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Metal sulfides are emerging as a promising anode material for sodium-ion batteries with high reversible capacities and fast reaction kinetics, but achieving long-cycling-life remains a great challenge. Here, taking cobalt sulfide as an example, its electrochemical sodium-ion storage failure phenomenon is first reported, which indicates that the battery cannot reach the cutoff voltage during charging. Detailed analyses demonstrate that such failure may originate from the dissolution and escape of polysulfide intermediates, further reacting with the released copper-ions from the current collector and inducing the occurrence of the shuttle effect. Based on the explored failure mechanism, a sulfur-doped carbon matrix with polar carbon sulfur bonds, which can firmly immobilize the dissolved polysulfides, is deliberately introduced into the Co 1−x S active particles (Co 1−x S/s-C) to improve their cycle stability. Consequently, the cycle life of the Co 1−x S/s-C anode for sodium-ion storage is extended from the original 125 to present 2000 cycles, even at high-rate current densities. Moreover, utilizing the carbon current collector instead of traditional copper can effectively delay the occurrence of the failure phenomenon. The present work promotes better fundamental understanding of the structural evolution of metal sulfide anodes during cycles, and the solution strategy can be extended to apply in other metal sulfides (ZnS, NiS).

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Transition‐metal sulfides (TMSs) are extensively investigated as anodes of low‐cost sodium‐ion batteries (SIBs) and potassium‐ion batteries (KIBs) due to their abundant resources and high theoretical capacity. However, their poor cyclability and low initial coulombic efficiency (ICE) in ester‐based electrolytes severely impede their application in SIBs and KIBs. To overcome these drawbacks, ether‐based electrolytes are considered as alternatives, but its fundamental principle remains rarely reported and poorly understood. Herein, the electrochemical performance of MoS2@C electrodes is explored using both carbonate and ether‐based solvents. The MoS2@C exhibits a higher ICE and Na/K‐ion storage capacity (a reversible specific capacity of 625 mAh g−1 with ICE of 80% for SIBs, and a capacity of 241 mAh g−1 with ICE of 81% for KIBs, respectively) in dimethyl ether (DME) electrolytes than in ethylene carbonate and diethylene carbonate (EC/DEC) electrolytes. Experimental measurements and theoretical calculation show that the DME electrolytes help to optimize the solid‐electrolyte interphase (SEI) composition, facilitate charge transport, reduce the energy barrier for Na/K‐ions migration and reinforcing geometry architecture, thus endowing excellent electrochemical performance. Importantly, this electrolyte optimization solution can be extended to other TMSs, such as Fe7S8@C anodes, demonstrating an exact match between the TMSs and DME electrolytes.

MoCl5–GICs are demonstrated as advanced anodes for simultaneously achieving high volumetric capacity and long cycle life in Na-ion and Li-ion batteries, owing to a strong charge transfer chemical doping effect and strong interaction force between MoCl5 and graphite layers.