The lack of cathode materials with high energy density has become a bottleneck for the development of low-cost lithium-ion batteries (LIBs). Here, we develop three-dimensional mesoporous carbon network (3DMCN)-coated LiFePO₄/Fe nanospheres (3DMCN-LFP/Fe-NSs) for tackling this problem. This new nanocomposite cathode is synthesized by using thermal conversion of natural alginate (ALG) biomass. In the heat-treated process, an ALG hydrogel with an “egg-box” structure was transformed into unique 3DMCN, while some of the iron ions were reduced to iron metal. The 3DMCN and Fe metal coating not only reduces the anisotropy and grain size of LiFePO₄, but also enhances its electron conductivity and lithium ion diffusion coefficient. This makes this specially designed nanocomposite give a remarkable synergistic effect for both lithium storage and transfer kinetics. After 256 cycles the discharge capacity (173 mA h g⁻¹) was still higher than the theoretical capacity of LiFePO₄, and its capacity retention rate is 99%. Even at a high current rate of 10C, the discharge energy density is still 6.2 times that of commercial LiFePO₄. More importantly, this nanocomposite is created through a simple and cost-effective approach. This work also opens a new vista for applying renewable biomass conversion technology to develop superior LIBs.