Effect of Metal Composition on the Electrochemical Properties of Lithium-Rich Positive Electrode Materials
Lithium-rich mixed transition metal (TM) oxide positive electrode materials such as Li[Li0.2Ni0.2Mn0.6]O2 and Li[Li0.2Mn0.54Ni0.13Co0.13]O2 are attractive due to their high reversible capacities. However, they suffer from some problems such as high irreversible capacity loss1, poor rate capability2 and voltage fade3. There is no clear understanding in the literature how the overall metal composition of the Li-rich material affects their electrochemical properties including the above mentioned issues. Hence we embark, in this study, on studying the effect of metal composition on the electrochemical properties of the Li-rich positive electrode materials.
A series of Ni(II)aMn(II)bCo(II)cCO3 precursors where a + b + c = 1 were made with co-precipitation synthesis using a continuously-stirring tank reactor (CSTR). Ni(II)aMn(II)bCo(II)cCO3 precursors were mixed with required amounts of Li2CO3and made into Li-rich positive electrode materials of the desired composition using solid-state synthesis at 900ᵒC in air. All the prepared materials were characterized by X-ray diffraction and their true densities were measured using a helium pycnometer. Coin-type cells were made from the synthesized positive electrode materials and Li metal anodes, which were then electrochemically tested under galvanostatic conditions at constant temperature, and their electrochemical properties were compared.
Results and Discussion
A Li-rich positive electrode material comprised of Li, Ni2+, Mn4+ and Co3+ can be made from a Ni(II)aMn(II)bCo(II)cCO3 precursor. By knowing the exact composition of Ni(II)aMn(II)bCo(II)cCO3 precursor, the theoretical formula of a Li-rich material can be calculated. For example, the Ni0.25Mn0.75 composition can be used to make Li1.2Ni0.2Mn0.6O2. Thus, the Ni-Mn-Co compositions that can be used to make all the possible Li-rich positive electrode materials were determined. Details results showing how the electrochemical behavior varies with overall metal composition will be presented.
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2. B. Xu , C. R. Fell , M. Chi , Y. S. Meng, Energy Environ. Sci.2011, 4 , 2223
3. Debasish Mohanty, Athena S. Sefat, Jianlin Li, Roberta A. Meisner, Adam J. Rondinone, E. Andrew Payzant, Daniel P. Abraham, David L. Wood, Claus Daniel, Phys.Chem.Chem.Phys., 2013, 15, 19496--19509