Understanding the Formation of Soluble Bases in Nickel-Rich Layered Materials and Their Reactivity Towards Electrolyte

Abstract

In the last years, LiNixMnyCozO2 (x+y+z = 1) layered materials have been widely used as positive electrode materials in lithium-ion batteries for electric car applications, because of their high energy density given by the nickel content. Unfortunately, their stability towards H2O and CO2 is compromised when x>0.5, favoring the formation of surface species commonly called soluble bases (SB). The presence of SB has shown to increase the cell’s impedance, decrease the capacity retention and evolve gas during functioning1-4. Therefore, in this work, accurate SB quantification methods were implemented on four high-nickel layered materials with the aim of further deciphering their formation mechanisms and reactivity towards electrolyte.The optimization of a procedure that allows the quantification of SB in the as-received and after H2O/CO2storage of the NMC532, 622, 811 and LiNi0.9Co0.1O2, will be shown through the complementary use of acid-base titration and inductively coupled plasma (ICP) techniques. We unveiled the presence and quantified different lithium and sodium salts. Moreover, thanks to additional TGA/MS analysis, we propose a novel NMC surface degradation mechanism upon H2O/CO2 storage that involves the presence of lithium and nickel-based surface species. Finally, through nuclear magnetic resonance technique (19F and 31P NMR), the chemical reactivity of these surface species towards a commercial LiPF6-based liquid electrolyte was studied at room and at high temperature, evidencing the presence of lithium fluorophosphate as major decomposition product.1. Liu, H. S., et al. (2006). "Investigation and improvement on the storage property of LiNi0.8Co0.2O2 as a cathode material for lithium-ion batteries." Journal of Power Sources 162(1): 644-650.2. Bi, Y. J., et al. (2016). "Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance." Rsc Advances 6(23): 19233-19237.3. Renfrew, S. E. and B. D. McCloskey (2017). "Residual Lithium Carbonate Predominantly Accounts for First Cycle CO2 and CO Outgassing of Li-Stoichiometric and Li-Rich Layered Transition-Metal Oxides." Journal of the American Chemical Society 139(49): 17853-17860.4. Mahne, N., et al. (2018). "Electrochemical Oxidation of Lithium Carbonate Generates Singlet Oxygen." Angewandte Chemie-International Edition 57(19): 5529-5533.