| Solid polymer electrolytes(hereafter abbreviated as SPEs)own several intrinsical superiorities over conventional non-aqueous liquid electrolytes,including no leakage,negligible volatility,and better designability,and so on.These merits are great benefits for effectively improving the safety of rechargeable lithium metal batteries(RLMBs,using metallic lithium as anode)under abuse and harsh conditions.Moreover,the utilization of SPEs tends to restrain the growth of lithium dendrites,thereby improving the cycling lifespan of RLMBs.Over the past 40 years,the research on SPEs has captured great attention in academics and industry.Generally,SPEs are formed by dissolving lithium salts into certain polymers containing Lewis-base groups in the main and/or side chains.To date,among various polymer matrices studied,poly(ethylene oxide)(PEO)is the most popular matrix for SPEs.This is majorly attributable to its several advantages over others as below:(1)the oxygen atom of ethylene oxide(–CH2CH2O–,EO)unit has a strong electron donating ability(Gutmann's donor number:22),which can effectively promote the dissolution and dissociation of various lithium salts;(2)the C–O bond in the EO unit has a high degrees of rotation freedoms,thus resulting in a low glass transition temperature(-64oC)and profound segmental mobility;and(3)it has a excellent thermal and chemical stabilities.However,PEO is a semi-crystalline polymer with a melting point of about65 oC,being above room temperature,and therefore its crystallization region(below melting point of 65 oC)severely impedes ion migrations,resulting in a low ionic conductivity of PEO-based SPEs in ambient temperature region(e.g.,ca.10-5 S cm-1for Li TFSI/PEO at 30 oC).In addition,the conformal contact between the PEO-based SPEs and electrode material is poor,resulting in high interfacial resistances.Therefore,it is of great significance to find alternative polymer matrices for the development of high-performing SPEs-based RLMBs.Bearing in mind the problems including low conductivities at ambient temperature and poor interfacial performances for the PEO-based SPEs,we conduct a systematical work on exploring the comb-like polymers containing oligo(ethylene oxide)side chains as polymer matrices of SPEs for RLMBs.Two kinds of acrylate,CH2=C(R)CO2–(CH2CH2O)n–CH3(n=9;hereafter abbreviated as H-MPEGA(R=H)and CH3-MPEGMA(R=CH3))are utilized as monomers,and are polymerized to the corresponding comb-like polymers containing oligo(ethylene oxide)side chains,P(H-MPEGA)and P(CH3-MPEGMA).The major difference in structure between the two comb-like polymers(i.e.,P(CH3-MPEGMA)and P(H-MPEGA))is that the acidic H atom of?-C of carbonyl group(i.e.,–CH2–CH–C(=O)–)is replaced by the–CH3 group(i.e.,–CH2–C(CH3)–C(=O)–).In this work,the impact of main chain structure of the two comb-like polymers containing oligo(ethylene oxide)side chains on the physical and electrochemical performances of SPEs are systematically investigated.The selection of the above two comb-like polymers containing oligo(ethylene oxide)side chain(–(CH2CH2O)nCH3),n=9)is conceived on the base of the following considerations:(1)the oligo(ethylene oxide)side chains with polar EO units can effectively promote the dissolution and dissociation of lithium salts;(2)the flexible oligo(ethylene oxide)as the side chain of the comb-like polymer is helpful to inhibit the crystallization of polymer matrices,thus enhancing ion transportation in the polymer matrices.The two comb-like polymers containing oligo(ethylene oxide)side chain,P(CH3-MPEGMA)and P(H-MPEGA),have been prepared by free radical polymerization in liquid solutions.Their corresponding lithium-ion conducting SPEs have been prepared by blending with lithium bis(trifluoromethanesulfonyl)imide(Li TFSI)or lithium bis(fluorosulfonyl)imide(Li FSI)in an appropriate molar ratio of[EO]/[Li+].With the traditional PEO-based SPEs(i.e.,Li TFSI/PEO and Li FSI/PEO)as baseline electrolytes,the effects of chemical structures of polymer matrices,the type and concentration of lithium salt,temperature and other factors on the fundamental physical and electrochemical properties of SPEs have been systematically investigated.The main results are summarized below:(1)Compared with conventional semi-crystalline PEO-based SPEs,the SPEs with the two comb-like P(CH3-MPEGMA)and P(H-MPEGA)are amorphous at room temperature and show lower Tg,which has the tendency as below:-55.2 oC(Li TFSI/P(H-MPEGA))<-52.1 oC(Li TFSI/P(CH3-MPEGMA))<-36.0 oC(Li TFSI/PEO)([EO]/[Li+]=20,[EO]/[Li+]by mole,the same as below).This suggests that(1)the comb-like polymers containing oligo(ethylene oxide)side chain can effectively inhibit the crystallization of PEO-based matrix,consequently increasing the content of amorphous zone,(2)compared with the H atom of P(H-MPEGA),the larger volume–CH3 in the backbone structure of P(CH3-MPEGMA)maybe reduce the the mobility of polymer backbones,and thus impeding ion migrations,causing a higher Tg.(2)Compared with conventional semi-crystalline PEO-based SPEs,the SPEs based on the two as-prepated comb-like polymers,the ionic conductivities of P(CH3-MPEGMA)and P(H-MPEGA)outweight approximately one order of magnitude in room temperature region for the same salt and salt concentration(e.g.,at 30 oC and[EO]/[Li+]=20 by mole),3.69×10-5 S cm-1(Li TFSI/P(CH3-MPEGMA))vs.7.93×10-5 S cm-1(Li TFSI/P(H-MPEGA)vs.9.27×10-6 S cm-1(Li TFSI/PEO)).This suggests that the comb-like polymers containing oligo(ethylene oxide)side chain can effectively inhibit the crystallization of EO units,consequently improving the ionic conductivities of SPE at room temperature,which is consistent with the above results of that the comb-like SPEs are amorphous state,and the traditional based on PEO of SPEs is semi-crystalline state at room temperature.(3)For both P(CH3-MPEGMA)and P(H-MPEGA)SPEs,replacing Li TFSI with Li FSI has negligible effect on the ionic conductivity(e.g.,30 oC,3.69×10-5 S cm-1(Li TFSI/P(CH3-MPEGMA),[EO]/[Li+]=20)vs.2.52×10-5 S cm-1(Li FSI/P(CH3-MPEGMA),[EO]/[Li+]=20;7.93×10-5 S cm-1(Li TFSI/P(H-MPEGA),[EO]/[Li+]=20)vs.6.76×10-5 S cm-1(Li FSI/P(P(H-MPEGA),[EO]/[Li+]=20).(4)In the three kinds of SPEs,including containing P(CH3-MPEGMA),P(H-MPEGA)and PEO,the SPEs with P(CH3-MPEGMA)and P(H-MPEGA)have more excellent chemical and electrochemical stabilities.Compared with the P(H-MPEGA)-based SPEs,the SPEs with P(CH3-MPEGMA)have more excellent chemical and electrochemical stabilities.At 80 oC and 0.1 m A cm-2,the Li0 symmetric cell using Li X/P(CH3-MPEGMA)and Li X/P(H-MPEGA)electrolyte(X=TFSI,FSI;[EO]/[Li+]=20)can cycle continuously more than 1000 h.As for the semi-crystalline PEO electrolyte systems,Li FSI/PEO and Li TFSI/PEO([EO]/[Li+]=20),they can cycle only253 h and 55 h respectively and have a short circuit.Noteworthy is that Li X/P(CH3-MPEGMA)(X=TFSI,FSI;[EO]/[Li+]=20)could sustain 1720 h.For another,in the Li0 symmetric battery with electrolyte Li X/P(H-MPEGA)(X=TFSI,FSI;[EO]/[Li+]=20),the polarization voltage gradually increases along with the cycle,which shows cumulative amplification effect and even short circuit.This indicates that the chemical structures of polymer matrices have a remarkable effect on the interface stability on the Li0|SPEs.It seems reasonable that the slightly acidic H atom of?-C of carbonyl group(i.e.,–CH2–CH–C(=O)–)in P(H-MPEGA)would have been reduced on the surface of lithium metal electrode,forming active species which could induce further degradations of other electrolyte components,and thereby reducing the chemical and electrochemical stabilities of Li0|SPE interphases.For the same polymer matrix,Li FSI can significantly improve the interfacial stability of between the SPEs and lithium metal electrode,and further extending the cycle life of RLMBs.However,it can't eliminate the side reaction caused by the acidic H atom,which suggests that the molecular structure in the polymer backbone has a significant influence on the interfacial stability.In summary,the ionic conductivity of the two comb-like polymer containing oligo(ethylene oxide)side chains are significantly improved at room temperature.The ionic conductivity of SPEs based-P(H-MPEGA)is slightly higher than that of SPEs based-P(CH3-MPEGMA),but the latter have more excellent chemical and electrochemical stabilities.This is mainly due to the fact that the active H of?-C conjointed the carbonyl group(C=O)in the P(H-MPEGA)is replaced by–CH3,which improves the oxidation resistance stability of the polymer structure.Therefore,P(CH3-MPEGMA)as SPEs matrix shows more excellent comprehensive properties,which indicates that the active H of the?-C conjointed the carbonyl group(C=O)should be replaced by a suitable substituent in the design and synthesis of a polymer matrix with acrylate and its derivatives as monomers.The results of this work is bebeficial to design and select polymer matrix with more excellent physical and chemical properties,and improve the performance of RLMBs at room temperature. |
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