FeS2 is considered as a high capacity electrode materials based on a conversion reaction mechanism, and mainly applied in primary batteries and rechargeable thermal Li-FeS2 batteries for decades. However, the widely application of FeS2 in rechargeable battery is still hindered by the low efficiency and poor cycle performance caused by the generation of elemental Fe and S during fully discharge and charge. Herein, we demonstrate a facile but effective strategy to improve the energy efficiency and cycling stability of Li-FeS2 battery by rational design of a novel micro/nano-structured FeS2 electrode, which exhibits as a microsphere constructed by numerous nanosheets. By optimizing the electrochemical experiments conditions, a high energy efficiency of 80.4% and a high reversible capacity of 216.8 mAh g(-1) retained after 730 cycles at the optimized voltage window of 1.0-2.4 V. The conversion process at higher (> 2.4 V) potentials to form FeSy and S has been suppressed as demonstrated by multiple characterizations. And the Li+ insertion/extraction reaction between Li2FeS2 and Li-2-xFeS(2) and the conversion reaction at lower potential range coexist and are highly reversible. The high reversibility could be mainly ascribed to the micro/nano-structured feature of FeS2 and the optimization of the voltage window. We believe that, our work could further enhance the fundamental understanding of battery reaction mechanism in the rechargeable Li-FeS2 battery.