Higher Capacity Lithium-Ion Batteries

RECHARGEABLE LITHIUM-ION batteries may get some extra zing, thanks to a new family of cathode materials that endow the batteries with greater charge storage capacity than the materials used in today's commercial lithium-ion batteries.

The new materials may lead to longer-lasting power sources for demanding applications such as cordless power tools and future plug-in hybrid electric vehicles, which feature a battery that can be recharged by connecting to the electric grid.

To improve the performance of lithium-ion batteries, electrochemists in several laboratories have been searching for high-energy-density substitutes for lithium cobalt oxide (LiCoO₂), the material commonly used as the battery cathode in portable electronic devices such as cell phones and laptop computers.

Battery developers discussed the merits of one group of candidates, a family of two-component, layered transition-metal oxides, on May 8 at the Electrochemical Society meeting in Chicago. "We find that these materials provide anomalously high charge capacity," reported Argonne National Laboratory's Christopher S. Johnson, who conducted the study with group leader and senior scientist Michael M. Thackeray and their coworkers. Johnson described the materials as disordered, complex composites that consist of nanocrystalline domains of Li₂MnO₃ and LiMO₂, in which M represents manganese, nickel, and cobalt.

In one example of anomalous performance, Johnson noted that a lithium cell containing a cathode composed of 70% Li₂MnO₃ and 30% LiMn_0.33Ni_0.33Co_0.33O₂ yielded an initial charge capacity in excess of 300 milliampere-hours per gram (mAh/g). That value gradually leveled off to approximately 261 mAh/g upon repeated charging and discharging. The theoretical capacity of that electrode, as calculated on the basis of the mass of the oxide incorporated in the cell, is only 242 mAh/g. For comparison, the theoretical charge capacity of LiCoO₂ is 274 mAh/g. However, as Johnson explained, LiCoO₂ is structurally unstable, which limits the usable charge capacity of the material to less than 150 mAh/g.

Offering an explanation for the "excess" charge capacity of the new electrode materials, Johnson proposed that during the initial charge cycle, as lithium and oxygen are extracted from the interior of the cathode, oxygen adsorbs weakly to the electrode surface. Then, during the discharge cycle, the adsorbed oxygen is reduced electrochemically, thereby providing a source of "extra" current. That explanation is based in part on mass spectrometry measurements that monitored oxygen evolution during charging.

The Argonne team also studied a cobalt-free material consisting of 30% Li₂MnO₃ and 70% LiMn_0.5Ni_0.5O₂ that yielded a charge capacity of 236 mAh/g (after the first charge-discharge cycle) and remained particularly stable over the course of a 40-cycle experiment.

The team plans to probe structural and mechanistic details of the new materials' behavior through the use of synchrotron-based X-ray absorption techniques and neutron diffraction methods.