Binary Cation Matrix Electrolyte and Its Effect on Solid Electrolyte Interphase Suppression and Evolution of Si Anode.

An unstable solid electrolyte interphase (SEI) has been recognized as one of the biggest challenges to commercializing silicon (Si) anodes for high-energy-density batteries. This work thoroughly investigates a binary cation matrix of Mg2++Li+ electrolyte and its role in SEI development, suppression, and evolution of a Si anode. Findings demonstrate that introducing Mg ions dramatically reduces the SEI growth before lithiation occurs, primarily due to the suppression of solvent reduction, particularly ethylene carbonate (EC) reduction. The Mg2+ alters the Li+ cation solvation environment as EC preferably participates in the oxophyllic Mg2+ solvation sheath, thereby altering the solvent reduction process, resulting in a distinct SEI formation mechanism. The initial SEI formation before lithiation is reduced by 70% in the electrolyte with the presence of Mg2+ cations. While the SEI continues to develop in the postlithiation, the inclusion of Mg ions results in an approximately 80% reduction in the postlithiation SEI growth. Continuous electrochemical cycling reveals that Mg2+ plays a crucial role in stabilizing the deep-lithiated Si phases, which effectively mitigates side reactions, resulting in controlled SEI growth and stable interphase while eliminating complex LixSiy formation. Mg ions promote the development of a notably more rigid and homogeneous SEI, characterized by a reduced dissipation (ΔD) in the Mg2++Li+ ion matrix compared to the solely Li+ system. This report reveals how the Mg2++Li+ ion matrix affects the SEI evolution, viscoelastic properties, and electrochemical behavior at the Si interface in real time, laying the groundwork for devising strategies to enhance the performance and longevity of Si-based next-generation battery systems.

Evolution of the Dynamic Solid Electrolyte Interphase in Mg Electrolytes for Rechargeable Mg-Ion Batteries.

Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode-electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg(TFSI)2 electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg2+ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg0, which maintains a MgF2-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg0. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.

In Situ Probing of Mass Exchange at the Solid Electrolyte Interphase in Aqueous and Nonaqueous Zn Electrolytes with EQCM-D.

Multivalent chemistry provides intriguing benefits of developing beyond lithium ion energy storage technologies and has drawn extensive research interests. Among the multivalent candidates, metallic zinc anodes offer an attractive high volumetric capacity at a low cost for designing the secondary ion batteries. However, the interfacial mass exchange at the Zn electrolyte/anode boundary is complicated. The least understood solid electrolyte interphase (SEI) occurs simultaneously with the reversible metal deposition, and its dynamic progression is unclear and difficult to capture. One major challenge to investigate such a dynamic interface is the lack of in situ analytical methods that offer direct mass transport information to reproduce the realistic battery operating conditions in an air-sensitive, nonaqueous electrolyte environment with a high iR drop. Work reported here reveals an in-depth analysis of the complex and dynamic SEI at the Zn electrolyte/electrode interface utilizing a multiharmonic quartz crystal microbalance with a dissipation method combined with the spectroscopic analysis. Key differences are observed for the SEI formation in the nonaqueous Zn(TFSI)2 electrolyte in contrast to the aqueous ZnCl2 electrolyte for reversible Zn deposition. A large disproportional loss of coulombs relative to the gravimetric mass change is prominently observed at the initial electrochemical cycles in the nonaqueous Zn electrolyte, and results suggest an in situ formation of an ionically permeable SEI layer that is compositionally featured with a rich content of organic S and N components. Further overtone-dependent dissipation analysis implies the changes in viscoelasticity at the electrode interface during the early SEI formation in the nonaqueous Zn(TFSI)2 electrolyte.