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 similar to 60 (similar to 165) mAhg(-1), similar to 32 (similar to 110) mAhg(-1) and similar to 22 (similar to 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 similar to 60% (x = 0) to similar 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 (similar to 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 similar to 0.2) is ideal for cathodes with increased cyclic stability at high C-rates.