| Among the candidate anodes of lithium ion battery,spinel Li4Ti5O12?LTO?has been widely studied as a promising alternative anode material,due to its:i)excellent cycle performance because of its“zero-strain”property during lithiation/delithiation process;ii)high flat potential of 1.55 V,which avoids SEI formation and provides a great safety improvement.Nevertheless,a low electronic conductivity(10-13 S cm-1)and Li+diffusion coefficient(10-9-10-13 cm2 s-1)of LTO restrict its electrochemical performance,especially high rate performance.What's more,the low theoretical specific capacity of LTO also makes it difficult to meet the demand of developed electric vehicles?EVs?and hybrid electric vehicles?HEVs?.In order to solve these problems,we firstly prepared compact LTO microspheres with abundant grain boundaries which can activate highly efficient lithium ion transportation,and then synthesized the carbon-based composite?like hard carbon and graphene?with a second component of LTO to further promote its specific capacity and extend its use in lithium metal battery.Firstly,we prepared compact LTO microspheres consist of densely packed primary nanoparticles.The as-prepared LTO microspheres possess a high tap density of 1.23 g cm-3 and an ultra-small specific surface area(2.40 m2 g-1).Impressively,the compact LTO microspheres present excellent electrochemical performance.At a high rate of 5 C,10 C and 20 C,the LTO microspheres show different specific capacity of 146.6 mA h g-1,138.2mA h g-1 and 111 mA h g-1,respectively.The capacity retention still remains 97.8%at 5C after 500 cycles.The real space distribution of lithium ions inside the compact LTO was revealed by using the scanning transmission electron microscopy with electron energy loss spectroscopy?STEM-EELS?to identify the function of grain boundaries for lithium ion transportation during lithiation.The abundant grain boundaries in compact LTO microspheres can form a highly efficient conductive network to preferentially transport the ions,which contributes to high volumetric and gravimetric energy density simultaneously.In addition,in order to get a composite with both high specific capacity and long cycle stability,we have used different methods like coprecipitation method and hydrothermal method to combine LTO and hard carbon.The LTO@hard carbon composite via coprecipitation method show good cycling performance,whose specific capacity can reach 184 mA h g-1at 10 C and the capacity retention still remains 91.8%after 300 cycles.The LTO/hard carbon composite via hydrothermal method show better specific capacity,it can reach 230 mA h g-1at 10 C but with a poor cycle stability.LTO was also discharged to 0.01 V to get higher specific capacity,and the graphene additive can improve its long cycle performance,the specific capacity of LTO/graphene discharged to 0 V at 10 C can reach 176.2 mA h g-1and the capacity retention still remains 88.5%after 500 cycles.At last,the LTO and graphene composite was used to modify the separator that was applied in lithium metal batteries.Graphene can induce the growth of Li dendrites while the planner LTO can protect the separator form damage.The modified layer can obviously improve the cycle performance of lithium metal batteries.It can stably cycle for 1400 h under the current density of 1 mA cm-2 without short circuit,even when the current density was elevated to 5 mA cm-2,the half cell can still stable cycle for 500 h.What's more,it can also improve the cycle performance of Li?LiFePO4full cells. |
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