Zr 掺杂对锂离子电池正极材料三元材料的影响

The Impact of Zr Substitution on the Structure,Electrochemical

Performance and Thermal Stability of Li[Ni1/3Mn1/32z Co1/3Zr z]O2

Wenbin Luo,a,b and J.R.Dahn a,*,z

a Dept.of Physics,Dalhousie University,Halifax,NS,Canada B3H3J5

b School of Metallurgical Science and Engineering,Central South University,Changsha,Hunan,410083,

People’s Republic of China

Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2(0z0.1)samples were prepared from hydroxide precursors.The hydroxide precursors were

heated with Li2CO3at900 C to prepare the oxides.X-ray diffraction results suggest that single phase samples can be prepared for

0z0.02.Chemical analysis using atomic absorption spectroscopy showed that the samples had some excess Li.Rietveld

refinements of X-ray diffraction data show that Zr substitution for Mn does not induce substantial cation mixing in the Li layer.

Electrochemical studies of single phase samples,Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2(0z0.02),show a small reduction in capacity

and no advantage in capacity retention as Zr is substituted for Mn.The impact of Zr substitution on the thermal stability of the

Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2samples was studied via accelerating rate calorimetry.The accelerating rate calorimetry results show

that the substitution of Zr into the layered structure of Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2does not improve the thermal stability of the

samples.

V C2011The Electrochemical Society.[DOI:10.1149/1.3552603]All rights reserved.

Manuscript submitted November7,2010;revised manuscript received January13,2011.Published February28,2011.

Dahn’s and Ohzuku’s groupsfirst reported the structure and elec-

trochemical performance of Li[Ni x Mn x Co1À2x]O2with x either

approximately equal to1/3or x¼1/3.1,2Much research has been

performed to optimize these and similar materials by new synthesis

methods,applying coatings,and including substituents for Ni,Mn,

and/or Co.

The impact of Al substitutions on the thermal stability of layered

lithium transition metal oxides is well known.The thermal stability

of Al-substituted Li[Co1Àz Al z]O2,3–5Li[Ni1Àz Al z]O2,6Li[Ni1/3Mn1/3

Co1/3Àz Al z]O2,7Li[Ni1/2Àz/2Mn1/2Àz/2Al z]O2,8and Li[Ni0.4Mn0.4-

Co0.2Àz Al z]O2,9improves as the Al content increases.However,the

specific capacity decreases at a rate of about-280mAh/g per z¼1

for all these samples,5so there is an important trade-off between ther-

mal stability,which impacts Li-ion cell safety,and energy density.

There have been reports that Mg substitution for Ni,Mn,or Co

in Li[Ni(1/3Àz)Mn(1/3Àz)Co(1/3Àz)Mg z]O2,Mg substitution for Ni in

Li[Ni(0.6Àx)Mg x Co0.25Mn0.15]O2,and substitutions of Mg and Ti for

Ni in Li[Ni1Àz Mg z/2Ti z/2]O2(Refs.10–12)all improve the thermal

stability of the charged electrode materials.The Dahn group studied

the effect of Al and Mg doping in LiCoO2and LiNi1/3Mn1/3Co1/

3O2.The results showed that Al substitution is more effective

than Mg substitution in improving the thermal stability of Li[Co1Àz M z]O2and Li[Ni1/3Mn1/3Co1/3Àz M z]O2samples.7,13,14 The Dahn group studied the effect of Mn and Mg cosubstutition in LiCoO2and LiNi1/3Mn1/3Co1/3O2.Simultaneous Mn and Mg substitu-tion for Co in LiCoO2and NMC-type(NMC is the abbreviation for LiNi1/3Mn1/3Co1/3O2)positive electrode materials is not beneficial.15,16 It is important to study the impact of other substituent atoms on the capacity and thermal stability of layered lithium transition metal oxides in a careful and systematic manner as was done for Mg in Ref.7.Oh et al.reported that the cycling performance of zirconium-doped LiNi0.8Co0.2O2cathode materials was superior to the pristine cathode,especially when they were tested at high voltage upper-cut-offs.17According to Sun et al.,Zr-doped Li[Ni0.45Co0.1Mn0.45Àx Zr x]O2(x¼0.02)shows better cycling performance and better ther-mal stability.The major exothermic reaction between the charged electrode material and electrolyte was delayed from252.1to 289.4 C.18Jeong et al.found that Zr-doped Li[Ni0.5Mn0.5]O2elec-trodes show about50mAh/g higher capacity than those of Li[Ni0.5Mn0.5]O2,and have a better thermal stability than the undoped electrode.19

Much of the work in the literature has been reported for only a few samples,not from a systematic series of samples from which trends,if any,can be clearly observed.When only a few samples are studied,it is possible that spurious conclusions can be reached.In this work,the synthesis,structure,and electrochemical properties of Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2(0z0.1)samples are characterized carefully.In addition,the reactivity of the charged electrode materials with electrolyte is measured using accelerating rate calorimetry.

Experimental

Material preparation.—A LiOH H2O(Sigma Aldrich,98%þ) solution and a mixed solution of Ni(NO3)2 6H2O(Sigma Aldrich, 97%þ),Mn(NO3)2 4H2O(Sigma Aldrich,97%þ),Co(NO3)2 6H2O(Sigma Aldrich,98%þ),and ZrO(NO3)2 x H2O[Sigma Aldrich,98%,x¼6.18determined by thermographic analysis and x-ray diffraction(XRD)tests]were simultaneously added over the course of about30min to a stirredflask using a two-channel peri-staltic pump(Masterflex C/L pump,Barnant Co.).The concentra-tions of the solutions were adjusted to set the Zr:(NiþMnþCoþZr) ratio,z.The precipitate was separated by centrifuging(Centra GP8R,International Equipment Company),washed with distilled water several times to remove any dissolved salts,and then was dried at80 C overnight.The dried precipitate was mixed with an appropriate amount of Li2CO3(Alfa Aesar,99%)and ground.The precursors were heated in air at900o C for3h.

Material characterization

X-ray diffraction.—XRD patterns were collected with a Siemens D5000diffractometer equipped with a Cu target x-ray tube and a diffracted beam monochromator.The Li[Ni1/3Mn1/3Àz Co1/3Zr z]O2 (0z0.1)samples were measured over a scattering angle range between10and90 using0.05 steps and a30s counting time.This long counting time was used so that small amounts of impurity phases could be detected.

Atomic absorption analysis.—Atomic absorption spectroscopy performed at the Minerals Engineering Center at Dalhousie University was used to measure the(Zr):(NiþMnþCoþZr)and Li:(NiþMnþCoþZr)ratio in the heat-treated oxide.The procedures used have been shown to give a Li:Co ratio of1.0:1.0for stoichiometric LiCoO2samples obtained from battery materials suppliers.

SEM testing.—A Hitachi S4700field-emission scanning elec-tron microscope(SEM)was used to image the materials.

Electrochemical testing.—Coin cells(23mm diameter and 2.5mm thick)were used for testing the electrochemical performance of

*Electrochemical Society Active Member z E-mail:jeff.dahn@dal.ca Journal of The Electrochemical Society,158(4)A428-A433(2011) 0013-4651/2011/158(4)/A428/6/$28.00V C The Electrochemical Society

A428