How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi_1/3Co_1/3Mn_1/3O₂Cathodes in Li-Ion Batteries

Abstract

The cycle life of LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NMC) based cells are significantly influenced by the choice of the negative electrode. Electrochemical testing and post mortem surface analysis are here used to investigate NMC electrodes cycled vs. either Li-metal, graphite or Li 4 Ti 5 O 12 (LTO) as negative electrodes. While NMC-LTO and NMC-graphite cells show small capacity fading over 200 cycles, NMC-Li-metal cell suffers from rapid capacity fading accompanied with an increased voltage hysteresis despite the almost unlimited access of lithium. X-ray absorption near edge structure (XANES) results show that no structural degradation occurs on the positive electrode even after >200 cycles, however, X-ray photoelectron spectroscopy (XPS) results shows that the composition of the surface layer formed on the NMC cathode in the NMC-Li-metal cell is largely different from that of the other NMC cathodes (cycled in the NMC-graphite or NMC-LTO cells). Furthermore, it is shown that the surface layer thickness on NMC increases with the number of cycles, caused by continuous electrolyte degradation products formed at the Li-metal negative electrode and then transferred to NMC positive electrode. Li-ion batteries (LiBs) are widely used in applications where rechargeability of high energy density storage units is required, such as portable devices including consumer electronics, vehicles and space applications. It is beneficial that the components are as light-weight as possible for portable devices, thereby decreasing the energy required to transport the device. One widely used positive electrode material contributing to high energy density is LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NMC), either operating at a higher potential or having a larger practical specific capacity than the classical LiMn 2 O 4 and LiFePO 4 materials.1,2 NMC is also less costly than the LiCoO 2 alternative due to the decreased cobalt content.In commercial cells, NMC electrodes are most often cycled vs. graphite negative electrodes which has a low working potential, enabling a large potential difference between the electrodes. However, the low working potential is outside of the electrochemical stability window of most common electrolytes, leading to formation of a solid electrolyte interphase (SEI) layer on the anode surface. A corresponding but thinner layer could also form on the side of the positive electrode depending on the working potential of the cathode. 3,4 Lithium is consumed during SEI formation, which results in a decrease in the cell capacity and an increase in the cell resistance. The problems caused by SEI layer formation can be resolved by changing the negative electrode to an electrode working at a higher potential -within the electrochemical stability window of the electrolytesuch as lithium titanate (Li 4 Ti 5 O 12 ; LTO). Although the increased working potential decreases the energy density of the cell, cycle life is generally increased. The Li-metal anode is another negative electrode that is commonly used as counter/reference electrode, at le...

Stabilizing lithium metal anode by octaphenyl polyoxyethylene-lithium complexation

“…In C 1s XPS spectrum (Fig. 4a), compared with untreated electrolyte, the electrolyte with OP-10 or PEGDME contains more different peaks, the visible peak at 286.7, 284.7, and 284 eV are found from the surface of Li electrode, which can be attributed to ethylene oxide group of OP-10/PEGDME, benzene group, and Li-C-O bond, respectively [42][43][44][45][46] . While in O 1s, the peak at 533.8 eV is assigned to the characteristic peak of Li-C-O, which is both observed in the cell using OP-10 or PEGDME as additives, but it could not observe in the cell using blank electrolyte 44 .…”

Analysis of the Solid Electrolyte Interphase on Hard Carbon Electrodes in Sodium‐Ion Batteries

“…Here, the main component at 284.6 eV is attributed to C−C sp 2[45–48] of the well‐ordered graphite sheets while the peaks at 285.2, 287.5 and 290.1 eV are ascribable to possible surface oxidation and/or defect formation (aliphatic, carbonyl/carboxylic and carbonate groups, respectively) probably due to the cracking of the large carbonaceous domains produced by the ball milling process. Two further components at 286.4 (dark green) and at 290.1 eV are assigned to CH 2 −CF 2 and to CF 2 −CH 2 and, during the curve fitting, the areal ratio between them has been kept equal to 1, according to the symmetrical alternation of CH 2 and CF 2 groups in the chemical structure of the Kynar . A barely visible component at 292.4 eV is associated with the CF 3 groups at the end of the chains of the binder and/or with the CF 3 of TFSI‐anion ,…”

Sulfolane‐Based Ethylene Carbonate‐Free Electrolytes for LiNi0.6Mn0.2Co0.2O2−Li4Ti5O12 Batteries

“…While previous studies have focused on the performance of sulfolane at high potentials or its compatibility with graphite electrodes, we here continue to explore its properties when used in mixtures with DMC by combining insights into the surface layer formation ability and the surface layer stability during battery cycling. To this end, LTO is used as negative electrode since lithium metal is unstable during long term cycling, graphite requires additives in sulfolane‐based electrolytes when LiPF 6 is used as salt, and LTO has previously achieved stable cycling in sulfolane electrolytes . During cycling, the cells were charged to 2.95 V vs .…”