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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 9 of 32

analyze the deposition and dissolution behavior of Li metal. In 1998, Osaka et al. employed in situ OM to
[47]
explore the effect of the electrolyte on the Li deposition morphology and anode cycling efficiency .
Arakawa et al. investigated the relationship between dead Li formation and the Li plating morphology by
[34]
recording the film attached to an optical microscope to obtain photographs . Subsequently, Yamaki et al.
employed in situ OM to probe the internal mechanism of deposition and dissolution of Li metal in
LiClO -PC and proposed the famous whisker model based on a mathematical analysis showing that
4
dendritic Li may grow from the current collector during deposition . Inevitably, stress from the bottom
[27]
regions can seriously destroy the SEI interphase, enabling Li whiskers to continuously grow in the form of
extrusion. Nevertheless, Steiger et al. proposed a different view of Li cycling behavior in a LiPF -EC/DMC
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electrolyte . They claimed that crystal defects control the Li deposition behavior, i.e., newly fresh Li atoms
[48]
are electrodeposited to the defective regions. In addition to defects, Li metal can also grow from grain
boundaries, kinks and chemical inhomogeneities, such as contaminants. Furthermore, they proved that
dendritic growth is based on the defect-controlled deposition mechanism, challenging the conventional
mechanism based on ion transport . Moreover, Shen et al. studied the impacts of current density,
[49]
magnetic field and electromigration on dendritic Li growth via in situ OM observation [50-52] . Although in situ
OM has shown its effectiveness in observing dendrite growth and battery failure, it still possesses some
drawbacks, the most important of which are the low resolution and poor image clarity in complicated
electrolyte circumstances .
[53]
Due to higher spatial resolution than in situ OM, in situ scanning electron microscopy (SEM) has been
widely applied to battery research. Krauskopf et al. investigated the Li growth kinetics on a garnet-type
solid-state electrolyte (SSE) at the microscale via in situ SEM . Motoyama et al. used in situ SEM to
[54]
[55]
observe the plating and stripping behavior of Li metal in a LiPON electrolyte . As shown in Figure 4A, a Li
island grows laterally over time without damaging the wolfram film. During the Li dissolution process, the
dome top of the island first begins to shrink and then the lateral bulges reduce gradually. In situ
electrochemical SEM (EC-SEM) provides a powerful tool to reveal the reaction mechanism in
electrochemical systems. For instance, Rong et al. found that the addition of single LiNO in a LiTFSI-
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DOL/DME electrolyte could hardly inhibit the Li dendrite formation and growth via EC-SEM .
[56]
As an advanced technology, in situ transmission electron microscopy (TEM) has been developed gradually.
With high energy electrons as the excitation source, the operation principle of in situ TEM is similar to that
of in situ SEM, but its spatial resolution is higher than in situ SEM. Furthermore, TEM can provide
[57]
chemical and structural information for electrodes at the nanoscale . Initially, Huang et al. reported an
“open” in situ TEM cell . However, ‘‘open’’ TEM inevitably has its own defects: (1) the electrode is in point
[58]
contact with the electrolyte, and thus the mode of Li ion diffusion is quite distinct from the actual cell;
+
(2) the strong electron beams are detrimental to some polymer electrolytes; and (3) some SSEs can cause
additional overpotential. A ‘‘closed’’ cell with a chamber was then designed to seal off the liquid electrolyte
[59]
[Figure 4B] to avoid the effect of the high vacuum on the system . Moreover, clear dynamic information
regarding Li stripping/plating processes can be obtained using high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM) . Gong et al. employed operando HAADF-STEM to
[60]
study Li electrodeposition and dissolution, which explained the F-enriched interface formation on Li
[61]
deposits . For the past few years, Li et al. upgraded Li dendrite images at the atomic scale via cryogenic-
electron microscopy technology (cryo-EM) [Figure 4C and D] . Wang et al. observed the phase transition
[62]
of ordered (crystalline) Li and disordered (glassy) Li via cryo-TEM, revealing the transient structural
evolution during Li deposition .
[63]