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

Abstract

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.

Correlating Mn Dissolution and Capacity Fade in LiMn0.8Fe0.2PO4/Graphite Cells During Cycling and Storage at Elevated Temperature

“…These cells cycled at C/3, 40 °C, up to 4.4 V using a protocol where a 24 h voltage hold was applied to the cells at top of charge of each cycle. 18 Thus, for NMC materials, the degree of Mn deposition is highly dependent on the cycling conditions. On the other hand, an LMO positive electrode can result in Mn dissolution values of 20-40 μg cm −2 at 80% capacity retention, even when blended with an NMC622 material.…”

“…Other Mn containing materials, such as LiMn 2 O 4 (LMO) and NMC, suffer from Mn dissolution from the positive electrode during cycling. [16][17][18] Transition metal (TM) dissolution is thought to proceed via an acid attack mechanism; HF is formed from water and LiPF 6 and can then cause the disproportionation of Mn 3+ into Mn 2+ and Mn 4+ , the former of which is soluble in the electrolyte. 19,20 The mechanism has previously been called into question, since Mn dissolution from LMO is most severe at top of charge, i.e., where there is less formal Mn 3+ .…”

Hybrid Dynamic Covalent Network-Based Protecting Layer for Stable Li-Metal Batteries

“…With the booming energy storage market of electronic devices, there is a growing demand for high-energy-density rechargeable batteries. − The energy density of lithium (Li) batteries is closely related to the theoretical specific capacity of electrodes and the voltage between the cathode and anode. The widely used anode in commercial Li-ion batteries (LIBs) is graphite with a theoretical specific capacity of 372 mAh g –1 , , which has reached the capacity limit. In contrast, the metallic Li with ultrahigh theoretical specific capacity (3860 mAh g –1 ) and low electrochemical potential (−3.04 V vs standard hydrogen electrode) is considered to be the most promising anode candidate for next-generation high-energy-density batteries. − However, before the commercialization of the Li electrode, there are several major challenges that should be overcome: (1) Because of the uneven deposition of Li + on the anode during the cycling process, the uncontrolled Li dendrites will penetrate the separator, leading to short circuits (or micro short circuits) of the batteries and even safety issues.…”