112
energy of the salt, such as BF 4
−
or PF 6
−
. Single-ion conductive polymers have been
developed to allow for large transference numbers. Mixed conductor composites
have found application in serving as advanced electrode materials, and in 2015, a
review specifically describing the types of polymer nanocomposites utilized was
published [92].
The use of inorganic fillers to improve the mechanical properties of polymers
and inhibit PEO crystallization to allow for ion transport at room temperature has
been applied yet again for lithium conductivity. In 1998, Croce, et al. demonstrated
that inorganic nanofillers can be used as a type of “solid plasticizer” and allowed for
room temperature conductivity. They utilized 13 nm TiO 2 nanoparticles and 5.8 nm
Al 2 O 3 , nanoparticles, resulting in large conductivity enhancements, ranging from
10
−3
and 10
−5
S cm
−1
versus the range 10
−4
to 10
−8
S cm
−1
for the ceramic-free electrolytes in the temperature range 30–80 °C. They also measured an average Li
+
transference number of ~0.6 in the 45–90 °C temperature range, which was the
highest at the time for PEO-LiX [4]. Similarly, silica nanoparticle/PEO composites
were described, whereby monodisperse 12 nm silica nanoparticles were grown in
situ. Conductivities of 1.2 × 10
−3
S cm
−1
at 60 °C and 4.4 × 10
−5
S cm
−1
at 30 °C were
reported. An electrochemical stability window up to 5.5 V was observed [93].
Ceramic particles are often considered to be inert fillers, yet their exact role
remains under investigation. The first reports simply used this method to improve
mechanical stability of the films. Wieczorek et al. suggested that small particles
(below 4 μm in size), allowed for enhanced interactions between the matrix and
surface, owing to the significant increase in surface area [94]. This was followed by
a study in 1998 describing the particle surfaces as binding sites for lithium ions,
which behave as hard Lewis acids [95]. Lewis acid–base reactions on the surface of
the particles are suggested to play an integral part in the conductivity. Further investigation by Croce, et al. demonstrated that the inorganic nanofillers behave as crosslinkers for the PEO and X-anions, stabilizing their separation and providing a
modified matrix for lithium ion transfer [96]. More recently, solid composite polymer electrolyte with Y 2 O 3 -doped ZrO 2 nanowires having positive-charged oxygen
vacancies resulted in 2-order-of-magnitude increase in lithium ion conductivity
[97]. In 2001, it was reported that ZnO nanoparticles (3.5 nm) having acetate groups
on the surface cooperate with the PEO segments and lithium ions to form crosslinked structures, decreasing the film’s crystallinity and enhancing conductivity [98].
Nanosized fillers have an advantage over bulk materials in that they disperse
more readily and reduce grain boundaries, while providing more free space for
polymeric chains to move. Yet, they may precipitate from the matrix at high concentrations. In order to circumvent this problem, researchers designed a threedimensional nanostructured hydrogel-derived Li 0.35 La 0.55 TiO 3 framework, which
was then used as a 3D nanofiller for PEO and displayed improved Li-ion conductivity up to 8.8 × 10
−5
S cm
−1
at room temperature [99].
The microstructure of the matrix is also of importance. A nanocomposite polymer electrolyte based on vinylidene difluoride-hexafluoropropylene copolymer
C. A. Bauer
energy of the salt, such as BF 4
−
or PF 6
−
. Single-ion conductive polymers have been
developed to allow for large transference numbers. Mixed conductor composites
have found application in serving as advanced electrode materials, and in 2015, a
review specifically describing the types of polymer nanocomposites utilized was
published [92].
The use of inorganic fillers to improve the mechanical properties of polymers
and inhibit PEO crystallization to allow for ion transport at room temperature has
been applied yet again for lithium conductivity. In 1998, Croce, et al. demonstrated
that inorganic nanofillers can be used as a type of “solid plasticizer” and allowed for
room temperature conductivity. They utilized 13 nm TiO 2 nanoparticles and 5.8 nm
Al 2 O 3 , nanoparticles, resulting in large conductivity enhancements, ranging from
10
−3
and 10
−5
S cm
−1
versus the range 10
−4
to 10
−8
S cm
−1
for the ceramic-free electrolytes in the temperature range 30–80 °C. They also measured an average Li
+
transference number of ~0.6 in the 45–90 °C temperature range, which was the
highest at the time for PEO-LiX [4]. Similarly, silica nanoparticle/PEO composites
were described, whereby monodisperse 12 nm silica nanoparticles were grown in
situ. Conductivities of 1.2 × 10
−3
S cm
−1
at 60 °C and 4.4 × 10
−5
S cm
−1
at 30 °C were
reported. An electrochemical stability window up to 5.5 V was observed [93].
Ceramic particles are often considered to be inert fillers, yet their exact role
remains under investigation. The first reports simply used this method to improve
mechanical stability of the films. Wieczorek et al. suggested that small particles
(below 4 μm in size), allowed for enhanced interactions between the matrix and
surface, owing to the significant increase in surface area [94]. This was followed by
a study in 1998 describing the particle surfaces as binding sites for lithium ions,
which behave as hard Lewis acids [95]. Lewis acid–base reactions on the surface of
the particles are suggested to play an integral part in the conductivity. Further investigation by Croce, et al. demonstrated that the inorganic nanofillers behave as crosslinkers for the PEO and X-anions, stabilizing their separation and providing a
modified matrix for lithium ion transfer [96]. More recently, solid composite polymer electrolyte with Y 2 O 3 -doped ZrO 2 nanowires having positive-charged oxygen
vacancies resulted in 2-order-of-magnitude increase in lithium ion conductivity
[97]. In 2001, it was reported that ZnO nanoparticles (3.5 nm) having acetate groups
on the surface cooperate with the PEO segments and lithium ions to form crosslinked structures, decreasing the film’s crystallinity and enhancing conductivity [98].
Nanosized fillers have an advantage over bulk materials in that they disperse
more readily and reduce grain boundaries, while providing more free space for
polymeric chains to move. Yet, they may precipitate from the matrix at high concentrations. In order to circumvent this problem, researchers designed a threedimensional nanostructured hydrogel-derived Li 0.35 La 0.55 TiO 3 framework, which
was then used as a 3D nanofiller for PEO and displayed improved Li-ion conductivity up to 8.8 × 10
−5
S cm
−1
at room temperature [99].
The microstructure of the matrix is also of importance. A nanocomposite polymer electrolyte based on vinylidene difluoride-hexafluoropropylene copolymer
C. A. Bauer