Optimal Voltage for Positive Effect of Lithium Difluorophosphate

To produce high-energy and simultaneously cost-effective lithium-ion batteries (LIB), layered oxides at higher charge voltage or lithium/manganese-rich layered oxides (LMR) are considered promising candidates as cathode active material. They have a high specific discharge capacity. However, this requires charging conditions up to high cathode potentials (>4.5 V vs. Li|Li+). These in turn lead to premature aging of the cells due to the release of oxygen, voltage drop and dissolution of transition metals. In cells with graphite anodes, the latter is particularly disadvantageous as it suffers from electrode crosstalk.

To prevent this and improve the lifetime of LIB, current research is using various electrolyte additives. Lithium difluorophosphate (LiDFP) plays a central role in suppressing crosstalk, as it reliably scavenges the dissolved transition metals. A study by MEET Battery Research Center at the University of Münster has now investigated the extent to which the positive effect on the lifetime depends on the voltage and concentrations.

Balance of Decomposition Products and Dissolved Transition Metals

When cells are charged up to 4.5 V, there is a balance between the decomposition of the additive and the dissolution of the transition metals from the cathode. The LiDFP decomposition product effectively complexes with the dissolved transition metals, prevents crosstalk, and thus the metal deposition on the anode, finally extending the cell-lifetime. At a voltage of 4.7 V, both LiDFP decomposition and transition metal dissolution are more pronounced. “However, due to the limited solubility of the decomposition products, their concentration in the electrolyte does not increase noticeably as it reaches a limit. This tips the balance, as there are not enough decomposition products to bind the larger amount of transition metals,” explains MEET scientist Anindityo Arifiadi. This excess deposits and damages the graphite anode and shortens the lifetime of the cell.

The results of the study allow the research team to better understand the potential and limitations of using LiDFP as an electrolyte additive for high energy LIB. Dr Johannes Kasnatscheew, Head of the Reserach Division Materials at MEET Battery Research Center, classifying the results: “Although the capacity degradation caused by the crosstalk at a moderately high charging voltage (4.5 V) is mitigated by the use of LiDFP, the main limiting factor for higher voltages is still the bulk degradation of the cathodes themselves.”

Entire Study Online Available

Detailed results of the study have been published by the authors Anindityo Arifiadi, Feleke Demelash, Tobias Brake, Christian Lechtenfeld, Sven Klein, Dr Simon Wiemers-Meyer and Dr Johannes Kasnatscheew, MEET Battery Research Center as well as Prof. Dr Martin Winter, MEET Battery Research Center and Helmholtz Institute Münster of Forschungszentrum Jülich, in the journal „Energy & Environmental Materials”.