We have synthesized spinel type cobalt-doped LiMn(2)O(4) (LiMn(2-y)Co(y)O(4), 0</=y</=0.367), a cathode material for a lithium-ion battery, with hierarchical sponge structures via the cobalt-doped MnCO(3) (Mn(1-x)Co(x)CO(3), 0</=x</=0.204) formed in an agar gel matrix. Biomimetic crystal growth in the gel matrix facilitates the generation of both an homogeneous solid solution and the hierarchical structures under ambient condition. The controlled composition and the hierarchical structure of the cobalt-doped MnCO(3) precursor played an important role in the formation of the cobalt-doped LiMn(2)O(4). The charge-discharge reversible stability of the resultant LiMn(1.947)Co(0.053)O(4) was improved to ca. 12 % loss of the discharge capacity after 100 cycles, while pure LiMn(2)O(4) showed 24 % loss of the discharge capacity after 100 cycles. The parallel control of the hierarchical structure and the composition in the precursor material through a biomimetic approach, promises the development of functional materials under mild conditions.