For a large number of applications, from automobiles to portable electronics, lithium-ion battery assembles have become the energy storage solution of choice. Lithium ion (Li-ion) battery cells are lightweight compared to other battery technology, which, combined with their relatively high energy density, makes them appropriate and potentially cost-effective for transport applications.
The first part of this post, Lithium Mining Today May Influence What You Drive In the Future, discussed the growing demand for lithium and other Li-ion battery raw materials as interest in hybrid and electric cars gains momentum, as well as current (hard rock mining, surface brines) and potential methods for obtaining lithium. Lithium is a common but poorly concentrated mineral, and there is considerable interest in increasing the supplies of raw materials needed to make lithium-ion batteries, and to develop them further.
Despite the popularity of lithium-ion batteries, there is still room for improvement in the performance of Li-ion cells, for example to increase energy density, reduce weight, decrease costs, and improve recharge times. Improving such characteristics involves developing improvements to at least one of the core components of the cell.
When operating, lithium stored in the anode is oxidized, and the Li+ ions created transport through the electrolyte and separator film to the cathode. In the cathode, it is the anion that is oxidized, creating a compound that can store the arriving lithium ions. When the cell is recharged after use, the flow of ions is in the opposite direction, and they are reduced back to lithium metal to be stored in the anode. The anode is typically made from graphite, with lithium intercalated into the graphite structure. The cathode is comprised of a lithium metal oxide, the exact composition of which varies depending upon the required characteristics of the cell. The most commonly used cathode materials are LiCoO2 (LCO – lithium-cobalt), LiMn2O4 (LMO – lithium-manganese), LiFePO4 (LFP – lithium-phosphate), and Li(NiMnCo)O2 (NMC – nickel manganese cobalt). These oxides change in stoichiometry depending on whether the cell is charged or discharged; i.e., if the flow of Li+ is to or from the cathode.
XPS Offers a Closer Look at Li-ion Battery Function
A by-product of the charge and discharge process is the formation of the solid-electrolyte interphase (SEI) layer on the anode. The formation and development of the SEI layer competes with the reversible lithium intercalation process, and over the lifetime of the battery the presence of the SEI will contribute to the lowering of capacity, and is a contributing factor to the ultimate failure of the cell. Understanding the SEI layer is an area of significant interest, so that it can be controlled and therefore improve cell performance. X-ray photoelectron spectroscopy (XPS) depth profiling offers a way of chemically characterizing the complex mix that makes up the interphase layer, allowing an identification of the chemistries that comprise the SEI. Read Analysis of Electrode Materials for Lithium Ion Batteries describing the use of an XPS system to investigate unused and cycled cathode samples and determine variations in lithium content.