A Northwestern University research team has found ways to stabilize a promising new high capacity cathode material—Li₄Mn₂O₅. An open-access paper on their work is published in the journal Science Advances.

Lithium-ion batteries shuttle lithium ions back and forth between the anode and the cathode. The cathode is made from a compound that comprises lithium ions, a transition metal and oxygen. The transition metal, typically cobalt, effectively stores and releases electrical energy when lithium ions move from the anode to the cathode and back. The capacity of the cathode is then limited by the number of electrons in the transition metal that can participate in the reaction.

There has been a significant research effort to improve the specific energy of LIBs for emerging applications such as electric vehicles. Conventional cathode materials used in LIBs are typically Li-containing transition metal (TM) oxides or phosphates (for example, LiCoO₂, LiFePO₄, and LiMn₂O₄) that can store (release) electrical energy via (de-)insertion of Li⁺ ions, accompanied by redox reactions of the TM cation. The specific capacity of the cathode is limited by the number of electrons per TM cation that can participate in the redox reaction. This exclusive dependence on the TM cations as the redox center in cathode materials typically used in LIBs has been challenged by the recent discovery of oxygen redox reactivity in Li-excess cathode materials.

… Recently, Freire et al. reported a new disordered rock salt–type Li-excess Li₄Mn₂O₅ cathode material with partially occupied cation and anion sites that exhibits a discharge capacity of 355 mA·hour g^-1 in the first cycle within an operating voltage window of 1.2 to 4.2 V versus Li/Li⁺. On subsequent cycling, the material is reported to preserve its rock salt structure with a discharge capacity of ~250 mA·hour g^-1.

… Here, we first give a detailed atomistic-level picture for the origin of the observed simultaneous anionic and cationic redox activity in this promising new high-capacity material.

—Yao et al.

The French research team first reported the large-capacity lithium-manganese-oxide compound in 2016. By replacing the traditional cobalt with less expensive manganese, the team developed a cheaper electrode with more than double the capacity. However, the battery’s performance degraded so significantly within the first two cycles that researchers did not consider it commercially viable. They also did not fully understand the chemical origin of the large capacity or the degradation.

After composing a detailed, atom-by-atom picture of the cathode, Christopher Wolverton, the Jerome B. Cohen Professor of Materials Science and Engineering in Northwestern’s McCormick School of Engineering, and his team discovered the reason behind the material’s high capacity: It forces oxygen to participate in the reaction process.

By using oxygen—in addition to the transition metal—to store and release electrical energy, the battery has a higher capacity to store and use more lithium.

Next, the Northwestern team turned its focus to stabilizing the battery in order to prevent its swift degradation.

Armed with the knowledge of the charging process, we used high-throughput computations to scan through the periodic table to find new ways to alloy this compound with other elements that could enhance the battery’s performance.

—Zhenpeng Yao, co-first author of the paper and a former Ph.D. student in Wolverton’s laboratory

The computations pinpointed two elements: chromium and vanadium. The team predicted that mixing either element with lithium-manganese-oxide will produce stable compounds that maintain the cathode’s unprecedented high capacity. Next, Wolverton and his collaborators will experimentally test these theoretical compounds in the laboratory.

Schematic illustration of the battery’s cathode structure in which lithium is red, oxygen is green, manganese is purple, chromium is dark blue and vanadium is light blue. Credit: Wolverton Research Group, Northwestern University. Click to enlarge.

This battery electrode has realized one of the highest-ever reported capacities for all transition-metal-oxide-based electrodes. It’s more than double the capacity of materials currently in your cell phone or laptop. This sort of high capacity would represent a large advancement to the goal of lithium-ion batteries for electric vehicles.

—Christopher Wolverton

This research was supported as a part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Science under award number DE-AC02-06CH11357.

Yao, currently a postdoctoral researcher at Harvard University, and Soo Kim, a postdoctoral researcher at the Massachusetts Institute of Technology, are both former members of Wolverton’s laboratory and served as the paper’s co-first authors.

Resources

• Zhenpeng Yao, Soo Kim, Jiangang He, Vinay I. Hegde and Chris Wolverton (2018) “Interplay of cation and anion redox in Li₄Mn₂O5 cathode material and prediction of improved Li₄(Mn,M)₂O₅ electrodes for Li-ion batteries” Science Advances Vol. 4, no. 5, eaao6754 doi: 10.1126/sciadv.aao6754