Thermal Stability Of Nickel Cobalt Manganate Lithium Battery And It's Modification

In recent years,with the negative influence of global environmental pollution and sharply reducing oil resource,lithium ion battery has been intensively used in new energy vehicles,due to that lithium ion battery is character of high-energy density,long life and no memory effect.Li?Ni_xCo_yMn_z?O₂?NCM?materials have character of low cost,high capacity and good circle performance,leading toward the direction of the power battery.Because of the scarce Co resource,the manufacture of high-nickel NCM is mainly in the market.With increasing nickel content,the capacity of NCM is getting higher,but the safety is conversely lower.Thus,in this paper,this dissertation adopted a way combining thermal experimental test and theoretical analysis of thermogenesis mechanism.Then on the basis of that,the resolution of preparing multi-function safe electrolyte was made up,in order to improve the safety and electrochemical performance of the high nickel NCM battery.First of all,the thermal stability and thermal degradation kinetics of materials were analyzed by C80 thermal test.What main conclusions are the NCM?x=1/3,0.5,0.6,0.8?electrolyte system and their full batteries produce more heat and release heat at lower temperature with the increasing nickel content.What' more,it also turns out that high-nickel content NCM has the worst thermal stability in comparison with other positive materials.Correspondingly,the whole heat production process of their full batteries is divided by three stages:the decomposition of the SEI from negative materials,melting of membrane,the reactions between positive materials and electrolyte.Once the temperature has been risen in a short time,the phenomena of thermal runaway may occur to the Li(Ni_0.8Co_0.1Mn_0.1)O₂ full battery.At the same time,on the basis of experiment results,the chemical kinetic parameter values of the system is getting smaller with increasing nickel content,which manifests that high nickel content system tends to chemical reaction easily.In further step,the structure stability was investigated via X-ray diffraction?XRD?measurements and X-ray photoelectron spectroscope?XPS?.At the same time,different nickel content NCM half cells were prepared for electrochemical test.Combining with thermal results,it can come to a conclusion that high nickel content has bad effects on the structure stability and thermal stability of materials.In details,increasing nickel content materials not only tend to structure collapse and heat release during lithium intercalation and deintercalation due to structural disorder.In addition,the structure of materials has changed during thermal test,the decompositionproduction are mainly Li_1-xMn₂O₄?LiNiO₂?manganese oxides?cobalt oxides?nickel oxides according to XRD analysis;also the valence of the Ni element and Ni⁴⁺transformation ascend with increasing nickel content.Thus,the electrolyte is easily to be oxidized by Ni⁴⁺ accompanied by the generation of heat;the EIS figure turns out that the surface interface on the cathode materials is thicker with more nickel content,leading to higher inner impedance which is relevant with the accumulation of irreversible heat inside the battery and incremental risk of battery runaway.At length,aiming at the high nickel content NCM materials,the safety electrolyte was prepared respectively,consisted of basal commercial electrolyte?1.0 mol/L LiPF₆/EC/DEC I:1,w:w?,ethylene carbonate?VC?,triphenyl phosphate?TPP?and commercial electrolyte,propylene sulfite?PS?,triphenyl phosphate?TPP?respectively.The measures demonstrate that the safety electrolyte comprised of commercial electrolyte,PS and TPP can not match with the high nickel content NCM materials,while in the other safety electrolyte,two functional additives produce synergistic effect.On the one hand,TPP can improve the thermal stability of the high nickel NCM battery.On the other hand,VC as a kind of film-forming agent enhances the cycle performance of the high nickel NCM battery.