Research On The Storage Aging Mechanisms And Life Extention Methods Of Lithium Ion Batteries At High State Of Charge

Lithium ion secondery battery with high capacity,exlent cycle and rate performances has been a nessarry power sourse in our daily life.Along with the development of technology,the battery lifetime,especially the calendar life,has attract much more attention.The storage life of lithium ion batteries is supposed to be longer than 10 years in some military uses.In this thesis,based on the research on the storage aging mechanism of lithium ion batteries at high state of charge?SOC?,a solid electrolyte interphase?SEI?depending model affecting the capacity loss rate was built.According to the model,the storage life of batteries was prolonged by adding electrolyte additives and forming functional layer on graphite anode.In the beginning,the aging phenomenons of the electrodes and interfaces after storage aging at high SOC were analyzed.The results showed that the capacity fading of active materiels in both electrode was exist,but the main cause of the capacity loss was the lithium loss in anode while the SEI growing was the main aging phenomenon.Based on the lithium loss mechanism,a SEI growing model was built and a simplied parameter estimation method was developed.Storage experiment of commercial batteries confirmed the precision of the model and life prediction showed that the model was more precise than the t^0.5 model.By the analysis of the anode lithium loss process,the electronic conductivity of SEI was thought to be the rate-depending step of lithium loss reaction during storage ageing at high SOC.To modify the electronic conductivity of SEI,electrolyte additives were used and their affections on the composition and thickness on the SEI were studied.Surface analysis showed that the addition of electrolyte additives could effciciently increase the thickness of SEI and decrease the content of components with high electronic conductivity.Storage experiment illustrated that electrolyte additive could prolong the storage life of batteries.Among the additives,adding 1%4,5-Dimethyl-1,3-dioxol-2-one expressed the most effective performance.The storage life at 55?was prolonged from92 to 337 days.Meanwhile,the results of LiBF₄ showed that the storage life would be shortened when the SEI was not well formed.The storage experiment of 18650 batteries showed that the storage life of batteries with effective additive would be prolonged from6 years to longer than 10 years.Forming functional layer on graphite or graphite anode would also retrain the electronic eonductivity of SEI.Polyvinyl alcohol?PVA?was coated on graphite by solution impregnation method.The PVA-Li generated during the intial charge-discharge would have appropriate Li⁺conductivity and lower electronic conductivity than normal SEI components.The storage expriments showed that the storage life was prolonged form92 days to 172 days with 4%content of PVA addition on anode.Meanwhile,LiPON layer with appropriate Li⁺conductivity and lower electronic conductivity was sputtered on graphite anode.The storage expriments showed that the storage life was prolonged from92 days to 319 days with a 800 nm thickness LiPON on graphite anode.The thesis illustrated that,by reducing the electron conductivity of SEI,the SEI growing would be restrained and the storage life of batteries would be prolonged.