全固态薄膜锂电池具有固态电解质层薄、 固固界面致密等特点, 可作为微小型设备的储能元件。 与传统锂离子电池相比, 全固态薄膜锂电池内部不含液态电解液, 反应与传质过程皆在固相中进行, 导致全固态薄膜锂电池的倍率性能一般较差。 为解决该问题, 该文基于磁控溅射和真空蒸镀技术制备了正极为钴酸锂、 固态电解质为锂磷氧氮(LiPON)、 负极为金属锂(Li)的全固态薄膜锂电池。 采用时频域配合和实验与仿真相结合的方法, 系统解析了影响全电池倍率性能的关键因素。 运用基于全电池倍率实验电压曲线的曲线平移分析方法及基于一维阻抗模型和阻抗谱的动力学参数辨识方法, 分析了电池内部不同部件、 不同物理过程对电池倍率性能的影响, 结合一维时域模型仿真结果得出如下结论： 电池中影响大倍率下放电总容量的主要限制因素为正极材料中的锂离子扩散过程, 放电末期正极扩散系数低是大倍率下放电容量衰减的主因; 影响瞬态放电功率的主要限制因素为固态电解质中锂离子的电迁移过程, 高固态电解质固相过电势是放电功率损失的主因。 基于上述结论, 该文提出了适当降低固态电解质薄膜厚度和缩短正极离子扩散路径等改进电池倍率性能的初步设计思路, 研究了一种全固态薄膜锂电池倍率性能的分析方法并得出了初步结论, 可用于进一步指导改进制备工艺。
[Objective] All-solid-state thin-film lithium batteries with advantages such as ultra-thin thickness, intimate interfacial contact, and simple structure have a promising prospect for application in portable and microdevices. Unlike the porous structures in conventional lithium-ion batteries, the electrode and electrolyte structures of all-solid-state thin-film lithium batteries are stacked in layers without any liquid electrolyte. Due to this structural layout, there is considerably high interface resistance between the electrode and electrolyte and relatively low ionic conductivity of the solid-state electrolyte, which lead to poor rate performance of the batteries when operated below a certain capacity demand.[Methods] Herein, an all-solid-state thin-film lithium battery with crystallization LiCoO2, amorphous LiPON, and lithium metal thin films have been fabricated via RF magnetron sputtering and high vacuum evaporation, respectively, while a lithium symmetric cell has been fabricated via electrochemical deposition. Through electrochemical experiments and physical models applied in the time and frequency domains, rate performance factors are systematically discussed and analyzed. Furthermore, in order to perform a detailed analysis of the rate performance of these thin-film batteries, it is necessary to obtain kinetic parameters corresponding to different physical and chemical processes of all the battery components. Here, electrochemical impedance spectroscopy (EIS) has been used to measure the parameters via the impedance spectrum of the Li1- xCoO2/LiPON/Li battery and the lithium symmetrical cell.[Results] The preliminary results of the electrochemical analysis method used on the voltage curve at different current rates showed that the diffusion process of lithium ions in the solid-state electrolyte or positive electrode was the origin of the main polarization causing low rate capacity. There was also a high rate of huge overpotential owing to the linear process of electron or ion migration. Based on the EIS under different lithium intercalation amounts in the positive lithium cobalt oxide and the one-dimensional frequency domain model, we obtained vital dynamic parameters of this battery. Moreover, a one-dimensional electrochemical time domain model with the dynamic parameters calculated above was introduced to further analyze the voltage curves. It was found that the mass transfer process in the solid-state electrolyte and the diffusion process in the positive electrode were the key physical and chemical processes of rate performance in the fabricated battery. Furthermore, the preliminary design was proposed to improve the rate performance of these batteries through an electrochemical model that reduced the thickness of solid-state electrolytes and shortened the diffusion path in the positive electrode.[Conclusions] This work provides an analytical method based on frequency and time domain physical models that are useful for accurately distinguishing and analyzing the impacts of various kinetic parameters of thin-film batteries. The method also allows for preliminary and practical conclusions for the rate performance of all-solid-state thin-film batteries. The mass transfer process in the solid electrolyte is the main factor affecting the power performance, while the diffusion process in the positive electrode is the main factor affecting the capacity performance. Knowing these parameters is helpful for fabricating and structuring the design of the battery.
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