| In lithium batteries,binder is an inactive component of electrodes and only represent a low weight percentage in the electrode.In terms of using function,binder is a vital component of the electrode.The performance of binders play critical role in lithium batteries,such as coating properties during preparation of slurry,good dispersion for negative particles and conductive carbon,mechanical properties and interface adhesive property in the electrode,good wettability and suitable swelling in electrolyte,and good chemical stability and electrochemical stability and so on.Lithium polysulfide dissolution and shuttling,volumetric dilatation of Si,structural changes of layered materials(NCM)need to be suppressed,and poor conductivity of material LiFePO4 needs to be improved,thus,the binders should have different functional requirements for different anode and cathode active materials.In order to reduce the influence of solvent of oily binders on the environment and human body,water soluble binders are used.And LiFePO4 has greater water tolerance than other positive active materials.DOPA is a biodegradable green environmentfriendly material with wide sources and low cost.In this study,the electrochemical performance of the LiFePO4 cathodes with modified DOPA binders was explored.The main contents and results are as follows:(1)A series of OPDA binders were prepared by modifying DOPA with OP 10(alkylphenol ethoxylates)and naming it OPDA(0.5),OPDA(1),OPDA(2),OPDA(5).The LiFePO4 cathode with OPDA(1)binder had the highest capacity retention of 91.2%after 100 cycles at the current density of 1 C and greater rate performance than CMC at the low current density.However,the cathode with OPDA(1)binder had lower charge transfer impedance than it with OPDA(0.5)binder,67.50 Ω and 66.73 Ω,respectively,and lower lithium ion diffusion coefficient than it with OPDA(2)binder,1.31×10-13 cm2/s and 1.88×10-13 cm2/s,respectively.In addition,OPDA(1)and OPDA(0.5)had the smallest polarization of 0.40 V(2)In order to form a good conductive cross-linking network,PEDOT:PSS,a conductive material with poor bonding performance,was used to cross-link nonconductive DOPA with good adhesive property in different proportions to prepare a series of PEDA binders,respectively PEDA(11),PEDA(21),PEDA(51).In LiFePO4 electrode,PEDA(51)had the best cycle performance and the capacity retention rate was still 98.7%after 100 cycles at 1 C.The capacity retention of the LiFePO4 electrode with PEDA(51)(78.9%)was the highest when the current density increased gradually from 0.2 C to 5 C,and the capacity recovery(101.8%)was the highest when the current density decreased from 5 C to 0.2 C.And the capacity retention of the cathode with PEDA(51)binder was 98.4%after 100 cycles when the current density were 2 C and 5 C.The lithium ion diffusion coefficient of the cathode with PEDA(51)binder was 3.13×10-13 cm2/s,while that of CMC was only 8.26×10-15 cm2/s.However,the polarization of the cathode with PEDA(51)was 0.36 V,while that of PEDA(11)was only 0.31 V(3)DLiH,DCaH,PDLiH and PDCaH conductive and crosslinked water soluble binders were prepared by cross-linking metal cations Li+and Ca2+with pure DOPA and PEDA binders.Among them,the LiFePO4 electrode with PDCaH binder had better cycling performance and rate performance.At the current density of 1 C,the capacity retention was 97.3%after 100 cycles.The capacity retention of the LiFePO4 electrode with PDCaH binder is 77.6%when the current density increased from 0.2 C to 5 C,and the recovery rate was 100.6%when the current density decreased from 5 C to 0.2 C.After 100 cycles,the initial capacity retention was 97.0%at 2 C and 97.7%at 5 C,respectively.The lowest charge transfer impedance(Rct)and lithium ion diffusion coefficient of cathode with PDCaH binder were 60.81 Ω and 3.04×10-13 cm2/s,respectively.But the polarization of cathode with PDCaH was slightly lower than that of PDLiH(0.28V,0.36V). |
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