(Invited) Electrochemical Characterization of Lithium-Ion Cells Based on Li1.2Ni0.15Mn0.55Co0.1O2 and Li4Ti5O12

In this presentation we will detail the electrochemical characteristics of lithium-ion cells containing Li_1.2Ni_0.15Mn_0.55Co_0.1O₂ (0.5Li₂MnO₃∙0.5LiNi_0.375Mn_0.375Co_0.25O₂)-based positive and Li₄Ti₅O₁₂-based negative electrodes. The Li_1.2Ni_0.15Mn_0.55Co_0.1O₂ compound is part of the lithium- and manganese- rich (LMR-NMC) family of layered oxides that display capacities exceeding 250 Ah-kg^-1.¹  To achieve these high capacities, the Li_1.2Ni_0.15Mn_0.55Co_0.1O₂ is typically cycled beyond the activation plateau at ~4.45V vs. Li/Li⁺, which is associated with irreversible structural changes in the oxide. Li₄Ti₅O₁₂ (LTO) negative electrodes are an alternative to graphite-based negative electrodes that are typically used in lithium-ion cells.² LTO reversibly transforms to Li₇Ti₅O₁₂, a rock salt-type compound during lithiation. The transformation occurs at ~1.55V vs. Li/Li⁺, well within the stability window of conventional carbonate-based electrolytes, and high enough to prevent Li-plating and allow use of aluminum current collectors. LTO-based electrodes show excellent good rate capability, enhanced thermal stability, minimal strain, and capacities to ~170 mAh-g^-1. Cells with LTO electrodes coupled to LiCoO₂, LiNi_0.8Co_0.15Al_0.5O₂, LiMn₂O₄, LiNi_1/3Co_1/3Mn_1/3O₂ and LiFePO₄ demonstrate improved cycling stability over their graphite-based counterparts.

All electrodes in this study are from the Cell Analysis, Modeling, and Prototyping (CAMP) facility at Argonne.³ The positive electrodes contained a coating of 92 wt% Li_1.2Ni_0.15Mn_0.55Co_0.1O₂, 4 wt% C45 carbons and 4 wt% PVDF binder on an Al current collector. The negative electrodes contained a coating of 87 wt% Li₄Ti₅O₁₂, 5 wt% C45 carbons and 8 wt% PVDF binder, also on an Al current collector. Electrochemical tests were conducted in 2032-type coin cells (1.6 cm² electrodes) housed in a 30°C temperature chamber. All cells were assembled in an argon-atmosphere glove box and contained Celgard 2325 separator and EC:EMC (3:7 by wt.) + 1.2 M LiPF₆ (aka Gen 2) electrolyte. The cells were cycled in 2 voltage ranges – 0.75-2.55 V (2.3-4.1 V vs. Li/Li⁺), which is below the oxide activation plateau, and 0.75-3.15 V (2.3-4.7 V vs. Li/Li⁺), which is above the oxide activation plateau. Cell capacity, voltage, energy and impedance data over 500 cycles were obtained during the testing period.

Some observations from our experiments are as follows:

• The Li_1.2Ni_0.15Mn_0.55Co_0.1O₂ is not ‘activated’ during the 0.75-2.55 V cycling. Therefore cell capacity is low, 70 mAh-g(oxide)^-1, and cell impedance is high. After 500 cycles, cell capacity is mostly unchanged but cell impedance rises, exclusively at the positive electrode.  Voltage fade, is any, is minimal even after long term cycling.
• The Li_1.2Ni_0.15Mn_0.55Co_0.1O₂ is ‘activated’ during the 0.75-3.15 V cycling, and the cell yields a significantly higher capacity, 230 mAh-g(oxide)^-1, which is mostly unchanged.  However, cell impedance rises and significant voltage fade is observed after 500 cycles; both features can be attributed to the positive electrode.

Additional experiments, including electrochemical measurements and physicochemical diagnostic studies on harvested electrodes and electrolytes will be detailed. The consequences of LTO-based negative electrodes on cell energy and performance degradation will be discussed during the presentation.

Acknowledgements

Support from the U.S. Department of Energy’s Office of Vehicle Technologies Program is gratefully acknowledged. We are grateful to S. Trask, B. Polzin, and A. Jansen from the CAMP Facility (Argonne), which is fully supported within the core funding of the Applied Battery Research (ABR) for Transportation Program.

References

1. Bettge et al., J. Electrochem. Soc. 160 (2013) A1-A10.
2. Trask et al., J. Power Sources 259 (2014) 233-244.
3. Abraham et al., Electrochimica Acta 51 (2005) 502-510.