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Mazzapioda et al. Energy Mater 2023;3:300019 https://dx.doi.org/10.20517/energymater.2023.03 Page 13 of 30

These findings evidence a direct correlation between the interfacial mechanical properties and the battery
performance, suggesting that the formation of a thin interphase offering pure ionic transport, together with
good mechanical properties, is necessary to stabilise the Li|ISE interface.

LI DENDRITE GROWTH IN INORGANIC SOLID ELECTROLYTES
The primary concern at the Li|ISE interface is the formation of Li dendrites, which causes several
detrimental effects, such as loss of interfacial contact, crack formation in ISEs, and formation of internal
[118]
short-circuit . Metallic lithium is a plastic and ductile metal with a Young’s modulus of 1.9-7.98 GPa and
a yield strength ranging from 0.41-2 MPa. According to the Monroe and Newman model, developed for
SPEs, Li dendrite growth can be successfully suppressed when SSEs possess Young’s modulus over two
[119]
times higher than that of Li metal . From this early model, it was expected that ISEs would be able to
prevent Li-dendrite growth because of their great robustness and high Young’s modulus. However, it was
recently demonstrated that Li dendrites penetrate into stiff ISEs on cycling, even under limited current
densities [120,121] . The Monroe-Newman model fails because it assumes both lithium and SE as pure elastic
materials, with the latter being free of defects and chemically stable against Li. In contrast, porosity and pre-
existing local inhomogeneous defects such as cracks, grain boundaries and void, are inevitable in ISEs, and
these defects play a key role in the propagation of Li dendrites. In fact, Porz et al. conducted a study on
amorphous Li S-P S , polycrystalline β-Li PS , LLZO (specifically Li La ZrTaO ), and single crystalline
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LLZO to understand the mechanism of Li dendrite growth. They confirmed that lithium dendrites grow
and propagate through the defects, pores, and cracks in crystalline materials, whereas no occurrence of
dendrites was observed solely in amorphous Li S-P S [122] .
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Shen et al. investigated the deposition of Li into the pores and cracks of LLZO processed at different
temperatures (1,050, 1,100, and 1,150 °C) using synchrotron X-ray tomography. As the sintering
temperature was increased from 1,050 to 1,150 °C, the porosity of LLZO decreased while the connectivity
between the porous region increased. They demonstrated that the interconnected pores facilitated lithium
transport along with undesirable dendrite growth within these microstructures, causing short circuits at
lower critical current densities (CCD). On the contrary, samples with disconnected pores showed higher
CCDs, indicating that, with regards to shorting, all microstructural features such as grain boundaries, pores
character, and density contribute to battery failure [Figure 5A] . Cheng et al. showed the occurrence of Li
[123]
dendrite propagation along Al-doped LLZO grain boundaries, which resulted in short-circuiting of
[124]
Li|LLZO|Li symmetric cells during cycling . These findings demonstrated a new intergranular type of Li
propagation mechanism besides the common transgranular type [Figure 5B].

To address the issue related to the pre-existing pore, Zheng et al. evaluated the addition of lithium-rich
additive Li Zr O (LZO) in garnet Ta-doped LLZO (Li La Zr Ta O ) to obtain high-density sintered
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LLZO. During the sintering process, LZO is decomposed into Li O and Li ZrO , which have two
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fundamental functions: Li O compensates Li-loss that occurs during the sintering, whereas Li ZrO fills up
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the voids between the grains, both enhancing the intergranular bonding and successfully improving the
dendrite-resisting ability of the ISE .
[125]
Han et al. investigated the formation of dendrites in SEs by monitoring the dynamic evolution of Li
concentration profiles within LLZO, amorphous Li PS and LiPON during Li plating. Among the three ISEs,
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no apparent changes in the Li concentration profile were detected in LiPON, while the deposition of Li
within LLZO and Li PS was observed. The origin of this phenomenon is considered to be correlated with
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the higher electronic conductivity of LLZO and LPS (10 -10 Scm ) than LiPON (10 -10 Scm )
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promoting the intergranular formation of Li dendrites . In addition, it was confirmed by Ping et al. that in
[126]