Solid-electrolyte interphase (SEI) plays a pivotal role in the performance of lithium metal anode, which is promising for high energy batteries. Understanding SEI composition is important but very challenging. Here we report on the use of synchrotron-based X-ray diffraction and pair distribution function analysis to identify and differentiate the two elusive components, LiH and LiF, in the SEI of lithium metal anodes. LiH is identified as a component of SEI in high abundance, and the possibility of its misidentification as LiF in the literature is discussed. LiF in the SEI is found to have different structural features from LiF in bulk phase, including larger lattice parameter and smaller grain size (<3 nm). These characteristics favor Li⁺ transport, and explain why an ionic insulator, like LiF, has been found to be a favored component for SEI. PDF analysis of the SEI reveals key amorphous components related to solvent or anion decomposition.

Nickel-rich layered cathode materials with high energy density are very promising for next generation batteries when coupled with lithium metal anode. However, the practical capacities accessible are far less than the theoretical value due to their structural instability during cycling, especially when charged at high voltages. Such challenge has been attributed to the concomitant electrolyte instability and cathode structural and morphological degradation. Therefore, various approaches have been developed through electrolyte engineering and bulk cathode material modifications. In this presentation, we will report a simple interphase engineering approach by using an ionic electrolyte additive, lithium difluorophosphate (LiDFP). Through theoretical calculation, we found that self-decomposition product PO₂F of LiDFP additive is adsorbed strongly and uniformly on the Ni-rich cathode, which is further decomposed to Li₃PO₄ and LiF through the catalytic effect of transition metals. The new interphasial chemistry contributed from LiDFP stabilizes the bulk structure of Ni-rich cathode materials and prevents the performance degradation by suppressing the dissolution of transition metals and undesired structural changes. Even after 200 cycles with charge limit as high as 4.8 V, no rock-salt phase can be detected in LiNi_0.76Co_0.10Mn_0.14O₂ (NMC76), and the cell consisting of NMC76 cathode and lithium metal anode retains 97% of the initial capacity (235 mAh/g). Various advanced characterizations were used, on both atomic and electrode levels, to reveal the mechanism how a new interphase stabilizes the layer structures in bulk electrode. The dissolution of nickel was identified to be most serious at high voltage, rather than the widely believed manganese dissolution. The transition metal dissolution can be effectively suppressed by LiDFP additive. Furthermore, the cathode surface protected by LiDFP-derived interphase is shown to regulate a more uniform lithium distribution within the bulk cathode particles and effectively mitigate the strain and crack formation. Machine-learning assisted nano-tomography discovered that such interphase-bulk interaction is most effective for those small-sized, spherically-shaped particles.

Acknowledgement:

The work done at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE through the Advanced Battery Materials Research (BMR) Program, (Battery500 Consortium), and Applied Battery Research for Transportation (ABRT) program under contract No. DE-SC0012704. This work used the resources of the Center for Functional Nanomaterials, a U.S. DOE office of Science User Facility, at Brookhaven National Laboratory, and beamlines 5-ID, 7-BM, 23-ID-2, and 28-ID-2 of the National Synchrotron Light Source II, a US DOE Office of Science user facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. The work done at Pacific Northwest National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) under contract No. DE-AC02-05CH11231. The work at ARL was performed under JCESR, an Energy Research Hub funded by Basic Energy Science, US Department of Energy.