Understanding the Role of Minor Molybdenum Doping in LiNi_0.5Co_0.2Mn_0.3O₂ Electrodes: from Structural and Surface Analyses and Theoretical Modeling to Practical Electrochemical Cells

Abstract

Doping LiNiCoMnO (NCM523) cathode material by small amount of Mo ions, around 1 mol %, affects pronouncedly its structure, surface properties, and electronic and electrochemical behavior. Cathodes comprising Mo-doped NCM523 exhibited in Li cells higher specific capacities, higher rate capabilities, lower capacity fading, and lower charge-transfer resistance that relates to a more stable electrode/solution interface due to doping. This, in turn, is ascribed to the fact that the Mo ions tend to concentrate more at the surface, as a result of a synthesis that always includes a necessary calcination, high-temperature stage. This phenomenon of the Mo dopant segregation at the surface in NCM523 material was discovered in the present work for the first time. It appears that Mo doping reduces the reactivity of the Ni-rich NCM cathode materials toward the standard electrolyte solutions of Li-ion batteries. Using density functional theory (DFT) calculations, we showed that Mo ions are preferably incorporated at Ni sites and that the doping increases the amount of Ni ions at the expense of Ni ions, due to charge compensation, in accord with X-ray absorption fine structure (XAFS) spectroscopy measurements. Furthermore, DFT calculations predicted Ni-O bond length distributions in good agreement with the XAFS results, supporting a model of partial substitution of Ni sites by molybdenum.

Review of Computational Studies of NCM Cathode Materials for Li‐ion Batteries

“…The commonly applied effective U parameters for Ni, Co and Mn are 5.96, 5.00 and 5.10 eV, respectively. For dopants, such as for Mo, the applied effective U is 5 eV …”

“…From calculation of the electronic structure, we can readily identify the spin state of the TMs in NCM . In pristine NCM, Ni ions are present in three different oxidation states, e. g. 2+, 3+, and 4+.…”

New Organic Complex for Lithium Layered Oxide Modification: Ultrathin Coating, High-Voltage, and Safety Performances

“…12,13 This is ascribed to its higher theoretical capacity (~276 mAh g -1 ) than those of LiFePO 4 (175 mAh g -1 ) and LiMn 2 O 4 (145 mAh g -1 ), respectively, and the superior cycle stability and lower cost than the commercial LiCoO 2 . [14][15][16][17] However, the practical capacities of most NCM are only around 155-170 mAh g -1 at present, because the charged cut-off voltage is restricted at 4.2 V for a good stability. Although a high capacity of ~200 mAh g -1 can be achieved as increasing the cut-off voltage (e.g., 4.5 V vs. graphite anode), 18,19 the capacity fading and safety concerns become more serious.…”

Selective TiO2 Nanolayer Coating by Polydopamine Modification for Highly Stable Ni‐Rich Layered Oxides

“…This leads to deterioration of the performance as well as a risk of heat generation and ignition, thereby significantly reducing the reliability of the battery. Recently, strategies of doping of foreign ions, or design of core–shell‐type gradient particles have been investigated by many researchers to solve the surface issues of Ni‐rich layered oxides. In addition, the surface‐coating method has been considered as one of the most common methods to solve the problem of the cathode material, which is vulnerable to surface degradation.…”