Stabilized Lithium, Manganese AB2O4 Spinel for Rechargeable Lithium Electrochemical Systems through A and B Site Doping

Wednesday, 4 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
Y. J. Howarth and T. B. Atwater (U.S. Army RDECOM CERDEC CP&I)
Lithium manganese spinel (LixMn2O4) has been extensively investigated as a safe and affordable lithium ion cathode material that can provide a great lattice for lithium intercalation. However, a major obstacle when utilizing LixMn2O4 cathodes is the loss of cycle capacity that occurs during cell charge and discharge. Lithium manganese oxide doped with potassium and cobalt produces a promising cathode material for rechargeable lithium-ion cells. Modifying the A site and B site of the spinel structure works to limit cathodic capacity fading by enhancing the robustness of the material. Successfully preventing degradation of the spinel matrix is key to maintaining the electrochemistry’s reversibility over prolonged cycling and abusive conditions.

LixKyMn2-zCozO4 was prepared by modifying a stabilized lithium manganese-based AB2O4 spinel material with potassium A site and cobalt B site dopants. This was done using a two-step method where a manganese mixed metal oxide precursor was synthesized and subsequently used in a solid state reaction. LixKyMn2-zCozO4 was synthesized as the active ingredient for the cathode with x=1, y=0.1, and both z=0.1 and 0.2, and was tested against metallic lithium to evaluate performance. In order to assess the robustness of the LixKyMn2-zCozO4 cathode material at both z=0.1 and z=0.2, a number of cycling tests were run and analyzed in comparison to each cell’s baseline performance. A 2 mA cycling test between 3.0 or 2.5 V and 4.5 V was run initially on all cells to establish baseline performance, and was run between high rate discharge tests, pulse cycling tests, and deep discharge tests in order to compare to the cell’s baseline. Different groups of cells were discharged at 2, 4, 8, 12, 16, and 20 mA, pulse charged and discharged for 1 second with a 1 second rest, and deep discharged down to 2.5 V in order to assess the reversibility of the LixKyMn2-zCozO4 cathode chemistry when compared back to baseline performance.

Through high-rate discharge, pulse cycling, and deep discharge tests, the electrochemistry of the cathode exhibited excellent reversibility and overall performance stability. For example, Figure 1 shows a Li \ LiK0.1Mn1.8Co0.2O4 cell that retains its full capacity when tested at the 2 mA cycling baseline after high discharge rates of 4, 12, 16, and 20 mA for 10 cycles at a time. Figure2 illustrates the maintenance of the base thermodynamics of the same cell after these high-rate discharges, exhibiting the stability of the LiK0.1Mn1.8Co0.2O4 chemistry. An Li \ LiK0.1Mn1.8Co0.2O4 cell was additionally shown to withstand a deep discharge test down to 2.5 V in Figure 3, and a pulse cycling test in Figure 4. Following each abusive test, the cell’s capacity returned to its previous value when cycled at baseline conditions, showing the stabilizing effect of the spinel’s modifications on capacity retention. Doping the LiMn2O4 spinel with potassium in the A site and cobalt in the B site proved to be a promising chemistry to prevent cathodic capacity fading in a lithium-ion cell, as well as a capable abuse tolerant option for use in rechargeable battery applications.