Structure-activity Relationship Of Cathode Materials For High-voltage And Manganese-rich Layered Batteries

Energy crisis and environment pollution have already become two vital issues to humanity,a major settlement to them is developing sustainable energy source.The secondary batteries have attracted great attentions as its eco-friendliness and energy-saving property compared with the traditional one.With the continuous forward of technology,the applications of Li-ion secondary battery will not only in the micro electrical products fields,but also apply to the large-scale power source fields such as the new energy automobile.The further study to the secondary battery with high energy density,high current density and assured safety is still necessary as a consequence of its unfitness for the needs of the large-scale power source application based on the recent research.In the system of secondary batteries,the quality of cathode materials is vital to energy desity,power desity and safety of the batteries.So it is urgently to exploit advance cathode materials which enrich outstanding electrical property,cycle stability as well as favorable safety.Manganese-rich layered oxides,which possess high specific discharge capacity,excellent cycle performance,wide charge/discharge voltage range,lower process cost,was considered as the most promising cathode material for LIBs and SIBs.This research is focus on the modification of cathode material M_xNi_yMn_zO₂,especially aimed at its drawbacks in several facets such as first efficiency,structural phase transmission,voltage decay,cycle life and so on.Definite modification methods were used to improve the electrochemistry properties as well as theory calculation was combined to analysis the bulk mechanism of structural performance promotion.So that the potential application of manganese-rich layered cathode materials could be extended in lithium/sodium ion secondary batteries.The research of this paper can mainly be divided into several parts below:（1）The effect of anion doping on structure of manganese-rich cathode materials.Aseriesofsulfurdopingmanganese-richcathodematerials Li_1.2Mn_0.6Ni_0.2O_2-xS_x（x=0,0.01,0.03,0.05）were synthetized successfully by co-precipitation method.The results of morphology and structure tests show that the lattice spacing of doped sample Li_1.2Mn_0.6Ni_0.2O_1.17S_0.03 is 0.472 nm,which is significantly larger than that of pristine sample Li_1.2Mn_0.6Ni_0.2O₂（0.452 nm）.Owing to the larger radius of sulfur atom,the arrangement of the doped samples are more orderly.The larger lattice spacing of（003）plays an important role on the decreasing barrier and increasing rate during the lithium ion diffusion process.Furthermore,the doped sample remains the same lattice spacing after 40 charge/discharge cycles,which totally results from the high structural stability of the layered material.From the theory aspect,electrochemistry test and VASP calculation explain the reason why Li_1.2Mn_0.6Ni_0.2O_1.17S_0.03 provides preferable cycle stability,high lithium ion diffusion rate and excellent first columbic efficiency of 96.06%.Furthermore,stable structure of Li_1.2Mn_0.6Ni_0.2O_1.17S_0.03 leads to higher discharge voltage plateau,which is beneficial to provide higher energy density during the electrochemical cycling in batteries.（2）Co-doping of anion and cation to regulate the structure of lithium/manganese-rich Li_1.2Ni_0.2Mn_0.6O₂.A series of anion/cation co-doping lithium/manganese-rich cathode materials were synthetized by co-precipitation method to explore the harmony of integrity between layers by co-doping anion/cation with large radius.To further analysis the morphology,structure and structural phase transmission during charge/discharge progress of these samples,several measurements such as SEM,TEM,HADDF-STEM,In-situ XRD were employed.The electrochemical properties were tested by electrochemical measurements such as cyclic voltammetry,constant current charging/discharging and AC impedance.As the results showing,the capacity retention of lithium ion battery increasing from 59.9%to 95.5%and over 180 mAh g^-1 specific capacity can be delivered at 5 C after co-doped by Cd and S.Voltage decay and structure collapse are suppressed at the same time.In this research,element S stabilized the layered oxygen structure,and Cd atoms improved the stability of TM and Li layers simultaneously.According to the DFT calculation results,Cd substituted for Ni site,which is prior to Li site,while the most difficult to replace is Mn.Furthermore,introduction of Cd lead to larger interlayer spacing,lower Femi level and energy of lithium/manganese-rich cathode material,so that the structural stability is improved significantly.（3）NaNi_0.5Mn_0.47Sn_0.03O₂,Sn-doped Mn rich cathode material,was synthesized by the facile sol-gel methods to figure out the influence of the large radius Sn atom doping to the structure of laminated material.As the results,the initial discharge capacity of NaNi_0.5Mn_0.47Sn_0.03O₂ is 190 mAh g^-1.At 1 C(384 mAh g^-1)current,the great capacity retention（85%）is displayed after 100 cycles based on Ni²⁺/Ni³⁺redox.Even at 10 C(3840 mAh g^-1),the discharge capacity of NaNi_0.5Mn_0.47Sn_0.03O₂is 60 mAh g^-1,meanwhile,the discharge capacity of NaNi_0.5Mn_0.5O₂ is about 0 mAh g^-1.HRTEM analysis shows that partial Sn doping can suppress the irreversible multiphase transformation of O3-NaNi_0.5Mn_0.5O₂ in the high voltage region,and maintain a highly reversible O3-P3 phase transition over a wide range of capacities,result in a high capacity retention and a favorable rate performance.