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Abstract Magnesium silicide (Mg2Si) is a new anode material candidate for Mg‐ion batteries due to the Earth‐abundance of Mg and Si and its high theoretical specific capacity of 1398 mAh/g. However, to date, no one reported its reversible Mg‐storage ability. Herein, we demonstrate for the first time the experimental visualization of a reversible electrochemical demagnesiation and magnasiation of Mg2Si film model electrode in a half‐cell with the electrolyte of PhMgCl/THF at room temperature. The Mg2+‐diffusivity, 1.3 × 10–18 cm2/s, of Mg2Si film model electrode determined with cyclic voltammetry is four orders of magnitude lower than that of Li+‐diffusivity of Mg2Si film electrode in a lithium cell, indicating a sluggish diffusion kinetics. Surface chemistry studies of the 1st discharged (demagnesiated) and the 1st cycled (re‐magnesiated) electrodes utilizing X‐ray photoelectron spectroscopy reveal that the surface of pristine Mg2Si is highly covered by various silicon oxides, and Mg2Si undergoes segregation and phase separation partly to Mg and Si along with the surface coverage of electrolyte decomposition products. Those events lower the homogeneity and connectivity of electrode composition, which limits the reaction reversibility. The data give insight into a new material design for Mg‐ion batteries.

State‐of‐the‐art lithium (Li)‐ion batteries employ silicon anode active material at a limited fraction while strongly relying on fluoroethylene carbonate (FEC) electrolyte additive exceeding 10 wt.% to enable stable cycling. The swelling issue of silicon in the aspect of solid electrolyte interphase (SEI) instability and a risk of safety hazards and high manufacturing cost due to FEC has motivated the authors to design a well‐working fluorinated additive substitute. High‐capacity cells employing nickel‐rich oxide cathode are pursued by operating at > 4.2 V versus Li/Li+, for which anodic stability of electrolyte is required. Herein, a highly effective new ambifunctional additive of icosafluoro‐15‐crown 5‐ether is proposed at a little fraction of 0.4 wt.% for the stabilized interfaces and reduced swelling of high capacity (3.5 mAh cm−2) 5 wt.% SiO‐graphite anode and LiNi0.88Co0.08Mn0.04O2 cathode. Utilizing together with a lowered fraction of FEC, reversible long 300 cycles at 4.35 V and 1 C (225 mA g−1) are achieved. Material characterization results reveal that such stabilization is derived from the surface passivation of both anode and cathode with perfluoro ether, LiF, and LixPFy species. The present study gives insight into electrolyte formulation design with lower cost and both‐side stabilization strategies for silicon and nickel‐rich active materials and their interfaces. [ABSTRACT FROM AUTHOR]