Insights About the Voltage Decay in Lithium- and Manganese-Rich Cathode Materials

摘要

Lithium- and manganese-rich cathode materials with the general formula Li1+xM1-xO2 (M = Mn, Ni, Co; x <= 0.2) are very promising cathode materials for lithium ion batteries, particularly due to their significantly higher specific capacity (>200 mAh g-1) compared to state of the art cathode materials.[1] Popular members of the Li rich family, like Li1.2Mn0.56Ni0.16Co0.08O2 and the Co-free compound Lil.2Mn0.6Ni0.2O2 have already been structurally and electrochemically investigated. Nevertheless, one of the major issues why these high voltage materials are not used in commercial battery cells yet is the gradual voltage decay during the process of cycling. This phenomenon of continuous loss of energy density is not fully understood, even though it is believed to be caused by mainly three effects, the continuous activation of Li2Mn03, a phase transformation from a layered to a spinel-like structure and polarization due to high temperatures. [2] The electrochemical phase transformation from a layered to a spinel-like structure has been reported from many groups. Considering the high manganese content in lithium-rich materials, the known Manganese dissolution in LiMn204 spinel oxides with higher voltages above 4 V in attendance of carbon content in the composite cathode [3] is an important factor in terms of cycle stability and voltage decay. In this work different approaches are used to investigate the effect on the voltage fading in Li-rich cathode materials. By using different calcination temperatures a lower voltage decay could be observed by using higher temperatures. Additionally the annealing time has an effect on the voltage decay. Other electrochemical studies in this work show that different voltage ranges and currents have a direct effect on the voltage fading as well.