Study On Process And Properties Of Low-chromium Passivation On Aluminum And Nickel-plated Copper Tabs Of Power Batteries

Metal tabs are one of the important components of lithium-ion power batteries,as the link of energy exchange between inside and outside of the battery during charge and discharge,its performance determines the safety of batteries during the process of using and storaging.In order to cope with the complex environment in lithium-ion batteries,it is necessary to improve the corrosion resistance and the electrical conductivity,the surface treatment of metal tabs should be carried out before using.Now,the surface of metal tabs are treated by high concentration of chromate,however,the use of hexavalent chromium is prohibited or restricted at home and abroad.Therefore,it is urgent to develop a low toxicity or non-toxic passivation process for the surface treatment of metal tabs instead of chromate passivation.Low-chromium passivation technology has lower hexavalent chromium content in the passivation film and solution than that in traditional chromium passivation,and the properties of the film are similar to chromate passivation film,so it is one of the most promising technologies to replace chromate passivation in the short term.In this paper,the feasibility of using chromate,trivalent chromium,molybdate and zirconium-titanium passivation systems on the surface of aluminum tabs was investigated.The results shown that the chromate passivation system is the most suitable system for the surface treatment of lithium-ion power batterie tabs.The optimum process range of low-chromium passivation on the surface of aluminum tabs and nickel-plated copper tabs was determined by orthogonal test:Cr O₃:0.2～0.3 g/L,NH₄HF₂:0.02～0.04 g/L,Na NO₃:0.03 g/L,K₃[Fe（CN）₆]:0.2 g/L,p H:2.8～3.2,temperature:40～45℃,film forming time:2～3 mins.After passivation,dried at 50℃for 15-20 mins,and left it at room temperature for 24h to form a film.The test result shown that low-chromium passivation film could improve the properties of aluminum and nickel-plated copper tabs,especially the electrical conductivity,but the bonding force and corrosion resistance were slightly worse than that of chromium passivation film.The elemental composition and film thickness of the low-chromium passivation film on aluminum and nickel-plated copper tabs were studied by SEM and EDS.The results shown that the low-chromium passivation film was mainly composed of base metal elements and Cr,Fe,N,O and F,which are uniformly distributed on the surface of the metal substrate.The low-chromium passivation film on the surface of the tab was very thin,and the thickness was less than0.2μm.The film formation process of the low-chromium passivation film on the surface of tabs was analyzed by XPS and electrochemical test.It was concluded that the main composition of low-chromium passivation film was Cr（OH）₃·Cr（OH）Cr O₄·Cr₂O₃·Cr₄[Fe（CN）₆]₃·Fe₄[Fe（CN）₆]₃·MF_n·M（OH）_n·MO_n/2.At the same time,it could be known that potassium ferricyanide acted as a film-forming accelerator and a film-forming substance during the film formation process of the low chromium passivation film.In order to improve the stability of the passivation solution and further improve the performance of the passivation film,the additives which can improve the stability of the passivation solution were screened,and the effect of the additives on the performance of the low chromium passivation film on the surface of tabs was studied.Through screening,it was determined that 0.025 g/L of additive P as a stabilizer was added to the low-chromium passivation solution.According to the test results of corrosion resistance,conductivity and bonding force,the aluminum and nickel-plated copper tabs prepared from the low-chromium passivation solution containing additive P have reduced corrosion current by 195 times and 44 times,the charge transfer resistance increased by 35times and 41 times,the resistivity decreased by 22.10%and 16.67%,and the binding force of the aluminum and nickel-plated copper tabs before soaking in the electrolyte increased by 95.50%and 1063.15%.The contact angle test results show that the wetting angles of the aluminum and nickel-plated copper tabs prepared from the low chromium passivation solution containing the additive P reached 71.3°and 65.5°.The above test results show that the additive P can improve the corrosion resistance,conductivity,adhesion,hydrophobicity and electrolyte resistance of the low-chromium passivation film on the surface of the tab.Compared with the tab prepared from a commercial passivation solution in Japan,the performance of tabs that prepared from the low-chromium passivation solution containing additive P is better.