Unraveling the Evolution of Transition Metals during Li Alloying- Dealloying by In-Operando Magnetometry Qingtao Xia, @ Xiangkun Li, @ Kai Wang,* Zhaohui Li, Hengjun Liu, Xia Wang, Wanneng Ye, Hongsen Li, Xiaoling Teng, Jinbo Pang, Qinghua Zhang, Chen Ge, Lin Gu, Guo-xing Miao, Shishen Yan, Han Hu,* and Qiang Li* Cite This: Chem. Mater. 2022, 34, 5852-5859 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: In view of the long-standing controversy over the reversibility of transition metals in Sn-based alloys as an anode for Li-ion batteries, an in situ real-time magnetic monitoring method was used to investigate the evolution of Sn-Co alloy during the electrochemical cycling. Sn-Co alloy film anodes with different compositions were prepared via magnetron sputtering without using binders and conductive additives. The magnetic responses showed that the Co particles liberated by Li insertion recombine fully with Sn during the delithiation to reform Sn-Co alloy into stannum-richer phases Sn 7 Co 3 . However, as the Co content increases, it can only recombine partially with Sn into cobalt-richer phases Sn 3 Co 7 . The unconverted Co particles may form a dense barrier layer and prevent the full reaction of Li with all the Sn in the anode, leading to lower capacities. In addition, we also showed that the Fe can recombine with Sn (Sb) during the delithiation in the Sn (Sb)-Fe alloy film anodes by operando magnetometry. These critical results shed light on understanding the reaction mechanism of transition metals and provide valuable insights toward the design of high-performance Sn (Sb)-based alloy anodes. 1. INTRODUCTION Although today’s portable energy storage market has almost been exclusively powered by the rechargeable Li-ion batteries (LIBs), 1-5 the accelerating demands for more advanced electric vehicles and portable electronics continue to stimulate extensive research on new electrode materials for breaking the current barriers in energy density and cell durability. Among various anode materials, metallic Sn has been widely considered as a promising anode material for its higher specific capacity than the widely used graphite electrodes. 6-10 However, the pulverization and aggregation induced by the large volumetric variations during the alloying-dealloying processes lead to poor cycling performance. To overcome these issues, inactive metal elements have been introduced to form Sn-M (M = Fe, Co, Ni, and Cu) alloy anode materials, 11-18 where the inert component acts as a buffer matrix to relieve the volume expansion of tin during cycling. Since Sony proposed the Sn-Co/C battery in 2005, 19 various Sn-M materials have been extensively investigated in both academia and industry. Especially, the magnetic elements (Fe, Co, Ni) are particularly promising due to their good electronic conductivity, high tap density, and gravimetric/volumetric capacity. 20-22 However, the mechanism of the reversibility of transition metals in Sn-M alloy is still a matter of debate, which restrains the rational design of reliable Sn-M anodes for lithium-ion batteries. The most widely accepted electrochemical process is a two- step electrochemical reaction, which involves an irreversible initial activation step [eq 1] followed by the main, reversible, electrochemical process [eq 2]. 23-25 − + → + Sn M Li M Li Sn 4.4 (1) ↔ + + + − Li Sn Sn 4.4Li 4.4e 4.4 (2) By means of ex situ X-ray diffraction (XRD), ex situ Mö ssbauer spectroscopy, and electron paramagnetic resonance spectroscopy, Nwokeke et al. claimed that iron nanoparticles are generated during the first discharge of FeSn 2 and preserved in the subsequent cycles. 26 However, using in situ XRD and in situ Mö ssbauer spectroscopy, Dahn et al. found that the Fe nanoparticles formed during discharge can recombine with Sn during delithiation. 27,28 Interestingly, Whittingham et al. revealed that some Fe particles still remain after the first charge by a combination of XRD, X-ray absorption spectros- copy (XAS), and magnetic measurements. 20 As for the Sn-Co alloy, Park et al. reported that CoSn 2 shows no recombination during Li extraction through ex situ XRD and extended X-ray absorption fine Structure (EXAFS) experiments. 15 In contrast, Han et al. showed a complete reversibility of CoSn 5 phase by means of ex situ XRD and ex situ XAFS. 22 Additionally, Lee et Received: February 26, 2022 Revised: June 21, 2022 Published: June 30, 2022 Article pubs.acs.org/cm © 2022 American Chemical Society 5852 https://doi.org/10.1021/acs.chemmater.2c00618 Chem. Mater. 2022, 34, 5852-5859 Downloaded via UNIV OF JINAN on July 14, 2022 at 02:35:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.