Dr. Mitsuharu Tabuchi
National Institute of Advanced Industrial Science and Technology, Japan.

Speech title: Synthesis of high-capacity Fe and/or Ti substituted Li₂MnO₃ positive electrode materials using carbothermal reduction method

Abstract: The R&D of positive electrode material is an attractive research subject, because it almost determines cell capacity and voltage of lithium-ion battery (LIB). Among them, Li-excess positive electrode material like LiMO2-Li2MnO3 solid solution (M=Ni1/2Mn1/2, Ni1/3Mn1/3Co1/3) is attractive for its high capacity (250 mAh/g) and acceptable discharge voltage (>3.5 V). To eliminate the rare elements like Co and Ni, novel LiFeO2-Li2MnO3 system has been discovered and developed as high-capacity material by optimizing wet chemical synthetic method. Its problem is low initial discharge voltage around 3 V. To work at the same voltage range (2.0-4.8 V) like previous system, a carbothermal reduction process was added after the coprecipitation-hydrothermal-calcination process. The aim of this process is reduce part of tetravalent Mn ion to trivalent one to raise discharge voltage and improve initial cycle efficiency. This process is an effective way to synthesize the “electrochemically-activated state” before electrochemical tests. Three target materials such as Fe or Ti or Fe and Ti substituted Li2MnO3 were selected. In the case of Ti substituted Li2MnO3 (Li1+x(Ti0.5Mn0.5)1-xO2,x<1/3,x>0), only tetravalent Mn ion was reduced to trivalent one partially by carbothermal reduction, The Ti valency remained 4+ state. The Li2MnO3-type structure (C2/m) unchanged after this process. The initial cycle efficiency was improved drastically from 53 to 68 % and the reduced sample has high initial discharge capacity (220 mAh/g). Better discharge capacity retention after 20th cycle was obtained (changed from 65 to 88%). Similar electrochemical performance improvement was obtained for other compositions like Li1+x(Fe0.2Ti0.2Mn0.6)1-xO2 and Li1+x(Fe0.3Mn0.7)1-xO2. These facts extend the possibility of practical use of these Co and Ni –free positive electrode materials suitable for large-scale and low-cost LIB. In the conference, electrochemical improvement using stepwise-charging method will be presented.