Li_(0.98)Ca_(0.02)Mn_(2)O_(4)的制备及其电化学性能

Preparation and electrochemical properties of Li_(0.98)Ca_(0.02)Mn_(2)O_(4)

本工作采用水热结合后续高温煅烧法制备单相的尖晶石型结构Li_(0.98)Ca_(0.02)Mn_(2)O_(4)。结果表明,Li_(0.98)Ca_(0.02)Mn_(2)O_(4)晶粒形成的小团聚体内有孔型通道,利于与电解液有效接触,增加正极材料的活性位点,有效缩短了Li+的扩散路径。电化学性能表明,Li_(0.98)Ca_(0.02)Mn_(2)O_(4)呈现出优异的倍率性能,并且在1 C的电流密度下,初始放电容量为117.5 mAh/g,是未掺杂LiMn_(2)O_(4)样品的1.4倍,150圈循环后,容量保持率为80%,1000圈循环后,仍可保持60%的容量。Ca掺入晶格后,使晶格膨胀,有利于提高Li+的扩散能力;经计算,Li_(0.98)Ca_(0.02)Mn_(2)O_(4)样品的扩散系数为2.5×10^(-11)cm^(2)/s,约为未掺杂LiMn_(2)O_(4)样品的1.6倍。

Many research focus on improving the electrochemical properties of LiMn_(2)O_(4) by chemical doping method.In cubic spinel structure LiMn_(2)O_(4),the diversity of doping elements and doping positions provides a wide space for improving performance.Doping at the 16d octahedral position occupied by Mn can effectively suppress the Jahn-Teller effect and maintain the stability of the structure.By comparison,using elements with large ion radius to dope at the 8a tetragonal position occupied by Li can enlarge the Li+diffusion channel and enhance the kinetics diffusion coefficient.In this work,pure phase of Li_(0.98)Ca_(0.02)Mn_(2)O_(4) was successfully synthesized using the hydrothermal method followed by annealing at 750℃for 5 h.The crystal structures and the morphologies of the products were analyzed by powder X-ray diffraction(XRD)and field emission scanning electron microscopy(FESEM).The electrochemical properties were characterized by galvanostatic charge/discharge experiments and electrochemical impedance spectroscope(EIS)tests.XRD analysis showed that the lattice constant increased by 0.12%in Ca-doped LiMn_(2)O_(4) and the expansion of the crystal cell was beneficial to improving the diffusion of Li+.The small aggregates with porous channels formed by stacking nanoparticles were observed by FESEM.The results showed that Li_(0.98)Ca_(0.02)Mn_(2)O_(4) exhibited the excellent rate capability with the larger discharge capacity at the relatively current rate range of 0.5 C~5 C.Especially,at 0.5 C,Li_(0.98)Ca_(0.02)Mn_(2)O_(4) delivered the first discharge capacity of 126 mAh/g,which was 17.8%higher than that of undoped LiMn_(2)O_(4) samples.The capacity retention of both samples was maintained at about 88.8%after 50 cycles.At 1 C,Li_(0.98)Ca_(0.02)Mn_(2)O_(4) still holded its high discharge capacity of 117.5 mAh/g and capacity retention of 90%after 50 cycles,80%after 150 cycles,and 60%after 1000 cycles.Undoped LiMn_(2)O_(4) sample had low capacity of 57.0 mAh/g,but the capacity retention reacheed 67%after 1000 cycles,indicating good cycle stability.The calculated kinetics diffusion coefficient of Li_(0.98)Ca_(0.02)Mn_(2)O_(4) was 2.5×10^(-11)cm^(2)/s,which was about 1.6 times of undoped sample.