Disposing of excessive decomposition and destructive intercalation of solvated Li+ in CNT-based flexible 3D Si anode of flexible battery

• A sieving-out strategy by replacing Li + with Na + cations in the electrolyte in the first cycle. • A sieving layer disposing of excessive decomposition and destructive intercalation of solvated Li + in CNT. • A significant increase in ICE at the first cycle is obtained for the as prepared 3D Si/CNT electrode. • Excellent cycle performance of the flexible 3D Si/CNT anode is due to the precise sieving layer. Carbon nanotubes (CNTs) suffer from low initial coulombic efficiency, large interface resistance and severe embrittlement when employed in flexible lithium-ion batteries (LIBs). We reveal in this paper that these issues are essentially caused by the excessive decomposition and destructive intercalation of solvated Li + due to the large electrochemical surface area and the defects in CNTs. We further demonstrate that Na + is much less solvated than Li + in carbonate-based electrolyte and the solvated Na + possesses poor affinity with graphitic carbon materials. Therefore, to simply replace solvated Li + with solvated Na + in the first cycle enables electrochemical deposition of an ultrathin NaF-containing solid electrolyte sieving layer on the CNTs, which isolates them from solvated Li + and suppresses both the extensive formation of thick bulky SEI (which causes large interfacial resistance) and the destructive intercalation of solvent molecules (which embrittles CNTs) when being employed in LIBs. Si/CNT electrode with such NaF-containing solid electrolyte sieving layer displays excellent flexibility and high specific capacity of 1042 mAh g −1 after 500 cycles, which is 3 times higher than those with copper foil or pristine CNTs as current collectors. This study have revealed the dramatic differences between solvated Na + and solvated Li + , and provided new insight into practical application of CNTs in flexible LIBs. A sieving-out strategy by replacing Li + with Na + cations in the electrolyte in the first cycle, which reveals dramatic difference of the dissolution and solvation behaviors between these two alkali cations in the same solvent enables practical application of CNT-based current collectors for flexible electrodes. And verify the above speculations by using a 3D freestanding Si/CNT network as an example.