Polymer-based electrolyte for Li-metal batteries

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Lithium-ion batteries: issues and challenges from electrolyte aspect

Lithium-ion batteries (LIBs) have been the successful electrochemical power sources since their first commercialization in 1991 by Sony Inc.2. In general, about 50 % of LIBs from the global battery market (in 2016) have been sold, in which 56 % were used for electric devices and mobile applications. Comparing to the other types of battery, LIBs provide high energy density, long lifetime, without the memory effects, low self-discharge, and existed in various sharps and sizes, as seen in Fig. 4.
Electrolyte does not determine the capacity (energy density or cyclability) but the safety, stability, and the current density of a system.8 The usable of electrode materials require high chemical stability of electrolyte since they are directly in contact with both electrodes. A basic electrolyte needs to be9:
(i) chemically stable with the electrode materials during cell’s operation,
(ii) electrochemically stable with a large potential window,
(iii) thermally stable: the melting and boiling points are higher than the operating temperature,
(iv) high ionic conductivity, low cost, low toxicity, less impact from the synthesis process, and based on the abundant elements.
Fig. 4 The shapes, sizes and components of LIBs in cylindrical (a), coin (b), prismatic (c) and flat (d) form.1
As seen in Fig. 5, during the discharge, Li+ ions from the graphite anode migrate through the electrolyte and insert in the intercalation cathode material thus produces the energy. Otherwise, when the cell is charging, Li+ ions from the positive electrode move through the electrolyte and remain in the opposite electrode. All along with these processes, electrons do not flow through the electrolyte but in the outer load. This technology helps to solve the growing of lithium dendritic during operation, which is the main issue of LMBs.
Fig. 5 Operation principle of rechargeable battery in discharge.
Electrolytes for LIBs are basically classified into 5 groups including the non-aqueous, aqueous, ionic liquid (IL), conducting polymer, and hybrid electrolyte, and most of the commercial LIBs are using the non-aqueous liquid electrolyte.
• Liquid electrolytes
Conventional non-aqueous liquid electrolyte is the mixture of lithium salt dissolved in organic solvents. In a LIB, the macro-porous separator impregnated with the electrolyte is sandwiched between two electrodes to prevent the short circuit and allow the migration of ions. This type of electrolyte offers many advantages due to the low-cost and high efficiency for ionic transport. The mixtures e.g. ethylene carbonate (EC), dimethyl carbonate (DMC), or ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) ensures good ionic conductivity of the electrolyte, meanwhile the corrosion prevention of current collectors and the narrow electrochemical window causing by these solvents are still challenged.10 Commonly, the electrolyte consisted of lithium hexafluorophosphate (LiPF6) in carbonate solvents is common in the commercial LIBs. Nevertheless, the demands of higher energy and safe electrolyte require new approaches for both lithium salts and organic solvents. Through which, the organic compounds contained high oxidation-potential fluorinated molecules become new nominees.
Although high ionic transfer in the liquid state, the safety issue of these electrolytes requires an improvement. The thermal runaway in LIB accompanies by a series of self-exothermic reactions, as described in Fig. 6 including the electrolyte oxidation, separator melt, SEI layer decomposition, electrodes breakdown, etc., and the flammability of organic solvents is an essential factor causing the critical damages.11 The most common lithium salt used for conventional liquid electrolyte i.e. LiPF6 is also unsafe due to its low thermal stability.
For safer LIBs, the approaches of lithium salts and solvents aspects such as (i) using the stable lithium salts, (ii) adding the stabilizer additives, (iii) developing the non-flammable solvents, as well as (iv) using the polymer conducting materials have been widely carried out. Following, the advancements of non-flammable solvents such as using the ionic liquids for dissolving lithium salt or the polymer-based solid electrolytes will be further discussed.

Ionic liquid-based electrolytes

Introduction

The first ionic liquids (ILs) have been discovered by Paul Walden in 1914.12 These molten salts (or molten oxides) compose of ions, that can be organic cations and organic/inorganic anions.13 During the 1990s, ILs having the melting point below 100 °C, usually called as ‘room temperature ionic liquids’ – RTILs, have been widely studied as green solvents for electrochemical applications.14 Various types of molten salt were investigated for energy storage due to the non-volatile, non-inflammable, and environmentally friendly characteristics, which were suitable for safer LIBs.15 ILs played the role of carbonate solvents in the liquid electrolyte for dissolving lithium salts and allowing the ions transfer between anode and cathode.16 They exhibit the liquid state in a wide range of temperatures, with negligible vapor pressure, and high thermal stability.17 The demand for high-security electrolyte makes ILs become one of the most promising solvents for future technology.18
The common IL cations and anions used for electrochemical applications are presented in Fig. 7 including the organic cations e.g. imidazolium, pyridinium, alkylammonium, alkylphosponium, alkylpyrrolidinium, etc., and a large choice from the inorganic anions e.g. the halides ([Cl-], [Br-], [I-]), [PF6-], [NO3-], [BF4-], to the organic anions e.g. [TFSI-], [Tf-]. Based on the literature survey, the choice of IL anion/cation were explored in different systems, including Li-ion, Na-ion, Li-O2 (air), Li-S, and Mg-ion batteries, etc.19 The physical/ electrochemical properties and therefore ionic conductivity of IL-based electrolytes are very sensitive to the choice of cation and anion. In this regard, ILs’ application usually depends on the system and its operated conditions.
Table 1 Several cation/anion combinations used for electrochemical application and their physical properties e.g. meting point (Tm), density (d), viscosity (ɳ), conductivity (σ) at RT.20
The effect of cation/anion natures composed with 1-butyl-3-methyl imidazolium cation [BMIm+] or bis(trifluoromethane)sulfonyl imide anion [TFSI-] on physical properties of IL are presented in Table 1. A strong dependence of cation/anion nature influences the melting point, density, viscosity, and conductive properties of the neat ILs. As reported in the literature, ILs contained the organic cations, such as 1,3-dialkylimidazolium, tetraalkylammonium, phosphonium, trialkylsulfonium, with the soft anions i.e. (PF6-), (BF4-), (CF3SO3-), or [(CF3SO2)2N-] were the promising candidates for electrochemical behaviors.21,22 The choice of IL displays an essential role for dissolving lithium salt, thermal stability, electrochemical behaviors, and the price of the resulted electrolytes. While the physical properties of the blends IL/salt have been investigated, the coordination shells and mechanism of conduction/ diffusion as a function of the coordinated cation were less reported. Investigating these behaviors is one of the main objectives of this work.

Physical and transport properties

Many factors can affect the properties of an IL including the combination of cation/anion nature, and its purity (the impurities content e.g. water or halides in IL).23,24 These properties were studied by a variety of techniques to measure thermal characteristics, viscosity, density, inter and intra molecular interaction (spectroscopy), ionic conductivity, self-diffusion coefficient, electrochemical stability, etc. Here, the essential physical and ionic transport properties of RTILs are discussed.
• Thermal properties
a) Degradation and melting temperature
Most of RTIL used for energy storage has high thermal stabilities. The degradation temperatures (Td) are usually higher than 400 °C, whatever the choice of cation/anion couple. However, the melting point (Tm) is dependent on IL structure, especially on the symmetry of cation, length of the alkyl chain, the charges distribution, as well as the anion nature.14
b) Glass transition temperature
The glass transition temperature (Tg) is an important factor that assigns the mobility of the charge carriers. A low Tg induces high mobility of ions corresponding to high ionic conductivity. When the cation is asymmetric, the Tg value is rather low due to the increase of free volume.
• Density
Density of the RTILs is usually between 1 and 1.6 (g.cm-3).14 The length of alkyl chain and the anion/cation natures significantly influence IL density. The density also varies with the anion nature in an ascending order of BF4- < Cl- ≈ PF6- < CF3SO3- < (CF3SO2)2N-, while the effect of cation is given by the order of pyrridium > imidazolium > pyrrolidinium > quaternary ammonium.25
• Viscosity
The biggest issue impeding diverse/various applications of ILs consists of the high viscosities, which prevent the mass transfer and ionic dissolution in IL. Viscosity of an IL is sensitive to the cation/anion natures, especially for the organic cations due to their large ionic size. A study of Bonhote et al.21 for the IL 1-alkyl-3methyl imidazolium TFSI, with the alkyl length of cation varied from methyl to butyl chain reported that viscosity decreased proportionally to the alkyl length. Thus, the anion gives also an effect on viscosity relating to the relative basicity and the possibility to form hydrogen bonds. The ILs based on small inorganic anions such as BF4- and PF6- are more viscous then the weakly basic TFSI- anion, in which the charge delocalizes over the sulfoxide groups.21 Viscosity data are frequently presented as a function of temperature and are approximately fitted to the Vogel-Tammann-Fulcher (VTF) model.
• Ionic conductivity
Ionic conductivity is an essential factor for electrochemical applications. The conductivities of pure ILs are between 0.1 to 18 mS.cm-1 at RT that are suitable for electrolyte behaviors. Moreover, the dissolution of salt in IL creates a ternary system which increasing the viscosity and reducing conductivity of the blend. The decrease of ionic conductivity is proportional to the salt concentration in IL. The evolution of conductivity with temperature follows a VTF model and shows an intimate correlation with the viscosity. For a given anion, conductivity of the organic cations vary in the order of 1-alkyl-3-methylimidazolium > N,N-dialkylpyrrolidinium > tetraalkylpyrrolidinium.26 The study of M. Vranes et al.27 about different physical properties of two ILs based TFSI i.e. 1-butyl-3-methylimidazolium [BMIm] and 1-butyl-3-methylpyrrolidinium [BMPyr] cations with [TFSI] anion, as a function of temperature showed that imidazolium-based IL exhibited lower density and viscosity than that of pyrrolidinium IL, and allowed higher ionic conductivity.
The ionicity of an IL is carried out by plotting the molar conductivity with the corresponded fluidity in the Walden plot. The diagrams of several imidazolium-based ILs are showed in Fig. 8.28 The deviation from the reference line, which represents the data of a diluted solution of KCl, helps to determine a ‘good’ or ‘poor’ IL. In the ideal case of KCl, the ions are completely dissociated, and how far from this reference line, how low the ionic dissociation.
• Electrochemical stability window
By definition, the electrochemical stability window is the range of potentials in which the electrolyte is neither reduced nor oxidized at the electrode surfaces. Most RTILs used for energy storages have wide electrochemical window, where the oxidation of anions occurs at high potential and the deposition of organic cations is found at lower voltage. Although the factors that can affect the electrochemical stability of IL i.e. its purity, the tiny variation in anodic potential limit was found for several common anions, such as BF4-, PF6-, Tf-, TFSI-, etc.25 Nevertheless, in the cathodic scan, the reduction potential was variable with the cation natures, for example: 1-alkyl-3-methylimidazolium cation was less stable in reduction than N,N-dialkylpyrrolidinium or tetraalkylpyrrolidinium cations.29 The non-haloaluminate ILs usually show good electrochemical stability, usually in range of 4.5 to 6.0 V.
• Cationic transference number
Another factor of interest is the suitable ionic transport of IL-based electrolytes, especially the alkali cations for battery applications. In principle, the blend of x molar LiX salt dissolved in (1-x) molar of A+X- ionic liquid, forms a composition of xLiX (1-x)AX, and the Li transference number is calculated following this relation30:
Where uLi, uA+, uX- are the ionic mobility of Li+, cation A+, and anion X-, respectively, tLi is given by the ratio between lithium mobility and the mobility of total charge species in the blend. As a conclusion, Li transference number in ILs is lower than in organic solvents due to higher viscosity of IL.
• Self – diffusion coefficient
The fundamental characteristics including the degree of ionic association, diffusion coefficient, and ion-ion interaction have not been clarified in the literature. The ionic diffusion coefficient (D) presents the mass of a substance diffusing through a unit surface in a unit time. Diffusion process is depended on the molecular size of ions, its properties, temperature, and environmental pressure. In fact, two common methods, such as electrochemical and pulsed-gradient spin-echo nuclear magnetic resonance (PFG-NMR) were used to measure the diffusion coefficient of ILs. In most of the studies, the NMR was used as a noninvasive method, in which the self-diffusion coefficient of each charged species was measured separately.
However, the Coulombic attractive force in IL would associate the ions to form ion pairs or ion aggregates, and the equilibrium of attractive/repulsive interactions of ions, at a defined temperature, always kept the charged species in balance. Meanwhile, the NMR measurements can only detect a nucleus but cannot distinguish the ions and their associated forms. The D values obtained by NMR method are an average of self-diffusion coefficient of ions and their associated ions.31
The relationships between coefficient diffusion, viscosity, and ionic conductivity are investigated using the Nernst-Einstein equation. In 2006, the cationic transport number, t+ = Dcation/(Dcation+ Danion), for the non-haloaluminate RTILs was found between 0.52 to 0.6 by Tokuda et al.32 This group also proved that the cations had higher diffusion coefficients than the anions at low temperature and approached at high temperature.

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Lithiated electrolytes based on IL

Some RTILs can dissolved the lithium salts, which is an addition point for the green electrolytes of LIBs.14 Actually, for an overall IL study, the physical properties and conductivity behaviors of the neat ILs or the binary systems combined a dissolved salt in IL were observed as a function of temperature, salt concentration, IL cation/anion natures, or the operational conditions, etc.16 Moreover, some of them using the molecular dynamic (MD) simulation to further determine the solvated shells of coordinated cation i.e. Li+ in IL. Here, a short literature review of the lithiated ILs for electrolyte aspect are reported.
The every first innovation in lithium batteries using imidazolium – TFSI ionic liquid with the LiTFSI salt as electrolyte has been reported in 1997.33 After that, various studies using the binary system IL/lithium salt as electrolyte for LIBs showed good cycling stability. As investigated in the literature, the salt concentration dissolved in IL gives the direct impacts on IL physical, conductivity, and diffusivity properties, and changes the final performance of a battery. In fact, low salt concentration can be not sufficient for supplying the lithium ions in electrolyte, but higher concentration of salt caused a failure in conductivity related to higher viscosity obtained.
Followed J. Pitawala et al.34, ionic conductivity of the binary system [Li][BMIm][TFSI] decreases which increasing the salt concentration due to higher viscosity. These results fitted well to the VTF equation. The presence of salt augments the Tg and the Tg values increase with the amount of Li+ doping. Thus, the solvation of Li+ ion in TFSI-based IL was presented by J-C. Lassegues et al.35 in 2006. In this work, the mixture of (1-x)1-ethyl-3-methylimidazolium bis(trifluoromethane) sulfonylimide [EMIm][TFSI], and x LiTFSI were characterized by Raman spectroscopy at different concentration of Li+, below x = 0.4. They showed that intensity of the bands indicated the transoid conformers of TFSI- (at 341 and 297 cm-1) decreases with the evolution of x, and that the bidentates Li+ coordinate with two oxygen atoms bounded to different sulfur atoms in the cisoid anion conformations at x ~ 0.2 to form [Li(TFSI)2]- complexes appear and increase.
Later, S. Duluard et al.37 studied the binary system [Li][BMIm][TFSI] with the salt faction x < 0.2, and showed that the transport properties decreased while increasing the value of x. In detail, all of the diffusion coefficients for Li+ and IL cation/anion decreased significantly at high salt fraction (Fig. 10). Moreover, the lithium coordination numbers, determined from the Raman spectrum of the IL-based electrolytes, were about two anions for one Li+ ion. The Li+ was tetrahedrally coordinated to four oxygen atoms of different sulfur atoms of two anions and formed [Li(TFSI)2]-anionic cluster. While increasing the salt concentration may induct to the formation of bigger ionic cluster such as [Lim(TFSI)n](n-m)-.
Fig. 10 The self-diffusion coefficient of proton, fluor, and lithium measured by PGSE-NMR for the blends of BMIm TFSI with LiTFSI as a function of salt concentration. 38
In 2010, A. Andriola et al.39 studied the dissolution enthalpy of [Li][BMIm][TFSI] system, using a ‘coffee-cup’ calorimeter, as described in Fig. 11. The salt LiTFSI was dissolved in IL using a magnetic stirring, and an isolated rubber on the top of the system. The temperature versus time was recorded at a constant pressure. They reported that the dissolution of LiTFSI in IL as solvent included three following processes: an endothermic breaking the attractions of LiTFSI salt, another endothermic breaking the attractions of IL ions and an exothermic related to the solvation process. Fig. 11 Schematic view of a ‘coffee-cup’ calorimeter and the dissolution process of LiTFSI in BMIm TFSI. 39
As considered, the solute LiTFSI and solvent share the same anion and that the Li+ ions are solvated by only the [TFSI-] anions. A value of -18.07 kJ.mol-1 of dissolution enthalpy indicated that solvation enthalpy of Li+ was higher than the crystal lattice enthalpy of LiTFSI meaning that each Li+ ion can be solvated by multiple [TFSI-] anions to form the [Li(TFSI)2]-, [Li(TFSI)3]2- or [Li(TFSI)4]3- complexes. At low concentration of salt, the anionic cluster [Li(TFSI)2]- seems to dominate in the blend IL/salt.
By both experimental and MD simulations, J-C. Lassegues et al.40 proved that not only in EMIm TFSI but also in the longer alkyl chain such as: BMIm TFSI and BMMIm TFSI doped LiTFSI, from low to medium Li+ concentration (under x = 0.2), the presence of [Li(TFSI)2]- complexes is confirmed40. At higher concentration, the formation of [Lim(TFSI)n](n-m)- aggregates with n = 2m – 1 were suggested. However, these aggregates provided the challenges to identify due to the limit of the IR and Raman spectroscopies.
The coordination of Li+ with TFSI-, studied by H. Liu et al.41, showed that each Li+ was coordinated with four oxygen atoms from different anions. At low molar fraction of lithium, the oxygen atoms from three TFSI- ions (two monodentate and one bidentate) coordinated with Li+ to form the [Li(TFSI)3]2- complexes. When increasing the concentration of lithium, about four anions were found around each Li+, which came from four monodentate TFSI- anions. These rigid structures limited the mobility of all charged species and leaded to the low diffusivity/conductivity of the combinations.
In brief, the coordination number of anions in the TFSI-based alkyl-immidazolium ILs change with the lithium concentration. The Li+ ion was coordinated with at less four oxygen atoms from different TFSI- anions.38 The aggregation of two lithium atoms with three anions was observed at low temperature and could be stable up to ten nanoseconds.42 Its presence increases the viscosity and resulting in a decrease of conductivity. However, the self-diffusion coefficients measured by PFG-NMR are not completely linked to the mobility of complexes but also the free ions and the aggregates, because this technique can not distinguish the difference between them.

Table of contents :

Chapter 1 Literature review
1. General context
1.1. Lithium metal batteries
1.2. Lithium-ion batteries: issues and challenges from electrolyte aspect
2. Ionic liquid electrolyte
2.1. Introduction
2.2. Physical and transport properties
2.3. Lithiated electrolytes based on IL
2.4. Ionic liquid doped alkali/alkaline-earth elements
3. Solid polymer electrolyte
3.1. Lithium salts
3.2. Poly(ethylene oxide) based electrolyte
3.3. Single-ion polymer electrolyte based on PEO
3.4. Application of SIPEs in all-solid-state lithium metal batteries
4. Electrolyte approaches for future rechargeable batteries
5. The aim of this work
References
Chapter 2 Ionic liquid-based electrolytes
1. Introduction
2. Salt solubilization
3. Effect of salt concentration
3.1. BMIm TFSI and the binary system [Li][BMIm][TFSI]
3.2. The binary system [Cs][BMIm][TFSI]
3.3. Conclusions
4. Alkali/ alkaline-earth based BMIm TFSI
4.1. Thermal characteristics
4.2. Density and viscosity
4.3. Ionic conductivity
4.4. Self-diffusion coefficient
4.5. Cisoid and Transoid TFSI conformers
5. Conclusions
References
Chapter 3 Polymer-based electrolyte for Li-metal batteries
1. Cross-linked single-ion conducting polymer (SICP)
1.1. Syntheses of ionic block (ionomers)
1.1.1. Synthesis of Ip-SO3
1.1.2. Synthesis of Ixp-SO3-db
1.1.3. Synthesis of Ip-TFSI
1.1.4. Synthesis of I1000 p-TFSI-db
1.2. Characterization
1.2.1. Molar mass and cation exchange efficiency
1.2.2. Cross-linking degree and NCC dispersion
1.2.3. Thermal properties
1.2.4. Conductive and transport properties
1.2.5. Electrochemical stability
1.2.6. Lithium plating/stripping test
1.2.7. Cycling tests
1.3. Conclusions
2. Multi-block copolymer
2.1. Synthesis of Coxp-SO3
2.1.1. Synthesis of FPES block
2.1.2. Synthesis of Cox p-SO3
2.2. Characterization
2.2.1. Thermal properties
2.2.2. Conductivity
2.2.3. Electrochemical stabilization
2.2.4. Lithium transference number
2.2.5. Lithium dendritic growth test
2.2.6. Cycling tests
3. Conclusions
References
Chapter 4 Impact of alkali cations on conductivity behavior of polymer electrolytes
1. Introduction
2. Cross-linked ionomer
2.1. Thermal properties
2.2. Conductivity
3. Multi-block copolymer
3.1. Thermal properties
3.2. Conductivity
4. Conclusion
References
Conclusions and perspectives
Annex
A. Synthesis part
1. Synthesis process
2. Polymer film casting
3. Ionic liquid/ salts preparation
B Characterization techniques
1. Spectroscopy
2. Thermal and physical properties
3. Electrochemical properties

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