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

Highlights: • The effects of Fe doping on the crystal structure of LMP were systematically investigated by NPD. • A flexible LIB was fabricated with LiMn_0.6Fe_0.4PO₄ cathode and PMMA-PI electrolyte. • A flexible TENG was integrated with the flexible LIB to demonstrate a prototype of wearable 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 LiMn_0.6Fe_0.4PO₄/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.