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Mu et al. Energy Mater 2022;2:200043 https://dx.doi.org/10.20517/energymater.2022.57 Page 3 of 16

Figure 1. Schematic illustration of Na loss during initial cycle.

FUNDAMENTALS AND STRATEGIES OF PRESODIATION
Similar to LIBs, the electrode materials from SIBs also suffer from Na loss during electrochemical cycles, as
shown in Figure 1, leading to the deterioration of capacity and energy density. In SIBs, the following three
factors are primarily responsible for the irreversible capacity loss:

(i) Formation of SEI from electrolyte decomposition. The electrolytes of SIBs mainly contain carbonate ester
solvent and sodium salt, which are prone to irreversible decomposition reactions at low potential to form
SEI film [44,45] , resulting in the reduction of Coulombic efficiency in the first cycle [44-47] . In particular, for an
alloy anode that undergoes a significant volume change during Na storage [48-50] , the SEI film continuously
splits and reconstructs during electrochemical cycling, leading to a further increase in Na consumption.

+
(ii) Capture of Na from structural defects. Generally, the anode material has several structural flaws and
sodiophilic functional groups that can permanently trap sodium ions and cause capacity deterioration [51,52] .
In particular, for anode materials with a high surface area or porous structure, the Na loss will be
+
promoted.

(iii) Na loss from side reactions. For instance, coordinated water is present in cathode materials, such as
Prussian blue, which is vulnerable to electrolyte side reactions at high potentials and results in Na
consumption . From the perspective of the Na-ion full cell, the SEI will develop on the surface of the
[53]
+
anode during the first charge cycle, which consumes partial Na from the energy storage system, causing a
slight capacity loss and a drop in energy density.