Control over porosity and exposed highly reactive facets is challenging in the area of materials science. Materials with high porosity and reactivity of exposed facets are favorable candidates in catalysis and energy storage. Here we demonstrate a facile template-free route to synthesize nanoporous LiMn₂O₄ nanosheets composed of single-crystalline LiMn₂O₄ nanorods with exposed {111} facets via an in situ lithiation of ultrathin MnO₂ nanosheets. Nearly 100% of the initial capacity can be retained after 500 cycles at a 1C discharge rate using the nanoporous nanosheets as a cathode, whereas at a discharge rate of 25C, the capacity retention is about 86% of the initial capacity after 500 cycles. The durable cycling performance and high capacity retention can be attributed to the intrinsic highly oriented crystallinity, two-dimensional (2D) nanoporosity and exposed {111} facet of the nanosheet cathode.