Preparation and electrochemical properties of Li0.98Ca0.02Mn2O4
Many research focus on improving the electrochemical properties of LiMn2O4 by chemical doping method.In cubic spinel structure LiMn2O4,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 Li0.98Ca0.02Mn2O4 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 LiMn2O4 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 Li0.98Ca0.02Mn2O4 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,Li0.98Ca0.02Mn2O4 delivered the first discharge capacity of 126 mAh/g,which was 17.8％higher than that of undoped LiMn2O4 samples.The capacity retention of both samples was maintained at about 88.8％after 50 cycles.At 1 C,Li0.98Ca0.02Mn2O4 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 LiMn2O4 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 Li0.98Ca0.02Mn2O4 was 2.5×10-11 cm2/s,which was about 1.6 times of undoped sample.
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