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A long cycle life, spherical-shaped Sn–Fe3O4@C ternary-phase composite consisting of nanosized tin (Sn), magnetite (Fe3O4), and graphite (C) was prepared via a two-step process using high-efficiency discharge plasma-assisted milling (P-milling).

affect the energy density and safety of devices. Also since the SEI permits ions to move freely and secludes the transit of electrons, it affects the kinetics of LIB reaction and the power density of the devices. [14][15][16] Recently, we reported a novel spherical Sn-Fe 3 O 4 @graphite composite prepared via a two-step process using high-efficiency discharge plasma-assisted milling (P-milling). [17] In this ternary composite, ultrafine Sn nanoparticles were tightly combined with nanosized Fe 3 O 4 , with graphite nanosheets coating the outside to form a multiscale spherical structure. This structure can enhance the structure stability and Li ions diffusion kinetics of the electrode during the electrochemical processes. Therefore, the Sn-Fe 3 O 4 @graphite composite anode demonstrates a reversible capacity of ≈750 mAh g −1 after 500 cycles between 0.01 and 3.0 V versus Li/Li + at 2000 mA g −1 . However, it has been observed that the specific capacity of electrode increased gradually with cycling. And this phenomenon was reported in previous publications on Sn-based [18][19][20] or Fe 3 O 4 [21,22] anodes, and also in almost the transition metal oxide-based anode materials. In some reports, this additional reversible capacity was even beyond the theoretical capacity calculated based on their reaction mechanism. [23,24] The phenomenon of capacity increasing in anode is very common, and many previous researchers have studied and explained this phenomenon. A pseudocapacitive mechanism has been proposed which Li can be stored in the interface of active materials to provide the additional capacity. [25,26] Another explanation accepted by more and more researchers was proposed by Tarascon and co-workers in 2002 based on study in CoO anode material by using transmission electron microscopy (TEM) and cyclic voltammetry. [27] A "polymer/gel-like film" was found to form on the surface of nanoparticles at low voltages (1.8-0.02 V) and vanish when the oxidation potential was increased above 2 V. In fact, this "polymer/gel-like film" is the organic component of the SEI film. The nanosized transition metal is considered to play a critical role in activating and promote the formation and dissolution of SEI film. Zhou and coworkers synthesized core-shell Fe@C microspheres, [24] which could provide a certain electrochemical capacity (667 mAh g −1 ). They also prepared Ni/C hierarchical composites, [28] which exhibited an unexpected reversible capacity of 1051 mAh g −1 .The Fe and Ni are regarded to be inert in the electrochemical process with Li. However, these composites present much higher capacities than that of pure C electrodes. This reveals The electrode-electrolyte interface plays a key role in the energy density and safety of the Li-ion battery. The interfaces evolution induces the capacity increasing in many metal oxide anodes and attracts much attention. Here, the origin of the capacity increasing is verified by our long-life Sn-Fe 3 O 4 @ graphite composite anode. By analyzing the electrochemical curves, the Coulombic e...

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