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)O6 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.