mater.scichina.com link.springer.com . . . . . . . . . . . . . . . . . . .Published online 18 December 2017 | https://doi.org/10.1007/s40843-017-9162-3Sci China Mater 2018, 61(5): 719–727

An Ostwald ripening route towards Ni-rich layeredcathode material with cobalt-rich surface for lithiumion batteryYan Li1, Xinhai Li1, Zhixing Wang1, Huajun Guo1 and Jiexi Wang1,2*

ABSTRACT An Ostwald ripening-based route is proposed toprepare Ni-rich layered cathodes with Co-rich surface for li-thium-ion batteries (LIBs). Commercially available Ni0.8Co0.1Mn0.1(OH)2 and spray pyrolysis derived porous Co3O4 are usedas mixed precursors. During the lithiation reaction processunder high-temperature, the porous Co3O4 microspheresscatter primary particles and spontaneously redeposit on thesurface of Ni-rich spheres according to Ostwald ripeningmechanism, forming the Ni-rich materials with Co-rich outerlayers. When evaluated as cathode for LIBs, the resultantmaterial shows ability to inhibit the cation disorder, relievesthe phase transition from H2 to H3 and diminishes side re-actions between the electrolyte and Ni-rich cathode material.As a result, the obtained material with Co-rich outer layersexhibits much more improved cycle and rate performancethan the material without Co-rich outer layers. Particularly,NCM-Co-1 (molar ratio of Ni0.8Co0.1Mn0.1(OH)2/Co3O4 is 60:1)delivers a reversible capacity of 159.2 mA h g−1 with 90.5%capacity retention after 200 cycles at 1 C. This strategy pro-vides a general and efficient way to produce gradient sub-stances and to address the surface problems of Ni-rich cathodematerials.

Keywords: nickel-rich layered cathode, cobalt-rich surface,Co3O4, ostwald ripening, lithium ion batteries

INTRODUCTIONEnergy crisis and environmental contamination havebecome increasingly serious accompanying economicboom in recent years [1,2]. New energy vehicle has beenbecoming more and more popular with rapid develop-ment and government support [3,4]. As one of the mainenergy resources, lithium ion battery has many ad-

vantages such as high energy density and long cycle life[5–19]. With the higher cruising range demands of ve-hicles, Ni-based lithium transition metal oxide cathodematerials with high energy density have become one ofthe currently significant research hotspots [20–23]. Ni-based cathode materials are capable of delivering capacityover 200 mA h g−1 and energy density of 800 W h kg−1

[24]. Unfortunately, the nickel-rich surface exacerbatesmoisture uptake and the side reactions with electrolyte,resulting in many problems such as gelation of thecathode slurry, capacity fade, structure deterioration andthermal instability [24–27]. In addition, these nickel-richcathodes suffer from gradual capacity fading with cyclingdue to their structural instability, which is ascribed to thesuccessive phase transitions from hexagonal 2 (H2) tohexagonal 3 (H3) phases when large amounts of Li+ areextracted from the host structure [28–33]. To solve theseproblems, various approaches including structure tuning[34–36], substitution [37–39], and surface coating [40–45] have been proposed. Metal oxides such as Al2O3

[46,47], ZrO2 [48], MgO [49], have been studied ascoating materials. The metal oxide layer is beneficial toprotecting the cathode materials from being exposed toelectrolyte directly, and thus reducing the side reactions.Nonetheless, the above-mentioned electrochemically in-active layer causes lower capacity and Li+ diffusion.Therefore, electron conducting materials such as carbonand graphene [50], lithium ion conductors such as Li2O·2B2O3 [51], Li3VO4 [42] and Li2ZrO3 [52,53], have beendeveloped as coating materials for Ni-based cathodematerials. Some lithium based materials have also beenused to react with residual impurities on the surface ofNi-rich cathode materials and form a uniform coating

1 School of Metallurgy and Environment, Central South University, Changsha 410083, China2 Powder Metallurgy Research Institute, Central South University, Changsha 410083, China* Corresponding author (email: wangjiexikeen@csu.edu.cn)

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layer on the bulk surface at the same time [54–56]. Inaddition, Co-shell and concentration-gradient Ni-richcathodes have been designed to improve the cycle life,structural and thermal stability of the Ni-rich cathodes[46,57–60]. Sun’s group first reported a novel high-ca-pacity and safe cathode material with an average com-position of Li[Ni0.68Co0.18Mn0.18]O2, in which each particleconsists of bulk material surrounded by a concentration-gradient outer layer [59]. However, many above-men-tioned approaches are complicated and the reactionconditions need controlling strictly. An effective, easilycontrolled and versatile synthesis method should beconstructed for Ni-rich cathodes modification.Ostwald ripening is an typical and general phenom-

enon in solid solutions or liquid sols which describes thechange of an inhomogeneous structure over time [61]. Inour previous researches, porous Ni-Co-Mn-O powdersare successfully prepared via spray pyrolysis [62–64]. Theporous Ni-Co-Mn-O powders are composed of numerousclosely packed primary particles, which would turn tosubmicron-sized particles after solid-state reaction withlithium salt. It seems that the binding force between theadjacent primary particles is too weak to withstand thereactivity. Based on the findings and Ostwald ripeningtheory, we propose a novel method to prepare Ni-richlayered cathodes with Co-rich surface. First, the porousCo3O4 microspheres are prepared from the solution ofcobalt chloride via spray pyrolysis. Then the Co3O4 wasmixed with Ni0.8Co0.1Mn0.1(OH)2 precursor powders andLiOH, followed by annealing in O2 to yield Ni-richcathode with Co-rich surface. Such Co-rich surface isexpected to prevent side reactions from moisture in airand electrolyte, and to improve the structure stability bysuppressing successive phase transitions from H2 to H3phases to some extent.

EXPERIMENTAL SECTION

Synthesis and characterization

Synthesis of Co3O4 microspheresThe Co3O4 microspheres were prepared by spray pyr-olysis in our previous paper [64]. The precursor solutionwas prepared by dissolving CoCl2·6H2O in distilled water.The concentration of cobalt chloride was 0.5 mol L−1. Theprecursor solution was aerosolized using a 1.75 MHz ul-trasonic nebulizer. The droplets stream was carried into a3-zone vertical furnace reactor by O2 with a flow rate of2 L min−1. The spray pyrolysis temperature was main-tained at 750 °C. The resulting powders were collected at

the reactor exit.

Synthesis of Ni-rich layered cathodes with Co-rich surfaceCommercially available Ni0.8Co0.1Mn0.1(OH)2 as the pre-cursor powder was mixed with Co3O4 in a molar ratio of60:1 and 30:1, respectively, and then mixed with an ap-propriate amount of LiOH (molar ratio of Li to transitionmetals was 1.05). The mixture was annealed at 780°C for15 h in O2. The samples are denoted as NCM-Co-1 andNCM-Co-2, respectively. For comparison, the pristineNi-rich cathode material (NCM) was also prepared withthe same process without the addition of Co3O4.The as-prepared cathode powders were characterized

by X-ray diffraction (XRD, Rint-2000, Rigaku). Particlemorphology was observed by a scanning electron mi-croscope (SEM, Sirion 200). The element distributions ofthe samples were analyzed by energy dispersive spectro-scopy (EDS) and liner canning analyses of particle crosssections.

Electrochemical measurementCathode film fabrication was performed according to theprocedures reported earlier [20]. The working electrodecomprises 80 wt.% cathode materials, 10 wt.% acetyleneblack and 10 wt.% polyvinylidene fluoride (PVDF).Electrochemical performance was evaluated in CR2025-type coin cells assembled in an argon-filled glove boxwith both the moisture and oxygen content below0.1 ppm. The electrolyte solution was 1 mol L−1 LiPF6 inan ethylene carbonate/ethyl methyl carbonate/dimethylcarbonate solution (EC:EMC:DMC=1:1:1, v/v/v). Cyclicvoltammetry (CV) was performed on a CHI 660A elec-trochemical workstation at a scan rate of 0.1 mV s−1 atroom temperature. The charge/discharge tests were per-formed on a NEWARE BTS-51 battery tester.

RESULTS AND DISCUSSIONThe formation of Ni-rich layered cathodes with Co-richsurface is schematically shown in Fig. 1. The sintering ofNi0.8Co0.1Mn0.1(OH)2 microspheres with LiOH yielded Ni-rich cathode material, while the porous Co3O4 scatteredprimary particles when it was reacted with LiOH. Ac-cording to Ostwald ripening theory, small crystals dis-solute and redeposit on the surface of larger crystals. Thusthe molten LiOH and primary particles of Co3O4 de-posited on the surface of Ni0.8Co0.1Mn0.1(OH)2, and finallyformed cathode material with Ni-rich core and Co-richouter layer after lithiation reaction.Fig. 2 shows the XRD patterns of NCM, NCM-Co-1

and NCM-Co-2, which exhibit a typical layered hex-

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agonal α-NaFeO2 structure with space group R3m. Thepatterns show clear split of the (006)/(102) and (108)/(110) peaks, indicating all samples have a highly orderedhexagonal structure. The lattice parameters a, c and V arecalculated based on the XRD and listed in Table 1. Thelattice parameter a, c and V decrease with the increase ofCo content, which is caused by smaller ionic radius ofCo3+ (0.545 Å) compared to that of Ni3+ (0.56 Å). Theratio of c/a and the intensity ratio of I(003)/I(104) are in-dicators for the extent of cation mixing. A higher ratio ofI(003)/I(104) indicates a lower degree of the cation mixing[65,66]. The ratios of c/a and I(003)/I(104) increase with the

increase of Co content, indicating lower cation disorderfor the Ni-rich layered cathode with Co-rich surface. Amore superior layered structure and lower degree of thecation mixing for Ni-rich cathodes with Co-rich surfacemay help to improve the electrochemical performance[67,68].The morphologies of Co3O4, Ni0.8Co0.1Mn0.1(OH)2,

NCM, NCM-Co-1 and NCM-Co-2 are imaged by SEM,as shown in Fig. 3. Fig. 3a shows that Co3O4 microspheresare developed from numerous interconnected primaryparticles. Fig. 3b indicates the Ni0.8Co0.1Mn0.1(OH)2 mi-crospheres are composed of densely aggregated andneedle-shape primary particles with a diameter of5–8 μm. After high temperature solid-state reaction withlithium salt, the as-prepared NCM particles (Fig. 3c, d)

Figure 1 Schematic diagram for the preparation of Ni-rich layeredcathodes with Co-rich surface.

Figure 2 XRD patterns and of NCM, NCM-Co-1 and NCM-Co-2.

Table 1 Lattice parameters of NCM, NCM-Co-1 and NCM-Co-2

Sample a (Å) c (Å) V (Å3) c/a I(003)/I(104)NCM 2.8703 14.1451 100.93 4.9281 1.68

NCM-Co-1 2.8657 14.1438 100.78 4.9355 1.72

NCM-Co-2 2.8634 14.1406 100.46 4.9384 1.81

Figure 3 SEM images of the samples: (a) Co3O4; (b) Ni0.8Co0.1Mn0.1

(OH)2; (c, d) NCM; (e, f) NCM-Co-1; (g, h) NCM-Co-2.

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keep the homogeneous spherical morphology and particlediameter. The surface of the pristine NCM is bare andclean. While for the NCM-Co-1 (Fig. 3e, f), many rock-shaped grains (marked by yellow circles) adhere to thesurfaces of the spheres to form a coating layer on thesurface of NCM. Such layer is presumed as the redepositCo-rich phase which is generated from the scattered co-balt species by Ostwald ripening. With the amount ofCo3O4 increasing, the rock-shaped grains on the surfaceof NCM-Co-2 (Fig. 3g, h) increase and the outer layerbecomes thicker than that of NCM-Co-1. There are alsosome fragmentized cobalt species distributed among theNCM-Co-1 and NCM-Co-2 samples, as indicated by thegreen arrows in Fig. 3e and g.The ICP testing results indicate chemical composition

of NCM, NCM-Co-1 and NCM-Co-2 is Li0.998Ni0.796-Co0.101Mn0.103O2, Li0.997Ni0.759Co0.144Mn0.097O2 and Li0.998-Ni0.723Co0.184Mn0.093O2, respectively, consistent with thestoichiometric composition LiNi0.8Co0.1Mn0.1O2, LiNi0.762-Co0.143Mn0.095O2 and LiNi0.727Co0.182Mn0.091O2. In order toestimate Co concentration at the surface of the Ni-richmicrospheres, linear scanning analyses of particle crosssections were conducted by EDS (Fig. 4). For pristineNCM (Fig. 4a, b), the Co and Ni element distributions areeven, and the relative elemental Ni:Co ratio across theNCM particle almost remains constant. For NCM-Co-1(Fig. 4c, d) and NCM-Co-2 particles (Fig. 4e, f), Ni andCo have homogeneous distributions at the center of the

particle. It is noteworthy that the distributions changeremarkably on the surface region of the particles, theconcentration of Co decreases from the surface to thecenter while the Ni distribution changes in the oppositetrend. These results indicate the formation of a Co-richphase at the surface of NCM-Co-1 and NCM-Co-2.Fig. 5 shows the electrochemical performance of NCM,

NCM-Co-1 and NCM-Co-2. Fig. 5a shows the CV curvesof the samples for the first cycle. In the positive sweep,NCM shows three oxidation peaks at 2.8–4.3 V, which arecaused by the phase transition from hexagonal tomonoclinic (H1 to M), monoclinic to hexagonal (M toH2) and hexagonal to hexagonal (H2 to H3) during theLi-ion extraction process [29–31,69]. The correspondingreversible behaviors are also clearly observed during thedischarge process, and the reduction peaks are lower thanthe oxidation peaks because of polarization. NCM-Co-1and NCM-Co-2 show very similar shapes and peak po-sitions with that of NCM. The phase transition from H2to H3 for NCM-Co-1 and NCM-Co-2 were significantlyrelieved, emphasizing that the Co-rich layer can improvethe structure stability of Ni-rich layered cathodes.Fig. 5b illustrates the first charge-discharge curves at 0.1

C. The as-prepared NCM, NCM-Co-1 and NCM-Co-2deliver an initial discharge capacity of 194.1, 190.9 and182.6 mA h g−1, respectively. The discharge capacity de-creases due to the lower Ni content, which is the mainredox species. The corresponding initial coulombic effi-

Figure 4 EDS linear scanning analyses of particle cross sections of the samples: (a, b) NCM; (c, d) NCM-Co-1; (e, f) NCM-Co-2.

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ciencies are 84.2 %, 85.8% and 86.4%, respectively. Theincrease of initial coulombic efficiency can be explainedas follows. The pristine NCM sample shows a voltageplatform from 4.2 to 4.3 V due to the phase transition ofH2-H3, corresponding to the distinct redox peak at4.23 V in the first CV curve. As a result, the sample de-livers a higher initial charge capacity. This indicates thatmore Li+ ions de-intercalate from the cathodes in the firstcharge process, leading to the collapse of crystal structure.While for the NCM-Co-1 and NCM-Co-2 with Co-sur-face, the introduction of cobalt relieves the phase transi-tion of H2-H3 to some extent, improves the structuralstability, and reduces the polarization in the first charge-discharge cycle. The initial coulombic efficiency improveswith the increasing of Co content. Cycle performances ofthe as-prepared NCM, NCM-Co-1 and NCM-Co-2cathodes at 1 C between 2.8 and 4.3 V are presented in

Fig. 5c. NCM-Co-1 and NCM-Co-2 deliver comparativelylower initial capacities than that of NCM. After 200 cyclesthe as-prepared NCM, NCM-Co-1 and NCM-Co-2 de-liver discharge capacities of 137.5, 159.2, and 151.8mA h g−1, corresponding to the capacity retention of76.4%, 90.5%, and 87.7%, respectively. The rapid capacityfade of NCM can be ascribed to the successive phasetransitions from H2 to H3 and the severe side reactions ofthe Ni-rich cathode surface with the electrolyte [68]. TheCo-rich surface of Ni-rich layer cathodes can play aneffective role in inhibiting the disorder of cations, en-hancing the structural stability, and diminishing side re-actions between the electrolyte and extremely unstableNi4+ in highly delithiated Ni-rich cathode material. Thus,the cycling performance of NCM-Co-1 improves due tothe chemical and structural stability endowed by the Co-rich surface.

Figure 5 Electrochemical performance of NCM, NCM-Co-1 and NCM-Co-2: (a) CV curves at the scan rate of 0.1 mV s−1; (b) first cycle charge/discharge profiles at 0.1 C (c) cycling performance at 1 C between 2.8 and 4.3 V and (d) rate capability in the rate range of 0.1–5 C.

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Fig. 5d shows the rate performance of the samples.Along with the increased current rates, the dischargecapacities of both samples are decreased. NCM delivershigher capacity at low current rates than NCM-Co-1 andNCM-Co-2. However, NCM-Co-1 presents better rateperformance than NCM, and exceeds the capacity at 5 C.The Co-rich surface plays a positive role in conductivityof lithium diffusion and improves rate performance ofNCM.Ni-rich cathode materials degrade faster than low Ni-

content materials upon exposure to air due to the sidereactions. To further compare the degradation of NCM,NCM-Co-1 and NCM-Co-2 during storage, the cyclingand rate performance of the three samples after exposingthem to air for 2 months are investigated. As shown inFig. 6, after 100 cycles, the as-prepared NCM, NCM-Co-1and NCM-Co-2 deliver discharge capacities of 132.9,143.9, and 141.4 mA h g−1, corresponding to the capacityretention of 75.4%, 83.2%, and 84.9%, respectively. Theresults confirm the Co-rich surface can prevent side re-actions from moisture in ambient air, thus delay the de-gradation of the cathodes.

CONCLUSIONSNi-rich layered cathodes with Co-rich surface were suc-cessfully prepared from Ni0.8Co0.1Mn0.1(OH)2, porousCo3O4 and LiOH via an Ostwald ripening-based route.The Co3O4 spheres were cracked and a Co-rich layer isthen coated on the surface of Ni-rich materials. Themodified samples displayed improved comprehensiveelectrochemical properties including higher capacity re-tention and better rate performance compared with thatof the pristine one, ascribed to their lower disorder ofcations, the suppression of phase transition from H2 to

H3, and the prevention of direct contact between theelectrolyte and Ni-rich cathodes. On the basis of theseresults, such an Ostwald ripening-based route could beconsidered as a viable surface modification method forimproving the electrochemical and thermal properties ofNi-rich layered cathodes.

Received 10 October 2017; accepted 17 November 2017;published online 18 December 2017

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Acknowledgements This work has been carried out with the financialsupport of the National Basic Research Program of China(2014CB643406), the National Natural Science Foundation of China(51674296, 51574287, 51704332), the National Postdoctoral Program forInnovative Talents (BX201700290), and the Fundamental ResearchFunds for the Central Universities of Central South University(2017zzts125).

Author contributions Li Y, Li X and Wang J designed and engineeredthe samples; Li Y performed the experiments; Li Y, Wang Z, Guo H andWang J performed the data analysis; Li Y and Wang J wrote the paper;All authors contributed to theoretical analysis and general discussion.

Conflict of interest The authors declare that they have no conflict ofinterest.

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Yan Li is now a PhD student at the School of Metallurgy and Environment, Central South University. She received herBSc degree (2012) and MSc degree (2015) in metallurgical engineering from Central South University (China). Herresearch interests include synthesis and modification of electrode materials for lithium ion battery, especially the Ni-richcathode materials.

Jiexi Wang received his BSc degree (2010) in Metallurgical Engineering and PhD degree (2015) in Physical Chemistry ofMetallurgy from Central South University (China). After working as a postdoctoral fellow at Hong Kong University ofScience & Technology and The University of Hong Kong, he started his independent research career as an AssociateProfessor at Central South University (China) in 2017. His research focuses on the green synthesis and application ofnonferrous-based materials and composites for energy storage, such as high-power/high-energy lithium/sodium ionbatteries, and supercapacitors. He has published about 80 SCI papers with ~1,600 citations (h-index=26).

奥斯特瓦尔德熟化法制备具有富钴表层的锂离子电池高镍正极材料李艳1, 李新海1, 王志兴1, 郭华军1, 王接喜1,2*

摘要 本文提出了一种基于奥斯特瓦尔德熟化制备具有富钴表面的高镍正极材料的方法. 采用了商业化的Ni0.8Co0.1Mn0.1(OH)2和喷雾热解制备了多孔Co3O4作为前驱体. 在高温固相反应锂化过程中, 多孔的Co3O4会分解成颗粒. 根据奥斯特瓦尔德熟化过程, 这些颗粒与锂盐反应并自发地沉积于高镍材料的表面, 形成具有富钴表层的高镍正极材料. 作为锂离子电池正极材料时, 改性后的材料中阳离子的混排得到抑制, H2到H3的相变程度降低, 并减少了电解液和高镍正极材料之间的副反应. 因此, 具有富钴表层的高镍正极材料相比于原始材料循环和倍率性能得到很大提升. NCM-Co-1 (Ni0.8Co0.1Mn0.1(OH)2/Co3O4的摩尔比为 60:1)在1 C下循环200次放电容量仍有159.2 mA h g−1, 容量保持率为90.5%. 本工作为制备梯度正极材料以及解决高镍正极材料的表面问题提供了一种通用而有效的方法.

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