Effect of Metal Composition on the Electrochemical Properties of Lithium-Rich Positive Electrode Materials

Introduction

 Lithium-rich mixed transition metal (TM) oxide positive electrode materials such as Li[Li_0.2Ni_0.2Mn_0.6]O₂ and Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]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.  

 Experiment

 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 Ni_0.25Mn_0.75 composition can be used to make Li_1.2Ni_0.2Mn_0.6O₂. 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.

References

 1. J. H. Kim, C. W. Park, and Y. K. Sun, Solid State Ionics, 164, 43 (2003).

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