简介概要

Decomposition mechanisms and non-isothermal kinetics of LiHC₂O₄·H₂O

来源期刊：Rare Metals2012年第6期

论文作者：CHOU Kuo-Chih

文章页码：615 - 620

摘要：The thermal decomposition process of LiHC2O4·H2O from 30 to 600 ℃ was investigated by the thermogravimetric and differential scanning calorimetry （TG-DSC）. The phases decomposited at different temperature were characterized by X-ray diffraction （XRD）, which indicated the decompositions at 150, 170, and 420℃, relating to LiHC2O4, Li2C2O4, Li2C2O4, and Li2CO3, respectively. Reaction mechanisms in the whole sintering process were determined, and the model fitting kinetic approaches were applied to data for non-isothermal thermal decomposition of LiHC2O4?H2O; finally, the kinetic parameters of each reaction were also calculated herein.

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Rare Metals 2012,31(06),615-620

Decomposition mechanisms and non-isothermal kinetics of LiHC₂O₄·H₂O

CHOU Kuo-Chih

School of Materials Science and Engineering, University of Science and Technology Beijing

作者简介：LIAN Fang,E-mail:lianfang@mater.ustb.edu.cn;

收稿日期：7 March 2012

基金：financially supported by the National“863”Program of China(No.2009AA03Z226);Project on the Integration of Industry,Education and Research of Guangdong Province(No.2011A090200012);the Fundamental Research Funds for the Central Universities(No.FRF-MP-12-005B);

Decomposition mechanisms and non-isothermal kinetics of LiHC₂O₄·H₂O

Abstract：

The thermal decomposition process of LiHC2O4·H2O from 30 to 600 ℃ was investigated by the thermogravimetric and differential scanning calorimetry (TG-DSC). The phases decomposited at different temperature were characterized by X-ray diffraction (XRD), which indicated the decompositions at 150, 170, and 420℃, relating to LiHC2O4, Li2C2O4, Li2C2O4, and Li2CO3, respectively. Reaction mechanisms in the whole sintering process were determined, and the model fitting kinetic approaches were applied to data for non-isothermal thermal decomposition of LiHC2O4?H2O; finally, the kinetic parameters of each reaction were also calculated herein.

Keyword：

apparent activation energy; non-isothermal; reaction mechanism; thermal decomposition;

Received： 7 March 2012

1 Introduction

During the past decades,extensive efforts were dedicated to lithium batteries because of the high energy and power density,high safety,long cycle life,and low cost[1-7].The cathode materials of lithium batteries were synthesized by various routes,such as ion exchange method[8-10],coprecipitation method[11-13],and solid-state method[14-17]Among the synthesis methods,low-heating solid-state synthesis was accepted as one of the most innovative methods due to the simplicity and low energy consuming.Recently some references reported that the cathodes were successfully prepared by the same or similar method[18-19].However the manufactured cathode with the same composition exhibited different values for discharge capacity and other electrochemical properties.In order to understand the mechanism behind the phenomenon and improve the electrochemical performance of cathodes,the structure has to be investigated,which depends critically on the synthesis process including the calcination process.Nevertheless,the decomposition process and corresponding intermediate product in sintering are still obscure due to multicomponent and the complex structure of precursor.In our previous research it is confirmed that the precursor consists of LiHC₂O₄·H₂O Li HC₂O₄,and chelate obtained by low-heat solid-state reaction.To understand the reaction mechanisms of calcination and the synthesis process,the decomposition process of Li HC₂O₄·H₂O and chelate was investigated herein.Moreover,decomposition products and non-isothermal kinetics of Li HC₂O₄·H₂O were also given.

2 Experimental

As previously reported[20],LiHC₂O₄·H₂O was prepared by dissolving H₂C₂O₄·2H₂O in hot water and adding an equivalent mole amount of Li₂CO₃.The thermal decomposition process of the LiHC₂O₄·H₂O in air was tested by TG-DSC(Germany,NETZSCH STA 449 C).Powder X-ray diffraction(XRD,Japan,Rigaku,D/max-RB,12kW)using Cu K?radiation was employed to identify the crystalline phase of the intermediate decomposition of Li HC₂O₄H₂O.

3 Results and discussion

3.1 TG-DSC and XRD analysis

Li HC₂O₄·H₂O was heated from 30 to 600?C at the heating rate of 5 K·min^-1,of which the TG-DSC curves are shown in Fig.1.There are obvious three mass loss stages in TG curve and values of mass loss in the different temperature range are given in Table 1.

According to the TG-DSC curves,reactions occur at 75150,and 416?C,respectively.To investigate the reaction mechanism and decomposition products in the whole sintering process,LiHC₂O₄·H₂O was sintered at three differen temperatures.The XRD patterns of decompositions are shown in Fig.2.The decomposition at 150?C is confirmed to be the dehydration of LiHC₂O₄·H₂O.The result is consistent with TG-DSC curves,and the sample heated to 170?C for 3 h is Li₂C₂O₄ pure phase.While there are several weak peaks in the XRD patterns(Fig.2)of the sample after heat treated at 420?C,which are indexed to the Li₂CO₃.A small amount of Li₂C₂O₄ begins to decompose,which can be confirmed from a weight loss at about 420?C in TG curve(Fig.2).The above analysis allows us to conclude the chemical reactions and corresponding theoretical weight loss value in the whole heating process listed in Table 1.Moreover,the non-isothermal kinetic analysis was applied to investigate the kinetic characteristics in sintering process.

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Table 1 Theoretical and experimental values of mass loss for each reaction

Fig.1 TG-DSC curves of thermal decomposition of LiHC2O4·H2O in air,obtained at the heating rate of 5 K·min-1

Fig.2 XRD patterns of decomposition products of LiHC2O4·H2O at different temperatures

3.2 Kinetic analysis

The normalized mass loss and conversionαwere typically calculated from the corresponding TG curves as

where m₀,m_T,and m_Teare initial sample mass,sample mass at the temperature T(the absolute temperature),and sample mass at the end of reaction,respectively.

The classical isothermal reaction rate equation is expressed as

where k is the reaction rate constant,and f(?)is the reaction mechanism function.

Generally,the relationship between reaction rate constan and reaction temperature should obey the Arrhenius law that is

where A is the frequency factor;(35)E is the apparent activation energy;R is the gas constant;and T is the reaction temperature(the absolute temperature).

When the temperature increase is linear,

whereβis the heating rate.

Combining Eqs.(2)~(3)yields Eq.(5)

d?d T(28)A?exp(-(35)Ea RT)?f(?)?(5)or

d?f(?)(28)A?exp(-(35)Ea RT)?d T?(6)

The integration form of Eq.(6)can be expressed as

where u(28)(35)Ea RT.For non-isothermal conditions,there are several relationships used to calculate Arrhenius parameters,each of which is based on an approximate form of the temperature integral that results from rearrangement and integration of Eq.(7).The Coats-Redfern[21]approximation of the p(u)function was applied in the study:

ln?_?g(?)T²?_?(28)

ln[AR?(1-2RT(35)Ea)(??(35)Ea)]-(35)Ea(RT)(31)(9)ln[AR(??(35)Ea)]-(35)Ea(RT)

According to Eq.(9),the plot of ²ln?g(?)T?_??versus1 T should be a straight line,and the apparent action energyof reaction(35)Ea is obtained.

3.3 Determination of the reaction model by Malek ap-proach

There is a single reaction within the desired temperature range,which concludes in Table 1.As reported in the Refs.[22-23],the model-fitting approach can be realized when the reaction proceeds via a single mechanism.The Malek method is reliable to determine the reaction kinetic model and is applied here.As follows,one obtains the master plots of theoretical calculation functions y(?)and experiment y?(?)as a function of?.

where g(?)and f(?)correspond to theoretical master plots of various functions.The knowledge of temperature as a function of?should be known in advance to draw the experiment master plots of y'(?)versus?from experimental date at a given heating rate.According to the Malek method,for a given?,the experimental values y'(?)and theoretically calculated values y(?)can be equivalent or a little derivation if the reaction mechanism can be given by the appropriate kinetic model.Table 2 shows the most probable reaction mechanism functions.

The non-isothermal kinetic data of dehydration in the first weight loss stage were simulated by 14 kinetic models listed in Table 2.It can be concluded from Fig.3 that the model 5is appropriate for describing the dehydration process(the first reaction),in conversion range of 0.05???0.9.The integral form of model 5,conversion?,and the corresponding T are substituted into Eq.(9),and the apparent activation energy of the first reaction(35)Ea₁=538 k J·mol^-1 can be obtained.

Fig.3(a)Plots of theoretical values y?(a)and experimental values y(a)versusαfor the dehydration of Li HC2O4·H2O;(b)Plots of ln[g(a)/T2]versus 1/T with model 5

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Table 2 Most probable reaction functions for three reactions

The same analysis is applied to the second and third reactions.It is shown that there are four appropriate models for the second reaction in Figs.4(a)and 5.To discern the kinetic model of the second reaction,the thermogravimetric data and four different kinetic models(2,3,9,and 10)are substituted into Eq.(9).The coefficient of determination(r²)of linear regression in Table 3 shows the best result of linearization procedure for process handled by model 3 with the apparent activation energy(35)Ea₂=292 kJ·mol^-1.

For the third reaction,Fig.6 shows that the experimental master plots of y'(?)versusαis in good agreement with the theoretical master plots of y(?)versusαcorresponding to model 14.In Fig.6(c),one can calculate the third reaction apparent energy,(35)Ea₃(28)54 kJ·mol^-1.

It has been known that precursor prepared by low-heating solid-state method consists of LiHC₂O₄·H₂O,LiHC₂O₄,and chelate.In view of the complex structure and thermal decomposition process of chelate,the investigation of Li HC₂O₄·H₂O thermal decomposition mechanism is very important in order to understand the decomposition mechanisms of precursor.The decomposition mechanism of Li HC₂O₄·H₂O and kinetic parameters corresponding to each reaction in sintering are given in the paper.The results suggest the thermal decomposition mechanism of precursor prepared by low-heating solid-state method and especially provide an important route for analysis of thermal decomposition process of other oxalates and dioxalates.The therma decomposition process of chelate would be discussed in the following investigation.

Fig.4(a)Plots of theoretical functions y?(a)and experiment y(a)versusαfor the second reaction;and(b)plots of ln[g(a)/T2]versus 1/T with model 9

Fig.5 Plots of??_??g Tνersus T²ln(?)(1/)with model 2,3,9,and 10

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Table 3 Coefficient of determination(r2)of linear regression with different models

Fig.6(a)Plots of theoretical functions y?(a)and experiment y(a)versusαfor third reaction;(b)Plots of theoretical functions y?(a)with model 14 and experiment y(a)versusαfor third reaction;(c)Plots of

4 Conclusion

The thermal decomposition process of Li HC₂O₄·H₂O from 30 to 600?C was studied by the TG-DSC.There are three decomposition reactions occurring at about 75,150and 416?C,and the decomposition products at 150,170and 420?C relate to Li HC₂O₄,Li₂C₂O₄,Li₂C₂O₄,and Li₂CO₃,respectively.

Three decomposition reactions can be concluded in the whole heating process as follows:

The reaction kinetic model 5,3,and 14 are confirmed to the corresponding first,second,and third weight loss stage from 30 to 600?C,of which the apparent activation energy is 532,292,and 54 kJ·mol^-1,respectively.

Acknowledgements

This work was financially supported by the National“863”Program of China(No.2009AA03Z226),Project on the Integration of Industry,Education and Research of Guangdong Province(No.2011A090200012)and the Fundamental Research Funds for the Central Universities(No.FRF-MP-12-005B).