Researchers led by a team at UC Berkeley have demonstrated high-capacity manganese-rich cathodes for advanced lithium-ion batteries. The work, reported in the journal Nature, shows a possible design approach for cathode materials that move away from the current reliance on nickel and cobalt—which are limited resources and are associated with safety problems. The work was a collaboration between scientists at UC Berkeley, Berkeley Lab, Argonne National Lab, MIT and UC Santa Cruz.

… it is remarkable that almost all Li-ion cathode materials rely on only two transition metals, Ni and Co, which are the electroactive elements in the layered-rocksalt cathode materials in the Li(Ni,Mn,Co)O₂ chemical space (NMCs). On one end of this compositional spectrum, LiCoO₂ dominates the electronics sector, whereas Ni-rich materials are of interest for the automotive sector. Although Mn [manganese] has been used in a spinel cathode, and Fe in the LiFePO₄ olivine, these compounds suffer from low energy density.

Given the limits of energy density that can be achieved with the layered NMCs and the potential resource constraints on cobalt, it is of interest to develop high-capacity cathode materials based on other redox metals. In particular, transition metals that can exchange two electrons are of interest for their ability to create high capacity, similar to the Ni²+/Ni⁴⁺ couple in NMC cathodes. Low cost and low toxicity make the Mn²⁺/Mn⁴⁺ couple particularly desirable for designing high-performance Li-ion batteries that are also inexpensive and eco-friendly.

… The development of a high-performance Li-ion cathode based on the Mn²⁺/Mn⁴⁺ couple requires a material that forms in its discharged state, contains enough Mn²⁺ and Li⁺ ions to provide high capacity and preferably crystallizes in a dense structure, such as the layered or disordered-rocksalt structure, to maximize its volumetric energy density. Introducing Mn²⁺ in the dense layered or disordered materials has been difficult, as the Li excess (x > 1 in Li_xTM_2−xO₂, where TM is transition metal) required to achieve high practical capacity demands a high average transition metal valence.

In this work, we demonstrate that high capacity (>300 mAh g^-1) and energy density (about 1,000 Wh kg⁻¹) can be achieved in disordered-rocksalt Li-rich intercalation cathodes from Mn²⁺/Mn⁴⁺ double redox combined with a small amount of O redox.

—Lee et al.

In 2014, the lab of senior author Gerbrand Ceder, professor in the Department of Materials Science and Engineering at Berkeley, discovered a way that cathodes can maintain a high energy density without using the layerered structure of current cathodes—a concept called disordered rock salts. The new study shows how manganese can work within this concept.

To deal with the resource issue of cobalt, you have to go away from this layeredness in cathodes. Disordering cathodes has allowed us to play with a lot more of the periodic table.

—Gerbrand Ceder

The new strategy combines high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn²⁺/Mn⁴⁺ double redox couple into lithium-excess cathode materials. The resulting material—Li₂Mn_2/3Nb_1/3O₂F—uses the combined presence of high-valent Nb⁵⁺ and low-valent F^- to set up the charge balance to incorporate Mn as Mn²⁺, leading to a very high theoretical Mn-redox capacity of 270 mAh g^-1—more than twice that of a typical Mn-based Li-rich cathode material.

Electrochemical performance of Li2Mn2/3Nb1/3O2F. a–d,Voltage profiles and capacity retention of Li2Mn2/3Nb1/3O2F under various cycling conditions: a, 1.5–4.6 V, 20 mA g−1; b, 1.5–4.8 V, 20 mA g−1; c, 1.5–5.0 V, 20 mA g−1; and d, 1.5–5.0 V, 10 mA g−1. e, The first-cycle voltage profiles of Li2Mn2/3Nb1/3O2F when cycled between 1.5 V and 5.0 V at 10, 20, 40, 100, 200, 400 and 1,000 mA g−1. f, The first-cycle and second-charge profiles of Li2Mn2/3Nb1/3O2F under different voltage windows: 1.5–4.6 V, 1.5–4.8 V and 1.5–5.0 V. All tests were conducted at room temperature. Lee et al. Click to enlarge.

The team also developed a second new material, Li₂Mn_1/2Ti_1/2O₂F, in which Ti⁴⁺ is the high-valent cationic species. This material delivers high capacities similar to Li₂Mn_2/3Nb_1/3O₂F.

Double redox couples are tremendously important for the development of advanced cathodes. Indeed, today’s modern NMC-based layered cathodes all rely to some extent on the Ni²⁺/Ni⁴⁺ double redox. With Li₂Mn_2/3Nb_1/3O₂F and Li₂Mn_1/2Ti_1/2O₂F, we have demonstrated that combined fluorination and high-valent cation substitution can introduce Mn²⁺/Mn⁴⁺ redox in a Li-excess disordered-rocksalt structure, which leads to high-capacity Mn-based Li-excess cathodes (capacity of >300 mAh g⁻¹, energy density of around 1,000 Wh kg⁻¹) without an excessive use of O redox. This discovery is important, as our strategy can be widely applied to design high-performance Mn-based Li-excess cathodes that do not suffer from structural degradation triggered by extensive O redox.

—Lee et al.

Resources

• Jinhyuk Lee, Daniil A. Kitchaev, Deok-Hwang Kwon, Chang-Wook Lee, Joseph K. Papp, Yi-Sheng Liu, Zhengyan Lun, Raphaële J. Clément, Tan Shi, Bryan D. McCloskey, Jinghua Guo, Mahalingam Balasubramanian & Gerbrand Ceder (2018) “Reversible Mn²⁺/Mn⁴⁺ double redox in lithium-excess cathode materials” Nature volume 556, pages 185–190 doi: 10.1038/s41586-018-0015-4