Electrolytic alloy-type anodes for metal-ion batteries

Alloy-type metals/alloys hold the promise of increasing the energy density of metal-ion batteries (MIBs) because of their theoretical high gravimetrical capacities. Semimetals and semimetal-analogs are typical alloy-type anodes. Currently, the large-scale extraction of semimetals (Si, Ge) and semimetal-analogs (Sb, Bi, Sn) by traditional metallurgical routes highly relies on using reducing agents (e.g., carbon, hydrogen, reactive metals), which consumes a large number of fossil fuels and produces greenhouse gas emissions. In addition, the common metallurgical methods for extracting semimetals involve relatively high operating temperatures and therefore produce bulk metal ingots solidified from the liquid metals. However, the commonly used electrode materials in batteries are fine powders. Thus, directly producing semimetal powders would be more energy efficient. In addition, semimetals are good candidates to host alkali/alkaline-earth ions through the alloying process because the electronegativity of semimetals is high. Therefore, preparing semimetal powders via an environment-sound manner is of great interest to provide sustainable anode materials for MIBs while reducing the ecological footprint. Low-cost and high-output capacity anode powder materials, as well as straightforward and environmental-benign synthetic methods, play key roles in enabling the energy conversion and storage technologies for real applications of MIBs. Electrochemical technologies offer new strategies to extract semimetals using electrons as the reducing agent that comes from renewable energies. Besides, the morphologies and structures of the electrolytic products can be rationally tailored by tuning the electrode potentials, electrolytes, and operating temperatures. In this regard, using the one-step green electrochemical method to prepare high-capacity and cheaper alloy-type metalloids for MIB anodes can fulfill the requirements for developing MIBs. This review critically overviews recent developments and advances in the electrochemical extraction of semimetals (Si, Ge) and semimetal-analogs (Sb, Bi, Sn) for MIBs, including basic electrochemical principles, thermodynamic analysis, manufacture strategies and applications in lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium-ion batteries (PIBs), magnesium-ion batteries (Mg-ion batteries), and liquid metal batteries (LMBs). It also presents challenges and prospects of employing electrochemical approaches for preparing alloy-type anode materials directly from inexpensive ore-originated feedstocks.