50
Therefore, in less than a decade, the research and development on ZIBs has been
very fruitful. From the abovementioned discussion the perspectives on the development of state-of-the-art ZIB cathodes can be summarized as follows:
1. Although high performances in terms of high zinc storage capacities and longterm cycling stabilities under high applied current densities are achieved for
ZIBs, there are challenges in identifying the underlying the electrochemical
reaction and the by-products. Also, it is worth noting that the monitoring of electrochemical reactions through low current drains under long-term cycling is
essential for such aqueous systems to be realized for practical stationary applications. At present, the electrochemical reactions proposed are contrasting and
highly debated. For example, in MnO 2 , the zinc storage mechanism was described
by Zn-intercalation/de-intercalation without or with additional phases and a conversion reaction, respectively. Even within the short time duration of writing and
publishing this chapter, additional reaction mechanisms based on reversible cointercalation reaction of H
+
/Zn
2+
, either sequentially or concordantly, combined
intercalation-conversion reactions have been proposed [54]. Further, the origin
and the role of discharged by-products on the electrochemical reaction remains
evasive. Hence, a balanced analysis of different real-time spectroscopic measurements based on electrochemical, X-ray, vibrational and NMR techniques in
combination with computational techniques can help to confirm the reaction
mechanism in ZIBs.
2. Special focus on the understanding of electrode degradation mechanism and
interface reactions related to the solid-electrolyte-interface layer in detail via in
situ electrochemical techniques including potentiostatic electrochemical impedance spectroscopy (PEIS) or galvanostatic intermittent titration technique
(GITT) during on-time cycling will be required to establish their influence on the
electrochemical reaction and shed more light on the aging issue of the ZIB during electrochemical reaction. Also, these techniques will help in answering many
questions in understanding the exact role of structural water in layered vanadium
cathodes.
3. Focus on identifying nanostructured materials based on phosphates or amorphous or organic/inorganic hybrid materials will also be the norm to achieve
further performance improvement in the ZIB battery system. The study on phosphates is motivated by their unique 3D open-framework structures with
transition- metal ions and covalently bonded polyanion (PO 4 )
3−
units. In addition,
they demonstrate high structural stability, tunable operating potential window
via altering local environment, high thermal stability and can accommodate oxidation reactions at high charge potentials [110]. Specially, the use of fluorophosphates can benefit in raising the operating potential window in aqueous ZIBs. On
the other hand, amorphous materials can evade macroscopic phase transitions
during electrochemical reaction and thereby preserve the electrode from abuse
related to volumetric changes and facilitate long-term electrode cycling
stability.
J. Kim et al.
Therefore, in less than a decade, the research and development on ZIBs has been
very fruitful. From the abovementioned discussion the perspectives on the development of state-of-the-art ZIB cathodes can be summarized as follows:
1. Although high performances in terms of high zinc storage capacities and longterm cycling stabilities under high applied current densities are achieved for
ZIBs, there are challenges in identifying the underlying the electrochemical
reaction and the by-products. Also, it is worth noting that the monitoring of electrochemical reactions through low current drains under long-term cycling is
essential for such aqueous systems to be realized for practical stationary applications. At present, the electrochemical reactions proposed are contrasting and
highly debated. For example, in MnO 2 , the zinc storage mechanism was described
by Zn-intercalation/de-intercalation without or with additional phases and a conversion reaction, respectively. Even within the short time duration of writing and
publishing this chapter, additional reaction mechanisms based on reversible cointercalation reaction of H
+
/Zn
2+
, either sequentially or concordantly, combined
intercalation-conversion reactions have been proposed [54]. Further, the origin
and the role of discharged by-products on the electrochemical reaction remains
evasive. Hence, a balanced analysis of different real-time spectroscopic measurements based on electrochemical, X-ray, vibrational and NMR techniques in
combination with computational techniques can help to confirm the reaction
mechanism in ZIBs.
2. Special focus on the understanding of electrode degradation mechanism and
interface reactions related to the solid-electrolyte-interface layer in detail via in
situ electrochemical techniques including potentiostatic electrochemical impedance spectroscopy (PEIS) or galvanostatic intermittent titration technique
(GITT) during on-time cycling will be required to establish their influence on the
electrochemical reaction and shed more light on the aging issue of the ZIB during electrochemical reaction. Also, these techniques will help in answering many
questions in understanding the exact role of structural water in layered vanadium
cathodes.
3. Focus on identifying nanostructured materials based on phosphates or amorphous or organic/inorganic hybrid materials will also be the norm to achieve
further performance improvement in the ZIB battery system. The study on phosphates is motivated by their unique 3D open-framework structures with
transition- metal ions and covalently bonded polyanion (PO 4 )
3−
units. In addition,
they demonstrate high structural stability, tunable operating potential window
via altering local environment, high thermal stability and can accommodate oxidation reactions at high charge potentials [110]. Specially, the use of fluorophosphates can benefit in raising the operating potential window in aqueous ZIBs. On
the other hand, amorphous materials can evade macroscopic phase transitions
during electrochemical reaction and thereby preserve the electrode from abuse
related to volumetric changes and facilitate long-term electrode cycling
stability.
J. Kim et al.