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Page 8 of 32 Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60

the electrode surface and can no longer participate in the electrode reactions) [Figure 3D]. The formation of
[34]
dead Li indicates an irreversible loss of battery capacity . Both uncontrollable dendritic Li growth and dead
Li are believed to be the reason for the low CE (less than < 99%) of Li deposition/dissolution in nonaqueous
[35]
electrolytes . In addition to the formation of dead Li, the newly exposed Li dendrites on the surface can be
rapidly corroded by the nonaqueous electrolyte, leading to the formation of a thick SEI with poor
conductivity. Due to the repeated generation/fracture of the unstable SEI during charge/discharge processes,
the exposed fresh Li reacts with the electrolyte continuously. Electrolyte depletion and electrochemical
corrosion of Li metal eventually lead to low CE and capacity decay [36,37] . Among the above-discussed issues
triggered by dendritic growth, safety concerns, including battery fires [Figure 3E] and even explosion
hazards [Figure 3F], which result from short circuit risks [Figure 3G], are also top priorities for developing
practical LMBs [38,39] .

Infinite relative volume change
Unlike insertion type anode materials (such as C, Si, Al, Ge and Sn), metallic Li is the conversion type anode
with an enormous volume change, far exceeding those of intercalated anodes, such as graphite (with the
volume change of 10%) and silicon (with the volume change of 400%) during each plating/stripping
process . The accommodative rate of the SEI cannot easily keep up with the expansion/contraction of the
[40]
anode during the plating/stripping of Li, eventually leading to damage to the SEI and dendrite growth. This
volume change can also induce the destruction of the electrode and electrolyte interfaces, resulting in a
decay in electrochemical performance. Actually, the pulverized and insulting LiH has been proposed as one
of the reasons for anode expansion of LMBs, which is formed by the parasitic reaction between Li metal and
H in batteries . Lu et al. found that at a current density of 1.5 mA cm , no evidence of separator
[41]
-2
2
penetration by dendritic Li could be found in the failed battery. Instead, expansive regions of Li/electrolyte
interface inhibited the contact among Li grains, leading to a porous and loose structure . The increased
[42]
resistance of this porous interphase was believed to be the true cause of the eventual battery failure.
Moreover, from a practical perspective, an areal capacity of at least 3 mAh cm is required for a single-sided
-2
commercial electrode, which corresponds to a relative thickness change of ~14.6 μm for lithium. For future
batteries, this value might be even higher, which means that the movement of the Li interface may reach
tens of microns during cycling, which imposes a serious challenge on the stability performance of the SEI.

CHARACTERIZATION OF LI METAL ANODES
There are a variety of detection and characterization techniques used to study the Li metal anode. Since Li is
extremely reactive, ex situ techniques often require very careful sample preparation and handling to
minimize damage to the electrode. Conversely, as typically nondestructive techniques, in situ techniques are
conducted with a cell in its fully assembled condition, thus furnishing dynamic cell information in non-
equilibrium states. In the past several years, there has been a rapid increase in research interest in in situ
technologies, including in situ morphology and composition characterization, to continuously capture
transient metastable information on Li metal anodes as a function of time.

Morphological and microstructural characterization of Li surface
In situ imaging techniques, including in situ optical image techniques, electron-based analysis, scanning
probe-based tools, X-ray and synchrotron-based analysis, magnetism-based techniques, neutron-based
analysis and other in situ techniques, have been widely applied to qualitatively study the composition of
Li-containing species [43-46] . Optical image and microscopy technologies are based on the light wave of the
object being reflected and projected by the detector to form the image. Therefore, optical microscopy (OM)
in conjunction with an in situ reaction cell and electrochemical workstation is the most widely used image
observation equipment. More importantly, it is the most direct, rapid, feasible and low-cost method to