Lithium Salts

In a mixture of organic based solvents, LiPF₆ is the dominant lithium salt still used in the majority of the LIBs manufactured today; however, the formation of the corrosive product at elevated temperatures are the main reason for their electrochemical degradation [13, 26]. Figure 6.2 represents the common lithium-salt structures, and

TABLE 6.1

Commonly Used Organic Solvents in LIBs and Their Structures and Properties [22-24]

Organic Solvents	Structure	Molecular Mass g mol'1	Melting point °C	Boiling point °C	Viscosity cP 25 °C	Density g cm -' 25 °C
EC		88.06	36.4	248	1.93	1.32
PC		102.08	-48.8	242	2.53	1.2
DMC		90.08	4.6	90	0.589	1.0632
DEC		118.13	-73.3	126	0.75	0.9690
EMC		104.10	-53	110	0.648	1.0063
Acetonitrile		41.05	-48.8	81.6	0.341	0.7768
V-BL		86.09	-43.1	204.8	1.73	1.99

Table 6.2 shows their respective flaws. With the aim of upgrading these prevailing hitches, efforts are being made to develop new' lithium salts that can operate on a long-term basis at higher temperatures. LiBOB is an unconventional salt with qualities such as low toxicity, good solubility in organic solvents, and good electrochemical and thermal stability, which makes them an attractive alternate lithium salt for

FIGURE 6.2 Chemical structures of commonly used lithium salts, (at Lithium hexa- fluorophosphate (LiPF₆), (b) lithium perchlorate (LiC10₄), (c) lithium tetrafluoroborate (LiBF,), (d) lithium hexafluoroarsenate (LiAsF₆), (e) lithium trifluoro methane sulpho- nate (Lithium triflate-LiCF,SO,), (f) lithium bisftrifluorosulphonylimide) (C,F₆LiN0₄S₂), (g) lithium tris(trilluoromethanesulphonyl) methide (C₄F₉LiO), (h) lithium bis(oxalate) borate (LiB[C₂0₄]₂), (i) lithium difluoro (oxalato) borate (C₂BF₂Li0₄), and (j) lithium difluoro(sulphato) borate (LiBF₂S0₄).

the LIB electrolyte system. These unique properties that contribute to form a stable and less resistive SEI can thereby prevent anodic corrosion at HTs. In comparison with LiPF₆, LiBF₄, and lithium bis(triflouromethane)sulphonimide (LiN(CF,S0₂)₂/ LiTFSI), this feature is ascribed to its bulky BOB anion in its structure. It is also evident from Table 6.3 that the higher decomposition temperature and its harmless decomposition products make LiBOB a thermally feasible lithium salt [27-29].

TABLE 6.2

Limitations of Common Lithium Salts Employed in Electrolytes for LIBs [35-38]

TABLE 6.3

Thermal Stability of Commonly Used Lithium Salts in Electrolytes for LIBs [37]

Lithium Salt	Structure	Melting Point (°C)
LiPF6		200
LiTFSI		234
LiBFj		310
LiBOB		>400

FIGURE 6.3 The nature and characteristics of different types of electrolytes employed in commercial LIBs.

Lithium diflouro(oxolato)borate (LiDFOB), an alternate to LiBOB, has stable fluorine atoms in its structure and has lower impedance values, stable SEI forming ability, and good performance at higher temperatures [30-33]. Recently, a novel salt named lithium diflouro(sulphato)borate (LiBF₂S0₄) has also found its place in the lithium salt genre as an excellent FIT candidate [34].

Polymer Electrolytes

Since the discovery of ion-conductive, solvent-free polymer electrolytes (PE) and their application in electrochemical systems [39,40], various efforts have been made to replace the flammable, volatile liquid systems as electrolytes in LIBs. PEs are basically comprised of a polymer matrix with a uniform dissolution of salts in them. In order to serve as an electrolyte in electrochemical storage systems, they should meet certain requirements, such as (i) good ionic conductivity in the range >10'⁴ S cm¹, (ii) good mechanical strength, (iii) wide electrochemical stability window (4-5 V vs. Li/Li⁺), (iv) good interfacial contact with the electrodes, and (v) good chemical and thermal stability. The classification of electrolytes based on their nature and characteristics are represented in the Figure 6.3. Without compromising the advantageous qualities of these systems, creating thermally feasible PEs is an exigency for high- end applications [41-48].

RTILs

Ionic liquids (ILs) are fused salts with melting points below 100° C, and its structure is made up of large cations with a delocalized anion. Because these low-temperature molten salts consisting of cations and anions exist in a liquid/molten state at room temperature (RT), they are also known as RTILs. These RTILs, which were accepted as an alternate option for organic solvents, found their practical platform of application in 1992 [49], even though they were reported earlier by Hurley et al. [50] and later by Wilkes et al. [51]. The factors that make these ILs a good component in LIB electrolyte systems are their nonflammability, meagre volatile nature, and good terms of stability chemically, electrochemically, and thermally. Figure 6.4 represents

FIGURE 6.4 The structures of cations and anions of ILs (a) imidazolium, (b) pyridinium, (c) piperidinium, (d) pyrrolidiniurn, (e) quaternary ammonium, (f) phosphonium, (g) sulphoniurn, (h) thiazolium cations, (i) bis(trifluorosulphonylirnide), (j) tris(trifluoromethanesulphonyl) methide, (k) hexafluorophosphate, (l) tetraborofluoride, and (m) trifluoro methane sulphonate anions.

the commonly used cations and anions of the ILs. With the aim to increase solubility of ILs with the lithium salts in the electrolyte systems, ILs are chosen with the same anion as that of the lithium salt. Thus, RTILs play a prominent role in the development of safe electrolytes when added with suitable components [52—55].