Application of lithium nonafluoro-1-butane sulfonate (nonaflate) based non-aqueous liquid electrolytes (NALE) in lithium-ion batteries

ABSTRACT

The non-aqueous electrolyte system comprising of the lithium nonafluoro-1-butane sulfonate (LiNfO) as a potential lithium ion-conducting salt in an equivalent binary mixture of propylene carbonate (PC) and 1, 2-dimethoxyethane (DME) as the solvent was explored for the lithium battery applications. The LiNfO based non-aqueous liquid electrolyte (NALE) system showed the highest ionic conductivity of 2.66 x 10^-3 S cm^-1 at ambient temperature, and a potential window stability of ~5 V. The lithium ion cells, Li/NALE//LiCoO₂ were fabricated with the proposed non-aqueous electrolyte. The cell with particular composition of electrolyte delivered a high specific discharge capacity of 154 mA h g^-1 at ambient temperature. The potential advantages of the proposed NALE are discussed in detail.

Keywords
electrolyte; lithium ionic conductivity; electrochemical properties; impedance spectroscopy

1. INTRODUCTION

Though new battery systems are evolving, the research on Lithium ion Batteries (LIBs) is an evergreen area of research interest due to its specific property of high voltage and high energy density [¹-³], which cannot be matched with other battery systems for portable electronic applications. So far, in LIBs, the electrolytes containing the LiPF₆ salt in EC/PC/DMC are mostly used due to its high conductivity and better electrochemical stability [⁴]. However, when the above electrolyte is used, the formation of HF during de-intercalation/intercalation leads to the poor performance of the cells [⁵] and hence for long storage times, it is unsuitable. Moreover, it also shows the thermal instability. Hence, considerable efforts have been made to replace LiPF₆ with other conventional lithium salts. ZHANG et al. [⁶], prepared a new family of Fluorosulfonimide anions, i.e., bis(fluorosulfonyl)imide, [(FSO₂)₂N]⁻, and (fluorosulfonyl) (perfluoroalkanesulfonyl)imide, [(FSO₂)(n-C_mF_2m+1SO₂)N]⁻ (m = 1, 2, 4, 6, 8), and their alkali metal salts. Lithium salts with fluorosulfonimide anions show good thermal, chemical, and electrochemical stability, and do not contain HF. However, the heavy corrosion behavior at the aluminium current collector on the cathode side (around 3.3 V vs Li/Li⁺) restricts its application range.

Fortunately, one of the newest fluoro-lithium salt, lithium nonafluoro-1-butane sulfonate (LiSO₃C₄F₉) has a weak cation-anion interaction and appreciably larger anion size compared to other leading lithium salts such as LiClO₄, LiSO₃CF₃, and LiPF₆ [⁷, ⁸]. The anionic size of the lithium salt has influenced the dissolution property, which in turn facilitate the number of charge carriers, diffusion coefficient and conductivity of the electrolytes [⁹]. Also, the proposed new lithium salt LiNfO possesses much greater thermal and electrochemical stabilities and will be the perfect alternate for LiPF₆ in commercial batteries.

But, in such non-aqueous liquid electrolytes (NALEs) system, the selection of the solvent for incorporating the salt plays a vital role on the electrochemical properties of LIBs. The high dielectric constant and low viscosity of solvents are generally desirable. Among the different carbonate based solvents, propylene carbonate (PC) exhibits excellent properties such as high dielectric constant (64.92) and wide electrochemical stability window [¹⁰, ¹¹]. Therefore, the blending of propylene carbonate with 1,2-dimethoxy ethane (DME) with the dielectric constant and the viscosity values of 7.2 and 0.46 cP respectively, may synergistically enhance the conductivity and electrochemical stability of LIBs [¹²]. Hence in the present investigation, we have chosen PC and DME as binary solvent mixture for LIBs. Few of the reports below show the trends in electrolyte research for Li/LiCoO₂ cells. KONG et al. [¹³], have reported that, the electrolyte with 1M LiPF₆ in dimethyl sulfone (DMS) as a solvent increases the capacity retention rate from the original 38.1% up to 87.1% after cycling for 300 times at 4.45 V for the Li/LiCoO₂ cell. Mostly Li/LiCoO₂ cells are cycled up to a high operating voltage of 4.0 V vs Li [¹⁴], and the above report shows that the voltage window of Li/LiCoO₂ cell could be enhanced by the selection of suitable electrolyte. HONG-BO HAN et al. [¹⁵], prepared the electrolyte of 1.0 M Lithium bis(fluorosulfonyl)imide LiFSI–EC/EMC (3:7, v/v) and compared the Li/LiCoO₂ cell performance with typical lithium salt of LiPF₆ in similar solvent concentration and reported the role of electrolyte. The main objective of the present work is to prepare a new category of NALEs systems for LIBs by varying the volume ratio of the binary solvents and lithium salt LiNfO. We observed that the prepared NALEs show better conductivity, electrochemical stability and cationic transference number. In addition, NALEs using LiNfO presented a promising performance in terms of cycle life and rate capability when tested in LIBs using Li//LiCoO₂ electrode.

2. MATERIALS AND METHODS

2.1 Materials

The precursor solvents such as anhydrous propylene carbonate (PC, 99.7 %), anhydrous dimethoxyethane (DME, 99.5 %) were purchased from Sigma Aldrich and were used without any further purification. The lithium salt LiNfO (95 %) was obtained from the TCI Chemicals. The lithium cobalt oxide material was obtained from Sigma Aldrich and used as such. A series of non-aqueous liquid electrolytes were prepared by dissolving 0.1 M LiNfO in the PC/DME binary solvent mixture (Table 1) and used for determining the Li//LiCoO₂ cell performance.

2.2 Coin cell Assembly

The cathode slurry was prepared by mixing the stoichiometric ratio of LiCoO₂, acetylene black and poly(vinylidene fluoride) (PVdF), in the ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) until smooth slurry was formed. The slurry was then coated on an aluminum foil using doctor blade coating machine. The coated aluminum sheet was dried in vacuum oven for 12 h at 120 °C and then pressed by a rolling machine and taken as the electrode. Lithium was used as both the reference and counter electrode in the half-cells. A monolayer polypropylene membrane (Celgard # PP2075, thickness of 20 μm, porosity of 48 %) was used as a potential cell separator. Both the preparation of NALEs and the fabrication of half-cells (both Swagelok and coin) were carried out in an argon filled glove box, where the moisture and oxygen level were maintained less than 1 ppm.

2.3 Electrical and Electrochemical studies

The conductivity of NALEs were measured using conductivity meter (Systronics, Model: conductivity meter-306) (cell constant K=1.0±10 %) with platinum electrodes. The linear sweep voltammetry (LSV) of the cell was done at a scan rate of 5 mV s^-1. The high conducting NALE was used to examine the cationic transference number using combination measurement of AC impedance and DC polarization techniques. For this measurement, the symmetric Swagelok cells (Liǀhigh conducting NALE electrolyteǀLi) were assembled using lithium electrodes. The impedance spectra were measured using dc polarization technique in the frequency range between 100 mHz and 100 kHz with a signal amplitude of 10 mV. For DC polarization measurement, the constant dc voltage of 20 mV was applied for 8 hours to reach the steady state current. The above characterization techniques of LSV, DC polarization technique, CV, Charge-Discharge and Electrochemical Impedance Spectroscopy (EIS) were carried out by using electrochemical workstation of Bio-Logic sp-300 (France).

3. RESULTS AND DISCUSSION

3.1 Electrical properties

The ionic conductivity of LiNfO/PC/DME NALEs were evaluated by AC ionic conductivity measurement. The ionic conductivity of 0.1 M solution of LiNfO in different volumetric ratio (by v %) of PC and DME binary solvents are shown in Figure 1 (a). As well as, its corresponding conductivity values and volumetric ratios of binary solvents are tabulated in the Table 1. As it can be seen from the Figure 1 (a) and the Table 1, the ionic conductivity of all the prepared NALEs is in the range of mS cm^-1. Amongst all the prepared NALEs, NALE 5 and NALE 1 shows the highest ionic conductivity at ambient temperature which is owing to the solvating ability of DME over PC (DME>PC) which in turn means the greater difference in donor number between DME (26.6) and PC (15.1) [¹⁶]. The obtained results for 0.1 M NALE solution are higher compared to other liquid electrolytes and strongly concurrent with the earlier reported results [¹⁷-¹⁹].

In order to validate the conductivity results as well as to confirm the migration of the ions in NALEs system, we have performed the chronoamperometry analysis for the high conducting sample NALE 1. The Nyquist impedance plot for NALE 1 before and after polarization is shown in Figure 1 (b) and the arrow indicates the direction of the increase in the frequency. It consists of two compressed semicircles at high and medium frequencies and an inclined peak at low frequency. The first semicircle touches Z' at medium frequency with highest resistivity while the second semicircle touches at higher frequency and lower resistivity. The compressed semicircle at high and medium frequencies may attribute to the grain boundary impedance in charge of the transfer process whereas the inclined peak at low frequency is related to the diffusion process. The resistances R_o and R_s were evaluated from the intercept of the semicrircle with real axis (Z′) of the Nyquist plot. The cationic transference number was calculated by means of following expression [²⁰] and mentioned in Table 2.

$$T_{L{i}^{+}}=\frac{I_S(\mathrm{\Delta }V-I_0{R}_0)}{I_0(\mathrm{\Delta }V-I_S{R}_S)}$$ (1)

Where ^‘I_o^’ and ^‘I_s^’ are initial and final steady state current, respectively (Figure 1 (c)). Here, ‘ΔV’ is the applied constant potential (20 mV). Further, it seems from the impedance plot that the resistance became higher after the polarization when compared to fresh cell which was due to the formation of an interfacial layer between the lithium electrode and electrolyte. Its corresponding equivalent circuit is shown as insert Figure 1 (b). The cationic transference number was determined using the above formula and it was found to be 0.39. The observed transference number value clearly demonstrates that the charge transport in NALE 1 is mainly due to the movable lithium ions and hence this electrolyte is a pretty potential candidate as electrolyte in LIBs. The obtained results are in concurrent with the earlier reported result by ZUGMANN et al. [²¹] for LiPF₆ in EC/DEC.

Table 1
Conductivity and Electrolyte stability potential for 0.1M of NALEs

NALE	RATIO OF PC: DME	CONDUCTIVITY mS cm-1	DECOMPOSITION POTENTIAL at 0.1 mA (VOLT)
NALE 1	1v%:1v%	2.65	5
NALE 2	1v%:2v%	2.55	4.6
NALE 3	1v%:3v%	2.26	4.5
NALE 4	2v%:1v%	2.40	4.9
NALE 5	2v%:3v%	2.66	4.7
NALE 6	3v%:1v%	2.18	5
NALE 7	3v%:2v%	2.54	5.3

Table 2
Cationic transference number from chronoamperometry studies for the NALE 1 electrolyte system

SAMPLE	Io	Is	Ro	Rs	$T_{L{i}^{+}}=\frac{I_s}{I_o}$	${T_{Li}}_{Li}=\frac{I_s(\mathrm{\Delta }V-I_o{R}_o)}{I_o(\mathrm{\Delta }V-I_s{R}_s)}$
NALE 1	0.45	0.18	2.72	2.96	0.4	0.39

Figure 1
(a) Conductivity plot of different NALEs system, (b) Impedance spectra before and after polarization measurement, (c) Chronoamperometry studies for the NALE 1 system and the (d) LSV curves of SS/NALE 1/Li cell

3.2 Electrochemical Stability

In order to find the electrochemical stability window of the prepared NALEs system, the coin cell (SS/NALEs/Li) was subjected to linear sweep voltammetry analysis. In general, the electrochemical stability window of liquid electrolyte system depends on the nature of the salt, solvent and electrode materials ²²-²⁴]. In order to determine the effect of solvent/solvent mixture in the NALEs system, the analysis was performed with similar types of electrodes. Hence, electrochemical stability window is expected to depend only on the nature of the salt and solvent. In the present investigation, we have optimized and fixed the cut-off current as 0.1 mA. Figure 1 (d) shows the linear sweep voltammogram of different NALEs which consists of various volumetric ratios of PC and DME. As it can be seen from the Figure 1 (d) that the I-V curve displays almost no residual current in the anodic decomposition voltage, postulating the high purity of the NALEs which is electrochemically sensitive to impurities. Henceforth, the anodic current detected in the curve elucidates the oxidation of NfO− which is the anion of the employed lithium salt [⁸]. The obtained decomposition potential of all the NALEs is given in the Table 1. Amongst, NALE 7 was electrochemically stable for a long time and shows the decomposition potential at 5.3 V. Whereas, in the case of NALE 2, NALE 3 and NALE 5 electrolytes the decomposition potential were 4.6, 4.5 and 4.7 V respectively. This is due to the fact that the solvent DME which is an ether compound and decomposed even at a lower potential than PC [²⁵, ²⁶]. The high conducting NALE 1 electrolyte showed a maximum stability voltage of 5 V, making it suitable for battery applications. The obtained results strongly suggest a way to utilize the NALEs system as potential candidates for the application in all state-of-the-art electrode materials for LIBs.

3.3 Cyclic Voltammetry analysis

In order to validate the reversible nature of prepared NALE system, the LiCoO₂/NALE 1/Li coin cell was subjected to cyclic voltammetry analysis. The resulting cyclic voltammogram is shown in the Figure 2.

Figure 2
(a) CV curve of Li//NALE 1// LiCoO₂ before and after cycling (Scan rate: 0.1 mV/s), (b) CV curve of Li//NALE 1// LiCoO₂ at different scan rates (c) Peak current (I_p) as a function of logarithmic scan rate for Li//NALE 1// LiCoO₂

The reason for choosing NALE 1 electrolyte is owing to its high ionic conductivity and potential stability as discussed so far. Figure 2 (a) represents the CV curve of fresh coin cell (i.e. before cycling) in the potential range between 3 and 4.3 V at room temperature at a scan rate 0.1 mV s^-1. In the case of fresh cell, the first-cycle anodic scan, the main peak occurs at 4.05 V (versus Li) due to de-intercalation of Li ions and the corresponding peak in cathodic scan was at 3.77 V due to intercalation of Li ions. It is widely recognized that the difference between the anodic and cathodic peak voltages (ΔV) relates to the degree of polarization [²⁷]. The ΔV value of 0.29 V was obtained for a fresh cell indicating good reversibility of the charge-discharge reaction. Favorably, the CV showed that, even after 90 cycles, the ΔV remains almost constant. Two more small peaks at 4.17 and 4.21 V were appeared in the CV of fresh cell during the anodic scan. The corresponding cathodic peaks were at 4.01, and 4.13 V. These can be assigned to the reversible phase transitions of Hexagonal (H) ↔ Monoclinic (M) in LiCoO₂. Similar behavior of CV of LiCoO₂ was reported in literatures [²⁸, ²⁹]. Whereas, the prolonged (after 90 cycles) cycled cell also shows a similar type of redox peak with slight changes in which the peaks are shifted towards left. The structural degradations of LiCoO₂ in the entire cycling lead to capacity fading in the cell [³⁰, ³¹]. In addition, it is interesting to note that the cycled cell has higher peak current than that of the fresh cell. However, the presence of clear oxidation and reduction peaks have proved the good reversibility of lithium ion over the fixed potential range [³², ³³]. The reversibility of the cell will be further strengthened by Laviron’s theory [³, ³⁴] as discussed below,

The CV response towards the electrochemical redox reaction of Li//NALE 1// LiCoO₂ at different scan rates between 0.1 and 3 mV s⁻¹ is displayed in Figure 2 (b). It seems that there is no alteration in the CV’s even at high scan rates indicating the fast intercalation –de intercalation behavior. Also, the value of anodic and cathodic currents increases with increase in scan rate and it doesn’t follow a linear shape as shown in Figure 2 (c). According to Laviron’s theory, the linear shape curves are obtained only at high scan rates (>20 mV s^-1) [⁷]. The ratio of anodic peak current to the cathodic peak current is nearly 1 and the positions of anodic and cathodic currents not shifted much as considerable potentials at different scan rates which are confirmed the good reversible reaction of the cell [³⁵]. Hence to conclude that the cathodic deposition and anodic oxidation are facetious at the electrolyte-electrode interfaces and so are investigative of lithium ion conduction in the NALE 1. The obtained results are further strengthened by charge discharge analysis which will be discussed later in the following sections.

3.4 Galvanostatic charge-discharge analysis

It is well recognized that the galvanostatic charge discharge measurement is the most trustworthy and precise method for evaluating the specific capacity of LIB’s components compared to both CV and impedance methods [³⁶, ³⁷]. Figure 3 (a) shows the discharge characteristic profiles obtained for LiCoO₂/NALE 1/Li coin cell at different C-rates such as 0.1, 0.2, 0.5 and 1 C. Since the rate capability is an important factor for battery performance, the coin cell comprising of NALE 1 electrolyte was subjected to a preconditioning cycle with a cut off voltage of charge and discharge between 3.0 and 4.3 V before the execution of the rate capability test at 0.1 C rates. As it can be observed from the Figure 3 (a) that the cycling performance at 0.1 C-rate with a discharge capacity of 154 mA h g^-1 is concededly good. Further, the discharge capacity at 0.2, 0.5 and 1 C rates were 143.12, 119.22 and 44.9 mA h g^-1, respectively for their first initial cycles. This was about 96 - 28% of the theoretical capacity of the active electrode LiCoO₂ (160 mA h g^-1) [³⁸, ³⁹]. The obtained results show that the discharge capacity and the efficiency decreased with increasing C- rate [⁴⁰, ⁴¹] and the capacity fading at higher current rates occur faster than at lower discharge rates as shown in Figure 3 (b). Additionally, the discharge profile clearly shows voltage plateau at ~ 4.1 V which corresponding to the phase transition of Hexagonal (H) ↔ Monoclinic (M) in LiCoO₂. They were observed as a minor peaks in the CV curves at 4.01 V and 4.13 V and these results are quite similar as reported by Szu Lui Tey [²⁹].

Further, the stability of the prepared NALE 1 electrolyte is equitably good. For instance, the cell run at 0.2 C rate between the voltage range of 3 and 4.3 V provides an initial discharge capacity of 143 mA h g^-1 and still maintained 31.41 mAh g^-1 capacity after 90 cycles as shown in Figure 3 (c). Its corresponding coulombic efficiency curve as displayed in Figure 3 (d) was 68 % during 1^st cycle and attained 98 % after 90^th cycle which clearly represents the better capacity retention nature of the cell. The capacity fade was mainly associated with the phase transitions of LiCoO₂ above 4.1 V [⁴², ⁴³] which has evident through the CV result. The results are consistent with the earlier work [³⁹]. Moreover, the coulombic efficiency values slowly increased and stabilized after few cycles. This may be due to the formation of electrode and electrolyte interfaces [²⁷].

Figure 3
(a) Discharge curves of Li/NALE 1/LiCoO₂ cell at different C-rates (b) rate capability of Li/NALE 1/LiCoO₂ cell (c) discharge curve of Li/NALE 1/LiCoO₂ cell at 0.2 C for 90 cycles (d) Columbic efficiency curve of Li/NALE 1/LiCoO₂ cell at 0.2 C rate

3.5 Electrochemical Impedance Spectroscopy

The electrochemical impedance, as well as kinetic behaviors of non-aqueous electrolytes, was analyzed using electrochemical impedance spectroscopy (EIS) in the frequency range of 100 mHz to 5 MHz with an alternating current signal amplitude of 10 mV at room temperature, and it was directly related to the charging and discharging process in LIB. Figure 4 (a) shows the Nyquist plot of the fresh cell (before cycling) using NALE 1 as an electrolyte at an open circuit voltage (OCV) of 2.9 V. Figure 4 (b) shows the Nyquist plot of the cell in the charged state (4.3 V) after 90 cycles. The fresh cell delivers a high impedance value of 223.62 ohm (Total cell resistance, R_all) with one semicircle at the higher frequency side and an inclined peak at lower frequency side. After the 90^th cycle, the impedance value of R_all was 171.35 ohm which consists of two semicircles at higher and middle-frequency range and peak at low-frequency side. Its corresponding equivalent circuit is given in figure 4 (c) and calculated parameters are mentioned in Table 3.

Table 3
Fitted resistance and capacitance values for the cell

SAMPLE	R1/Ω	R2/Ω	R3/Ω	RALL/Ω	CPE1/µF	CPE2/mF	CPE3/mF
Fresh cell (Before cycling)	19.62	204	-	223.62	8.76	-	16.55
After 90 cycle	17.41	30.64	123.3	171.35	7.52	0.27	10.16

Here, R1 is the resistance of the electrolyte and cell components. The semicircle in the high-frequency range was attributed to the surface-film (sf) resistance and the charge-transfer (ct) resistance. However, only one semicircle was seen in the higher to middle frequency range indicating the effects due to sf and ct are not separable and mentioned as R2. The semicircle in the low-frequency range was attributed to the bulk (b) resistance and assigned as R3. A constant phase element (CPE) was used in the equivalent circuit instead of pure capacitance due to the observation of depressed semicircle, which indicates the inhomogeneous surface of the cathode material. The respective constant phase element of CPE1 represents the combined effect of surface-film and double-layer capacitance and CPE2 in the cycled cell represent the bulk capacitance. The CPE3 in the cycled cell was exhibited at very low frequencies and was associated with the Li-ion diffusion process in the cathode. The proposed equivalent circuit and the assignment of resistances and capacitances are consistent with the interpretation of impedance spectra on electrode materials [²⁸].

The fitted data result shows that the value of R1 for fresh cell was 19.62 ohm which slightly decreased to 17.41 ohm after 90 cycles. In the meantime, in fresh cells, the impedance of R2 corresponding to the high-frequency semicircle was generally attributed to the passivating surface-film (Solid Electrolyte Interface, SEI) that forms on the cathode. In cycled cells, the R2 reached 30.64 ohm which was smaller as compared to that of 204 Ohm for a fresh cell. This was due to the ‘formation’ of interface between electrolyte and electrode [¹⁴]. The bulk resistance of R3 was observed as 123.3 ohm. In cycled cell, the capacitance of CPE1 and CPE2 values were in 7.52 µF and 0.27 mF, respectively. The CPE1 value (in the order of mF) was much higher as compared to CPE2 (in the order of µF), because this includes not only the bulk impedance of the electrode material but also the electrolyte trapped in the electrode pores [⁴⁴].

SHANGGUAN et al. [⁴⁵], have studied a similar trend in impedance behavior on Li|LiCoO₂ cells with LiFSI and LiPF₆ after 6 cycles at 0.5 C rate at ambient temperature. The obtained results show the value of 1.5 Ohm (R_el), 113.5 Ohm (R_sf) and 105.8 Ohm (R_ct) for the LiFSI-based cell and 8 Ohm (R_el), 95.7 Ohm (R_sf) and 72.4 Ohm (Rct) for the LiPF₆-based cell. However, in the present investigation, interestingly the surface film resistance was very low even after 90 cycles, which points out its good capacity retention and lower polarization [⁴⁵]. Based on our investigations, the new salt LiNfO based NALE 1 could be the potential candidate as electrolyte for LIBs and better alternative for commercial LiPF₆ based liquid electrolytes in the near future.

Figure 4
(a, b) Nyquist impedance plot of Li/NALE 1/LiCoO₂ cell before and after cycling (c) its corresponding equivalent circuit

4. CONCLUSIONS

The novel NALE electrolyte made from 0.1M LiNfO in the PC/DME binary solvent mixture shows the maximum ionic conductivity of 2.66 x 10^-3 S cm^-1 at ambient temperature for PC: DME ratio of 1:1. The above electrolyte shows the high electrochemical stability window of nearly 5 V making it as most suitable for high voltage applications. The Li/NALE 1/LiCoO₂ cell delivered a very high discharge capacity of 154 mA h g^-1 at 0.1 C with better rate capability. This work opens up the possibility of widespread application of proposed LiNfO based non-aqueous liquid electrolytes in high voltage LIBs.

ACKNOWLEDGEMENTS

Hirankumar Gurusamy acknowledges BRNS, Department of Atomic Energy, Government of India for the financial support through the project (sanction number 2012/34/71/BRNS).

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Publication Dates

• Publication in this collection
  16 May 2022
• Date of issue
  2022

History

• Received
  25 May 2021
• Accepted
  25 Oct 2021