Electrolytes for High Temperature Lithium-Ion Batteries: Electric Vehicles and Heavy-Duty Applications

Leya Rose Raphael, Neethu T.M. Balakrishnan, Akhila Das, Nikhil Medhavi, Jabeen Fatima M. J., Jou-Hyeon Ahn, Prasanth Raghavan

Introduction: Background and Driving Forces

“Our house is on fire.” These were the words remarked by a 16-year-old environmental activist, Greta Thunberg, that pinpoints the climate crisis our world is currently facing [1]. This irreversible climate change has been the result of various factors among which fueled vehicle emissions has contributed greatly. Setting aside the folly and ignorance on this matter, a great deal of effort has been put forward in both research and application to recuperate. The sustainable energy foundation will be strong and efficacious if the storage technologies of chemical, electrical, mechanical, electrochemical origin are robust. Out of these systems, the electrochemical storage of energy using secondary batteries are at the forefront. For nearly three decades, lithium-ion batteries (LIBs) have found a prominent place among the electrification of vehicles and powering of utility grids and consumer electronics, replacing other battery systems, such as nickel-cadmium and nickel metal hydride (NiMH), because of their non-toxic nature , higher energy, and low-cost [2]. In order to replace fos- sil-fuel-based vehicles with highly efficient and reliable electric vehicles (EV) and/ or hybrid electric vehicles (HEV), safety of the energy storage system is considered a basic requirement. The major drawback of LIBs in EVs is the thermal decomposition of the battery components due to overcharging and/or overheating. Even though many measures are adopted to avoid these, the improvisation of the battery system itself can yield better results, which can be made possible by using components that can withstand high temperature (HT) [3]. Being the only liquid component in conventional LIBs, it was easily established that the electrolyte is very unsafe when subjected to extreme conditions [4]. This is evident from various unfortunate incidents of recalls of mobile phones and laptops and explosions during transportation and in EVs [5-8]. The thermal runaway triggered by electrolytes at higher temperatures can easily lead to the downfall of LIB technology, which offers high power density, energy density, and high capacity when handled with skill (Figure 6.1) [9,10] et al. et al.. This chapter discusses electrolytes’ existing deteriorating chemistry and their improvisational materialistic aspects in the LIB research.

The Role of Electrolytes

The choice of electrolytes is of paramount importance because it is the only component physically in contact with the other components of an electrochemical system. A good electrolyte is considered to be electrically insulating and ionically conducting. The ionic transport should be high at higher temperatures and should be negligible during storage conditions in order to avoid self-discharge. To steer clear of electrolyte degradation, the electrolyte should be stable within its electrodes’ potential, that is, it should have a wide electrochemical stability window. The electrolyte should maintain inert reactivity toward the other cell components, too. In terms of producing thermally safe LIB technology, the electrolytes primarily should be compatible with the evolving electrodes, which can be altered by understanding its composition [11-13].

Electrolyte Composition of LIBs

Typically, the LIB electrolytes are composed of nonaqueous organic solvents, including ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or their mixtures with a lithium salt, such as lithium hexaflourophospahte (LiPF6), lithium tetraflourobo- rate (LiBF4), lithium bis(oxolato)borate (LiBOB), or lithium perchlorate (LiCKX,). However, various new electrolyte formulations are being considered that use polymers and room-temperature ionic liquids (RTILs) for advanced performance and improvements [14-16].

Temperature dependence of LIB applications. Adapted and reproduced with permission from reference [10]. Copyright 2016 Royal Society of Chemistry

FIGURE 6.1 Temperature dependence of LIB applications. Adapted and reproduced with permission from reference [10]. Copyright 2016 Royal Society of Chemistry.

Organic Solvents

The utilization of organic solvents, specifically, their optimized combinations with lithium salts and/or the addition of additives to improve their dissolution in lithium salts are quite compelling [17, 18]. The reasons behind this being their reduced viscosity, good dielectric constant, and good ionic conductivity. But they are limited in operating successfully without any thermal runaway at higher temperatures due to their low volatility and low boiling point (Table 6.1) [19-21].

Efforts are being made to either improve the performance of the existing organic electrolyte systems or supplant them with better ones. Due to high dielectric constant, better flash point, and boiling points, linear sulfites, such as dimethyl sulfite (DMS) and diethyl sulfite (DES), are found to form better solid electrolyte interphase (SEI) compared to alkyl carbonates. The use of the organic solvent, sulfolane with DMS-DES solvents exhibited excellent electrochemical performance with lithium bis(oxolato)borate (LiBOB) salt at 60° C [25].

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