Designing High-Performance Lithium-Ion Batteries with Core-Shell NMC Morphology

In the continuous search for portable energy sources, active materials for lithium-ion batteries have come into the spotlight, sparking intensive research into promising cathode materials. Currently, one of the most popular cathode materials for lithium-ion batteries is lithium nickel manganese cobalt oxide, described by the formula Li(Ni_xMn_yCo_z)O₂. Its attractiveness lies in its high energy density and relatively cost-effective constituent elements. However, there is no middle ground in NMC composition, as both the electrochemical and safety properties of batteries depend significantly on each constituent element. Increasing the Ni concentration enhances the discharge capacity, making it a promising candidate. However, it also induces thermally triggered phase transitions, negatively impacting cycling retention and structural stability upon long cycling [1]. Recently, a beneficial improvement seems to be creation of a cathode material with gradient structure. This involves having a high nickel concentration in the grain interior and increasing manganese and cobalt content towards the outer part. This arrangement secures the high-capacity nickel-rich core with a stable nickel-poor shell [2]. Both high discharge capacity, stability, and safety are highly relevant.<br/>This research focuses on developing a synthesis route for core-shell NMC with differentiated nickel content. This significant change in the approach to NMC material synthesis aims to provide high capacity without loss in cycling stability. Moreover, to optimize the effect of increasing material stability, we investigated and cross-referenced shell variants differing in Co/Mn content. Gradient structures with high-manganese outer content are well-known, but initial indications suggest that preparing core-shell or gradient NMC with a Co-rich outer layer results in even better stability. Studies have shown that Co-rich material is more stable than Mn-rich and prevents primary grains from cracking and degradation [3].<br/>The high-nickel core precursors were prepared using the coprecipitation method to form the complex metal hydroxide Ni_xMn_yCo_z(OH)₂ in a Couette-Taylor flow reactor, which allowed for maximum control of the synthesis process. Following this, a method for shell deposition was developed. The resulting core-shell precursors were then annealed to produce the lithiated powders. This careful precursor synthesis strategy and the subsequent calcination of the final product ensured reliable and reproducible results.<br/>To explore the properties of the materials and conduct a comprehensive analysis of their differences, we employed a variety of methods. Scanning electron microscopy was used to examine the morphologies of the prepared powders both before and after the shell coating step. X-ray powder diffraction allowed to characterize the crystal structure of the prepared materials, providing insight into their crystallographic parameters and phases present. Raman spectroscopy was employed to examine the structural homogeneity core and shell and detect post-synthesis contamination of the material. Moreover, to study the shell creation route and the influence of composition on material properties, we investigated its post-cycling morphology and structure.<br/><br/>A detailed analysis will be presented during the conference. This work was funded by the National Science Center in Poland through the Sonata 17 programme (No. UMO-2021/43/D/ST5/03094).<br/><br/>[1] Entwistle, T., et al. "Co-precipitation synthesis of nickel-rich cathodes for Li-ion batteries." Energy Reports 8, 67–73 (2022).<br/>[2] Hou, P., et al. "Core-shell and concentration-gradient cathodes prepared via co-precipitation reaction for advanced lithium-ion batteries." J. Mater. Chem. A 5, 4254–4279 (2017).<br/>[3] Liu, Tongchao, et al. "Rational design of mechanically robust Ni-rich cathode materials via concentration gradient strategy." <i>Nature communications</i> 12.1 (2021): 6024.