When 2:30 PM - 5:00 PM Jun 09, 2021
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PhD Defense: “Novel Solid Electrolytes Derived from Polymer Syntheses"

Xinyu (Miranda) Zhang

Lithium-ion batteries (LIBs) have become the most demanding electrochemical storage system owing to their high energy densities (~250 Wh/kg), providing an escape from consumption of nonrenewable energy sources. However, current LIBs with flammable organic liquid electrolytes suffer from poor electrochemical and thermal stabilities that can lead to severe thermal runaway accidents. All-solid-state batteries (ASSBs) are regarded as a fundamental solution to address the safety issue by using solid-state electrolytes (SSEs). Therefore, we explore novel polymeric SSE materials and develop new synthesis methods with low-cost, scalable and environmentally friendly features.  

We investigate polymer precursor electrolytes that offer properties similar or superior to lithium phosphorous oxynitride (LiPON) glasses. LiPON glasses form lithium dendrite impenetrable interfaces at electrodes offering the potential to replace liquid electrolytes. Unfortunately, to date such materials are introduced only via gas phase deposition. We demonstrate the design and synthesis of easily scaled, low-temperature, low-cost, solution processable inorganic polymers containing LiPON/LiSiPON elements. OPCl3 and (Cl2P=N)3 provide starting points for elaboration using MNH2 (M = Li/Na) or (Me3Si)2NH producing OP(NH2)3-x(NH)x, OP(NH2)3-x(NHSiMe3)x and (P=N)3(NHSiMe3)6-x(NH)x. It is followed by lithiation with selected amounts of LiNH2 providing varying degrees of lithiation. Produced oligomers/polymers show higher N/P ratios (1-3) than traditional LiPON glasses (≤0.9) correlating with enhanced ionic conductivities. Li6SiPON and Li2SiPHN precursors offer highest ambient conductivities of 3.3 × 10-5 and 2.7 × 10-4 S/cm after treating at 400°C/2 h, respectively. These polymer precursor solutions can directly coat on Celgard separators and dried (90°C/12 h/vac) prior to cell assembly. A nearly all-solid-state Li-S battery assembled using celgard/Li6SiPON exhibits a reversible capacity of 750 mAh/gsulfur and excellent cycle performance at 0.25 and 0.5 C rate over 120 cycles at room temperature. They also form solid solutions with PEO (60 wt.%) forming composite SSE films for ASSBs, which exhibits discharge capacities of 1000 mAh/gsulfur at 0.25 C and 800 mAh/gsulfur at 1 C retaining coulombic efficiency of ~100 % over 100 cycles.

We also studied another set of LixSiON (Li/Si = x = 2, 4, 6) polymer precursors derived from rice hull ash (RHA), an agricultural waste, providing a green route towards ASSBs. Silica, ~90 wt. % in RHA, can be catalytically dissolved (20-40 wt.%) in hexylene glycol and distilled directly as spirosiloxane [(C6H14O2)2Si, SP] at 200°C. SP can be lithiated using LiNH2 to produce LixSiON oligomers/polymers, and the Li content can be easily controlled by LiNH2 amounts correlating with Li+ conductivity. Li6SiON exhibits the highest ambient conductivity of 8.5 × 10-6 S/cm after treating at 200°C/2 h. Similar to LiPON-like polymer precursors, LixSiON coated Celgard enable nearly ASSB assembly delivering a reversible capacity of ~725 mAh/g at 0.5 C over 50 cycles.


For Zoom link, please email Kristen at krisfres@umich.edu