Spatially Resolved Operando X-Ray Absorption Spectroscopy in NCA/Graphite to Quantify the Potential-Dependent Transition Metal Dissolution and Its Effect on Capacity Fading

Abstract

Layered transition metal oxides like NCAs (LiNixCoyAlzO2, with x+y+z=1) and NCMs (LiNixCoyMnzO2, with x+y+z=1) are used as cathode active materials (CAMs) for high energy Li-ion batteries due to their high capacity. However, at high upper cut-off potentials, those CAMs suffer from structural instabilities, resulting in severe capacity fading and thus limiting the accessible capacity that can be obtained. Possible causes for the capacity fade at high cut-off potentials and high state-of-charge (SOC) include the (electro)chemical oxidation of the electrolyte oxidation and transition metal (TM) dissolution from the CAM surface.1 Furthermore, layered TM-oxides are known to release lattice oxygen from the near-surface region at high SOC (i.e., at ≈80% SOC when referenced to the total amount of lithium), resulting in reactive oxygen species that induce electrolyte oxidation and HF formation.2 This release of lattice oxygen results in a surface reconstruction from the pristine layered structure to a more resistive spinel- or rocksalt-like structure, thereby inducing an impedance build-up on the cathode. Diffusion of dissolved transition metals to the anode and their subsequent deposition on the anode active material particles can also have a severe effect on cell aging, as the accumulation of metal species on the graphite anode has shown to catalyze the degradation of the protective anode solid/electrolyte interphase (SEI), eventually resulting in the loss of active lithium and in an anode impedance growth. Since the dissolution of manganese is considered to have the most detrimental effect on the anode SEI compared to cobalt and nickel,3 manganese-free NCAs (e.g., LiNi0.8Co0.15Al0.05O2) might have an advantage over manganese-containing NCMs.In this study, we will examine the potential-dependent dissolution of Ni and Co in NCA/graphite cells using operando XAS, and compare it to the potential-dependent dissolution of Ni, Co, and Mn from LiNi0.6Co0.2Mn0.2O2 (NMC622) that we had determined previously by operando XAS.4 Owing to the specially designed geometry of the operando XAS cell,5 we can spectroscopically access and independently investigate the concentration and oxidation state of transition metals, both dissolved in the electrolyte and deposited within the graphite anode. This is illustrated for an NCA/graphite cell in Figure 1. We will also examine the effect of lattice oxygen release from NCA on the NCA/graphite full-cell performance by applying different techniques: We employ a three-electrode Swagelok® type T-cell with a gold wire micro reference electrode (µ-GWRE)6 to quantify the anode and the cathode impedance over the course of 100 cycles as a function of the upper cutoff voltage. In addition, on-line electrochemical mass spectrometry (OEMS)7 is applied to detect the onset SOC for the release of lattice oxygen. From these comparisons, we aim to get a detailed understanding about the influence of transition metal dissolution from NCA on capacity fade and cycle life.