Controlled Synthesis And Superior Electrochemical Performance Of Anode Materials For Lithium Ion Batteries

Lithium ion batteries?LIBs?fall short of the demands for powering advanced hybrid electric vehicles and plug-in electric vehicles because of their relatively low energy and power densities.Due to their high surface area,nanostructured materials can provide high Li-ion flux across the interface,short diffusion pathways for both Li ions and electrons,abundant active sites for Li storage,and high freedom for volume change during charge/discharge process to enhance the structural stability of the electrodes.All of these aspects could ensure high specific capacity,fast charge and discharge,good rate capability,and stable cycling retention for the battery.In this thesis,we choose three categories of anode materials for LIBs.By adjusting their microstructure and composition,we have synthesized their nanomaterials or hybrid materials with different components.The first one is nano-carbons,and the improved storage performance is closely related to their surface area,crystallinity as well as microstructure.The second category is Ge-based materials.Downsizing these bulk materials to the nanoscale,dispersing these elements into electrochemically active and/or inactive matrices and preparing Ge-based oxides can greatly relieve the volume expansion and improve the electrochemical performance of the Ge-based materials.The third category is metal oxides.Preparing nano-sized or composite materials is one effective method to prevent the pulverization of the metal oxide electrode.On account of the above discussion,we prepared various nanostructured materials with high specific capacity,and finally achieved excellent electrochemical performance by optimizing their composition and morphology.In addition,we also explore Li-storage mechanism of these materials.The details are as follows.?1?A mechanically robust three-dimensional?3D?conductive porous network was prepared through freeze drying and KOH activation.Its hierarchically porous structure and high surface area can greatly enhance the contact area of the electrode-electrolyte,decrease the diffusion resistance of lithium ions,shorten the diffusion length of lithium ions and provide a solid and continuous pathway for electron transport.As an anode material for LIBs,it can deliver a specific capacity of over 857.6 mAh g^-1 after 100 cycles at 100 m A g^-1.?2?Three types of Ge-based hybrids were designed and synthesized by a facile electrospinning method followed by a thermal treatment.First,germanium clusters are homogeneously encapsulated into porous nitrogen-doped carbon nanofibers?N-CNFs?to form Ge/N-CNFs hybrids.The Ge/N-CNFs hybrids have some unique advantages,such as uniform distribution of the germanium clusters,porous carbon nanofibers,and Ge-N chemical bonds.These features can not only alleviate the large volume changes of germanium during the discharge–charge process,but also increase the contact area between the electrode and the electrolyte.Second,an interlaced Zn₂GeO₄ nanofiber network with continuous and interpenetrated mesoporous structure was prepared.The mesoporous structure in Zn₂GeO₄ nanofibers is directly in situ constructed by the decomposition of polyvinylpyrolidone,while the interlaced nanofiber network is achieved by the mutual fusion of the junctions between nanofibers in higher calcination temperature.Third,CuGeO₃ hollow nanotubes were synthesized and they can decompose and in situ form Cu particles during the first discharge process.The formed Cu can serve as an efficient catalyst to promote the decomposition of Li₂O and thus release additional Li⁺,leading to extra capacity.Moreover,the Cu particles can ensure good electron transport in the hollow nanotubes and thus increase the rate performance.All of the above Ge-based materials exhibited high lithium storage capacity and remarkable cycling performance when being used as anode materials for LIBs.?3?Cd₂SnO₄–SnO₂ hybrid micro-cubes assembled from fine nanoparticles were successfully prepared through a facile hydrothermal method followed by a thermal treatment.Compared to pristine Cd₂SnO₄ and SnO₂,the micro-cubic Cd₂SnO₄–SnO₂hybrid exhibited an enhanced reversible capacity,which can be attributed to the synergetic effect between the small diffusion length in the nanoparticle building blocks and the hybrid structural features.?4?A freeze-drying-assisted general strategy is developed to synthesize MMoO₄?M=Ni,Co,Mn?mesoporous nanosheet-like structure with high porosity and enhanced specific surface area.For example,the NiMoO₄ exhibits significantly improved specific capacity of 1104.8 mAh g^-1.Furthermore,a LiFePO₄/NiMoO₄ full cell demonstrates a high specific capacity of 152.1 mAh g^-1 at 0.1 C and a stable operation for over 60 charge-discharge cycles.?4?Mesoporous MoO₃-CoMoO₄ hybrid microspheres?MP-Mo-Co-HMS?has been designed and fabricated via a facile hydrothermal method followed by thermal treatment.The introduction of CoMoO₄ in MoO₃ will result in the formation of Co nanoparticles during the discharge process,which could not only guarantee the reversible formation/decomposition of Li₂O,but also prevent the aggregation of Mo nano-grains upon repeated cycling.When evaluated as an anode material for LIBs,the resultant product displays excellent lithium storage performance in terms of specific capacity,cyclingstabilityandratecapability.Moreover,the LiFePO₄/MP-Mo-Co-HMS full cell displays high capacities up to 155.7 mAh g^-1 at0.1 C and 90.75 mAh g^-1 at 1.0 C after 70 cycles.