335. Performance of a Novel In-Situ Converted Additive for High Voltage Li-ion Pouch Cells

Saad Azam, Quinton Meisner, Connor Aiken, Wentao Song, Qian Liu, Dong-Joo Yoo, Ahmed Eldesoky, Zhengcheng Zhang and Jeff R Dahn; Journal of The Electrochemical Society; (2022); DOI: 10.1149/1945-7111/ac9c36 (open access)

In search of new classes of additives for high-voltage NMC/graphite lithium-ion cells, the precursor additive bis(trimethylsilyl) malonate (bTMSM) is shown to be activated via a spontaneous reaction with LiPF6 and LiBF4 salts in carbonate-based electrolyte to form lithium tetrafluoro(malonato)phosphate (LiTFMP), and lithium difluoro(malonato)borate (LiDFMB), respectively. The reaction schemes and rates were studied via nuclear magnetic resonance spectroscopy and gas chromatograph mass spectrometry. The effects of LiTFMP and LiDFMB on high-voltage electrochemical performance were then examined up to 4.5 V in Li[Ni0.4Mn0.4Co0.16]O2 (NMC442)/graphite and Li[Ni0.6Mn0.4Co0.0]O2 (NMC/640)/graphite pouch cells using aggressive voltage-hold cycling, long-term charge/discharge cycling, storage experiments, electrochemical impedance spectroscopy, and gas evolution measurements. While in-situ converted additives suffer from gassing issues due to the presence of trimethylfluorosilane gas, a side product of the in-situ reaction of bTMSM with LiPF6, the cycling and storage capability for the activated additives under study shows competitive performance and controlled impedance when compared to other well-known high voltage additives. Micro x-ray fluorescence spectroscopy confirmed that LiTFMP successfully minimizes the rate of transition metal deposition on the surface of graphite apparently by forming a protective agent at the cathode surface, hence allowing for improved cycling performance at high voltages.