Controlled Release of Encapsulated Additives for Enhanced Performance of Lithium-Ion Batteries

Autonomous strategies are developed for controlled release of encapsulated vinylene carbonate (VC) to promote beneficial interfacial reactions for SEI (Solid-Electrolyte Interphase) formation. Typically, VC is incorporated in a lithium-ion battery when it is manufactured but its initial concentration is usually limited to a maximum of a few wt% to avoid increased cell resistance due to a reaction of surplus VC. This initial concentration of VC is rapidly consumed during the initial charge and discharge cycles and cannot be utilized effectively for subsequent cycles. In our approach, microcapsules containing VC additive are designed for release of the additive into electrolyte over time. The diffusion-based release of VC from the microcapsules into electrolyte leads to a relatively low VC concentration during initial charge and discharge cycles, and additional VC is available for subsequent cycles.

The VC microcapsules are successfully prepared by a solvent-exchange method that allows VC to diffuse through the microcapsule shell-wall at elevated temperature. VC concentration profiles in electrolyte at three different environments (pouch cell with 2wt% VC, pouch cell with 5wt% VC microcapsules, and electrolyte with 5wt% VC microcapsules) were evaluated at room temperature by taking aliquots of electrolyte at fixed intervals and measuring the VC concentration by NMR. In pouch cells where 2wt% VC liquid was added directly to electrolyte (not encapsulated), the VC concentration decreased significantly after the first cycle at C/10 rate. In great contrast, in pouch cells that contained VC microcapsules, the VC concentration increased from 0 to 3wt% due to timed-release of encapsulated VC in electrolyte.

To evaluate the electrochemical performance for the additive release from microcapsules, we conducted EIS (Electrochemical Impedance Spectroscopy), cycling tests at various C-rates, as well as long-term cycling tests for pouch cells containing no VC (0wt%), VC liquid (5wt% not encapsulated) and VC microcapsules (5wt%). Comparison of the impedance spectra (Fig. 1a) showed that interfacial resistance for pouch cells with 5wt% VC microcapsules was much lower than pouch cells with same amount of VC liquid added directly, which results from lower initial VC concentration in pouch cell with VC microcapsules. In cycling tests at various C-rates, the pouch cell with 5wt% VC microcapsules has even better rate capability, where its discharge capacity was over 2.5 times higher at 5C-rate compared to pouch cell with 5wt% VC liquid. After 400 cycles at 1C-rate (Fig. 1b), the pouch cell with VC microcapsules showed similar capacity retention as the pouch cell including same amount of VC liquid (not encapsulated). This result provides preliminary evidence that the rate capability of lithium-ion batteries can be enhanced without trade-off for capacity retention through time release of microencapsulated additives.