Improving Performance of LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials for Lithium-Ion Batteries by Doping with Molybdenum-Ions: Theoretical and Experimental Studies

Abstract

The work reported herein is an important continuation of our recent experimental and computational studies on Li­[Ni x Co y Mn z ]­O2 (x + y + z = 1) cathode materials for Li-ion batteries, containing minor amounts of multivalent cationic dopants like Al3+, Zr4+, W6+, Mo6+. On the basis of DFT calculations for LiNi0.8Co0.1­Mn0.1O2, it was concluded that Mo6+ cations preferably substitute Ni cations in the layered structure due to the lowest substitution energy compared to Li, Co, and Mn. It was established that the electrochemical behavior of LiNi0.8Co0.1­Mn0.1O2 as a positive electrode material for Li-ion batteries can be substantially improved by doping with 1–3 mol % of Mo6+, in terms of lowering the irreversible capacity loss during the first cycle, increasing discharge capacity and rate capability, decreasing capacity fade upon prolonged cycling, and lowering the voltage hysteresis and charge-transfer resistance. The latter is attributed to the presence of additional conduction bands near the Fermi level of the doped materials, which facilitate Li-ions and electron transfer within the doped material. This is expressed by a lower charge-transfer resistance of Mo-doped electrodes as shown by impedance spectroscopy studies. We also discovered unique segregation phenomena, in which the surface concentration of the transition metals and dopant differs from that of the bulk. This near surface segregation of the Mo-dopant seems to have a stabilization effect on these cathode materials.

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 …”

“…Further, Susai et al. recently showed discharge capacity of 185–195 mAh g −1 for undoped and 1–3 mol % Mo‐doped NCM811 in the voltage window of 2.8 to 4.3 V . They also showed that Mo‐doped NCM811 has reduced fading upon cycling from a combined experimental and computational approach.…”

The Effects of Reversibility of H2-H3 Phase Transition on Ni-Rich Layered Oxide Cathode for High-Energy Lithium-Ion Batteries

“…Such severe structural collapse affected by mechanical strain associated with the poor irreversibility of the H2-H3 phase transition during cycling. Cations substitutions have been regarded as promising way to overcome these challenges and enhance the structural stability of Ni-rich materials (Xia et al, 2018; Li et al, 2019; Susai et al, 2019; Weigel et al, 2019). Wu et al (2019b) demonstrated that by doping Ti 4+ in LiNi 0.9 Co 0.1 O 2 materials, the improved reversibility of the H2-H3 phase transitions and the lossless H3 phase suppress the generation of microcracks and structural degradations.…”

Surface Modification Strategies for Improving the Cycling Performance of Ni‐Rich Cathode Materials

“…The procedure induces diffusion of Nb 5+ into the secondary particles and creates a gradient from about 4 at‐% at the surface to below 1 at‐% within the first 100 nm below the surface. In this work, the Nb 5+ has been hypothesized to occupy the Li sites, which is in contrast to prior experiments on similar sized and charged Mo 6+ , found primarily at the surface of NCM811 …”

“…In a similar study, 1 and 3 mol‐% Mo‐doped NCM811 were synthesized from the transition‐metal nitrates, (NH 4 ) 6 Mo 7 O 24 · 4H 2 O and sucrose in a sol‐gel process, then heated to self‐ignition and calcined . The individual primary particles were between 400 nm and 900 nm in the pristine material and 200–500 nm in the doped samples, indicating inhibition of particle growth by the dopant.…”