PEG高温固相法合成-英文

Journal of Alloys and Compounds456(2008)
461–465
A soft chemistry synthesis routine for LiFePO4–C
using a novel carbon source
L.N.Wang a,X.C.Zhan a,Z.G.Zhang a,K.L.Zhang a,b,∗
a College of Chemistry and Molecular Sciences,Wuhan University,Wuhan430072,China
b Centre of Nanoscience and Nanotechnology Research,Wuhan University,Wuhan430072,China
Received17August2006;received in revised form13February2007;accepted20February2007
Available online23February2007
Abstract
As a novel carbon source,polyethylene glycol(PEG;mean molecular weight of10,000)was adopted to prepare LiFePO4–C cathode material by a very simple soft chemistry method,which was called rheological phase reaction by us.X-ray diffraction(XRD)reveal that the crystallized LiFePO4powder can be synthesized easily by rheological phase reaction at a relative low temperature(400◦C).Scanning electron microscopy (SEM)results indicate that after the decomposition of PEG,active material and porous structure carbon were left in the resultant products.An initial discharge capacity of162and139mAh g−1was achieved for500◦C sample at room temperature with C rates of0.06C(10mA g−1)and1C (170mA g−1),respectively.The satisfactory initial discharge capacity and superior rate capacity should attribute to the porous conductive carbon structure and the soft synthesis process.
©2007Elsevier B.V.All rights reserved.
Keywords:Cathode material;LiFePO4;PEG;Rheological phase reaction
1.Introduction
Due to its overwhelming advantages of low toxicity,good thermal stability and relatively high theoretical capacity,olivine type LiFePO4,which wasfirst introduced as a lithium batteries cathode material by Padhi et al.appears as a potential candidate to be used as positive electrode in next generation of Li-ion batteries[1–5].
As a material,however,LiFePO4is an insulator,which seri-ously limits its rate capability in lithium cells.It shows high electrochemical properties only at low charge–discharge rates owing to its low electronic conductivity and low lithium ion motion ability[6,7].Therefore,research on this insulating com-pound was up to now mostly devoted to enhance the composite’s conductivity by metal doping[8,9]or coating with the electron-ically conductive like carbon,metal and metal oxide[10–12].
Other possible means of improving the rate performance of LiFePO4materials are those of enhancing its ionic/electronic conductivity by optimization of particles with suitable prepara-tion procedures.In addition to the traditional solid-state reaction
∗Corresponding author.Tel.:+862787218484;fax:+862768754067.
E-mail address:klzhang@(K.L.Zhang).synthesis routine,alternative synthesis processes including sol–gel,hydrothermal,co-precipitation,microwave heating,etc. have developed continually[11,13–15].
In this work,an example of preparing LiFePO4material with nano-sizedfine particles by employing a novel carbon source of polyethylene glycol(PEG)with a very simple soft chem-istry method—the rheological phase reaction method[16–19]is provided.The solid reactants are fully mixed in a proper molar ratio,made up by adding the required amount of water or other solvent to a solid–liquid rheological body in which the solid particles and liquid substance are uniformly distributed.Then after reaction under suitable conditions,the product is obtained. Under the solid–liquid rheological state,many substances have new reaction properties.The resultant powders prepared by this method often show excellent electrochemical performance. The microstructure and the rate performance of the as-prepared LiFePO4–C were investigated.Also,an excellent organic carbon source—PEG was introduced.
2.Experimental
Li2CO3,FeC2O4·2H2O,NH4H2PO4and polyethylene glycol(mean molec-ular weight of10,000)powders were used as the starting materials by rheological phase reaction.They were mixed thoroughly by grinding.Then appropriate
0925-8388/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.jallcom.2007.02.103
462L.N.Wang et al./Journal of Alloys and Compounds 456(2008)461–465
amount of de-ionized water was added to get a rheological body,which was the precursor of LiFePO 4–C.Finally,the resulting precursor was heated in Ar atmosphere to get the powders of LiFePO 4–C.
Thermal decomposition and crystallization temperatures of the rheological body precursor were investigated by thermogravimetry and different thermal analysis (TG/DTA),which were performed between ambient and 1000◦C with heating rate of 20◦C min −1under the flowing Ar.X-ray diffraction (XRD)pro-files of the sample were carried out on a Shimadzu XRD-6000diffractometer using Cu K ␣1radiation .The particles size distribution was determined by the optical particle size analyzer (Mastersizer 2000,England).The morphology was observed using scanning electron microscopy (SEM)with Sirion FEG SEM.Elemental composition of the compound powder (Li,Fe and P)was determined by inductively coupled plasma (ICP)with atomic emission spectroscopy (ICP-AES,model IRIS,TJA,USA),and the amount of carbon was determined by element analyzer (FLASH 112SERIES,Italy).EDAX was also used to further examine the elemental composition.
For the cell measurement,the prepared sample was examined by a simulated cell system.The LiFePO 4–C material was mixed with 20%carbon black and 5%PTFE.The mixture was pressed onto nickel grid as the cathode,pure lithium as the anode,1mol/L LiClO 4(EC:DMC =1:1)as the electrolyte,a Celguard 2400(American)micro-porous membrane for the separator.The assemblies of the cell prototypes were carried out in an Ar-filled glove box (Mikrouna,Super 1220/750,China)with both water and oxygen concentrations less than 5ppm.The cathode performance was examined by a Neware instrument.The cells were charge–discharged between 2.0and 4.4V using different current density at room temperature.The charge and discharge rates were equal under a given current density.
3.Results and discussion
Themal gravimetry and different thermal analysis (TG/DTA)was used to determine the temperature for the preparation of precursor.Fig.1shows TG/DTA curves performed on LiFePO 4precursor,in which the content of PEG is 13wt.%.The curves show three mass main stages.While the first one attributed to the release of little physical adsorbed water and some crystal water is found from ambient to 183◦C.The steep weight loss,which occurs between 183–307and 307–404◦C can be ascribed to the decomposition of reactants,with carbonization of PEG and crystallization of LiFePO 4.In this stage,two endothermic peaks were also found at 283.2and 390.8◦C in the DTA curve,respec-tively.Finally,the slow and gradual form weight loss above 404◦C can be associated with the decomposition of little
rem-
Fig.1.TG/DTA curves performed on LiFePO 4precursor with addition of 13wt.%
PEG.
Fig.2.XRD patterns of LiFePO 4powders prepared at different temperatures.The asterisks sign reflection peaks of Li 3PO 4as minor impurity phase.
nant PEG and the oxidation of carbon,which produces carbon monoxide or carbon dioxide.Correspondingly,one endothermic peak at 431.5◦C and an exothermic peak around 717.9◦C were found in the DTA curve.By following the results of TG/DTA,the precursor was heat-treated at temperatures between 400and 700◦C for 12h in the Ar flow.
The X-ray diffraction patterns of the LiFePO 4powders heat-treated for 12h at different temperatures are shown in Fig.2.For the samples synthesized at 400and 500◦C,all peaks can be indexed as pure and well-crystallized LiFePO 4phase with an ordered olivine structure and a space group of Pnma .But a minor impurity phase,which is lithium ortho -phosphate Li 3PO 4appeared when the temperature increased to 600and 700◦C.However,no evidence of diffraction peaks for crystalline carbon (graphite)appeared in the diffraction pattern throughout the tem-perature range,which indicates that the carbon generated from PEG is amorphous carbon and its presence does not influence the structure of LiFePO 4.It is found the patterns of the LiFePO 4synthesized at different temperatures are mostly similar except that the peaks gradually sharpen with increasing temperature.There are only weak X-ray diffraction peaks observed at 400◦C,and the relative intensity of the X-ray diffraction patterns of the LiFePO 4powders becomes the strongest when the temperature is up to 700◦C,which indicates an increase of crystallinity as may occur from growth of grain size,ordering of local structure,and release of lattice strain [20].
The morphology for LiFePO 4–C powders,which was syn-thesized by sintering the precursor at different temperatures,was observed on SEM,as shown in Fig.3.We can see that the LiFePO 4is embedded in the porous structure of carbon,which reduced from the decomposition of PEG.The porous structure of carbon would prohibit the particle size to grow during heat-ing and the conductivity of LiFePO 4–C compounds would be enhanced due to the well-distributed LiFePO 4powder in the porous carbon.But the difference among the 400–600◦C prod-ucts is still obvious.Fig.3a and b show the morphology of the
L.N.Wang et al./Journal of Alloys and Compounds456(2008)461–465
463
Fig.3.SEM morphology of LiFePO4–C powders prepared at different temperatures:(a)400◦C,(b)500◦C,(c)600◦C and(d)700◦C.
400and500◦C samples,respectively,that a lot of independent nano-sized particles pack closely between the porous structure of carbon.When the preparing temperature was enhanced to 600◦C,the interface of particles disappeared partially and the relatively larger particles can be observed,as shown in Fig.3c. With the preparing temperature further increasing to700◦C,the size of the particles become even bigger.This phenomenon is in agreement with the result obtained from XRD patterns,in which the particle size increases with increasing the heat-treatment temperature.
The particles size distribution of LiFePO4–C powders pre-pared at different temperatures,as shown in Fig.4,was measured by the optical particle size analyzer(Mastersizer2000,Eng-land).The value at50%cumulative population(d50%)represents the average particle size.One can see that with the tempera-ture increasing from500to700◦C,the average particle size was increasing from0.324to3.814␮m,which was consistent with results of the XRD and SEM patterns.However,the d50% value of400◦C sample(0.325␮m)is a little larger than500◦C sample and the reason can be attributed to that too small parti-cles induce more agglomerated particles of400◦C sample.The particle distribution has two regions for all samples at differ-ent temperatures.The population peaks around smaller particle size are LiFePO4–C powders and the other population peaks at lager particle size region can be attributed to agglomerated particles that were not dispersed perfectly.In addition to crys-tallinity,the particle morphology,particle size and particle size distribution of cathode materials are of great importance to the performance of battery.The particle size distribution of500◦C sample is more uniform than other samples because the main particle distribution of500◦C powders is in nano-sized region, which will effect its electrochemical performance,as shown in Fig.5,it can be seen the sample synthesized at500◦C for12h yields the highest specific capacity,157mAh g−1,when the volt-age of the four samples range from2.0to4.4V at the same current density,10mA g−1,at room temperature.Besides
the Fig.4.Particle size distribution of LiFePO4–C powders prepared at different temperatures.
464L.N.Wang et al./Journal of Alloys and Compounds 456(2008)
461–465
Fig.5.The second charge–discharge curves of LiFePO 4–C composites obtained at different heating temperatures.
less uniform particle size distribution,the less full-crystallized structure for 400◦C sample and larger particle size for 600and 700◦C samples should be main reasons that prohibit these sam-ples to have better electrochemical performance,respectively,compared with the sample synthesized at 500◦C.The result further confirmed previous reports that the combination of a fine particle size and full crystallinity would be important for LiFePO 4to have a good electrochemical property [5,21].The optimized temperature for preparing the LiFePO4–C cathode material is 500◦C from above analysis,however,this sample shows a comparative ratio of capacity below 3V and the same above 4.3V ,the reason is not clear presently and further work is in progress by our group.
The elemental analysis of the 500◦C product was performed by the EDAX technique (see Fig.6).EDAX results were inter-preted in terms of the atomic percentage and prove the presence of Fe,P,O and C in the product.But the EDAX only presents the surface elemental analysis of the compound and does not pro-vide any information on lithium.Therefore,inductively coupled plasma-atomic emission spectroscopy analysis was conducted.The amount of Li,Fe,P is 4.22,32.4and 18.8wt.%according to ICP result,corresponding to 0.980:0.952:1.00,respectively,at a molar ratio.However,the result indicates that the sample is Fe and Li deficient,and deviate from their theoretical
value,
Fig.6.EDAX pattern of the LiFePO 4–C composite synthesized at 500◦
C.
Fig.7.The rate performance of the LiFePO 4–C compound prepared at 500◦C.
revealing that some lithium mass loss occur during the heating process.As far as the Fe deficient,the conversion of ferric ions to ferrous ions was possibly occurred during the synthesis process,although,there are no visible ferrous ions impurity diffraction peaks in XRD patterns of the sample.There is because XRD cannot detect the impurity at levels below 5%[4]or because the impurity grains are too small to be detected.The amount of carbon in the LiFePO 4–C composite is 3.22wt.%through the element analysis by element analyzer.
One of the serious drawbacks of olivine LiFePO 4is poor rate capacity.In order to determine the rate performance and cycle performance of the 500◦C product prepared by rheo-logical phase reaction,we performed charge–discharge cycling of tests on LiFePO 4–C electrodes at different rates.The best sample in our experiments exhibits relatively good rate per-formance,as shown in Fig.7.In this experiment,charge and discharge rates under a specific current density were equal and all cells were charged to 4.4V and discharged to 2.0V at given rates.The as-prepared LiFePO 4electrode demonstrated a high specific discharge capacity of 139mAh g −1at the current den-sity of 170mA g −1(1C).When the applied current density is 900mA g −1(5.3C),the delivered capacity is 91mAh g −1.The LiFePO 4electrode still exhibits 69mAh g −1when the applied current density increased to 1500mA g −1(8.8C).The result is better than expected,which may be due to the enhanced elec-tronic conductivity of the material resulting from carbon coating and the soft synthesis process.The individual LiFePO 4particles are connected by a network,amorphous nanometer sized carbon webs in LiFePO 4–C composite exist,wrapping and connecting the LiFePO 4particles.And there is a possibility that porous car-bon webs can also reside in the interior part of the particles,as Chung et al.reported [22].In addition,rheological phase reac-tion is a simple and low cost method,which would be applicable to other electrode materials.
Fig.8shows the cycle performance of the carbon-coated LiFePO 4cycles at various charge–discharge rates.At the charge–discharge rate of 170mA g −1(1C),the specific capac-ity of the composite cathode decreases from 139mAh g −1at the first cycle slightly to 135mAh g −1at the 25th cycle,showing an
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465
Fig.8.Cycle performance of the LiFePO4–C compound prepared at500◦C with various applying current.
excellent cycling stability.But when the current density is too high,for example,1500mA g−1,the specific capacity decreased still rapidly with the cycling of the cell.
4.Conclusions
With a view of ourfindings,the simple,one step rheolog-ical phase reaction has been developed to directly synthesize LiFePO4–C powders,using PEG as the carbon source.The decomposition of PEG during the synthesis process controls the morphology and size of the powder and the porous structure of carbon improves the conductivity of the material.Different heat temperature also influenced the particle size,crystallinity of the product and the electrochemical performance.The product pre-pared at500◦C for12h yields smaller,more uniform particles and better electrochemical performance. Acknowledgements
This work was supported by the National Natural Science Foundation of China(No.20071026).The authors are grateful to the Centre of Nanoscience and Nanotechnology Research of Wuhan University for their experimental assistance. References
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