Coordination complexes with multi electron transfers as ion-battery electrodes
Résumé
The increasing use of batteries in wireless devices and electric vehicles considerably accelerated the
demand for Li-ion batteries (LIBs). Over the past decades, the development of Li-based electrode
materials such as LiFePO4 (LFP) or Li(Ni, Mn, Co)O2 (NMC) allowed to reach high capacities (up to 200
mAh.g-1) and good cycling stabilities, but their low power limits their fast-charge capabilities. In contrast,
electrode materials like Li4Ti5O12 (LTO) harbor fast charging performances to the detriment of their
specific capacity (~140 mAh.g-1). Overall these materials suffer from several drawbacks: poor versatility,
toxicity and high cost due to lithium that represents a geostrategic resource.1 In this sense, the search for
original electrode materials that tackle the aforementioned challenges while ensuring eco-compatible
requirements is needed.
Alternatively, various redox active materials based on purely organic or inorganic compounds have been
explored. Thanks to multistep synthesis, organic materials are easy to tune allowing the preparation of
species with multi-electrons transfers. Their redox activity is not limited by the counterion, which permits
a great versatility regarding the electrolyte salt. Also, the fast electron exchanges of these species offer
high-rate charge/discharge performances, a famous example being nitroxide radical polymers. However,
their electrical conductivity, cycling performances and potentials remain limited. Quite the contrary,
inorganic redox materials are superior on these points and guarantee high specific capacities and
insolubility in the electrolyte. Researchers investigated for example LiNi0,5Mn1,5O4 (LNMO) spinel as
possible high voltage cathode materials, keeping a correct capacity (~150 mAh.g-1). Nevertheless these
materials are not versatile and have limited fast charging performances.
Supposedly, mixing the two approaches would result in a material that gathers the advantages of each.
Such a combination could be obtained through coordination complexes (metal ion and organic ligand) but
surprisingly, this strategy has only been scarcely investigated. Coordination complexes features a high
chemical stability in redox processes as well as a good reversibility in electrochemical systems. Each key
parameters such as capacity, power, electrochemical window, potential, can be optimized by tailoring the
complexes at the molecular scale by playing on the metal ion(s) and/or the ligand part.2 The perspective
to polymerize or graft them on surfaces thanks to adequate functional groups, offers an easy solution to
overcome the solubility issues in the electrolyte. Regarding these features, coordination complexes appear
as good candidates as redox-active electrode materials.
Recently, we envisioned that complexes from earth-abundant metal ions (Ni, Cu, Fe…) and redox noninnocent
tetradentate ligands based on o-phenylenediamines derivatives3 could act as electrode materials,
while being an eco-compatible answer to the aforementioned challenges. Herein, our latest results will be
presented.
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