Electrochemical Performance of Full Concentration Gradient Cathode for Lithium-Ion Batteries

Worldwide effort is being devoted to development of lithium-ion batteries for consumer devices, plug-in hybrid electric vehicles (PHEVs) and all-electric vehicles (EV), which require higher energy, increased safety, and lower cost. Most commercial lithium-ion batteries use a layered LiCoO₂ as the cathode material, but LiCoO₂has a limited practical capacity of about 140 mAh/g. A high capacity cathode material with high packing density is thus being sought to improve the energy density of lithium-ion batteries with improved safety.

Recently, some novel design structures cathode material are reported, such as core-shell, gradient material. First, a co-precipitation process is used to synthesize core-shell or gradient structure spherical precursor, then using this precursor to prepare a same structure cathode material. Using this new process, it is easy to control the elements distribution inside of single particles. This concept make it is possible to achieve the high capacity, cycling life and safety for a cathode material. So the composition design is based on those, high Ni in bulk material for high capacity and high Mn on the surface for safety with certain Co are investigated.

In this study, we investigated a full concentration gradient material, and the average composition is closed to Li_1.0Ni_0.6Co_0.2Mn_0.2O₂. The material was synthesized by a solid-state reaction from full concentration gradient spherical precursor Ni_0.6Co_0.2Mn_0.2(OH)₂ and LiOH precursors. The full concentration gradient spherical precursor Ni_0.6Co_0.2Mn_0.2(OH)₂was prepared by a co-precipitation process. It exhibited uniform spherical particle shape with 15-mm diameter and high tap density of 2.7 g/cc. The electrochemical performance of the material was evaluated in a coin-cell. When the current density was 16 mA/g (C/10), the discharge capacities were about 191 mAh/g.

The significant improvement in tap density and the high capacity of this material makes it a potential cathode material for high energy battery application.

Acknowledgments

The authors acknowledge the financial support of the U.S. Department of Energy, FreedomCAR & Vehicle Technologies Office, under Contract No. DE-AC02-06CH11357.