Chinese Chemical Letters  2016, Vol. 27 Issue (06): 864-866   PDF    
Citation
Zhu Bin-He, Ni Cheng-Zhu, Wang Na-Ni, Zhao Xun-Yan, Guo Wei-Qiang, Zhang Pei-Min, Zhang Jia-Jie, Chen Su-Qing, Lv Wei-De, Zhu Yan.Determination of anions in lithium-ion batteries electrolyte by ion chromatography[J]. Chin. Chem. Lett., 2016,27(06): 864-866  
Determination of anions in lithium-ion batteries electrolyte by ion chromatography
Zhu Bin-Hea, Ni Cheng-Zhua, Wang Na-Nia,b, Zhao Xun-Yana, Guo Wei-Qianga, Zhang Pei-Mina, Zhang Jia-Jiea, Chen Su-Qingc, Lv Wei-Ded, Zhu Yana     
a Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028, China ;
b Department of Medicine, Zhejiang Academy of Traditional Chinese Medicine, Hangzhou 310007, China ;
c School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 317000, China ;
d Hangzhou Vocational & Technical College, Hangzhou 310018, China
Received Aug 17, 2015, Received in revised form Oct 16, 2015, Accepted Oct 16, 2015, Available online Jun 25, 2016
* Corresponding author. wnn8511@163.com (N.-N. Wang),; zhuyan@zju.edu.cn (Y. Zhu)
Abstract: A sensitive and accurate method based on ion chromatography was established for determination of five lithium salts in lithium-ion batteries electrolytes. Chromatographic analyses were carried out on an anion exchange column at flow rate of 1 mL/min. Under the optimal conditions, five target anions (BF4-, PF6-, TFSI-, BOB- and FSI-) exhibited satisfactory linearity with a correlation coefficient of 0.9996. The relative standard derivations of the target anions were less than less than 0.94% (n = 7). The limits of detections were in the range of 0.068-0.29 mg/L with average spiked recoveries ranging from 96.8% to 105.1%.
Key words: Ion chromatography     Lithium salts     Lithium-ion batteries electrolyte    
1. Introduction

Alkaline batteries,lead-acid batteries,and lithium-ion batteries are commonly used for industrial applications and portable utilities [1]. Compared with other batteries,lithium-ion batteries feature long lifetime,environmental friendliness,and high power density [2]. The electrolyte plays an important role in lithium-ion batteries. It consists of lithium salts and organic solvents. Five lithium salts,including lithium tetrafluoroborate (LiBF4) [3],lithium hexafluorophosphate (LiPF6) [4],lithium bis(oxalate)borate (LiBOB) [5],lithium bistrifluoromethanesulfonimide (LiTFSI),and lithium bis(fluorosulfonyl)imide (LiFSI) [6],are most widely used in lithium-ion batteries as electrolytes. The species and quantity of lithium salts in the electrolyte determine the efficiency of ion transport and the conduction current. For example,when the lithium salt concentration was between 0.9 mol/L and 1.5 mol/L,higher concentrations allowed for heavy load discharge,but the rate of discharge at intermediate voltages decreased above ion concentrations of 1.4 mol/L [7]. Some lithium salts (such as LiPF6) have thermal instability and moisture sensitivity which causes them to easily decompose into other compounds in the working process of the batteries. Therefore,it is important to analyze the species and concentration of lithium salts in lithium ion battery electrolytes [8].

Several approaches to analyze lithium salts have been published,such as 1H NMR [9],X-ray diffraction (XRD),infrared (IR) methods and so on. However,they are either expensive or limited only to pure substances. Ion chromatography (IC) is regarded as a very useful analytical technique which can be used for analysis of complex ions with the great advantage of convenience and sensitivity. Until now,no articles have been reported about the simultaneous analysis of the five aforementioned lithium salts. Our work proposed a novel method for simultaneous determination of five anions in lithium-ion electrolyte by IC.

2. Experimental

Reagents,stock solutions,calibration standards and sample: LiBF4,LiPF6,LiBOB,LiTFSI,LiFSI,and the samples were provided by Power Systems Company (Huzhou,China). All of them were of analytical grade. Acetonitrile and methanol were of HPLC-grade (Merck & Co Inc,New,Jersey,USA). The water employed in all experiments was supplied by a Milli-Q water purification system from Millipore (Molsheim,France),18.2 (MV cm). The single standard stock solutions contained 1000 (mg/L) of BF4-,PF6-,TFSI-,BOB-,and FSI-. Mixed standard solutions were prepared by appropriate dilution of the stock solutions with deionized water.

Sample preparation: The sample was diluted with water by 1000 times.

Chromatographic system: The chromatographic system was ICS-2100 (Thermo Fisher model) equipped with an isocratic pump,a thermostat column compartment,a six-port valve,a 25 mL sampling loop,and conductivity detection (DS60). The separation was performed on an IonPac AG22 guard column and an IonPac AS22 separation column. System control and data collection were run with Chromeleon 6.80 chromatogram workstation. The system was operated under isocratic mode of carbonate (3 mmol/L Na2CO3 + 1 mmol/L NaHCO3) and acetonitrile (73:27,v/v),with a flow of 1 mL/min.

3. Results and discussion 3.1. Optimization of chromatographic parameter

IC analysis of lithium salts was carried out with an anion exchange column. Mobile phase was optimized in this study. Carbonate (4.5mmol/L Na2CO3 + 1.5mmol/L NaHCO3) was employed for separating the five anions. The results showed that PF6-,TFSI-,FSI- had broad peaks and could not be separated out within 60 min. Since organic solvents can modify the polarity of the mobile phase,they were added to shorten the analysis time and improve the peak shape.We examined the influence of the amount of acetonitrile on retention times of peaks. Its content in the mobile phasewas varied in the range of 25%-30% (v/v). The retention times decreased with increasing acetonitrile concentration (shown in Fig. 1). However,when the concentration was higher than 27%,the peaks of PF6- ,TFSI- overlapped together. Therefore,27% acetonitrile was chosen. The amount of carbonate was also examined (shown in Fig. 1). There was a significant increase in retention time of BOB- as compared to the other four anions on decreasing the amount of carbonate. Thus carbonate (3mmol/L Na2CO3 + 1 mmol/L NaHCO3) and acetonitrile (73:27,v/v) with flowrate of 1 mL/min was applied in the experiment. The five ions (BF4-,PF6-,TFSI-,BOB- and FSI-) were well separated under this condition.

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Figure 1. The retention times of 1-BF4-, 2-PF6-, 3-TFSI-, 4-BOB-, 5-FSI-. (A) The effect of different concentrations of acetonitrile in the eluent (a) 30% acetonitrile; (b) 29% acetonitrile; (c) 28% acetonitrile; (d) 27% acetonitrile; (e) 26% acetonitrile; (f) 25% acetonitrile; (B) The effect of different concentrations of carbonate in the eluent (g) 3 mmol/L Na2CO3 + 1 mmol/L NaHCO3; (h) 3.6 mmol/L Na2CO3 + 1.2 mmol/L NaHCO3; (i) 4.5 mmol/L Na2CO3 + 1.5 mmol/L NaHCO3.

From Fig. 2,we can see that there are three peaks unidentified. Terborg [8] and Kraft [10] showed us that PF6 - can be easily decomposed. So these peaks include peak of organic solvent and degradation products. Its degradation rate is closely related to the temperature and the solvent. As a result,everything used in the experiment need to be prepared when it will be used.

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Figure 2. The retention times of 1-BF4-; 2-PF6-; 3-TFSI-; 4-BOB-; 5-FSI- (a)chromatogram of the mixed standard solution (BF4-: 25 mg/L; PF6-: 100 mg/L;TFSI-: 100 mg/L; BOB-: 25 mg/L; FSI-: 100 mg/L); (b) Chromatogram of sample; (c) Chromatogram of sample spiked at 5 mg/L BF4-, 40 mg/L PF6-, 5 mg/L TFSI-, 5 mg/LBOB-, 10 mg/L FSI-.

3.2. Method validation and real sample analysis

Under the optimized conditions above,a series of mixed standard solutions of various concentrations were analyzed. The data for the work,such as linear ranges,limits of detection (LODs) and relative standard deviations (RSDs) are summarized in Table 1. Calibration curves for each anion showed excellent linearity with correlation coefficient r - 0.9997. LODs were in the range of 0.068-0.29 mg/L based on the signal-to-noise ratio of three (S/N = 3). The RSDs of peak area were all less than 2.5% for seven replicate analyses of a mixed standard solution (shown in Table 1).

Table 1
Calibration parameters reproducibility of the method

3.3. Real sample analysis

The developed method was applied to analyze five lithium salts in real samples. The concentrations of the analytes were calculated by referring to the calibration equations. The results are shown in Table 2.

Table 2
Determination result and spiked recoveries in real sample

4. Conclusion

This work shows the determination of five lithium salts in lithium-ion battery electrolytes with IC for the first time. The performance of the proposed method was investigated with respect to repeatability,reproducibility,and linearity. It showed good results,which indicated determination of lithium salts in lithium-ion batteries electrolyte by ion chromatography is a lowcost,simple,and effective approach.

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