This presentation summarizes the recent studies in Bar-Ilan University of materials for positive electrodes of Li-ion batteries. The materials included lithiated oxides of Ni-based family Li[Ni-Co-Mn]O₂ (layered, R-3m space group) and of Li,Mn-rich layered-layered xLi₂MnO₃^.(1-x)LiMO₂ (M=Ni, Co, Mn), 0<x<1. The main questions studied were lattice doping by Al³⁺, Zr⁴⁺ of the above materials in relation to the electrode behavior, structural stability, and thermal reactions [1, 2]. For Li[Ni_0.5Co_0.2Mn_0.3]O₂ materials, using density functional theory calculations, we have shown that Al³⁺ doping is preferred at Ni sites and the thermodynamic preference follows the order: Ni>Co>Mn. It was concluded from XPS studies that the modified stable and less resistive interface on the Al-doped particles comprised the Li⁺-ion conducting centers like LiAlO₂, AlF₃, etc., which promote, to some extent, the Li⁺ transport to the bulk and therefore facilitate the electrochemical reactions [2]. We discuss also the following issues: ab-initio calculations of the preferential substitution of Zr⁴⁺ at Mn, Co or Ni sites; influence of Zr⁴⁺ and Mo⁶⁺ ions on the Li⁺/Ni²⁺ mixing, charge distribution, the lattice constants, as well as partial layered-to-spinel structural transformation and thermal characteristics of cathode materials in reactions with solutions.

In case of Li,Mn-rich cathode materials, we used thin surface coatings, as well as gas treatment with ammonia at 400 °C to stabilize these materials. The AlF₃-coated electrodes exhibit stable charge-discharge behavior providing higher capacities and lower fade upon prolonged cycling at 60 °C. It has been found that electrodes comprising the AlF₃-coated material exhibited higher reversible capacities of ∼250 mAh/g at a C/5 rate, more stable cycling behavior, higher lithium storage capability at 60 ^oC, and lower impedance measured during Li-deinteraclation comparing to electrodes prepared from the uncoated material. An important finding is that Li_x[MnNiCo]O₂ /AlF₃ materials revealed much higher thermal stability both in the pristine (lithiated) and cycled (delithiated) states than their uncoated counterparts [1]. Ammonia treatment of Li,Mn-rich materials for 2 h improves discharge capacity, lowers capacity and mean voltage fading during cycling. The mechanism of the ammonia treatment will be discussed. Further work will explore full cell studies with graphite anodes to confirm if NH₃ treatment can indeed improve the likelihood of commercialization of the above materials [3].

Fig : 1 Discharge capacity of the electrodes comprising NH₃-treated and untreated Li,Mn-rich 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂ materials. The treatment was performed for 1, 2 and 4 hours, as indicated.

References :

[1] S. F. Amalraj, M. Talianker, B. Markovsky et al. J. Electrochem. Soc. 160 (2013) A2220.

[2] D. Aurbach, O. Srur-Lavi, C. Ghanty et al. J. Electrochem. Soc. 162 (2015) A1014.

[3] Evan M. Erickson, Hadar Sclar, Florian Schipper, B. Markovsky et al., Adv. Energy Mater., 7, (2017) 1700708