Fabrication And Investigation Of LiFePO₄ Power Battery Composite By Using New Phosphorous Sources For Electric Vehicle

The development of electric vehicles?EVs?has been greatly promoted due to a greater concern for environmental protection and energy saving.Lithium-ion batteries?LIBs?play a crucial role in the energy storage materials for EVs.As one of the most promising cathode materials in LIBs,the olivine structured LiFePO ₄?LFP?has received considerable attention due to a high theoretical specific capacity,excellent cycle performance,good thermal stability,low cost,environmental friendliness.However,the high rate performance and the application of the LFP materials are significantly restricted by the sluggish kinetics of lithium-ion diffusion and intrinsically low electronic conductivity.Therefore,in this present work,in order to overcome the LFP's drawbacks mentioned above,improve the performance of the LFP and further promote its application in the power batteries for EVs,the LiFePO₄@Carbon?LFP@C?composite,LiFePO₄@Multi-walled carbon nanotubes?LFP@MWCNTs?composite and the LiFePO₄@Graphene?LFP@G?composite have been fabricated by using three different structures organic phosphorus acid as new phosphorus sources,respectively.Also all of the fabricated LFP@C,LFP@MWCNTs and LFP@G composite exhibit uniform nanosized structure,good electrical conductivity and excellent charge-discharge performance at high current.Furthermore the realationship among in the microstructure,technology and electrochemical properties and the interaction mechanism of the fabricated composites were also investigated,which is expected to provide theoretical guidance for prepar ing the LFP materials with excellent performance.The main contents of this dissertation are as follows:?1?Bis?Hexamethylene Triamine Penta?Methylene Phosphonic Acid???BHMTPMPA?due to its high phosphorus,carbon content and especially has some chelation functional groups in the molecule,has been used as a new and environmentally friendly phosphorus source to fabricate the LFP@C nanocomposites.The LFP@C are nanoscale composite with carbon in-situ formed on around the LFP.The XRD and SEM analyses show that the prepared LFP@C exhibit s single phase of LiFePO₄ with an ordered olivine structure and an uniform particle size of about 300nm.The electrochemical tests also verify that the fabricated LFP@C nanocomposite exhibits a high reversible capacity of 166.8 mAh·g^-1 at 0.2 C and present an excellent rate capacity of 102.3 mAh·g^-1?capacity retention:60.2%?at 10 C,which verify an excellent high current discharge characteristics and can meet the speed change and acceleration requirements of the electric vehicle.?2?The present work has developed a new method of fabricating novel 3D nanostructured LFP@MWCNTs composite by using risedronic acid?RDA?as a new phosphorus source and then elucidated the mechanism of structure formation and the enhancement mechanism of Li⁺diffusion and electronic conductivity.The fabricated LFP@MWCNTs nanocomposite exhibits an ordered olivine crystal structure,uniform nanoscale long-short rod-like structure with a particle size range between 100²00nm and an in-situ coiled MWCNTs with a tube width of¹6 nm,which wrap and encrust around LFP particles to form a 3D nano-network microstructure.The electrochemical tests results also show that the LFP@MWCNTs composite delivers an enhanced reversible capacity of 168.5 mAh·g^-1 at 0.2 C,high capacity retention of 76.5%even at 10 C after 800th cycles.Additionally,the Li⁺diffusion coefficient(D⁺_Li)of the fabricated LFP@MWCNTs is of 5.28×10^-11 cm²·s^-1,which is almost 3 orders of magnitude higher than that of pure LiFePO₄.The formation mechanism of the 3D nano-network structure and enhanced electrochemical performance of the LFP@MWCNTs nanocomposite is mainly attributed to:1)Firstly,the N atoms in the?-deficient pyridine-ring of RDA can react with the Fe²⁺during the synthesis process of the LFP.This is beneficial to prevent the particles overgrowth of the LiFePO ₄ in nucleation&growth process through the steric hinerance induced from the?-deficient pyridine.2)Secondly,the C-OH or O=P-OH groups in the RDA molecules can react with some oxidized groups?such as C-OH,-C=O or-COOH?in the functionalized MWCNTs then form into a precursor.So,the RDA acts as a“dual-bridge”effect between the Fe²⁺and functionalized carbon nanotubes due to its dual reaction with the Fe²⁺and functionalized carbon nanotubes.This“dual-bridge”effect can effectively improve the interface bonding between LFP and carbon nanotubes,and further finally form a 3D nanostructures of the LFP@MWCNTs nanocomposite.The 3D nano-network can promote the electronic conductivity in between the LFP nanoparticles through wrapped MWCNTs and also accelerate the lithium ion diffusion owing to the uniformly nano-sized long-short rod-like particles of LiFePO₄,and finally result in good electrochemical performances of the fabricated LiFePO₄@MWCNTs nanocomposite.?3?The ethylenebis?nitrilodimethylene?tetraphosphonic acid?EDTMPA?with some reducing functionalized groups,which can be instead of conventional hydrazine hydrate,has been applied to reduce the graphene oxide into the graphene nanosheets.The results indicate that the graphene here is in a form of fold nanoshee ts with an interplanar spacing of 0.38 nm and few layers verified by the 2D characteristic peak(2700 cm^-1)in the Raman spectra.This fabrication method can be used to fabricate the LFP material by using the EDTMPA as new phosphorus source and simultaneou sly in-situ reduce,grow and wrap gaphene onto the the LFP particles to form a LFP@G compoite?4?In this work,special core-shell structure LFP@G nanocomposites are fabricated by using the EDTMPA as the new phosphorus source and the formation mechanism of the special microstructure is also elucidated.The results of the SEM and TEM analysis show that the LFP@G exhibits a nanosphere core-shell structure,in which the LFP?⁵0-100 nm?particles act as the“core”and the in-situ growed nano-sized graphene sheets?²-5 nm?act as the“shell”that uniformly covers around the LFP nanoparticles to form a good electronic diffusion network between LFP particles.The LFP@G nanocomposites also exhibits a high reversible capacity of167.4 mAh g^-1 at 0.2 C rate,high capacity retention of 84.1%even at 10 C after 800th cycles,improved Li⁺diffusion coefficient(1.12×10^-10 cm²·s^-1,⁴ orders of magnitude higher than that of pure LiFePO₄),enhanced electric conductivity(2.95×10^-1 S²·cm^-1,⁸ orders of magnitude higher than that of pure LiFePO₄)and excellent rate and cycle performance in low temperature,indicating a great excellent electrochemical properties.The formation mechanism of core-shell structure of the prepared LFP@G nanocomposites can be described as:the EDTMPA can coordinate with the Fe²⁺and then control the growth of the LFP particles in the nucleation process.Also,the graphene nanosheets can in-situ grow and uniformly wrap around the LFP particles to form a unique core-shell structure of the LFP@G during the sintering process because of the reduction functional groups in the EDTMPA.The core-shell structure can benefit the electron transportation in between the LFP nanoparticles and form a uniform lithium ion immigration path,and then finally results in g ood electrochemical performance of LFP@G nanocomposites.?5?This work also tries to develop a new method to explore the grain growth kinetics of the fabricated LFP@C particles during the sintering process.The crystal growth thermodynamics and activation energy of the fabricated LFP@C are investigated by using the experimental results of particles size distribution and the fundamental theory of Arrhenius equation.The results indicate that the activation energy of the prepared LFP@C composite is lower than th at of 3.82 kJ·mol^-1 when the sintering temperature is within the range of 600-800?,which suggests that the crystal growth kinetics of the LFP@C particles is diffusion-controlled.The diffusion-controlled mechanism is resulted from the mutual effects of ch elation with Fe²⁺cations,in-situ form carbon layers and large concentration of hard aggregates due to the use of organic phosphorous source?BHMTPMPA?.In summary,in order to over the shortcomings of LFP materials,three different kinds of organic phosphorus sources were designed and used as new phosphorus source to fabricate LFP@C,LFP@MWCNTs and LFP@G power batteries materials for electric vehicle.The effects of the designed organic phosphorus sources on the microstructure,electrochemical properties,formation mechanism of special morphology and sintering mechanism were also systematically investigated.All of the fabricated composite has the advantages of nanosized scale,good conductivity,high Li⁺diffusion coefficient,high current discharge capacity and long cycle life,which can provide some theoretical and technical support and then be beneficial for the application of power battery materials for electric vehicles...