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Electrochimica Acta 85 (2012) 605– 611

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

uO-coated Li[Ni0.5Co0.2Mn0.3]O2 cathode material with improved cyclingerformance at high rates

ing Liua,b, Shi-Xi Zhaob,∗, Kezhen Wanga,b, Ce-Wen Nana,b

Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, ChinaDepartment of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China

r t i c l e i n f o

rticle history:eceived 7 May 2012eceived in revised form 23 August 2012ccepted 24 August 2012vailable online xxx

a b s t r a c t

CuO was used to modify the surface of Li[Ni0.5Co0.2Mn0.3]O2 cathode material. The structure and electro-chemical properties of the CuO-coated Li[Ni0.5Co0.2Mn0.3]O2 were investigated using X-ray diffraction,scanning electron microscope, and charge/discharge tests. The results showed that the CuO coatedLi[Ni0.5Co0.2Mn0.3]O2 cathode exhibited an improved rate capability at room and elevated temperature athigh rates. The 2.0 wt.% CuO coated sample had the capacity retention of higher than 89%, and high capac-

eywords:ithium ion batteriesathode materiali[Ni0.5Co0.2Mn0.3]O2

uO coating

ity of 179.7 mAh g−1 at 5C, in comparison with the capacity retention of 60% and capacity of 161.5 mAh g−1

for the pristine one at elevated temperature. The cyclic voltammograms and impedance spectra resultsrevealed that the CuO coating reduced the polarization and improved the electrochemical activity of cath-ode. Thus the CuO-coated Li[Ni0.5Co0.2Mn0.3]O2 shows a potential lithium ion batteries for high powerapplications.

igh rates

. Introduction

Recently, Li[Ni, Co, Mn]O2 has been considered to be one ofhe most attractive cathode materials due to its high capacity,afety and low cost. The Li[Ni, Co, Mn]O2 cathode, which can offeri4+/Ni2+ and Co4+/Co3+ redox couple during electrochemical reac-

ion, exhibits much higher capacity (>170 mAh g−1) compared tohe currently used LiCoO2 cathodes(135 mAh g−1) [1]. However, itas been reported that the poor electrochemical properties of lay-red Li[Ni, Co, Mn]O2 at high voltage (>4.6 V) due to the polarizationffect and the electrolyte decomposition [2–4]. Moreover, even atpper voltage limits of 4.3–4.5 V, capacity fading is still observedpon cycling [5–7]. An alternate approach to improve electrochem-

cal performance is to change the surface properties of the cathodeaterial by coating its particles with some metal oxides [8–11]

o avoid the unwanted reactions on the surface and protect theulk. For example, the cycle-life performance of LiCoO2 and LiFePO4an be improved by a CuO coating with a reasonable high Li iononductivity [12,13].

In this article, we examine the effect of surface modificationf the Li[Ni0.5Co0.2Mn0.3]O2 by CuO coating on the electrochemi-

al performance at room and elevated temperature. The treatments expected to affect the cycleability of cathode at high rates.he preparation, structure and electrochemical performance of

∗ Corresponding author. Tel.: +86 0755 26036372; fax: +86 0755 26036372.E-mail address: zhaosx@sz.tsinghua.edu.cn (S.-X. Zhao).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.08.101

© 2012 Elsevier Ltd. All rights reserved.

the surface-treated Li[Ni0.5Co0.2Mn0.3]O2 cathode material are dis-cussed in comparison with the pristine one.

2. Experimental

Commercially available spherical Li[Ni0.5Co0.2Mn0.3]O2 powder(Sxtrcn Co., China) was utilized as pristine sample. Reagent gradechemical of Cu(Ac)2·H2O(AR) was used as starting materials. Forcoating CuO on the Li[Ni0.5Co0.2Mn0.3]O2 surface, Cu(Ac)2·H2O wasdissolved in the methanol. To compare the influence of Cu2+ con-centration in the coating solution on the electrochemical propertiesof surface-modified Li[Ni0.5Co0.2Mn0.3]O2, the volume of coatingsolution was fixed, while the amount of CuO was varied from0.5 to 4.0 wt.% of the Li[Ni0.5Co0.2Mn0.3]O2 powders. CommercialLi[Ni0.5Co0.2Mn0.3]O2 powders were added into the coating solu-tion and then stirred at room temperature for 4 h, in successionconstantly stirred at 80 ◦C to evaporate solvent over. After coat-ing, the resulting sample was dried at 110 ◦C for 2 h and heatedat 500 ◦C for 4 h in air to obtain the surface-modified sphericalLi[Ni0.5Co0.2Mn0.3]O2 powders.

The pristine and CuO coated samples were characterized byPowder X-ray Diffraction (D/MAX-2500, Rigaku, Japan) to iden-tify the crystalline phase, and by field-emission scanning electron

microscopy (MIRA3XMH, TESCAN, Czech) for morphologies. Energydispersive X-ray spectroscopy (EDS) was employed to analyze thesurface composition and element distribution of cathode materi-als. An inductively coupled plasma-atomic emission spectrometer

606 T. Liu et al. / Electrochimica Acta 85 (2012) 605– 611

Table 1ICP-AES results of CuO-coated Li[Ni0.5Co0.2Mn0.3]O2 powders.

Sample designation CuO (wt.%)

Design valuea ICP-AES measured value

0.5 wt.%-CuO 0.50 0.471.5 wt.%-CuO 1.50 1.582.0 wt.%-CuO 2.00 2.082.5 wt.%-CuO 2.50 2.643.0 wt.%-CuO 3.00 3.124.0 wt.%-CuO 4.00 4.15

a

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10C rate is only 22%. However, the 2.0 wt.% CuO coated sam-

The amount of CuO coating is evaluated with respect to pristinei[Ni0.5Co0.2Mn0.3]O2.

ICP-AES, Optima 3000DV, Perkin Elmer, USA) was used to evaluatehe amount of CuO coated on the Li[Ni0.5Co0.2Mn0.3]O2 powders.

Electrochemical performances were evaluated with CR2032oin cells. The coin cells were assembled with fabricated cath-de, lithium foil anode, 1 M LiPF6 in ethylene carbonate/diethylarbonate/ethyl methyl carbonate (SHANSHANTECH Co., China)lectrolyte, and a porous polypropylene separator (Celgard 2400,elgard Inc., USA). We used a slurry of 80 wt.% prepared pow-ers, 10 wt.% acetylene black, and 10 wt.% polyvinylidene fluoridePVDF) in N-methyl pyrrolidone (NMP) solvent. This mixture wasoated onto an aluminum foil current collector and dried at 150 ◦Cor 12 h. The coated cathode foil was cut into circular discs of5 mm in diameter. The cells were assembled in an argon-filledlove box (with O2 and H2O levels below 5 ppm). Charge/dischargeycling were galvanostatically carried out using a battery test sys-em (C2001A, LAND, China) at rates of 1C–10C (1C = 190 mA g−1).IS measurements were conducted (CHI660D, CHENHUA, China)n the frequency range of 100 kHz to 1 mHz with an AC voltagemplitude of 5 mV. All the EIS data were collected after discharginghe cells to 3.0 V at 1C. Cyclic voltammograms were run on a VMP

ultipotentiostat (VMP3, Biologic Science Instruments, France) at scan rate of 0.1 mV s−1 between 3.0 and 4.6 V.

. Results and discussion

The ICP-AES analyzed concentration of Cu2+ coated oni[Ni0.5Co0.2Mn0.3]O2 is recorded in Table 1. The nominal compo-ition of CuO in the initial design for synthesis is also listed inable 1. With the concentration between these two values, it isble to control the quantity of coated CuO precisely in the adoptedrocess.

Fig. 1 exhibits the powder XRD patterns of the CuO coated andristine Li[Ni0.5Co0.2Mn0.3]O2 samples. All the observed peaks forristine powder can be indexed to a hexagonal �-NaFeO2 structureith space group R3m [14]. The XRD patterns appear to be almost

dentical for all the materials. However, the crystalline phase of theoating is clearly detected in the diffraction patterns of coated sam-les which the quantity of coating is over 0.5 wt.%. The small peakt 2� = 35.495◦ and 38.730◦ corresponds to (0 0 2) and (1 1 1) reflec-ion peak of CuO phase (JCPDS no. 45-0937); which indicates thathe coating of samples heat-treated at 500 ◦C is a crystalline phasef CuO. As seen in Fig. 1(b), the intensity of the two CuO reflectioneak increases with the quantity of coating. This is consistent withhe result of ICP analysis in Table 1. Apart from the small peaks asso-iated with the coating material, a structural change in the pristineowder due to surface coating and heat-treatment is not observed

n the XRD patterns.As seen from SEM images of pristine and CuO coated powders

hown in Fig. 2, the pristine powder appears as 5–10 �m spheresomposed of 0.3–0.5 �m polyhedral primary particles. The surfacesf the pristine Li[Ni0.5Co0.2Mn0.3]O2 particle are smooth and clean.

Fig. 1. XRD patterns of pristine and CuO coated Li[Ni0.5Co0.2Mn0.3]O2 powders.

In comparison, the surfaces of the coated sample become a bitcoarse, covered with some nano-particles 10–60 nm in size.

Fig. 3 exhibits the element mapping of the 2.0 wt.% CuO-coatedLi[Ni0.5Co0.2Mn0.3]O2 particles. Mapping of copper has the sameshape as the cathode material particle, indicating that there is aCuO coating on the surface of the cathode material and the coatingis uniform. This distribution, serving as a protection material wouldbe favorable for improving cycling performance.

In order to investigate the electrochemical properties of thesamples, we adopt noncontinuous rates-alteration cycling styleto check the electrochemical performance for the cathode mate-rial. Charge/discharge cycling behavior of various amounts CuOcoated Li[Ni0.5Co0.2Mn0.3]O2 are compared with that of the pris-tine sample. Fig. 4 shows the rate capability tests for the samplesat 25 ◦C with increasing rates. The pristine and coated samplesshowed a similar discharge capacity at low C rates. However,as the C rate increases, the 2.0 wt.% coated sample exhibits anoticeably higher discharge capacity than the pristine and othersamples, which implies enhancement of the rate capability. Forinstance, the capacity retention for the pristine electrode at the

ple exhibits as high as 46% capacity retention under the samemeasurement condition. It is worth noting that the dependenceupon the amount of coating material. When the amount of

T. Liu et al. / Electrochimica Acta 85 (2012) 605– 611 607

F s: (a) 0

ceai

ig. 2. SEM images of the various amount CuO coated Li[Ni0.5Co0.2Mn0.3]O2 powder

oating material was more or less than 2.0 wt.%, the positiveffect of the coating on the rate capability was lower. The 3.0nd 4.0 wt.% CuO coated samples show inferior discharge capac-ty to the pristine sample at all C rates. The improved performance

.0 wt.%; (b) 0.5 wt.%; (c) 1.5 wt.%; (d) 2.0 wt.%; (e) 2.5 wt.%; (f) 3.0 wt.%; (g) 4.0 wt.%.

may be due to the protection of the electrode surface by theCuO coating from electrolyte corrosion. Moreover, CuO coatingsmay also have acted as an efficient transportation medium of Li+

between cathode and electrolyte. It is reported that layer LixCuO2

608 T. Liu et al. / Electrochimica Acta 85 (2012) 605– 611

uO coa

ccpo

gm3etiftl

Fig. 3. EDS images of the 2.0 wt.% C

ompounds can be formed with CuO and Li2O and exhibit electro-hemical activity during lithiation/delithiation cycling [15]. Theseroperties lead to increased rate capability of the CuO coated cath-de.

In practice, a lower coating level of 0.5 wt.% would be advanta-eous, but cannot achieve a sufficiently compact coating on the coreaterial. However, when the coating level is increased further to

.0 wt.%, the cycle stability and initial capacity declined. The pres-nce of excess coating material between the particles may lowerhe core material’s electronic conductivity. It is clear that 2.0 wt.%

s optimum to form a uniform coating and result in best cycling per-ormance. Since the cathode material is exposed to the electrolyte,he cathode material may have a reaction with the electrolyte andead to a capacity loss. Therefore, the optimum level of 2.0 wt.% CuO,

ted Li[Ni0.5Co0.2Mn0.3]O2 powders.

could prevent direct contact between the cathode particles and theelectrolyte, and thereby decrease the capacity loss and retain thecycle stability.

Fig. 5 shows the discharge capacity of the pristine and 2.0 wt.%CuO-coated samples as a function of cycle number at 60 ◦C inlithium cells at a rate of 5C. The result shows a vivid differencebetween the pristine and coated samples. The initial dischargecapacity of the CuO-coated Li[Ni0.5Co0.2Mn0.3]O2 is 179.7 mA h g−1,while that of the pristine sample is only 161.5 mA h g−1. More-over, the cycling behavior of the CuO-coated Li[Ni0.5Co0.2Mn0.3]O2

greatly improved, showing a capacity retention of 89% after 50cycles, while the pristine electrode shows a gradual decrease incapacity, leading to a capacity retention of only 60% during the samecycling period.

T. Liu et al. / Electrochimica Acta 85 (2012) 605– 611 609

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ig. 4. Discharge capacities and cyclic performances of pristine and CuO coatedi[Ni0.5Co0.2Mn0.3]O2 cathode in the voltage range of 3.0–4.6 V at different rates.

It is known that chemical or structural instability begins in theigh voltage range (i.e., above 4.6 V) for the Li[Ni, Co, Mn]O2 cathode16]. In order to illustrate the effects of the coating on the dischargeapacity and cycling behavior under severe measurement condi-ions, the upper cut-off voltage is increased to 4.8 V. Fig. 6 shows theischarge capacity and cyclic performances of the cathodes in theoltage range of 4.8–3.0 V at a 1C rate at 25 ◦C and 60 ◦C, revealinghat the cyclic performance under a high cut-off voltage is improvedy the surface coating. For a pristine electrode, the capacity reten-ion during 50 cycles is 39% at 25 ◦C and 43% at 60 ◦C. In contrast,he capacity retention increases to 43% at 25 ◦C and 51% at 60 ◦C forhe 2.0 wt.% CuO-coated sample. Although the CuO coating does noterfectly suppress the instability and the rapid fading of dischargeapacity in the high cut-off voltage range, but the coating treatmentan slow down the structure and performance deteriorating.

Impedance spectra of the cell containing samples are measuredo confirm the coating effect. Fig. 7 compare the Nyquist plots ofhe 2.0 wt.% CuO coated and pristine samples (after 50 and 100ycles). In general, the impedance spectra for a lithium batteryest cell shows two semicircular curves and a line inclined at a

onstant angle to the real axis. Rs represents the impedance of solu-ion. A high-frequency semicircle represents the impedance (Rsf)ue to a solid-state interface layer formed on the surface of the

ig. 5. Discharge capacity of pristine and 2.0 wt.% CuO-coated Li[Ni0.5Co0.2Mn0.3]O2

ells at a rate of 5C between 3.0 and 4.6 V at 60 ◦C.

Fig. 6. Cyclic performances of pristine and 2.0 wt.% CuO-coatedLi[Ni0.5Co0.2Mn0.3]O2 electrodes in voltage range of 4.8–3.0 V at 1C at 25 ◦C(a) and 60 ◦C (b).

electrodes. The intermediate-frequency semicircle represents thecharge-transfer resistance (Rct) in the electrode/electrolyte inter-face. The slope in the low frequency refers to lithium-ion diffusionin the bulk material [17].

We used Z-View fitting procedure to simulate the impedancedata applying the inset equivalent circuit in Fig. 7. In fact we couldfind that the first semi-circle was integrated by the two circles.That is maybe the effects of negative electrode, so we adopt thenew equivalent circuit to simulate the data. In our equivalent cir-cuit, Rs represents the impedance of solution, R1 represents thecharge-transfer resistance of lithium corresponding to the neg-ative electrode. Rf represents the impedance of the impedancedue to a solid-state interface layer formed on the surface of thecathode. Rct represents the charge-transfer resistance in the elec-trode/electrolyte interface. As shown in Table 2, the R1 and Rf forboth electrodes are relatively small compared to Rct and remainalmost stable during cycling. It is noticed that Rct for both electrodesrapidly increases with cycling: the Rct value of the pristine sampleis 640 � after 50 cycles, increasing to 1493 � after 100 cycles, whileit is 573 � after 50 cycles and 679 � after 100 cycles for the coatedsample. In this regard, CuO coating is very effective in decreasing

the side reactions. CuO coating may enhance the lithium ion con-ductivity of the sample’s surface. This function can facilitate themigration of lithium ion at the surface of cathode. And also theimproved surface lithium ion conductivity decreases the values of

610 T. Liu et al. / Electrochimica Acta 85 (2012) 605– 611

ated

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Fig. 7. Electrochemical impedance spectra of the 2.0 wt.% CuO (a) co

ct. These results corroborate the findings regarding the enhancedate capability of the CuO coated cathode. Based on these data, weelieve that appropriate CuO coating is helpful for inhibiting theide reaction between the cathode and electrolyte associated withhe increase of lithium ion conductivity.

A cyclic voltammetry study is carried out to evaluatehe cathode performance of 2.0 wt.% CuO coated and pristinei[Ni0.5Co0.2Mn0.3]O2 materials in the potential region 3.0–4.6 V.ig. 8 shows the cyclic voltammograms of 2.0 wt.% CuO coated andristine Li[Ni0.5Co0.2Mn0.3]O2, respectively. Only one anodic peak

s found in the CV profiles of CuO coated and pristine samples, indi-ating only Ni2+/Ni4+ process is involved. In the previous research,nly one anodic peak was observed upon the transition of Ni2+ toi4+ in the CVs [18,19]. This is because the Jahn–Teller distortion of

able 2imulated parameters using equivalent circuit in Fig. 7.

50th

RS (�) R1 (�) Rf (�) Rct (

2.0% coated 7 6 10 573Pristine 8 6 16 640

and pristine (b) Li[Ni0.5Co0.2Mn0.3]O2 cathode after different cycles.

Ni3+(d7) in NiO6 octahedra leads to the direct oxidation of Ni2+ toNi4+[5].

For the pristine sample, the first cycle anodic peak is cen-tered at potentials 3.983 V (vs. Li+/Li) that correspond to thedelithiation from the lattice. The cathodic peaks is centered at3.678 V, which corresponds to the intercalation of the Li ion.The major anodic/cathodic peaks at ∼3.8 V is assigned to theoxidation/reduction of Ni2+/Ni4+ accompanying the (de)lithiationprocesses. The initial shift in the first cycle anodic peak poten-tial is an indication of the electrochemical alteration in the active

materials. This behavior is believed to be due to initial activa-tion and stabilization. No considerable change in the position orintensity of the peak is observed during cycling. This indicates thatthe initial layered crystal structure is maintained with no phase

100th

�) RS (�) R1 (�) Rf (�) Rct (�)

8 5 18 679 4 9 60 1493

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ig. 8. Cyclic voltammetry of the 2.0 wt.% CuO (a) coated and pristine (b)i[Ni0.5Co0.2Mn0.3]O2 cathode between 3.0 and 4.6 V at a scan rate of 0.1 mV s−1.

ransitions. In the following scans, compared to the pristine sam-le, the major oxidation peak of the coated sample shifts to a loweroltage; the reduction peak shifts to a higher voltage. The poten-ial difference between the oxidation peak and the reduction peakecreases for the coated samples, it is advantageous to decreasehe interfacial polarization of cathodes. The current intensity of

he anodic peak increases with cycle numbers for the CuO coatedample while decreases for the pristine one. All these results sug-est that the reversibility of the CuO coated Li[Ni0.5Co0.2Mn0.3]O2athode is much better than that of the pristine one.

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cta 85 (2012) 605– 611 611

4. Conclusions

CuO has been successfully coated on the surface ofLi[Ni0.5Co0.2Mn0.3]O2 material. The coating modifies the sur-face characteristics of electrode material and suppresses theside reactions between electrode and electrolyte. At high rates,the 2.0 wt.% CuO coated Li[Ni0.5Co0.2Mn0.3]O2 cathode exhibitsmuch enhanced rate capability and cycling performance com-pared to the pristine sample at room and elevated temperature.This enhancement can be explained that CuO coating acts as aneffective lithium-ion conductor, as well as a protective materialagainst corrosion from electrolyte. As a result, enhanced dischargecapacity and better capacity retention are obtained. Thus anappropriate CuO coating is a promising method to overcome theexisting problems of layer Li[Ni, Co, Mn]O2 cathode for high powerapplications.

Acknowledgements

This work was supported by the Research Projects of NationalNSFC (No. 51172124). The authors would like to thank XiaochongZhao and Xuefeng Fan for helpful discussion. The authors wouldalso like to thank Shoue Ma for graphs processing.

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