Study On Lithium Storage Mechanism And Modification Of Lithium-Rich Cathode Materials For Lithium Ion Batteries

Due to the unique two-phase structure,Li-rich cathodes can realize multi-electron transfer under the combined action of anion and cation redox,which can release capacity of over 250 m Ah g^-1.However,the problems such as voltage hysteresis,poor rate and cycle property,rapid decay of voltage and capacity of Li-rich cathodes cannot be ignored.In this paper,the internal relationship between voltage hysteresis and charge compensation,as well as structure evolution is decoupled,and the electrochemical properties of Li-rich cathodes were optimized by chemical composition design,local structure optimization and surface modification.Through a variety of physical and electrochemical characterization,the interaction between the crystal structure,local electronic structure,spatial distribution of elemental chemical states and the electrochemical properties of materials are gradually studied in depth.The specific content is as follows:(1)A series of electrochemical and physical characterization are conducted on Li-rich cathodes to probe the formation processes of voltage hysteresis,and to explore the internal relationship between voltage hysteresis and anionic/cationic charge compensation process,structure evolution,and transition metal(TM)migration.The results show that the voltage hysteresis is caused by the reduction of Mn^4+/3+,lagging reduction of Ni^4+/2+/Co^4+/3+and O^n-/2-at lower voltage.The anion/cation redox sequence,structure evolution,and TM ion migration,vary from oxidation to reduction processes,resulting in path dependence and hysteresis.An effective strategy has been proposed that fine-modulating structure by altering Li/O ratio can tune the anionic redox chemistry.This eventually improves its electrochemical properties.(2)The high-Ni Li-rich cathodes,(Compared with Li-rich cathodes,the operating voltage increases nearly 300 m V,and the capacity retention rises nearly 55%in full-cell)with high operating voltage,low voltage and capacity fade,have been successfully prepared by regulating chemical composition and synthesis conditions.Ni^2+/3+/4+has a higher and stable redox reaction potential.Therefore higher O-Ni^2+/3+content with high redox reaction potential in Li-rich cathodes increases the oxidation state of TM ions,reduces the low potential Mn^3+/4+redox couple,improves operating voltage and stability of voltage and capacity.By theoretical calculations,we reveal that regulating chemical composition shifts the TM(3d)-O(2p)band and the non-bonding O 2p band to lower energy levels,leads to an increase in the redox reaction potential.What's more,through the investigation of the synthesis conditions,the best conditions for the preparation of high-Ni Li-rich cathodes:the ratio of Li/O is 1.1/2;Oxygen atmosphere sintering;The optimum sintering condition is preheating 5h at 600?and sintering 12h at 850?.(3)The self-reconstruction strategy is proposed to optimize surface chemistry and structure of high-Ni Li-rich cathodes by PH₃treatment at low temperature(300?),and uniformly formed a high voltage resistant multifunctional surface protective layer.It is found that the protective layer is composed of cation disordered phase and LiTMPO₄-like(TM:Ni,Co,Mn)phase,and perfectly combined with bulk structure,which effectively inhibits the oxygen release and Mn dissolution,and plays a significant role in improving its long-cycle electrochemical performance.The modified electrode has capacity retention of 84.1%after 250 cycles at 1C,and the voltage decay per cycle is only 0.0012V.(4)Using synchrotron STXM-Ptychography technology with a spatial resolution of around 5.6 nm to imaging degradation heterogeneities and interplay among components in degraded Li-rich cathodes.According to the analyses of soft X-ray absorption spectrum at Mn,Ni,O and F edge and the spatial distribution of elemental chemical state,its found that F ions incorporated into the lattice during long cycles,and correlate with the Mn dissolution and O loss,induced degradation of the structure and electrochemical performance.The reconstruction imaging at high resolution directly visualized scattered LiF particles(100-500 nm)and Mn F₂layer between the primary particles of the degraded electrode,which consumed of effective lithium,while impeding the transport of electrons and ions,leading to the decline of electrochemical performance.The importance of surface modification technology in electrode interface protection was further verified according to the element distribution characteristics of the surface region and core region of degraded secondary particles.