Electrolyte Reactions of the LIBs

With the advent of the EV and HEV, LIBs must be operable at challenging conditions, especially if they are prone to damage even at normal conditions. The LIB systems are sensitive at temperatures near 100° C at which point several heatgenerating, irreversible exothermic reactions take place. These complex processes of heat generation inside the cell can trigger more inimical reactions that can even disseminate to the nearby cells. The rapid increase of the internal battery temperature should be suppressed, which is potentially dangerous. Various studies have been carried out to understand the thermal runaway dynamics from which they have found the electrolytes guilty. This is governed by the fact that the interactions and the conditions of the LIB components with the electrolyte, other than the characteristics of the materials incorporated, do reflect in its performance [9, 56-60].

Thermal Decomposition of Electrolytes

LiPF6 has been the widely used salt, and this is unlikely to change until and unless an optimized lithium salt is formulated. This lithium salt undergoes dissociation, and their products are found to react with the organic electrolytes that are omnipresent in the conventional electrolyte systems. (Equation 6.1)

The dissociation product, PF5, is a strong Lewis acid and readily reacts with the conventional organic solvents even in the small amount of moisture content. (Equation 6.2-6.3) [27, 61]

The proposed mechanism of the decomposition of the LiPF6 salt with organic electrolytes such as EC and DEC is shown in Figure 6.5. Their decomposition pathway at elevated temperatures is shown in Figure 6.6. The compounds, such as hydrogen fluoride, lithium fluoride, carbon dioxide, ethylene, alkyl flourides, dialkyl ethers, fluorophosphates, flourophosphoric acids, and oligoethylene oxides are found to be the thermal decomposition products of the electrolytes [26, 62, 63].

The thermal decomposition of LiPF in carbonyl solvents,

FIGURE 6.5 The thermal decomposition of LiPF6 in carbonyl solvents, (a) Diethyl carbonate (DEC) and (b) ethylene carbonate (EC). Adapted and reproduced with permission from reference [26]. Copyright 2005 IOP Science.

The decomposition pathway of LiPF at HTs. Adapted and reproduced with permission from reference [63]. Copyright 2014 Elsevier

FIGURE 6.6 The decomposition pathway of LiPF6 at HTs. Adapted and reproduced with permission from reference [63]. Copyright 2014 Elsevier.

Thermal Reactions of Electrolytes with Electrode Surface

The electrolytes’ thermal reactions with the electrode materials at its interface are considered to be a lethal cause for the prevailing thermal instability of LIBs. As a result of the decomposition due to the initial reaction of the anode with the electrolyte consisting of the salt, organic solvent(s), and additives, if present, form a stable SEI. It is composed of lithium alkyl carbonate (LiOCOOR), lithium alkoxide (ROLi), polycarbonates, and ethers formed as a result of the decomposition of the electrolyte system. Even though the role of the SEI is to prevent the further reduction of the anode with the electrolyte, which it does not fail to do, it is, however, unstable at higher temperatures. The thermal stability of the SEI is affected by the attack of PF, that is formed as a result of the decomposition of the LiPF6 salt. Upon aging, the SEI thickens, and a change in composition is observed. The passivation layer, which contains higher concentrations of several fluorine and phosphorus species, such as lithium fluoride (LiF), lithium fluorophosphates (LixPFy and LixPFyOz), when thermally abused, increases the cell impedance. This formation of nonuniform impedance values are also portrayed by the cathode-electrolyte interface, which contributes to the power fade in the LIB system [26, 62, 64].

Thermally Stable Electrolytes

The electrolyte components mentioned above that are incorporated in the LIB electrolyte system have their own role to play in the HT paradigm. Recently, Wang et al. [65] developed a satisfactory flame-retardant solvent as a nonflammable electrolyte to replace conventional organic solvents. This highly concentrated electrolyte, composed of voguish flame retardant, trimethyl phosphate (TMP) and lithium bis(flourosulphonyl)imide (LiFSI) salt, formed an unusual passivation layer (Figure 6.7), which showed negligible degradation; therefore, it is a leap toward safe battery electrolytes.

The action of additives on the electrolyte system have demonstrated a significant difference over the additive-free electrolyte at a temperature of 55° C. This HT cyclability was shown by a maleimide-based branched oligomer with the conventional electrodes and electrolytes, such as LiCoO, (LCO), mesocarbon microbead (MCMB) and LiPF6 in EC:DEC [66]. With respect to the ability to stabilize LiPF6, a similar study involving a benzimidazole anion in the lithium salt as the additive portrayed a good battery performance at 60° C. This anion took part in a Lewis acid- base reaction with PFS that suppressed the unwanted side reactions and helped in the formation of a stable SEI. This additive salt showed no difference in its performance with the change in the cathode component [67, 68]. The borate-based salt, LiDFOB were reported capable of improving cycling performances at elevated temperatures of 60° C along with the suppression of thermal deterioration due to its synergistic phenomenon with LiPF6 with different types of electrodes [30, 33]. Lithium tetra- flouro oxolato phosphate (LTFOP) is a similar salt that is used as an electrolyte additive that shows beneficiary results at 55° C along with good capacity retention [69]. Several lithium salts with good cycling performance, rate performance and SEI forming ability at elevated temperatures of 60° C [70] and 80° C, such as lithium

Electrolyte design concept for a safer battery,

FIGURE 6.7 Electrolyte design concept for a safer battery, (a) Schematic diagram of battery explosion caused by the ejection of flammable electrolyte vapor heated following thermal runway, (b) intercalation behavior of cations into a carbonaceous anode in various electrolytes. A conventional EC-based electrolyte passivates the anode via preferential reduction of the EC solvent over anions, but its high flammability poses a severe safety risk. An electrolyte with nonflammable solvents (conventional nonflammable electrolyte) generally cannot passivate the anode to cause continuous solvent decomposition or solvent cointercalation. A concentrated electrolyte with nonflammable solvents can effectively passivate the anode via the formation of salt-derived SEI while functioning as a fire-extinguishing material. Adapted and reproduced with permission from reference [65]. Copyright 2018 Springer Nature.

dodecaflourododecaborate (Li^B^F^/L^DFB) and LiBF2S04, were investigated to replace LiPF6 [34].

The ionic conductivity, which is a major criterion for the electrolytes in LIBs, has been found to be satisfactory for polymer-based and single-ion materials. This novel polymer electrolyte, lithium polyvinyl alcohol oxalate borate (LiPVAOB), had an increase in ionic conductivity with respect to temperature in its Arrhenius plot [32]. There has been special interest in a new class of lithium-ion conductors based on rare earth elements that show good conductivity at elevated temperatures of above 600° C. Lithium samarium holmium silicate is one such material with good ionic conductivity at 850° C with the possibility to replace conventional electrolytes for HT applications [71]. To address the thermal runaway problems in LIBs, which use liquid electrolytes susceptible to leakage, researchers are investigating solid-state lithium systems using unconventional electrolytes, especially over a wide temperature range. Lithium phosphorous oxynitride (LiPON) is one such aberrant material over the traditional ones that has shown increased conductivity at higher temperatures when studied in the range 20° C-200° C [72].

Novel developments in the use of polymer materials as solid electrolytes, with optimum ionic conductivity, has had success, making them also suitable as HT materials in LIBs. One such material is a graft copolymer electrolyte made of poly(oxyethylene) methacrylate-g-poly(dimethyl siloxane) (POEM-g-PDMS) with lithium triflate salt, which showed good charge-discharge cycling at 120° C [73]. The study on polymer blends, such as polyethylene oxide/polyvinylidene diflouride (PEO/PVdF), in lithium salt has also resulted in finding good substitutes for liquid systems with amiable performance in ionic conductivity above room temperature [74]. The compatibility of polymer-based nonflammable material, such as perflouri- nated analogue of PEO with the (LiTFSI), was studied and proved to be an excellent candidate for intrinsically safe electrolyte system in LIB [75]. Along with good ionic conductivity, a stable electrochemical stability window was displayed by a polycarbonate-based material at 60° C [76].

The gel polymer electrolytes (GPE) are considered as an effective electrolyte alternative due to its various advantages [Figure 6.3). A PVdF-based GPE exhibited a good thermal stability individually at 150° C and, when coupled with non- conventional electrodes, it was extended up to 200° C [77]. A modification on this system using a nonflammable material, phosphonate-based copolymer, have also been reported recently, which are intrinsically safer than PVdF-based GPE [78]. Shin et al. [79] prepared a polyethylene glycol (PEG) cross-linked with poly(vinyl pyridine)-PEG-poly(vinyl pyridine) (PVP-PEG-PVP) copolymers with a good ionic conductivity at 80° C. This conductivity increase was found to be 33 times the value of polyfvinylidene fluoride-co-hexafluoropropylene)(PVdF-co-HFP)-based electrolytes, w'hich makes these PEG-based GPEs superior in terms of thermomechanical properties. Recently, another triblock polymer of polystyrene-poly(ethylene oxide)- polystyrene (PS-PEO-PS) on PVdF exhibited good thermal dimensional stability with little shrinkage at temperature up to 260° C [80]. Considering that GPE lacks mechanical stability, Lv et al. [81] fabricated succinonitrile (SN) containing GPE with a polyurethane acrylate (PUA) skeleton with LiTFSI salt. This electrolyte showed mechanical robustness along with good ionic conductivity at the temperature of 50° C compared to the normal electrolytes.

The addition of fillers has also contributed to the thermal stability of polymer- based electrolytes [82]. Composite polymer electrolytes (CPE) with efficient fillers, such as titanium dioxide, alumina, and lithium aluminate, have shown good transport properties at temperatures in the range of 80° C-90° C with PEO matrix [83]. The use of the heat resister polytetraflouroethylene (PTFE) as filler in CPEs in the form LiBOB-SN-PEO-PTFE and LiCF,SOrPEO-PTFE tend to increase the thermal feasibility at 160° C with good ionic conductivity [84]. A CPE consisting of cyanoethyl-(3- polyvinyl alcohol (PVA-(3-CN) with optimum amount of PC operated well at 120° C in LFP/Li half cells [85]. Apart from the ion-transporting ability, the electrolytes can also function as separators if constructed in a manner consistent with the abusive condition that the battery undergoes. A GPE with inorganic nanoparticles, such as SiO, and A1,03, were found to overcome the problem of thermal shrinkage in the existing separators [86]. The silica nanoparticle has proven to be a good electrolyte additive with greater than 80% capacity retention in half-cell study at 60° C [87].

The IL-based electrolytes have found a role in replacing the conventional electrolytes with similar electrochemical performance because of their nonvolatile nature and good ionic conductivity. Several imide-based ILs, such as N-butyl-N- methylpyrrolidinium bis(triflourosulphonyl)imide (PYR14TFSI) [88], N-propyl- N-methylpyrrolidinium bis(triflourosulphonyl)imide (PYR,_,TFSI) [89], and 1-butyl-l-methylpiperidinium bis(triflourosulphonyk)imide (Pipl4TFSI) [90] in LiTFSI salt, have portrayed good cycling ability over the temperature range of 60° C-80° C. This has been accomplished not only with lithium iron phosphate (LFP) electrodes but also with the unconventional tin oxide (SnO,) electrode. Combining ILs with poor capacity have shown no short-circuiting at 50° C, making them a safe system [91]. A practically adequate binary system composed of ILs and organic solvents has proven its workability from RT to 60° C [92]. The temperature effect of binary IL has reported an increase in charge-discharge specific capacity with temperature [93].

The compatibility between IL and polymer matrix has been explored since a long time, in which systems without thermal degradation at elevated temperatures up to 205 °C have been studied with conductivities upto 4.1x10-- S cnv1 [94]. The ionic conductivity of an imide IL based GPEs increased with rise in temperature from 25 °C to 95 °C from 0.64xl0-3 S cnv1 to 4.8xlO-J S cm-1 [95]. The electrolyte systems with imidazolium IL with different polymers have showed thermal stabilities near 300 °C which proves themselves to function as non-flammable safe electrolytes [96, 97]. An ionogel made up of IL, its lithium salt, and silane coupling agent, is not only reported to be stable at 195 °C but also exhibits high capacities till temperatures upto 90 °C [98].

The conventional cathode for LIB system has been lithium cobaltate (LCO), ever since its discovery by Yoshino et al. [99] and into its commercialization in 1991 [100]. However, LFP have attracted attention due to its high power capability, nontoxic nature, and thermal stability [101]. The study of IL-based GPE with LFP cathode and lithium metal as an anode has reflected appreciable ionic conductivities from RT to 50° C along with no significant discharge capacity loss [102]. A similar electrolyte with a different IL showed thermal stability with good ionic conductivity up to 80° C in addition to the good cycling performance at 60° C [103]. Although the GPEs tend to restrict the fluidic nature of ILs, their mechanical instability is addressed by the use of ceramic fillers. The incorporation of dry bentonite clay [104] and hexagonal boron nitride (h-BN) [105] particles in piperidinium-based IL exhibited good wetting properties along with an increase in the electrochemical stability window. These were studied with LTO half-cells with a wide thermal stability from RT to 120° C and 150° C with extended cycling stability (Figure 6.8), ensuring their practical usage under extreme requirements. The development of HT electrolytes in LIBs has expanded to various dimensions in which 3D printing of the electrolytes are even explored [106].

The cycling stability of Li/LTO half cells assembled with

FIGURE 6.8 The cycling stability of Li/LTO half cells assembled with (a) bentonite clay (cell tested at 120° C at current density of C/3) and (b) h-BN- (cell tested at 150° C at current density of C/2) based Quasi Solid State Electrolyte (QSSEs). Although there are changes in the electrode kinetics over time, the electrolyte is electrochemically stable, and the capacity fade is negligible. Adapted and reproduced with permission from reference [104, 105]. Copyright 2015 American Chemical Society and 2016 Elsevier.

 
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