\makeatletter
\@ifundefined{HCode}
{\documentclass[CRPHYS,Unicode,screen,biblatex,published]{cedram}
\addbibresource{crphys20251034.bib}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\sfrac#1#2{{#1}/{#1}}
\usepackage{amssymb}
\usepackage{upgreek}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\def\tabnote#1{\vskip4pt\parbox{.90\linewidth}{#1}}
\def\ndash{\text{--}}
\def\hyphen{\text{-}}
\RequirePackage{etoolbox}
\usepackage{amsmath}
\usepackage{marvosym}
\usepackage{mathtools}
\usepackage{tipa}
\csdef{Seqnsplit}{\\}
\def\tbody{\noalign{\relax}\hline}
\def\jobid{crphys20251034}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\def\xsection{}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\def\refinput#1{}
\def\0{\phantom{0}}
\def\sfrac#1#2{{#1}/{#2}}
\def\stfrac#1#2{({#1})/{#2}}
\def\stofrac#1#2{(({#1})/{#2})}
\def\back#1{}
\def\botline{\\\hline}
\def\rmpi{\uppi}
\def\og{\guillemotleft}
\def\fg{\guillemotright}
\def\mathbi#1{\text{\textbf{\textit{#1}}}}
\def\xmorerows#1#2{#2}
\DOI{10.5802/crphys.279}
\datereceived{2025-12-30}
\daterevised{2026-02-20}
\dateaccepted{2026-03-10}
\ItHasTeXPublished
\makeatletter
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\gappto{\UrlBreaks}{\UrlOrds}
\makeatother
}
{\documentclass[crphys]{article}
\def\CDRdoi{10.5802/crphys.279}
\let\newline\break
\def\Hfill{}
\usepackage[T1]{fontenc} 
\usepackage{marvosym}
\usepackage{mathtools}
\usepackage{tipa}
\makeatletter
\def\sfrac#1#2{{#1}/{#1}}
\def\CDRsupplementaryTwotypes[#1]#2#3{}
\def\xmorerows#1#2{\morerows{#1}{#2}}
}
\makeatother

\usepackage{upgreek}

\dateposted{2026-04-16}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\title{Chalcogenide-based materials for batteries: state of the art and
research perspectives}

\alttitle{Mat\'eriaux chalcog\'enures pour batteries : \'etat de l'art et
perspectives de recherche}

\author{\firstname{Andrea} \lastname{Piarristeguy}\CDRorcid{0000-0002-8922-4566}}
\address{ICGM, Univ. de Montpellier, CNRS, ENSCM, Montpellier, France}
\email[A. Piarristeguy]{andrea.piarristeguy@umontpellier.fr}

\author{\firstname{Virginie} \lastname{Viallet}\CDRorcid{0000-0002-3853-6287}\IsCorresp}
\address{Laboratoire de R\'{e}activit\'{e} et de Chimie des Solides,
UMR CNRS 7314, Universit\'{e} de Picardie Jules Verne, Cedex 1, 80039,
Amiens, France}
\address{RS2E, R\'{e}seau Fran\c{c}ais sur le Stockage Electrochimique de l'Energie, FRCNRS 3459, Cedex 1, 80039, Amiens, France}
\email[V. Viallet]{virginie.viallet@u-picardie.fr}

\begin{abstract}
In  the context of the energy transition and the growing demand for
electrochemical energy storage, the development of efficient and
sustainable battery materials is a major scientific and technological
challenge. Chalcogenide-based materials, including sulfides, selenides,
and tellurides, have attracted increasing attention due to their
versatile structural and electronic properties.

This article provides a concise overview of the role of chalcogenides
in battery technologies, from their historical contribution to the
emergence of lithium-ion batteries to their current applications as
intercalation and conversion electrodes, nanostructured negative
electrodes, solid electrolytes, and lithium-rich positive electrodes in
solid-state batteries. Particular emphasis is placed on the
relationships between composition, structure, electrochemical
mechanisms, and performance. Finally, the potential of chalcogenide
materials for post-lithium battery technologies, such as sodium-,
potassium-, and magnesium-ion systems, is discussed, highlighting
remaining challenges and future research perspectives.
%\vspace*{-3pt}
\end{abstract}

\begin{altabstract}
Dans le contexte de la transition \'energ\'etique et de la demande
croissante en mati\`ere de stockage \'electrochimique de l'\'energie, le
d\'eveloppement de mat\'eriaux efficaces et durables pour les batteries
repr\'esente un d\'efi scientifique et technologique majeur. Les mat\'eriaux
chalcog\'enures, que ce soient les sulfures, les s\'el\'eniures et les
tellurures, suscitent un int\'er\^et croissant en raison de leurs
propri\'et\'es structurelles et \'electroniques polyvalentes.

Cet article pr\'esente un aper\c cu concis du r\^ole des chalcog\'enures dans
les technologies des batteries, depuis leur contribution historique \`a
l'\'emergence des batteries lithium-ion jusqu'\`a leurs applications
actuelles en tant qu'\'electrodes d'intercalation et de conversion,
qu\'electrodes n\'egatives nanostructur\'ees, qu\'electrolytes solides et
qu\'electrodes positives riches en lithium dans les batteries \`a l'\'etat
solide. L'accent est particuli\`erement mis sur les relations entre la
composition, la structure, les m\'ecanismes \'electrochimiques et les
performances. Enfin, le potentiel des mat\'eriaux chalcog\'enures pour les
technologies de batteries post-lithium, telles que les syst\`emes \`a ions
sodium, potassium et magn\'esium, est examin\'e, en soulignant les d\'efis
qui restent \`a relever et les perspectives de recherche futures.
\end{altabstract}

\keywords{\kwd{Chalcogenide materials}
\kwd{Lithium-ion batteries}
\kwd{Solid-state batteries}
\kwd{Solid electrolytes}
\kwd{Electrochemical energy storage}
\kwd{Post-lithium batteries}}

\altkeywords{\kwd{Mat\'eriaux chalcog\'enures}
\kwd{Batteries lithium-ion}
\kwd{Batteries tout solide}
\kwd{\'Electrolytes solides}
\kwd{Stokage d'\'energie \'electrochimique}
\kwd{Batteries post-lithium}}

%\thanks{UMICORE}

%\input{CR-pagedemetas} 

\maketitle

\end{noXML}

%\vspace*{-1pt}

\section{Introduction}\label{sec1}

%\vspace*{-3pt}

In the current context of the energy transition and the sustained
growth in demand for electrochemical energy storage devices, battery
materials represent a strategic field of research and
innovation~\cite{1,2}. This dynamic is driven by the simultaneous rise
of renewable energies, electromobility, and portable technologies,
which impose ever more stringent requirements in terms of energy
density, lifetime, and environmental sustainability.

Today, various battery technologies are available, each one addressing
specific needs depending on the field of application~\cite{3}. 
Lead-acid accumulators, a historical technology still widely used, are
notably employed in automotive ignition systems and uninterruptible
power supplies (UPS). Nickel-metal hydride (Ni--MH) batteries maintain a
significant presence in small-power electronic devices such as electric
shavers, cameras, pagers, and portable medical equipment. Lithium-ion
accumulators, in turn, dominate almost all markets, ranging from
smartwatches and mobile phones to laptops, drones, and electric
vehicles. Nickel-cadmium (Ni--Cd) batteries, although in decline due to
their toxicity and environmental regulations, are still found in
certain portable power tools, model-making applications, and
specialized photographic equipment.

Market projections for the 2015--2030 period, based on industrial data
and sectoral analyses, indicate that lead- and lithium-based
technologies are expected to remain predominant at least until 2030.
The most dynamic sectors include lithium for electric vehicles, lithium
for consumer electronics, and lead-acid for stationary and automotive
applications. However, this scenario raises several critical issues. On
one hand, the rising price of lithium is exacerbated by the progressive
depletion of exploitable deposits and by the increasing costs of
extraction, which requires substantial water resources and can generate
significant environmental impacts. On the other hand, the growing
global demand calls for the development of more efficient batteries,
capable of supporting a higher number of charge--discharge cycles, while
incorporating more abundant, less expensive, and more environmentally
friendly materials.

In response to these challenges, the search for alternatives to
conventional lithium-ion accumulators has turned toward emerging
chemistries. Among them, materials derived from group 16 of the
periodic table---sulfur, selenium, and tellurium---are attracting
increasing attention due to their remarkable structural and electronic
properties. Chalcogenides, in particular, stand out for their ability
to form flexible covalent networks, their tunable electrical
conductivity, and their chemical stability, all favorable
characteristics for electrochemical energy storage. Several recent
studies, such as those reported by Pruthvija~\cite{4}, demonstrate
their potential in advanced battery architectures, including
solid-state and hybrid configurations, aiming to combine high energy
density, enhanced safety, and extended operational lifetime.

These preliminary works open new research directions, where the
selection and optimization of chalcogenide-based materials could
simultaneously address the requirements of performance, durability, and
resource availability, while remaining consistent with the economic and
environmental constraints of the global battery market. In this
article, we aim to assess the potential of selected chalcogenide
materials for applications in electrochemical energy storage, through
an approach combining structural and physicochemical characterization,
analysis of electrochemical performance, and investigation of their
behavior during charge--discharge cycling. The objective is to highlight
the relationships between composition, structure, and functional
properties, in order to identify the key parameters likely to optimize
performance and stability. Through this approach, we seek to contribute
to the definition of new battery design strategies based on more
abundant and sustainable resources, while maintaining performance
comparable to-or even superior to-that of current lithium-ion
technologies.

\section{Chalcogenide materials and the emergence of lithium-ion
batteries: a historical perspective}

The award of the 2019 Nobel Prize in Chemistry to Stanley Whittingham,
John B. Goodenough, and Akira Yoshino recognized a series of
discoveries that led to the development of lithium-ion batteries.

The development of electrochemical devices began in the 19th century
with the voltaic pile (Cu/Zn, 1801), followed by the first fuel cells
(1839) and lead-acid batteries (1859) as illustrated in
Figure~\ref{fig1}. The latter played a decisive role in the early
industrial electrification. The 20th century saw the emergence of new
chemistries: Ni--Zn (1901), Ni--Cd (1930), Ni--H$_{2}$ (1970), and
Ni--MH (1975) systems. Although robust and well-suited for certain
specific applications (notably space, in the case of Ni--H$_{2}$),
these technologies were limited by their intrinsic energy density.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Timeline of the development history for batteries.}
\end{figure}

Lithium, owing to its low atomic mass ($M =
6.94$~g${\cdot}$mol$^{-1}$) and very negative electrochemical
potential, emerged as an ideal candidate for energy storage systems.
The fundamental reaction Li\ ${\leftrightharpoons}$\ $\mathrm{Li}^{+}
+ \mathrm{e}^{-}$ confers a theoretical specific capacity of 3860
Ah${\cdot}$kg$^{-1}$. As early as 1899, attempts were made to use
metallic lithium as negative electrode. However, the formation of
dendrites during charge--discharge cycles led to internal short
circuits, raising major safety concerns.\looseness=1

In 1976, Stanley Whittingham (Exxon)~\cite{5,6}  demonstrated the
potential of layered transition-metal dichalcogenides (TiS$_{2}$,
TaS$_{2}$, TiS$_{3}$, NbSe$_{3}$) as positive electrode materials.
These compounds allow the reversible intercalation of lithium ions
between their layers, thus realizing the first functional lithium
battery. This discovery marked a foundational milestone, introducing
the concept of host materials for ion insertion---the cornerstone of
lithium-ion chemistry.

From 1978 onward, Don Murphy (Bell Laboratories) and other researchers
initiated the transition from metallic lithium systems to lithium-ion
systems, in which lithium ions shuttle between intercalation-type
positive and negative electrodes without metallic deposition. In 1979,
Michel Armand proposed the concept of a lithium-polymer 
battery~\cite{7,8}, which was later explored by Hydro-Qu\'{e}bec and
eventually developed industrially several decades later.

In 1980, John B. Goodenough identified LiCoO$_{2}$ as a layered
positive electrode material~\cite{9}. This compound doubled the
operating potential compared with chalcogenide-based positive
electrodes and significantly enhanced energy density. Finally, in 1985,
Akira Yoshino introduced graphite as negative electrode
material~\cite{10}.

An initial industrial attempt at lithium-metal batteries was made in
1985 by Moly-Energy (250 Wh${\cdot}$kg$^{-1}$). However, problems
associated with dendrite formation and safety concerns limited their
widespread use. In 1990, Sony launched the first commercial lithium-ion
battery (LiCoO$_{2}$/graphite), which quickly outperformed competing
technologies (Ni--Cd, Ni--MH, Pb) in terms of energy density, lifetime,
and safety. This milestone marked the turning point toward the massive
adoption of Li-ion systems.


In the early 2000s, lithium-polymer batteries reached the market,
extending the pioneering research initiated by Armand more than twenty
years earlier. For further details, see Yoshino's retrospective and
the comprehensive reviews discussing the challenges, recent advances,
and positive electrode materials in lithium-ion
batteries~\cite{1,11,12,13}.

\section{Conventional lithium batteries with liquid electrolytes}\label{sec3}

\subsection{The role of layered chalcogenide materials in the evolution
of intercalation positive electrodes}\label{sec31}

Transition-metal dichalcogenides and trichalcogenides such as
TiS$_{2}$, TiS$_{3}$, NbSe$_{3}$, and VSe$_{2}$ were the first positive
electrode materials investigated during the development of lithium
batteries. Owing to their layered structure, which facilitates
lithium-ion intercalation, these compounds paved the way for modern
lithium-ion battery technology.

The development of lithium-ion batteries is inseparable from the search
for host materials capable of reversibly accommodating lithium ions. As
early as the 1970s, attention turned to layered dichalcogenides with
the general formula MX$_{2}$, where M is a transition metal and X a
chalcogen (sulfur or selenium). Their lamellar structure provided an
ideal framework for reversible lithium insertion and extraction without
significant disruption of the crystal lattice. These properties placed
them at the center of the research carried out at Stanford and Exxon
between 1973 and 1976, which led to the identification of TiS$_{2}$ as
a reference positive electrode material and to the exploration of
trichalcogenides such as TiS$_{3}$ and
NbSe$_{3}$~\cite{5,14,14a,15,16}.

The electrochemical behavior of TiS$_{3}$ has been studied in detail.
This compound can be described as TiS(S--S), highlighting the presence
of a polysulfide group that can react with lithium. During
intercalation, the first step corresponds to the cleavage of the S--S
bond, leading to the formation of Li$_{2}$TiS$_{3}$ through an
irreversible process. In a second step, titanium is reduced from
oxidation state $+$4 to $+$3, producing a reaction similar to the one
observed in TiS$_{2}$. Unlike the first, this second step is
reversible. However, TiS$_{3}$ exhibits only partial reversibility, due
to a change in titanium coordination from trigonal prismatic to
octahedral upon lithiation. This structural change represents a major
limitation for practical use.

NbSe$_{3}$, on the other hand, exhibits reversible electrochemical
behavior. Upon reaction with lithium, it can accommodate up to three
lithium ions per formula unit, corresponding to a fully reversible
global reaction. Compared with TiS$_{3}$, NbSe$_{3}$ thus showed
greater potential as a positive electrode material. However, the
structural complexity of this trichalcogenide, together with
limitations in its electrical conductivity, hindered its industrial
development~\cite{17}.

Among chalcogenide materials, TiS$_{2}$ remains the most emblematic. In
1976, Stanley Whittingham proposed TiS$_{2}$ as an intercalation
positive electrode. The first Li/TiS$_{2}$ battery prototypes,
developed by Exxon in the late 1970s, used a liquid electrolyte
consisting of lithium perchlorate dissolved in dioxolane and metallic
lithium as the negative electrode. The latter was soon replaced by a
Li--Al alloy to limit dendrite growth and improve safety~\cite{18}.

These cells exhibited an average voltage of about 2 V and a theoretical
specific capacity of 239~mAh${\cdot}$g$^{-1}$. Most notably, they
offered exceptional longevity, with more than one thousand cycles
achievable---a major advance over previous systems. The excellent
performance of TiS$_{2}$ stemmed from its compact hexagonal layered
structure, in which Ti$^{4+}$ ions occupy octahedral sites between
sulfur layers. Lithium intercalates continuously into these interlayer
spaces without requiring the nucleation of new phases. This
single-phase mechanism results in a characteristic sloping
voltage-capacity curve, indicative of reversible intercalation across
the full composition range from TiS$_{2}$ to LiTiS$_{2}$, as
illustrated in Figure~\ref{fig2}~\cite{18}.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Layered structure of LiTiS$_{2}$
showing hexagonal close-packed S lattice with Ti ions in octahedral
sites between alternating sulfur sheets. Discharge/charge curve of
TiS$_{2}$ in lithium cells (76th cycle at
10 mA${\cdot}$cm$^{-2}$).}
\end{figure}

The efficiency of TiS$_{2}$, however, strongly depended on synthesis
conditions. Below 600~\textdegree C, a stoichiometric compound
favorable to lithium intercalation was obtained. Above this
temperature, non-stoichiometric sulfides of the Ti$_{1+y}$S$_{2}$ type
formed, with excess titanium atoms occupying van der Waals gaps. This
structural disorder reduced ionic mobility and hindered lithium
intercalation, thereby degrading electrochemical performance~\cite{18}.

Most transition-metal dichalcogenides exhibit electrochemical behavior
similar to that of TiS$_{2}$, characterized by single-phase and
reversible lithium intercalation. A notable exception is~VSe$_{2}$.
Unlike TiS$_{2}$, this compound exhibits a biphasic behavior during
lithiation. The reaction proceeds through successive steps: first the
formation of Li$_{0.25}$VSe$_{2}$, followed by LiVSe$_{2}$,
and finally Li$_{2}$VSe$_{2}$, as illustrated in Figure~\ref{fig3}. This
sequence of transitions leads to distinct voltage plateaus during
charge and discharge. This atypical behavior is related to a particular
crystallographic c/a ratio and to the unusual coordination of vanadium.
Whereas group V elements generally adopt trigonal prismatic
coordination with sulfur or selenium, vanadium assumes octahedral
coordination in VSe$_{2}$, resulting in distinct electrochemical
behavior~\cite{19}.\looseness=1

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Electrochemical insertion of lithium into vanadium
diselenide, showing reaction of two lithium (behavior on cycling at 2
mA${\cdot}$cm$^{-2}$).}
\end{figure}

In parallel with lithium-based systems, research from 1978 also
explored sodium-ion batteries, with Whittingham proposing
transition-metal disulfides such as TiS$_{2}$ and TaS$_{2}$ as positive
\mbox{electrode} materials. However, sodium chemistry introduced additional
complexity. In Na$_{x}$TiS$_{2}$ and Na$_{x}$TaS$_{2}$ compounds,
structural transitions occur as a function of sodium content. At low
sodium concentrations, trigonal prismatic coordination is favored,
while at higher contents ($x\rightarrow1$), octahedral coordination
becomes predominant. These successive structural transitions severely
hindered electrochemical reversibility and limited the durability of
these systems, preventing their adoption as viable alternatives to
lithium-ion batteries.

Nevertheless, these studies laid the foundations to the development of
layered transition-metal oxides, which enabled the commercialization of
the first safe and high-performance lithium-ion batteries. Among the
main intercalation positive electrode materials are LiCoO$_{2}$ (LCO),
developed by Sony and commercialized in 1991; LiTiS$_{2}$ (LTS),
studied at Exxon; mixed oxides of the NMC type
(LiNi$_{0.33}$Mn$_{0.33}$Co$_{0.33}$O$_{2}$ or NMC111),
widely used in today's battery market; and NCA
(LiNi$_{0.8}$Co$_{0.15}$Al$_{0.05}$O$_{2}$), notably employed
by Panasonic in Tesla vehicle batteries. Other examples include the
spinel LiMn$_{2}$O$_{4}$ (LMO) and the olivine LiFePO$_{4}$ (LFP),
which offer more stable and cost-effective alternatives. These layered
transition-metal oxides exhibit higher operating voltages, while
polyanionic compounds such as LFP provide additional advantages in
terms of thermal stability and safety.

Thus, although progressively replaced by these newer families of
materials, chalcogenides played a decisive role in the emergence of a
technology that today dominates the landscape of electrochemical energy
storage. The optimization of intercalation positive electrodes remains
a major scientific and industrial challenge, particularly in view of
the growing demands associated with electric mobility and the
integration of renewable energy sources.

\subsection{Chalcogenide materials as conversion electrodes}

In addition to intercalation-type electrodes, which enabled the
development of lithium-ion batteries, so-called \textit{conversion
electrodes} have attracted increasing attention, particularly in
lithium-metal systems. Unlike intercalation materials---where lithium
ions are inserted into a preserved host lattice---conversion electrode
undergo a solid-state redox reaction involving significant structural
rearrangement and the breaking and recombination of chemical bonds.
This mechanism enables much higher specific capacities but also
introduces major challenges in terms of cyclability, conductivity, and
stability.

Conversion-type materials include a variety of chalcogenides and
halides for transition metals. The general principle of these
electrodes relies on a complete redox reaction, such as:  CoS $+$
2Li$^{+}+ 2\mathrm{e}^{-}\leftrightharpoons \mathrm{Co}_2 +
\mathrm{Li}_{2}$S  for cobalt sulfide, or FeF$_{3} + 3\mathrm{Li}^{+} +
3\mathrm{e}^{-} \leftrightharpoons \mathrm{Fe}_2 + 3$LiF for iron
fluoride~\cite{20}.  Conversion reactions can also involve the
elemental chalcogens themselves: sulfur, selenium, and tellurium can
form Li$_{2}$S, Li$_{2}$Se, and Li$_{2}$Te during discharge,
respectively. In all cases, these reactions are accompanied by profound
structural reorganization, distinguishing these electrodes from
conventional intercalation hosts. Figure~\ref{fig4} shows the
approximate range of average discharge potentials and specific capacity
of some of the most common conversion-type
cathodes~\cite{20}.\looseness=-1

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Approximate range of average discharge potentials
and specific capacity of some of the most common conversion-type
cathodes (theoretical).}
\end{figure}

Among conversion materials, sulfur has been the most extensively
studied due to its exceptionally high theoretical specific capacity of
1671~mAh${\cdot}$g$^{-1}$, its natural abundance in the Earth's crust,
and its low cost. The fundamental electrochemical reaction corresponds
to the reduction of elemental sulfur to Li$_{2}$S, according to the
equation 1/8 S$_{8} + 2\mathrm{Li}^{+} + 2\mathrm{e}^{-}\leftrightharpoons
\mathrm{Li}_{2}$S. The discharge profile exhibits long, nearly flat voltage
plateaus, characteristic of favorable solid-solid reaction kinetics.
However, the use of sulfur poses several challenges. First, sulfur is
an electronic insulator with an extremely low intrinsic conductivity
(${\approx}5\times10^{-27}$ mS${\cdot}$cm$^{-1}$ at 25~\textdegree
C), requiring the formation of composites with conductive carbon and
polymeric binders. Second, both sulfur and its reaction product
Li$_{2}$S undergo a large volume change (${\sim}$80\%), which
destabilizes conventional composite electrodes. Finally, a major issue
arises from the dissolution of {soluble} lithium {polysulfide}
intermediates into the electrolyte, leading to rapid capacity fading
and poor {coulombic} efficiency~\cite{20}.

Until around 2009, these limitations resulted in very poor capacity
retention. Since then, numerous strategies have been explored to
address them. Encapsulating sulfur within hollow structures with
internal void volume---such as TiO$_{2}$, carbon, or reduced graphene
oxide shells---has proven effective in accommodating volume changes.
Polymer hosts such as polyvinylpyrrolidone have also been employed.
These architectures have enabled cycle lifetimes exceeding 1000 cycles
in half-cell configurations~\cite{21}.

In parallel, electrolyte engineering approaches have been developed,
including the use of additives such as LiNO$_{3}$ or P$_{2}$S$_{5}$ to
promote the formation of a protective solid-electrolyte interphase
(SEI) on the lithium negative electrode. The use of high-molarity or
solid-state electrolytes has also been investigated to suppress
polysulfide dissolution.

From 2015 onwards, selenium and tellurium have received growing
attention. Selenium exhibits a theoretical specific capacity of 678
mAh${\cdot}$g$^{-1}$ and tellurium 420~mAh${\cdot}$g$^{-1}$.
Although these values are lower than that of sulfur, their volumetric
energy densities are considerably higher, reaching 3254
mAh${\cdot}$cm$^{-3}$ for selenium and 2621~mAh${\cdot}$cm$^{-3}$
for tellurium in the fully lithiated state (3467
mAh${\cdot}$cm$^{-3}$ for sulfur)~\cite{20}. Another key advantage
of these materials lies in their much higher electronic conductivity
compared to sulfur, resulting in better active material utilization and
faster reaction kinetics.

Nevertheless, selenium suffers from the dissolution of lithium
polyselenide intermediates, leading to rapid capacity fading and low
coulombic efficiency, similar to sulfur. Tellurium, on the other hand,
does not appear to suffer from this issue but presents other drawbacks.
While its volume change upon lithiation is lower (35--40\%), its high
cost and relative scarcity in the Earth's crust make its large-scale
application unrealistic. Consequently, selenium and tellurium have
mainly been investigated as model materials or for niche applications.
To enhance their stability, both elements have been incorporated into
conductive matrices---especially porous carbons or doped graphene--to
limit dissolution effects and maintain electrical contact.

\subsection{Chalcogenides as active material for negative electodes:
nanostructures and\newline composites}

The development of lithium-ion batteries (LIBs) has been strongly
driven by the search for electrode materials offering both high
specific capacity and satisfactory electrochemical stability.
Conventional graphite negative electrode, which operate via lithium
intercalation, exhibit a theoretical capacity of 
372~mAh${\cdot}$g$^{-1}$, while positive electrode materials such as
LiCoO$_{2}$ reach about 135~mAh${\cdot}$g$^{-1}$, as established in
the earliest studies on commercialized  batteries~\cite{22}. Despite
their technological success, these materials remain limited in terms of
energy density and stability under high-rate cycling, which has
stimulated the exploration of alternative compounds among which
transition-metal chalcogenides are gaining increasing
attention~\cite{20}.\looseness=1

When synthesized as electrode materials, chalcogenides exhibit
remarkable electrochemical properties, particularly their ability to
promote redox reactions while minimizing self-discharge~\cite{4}. Their
structural versatility-often achieved through nanostructuring
techniques- confers high porosity and good ionic conductivity, both of
which are essential for fast charge--discharge processes. Their
performance can be further enhanced by doping with metals such as
copper, aluminum, or cobalt, which improve electronic conductivity.
Complementarily, doping with semiconductors such as germanium or
silicon promotes the formation of polycrystalline structures that are
particularly active as electrodes~\cite{4}.

A wide variety of metal sulfides have been investigated as negative
electrode materials. For example, Bi$_{2}$S$_{3}$ obtained by
mechanical milling exhibits an initial charge capacity of 1146
mAh${\cdot}$g$^{-1}$ and a discharge capacity of 746
mAh${\cdot}$g$^{-1}$ after 50 cycles, whereas a one-step synthesized
CuS--Cu$_{1.8}$S system retains a more modest capacity of 320
mAh${\cdot}$g$^{-1}$ after 500 cycles. Other sulfides, such as
WS$_{2}$ and MoS$_{2}$-graphene composites prepared via hydrothermal
routes, reach initial capacities of 900 and 1157
mAh${\cdot}$g$^{-1}$, respectively, with the MoS$_{2}$-graphene
composite maintaining 1066~mAh${\cdot}$g$^{-1}$ after 100 cycles.
These results highlight the potential of sulfide-based
nanostructures-especially when combined with conductive additives such
as graphene-to overcome the intrinsic limitations of conventional
graphite negative electrode~\cite{4}.\looseness=1

Metal selenides have also attracted growing attention as they overcome
some of the limitations associated with sulfur-based electrodes. Their
superior electronic conductivity, shorter ionic diffusion lengths, and
additional properties such as hydrophilicity, photoconductivity, and
photovoltaic response make them attractive for electrochemical energy
storage. For instance, solvothermally synthesized SnSe-C composites
exhibit an initial capacity of about 1098~mAh${\cdot}$g$^{-1}$ and
retain approximately 707~mAh${\cdot}$g$^{-1}$ after 50 cycles, while
CoSe$_{2}$-CNT composites show excellent stability, maintaining 1405
mAh${\cdot}$g$^{-1}$ after 300 cycles. Other systems, such as
Sb$_{2}$Se$_{3}$ thin films prepared by pulsed laser deposition, retain
around 605~mAh${\cdot}$g$^{-1}$ after 100 cycles, confirming the
potential of selenides as high-performance negative electrode
materials~\cite{4}.

Although less explored, metal tellurides have emerged as particularly
promising candidates owing to their intrinsic electronic conductivity,
high volumetric energy density, and large volumetric specific capacity.
For example, CuTe obtained by solvothermal synthesis has shown
remarkable stability up to 5000 cycles, albeit with moderate capacity,
while Te--C composites prepared by various methods---including
melt-diffusion and high-energy ball milling (or high energy mechanical
milling---HEMM)---have demonstrated initial capacities exceeding 
1000~mAh${\cdot}$g$^{-1}$, with retention depending on synthesis
conditions~\cite{4}. These findings underline the advantages of
tellurium for potential use as both negative and positive electrode
materials in advanced LIB systems. Table~\ref{tab1} tabulates metal
sulfides, selenides, and tellurides characterized as negative electrode
materials with their performance metrics.

%tab1
\begin{table}
\caption{\label{tab1}Metal-sulfide, metal-selenide, metal-telluride
used as negative electrodes~\cite{4}\vspace*{-3pt}}
\fontsize{9}{11.5}\selectfont\tabcolsep4pt
\begin{tabular}{llcccc}
\thead
Composition & \parbox[t]{2cm}{\centering Synthesis technique} & 
\parbox[t]{2cm}{\centering Initial charge capacity
(mAh${\cdot}$g$^{-1}$)} & \parbox[t]{1.2cm}{\centering Current density} &
\parbox[t]{2cm}{\centering Discharge capacity (mAh${\cdot}$g$^{-1}$)} &
\parbox[t]{1.5cm}{\centering Cycle number}\vspace*{2pt}\\
\endthead
Bi$_{2}$S$_{3}$--C & Ball milling & 1146 & 100 mA${\cdot}$g$^{-1}$ & 746 & 50\\
CuS--Cu$_{1.8}$S & One-pot & 620 & 4C &320  &500 \\
WS$_{2}$  & Hydrothermal & 900 & 47.5 mA${\cdot}$g$^{-1}$ & -- & 20\\
MoS$_{2}$--S$_{2}$-graphene & Hydrothermal & 1157 & 100 mA${\cdot}$g$^{-1}$ & 1066 & 100\\
SnSe-C & \parbox[t]{2cm}{\raggedright  Grinding and solvothermal}\vspace*{4pt}  & 1097.6 & 50 mA${\cdot}$g$^{-1}$ & 707 & 50\vspace*{2pt}\\
Sb$_{2}$Se$_{3}$ & \parbox[t]{2cm}{\raggedright  Pulsed laser deposition} & 624 & 500 mA${\cdot}$g$^{-1}$ & 605.1 & 100\vspace*{2pt}\\
CoSe$_{2}$-CNT & Hydrothermal & 845 & 200 mA${\cdot}$g$^{-1}$ & 1405 &300 \\
FeSe--C &  One-pot  & 390 & 40 mA${\cdot}$g$^{-1}$ & 340 & 40\\
MoSe$_{2}$-rGO & Hydrothermal & 550 & 0.1 C & 523 & 5\\
Se--Mo with CNT & Hydrothermal & 1008.7 & 7000 mA${\cdot}$g$^{-1}$ & 607 & 40\\
CuTe &   Solvothermal &229 & 400 mA${\cdot}$g$^{-1}$ & 180 & 5000\\
Te--C & Melting diffusion & -- & 0.1 C & 429.6 & 100\\
Te--C & HEMM & 1088 & 10 mA${\cdot}$g$^{-1}$ & 740 & 100
\botline
\end{tabular}
\vspace*{-3pt}
\end{table}

\section{Solid-state batteries}

\vspace*{-3pt}

Among the various battery chemistries currently available, solid-state
batteries (SSBs) have emerged as a promising alternative to
conventional lithium-ion batteries (LIBs) employing liquid
electrolytes. The latter are intrinsically limited by several factors,
notably by use of flammable organic liquid electrolytes and graphite
negative electrode with a restricted specific capacity of 
372~mAh${\cdot}$g$^{-1}$. By contrast, metallic lithium-with its very low
density (0.59 g${\cdot}$cm$^{-3}$), high \mbox{theoretical} capacity (3860
mAh${\cdot}$g$^{-1}$), and extremely negative electrochemical
potential-represents an ideal negative electrode material for achieving
significantly higher gravimetric and volumetric energy
densities~\cite{22}.

Positive electrodes used in conventional batteries, such as
LiCoPO$_{4}$ (4.8 V vs.\ Li/Li$^{+}$, 801 Wh${\cdot}$kg$^{-1}$) or
LiNi$_{0.5}$Mn$_{1.5}$O$_{4}$ (4.7~V vs.\ Li/Li$^{+}$, 690
Wh${\cdot}$kg$^{-1}$), already offer high electrochemical potentials.
However, their performances are inherently constrained by the chemical
and electrochemical stability of the liquid electrolyte. Linear organic
solvents (diethyl carbonate, dimethyl carbonate, ethyl methyl
carbonate) or cyclic ones (ethylene carbonate, propylene carbonate,
butylene carbonate, ${\upgamma}$-butyrolactone), combined with
conducting salts such as LiPF$_{6}$ or LiClO$_{4}$, present significant
risks of leakage, evaporation, and particularly 
flammability~\cite{23}. Notable incidents, including failures in laptop
batteries (Sony, Dell, Apple) and in the Boeing 787 Dreamliner,
illustrate these safety limitations~\cite{24}.

The transition to solid-state batteries aims to overcome these
constraints. By replacing the liquid electrolyte with a nonflammable
solid electrolyte and substituting graphite with metallic lithium, it
becomes possible to simultaneously enhance safety and energy density
while reducing system volume and weight~\cite{22}, as illustrated in
Figure~\ref{fig5}. This development also opens the door to the use of new
families of materials, including chalcogenides, whose electronic and
ionic properties are particularly attractive for high-performance solid
electrolytes.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Comparative diagrams of a conventional lithium-ion
battery (LIB in the center) and a solid-state battery with lithium
metal (LiM-SSB on the left) or a composite negative electrode (Li-SSB
on the right). W$_{\mathrm{vol}}$ and W$_{\mathrm{grav}}$ are the
volumetric and gravimetric energy densities,
respectively~\cite{22}.}
\end{figure}


The expected benefits of SSBs include enhanced safety due to the
absence of organic solvents, improved thermal stability, and
suitability for extreme environments, such as deserts or drilling
sites. The reduction in module weight and the elimination of
components, notably the separator, further facilitate integration into
electric vehicles and other embedded applications~\cite{25}.\looseness=1

Chalcogenide materials---mainly sulfides, selenides, and tellurides---play
an increasingly important role in solid-electrolyte research. Their
appeal lies in a combination of structural and \mbox{electronic} properties
favorable to ionic conduction. Unlike oxides, which generally offer
high chemical stability but limited ionic conductivity, chalcogenides
provide high ionic conductivities (up to 10 mS${\cdot}$cm$^{-1}$ at
room temperature for certain sulfides such as
Li$_{10}$GeP$_{2}$S$_{12}$), comparable to or exceeding those
of liquid electrolytes~\cite{26,27}. 

Sulfides are the most extensively studied in this context. Their
relatively open crystal frameworks facilitate lithium-ion diffusion,
enabling conductivities approaching 10 mS${\cdot}$cm$^{-1}$, making
them excellent candidates for solid electrolytes. Compounds such as
amorphous or crystalline Li$_{2}$S--P$_{2}$S$_{5}$ offer a good balance
between conductivity and mechanical flexibility, accommodating
volumetric changes during cycling. However, their sensitivity to
moisture and the formation of toxic H$_{2}$S upon degradation pose
challenges for industrial-scale implementation~\cite{27}.\looseness=1

Selenides and tellurides, less studied than sulfides, are gaining
attention due to their higher electronic conductivity and improved
chemical stability at electrode interfaces. Selenides, for instance,
can enhance the interface with nickel-rich positive electrodes by
reducing interfacial resistance, a key technological bottleneck in
SSBs. Nevertheless, their higher cost and greater atomic mass limit
competitiveness compared to sulfides, especially in applications where
gravimetric energy density is critical.

A major remaining challenge is interfacial instability between
electrodes and solid electrolytes. Chalcogenides, despite their
promising conductivity, often undergo electrochemical degradation at
the high potentials of positive electrodes (${>}$4~V vs.\ 
Li/Li$^{+}$). Strategies to mitigate these issues include protective
electrode coatings, the design of stabilized chalcogenide compositions,
and optimized sintering and shaping processes for solid
electrolytes~\cite{26}.

Finally, lithium dendrite growth remains a critical obstacle in
lithium-metal SSBs. Chalcogenide materials, owing to their superior
plasticity relative to oxides, could potentially suppress dendrite
propagation through more uniform mechanical stress distribution.
However, a comprehensive understanding of lithium/chalcogenide
electrolyte interactions is essential to ensure long-term device
durability~\cite{27}.

\subsection{Chalcogenide materials as solid electrolytes}

Inorganic solid electrolytes are at the core of solid-state battery
(SSB) development, which aims to overcome the safety and performance
limitations of liquid electrolytes. Among the various material families
investigated, chalcogenides occupy a prominent position. In practice,
the literature shows that sulfides are almost exclusively used as solid
electrolytes, both for lithium-ion and sodium-ion 
batteries~\cite{28,29}, as illustrated in  Figure~\ref{fig6}~\cite{30}.
These materials exhibit high ionic conductivity on the order of 
1~mS${\cdot}$cm$^{-1}$, favoring ductility for cold-press assembly, and
interesting mechanical compatibility~\cite{31}. However, they suffer
from moisture sensitivity, a limited electrochemical stability window,
and parasitic reactions with electrode materials and metallic lithium.

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Developing trend of solid-state sulfide
electrolytes~\cite{30}.}
\end{figure}

Among sulfides, several families stand out. Solid electrolytes based on
Li$_{10}$GeP$_{2}$S$_{12}$ (LGPS), argyrodites of the type
Li$_{6}$PS$_{5}$X (X $=$ Cl, Br, I), and glassy or crystalline
derivatives of Li$_{3}$PS$_{4}$ have been extensively 
studied~\cite{32,33}. Li$_{3}$PS$_{4}$ exists in amorphous,
glass-ceramic, or crystalline forms and has been the subject of
numerous fundamental studies and recent patents~\cite{34,35,36,37,38}.

Another important family is the argyrodites. Initially discovered in
1886 with the compound Ag$_{8}$GeS$_{6}$, these materials have
attracted renewed attention since 2008 with the identification of
Li-rich phases of the type Li$_{6}$PS$_{5}$X, exhibiting remarkable
ionic conductivity ~\cite{39}. These
argyrodites adopt orthorhombic or cubic crystal structures depending on
composition and temperature, with ionic conductivities ranging from
10$^{-3}$ mS${\cdot}$cm$^{-1}$ in the orthorhombic
phase to ${\sim}$1 mS${\cdot}$cm$^{-1}$ in the cubic phase at room
temperature~\cite{40}. Their high
chemical flexibility allows aliovalent substitutions at various
crystallographic sites to increase charge carrier concentration and
expand diffusion pathways. For instance, tetravalent cations such as
Si$^{4+}$, Sn$^{4+}$, or Ge$^{4+}$ can partially replace
phosphorus, while halides (Cl$^{-}$, Br$^{-}$, I$^{-}$, F$^{-}$),
O$^{2-}$, or Se$^{2-}$ can substitute for sulfur, yielding a
softer and more polarizable lattice. This chemical engineering strategy
has led to compositions with significantly enhanced conductivities,
reaching up to 10 mS${\cdot}$cm$^{-1}$ at 25~\textdegree C, such as
the recently developed
Li$_{5.4}$PS$_{4.4}$BrCl$_{0.6}$
electrolyte, surpassing commercial Li$_{6}$PS$_{5}$Cl 
(${\sim}$1.5 mS${\cdot}$cm$^{-1}$)~\cite{41}.


These advances translate into remarkable electrochemical performance.
Cells using Li$_{6}$PS$_{5}$Cl or its derivatives exhibit low
irreversible capacity, low polarization, and stable reversible \mbox{capacity}
even after several hundred cycles at high current density, highlighting
their potential for industrial-scale
integration~\cite{41,42}.

\subsection{Lithium-rich sulfides as positive electrode materials}

Beyond their well-established role as solid electrolytes, sulfides can
also be considered as active positive electrode materials in
solid-state batteries. This strategy relies on the use of lithiated
\mbox{sulfides}, either alone or in combination with oxide-based
lithium-active materials, to exploit their chemical richness and ionic
conductivity. However, the direct integration of these materials faces
significant interfacial challenges. Indeed, the difference in lithium
electrochemical potential between oxides based positive electrode and
sulfide solid electrolytes leads, upon contact, to undesired chemical
reactions or the formation of a lithium-depleted interfacial layer, a
phenomenon known as the ``space-charge layer effect.'' A widely adopted
solution to mitigate this issue is the insertion of an interlayer
between the positive electrode material and the solid electrolyte. The
coatings used must possess dual functionality: electronic insulation
while maintaining ionic conductivity. Compounds identified as suitable
coatings include LiNbO$_{3}$, ZrO$_{2}$, Li$_{4}$Ti$_{5}$O$_{12}$,
Al$_{2}$O$_{3}$, and LiTaO$_{3}$, which ensure chemical compatibility
and enhanced interfacial\break stability.

A particularly promising example of a sulfide positive electrode
material is the lithium-rich compound
Li$_{1.13}$Ti$_{0.57}$Fe$_{0.3}$S$_{2}$ (LTFS), recently
studied by Marchini \etal~\cite{43}. 
Its synthesis is carried out via a solid-state reaction from a mixture
of Li$_{2}$S, TiS$_{2}$, and FeS$_{2}$, sealed in a quartz tube under
inert atmosphere, and annealed at 750~\textdegree C for 36~h, followed
by rapid quenching in water. The resulting product consists of
pseudo-spherical monolithic particles of 5--20~${\upmu}$m, forming a
single phase crystallizing in the R$\overline{3}$m space group, with
lattice parameters $a = 3.53~\AA{}$ and $c = 18.09~\AA{}$. This hexagonal
structure is analogous to lithium-rich layered oxide phases of the type
Li$_{1+y}$M$_{1-y}$O$_{2}$ but possesses a larger lattice
capable of accommodating the bulkier sulfide ligands (S$^{2-}$).
The structure is cation-disordered, with iron atoms sharing sites with
titanium and lithium in the metal layers.

LTFS has a theoretical capacity of 261~mAh${\cdot}$g$^{-1}$,
attributable to mixed redox activity combining both cationic
Fe$^{2+}$/Fe$^{3+}$ and anionic S$^{2-}$/S$^{n-}$ 
($n <2$) couples. Integrated into a solid-state cell of the type
LTFS $+ {\upbeta}\hyphen \mathrm{Li}_{3}\mathrm{PS}_{4}|
\upbeta\hyphen \mathrm{Li}_{3}\mathrm{PS}_{4}|$InLi, 
this material exhibited promising electrochemical properties. The
assembly of the cell is facilitated by the low mechanical rigidity of
sulfides, whose reduced Young's modulus allows good densification under
pressure. Scanning electron microscopy (SEM) and Energy Dispersive
Spectroscopy (EDS) mapping reveal a continuous and homogeneous
interface between the composite positive electrode and electrolyte,
with uniform distribution of titanium and phosphorus, indicating good
mixing between LTFS and ${\upbeta}\hyphen \mathrm{Li}_{3}\mathrm{PS}_{4}$ and effective
electronic percolation.

Galvanostatic measurements at C/25 show an initial charge with a long
redox plateau of around 2.1~V (2.7~V vs.\ Li/Li$^{+}$), accompanied by
significant irreversible capacity loss, with an initial coulombic
efficiency of 69\% and a loss of ${\sim}$50~mAh${\cdot}$g$^{-1}$. In
the second cycle, a reversible capacity of 120~mAh${\cdot}$g$^{-1}$
is obtained at room temperature, increasing to
140~mAh${\cdot}$g$^{-1}$ at 100~\textdegree C. However, using lithium
alloys as negative electrodes, although functional, is impractical due
to their high cost, weight, and the lower cell voltage compared to
metallic lithium.

Experiments with LTFS $+\upbeta\hyphen \mathrm{Li}_{3}\mathrm{PS}_{4}|\upbeta\hyphen \mathrm{Li}_{3}\mathrm{PS}_{4}|$ Li cells yielded
better results, with dense, continuous electrode structures, no visible
cracks, and satisfactory interfacial contact. Galvanostatic
charge--discharge cycles at C/50 exhibited low polarization (85~mV at
120~mAh${\cdot}$g$^{-1}$) and excellent coulombic efficiency above of
99\% as illustrated in Figure~\ref{fig7}. After 10 cycles, the reversible
capacity reached 214~mAh${\cdot}$g$^{-1}$, corresponding to 83\% of
the theoretical capacity, or nearly one lithium ion per formula unit of
LTFS. These performances demonstrate improved capacity \mbox{retention} and
enhanced stability in cells using metallic lithium negative electrodes
compared to InLi alloy-based cells.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}First and tenth galvanostatic charge/discharge
cycles and discharge capacity and Coulombic efficiency obtained at C/50
and room temperature of LTFS $+ \upbeta\hyphen
\mathrm{Li}_{3}\mathrm{PS}_{4}$ (70--30 wt\%)  $|{\upbeta}\hyphen
\mathrm{Li}_{3}\mathrm{PS}_{4}|$ Li battery~\cite{43}.} 
\end{figure}

In summary, the use of lithium-rich sulfides as active positive
electrode materials represents a highly promising approach for
solid-state batteries. The LTFS compound illustrates this strategy by
combining a crystal structure suited to sulfide ligands, mixed redox
activity, and favorable integration with Li$_{3}$PS$_{4}$-type solid
electrolytes. While challenges remain, particularly regarding interface
optimization and reduction of initial irreversible losses, these
results pave the way for a new generation of high-capacity positive
electrodes capable of competing with the best current oxide-based
solutions.

\section{Chalcogenide materials as alternatives to Li-ion batteries}

The growing demand for energy storage solutions has driven intensive
research into alternatives to lithium-ion batteries (LIBs),
particularly due to the limited availability of lithium and the need
for more sustainable and cost-effective systems. Among the explored
options, sodium-ion (SIBs), potassium-ion (PIBs), and magnesium-ion
batteries (MIBs) emerge as promising technologies due to the abundance
and favorable geographic distribution of these elements.

For sodium-ion batteries, sodium offers the advantage of being more
abundant and less expensive than lithium while sharing a similar
operating mechanism. However, the larger ionic radius of Na$^{+}$
(1.02~\AA{} vs.\ 0.76~\AA{} for Li$^{+}$) slows ion transport and
electrochemical kinetics, which can reduce overall performance and
introduce safety issues such as explosion or corrosion~\cite{44}. To
compensate these limitations, chalcogenide materials have been
extensively investigated as negative electrode materials. Their high
storage capacity is, however, counterbalanced by low electrical
conductivity and significant volumetric expansion during
sodiation/desodiation cycles. Various modification strategies,
including carbon doping, metal substitution, and the synthesis of
hybrid nanostructured architectures, have been developed to enhance
their performance~\cite{45,46}. Reducing particle size to the nanoscale
significantly increases the electrode--electrolyte interfacial area and
accommodates better mechanical stresses associated with \mbox{volume} changes.
\mbox{Two-dimensional} layered dichalcogenides (TMDs) such as MoS$_{2}$,
VS$_{2}$, or CoS$_{2}$, with their expanded interlayer spacing and high
theoretical capacities, provide an ideal platform for rapid and
reversible Na$^{+}$ intercalation~\cite{44}. Their electrochemical
mechanism occurs in two steps: an initial intercalation at relatively
high potential followed by a lower-potential conversion reaction,
leading to the formation of sodium sulfides and elemental
metals~\cite{45,46}. Beyond MoS$_{2}$ and VS$_{2}$, SnS$_{x}$-based
materials stand out due to their high capacity, abundance, and low
toxicity, making them an attractive option for SIBs. Nanoscale
architectures, such as VS$_{2}$ ``flower-like'' morphologies or
MoS$_{2}$ nanosheets, highlight the importance of surface engineering
strategies~\cite{44}.

Among the various configurations of sodium-based batteries (SBs),
solid-state ones have also emerged as a particularly attractive option.
A wide variety of inorganic solid electrolytes for sodium-ion
solid-state batteries includes chalcogenide-based compounds such as the
Na$_{3}$MS$_{4}$ series~\cite{47,48,49,50}, the Na$_{3}$MSe$_{4}$ (M $=$ P, Sb) 
family,~\cite{51,52} and the Na$_{7}$P$_{3}$X$_{11}$ (X $=$ O, S, Se) 
series~\cite{53}. Recent studies also explore the
development of glassy, glass-ceramic, and ceramic sulfide electrolytes,
notably in Na$_{2}$S--GeS$_{2}$~\cite{54} and Na$_{2}$S--SiS$_{2}$ 
systems~\cite{55,56}. Na$_{3}$PS$_{4}$ is the most studied representative. This
material has recently undergone a reevaluation of its thermodynamic
properties, revealing a high-temperature polymorph
(${\upgamma}$-Na$_{3}$PS$_{4}$) characterized by plastic and
mesomorphic behavior, along with a local tetragonal structure within a
cubic phase~\cite{57,58}. Under pressure, its structural
and conductive properties further evolve, providing insight into its
stability and potential as a solid electrolyte for sodium-ion 
batteries~\cite{59}. 

Chalcogenide electrolytes represent attractive and cost-effective
alternatives to oxide electrolytes, as they can be synthesized at
relatively low temperatures. Nevertheless, they often suffer from poor
air stability and the presence of voids, which can adversely affect
ionic conductivity. To address these limitations, several modification
strategies have been investigated, including cation doping at the P
site, halogen substitution at the S site, the formation of hybrid
organic--inorganic composites such as polyethylene oxide-based
Na$_{3}$PS$_{4}$, high-temperature thermal treatments to induce
favorable crystal phase transformations, as well as ceramization and
vitrification processes. These approaches have proven effective in
enhancing the stability, electrochemical performance, and mechanical
properties of chalcogenide-based solid electrolytes for next-generation
solid-state batteries~\cite{60,61,62,63,64,65,66}.

Potassium-ion batteries represent another promising alternative. With a
redox potential for K$^{+}$/K (${-}$2.93~V vs.\ SHE) close to that of
Li$^{+}$/Li (${-}$3.04~V vs.\ SHE), PIBs can achieve operating voltages
comparable to or even higher than those of LIBs and SIBs. Potassium,
being a weaker Lewis acid and exhibiting higher ionic conductivity,
facilitates reversible intercalation into graphite, unlike sodium,
which forms irreversible
intercalation~\cite{44}. Transition metal dichalcogenides such as
MoS$_{2}$, SnS$_{2}$, and Sb$_{2}$S$_{3}$, with low energy barriers and
high specific capacity, are considered prime candidates for PIB
negative electrode materials. Their mechanisms involve either a simple
intercalation or a combination of conversion/alloying, particularly for
chalcogenides containing group 14 and 15 metals (Sb$_{2}$S$_{3}$,
SnS$_{2}$, GeSe).

Magnesium-ion batteries have also attracted growing interest, mainly
due to the intrinsic safety and abundance of magnesium. However, the
divalent nature of Mg$^{2+}$ ions presents challenges in terms of
diffusion and reversibility. Innovative approaches include
dual-electrolyte systems, combining co-intercalation of Mg$^{2+}$
with monovalent cations (Li$^{+}$ or Na$^{+}$) to improve kinetics.
TiS$_{2}$- and WS$_{2}$-based materials have shown promising potential
as electrodes due to their ability to accommodate Mg$^{2+}$ ions
within their lattice~\cite{67,68}.

In conclusion, transition metal chalcogenides represent a strategic
family of materials for the development of ``post-lithium'' energy
storage technologies. Current research focuses on optimizing their
structural and electronic properties, particularly via nanostructuring
and multi-metal approaches, to address challenges related to
conductivity, cycling stability, and \mbox{energy} density. These advances
highlight the potential of chalcogenides to play a key role in the
energy transition and in the development of more sustainable,
large-scale energy storage\break systems.

\section{Conclusion}

Research on chalcogenide materials has demonstrated their significant
potential in the development of next-generation electrochemical energy
storage systems, particularly as electrodes in rechargeable batteries.
The history of these materials dates back to the early work on titanium
disulfide (TiS$_{2}$), which was first used in 1970 and commercialized
by Exxon in 1977 as an intercalation material for lithium batteries.
This pioneering breakthrough paved the way for systematic exploration
of chalcogenide materials across various configurations, ranging from
liquid electrolyte systems to solid-state architectures. Titanium
disulfide exemplifies the key role of layered materials capable of
reversible intercalation of alkali ions, delivering a cell voltage of
approximately 2~V in lithium-based systems.

Subsequently, research expanded to systems based on sulfur and its
analogues, selenium and tellurium, which enable conversion reactions
involving S$_{2}$, Li$_{2}$S, Se, or Te. These reactions provide high
specific capacities, but their integration requires substantial
optimization of electrode and electrolyte formulations. In the field of
solid electrolytes, sulfides have become benchmark materials, with
families such as Li$_{10}$SnP$_{2}$S$_{12}$ argyrodites,
Li$_{3}$PS$_{4}$, and mixed phases
Li$_{4}$PS$_{4}$X$_{1-y}$X$^{\prime}_{y}$ (X $=$ Cl, Br, I),
while for sodium-ion batteries, Na$_{3}$PS$_{4}$ represents a promising
option. These~sulfide-based solid electrolytes are distinguished by
high ionic conductivity and compatibility with chalcogenide electrodes,
paving the way for the development of solid-state batteries (SSBs).

Metal-chalcogen compounds, including sulfides, selenides, and
tellurides, are also proving to be relevant as positive electrode
materials. Lithium-rich compositions such as
Li$_{1.13}$Ti$_{0.57}$Fe$_{0.3}$S$_{2}$ (LTFS) illustrate the
potential of high-energy-density sulfide positive electrodes. At the
same time, the emergence of two-dimensional layered structures has
reinforced interest in these materials, particularly for lithium
alternatives such as sodium-ion, potassium-ion, and magnesium-ion
systems, where chalcogenides provide stable and reversible reaction
pathways.

Future perspectives in this field include optimizing synthesis routes
and characterization techniques (XRD, Raman spectroscopy, SEM, as well
as operando studies), improving solid-state battery assembly methods
through cold or hot pressing (Spark Plasma Sintering---SPS---or Pulsed
Electric Current Sintering---PECS), and developing prototypes
incorporating multiscale models linking structure, kinetics, and
performance. Significant efforts are still required in composite
positive electrode formulation, the selection of conductive additives,
negative electrode protection, and material recycling. The diversity of
possible approaches, from liquid-electrolyte systems to lithium-metal-
or composite-electrode-based solid-state architectures, highlights the
considerable room for improvement. Optimizing chalcogenide materials,
while considering environmental impact and recyclability, opens the
path toward a new generation of energy storage systems that are more
efficient, safer, and sustainable.

\section*{Acknowledgments}

The authors gratefully acknowledge colleagues and graduate students
from ICGM and LRCS. The authors are grateful for the support of
Fondation MAIF, UMICORE and the Research Group GDR Chalco (Groupement
de recherche, CHALCO Mat\'eriaux chalgon\'erures: Recherche,
D\'eveloppement et Innovation). 

\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.

\back{}

\printbibliography
\refinput{crphys20251034-reference.tex}

\end{document}