Electrochemical property of LiFePO4/C composite cathode with different carbon sources
Shenyang Ligong University
作者简介:*Xi Kun Li,e-mail:kunxi@163.com;
收稿日期:16 July 2015
基金:financially supported by the National Natural Science Foundation of China (No.51274143);
Electrochemical property of LiFePO4/C composite cathode with different carbon sources
Xin Li Yu Zhi Jiang Xi Kun Li Hai Xia Jiang Jun Long Liu Jun Feng Shi Bin Lin Xin Guan
Shenyang Ligong University
Abstract:
To improve the performance of battery cathode materials that consist of carbonaceous organic material,carbon coatings on lithium iron phosphate (LiFePO4/C)materials were synthesized by different carbon sources.LiFePO4/C was synthesized by a combination method of sol-gel and gas-phase diffused permeation. LiFeO4/C materials were prepared by coating different carbon contents. High-performance composite materials were prepared by combining carbon with element doped by two modified methods. The elements of Fe and C came from Fe3+and sucrose, glucose, citric acid. Thermogravimetrydifferential thermal analysis (TG-DTA), X-ray diffractometer (XRD), scanning electron microscope (SEM),cycle voltammetry (CV), and charge-discharge test were used to characterize and test the surface morphology,structure, and electrochemical performance. The results show that LiFePO4/C synthesized with sucrose has higher specific discharge capacity than the other materials. The specific discharge capacity of this material is 84.27 mAh·g-1. The capacity retention could attain 94%of the initial discharge capacity after 30 cycles, showing good electrochemical performance.
Keyword:
Solgel; Gas-phase diffused permeation; LiFePO4/C; Sucrose; Citric acid;
Received: 16 July 2015
1 Introduction
Since LiFePO4 was reported
The solid-phase method is limited for the uneven phase,irregular crystals,large sizes,long period,and so on.Coating the surface of LiFePO4 can improve the conductivity among the particles and reduce the particle size to achieve the purpose of improving the electrochemical performance of LiFePO4.S tudies have shown that coating the carbon surface can improve the high current discharge performance of the samples
Gas-phase diffusion permeation method originates from multi-component rare earth infiltration method which is the technology based on the basic standpoint of physical chemistry,using special structural characteristics and chemical properties of rare earth elements to modify metal surface.The earliest multi-element rare earth penetration method focused on rare earth permeating treatment
2 Experimental
2.1 Reagents and instruments
Ferric nitrate (Fe(NO3)3·9H2O)(analytically pure),lithium nitrate (LiNO3)(analytically pure),mono-ammonium phosphate[MAP](NH4H2PO4)(analytically pure),sucrose(analytically pure),glucose (analytically pure),citric acid(analytically pure),and methanol (analytically pure) were purchased from Sinopharm Chemical Reagent Ltd.,and lithium tablets (Beijing Research Institute of Nonferrous Metals),acetylene black (Shanghai Haotian Chemical Co.,Ltd.),and polyvinylidene fluoride PVDF (industrial grade,Shanghai Hersbit Chemical Co.Ltd.) were also purchased.
S ample gas-phase diffusion experiment was carried out in gas-phase diffusion furnace (XMT-101).High-temperature thermal gravimetric (TG,ZRY-2P,Shanghai Precision and Scientific Instrument Co.Ltd.) analyzer and high temperature difference thermal analyzer (DTA,CRY-2P) were used to confirm the vapor diffusion temperature of the material;X-ray diffractometer (XRD,D/max-2000,Japan Science Company) was used to do the material phase analysis;scanning electron microscope (SEM,S-4800,Japan Hitachi company) was used to analyze surface topography of the material;battery assembly was carried out in the STX-type vacuum glove box;charge and discharge tests were completed in highly accurate test system of battery performance(BTS-5 V/2 mA,Shenzhen Xinweier Electronic Co.Ltd.)and electrochemical workstation (CHI 604C,Shanghai Chenhua Company);data acquisition and process were completed by Xinwei data processing software.
2.2 Experimental process
2.2.1 Sample preparation
LiFePO4/C composite cathode materials were prepared by sol-gel and gas-phase diffusion permeation method:the stoichiometric ratio of LiNO3.H2O,Fe (NO3)3·9H2O,and NH4H2PO4 was 1:1:1.They were dissolved in the solution that mixed water with ethanol,either without carbon source or adding soluble carbon source (glucose,citric acid,and sucrose).Then,they were mixed evenly with a magnetic stirrer,continuing at 80℃for about 80 min until the sol appeared,then dried under 80℃,and the precursor of LiFePO4/C was obtained.The diffusion experiments of LiFePO4/C precursor were carried out in a small crucible gas-phase permeability furnace that we made ourselves,and the temperature was controlled by precise temperature controller (XMT-101).
Diffusion experiment
2.2.2 Battery assembly
Battery assembly process was conducted in a glove box filled with argon gas.Active substance acetylene black and polyvinylidene fluoride (PVDF) were weighed with the ratio of 75:15:10 (mass ratio).The mixed substances were dissolved in N-methyl pyrrolidone (NMP) and were evenly applied to the aluminum foil.After dried in a vacuum drying box under 120℃,it was punched into card,tablet,and detect battery.The electrode was lithium metal wafer with a diameter of 12 mm,a thickness of 100μm.The diaphragm was polypropylene with a diameter of 16 mm,a thickness of 16μm;the electrolyte consisted of ethylene carbonate (EC) and dimethyl carbonate (DMC),the volume ratio of which was EC:DMC=1:1 and the concentration of LiPF6 was 1 mol·L-1.
2.2.3 Cycle voltammetry (CV) characterization
The assembled simulation battery prepared by LiFePC4/C material mixed 70%sucrose was scanned at voltage range of 2.5-4.3 V to test CV curve of the sample with scanning rate of 0.1 mV·s-1.In the experiment,electrochemical workstation (CHI604C,Shanghai Chen Hua Company)was used.Test condition was two-electrode system.Auxiliary electrode and reference electrode were both lithium.The electrode was active sample electrode.
3 Results and discussion
3.1 Determination of gas-phase diffusion permeation temperature
LiFePO4/C precursor without carbon source was tested by thermogravimetry-differential thermal analysis (TG-DTA)from room temperature to 800℃,and the heating rate was5℃·min-1.The results were shown in Fig.1.It can be seen from the TG curve that the sample was at great loss from 80 to 600℃.The loss mainly occurred in three stages:the first stage was from room temperature to178℃,mainly removing the adsorbed water in the gel;the second stage was from 178 to 346℃,which was mainly because
The TG-DTA test of LiFePO4/C precursor with glucose as carbon source was carried out from room temperature to800℃,the heating rate was 5℃·min-1 and the test results were shown in Fig.2.According to the TG curve,the sample had obvious weight loss from 80 to 600℃.Weight loss mainly occurred in three stages:the first stage was from room temperature to 178℃,mainly removing the adsorbed water in the gel;the second stage was from178 to 246℃,which was mainly caused by glucose decomposition;the third stage was from 246 to 600℃,and it was the main process of LiFePO4/C crystal type changes.No weight loss after 600℃means that the sample no longer had decomposition reaction after that temperature.According to the results of TG-DTA test,the diffusion temperature of LiFePO4/C precursor with glucose as carbon source is 700℃.
Fig.1 TG-DTA curves of LiFePO4 precursor
The TG-DTA test of LiFePO4/C precursor with 50%sucrose as carbon source was carried out from room temperature to 800℃,the heating rate was 5℃·min-1.The test results were shown in Fig.3.According to the TG curve,the sample had obvious weight loss from 80 to600℃.Weight loss mainly occurred in three stages:the first stage was from room temperature to 125℃,mainly resulting from removal of the adsorbed water in the gel;the second stage was from 125 to 330℃,mainly caused by sucrose decomposition;the third stage was from 330 to610℃,which was the main process of LiFePO4/C crystal type changes.No weight loss after 610℃shows that the sample no longer had decomposition reaction after that temperature.According to the test results of TG-DTA,the diffusion temperature of LiFePO4/C precursor with sucrose as carbon source is 700℃.
The TG-DTA test of LiFePO4/C precursor with 50%citric acid as carbon source was carried out from room temperature to 800℃,and the heating rate was5℃·min-1.The test results were shown in Fig.4.According to the TG curve,the sample had obvious weight loss from 80 to 600℃.Weight loss mainly occurred in three stages:the first stage was from room temperature to160℃,mainly removing the adsorbed water in the gel;the second stage was from 160 to 350℃,mainly caused by citric acid decomposition;the third stage was from 350 to590℃,which was the main process of LiFePO4/C crystal type changes.No weight loss after 600℃shows that the sample no longer had decomposition reaction after that temperature.According to the results of TG-DTA test,the diffusion temperature of LiFePOz4/C precursor with citric acid as carbon source is 700℃.
Fig.2 TG-DTA curves of LiFePO4/C precursor with glucose
Fig.3 TG-DTA curves of LiFePO4/C precursor with sucrose
Fig.4 TG-DTA curves of LiFePO4/C precursor with citric acid
Through the TG-DTA diagrams,it can be seen that different carbon source types determine different degrees of penetration.
3.2 XRD analysis
XRD patterns of the LiFePO4/C sample prepared without carbon source and the LiFePO4/C sample used glucose,citric acid,and sucrose as carbon source are shown in Fig.5.Compared with the standard spectrum (JCPD40-1499),all diffraction peaks were indexed to olivine structure.The results show that LiFePO4/C prepared with different carbon sources have pure phase.No diffraction peaks appeared means that carbon exists in the form of amorphous carbon.Three peaks of all the samples were observed between 20°and 40°,among which LiFePO4/C samples used citric acid as carbon source are the strongest.Angles of strong diffraction peaks of samples prepared with these four carbon sources are almost consistent.
Fig.5 XRD patterns of LiFePO4/C synthesized by different carbon
3.3 SEM morphology analysis
The morphologies of LiFePO4/C samples with different carbon sources are shown in Fig.6.From the chart,the grain size of the prepared sample is about 1μm with carbon coated.Samples without carbon source among the precursors are hardened to a certain extent.However,samples prepared by adding different carbon sources(glucose,sucrose and citric acid) to precursors have even particles.LiFePO4/C sample with sucrose as the carbon source is the best among them as Fig.6c shows
3.4 Charge and discharge test
In order to study the effects of different carbon sources on the electrochemical properties of LiFePO4/C samples,the LiFePO4/C samples of button-type battery which were prepared with different carbon sources which were tested by charging and discharging.The first charge and discharge curves and cycle performance curves of samples without carbon source and with different carbon sources under0.5℃are shown in Figs.7 and 8.According to the Figures,the initial specific discharge capacity of LiFePO4/C samples used glucose as carbon source is 64.80 mAh·g-1and the initial discharge efficiency is 69.8%.As for LiFePO4/C samples used citric acid as carbon source,the initial specific discharge capacity is 15.0 mAh·g-1.The first specific discharge capacity of LiFePO4/C samples prepared with sucrose as carbon source is 84.27 mAh·g-1,and the initial discharge efficiency is 71.2%,higher than the initial efficiency (69.8%) of those used glucose as carbon source.The cycle performance is good,as well.After 30 cycles,the capacity retention rate is 94%,so LiFePO4/C material is chosen as adding sucrose during the preparation of precursor.
Fig.6 SEM images of LiFePO4/C synthesized by different carbon sources:a without carbon source,b glucose,c sucrose,and d citric acid
Fig.7 First discharge curves of LiFePO4/C synthesized by different carbon sources
Fig.8 Cycle performance curves of LiFePO4/C synthesized by different carbon sources
3.5 CV analysis
The symmetry of CV curve and redox peak area are important references of material cycle performance and the reversible capacity size.The better the symmetry of material circulation is,the greater the performance and the bigger the reversible capacity.The better the uniform of redox peak size is,the better cycle performance of the material and the greater reversible capacity of the assembled simulation battery.The curves of samples added with75%sucrose are consistent with the peak of charge and discharge.The charge peak is at 3.6 V,discharge peak is at3.25 V.The peak sharpness is better and the discharge peak area is consistent with charge peak area,showing the good reversible capacity and cycle performance of the material,as shown in Fig.9.
Fig.9 CV curve of LiFePO4/C synthesized by 70 wt%sucrose
4 Conclusion
Pure-phase LiFePO4/C materials were prepared by sol-gel and gas-phase diffusion permeation method.LiFePO4/C precursor material prepared with glucose,sucrose,and citric acid as carbon source was diffused in diffusion furnace under 700℃,the diffusion time was 7 h.XRD,SEM,and charge-discharge test were used to analyze LiFePO4/C materials prepared with different carbon sources.The results show that LiFePO4/C material prepared with sucrose as carbon source has even particles with~1μm in diameter.The initial specific discharge capacity of the material is the highest at 84.27 mAh·g-1 and has good cycle performance.After 30 cycles,the capacity retention rate is 94%,so the choice of carbon source for preparing LiFePO4/C materials is sucrose.Owing to small electronic conductivity of LiFePO4 and low speed of diffusion of lithium ion,the volume energy density is low,which severely affects the utility of the material.Therefore,using different methods to increase the conductivity of the LiFePO4 material is the main research direction.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51274143).
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