Battery Degradation and Lifetime – Studies within the Faraday Institution on NMC811/Graphite Full Cells
Authors: Hall David S. , Jervis Rhodri , Piper Louis F.J. , Kersting Alexandra L. , Grey Clare P. 
Published: July 2022

Lithium-ion batteries (LIBs) find use in a wide range of applications, each of which has its own design specifications and practical requirements. With regards to the role of LIBs in mitigating carbon emissions, and therefore climate change, it is desirable to support the rapidly growing adoption of electric vehicles and renewable grid-storage systems via development of higher energy density, lower cost, and improved rate capability. However, the design of energy-dense, low-cobalt, and/or high-rate cell chemistries is impeded by inherent trade-offs with cycling and calendar lifetimes. A key goal for the automotive and utilities industries is therefore to predict battery lifetime for new cell designs and chemistries at a very high level of confidence, for example through improved understanding of the physical and chemical processes that determine the state of health of battery systems.

As part of the Faraday Institution, the UK’s independent institute for electrochemical energy storage technology, the Battery Degradation Project has built new understanding of the underlying physical and chemical processes that can lead to degradation in energy-dense NMC811/graphite lithium-ion cells as a model system. Led by the University of Cambridge (Clare Grey PI) in collaboration with eight UK universities, the research consortium is working closely with industry partners to create a new hub for lithium-ion battery research and to address key challenges and opportunities in the field. This presentation will give an overview of the research consortium’s diverse membership, key milestones, and technical progress.

To date, the consortium has been applying a variety of analytical techniques to study degradation processes in NMC811/graphite cells. For example, electrochemical testing and operando solid-state 7Li nuclear magnetic resonance spectroscopy (NMR) were combined to monitor processes in both electrodes individually, including Li-ion mobility and its changes with temperature.1 The method is now being applied to understand how the dynamics are affected by long-term structural damage to the NMC811 material. A series of differential voltage analysis experiments have been paired with operando X-ray diffraction measurements to propose the mechanism behind a critical ‘turning point’ in NMC811/graphite cells, following which degradation accelerates significantly.2 New operando cell designs have been developed to measure changes in cell pressure,3 which are being paired with solution NMR spectroscopy,4 and differential electrochemical mass spectrometry to quantify electrolyte oxidation. The role and rate of transition-metal dissolution in cells under stressed cycling conditions, namely cycling at high temperature (60 °C) and high upper cut-off voltages (4.4, 4.6 V), has been investigated.5 Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) measurements were coupled with time-of-flight secondary-ion mass spectrometry (Tof-SIMS) to observe the Al-containing phases at NMC811 surfaces and grain boundaries.7 New spectroscopic methods are also being developed, including Kerr-gated Raman, which allows sensitive measurements of electrode materials and electrolytes with lower background signal than conventional Raman spectroscopy.6 Finally, X-ray computed tomography methods have been developed that enable operando imaging and spectroscopic mapping of heterogeneities at a sub-particle length scale and across large areas of electrodes, which are being applied to study the origins of microstructural defects, cracking, and redox activity during charging, cycling, or storage.8,9

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