A key property of a polymer electrolyte is the conductivity [
20,
23,
24,
28,
54]. The performance of the polymer electrolyte is greatly determined by the polymer structure as it constitutes the matrix for ion transport. The mobility of polymer chains [
55] and the interactions of lithium ions [
56,
57] within the polymer matrix greatly determine the conduction behavior of polymer electrolytes. The second factor will be discussed in detail in
Section 3.5. Regarding the first factor, for high molecular weight polymer-based electrolytes, the amorphous polymer domains [
58,
59] account primarily for the ion transport whereas the crystalline counterparts hinder ion movement. (Note that ion transport in crystalline domains has been reported in low molecular weight PEO [
60,
61], but the discussion in this review focuses on amorphous conduction.) The mobility of the polymer chains also affects ion conduction. In the first three sub-sections of Chapter 3 we review the effects of nanoparticles on the polymer chain (1) structure; (2) conformation; and (3) segmental movements.
3.1. Effect of Nanoparticles on Polymer Crystallinity
Given that the structure of polymer electrolytes (e.g., amorphous or crystalline) plays a prominent role in facilitating ion transport, the effect of nanoparticles on the fraction of amorphous domains in CPEs becomes critical. We start by discussing the crystallinity of polymer-nanoparticle systems in the absence of lithium salt. The crystallinity of PEO (molecular weight = 100,000 g/mol, polydispersity index
Mw/
Mn = 2.4)-sodium montmorillonite (cation exchange capacity 92.6 mmol/100 g, particle size not reported) hybrids [
62] (without lithium salt) is presented in
Figure 3. When PEO < 70 wt %, the crystalline peaks of X-ray diffractograms all disappear, which indicates that the hybrid system is almost amorphous. However, when PEO > 70 wt %, the PEO crystallinity seems rather constant and independent of the PEO content. The results were explained on the basis of two opposing effects: [
62] crystallization enhancement by the addition of inorganic fillers, and crystallization hindrance by the coordination of alkali cations with polymer chains.
When nanoparticles are added into a binary system of polymer with lithium salt, a different behavior is observed. For example, CdO nanoparticles (2.5 nm diameter) were added to PEO (400,000 g/mol)–LiI electrolyte at a fixed molar ratio of EO/Li
+ = 12 (the CdO content in the system was 0.05–0.20 wt %) [
63]. Because of the LiI in the system, the crystallinity of the polymer was already lowered compared to the neat polymer (
Table 1) because of the coordination of alkali cations with polymer chains. The introduction of CdO at first decreased the crystallinity [
63]. A minimum value of crystallinity was achieved at 0.10 wt % CdO. Further addition of CdO led to a crystallinity increase. This result is comparable with the following example, in which the EO/Li
+ ratio was set as a variable [
64]. In PEO (300,000 g/mol)–LiClO
4, a fixed amount of 10 wt % SiO
2 (diameter = 10 nm, Specific Surface Area = 956 m
2/g) was introduced, which reduced crystallinity from 48.5% for the SiO
2-free sample to 44.3% in the temperature range −75–100 °C. As the EO/Li
+ molar ratio increased to 12, the polymer turned all amorphous.
The effect of CdO nanoparticles on crystallinity has also been evidenced in the Field Emission Scanning Electron Microscope (FE-SEM) images of spherulites in PEO-LiI-CdO CPEs. The average diameter of spherulites for the additive-free PEO-LiI electrolytes is around 120 μm. When CdO was added into PEO-LiI electrolytes, the PEO spherulites increased in number and decreased in average size to about 50 μm. This indicates that CdO addition is beneficial for the reduction of the PEO degree of crystallinity [
65,
66]. As for the reason, CdO serves as a nucleation center for spherulite initiation and because the number the sperulites increases, the size of an average spherulite is reduced. However, it was noticed that as the CdO concentration increased, the spherulite size would not change appreciably which might be explained in that not all CdO added could effectively act as nucleation sites. Aside from this, the variation of the amorphous phase content was dramatic.
The importance of polymer crystallinity in the study of CPEs cannot be overrated. Crystallinity is affected by temperature, amount of lithium salt and nanoparticle changes [
26,
62,
63,
64,
67,
68]. Crystallinity underlies the performance of CPEs such as conductivity and mechanical strength.
3.2. Effect of Nanoparticles on Polymer Chain Conformation
The polymer chain conformation influences the dynamics of the lithium ions coordinated with the polymer backbones [
69] because Li
+ ions exhibit different binding energies with the different conformers [
70]. The polymer chain conformation can change due to nanoparticle addition as shown by Raman [
62] and FTIR spectroscopy [
67,
71,
72]. In the 700–1600 cm
−1 Raman spectra of a PEO-sodium montmorillonite system, there are four characteristic regions corresponding to four types of vibration bands: CH
2 rocking vibrations, C–O–C stretching vibrations, CH
2 twisting vibrations and CH
2 bending vibrations [
62]. As temperature changed or the PEO content of the system varied, the intensity in the CH
2 rocking vibrations region (750–970 cm
−1) changed, suggesting conformation changes of polymer chains [
62]. As shown in
Figure 4b, the gauche conformation in neat polymer accounts for 55%–60% in the temperature range 75–180 °C. When sodium-activated montmorillonite (Na
+-MMT) (cation exchange capacity 92.6 mmol/100 g) was added, the gauche percentage increased to 72% at ambient temperature [
62]. Because salt may form transient crosslinks with polymer chains [
36], the effect of salt on the polymer chain conformation merits further investigation in the presence and in the absence of nano-sized additives.
Conformational perturbations would be expected to affect the dynamics of Li
+ ion coordination to the PEO backbone and ion mobility [
69], which lead to conductivity changes. As for a quantitative analysis of polymer chain conformation effects on polymer electrolyte conductivity, molecular dynamic simulation studies are available [
73,
74] but experimental evidence appears difficult to obtain. This is because the conformation change is brought by certain experimental conditions, e.g., temperature variation or nanoparticle addition, which possibly exert a more significant effect on conductivity. Thus, it is hard to isolate the contribution of conformation change to the ionic conductivity.
In addition to Raman, Fourier Transform Infrared (FTIR) spectroscopy can also be employed to obtain information on polymer chain conformation in CPEs [
67,
72]. In the PEO–NaClO
4 (EO:Na
+ = 25) system, results have been presented with a deconvolution of C–O–C within the wavelength range of 950–1250 cm
−1 [
67].
Figure 5a–c shows FTIR spectra for PEO–NaClO
4 (EO:Na
+ = 25) and for the same system but with 4.5 and 7.6 nm ZrO
2 (5 wt %) addition. When sodium salt and nanoparticles were added, the main absorption band of PEO in the polymer-salt complex shifted from 1112 cm
−1 to a lower value of 1108 cm
−1, and its full width at half maximum (FWHM) broadened to 53 cm
−1. This suggested the coordination of ions with ether oxygens. The peak at the 1059 cm
−1 position associated with the crystalline structure in pristine PEO almost disappeared, indicating a decrease of crystallinity.
Figure 5d shows the FTIR spectra of C–O–C vibrational mode of pristine PEO. Within the fitted spectral region of 1200–1000 cm
−1 of the FTIR spectrum, there are six peaks in the positions of 1014, 1033, 1059, 1094, 1112, and 1148 cm
−1, which correspond to C–O–C symmetric and asymmetric modes. The maximum of the peak is at 1112 cm
−1 with FWHM = 29 cm
−1. The changes of the spectra revealed that different interactions within the system caused the deformation on the C–O–C bond angle and changes in the stretching module, which led to changes of the integrated area ratio as shown in
Figure 5e.
Although its effect is not as strong as crystallinity, the chain conformation responds to various conditions, e.g., particle size and temperature change. This conformation analysis contributes to a better understanding of composite polymer electrolytes.
3.3. Effect of Nanoparticles on Polymer Chain Segmental Movement
Following the discussion about “static” chain conformation in
Section 3.2, we address here the polymer segmental motion as a dynamic counterpart with an aim to understand how the polymer matrix facilitates ion movement. In order to study the effect of nanoparticles on polymer chain segmental mobility, nuclear magnetic resonance (NMR) has been employed to analyze two characteristic temperatures, the glass transition temperature,
TgNMR, and the temperature which corresponds to the maximum spin-lattice relaxation rate,
Tmax [
75]. Additionally, the line widths (Δ
ν) and the spin-lattice relaxation times (
T1) of the
1H,
13C, and
7Li nuclei as affected by temperature have been employed to study the mobility of the ions and the polymer chains [
76,
77].
Different samples of PEO/silica with lithium salts have been prepared and denoted as [
X]
n[
Y]-
Z, where
X is the polymer weight percentage,
n is the average number of repeating units in polymer chains,
Y is the ratio of ether oxygens to Li
+,
Z is either I or II, standing for non-bonded (i.e., each silicate group only bonded to each other through oxygen bridge in the silica phase) or bonded (silicate group bonded to polymer chains by covalent bonds) complex structure, respectively [
76]. As temperature changes, the
7Li NMR line width (FWHM, Δ
ν) would undergo narrowing, which is commonly associated with chain motion increase. The corresponding temperature is close to the
Tg value obtained from DSC and thus is denoted as
TgNMR. Another temperature that corresponds to the maximum spin-lattice relaxation rate is denoted as
Tmax, where transverse local field fluctuations suffice to maximize the rate of spin-lattice relaxation at the Larmor frequency [
76]. These two parameters, as listed in
Table 2, have been employed to analyze polymer chain mobility.
The results from
Table 2 can be summarized as follows. For non-bonded complexes of type I: (1) in Series 1, decreased
TgNMR and
Tmax when
X > 80 indicate a reduced mobility of the polymer domains adjacent to silica clusters; (2) in Series 2, longer chain length increased chain hindrance and thus increased
TgNMR and
Tmax [
78]; (3) in Series 3, upon addition of silica,
TgNMR and
Tmax remain approximately constant for samples with different amount of lithium salt, the result being different from those systems without silica [
79]. Thus, it can be concluded that polymer-silica interactions weakened the effect of lithium salt on polymer chain mobility. For bonded complexes of type II: in Series 4, salt addition led to increased
TgNMR and
Tmax as expected, since lithium ions form transient crosslinks with polymer chains that reduce mobility.
A typical example that demonstrates the effectiveness of NMR in providing insights on the conductivity change follows.
Figure 6 is a plot of
7Li line width without
1H decoupling (Δ
ν) and
7Li spin-lattice relaxation rate (
T−1) versus temperature for three different nanocomposites: [58]
12[4]-I, [58]
20[4]-I, and [76]
17[4]-II. We can see an obvious
TgNMR and
Tmax increase of bonded sample of type II as shown by a shift of the Δν and
T−1 plot to the higher temperature, in contrast to non-bonded sample of type I. This suggests that covalent bonding between silica particle and the PEO chains hinders the polymer chain motion and also the Li
+ mobility, accompanied by an ionic conductivity drop [
76]. This result supports the general conclusion that polymer electrolytes containing nanoparticles (for the case of non-bonded samples) exhibit improved conductivity. The chemical bonding between nanoparticles and polymer chain does not always increase the CPE ionic conductivity.
3.4. Effect of Nanoparticles on Polymer Self-Assembly and Anisotropic Conductivity
We are interested in polymer self-assembly in composite polymer electrolytes because (i) nanoparticles in the system affect the micro-phase separation of the polymer matrix [
80,
81,
82] and (ii) polymer self-assembly (microphase separation or phase transition) creates different environments for ion conduction and both ionic and electric conductivities vary accordingly. We address this topic here briefly; however, extensive efforts have been devoted to block copolymer thermodynamics [
5,
83,
84,
85] in the melt state [
86,
87,
88,
89,
90] and in solution [
90,
91,
92,
93]. The introduction of nanoparticles into block copolymer electrolyte systems typically leads to an increase of the segregation strength (χN) to generate different microphase separated morphologies [
94,
95,
96]. Block copolymers can change from the disordered state to form spherical, cylindrical, lamellar, gyroid or other morphologies depending on factors such as the block ratio, solvent medium, and hydrogen bonding of the ligands [
97] with polymer chains [
98,
99,
100]. Here we highlight (1) the effect of nanoparticles on the segregation of block copolymer-salt electrolytes [
101] and (2) how self-assembly can affect the performance of CPEs.
Anisotropic ion transport behavior was reported in microphase-segregated electrolyte membranes of (PEO
114)-
b-poly(methyl acrylate) with azobenzene mesogen [PMA(Az)
47]–LiCF
3SO
3 as shown in
Figure 7 [
102]. Upon comparing the perpendicular conductivity (where cylindrical domains of PEO are perpendicular to the electrodes, thus current moves along the cylindrical domains of PEO) with the parallel conductivity (cylindrical domains of PEO are parallel to electrodes, thus current transverses the cylindrical domains of PEO), it was found that the perpendicular conductivity σ
⊥ facilitated by a perpendicularly aligned PEO cylindrical array was always higher than the parallel conductivity σ
∥. The parallel conductivity σ
∥ exhibited a monotonic increase as the temperature increased for samples with EO:Li
+ = 20:1 and 4:1. The perpendicular σ
⊥ attained a maximum at around 377 K, followed by a drop [
102].
Similarly, for a poly(ethylene oxide)-
b-6-(4′-cyanobiphenyl-4-yloxy)-hexyl methacrylate) (PEO-b-PMA/CB) polymer matrix doped with LiClO
4 (EO:Li
+ = 120:1), a magnetic field was used to induce micro-domains of highly aligned hexagonally packed cylinders [
103]. The conductivity of parallel PEO cylinders (σ
∥) was lower than that of randomly oriented sample (σ
rand) by more than one order of magnitude at room temperature. The perpendicular PEO cylinder conductivity (σ
⊥) was higher than that of a randomly oriented sample by one order of magnitude as shown in
Figure 8a. In
Figure 8b, σ
⊥, σ
∥ and σ
rand overlapped at high temperatures until
TODT, i.e., the temperature where order-disorder transition (ODT) occurred. When
T <
TODT, σ
⊥, σ
∥ and σ
rand responded differently with temperature increase. The origin of distinct behaviors of conductivity in aligned samples below
TODT was still unclear [
103].
Further research can examine the effect of nanoparticles on other micro-phase separated morphologies, e.g., aligned lamellae or bicontinuous structures. The number of nanoparticles that could be accommodated in the polymer matrix remains unresolved. It is postulated that a stronger interaction between ligand and the polymer chains allows a higher incorporation level and the ordered polymer structure can be maintained at a higher content of additive [
97]. In closing, the effect of nanoparticles on block copolymer segregation and the anisotropic conductivity of block copolymer electrolytes is a topic of continuous interest [
101,
104].
As mentioned at the beginning of Chapter 3, atom/ion/nanoparticle interactions within the polymer matrix greatly affect the conduction behavior of polymer electrolytes [
56,
57]. After the discussion about the polymer matrix structure, polymer chain conformation and segmental movement, and block copolymer self-assembly, we now proceed to discuss the interactions between atoms, ions, and nanoparticles inside the polymer matrix. Thus, in what follows, we address (1) interactions within CPEs in
Section 3.5; (2) lithium salt dissociation versus ion pairing and ion aggregates in
Section 3.6; and (3) cation transference number
t+ as the contribution of lithium ion to the charge transport in
Section 3.7.
3.5. Interaction of Nanoparticles with Polymer Chains
Ion transport can be either hindered or facilitated by interacting polymer chains and nanoparticles. The introduction of nanoparticles increased conductivity as reported by many authors [
22,
23,
24,
26]. Oxygen vacancies at the surface of nanoparticles (e.g., SnO
2) are thought to act as Lewis acids to coordinate with either ether oxygens of polymer chains or anions of lithium salt as Lewis bases. Based on this understanding, we need to obtain information about the “oxygen vacancy percentage” on the surface of nanoparticles. Such information can be obtained from X-ray photoelectron spectroscopy (XPS). According to XPS spectra of oxygen of SnO
2 nanoparticle for the system PEO (600,000 g/mol)–LiClO
4 (EO:Li
+ = 8)–SnO
2 (
d = 3–4 nm, weight ratio of SnO
2:PEO = 0.05, 0.10, 0.15, 0.20) [
105], two types of oxygens are seen: (1) structural O, from Sn–O bond and (2) absorbed O, from O
2 and CO
2 in the atmosphere. By peak deconvolution, the percentage of structural O in the systems and its ratio to Sn can be estimated (
Table 3) [
105]. The theoretical Sn/O ratio is 0.5 for SnO
2. Assuming the Sn atom amount in each sample is 100, the results in
Table 3 show much larger Sn/O value. This indicates many oxygen vacancies present on the surface of SnO
2 nanoparticles [
105].
Figure 9 illustrates the interaction of nanoparticles within polymer electrolytes. The oxygen vacancies on the nanoparticle surface (as Lewis acid) coordinate in two ways: (1) with ether oxygens of the polymer chain, to hinder PEO crystallization and produce higher amorphous fraction; and (2) with oxygens from anions of lithium salt (LiClO
4) [
105,
106,
107,
108], to reduce the ion pair (Li
+–ClO
4−) and release more free Li
+ ions. Both effects contribute to enhance the conductivity of the CPEs. After the “oxygen vacancy percentage” on the surface of nanoparticles has been obtained, this notion can be combined with the topic of “free ion percentage” in
Section 3.6 for more detailed interaction analysis.
In order to improve the details of the scheme shown in
Figure 9, the distance between lithium ions and carbons of polymer chains (Li
+–Carbon distance) and the coordination number (the number of oxygens coordinated with one lithium ion) are required. Rotational Echo Double Resonance (REDOR) has been employed to measure dipolar and quadrupolar coupling values between NMR active nuclei to determine intermolecular distances in the solid state [
109,
110]. For example,
7Li–
13C quadrupolar signals indicated that Li–C distance = 3.14 Å [
111]. This result compared favorably with that of 2.23–4.27 Å obtained from neutron powder diffraction [
112].
3.6. Single Ions versus Ion Pairs and Ion Aggregates within Composite Polymer Electrolytes
After the discussion of interactions between nanoparticles and binary polymer electrolytes, we now focus on the lithium salt. In the polymer electrolyte, lithium salt ions exist in the forms of (1) single ions; (2) ion pairs and (3) ion aggregates [
56,
57] but only single ions contribute to the ionic conductivity. Thus it is desirable to enhance the ion dissociation in the polymer matrix in order to improve the CPE performance. The percentage of free lithium ions can be studied by FTIR, Raman and NMR [
67,
110,
113].
The addition of nanoparticles into polymer electrolytes can enhance the salt dissociation to produce more free cations for conductivity. The percentage of free anions in PEO (1,000,000 g/mol)–NaClO
4 (EO:Na
+ = 25) system was 68% [
67]. When 5 wt % of ZrO
2 (
d = 4.5 nm) was added, this percentage increased to 81% as obtained from the FTIR peak integration ratio. However, larger ZrO
2 particle size (
d = 7.6 nm) caused a slight decrease in the percentage of free anions. This was attributed to stronger Lewis acid-base interaction resulting from comparatively larger surface area of 4.5 nm ZrO
2, which produce more free ClO
4− ions in the solid polymer electrolyte system [
67]. Fitted Gaussian-Lorentzian peak of the ClO
4− of FTIR pattern [
67] is shown in
Figure 10a: the ClO
4− band has been well fitted into two peaks centered at wavenumber of 624 and 633 cm
−1 corresponding to free anions and contact ion pairs, respectively [
114,
115].
The free ion percentage is affected not only by nanoparticle addition but also the lithium salt concentration.
Figure 10b shows Raman spectra of the anion symmetric stretching mode for ClO
4− for different salt concentration in the system poly(propylene glycol) (4000 g/mol)–LiClO
4 (O:Li = 30, 10, 8, 5) [
113]. The fraction of ion pairs increased with salt amount for all concentrations.
The effect of different anions on lithium salt dissociation has been studied by NMR which also provides estimates about ion pairing and contact between certain solvent moieties by homo- or hetero-Nuclear Overhauser Effect measurements (abbreviated as NOESY or HOESY) [
110]. The HOESY methodology can be used, for example, to study the ion-pairing between LiBF
4 and LiPF
6 in PEO melt.
Figure 10c presents a comparison of the NOE effect for both LiBF
4 and LiPF
6 in block oligomer C
5H
11NHCONH(CH
2CH
2O)
11NHCONHC
5H
11. The bell-shaped profile of LiBF
4 salt indicated strong ion-pair formation in the 100–500 ms timescale; while the quasi null Nuclear Overhauser Effect (NOE) effect for LiPF
6 ions revealed the much stronger dissociation of this salt in the PEO matrix [
110].
The free ion percentage is important in CPEs because the free ions contribute to ion conduction whereas ions pairs and ion aggregates do not. Research on this topic continues [
116].
3.7. Effect of Nanoparticles on Transference Number of Composite Polymer Electrolytes
Following the discussion of polymer structure and interactions in CPEs, we proceed to address a parameter that is related to the ion conduction inside the polymer matrix. The transference number directly describes the charge transport and thus the current of a specific ion [
117]. Specifically, the lithium ion transference number
t+ indicates the fraction of the current carried by the cation (Li
+) in the electrolytes. It is desirable to achieve a high
t+ in order to enhance the electrode reaction kinetics and to eliminate the concentration gradients within the battery so that the internal voltage drop could be lowered and the output current increased [
64]. The cation transference number is most commonly calculated by Equation (8) [
118,
119].
In Equation (8),
t+ is the transference number of cations.
D+ and
D− stand for the cation and anion self-diffusion coefficients, μ
+ and μ
− are the mobility [
120,
121] of the cation and anion, respectively.
High lithium transference number (
tLi+) at ambient temperature contributes to efficient battery performance [
122,
123]. In view of the importance of
t+, we discuss here two factors that influence
t+: electrolyte state of matter (liquid, gel or solid) and nanoparticle surface properties (acidic/basic/neutral).
The transference number of ions (
t+ and
t−) can be obtained from AC impedance spectroscopy [
124,
125], DC polarization electrochemical method [
126,
127], potentiometric measurements [
128], and NMR [
118,
129,
130,
131]. A comparative study of these methods has been presented [
132].
Table 4 gives
t+ values for different systems [
133]. In these samples, PEO (4 × 10
6 g/mol) is denoted as 4mPEO and the organic solvents used were ethylene carbonate (EC) and diethyl carbonate (DEC), EO:Li = 10:1. The size of the nano-sized fumed silica added (content = 10 wt %) was not reported.
We can see in
Table 4 that the highest value of
t+NMR was 0.52 for the organic solvent-based liquid electrolyte of 1 M LiB
4 in ethylene carbonate and diethyl carbonate mixture (EC–DEC), while in the Gel Polymer Electrolytes (GPE)
t+ decreased to 0.14 and 0.11 for the system of G4mPEO (GPE with 4mPEO matrix swelled by EC-DEC), without and with 10 wt % fumed silica, respectively. Thus, the fumed silica decreased the
t+. Also, the lower
t+ matched the result [
133] of lower conductivity for GPE at room temperature. Note that the
t+pol values obtained from electrochemical analysis were different from the
t+NMR from NMR, something that was ascribed to the motive forces of these two techniques [
133]. Moreover, as shown in
Figure 11, the larger values of the self-diffusion coefficients of the anion are higher than those of Li
+, which indicates faster diffusion of the anion. This has been attributed to the interaction of Li
+ with the polymer matrix [
133].
The second factor affecting
t+ is the active sites on the nanoparticle surface. For the system PEO–LiCF
3SO
3 (EO:Li
+ = 20)–10 wt % Al
2O
3 (basic, neutral or acidic,
d = 5.8 nm), the transference number
t+ increased in the sequence of updoped (
t+ = 0.46) < basic Al
2O
3 (
t+ = 0.48) < neutral Al
2O
3 (
t+ = 0.54) < acidic Al
2O
3 (
t+ = 0.63) [
134]. As for the explanation, the hydrogens of acidic ceramic surface (Lewis acid) form hydrogen bonds with the lithium salt anions and the ether oxygens (Lewis base) [
134], which promote salt dissociation and also decrease the PEO crystallinity [
123,
134]. In this way, the transference number
t+ increased. As for the neutral and basic Al
2O
3, the number of Lewis acid sites decreased leading to weaker increase in
t+. This discussion would be more informative if the number of acidic sites on the surface could be quantified in combination with oxygen vacancy analysis. This would reveal the “efficiency of acidic site” on the
t+ increase.
We discuss above factors affecting the lithium transference number
t+. However, how is
t+ related to conductivity? Conductivity and
tLi+ results have been compared for CPEs, with the additive being ionically active/inert SiO
2 particles (active SiO
2 was mesoporous silica SBA-15 absorbing plasticizers of ethylene carbonate (EC)/propylene carbonate (PC); inert SiO
2 was mesoporous silica SBA-15 without plasticizers) and organic solvent additive. As seen in
Figure 12, for PEO (300,000 g/mol)–LiClO
4 (EO:Li
+ = 16)–SiO
2 (1000 m
2·g
−1), the conductivity initially increased with addition of active SiO
2, attained a maximum at 10 wt % active SBA-15 at a value of about 2.4 × 10
−5 S∙cm
−1, followed by a decline with further loading of active SBA-15 [
64].
As a comparison, at the same optimum content of 10 wt % active SiO
2,
tLi+ reaches a maximum value of 0.54 [
64]. This has been attributed to the competition of –OH on the surface of SBA-15 with Li
+, both as Lewis acid, to coordinate with ether oxygens on the PEO chains and ClO
4− anions, both as Lewis base, which led to the promotion of desired Li
+ transport by occupying more Lewis base coordination sites and thus
tLi+ was enhanced. In the range of
tLi+ increase with SBA-15 doping lower than 10 wt %, there are not enough –OH sites available for Lewis acid-base interaction. At content higher than 10 wt %, the trend of decreasing
tLi+ could be ascribed to the aggregation of nanoparticles [
64]. The simultaneous increase of
tLi+ and conductivity from 0–10 wt % active SBA-15 loading indicates an effective way to increase conductivity by improving
tLi+. The increase of both
tLi+ and conductivity is required [
135] for achieving high performance in lithium-ion batteries.