Designed synergetic effect of electrolyte additives to improve

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Designed synergetic effect of electrolyte additives to improve interfacial chemistry of MCMB electrode in propylene carbonatebased electrolyte for enhanced low and room temperature performance Aselefech Sorsa Wotango, Wei-Nien Su, Atetegeb Meazah Haregewoin, HungMing Chen, Ju-Hsiang Cheng, Ming-Hsien Lin, Chia-Hsin Wang, and Bing-Joe Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02185 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Applied Materials & Interfaces

Designed Synergetic Effect of Electrolyte Additives to Improve Interfacial Chemistry of MCMB Electrode in Propylene Carbonate-Based Electrolyte for Enhanced Low and Room Temperature Performance Aselefech Sorsa Wotango,1 Wei-Nien Su,2 Atetegeb Meazah Haregewoin,1 Hung-Ming Chen,1 JuHsiang Cheng,1 Ming-Hsien Lin,1,3 Chia-Hsin Wang4 and Bing Joe Hwang1,4,＊ 1

Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

2

Nanoelectrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan

3

Department of Chemical & Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Dasi, Taoyuan 335, Taiwan 4

National Synchrotron Radiation Research Center, Hsin-chu, Taiwan

*Corresponding Author. E-mail: [email protected]

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Abstract The performance of lithium ion batteries rapidly falls at lower temperatures due to decreasing conductivity of electrolytes and Solid Electrolyte Interphase (SEI) on graphite anode. Hence, it limits the practical use of lithium ion batteries at sub-zero temperatures and also affects the development of lithium ion batteries for widespread applications. The SEI formed on the graphite surface is very influential in determining the performance of the battery. Herein, a new electrolyte additive, 4Chloromethyl-1,3,2-dioxathiolane-2-oxide (CMDO), is prepared to improve the properties of commonly used electrolyte constituents - ethylene carbonate (EC), and fluoroethylene carbonate (FEC). The formation of an efficient passivation layer in propylene carbonate (PC) -based electrolyte for MCMB electrode was investigated. The addition of CMDO resulted in a much less irreversible capacity loss and induces thin SEI formation. However, the combination of the three additives played a key role to enhance reversible capacity of MCMB electrode at lower or ambient temperature. The electrochemical measurement analysis showed that the SEI formed from a mixture of the three additives gave better intercalation-deintercalation of lithium ions.

Keywords: electrolyte additive, solid electrolyte interphase, 4-Chloromethyl-1,3,2-dioxathiolane-2oxide, fluoroethylene carbonate, MCMB electrode, propylene carbonate


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1. Introduction Graphite, with a theoretical specific capacity of 372 mAh g−1 has been the most widely used anode material in the commercial lithium ion batteries owing to a number of advantages including, low irreversible capacity, low volume expansion during lithiation, high Coulombic efficiency, good cycle life, good electronic conductivity, etc.1-3 However, lithium ion batteries exhibit poor performance at sub-zero temperatures and as studied by many literatures the reason for poor low temperature performance was found mainly due to graphite anode and electrolyte.4,

5

More importantly, many

developments of future applications of Li-ion batteries demand better low temperature performance and higher power density.6 For this reason, many studies have been done to alleviate the problem, mostly by optimizing electrolyte solvents. Electrolyte solvents not only affect the conductivity of electrolytes but also participate in the formation of solid electrolyte interphase layer and which has a great role on the performance of graphite electrode.6-8 The typical electrolyte system of a conventional lithium-ion battery is lithium salt in a mixture of organic carbonate-based solvents. To satisfy various and contradicting requirements, the solvents are usually composed of linear and cyclic carbonate solvents. Among all solvents, ethylene carbonate (EC) forms stable and effective SEI on graphite surface. Because of this advantage, EC is used in commercial lithium ion batteries as one of the necessary solvent in mixture of co-solvents.9 However, EC has high melting point (36.4 °C), and it causes the conductivity of electrolyte solutions to fall rapidly at lower temperature. Apparently, it limits the performance of lithium ion battery at low temperatures. On the contrary, propylene carbonate (PC) has low melting point (−48.8ºC) and been considered as the most promising candidate solvent to improve low temperature performance. However, PC decomposes on graphite electrode surface and co-intercalates with lithium ions into graphite layer causing exfoliation of graphite layers, which affects the reversible capacity and stability of graphite electrode.10 When low EC-content is used with high PC-content to optimize the conductivity of



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electrolyte at low temperature, PC completely suppresses crystallization of EC. However, unfortunately EC cannot suppress completely the co-intercalation of PC into graphite electrode.11 Nevertheless, PC is an attractive solvent due to its wide temperature range. The common approach to improve performance at low temperatures is the formulation of electrolyte solvents by optimizing the conductivity of electrolyte and SEI. Smart et al. studied a number of mixture of carbonate based solvents without PC solvent for performance improvement at low temperature and reported that quaternary carbonate based solvents gave better performance.12 With and without PC in the mixture of carbonate based solvents was studied by Zhang et al. and the authors found that resistance of SEI increased for PC based electrolyte much slowly than without PC with decreased temperature.7 Recently, elemental sulfur as an additive on graphite electrode and sulfite containing electrolyte solvents on MCMB electrode have been studied for low temperature performance and showed that the sulfur-derived SEI layer was less resistive.13, 14 Several research studies were done to understand the reason for poor performance at low temperature which mainly attributed to carbon electrode activity than cathode electrode,4, 5 low ionic conductivity of the electrolyte,12, 15 resistance of SEI formed on the graphite surface,6 limited diffusivity of lithium ions within graphite anode,5 and increased charge-transfer resistance on the electrolyte–electrode interfaces,7, 13 or combined effects. Although there are many studies done, the performance of lithium ion battery at low temperature is still poor. Thus, it is necessary to improve the performance of graphite electrode at low temperature. PC solvent is reported to improve lower temperature performance.16, 17 However, it needs co-solvents to decrease its viscosity. On the other hand, dimethyl carbonate (DMC) was found as one the potential solvent in a mixture of solvents for low temperature performance improvement.12,

15, 18

Herein, we formulated

electrolyte solvents considering the advantage of PC as low melting point solvent to increase the liquid range and DMC as a low viscosity solvent to optimize the conductivity of electrolyte. However, instead of using EC as a co-solvent, it is used as additive content to improve the properties of SEI and to 4 ACS Paragon Plus Environment

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circumvent the adverse effects due to poorer electrolyte conductivity caused by low temperatures.19 Butyl sultone,20 vinyl sulfones,21 prop-1-ene-1,3-sultone,22 etc., have been reported to have lower LUMO energy levels which allow them to be reduced prior to PC, so SEI film forms first on graphite electrode. Other SEI former additives like vinyl carbonate,23 N-methylacetamide,24 and maleimidebased additives25 were also suggested to suppress PC co-intercalation. Sulfite functional group containing additives and halogen substituted additives are known as good SEI former additives on graphite electrode.23, 26, 27 In addition, sulfur derived SEI and chlorine atom containing additive are reported to improve low temperature performance of graphite electrode.13,

14, 27

Based on the

considerations and requirements of PC-based electrolyte, 4-(Chloromethyl)-1,3,2-dioxathiolane 2-oxide (CMDO), a sulfur-containing compound with similar structure to sultones, seems to be a reasonable additive and the material has not been used and studied for this purpose. CMDO has sulfite functional group and chlorine substituent on the structure and is designated to utilize the advantages of sulfite functional group and chlorine atom. Moreover, it is interesting to know if the interfacial chemistry of MCMB electrode in PC-based electrolyte can be further improved through a combination of different additives, especially at lower temperatures. SEI-forming additives on the MCMB surface and their synergetic effect were designed to promote the desired properties of SEI. The additives of interests are 4-(Chloromethyl)-1,3,2-dioxathiolane 2-oxide (CMDO), ethylene carbonate (EC), and fluoroethylene carbonate (FEC).

2. Experimental section 2.1.

Synthesis of additive

4-Chloromethyl-(1,3,2)dioxathiolane-2-oxide (CMDO) as one of the necessary additive was synthesized.28 To a solution of 3-chloro-1,2-dihydroxypropane in dichloromethane an equimolar amount of thionyl chloride was added dropwise, then the resulting mixture was refluxed for 2 h. 5 ACS Paragon Plus Environment


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Subsequently the solvent was evaporated to collect the oil product, which was purified by silica gel column chromatography (ethyl acetate: hexane, 1:1) and dried in vacuum oven for overnight to avoid any solvent residue. 1H and

13

C NMR spectra of the oil product (CMDO) were recorded on a Bruker

AVIII HD 600NMR spectrometer using CDCl3 solvent and the chemical shifts were observed 1

according to literature reported.29 (2S, 4S): H NMR δ: 4.78–4.68 (m, 1H, OCH), 4.58–4.43 (m, 2H, 13

OCH ), 3.81 (d, 2H, CH Cl). 2 2

C NMR δ: 80.93 (CH), 69.98 (OCH2), 43.35 (CH2Cl). (2R,4S): 1H

NMR δ: 5.14–5.04 (m, 1H, OCH), 4.66 (dd, 1H, OCH ), 4.32 (dd, 1H, OCH , 3.62(d, 2H, CH2Cl, 3J 2 2 5.4 Hz). 13C NMR δ: 78.83 (CH), 68.94 (OCH ), 42.56(CH Cl). ESI-MS: m/z = 157 [M + H] + 2

2.2.

2

Preparation of electrolyte and electrode

The baseline (B) electrolyte was 1 M LiPF6 in PC: DMC (1:1, v/v), formulated from commercially available PC (Acros Organics, 99.5%), DMC (Acros Organics, 99%), and LiPF6 (Aldrich, 99.99%). The mixing ratio of PC to DMC was decided after some preliminary trials to balance the requirements of solution conductivity and improved convenience of handling. The three additives were FEC (Aldrich, 99%), EC (Acros Organics, 99+ %), and CMDO as prepared. The combinations of additives - 2 vol% CMDO as” 2C”, 3 vol% EC as “3E” as well as 5 vol% FEC as “5F” with the baseline electrolyte (B) were compared. The sample electrolyte containing and 3 vol% EC and 5 vol% FEC additives were denoted as B3E5F. Complete electrolyte formulations are listed in Table 1. All electrolyte solvents were dried over molecular sieve (4Å). In electrolyte around 20 ppm water was detected by Karl Fischer titration. The graphite electrode was composed of mesocarbon microbeads (MCMB-2528, Osaka Gas), Super-P conductive carbon and polyvinylidene fluoride (PVdF) binder (weight ratio 93:3:4). MCMB powders were mixed with Super P conductive carbon and dispersed in the mixture of PVdF and NMethyl-2-pyrrolidinone (NMP). Then the slurry was coated on Cu foil and dried under vacuum oven 6 ACS Paragon Plus Environment

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for overnight to evaporate NMP. Active material loading was kept as ~5.4 mg cm-2. Glass fiber (GF) was used as a separator.

Table 1. Electrolyte formulation used in this work. All additives were added to electrolyte by volume percent. Electrolyte notation B B2C B5F B2C5F B3E5F B2C3E5F

2.3.

Formulation 1M LiPF6 PC:DMC(1:1) B + 2 vol% CMDO B + 5 vol% FEC B + 2 vol% CMDO + 5 vol% FEC B + 3 vol% EC + 5 vol% FEC B + 2 vol% CMDO + 3 vol% EC + 5 vol% FEC

Electrochemical Cycling

For electrochemical measurement the coin cells 2032-type were assembled in an argon filled glovebox (Unilab, Mbraun) with less than 1 ppm of oxygen and water vapor. The Li/MCMB half-cells were measured using a battery cycler in galvanostatic mode (Arbin, BT-2000). The coin cells were charged and discharged in a potential window between 0.01 to 3 V under controlled temperature. For cycling performance at room temperature (RT) and low temperature (-10 ᴼC) was tested at 0.1 C (C= 372 mAh/g). Rate capability was evaluated at RT with various current densities. Cyclic voltammetry (CV) was performed using coin cells with MCMB as the working electrode and Li foil as the counter and reference electrode. CV was measured with the potential window from 0.01 to 3 V vs Li/Li+ at scan rate of 0.1 mV/s (Bio-Logic, VMP3). Electrochemical impedance spectroscopy (EIS) tests were carried out on a potentiostat with frequency range from 1 MHz to 10 mHz and AC amplitude was set as 5 mV.

2.4.

Surface analysis

For surface analysis the cells after cycling were dismantled in an argon filled glovebox and rinsed with diethyl carbonate (DEC) to remove the residual electrolyte. The washed electrodes dried at room



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temperature inside glovebox. X-ray photoelectron spectroscopy was performed by using 0.05 eV step and 20 eV pass energy at NSRRC. All XPS spectra were calibrated based on the C-H (hydrocarbon) binding energy at 286.0 eV. The MCMB electrodes before cycling, after cycling with and without additives were investigated by scanning electron microscopy (SEM, JEOL JSM-6500F).

3. Results and discussion The electrochemical reductive potential of CMDO, EC, and FEC additives were investigated by CV measurements. The first scan of Li/MCMB cells in B, B2C, B5F, B2C5F, B3E5F, and B2C3E5F electrolytes are shown in Figure 1. Figure 1a displays the first scan CV of baseline electrolyte without additive. During the cathodic sweep the onset of electrolyte reduction started near 0.7 V vs Li/Li+, suggesting that the PC decomposition more likely started at this potential. There is a large irreversible peak near 0.5 V vs Li/Li+ in the cathodic sweep, which corresponds to PC decomposition and exfoliation of the graphite as a result of co-intercalation of PC with solvated Li+.30 The absence of observable anodic peak suggested that electrolyte reduction was irreversible and no lithium ion deintercalation occurred. Regarding cell containing B2C electrolyte in Figure 1b, there is a large peak appeared around 1.7 V during cathodic sweep, which is certainly ascribed to the decomposition of CMDO. The disappearance of subsequent PC decomposition suggested that CMDO decomposition products formed a protecting surface layer, which prevented the decomposition of electrolyte and intercalation of Li+ solvated with PC. CMDO has a similar sulfite functional group and cyclic structure to ES and VES, where the reductive decomposition peaks of ethylene sulfite (ES) and vinyl ethylene sulfite (VES) were found at 1.9 and near 1.5 V vs Li/Li+, respectively.30, 31 The decomposition of FEC in the cell containing B5F electrolyte is shown in the inset of Figure 1b which shows only FEC decomposition peak, was appeared around 1.18 V vs Li/Li+, whereas FEC decomposition was observed on graphite electrode in PC based electrolyte near 1.1 V vs Li/Li+ in literature. 32, 33 However, the SEI formed from 5% FEC could not completely suppress the further


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decomposition of PC around 0.25 - 0.7 V vs Li/Li+ showing higher irreversible electrolyte decompositions. In the cell containing B2C5F electrolyte (Figure 1b), as observed in the inset with violet color, there are two cathodic peaks with the decomposition of CMDO prior to FEC indicating that the SEI formed in this system effectively suppressed the decomposition of electrolyte, when compared to the cell containing B5F electrolyte. The decomposition potentials of FEC and CMDO slightly shifted to lower potential than the individual ones, when they were mixed. It suggests that the reduction potential depends on the medium of electrolyte. The addition of 3% EC in the cell containing B3E5F electrolyte also minimized the electrolyte decomposition compared to the cell containing FEC additive alone (B5F electrolyte). Shen. et al. studied in situ formation of SEI on HOPG electrode in FEC/DMC and EC/DMC electrolyte systems and they found that FEC system formed thick SEI compared to EC system.34 This result suggests that more electrolyte decomposition occurred in FEC system than EC system. As shown in the following results in our study, only 5% FEC additive in PC system also resulted in excessive electrolyte decomposition. However, the addition of 3% EC to 5% FEC additive effectively minimized the electrolyte decomposition.

(a)

(b)

Figure 1. Comparison of first scan CV of Li/MCMB cells with electrolyte (a) B, and (b) B2C, B5F, B2C5F, B3E5F, and B2C3E5F, at scan rate of 0.1 mV/s.



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As seen in the inset (with black color) of Figure 1(b), there are two peaks observable for the cell containing B2C3E5F electrolyte. The larger reduction peak near 1.25 V vs Li/Li+ corresponds to the reduction of CMDO. In other studies it is reported that FEC decomposes reductively prior to EC, 35 so the peak appeared near 0.9 V vs Li/Li+ apparently corresponds to the decomposition of FEC. However, the EC decomposition peak expected to occur after FEC decomposition was not observed, this might be due to a too small peak. On the second cathodic sweep for all cells containing additives as seen in Figure S4, the disappearance of the cathodic peak at the additives reduction potential indicated that the reduction products during the first scan had passivated the MCMB electrode. Figure 2 displays Li/MCMB half-cells tested at RT with five different electrolytes. As shown in Figure 2a, from the first discharge of baseline (B) electrolyte the potential plateau appeared near 0.7 V and remained at this potential, which is due to co-intercalation of PC with solvated Li+ into graphite layer resulting graphite exfoliation.36,

37

From the first discharge potential plateau for cells containing

additives, excessive SEI formation was occurred for the cells without CMDO additive suggesting that maximal amount of electrolyte was decomposed, leading to a thick SEI layer. However, in the cells containing CMDO additive the plateau apparently decreased indicating that irreversible capacity loss was effectively minimized. The size of potential plateau in the first discharge represents the amount of irreversible capacity.26 Compared to the cell containing only FEC additive (B5F electrolyte), the presence of 3% EC in the cell containing B3E5F electrolyte slightly increased the 1st cycle Coulombic efficiency from 80.0% to 82.5%. However, the presence of 2% CMDO in the cells containing B2C5F and B2C3E5F electrolyte significantly improved the 1st cycle Coulombic efficiency as summarized in Table 2. It is to note that in a Li/MCMB half-cell the consumption of lithium ions for the formation of SEI may not affect the capacity of the cell. However, for lithium ion cells, where limited lithium ions are provided from cathode, the irreversible consumption of lithium ions significantly affects the capacity of cells. The prevention of lithium ion consumption is very important for lithium ion cells in practice. 10 ACS Paragon Plus Environment

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From the charge capacity retention shown in Figure 2b, the cell containing the mixture of the three additives showed superior performance than the cell containing either FEC additive alone or binary additive systems. On the other hand, the cell containing B5F electrolyte showed poor capacity retention, though the performance at the beginning comparable to others cell. The highest capacity decay in the cell containing only FEC additive suggests that the formation of porous SEI which may cause continuous electrolyte decomposition and consumption of the active mass. On the contrary, the cells containing CMDO additive showed good capacity retention than the cells without CMDO additives. Figure 2a and 2b results suggest that CMDO additive not only induced thin SEI but also formed compact SEI which minimized the continuous active mass consumption. The presence of 3% EC in the cell containing B3E5F electrolyte also reduced the capacity decay compared to the cell containing B5F electrolyte. Rate performance of Li/MCMB is shown in Figure 2c. The cell containing B5F electrolyte delivered lower rate performance than other cells indicating that retarded the lithium ion transport might be caused by the thick SEI formation. When the discharging rate increased to 0.2 C all cells capacity initially dropped but with cycling the capacity increased, this might be due to some delayed wetting effect of the porous PVdF in graphite electrode.26 The presence of 3% EC in the cell containing B3E5F electrolyte improved the rate performance of the cell compared to the cell containing B5F electrolyte. The cells containing CMDO additive showed better rate performance than the cells without CMDO.



(a)

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(b)

(c)

Figure 2. (a) Initial charge/discharge curve, (b) charge capacity retention and (c) rate capability test of Li/MCMB cells, which are tested at RT in the electrolytes of B (only the first discharge curve), B5F, B2C5F, B3E5F and B2C3E5F.

In order to evaluate the effect of the SEI on the low temperature performance of Li/MCMB, cells with four different electrolyte systems were analyzed at -10 ᴼC. From the first cycle charge and discharge curve in Figure 3a, the cell containing B3E5F electrolyte showed higher capacity than the other cells. The presence of 3% EC in the cell containing B3E5F electrolyte significantly improved the first cycle capacity and Coulombic efficiency of the cell compared to the cell containing B5F electrolyte (Table 2). In the first cycle, cells with CMDO additive showed lower lithiation and de-lithiation capacity than 12 ACS Paragon Plus Environment

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cells without CMDO additive (Table 2). However, the cells containing CMDO additive showed significantly improved first cycle Coulombic efficiency as listed in Table 2. The de-lithiation capacity depends on the Li+ ions inserted into the graphite electrode and consumed for SEI formation. However, the lower lithiation capacity for CDMO containing cells may suggests that the higher cell polarization in these cells.13,

38

Figure 3b shows cycling performance of Li/MCMB half-cells at -10 ᴼC. The

performance of cell containing B2C5F electrolyte was worse than others. The addition of 2% CMDO in the cell containing B2C5F electrolyte decreased the performance of the cell compared to the cell containing B5F electrolyte at -10 ᴼC. Although the cell with B3E5F electrolyte gave almost comparable cycling performance to that of the cell containing B2C3E5F electrolyte (Figure 3b), it should be noticed that the Coulombic efficiency of the cell containing B2C3E5F electrolyte was better than the cell containing B3E5F electrolyte suggesting that the former had better passivation of electrode surface. As seen in Figure 3b, in the initial cycles the capacity of all the cells increased continuously and the increase might be attributed to gradual stabilization/modification of SEI upon cycling, which resulted in decreased polarization.39, 40



(a)

(c)

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-10 ᴼC

RT

(b)

(d)

-10 ᴼC

Figure 3. (a) Initial cycle voltage profile, (b) charge capacity retention and (c) Coulombic efficiency of Li/MCMB cells, which are tested in B5F, B2C5F, B3E5F and B2C3E5F electrolytes at -10 ᴼC and (d) capacity retention of cells containing B3E5F and B2C3E5F electrolyte after five formation cycles at RT were taken to -10 ᴼC.

In order to see temperature effect of the formation cycles on the overall performance of Li/MCMB at 10 ᴼC, cells containing B3E5F and B2C3E5F electrolyte were run at RT for the first five formation cycles and then taken to -10 ᴼC. Figure 3d shows cycling performance of cells containing B3E5F and B2C3E5F. When the coin cells were taken from RT to -10 ᴼC the capacity gradually dropped. However, 14 ACS Paragon Plus Environment

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starting from some cycles the capacity increased steadily until it reached a stable state. Comparing the cyclic performance of Li/MCMB at -10 ᴼC with different temperatures of the formation cycles, the capacity is almost similar for cell containing B3E5F electrolyte and slightly higher for cell containing B2C3E5F electrolyte when formation cycles run at RT. This may suggest that for CMDO additive better SEI formation occur at RT. The Coulombic efficiency of the same test can be seen in Figure S5. In comparison, the addition of EC gave better characteristics of SEI for LT performance so that the cell containing EC electrolytes showed better performance at LT, whereas the addition of CMDO gave better characteristic of SEI for RT and rate performance. It is noted that the mixture of three additives gave superior performance in all cases at LT, RT and rate capability performance test.

Table 2. Comparison of the 1st cycle capacity and Coulombic efficiency of Li/MCMB cells in four different electrolyte systems at different temperature conditions. Conditions

At -10 ᴼC

At 25 ᴼC

Electrolytes

1st Cycle cha.

1st Cycle discha.

1st Cycle Coulombic

capacity (mAh g-1)

capacity (mAh g-1)

efficiency (%)

B5F

80.7

241.6

33.4

B2C5F

14.0

26.4

53.0

B3E5F

126.3

251.0

50.3

B2C3E5F

69.7

90.5

77.0

B5F

324.0

405.4

80.0

B2C5F

328.0

353.5

92.8

B3E5F

320.6

388.8

82.5

B2C3E5F

329.8

356.5

92.5

Electrochemical Impedance Spectroscopy (EIS) was carried out to observe the interface resistance. The EIS spectra consist of two overlapped semicircles at high and medium frequency region, and a slopping straight line at lower frequency region. The high frequency semicircle represents the resistance for 15 ACS Paragon Plus Environment


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migration of lithium ion through SEI, the medium frequency semicircle represents the charge transfer at the electrode-electrolyte interface, and the slopping line at lower frequency due to diffusion process with in the graphite layer.41 Figure 4 shows Nyquist plots of Li/MCMB half-cells after five formation cycles and 45th cycle rate performance test with four different electrolyte systems at RT. As seen in the inset of Figure 4a, after 5th cycle the semicircle at high frequency for the cells containing CMDO additive showed smaller than that of the cells without CMDO additive suggesting that less SEI resistance for cells containing CMDO additive. However, after 45 cycles it is seen in the inset of Figure 4b, the high frequency semicircle for the cell containing B2C3E5F electrolyte gradually increased and seems to divide into two separated semicircles suggesting that surface resistance was changed.42 After the 45th cycle the semicircle at high frequency as seen in the inset of Figure 4b nearly unchanged for the cell containing B5F electrolyte but decreased for the cell containing B3E5F electrolyte. After the 5th and 45th cycle the large semicircle was appeared in the medium frequency region for the cells without CMDO additive. Interestingly, semicircle in the medium frequency region was not apparent for cells containing CMDO. The charge transfer resistance was smaller for the cell containing B3E5F electrolyte after five formation cycle compared to the cell containing B5F electrolyte. However, after the 45th cycle the charge transfer resistance for the cell containing B3E5F electrolyte becomes slightly higher than that of the cell containing B5F electrolyte. The rate capability performance of the cell containing B3E5F electrolyte was higher than the cell containing B5F electrolyte for C-rate from 0.1 to 0.3 C. However, the rate performance of the cell containing B3E5F electrolyte dropped at 1 C even became less than that of the cell containing B5F electrolyte, this fact is consistent with the observed increasing charge transfer resistance of the cell containing B3E5F electrolyte compared to the cell containing B5F electrolyte after the 45th cycle. After five formation cycles, the electrolyte resistance in the very high frequency region seemed almost similar for all cells. However, after 45th cycle, the electrolyte resistance slightly increased in the cells 16 ACS Paragon Plus Environment

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without EC additive and remained the same for the cells with EC additive suggesting that the presence of EC in these electrolyte systems gave beneficiary effect for electrolyte conductivity and which confirms the high conductivity of EC.43

(a)

(b)

Figure 4. Nyquist plots of Li/MCMB cells (a) after five formation cycles with 0.1C and (b) after 45th cycle rate capability performance, in B5F, B2C5F, B3E5F and B2C3E5F electrolyte systems which was taken at RT. Inset shows an enlarged part in the higher frequency region.

The impedance spectra of Li/MCMB were measured with four different electrolyte systems at -10 ºC. As seen in the inset of Figure S6a, for the impedance measured after five formation cycles the semicircle at high frequency not that much difference in all cells. On the other hand, as seen in Figure 3b, the capacity of all cells was increasing continuously at the initial cycling. After 35th cycle, shown in Figure S6b, the SEI impedance showed in the following order B2C3E5FB2C5F, the less SEI resistance showed the better performance. In general as can be seen from Figure S6, the SEI resistance seems



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decreasing through cycling for the cells cycled at -10 ᴼC. Therefore, the improved low temperature performance of the Li/MCMB may be attributed to the less SEI resistance. The SEM images of graphite electrode surfaces before and after cycling with different electrolyte systems were performed. Figure 5a exhibits the SEM image of MCMB electrode before cycling which looks uncovered surface. The morphology of cycled MCMB electrode with baseline electrolyte apparently changed due to PC co-intercalation with solvated Li ions which resulted in graphite exfoliation, as seen in Figure 5b. Similar result was observed for MCMB electrode with PC solvent system in other studies.44 Figure 5c shows the MCMB surface cycled in B5F electrolyte covered with rough and thick surface layer, which is consistent with the first discharge potential plateau was observed with more electrolyte decomposition for the cell containing B5F electrolyte in Figure 2a. It is apparent that the MCMB surface cycled in electrolyte containing B2C5F coated with a thin and dense layer (Figure 5d). However, the surface of MCMB electrode cycled in the mixture of the three additives appeared uniformly coated and smoother than others as seen in Figure 5e.


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(a)

10 µm (c)

10 µm

(b)

10 µm (d)

10 µm

(e)

10 µm

Figure 5. SEM images of MCMB electrodes: (a) before cycling and (b) after 1st discharge in baseline electrolyte and (c) after 35th cycle at -10º C in B5F electrolyte, (d) in B2C5F electrolyte and (e) in B2C3E5F electrolyte. The inset shows x 2200 magnification.



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To investigate the surface composition of MCMB electrode in different electrolyte systems, XPS was performed after initial formation cycle at RT. Figure 6 shows the C 1s, O 1s and F 1s XPS spectra obtained from MCMB surface in electrolyte of B5F, B2C5F and B2C3E5F. All C 1s spectra from MCMB surface in three different electrolytes revealed peaks around 284.2, 286.0, 287.6, 289.8, and 291.6 eV. A very small peak at 284.2 eV is attributed to graphite. The appearance of a very small peak of graphite indicates that the surface of MCMB was covered by a passivation layer. A broad peak centered at 286.0 eV which is superimposed peaks associated with hydrocarbon contamination of C-H/C-C peak overlapping with SEI-polymeric species peak.45-47 Polypropylene glycol-type(PPG) oligomers was identified in electrolyte solvents with PC/DMC system as reported elsewhere.48 Another possibility is that the decomposition of FEC likely to give polymeric products.49 The peak at 287.6 eV is a group of superimposed peaks attributed to carbon atoms bonded to one oxygen/fluorine/chlorine atom.50 The peak around 289.8 eV is assigned to CO3-like carbon atoms due to the presence of carbonate species such as ROCO2Li or Li2CO3 formed from alkyl carbonates which are likely the decomposition products of FEC, EC, DMC, or PC. The peak observed at 291.6 eV is the characteristic of carbon bonded to fluorine atoms in PVdF binder. However, this peak intensity is slightly higher than that of graphite peak, maybe the binder reacted with electrolyte and the species in this environment became higher on the electrode surface.51 Comparing the relative peak intensity in the three electrolyte systems showed that the highest peak of carbon species were found on electrode surface cycled in B5F electrolyte. This suggests more electrolyte decomposition occurred in this system and is likely due to poor passivation of the SEI formed in this system. On the contrary, electrode surface cycled in B2C3E5F electrolyte showed the lowest peak intensity implying minimal electrolyte decomposition in this system, as well as less binder side reactions and alkyl carbonates. Similarly, all O 1s spectra exhibit two peaks with the characteristic of main peak at 532.8 eV and the small peak at 534.6 eV. The main broad peak at 532.8 eV may include superimposed peaks of the carbonates decomposition products RCO3Li, C=O and Li-O-H for the three electrolyte systems.52, 53 20 ACS Paragon Plus Environment

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However, in the case of B2C5F and B2C3E5F electrolyte systems, the MCMB surfaces might contain additional peaks due to SOx overlapped in this region.30 The appearance of second peak at 534.6 eV corresponds to oxygen atom bonded to two carbon atoms, which is likely ascribed to the decomposition products of electrolyte. All F 1s spectra consist of two peaks at 686.5 and 688.8 eV, which are attributed to LiF and LiPF6 overlapping with the signal of fluorine in PVdF, respectively. The peak at 686.5 eV was observed for the presence of LiF product.46, 51 LiF was found to be the main decomposition products of FEC and LiPF6.34, 53, 54 The decomposition mechanism of FEC is proposed to occur through the elimination of HF to produce vinylene carbonate (VC).49, 55, 56 The HF reacts with lithium alkoxide species on the SEI to form LiF whereas VC undergoes reductive decomposition on electrode surface to give polymeric carbonate species.49 From C 1s, O 1s and F 1s XPS spectra analysis of the MCMB surface in the three electrolyte systems B5F, B2C5F and B2C3E5F showed that the peak binding energy appeared similar but the intensity of the peaks were quite different. The addition of CMDO to B2C5F reduced electrolyte decomposition on the surface compared to the application of FEC alone. Furthermore, the mixture of three additives minimized the electrolyte decomposition more significantly compared to either binary or single additive systems.



C 1s in B5F

O 1s in B5F

F 1s in B5F

(a)

(b)

(c)

C 1s in B2C5F

(d)

O 1s in B2C5F

(e)

F 1s in B2C5F

(f)

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C 1s in B2C3E5F

O 1s in B2C3E5F

F 1s in B2C3E5F

(g)

(h)

(i)

Figure 6. C 1s, O 1s and F 1s XPS spectra of MCMB electrode surface after 1st formation cycle at RT: (a), (b), (c) in electrolyte of B5F, (d), (e), (f) in electrolyte of B2C5F and (g), (h), (i) in electrolyte of B2C3E5F.

The XPS spectra of S 2p, and Cl 2p are shown in Figure 8. Since CMDO is the only source of S and Cl in cell containing B2C5F and B2C3E5F electrolyte, only samples B2C5F and B2C3E5F are presented. XPS spectra in Figure 7 provide clear evidence for the participation of CMDO decomposition product in the SEI formation. Two S 2p peaks at 171.6 and 163.6 eV are observed on the MCMB electrode 22 ACS Paragon Plus Environment

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surface, corresponding to sulfur oxide and sulfur species.30, 57, 58 The Cl 2p spectrum shows two peak at 200.0 and 196.9 eV. The main peak at 200.0 eV attributed to C-Cl bonds and a weak peak at 196.9 eV can be attributed to adsorbed chlorine on the surface or possibly trace LiCl.

54, 59, 60

The MCMB

electrode surface cycled with B2C5F electrolyte exhibits more intense Cl and S 2p peaks compared to MCMB electrode surface cycled with B2C3E5F electrolyte. This is probably because of the SEI grown in the mixture of the three additives is denser than the SEI-derived in the presence of two additives. Therefore, does block more the inner part of the SEI in the three additives system. It is suggested form CV results in Figure 1b, CMDO decomposes prior to FEC and EC at higher reductive potential and thus the decomposed products from CMDO are likely to form the inner part of the SEI. The EC decomposed products possibly form the outer part of the SEI in B2C3E5F. The presence of EC may also reduce the decomposition of other electrolytes, compared to B2C5F electrolyte system.

S 2p in B2C5F

(a)

S 2p in B2C3E5F

(c)

Cl 2p in B2C5F

(b)

Cl 2p in B2C3E5F

(d)

Figure 7. S 2p and Cl 2p XPS spectra of MCMB electrode surface after 1st formation cycle at RT: (a), (b) in electrolyte of B2C5F and (c), (d) electrolyte of B2C3E5F. 23 ACS Paragon Plus Environment


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The reductive decomposition mechanism of carbonates (such as FEC, EC, PC, etc.) is reported to occur via one electron reduction pathway.47, 49 Sulfite has an analogous structure with carbonate except the sulfur atom replaced for the carbonyl carbon atom. One electron reduction of carbonate produces radical intermediates with the major product likely to be the more stable radical intermediate. Radicals are highly reactive because they seek an additional electron to complete its octet.61 For this reason, in the second step of the reaction the intermediate radicals undergo subsequent one electron reduction to give stable products. Based on the previous computational and experimental studies on the reduction mechanism of sulfite functional group, as well as based on our XPS results the possible a one electron reduction pathway of CMDO is proposed in Figure 8. There is the possibility for the formation of two radical intermediates in the decomposition of CMDO. Radicals are considered as an electron deficiency because they are seeking a further electron to fulfill octet. Secondary radicals are attached with two alkyl (inductively electron donating) group but primary radicals attached with one alkyl group. Secondary radical more stabilized by two alkyl groups compared to primary radical because of this reason secondary radical intermediate will be the major intermediate product. The reductive decomposition of sulfite leads to the formation of ROSO2Li, Li2SO3, Li2S, Li2O and derivatives of these sulfate groups.30, 62 Furthermore, other products such as RCl, and LiCl are also expected for the decomposition of CMDO. Without any addition, the intercalation of solvated lithium ions into MCMB would cause the degraded or even exfoliated material. Although FEC is very well-known SEI forming additive on the anode surface of lithium ion batteries, we found that FEC in the PC electrolyte systems induced thick SEI formation. In addition, other studies also reported FEC electrolyte system formed thick SEI relative to EC electrolyte system on graphite electrode.34 The same phenomena was similarly observed on silicon electrode.63 The addition of EC or CMDO along with FEC could only bring limited improvement. It would be more beneficial to combine electrolyte additives like CMDO and EC, together with FEC to optimize the SEI formation to achieve a thinner


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and conductive SEI layer, especially under critical conditions such as low temperatures. The overall schematic representation of the surface phenomena in different additive systems is depicted in Figure 9.

Figure 8. Possible lithium-assisted one electron reduction pathways of CMDO.



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Figure 9. Effects of the application of various additives.

4. Conclusion The mixture of three electrolyte additives CMDO, EC, and FEC was formulated as effective SEI former for graphite electrode in PC-based electrolyte system. Enhanced cycling performance of MCMB electrode was obtained at -10 ᴼC and ambient temperature. In addition, the rate capability of MCMB electrode was significantly improved. The mixture of the three additives drastically improved the first cycle Coulombic efficiency compared to FEC additive alone from 33.4% to 77.0% at -10 ᴼC and from 80.0% to 92.5% at RT. The binary additives systems showed better performance than only FEC additive system. The cells containing CMDO additive gave improved performance at RT and in rate capability than cells without. On the other hand, EC additive showed the capability to improve performance at -10 ᴼC. However, the cells containing the mixture of the three additives showed superior performance in all tests. We believe that the result of this study will have great value for developing electrolyte additives and to improve performance of lithium ion batteries, especially for graphite electrode with PC-based electrolyte used at low temperatures.


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Supporting Information Following information is available via the ACS Publications website at DOI:

1

H and

13

C NMR; Electrospray Ionization Mass Spectrometer (ESI-MS) results of 4-Chloromethyl-

1,3,2-dioxathiolane-2-oxide (CMDO); cyclic voltammograms and Coulombic efficiency of Li/MCMB cells with various electrolytes; EIS measurements of Li/MCMB cycled at -10 ᴼC

Acknowledgements The financial support from the Ministry of Science and Technology (MoST) (106-2923-E－011-005, 105-3113-E-011-001, 105-ET-E-011-004-ET, 104-2923-M-011-002-MY3, 104-2911-1-011-505-MY2, 103-2221-E-011-156-MY3), the Ministry of Economic Affairs (MoEA) (101-EC-17-A-08-S1-183), the Top University Projects (100H45140), the Global Networking Talent 3.0 Plan (NTUST 104DI005) from the Ministry of Education of Taiwan, Taiwan’s Deep Decarbonization Pathways toward a Sustainable Society Project (106-0210-02-11-03) from Academia Sinica as well as the facilities of support from National Taiwan University of Science

and Technology (NTUST) and National

Synchrotron Radiation Research Centre (NSRRC).



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Graphical Abstract


Designed synergetic effect of electrolyte additives to improve

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