Synthesis and rate performance of monolithic macroporous carbon electrodes for lithium-ion secondary batteries

Three-dimensionally ordered macroporous (3DOM) materials are composed of well-interconnected pore and wall structures with wall thicknesses of a few tens of nanometers. These characteristics can be applied to enhance the rate performance of lithium-ion secondary batteries. 3DOM monoliths of hard carbon have been synthesized via a resorcinol-formaldehyde sol-gel process using poly(methyl methacrylate) colloidal-crystal templates, and the rate performance of 3DOM carbon electrodes for lithium-ion secondary batteries has been evaluated. The advantages of monolithic 3DOM carbon electrodes are: 1) solid-state diffusion lengths for lithium ions of the order of a few tens of nanometers, 2) a large number of active sites for charge-transfer reactions because of the material's high surface area, 3) reasonable electrical conductivity of 3DOM carbon due to a well-interconnected wall structure, 4) high ionic conductivity of the electrolyte within the 3DOM carbon matrix, and 5) no need for a binder and/or a conducting agent. These factors lead to significantly improved rate performance compared to a similar but non-templated carbon electrode and compared to an electrode prepared from spherical carbon with binder. To increase the energy density of 3DOM carbon, tin oxide nanoparticles have been coated on the surface of 3DOM carbon by thermal decomposition of tin sulfate, because the specific capacity of tin oxide is larger than that of carbon. The initial specific capacity of SnO ₂-coated 3DOM carbon increases compared to that of 3DOM carbon, resulting in a higher energy density of the modified 3DOM carbon. However, the specific capacity decreases as cycling proceeds, apparently because lithium-tin alloy nanoparticles were detached from the carbon support by volume changes during charge-discharge processes. The rate performance of SnO ₂-coated 3DOM carbon is improved compared to 3DOM carbon.