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reactions), indicating the existence of undiscovered extra charge reservoirs inside the system. [6-9] To shed light on this surprising behavior, Tarascon and co-workers proposed that it is the reversible formation/dissolution of polymeric films around the reduced Co nanoparticles that leads to the unusually large capacity, [10,11] which has been widely accepted. [6,7,12] However, using in situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses, Kim et al. demonstrated that the electrolyte decomposition cannot make major contributions to the extra capacity in RuO 2 LIBs. [13] In addition, Maier and co-workers presented an interfacial charge storage mechanism between Li salts and transition-metal nanocrystals. [14-16] Just recently, we demonstrated that the surface capacitance on metal nanoparticles involving spin-polarized electrons is the dominant source of the extra capacity in Fe 3 O 4 LIBs. [17-19] Therefore, the possible contribution of polymeric/gel films to the unusual capacity in CoO deserves to be revisited and clarified in more details, which can foster innovations in modern battery technologies based on this new storage mechanism.

In spite of the excellent electrochemical performance in lithiumion batteries (LIBs), transition-metal compounds usually show inferior capacity and cyclability in sodium-ion batteries (SIBs), implying different reaction schemes between these two types of systems. Herein, coupling operando magnetometry with electrochemical measurement, we peformed a comprehensive investigation on the intrinsic relationship between the ion-embedding mechanisms and the electrochemical properties of the typical FeS 2 /Na (Li) cells. Operando magnetometry together with ex-situ transmission electron microscopy (TEM) measurement reveal that only part of FeS 2 is involved in the conversion reaction process, while the unreactive parts form "inactive cores" that lead to the low capacity. Through quantification with Langevin fitting, we further show that the size of the iron grains produced by the conversion reaction are much smaller in SIBs than that in LIBs, which may lead to more serious pulverization, thereby resulting in worse cycle performance. The underlying reason for the above two above phenomena in SIBs is the sluggish kinetics caused by the larger Na-ion radius. Our work paves a new way for the investigation of novel SIB materials with high capacity and long durability.

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In view of the long-standing controversy over the reversibility of transition metals in Sn-based alloys as an anode for Li-ion batteries, an in situ real-time magnetic monitoring method was used to investigate the evolution of Sn−Co alloy during the electrochemical cycling. Sn−Co alloy film anodes with different compositions were prepared via magnetron sputtering without using binders and conductive additives. The magnetic responses showed that the Co particles liberated by Li insertion recombine fully with Sn during the delithiation to reform Sn−Co alloy into stannum-richer phases Sn 7 Co 3 . However, as the Co content increases, it can only recombine partially with Sn into cobalt-richer phases Sn 3 Co 7 . The unconverted Co particles may form a dense barrier layer and prevent the full reaction of Li with all the Sn in the anode, leading to lower capacities. In addition, we also showed that the Fe can recombine with Sn (Sb) during the delithiation in the Sn (Sb)−Fe alloy film anodes by operando magnetometry. These critical results shed light on understanding the reaction mechanism of transition metals and provide valuable insights toward the design of high-performance Sn (Sb)-based alloy anodes.