Phase-separating Active Materials in Lithium-ion Batteries: Implications for Fast-charging and Material Characterisation

Some active materials used in lithium-ion battery (LIB) electrodes undergo phase separation into Li-rich and Li-poor phases upon lithium intercalation. Typical examples include LiFePO4 (LFP) at the cathode and graphite at the anode. While phase separation enables useful features, such as a constant equilibrium potential as a function of state-of-charge, such behaviour poses challenges during material characterisation and for high-rate operation. This study provides a perspective of phase separation in LIB active materials by combining non-equilibrium thermodynamics principles [1] with in-operando techniques [2]. The results show that classical techniques used to estimate solid-state diffusion coefficients, such as the galvanostatic intermittent titration technique (GITT) [3], must be revisited for this class of materials. In fact, although the rapid equilibration of the interface between Li-rich and Li-poor phases within a particle leads to a quick voltage relaxation, the solid-state diffusivity can be significantly lower than what this fast dynamics may suggest. This has significant implications on the distribution of lithium within secondary particles because, upon fast lithiation, the Li-rich phase grows at the particle surface and prevents further lithiation. This is especially critical for graphite anodes, since the Li-rich phase (also known as stage I) at the particle surface is the primary cause for the plating of metallic lithium outside the particle, as quantified in in-operando experiments [2]. Nevertheless, experiments also show that Li plating is not irreversible and part of the plated lithium can be stripped and back-intercalated in graphite when the current is interrupted. This discovery opens opportunities for alternative fast-charging protocols as long as the particle size distribution is well controlled to prevent excessive plating on small particles. On the other hand, experimental characterisation and simulation of a disordered carbon, which does not undergo phase separation, reveal an effectively faster solid-state diffusion and a more significant resistance to lithium plating even at high C-rate [4], thus enabling for a comprehensive comparison between solid-solution and phase-separating active materials when it comes to characterisation and fast-charging capabilities. References [1] 10.1021/ar300145c. Bazant, M.Z. Theory of Chemical Kinetics and Charge Transfer Based on Nonequilibrium Thermodynamics. Acc. Chem. Res. 2013, 46, 1144-1160 [2] 10.1038/s41467-023-40574-6. Lu, X.; Lagnoni, M.; Bertei, A.; Das, S.; Owen, R.E.; Li, Q.; O'Regan, K.; Wade, A.; Finegan, D.P.; Kendrick, E.; Bazant, M.Z.; Brett, D.J.L.; Shearing, P.R. Multiscale Dynamics of Charging and Plating in Graphite Electrodes Coupling Operando Microscopy and Phase-field Modelling. Nat. Commun. 2023, 14, 5127 [3] 10.1149/1.2133112. Weppner, W.; Huggins, R.A. Determination of the Kinetic Parameters of Mixed-Conducting Electrodes and Application to the System Li3Sb. J. Electrochem. Soc. 1977, 124, 1569-1578 [4] 10.1021/acsaem.3c01280. Ahn, S.; Lagnoni, M.; Yuan, Y.; Ogarev, A.; Vavrinyuk, E.; Voynov, G.; Barrett, E.; Pelli, A.; Atrashchenko, A.; Platonov, A.; Gurevich, S.; Gorokhov, M.; Rupasov, D.; Robertson, A.W.; House, R.A.; Johnson, L.R.; Bertei, A.; Chernyshov, D.V. Chemical Origins of a Fast-Charge Performance in Disordered Carbon Anodes. ACS Appl. Energy Mater. 2023, 6, 8455-8465 Acknowledgements This study received funding from the National Recovery and Resilience Plan, Mission 4 Component 2 Investment 1.3 - Call for tender No. 1561 of 11.10.2022 of Ministero dell’Università e della Ricerca, according to attachment E of Decree No. 1561/2022, Project title “Network 4 Energy Sustainable Transition – NEST”, CUP I53C22001450006; funded by the European Union–NextGenerationEU. This paper reflects only the authors’ views and opinions; neither the European Union nor the European Commission can be considered responsible for them.