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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 11 of 32

[67]
SEI region via an in situ SECM device [Figure 4F] . As shown in Figure 4G, it can be found that the gap
between the minimum and maximum currents gradually increases with the enhancement of applied current
during cycling. The SEI will be increasingly challenged by the heavy current due to the continually changing
surface shape of metallic Li in the subsequent cycles; thus, it can be applied to detect the appearance of
lithium protrusions. Furthermore, laser scanning confocal microscopy (LSCM) and scanning tunnelling
[68]
microscopy (STM) have also been exploited to investigate Li metal anodes .
In situ X-ray imaging technologies, such as transmission X-ray microscopy (TXM) and X-ray tomography
(XRT), can establish real-time three-dimensional (3D) images of Li deposits without damaging cells.
Considering the high energy of X-rays, TXM can be employed to study thick samples. Using in situ TXM,
Cheng et al. first revealed the dynamics of dendritic Li growth in 1 M LiPF -ethylene carbonate (EC)/diethyl
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carbonate (DEC) . Notably, as a brand-new 3D detection technology, XRT is capable of investigating the
[69]
inner electrode structure without slicing treatment. It photographs electrodes from different angles by
rotating samples and then reconstructs 3D images through remodeling [Figure 4H] . Harry et al. reported
[70]
that dendritic Li was mainly distributed inside the electrode at the initial stage, i.e., below the interface of the
electrode/polymer electrolyte [71,72] . Crystal impurities and the subsurface structure are the nucleation seeds of
Li dendrites. Subsequently, Li branches pierce the electrolyte and reach the cathode side, eventually leading
to short circuit of the battery [Figure 4I]. Yu et al. systematically estimated the impact of various electrolyte
systems on the plating and stripping processes of Li metal via synchrotron-based X-ray imaging
technology . They proposed that the excessive overpotential generated a higher nucleation density, leading
[45]
[73]
to smaller nuclei during the charge process, which is consistent with the previously reported result .
In situ nuclear magnetic resonance (NMR) is commonly used to monitor the component variation of the
electrolyte, electrode and interface. As a noninvasive imaging technique developed from in situ NMR,
in situ nuclear magnetic resonance imaging (MRI) can achieve sufficient temporal and spatial resolution of
the Li chemical shift. Ilott et al. reported the utilization of indirect MRI with 3D imaging to investigate the
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microscopic structure of Li metal, showing that dendritic growth was distorted rather than unidirectional
[74]
until the short circuit was formed . Because this method not depending on measuring Li directly, it is also
suitable for several metallic dendrites, such as sodium, zinc and magnesium anodes. In situ electron
paramagnetic resonance (EPR), in which the unpaired electron spins rather than nuclear spins are present
in EPR when applying a magnetic field, is an emerging technique in Li metal anode research. Sathiya et al.
first observed uneven Li deposition with a LiPF -EC/PC/DMC electrolyte in a Li Ru Sn O /Li battery by
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0.25
3
2
0.75
in situ EPR . Wandt et al. found that the formation of microstructured (mossy/dendritic) Li was greatly
[75]
reduced after the addition of FEC via in situ EPR and ex situ SEM .
[76]
Neutron radiography and tomography (NR/NT), whose imaging principles are based on the neutron
absorption gradient determined by the permeability and absorption of sample composition and density, are
typical 2D and 3D imaging technologies, respectively. Recently, Song et al. investigated the distribution and
growth mechanism of Li dendrites by in situ NR/NT techniques . Combined with electrochemical tests, it
[77]
was confirmed that the dendritic Li growth between the anode and LiMn O contributed to the short circuit.
2
4
Sun et al. used in situ NT combined with in situ XRT to demonstrate the relationship between the overall
electrochemical performance of a Li-O battery and the evolution of Li morphology, which fully revealed
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that the irreversible conversion of Li metal from the bulk structure to porous state . This work has
[78]
advanced the mechanism of battery failure, which contributes to new insights into future battery design.