Abstract
Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) is one of the high-energy positive electrode (cathode) materials for next generation Li-ion batteries. However, compared to the structurally similar LiNi1/3Mn1/3Co1/3O2 (NMC111), it can suffer from a shorter lifetime due to its higher surface reactivity. This work studied and compared the formation of surface contaminations on NMC811 and NMC111 when stored under ambient conditions using electrochemical cycling, Raman spectroscopy, and X-ray photoelectron spectroscopy. NMC811 was found to develop a surface layer of up to ∼10 nm thickness that was mostly composed of nickel carbonate species mixed with minor quantities of hydroxide and water after ambient storage for 1 year, while no significant changes were observed on the NMC111 surface. The amount of carbonate species was quantified by gas chromatographic (GC) detection of carbon dioxide generated when the NMC particles were dispersed in hydrochloric acid. Surface impurity species formed on NMC811 upon ambient storage not only lead to a significant delithiation voltage peak in the first charge, but also markedly reduce the cycling stability of NMC811-graphite cells due to significantly growing polarization of the NMC811 electrode.
Highlights
Secondary Li-ion batteries are alternatives to the combustion engine in vehicles, paving the way to electromobility
We can distinguish three processes that can be responsible for the presence of surface carbonates and hydroxides, which include i) residual impurities stemming from unreacted precursors during synthesis, ii) a higher equilibrium coverage of surface carbonates/hydroxides required to stabilize the surface of Ni-rich materials after the synthesis process, and/or iii) impurities formed during ambient storage
Using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and analysis of the carbonate content by gas chromatography (GC) we show that NMC811 is, in contrast to NMC111, sensitive when stored under ambient conditions, forming surface species mainly composed of nickel carbonate mixed with minor amounts of hydroxide and water
Summary
Secondary Li-ion batteries are alternatives to the combustion engine in vehicles, paving the way to electromobility. Andre et al have reported that in order to reach a driving range of 300 miles, the specific energy of today’s Li-ion batteries needs to be increased to ∼750 Wh/kg on a cathode active material (CAM) level, corresponding, e.g., to ∼200 mAh/g at an average voltage of ∼3.8 V.1 This demands the development and application of advanced positive electrode (cathode) materials and at the same time requires further improvements with respect to durability and costs for mass market penetration.[1,2] Layered lithium nickel manganese cobalt oxide (LiNixMnyCozO2, generally referred to as NMC) is one of the most promising classes of positive electrode materials with LiNi1/3Mn1/3Co1/3O2 (NMC111) being already commercialized for automotive applications.[1,3] its reversible capacity only reaches up to ∼160 mAh/gNMC4–7 when cycled at 25◦C up to 4.2 V cell voltage in NMC111-graphite full cells (i.e., ∼4.3 V vs Li/Li+ at discharge rates of 0.1 C) or 4.4 V cell voltage (i.e., ∼4.5 V vs Li/Li+ at 1 C). Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract)
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