Impacts Of Kinetics On Reversibility Of Conversion Reactions

With the wide application of the lithium ion batteries in portable electronic devices and electric vehicles,the demands of market keep increasing for higher energy density,longer cycling life and so on.Electrode material based on the insertion reaction?such as LiCoO₂ as the cathode and graphite as the anode?cannot meet the requirements any more.In addition,considering the limited abundance of lithium in the earth,it is necessary to study sodium ion batteries beforehand,as an alternative to the lithium ion batteries in large-scale energy storage.The electrode material based on conversion reactions usually has a theoretical capacity several times higher than those by the insertion mechanism.In-depth understanding of the conversion reaction and search for solutions to the above issues are critical in developing new materials and optimizing their application.In this dissertation,composite materials are prepared with porous carbon as the conducting host and their conversion reaction mechanisms are comprehensively studied.We clarify that the conversion reaction of nano-MoS₂ can be reversible in a carbon matrix and the extent of reversibility is highly dependent on the dynamic properties of the composite.In addition,hexagonal NaMoS₂ is recognized as a new phase in the early stage of Na extraction from the reduction products of MoS₂,Mo+Na₂S,on the basis of high-resolution transmission electron microscopy?HRTEM?characterization and first-principles calculations.These findings enrich the understanding of reaction mechanism of Mo S₂ upon Na storage and removal and are helpful to the design and applications of the transition metal sulfides.By proof of experiments and calculations,the MoS₂/C composite has a similar reaction path in lithium ion storage process,compared to sodium ion storage process.Because the lithium ion radius is smaller,the layer distance of corresponding oxidation product LiMoS₂ is near to MoS₂.So that it is hard to distinguish them by HRTEM.By controlling discharge depth,the voltages corresponding to different oxidation reaction peaks are tested by CV.The formation of LiMoS₂,or the reversibility of MoS₂ in lithium storage,is proved indirectly.The silicon?Si?-based anode materials have high Li-storage capacities but poor cycling performances.With the buffering effect of the amorphous products such as Li₄SiO₄ and Li₂O,silica?Si O₂?shows improved cycling stability but still cannot meet the requirements of commercial Li-ion batteries?LIBs?due to its significant volume variation during discharge and recharge.Amorphous silica was uniformly embedded in tunnel-structured mesoporous carbon,with no free or aggregated silica or carbon particles in the cavity of the tunnels.The tunnel provides sufficient space for the volume expansion of SiO₂,while the carbon skeleton forms electric conducting paths for the electrochemical reaction/activation of the silica.Therefore,impregnation of inactive and insulating SiO₂ particles in active and conducting mesoporous carbon?SiO₂@MC?is beneficial for improving the electrochemical activity and cycling performance of the former as an LIB anode.As a result,the SiO₂@MC composite delivers a reversible capacity of⁶00 mAh g^-1.This impregnation concept can be applied in the structural design of composite electrodes for other secondary batteries and preparation of high-efficiency catalysts.Additionally,phase-pure layer-structured LiNi_0.85Co_0.15O₂ was synthesized by solid state reaction.Different from the normal cycling,the discharging potential was extended to 1.35V.A couple of discharge and recharge plateaus are observed at 1.8 V and 2.4 V?low-potential plateaus?,respectively,below 3.0 V.In the subsequent cycling,the length of the normal discharge potential plateau at 3.75 V?high-potential plateau?keeps decreasing but that of the low-potential plateau keeps increasing.Interestingly the addition of the discharge capacities corresponding to these two discharge plateaus increases slightly with cycling.Further structural characterization is in progress and its output is expected to provide some interesting guidance to the evaluation and application of the layer-structured cathode materials,especially the Ni-rich layered materials.