The Department for Transport of UK government announced to ban petrol and diesel vehicles by 2030 to facilitate Net Zero strategy. Being a major part of transportation electrification, the electric vehicle (EV) market is growing quickly; there are 190,727 new registrations of pure-EVs in 2021, 76.3% increase compared to 2020. Despite such success, the driving range and fast charge capability of EVs are recognised as predominant factors limiting further market penetration. Unfortunately, the physics of these requirements results in a trade-off of the lithium-ion battery design strategy. For instance, cells with high energy density provide maximum range but cannot deliver fast charging, because thicker electrodes suffer more acutely from the concentration polarisation across the electrode due to the slow ionic transport. Likewise, cells with high power density are capable of fast charging, but suffer from low mileage. More impetus in fundamental studies on physical processes of battery and the interplay between microstructure and performance are needed to eliminate range anxiety and charge-time trauma of EVs.
Graphite/silicon composite electrode is regarded as one of the most promising candidates for next-generation automotive LiBs due to its high energy density. However, it suffers from the major drawbacks such as (1) volume expansion, cracking and pulverization of Si particles; (2) fast decay of capacity due to side reactions, consuming electrolyte rapidly. There is great potential to mitigate the degradation mechanisms by improved compositional and structural design based on better understanding of the ambiguous synergistic effect between the two types of particles. Moreover, lithium plating on the graphitic negative electrode is regarded as the foremost safety concern restricting the fast charge capability, leading to the consumption of lithium, electrolyte decomposition, formation of lithium dendrite and even thermal runaway. Therefore, it is critical to suppress lithium plating employing electrode design, manufacturing and rational protocols to address the longstanding challenge of battery fast charging.
In this project, we aim to develop scalable and widely applicable innovations to facilitate the advancement of battery technologies for transport electrification. Correlative in operando experiment coupling the chemical, structure, crystallographic and electrochemical information from 2D to 4D will be conducted to elucidate the failure mechanisms of the graphite/Si composite electrode at the micrometer scale, particularly the synergistic dynamics of charge transfer, lithiation and deformation. Structural evolution is characterised as a function of SOC, C-rates and Si content, and linked to the capacity decay. Advanced 3D microstructure-resolved electro-chemo-mechanical model will be developed to analyse the performance limiting mechanisms, the impact of microstructural evolution on the reaction heterogeneities and predict the cycle life; in operando experiment and 3D microstructure-resolved phase field modelling will be employed to reveal the interplay between 3D microstructure of the electrode with the phase separation phenomenon, spatial dynamics of lithiation and plating. In addition, the physical processes of the relaxation behaviour, such as lithium exchange and redistribution will be elucidated by the 3D model, which will provide valuable guidelines for the refinement of fast charge protocols in terms of the timing and period of the rest steps. Finally, building on the insights of the study above, graphite/Si composite electrodes with novel structures will be fabricated, aiming to achieve at least 280 Wh kg-1 at the cell level with 20 mins charging for 50% of the capacity, corresponding to 15% increase in energy density and over 30% decrease of charging time compared to the commercial cells; an advanced physics-based fast charge protocol will be delivered to mitigate the plating risk and capacity fade.
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