in the XRD presented in Fig. 8. Similar to Figs. 1 and 7, all thepeaks can be indexed to the typical layered hexagonal α-NaFeO2

structure, and the pattern clearly indicates the formation of a high-ly ordered layered structure. The XRD results imply that if the flowrate of air is high enough to maintain the desired O2 concentra-tion for the synthesis, the reaction with CO2 to form Li2CO3 canbe suppressed. The Li[Ni0.8Co0.1Mn0.1]O2 powders (a, b, c and d) cal-cined in different atmospheres show similar morphology as the SGsample in Fig. 2, indicating that the different calcining atmospheresdid not lead to any other observable changes in the morphologies ofthe samples.

Fig. 6. The cyclic voltammetry of Li[Ni0.8Co0.1Mn0.1]O2 synthesized via sol–gel (SG)and co-precipitation (CP) route.

Fig. 9 shows the initial charge–discharge curves of Li/Li[Ni0.8Co0.1Mn0.1]O2 cells at a constant current density of 18 mA·g−1 (0.1 C) be-tween 2.5 and 4.3 V at room temperature. Table 3 compares the dis-charge capacity and columbic efficiency of the samples. All thesesamples were calcined at a flow rate of 3.5 L·h−1. As shown, with theincreasing O2 concentration in the calcination atmosphere, the dis-charge capacity and columbic efficiency of the samples are all improved.Sample c (calcined in 75% O2 + 25%N2) and sample d (calcined in pureO2) show similar charge–discharge curves, and they deliver dischargecapacities of 198.9 and 200.1 mAh·g−1, with the columbic efficiencyof 89.3% and 91.4%, respectively. The samples calcined in a low O2 con-centration (50% O2 + 50% N2) as well as in air, show lower dischargecapacity and lower columbic efficiency than those calcined in high O2

concentration atmospheres.

Fig. 8. X-ray diffraction patterns of Li[Ni0.8Co0.1Mn0.1]O2 powder synthesized in air at aflow rate of 10 L·h−1.

Fig. 9. The initial charge and discharge curves of Li[Ni0.8Co0.1Mn0.1]O2 synthesized indifferent calcination atmospheres (3.5 L·h−1): (a) air; (b) 50% O2 + 50% N2; (c) 75%O2 + 25% N2; and (d) 100% O2.

Fig. 10. The rate capability of Li[Ni0.8Co0.1Mn0.1]O2 synthesized in different calcinationatmospheres (3.5 L·h−1): (a) air; (b) 50% O2 + 50% N2; (c) 75% O2 + 25% N2; and(d) 100% O2.

110 H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

The rate capability performances of the Li[Ni0.8Co0.1Mn0.1]O2 sam-ples (a, b, c and d) synthesized in different atmospheres are comparedin Fig. 10. Samples d and c show similar rate capability when chargedand discharged at different current rates up to 1 C. While sample ashows the poorest electrochemical performance, the trends are identi-cal to the initial charge–discharge results discussed above.

The electrochemical results indicate that when calcined at highO2 concentration (pure O2 or 75% O2 + 25% N2 at a flow rate of3.5 L·h−1) a sufficient amount of oxygen is supplied, whereas whencalcined in the flows of 50% O2 + 50% N2 or air (3.5 L·h−1), the supplyof oxygen was insufficient.

The initial charge–discharge curves of the samples calcined in air at ahigh flow rate of 10 L·h−1, which are compared to the sample calcinedin pure oxygen and air at theflow rate of 3.5 L·h−1, are shown in Fig. 11.The cells were charged and discharged between 2.5 and 4.3 V at a cur-rent rate of 0.1 C (18 mA·g−1). Sample e (synthesized in air at a gasflow of 10 L·h−1) delivers a discharge capacity of 200.0 mAh·g−1

with a columbic efficiency of 90.2% which is much better than samplea (synthesized in air at a gas flow of 3.5 L·h−1), and is similar to sampled (synthesized in pure O2 at a gas flow of 3.5 L·h−1).

Fig. 12 shows the discharge capacities of the prepared samples as afunction of C-rate between 2.5 and 4.3 V. As illustrated in Fig. 12, therate capability for the Li[Ni0.8Co0.1Mn0.1]O2 materials calcined in air ata flow rate of 10 L·h−1 and in pure oxygen at a flow rate of 3.5 L·h−1

is quite similar from each other even up to 5 C rate. And the rate capa-bility of these two samples (d and e) are better than the sample a (cal-cined in air with a low flow rate of 3.5 L·h−1). The results of both thereversible capacity and the rate capability test show that the sampleobtained in air at a high flow rate (10 L·h−1) delivers nearly identicalreversible capacity and rate capability compared to the sample obtainedin pure oxygen.

Table 3Comparison of the discharge capacity and the columbic efficiency of the samples obtainedby sol–gel synthesis and in different calcination atmospheres.

Sample Calcinationatmosphere

Flow rate(L·h−1)

1st dischargecapacity (mAh·g−1)

Columbicefficiency

(a) Air 3.5 184.9 84.5%(b) 50% O2 + 50% N2 3.5 192.2 87.8%(c) 75% O2 + 25% N2 3.5 198.9 89.3%(d) 100% O2 3.5 200.1 91.4%(e) Air 10 200.0 90.2%

4. Conclusion

In conclusion, we reported high capacity nickel-based Li[Ni0.8Co0.1Mn0.1]O2 cathode materials synthesized by sol–gel and co-precipitation methods. The effects of different synthesis methods onthe crystal structure,morphology, BET surface and electrochemical prop-erties of thematerials have been investigated. The two samples preparedby the different methods show good layered characteristics and highcrystallinity. However, from Rietveld refinements of XRD data, it wasfound that the sample synthesized by the sol–gel method presentedlower site exchange between Ni2+ and Li+ (2.44%) compared to theco-precipitation sample (4.02%). Also, the sol–gel sample shows less ag-gregation and larger BET surface. An electrochemical study showed thatthe sample prepared by the sol–gel method has better electrochemicalproperties, with a higher initial discharge capacity of 200.1 mAh·g−1

and a higher capacity retention of 82.2% after 50 cycles at a cycling rateof 0.5 C, as well as better capability at 5 C. The better electrochemicalproperties of the sol–gel sample may be ascribed to the lower cation(Li/Ni) disorder in addition to the higher surface area that can providebetter contact between the electrolyte and the active material.

Fig. 11. The initial charge and discharge curves of Li[Ni0.8Co0.1Mn0.1]O2 synthesized in airat aflow rate of 10 L·h−1 (e), compared to the samples calcined in air (a) and pure oxygen(d) at the flow rate of 3.5 L·h−1.

Fig. 12. The rate capability of Li[Ni0.8Co0.1Mn0.1]O2 synthesized in air at a flow rate of10 L·h−1 (e), compared to the samples calcined in air (a) and pure oxygen (d) at theflow rate of 3.5 L·h−1.

111H. Lu et al. / Solid State Ionics 249–250 (2013) 105–111

We also reported the effect of calcination atmosphere on the struc-ture and electrochemical performance of the Li[Ni0.8Co0.1Mn0.1]O2 cath-ode materials. XRD results revealed that when calcined in a low flowrate of air, some Li2CO3 impurities formed through the reaction of thesample with CO2 in the air. However, when calcined in a high flowrate of air, the impurity peaks disappeared. Electrochemical perfor-mance showed that with increased O2 concentration in the calcinationatmosphere, the capacity and columbic efficiency of the samples wereall improved. At the same time, increasing the flow rate of air alsohelped to improve the electrochemical performance. The sampleobtained in air at a high flow rate (10 L·h−1) delivered nearly identicalreversible capacity and rate capability compared to the sample obtainedin pure oxygen. The results obtained in this work indicated that it wasfeasible to synthesize Li[Ni0.8Co0.1Mn0.1]O2 cathode materials withgood electrochemical performance in high flow air atmosphere, whichmay help in reducing preparation cost than using pure O2 atmosphere.

The Li[Ni0.8Co0.1Mn0.1]O2 material is attractive as a positive electrodematerial for high power energy lithium-ion batteries, and this workshows that low cost and environmentally friendly sol–gel synthesis is avery promising route to achieve high performance cathodes. However,the cycling stability of Li[Ni0.8Co0.1Mn0.1]O2 material still needs further im-provement. Methods for achieving this will be explored in a future study.

Acknowledgment

This work was carried out at the Department of Materials Scienceand Engineering, Norwegian University of Science and Technology(NTNU) under the financial support from the grant with the NorwegianResearch Council and SINTEF/NTNU (grant no. 208773/E20).

Pei Na Kui and Dr. Julian Tolchard at NTNU are gratefully acknowl-edged for their experimental assistance and valuable discussions.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.ssi.2013.07.023.

References

[1] T. Ohzuku, Y. Makimura, Chem. Lett. 30 (2001) 642.[2] Z. Lu, D.D. MacNeil, J.R. Dahn, J. Sol–Gel Sci. Technol. 4 (2001) A200.[3] Y. Makimura, T. Ohzuku, J. Power Sources 156 (2003) 119.[4] X.Y. Zhang, W.J. Jiang, L. Qi, et al., J. Power Sources 195 (2010) 1292.[5] S.H. Park, H.S. Shin, Y.K. Sun, et al., Chem. Mater. 17 (2005) 6.[6] Y. Idemoto, T. Matsui, Solid State Ionics 179 (2008) 625.[7] K.B. Kamel, N. Amdouni, M. Dhahbi, et al., J. Sol–Gel Sci. Technol. 59 (2011) 358.[8] S.W. Cho, K.S. Ryu, Mater. Chem. Phys. 135 (2012) 533.[9] S.H. Park, C.S. Yoon, Y.K. Sun, et al., Electrochim. Acta 49 (2004) 557.

[10] S.W. Oh, S.H. Park, Y.K. Sun, et al., Solid State Ionics 171 (2004) 167.[11] K.S. Lee, S.T. Myung, K. Amine, et al., J. Electrochem. Soc. 154 (2007) A971.[12] J.J. Saavedra-Arias, N.K. Karan, D.K. Pradhan, et al., J. Power Sources 183 (2008)

761.[13] C.C. Chen, C.J. Wang, B.J. Hwang, J. Power Sources 146 (2005) 626.[14] J. Cho, H.S. Jung, Y.C. Park, et al., J. Electrochem. Soc. 147 (2000) 15.[15] E. Zhecheva, R. Stoyanova, R. Alcantara, et al., Appl. Chem. 74 (2002) 1885.[16] V. Bianchi, S. Bach, C. Belhomme, et al., Electrochim. Acta 46 (2001) 999.[17] G. Hu, W. Liu, Z. Peng, et al., J. Power Sources 198 (2012) 258.[18] J. Eom, M.G. Kim, J. Cho, J. Electrochem. Soc. 155 (2008) A239.[19] H.S. Liu, Z.R. Zhang, Z.L. Gong, et al., J. Sol–Gel Sci. Technol. 7 (2004) A190.[20] K. Shizuka, C. Kiyohara, K. Shima, et al., J. Power Sources 166 (2007) 233.[21] L.J. Li, X.H. Li, Z.X. Wang, Powder Technol. 206 (2011) 353.[22] S.U. Woo, B.C. Park, S.T. Myung, et al., J. Electrochem. Soc. 154 (2007) A649.[23] B. Zhang, L. Li, J. Zheng, et al., J. Alloys Compd. 520 (2012) 190.[24] M.H. Kim, H.S. Shin, D. Shin, et al., J. Power Sources 159 (2006) 1328.[25] S.W. Woo, S.T. Myung, H. Bang, et al., Electrochim. Acta 54 (2009) 3851.[26] R.J. Gummow, M.M. Thackeray, W.I.F. David, et al., Mater. Res. Bull. 27 (1992) 327.[27] W. Li, J.N. Reimer, J.R. Dhan, Solid State Ionics 67 (1993) 123.[28] H. Arai, S. Okada, H. Ohtsuka, et al., Solid State Ionics 80 (1995) 261.[29] R. Alcántara, P. Lavela, J.L. Tirado, Chem. Mater. 9 (1997) 2145.[30] M.Y. Song, D.S. Lee, H.R. Park, Mater. Res. Bull. 47 (2012) 1021.[31] Y.R. Bak, Y.M. Chung, J.H. Ju, et al., J. New Mater. Electrochem. Syst. 14 (2011)

203.[32] D.P. Abraham, R.D. Twesten, M. Balasubramanian, et al., J. Electrochem. Soc. 150

(2003) A1450.[33] I. Belharouak, W. Lu, D. Vissers, et al., Electrochem. Commun. 8 (2006) 329.