Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

Abstract

The discovery of facile Li transport in disordered, Li-excess rocksalt materials has opened a vast new chemical space for the development of high energy density, low cost Li-ion cathodes. We develop a strategy for obtaining optimized compositions within this class of materials, exhibiting high capacity and energy density as well as good reversibility, by using a combination of low-valence transition metal redox and a high-valence redox active charge compensator, as well as fluorine substitution for oxygen.Furthermore, we identify a new constraint on high-performance compositions by demonstrating the necessity of excess Li capacity as a means of counteracting high-voltage tetrahedral Li formation, Li-binding by fluorine and the associated irreversibility. Specifically, we demonstrate that 10-12% of Li capacity is lost due to tetrahedral Li formation, and 0.4-0.8 Li per F dopant is made inaccessible at moderate voltages due to Li-F binding. We demonstrate the success of this strategy by realizing a series of high-performance disordered oxyfluoride cathode materials based on Mn 2+/4+ and V 4+/5+ redox. Broader contextElectrochemical energy storage is a key component of modern energy systems, providing portable power to devices ranging from personal electronics to electric vehicles, and enabling grid-scale mitigation of the fluctuating availability of renewable energy sources. The central role of energy storage systems motivates the search for, and optimization of, low-cost, environmentally-benign materials which can reversibly provide high energy density. Cathode materials, which are presently the performance-limiting components in state-of-the-art Li-ion batteries, have been traditionally limited to Ni and Co-based layered oxides. The recent discovery of Li-percolation in disordered rocksalts has expanded the structural space of materials which may serve as a Li-ion electrode, while the demonstration of Mn 2+/4+ cathode electrochemistry and disordered rocksalt fluorination has opened to door to the use of cheap, environmentally-friendly chemistries. Here, we build on these demonstrations to derive optimization rules for designing disordered rocksalt oxyfluoride cathodes and provide an example of an optimized series of cathode materials.

Improved Cycling Performance of Li‐Excess Cation‐Disordered Cathode Materials upon Fluorine Substitution

“…[10,19] Within the LiF-LiMnO 2 -LiNbO 3 system, the cluster expansion consisting of pair interactions up to 7.1 Å, triplet interactions up to 4.0 Å, and quadruplet interactions up to 4.0 Å based on a primitive rock salt lattice was constructed using DFT-calculated energies. [10,19] Within the LiF-LiMnO 2 -LiNbO 3 system, the cluster expansion consisting of pair interactions up to 7.1 Å, triplet interactions up to 4.0 Å, and quadruplet interactions up to 4.0 Å based on a primitive rock salt lattice was constructed using DFT-calculated energies.…”

“…d. Recent work [10] has demonstrated that the strong LiF bond limits the amount of Li that can be removed from the coordination shell of a F − anion. d. Recent work [10] has demonstrated that the strong LiF bond limits the amount of Li that can be removed from the coordination shell of a F − anion.…”

“…A cluster expansion approach [10,19,32] was used to generate 20 snapshots of atomic arrangements (distributions of Li, Mn, Nb on the cation sites, and distributions of O and F on the anion sites of the rock salt structure, for both LMNO and LMF10). A cluster expansion approach [10,19,32] was used to generate 20 snapshots of atomic arrangements (distributions of Li, Mn, Nb on the cation sites, and distributions of O and F on the anion sites of the rock salt structure, for both LMNO and LMF10).…”

“…Fe-Nb-and Fe-Ti-based DRXs have been reported, but in general show lower discharge energy densities compared with Mn redox systems. [24] The recently reported Li 2 Mn 2/3 Nb 1/3 O 2 F/ Li 2 Mn 1/2 Ti 1/2 O 2 F [5] and Li 1.2 Mn 0.2 V 0.6 O 2 [10] compounds prepared by mechanochemical synthesis rely mainly on Mn 2+ / Mn 4+ redox (for Li 1.2 Mn 0.2 V 0.6 O 2 , V 4+ /V 5+ redox as well) to achieve high capacities over 300 mAh g −1 (room temperature, 20 mA g −1 ). Li 1.3 Mn 0.4 Nb 0.3 O 2 prepared using a traditional solid-state synthesis method exhibits a large initial capacity of >290 mAh g −1 (60 °C, 10 mA g −1 ) based partially on Mn 3+ /Mn 4+ redox in addition to a large amount of oxygen redox, but suffers from substantial capacity fading upon extended cycling.…”

“…Cobalt in particular is expensive and relatively scarce compared to other 3d transition metals, such as Fe or Mn. [6,7] Lifting the requirement that cations form an ordered (layered) structure enables the use of various transition metal (TM) redox centers, including Mn 3+ /Mn 4+ , [8,9] Mn 2+ /Mn 4+ , [5,10] Cr 3+ /Cr 5+ , [6,11] Mo 3+ /Mo 6+ , [12] and V 3+ /V 5+ . [6,7] Lifting the requirement that cations form an ordered (layered) structure enables the use of various transition metal (TM) redox centers, including Mn 3+ /Mn 4+ , [8,9] Mn 2+ /Mn 4+ , [5,10] Cr 3+ /Cr 5+ , [6,11] Mo 3+ /Mo 6+ , [12] and V 3+ /V 5+ .…”