Improved Electrochemical Properties of LiMn2O4-Based Cathode Material Co-Modified by Mg-Doping and Octahedral Morphology

Efficient enhancement on crystallization and electrochemical performance of LiMn2O4 by recalcination treatment

Spinel LiMn₂O₄ cathode material was obtained by a recalcination treatment, which exhibits excellent crystallization and electrochemical performance. A series of test and analysis results revealed that the performance enhancement of as-prepared sample is related to the crystal structure, morphology and electrochemical properties. Owing to the recalcination treatment, the spinel LiMn₂O₄ presents a truncated-octahedral morphology with selective growth of the (110) and (100) crystal planes, which would effectively inhibit manganese dissolution. Moreover, the optimized sample exhibits a better crystallinity and electrochemical reversibility than that of pristine sample, which can provide a faster Li ion de-intercalation/intercalation kinetics. Hence, the spinel LiMn₂O₄ cathode material delivers a high initial discharge capacity of 112.3 mAh·g^-1 with a good capacity retention of 90.3% after 500 cycles and an excellent rate performance. This study constructed a facile and meaningful method to prepare spinel LiMn₂O₄ cathode material, which may facilitate the development of lithium-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.

Enhancing Surface Chemical Stability of LiMn2 O4 Cathode by Strontium Enrichment at Grain Boundaries

LiMn₂ O₄ (LMO) cathodes suffer from limited cycle life, resulting from Mn dissolution and side reactions between electrode and electrolyte. In this study, Sr-modified LMO is prepared by using a simple strategy. The nature and position of large-radius Sr ions are investigated, alongside their influence on the structural stability of the bulk. SrMnO₃ (SMO) is found to be enriched at grain boundaries of LMO, with Mn-O-Sr bonds forming at the SMO/LMO interface. Furthermore, stable SMO alleviates the migration of Mn ions in LMO associated with structural integrity and suppresses side reactions between the electrode and electrolyte. The modified LMO cathodes maintain their structural integrity and display improved rate performance and cycling stability under harsh conditions. Remarkably, the discharge capacity of a Sr-modified LMO||Li half-cell maintains 94.8 % at 25 °C and 79.6 % at 55 °C after 500 cycles. Consequently, enrichment of strontium at grain boundaries presents a promising strategy for developing cathodes for long-term use.

First-Principles Simulations for the Surface Evolution and Mn Dissolution in the Fully Delithiated Spinel LiMn2O4

The interfacial stability between the cathode and electrolyte is an essential issue in the development of high-energy-density and long-life lithium-ion batteries. The deterioration of capacity dominated by Mn dissolution makes LiMn₂O₄ a representative case for studying the evolution of interfaces. Here, we use the ab initio molecular dynamics (AIMD) method to simulate the interface reaction between the ethylene carbonate (EC) molecules and the (110) surface of completely delithiated LiMn₂O₄ where most severe Mn dissolution is observed in the experiment. It is found that the intrinsic oxygen loss on the surface will drive the initial migration of surface Mn atoms to the electrolyte while reducing them. The EC molecules will decompose after transferring electrons to the surface Mn atoms, and its decomposition products further promote the Mn dissolution. In addition, oxygen loss and EC decomposition are in a competitive relationship when transferring electrons to the surface Mn atoms. This work provides a complete picture of the step-by-step dissolution of Mn atoms along with the interfacial evolution in the spinel LiMn₂O₄ system and also provides a scope for the study of transition-metal dissolution in other cathode materials and electrolytes.