Structural and electrochemical properties of LiMn0.6Fe0.4PO4 as a cathode material for flexible lithium-ion batteries and self-charging power pack

Cathode materials with low-cost, environment-friendly, high energy density are critical for lithium-ion batteries (LIBs). Here, the effects of Fe doping on the structure of LiMnPO4 (LMP) are investigated by neutron powder diffraction (NPD). The prepared LiMn0.6Fe0.4PO4/carbon (LMFP/C) shows a higher specific capacity of 90 mAh g(-1) at a current density of 1 C, which is about 5 times of that of LiMnPO4/C. It also shows excellent cycling performance for 1000 cycles. The improved electrochemical performance is ascribed to the higher octahedral distortion of (Mn, Fe)O-6 and an easiness for Li diffusion due to much less anisotropic ellipsoids for Li in LMFP. We further fabricated a flexible LIB with LMFP/C cathode and an in-situ polymerized electrolyte, which exhibits excellent flexibility and cyclability. The cell shows no obvious performance degradation after bending for 300 times. Moreover, a flexible triboelectric nanogenerator (TENG) was coupled with the flexible cell to form a wearable self-charging power pack. The TENG can harvest mechanical energy and convert it into electrical form, charge the battery to supply energy for a flexible electrochromic membrane. The open circuit voltage (V-OC) of the flexible LIB increases from 3.32 V to 3.51 V in about 20 min through daily human motion.