| All-solid-state Li metal batteries（ASSBs）have been considered to be the holy grail of the next generation of energy storage devices,owing to the superiority of safety and high energy density.Notice that the ions transfer behavior of composite polymer electrolytes（CPEs）has an important impact on the performance of ASSBs,which is affected by various factors such as external physical fields（heating,pressing,etc.）and their own composition（polymer matrix,lithium salts,and inorganic fillers）.In this work,the PCL/LiTFSI solid state electrolyte（SSE）was prepared by adding LiTFSI into environmentally friendly polycaprolactone（PCL）.The effects of thermal-mechanical fields and inorganic fillers（Al2O3）on the ion transport mechanism of PCL based SSEs were studied using various characterization techniques（e.g.,polarized light microscopy,AC impedance spectroscopy,and solid-state nuclear magnetic resonance technology）.Besides,the interfacial ions transport behavior of CPEs was investigated systematically.The main research results are as follows:（1）The effect of LiTFSI on the ionic conductivity of PCL/LiTFSI SSE was studied,and the results showed that maximum ionic conductivity(8.88×10-6 S·cm-1)was obtained when the content of LiTFSI arrived at 20 wt%,due to the significantly reduce of the crystallinity for PCL.Then,the effects of different temperatures（16℃,25℃,30℃）at 10 MPa on the ions transport mechanism of PCL/LiTFSI（20%）were studied.Under the thermal-mechanical fields,the crystallinity of PCL decreases,whereas the ionic conductivity increases to the maximum and then gradually decreases.The spin-lattice relaxation time（T1）increases from 609.3 ms at 24 h to 618.9 ms at 200 h.The spin-spin relaxation time（T2）decreases from 2.00 ms at 24 h to 1.73 ms at 200 h.The results indicates that the rigidity of the PCL segments increases and the motion speed of the polymer chains decreases,which hinders the transport of lithium ions,leading to a decrease in ionic conductivity.Furthermore,the effects of different pressures（5 MPa,10MPa,15 MPa）at 25℃on the ions transport mechanism of PCL/LiTFSI（20%）were studied.The ionic conductivity at 10 MPa increases to a maximum of 2.93×10-5 S·cm-1,and then decreases,while the values are still much larger than that at 5 MPa(2.93×10-5S·cm-1).The T2 increases from 1.50 ms at 5 MPa to 1.73 ms at 10 MPa with the holding time of 200 h,indicating that the proton spacing of PCL is increased,and the mobility of local chain segment in polymer matrix is enhanced.As a result,the ionic conductivity at10 MPa is much higher than that at 5 MPa.The ionic conductivity at 15 MPa is lower than that at 10 MPa,which can be attributed to the facts that the polymer chains are distorted and the free volume is compressed at 15 MPa.The change of polymer matrix finally limited the lithium ions transport through the polymer chains,and thus decreasing the ionic conductivity.（2）Different sized（30,100,200,400 nm）nano-Al2O3 particles were added into the PCL/LiTFSI matrix to prepare the PCL/LiTFSI-Al2O3 CPEs.The spherulite size of PCL decreases with the increase of nano Al2O3 particle size,while the crystallinity of PCL does not change significantly.Polymer spherulites are formed by stacking polymer chips.The spherulite size changes significantly,while the content of crystal zone changes little,indicating that the chain folding structure has been changed.This change makes CPE obtain the highest ionic conductivity(4.87×10-5 S·cm-1)and lithium ions transfer number（0.65）when the nano Al2O3 particle size is 30 nm（CPE-30）.Beneficial from the high ionic conductivity and ions transfer number,the LiFePO4//CPEs-30//Li battery obtained high capacity-retention of 99.2%after 300 cycles at 1.0 C,and the Li//CPEs-30//Li symmetrical battery worked stably for 500 h at 0.1 m A·cm-2.The T1 and T2 of CPEs-30and CPEs-200 were studied by solid state NMR test.It is found that the T1 increases from543.9 ms in CPEs-30 to 577.5 ms in CPEs-200,since the T2 decreases from 2.669 ms in CPEs-30 to 2.051 ms in CPEs-200,indicating a faster-moving segment and a looser chain folding structure in CPEs-30.Therefore,the CPEs-30 obtained excellent electrochemical properties and battery performances.（3）The interfacial ions transport behavior in ASSBs was studied when nano-Al2O3particles were selected as inert fillers.It is found that the CPEs-A20 obtained the highest ionic conductivity(1.94×10-5 S·cm-1)and lithium ions transfer number（0.65）,in which the content of nano-Al2O3 particles is 20 wt%.This high lithium ions transfer number indicates a uniform plating and striping process of Li+ions.The LFP//CPEs-A20//Li and LFP//CPEs-A30//Li batteries showed excellent cycle stability with capacity retentions of98.1%and 100%at 1.0 C,respectively.The excellent cycle stability confirms that a stable interface layer is formed during cycles.Li//CPEs-A20//Li symmetric battery can be operated stably for 1400 h at 0.05 m A·cm-2,implies that CPEs-Al2O3 electrolyte is stable against Li metal.The surface morphology of Li metal after cycling was characterized by SEM.The lithium ions plate uniformly on the surface of Li metal without any Li dendrites.The element composition and valence state of Li metal surface were studied by XPS.The results showed that with the addition of nano-Al2O3,an in-situ Li-Al-O interface layer is formed during battery cycling,which can improve the battery cycle stability and inhibit the formation of Li dendrites.The present studies indicate that under the thermal-mechanical coupling fields,PCL obtains faster local migration of polymer chains and higher ionic conductivity.With the addition of small sized Al2O3 particles,PCL obtains looser chain folding structure and faster migration of lithium ions.In addition,an in-situ Li-Al-O interface layer forms during cycling in ASSBs,which stabilizes the ions transport and inhibits lithium dendrites.The above research provides new ideas for regulating the ions transport performance of CPEs,improving the stability of the electrode/electrolyte interface,and inhibiting the growth of Li dendrites. |
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