air and organic electrolytes. [1c,e,7] Furthermore, the cost of Mg metal is 30 times lower than that of Li. [8] Therefore, magnesium-sulfur (Mg-S) batteries theoretically offer a higher volumetric energy density, greater safety, and much lower cost than Li-S batteries.The development of Mg-S batteries is still in its infancy and is limited by several challenges, including high cell polarization, low capacity, and rapid capacity fading. One of the most critical challenges is the high cell polarization (>1000 mV overpotential), [8,9] which has been widely attributed to the sluggish kinetics and passivation of Mg anodes. [9a,10] Substantial effort has been made to develop electrolytes that allow fast and reversible electroplating/stripping of divalent magnesium and are compatible with sulfur cathodes. [1e,7] Commonly used chloride-and boron-based electrolytes have successfully reduced the polarization potential of Mg electroplating/stripping to ≈200 mV. However, the overpotential of Mg-S cells is still high (>1000 mV in chloride-based electrolytes [6a,7,11] and ≈600 mV in boron-based electrolytes [12] ), leading to low energy efficiency (≈47% in chloride-based electrolytes and ≈56% in boron-based electrolytes). Other challenges such as low capacity and rapid capacity fading have been attributed to the loss of soluble polysulfide. [7,13] Sulfur reactions in Mg-S batteries are reported to follow similar reaction pathways with that in Li-S batteries. During discharge, the elemental sulfur is reduced to soluble polysulfides, first to long-chain polysulfides followed by short-chain polysulfides via chain-shortening reactions. The short-chain polysulfides subsequently reduce to solid sulfide and precipitates onto the cathode. Meanwhile, part of the polysulfides could diffuse to the anode, resulting in irreversible loss. [6b,13d,14] Consequently, similar to Li-S batteries, addressing the loss of polysulfides is considered the priority in Mg-S batteries. To overcome these challenges, interlayer and host materials [13a,15] have been designed according to the strategies developed for Li-S batteries. [16] These approaches increase the first discharge capacity to ≈1000 mAh g −1 below 0.1 C, which is lower than that of Li-S batteries (1400-1600 mAh g −1 at a C rate of ≥0.1 C). Despite considerable efforts, the performance of Mg-S cells is still far from satisfactory, which raises the question of what truly limits the reversibility of Mg-S batteries.In this work, we reveal that the sluggish sulfur reaction is the main origin of poor Mg-S cell performance. Exploiting Magnesium-sulfur batteries promise a higher theoretical volumetric energy density, improved safety, and lower cost compared to lithium-sulfur batteries. However, Mg-S batteries suffer from poor cycle life and low energy efficiency. Here, it is revealed that Mg-S reactions are dominated by "solid-solid" reactions due to much lower polysulfide solubility in the presence of Mg 2+ compared to that of Li + in 1,2-dimethoxyethane (DME)-based electrolyte, leading ...

insufficient reduction stability are prone to be reduced at the anode side thus not compatible with Mg anode. [7][8][9] Grignard reagents (RMgX, R: organic group; X: halogen ligand) with reductionresistant property first enabled reversible Mg reaction and prevented the formation of passivation film. [10,11] The highly reactive Grignard reagents can serve as scavengers to remove contaminants (such as water or O 2 ) thus suppressing their chemical reactions with fresh Mg 0 . [7] However, the low oxidation stability (≈1.5 V vs Mg/Mg 2+ ) of Grignard reagents hampers their practical utilization. [12] To widen the electrochemical window of Grignard reagents, Aurbach et al. [13] designed a family of solutions prepared by in situ reacting organo-magnesium compounds (R 2 Mg) with organic halo aluminum compounds (AlCl 3−n R n ) in ethereal solvents. Among all the candidates, dichloro-complex (DCC) electrolytes obtained via reacting din-butylmagnesium (Bu 2 Mg) and ethylaluminum dichloride (EtAlCl 2 ) (1:2) in THF achieved ≈100% coulombic efficiency, high ionic conductivity (1.4 mS cm -1 ) and extended electrochemical window up to 2.4 V. [13] All phenyl complex (APC) electrolyte with no β-located hydrogen obtained via the reaction between phenylmagnesium chloride (PhMgCl) and AlCl 3 achieved promising properties, including high oxidation stability (>3.3 V), high coulombic efficiency (≈100%), and low overpotential (<200 mV). [14] Although highly reversible Mg reaction was achieved with Grignard-based electrolytes, high-energy and high-power RMBs were still difficult to be achieved because of the inferior cathode compatibility. [15,16] Several intercalation cathodes, such as Mo 6 S 8 , were reported to exhibit high reversibility in Grignardbased electrolytes, but the high energy barrier for Mg 2+ desolvation at the electrode/electrolyte interphase [17][18][19] and sluggish ion diffusion [20,21] hamper the Mg 2+ storage at room temperature. Electrophilic organic polymers and conversion cathodes promise to achieve high-power and high-energy Mg storage but are prone to be attacked by the nucleophilic components in Grignard-based electrolytes. [22][23][24] Furthermore, the chloride-containing dimer cations in Grignard-based electrolytes also raise concerns about corrosion toward non-noble metals, excluding the utilization of conventional low-cost current collectors, such as aluminum. [25] The sensitive precursors and additional purification pretreatments also impede the practical usage of Grignard-based electrolytes. [26] Consequently, it is still challenging to screen an Mg 2+ conductive electrolyte compatible with both anode and cathode. 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 Mg 0 , and mechanisms of...

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Magnesium batteries suffer from low power density and poor cycle life due to severe Mg passivation. Using nucleophilic electrolytes is effective to stabilize the Mg anode, but it prohibits the use of organic and conversion cathodes due to chemical reactivity. Here, we report an effective non-passivating anion additive, the reduced perylene diimide–ethylene diamine (rPDI), to enable fast and reversible Mg deposition/dissolution in a simple non-nucleophilic electrolyte. The rPDI additive exhibits higher adsorption energy than the TFSI– salt on the Mg surface, preventing TFSI– decomposition and Mg anode passivation. Using 0.2 mM rPDI additives, we demonstrated a Mg–organic full cell achieving a high power density (2.0 mW cm–2) and a stable cycle life (>200 cycles). Our study provides a facile and effective strategy for non-nucleophilic electrolytes, enabling the combination of Mg anode with a wide variety of organic and conversion cathodes beyond intercalation chemistries.

Nickel(II) chloride materials were synthesized via a novel two-step variable-temperature method for the use as a cathode material in Li-B/NiCl2 cells with the LiCl-LiBrLiF electrolyte. The influence of temperature on its structure, surface morphology, and electrochemical performance was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical measurements of single cells. XRD results showed that after pre-dehydration for 2 h at 270°C followed by sintering for 5 h at 600°C, the crystal water in nickel chloride hexahydrate could be removed effectively. The SEM results showed that particles recombined to form larger coarse particles and presented a layered structure. Discharge tests showed that the 600°C-treated materials demonstrated remarkable specific c apacities of 210.42 and 242.84 mA h g −1 at constant currents of 0.5 and 2.0 A, respectively. Therefore, the Li-B/NiCl2 thermal battery showed excellent discharge performance. The present work demonstrates that NiCl2 is a promising cathode material for thermal batteries and this two-step variable-temperature method is a simple and useful method for the fabrication of NiCl2 materials.