Microcapsule-based strategies for enhanced performance of lithium-ion batteries

Lithium-ion batteries;Vinylene carbonate;Battery additives;Microcapsules;Time-release;Capsule stability;Mechanical triggering;Si composite electrodes;In-situ fluorescence imaging

Lim, Tae Wook

关键词: Lithium-ion batteries;    Vinylene carbonate;    Battery additives;    Microcapsules;    Time-release;    Capsule stability;    Mechanical triggering;    Si composite electrodes;    In-situ fluorescence imaging;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/98155/LIM-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Capacity fade of lithium-ion batteries is mainly accompanied by the degradation of solid electrolyte interphase (SEI) and the damage of electrical interfaces in electrodes. In this dissertation, two different microcapsule-based strategies to address degradation and enhance battery performance are developed: (1) time-release of encapsulated battery additive, vinylene carbonate (VC) and (2) mechanical triggering of microcapsules embedded in battery electrodes.Time-release of encapsulated battery additives aims to promote a beneficial SEI layer at the interface between electrode and electrolyte by controlled delivery of battery additives. To prepare microcapsules filled with high loading amount of VC in the core, we design a new microencapsulation method based on solvent-exchange technique which allows VC to diffuse into the core of as-prepared microcapsules at elevated temperature. The release profile of VC microcapsules is evaluated using 1H-NMR method in electrolyte, as well as in operating pouch cells. The microcapsules rapidly release VC for SEI formation then more slowly release the remaining VC to stabilize the interface for subsequent cycles. In pouch cells with encapsulated VC (5 wt% VC), the VC concentration in electrolyte remains lower than for pouch cells where the same amount of VC is directly added to electrolyte. Time-release of encapsulated VC for enhanced performance is investigated in both NCA/graphite and LMO/graphite batteries. Electrochemical Impedance Spectroscopy (EIS) and cycling at different rates are conducted for pouch cells with varying VC additive (0, 2 and 5 wt%) and VC microcapsules (5 wt% VC). In NCA/graphite batteries, pouch cells with 5 wt% VC additive (both non-encapsulated and microencapsulated VC) show improved capacity retention over 400 cycles at 1C-rate, compared to that without VC additive. In the case of directly added VC, a high initial concentration of VC in the pouch cell decreases the rate capability of the battery due to increased interfacial resistance. In contrast, time release of microencapsulated VC increases discharge capacity 2.5 times at 5C-rate compared to the non-encapsulated VC system. In LMO/graphite batteries, the pouch cells with encapsulated VC also exhibit cycling stability while maintaining lower cell resistance compared to pouch cell with directly-added VC at the same loading. We also conduct EIS measurements using pouch cells with lithium metal reference to determine the effect of encapsulated VC on the cell resistance of the anode and cathode, separately. ICP-MS analysis is performed to measure Mn deposition on anode and Mn dissolution in electrolyte. The amount of Mn deposited on anode relies on the VC concentration as well as water or HF traces in electrolyte. In the presence of HMDS additives which can remove water and HF in electrolyte, the pouch cell with encapsulated VC (5 wt% VC) exhibits lower Mn deposition than the pouch cells with directly-added VC (0, 2 and 5 wt%). Release of microcapsule core content is also demonstrated by mechanical triggering of microcapsules embedded in electrodes upon cycling. We first evaluate capsule stability by quantifying core release in four different battery environments: capsules dispersed in electrolyte with and without cycling, and capsules embedded in electrodes immersed in electrolyte with and without cycling. 1,2-dichlorobenzene (DCB)-filled PU/PUF microcapsules are prepared by varying the amount of PU and UF. Capsule thermal stability, and DCB core release in electrolyte are evaluated by TGA and 1H-NMR to identify the most suitable microcapsules for the battery environment. Stable microcapsules survive electrode fabrication process and are successfully incorporated into graphite and silicon electrodes. For in-situ measurement of capsule damage in electrodes upon cycling, an in-situ imaging technique is developed using microcapsules with fluorescence dye in the core. Little change in fluorescence is observed for graphite electrodes, indicating no capsule rupture. For silicon electrodes, integrated fluorescence intensity significantly decreases by the tenth cycle. Capsule damage is further investigated by SEM and GC-MS analysis for cycled electrode and electrolyte. SEM images reveal that embedded microcapsules are ruptured by the large volume change that occurs upon cycling in silicon electrodes. The amount of capsule damages is also quantified by GC-MS to measure the amount of DCB core release in electrolyte after pouch cell cycling.