By Prof. Xinping Qiu, Tsinghua University, China.
Lithium-ion batteries have been widely applied in electric vehicles (EVs) in recent years. Limitation of cobalt resource is becoming the obstacle for application of LIBs in EVs. It is urgent to develop new cobalt-free cathode material. Here, we studied a low-cost, cobalt-free Fe- and Ni-substituted Li-rich Mn-based layered material Li-Ni-Fe-Mn-O, Li1.16Ni0.19Fe0.18Mn0.46O2 (NFM). It delivered a reversible capacity of 214.3 mAh g-1 at the initial cycle at a rate of 0.1 C and its average voltage for first discharge was 3.56 (V vs. Li/Li+). After 50 cycles, the capacity retention was 81.2% and the average discharge voltage decreased only 0.13 V. Results of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculation revealed that the capacity of NFM is derived from the reversible redox couples of Ni2+/Ni4+, Fe3+/Fe4+ and 2O2-/O2n-.
We also found that the low coulombic efficiency of NFM during cycling in the conventional electrolyte is related to the dissolution of Fe. By replacing ethylene carbonate (EC) with fluoroethylene carbonate (FEC), a cathode electrolyte interfacial (CEI) film with 10-20 nm thickness formed on NFM. The film effectively suppresses the phase transition during cycling, which improved the coulombic efficiency of NFM from 93% to 99%, and it exhibited better rate performance.
Xinping Qiu holds a B.S. and a Ph.D. from the University of Science and Technology Beijing in 1988 and 1994, respectively. He is a distinguished professor at Tsinghua University the top University in China. He is also the deputy director of the US/China clean energy research center from china side. His research interests focused on the advanced power sources, such as lithium-ion batteries, direct methanol fuel cells and battery packages for electric storage. The main directions include new electrode materials for lithium-ion battery, new catalysts for fuel cell, polymer electrolytes, electrode kinetics, the structure of porous electrode and new techniques for battery characterization. He holds many awards during his carrier including the International Lithium Battery association research award and the international battery association award.