Electrochemical Window
Apart from ionic conductivity, the electrochemical window of QSSE would be a critical parameter, as it decides the total operational potential window of LIBs. Generally, it can be defined as the difference between the oxidation and reduction reaction potentially present in the system. For an efficient QSSE, the fundamental necessity is to be inert to the anode and the cathode, which means that the oxidation potential must be higher than the embedding potential of Li+ in the cathode, and the reduction potential must be lower than the Li metal at the anode. The operational potential window is measured by linear or cyclic voltammetry sweeps. In general, QSSE-based devices find the electrochemical operational window of 3 V-4 V vs. Li/ Li+, but, in some cases, it also exceeds even 4.5 V [6].
Cationic Transport Number (t+)
For an efficient QSSE, the Li+ transference number has to be close to unity. Generally, it is cation mobility measurement relative to the anion species in the salt of QSSE, and suppressing the mobility of anions can yield a prominent increase in the Li+ transport number. In order to achieve the same, the concept of anchoring of anion, using a polymeric backbone, has been developed and is the most common method for single-ion conducting QSSEs [6]. Another approach is the introduction of anion receptors that undergo selective complex formation with anionic species in the electrolyte system. Therefore, a Li+ transport number close to unity will lead to an effective reduction in the concentration polarization of QSSE during the charge- discharge cycles, yielding high power densities. In general, for a simple binary electrolyte system dissociated into Li+ and X' ions, Li+-transport number can be given as shown in Equation 5.3,
Where, t+ is the cationic transfer number, and I,* and // represent current observed by Li+ and X- ions, respectively, when electrolytes are embedded between blocking electrodes.
Chemical and Thermal Stability
An important aspect of the QSSE is that it must be chemically inert toward the other components of LIBs, such as anode, cathode, separator, current collectors, and various additive materials and do not undergo any side reactions during its operational period. Apart from chemical stability, excellent thermal stability is also a desirable parameter, as LIBs must withstand the heat generated due to short circuit, overcharge, improper operation, or external thermal abuse. Therefore, thermally stable matrices will prevent the decomposition, evaporation, melting, and side reactions of the species present within the QSSE. These parameters can be evaluated using various thermal analyses, such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis, with the temperature range of -50° C-70° C, prior the application, where parameters, such as decomposition temperature (Td), glass transition temperature (T), melting point (TJ, and phase transition of QSSE, are determined for thermal stability of LIBs.
Porosity and Electrolyte Uptake
The porosity is defined as the ratio of the volume occupied by voids to the total geometric volume [7], as it is directly proportional to the electrolyte uptake. An increase in porosity simultaneously increases electrolyte uptake, which yields to high ionic conductivity, but simultaneously changes mechanical properties, too. Therefore, it is crucial to balance the porosity and electrolyte uptake, which can possess good mechanical strength, too. The uptake can be expressed as shown in Equation 5.4
where, W, is the weight after electrolyte uptake, and W0 is the weight before uptake [8].
Mechanical Robustness
The high mechanical strength of QSSEs is again a critical parameter, as they must withstand various mechanical stress/strain during large-scale manufacturing, cell assembly, storage, and real-scale applications, especially for flexible LIBs on the market. Therefore, various mechanical properties of QSSEs are investigated prior to their application. Generally, cross-linkable components are preferred over linear polymer chains because of good mechanical strength with high ionic conductivity [9].
Interface with Electrode Materials
It is essential to understand the compatibility and the reactions between the electrode-electrolyte interface for an efficient device to have long cycling life and safety.
For efficient LIBs, efforts are ongoing to form stable an electrode-electrolyte interface and dendrite suppression, which is conductive to Li+, yielding high voltage efficiency, long cycle life, and safety. In practice, various advanced techniques are used to investigate the interface mechanism.
As discussed in the previous section, the QSSE provides more significant potential as it combines the fundamental properties of liquid and solid electrolytes, which results in high performing LIBs. Specifically, a fixed quantity of polymer-host matrix is chosen with Li salt with an appropriate solvent that yields better QSSEs. In general, Li+-transport number and conductivity are typically governed by liquid electrolytes, while mechanical and morphological properties are governed by polymeric matrix. Although, disputes are observed in some special cases in which researchers find that polymer matrix contributes to Li+ transport number [10]. The concept of trapping alkali metal salt into polymer matrices, such as polyvinylformal (PVF) and PAN, resulting in quasi state was reported in 1975. After consecutive efforts, utilization of QSSEs into LIBs specially entailing polyfvinylidene fluoride)-co-hexafluoro propylene (PVdF-co-HFP) with lithium iron phosphate (LiPF6, simply LFP) electrolyte were reported [11]. In recent years, many reports using various polymer matrices (as given in Figure 5.3) have come up using PEO, PVdF, PMMA, PAN, polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyethylene glycol diacrylate (PEGDA), PVdF -co-HFP, and naturally available matrix, such as cellulose and other inorganic fillers [4, 6].
However, the addition of these plasticizers bring shortcomings, such as low ionic conductivity and LP-transport number, when compared to liquid electrolytes, but
FIGURE 5.3 Various polymer matrices used to prepare QSSEs for LIBs.
they show improvements in other parameters, such as mechanical strength, thermal stability, and chemical stability. Additionally, because they are leakage-free and have better electrode-electrolyte interface contact with comparable conductivity and transport number, they are suitable for the application of QSSE into LIBs. Efforts are being made to overcome the existing limitations of QSSEs by adding inorganic fillers, cross-linking polymers, and introducing anisotropy, single-ion conduction polymers as well as adding chemically bonded anion of Li-salt to improve the structural design of polymers for LIBs. In the following sections, some well-known QSSEs with different polymer matrix are discussed with their properties and mechanism.
