Page 88 - Read Online

Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 21 of 32

[163]
formed in HCEs [Figure 8E] . However, an ultrahigh concentration of Li salt commonly sacrifices the
ionic conductivity of electrolytes, leading to batteries with high costs. To maintain the favorable solvation
structure, non-solvating fluorinated ethers with weak interactions with cations have been innovatively
adopted to decorate the electrolyte, which decreases the apparent concentration of the lithium salt and
retains the same solvation structure as that in the HCEs. Therefore, local HCEs that function similarly to
HCEs but with reduced viscosity, improved ionic conductivity and reduced costs have been reported.
Ding et al. reported that a type of non-solvating and low-dielectric (NL) cosolvent can intrinsically facilitate
the anion-cation interaction and thus promote the proportion of AGGs via model investigations
[Figure 8F] .
[164]

Unlike both nucleophilic and electrophilic aqueous solvents, the electrolyte solvents employed in LMB
systems are nucleophilic to prevent active protonation reactions from occurring. Furthermore, Li salts
possessing anions with large ionic radii are generally adopted to guarantee ideal solubility in organic
solvents with small dielectric constants. The delocalized charges and large radii of the anions both weaken
the interaction between cations and solvent molecules. As a result, in most cases, the anions in electrolytes
are conventionally considered to be unsolvated and the solvation of anions is rarely investigated.
Nevertheless, the solvation behavior of anions inevitably influences the stability of anions and solvent
molecules, the anion diffusivity and the transference number of cations. Furthermore, solvation might
achieve the stability of the radicals in the electrolyte, leading to the inhibition of parasitic interfacial
reactions. Solvents with a high acceptor number (AN) are expected to interact strongly with anions in the
electrolyte. For instance, tris(pentafluorophenyl)borane (TPFPB) with the electron-deficient B element
acting as the high AN solvent could strongly interact with PF and BF , with free energy variations of -12.7
4-
6-
-1
and -24.4 kcal mol , respectively [Figure 8G] . These free energy variations are significantly more obvious
[165]
than those between routine ester solvents and anions, which are mostly achieved by H-F interactions
[Figure 8H]. In addition, cyclic carbonates rather than linear counterparts are favorable due to the greater
polarity with the ring constraint. The effect of anions on the solvent molecule stability is contrary to that of
cations, which improves the reductive stability and reduces the oxidative stability. In addition, the anion-
solvent complexes lower the mobility of anions, thus increasing the transference number of the cations,
which is conducive to the electrochemical rate performance. Furthermore, the anion solvation promotes salt
dissociation and increases the solubility of the salt.

Replacing flammable liquid electrolytes with non-flammable SSEs is a feasible method to improve the safety
of batteries. Current SSEs can be divided into three categories: polymer SSEs; inorganic ceramic SSEs;
polymer/ceramic composite SSEs. Polymer electrolytes exhibit better flexibility and thus better interfacial
contact with the Li electrode. However, their relatively low ionic conductivity and narrow electrochemical
window limit their application in high-voltage batteries. Ceramic electrolytes usually possess desirably high
moduli for dendrite suppression and high ionic conductivity. However, most inorganic ceramics are fragile,
which restricts their application in mass production. Therefore, polymer/ceramic composite electrolytes are
promising for achieving high conductivity and proper flexibility in practical LMBs. Because of the high
ionic conductivity and thermal stability of PEO, it is frequently used to fabricate polymer/ceramic
composite electrolytes. PEO-in-ceramic electrolytes (80% ceramic) , polyvinylidenfluoride-
[166]
hexafluoropropylene/Li Al Ti (PO ) [167,168] , Li P S -PEO-LiClO 4 [169] , Li La 0.557 TiO nanofiber-enhanced
0.3
1.7
4 3
3
0.33
7 3 11
1.3
PEO and Li La Zr Ta O /PEO/succinonitrile have been successfully used as novel composite
[170]
[171]
12
0.6
1.4
3
6.4
electrolytes.
Separator modification
The most direct and simple approach to protect LMBs from short circuit is blocking the growth of Li
dendrites. However, conventional PP- and PE-derived separators can be easily pierced by sharp dendritic