Study On The Modification And Optimization Of Ni-rich Ternary Cathode Materials For Lithium-ion Batteries

Although energy has provided an important driving force for the development of our science and technology,the pollution from the use of traditional fossil fuels（coal,oil and natural gas）has already caused serious impacts on human health and the natural environment.Therefore,exploring and utilizing sustainable clean energy to get rid of the dependence on conventional fossil fuels has become our consensus.Owing to their distinct advantages such as high energy density and long cycle life,lithium-ion batteries have been widely used in the storage of sustainable clean energy as well as for the applications of a large variety of areas including mobile electronic equipment,electric vehicles and aerospace.Cathode materials play an important role in determing the performances and are indeed a limiting factor in restricting the applications of lithium-ion batteries.Among various cathode materials that have been marketed,Ni-rich ternary cathode materials have been the first choice for next-generation high-performance lithium-ion batteries largely due to their higher capacity.Nevertheless,the large-scale practical applications of these materials have been severely hindered by their inherent shortcomings including insufficient cycle life,relatively poor rate capability,and poor safety.This,therefore,indicates a strong need for appropriate modification means to overcome these disadvantages.It has been proven previously that appropriate bulk doping and surface coating can efficiently improve the performances of Ni-rich ternary cathode materials.Incorporating these streategies together,we proposed and investigated a series of in-situ modification approaches in this thesis,with which significantly enhanced performances have been achieved for Ni-rich ternary cathode materials.Specifically,the rsearch of these novel approaches on Ni-rich ternary cathode materials Li Ni_0.5Co_0.2Mn_0.3O₂（NCM523）and Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）was comprehensively carried out and optimized in four parts as follows:1.In Chapetr 3,we developed a facile in-situ approach to co-modify NCM523cathode material with simultaneous lithium aluminum oxide（Li Al O₂）coating and Al doping.Without any additional steps and extra reagents,the co-modification begins during the preparation of the hydroxide precursor of NCM523 and ends upon the calcination of the cathode material.Combining the synergistic effects from Li Al O₂coating and Al doping,the resultant Al-modified NCM523 possesses a robust protective layer as well as a well-ordered layered structure with enlarged lattice spacing,restrained cation mixing,and concentration-gradient-distributed Ni,thus exhibiting significantly enhanced environmental and thermal stabilities and electrochemical performances over its pristine counterpart.At a high active material content of 94.0 wt.%and a high mass loading of 19.5 mg cm^-2 for the testing electrode,the optimized Al-modified NCM523shows a high discharge capacity(168.4 m Ah g^-1 at 0.1 C),a high rate capability（capacity retains 67.8%at 3.0 C vs.0.1 C）,and a long cycle life（capacity retains 62.3%after charge/discharge at 0.5 C for 120 cycles）.2.In Chapetr 4,we further introduced titanium（Ti）as a doping agent into NCM523to develop a Ti/Al co-modification approach,with which performances of NCM523 have been further improved.Combining the synergistic effects from Li Al O₂ coating and Ti/Al co-doping,the resultant Ti/Al co-modified NCM523 possesses a robust protective layer as well as a well-ordered layered structure with enlarged lattice spacing and restrained cation mixing,thus exhibiting significantly enhanced environmental and thermal stabilities and electrochemical performances over its pristine,single Ti-doped or single Al-modified counterparts.At a high active material content of 94.0 wt.%and a high mass loading of 19.5 mg cm^-2 for the testing electrode,the Ti/Al co-modified NCM523 shows a high discharge capacity(160.90 m Ah g^-1 at 0.1 C),a high rate capability（capacity retains 76.55%at 3.0 C vs.0.1 C）,and a long cycle life（capacity retains 85.83%after charge/discharge at 0.5 C for 120 cycles in half cell testing,and capacity retains 95.10%after charge/discharge at 0.5 C for 200 cycles in full cell testing）.3.In Chapetr 5,the in-situ Al co-modification approach developed for NCM523cathode material was verified with a higher-Ni-content ternary cathode material,i.e.,simultaneoussly Li Al O₂-coating and Al-doping NCM811.Specifically,without any additional steps and extra reagents,the co-modification begins during the preparation of the hydroxide precursor of NCM811 and ends upon the calcination of the cathode material.Combining the synergistic effects from Li Al O₂ coating and Al doping,the resultant Al-modified NCM811 possesses a robust protective layer as well as a well-ordered layered structure with enlarged lattice spacing,restrained cation mixing,and concentration-gradient-distributed Ni,thus exhibiting significantly enhanced environmental and thermal stabilities and electrochemical performances over its pristine counterpart.At a high active material content of 90.0 wt.%and a high mass loading of 15.6 mg cm^-2 for the testing electrode,the optimized Al-modified NCM811 shows a high discharge capacity(197.25m Ah g^-1 at 0.1 C),a high rate capability（capacity retains 70.5%at 3.0 C vs.0.1 C）,and a long cycle life（capacity retains 66.40%after charge/discharge at 1.0 C for 100 cycles）.4.In Chapetr 6,we further introduced boron（B）as a doping agent into NCM811 to develop a B/Al co-modification approach,with which performances of NCM811 have been further improved.Combining the synergistic effects from Li Al O₂ coating and B/Al co-doping,the resultant B/Al co-modified NCM811 possesses a robust protective layer as well as a well-ordered layered structure with enlarged lattice spacing and concentration-gradient-distributed Ni,thus exhibiting significantly enhanced environmental and thermal stabilities and electrochemical performances over its pristine,single B-doped or single Al-modified counterparts.At a high active material content of90.0 wt.%and a high mass loading of 15.6 mg cm^-2 for the testing electrode,the B/Al co-modified NCM811 shows a high discharge capacity(191.96 m Ah g^-1 at 0.1 C),a high rate capability（capacity retains 64.78%at 3.0 C vs.0.1 C）,and a long cycle life（capacity retains 85.46%after charge/discharge at 1.0 C for 100 cycles in half cell testing,and capacity retains 86.00%after charge/discharge at 1.0 C for 300 cycles in full cell testing）.In summary,during the reaserch of the present work,we have developed a series of unique in-situ modification approaches,i.e.,simultaneous Li Al O₂ coating and Al doping（or Li Al O₂ coating and Ti/Al doping,or Li Al O₂ coating and B/Al doping）,to significantly enhance the environmental and thermal stabilities and electrochemical performances for Ni-rich NCM cathode materials.Apart from NCM523 and NCM811 tested,other NCM cathode materials can also be co-modified by these approaches.More broadly,beyond the doping elements of Al,Ti,and B that have been investigated,the novel in-situ modification approaches developed in this work may be further extended to utilize other modification chemistries to co-modify NCMs and other cathode materials in situ for enhancing their stabilities and electrochemical performances.Therefore,our research of the present work would stand for a remarkable instructive significance to the modification and optimization of cathode materials in general for lithium-ion batteries.