From 2D to 4D: correlative imaging and modelling for next-generation automotive lithium-ion batteries

英国政府运输部宣布到2030年禁止汽油和柴油车辆，以促进净零战略。作为交通运输电气化的重要组成部分，电动汽车（EV）市场增长迅速；2021年，纯电动汽车新注册量为190727辆，比2020年增长76.3%。尽管取得了这样的成功，但电动汽车的续驶里程和快速充电能力被认为是限制市场进一步渗透的主要因素。不幸的是，这些物理要求导致了锂离子电池设计策略的权衡。例如，高能量密度的电池可以提供最大的充电范围，但不能提供快速充电，因为由于离子传输缓慢，较厚的电极更容易受到电极上浓度极化的影响。同样，高功率密度的电池可以快速充电，但续航里程较低。为了消除电动汽车的续航里程焦虑和充电时间创伤，需要加大对电池物理过程以及微观结构与性能相互作用的基础研究力度。石墨/硅复合电极因其高能量密度被认为是下一代汽车用锂离子电池最有前途的候选者之一。然而，它的主要缺点是：(1)硅颗粒的体积膨胀、开裂和粉碎；(2)由于副反应，容量衰减快，电解液消耗快。在更好地理解两种颗粒之间模糊的协同效应的基础上，通过改进组成和结构设计，有很大的潜力来减轻降解机制。石墨负极镀锂是制约快速充电能力的首要安全问题，会导致锂的消耗、电解液的分解、锂枝晶的形成甚至热失控。因此，通过电极设计、制造和合理的工艺方案来抑制锂电镀是解决电池快速充电长期挑战的关键。在这个项目中，我们的目标是开发可扩展和广泛适用的创新，以促进运输电气化电池技术的进步。通过二维到四维的化学、结构、晶体学和电化学信息耦合的相关操作实验，阐明石墨/硅复合电极在微米尺度上的失效机理，特别是电荷转移、锂化和变形的协同动力学。结构演化的特征是有机碳、碳率和硅含量的函数，并与容量衰减有关。建立先进的三维微结构分辨电化学力学模型，分析性能限制机制、微结构演变对反应非均质性的影响以及预测循环寿命；在operando实验和三维微观结构分辨相场建模将揭示电极的三维微观结构与相分离现象，锂化和电镀的空间动力学之间的相互作用。此外，弛豫行为的物理过程，如锂交换和再分配将通过3D模型来阐明，这将为在其余步骤的时间和周期方面改进快速充电协议提供有价值的指导。最后，在上述研究的基础上，将制造具有新型结构的石墨/硅复合电极，目标是在电池级实现至少280 Wh kg-1，充电20分钟，容量为50%，与商业电池相比，能量密度增加15%，充电时间减少30%以上；将提供一种先进的基于物理的快速充电协议，以减轻电镀风险和容量衰减。

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.