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 article number 2006629, Hongsen Li, Yunze Long, Qiang Li, and co‐workers use an advanced operando magnetometry technique to probe the charge‐storage mechanism of CoO lithium‐ion batteries, showing that the anomalous discharge capacity in this particular system is associated with both the reversible formation of a spin capacitor and the growth of a polymeric film at low voltages. The key catalytic role of metallic Co in assisting the polymeric film formation is demonstrated, which is helpful for the design of new types of energy‐storage devices and improving the energy density for future battery designs.

The performance of electrode materials depends intensively on the lithium (Li)-ion storage mechanisms correlating ultimately with the Coulombic efficiency, reversible capacity, and morphology variation of electrode material upon cycling. Transition metal nitrides anode materials have exhibited high-energy density and superior rate capability; however, the intrinsic mechanism is largely unexplored and still unclear. Here, a typical 3D porous Fe 2 N micro-coral anode is prepared and, an intercalation-conversion-heterogeneity hybrid Li-ion storage mechanism that is beyond the conventional intercalation or conversion reaction is revealed through various characterization techniques and thermodynamic analysis. Interestingly, using advanced in situ magnetometry, the ratio (ca. 24.4%) of the part where conversion reaction occurs to the entire Fe 2 N can further be quantified. By rationally constructing a Li-ion capacitor comprising 3D porous Fe 2 N micro-corals anode and commercial AC cathode, the hybrid full device delivers a high energy-density (157 Wh kg −1 ) and high power-density (20 000 W kg −1 ), as well as outstanding cycling stability (93.5% capacitance retention after 5000 cycles). This research provides an original and insightful method to confirm the reaction mechanism of material related to transition metals and a fundamental basis for emerging fast charging electrode materials to be efficiently explored for a next-generation battery.

Aluminum is the most abundant metal element in the Earth's crust, thus developing the rechargeable aluminum‐ion batteries (AIBs) provides an ideal opportunity to realize cells with pleasing energy‐to‐price ratios. However, the further development of AIBs is plagued by the scarcity of suitable positive electrode materials. Here, for the first time, a tin‐based alloy positive electrode material for AIBs, Co3Sn2 wrapped with graphene oxide (Co3Sn2@GO composite) is well‐designed and investigated to understand the aluminum storage behavior. A series of experimental measurements and theoretical calculations results reveal that a novel “bimetallic activated center alloying reaction” aluminum storage mechanism is occurred on the prepared Co3Sn2 positive electrode. The reversible alloying/de‐alloying process in AlCl3/[EMIm]Cl ionic liquid, where both Co and Sn in Co3Sn2 alloys react electrochemically with Al3+ to form AlxSn and AlyCo is first put forward. This study delineates new insights on the aluminum storage mechanism, which may guide to ultimately exploit the energy benefits of “bimetallic activated center alloying redox”.

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