快速锂化过程中考虑自由体积变化的非晶硅粘塑性力化耦合模型研究

It is known that the mechanical degradation tends to happen in amorphous silicon electrode during the processes of fast charging. Fast charging may lead to the atoms rearrangement, and the micro-defect in amorphous silicon electrode. The effect of micro-defect is neglected in the conventional continuum mechanics, so the stress evolution and lithiation process is not precisely analyzed. In order to overcome this issue, the concept of free volume is introduced to define the micro-defect in amorphous silicon during the process of lithiation, which can be used to establish a new chemo-mechanical model. The specific research content includes three aspects: (1) The relationship between free volume and plastic flow is analyzed, and the relationship between free volume and plastic strain rate are determined. (2) The variation of free volume is investigated with considering the effect of lithiation rate. (3) In order to established new chemo-mechanical coupling model, the coupling relation among the lithiation process, stress evolution process and variation of free volume is studied. The provided theoretical model are validated by simple experiment for the application of high performance electrode material. This project can provide scientific basis for the structural design of amorphous silicon electrode, which may solve the issue of mechanical degradation during the process of fast charging.

非晶硅负极材料在锂电池快速充电条件下极易发生力学退化，导致不可逆容量衰退，缩短高性能锂电池使用寿命。快速充电条件下非晶硅中原子重新排列并极易产生微观缺陷，但传统连续介质力学模型未引入材料微观缺陷，无法准确分析电极材料的应力演化过程和锂化动力学过程。为解决这一问题，本项目采用自由体积定义非晶硅负极材料在锂化过程中的微观缺陷，并以此为基础建立新的力学-化学耦合模型。具体研究内容叙述涵括三个方面：（1）研究锂化过程中自由体积改变对塑性流动的影响，确定微观自由体积和宏观塑性应变率之间的关系；（2）将锂化速率考虑在内，研究微观自由体积变化规律；（3）研究锂化、应力演化和自由体积变化之间的耦合关系，建立新的粘塑性力化学耦合模型，并通过实验验证其正确性。本项目为非晶硅负极材料的结构设计提供必要的基础性科学依据，推动解决其在快速充电条件下力学退化的难题。

硅负极材料在快速充放电条件下易发生力学退化，导致不可逆容量衰退并缩短锂电池使用寿命。Si 负极材料从晶体相到非晶相的相转变会产生亚稳态非晶结构，其中涉及反应界面的演化、自限性锂化和粘塑性变形等。本项目基于非线性连续介质力学，粘塑性自由体积理论和扩散学建立非晶硅粘塑性力化耦合模型，以此分析电极材料在锂化过程中的应力演化和锂化动力学过程。首先结合前沿热点，研究了基于深度学习的锂电池电极材料扩散反应问题，包括线弹性材料和大变形硅负极材料，为深度学习解决力化耦合问题提供了一种独特的方法；其次进一步研究了损伤状态变量、局部浓度和溶质原子浓度梯度之间的相关性，提出了大变形硅负极材料的损伤模型；最后根据自由体积理论，建立微观自由体积和宏观塑性应变率之间的关系，通过自由体积生成速率和湮灭速率达到平衡态时确定自由体积变化规律，应用Cahn-Hilliard相场理论描述相界面产生的不均匀扩散过程，分析了薄膜硅负极材料和硅纳米线的应力演化。结果表明本项目提出的力化耦合模型能准确分析硅负极材料的应力和反应界面演化过程，以此能预测电极材料的力学退化和损伤演化，具有商业推广价值。本项目为硅负极材料的结构设计提供必要的基础性科学依据，推动解决其在快速充电条件下力学退化的难题。