In lithium-ion batteries (LIBs), during charge-discharge cycles, both Li metal and organic electrolyte become unstable. The continuous deposition and stripping of the Li metal results in a large structural change while dendrite growth worsen the situation. Decomposition of the organic electrolyte at the anode leads to the formation of a solid electrolyte interphase (SEI) layer consisting of organic and inorganic components. The changes in the SEI layer as well as the growth directions of the dendrites could alter the efficiency of the system. Since the Li containing electrode material, organic electrolyte and the SEI layer are chemically reactive and sensitive to electron-beam irradiation, it is hard to characterize them using transmission electron microscopy. These attributes poses difficulty in identification of failure mechanism of LIB.
Researchers at the Stanford University, USA, ShanghaiTech University, China, Universität Erlangen–Nürnberg, Germany, National Accelerator Laboratory, USA have developed a cryo-transfer method (Fig. 1) based on cyro-electron microscopy (cyro-EM) and demonstrated that it would be possible to obtain atomic-resolution images of sensitive battery materials in their native state (Yuzhang Li et al., Science, 358, Issue 6362, 2017, pp. 506-510).
Fig. 1 Preserving and stabilizing Li metal by cryo-transfer method: (a) Li metal dendrites are electrochemically deposited directly onto a Cu TEM grid and then plunged into liquid N2 after battery disassembly; and (b) The specimen is then placed onto the cryo-TEM holder while still immersed in liquid nitrogen and isolated from the environment by a closed shutter. During insertion into the TEM column, temperature is not increased > –170 °C, and the shutter prevents air exposure to the Li metal.
The cryo-TEM and cyro-SEM images of the electrodeposited Li metal dendrites (Figs. 2(a) and 2(b)) reveal that the dendrite structure is preserved during the cryo-transfer method. Time time-lapse images obtained under constant electron-beam irradiation (~50 e Å–2 s–1) in cryogenic conditions (Figs. 2(c), 2(d) and 2(e)) show no signs of damage in the dendrite morphology even after 10 min. The lack of reactivity of the Li metal with liquid N2, helps the dendrites to retain their electrochemical state so that the relevant structural and chemical information could be obtained. The inferences made in this study reveal that in carbonate-based electrolyte, Li metal dendrites grow as single-crystalline nanowires along three primary growth directions: <111>, <110>, and <211> (Figs. 2(f), 2(g) and 2(h)) with 49% growth along the <111> direction, followed by 32% along <211> and 19% along <110> direction. In spite of growing as single-crystalline nanowires along a linear direction, the Li metal dendrites often change their growth directions.
Fig. 2 (a) Cryo-TEM and (b) Cryo-SEM images of Li metal dendrites depicting that the morphology is preserved by the cryo-transfer method; (c to e) time-lapse images of Li dendrite; (f to h) growth of Li metal dendrites along: (f) <111>; (g) <110>; and (h) <211> directions.
The methodology described in this work is likely to provide a complete understanding of the failure mechanisms in high-energy batteries.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Yuzhang Li et al., Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy, Science, 358, Issue 6362, pp. 506-510.
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