Advances in Electrolytes for LIBs
Development of Electrolytes for LIBs
Research and development of LIBs began since the early 1980s , while the first commercial applications appeared in 1990 . A large number of applications of LIBs are used for electronic products and electric vehicles due to high specific energy density, high working voltage, low self-discharge rate, fast charge/discharge, long lifetime, and no memory effect [3-5]. However, it is noted that volatile and flammable liquid organic solvents widely used as electrolyte solutions in LIBs are prone to cause safety problems during cycling due to the dendrite growth of metallic Li anode . Hence, the utilization of solid electrolytes is expected to enhance the safety of LIBs by the almost complete elimination of the growth of Li dendrites [7-9].
FIGURE 13.6 Schematic diagram of PEM of hydrogen fuel cells.
Additionally, liquid organic solvents will result in irreversible capacity losses due to the formation of stable solid electrolyte interphase to hinder the increase in cycle life and limit the temperature window, which pose severe safety concerns on LIBs . All solid-state electrolyte (SSE) materials have been attracted increasing interests, which mainly include inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and organic-inorganic hybrid composite electrolytes .
Inorganic Solid Electrolytes (ISEs)
Among all SSE materials, ISEs are classified into oxide-based, sulfide-based, etc [11-16]. Figure 13.7 shows the crystal structure of parent garnet-like Li5La,M2012 . It is found that lithium ion transference number of ISEs is almost unity and its ionic conductivity is almost comparable to that of organic liquid electrolyte . Also, there are still two main challenges for achieving high-performance ISEs [18,19]: The first one is how to create favorable solid-solid interface between electrode and electrolyte, and the second one is how to obtain high ionic conductivity at room temperature (e.g., above 1CH S cm1).
Due to their high ionic conductivity and adequate mechanical features for lamination, sulfide composites have attracted an increasing attention as solid electrolytes used in all-solid-state batteries. Their smaller electronegativity and binding energy to Li ions and bigger atomic radius provide high ionic conductivity and make them more attractive for practical applications. In recent years, noticeable efforts have been made to develop high-performance sulfide SSEs. The improvement of the ionic conductivity of LISICON-type SSEs focuses on the replacement of oxide by sulfur in the framework, which is referred to as thio-LISICON . Since the radius of
FIGURE 13.7 Crystal structure of parent garnet-like Li5La,M20,2 . (Reproduced with permission with Elsevier publisher.)
S2~ is higher than that of 02~, this substitution can significantly enlarge the size of Li+ transport bottlenecks. Besides, S2~ has better polarization capability than O2-, thus weakening the interaction between skeleton and Li+ ions. Therefore, thio-based materials can achieve high ionic conductivity. It is reported that Li3 25Ge0 25P075S4 possesses the high conductivity of 2.2 x 10~3 S cm-1 at room temperature, high electrochemical stability, and no phase transition up to 500°C .
As is known, most sulfide SSEs are derived from the Li-P-S systems. Among these different sulfide materials, the LGPS family has shown superior ionic conductivity (up to 2.5 x К)-2 S cm-1 at room temperature), even higher than the conventional liquid electrolytes . Thus, sulfide-based all solid-state LIBs have a great potential in electrode/electrolyte synthesis and cell fabrication methods.
Solid Polymer Electrolytes (SPEs)
SPEs are the promising electrolyte materials, which are extensively used in electrochemical devices, especially polymer LIBs. They also exhibit potential applications in flexible and wearable devices. Generally, SPEs are composed of polymer host as solid matrix along with alkali metal salt without the addition of organic liquid solvents . SPEs have no leakage of electrolytes, low flammability, good flexibility, and safety as compared to the conventional liquid electrolytes. It is noteworthy that the stable contact between the electrode and the electrolyte greatly enhances the interfacial impedance due to the strong adhesive property on the surface of electrodes [23,24]. Besides, SPE process is flexible. In addition, SPEs of LIBs are required for good mechanical strength, excellent flame retardancy, superior thermal stability, high ionic conductivity at ambient temperature, and wide electrochemical window [25-28]. However, at a higher temperature, the chemical reaction will occur at the interfaces between polymer electrolyte materials and electrodes. Thus, the increased interfacial impedance will lead to the deterioration of LIBs.
The research and development of all solid-state polymer electrolyte materials aim to enhance high ionic conductivity at low temperature. As suggested by Yuan et al. , the flexible PAN-PEO copolymer of SPEs shows a high ionic conductivity of 6.79 x 10~4 S cm-1 at 25°C and an electrochemical stability of 4.8 V vs. Li+/Li, as shown in Figure 13.8. Besides, it is noted that Li-dendrite growth in the charging process of Li batteries can be effectively inhibited by PAN .
Kim et al.  demonstrated a shape-deformable and thermally stable plastic crystal composite polymer electrolyte (PC-CPE), with the high ionic conductivity of 1.02 x 10-3 S cm-1 at room temperature only slightly lower than that of the carbonate-based liquid electrolyte, which is attributed to its high diffusivity, plasticity, solvating power, and well-interconnected ion-conductive channels, as shown in Figure 13.9. Meanwhile, the electrochemical window of the PC-CPE up to 5.0 V vs. Li+/Li, indicates the potential application to high-voltage batteries. Also, due to its
FIGURE 13.8 (a) Photograph of PAN-PEOSPE film; (b) Arrhenius plot of PAN-PEO
copolymer SPE film doped with (1) 0wt%, (2) 10wt%, and (3) 20wt% of PEO (Mn!4
3,000,000) . (Reproduced with permission with Elsevier publisher.)
FIGURE 13.9 Structural characterization of PC-CPE . (Reproduced with permission from Royal Society of Chemistry (RSC).)
unique chemical/structural peculiarity, PC-CPE significantly improves the mechanical flexibility and thermal stability. Notably, the cell incorporating the self-standing PC-CPE delivered the stable charge/discharge behavior without suffering from safety problems, even after exposure to thermal shock condition .
Organic–Inorganic Hybrid Composite Electrolytes
Organic-inorganic hybrid composite electrolytes integrate the merits of organic polymers and inorganic ceramics, as shown in Figure 13.10. The improvements in mechanical properties, ionic conductivity, and interfacial stability of the polymer electrolytes are achieved by using inorganic materials filled into a polymer substrate. The polymer electrolytes are generally prepared by dispersing inorganic fillers, such as inert ceramic fillers (A120,, Si02, TiO,, etc.), ferroelectric ceramic fillers (BaTiO,, PbTiO,, and LiNbO,), carbon nanotubes (CNTs), and fast ionic conductors, into the polymer matrix [6,30].
FIGURE 13.10 Characteristics of organic-inorganic composite solid electrolytes.
In organic-inorganic hybrid composite electrolytes, the fillers provide a support matrix to retain an overall solid structure, even at high temperature. It is known that the ionic conductivity of the composite electrolytes increases with the decreasing particle size because the nanoparticles reduce the crystallinity of the polymer-salt system. Another reason for the enhanced ionic conductivity is the percolating interfacial effect: Anions adsorb on the surface of fillers, then break up the ion pair, and lead to increased interfacial ionic conductivity. Thus, the composite electrolytes are considered as high energy and safe rechargeable LIB materials.
It has been found that nanosized ceramic powders incorporated into the polymer- based electrolytes not only act as solid plasticizers to inhibit crystallization kinetics, but also show the effects on solvent and cationic mobility. As suggested by Sun et al. , the addition of ferroelectric materials such as BaTiO,, PbTiO,, and LiNbO, into the PEO-Lix polymer electrolyte significantly enhances the ionic conductivity. This phenomenon is related to the association of anions with lithium cations and the spontaneous polarization of ferroelectric ceramic particles, due to their particular crystal structure. A combination of rutile and barium titanate decreases the interfacial resistance between the lithium electrode and the composite polymer electrolyte. The composition of the electrolyte can be optimized by a proper choice of the type and morphology of ferroelectric ceramics. Moreover, all the electrolytes studied show the decomposition potentials higher than 4 V vs. Li/Li+. This might lead to the development of a polymer electrolyte having a true solid-state configuration and thus good mechanical properties, combined with high conductivity and low interfacial resistance with a lithium metal electrode. Hence, it is believed that the ferroelectric materials and the PEO-Lix polymer hybrid composite electrolytes are suitable candidates for the practical applications .
ISEs, like fast ionic conductors, have presented many potential advantages, such as no electrolyte leakage, high electrochemical stability, nonflammability, and high thermal stability . Organic-inorganic hybrid composite electrolytes are composed of ISEs and flexible polymer materials that can synergistically combine the beneficial properties of both glass ceramics and polymers . Kobayashi et al.  proposed a composite concept in which a ceramic electrolyte is placed at the positive electrode interface and a polymer electrolyte at the negative electrode interface . Ethylene oxide co-2-(2-methoxyethoxy) ethyl ether-LiBF4 polymer film was placed between (Li, La)TiO, and Li metal, and showed relatively high lithium ion conductivity, typically 1(H S cm-1 at 22°C. Moreover, the all-solid-state battery [LiMn204/(Li. La)TiO,/ dry polymer/Li] showed good cycle characteristics at 60°C (Figure 13.11). It is demonstrated that this new composite method should be a promising electrolyte applied in solid-state batteries based on lithium metal electrode.