| Lithium ion batteries are highly attractvie in extensive applications due to their high energy density, high working voltage, low self discharge, and environmental friendliness. Generally speaking, all the available anode materials proceed on three lithium storage mechanisms:insertion/extraction (I) represented by graphite, Li4Ti5O12and TiO2; alloying/de-alloying (II) represented by Si and Sn; conversion reactions (III) represented by transition metal oxides. Although these three mechanisms have been widely used to describe and evaluate the lithium storage capability of active materials, they can not explain all the electrochemical behaviors for lithium storage, especially the extra capacity phenomenon which intensively exists in transition metal oxide anodes.Based on the three mechanisms, we further proposed a new mode for lithium storage, i.e., electrochemical catalytic conversion mechanism. This mechanism enlightened us on exploiting various untapped materials as advanced lithium storage anodes, including transition metal carbonates, transition metal-containing organic compounds and Prussian blue analogues. However, the intrinsically poor electronic conductivity and the huge volume swing during the repeated charge/discharge processes seriously hamper their electrochemical performances and practical applications. Therefore, we combined nanotechnology and composite methods to prepare nanocomposites with unique structures. Simultaneously, various characterizations (such as X-ray diffraction, thermal gravity, Raman spectrum, scanning electron microscopy, transmission electron microscopy, element analysis, X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer) and electrochemical measurements (such as cyclic voltammetry, electrochemical impedance spectroscopy and cycling tests) were performed to investigate their components, morphologies, structures and electrochemical performances. All the results focused to further illustrate the newly-proposed mechanism for lithium storage and explain the extra capacity phenomenon in transition metal oxides.To study the lithium storage mechanism of transition metal carbonates, we prepared uniform MCO3(M=Co, Fe, Mn) microparticles. To maximize their lithium storage performance, we further prrepared MCO3/graphene composites with much small MCO3particles and better electronic conductivity. When tested as anodes, the samples presented much higher specific capacities than the theoretical values based on the conversion mechanism. The unexpected lithium storage behaviors were systematically investigated by combining experiments and computations. As a result, we preliminarily proposed a different mode for lithium storage, i.e., electrochemical catalytic conversion mechanism. Both M and C elements in MCO3anticipated in the electron transfer and energy storage. More importantly, metal nanoparticles functioned as important catalysts in the reversible conversion reactions. Based on the mechanism, MCO3can reversibly store and release considerable specific capacities over1300mAh g-1, much higher than the theoretical values of the corresponding transition metal oxides (MO).To verify the role of metal nanoparticles in the electrochemical lithium storage behaviors, we designed a facile, eco-friendly, and low-cost sol-gel method to fabricate nanosheet-assembled Ni/C composites and core-shell Fe@Fe3C/C nanocomposites. When tested as anodes, these samples exhibited obvious extra capacity phenomenon. Further, the formation mechanism was systematically investigated for the unique hierarchical structure and the origin of the extra capacity by using a diversity of characterizations and electrochemical tests. Ni and Fe@Fe3C nanoparticles cannot directly react with Li+in the energy storage process; instead, they served as highly-efficient electrocatalysts to activate or promote the reversible formation and decomposition of some components in solid electrolyte interface (SEI) films.To investigate the origin of the extra capacity phenomenon widely existing in transition metal oxide (MO) anodes, we synthesized hierarchical core-shell Fe3O4@C microspheres and MnO@C nanorods. Profiting from the superior structures, Fe3O4@C microspheres presented a significantly extra capacity phenomenon. Here Fe0nanoparticles generated from Fe3O4functioned as essential electrocatalysts on the reversible formation of decomposition of some SEI components (especially Li2CO3). Furthermore, the electrochemical lithium storage behaviors of M/C nanosheets (M=Co, Ni, Cu) and hierarchical core-shell MnO@C nanorods also demonstrated the credibility and generality of this explanation.To further verify the role of SEI components and transition metal nanoparticles in the charge/discharge processes of lithium ion batteries, we introduced transition metal-containing organic compounds as anode materials for lithium storage. Ultra thin COC/graphene nanosheets (Co-containing organic compounds, COC) were synthesized through a facile solvothermal route and demonstrated a very high specific capacity (～900mAh g-1after120cycles) and good reversibility. Besides the reversible conversion between Co2+and Co0, high-valence C (such as carboxyl and carbonyl groups) also anticipated in the electron transfer and reversibly reduced to other C with lower valences.To verify the newly-proposed mechanism and the explanation of the extra capacity phenomenon, Prussian blue analogues (PBA) were finnaly introduced as lithium ion battery anodes. The large bulk particles and the intrinsically poor electronic conductivity of PBAs seriously restrict their lithium storage capability. To improve the electrochemical performance, we prepared various Co-PBA microparticles and Cu-PBA/graphene nanocomposites. These obtained samples exhibited high capacities and good cyclic stability, indicating that PBAs were potential materials for lithium storage. Simultaneously, the obvious extra capacity phenomenon verified the newly-proposed Li storage mechanism and the role of transition metal nanoparticles from another direction.The viewpoints in this work are valuable for both theories and applications.In theories, this work helps us to further understand the lithium storage mechanism and the origin of the extra capacity, and discloses new ways for exploiting advanced active materials and electrolyte systems for lithium ion batteries and even new energy storage devices. In applications, this work introduces some promising anode materials, illustrates the important role of SEI components and transition metal nanoparticles in electrochemical lithium storage, and resultantly enlightens us on improving the commercialized graphite and electrolytes. |
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