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Start of funding 01.01.2015

Multiscale characterisation of NMC-based electrodes in capillary Li-ion microbatteries

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Lithium ion batteries are became a major energy storage media in the field of portable electronics and electric vehicles. Despite its overall spreading the Li-ion technology possesses severe drawbacks, e.g. on issues concerning safety, stability of electrode materials, capacity improvements etc. Modern Li-ion cell is a closed electrochemical system of high complexity, having numerous degrees of freedom with various interactions and couplings. This fact along with the need for elimination of possible risks of materials oxidation, electrolyte evaporation or cell charge changes make experimental studies under real cell operating conditions highly relevant. The current project aims a development and test of capillary-based Li-ion electrochemical cell suitable for imaging, spectroscopy, diffraction and PDF studies using lab X-ray and synchrotron radiation.

Final report:
The rapid progress in portable electronics and electric vehicles has led to a growing demand for rechargeable batteries with higher energy density, smaller size, reduced weight, and lower cost. Among existing energy storage technologies, lithium-ion batteries (LIBs) have become the predominant choice due to their high operating voltage, excellent energy density, long cycle life, and reliable storage characteristics. Despite these advantages, several fundamental challenges remain unresolved. Issues related to safety, electrode material stability, and capacity limitations hinder further improvement. It is now widely recognised that the materials aspect - particularly electrode materials and electrolytes - represents the primary bottleneck for advancing current Li-ion technology. Among the available cathode materials for LIBs, delafossite LiCoO2-based compounds doped with nickel and manganese (NMC) or nickel and aluminium (NCA) have demonstrated promising electrochemical performance, improved safety, and enhanced cycle stability. However, the presence of cation (anti-site) mixing, especially in Ni-containing NMC compositions, significantly limits their performance. Cation mixing has long been considered a thermodynamically favoured structural feature, intrinsic to the layered oxide lattice. Recent studies [1] have revealed that cation disorder evolves as a function of the cell’s state-of-charge (SOC), suggesting that this process is dynamic rather than static. This finding emphasises the need for systematic and comprehensive investigations of NMC/NCA crystal structures and local atomic ordering under realistic operating conditions, which remains an experimental challenge. Modern Li-ion cells constitute closed electrochemical systems, making non-destructive characterization methods essential for accurately probing material behavior without inducing artefacts such as oxidation, electrolyte evaporation, or unintended charge variations. Synchrotron-based X-ray scattering techniques provide powerful means to study materials as a function of SOC and state-of-health (SOH). However, the macroscopic dimensions of conventional Li-ion cells-both commercial and laboratory-scale-limit the achievable data quality in in-situ X-ray scattering experiments. Although several in-situ cell designs have been proposed in literature, each approach exhibits specific advantages and limitations.

A series of combined synchrotron/X-ray fluorescence studies was performed at Advanced Light Sources in Berkeley, where the structural changes of NCA/NMC cathodes were monitored in situ. Traces of LiPF6 were observed along with the appearance of long-range order reflections at lower temperatures, which was attributed to the diffraction signal from frozen electrolyte.

All this turned to look back to neutron scattering: two pivotal studies - Mühlbauer et al. [3] and Petz et al. [4] employ advanced neutron diffraction techniques to non-destructively probe ageing effects on the graphite anode and electrolyte in NCA-graphite (NCA|C) 18650 cells. These papers reveal inhomogeneous lithium and electrolyte distributions, correlating them to fatigue states. The Ref. 3 focuses on spatial mapping at low temperatures, while the Ref. 4 extends this with thermal imaging, calorimetry, and chemical analysis, identifying conducting salt decomposition as a primary degradation driver.

A series of NCR18650B cells (3.4 Ah nominal capacity) were cycled aggressively (1.675 A charge at C/2, 6 A discharge at ~2C) up to 1392 cycles, leading to rapid fatigue. Capacity dropped to 80% after ~110 cycles and 60% after 400 cycles, with accelerated fading beyond 600 cycles due to rising internal resistance (from ~50 mO to over 100 mO at SOC=0%).

Spatially-resolved neutron powder diffraction at 150 K and 300 K mapped lithium in the graphite anode (LixC6) and frozen electrolyte. At low temperatures, the electrolyte crystallises below ~260 K, enabling quantification via a new (002) reflection at ~28.8° 2?. Lithium concentration x in the fully charged state was derived from (001) LiC6 and (002) LiC12 reflections. In Ref. 4, we examined the same NCR18650B cells at intermediate fatigue states (120-1112 cycles), integrating neutron diffraction with infrared thermal imaging, quasi-adiabatic calorimetry, and ex-situ electrolyte analysis (GC-MS, IC). Surface temperature profiles were obtained using thermal/infrared imaging during 1C/2C discharge and 0.5C charge, showing increased heating in aged cells (e.g., +40% at 1C for a 600-cycle cell) due to resistance rise. Cooling/decay times lengthened with ageing, suggesting electrolyte dry-out reduces thermal conductivity. Non-uniform radial gradients correlated with lithium distributions. Low-temperature DTA (150-300 K) on cells revealed evolving endothermic signals, indicating composition changes, where synthetic electrolytes (DMC:EMC:EC=72:5:23 with varying LiPF6) mimicked this. Harvested electrolyte has been found stable in solvents (no aging correlation), but LiPF6 decomposed (~25% loss from 1.48 to 1.13 mol/L after 1000 cycles), producing traces of DEC, DMDOHC, etc. This drives power fade via increased charge transfer resistance.

A direct correlation of mobile lithium loss (~16%) with salt decomposition (~24%) was established, proposing artificial lithium sources (e.g., overlithiated cathodes) to stabilise inventory. These insights inform battery design, e.g., optimising electrolytes for salt stability or layouts minimising gradients. Future work could explore milder cycling or additives to mitigate losses, enhancing longevity for EVs and renewables.

[1] O. Dolotko et al., J. Power Sources 255 (2014) 197-203
[2] T. Hölderle et al., J. Power Sources 564 (2023) 232799
[3] M.J. Mühlbauer et al., J. Power Sources 475 (2020) 228690
[4] D. Petz et al., Adv. Energy Mater. 12 (2022) 2201652