This article discusses the analysis of the surface of lithium-ion battery electrodes using the Thermo Scientific™ K-Alpha™+ XPS system (Figure 1).
The lithium-ion battery electrodes are air sensitive in nature and therefore should not be exposed to ambient atmosphere.
Their samples can be safely transported from a glove box to the instrument without being exposed to ambient atmosphere using the K-Alpha+ vacuum transfer module.
Therefore, the surface can be taken as an exact representative of the electrode material as present in the cell.
Lithium-ion (Li-ion) battery assembles are increasingly becoming the energy storage system of choice in a variety of applications, from portable electronics to automobiles. The weight of Li-ion battery cells is less than other battery technologies, making them suitable for transport applications. This quality in conjunction with their relatively high energy density offsets their higher cost.
Figure 2 shows a Li-ion cell’s core components, and at least one of them needs to be improved in order to achieve improved Li-on cell performance, for instance improved energy density, improved recharge times, and reduced weight and cost.
During operation, Li+ ions are generated through oxidation of lithium stored in the anode. They then travel towards the cathode through the electrolyte and separator film. At the cathode, a compound is created when anion is oxidized and stores the arriving Li+ ions. During recharging, the ion flow is reversed and reduction of ions occurs to store them back as lithium metal in the anode.
The anode typically has a graphite structure intercalated with lithium. The cathode consists of a lithium metal oxide with compositions that vary based on the desirable characteristics of the cell. Li(NiMnCo)O2 (NMC – nickel manganese cobalt), LiMn2O4 (LMO – lithium-manganese), and LiCoO2 (LCO – lithium-cobalt) are some of the most widely used cathode materials. The stoichiometry of these oxides changes in response to charging and discharging of the cell (direction of Li+ ion flow).
A solid-electrolyte interphase (SEI) layer is formed on the anode as a by-product of the charge and discharge process, affecting the reversible lithium intercalation process. The battery capacity is lowered when the SEI layer is present, eventually leading to the failure of the cell.
Therefore, it is crucial to gain insights into the SEI layer in order to control its formation to achieve improved cell performance. The complex mix making up the interphase layer can be chemically characterized with XPS depth profiling, enabling to determine the chemistries of the SEI layer.
Considering the highly air- and moisture-sensitive nature of lithium, the electrode materials must be transferred into the K-Alpha+ XPS system without being exposed to air for a successful analysis.
To achieve this, the samples are fed into the K-Alpha+ Vacuum Transfer Module (VTM) in a glove box. As shown in Figure 3, after evacuating the VTM in the glove box antechamber, it is transferred to the XPS system.
Figure 3. The vacuum transfer module allows samples that have been prepared in an inert environment to be transferred into the spectrometer chamber without exposure to air.
Since the air pressure holds the VTM together, the VTM automatically opens in the K-Alpha+ XPS system during the pump-down cycle and is therefore incorporated into the standard, automated, sample transfer process.
This experiment analyzed two cathode samples, of which one was a pristine, unused cathode and the other was from a cell that had been subjected to several charge/discharge cycles and was in the charged state when dismantling the cell.
Figure 4 shows the survey spectra acquired from the as received cathode sample. A binder medium is used to hold the cathode material (Li(NixMnyCoz)O2) together.
The binder is a mixture of polymers containing oxygen and fluorine and can be apparently observed for the pristine sample as a considerable amount of residue on the cathode surface.
If the binder residue transports through the electrolyte, a surface layer is created due to the reaction of the binder residue. This layer acts a barrier to ion transport.
The presence of the binder can still be observed on the cycled cathode, which also shows the presence of residue from the electrolyte at the cathode surface. The composition variation for the NMC components of the two samples other than oxygen is presented in Figure 5.
Although the relative intensities of the Co, Mn, and Ni components are identical for the two samples, the amount of Li measured is roughly 40% of that observed in the pristine cathode, as expected for a sample taken from a charged cell due to depleted lithium level in the cathode caused by the movement of Li+ ions towards the anode.
The results clearly demonstrate the advantage of using the combination of the K-Alpha+ XPS system and the vacuum transfer module in the analysis of Li-ion battery components. The expected change in lithium content was determined from the analysis of cycled and unused cathode samples.