Enhancing Lithium Manganese Oxide Electrochemical Behavior by Doping and Surface Modifications

Unraveling the Mechanism and Practical Implications of the Sol-Gel Synthesis of Spinel LiMn2O4 as a Cathode Material for Li-Ion Batteries: Critical Effects of Cation Distribution at the Matrix Level

Spinel LiMn₂O₄ (LMO) is a state-of-the-art cathode material for Li-ion batteries. However, the operating voltage and battery life of spinel LMO needs to be improved for application in various modern technologies. Modifying the composition of the spinel LMO material alters its electronic structure, thereby increasing its operating voltage. Additionally, modifying the microstructure of the spinel LMO by controlling the size and distribution of the particles can improve its electrochemical properties. In this study, we elucidate the sol-gel synthesis mechanisms of two common types of sol-gels (modified and unmodified metal complexes)-chelate gel and organic polymeric gel-and investigate their structural and morphological properties and electrochemical performances. This study highlights that uniform distribution of cations during sol-gel formation is important for the growth of LMO crystals. Furthermore, a homogeneous multicomponent sol-gel, necessary to ensure that no conflicting morphologies and structures would degrade the electrochemical performances, can be obtained when the sol-gel has a polymer-like structure and uniformly bound ions; this can be achieved by using additional multifunctional reagents, namely cross-linkers.

Core-shell structure of LiMn2O4 cathode material reduces phase transition and Mn dissolution in Li-ion batteries

Although the LiMn₂O₄ cathode can provide high nominal cell voltage, high thermal stability, low toxicity, and good safety in Li-ion batteries, it still suffers from capacity fading caused by the combination of structural transformation and transition metal dissolution. Herein, a carbon-coated LiMn₂O₄ cathode with core@shell structure (LMO@C) was therefore produced using a mechanofusion method. The LMO@C exhibits higher cycling stability as compared to the pristine LiMn₂O₄ (P-LMO) due to its high conductivity reducing impedance growth and phase transition. The carbon shell can reduce direct contact between the electrolyte and the cathode reducing side reactions and Mn dissolution. Thus, the cylindrical cell of LMO@C//graphite provides higher capacity retention after 900 cycles at 1 C. The amount of dissoluted Mn for the LMO@C is almost 2 times lower than that of the P-LMO after 200 cycles. Moreover, the LMO@C shows smaller change in lattice parameter or phase transition than P-LMO, indicating to the suppression of λ-MnO₂ phase from the mixed phase of Li_1-δMn₂O₄ + λ-MnO₂ when Li-delithiation at highly charged state leading to an improved cycling reversibility. This work provides both fundamental understanding and manufacturing scale demonstration for practical 18650 Li-ion batteries.

Screening LiMn2O4 Surface Modification Schemes under Theoretical Guidance

Mn dissolution is one of the most important factors for the failure of LiMn₂O₄ batteries. Doping has been widely adopted in the modification of LiMn₂O₄ cathodes; however, there is still a lack of theoretical guidance on screening the dopants. Here, through first-principles calculations, we systematically investigated the effects of all 3, 4d transition metals as well as Mg, Ca, Sr, Al, Ga, and In on the surface oxygen stability of LiMn₂O₄ cathodes, which has been proved to be correlated with the stability of the surface Mn atoms. Six competitive dopants, namely Nb, Ru, Mo, V, Tc, and Ti, were screened out. Besides, for three dopants in low valence states (Mg, Cu, and Zn), their Li-site doping can more effectively stabilize the surface oxygen atoms compared with Mn-site doping. Finally, we synthesized LiMn₂O₄ samples with Mg, Mo, and Nb surface doping to validate the rationality of the computational results. We found that particle morphology should also be considered in addition to surface oxygen stability for controlling Mn dissolution. Moreover, the electrochemical performance of LiMn₂O₄ batteries is a more complex issue and cannot be solely regulated by Mn dissolution. During the experiments, we have explored novel efficient binary chromogenic reagents for ultraviolet-visible spectroscopy analysis that can be used for rapid and low-cost Mn dissolution detection. This work provides a paradigm for the systematic design of the surface modification of the LiMn₂O₄ cathode under theoretical guidance.

Improved Cycling Performance and High Rate Capacity of LiNi0.8Co0.1Mn0.1O2 Cathode Achieved by Al(PO3)3 Modification via Dry Coating Ball Milling

LiNi0.8Co0.1Mn0.1O2 (NCM811) has attracted extensive attention as a promising cathode of lithium-ion batteries (LIBs) in next-generation electric vehicles, as the NCM811 sample possesses a high energy density and a price advantage. In this work, NCM811 was modified with an Al(PO3)3 precursor using the dry ball milling method followed by heat treatment to enable commercial development both at room temperature and a higher temperature. Compared with the unmodified NCM811 sample with the capacity retention of 68.70%, after Al(PO3)3 modification, the NCM811 sample heated to 500 °C exhibited a super capacity retention ratio of 93.88% after 200 charging–discharging cycles with the initial discharge capacity of 178.1 mAh g−1 at 1 C. Additionally, after Al(PO3)3 modification, the NCM811 sample heated to 500 °C showed much improved rate performance compared to bare NCM811 at the current density of 5 C. The enhanced electrochemical performance after cycling was due to the decreased charge transfer resistance and increased Li+ transmission, which were confirmed via electrochemical impedance spectra (EIS). The NCM electrodes showed improved structural stability as layered structures after Al(PO3)3 modification, consistent with the improved cycling performance. This work revealed that LiNi0.8Co0.1Mn0.1O2 material with phosphide coating can be constructed using a simple ball milling method, which is feasible for obtaining high-performance electrode materials.