Degradation Mechanism Analysis of Lithium-Ion Battery Full-Cells

Lithium-ion batteries (LIBs) comprising a layered LiMO2 cathode (with M being mainly a mixture of Ni, Co, Mn, Al) and a graphite anode are generally considered to be the electrochemical storage technology of choice for hybrid electric and full electric vehicles. However, further enhancement in the energy and power densities of LIBs is necessary to fulfill the requirements imposed by such applications. One major strategy that is currently pursued to improve the energy density is to increase the content of redox active Ni in the cathode active material. Unfortunately, particularly high-Ni LIBs suffer from various mechanisms of capacity loss that significantly restrict their lifetime.

Our research goals are to identify and understand the degradation processes taking place in full-cells using Ni-rich cathodes during long-term operation in order to tailor the chemistry of cell components and improve the overall performance of LiMO2/graphite-based LIBs.

Advanced Materials Characterization

Important deactivation pathways are phase transformations and crystallographic volume changes in the active materials that lead to particle fracture and formation of highly reactive surface. The structural changes of the electrode materials such as evolution of lattice parameters and unit cell volume are analyzed during cell cycling by use of operando X-ray diffraction. In combination with galvanostatic charge/discharge measurements, optimal cycling conditions for specific cathode compositions can be assessed and structure-related long-term aging effects are identified.

Figure 1: Time-resolved structural investigation of NCM851005 cathode material during electrochemical cycling via operando X-ray diffraction.

Interlinked Ageing Effects Between Cathode and Anode

In addition, our studies seek to understand the different electrochemical mechanisms involved in the modification of the anode/electrolyte interface. In particular, the leaching of transition metals (TMs) from the cathode matrix is of interest. TM ions diffuse through the separator and subsequently deposit at the graphite anode, thus disturbing the lithium intercalation. An in-depth evaluation is carried out using three-electrode measurements in order to interpret the processes occurring at cathode and anode separately. Moreover, inductively coupled plasma mass spectrometry, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy are used to characterize the graphite and reveal the location of the TM enrichments.

Figure 2: Transition metal deposition at the graphite anode.

Bulk Doping and Coating of Cathode Active Materials

A simple way to improve the performance of layered LiMO2 cathode materials is to dope and/or coat them. To this end, we use both solution chemistry strategies and atomic layer deposition (ALD). The modified materials are characterized using state-of-the-art techniques and their electrochemical performance is evaluated in long-term cycling experiments. Ex-situ analysis of cycled cells allows for better understanding of the prevailing degradation mechanisms and how protective coatings and specific dopants affect/alter these processes.

Figure 3: Coating of cathode active material as one strategy to improve the long-term performance of LIB full-cells.