原子层沉积MgO薄膜改性LiNi0.6Co0.2Mn0.2O2

寇华日，李喜飞，刘 文，鄯 慧，颜 波，丁书江

(1.天津师范大学物理与材料学院 天津 西青区 300387；2.西安理工大学先进能源与器件中心 西安 710048；3.西安交通大学深圳研究院 广东 深圳 518057；4.西安交通大学理学院 西安 710049)

A lithium ion battery system has been known as one of the most promising energy storage devices to address the energy crisis owing to its high energy density,high voltage and long cycling life.For the applications of electric vehicles(EVs) and plug-in hybrid electric vehicles(HEVs),lithium ion batteries(LIBs) are required with high reversible capacities at high cutoff potentials and good cycling stabilities.Currently,the commercial LiCoO2cathode may not satisfy the requirements of EVs and HEVs.Because of integrating the advantages of LiCoO2,LiNiO2and LiMnO2,the LiNixCoyMn1−x−yO2cathode material has been an available candidate to build high performance LIBs.

To promote the lithium ion storage performance of LiNi1−x−yCoxMnyO2,many kinds of LiNi1−x−yCoxMnyO2materials with different ratios of Ni,Co and Mn have been studied[1-4].As a type of nickel-rich LiNixCoyMn1−x−yO2(x＞0.5) materials,LiNi0.6Co0.2Mn0.2O2has attracted a lot of attention for its high reversible capacity.However,the dissolution of Ni,Co and Mn ions into the electrolyte occurs upon cycling,causing severe structural damages and degradations of the material stabilities[5-9].Furthermore,it was demonstrated in a number of studies[10-12]that a high cutoff potential can lead to a high energy density,but an aggravating dissolution of active ions,especially at potentials above 4.3 V.

To address these challenges,a modification of LiNixCoyMn1−x−yO2cathode is demanded.And a great deal of strategies have been tried to improve the properties of LiNixCoyMn1−x−yO2cathodes,such as ion doping[13-17],surface coating[1,4,18-19],surface reconstruction[20-22],size and shape control[23-25].Among these methods,surface coating with metal oxides has been verified to be efficient in improving the cathode electrochemical performances,such as Al2O3[26-30],MgO[31-35],TiO2[36-38]and ZnO[39-41].Due to the hexagonal structure of MgO,a MgO coating layer has negligible effect on Li ion transition[35].Moreover,the Mg2+ions can diffuse into the interslab space under the pillaring effect with enhancing the structural stability[42-44].As a result,the MgO coating shows unique advantages comparing with the other metal oxides.To promote the performances of LiNi0.6Co0.2Mn0.2O2cathodes,moreover,it is essential to obtain a conformal and ultrathin MgO coating layer.But it is of difficulty for the traditional methods,such as sol-gel and wet chemical methods,to produce an ultrathin layer onto the LiNi0.6Co0.2Mn0.2O2.Therefore,how to design an ultrathin MgO coating onto LiNi0.6Co0.2Mn0.2O2has still been challenging.

Fortunately,an atomic layer deposition(ALD) as an advanced coating method can conveniently synthesize ultrathin films on many kinds of materials[45-49].The thicknesses of coating layers can be precisely controlled by regulating numbers of deposition reactions.Moreover,our previous results indicated that the electrode modification can well maintain the conductivity network,differing from the powder coating[28,50].Interestingly,the whole electrode modification is easy to conduct via ALD,which is challenging for the traditional methods.

In this work,LiNi0.6Co0.2Mn0.2O2cathodes coated with ultrathin MgO layer were successfully designed.Remarkably,the electrochemical performances of LiNi0.6Co0.2Mn0.2O2were much improved due to the effective protection of MgO coating from the electrolyte.

1 Experimental

1.1 Material Synthesis

All the reagents were used with no further purification.A mixture of commercial LiNi0.6Co0.2Mn0.2O2,acetylene black and Polyvinylidene Fluoride(PVDF) were well mixed in appropriate amount of 1-Methyl-2-pyrrolidinone(NMP) at a mass ratio of 8:1:1.The resultant slurry was spread on the aluminum foil and dried in a coven at 80 °C for overnight,and the cathode electrodes were obtained.The direct deposition of MgO onto the cathode electrodes was conducted via ALD R200 Advanced system(Picosun,Finland).In the ALD process of MgO,Bisclopentadienyl magnesium(MSDS) and H2O were used as the precursor of Mg and oxidizer,respectively.The deposition temperature was set at 200 °C.A typical cycle of MgO deposition was conducted with the following steps: 1) pulsing the MSDS into the reactor for 1.6 s; 2) evacuating the excess MSDS for 5 s;3) pulsing the H2O into the reactor for 0.1 s; 4)evacuating the excess H2O and by product for 8 s.The cathode electrodes with different thicknesses of MgO coating layers were acquired by depositing for 2,5 and 10 ALD cycles,which were marked as NCM-2c,NCM-5c and NCM-10c,respectively.For comparison,the pristine LiNi0.6Co0.2Mn0.2O2electrode was noted as NCM-0c.

1.2 Physical Characterization

NCM-0c and NCM-10c were characterized by X-ray diffraction(XRD,D8 Advance of Bruker,Germany) with Cu-Kαradiation to obtain the crystallinity phases between scattering angles(2θ) of 10°～80° at a scanning rate of 10° min−1.The morphologies of the samples were observed by the scanning electron microscopy(SEM,SU8010,Hitachi) and high-resolution transmission electron microscopy(HRTEM,TecnaiG2F30,FEI),and the elemental mapping was acquired by SEM(SU8010,Hitachi) with EDAX.The particle size distribution of NCM-0c was obtained via laser particle sizer(Mastersizer 3000,Malvern).The elemental information of the NCM-10c electrode was collected by X-ray photoelectron spectroscopy(XPS,VG ESCALAB MK II).

1.3 Electrochemical Performance

The electrodes obtained were used as working electrodes,and the lithium metal slices acted as both counter and reference electrodes.The electrolyte used in the cells contains 1 M LiPF6in an ethylene carbonate(EC) and dimethyl carbonate(DMC)mixture(1:1 in volume).The cells were assembled in a glove box with moisture and oxygen contents less than 0.1 ppm.The galvanostatical tests were conducted on a Land battery tester(LANHE CT2001A) using CR2032-type coin cells with the potential ranges of 2.7～4.5 V and 2.7～4.7 V.Cyclic voltammogram(CV) measurements were performed using Princeton Applied Research VersaSTAT4 in the voltage range of 2.7～4.5 V(vs.Li/Li+) at a scan rate of 0.1 mV·s−1.Electrochemical impedance spectroscopy(EIS) was also carried out on Princeton Applied Research VersaSTAT4 at an amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz.All the electrochemical experiments were conducted at room temperature.

2 Results and Discussion

The typical SEM images of NCM-0c,NCM-2c,NCM-5c and NCM-10c are shown in Fig.1.As seen in the Fig.1a,the NCM-0c is combined by primary particles with relatively smooth surface.The NCM-2c,NCM-5c and NCM-10c show uniform MgO coating layers on the surface.Even on the surface of NCM-2c,the dense coating layer may be discerned.And it can also be observed that the amount of MgO increases with increasing ALD cycles.As shown in Fig.1b～d,the MgO films are compact and conformal which is the outstanding advantage of ALD derived coatings.This kind of ultrathin film can provide a perfect protection for the LiNi0.6Co0.2Mn0.2O2material.The HRTEM image(Fig.1e) of NCM-10c displays a thin MgO layer with the thickness of about 3 nm.The low crystallization of conformal-coating layer in Fig.1e may own to the low conducting temperature[51].The size distribution curve in Fig.1f indicates that the particle diameter of NCM-0c is about 10 μm.The distributions of Ni,Co,Mn,O,Mg,F,C are well coincided in the images.It verifies that the MgO film is successfully deposited on the electrode slice.And in the Energy dispersive spectroscopy profile,Mg peak at around 1.3 keV further proves the existence of MgO on the electrode surface.The surface element contents are summarized.It can be found that Mg weight ratio is only 0.47%.The limited Mg amount suggests ALD-derived thin MgO layers onto NCM-10c.

The XRD patterns of NCM-0c and NCM-10c are shown in Fig.2a.Both two patterns are well consistent with the hexagonal α-NaFeO2layered structure with space group R-3 m.The distinct peaks of(003),(101)and(104) planes are discerned,and no obvious differences can be found by comparing both patterns.This may be attributed to the ultrathin MgO layer exceeding the resolution of XRD.It may also own to the relatively low crystallinity of MgO synthesized at 200 °C.The chemical bonding states of the elements in NCM-10c electrode are characterized by XPS.In Fig.2b,the strong peaks of C 1s and F 1s are found owing to the existence of the PVDF and acetylene black in the cathode electrodes.As shown in Fig.2c,the two spinorbit peaks at 857.5 eV and 878.2 eV are assigned to 2p3/2 and 2p1/2 of Ni in the LiNi0.6Co0.2Mn0.2O2,respectively.And the peaks at 779.7 eV and 795.2 eV correspond to 2p3/2 and 2p1/2 of Co,respectively.Similarly,the spin-orbit peaks of Mn 2p are located at 641.9 eV and 653.5 eV,suggesting the bonding energy of Mn-O in the cathode.Importantly,the signal of Mg 1s is obvious in the spectrum of NCM-10c in Fig.2f,and the peak at 1 303.1 eV verifies the successful deposition of MgO film.

The CV curves of NCM-0c,NCM-2c,NCM-5c and NCM-10c are expressed in Fig.3.The peaks in anodic process correspond to the Li+extraction from electrodes,while the insertion causes the cathodic peaks[8-9].With increasing the thicknesses of MgO layer,the positions of the initial anodic peaks shift from 3.89 V to 3.95 V.The polarization is caused by the MgO coating layer with poor conductivity.And this phenomenon can also be found in the following CV curves,for instance,in the 4th cycles the anodic peaks of NCM-0c,NCM-2c,NCM-5c and NCM-10c are located at 3.77 V,3.78 V,3.79 V and 3.80 V,respectively.Interestingly,all the electrodes show similar cathodic peak positioned at about 3.69 V.It can be inferred that the MgO coating layer exhibits a weak blocking effect to the lithium ion insertion after the initial cycle.

The charge-discharge profiles of NCM-0c and NCM-2c in the 1st,5th,and 50th cycles are summarized in Fig.3e and 3f.It can be discerned that the specific capacities of NCM-0c show a dramatic degradation during the cycling process,for instance,the charge/discharge capacities decrease from 174/173 mAh·g−1in the 5th cycle to 155/153 mAh·g−1in the 50th cycle.In contrast,the NCM-2c displays good cycling stabilities with high reversible capacities of 177/175 mAh·g−1and 170/169 mAh·g−1at the same test conditions.The initial charge platform of NCM-0c is about 3.75 V in Fig.3e,while that of NCM-2c is about 3.80 V in Fig.3f.The polarization is caused by MgO coating layer which is an insulting material to the electron transfer.The discharge platforms of NCM-0c and NCM-2c are 3.71 V,which agrees with the CV curves in Fig.3a and 3b.

The cycling performances of NCM-0c,NCM-2c,NCM-5c,and NCM-10c are compared at a rate of 1C(224 mA·g−1) in Fig.4a.The initial charge/discharge specific capacities of NCM-0c,NCM-2c,NCM-5c and NCM-10c are 206.8/164.1 mAh·g−1,218.2/170 mAh·g−1,212.3/163.3 mAh·g−1and 268.1/160.1 mAh·g−1,respectively.It can be found that the initial coulombic efficiencies of NCM-0c,NCM-2c and NCM-5c are all about 78%,while NCM-10c expresses a coulombic efficiency of 69%.This suggests the thick MgO coating may be detrimental to the reversibility of cathode electrodes.The NCM-2c shows the highest initial specific capacity among the electrodes.After 100 cycles in the voltage range of 2.7～4.5 V,NCM-2c still exhibits the best capacity retention with a specific capacity of 156.9 mAh·g−1.Meanwhile,that of NCM-0c degrades dramatically from 164.1 mAh·g−1to 131.3 mAh·g−1.As mentioned earlier,the energy density of LIBs can be promoted by increasing the cutoff potentials.To get the performances of LiNi0.6Co0.2Mn0.2O2with MgO coating layer,the electrodes are also tested in a voltage range from 2.7～4.7 V.And the data are shown in Fig.4b and 4f.The similar phenomena occur,when the cutoff potential increases to 4.7 V.As expected,the high cutoff potential leads to high specific capacities with low capacity retention.The initial discharge capacity of NCM-2c increases to 200 mAh·g−1from 170 mAh·g−1by rising the cutoff potential from 4.5 V～4.7 V,and a specific capacity of 158 mAh·g−1is obtained after 100 cycles.Similar to the cycling performances at a cutoff potential of 4.5 V,NCM-5c,NCM-10c and NCM-0c successively deliver specific capacities of 151.8 mAh·g−1,146.9 mAh·g−1and 144 mAh·g−1,respectively.

As the reflections of electrodes stabilities,capacity retentions are exhibited in Fig.4c and 4d corresponding to cycling voltage ranges of 2.7～4.5 V and 2.7～4.7 V.In agreement with Fig.4a,NCM-0c shows the lowest retention among these electrodes in Fig.4c.After 95 cycles,only 80% of specific capacity is retained in a voltage range of 2.7～4.5 V.In comparison,the retention rate of NCM-2c is 93% after 100 cycles.Interestingly,all of electrodes display retention rates higher than 100% in first 20 cycles attributing to the activation process of cathode materials.When the cutoff potential increases to 4.7 V(Fig.4d),all electrodes show close retention rates.This may be due to severe side-reactions at high potential damaging the surface structure of cathodes.However,after 100 cycles,NCM-2c still shows the highest retention rate.And the high retention rates of NCM-10c in the first 10 cycles verifies a good protection of MgO layer for electrodes.

The rate capabilities of different electrodes in a voltage range of 2.7～4.5 V are shown in Fig.4e.NCM-2c and NCM-5c display higher capacities than NCM-0c at the rates of 1C,2C and 5C.Notably,after 60 cycles at different current densities,the NCM-2c delivers a capacity of 167 mAh·g−1,which is 96% of the specific capacity after 10 cycles at 1C,while the retention rate values of NCM-0c,NCM-5c and NCM-10c are 90%,92% and 75%,respectively.Moreover,the NCM-10c exhibits the worst performance among these electrodes,especially at high current densities.This may be due to the thick MgO layer with poor conductivity detrimental to the lithium ion transfer.When the electrodes were cycled in the voltage range of 2.7～4.7 V,the NCM-2c also shows good rate capability at each current density,especially after cycling at a rate of 10C in Fig.4f.The reversible capacity of NCM-2c is about 130 mAh·g−1at 10C,while the capacities of the other electrodes are much lower than 100 mAh·g−1.After cycling at 1C,2C,5C,10C and 5C,all the electrodes with MgO coating layers exhibit higher specific capacities than NCM-0c.It can be inferred that the MgO coating layers can decrease the corrosion of electrolyte at high rates.Moreover,the rate capabilities of NCM-5c and NCM-10c are worse than NCM-0c at 10C.The similar phenomenon was also reported in the previous work[28].

To investigate the influences of MgO coating layers on the evolution of cathodes,the EIS of NCM-0c and NCM-2c is tested after 5,10,20 and 50 cycles.The Nyquist plots of NCM-0c and NCM-2c are shown in Fig.5a and 5b.The equivalent circuit in Fig.5d is used to simulate the EIS.The resistance of cell system is represented by Rs,including the electrolyte resistance and various connecting resistances between the different parts in cells.Rsfand CPEsfcorresponding to the first semicircle of Nyquist in high frequency are used to simulate the resistances in the surface of electrodes.The change of first semicircle in Nyquist can reflect the surface stability of electrodes.Rctand CPEctrepresent the resistances caused by charge transfer reactions,which are related to the second semicircle in Nyquist.Wsis attributed to the diffusion process of Li ions in the cathode material,reflecting by the inclined line in low frequency.As seen in Fig.5a and b,the first Nyquist semicircles of NCM-0c dramatically change with cycles,while those of NCM-2c show a relatively good coincidence.This suggests a better stability of NCM-2c than NCM-0c.The Rctvalue reflects a stable surface structure and verifies the protection effect of MgO coated on the electrode surface.The fitting Rctresults of Nyquist plots are summarized in Fig.5c.It is obvious that the Rctvalues of NCM-0c are more than those of NCM-2c after 5,10,20 and 50 cycles.Furthermore,the Rctof NCM-2c is more stable than that of NCM-0c.Especially,after 5 cycles,Rctof NCM-0c is close to 150 Ω,while the Rctof NCM-2c is only 75 Ω.These are good evidences to prove that the cathode structure is stabilized by the MgO coating layers.

3 Conclusion

In this study,for the first time,the electrochemical performances of LiNi0.6Co0.2Mn0.2O2material were improved by depositing ultrathin MgO layers onto electrodes.By preciously controlling the MgO thickness,it was found that 2-ALD- cycle deposition of MgO on LiNi0.6Co0.2Mn0.2O2electrodes can efficiently promote the cycling performance and rate capability.The electrochemical performance promotion owns to the stabilization and protection effects of ALD derived ultrathin MgO coating layer.Our results exhibit the potential of ALD derived MgO for improvement of cathode performance for LIBs.

4 Acknowledgments

This research was supported by Academic Innovation Funding of Tianjin Normal University(52XC1404) and Training Plan of Leader Talent of University in Tianjin.