Effect Of Isocyanate Based Electrolyte Additive On Performance Of Lithium Battery

With the rapid development of the lithium-ion batteries industry,Li Ni_xCo_yMn_1-x-yO₂（NCM）has attracted wide attention due to its advantage of high energy density.However,Li Ni_xCo_yMn_1-x-yO₂（NCM）possesses a series of problems such as transition metal ion dissolution in the processes of lithiation/delithiation,structural instability under high voltage,and violent chemical reactions between electrolyte and electrode materials.To address the above problems,some strategies have been proposed,such as materials coating,element doping modification and electrolyte modification,among which,electrolyte modification is essential to achieve high voltage and high energy density lithium-ion batteries.Introduction additive into the electrolyte,as one of the simplest and most effective approaches of electrolyte modification,has become a hot topics of current research.Nevertheless,there are still many deficiencies in the available electrolyte additive systems,therefore it is urgent to develop new additives.It has been proved that the isocyanate based additives can eliminate trace HF and H₂O in electrolytes and reduce the decomposition of electrolyte,improving battery performance.However,there are few researches on this kind of additives.In this paper,a new additive 4-fluorophenyl isocyanate（hereinafter referred to as 4-FBC）is obtained after introduction F atom to increase the polarity of isocyanate based molecules and screening with theoretical calculation of isocyanate based molecules.The electrochemical properties of medium nickel Li Ni_xCo_yMn_1-x-yO₂（0.5≤x≤0.6）,high nickel Li Ni_xCo_yMn_1-x-yO₂（0.6<x<0.9）and ultra-high nickel Li Ni_xCo_yMn_1-x-yO₂（x≥0.9）cathode materials have been investigated and the mechanism has been explored.The main research content of this paper is as follows.（1）The dissolved positive transition metal ions deposition onto the negative surface is considered to be one of the main reasons for the degradation of lithium-ion batteries performance.4-FBC has a lower lowest unoccupied molecular orbital（LUMO）and a higher occupied molecular orbital（HOMO）than the solvent in the base electrolyte（without 4-FBC additives）,which can preferentially gain and lose electrons during the first charge and discharge processes to generate a more stable interfacial film.According to this result,NCM523/Si@C soft pack full battery was assembled to consider the effects of 4-FBC on the dissolution of transition metal ions in cathode materials.The results showed that,compared with the basic electrolytic liquid phase,the addition of 1 wt%4-FBC reduced the content of Mn dissolving to the cathode electrode from 53.43 ppm to 18.38 ppm（a decrease of 65.60%）,which greatly stabilize the positive electrode materials structure and improve the cycle performance of the soft pack battery.At the same time,the introduction of 4-FBC can also enhance the high temperature stability of the electrolyte and increase the wettability of the electrolyte to the materials.（2）LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）is one of the mainstream products in the lithium battery cathode materials market,and has been favored by many consumers.However,its structural instability cycled under high voltage and severe interfacial reaction with electrolyte limit its development in the high energy density materials market.The effect of 4-FBC on the properties of NCM811 materials at high voltage is studied.The results show that the specific capacity at 4.5V（213.8 m Ah/g）is 15.88%higher than that at 4.3V（184.5 m Ah/g）.When cycled under 4.3V,2 wt%4-FBC improve the performance greatest.After 200 cycles,the capacity of 2 wt%4-FBC based battery only decreases by 25.74%,while the capacity of the basic electrolyte decreases by 36.83%.When the cut-off voltage is 4.5V,1 wt%4-FBC enhance the performance most.After 200 cycles,the capacity of 1 wt%4-FBC the based battery decreases by38.12%,while the capacity of the basic electrolyte decreases by 53.93%.High voltage will lead to rapid attenuation of battery capacity,but the introduction of 4-FBC can effectively alleviate the attenuation of battery capacity.TEM tests show that the addition of 4-FBC could form a thin and uniform CEI layer on the electrode surface,which inhibit the interface reaction between electrode materials and electrolyte at high voltage.Ex-site XRD results show that 4-FBC can stabilize the layered structure at high voltage,which is conducive to the improvement of battery performance.（3）LiNi_xCo_yMn_1-x-yO₂（x≥0.9）has a broad application prospect in the future lithium battery market due to its higher specific capacity.However,the highly active nickel in ultra-high nickel will induce serious decomposition of electrolyte,resulting in rapid capacity attenuation in the cycle process,which limits its large-scale application.In this paper,Li Ni_0.9Co_0.05Mn_0.05O₂（NCM955）is studied the effect of 4-FBC on the electrochemical performance of NCM955 materials.It was found that the capacity of the battery with basic electrolyte decays to 184.6 m Ah/g after 70 cycles.By contrast,1 wt%4-FBC based battery still shows high capacity of 193.0 m Ah/g,In implying the addition of additive 4-FBC can improve the cycle stability of NCM955materials.In summary,the new additive 4-FBC proposed in this research generates stable CEI film on the surface of the positive electrode materials,which can reduce the dissolution of transition metal ions,inhibit the decomposition of electrolyte,and stabilize the structure of the positive electrode materials.Through this research,we proposed a new idea of development of electrolyte additive,which is expected to promote the further development of high energy density lithium-ion batteries.