Page 57 - Read Online

Page 6 of 16 Mu et al. Energy Mater 2022;2:200043 https://dx.doi.org/10.20517/energymater.2022.57

into sodium-ion full-cell systems. The catalytically-driven sodium-ion compensation was monitored by
in-situ synchrotron X-ray diffraction (XRD). It was concluded that the Na O@Ru@G electrode employed in
2
a full-cell system not only provided extra cyclable sodium ions but also efficiently alleviated the continuous
phase transformation of cathode materials. The extra cyclable sodium ions in the full-cell system can inhibit
the surface phase transformation and alleviate the transition metal dissolution in the electrolyte. All of these
can protect the hard carbon (HC) anode from increased resistance by suppressing the dissolution-
migration-deposition process. After presodiation treatment for a Na[Li Mn Ni Cu Mg ]O //HC
0.05
0.50
0.05
0.10
2
0.30
pouch cell, the initial Coulombic efficiency and energy density can reach 90.0% and 295 Wh kg ,
-1
respectively, and the cycle performance is also markedly improved. Moreover, this method also addresses
the issues of the decomposition of cathode additives and the release of produced gas and residues.
In addition to experimental investigations, theoretical calculations are also useful tools for developing
presodiation techniques. Zou et al. calculated the optimal binding energy of O-M (M = Li, Na or K) bonds
in metal carboxylates [67-69] . After an in-depth analysis of the experimental results and density functional
theory calculations, it was found that the cathode additive decomposition caused by irreversible
decarboxylation is determined by the O-M (M = Li, Na or K) bond energy, which can be further affected by
the electronic structure of the substituent and hardness/softness adjustment of metal elements.
Furthermore, the bonding strength of O-M bonds can be regulated by the electron-donating effect of
substituents and the low charge density of cations, resulting in a lower electrochemical oxidation potential.

The presodiation process by introducing Na-containing cathode additives has many advantages. First, it is
straightforward and the total cost is determined by the cost of the additive substance, which is easy to
commercialize and industrialize. Second, cathode additives have excellent environmental adaptability and
high compatibility with current battery manufacturing technologies. However, there are still some
remaining challenges facing cathode presodiation. For instance, the impact of cathode additive residues and
emitted gases on the overall battery system is still not well understood at present. In particular, the released
gases are likely to change the microstructure of cathode materials, which may have a significant influence on
the long-time operation of the battery system.

Self-presodiation cathode materials
The introduction of sacrificial additives results in an increase in cathode mass and the inevitable generation
[60]
of gases and byproducts, which restrains their commercial and industrial application . To overcome the
above-mentioned problems, the researchers proposed a Na-rich cathode as an alternative approach towards
presodiation. The Na-rich cathode is a solid solution including supersaturated Na, which can be irreversibly
released to the electrolytes during cycling, compensating for active Na loss. As shown in Figure 3A, the self-
presodiation cathode compound O3-type Na Cu Ni Fe Mn Ti O was prepared by the quenching
2
0.48
0.11
0.9
0.30
0.10
0.11
treatment, which can retain a high sodium content (nearly 0.9) in the crystal structure by inhibiting the
+
precipitation of carbonate. The quenched materials maintain high Mn and Na contents, which can
3+
compensate for Na consumption during initial charging by releasing Na activated by Mn oxidation. Other
3+
+
transition metals are employed to supply capacity for subsequent cycles. In contrast, the structural evolution
of the naturally cooled cathode material was investigated by in-situ temperature-variable XRD, indicating
that the Na CO layer formed on the surface of the cathode particles, accompanied by a large amount of
3
2
Mn oxidation caused by the reaction between Na precipitated from the layered oxide lattice and CO
3+
+
2
molecules in the air. The quenching procedure could significantly suppress the emergence of surface
carbonates and preserve the long-range structure of Na Cu Ni Fe Mn Ti O , particularly the lattice
2
0.11
0.48
0.11
0.9
0.30
0.10
oxygen array architecture. Paired with a commercially available HC anode in Na-ion full cell, the quenching
-1
cathode delivered a higher energy density of 256 Wh kg , representing a ~9.9% increase compared with that
of the naturally cooled cathode .
[70]