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S C I E N T I F I C H I G H L I G H T S

8 2 H I G H L I G H T S 2 0 2 2 I

Microstructuring to improve the thermal stability of GeSn layers

Nano X-ray fluorescence was used to characterise layers of a microstructured germanium-tin alloy before and after annealing at different temperatures and durations. The increased thermal stability of the microstructured alloy allows higher-temperature annealing, which could improve its performance in optoelectronic devices.

Direct bandgap group-IV semiconductor materials are needed for optical interconnects and monolithic photonics on silicon (Si). However, Si and Germanium (Ge) are both

indirect bandgap semiconductors. Unlike Si, Ge can be turned into a direct bandgap material by alloying it with tin (Sn). Crystalline Ge1-xSnx turns into a direct bandgap material for Sn concentrations higher than 6.5% [1]. There is huge interest in the manufacturing of high Sn-content Ge1-xSnx layers due to its technological applications (LEDs, lasers, photo-detection, field-effect transistors, etc.). However, the main challenge of increasing the Sn content in Ge1-xSnx is to preserve the crystalline and compositional quality of the material despite the increased lattice distortion introduced by Sn atoms into the Ge lattice. Moreover, the very low solid solubility limit (less than 1%) of Sn into Ge generates metallic Sn precipitates in relatively low Sn-content layers. High Sn-content layers

Fig. 72: Sn distribution in (a) fully segregated 16.9% and (b) 12.7%

blanket layers after annealing. c) Scanning electron microscopy

(SEM) top and lateral views of a Ge1-xSnx micro-disk 8 μm in

diameter. d,e) Low- and high- resolution Sn distribution maps of 10-μm diameter micro-disks

made from layers (f) P08 and (g) P07, annealed simultaneously with

blanket layers. Scale bars: 2 μm.

PRINCIPAL PUBLICATION AND AUTHORS

Additive engineering for robust interphases to stabilize high-Ni layered structures at ultra-high voltage of 4.8V, S. Tan (a), Z. Shadike (a), J. Li (b), X. Wang (a), Y. Yang (a), R. Lin (a), A. Cresce (c), J. Hu (d), A. Hunt (a), I. Waluyo (a), L. Ma (a), F. Monaco (e), P. Cloetens (e), J. Xiao (d, f), Y. Liu (b), X.-Q Yang. (a), K. Xu (c), E. Hu (a), Nat. Energy 7, 484-494 (2022); https:/doi.org/10.1038/s41560-022-01020-x (a) Brookhaven National Laboratory, Upton (USA) (b) SLAC National Accelerator Laboratory, Menlo Park (USA) (c) US Army Research Laboratory, Adelphi (USA) (d) Pacific Northwest National Laboratory, Richland (USA) (e) ESRF (f) University of Washington, Seattle (USA)

REFERENCES

[1] Y. Mao et al., Adv. Funct. Mater. 29, 1900247 (2019). [2] Z. Jiang et al., Nat. Commun. 11, 2310 (2020).

In addition, X-ray nanotomography was utilised for studying crack formation in the bulk cathode materials after high voltage cycling. Figure 71a shows that fewer cracks were formed and fewer secondary particles broke down in the cathode cycled with LiDFP-containing electrolyte. The statistical results (Figures 71b and 71c) collected on a large number of particles provide convincing evidence for the better structural stability at particle-level from the LiDFP-derived interphase.

This study suggests that surface and bulk are not two independent aspects for battery materials. A well- protected cathode surface facilitates uniform lithium distribution within the bulk and mitigates strain and crack issues. The fact that polycrystalline high-Ni layered cathode can be stabilised at high voltage suggests that single crystals may not be the only solution to the notorious crack problem facing these materials. Electrolyte engineering through applying additive in an already-matured electrolyte system is still an effective and efficient way to improve battery performance.