| With the development of the demand of practical application,the energy density of current lithium-ion batteries has been difficult to meet the demand.It is urgent to develop active materials with high energy density.Silicon based anode material has high theoretical energy density,but its drastic volume change in the process of charge and discharge leads to short cycle life,which limits its application.Therefore,from the perspective of electrode binder,considering with the characteristics of large volume expansion of silicon-based negative electrode material and its peculiar surface properties,the molecular structure of the binder is redesigned,and a series of copolymerized modified binders are prepared.The influence of molecular structure on the properties of the binder and corresponding electrode is studied,and the molecular structure design scheme is subsequently adjusted accordingly.After many explorations and adjustments,a binder which can effectively prolong the cycle life of silicon-based negative electrode is obtained.The specific research contents are as follows:（1）Polyamide imide has high mechanical strength but poor flexibility,which cannot match the huge volume expansion of silicon cathode material.In order to improve its flexibility,a flexible chain modified polyamide imide binder is prepared by copolymerizing short chain polyethylene glycol with the polyamide imide,and the effects of polyethylene glycol chains with different molecular weights on mechanical properties of binders and the electrochemical properties of the corresponding silicon negative electrodes are studied.It is found that when the molecular weight of polyethylene glycol is 200 g mol-1,the binder exhibits the best comprehensive properties,and its elongation at break is 7.0%,which is 26%higher than the binder before modification.The modified binder can inhibit the cracking of the electrode and improve the cycle performance of the silicon negative electrode.After 200 cycles at 0.2 C,it can still maintain a reversible specific capacity of up to 2235.5 m Ah g-1,and the capacity retention rate is 74.8%,which is significantly improved compared with the binder before modification.（2）The monomer MDOPA which contains catechol group is synthesized from the low-cost levodopa,and a series of polymers PMDOPA with different molecular weight and containing high content of catechol groups is polymerized and used as the binder silicon-based negative electrode.The effects of molecular weight and catechol group on the bonding properties of the binder and the electrochemical properties of the corresponding silicon negative electrode are studied.With the increase of molecular weight,the adhesive force of PMDOPA also increases.The peel strength of the electrode of PMDOPA-20 binder,whose weight average molecular weight is 20.8×104 g mol-1,can reach 1.08 N cm-1,which is 2.77 times high as that of the polyacrylic acid（PAA）electrode.The extremely high adhesive force of PMDOPA-20can inhibit the electrode cracking and prolong the cycle life of silicon negative electrode.The cycling life of PMDOPA electrode at 0.2 C can be up to 665 times,far exceeding the cycle life of 375 times of the PAA electrode.In addition,the rate performance of PMDOPA-20 electrode is also better than that of PAA electrode.（3）A low-cost polymer PMAA-AM-MDOPA which contains catechol groups is prepared by copolymerizing methacrylic acid,acrylamide and MDOPA.The copolymerization modification of PAA binder is preliminarily explored.It is found that the addition of MDOPA will reduce the molecular weight of the polymer but can improve its bonding strength.Under a strict addition amount of binder,the peel strength of the silicon-based negative electrode prepared with PMAA-AM-MDOPA-10 binder can still reach up to 2.7 N cm-1,which is 69%higher than that of PMAA-AM,and the cycle performance of the corresponding electrode is also improved by 11%.However,due to the low molecular weight of the prepared PMAA-AM-MDOPA,the performance of PMAA-AM-MDOPA still has a certain gap compared with the high molecular weight PAA.Therefore,the preparation method of the binder needs to be optimized to improve the molecular weight of the polymer.（4）A flexible and thermally crosslink-able copolymer PAA-HEA-NMA（PAHN）is prepared by copolymerization with acrylic acid,hydroxyethyl acrylate and N-hydroxy methyl acrylamide.Its viscosity is improved by optimizing and adjusting the polymerization process.The binder has good water solubility and can be crosslinked after heating.The cross-linked silicon-based negative electrode thus can be prepared without affecting the preparation process of electrode slurry.The electrode exhibits a high peel strength up to 2.33 N cm-1 even under a strict addition amount of binder.At the same time,the binder still has good flexibility after crosslinking.In the nano indentation test,when the load is 100 m N,the indentation depth of PAHN3 and PAHN5 binder is 3118 nm and 2479 nm respectively,which is 60%and 28%higher than that of PAA binder.This good flexibility and thermal crosslinking characteristics can effectively maintain the stability of electrode structure and inhibit the growth of SEI film,thus improving the capacity retention rate and cycle life of the commercial silicon-based negative active material in the long-term cycling.Before the capacity attenuated to 80%,the cycle life of PAHN3 and PAHN5 electrodes are 155 and 161 times,which is 74%and 81%higher than that of PAA electrode respectively.（5）Finally,based on the previous experimental results and design ideas,a quaternary copolymer PAA-HEA-MDOPA-NMA（PAHMN）with good flexibility,high bonding strength and thermal crosslinking ability is designed and prepared.These characteristics can effectively inhibit the shedding of active material in the silicon-based negative electrode during long-term cycling,which contribution to excellent cycle performance.The electrode can cycle 193 times before the specific capacity decays to 80%of the nominal specific capacity,which is 117%and20%higher than the PAA binder and PAHN5 binder respectively. |
PDF Full Text Request