The increasing demand for high-capacity batteries, driven by the proliferation of portable electronic devices, necessitates the development of advanced cathode materials. LiCoO₂ (LCO) stands out due to its superior energy density, making it a promising candidate for high-capacity lithium-ion batteries. While LCO boasts a theoretical capacity of 274 mAh g⁻¹, its practical application is hampered by degradation at high voltages (above 4.55 V vs Li⁺/Li), leading to reduced capacity and lifespan. This degradation stems from several factors, including side reactions on the LCO surface, Co dissolution, oxygen release, crystal structure changes, and crack formation. Various techniques like elemental doping, surface coating, and electrolyte improvement have been explored to mitigate these issues. LiCoO₂ undergoes phase transitions (O3 to H1-3 and then to O1) at high voltages, involving structural changes that cause stress, cracking, and crystallinity degradation. Preventing this O3 to H1-3 transition is crucial for stable cycling at voltages exceeding 4.55 V vs Li⁺/Li. Magnesium doping and surface coating have shown promise in improving high-voltage performance by acting as pillars between CoO₂ slabs, enhancing structural stability. However, achieving satisfactory electrochemical reversibility at high voltages remains a challenge. This study aims to address this limitation by proposing a novel method for integrating Mg into LCO through treatment with molten fluoride salt, enhancing its electrochemical stability and suppressing the detrimental phase transition.

Existing literature extensively documents the degradation mechanisms of LiCoO₂ at high voltages. Studies have highlighted the role of Co dissolution (Amatucci et al., 1996), oxygen release (Sharifi-Asl et al., 2019), and crack formation (Jiang et al., 2020) in capacity fade. Numerous attempts have been made to improve LCO's high-voltage performance through elemental doping (Liu et al., 2017; Zhang et al., 2019; Kong et al., 2021), particularly with Mg, which has been shown to enhance stability by acting as a pillar between CoO₂ slabs (Shim et al., 2014; Orikasa et al., 2014). Surface coating strategies employing various materials like MgO (Shim et al., 2014; Taguchi et al., 2016), Al₂O₃ (Cho et al., 2000; Lee et al., 2004), and TiO₂ (Moon et al., 2019) have also been investigated to improve interfacial stability and suppress side reactions. Electrolyte modifications with additives like vinylene carbonate (VC) and 1,3-propanesultone (PS) have been explored to enhance SEI formation and stability (Zhang et al., 2015; Wu et al., 2018). However, despite these efforts, achieving stable and reversible high-voltage cycling of LCO remains a significant challenge. The current study builds upon this existing body of research by introducing a novel approach that combines both Mg doping and surface coating in a controlled manner.

The study synthesized Mg, Ni, and Al-modified LiCoO₂ (MFNA-LCO) using a molten fluoride salt (MgF₂-LiF) as a reaction accelerator. The synthesis process involved two heating steps. In the first step, commercially available LCO powder was preheated at 850°C for 2 hours in an oxygen atmosphere. Then, MgF₂ and LiF were added (LCO:MgF₂:LiF = 1:0.01:0.003 molar ratio), and the mixture was heated at 900°C for 20 hours in oxygen. In the second step, Ni(OH)₂ and Al(OH)₃ were added (LCO:Ni(OH)₂:Al(OH)₃ = 1:0.005:0.005 molar ratio), and the mixture was heated at 850°C for 10 hours in oxygen. Various control samples (M-LCO, MF-LCO, etc.) were synthesized by varying the added elements and their ratios to investigate the individual effects of Mg, Ni, Al and the fluoride salt. The molten fluoride salt facilitates Mg diffusion and doping into the LCO bulk. Differential scanning calorimetry (DSC) confirmed the eutectic point of MgF₂-LiF at 731.4°C, ensuring the formation of the molten salt during the synthesis process. Characterization techniques included powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) elemental mapping, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), nanobeam electron diffraction (NBED), and X-ray photoelectron spectroscopy (XPS). Electrochemical performance was evaluated using both coin-type half-cells and pouch-type full-cells with various electrolytes (including standard and fluorinated electrolytes). Cycling performance, rate capability, and charge/discharge curves were analyzed to assess the effectiveness of the modification. Ex situ XRD measurements were performed on both charged and discharged electrodes at various cycle numbers to track phase transitions. Rietveld refinement was used to analyze the XRD patterns and determine the lattice parameters.

The molten fluoride salt synthesis method successfully incorporated Mg, Ni, and Al into the LCO structure. SEM analysis revealed a smoother surface morphology for MFNA-LCO compared to pristine LCO. EDX and STEM-EDX mapping confirmed the uniform distribution of Mg, Ni, and Al, particularly enriched at the particle surface forming a ~1 nm thick rock salt shell coherently bonded to the layered rock salt core. XPS analysis indicated a higher surface concentration of Mg and F than initially added, suggesting surface enrichment and the formation of an oxyfluoride layer. Ex situ XRD analysis revealed that the MFNA-LCO undergoes a phase transition to a compressed O3 phase (“O3’ phase”) at 4.7 V without transforming to the harmful H1-3 phase observed in pristine LCO. This O3’ phase is characterized by a smaller unit cell volume compared to the conventional O3 phase. Electrochemically, MFNA-LCO exhibited significantly higher capacity retention than pristine LCO at both 4.6 V (96.4% after 100 cycles) and 4.7 V (72.7% after 100 cycles). The high Coulombic efficiency suggests that the surface coating layer effectively suppressed electrolyte decomposition. Ex situ XRD analysis of cycled electrodes showed that MFNA-LCO maintained high crystallinity and prevented the formation of cracks, unlike the pristine LCO which showed significant degradation, crack formation and the generation of rock salt and spinel phases on the surface after cycling. The superior performance of MFNA-LCO is attributed to the pillar effect of Mg within the Li layers and the protective Mg-rich rock salt surface layer which hinders the gliding of the CoO₂ slabs, preventing the detrimental O3 to H1-3 phase transition and maintaining the structural integrity of the cathode. Comparison studies with other modified LCO samples highlighted the importance of both Mg doping and the combined effects of Mg, Ni, and Al in achieving high performance and high coulombic efficiency.

The findings demonstrate a novel and effective strategy to mitigate the degradation of LiCoO₂ at high voltages by employing molten fluoride salts to facilitate Mg doping and surface modification. The formation of a coherent rock salt shell on the surface of the LiCoO₂ particles combined with Mg doping into the bulk material serves dual purpose. The Mg doping inhibits the harmful phase transition from O3 to H1-3, which is responsible for crack formation and capacity fading. The rock salt surface layer passivates the electrode surface and suppresses electrolyte decomposition, further contributing to improved cycling stability. This combined approach outperforms previous strategies using either Mg doping or surface coating alone, showcasing the synergistic effect of both. This work highlights the importance of understanding and controlling phase transitions in cathode materials. The ability to suppress the detrimental H1-3 phase transition allows LCO cathodes to operate stably at higher voltages, leading to increased energy density. The results have significant implications for developing high-energy-density Li-ion batteries and provide valuable insights into fundamental degradation mechanisms in other cathode materials susceptible to phase transition during cycling.

This study successfully demonstrated a molten fluoride salt-mediated synthesis of Mg, Ni, and Al modified LiCoO₂ (MFNA-LCO), resulting in a significantly improved high-voltage performance. The use of the molten fluoride salt facilitated Mg doping and surface coating, suppressing the harmful phase transition to the H1-3 phase and promoting the formation of a new O3’ phase. The enhanced electrochemical properties, including high capacity retention, stable cycling, and reduced crack formation, provide a pathway towards ultra-high energy density lithium-ion batteries. Future research could explore the optimization of the molten salt composition and explore the application of this approach to other cathode materials prone to similar phase transitions.

The study focused on a specific synthesis method and a limited set of dopants. The long synthesis time at high temperatures might pose scalability challenges for industrial applications. Further research is needed to explore other potential dopants or additives and to optimize the synthesis process for improved efficiency and cost-effectiveness. The long-term cycling stability over multiple thousands of cycles needs to be further investigated to confirm the long-term stability of the MFNA-LCO cathode material.