Mn substitution controlled Li-diffusion in single crystalline nanotubular LiFePO4 high rate-capability cathodes: Experimental and theoretical studies

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Carbon-coated single crystalline nanotubular (NT) and nanoparticular (NP) LiFe1-xMnxPO4 (x = 0, 0.2, and 0.5) cathodes are fabricated to study the effect of compositional and microstructural changes on Li+ diffusion and electrochemical properties. Insight in to the compositional effect on Li+ diffusion is obtained from DFT facilitated climbing image nudged elastic band (CI-NEB) simulations. NT cathodes exhibit exceptionally good discharge capacities ∼60 (∼165) mAhg−1, ∼32 (∼110) mAhg−1 and ∼22 (∼82) mAhg−1 at 25C (1C) rate for x = 0, 0.2, 0.5, respectively. NP cathodes show capacity <5 mAhg−1 at 5C/2C-rate. The high-rate capability with two orders larger diffusion coefficient in nanotubes is due to improved access to Li+ intercalation channels. Whereas, nanoparticles are agglomerated, making b-axis inaccessible. While, Mn substitution affects the discharge capacity, it significantly improves capacity retention from ∼60% (x = 0) to ∼88% (x = 0.2) measured over 1000 cycles at ≥5C. From CI-NEB calculations we infer that Mn increases the activation barrier in its neighbourhood, thereby creating a steep potential hump (∼0.15 eV) for the Li+ diffusion. This largely impedes the ion transport and accounts for the steep loss of discharge capacity. We observe, while single crystalline nanotubular LiFePO4 are useful for high power density applications, Mn substitution in small quantities (x∼0.2) is ideal for cathodes with increased cyclic stability at high C-rates.