Application Of Li₄Ti₅O₁₂ In Lithium Ion And Lithium Sulfur Batteries

Spinel Li₄Ti₅O₁₂,known as “zero strain” anode,its crystal structure is almost no changed after Li+ insertion or extraction and considered as o ne of the ideal materials to replace commercial graphite/carbon due to its excellent cycleability,long and flat charge/discharge plateau and higher Li+ diffusion coefficient.However,there are two disadvantages on Li₄Ti₅O₁₂ anode compared to graphite/carbon.First,its theoretical capacity is low as 175 mAh/g?1³V vs.Li+/Li?.Second,the voltage plateau is higher?1.55 V vs.Li+/Li?,which leads to a low discharge potential of full cells.Here,in order to improve its specific capacity and decrease the charge/discharge plateau,Li₄Ti₅O₁₂ composites were prepared by adding Sn,Sb and Bi,whose theoretical capacity is higher and plateau potential is lower.Moreover,as a fast Li+ conductor,the effect on electrochemical performances of Li₄Ti₅O₁₂ on lithium ion batteries anode and on lithium sulfur batteries cathode were investigated.Spherical carbon coated Li₄Ti₅O₁₂/Sn with large particle size was prepared by a reasonable synthesis route,which was named as Li₄Ti₅O₁₂/Sn/C.The physical and electrochemcial properties were studied via many strategies.The results indicate that when Sn content was 11.15%,the charge/discharge capacity of compound was increased slightly and the charge/discharge plateau was switched to lower voltage.However,Sn content on t he composite was hard to increase further due to the large particle size of spheres,which leaded to moderate cyclability and rate property.In order to improve the reversible capacity and rate property further,Li₄Ti₅O₁₂/M/C?M=Sn,Sb and Bi?composites with small particle size were designed and synthesized.Their morphology,component and the relationship between their physical and electrochemical properties were studied systemically.Furthermore,the contribution rate on total reversible capacity of each component in the composites was also calculated.The results show that Li₄Ti₅O₁₂/M/C with small particle size can deliver a h igher reversible capacity,lower charge/discharge plateau and exhibit an excellent rate property compared to Li₄Ti₅O₁₂/Sn/C with large particle size.When the content of Sn,Sb and Bi in the composites was 25.59%,23.23% and 34.69%,the reversibility capacity reaches to 308 mAh/g,235 mAh/g and 239 mAh/g after 500 cycles at a current density of 200 mA/g.The value of Li₄Ti₅O₁₂/C was 205 mAh/g.Meanwhile,the major capacity lied in the voltage range of 0.5¹.5V,and the capacity recover rate was higher than 90% when the rate cycle current density was switched to 50 mA/g from 1000 mA/g.Li₄Ti₅O₁₂ is a fast Li+ conductor in room temperature so that the effect of Li₄Ti₅O₁₂ on electrochemical performances of Sn@C anode was investigated.When Li₄Ti₅O₁₂ content in(nano-Sn/nano-Li₄Ti₅O₁₂)@C [named as?Sn/L?@C] was 33.1% and 17.6%,the lithium ion diffusion cofficient of?Sn/L?@C was higher as 6.09×10-8 cm2/s and 9.47×10-9 cm2/s,and the reversible capacity was higher as 330.6 mAh/g and 381.2 mAh/g after 350 cycles with a low capacity lost per cycle of 0.081% and 0.094% at a current density of 500 mA/g,while the corresponding value of Sn@C was 7.54×10-10 cm2/s,219 m Ah/g and 0.160%.The above results indicate that Li₄Ti₅O₁₂ as a fast Li+ conductor can improve effectively the reversible capacity and the cyclability of Sn@C anode.It is well-known that the sulfur host should possess the merits of fast lithium ion conduction and electronic conduction.Here,Li₄Ti₅O₁₂ was combined with carbon paper to construct a sulfur host.When Li₄Ti₅O₁₂ content was 25.69%,the lithium ion diffusion cofficient of S/Li₄Ti₅O₁₂ was two orders of magnitude higher than S/C,which reached 1.23×10-8 cm2/s.At 0.5C and after 700 cycles,the reversible capacity of S/Li₄Ti₅O₁₂ was 615 mAh/g and the capacity lost per cycle was 0.0197%,while the corresponding value of S/C was 436 mAh/g and 0.0598%.However,the sulfur in S/Li₄Ti₅O₁₂ was exposed,leading to low sulfur utilization rate ultimately.In order to improve the sulfur utilization rate,(volid/Li₄Ti₅O₁₂)@C spheres were designed and synthesized as the sulfur host.Li₄Ti₅O₁₂ here acted as fast lithium ion conductor as well as lithium polysulfide adsorbent,while carbon shell acted as electronic conductor and blocked the inside sulfur outflow.Furthermore,Li₄Ti₅O₁₂ was dispersed evenly within carbon shells and can hinder the aggregation of Li2S2 and Li2 S.The results show that the mechanism of Li2S4 and Li2S6 adsorbed on Li₄Ti₅O₁₂ surface was forming Ti-S bond between Li2S4 or Li2S6 and Li₄Ti₅O₁₂.When Li₄Ti₅O₁₂ content was 26.59%,the lithium ion diffusion cofficient reached to 2.55×10-10 cm2/s,which was improved one order of magnitude compared to S@C,while the value of S@C was 3.84×10-9 cm2/s.At 2C and after 1000 cycles,the reversible capacity of(S/Li₄Ti₅O₁₂)@C was higher as 600 mAh/g,exhibiting an excellent cycle property.