Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries

Fine nanostructure design of metal chalcogenide conversion-based cathode materials for rechargeable magnesium batteries

Magnesium-ion batteries (MIBs) a strong candidate to set off the second-generation energy storage boom due to their double charge transfer and dendrite-free advantages. However, the strong coulombic force and the huge diffusion energy barrier between Mg2+ and the electrode material have led to need for a cathode material that can enable the rapid and reversible de-insertion of Mg2+. So far, researchers have found that the sulfur-converted cathode materials have a greater application prospect due to the advantages of low price and high specific capacity, etc. Based on these advantages, it is possible to achieve the goal of increasing the magnesium storage capacity and cycling stability by reasonable modification of crystal or morphology. In this review, we focus on the application of a variety of sulfur-converted cathode materials in MIBs in recent years from the perspective of microstructural design, and provide an outlook on current challenges and future development.

The role of electrocatalytic materials for developing post-lithium metal||sulfur batteries

The exploration of post-Lithium (Li) metals, such as Sodium (Na), Potassium (K), Magnesium (Mg), Calcium (Ca), Aluminum (Al), and Zinc (Zn), for electrochemical energy storage has been driven by the limited availability of Li and the higher theoretical specific energies compared to the state-of-the-art Li-ion batteries. Post-Li metal||S batteries have emerged as a promising system for practical applications. Yet, the insufficient understanding of quantitative cell parameters and the mechanisms of sulfur electrocatalytic conversion hinder the advancement of these battery technologies. This perspective offers a comprehensive analysis of electrode parameters, including S mass loading, S content, electrolyte/S ratio, and negative/positive electrode capacity ratio, in establishing the specific energy (Wh kg-1) of post-Li metal||S batteries. Additionally, we critically evaluate the progress in investigating electrochemical sulfur conversion via homogeneous and heterogeneous electrocatalytic approaches in both non-aqueous Na/K/Mg/Ca/Al||S and aqueous Zn||S batteries. Lastly, we provide a critical outlook on potential research directions for designing practical post-Li metal||S batteries.

Effect of long range interactions on the reduction of divalent ions in N,O-chelating solvents

Local processes inside the ion solvation shell are believed to be the main factor affecting ion reduction in battery electrolytes. Much less attention is devoted to the interaction between the ion and molecules outside the shell. We demonstrate that in recently developed divalent batteries, long range ion/solvent and ion/electrode interactions significantly affect the reduction of ions. This effect is caused by the combination of low permittivity solvents, compact solvation shells, and high charge of Mg ions (compared with Li), leading to an effect of up to 1 eV. We establish a connection between our findings and recent experiments, highlighting the potential impact of this effect on battery performance. Additionally, we warn against arbitrarily choosing the dielectric permittivity in cluster-continuum models used for simulations, as even minor uncertainties may lead to significant variations in simulation results for divalent ions.

Effect of Polysulfide Speciation on Mg Anode Passivation in Mg-S Batteries

Mg-S batteries are a promising next-generation system for beyond conventional Li-ion chemistry. The Mg-S architecture pairs a Mg metal anode with an inexpensive, high-capacity S8 cathode. However, S8-based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced Sx2- species generated at the cathode diffuse to and react with the anode. While dissolved polysulfides may undergo reactions to form Li+-permeable layers in Li-S systems, the interfaces on Mg anodes are passivating. In this work, we probe the reactivity of various Mg polysulfide solutions at the Mg anode interface. Mg polysulfide solutions are prepared without any chelating agents to closely mimic conditions in a Mg-S cell. The polysulfides are synthesized by reacting Mg metal and S8 in electrolyte, and the speciation is controlled by varying the Mg:S precursor ratio. S-poor precursor ratios produce magnesium polysulfide solutions with a higher proportion of short-chain polysulfides that react at the Mg anode faster than the longer-chain analogues. Anode passivation can be slowed by shifting the polysulfide equilibria toward longer-chain polysulfides through addition of S8.

Electrolyte and Interphase Design for Magnesium Anode: Major Challenges and Perspectives

The rechargeable magnesium battery (RMB) is regarded as a high-energy, safe, and cost-effective alternative for conventional batteries. Unfortunately, the passivation and uneven Mg growth not only raise the voltage hysteresis but also shorten the cycle life of RMBs. In this review, Mg passivation induced by electrolytes/contaminants, growth patterns of high dimensional Mg⁰ , and mechanisms of Mg anode degradation are discussed. The recent efforts on suppressing electrolyte decomposition and uneven Mg growth including electrolyte/interphase modifications through additives, weakly coordinating anions, artificial interphases, and 3D magnesiophilic hosts are summarized. Finally, the future directions in stabilizing Mg anode and realizing high-performance RMBs are highlighted.