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\DOI{10.5802/crchim.394}
\datereceived{2024-09-17}
\daterevised{2024-12-03}
\dateaccepted{2025-03-17}
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\dateposted{2025-06-04}
\begin{document}

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\title{Fluorinated materials in electrochemical storage and conversion
devices: assessment of advantages and disadvantages}

\alttitle{Mat\'{e}riaux fluor\'{e}s dans les dispositifs
\'{e}lectrochimiques de stockage et de conversion : \'{e}valuation des
avantages et des inconv\'{e}nients}

\author{\firstname{Yunfan} \lastname{Shao}\CDRorcid{0000-0002-2770-7036}}
\address{Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble
INP, LEPMI, 38000 Grenoble, France}

\author{\firstname{Lysandre} \lastname{Hourdin}}
\addressSameAs{1}{Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS,
Grenoble INP, LEPMI, 38000 Grenoble, France}

\author{\firstname{Jean-Yves} \lastname{Sanchez}\CDRorcid{0000-0003-1840-4647}}
\addressSameAs{1}{Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS,
Grenoble INP, LEPMI, 38000 Grenoble, France}
\email[J.-Y. Sanchez]{jean-yves.sanchez@grenoble-inp.fr}

\author{\firstname{Cristina} \lastname{Iojoiu}\CDRorcid{0000-0002-5823-8025}\IsCorresp}
\addressSameAs{1}{Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS,
Grenoble INP, LEPMI, 38000 Grenoble, France}
\email[C. Iojoiu]{cristina.iojoiu@grenoble-inp.fr}

\shortrunauthors

\keywords{\kwd{Lithium salts}\kwd{Fuel cells}\kwd{Fluorinated solvents}\kwd{Fluorinated polymers}\kwd{Lithium batteries}}

\altkeywords{\kwd{Sels de lithium}\kwd{Piles \`{a} combustible}\kwd{Solvants fluor\'{e}s}\kwd{Polym\`{e}res fluor\'{e}s}\kwd{Batteries au lithium}}

\begin{abstract}
Electrochemical energy storage and conversion systems, such as
rechargeable lithium batteries and fuel cells, are considered to be
nexuses that link chemical and electrical energies. To achieve
efficient, safe, and sustainable energy storage and conversion, many
new materials must be designed, synthesized, and tested. Among these,
fluorinated organic and inorganic materials play a key role. The highly
electronegative fluorine atoms give these materials exceptional
stability against degradation, as well as improved performance in
electrochemical processes and the development of next-generation
solid-state batteries, PEMFC and electrolyzers. In this article, we
provide a brief overview of our recent work on fluorinated materials
developed at the Laboratory of Electrochemistry and Physical-Chemistry
of Materials and Interfaces (LEPMI, Grenoble). These include new salts,
solvents, separators, binders, polymer electrolytes, etc.\ for batteries
and fuel cells. We hope that this article will raise awareness of the
importance of fluorinated synthons, molecules and materials in
electrochemical energy sources, especially given the serious
environmental concerns surrounding PFAS compounds.
\end{abstract}

\begin{altabstract}
Les syst\`{e}mes \'{e}lectrochimiques de stockage et de conversion de
l'\'{e}nergie, comme les batteries rechargeables au lithium ou les
piles \`{a} combustible, relient l'\'{e}nergie chimique \`{a}
l'\'{e}nergie \'{e}lectrique. Pour qu'ils soient efficaces,
s\'{e}curitaires et durables, il faut concevoir, synth\'{e}tiser et
tester plusieurs nouveaux mat\'{e}riaux. Parmi ceux-ci, les
mat\'{e}riaux organiques et inorganiques fluor\'{e}s rev\^{e}tent une
importance capitale. Les atomes de fluor, qui sont tr\`{e}s
\'{e}lectron\'{e}gatifs, conf\`{e}rent \`{a} ces mat\'{e}riaux une
grande stabilit\'{e} pour r\'{e}sister \`{a} la d\'{e}gradation,
am\'{e}liorer les performances dans le processus \'{e}lectrochimique et
favoriser l'av\`{e}nement de la prochaine g\'{e}n\'{e}ration de
batteries {\og} tout solide {\fg}, de piles \`{a} combustible et
d'\'{e}lectrolyseurs. Dans cet article, nous pr\'{e}sentons
bri\`{e}vement nos r\'{e}cents travaux sur les mat\'{e}riaux
fluor\'{e}s d\'{e}velopp\'{e}s ou caract\'{e}ris\'{e}s au sein du
Laboratoire d'\'{E}lectrochimie et de Physico-chimie des Mat\'{e}riaux
et des Interfaces (LEPMI, Grenoble). Nous abordons notamment les
nouveaux sels, solvants, s\'{e}parateurs, liants, \'{e}lectrolytes
polym\`{e}res, etc., destin\'{e}s aux batteries et aux piles \`{a}
combustible. Nous esp\'{e}rons que cet article attirera l'attention sur
l'importance des synthons, des mol\'{e}cules et des mat\'{e}riaux
fluor\'{e}s dans les sources d'\'{e}nergie \'{e}lectrochimique, en
particulier dans le contexte des graves pr\'{e}occupations
environnementales li\'{e}es aux compos\'{e}s PFAS.
\end{altabstract}

\maketitle

{\vspace*{10pt}}

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\section{Introduction}

In the context of addressing the challenges of climate change and
energy crises, the energy storage and conversion industry is
experiencing rapid growth. Many new synthetic chemicals are used in the
production of electrochemical systems, making it a technology-intensive
sector. Fluorinated organic and inorganic materials are widely used in
electrochemical energy sources, including electrochemical storage
devices (batteries, supercapacitors) and electrochemical conversion
devices (fuel cells).\looseness=1 

\begin{figure*}
\includegraphics{fig01}
\vspace*{-1pt}
\caption{\label{fig1}Schematic representation of (left) a Li-ion
battery (graphite as negative electrode) and (right) a Li-metal battery
(Li metal as negative electrode). The main components containing
fluorinated compounds are presented.}
\vspace*{-2pt}
\end{figure*}

Rechargeable lithium-ion batteries (LIBs) are electrochemical cells
that convert chemical energy into electrical energy. They have several
advantages over other battery chemistries.\ For example, the voltage
between their two electrodes is much higher, ranging from 3 to 4~volts,
versus 1.5 to 2.2~volts for other batteries.\ This provides better
performance in terms of energy and power density~\cite{1}.\ The battery
consists of two electrodes: one positive and one negative An
ion-conducting membrane separates them (\mbox{Figure}~\ref{fig1}).\ In
LIBs, both the negative and positive electrodes consist of composite
electrodes, with materials such as graphite, iron phosphate, cobalt
oxide, etc.\ These composite electrodes also contain carbon black, an
electrolyte, and a polymeric binder.\ In between, a membrane is either a
porous polymer separator filled with liquid electrolyte, or a dense
ion-conducting polymeric film{\break} (sometimes swelled by a liquid
electrolyte). It's important to note that liquid-free electrolytes are
used in all-solid-state batteries based on ceramic or polymeric
electrolytes, allowing lithium metal (Li) to be used as a negative
electrode.

The key requirements for the materials used in electrochemical systems,
such as electrolytes, separators, binders, etc., include high ionic
conductivity, a wide electrochemical stability window, and good
chemical and thermal stability~\cite{2}. Fluorine-containing organic
compounds meet almost all of these requirements, especially high anodic
stability. Currently, the recognized negative environmental impact of
perfluoroalkyl and polyfluoroalkyl substances (PFAS)~\cite{3,4} is
offset by their positive effects on energy and power density, life
span, and safety. Therefore, they still play an important role in the
development of batteries.

In the Proton Exchange Membrane Fuel Cells (PEMFC), fluorinated
compounds are of utmost importance in energy conversion systems. PEMFCs
directly produce electrical energy by converting the chemical energy
stored in fuels, such as hydrogen (H$_{2}$) and methanol
(CH$_{3}$OH)~\cite{5,6}, with a low environmental impact due to their
low or even zero greenhouse gas emissions (Figure~\ref{fig2}a). It also
has high theoretical (83\%) and practical (${\sim}$50\%) efficiency,
making it the most developed and most promising fuel cell. The PEMFC
requires low maintenance costs, thanks to its low operating temperature
(around 80~{\textdegree}C) and quick start-up, which makes it suitable
for a wide range of applications, from stationary use to
electromobility. It is often used in hybrid form with LIBs~\cite{7}.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}(a)~Schematic representation of a PEMFC. (b)~PFSA,
e.g., Nafion and its chemical structure and morphology.}
\vspace*{-2pt}
\end{figure*}

In this system, the membrane, sandwiched between the two electrodes,
allows protons to transfer from the anode to the cathode while
preventing the transfer of reactant gases. Both electrodes contain
binders and electrolytes based on perfluorosulfonic acids (PFSA), such
as Nafion{\texttrademark} or Aquivion{\texttrademark}. The advantages
of these polymers come from their unique chemical structure, which is
made up of a highly hydrophobic, chemically stable perfluorinated
backbone, i.e., polytetrafluoroethylene (PTFE), and perfluorinated
ether side chains that end in a sulfonic acid, i.e., a highly
hydrophilic superacid. This structure induces a strong thermodynamic
incompatibility between the backbone and side chain, which results in a
specific nanoscale \mbox{self-organization} in the presence of
water~\cite{8}. This, in turn, leads to the formation of well-defined
nano-separated hydrophilic and hydrophobic phases and percolated
nanodomains, as shown in Figure~\ref{fig2}b for an example of Nafion.

The LEPMI has pioneered the development and application of lithium
salts, polymer electrolytes, and the study of the impact of different
commercial or homemade fluorinated or partially fluorinated compounds
on the electrochemical performance of the system. 

In this article, we have selected a few examples from among many others
to illustrate the range of studied materials and their effect on
electrochemical properties. 

In a first part, we focused on the various components of lithium-ion
batteries (binder, solvent, salt, polymer electrolyte). In the final
section, we present briefly some alternatives to PFSA for use in fuel
cells. 

\section{Lithium-ion batteries (LIBs)} \label{sec2}
\subsection{Fluoropolymers as binders for composite{\hfill\break}
electrodes} \label{sec2.1}

The binder material in electrodes, even though it is considered inert,
plays a crucial role in maintaining structural integrity, improving
interconnectivity within the electrode, enhancing adhesion to the
current collector, and forming the solid--electrolyte interface (SEI)
during the initial battery charging cycles~\cite{9,10}. This polymeric
binder must have excellent electrochemical stability and be compatible
with the solid powders (active materials and conductive fillers), and
also be insoluble in the electrolytes. Nevertheless, it must have a
sufficient affinity for the electrolyte so that it is well impregnated,
allowing the composite electrode to have good ionic conductivity.
Fluorinated binders exhibit better electrochemical stability thanks to
their superior resistance to oxidation compared to non-fluorinated
binders~\cite{10}.\looseness=1

The binders most commonly used today are based on PVdF (polyvinylidene
fluoride) and its copolymers.\ Among these, PVdF--HFP poly \mbox{(vinylidene}
fluoride-co-hexafluoropropene) is the most widely used. Since
hexafluoropropene (HFP) cannot homopolymerize, its well-defined
distribution along the chains must be spaced by the PVdF moieties. The
PVdF and PVdF-based copolymers have excellent anodic stability and are
used extensively as binders for various cathode materials. Due to the
disorder caused by the inclusion of the HFP monomer, the~crystallinity
and melting temperature ($T_{\mathrm{c}}$ and $T_{\mathrm{m}}$, respectively) decrease
with increasing HFP concentration, while the glass transition
temperature ($T_{\mathrm{g}}$) increases. However, the stability of PVDF and
PVdF-based copolymers in alkaline conditions is limited, where they
undergo a dehydrofluorination reaction by successive elimination.
Despite this instability, PVdF-based binders have been extensively used
in composite negative electrodes. There is, however, a trend to replace
them with carboxymethyl cellulose (CMC)~\cite{11}. In order to improve
the interaction \mbox{between} the binder and the active electrode
material, some copolymers bearing polar protic groups, such as PVdF
grafted by vinyl phenol, were also used~\cite{12}.

In composite electrodes, the goal is to maximize the content of active
material, i.e., specific \mbox{capacity}, while minimizing the content
of binder and \mbox{conductive} carbon. This must be done without
compromising mechanical strength or electronic conductivity. This can
be achieved by optimizing the performance of the PVdF binder,
particularly its molecular weight distribution. Increasing $M_{w}$ and
$M_{n}$ will improve both the mechanical strength of the composite
electrodes and the binder's adhesion to the active material and the
carbon black.

\subsection{Fluorinated organic solvent and porous{\hfill\break}
separators} \label{sec2.2}
\subsubsection{Fluorinated organic solvent} \label{sec2.2.1}

The conventional electrolytes used in LIBs are based on lithium salts
dissolved in organic or polymer solvents. The basic function of an
electrolyte is to transport ions while blocking electron conduction
between the two electrodes. In the case of liquid electrolytes, the
salts are typically dissolved in organic carbonate-based solvents, such
as cyclic carbonates: ethylene carbonate (EC), propylene carbonate
(PC), and linear carbonates: dimethyl carbonate (DMC), diethyl
carbonate (DEC), etc.\ (Figure~\ref{fig3}a). Cyclic carbonates (EC, PC)
offer several benefits. They have a high dielectric constant, making it
possible to use highly concentrated lithium salt solutions. However,
they also have a high viscosity and melting point, especially in the
case of EC. This can affect ionic conductivity. Therefore, these
carbonates are typically mixed with linear carbonates, such as DMC,
DEC, or ethyl-methyl carbonate (EMC), which exhibit lower viscosity and
dielectric constants. This allows them to optimize the electrolyte's
properties. In addition to carbonates, electrolytes based on ethers or
acetals are also widely used, especially in alternative battery
systems. For example, dioxolane (DOL, acetal) and dimethoxyethane (DME,
ether) are widely used in lithium--sulfur batteries. These
low-viscosity electrolytes have good ionic conductivity, but they have
low flash point Fp, low boiling temperature, and limited anodic
stability.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}(a)~Usual organic solvents used in liquid
electrolytes. (b)~Fluorinated solvents.}
\end{figure*}

Fluorinated or partially fluorinated organic solvents
(Figure~\ref{fig3}b) are also widely used in lithium batteries because
fluorine atoms exhibit unique high electronegativity and low
polarizability. In comparison with fully fluorinated organic solvents,
partially fluorinated organic solvents (such as fluoroethylene
carbonate [FEC]) have fairly high polarity, although less fluorine
substitution~\cite{13}. Multiple studies~\cite{14,15,16} have
demonstrated that replacing hydrogen atoms with fluorine can
effectively reduce the HOMO and LUMO energy levels, thereby improving
anodic stability, but decreasing cathodic stability due to the
electron-withdrawing inductive effect of the fluorine
atoms~\cite{15,17}. Compared to nonfluorinated organic solvents, the
fluorinated solvents and cosolvents show higher anodic stability, with
an oxidation limit that extends to 4.3--4.5~V versus Li$^{+}$/Li. In
addition, they are much less flammable. Additional to the improved
anodic stability, the increase in the reduction potential (decrease in
cathodic stability) can lead to the formation of an effective SEI,
which prevents further decomposition of the electrolyte on the negative
electrode's surface, resulting in an improved cycle life. In the
fluoroethylene carbonate (FEC) solvent, the presence of fluorine has a
positive effect in the SEI formation and composition~\cite{18}. The
C--F bond is broken leading to LiF and vinylene carbonate
(VC)~\cite{19}. Additionally, partially fluorinated acyclic carbonate
solvent mixtures overcome the recurring corrosion of aluminum current
collectors, which typically occurs at around 3.7--3.8~V versus
Li/Li$^{+}$ with lithium salts such as triflate and sulfonimide
(LiTFSI)~\cite{20}. The same is true for replacing expensive
fluorinated diethyl carbonates with cheaper fluorinated
carbamates~\cite{14}. Beyond the previous advantages of fluorinated
solvents, we should note that they have fairly limited salt solubility.
To ensure a good salt concentration in the electrolyte, these solvents
must be mixed with non-fluorinated ones.

\subsubsection{Porous separators based on PVdF copolymers}
\label{sec2.2.2}

According to IUPAC nomenclature rules, since they have an average pore
diameter of over 100~nm, these porous separators are called
``macroporous'' rather than ``microporous''. The usual separators are
made of polyolefin, either based on polyethylene (PE), such as
Solupor{\texttrademark}, or polypropylene (PP), such as
Celgard{\texttrademark}. Their high crystallinity ensures their
mechanical strength, which starts to decrease when the temperature
approaches their $T_{f}$. Copolymers of PVdF--HFP, due to their lower
crystallinity and high electrolyte uptake, are not suitable for making
macroporous separators for LIBs because they lose their mechanical
properties~\cite{21}. Therefore, we investigated in detail the
physical--chemical behavior of PVdF homopolymers swelled by individual
solvents e.g., EC, PC, DMC, DEC and by solvent mixtures e.g.\ EC, DMC,
last by lithium electrolytes \mbox{\cite{22,23,24,25,26}}. We also
studied their ions mobility~\cite{27}. The sets of porous PVdF/liquid
electrolyte were found to markedly improve LIBs~\cite{28,29}. This
clear improvement can be attributed to several factors. First, it may
result from the better filling of PVdF pores by the polar, hydrophilic
liquid electrolytes compared to traditional separators made of PE
or~PP.{\break} Second, the~higher ionic conductivity, as indicated by
the lower McMullins number, $N_{\mathrm{M}}$, also contributes. Third, the
better compatibility between the electrode binder and the separator is
another factor. However, there are drawbacks. For example, the porous
separator weakens due to the swelling of the inter-pore polymer by the
electrolyte. In order to counteract this weakening, we chose to
strengthen the macroporous separator with crystalline nanocellulose
(CNC or NCC). NCC has been found to significantly reinforce the porous
PVdF without sacrificing ionic conductivity~\cite{30}. Furthermore, a
PVdF/NCC macroporous separator 20~$\upmu$m thick showed better{\break}
performance in both half and full cells than the commercially available
Celgard2400$^{\text{\textregistered}}$~\cite{31}. Another energy
storage device, namely supercapacitors, has been investigated. In these
devices, the electrolyte is an acetonitrile solution of
tetraalkyl-ammonium tetrafluoroborate. Despite this, PVdF separators
still experienced the same mechanical problems as those in
LIBs~\cite{32}. Interestingly, a neat reinforcement was observed when
performing the phase inversion on woven tissues made of nylon and
polyethylene terephthalate (PET). However, these available tissues are
too thick, which compromises the ionic conductance~\cite{33}. Another
option is to modify the pore \mbox{surface} while partially
crosslinking the separator using gamma irradiation~\cite{34}. Despite
its advantages in terms of conductivity and anodic stability, the high
cost of PVdF makes it uncompetitive for supercapacitor markets.
Electrospinning~\cite{35} is an alternative process that produces
well-defined fibers, but its industrial scalability is currently
questionable.\looseness=1

\section{Lithium metal polymer batteries} \label{sec3}
\subsection{Lithium salts and polymer electrolytes} \label{sec3.1}

Although liquid electrolytes are currently the standard in the battery
industry, conventional liquid electrolytes pose serious safety issues.
During battery failure, thermal runaway can be triggered by electrical,
mechanical, thermal abuse, or a latent defect inside the
cell~\cite{36}. Liquid electrolytes without mechanical strength clearly
cannot protect batteries against these three types of abuse. Mechanical
abuse can cause electrolyte leaks, which can lead to overcharging or
overdischarging, or even internal short circuits. Additionally, a major
trend in increasing energy density is to replace the carbon-based
active material (graphite, hard and soft carbon, etc.)~\cite{37} in the
negative electrodes by lithium metal, owing to its exceptionally high
capacity of 3840~mAh${\cdot}$g$^{-1}$ and very low redox potential of
${-}$3.04~V versus SHE~\cite{38,39}.

However, the standard liquid organic electrolytes do not form a stable
solid--electrolyte interphase (SEI) on the surface of the lithium
metal, even with fluorinated solvents. This results in continuous
electrolyte decomposition, the formation of a thick, resistive SEI, and
significant lithium loss. In the worst cases, a highly inhomogeneous
lithium deposition can occur, which can eventually lead to dendrite
growth that threatens the battery's safety~\cite{40}.

Therefore, identifying viable alternative electrolyte systems is of the
utmost importance, as it will enable the transition from LIBs to
lithium metal batteries (LMBs). Among all the possible candidates,
polymer electrolytes are generally considered to be the most promising
alternatives to the current organic liquid electrolytes, as they would
increase battery safety and allow the use of lithium metal as the
negative. Polymer electrolytes can be considered as simply a polymer
mixed with some salt, where the polymer acts as a solvent endowed with
mechanical strength and the salt is the source of charge
\mbox{carriers}, i.e., ions. With the polymer matrix, the electrolyte
gains a better mechanical strength and provides a neat safe improvement
compared to liquid electrolytes.

In fact, polymer electrolytes have been one of the hotspots in the
field of energy storage materials. To classify these electrolytes,
various standards can be used. One way is to differentiate between two
types of polymers. Dual ion-conducting polymers are blends of salt and
polymer, while single-ion-conducting polymers (SIPE) have fixed anionic
functions, such as being bound to the polymer chain. The other way is
more closely related to the role of the polymer. It can play the role
of the solvent and thus transports the ions by itself (dry or solvating
polymer electrolyte), or an organic solvent is added (plasticized or
gelled polymer electrolytes). 

In our group, we developed different fluorinated and partially
fluorinated lithium salts. Our goal was to improve the dual polymer
electrolyte performance as well as different types of SIPE. Examples
will be discussed in the next section.

\begin{figure*}
\includegraphics{fig04}
{\vspace*{-.15pc}}
\caption{\label{fig4}Energy profile and schematic illustration of the
ion transport mechanism of polymer electrolytes.}
{\vspace*{-.45pc}}
\end{figure*}

\subsubsection{Salts and dry dual-polymer electrolytes}
\label{sec3.1.1}

The ion conductivity in solvent-free or ``dry'' polymer electrolytes,
especially those based on poly(ethylene oxide) (PEO, trivial
nomenclature) or poly(oxyethylene) (POE, IUPAC nomenclature) blended
with a lithium salt, takes place in the amorphous phase via the
segmental motion of the polymer chains that coordinate the Li$^{+}$
cations. The mechanism of this ion transport is usually described by
the free volume model, as illustrated in Figure~\ref{fig4} \cite{41}.
Above the $T_{\mathrm{g}}$, polymer chains in an amorphous state have local
segmental movement, which results in the availability of free volume in
the immediate vicinity of the moving chain segments. This free volume
allows an intermolecular coordination of lithium cations and, due to
the polymer segmental motion, Li$^{+}$ could jump from one polymer
chain to another or along a single polymer chain by switching the
coordination sites in the presence of an electric field (``hopping''
mechanism), i.e., the transfer of Li$^{+}$ from one coordination site
to another occurs due to the sufficiently reduced energy barrier
(Figure~\ref{fig4}) \mbox{\cite{42,43,44}}.\looseness=-1

In an electrolyte, both cations and anions can act as charge carriers
and conduct the current. We can consider the total current $I$ as the
sum of the currents carried by the cations $i^{+}$ and the current
conducted by the anions $i^{-}$~\cite{45}. Therefore, the cation
transference number $t^{+}$ is defined as:
{$$
t^{+}=\frac{i^{+}}{i^{+}+i^{-}}.
$$}\unskip
In lithium batteries, energy storage and release occur as lithium ions
migrate between the two electrodes. Therefore, the lithium transference
number indicates the efficiency of lithium transportation, which is
essential for battery performance.

Lithium salts are one of the key components of the electrolyte. They
provide lithium ions, but also, due to their nature and structure,
affect ionic conductivity (through dissociation and transference
numbers). The physical--chemical and electrochemical properties of these
salts influence the overall performance of a battery. The essential
requirements of a good lithium salt in an electrolyte are complete
dissolution and dissociation in a nonaqueous solvent, high degradation
temperatures and a broad \mbox{electrochemical} stability window (ESW).
It is known the only lithium salts, which are cheap and intrinsically
stable in reduction, are the lithium halides. Among them, LiF shows
also a high anodic stability but it exhibits a very poor solubility in
aprotic solvents. Both LiCl and LiBr lead to poorly conductive
electrolytes, whereas LiI, although the most conductive, lacks
sufficient anodic stability. Indeed, to achieve sufficient dissociation
in aprotic polar solvents (liquid mixtures of carbonates) and poorly
polar ones (tetraglyme and PEO), both having low acceptor numbers (AN),
the anion's basicity must be as low as possible, meaning that the
anions must be the conjugated bases of very strong acids, i.e.,
superacids. Due to the strong electron-withdrawing properties of
fluorine and its ability to delocalize anions charges, almost all known
salts are fluorinated. These fluorinated salts exhibit high solubility
in aprotic solvents. They play a significant role in LIBs and LMBs,
especially in the formation of a stable SEI or Cathode Electrolyte
Interface (CEI)~\cite{46}. They can also reduce the surface tension of
the electrolyte, enhancing the wettability of both porous separators
and composite electrodes.

In the current state of knowledge, the most studied salts are
LiPF$_{6}$, LiBF$_{4}$, LiTFSI (LiN[CF$_{3}$SO$_{2}$]$_{2}$), LiFSI,
LiClO$_{4}$, and LiTf (LiCF$_{3}$SO$_{3}$), among others
(Figure~\ref{fig5}a). LiPF$_{6}$ is the most widely used salt in liquid
electrolytes because of its excellent solubility in organic solvents,
high ionic conductivity (up to $10^{-2}$~S${\cdot}$cm$^{-1}$ at room
temperature), and high anodic stability (over 4.2~V versus
Li/Li$^{+}$). The $\mathrm{PF}_{6}^{-}$ anion has been proved to form a
protective layer on the positive current collector, made of aluminum,
that prevents further corrosion~\cite{47,48}. However, LiPF$_{6}$ is
not stable above 50~{\textdegree}C, nor tolerant to traces of moisture,
which leads to the production of HF. Its instability makes it
questionable for use in rechargeable batteries. The $\mathrm{XF}_{n}^{-}$ anions
(X~${=}$~B, As, P, Sb) are formed through a dative bond between F$^{-}$
and the vacant orbital of the strong Lewis acid XF$_{n-1}$. As a
result of the weakening of coordination bonds compared to classical
covalent bonds, these anions undergo reversible decomposition:
$\mathrm{XF}_{n}^{-} \rightarrow \mathrm{F}^{-} +
\mathrm{XF}_{n-1}$~\cite{49,49a}. The resulting release of strong Lewis
acids makes them unsuitable for use in polymer electrolytes, whose host
polymers have a highly sensitive polyether backbone to chain-breaking
induced by strong Lewis acids. Thus, to improve the ionic conductivity
while maintaining the mechanical stability of the polymer, we proposed
about 34 years ago to replace lithium perchlorate and lithium triflate
with salts based on large, highly delocalized anions, such as
(i)~lithium trifluoromethanesulfonyl imide, LiTFSI,
LiN[(CF$_{3}$SO$_{2}$)$_{2}$] \cite{50} and (ii)~the carbanion-based
salt, methide, LiC[(CF$_{3}$SO$_{2}$)$_{3}$] \cite{51}. Despite its
attractive performance, the methide salt suffers from its high
molecular weight\break (411~g${\cdot}$mol$^{-1}$).

\begin{figure*}
\includegraphics{fig05}
{\vspace*{-.2pc}}
\caption{\label{fig5}(a)~Commercially available lithium salts.
(b)~Aryl-based lithium salts developed at LEPMI.}
{\vspace*{-.5pc}}
\end{figure*}

Recently, we developed and studied various aromatic salts. Paillard
{et~al.}\ reported on the performance of the lithium pentafluorobenzene
sulfonate, C$_{6}$F$_{5}$SO$_{3}$Li, hosted by high molecular weight
poly(oxyethylene) POE~\cite{52}. The POE/C$_{6}$F$_{5}$SO$_{3}$Li
complexes display moderate conductivities and high cationic
transference numbers, $t^{+}$, within a limited concentration range, as
the solubility of the salt is limited. Surprisingly, however, an
increase in strength was observed above the melting point of the
electrolyte. Transient physical crosslinks involving interactions
between oxygen's lone pairs and electronic impoverishment of the
C$_{6}$F$_{5}$SO$_{3}$Li aryl moiety, induced by the five fluorine
substituents, were assumed. Aryl anions with a general formula of
C$_{6}$H$_{5}$--SO$_{x}$--(CF$_{2}$)$_{n}$--A$^{-}$ were prepared using
multistep reactions~\cite{53,54}. In this formula, $x = 0$ (sulfide), 1
(sulfoxide) or 2 (sulfone), and $n = 1$ or~2. A$^{-}$ is either a
sulfonate ($\mathrm{SO}_{3}^{-}$, \textbf{1--7}) (Figure~\ref{fig5}b) or a
sulfonamide (\mbox{CF$_{3}$--SO$_{2}$--N$^{-}$}, \textbf{8--9}). This
leads to a wide variety of salts (Figure~\ref{fig5}b). Their
performance (conductivity, $t^{+}$, and electrochemical stability
window) in the configuration of solvent-free polymer
electrolytes~\cite{55,56} are very interesting, especially because,
thanks to their aryl moiety, these salts can be easily converted into
ionic monomers, such as ionic styrene~\cite{57}. To reduce the fluorine
content of TFSI anions, while maintaining an extended negative charge
delocalization, we synthesized anilinyl-sulfonamide salts
\mbox{(\textbf{10--15})} using simple and inexpensive
chemistry~\cite{58}. In this series, the fluorine content per mole of
salt of the best lithium salt is~3, compared to 6 for LiTFSI. These
salts exhibit cationic transport numbers greater than those of LiTFSI
(Table~\ref{tab1}).\looseness=-1

\begin{table}%tab1
\caption{\label{tab1}Ionic conductivity and lithium transference
numbers of the POE/salt electrolytes with an EO/Li ratio of~30}
\tabcolsep3.7pt\fontsize{9.7}{12}\selectfont
\begin{tabular}{ccccc}
\thead
Salt &amp; \parbox[t]{4pc}{\centering Temperature ({\textdegree}C)} &amp; 
\parbox[t]{6.2pc}{\centering Ionic conductivity (mS${\cdot}$cm$^{-1}$)} &amp; 
$t^{+}$ &amp; Ref.\vspace*{2pt}\\
\endthead
LiTFSI &amp; 70 &amp; 0.63\0 &amp; 0.17 &amp; \cite{58}{\vspace*{4pt}}\\
\01 &amp; \morerows{6}{70} &amp; 0.087 &amp; 0.49 &amp; \morerows{6}{\cite{56}}\\
\02 &amp;&amp; 0.12\0 &amp; 0.54 &amp;\\
\03 &amp;&amp; 0.18\0 &amp; 0.48 &amp;\\
\04 &amp;&amp; 0.11\0 &amp; 0.39 &amp;\\
\05 &amp;&amp; - &amp; 0.35 &amp;\\
\06 &amp;&amp; 0.088 &amp; 0.47 &amp;\\
\07 &amp;&amp; 0.086 &amp; 0.35 &amp;{\vspace*{6pt}}\\
\08 &amp; \morerows{1}{80} &amp; 0.36\0 &amp; 0.21 &amp; \morerows{1}{\cite{59}}\\
\09 &amp;&amp; 0.21\0 &amp; 0.25 &amp;{\vspace*{6pt}}\\
10 &amp; \morerows{5}{70} &amp; 0.27\0 &amp; 0.4\0 &amp; \morerows{5}{\cite{58}}\\
11 &amp;&amp; 0.31\0 &amp; 0.35 &amp;\\
12 &amp;&amp; 0.31\0 &amp; 0.27 &amp;\\
13 &amp;&amp; 0.38\0 &amp; 0.23 &amp;\\
14 &amp;&amp; 0.48\0 &amp; 0.20 &amp;\\
15 &amp;&amp; 0.42\0 &amp; 0.18 &amp;
\botline
\end{tabular}
\end{table}

\subsection{Single ion-conducting polymer electrolyte{\hfill\break}
(SIPE)} \label{sec3.2}

The SIPE can be a polyelectrolyte (each repeat unit is ionic) or an
ionic copolymer, called an ionomer, in which the anionic function is
fixed by covalently bonding it to the polymer chain, and the free
counterion is Li$^{+}$. In the SIPE, the lithium transference number is
greatly improved because the anionic groups are immobilized on the
polymer chain through covalent bonds. A low Li$^{+}$ transference
number (e.g.,~for POE/LiTFSI, the $t^{+}$ is usually lower
than~0.2)~\cite{60} \mbox{indicates} that most of the ionic motion is
due to the anion, which tends to migrate in the opposite direction of
the lithium ion. It then builds up a concentration gradient and
accumulate on the electrode surface. Due to the immobilization of the
anion, SIPEs eliminate the gradient, thereby avoiding reverse
polarization.\ This improves battery \mbox{performance} at high current
densities~\cite{61}. The lack of concentration gradient could improve
the performance of the battery in several ways, especially in terms of
homogeneity, stability, and safety.

Indeed, if lithium metal is used as a negative electrode, the
concentration gradient can lead to lithium dendrite growth, which can
compromise the safety of the battery. Chazalviel's model~\cite{62}
suggests that the ramified metallic electrodeposits in dilute salt
solutions are mainly driven by the space charge that tends to form due
to anion depletion near the cathode. Another theoretical analysis was
developed by Tikekar {et~al.}~\cite{63}, which predicts that tethering
anions can effectively curb dendrite growth and improve electrolyte
conductivity at large overpotentials, especially at high current
density~\cite{64}. Therefore, SIPEs with a high lithium transference
number could lead to uniform deposition of dendrite-free lithium. This
is very promising for LMBs with higher current densities.

However, the ionic conductivity of SIPEs is generally insufficient at
low temperatures, even after considering the effect of such an
increased lithium transference number. Another point is that,
generally, block copolymers or random copolymers with inhomogeneous
microstructures can undergo microphase segregation into ion-rich and
ion-poor phases. This leads to an inhomogeneous distribution of anions.
At LEPMI, we developed new SIPEs to overcome these challenges, allowing
SIPE-based batteries to reach competitive energy and power densities.
We have a large activity in designing solvating SIPEs, some of them at
pilot scale, and gelled SIPEs. These allow for the first time the use
of high potential positive electrodes, such as lithium nickel manganese
cobalt oxide (LiNi$_{x}$Mn$_{y}$Co$_{1-x-y}$O$_{2}$).

\subsubsection{Dry SIPEs} \label{sec3.2.1}

To create a high-performance dry SIPE, we must design polymers with
high dielectric constants or/and donor numbers, as well as good polymer
chain mobility (e.g., POE). In addition, homogeneous distribution of Li
salt along the chain polymer and good mechanical strength in the final
membranes are essential.

To generate monomer-based salts that could be easily polymerized by
polycondensation or free-radical polymerization, we chose aryl-based
salts whose aryl groups can be easily modified into various functional
moieties, allowing them to be (co)polymerized~\cite{48,65}.

\begin{figure*}
{\vspace*{-.2pc}}
\includegraphics{fig06}
{\vspace*{-.2pc}}
\caption{\label{fig6}(a)~Schematic illustration of desired properties
for an ideal SIPE. (b)~An example of solvating SIPE with 3D structures
resulting from linear SIPE bearing double bonds. (c)~An example of
solvating SIPE with linear/branch structures.}
{\vspace*{-.2pc}}
\end{figure*}

Two examples are shown in Figure~\ref{fig6}b. The first SIPE was
obtained by polycondensation of a difluorinated aryl monomer bearing
perfluorosulfonate functions with poly(ethylene glycol) (PEG) of
different chain lengths. The distance between two Li salts is fixed by
the length of PEG, which allows us obtaining SIPE with a very
homogeneous salt distribution. In order to obtain membranes with both
high chain mobility and good mechanical properties, we introduce a few
double bonds into the copolymer, which can be crosslinked by UV
irradiation. To boost the mechanical properties, a small amount of NCC
is added during the membrane manufacturing. Finally, membranes with
excellent thermal/electrochemical stability and excellent compatibility
with lithium metal were obtained. This process was developed on a pilot
scale at LEPMI as part of a maturation project (E2PS) and published
recently~\cite{66}.

For the second route, SIPEs with improved conductivities
(Figure~\ref{fig7}a) were developed by using a donor--acceptor
copolymerization mechanism of an ionic monomer bearing a vinyl ether
function and a PEG monomer functionalized with a maleate group. This
led to a linear ``brush'' structure with many short PEG side chains,
which significantly increased the overall chain mobility. This type of
SIPE allowed a battery to operate at a relatively low temperature of
40~{\textdegree}C with very good performance~\cite{67}.

\begin{figure*}
{\vspace*{-.1pc}}
\includegraphics{fig07}
{\vspace*{-.15pc}}
\caption{\label{fig7}(a)~Conductivities of SIPE crosslinked (3D) and
linear SIPEs. (b)~Li stripping/plating experiments at current density
of 0.1~mA${\cdot}$cm$^{-2}$, conducted on Li|SIPE|Li cells at
80~{\textdegree}C.}
{\vspace*{-.2pc}}
\end{figure*}

Lithium plating/stripping tests, which are widely used to demonstrate
the compatibility of the electrolyte with lithium metal electrodes,
were conducted on symmetric Li|SIPE|Li cells for both SIPEs that are
widely used to demonstrate the compatibility of the electrolyte with
lithium-metal electrodes. The results presented in Figure~\ref{fig7}b
show that the cell potential remains stable for 6 days at
80~{\textdegree}C without any signs of an unstable interface or of
dendrite formation. The overpotential remained constant and showed a
perfect rectangular shape, which indicates homogenous plating and
stripping of lithium without concentration polarization, a continuous
evolution of the initially formed interphase and/or dendrite
formation~\cite{68,69}.

\subsubsection{Gelled SIPEs} \label{sec3.2.2}

As we saw in the previous section, SIPEs have many advantages when used
with LMBs.\ They \mbox{perform} well at temperatures above
40~{\textdegree}C, thanks to their strong dependence on the segmental
motion of the polymer chain. Additionally, the POE solvating polymer
backbone limits the anodic electrochemical stability to about 4~V. 

In 2018, we developed an approach to overcome conductivity and
electrochemical stability issues by synthesizing a single-ion
conducting multiblock copolymer electrolyte based on poly(arylene ether
sulfone) via polycondensation and the subsequent introduction of small
organic carbonate molecules (such as ethylene carbonate [EC] or
propylene carbonate [PC]) (Figure~\ref{fig8})~\cite{70}. The
alternating multiblock structures, which have lithiated side chains
attached to the polymer backbone and a hydrophobic block, can form an
ionophobic--ionophilic nanophase-separated morphology. The EC or PC
acts as a plasticizer for the ionophilic domains, it also dissociates
the lithium ions and transports them from one coordination site to
another~\cite{70}. In this SIPE, the ionophilic domains enable charge
transport, while the ionophobic domains guarantee mechanical stability
and the self-standing properties of the SIPE membrane, even when it
contains more than 50\% solvent. The temperature dependence of the
ionic conductivity for the SI-Sx (Sx corresponds to \% of EC) is
plotted in Figure~\ref{fig8}c. It is compared to the data for
Celgard$^{\text{\textregistered}}$ membranes impregnated with a liquid
electrolyte containing a salt similar to the site chain of SI
(LiPBTFSI/EC) or LiTFSI/EC. For the SIPE membranes', increasing the EC
content leads to a continuous improvement in lithium conductivity. This
improvement can be attributed to (i)~better percolation of the
ionophilic domains, (ii)~enhanced lithium cation concentration
(dissociation), (iii)~decreased local viscosity, allowing faster
lithium-ion motion, and (iv)~a better electrolyte--electrode interface.
The membranes with an EC content exceeding 50~wt\% exhibit better
conductivities than the liquid references across the entire temperature
range. Remarkably, the conductivity above 30~{\textdegree}C is higher
than $10^{-3}$~S${\cdot}$cm$^{-1}$, which is close to the conductivity of
conventional liquid electrolytes.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Example of single-ion conducting block copolymer:
(a)~molecular structure (SI-). (b)~Scheme of phase separation
(ionophilic--ionophobic domains, EC plasticized only with ionophilic
domains). (c)~Conductivities as a function of temperatures (SI-SX, X
correspond to \% of EC). (d)~Li stripping/plating experiments at
2~mA${\cdot}$cm$^{-2}$ current density, conducted on Li|SIPE70|Li cells
at 20~{\textdegree}C.}
{\vspace*{-.1pc}}
\end{figure*}

Figure~\ref{fig7}d compares the voltage profiles of symmetrical Li/Li
cells using either the liquid reference electrolyte (1M LiTFSI in PC)
within a thick glass fiber separator or a SIPE containing 70\% of PC.
The two systems behave very differently. As can be seen with the liquid
reference electrolyte, the overvoltage first decreases, which might be
due to an increase in the surface area of lithium, but then continues
to rise. This continuous increase can be explained by the formation of
mossy lithium during plating.\ Upon lithium stripping, it leaves behind
electronically disconnected lithium residues mixed with electrolyte
reduction products, which end up forming a mixed layer that slows down
lithium transport~\cite{71,72}.{\break} The~voltage profiles of cells
using a SIPE (SI-S70 [SI10/5 70 PC]) membrane are very different,
exhibiting a square shape with no evidence of internal resistance
change during plating/stripping, either at the start or at the end of
the cycling.

Remarkably, due to the wide electrochemical stability window of the
SIPE, very stable long-term lithium plating and stripping were
observed, even at high current densities.\ Successful cycling of
Li|SIPE|NMC$_{811}$~\cite{73} and Li|SIPE|NMC$_{622}$~\cite{74} cells
at low temperature (up to 0~{\textdegree}C) was observed. Recently we
investigated the influence of the polymer backbone
fluorination~\cite{75} and we demonstrated an enhanced electrochemical
stability for more fluorinated ones. This improved stability was
attributed to the formation of a stable SEI with lithium metal
electrodes, which results from a fluorine-richer \mbox{interphase}.

\section{Fuel cells: partially fluorinated aromatic{\hfill\break}
membranes for fuel cells} \label{sec4}

The current polymers used in the PEM membrane and the ionomeric binder
used in the catalyst layer belong to the PFSAs (perfluorosulfonic
acids). They are made from tetrafluoroethylene, a very controlled
explosive monomer, which cannot be shipped and must be prepared
on-site. As a result, the synthesis of PFSA ionomers is limited to only
a few research laboratories around the world. Although successful, this
method suffers from high costs. In addition, the rate of gas
(H$_{2}$/O$_{2}$) permeation through PFSA membranes is excessively
large, due to the high solubility of the gases in the perfluorinated
domains. This compromises the purity of gases at the corresponding
electrodes. The drawbacks of PFSA ionomers and membranes alluded to
above have led to a large body of research devoted to alternative
hydrocarbon-based solid polymer electrolytes. However, the
literature on hydrocarbon (HC) ionomers in the catalyst layer is
scarce, and surprisingly, there are very few reports in the
peer-reviewed literature of membrane electrode assemblies (MEA) in
which both membrane and ionomer are HC-based. Ionomers based on
multiblock hydrocarbon copolymers (HCMC) are known for their excellent
thermomechanical properties, easy synthesis/modification, and low cost,
etc.~\cite{76,77,78}. However, most HCMC are functionalized with a
strong acid, such as aromatic sulfonic acid. They are often designed to
have a high ionic exchange capacity (IEC) in order to form proton
exchange membranes (PEM) with high proton conductivity. However, these
types of PEM have high conductivity at high relative humidity, but for
low relative humidity, they suffer from an inability to hold sufficient
water, which causes a significant drop in {conductivity.} Another problem
is the \mbox{limited} solubility of HCMCs in high-boiling, aprotic solvents,
such as N,N-dimethylformamide, and poor solubility in the preferred
solvent for preparing catalyst inks, namely low-boiling point alcohols,
water, and alcohol--water mixtures. The use of low-boiling, aprotic
solvents during catalyst layer deposition is claimed to be necessary
for the manufacture of scaled-up fuel cells with high
performance~\cite{79}.\looseness=-1

Our strategy to address these issues, at LEPMI, was to synthesize HCMC
by grafting, instead of aromatic sulfonic acid, a superacid i.e., a
perfluorosulfonic acid that is known to be able to easily dissociate at
low relative humidity. We have shown that these types of relatively
low-IEC ionomers possess (i)~a hydrophilic/hydrophobic
nanophase-separated morphology, with hydration causing a size evolution
of the hydrophilic phase similar to that of benchmark PFSA
materials~\cite{80} and (ii)~a better proton conductivity than
commercial PFSA ionomer membranes. Another positive point about
choosing this kind of ionomer is that they are soluble in the suitable
solvents for catalyst layer formulation, namely protic solvents (e.g.,
methanol, isopropanol).

Different approaches were studied for the synthesis of these ionomers.
These include: (i)~the synthesis of a backbone via polycondensation,
followed by bromination and Ullmann coupling reactions (Gen~1,
Figure~\ref{fig9}); (ii)~the synthesis of a brominated backbone via
polycondensation, followed by coupling reactions (applied to all
ionomers) (Gen~2 and~3, Figure~\ref{fig9}); and (iii)~the direct
synthesis of ionomers via bottom-up polycondensation reactions of
already functionalized monomers. The final method involves the
synthesis and purification of functional monomers bearing
perfluorosulfonic acid. It might ensure better control over polymer
microstructure. However, this is experimentally very challenging, as a
polycondensation reaction requires highly pure monomers, which cannot
be easily achieved using current chromatographic techniques (e.g.,
preparative HPLC). Therefore, for upscaling, the second method is the
best.\looseness=-1

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}Chemical structure of (a)~Gen1-$xy$,
(b)~Gen2-$xy$, and (c)~Gen3-$xy$ ionomers, where $x$ and $y$ are the
lengths (g/mole) of hydrophobic and ionic block backbone.}
{\vspace*{-.45pc}}
\end{figure*}

The presence of well-separated hydrophobic and hydrophilic phases was
established through thermal analysis (DSC), mechanical analysis (DMA),
morphological characterizations using microscopy (AFM, FESEM) and small
angle neutron scattering (SANS) techniques. The structural organization
at different length scales, as well as the characteristic distances in
relation to the expected block size of HCMC ionomer, strongly supports
the formation of multiblock copolymers with an acceptable
polydispersity for both blocks. 

We have developed several ionomers with various backbone structures and
side chains. We modified the structure of the backbone
(Figure~\ref{fig9}) and the density of perfluorosulfonic acid functions
to achieve moderate ion exchange capacity, water sorption, and improved
proton conductivity. With the block copolymer from Gen3, we obtained
membranes with improved mechanical and thermal properties, higher
conductivity at high relative humidity (RH) and similar conductivity at
low RH as Nafion.

\subsection{Membrane durability} 

HCMC is not considered stable in the presence of free radicals, which
has hindered its use as an alternative to the PFSA ionomer in catalyst
layers and membranes for commercial fuel cells. However, recent
research showed that our HCMC membranes with perfluorosulfonic side
chains were more stable than Nafion (N211) membranes~\cite{77}. For
example, the Gen1-1515 (In1515)-based MEAs underwent accelerated stress
testing, where fuel cells were exposed to highly oxidizing conditions
of open circuit voltage at 30\% RH, 90~{\textdegree}C, with
H$_{2}$/Air, and zero backpressure. Figure~\ref{fig10} shows the
decrease in open circuit voltage (OCV) over time, which is indicative
of membrane degradation. Under the accelerated stress conditions, the
initial OCV of the In1515 membrane-based MEAs was 0.978~V and remained
above 0.94~V after 150~h and above 0.88~V after 400~h. For comparison,
the Nafion N211-based MEAs deteriorated much more quickly; after 150~h
of operation under the same conditions, the OCV dropped
rapidly~\cite{81}.

\begin{figure}
\includegraphics{fig10}
{\vspace*{-.15pc}}
\caption{\label{fig10}Accelerated stress test (AST) on
membrane--electrode assemblies via an open-circuit voltage hold at 30\%
RH, 90~{\textdegree}C, H$_{2}$/Air, zero backpressure (Gen1 - 15,15).
Spikes represent times where the OCV stress test was interrupted to
obtain polarization curves and gas cross over current
density~\cite{81}.}
{\vspace*{-.35pc}}
\end{figure}

In conclusion, assembling these electrodes with aromatic ionomer
membranes, which are less permeable to oxygen and more conductive than
Nafion$^{\text{\textregistered}}$, has led to more durable and
efficient PEMFCs. Realizing ``all-aromatic'' PEMFCs that can compete
with current technologies based on Nafion$^{\text{\textregistered}}$
constitutes a major scientific and technological advance in a highly
competitive field. In-depth \mbox{studies} on the impact of ionomer structure
on membrane morphology and its functional properties help us understand
the mechanisms of conduction and mass transfer in this highly complex
environment. Additionally, we conducted a detailed study of the
organization of the ionomer within the electrode, in order to
understand its interactions with carbon and platinum, which partly
determine its catalytic performance. 

\section{Conclusions and perspectives} \label{sec5}

Fluorine-based chemicals are ubiquitous in electrochemical energy
storage and conversion. They are undoubtedly key components of these
technologies, especially in connection with the development of
renewable energy sources. For example, solar photovoltaic systems and
wind turbines are subject to daily cycles and must be able to store and
release this intermittent energy efficiently. Storage can be guaranteed
with batteries, but the excess energy from renewable sources can also
be used, for example, to produce carbon monoxide-free hydrogen (by
electrolysis) that can then be used to power PEMFC-based electric or
plug-in vehicles. The use of fluorinated chemicals raises questions
regarding their role in ensuring the safety and performance of
batteries, PEMFCs, and electrolyzers, all of which are essential
components of electrochemical energy systems.

\looseness=-1
However, despite progress in salts, ionic monomers, and related
ionomers, further work is needed to develop new fluorinated synthons,
precursors of salts exhibiting increased ion-pair dissociation.
Regarding fluorinated binders, an effort must be made on two fronts.
Firstly, they must be turned into ionomers. Secondly, they must be able
to build thick electrodes. This will optimize their areal capacity.
Additionally, in PEMFC, while the durability of hydrocarbon-based
polymer membranes is proven, their stability under harsh operating
conditions, especially in the cathode, needs to be fully
investigated.\ Finally, new fluorinated molecules and materials must be
invented to improve safety, performance and environmental impact.
Fluorine chemistry differs from organic chemistry, so these inventions
would benefit greatly from collaboration between organic chemists and
fluorine chemists.

Nonetheless, we must pay close attention to minimizing the content of
so-called ``immortal residues'' by (1)~efficiently controlling waste,
and (2)~producing only what is needed. In other words, we must minimize
the amount of F-products by taking advantage of recycling. Indeed, the
high cost of fluorinated chemicals makes battery and PEMFC recycling
\mbox{particularly} attractive. The aim is to efficiently reuse these
valuable chemicals, whether for incorporation into the same technology
or for use in less demanding applications, such as non-rechargeable
batteries or supercapacitors. We have already proven the efficient
recycling of PVdF binders and stable salts like LiTFSI in an EU project
(AMELIE). In the future, designing stable, soluble salts and polymers
that can be easily recycled using a low-energy-demanding process is
extremely important for decreasing the environmental impact. For
example, unidimensional ionomers are preferable to crosslinked ones, as
they can be reused in the same or different applications.

{\vspace*{-4pt}}

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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