Synthesis of Single-Crystal Disordered Rock-Salt Li-Ion Cathode Materials Enabled by Nucleation and Growth Control

Lithium-rich manganese-based disordered rock salt oxides and oxyfluorides (Mn-DRX) are emerging as high-performance, cost-effective alternatives to cobalt- and nickel-based cathodes in Li-ion batteries. However, current synthesis techniques consistently produce pulverized particles with poor crystallinity and uncontrolled microstructures, primarily due to the necessary pulverization step to achieve suitable particle sizes for DRX cycling. The poor crystallinity, along with the uncontrolled microstructure, accelerates performance degradation and impedes secondary particle processing.
This work presents a novel synthesis method that effectively preserves high crystallinity while offering precise control over microstructure. This is achieved by promoting nucleation and suppressing excessive particle growth and agglomeration during molten salt synthesis. Using this approach, Li_1.2Mn_0.4Ti_0.4O₂ (LMTO), representing a Mn-DRX material, was synthesized with single particle sizes below 200 nm. The resulting material exhibited enhanced electrochemical performance, including improved capacity retention of 84.3% over 100 cycles at ~200 mAh/g and minimal discharge voltage decay, with only a 4.8 mV loss per cycle. In contrast, LMTO synthesized via conventional solid-state methods followed by pulverization (PS method) showed inferior performance, with only 38.1% capacity retention and a 7.5 mV loss per cycle.
This study highlights how nucleation-promoting and growth-limiting molten-salt synthesis (NM method) effectively controls particle size. It also emphasizes the importance of each step in the NM method, including calcination, annealing, washing, and reinsertion, as revealed by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) mapping, X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), and particle size distribution (PSD) analyses.
Additionally, it examines the role of single LMTO particles synthesized via the NM method (NM-LMTO) in enhancing electrode performance compared to LMTO synthesized using the PS method (PS-LMTO), utilizing characterization techniques such as cross-sectional scanning electron microscopy (SEM), XRD, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and ICP-OES of the electrode films before and after cycling. The results indicate that the improved performance is due to the uniform distribution of finely dispersed NM-LMTO particles within the electrode film, compared to the uneven distribution seen with PS-LMTO.
In conclusion, the NM method paves the way for advancements in developing high-performance, nickel- and cobalt-free DRX cathode materials for Li-ion batteries, with significant performance retention enhancements achieved through precise particle microstructure control.