Structural Optimization Design And Performance Study Of Low-Nickel Lithium-Rich Manganese-Based Cathode

In recent years,with the rapid development of new energy electric vehicles,the ability to match lithium-ion batteries（LIBs）,which are the core energy supply component of new energy vehicles,has become a focus of attention.Although the common commercial LIBs cathode materials（Li Ni_xMn_yCo_zO₂,Li₂MnO₄ and Li Fe PO₄）have been successfully commercialized,their low discharge capacity(<200 m Ah g^-1)cannot meet the demand for range.As a result,Li-rich Mn-based cathode materials are gaining attention in the academic world for their high specific capacity(≈300 m Ah g^-1),low cost and high environmental friendliness.However,the undesirable cycle stability,significant phase changes during charging/discharging,and severe voltage degradation behind the high capacity have restricted the commercialization of this material.Therefore,this paper investigates the properties of Li-rich Mn-based cathode materials from a molecular/atomic microscopic perspective,and uses the CASTEP module in Material Studio software to carry out relevant theoretical calculations on the materials before and after modification.Based on the guidance of theoretical calculations,this paper focuses on improving the cycle life and multiplication capacity of Li-rich Mn-based materials with mitigation of voltage decay and suppression of phase transformation,optimizes the synthesis route of Li-rich Mn-based precursors,proposes a double ion-dense strategy and a one-step gas-solid treatment strategy to improve the electrochemical performance of Li-rich Mn-based cathode materials,and focuses on the mechanism of action of the two modification strategies.Details of the study are as follows:（1）In this paper,the crystal model of Li₂MnO₃ was constructed in the CASTEP module of Material Studio software,and single-point energy calculations were carried out after optimizing the crystal structure to obtain the energy band information of the material,and the electrical conductivity of the Li-rich Mn-based cathode material was analyzed from the perspective of first principles.In this paper,Li-rich Mn-based precursors were prepared by co-precipitation and the important parameters（ammonia concentration,p H,reaction time,etc.）affecting the synthesis of the precursors were determined by various test characterization methods.In addition,electrochemical test methods have been used to explore the effects of conditions such as lithium carbonate addition,sintering temperature and sintering time on the capacity of the material and its cycling stability.The Li-rich Mn-based prepared under optimized conditions has a first discharge capacity of 263.8 m Ah g^-1,a first coulombic efficiency（ICE）of 74.9%and a capacity retention rate of 83.2%after 200cycles.（2）The results of the Density functional theory（DFT）calculations show that the band gap spacing of the lithium-rich manganese cathode material is significantly reduced after Mg²⁺and Cl^-doping,indicating that the introduction of Mg²⁺and Cl^-has a significant effect on the enhancement of the electrical conductivity of the material.On this basis,we have achieved a double ion densification strategy for the modification of the material using Mg Cl₂as the modifying source.The resulting sample exhibits a first discharge specific capacity of296 m Ah g^-1 at a current density of 0.1 C.The results of the particle cross-sectional characterization（FIB）show that the modified material becomes more compact internally.In addition,the outer regions of the modified material maintain a good lamellar structure after cycling due to the suppression of the phase transition from the lamellar to spinel phase by cations（Mg）and anions（Cl）.（3）Density functional theory calculations show that the introduction of oxygen vacancies and Fe³⁺can effectively alleviate the excessive band gap of the pristine material.Based on this,we have achieved a one-step gas-solidification treatment of Li-rich Mn-based materials by means ofFe（N H₄）₂`（SO₄）₂（FAS）,in which the introduction of Fe and the construction of oxygen vacancies are achieved through the generation of Fe₂O₃ and NH₃ by thermal decomposition reactions during the formation of layered Li-rich oxides.The shift to the left of the（003）peak in the XRD results provides evidence for the introduction of Fe³⁺,ensuring the transport of Li ions and achieving improved multiplicity performance(10C=117.36 m Ah g^-1),while the increased oxygen vacancies characterized by EPR spectroscopy and the suppression of irreversible O₂ release by the more energetic Fe-O bonds contribute to the improved ICE of the material（79.31%）.Electrochemical tests show that under room temperature,the modified material still has a capacity retention of 80.04%after 300 cycles,which is better than the original material.