Improved cycle performance of LiMn2O4 cathode material for aqueous rechargeable lithium battery

Improved cycle performance of LiMn2O4cathode material for aqueous rechargeable lithium battery by LaF3coating

Qing Zhu a,b,Shuai Zheng a,Xuwu Lu a,Yi Wan b,Quanqi Chen a,*,Jianwen Yang a,

Ling-zhi Zhang a,Zhouguang Lu b,**

a College of Chemistry and Bioengineering,Guilin University of Technology,Guilin541004,China

b Department of Materials Science&Engineering,South University of Science and Technology of China,Shenzhen518055,China

a r t i c l e i n f o

Article history:

Received18July2015

Received in revised form

9September2015

Accepted11September2015 Available online14September2015

Keywords:

Aqueous rechargeable lithium battery (ARLB)

Spinel LiMn2O4

LaF3coating

Cathode a b s t r a c t

LaF3-coated LiMn2O4composites were prepared by a simple co-precipitation method using pristine LiMn2O4,La(NO3)3$6H2O,NH4F and polyvinyl pyrrolidone(PVP)as raw materials and the physical and electrochemical properties of the composites were investigated.The results indicate that LaF3coating is bene?cial to improving the electrochemical performance of LiMn2O4used as a cathode material for aqueous rechargeable lithium battery(ARLB)in1mol/L LiNO3solution.The optimal LaF3-coated LiMn2O4 composite(L-3-LMO)containing3wt%LaF3exhibits much improved electrochemical performance as compared with the pristine LiMn2O4.When galvanostatically charged and discharged at2C (296mA gà1),L-3-LMO displays initial discharge capacity of118.4mAh gà1and capacity retention of 99.7%after50cycles,while the pristine LiMn2O4presents initial discharge capacity of109.2mAh gà1and capacity retention of89.4%.Even at high current density of10C,L-3-LMO delivers discharge capacity of 109.5mAh gà1and the capacity remains107.9mAh gà1after100cycles,while the highest discharge capacity of pristine LiMn2O4is only101.6mAh gà1and the capacity is as low as86.3mAh gà1after100 cycles.The effects of LaF3coating on Litdiffusion coef?cient,dissolution of manganese ion of electro-active materials are also investigated.

?2015Elsevier B.V.All rights reserved.

1.Introduction

Since commercialized by Sony in1991[1],non-aqueous lithium-ion battery has been attractive for its high energy density and long cycle life and widely used to power portable electronic equipments such as laptop,camera and cellar phone.However,its?ammable organic electrolytes limit its large-scale applications in energy storage systems due to potential safety issues resulting from the ?ammable solvents and lower power density originating from the lower conductivity of organic electrolytes[2e5].While its coun-terpart,aqueous rechargeable lithium battery(ARLB)containing aqueous electrolyte,has similar mechanism of energy storage and conversion[4,6],and recently has been regarded as one kind of the most competitive storage systems due to its safety,high power density,environmental friendliness and low cost since it was proposed by Dahn in1994[7].Unfortunately,the cycle perfor-mance of ARLB is much inferior to that of non-aqueous lithium battery with the same electrodes,and it was usually attributed to the dissolution of electroactive materials and side reactions in aqueous electrolytes.

Compared with other studied cathode materials for ARLBs such as LiNiO2[8],LiNi0.8Co0.2O2[9],LiCoO2[10,11],LiCo1/3Ni1/3Mn1/3O2 [12],Li1.2Co0.3Mn0.5O2[13],Li1.05Cr0.10Mn1.85O4[14],LiFePO4 [15e17],LiFe0.6Mn0.4PO4[18],LiFe0.5Mn0.5PO4[19],LiFe1àx V x PO4 [20],Li3V2(PO4)3[21]and LiMnPO4[22],the spinel LiMn2O4 recently has been regarded as one of the most promising cathodes because of its high energy density,low cost,safety,environmental affability and high rate capability.But the dissolution of Mn ion into the electrolyte and structural distortion result in poor cycle life,so the cycling performance could be improved by changing the morphology and size of LiMn2O4[23e29].But the effects of surface modi?cation on the electrochemical performance of LiMn2O4 cathode for ARLBs have been rarely studied.Previous reports have shown that the LaF3coating can greatly improve the cycle perfor-mance of LiMn2O4as a cathode material for non-aqueous lithium-

*Corresponding author.

**Corresponding author.

E-mail addresses:quanqi.chen@https://www.360docs.net/doc/1217545491.html,,quanqi.chen@https://www.360docs.net/doc/1217545491.html,(Q.Chen), luzg@https://www.360docs.net/doc/1217545491.html,(Z.

Lu).

Contents lists available at ScienceDirect

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0925-8388/?2015Elsevier B.V.All rights reserved.

Journal of Alloys and Compounds654(2016)384e391

ion batteries[30,31],and this electrochemical improvement may be related to the special properties of LaF3crystal such as chemical stability,insolubility and fast ion conductor[32e34].In this work, we presented that LaF3coating onto the surface of LiMn2O4by a modi?ed co-precipitation method can also substantially improve the electrochemical performance of LiMn2O4in1mol/L LiNO3 aqueous solution,and the possible reasons for the improvement are also discussed.

2.Experimental

2.1.Preparation of materials

The pristine LiMn2O4samples were synthesized by calcination of precursors which were prepared via a facile precipitation pro-cess.A certain amount of Li2CO3solution was slowly added to a reaction vessel containing the corresponding Mn(CH3COO)2solu-tion,in which the molar ratio of Mn to Li is2:1.05,with vigorous stirring at80 C in a thermostatic bath for about6h to obtain yellow-brown gel.The resulted gel was dried and sintered in air at 350 C for6h.Then the resulted product was pelletized,calcinated in air at800 C for20h and cooled to room temperature to get pristine LiMn2O4.The LaF3coated LiMn2O4samples were prepared by a modi?ed co-precipitation method[30]and the typical proce-dure is as follows:1g of as-prepared pristine LiMn2O4powder was added to100mL polyvinyl pyrrolidone(PVP)(Alfa Aesar, M w?1,300,000)solution(0.02g/mL)with robust agitation for2h at60 C,then10mL La(NO3)3solution was poured into the above suspension with continuous stirring for1h,subsequently a stoi-chiometry amount of NH4F solution was added dropwise slowly to the suspension with agitation for2h.The resulted suspension was aged at room temperature for2h,?ltered and washed to obtain a black deposit coated by white?occulation.The resulted deposit was dried,sintered in air at400 C for2h and cooled to room temperature to obtain LaF3coated LiMn2O4.The coated LiMn2O4 samples contain different contents of LaF3and the expected weight ratios of LaF3to LiMn2O4are1wt%,3wt%and5wt%,respectively, and the corresponding coated samples are denoted as L-1-LMO,L-3-LMO and L-5-LMO,respectively.

2.2.Characterizations of materials

The phase identi?cation of pristine and LaF3coated LiMn2O4 samples was performed by a D/Max III X-ray diffractometer(XRD) with Cu K a radiation(l?0.15418nm),and the cell parameters of samples were analyzed by Jade 5.0,an XRD analysis software. Microstructure of coated and pristine LiMn2O4samples was investigated by?eld emission scanning electron microscopy (FESEM,Hitachi S-4800)and transmission electron microscopy (TEM,JEM-2100F).

The electrochemical tests of the samples were carried out using three-electrode cells.In all cells,platinum foil and saturated calomel electrode(SCE)served as counter and reference elec-trodes,respectively,LiMn2O4or coated LiMn2O4electrodes were used as working electrodes,and the electrolyte was1mol/L LiNO3 aqueous solution.The working electrode was prepared by coating slurry of active material,acetylene black,polyvinylidene?uoride (PVDF)and N-methylpyrrolidone(NMP)on the nickel mesh col-lector.After dried at120 C,the resulted electrode consisted of active material,acetylene black and PVDF with a weight ratio of 80:10:10,and the electrode was pressed with a pressure of 600e800kg/cm2.The cells were galvanostatically charged/dis-charged at different current rates(1C?148mA gà1)in the voltage range of0e1.1V(vs.SCE)using Neware battery test system (Shengzhen,China).The speci?c capacity of electrodes was calculated based on the total mass of LiMn2O4and LaF3.The measurements of electrochemical impedance spectroscopy(EIS) and cyclic voltammetry were conducted on an IM6ex electro-chemistry workstation(Zahner-Elektrick,German).

3.Results and discussion

The XRD patterns of bare and LaF3coated LiMn2O4samples are shown in Fig.1.It can be seen that XRD patterns of all samples except L-5-LMO are consistent with that of standard spinel LiMn2O4 (PDF:35-0782),similar to our previous report[30].Absence of LaF3 diffraction peaks in XRD patterns of L-1-LMO and L-3-LMO is due to the low contents of LaF3in these two samples.While the XRD patterns of L-5-LMO displays apparent diffraction peaks of LaF3, indicating that LaF3crystals exist in the coated LiMn2O4samples. The cell parameter a of pristine LiMn2O4,L-1-LMO,L-3-LMO and L-5-LMO is0.8247,0.8244,0.8241and0.8243nm,respectively,in a good agreement with our previous report[30],suggesting that the addition of PVP in the preparation process does not in?uence the spinel structure of LiMn2O4.

The SEM images of bare and LaF3coated LiMn2O4samples presented in Fig.2show that surface of pristine LiMn2O4is angular and smooth,while the coated LiMn2O4samples display coarser surface,indicating that surface of LiMn2O4has been successfully coated by LaF3.To further investigate the microstructure of sam-ples,TEM tests were conducted and the TEM images are depicted in Fig.3.Fig.3a shows the smooth surface of pristine LiMn2O4,in accordance with SEM image in Fig.2a.Fig.3b,c and d indicate that pristine LiMn2O4particles are covered by nanoparticles of LaF3and the ratio of coated surface to the total surface of LiMn2O4seems to increase with the fraction of LaF3in reference to LiMn2O4.More-over,the LaF3nanoparticles are distributed uniformly on the sur-face of LiMn2O4particles,which may be ascribed to the help of PVP during the precipitation of LaF3,similar to the effect of PVP on the uniform coating of nickel oxide on the MnO2[35].The insoluble LaF3coating facilitates LiMn2O4particles to reduce the exposed area to the electrolyte,in favor of decrease in side reactions and the dissolution of active materials,which may greatly improve the cyclability of LiMn2O4.

Fig.4depicts the charge and discharge pro?les of pristine and LaF3coated LiMn2O4electrodes in1mol/L LiNO3solution at a current rate of2C(296mA gà1)in the voltage range of0e1.1V(vs. SCE).It is apparent that all charge/discharge curves are of the

same Fig.1.XRD patterns of pristine and LaF3coated LiMn2O4samples.

Q.Zhu et al./Journal of Alloys and Compounds654(2016)384e391385

shape,suggesting that LaF 3coating does not change the mechanism of Li tintercalation/extraction in spinel LiMn 2O 4during cycling in LiNO 3solution.All the charge voltage plateaus are located at about 0.67and 0.83V (vs .SCE),corresponding to 3.98and 4.14V (vs .Li t/Li),respectively,for the charge voltage plateaus of LiMn 2O 4in organic electrolyte,close to those of LaF 3coated LiMn 2O 4at 0.2C in organic electrolyte [31].While the discharge voltage plateaus are 0.77and 0.65V (vs .SCE),in accordance with the discharge voltage plateaus of 4.08and 3.96V (vs .Li t/Li)for LiMn 2O 4in organic electrolyte,approaching to 4.1and 3.9V (vs .Li t/Li)for discharge voltage plateaus of LiMn 2O 4at 0.2C in organic electrolyte [31],respectively.The results further reveal that LiMn 2O 4possesses the

same mechanism of Li textraction/intercalation in both aqueous and non-aqueous electrolytes.

The pristine LiMn 2O 4shows the initial discharge capacity of 109.2mAh g à1and remains capacity of 97.6mAh g à1after 50cy-cles,corresponding to a capacity retention of 89.4%.While the LaF 3coated LiMn 2O 4samples,L-1-LMO,L-3-LMO and L-5-LMO,exhibit higher initial capacities of 112.9,118.4and 111.5mAh g à1,respec-tively,and the corresponding capacity retention rates are 94.1%,99.7%and 93.4%after 50cycles,respectively,much higher than that of the pristine LiMn 2O 4.It is clearly observed that both the capacity and cycle performance improve with the weight ratio of LaF 3to LiMn 2O 4increasing from 1wt%to 3wt%,then deteriorate as

the

Fig.2.SEM images of (a)pristine LiMn 2O 4,(b)L-1-LMO,(c)L-3-LMO and (d)

L-5-LMO.

Fig.3.TEM images of (a)pristine LiMn 2O 4,(b)L-1-LMO,(c)L-3-LMO and (d)L-5-LMO.

Q.Zhu et al./Journal of Alloys and Compounds 654(2016)384e 391

386

ratio reaches 5wt%,demonstrating that the optimal content of LaF 3is 3wt%in reference to LiMn 2O 4in our work.The improvement of cycle stability after coating LaF 3is ascribed to the fact that insoluble LaF 3coating prevents LiMn 2O 4from being exposed to LiNO 3elec-trolyte,largely reducing the dissolution and side reactions of LiMn 2O 4.Insuf ?cient coating resulting from low content of LaF 3can not cover the surface of LiMn 2O 4completely,which is not favorable for further suppression of the dissolution and side reactions of LiMn 2O 4.While the excess of LaF 3forms thick coating and sequentially plays adverse effects on the rapid penetration of Li t,leading to the higher polarization and impaired electrochemical performance.Therefore,compared with low content of LaF 3(1wt%)and excess of LaF 3(5wt%),3wt%LaF 3coating is the best amount.The phenomenon that the capacity of LiMn 2O 4increases after coated by LaF 3is similar to the result of AlF 3coated Li 1.17Ni 0.25Mn 0.58O 2[36].However,the reasons for that enhance-ment of capacity of LaF 3coated LiMn 2O 4are unclear.We speculate that thin coating of LaF 3,which has large channels for the trans-portation of ?uoride (r ?0.133nm ),may favor diffusion of Li tto/off the surface of LiMn 2O 4and facilitate dissociation of hydrated Li tat the interface between electrode and aqueous electrolyte and be in favor of intercalation/extraction of Li tinto/from LiMn 2O 4,resulting in higher capacity of LaF 3coated LiMn 2O 4.In contrast with elec-trodes in organic electrolyte,LaF 3coated LiMn 2O 4electrodes in LiNO 3solution exhibit higher capacity than that of pristine LiMn 2O 4,which may be caused by the quite different interfacial properties of LiMn 2O 4/LaF 3/organic electrolyte and LiMn 2O 4/LaF 3/LiNO 3solution,and the different radii of solvated Li t(0.37nm )[37]and hydrated Li t(0.237nm )[38].

The cycle performance of pristine and LaF 3coated LiMn 2O 4samples in the voltage range of 0e 1.1V (vs .SCE)at 5C and 10C are shown in Fig.5.We can see that the LaF 3coated LiMn 2O 4electrodes present much better cycle performance than bare LiMn 2O 4elec-trode and L-3-LMO shows the highest capacity and the best cycle performance among all samples.Fig.5a demonstrates that the initial discharge capacities at 5C of pristine LiMn 2O 4,L-1-LMO,L-3-LMO and L-5-LMO are 108.6,112.4,117.2and 107.8mAh g à1,respectively,and the corresponding capacity retention rates are 82.8%,93.1%,98.2%and 87.4%,respectively.It is clear that the cycle performance was enhanced after LaF 3coating and the optimized amount of LaF 3coating was 3wt%.Fig.5b shows that the capacities of samples at 10C are lower than those at 5C due to the larger polarization at a higher current density,while L-3-LMO still ex-hibits the highest capacity of 109.5mAh g à1and remains capacity of 107.9mAh g à1after 100cycles among all the samples,much better than the highest capacity of 101.6mAh g à1and capacity of 86.3mAh g à1of 100th cycle for pristine LiMn 2O 4.As a result,the sample with 3wt%LaF 3coating (L-3-LMO)exhibits the most promising electrochemical performance with a good combination of high speci ?c capacity,high rate capability and excellent cyclability.

To further investigate the effects of LaF 3coating on the elec-trochemical properties of LiMn 2O 4in 1mol/L LiNO 3solution,cyclic voltammetry was carried out at a scan rate of 0.2mV s à1in the potential range of 0e 1.1V (vs .SCE),and the cyclic voltammograms of pristine LiMn 2O 4and L-3-LMO are shown in Fig.6.It can be seen that two electrodes display two well-de ?ned redox peaks and similar shape of curves,further implying that LaF 3coating does not alter the mechanism of Li tinsertion/extraction.The pristine LiMn 2O 4shows two pairs of redox peaks,located at 0.769/0.901and 0.688/0.810V (vs .SCE),corresponding to two steps of two-phase reactions during electrochemical cycling,in a good agreement with the results of Fig.4.In case of L-3-LMO,two pairs of redox peaks situated at 0.761/0.892and 0.689/0.813V (vs .SCE)are observed.It can be seen that the anodic peak potentials of L-3-LMO are lower than those of LiMn 2O 4,while the cathodic peak potentials are on the contrary,demonstrating that intercalation/extraction of Li tin L-3-LMO requires lower energy than that in pristine LiMn 2O

4

Fig.4.Initial and 50th charge/discharge pro ?les of (a)pristine LiMn 2O 4,(b)L-1-LMO,(c)L-3-LMO and (d)L-5-LMO in 1mol/L LiNO 3aqueous solution at 2C (296mA g à1)in the voltage range of 0e 1.1V (vs .SCE).

Q.Zhu et al./Journal of Alloys and Compounds 654(2016)384e 391387

and LaF 3coating is helpful to improve the electrochemical perfor-mance of LiMn 2O 4.

It is well known that Li tdiffusion is a crucial step to in ?uence the electrochemical performance of electroactive materials.To

understand the effects of LaF 3coating on the diffusion of Li t,the diffusion coef ?cient of Li t,D Li ,was determined by a cyclic vol-tammetry method and the value was calculated based on the following Eq.1[39,40].

i p ?2:69?105n 3=2AD 1=2

Li C 0Li n

1=2

(1)

where i p ,n ,A ,D Li ,C 0Li

and n are peak current (ampere),charge-transfer number,contact area between electrode and electrolyte (cm 2),diffusion coef ?cient of Li t,concentration of Li tin the elec-troactive materials (mol cm à3),and potential scan rate (V s à1),respectively.

Fig.7presents the cyclic voltammograms (CVs)of bare LiMn 2O 4and L-3-LMO electrodes in 1mol/L LiNO 3electrolyte at different scan rates of 0.05,0.1,0.2and 0.5mV s à1in the potential region of 0e 1.1V (vs .SCE).It can be seen that oxidative peak potentials shift to higher values and reductive peak potentials move to lower ones with scan rates increasing,which results from the larger polariza-tion at higher scan rates.Although the scan rates increase to 0.5mV s à1,both uncoated LiMn 2O 4and L-3-LMO in 1mol/L elec-trolyte exhibit more well-de ?ned CV peaks than LiMn 2O 4in organic electrolytes,suggesting that LiMn 2O 4samples have smaller polar-ization in aqueous electrolytes than in organic electrolytes,and LiMn 2O 4samples display better rate capability in aqueous elec-trolytes than in organic electrolytes.The relationship of peak cur-rent i p with square root of scan rate v 1/2derived from Fig.7a and b

is

Fig.5.Cycle performance of pristine and LaF 3coated LiMn 2O 4electrodes in 1mol/L LiNO 3aqueous solution at (a)5C and (b)10C in the voltage range of 0e 1.1V (vs .

SCE).

Fig.6.Cyclic voltammograms of pristine LiMn 2O 4and L-3-LMO at a scan rate of 0.2mV s à1in the potential range of 0e 1.1V (vs .

SCE).

Fig.7.Cyclic voltammograms of (a)pristine LiMn 2O 4and (b)L-3-LMO in 1mol/L LiNO 3solution at different scan rates in the potential range of 0e 1.1V (vs .SCE).

Q.Zhu et al./Journal of Alloys and Compounds 654(2016)384e 391

388

depicted in Fig.8a and Fig.8b,respectively.The linear relationship of i p and v 1/2suggests that the insertion and extraction of Li tin LiMn 2O 4as electrodes for ARLBs is kinetically limited by Li tdiffu-sion [40],and it is right to calculate the diffusion coef ?cient of Li taccording to Eq.1.The diffusion coef ?cients of Li tcalculated by ?rst oxidation peak O1,second oxidation peak O2,?rst reductive peak R1and second reductive peak R2shown in Fig.7are labeled as D O1,D O2,D R1and D R2,respectively,and the results are listed in Table 1.The results show that the diffusion coef ?cients of Li tresulted from different current peaks of L-3-LMO at least are 20%higher than corresponding ones of bare LiMn 2O 4,which explains why L-3-LMO has better electrochemical performance than pristine LiMn 2O 4,and demonstrates that LaF 3coating favors improvement of electro-chemical performance of LiMn 2O 4.The effects that LaF 3coating speeds up the Li tdiffusion of LiMn 2O 4may resemble to in ?uence of Al 2O 3coating on the improved Li tdiffusion of LiNi 1/3Co 1/3Mn 1/3O 2[41].LaF 3coating may change surface microstructure of LiMn 2O 4,and the interface of LaF 3/LiMn 2O 4may alter the internal electric ?eld of LiMn 2O 4and decrease the activation energy of charge transfer reaction,leading to higher diffusion coef ?cients of Li tin LiMn 2O 4.

Fig.9a and b depict the impedance behavior of pristine LiMn 2O 4and L-3-LMO at 1st and 100th fully charged state when cycled at 10C,and the two samples exhibit typical Nyquist characteristics.All Nyquist plots consist of an intercept in the high

frequency

Fig.8.Relationship between peak current (i p )and square root of scan rate (n 1/2

)for (a)

pristine LiMn 2O 4and (b)L-3-LMO.

Table 1

Diffusion coef ?cients of Li tof LiMn 2O 4and L-3-LMO calculated by different current peaks.

Active material D O1/cm 2s à1D O2/cm 2s à1D R1/cm 2s à1D R2/cm 2s à1LiMn 2O 4 2.94?10à8 3.36?10à8 2.85?10à8 2.23?10à8L-3-LMO

3.67?10à8

4.06?10à8

3.91?10à8

2.82?10à

8

Fig.9.Nyquist plots of 1st and 100th fully charged (a)pristine LiMn 2O 4and (b)L-3-LMO (inset is the Nyquist plots of 1st fully charged electrodes),and (c)equivalent circuit used for ?tting Nyquist plots.

Q.Zhu et al./Journal of Alloys and Compounds 654(2016)384e 391389

region(105e8.25?104Hz),a small incomplete semicircle in high frequency region(8.25?104e4.64?104Hz)and a large semicircle in middle-low frequency region(4.64?104e0.261Hz),and an inclined line in the low frequency region(0.261e0.01Hz).The intercept impedance on the Z0real axis stands for ohmic resistance (R s),corresponding to the total resistance of electrolyte,current collector and electric leads.The small incomplete semicircle rep-resents the formation of layer on the surface of electrodes.The large semicircle represents the charge transfer process on the electrode interface,demonstrating the lithium transfer rate parameters as well as capacitance of the electrolyte/electrode surface double-layer.The inclined line illustrates the Warburg impedance(Z w), which is related with Litdiffusion in active particles.The constant phase element(CPE)and C are on the behalf of double layer capacitance.The equivalent circuit shown in the Fig.9c,similar to the equivalent circuit of previous report[26],is used to?t Nyquist plots and the?tting results are listed in Table2.The small difference in R s demonstrates that existence of LaF3coating does not change the contact resistance signi?cantly.R SEI,resistance of surface?lm on electroactive material,for pristine LiMn2O4is higher than that one for L-3-LMO,suggesting that LaF3coating can suppress the increase of?lm resistance.R ct of bare LiMn2O4increases from6.6to 142.7U with increasing cycles,while the R ct of L-3-LMO is improved

from0.81to10.11U,which means that charge-transfer resistance of L-3-LMO and the increase of R ct in electrochemical process are smaller than those of uncoated LiMn2O4,demonstrating that the LaF3coated LiMn2O4is of higher rates of electrochemical reaction than uncoated LiMn2O4.

In order to investigate the in?uence of LaF3coating on reducing the dissolution of LiMn2O4in1mol/L LiNO3solution during cycling, the concentration of Mn ion in LiNO3was determined by induc-tively coupled plasma-atomic emission spectroscopy(ICP-OES)af-ter pristine LiMn2O4and L-3-LMO electrodes were cycled at2C for 50cycles.The cycled electrolyte was concentrated to15mL and then was tested.The Mn ion concentration of cycled electrolyte for uncoated LiMn2O4electrode with10.3mg of mixture of LiMn2O4, PVDF and acetylene black is0.79mg/L,corresponding to the dissolution of about0.19wt%of LiMn2O4.While the Mn ion con-centration of cycled electrolyte for L-3-LMO electrode with10.9mg of mixture of L-3-LMO,PVDF and acetylene black is0.22mg/L,in accordance with the dissolution of about0.06wt%of LiMn2O4.The difference of Mn ion concentration in LiNO3solution con?rms that LaF3coating can effectively decrease the contact area between LiMn2O4and aqueous electrolyte and results in the decrease in dissolution of LiMn2O4,hence to probably improve the cycle per-formance of LiMn2O4in LiNO3electrolyte.

To investigate the change in crystal structure of electroactive materials after cycling,XRD tests were conducted on the fresh and cycled electrodes.Both the LiMn2O4and L-3-LMO cycled electrodes were cycled at2C for50cycles.XRD patterns of fresh and cycled LiMn2O4and L-3-LMO electrodes shown in Fig.10suggest that the diffraction peak density of LiMn2O4in cycled electrodes is some-what lower than that of LiMn2O4in fresh electrodes,but LiMn2O4in both uncoated and coated electrodes remains spinel structure,and both electrodes do not exhibit apparent differences in LiMn2O4 spinel structure.It seems that effect of LaF3coating on the structure change in LiMn2O4is not obvious,which may result from the fact that a so small amount of dissolution of LiMn2O4is not re?ected in XRD patterns.

4.Conclusions

LaF3component was homogeneously coated onto the surface of LiMn2O4particles by a modi?ed co-precipitation method.As cathodes for aqueous rechargeable lithium batteries,all coated samples in1mol/L LiNO3solution exhibit better electrochemical performance than the pristine LiMn2O4.The optimal weight ratio of LaF3to LiMn2O4is3%.The analysis of diffusion coef?cients of Litindicates that LaF3coating is helpful to improve the rate of Litdiffusion.The investigation of manganese ion concentration in the cycled electrolyte and SEM images of the cycled electrodes also suggest that LaF3coating plays important roles in enhancing the electrochemical performance of LiMn2O4in aqueous electrolytes. The results imply that LaF3coating can be further applied to other cathodes and anodes considered in aqueous rechargeable lithium batteries.

Acknowledgments

The authors gratefully acknowledge?nancial support from the Natural Science Foundation of Guangxi Province (No.2013GXNSFAA019304),the National Natural Science Founda-tion of China(No.51364007),the Shenzhen Peacock Plan (KQCX20140522150815065),the Science and Technology Innova-tion Foundation for the Undergraduates of SUSTC(2014S07). References

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Table2

Fitting results of Nyquist plots for different charged state of LiMn2O4and L-3-LMO.

R s(U)R SEI(U)R ct(U)

1st fully charged LiMn2O4 2.40 2.26 6.63 1st fully charged L-3-LMO 2.440.430.81 100th fully charged LiMn2O4 2.65 2.73142.7 100th fully charged L-3-LMO 2.500.45

10.11Fig.10.XRD patterns of(a)fresh LiMn2O4electrode,(b)LiMn2O4electrode cycled at 2C for50cycles,(c)fresh L-3-LMO electrode,and(d)L-3-LMO electrode cycled at2C for50cycles.

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