Monday, April 16, 2018

Researchers Demonstrate High-Capacity Mn-rich Li-ion Cathodes; a Design Pathway Away from Cobalt and Nickel

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. (Credit: Lee et al.)  Click to Enlarge.
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 LixTM2−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.

Read more at Researchers Demonstrate High-Capacity Mn-rich Li-ion Cathodes; a Design Pathway Away from Cobalt and Nickel

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