All-solid flexible fiber-shaped lithium ion batteries

Abstract: We propose fabrication of the fiber-shaped lithium ion batteries assembled by twisting a cathode filament together with an anode filament. The cathode filament is fabricated by depositing a LiFePO4 (LFP)-composite layer onto a steel-filled polyester conductive thread (SPCT). As anode filaments, we propose several scenarios including a Li4Ti5O12 (LTO)-composite coated SPCT (dip-and-dry deposition), a tin-coated SPCT (PVD deposition) as well as a bare tin wire. An electrolyte composite layer consisting of LiPF6 and polyethylene oxide (PEO) is then deposited onto both the anode and cathode filament before the battery assembly. By twisting the cathode filament and anode filament together using a customized jig, the batteries are then assembled. The open-circuit voltage is found to be ~ 2.3 V for the battery using the LTO@SPCT anode, and ~3.3 V for the battery using the tin@SPCT anode and the tin wire anode. Charge-discharge tests are carried out at different C rates for each battery sample. Experimental results suggest that the LIBs using the LTO@SPCT anode, the tin@SPCT anode and the bare tin wire anode could achieve a specific capacity of ~64, ~67, and ~96 mAh/g, respectively, when charge-discharged at 0.5-C rate. The battery could retain well its capacity after 80 charge-discharge cycles. During operation of all the batteries reported in this paper, their coulombic efficiency remained above 80%. Among the advantages of the proposed LIB are light weight, ease of fabrication, high specific capacitance, high energy density, and good durability. Finally, employing cheap and commercially-available steel-filled polyester threads as a base material in our batteries, makes them potentially suitable for integration into wearables using various standard textile manufacturing techniques.