Li_0.35La_0.55TiO₃ Composite Structure And Its Effect On The Coupling Process Of Electrodes In Li-ion Battery

In the system of Li-ion battery electrode,the reversible electrochemical reaction is a prerequisite to achieve the repeated lithiation/delithiation process for the electrode material,in which the transport of Li-ions and electrons plays a vital role.In the electrodes,both the transport of Li-ions and electrons achieves in different systems.Slow kinetic condition will cause electrode polarization and capacity attenuation,which restricts the improvement of performance in electrode.If an electron&ion coupling conduction structure can be constructed,a unified conduction path of electron and ion will be achieved,which will enhance the kinetic transport condition in electrodes and improve the electrochemical performance effectively.Based on the new idea,a core-shell nanofiber structure of Li_0.35La_0.55Ti O₃@C has been designed,and two kinds of kinetics sensitive material NCA(Li Ni_0.8Co_0.15Al_0.05O₂)cathode and Sn O₂ anode were researched as demonstration electrodes.Via the regulation of kinetic condition in NCA and Sn O₂,the effect of electron/ion coupling on the electrode reaction process is studied.For NCA cathode,a carbon-coated lithium lanthanum titanate nanofiber(Li_0.35La_0.55Ti O₃@C)material was prepared by electrospinning and hydrothermal process,and the Li-ion/electron coupling conductive matrix LLTO@C nanofibers were introduced to polyvinylidene difluoride to provide a coupling path for Li-ions&electron in the binder and improve the transport kinetics.According to the results,with the modification of LLTO@C,NCA-20L@C shown the suppressed impedance and polarization.The Li-ion diffusion coefficient(D⁺_Li)in the cathode increased from respectively,the capacity retention rates increased to 77.2%and 68.9%,respectively,indicating that the modification of LLTO@C improved the cycle stability significantly.Based on the evolution of the structure and electrochemical state of the NCA surface after 200 cycles at 5 C,we analyzed the influence of kinetic condition on the structure and performance of NCA.In the NCA-pristine cathode,a large scale of lattice broken regions were formed continuously,which was attributed to the phase transition from H₂ to H₃.The high density edge-dislocation walls led to the formation of lattice damaged regions,which caused degradation of the lattice structure of NCA finally.Moreover,the formation of rock-salt phase coating layer continuously on the surface of NCA was responsible for the capacity loss.In contrast,for NCA-20L@C cathode,with the enhance of ion/electron coupling conduction kinetics,the dislocation and cation mixing only occurred within a few atomic scales.On the surface of NCA,only“Ni O”pitting occurred under 2?4 nm scales,which indicating that ion/electron coupling mechanism is help to regulate the kinetics and retard the generation of the rock-salt phase,and finally a preeminent capacity retention was obtained.For SnO₂ anode,the Li_0.35La_0.55Ti O₃@Sn O₂@C composite anode was prepared by electrospinning and hydrothermal process in this thesis.The designed composite structure is help to enhance the kinetic condition during alloying process of Sn O₂ and improve the structural stability of the anode.According to the results,compared with Sn O₂ nanotube,this excellent core-shell composite anode possessed outstanding cycling stability and shown a higher capacity retention of 673.4 m Ah g^-1 after 100cycles at 0.2 C.The LLTO@Sn O₂@C composite anode has a core-shell structure with the LLTO nanosheets distributing inside the Sn O₂ nanotubes and the nano carbon layers coating outside.The supporting of LLTO nanosheets inside and the coating of carbon layers outside form a synergistic structure of working“in”and collaborating“out”for Sn O₂ nanotube.On the one hand,the synergistic effect of LLTO and carbon enhances the conductivity of Li-ion and electron in anode,which achieves an improved reversibility of alloying reaction.On the other hand,the core-shell coating structure inhibits the the volume expansion and material agglomeration of Sn O₂during the alloying process,which delays the degradation of the anode structure and achieves excellent cycle stability.The regulating mechanism of Li-ion/electron coupling effect on electrode helps to achieve more stable and safe electrochemical performance,which provides a new idea to the development of safe high-energy density electrode materials.