Nanostructured LiMn1-xFePO4 (0 <= x <= 1) with in-situ carbon coating are synthesized via a simple and facile polymer-assisted mechanical activation (PAMA). Effectiveness of this synthesis route has been evaluated using two types of liquid polymers, polyethylene glycol (PEG 400) and polypropylene glycol (PPG 2000). In particular, LiMn1-xFexPO4/C nanoparticles (similar to 60 nm) with 1-2 nm thin carbon coating have been successfully achieved with the assistance of PPG 2000. The LiMn1-xFexPO4/C composites with only 1.8 wt% carbon content have demonstrated high specific capacities and superior discharge rate capability. Mn rich LiMn1-xFexPO4 (x = 0.25) can still deliver 157 mAh g(-1) at 0.1C, about 120 mAh g(-1) at 10C and about 90 rnAh g(-1) at 20C rate. All of the LiMn(1-x)FexPO(4)/C composites also reveal good cycleability at both room temperature (25 degrees C) and elevated temperature (55 degrees C). Furthermore, the effects of Fe content on the PAMA synthesized LiMn1-xFexPO4/C composites have been systemically investigated with various compositions of x = 0, 0.25, 0.5, 0.75 and 1, toward the optimized battery performances. Among all the investigated compositions, LiMn0.75Fe0 25PO4/C provides the highest energy and power density at low current rates, while LiMn0.5Fe0.5PO4/C may be the ideal option for practical large-scale applications, from a comprehensive view of power and energy density, fast charge capability and durability. (C) 2013 The Electrochemical Society. All rights reserved.