Study On Grain Boundary Conductivity And Power Battery Degradation Mechanism Of High Density Ternary Cathode Material

In recent years,Lithium-ion batteries(LIB)have received more and more extensive appliciation owing to the rapid development of new energy and electric vehicle industries.Among the many kinds of commercial cathode materials,ternary cathode LiNi0.8Co0.15Al0.05O2(NCA)is one of the most promising material to reach the requirement of 300 Wh/kg energy density for power battery due to the high reversible specific capacity,high energy density,high rate capacity,long cyclic lifeterm and low cost of NCA cathode.The NCA therefore is receiving growing concern and research in academic and industrial fields.Nevertheless,the mechanism of compact NCA seconday particles high rate discharging performance is not clearly studied,especially the effect of anundant grain boundaries on the ion transportation inside the microsize compact NCA particles.Besides,the deterioration mechanism of NCA power battery under various application conditions still need further investigation.For the issues mentioned above,the research work in this treatise is carried out from two aspects of fundamental research and industrial application.Firstly,the effect of grain boundaries on ion transportation mechanism of compact NCA secondary particles was investiaged.The NCA particles with reduced grain size(1 ?m)and grain boundary density were achieved via ball milling.It is found that the primitive and milled NCA presented similar high rate discharging performance.After that,graphene conductive additives were introduced into the NCA electrode to minimize the electric conductivity difference and extend the ionic transfer path,finding that primitive NCA with compact structure still displayed reasonable rate performance,whereas milled NCA with reduced grain size decreased severely.These results demonstrate that lithium ion can transport rapidly inside the grain boundary network of micro size NCA particle.The grain boundaries can establish an excellent ion transportantion network,achieving outstanding rate performance for compact high density NCA secondary particles.Seondly,the deterioration mechanism of NCA/graphite-SiO2 power battery was studied and the NCA/graphite-SiO2 power batteries were tested under various temperature(25 °C,45 °C,65 °C)and discharge rate(1 C,3 C)in a long term.Results show that the long-term cycling behavior is strongly affected by temperature,whereas negligibly affected by discharge rate.After long term high temperature cycling,the reversible lithium loss from NCA cathode is quite serious,but the the structure integrity and stability are not completely destoryed,and the lost capacity can be recoveried.However,graphite-SiOx lost most of its capacity and cannot recover,and the structure degraded seriously.During high temperature cycling,the side reaction between NCA and electrolyte is relatively mild,whereas there are much severe side reaction between graphite-SiOx and electrolyte.A lot of lithium alkyl carbonates,fluorophosphates and oxygenated species are observed at the surface of graphite-SiOx due to the decomposition of electrolyte at high temperature.These species are deposited on the separator and blocks the pores and hinder the ion transportation,resulting in a great increase of battery resistance and sudden loss of battery capacity.