Case Study: Characterisation of NMC electodes used in Li-ion batteries
Finden Ltd, Electrochemical Innovation Lab (EIL) from the UCL Chemical Engineering, Johnson Matthey, the Faraday Institution, NREL, UCL Chemistry and ESRF.
Environmental considerations and a further electrification of the global automotive fleet and energy grid systems are driving the need for robust electrochemical energy storage devices. While lithium-ion (Li-ion) batteries have emerged over the last few decades as the prime choice for powering portable consumer electronics, they are also being employed for a wider range of applications due to their high energy densities and specific capacities. Since its discovery, lithium cobalt oxide (LCO) has been one of the most commonly used materials in Li-ion electrochemical storage devices and has been employed in numerous applications. High cobalt costs and its inherent toxicity have driven the exploration of other chemistries with reduced Co content, such as lithium nickel manganese cobalt oxide (NMC), where the slightly lower capacity is compensated by excellent power and low self-heating during cycling. However, to meet the demands of modern applications, improvements are required to increase the cycle life, capacity and safety of these materials.
Intercalation battery cathodes are formed by transition metal oxide solid matrices from which Li-ions travel from and intercalate into a carbonaceous anode during charging (and vice versa during discharge). Repeated intercalation of Li-ions during electrochemical cycling can lead to internal stresses within particles of active material due to cyclical expansion and shrinking. Furthermore, Li-ion concentration gradients within individual particles, caused by uneven active lithium loss during extended cycling can contribute to these internal stresses, which ultimately may result in particle cracking. An approach to increase the capacity per cycle with intercalation electrode materials consists in forcing a larger number of Li-ions to participate in the electrochemical process by cycling to a higher upper cut-off voltage, which corresponds to a greater fraction of Li removal from the transition metal oxide crystal structure at higher voltages. This not only accentuates the aforementioned phenomena, but is also likely to affect the stability of the electrolyte over extended cycling. Moreover, cycling at high voltages can cause the release of oxygen from the cathode lattice, which then reacts with the electrolyte, and transition metal dissolution, which can subsequently deposit onto the anode increasing its resistivity and reducing the utilizable capacity.
Li-ion battery cells with LiNi0.33Mn0.33Co0.33O2 and Graphite electrodes
We have applied the XRD-CT technique to investigate NMC electrodes cycled at different voltages using a 1 μm X-ray beam. A wealth of information was obtained from analysing 3D slices taken from electrodes harvested from previously cycled cells; the effect of two upper cut-off voltages, namely 4.2 and 4.7 V. Through full profile analysis of the XRD data contained in the datasets, a localized map of the lattice parameter and unit cell volume was obtained. XRD-CT allowed us to investigate the expansion along the c-axis and shrinkage along the a-axis as a result of extended cycling. The crystal structure of the electrodes cycled to 4.7 V underwent larger shrinkage. The overall capacity fade observed across the different cells was thought to be caused by a combination of poor cell sealing, electrolyte and cathode degradation. When individual particles were analysed, no distribution that correlates local shrinkage to radial depth within the particle was identified. This was attributed to the highly heterogeneous internal structure of the secondary particles, which was also examined with FIB-SEM, revealing extensive cracking as a result of high voltage cycling.
The XRD-CT technique facilitates the detection of parts of the NMC electrode that have inhomogeneous lattice parameters that deviate from the bulk of the sample, further highlighting the effectiveness of the technique as a diagnostic tool, bridging the gap between crystal structure and electrochemical performance.
Exploring cycling induced crystallographic change in NMC with X-ray diffraction computed tomography , Phys. Chem. Chem. Phys., 2020, Advance Article, DOI: https://doi.org/10.1039/D0CP01851A