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Electrochimica Acta 109 (2013) 52– 58

Contents lists available at ScienceDirect

Electrochimica Acta

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

aF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials for Li-ionatteries

iaoyu Liu, Jali Liu, Tao Huang ∗, Aishui Yu ∗

epartment of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis andnnovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China

r t i c l e i n f o

rticle history:eceived 18 April 2013eceived in revised form 25 June 2013ccepted 10 July 2013vailable online 22 July 2013

eywords:ithium-ion battery

a b s t r a c t

Li-rich cathode material Li1.2Mn0.54Ni0.13Co0.13O2 is prepared by a sol–gel method and coated with CaF2

layer via a wet chemical process. The pristine and CaF2-coated samples are characterized by X-raydiffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).An amorphous nanolayer coating of CaF2 is obtained on the surface of layered pristine material. TheCaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 material exhibits excellent electrochemical performance. The ini-tial coulombic efficiency is enhanced to 89.6% with high initial discharge capacity of 277.3 mAh g−1 afterCaF2 coating. Galvanostatic charge–discharge tests at 0.2 C display faster activation of Li2MnO3 phase and

i-rich cathode materialurface coatingmproved electrochemical performance

higher capacity retention of 91.2% after 80 cycles for CaF2-coated material. Meanwhile it also shows higherrate capability with the capacity of 141.5 mAh g−1 at the 3 C-rate and stable cyclic performance above190 mAh g−1 after 100 cycles at the 1 C-rate. The analysis of dQ/dV plots and electrochemical impedancespectroscopy (EIS) indicates that the obvious improvement of CaF2 coating is mainly attributed to theaccelerated phase transformation from layered phase to spinel phase and stable electrolyte/electrode

to the

interfacial structure due

. Introduction

Lithium ion battery with high energy density and high powerensity has been considered as the promising power sources forlectric vehicles (EV) and hybrid electric vehicles (HEVs) [1,2]. How-ver, the conventional cathode materials, such as LiCoO2, LiMn2O4nd LiFePO4, cannot meet the requirements of high energy densityue to its limited specific capacity. Recently, Li-rich Mn-based lay-red solid-solution system Li2MnO3-LiMO2 (M = Co, Ni, Mn1/2Ni1/2,n1/3Ni1/3Co1/3) has drawn much attention for its higher capac-

ty over 250 mAh g−1 with significantly reduced cost and toxicityompared to the LiCoO2 cathode [3,4]. Different from conven-ional cathode materials, Li-rich cathode materials exhibit a longoltage plateau at about 4.5 V in the first charging process, corre-ponding to the delithiation process accompanied with oxidationf O2− and subsequent evolution of oxygen gas [5–7]. This processs regarded as the activation of electrochemical inactive Li2MnO3hase and responsible for the unusually high discharge capacity.owever, several drawbacks hinder extensive commercial appli-

ation of Li-rich cathode materials. The first problem is the hugerreversible capacity loss (ICL) in the first cycle which is attributedo the extraction of Li2O followed by an elimination of the oxide

∗ Corresponding authors. Tel.: +86 21 51630320; fax: +86 21 51630320.E-mail address: asyu@fudan.edu.cn (A. Yu).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.069

suppression of the electrolyte decomposition.© 2013 Elsevier Ltd. All rights reserved.

ion vacancies from the structure during the first charge, leadingto fewer insertion–extraction sites of lithium ions in the dischargeprocess [8]. Furthermore, poor cyclic performance and rate capa-bility due to complicated structural evolution and formation ofsolid–electrolyte interfacial (SEI) layer at higher potential are alsopractical problems to solve urgently.

One common approach to improve the electrochemical perfor-mance of the Li-rich cathode material is to modify the cathodesurface with materials inert against the electrolyte. Surface modi-fied (1−z)Li[Li1/3Mn2/3]O2-(z)Li[Mn0.5−yNi0.5−yCo2y]O2 with seriesof metal oxide, such as Al2O3, ZnO, ZrO, CeO2, show lower ICL inthe first cycle and higher discharge capacity than pristine samples[9]. Double-layer surface modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2,reported by Manthiram, exhibits better rate capability and capac-ity retention [8,10]. In recent years, AlF3 is considered to be one ofthe most promising coating materials to improve electrochemicalperformance of Li-rich cathode, due to its high stability [11–15].The existence of the AlF3 coating layer is believed to suppress theHF corrosion which is responsible for better cycle stability. Fur-thermore, it is also proposed that AlF3 could induce the phasetransformation of the Li-rich cathode, from layered phase to spinelphase, which is beneficial for rate performance [13]. In this paper,

CaF2 was first introduced and utilized as the coating materialfor Li-rich cathode Li1.2Mn0.54Ni0.13Co0.13O2 to further investigatethe role of metal fluoride coating. Compared to AlF3, CaF2 showsbetter thermal stability and equally inert behavior in electrolyte

imica Acta 109 (2013) 52– 58 53

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16,17]. A comparison of pristine and CaF2-coated Li-rich cathodei1.2Mn0.54Ni0.13Co0.13O2 on structure and electrochemical per-ormance was conducted. As cathode of lithium ion battery, theaF2-coated Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 demonstrates higherapacity retention, rate capability and more stable cyclic perfor-ance.

. Experimental

.1. Preparation and characterization

The Li-rich layered oxide, Li1.2Mn0.54Ni0.13Co0.13O2 was syn-hesized by a sol–gel process with citric acid as chelating agent.toichiometric amounts of Li(CH3COO)·2H2O, Ni(CH3COO)2·4H2Ond Mn(CH3COO)2·4H2O were dissolved in distilled water andtirred continuously. The ratio of metal: chelating agent was 1:1.fter adjusting the pH value to 7–8 with ammonium hydroxide, theolution was evaporated at 80 ◦C until a viscous violet gel emerged.he gel was dried in a vacuum oven at 120 ◦C for 12 h. The precursorowders were decomposed at 450 ◦C for 5 h. The obtained pow-ers were compressed in the form of circular pellets and calcinedt 900 ◦C for 12 h in air to obtain the final product.

A chemical deposition method was used to obtainaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2. The as-preparedi1.2Mn0.54Ni0.13Co0.13O2 powders was immersed in the Ca(NO3)3ilute aqueous solution. Then the solution was heated to 80 ◦C andtirred vigorously. NH4F dilute solution was then added into theolution drop by drop. The molar ratio of Ca to F was controlledo be 1:2, and the designed amount of CaF2 was 2 wt.% of thei-rich layered oxide. Continuous stirring was performed till theixed solution was evaporated to dryness. The obtained powdersere heated at 450 ◦C in the flowing nitrogen for 4 h to obtain theaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2.

The structure of the as-prepared powders was confirmed by-ray diffraction (XRD, Bruker D8 Advance, Cu K� radiation,

= 1.5406 A) with a range of 2� from 10◦ to 80◦at a scan ratef 1 min−1. The morphology and composition of the samplesere investigated by scanning electron microscopy (SEM, JEOL

SM-6390) and field-emission scanning electron microscopy (FE-EM, Hitachi S-4800) equipped with an energy-dispersive X-raypectrometer (EDX). The CaF2 coating layer was analyzed by trans-ission electron microscopy (TEM, JEM-2100F).

.2. Electrochemical measurements

The working electrodes were consisted of 70 wt.% as-preparedowders, 20 wt.% carbon conductive agents (super P) and 10 wt.%olytetrafluoroethylene (PTFE) and compressed onto the alu-inum nets. The total mass of the active electrode material is about

–5 mg and the electrode surface area is 0.785 cm2 (˚10 mm). Thelectrodes were dried overnight at 80 ◦C in a vacuum oven prior tose. The A metallic lithium foil was used as an anode, 1 M LiPF6 inthylene carbonate (EC)–dimethyl carbonate (DMC)–diethyl car-onate (DEC) (1:1:1 in volume) was used as the electrolyte, and

polypropylene micro-porous film (Cellgard 2300) served as theeparator. Coin-type (CR2032) half-cells were assembled in anrgon-filled glove box (Mikarouna, Superstar 1220/750/900). Thealvanostatic discharge–charge measurements were performed onattery test system (Land CT2001A, Wuhan Jinnuo Electronic Co.td.) between 2.0 and 4.8 V (vs. Li+/Li) at different rates at room tem-erature. EIS measurements were performed on an electrochemical

orkstation (CH Instrument 660A, CHI Company). Impedance spec-

ra of the pristine and CaF2-coated samples were tested at the openircuit potential before charge and after cycle test, respectively. Themplitude of the AC signal was 5 mV over a frequency range from

2θ

Fig. 1. XRD patterns of (a) pristine and (b) CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2.

100 kHz to 10 mHz. The EIS results were simulated using ZPVIEWsoftware.

3. Results and discussion

3.1. Structure and morphology of pristine and CaF2-coatedLi1.2Mn0.54Ni0.13Co0.13O2

XRD patterns of pristine and CaF2-coatedLi1.2Mn0.54Ni0.13Co0.13O2 are shown in Fig. 1(a) and (b) respec-tively. All the diffraction peaks of both samples, except weak peaksbetween 2� = 20–25◦, are indexed to �-NaFeO2 hexagonal typestructure with a space group symmetry of R 3 m. Clear separationof adjacent peaks of (0 0 6)/(0 1 2) and (1 0 8)/(1 1 0) indicates thatthe samples have a well crystalline layered structure. Weak peakslocated between 2� = 20–25◦ show the presence of monoclinicLi2MnO3 phase with a space group symmetry of C2/m. These peaksare attributed to the short-range Li–Mn cation ordering in thetransition metal layers [18,19]. The low intensity results from thedisorder of LiMn6 clusters with addition of Ni and Co [20]. There isno peak indexed to CaF2 in XRD patterns, resulting from the lowquantity and poor crystalline of coating CaF2 material.

SEM images and EDX patterns are shown in Fig. 2. Both pris-tine and CaF2-coated samples exist as particles of 200–400 nm withlittle agglomeration (Fig. 2(a) and (b)), indicating no difference inmorphology after CaF2 coating. At a high magnification, we canclearly see that the surface of CaF2-coated samples becomes rough(Fig. 2(c)). In order to prove the homogeneity of the coating, EDXtest is carried out on CaF2-coated samples and the results showthe presence of Ca and F elements besides O, Mn, Ni, Co elements(Fig. 2(d)).

Fig. 3 shows TEM images of pristine and CaF2-coated samples.The pristine sample has a clear grain edge while the coated samplehas a much rougher edge (Fig. 3(a) and (b)). Both samples exhibitcontinuous interference fringe spacing of 0.47 nm correspondingto the (0 0 1) and (0 0 3) lattice fringes of the monoclinic Li2MnO3and hexagonal layered LiMO2 phase. A clear boundary separatesthe 2–7 nm thick amorphous coating layer from the bulk crys-talline phase in Fig. 3(d), indicating that CaF2 coating was successfulachieved on the surface of Li1.2Mn0.54Ni0.13Co0.13O2.

3.2. Electrochemical properties of pristine and CaF2-coated

Li1.2Mn0.54Ni0.13Co0.13O2

The initial galvanostatic charge–discharge profiles of pris-tine and CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 material at 0.05 C

54 X. Liu et al. / Electrochimica Acta 109 (2013) 52– 58

F 4Ni0.1

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ig. 2. Low magnification SEM images of (a) pristine and (b) CaF2-coated Li1.2Mn0.5

i1.2Mn0.54Ni0.13Co0.13O2.

1 C = 250 mA g−1) are presented in Fig. 4. Both samples show a longlateau at about 4.5 V during the first charge process. The mech-

nism of the 4.5 V plateau is complicated and most researcherspeculate that it is related to the removal of oxygen with fur-her delithiation and migration of transition metal ions from the

Fig. 3. TEM images of (a) pristine and (b) CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 and

3Co0.13O2; (c) high magnification SEM images and (d) EDX patterns of CaF2-coated

surface to the bulk [5,21]. The initial charge curve of CaF2-coatedsample shows a shorter plateau compared to pristine sample, lead-

ing to lower initial charge capacity (362.9 mAh g−1 for pristinesample and 319.0 mAh g−1 for CaF2-coated sample). This can beattributed to the strong F O bonds which suppress the oxygen

HRTEM images of (c) pristine and (d) CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2.

X. Liu et al. / Electrochimica Acta 109 (2013) 52– 58 55

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0 20 40 60 800

50

100

150

200

250

300CaF

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ig. 4. Charge–discharge curves of pristine and CaF2-coatedi1.2Mn0.54Ni0.13Co0.13O2 at 0.05 C and dQ/dV plots of the circled part (inset).

acancies migration and reduce the activity of the evolved oxy-en species, leading to less oxygen removal and less electrolytexidation during the initial charge process [22,23]. The first dis-harge capacity of both samples are similar (275.8 mAh g−1 forristine sample and 277.3 mAh g−1 for CaF2-coated sample) and theoulombic efficiency of pristine and CaF2-coated samples are 76.0%nd 86.9% respectively. The higher coulombic efficiency of CaF2-oated samples can be explained as follows: (a) the CaF2 coatingayer can be regarded as a buffer layer to promote the formation ofnactive O2 molecules, preventing the side reaction of electrolytexidation caused by active oxygen species [23]; (b) as exhibitedn the inserted plot of Fig. 4, the initial discharge dQ/dV curve ofaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 shows a small peak at 2.8 V

ndexed to the spinel phase of lithium manganese oxide and thehase transformation from layered structure to spinel structure can

ncrease initial coulombic efficiency, similar to AlF3 coating effecteported by Sun et al. [13]. The results indicate that CaF2 coatingn Li1.2Mn0.54Ni0.13Co0.13O2 cathode material increases the initialoulombic efficiency and induces the phase transformation at a lowurrent density.

The cyclic performances of pristine and CaF2-coated samplesre investigated at a higher current density of 0.2 C to suppress theecomposition of electrolyte. The first three discharge curves ofoth samples at 0.2 C are exhibited in Fig. 5. The charge processesre also performed at 0.2 C. The discharge capacity of both samplesncrease gradually during the first three cycles. This is attributedo the stretch of the plateau under 3.5 V, illuminating that itakes several cycles to activate the Li2MnO3 phase completely at.2 C. However, the changes of discharge capacity and dischargeurves are steeper for pristine samples, showing more inactiveomponent left after initial charge process. While for CaF2-coatedamples, the initial discharge capacity is much closer to thethers, indicating that CaF2 coating can accelerate the activation ofi2MnO3 phase during the initial charge process at a higher chargeurrent density. The cyclic performances of both samples at 0.2 Cre shown in Fig. 6. After a few activating cycles, the dischargeapacity of both samples reaches the maximum, 259 mAh g−1 forristine Li1.2Mn0.54Ni0.13Co0.13O2 material and 250 mAh g−1 foraF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 material. Yet after 80 cycleshe discharge capacity of the pristine material is only 155 mAh g−1

hile the CaF -coated material still has a discharge capacity of

228 mAh g−1. The capacity retention rate with respect to theaximum of discharge capacity is 59.8% for pristine samples and

1.2% for CaF2-coated samples. High capacity retention rate of

Fig. 5. Initial three discharge curves of (a) pristine and (b) CaF2-coatedLi1.2Mn0.54Ni0.13Co0.13O2 at 0.2 C.

fluoride-coated conventional layered cathode is usually attributedto the protection of coating layer which prevents the inneroxide from reacting with HF in the electrolyte. However, someresearchers recently reported that AlF3 coating on Li-rich cathodecan have a great effect on the bulk structure of cathode, especiallyon the phase transformation [13,14]. Thus the mechanism ofgood cyclic performance of CaF -coated Li Mn Ni Co O

cycle n umber

Fig. 6. Cyclic stability of pristine and CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 at 0.2 C.

56 X. Liu et al. / Electrochimica Acta 109 (2013) 52– 58

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0 5 10 15 20 25 300

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0.5 C

To investigate the origin of the improved electrochemical per-formance of CaF2-coated sample, EIS patterns of both samples wereperformed. The measurements were carried out before charge and

0

50

100

150

200

250

300

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coated

pristine

cap

aci

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ig. 7. dQ/dV plots of pristine (dash) and CaF2-coated (solid)i1.2Mn0.54Ni0.13Co0.13O2 during (a) the 3rd cycle and (b) the 50th cycle at.2 C.

nvestigated. We choose the third cycle as reference to ignore theifferent activation rates during the initial two cycles. The dQ/dVlots of the third cycle are shown in Fig. 7(a). The peaks over 3.6 Vre attributed to the oxidation and reduction of Co3+/Co4+ andi2+/Ni4+. The reduction of Mn4+ in the layered MnO2 componentreated after Li2MnO3 activation occurs at 3.3–3.4 V. The plots ofaF2-coated samples show a shift to lower potential at oxidationeaks and a shift to higher potential at reduction peaks, indicating

smaller polarization after CaF2 coating. The dQ/dV plots of the0th cycle are presented in Fig. 7(b). The area under curves isuch larger for CaF2 coated material, consistent with its good

apacity retention. The intensity of reduction peak at about 3.3 V isecreased obviously and a new reduction peak appears below 3.0 Vorresponding to spinel-like phase of lithium manganese oxide.his phenomenon keeps in line with the mechanism of phaseransformation due to migration of transition metal ions from theransition metal layer to the lithium layer during cycles [24–27].n the dQ/dV plot of CaF2-coated sample, the new reduction peaks much stronger than the reduction peak at 3.3 V, indicating littleayered phase of lithium manganese oxide left after 50 cycles. Onhe contrary, the reduction peak at 3.3 V remains stronger thanhe reduction peak below 3.0 V for pristine samples. The resultsllustrate that CaF2 coating could accelerate the phase trans-ormation of activated Li2MnO3 phase during charge–dischargerocesses. The faster phase transformation can be associated

o the larger amount of inactive O2 molecules and more stablenterfacial structure due to the presence of mitigating fluorideayer. Though the phase transformation is usually regarded as

drawback to cause capacity deterioration, some researchers

Fig. 8. Cyclic performance of pristine and CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 atvarious charge and discharge rates.

consider spinel-like phase to be a framework structure for stablecycling [13,26]. However, the increase of spinel-like phase reducesthe average discharge voltage, leading to lower power density andobvious hysteresis upon cycles in the full cell [28].

The comparison of high rate capability between CaF2-coatedand pristine samples is exhibited in Figs. 8 and 9.The cyclic per-formance at various rates from 0.1 C to 3 C for each 5 cycles areshown in Fig. 8. No major change in discharge capacity is observedat 0.1 C for both samples. Yet the capacity difference between theCaF2-coated cathode and the pristine cathode increases graduallywith the rate value. At the 3 C-rate, the discharge capacity of CaF2-coated cathode is 141.5 mAh g−1 while that of pristine cathode isonly 85 mAh g−1. The better cyclic stability for CaF2-coated cathodeat the 1 C-rate is exhibited in Fig. 10. The initial discharge capac-ity of CaF2 coated sample at 1 C reaches 216 mAh g−1 in contrastto 172 mAh g−1 for pristine sample. After 100 cycles, the dischargecapacity of 191 mAh g−1 still remains for the CaF2-coated sample.While for the pristine sample, the discharge capacity is lower than130 mAh g−1 in the 100th cycle.

0 20 40 60 80 100

cycle number

Fig. 9. Cyclic stability of pristine and CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 at 1 C.

X. Liu et al. / Electrochimica

Fig. 10. Electrochemical impedance spectroscopy (EIS) and the used equivalent cir-c(a

atsTriatTcw7ivcasecc

4

i

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uit (inset) of pristine and CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 (a) before chargeb) after 30 cycles at 0.2 C. The symbols and lines correspond to the experimentalnd simulated data, respectively.

fter 30 cycles at 0.2 C. The experimental results were fitted withhe equivalent circuits inset in Fig. 10. An intermediate-frequencyemicircle and a low-frequency tail are observed in Fig. 10(a).he intermediate-frequency semicircle is related to charge transferesistance (Rct) while the low-frequency tail is associated with Li+

on diffusion process in the solid phase of electrode. After 30 cycles,n additional high-frequency semicircle, ascribed to formation ofhe passivating surface film upon cycling, is observed in Fig. 10(b).he fitted results show obviously decreased Rct values after CaF2oating. Before charge, the Rct value of pristine cathode is 154.8 �hile the CaF2-coated cathode exhibits much smaller Rct value of

5.1 �. After 30 cycles, Rct values of both samples decrease dramat-cally, resulted from electrochemical activation [14,29,30]. The Rct

alue of pristine cathode is 44.6 � in contrast to 15.3 � for CaF2-oated cathode after cycle test. As mentioned above, CaF2 acts as

role of buffer layer to reduce the activity of extracted oxygenpecies. Consequently oxidation of electrolyte and degradation oflectrolyte/electrode interface are suppressed effectively by CaF2oating, which leads to decreased Rct value and improved electro-hemical performance.

. Conclusion

The present paper shows distinguishable effects of CaF2 coat-ng for the Li-rich cathode material Li1.2Mn0.54Ni0.13Co0.13O2.

[

[

Acta 109 (2013) 52– 58 57

Through a sol–gel method and wet chemical process, an amor-phous CaF2 layer with a thickness of 2–7 nm is successfully coatedon the surface of the pristine sample. Compared with the pris-tine Li1.2Mn0.54Ni0.13Co0.13O2, the CaF2-coated sample has theobviously improved electrochemical performance. The coulombicefficiency of the first charge–discharge process is enhanced from76.0% to 86.9% after CaF2 coating with almost unchanged dis-charge capacity of 277.3 mAh g−1 at 0.05 C. The CaF2-coated sampleexhibits superior capacity retention rate of 91.2% at 0.2 C after80 cycles. Furthermore, a discharge capacity of 141.5 mAh g−1 isobtained at 3 C and 190 mAh g−1 remains after 100 cycles at 1 Cfor the CaF2-coated sample, much higher than the pristine sam-ple. The excellent initial coulombic efficiency, cyclic stability andrate capability can be attributed to the suppression of electrolytedecomposition, stable electrolyte/electrode interfacial structureand accelerated phase transformation from the initial layered phaseto spinel phase. Therefore, we believe that the obtained CaF2-coated Li1.2Mn0.54Ni0.13Co0.13O2 should be one of ideal candidatesfor expansion of Li-ion battery technology in electric vehicles andhybrid electric vehicles.

Acknowledgements

The authors acknowledge funding supports from 973 Pro-gram (2013CB934103) and Science & Technology Commission ofShanghai Municipality (12dz1200402 & 08DZ2270500), China.

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