Ï¡ÓнðÊô(Ó¢ÎÄ°æ) 2019,38(03),189-198

Electrochemical performance of Li-rich cathode material,0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O₂ microspheres with F-doping

Ting Liu Shi-Xi Zhao Lu-Lu Gou Xia Wu Ce-Wen Nan

Graduate School at Shenzhen,Tsinghua University

School of Materials Science and Engineering,Tsinghua University

×÷Õß¼ò½é£º*Shi-Xi Zhao,e-mail:zhaosx@sz.tsinghua.edu.cn;

ÊÕ¸åÈÕÆÚ£º24 September 2017

»ù½ð£ºfinancially supported by the National Natural Science Foundation of China (No. 51372136);the NSFC-Guangdong United Fund (No. U1401246);

Electrochemical performance of Li-rich cathode material,0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O₂ microspheres with F-doping

Ting Liu Shi-Xi Zhao Lu-Lu Gou Xia Wu Ce-Wen Nan

Graduate School at Shenzhen,Tsinghua University

School of Materials Science and Engineering,Tsinghua University

Abstract£º

Layered F-doped cathode materials 0.3 Li₂ MnO₃-0.7 LiMn_1/3Ni_1/3CO_1/3)O_2-xF_x(x = 0, 0.01, 0.02, 0.03, 0.04,0.05) microspheres made up of nanosized primary grains were prepared through co-precipitation method. The sample of x = 0.02 demonstrates a large discharge capacity of226 mAh g^-1 over 100 cycles at 0.1 C and excellent rate performance with discharge capacity of 96 mAh g-1 at 5.0 C and room temperature. Particularly, this material shows much enhanced electrochemical performances even at high temperature of 55 ¡æ. It delivers a quite high discharge capacity of 233.7 mAh¡¤g^-1 at 1.0 C with capacity retention as high as 97.9% after 100 cycles. The results demonstrate that the fluorine incorporation stabilizes the cathode structure and maintains stable interfacial resistances.

Keyword£º

Lithium-ion battery; Cathode materials; 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x; F-doped;

Received£º 24 September 2017

1 Introduction

The issues of cathode materials involving high discharge capacity and high discharge voltage platform are becoming a hot research topic,especially in the application field of blade electric vehicles (BEVs).As the largest consumer market,Chinese government had published the road map for the development of new energy vehicles.The energy density of inpidual lithium battery pack was set up to350 Wh¡¤kg^-1 to satisfy over 300 miles range for BEVs before 2020 [ 1] .Conventional transition metal oxide cathodes,e.g.,LiCoO₂,LiMn₂O₄ or LiFePO₄,are insufficient to deliver high capacities to satisfy the range requirements for BEVs.The lithium-rich materials,referring to the Li₂Mn0₃-stabilized LiMO₂ (M=Mn,Ni,Co,etc.),are being vastly investigated with an aim to meet urgent demands for higher energy density of rechargeable batteries and larger cruise range of BEVs.The broad prospect of lithium-rich materials lies in the activation process undergone over 4.6 V versus Li/Li⁺during initial charging,resulting in an unprecedentedly high operating voltage and capacities of 280 mAh¡¤g^-1 [ 2, 3, 4, 5, 6, 7] .It has the potential to revolutionize the transportation industry,especially if it is paired with a high capacity anode material such as lithium or silicon.The resulting full cell is proposed to truly realize the high energy density of batteries,supplying the large cruise range of BEVs while lowering the costs.

However,its potential commercialization has been compromised for the signature drawbacks:the degradation of cell voltage during cycling and structural instability referring to the oxygen loss during activation of Li₂Mn0₃ [ 8, 9, 10, 11, 12] .In Li_1+xMn_1-x-y-zNi_yCo_zO₂ structures,closepacked two-component,monoclinic Li₂Mn0₃ and layered trigonal LiMO₂ (M=Mn,Ni,Co) is nominally integrated by skeletal O.During initial charge,when charged above4.6 V,Li₂MnO₃ is activated to simultaneously remove Li and O in the form of Li₂0,leading to the oxygen loss from the original layered lattice.Meanwhile,transition metal(M) cations filling Li sites upon discharge give rise to the transformation from layered to spinel phase.The result of these structural changes is the loss of Li intercalation sites and the formation of a spinel phase ( ) with a significantly lower operating voltage compared to that of the initial layered material,with the changes becoming more severe with cycling.Amount of efforts has been adopted to improve the lithium-rich cathode electrochemical performance.The measures are focused on the surface modification [ 13, 14, 15, 16] ,ion substitution [ 11, 17, 18, 19] and morphology manipulation of particles [ 20, 21, 22] .

Fig.1 Schematic illustration for formation of 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05) microspheres

In response to demands for higher energy density of batteries applied in BEV s,a facile approach was introduced to synthesize spherical Li-rich layered cathode.The method simplified the procedure for precursor preparation by averting the ammonia addition and delicate manipulation of pH commonly used.It was economical and easily handy.Moreover,fluorine incorporation was explored to evaluate the effects on the structural stabilization and the improvement of electrochemical performance for the layered cathode.

2 Experimental

2.1 Sample preparation

0.3Li₂MnO₃-07LiMn_1/3Ni_1/3Co_1/3O_2-xF_x(x=0,0.01,0.02,0.03,0.04,0.05) cathode materials were obtained using (Ni_7/30Co_7/30Mn_16/30)CO₃ as transition metal precursor.The spherical (Ni_7/30Co_7/30Mn_16/30)CO₃ precursor was prepared by co-precipitation method.Mixed transition metal solution (1 mol¡¤L^-1)(molar ratio of NiSO₄,CoSO₄and MnSO₄ were 7:7:16) was pumped into a continuous stirred reactor.Then,0.2 mol¡¤L^-1 Na₂CO₃ solution was added as precipitant drop by drop.The precipitate was then aged at 80¡æfor 12 h.After successional process of washing,filtration and drying three times,the final (Ni_7/30Co_7/30Mn_16/30)CO₃ precursor was obtained.Then,the resultant (Ni_7/30Co_7/30Mn_16/30)CO₃ powder was mixed with a stoichiometric amount of LiOH and LiF.The lithium-rich cathode was obtained after processing mixture calcination firstly at 500¡æfor 5 h and then at 850¡æfor 20 h under air.The scheme for preparation of 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05)microspheres is schematically illustrated in Fig.1.

2.2 Structural characterization

The chemical compositions of cathode materials were examined using inductively coupled plasma (ICP,Optima3000DV,Perkin-Elmer,USA).X-ray diffraction (XRD,Rint-2000 V/PC,Rigaku,Japan) patterns were collected in a 2¦Èrange of 10¡ã-80¡ã.The lattice parameters were refined by the Rietveld method.The morphology of the as-synthesized samples was observed by scanning electron microscope (SEM,S-4800,Hitachi,Japan).

2.3 Cell assembly and electrochemical performance testing

Electrochemical performances were evaluated using R2032-type coin cells,and Li metal was acted as negative electrodes.The composite cathode slurry consisted of80 wt%cathode materials,10 wt%super P,and 10 wt%PVDF,was c as ted onto an aluminum foil.The electrolyte solution was 1 mol¡¤L^-1 LiPF₆ in ethylene carbonate (EC):dimethyl carbonate (DMC)=1:1.The assembled cells were tested using LAND CT2001A in the voltage range of2.5-4.8 V.Cyclic voltammetry (CV) measurements were tested with a Shanghai Chenhua CHI660D electrochemical analyzer between 2.5 and 4.8 V at a sweep rate of0.1 mV¡¤s^-1.The electrochemical impedance spectroscopy(EIS) was examined with a 5-mV amplified voltage over the frequency range of 0.01-100 kHz.

Fig.2 XRD pattern of (Mn_16/30Co_7/30Ni_7/30)CO₃ powders

3 Results and discussion

XRD pattern of the carbonate precursor corresponding to R3c space group is shown in Fig.2.The broad diffraction peaks indicate small size of grains for precursor.The nominal formula of precursor is examined as (Mn_16/30Co_7/30Ni_7/30)CO₃ from ICP tests.Figure 3 shows SEM images of (Mn_16/30Co_7/30Ni_7/30)CO₃ powder.The (Mn_16/30Co_7/30Ni_7/30)CO₃ powder appears as 3-4¦Ìm spheres composed of primary nanoparticles with its diameter smaller than10 nm.

Figure 4 shows XRD patterns of 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_l/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05).The patterns are indexed to the space group of except for the super-lattice peaks referring to the shortrange LiMn₆ cation ordering between 20¡ãand 25¡ã [ 23] .The result suggests that the prepared 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05) cathodes are composed of intergrowth of LiMO₂ (M=Mn,Ni,Co) and Li₂MnO₃ C2/m phases.The peak shift in right side of this figure indicates that F is successfully doped into the structural lattice [ 24, 25] .

Fig.3 SEM images of (Mn_16/30Co_7/30Ni_7/30)CO₃ powders with a low and b high magnifications

Fig.4 XRD patterns of 0.3Li₂MnO_3-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05) powders

Fig.5 Variation in lattice parameters as a function of F amount in0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05)

Lattice parameters obtained by Rietveld refinement are shown in Fig.5.The lattice constants of both a-and c-axes increase after F-doping,and the extent enlarges with increment of F content.Actually,the effective ionic radius of F^-(0.133 nm) is smaller than that of O^2-(0.140 nm),and substitution of F for O would lead to the decrease in lattice parameter.However,the lattice parameters of both a-and c-axes increase in the experiment.Similar results were reported in the F-doped Li[Li_0,2Mn_0.54Ni_0.13Co_0.13]O₂by Zheng et al. [ 25] and Li(Ni_0.5Mn_0,5)O₂ by Kang and Amine [ 26] .The stronger bonding between Li and F would result in repulsive force in the oxide matrix and lead to the lattice expansion along a-and c-axes.Indeed,it exhibits that the variation of lattice parameters is in accordance with the increase of F content,fitting well with the peak shift in XRD patterns.

Figure 6 shows SEM images of all the finial products.As shown in Fig.6a1-f₁,all the powders including pristine and F-doped samples exhibit an average diameter of3-4¦Ìm.Higher magnification SEM images in Fig.6a₂-f₂indicates that all finial products are composed of nanopolyhedral primary grains which are favor for Li⁺migration.As shown in Fig.6a₃-f₃,the size of primary grains is30-80 nm for the F-free sample,while it increases from50-100 to 200-300 nm along with the increase of F content for F-doped samples.The result is consistent with reports about fluorine doping [ 25, 27] .The reason may be that the increase of fluorine content is beneficial for crystals to grow up.Besides,fluorine incorporation also affects the surface morphology of the particles.The surfaces of the product particles (for x=0.01 and 0.02) are smooth and clean,whereas,as F content increases,the surfaces of other F-doped samples become coarse,covered with a small number of nanoparticles with 10-60 nm in size.

Galvanostatic test was used to investigate the electrochemical performance of F-doped 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0.01,0.02,0.03,0.04,0.05)(the corresponding cell is denoted as F1-F5 hereafter,respectively) and the pristine samples.The tested current was set at 0.1 C during 2.5-4.8 V.In the initial charge curves(Fig.7a),two voltage plateaus are distinctly observed,one below 4.4 V belongs to lithium deintercalation from the LiMO₂ (M=Mn,Ni,Co) component,while the other above4.4 V is associated with lithium deintercalation from Li₂MnO₃ component [ 23] .Nonetheless,the charge plateau becomes ambiguous in the second cycle in Fig.7b,indicating that structure rearrangement occurs during lithiation/delithiation process [ 28] .The initial discharge capacity of pristine,F2 and F5 samples is 252.2,254.3 and 246.4mAh.g^-1,respectively.Also,a certain amount of capacity loss is observed for each sample,attributed to the irreversible structure change caused by lithium extraction in the form of Li₂O above 4.4 V [ 29, 30] .The irreversible capacity loss for each sample is 152.2,131.2 and120.9 mA.g^-1 in the initial cycle,respectively.Obviously,the pristine sample suffers the largest initial capacity loss and fluorine incorporation functions effectively in inhibiting the loss.As shown in Fig.7,the voltage fading starts to appear during the following cycles and is obviously observed especially at the 50th and 100th cycles.Nevertheless,the F2 sample still exhibits the highest capacity at all the cycles,e.g.,with the discharge capacities of 254.3,254.1,239.3 and 226 mAh¡¤g^-1 for the initial,second,50th and 100th cycles.Definitely,F-incorporation is beneficial for decreasing the irreversible capacity loss.

Figure 8a shows cycling performance of all the samples examined at 0.1C.After 100 cycles under room temperature,the remaining discharge capacity for the pristine and F1-F5 samples is 190.5,210.6,226.0,211.5,213.2 and212.3 mAh¡¤g^-1,respectively.Indeed,the F-incorporated samples deliver an enhanced stability with higher reversible capacity than the pristine one.Figure 8b displays the rate performance for all the samples at different rates.The cell was cycled at 0.1C for 5 cycles firstly,followed by varied rates from 0.2 to 5.0C.Even under the high tested rate of 5.0C,the F2 sample still demonstrates an excellent cycling performance with the largest reversible discharge capacity of 96.0 mAh.g^-1.When the tested rate returns to0.1C,the reversible capacity of F2 sample is as high as 240mAh.g^-1,exhibiting stabilized performance with capacity loss less than 15 mAh¡¤g^-1 after 30 cycles.The effects of tested temperature were also evaluated.As shown in Fig.8c,d,the cells were tested under 25 or 55¡æat the rate of 1.0C.At 25¡æ,the F2 sample delivers the highest discharge capacity,as well as the slowest capacity fading rates with the capacity retention as high as 88.2%after 100cycles.On the contrary,the pristine and other F-doped samples show inferior cycling performance with lower discharge capacities.Similar improvement is also observed when the tested temperature increases to 55¡æ.The pristine and F5 samples obtain initial discharge capacities of192.1 and 206.4 mAh¡¤g^-1,respectively,which are significantly lower than those for other F-doped samples.Moreover,they also exhibit worse performance with just capacity retentions of 83.7%and 65.6%after 100 cycles,compared with that of 97.9%for F2 sample.

Fig.6 SEM images of various amount F-doped 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3CO_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05) cathode materials:a_1-a3 x=0,b₁-b₃ x=0.01,c₁-c₃ x=0.02,d₁-d₃ x=0.03,e₁-e₃ x=0.04 and f₁-f₃x=0.05

Fig.7 Charge/discharge curves of various 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.02,0.05) in voltage range of 2.5-4.8 V for a first cycle,b second cycle,c 50th cycle and d 100th cycle

CV test was applied to evaluate the electrochemical performance of F2 and the pristine samples during2.5-4.8 V.Figure 9 shows CV curves of F2 and the pristine one at first,second and 50th cycle with scan rate of0.1 mV¡¤s^-1.In the first cycle (Fig.9a),there are two charge peaks located at¡«4.0 and¡«4.6 V for both samples.The former is associated with Li⁺deintercalation from LiMO₂ (M=Mn,Ni,Co) component,whereas the latter at 4.6 V is associated with the removal of Li₂O for Li₂MnO₃ component.In the negative scan,there lie three peaks for both samples,of which¡«4.4 and¡«3.7 V relate to the reduction of Ni⁴⁺and Co⁴⁺and¡«3.3 V to Mn⁴⁺reduction.However,the location of the peaks on CV curves has been significantly varied in the second cycling.The strongest peak at¡«4.6 V disappears,and a weak peak emerges at 4.5 V.According to the early reported works,the oxidation of Ni²⁺and Co³⁺can only be observed above 3.5 V [ 23, 31] .Therefore,the two oxidation peaks in the second cycle at¡«3.8 and¡«4.5 V are,respectively,related to the oxidation of Ni²⁺and Co³⁺.The redox below 3.5 V can still be observed in the second scan.In the 50th scan,noticeable differences are observed:(1) a new shoulder marked as#emerges in the CV curve of the pristine sample;(2) for F2 sample,the major oxidation peak shifts to a lower voltage (<3.8 V),together with a contrary tendency in location change for the reduction peak.This will result in the reduced potential difference and reduction of polarization.The new shoulder (<3.5 V)marked as#for pristine sample could be ascribed to fthe oxidation of Mn³⁺,which may lead to the instability of the layered structure.The tested results reveal that the F2sample exhibits promoted symmetric redox peaks than the pristine one,suggesting that an enhanced electrochemical reversibility of Li-insertion/extraction is obtained by an appropriate fluorine incorporation.

EIS was performed to confirm the effect of fluorine incorporation.All cells were tested at fully charged state under 0.2C cycling.The obtained impedance spectra of the pristine and F2 samples are presented in Fig.10,in which a high-frequency semicircle refers to electrode surface resistance (R_f) and intermediate frequency semicircle refers to the charge transfer resistance (R_ct) originated from electrode/electrolyte interface [ 31, 32] .Values of R_f and R_ct are obtained from the equivalent circuit in Fig.10 and listed in Table 1.After 50 cycles,R_f value of F-free sample increases from 45.05 (first cycle) to 196.80¦¸(50th cycle)while that of F2 sample changes from 27.28 (first cycle) to131.30¦¸(50th cycle).The decreased R_f and R_ct values of F2 sample reveal that fluorine incorporation could inhibit side interaction between cathode and electrolyte and promote the interfacial migration of Li⁺.

Fig.8 a Cycling performance of electrodes in voltage range of 2.5-4.8 V at a charge and discharge current density of 0.1C;b rate performance cycled between 2.5 and 4.8 V;c cycling performance of electrodes in voltage range of 2.5-4.8 V at a charge and discharge current density of1.0C;d cycling performance of electrodes in voltage range of 2.5-4.8 V at a charge and discharge current density of 1.0C at 55¡æ

Fig.9 CV curves of 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.02) for a first cycle,b second cycle and c 50th cycle,where¢ñstanding for redox peak of LiMO₂ (M=Mn,Ni,Co) component,¢òfor redox peak of Li₂MnO₃ component,¡°asterisk¡±for redox reaction of Ni²⁺/Ni⁴⁺,¡°circle¡±for redox reaction of Co³⁺/Co⁴⁺,¡°square¡±for redox reaction of Mn³⁺/Mn⁴⁺

MPV,defined as cell voltage at semi-discharge state,refers to the polarization and indicates extent of phase transformation from layered to spinel structure [ 25, 33] .It can be observed from Fig.11a that MPV values of all the samples decrease with the increment of cycle numbers.F-doped samples show higher MPV value than that of pristine one,attributed to the reduced activation of Li₂MnO₃ component [ 33] .In the first cycle,as F content increases,MPV values of the pristine and Fl-F5 samples are 3.60,3.61,3.65,3.68,3.68 and 3.72 V,respectively.With cycle number increasing,MPV values decrease more obviously for the pristine and F5 sample than others.At100th cycle,MPV values of the pristine and F1-F5 samples are 3.07,3.23,3.25,3.24,3.23 and 3.13 V,respectively.MPV values of pristine and F5 samples are much lower than those of others.Furthermore,higher MPV retention is obtained for F-doped sample than pristine one except for F5 samples,as depicted in Fig.11b.Therefore,it can be drawn a conclusion that an appropriate F content is good for promoting the electrochemical performance of the electrodes.The result firmly supports the point that fluorine incorporation can effectively enable the structure stability by inhibiting the layered-to-spinel phase transformation for the lithium-rich cathode.

Fig.10 EIS plots of pristine sample and F-doped one for x=0.02:a first cycle charge to 4.8 V and b 50th charge to 4.8 V;c equivalent circuit for fitting process,where R_s being resistance of electrolyte,R_ct being charge transfer resistance,CPE₁ and CPE₂ being constant phase elements,and W₁ being Warburg impedance

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Table 1 Surface film resistance (R_f) and charge transfer resistance (R_ct) of pristine and F-doped of x=0.02 samples in the first and 50th cycles

Fig.11 a Midpoint voltage (MPV) and b corresponding MPV retention for 0.3Li₂MnO_3-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05)

4 Conclusion

F-doped 0.3Li₂MnO₃-0.7LiMn_1/3Ni_1/3Co_1/3O_2-xF_x (x=0,0.01,0.02,0.03,0.04,0.05) microspheres were successfully synthesized through a facile route using flocculent(Mn_16/30Co_7/30Ni_7/30)CO₃ spherical precursor.Fluorine incorporation during the synthesis process is found to be beneficial for the enhanced electrochemical performance of cathode by providing a robust and stabilized interface with the electrolyte,along with the restriction of layered-spinel phase transformation testified with higher MPV values for F-doped samples.The sample of x=0.02 exhibits a discharge capacity as high as 226.0 mAh¡¤g^-1 over 100 cycles at room temperature and 233.7 mAh¡¤g^-1 over 100 cycles at elevated temperature.Also,it exhibits much enhanced rate capacity of about 96 mAh¡¤g^-1 at 5.0C.Owing to its simplicity and applicability,it is easy to realize high-performance lithium-rich cathode materials for practical battery applications.

²Î¿¼ÎÄÏ×

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