Interface Optimization Of Ni-Rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathodes With Enhanced Lithium Storage

Originating from the low cost and high energy density,the Ni-rich cathode material has been one of the best cathode materials for high energy density lithium ion batteries.However,its obvious interface problem seriously restricts its lithium storage for the high performance cathode materials.In this dissertation,the optimization of the surface modification of the LiNi0.8Mn0.1Co0.1O2(NCM)cathode has been utilized to address this challenge.It focuses on decreasing capacity/voltage decay via some strategies,such as,coating and surface doping.The mechanism of interface reaction and phase transitions is further explored during lithium storage.More importantly,upon cycling the intrinsic mechanism of the thermodynamics and kinetics for the interface modified cathodes is elucidated.The detailed work in this dissertation is as following.(1)Given the physical properties of the different electrical conductivity of the non-solid electrolyte(SnO2)and solid electrolyte(Li2SnO3),two types of coating layers on the surface of LiNi0.8Mn0.1Co0.1O2(NCM)were constructed by a sol-gel thermal assisted method.The results show that,compared with SnO2,Li2SnO3 not only effectively protects the active material from the erosion of the electrolyte,but also acts as a Li+conductor to construct stable Li+diffusion channels at the surface of NCM,which greatly improves the reversibility of phase transitions of the cathode for lithium storage performance and also optimizes its rate capabilities.At the current densities of 0.25,0.5,1.25,2.5 and 6.25 C,the capacity of NCM was increased from the original 202,190,174,161 and 141 mAh g-1 to 217,208,191,176 and 150 mAh g-1 after the modification.(2)A functional phosphorization interface was constructed on NCM surface by a two-step surface phosphating and passivation method through a physicochemical vapor deposition,which acts as a stable Li+migration path,enhances the stability of the cathode/electrolyte interface and suppresses interfacial side reactions,such as intergranular cracking of the active material and irreversible phase transitions from layered structure to spinel and even to salt rock phase.The results show that this passivated interface significantly improves the capacity retention and reduces operating voltage decay of the cathode at a high cutoff voltage of 4.5 V.The capacity retention of NCM increases from 73.0%capacity to 92.0%after 100 cycles at a current density of 0.25 C.The capacity can reache 115.7 mAh g-1 after 400 cycles at 2.5 C,significantly higher than that of the pristine NCM(88.2 mAh g-1).(3)A kind of Se-doped surface was designed for NCM by adopting physicochemical vapor deposition stategy.After partially replacing O sites in NCM,Se increases the lattice d-spacing of the NCM to form a surface interface with the thick of 2 nm.This interface layer not only strengthens the stability of the cathode/electrolyte interface,but also significantly improves the electronic and ionic conductivity of the NCM.After 500 cycles at the current of 2.5 C,the specific capacity of 135 mAh g-1 is achieved for Se-doped NCM,much higher than that of 80 mAh g-1 of the pristine NCM.More importantly,this work elucidates the bidirectional discontinuous diffusion mechanism of lithium ions in layered cathode materials for the fist time.(4)To investigate the influence of surface defects on the structure and performance of NCM,an interface of oxygen defects was conducted though the physicochemical vapor deposition method in(3)by combining the interaction mechanism of the gas-solid interface between Se gas and NCM.The presence of oxygen defects accelerates the occurrence of irreversible phase transitions to form the strong spinel-like phase on the surface of NCM at the early cycling,which greatly eliminates the production of the further irreversible phases and improves the cycle life of the NCM.Its capacity can be up to 145 mAh g-1 after 500 cycles at a low current density of 0.25 C(the capacity retention rate up to 80.6%and the attenuation rate of the average operating voltage from 3.847 to 3.7 V after 500 cycles down to 0.0076%per cycle).After 200 cycles,the capacity of the pristine NCM is only 113 mAh g-1,with a retention rate of 54.6%.This spinel-like phase limits the formation of inactive NiO rock salt phase,provides a stable path for Li+transport,and most importantly,almost eliminates the problem of the voltage decay in the lithium storage process of the NCM.