Investigation Of Polyacrylic Acid-based Binders For Silicon Anodes In Lithium-ion Batteries

As its high theoretical mass specific capacity of 4200 mAh·g^-1,low lithiation/delithiation potential,abundant in earth and safe,silicon is one of the most promising anode materials for next generation high energy density lithium-ion batteries.Binder,as one of the important components of the negative electrode of battery,affects the electrochemical performance of the electrode.There are still some problems in commercial binders,such as weak bonding strength,low energy density,and unsatisfactory cycle stability and cycle life.Therefore,we have designed two new water-soluble polyacrylic acid?PAA?based binders to improve the electrochemical performance of lithium-ion batteries.The main contents are as follows:?1?Polymer binder?PAA-GA?with excellent mechanical and self-healing properties was synthesized by a one-step coupling method of commercial PAA with glycinamide,which contains double amide bonds.The physical and chemical properties and electrochemical properties of PAA and PAA-GA binders were investigated.The swelling and rheological tests showed that the swelling ratio of PAA-GA in electrolyte was only 4.52%,and PAA-GA was self-healing.The 180^o peeling tests and nano-indentation tests illustrated that the average bonding strength of Si/PAA-GA electrode was 1.3N,and its mechanical properties were better than Si/PAA electrode.The electrochemical tests showed that the reversible specific capacity and cycle stability of Si/PAA-GA electrode were significantly improved.The reversible specific capacity of Si/PAA-GA electrode was 2597 mAh·g^-1 and the capacity retention was81.0%after 285 cycles at a current density of 0.2C.Notably,the mass specific capacity was around 991 mAh·g^-1 even at a high current density of 5C.It demonstrates that double hydrogen bonds in the PAA-GA are beneficial to maintain the stability of the electrode structure,alleviate the volume expansion of silicon particles,increase the reversible specific capacity of the electrode,and improve the long-term cycle stability of lithium-ion batteries.?2?The commercial waterborne polyurethane?PU?with non-pollution,good film-forming and toughness,and strong adhesion was blended with PAA-GA to obtain a binder with excellent mechanical properties and toughness.The physical and chemical properties and electrochemical properties of PAA,PAA-GA and complex binder were studied.The mass ratio of PU versus PAA-GA was 0.5:9.5(PU_0.5?PAA-GA?_9.5).The 180^o peeling tests showed that the average bonding force of Si/PU_0.5?PAA-GA?_9.5 electrode was about 0.35N·mm^-1,and the bonding strength was the highest.The swelling tests demonstrated that the swelling ratio of PU_0.5?PAA-GA?_9.5 in electrolyte was 12.55%,which slightly higher than PAA-GA and significantly lower than PAA.Nano-indentation tests showed that Si/PU_0.5?PAA-GA?_9.5 electrode had the highest hardness and elastic modulus values,indicting better mechanical properties.The electrochemical tests showed that the initial coulombic efficiency of Si/PU_0.5?PAA-GA?_9.5 electrode was 84.5%,which was higher than Si/PAA and Si/PAA-GA electrodes.The specific capacity was still 1699 mAh·g^-1 and the capacity retention was 61.2%at a current density of 0.5C after 500 cycles.PU has high elasticity and high tensile properties,which can effectively adapt to the volume expansion of silicon particles,illustrating that proper toughness of binder can promote the electrochemical performance of lithium-ion batteries.