Synthesis Of Lithium Titanate Nanocomposites And Studies On Their Lithium Storage Properties

Lithium ion battery is the most promising green energy in the 21 st century,with the advantages of high working voltage,long cycle life and no memory effect,etc.,so it is widely used in mobile phones,computers and other portable electronic equipment and pure electric cars,hybrid electric cars.At present,the most successful commercial anode material is graphite and other carbon-based materials,but the redox potential of carbon materials is close to that of lithium metal,and lithium dendrites are easy to be generated under charging and discharging process,which will degrade the cycling performance of the battery.In addition,lithium dendrites are easy to cause short circuit of the battery,leading to serious safety risks.Among the anode materials,spinel lithium titanate(Li4Ti5O12)has attracted much more attention.Li4Ti5O12 possesses high charging and discharging platform(1.55 V vs Li+/Li),which can effectively avoid the formation of lithium dendrites,thus showing higher safety.In the process of lithiation/delithiation,the volume of Li4Ti5O12 hardly changes,showing high cycling reversibility,thus Li4Ti5O12 is called "zero strain" material.However,poor conductivity and low lithium ion diffusion coefficient limit the application of Li4Ti5O12 material in the field of high-power batteries.Based on the above problems,this paper developed the strategies of regulating the morphology and structure of samples and synthesizing composites with conductive carbon materials to improve the electrochemical properties of Li4Ti5O12.We synthesize Li4Ti5O12 nanosheets by a simple hydrothermal method,then mixing it with carbon source and followed by further calcination under Argon to obtain the carbon-coated Li4Ti5O12 composite.The effects of calcination temperature and carbon source on the morphology and electrochemical properties were investigated.When L-cysteine was employed as carbon source under the calcination temperature of 600 °C,the morphology of Li4Ti5O12 was tuned by Ti-C bond formation during carbonization process,accompanied by a change in the original orientation growth of the Li4Ti5O12 lattice plane.Consequently,Li4Ti5O12 was transformed from nanosheets to nanoparticles with nitrogen and sulfur atoms doped carbon layer coated on the surface,the composite was denoted as LTO/NSC.The nitrogen and sulfur atoms co-doped carbon layer is conducive to shorten the electron transmission path,improving the lithium storage performance of the material.In addition,it can effectively avoid the agglomeration of Li4Ti5O12 nanoparticles,leading to much more stable structure,this conclusion is confirmed by SEM tests of the detached batteries.Compared to LTO nanosheets,LTO/NSC nanoparticles shows significantly improved electrochemical performance,such as high specific capacity(discharge capacity of 183 m Ah g-1 at 0.1 C),excellent rate performance(discharge capacity of 122 m Ah g-1 at 10 C),and stable cycling performance(capacity retention of 96.3 % after 200 cycles at 1 C).CV and EIS tests results also reflect the excellent dynamic performance of the material.Carbon materials are the most common materials used to modify the electrodes of lithium ion batteries,but there are still few reports on the modification of carbon quantum dots.Carbon quantum dots(CQDs)have small size and large specific surface area,which is beneficial to improve the specific surface area of the host material.Hetero atom doping can generate defects on the carbon quantum dots surface,which is conducive to the diffusion of lithium ions and further improve the conductivity of the material.Thus,we synthesized nitrogen atom doped carbon quantum dots by microwave method and employ lysine as carbon source.The carbon quantum dots were loaded on the surface of Li4Ti5O12 nanosheets by solvent thermal reaction and further calcination under argon/hydrogen gas,obtaining LTO/CQDs composite.We explored the influence of different carbon quantum dots content on the electrochemical properties of the materials.When the loading mass is 4.2 wt.%,LTO/CQDs composite shows high specific capacity(discharge capacity of 186.1 m Ah g-1 at 0.1 C),the excellent rate of performance(10 C discharge capacity of 102.8 m Ah g-1 at 10 C),stable cycle performance(capacity retention rate of 97.6% after 100 cycles,86.3% after 500 cycles at 1C).In addition,its excellent interface dynamics performance was verified by CV and EIS tests.