Hydrothermal Modification And Related Mechanism Of Lithium Zinc Titanate Anode Material

The ever-increasing demand for energy urges the rapid development of lithium-ion batteries.Li2ZnTi3O8(LZTO)material is considered to be one of the new candidates for anode materials because of its high safety,working at high current with high cyclability compared to the commercial graphite anode.However,LZTO exhibits low electronic and ionic conductivities,influencing to give play to the performance.To address these issues,several hydrothermal modification strategies are proposed in this work.Meanwhile,the structural evolution of LZTO during cycling was analyzed,and the lithium storage mechanism of LZTO was further clarified.The main content is as follow:(1)LZTO was hydrothermally modified with melamine(MA)at 200℃ for 12 h.The modified products demonstrate alleviated polarization,high capacity and excellent cyclability.At a mass ratio of 1:0.05 for LZTO to melamine,the product reveals superior Li-ion diffusion,low impedance,and optimal rate performance.Combined the performance with structure and composition,the MA coating gradually evolves into solid electrolytes of Li3N and LiNxOy with high ionic conductivity in the initial lithiation process,meanwhile the oxygen vacancies introduced in LZTO accompanied with the occurrence of LiNxOy facilitate electron and ion conductance,thus responsible for the promoted electrochemical performance of LZTO.More importantly,by virtue of the enhanced cyclability of the MA-modified LZTO,the structural reorganization of LZTO during the cycling was further investigated.(2)LZTO was modified by hydrothermal treatment with trithiocyanuric acid(TCA)at 150℃for 12 h.The two isomers of keto-and enol-TCA undergo different evolutions in the hydrothermal and electrochemical processes.For the enol-TCA in the hydrothermal process,the HS-bonds close to LZTO react with LZTO to introduce superficial S-doping to facilitate electron conductance,and the others oxidize into-SO3-groups which transform into-SO3Li groups in lithiation process to favor Li+migration.The keto TCA is stable in the hydrothermal process,yet the C=S groups oxidize into-SO3-groups in anodic process and yield-SO3Li groups in subsequent cathodic process for Li+diffusion.Despite these changes,the six-membered cyclic structures in both the keto and enol TCA are stable and coordinate with LZTO to create coating layer.The organic coating with good mechanical performance and strong interaction with PVDF protects LZTO from electrolyte corrosion and side reactions.Therefore,the TCA-modified LZTO demonstrates accelerated Li-ion migration and electron transfer,significantly enhanced rate capability and cycling stability.(3)When the TCA-coated LZTO obtained by the hydrothermal treatment at 120℃ for 12 h was further sintered at 650℃ for 5 h,it was evolved into carbon-coated LZTO with superficial sulfur-doping.The carbon coating not only provides a faster electron transport path for the LZTO material,but also avoids direct contact between the LZTO particles and electrolytes,reducing the occurrence of side reactions;the lattice distortion near the surface of LZTO resulted from the S-doping could favor Li+migration.The ZnS formed during the sintering also reversibly participated in the electrochemical cycle,providing additional capacity.By virtue of electrochemical analysis,the S-doping greatly accelerates the structural reorganization of LZTO.As a result,the modified LZTO exhibits alleviated polarization,low charge transfer resistance,increased initial coulombic efficiency,improved electrochemical kinetics,and excellent electrochemical performance.