In Situ X-ray Absorption Studies of Cathode Materials for Rechargeable Lithium-Ion Batteries

Rechargeable batteries with high energy and power density are in great demand as energy sources for various purposes; e.g., portable telecommunication, computer equipment, hybrid electric vehicles, etc. Lithiumion batteries are the most promising candidates to fulfill such needs due to their intrinsic high discharge voltage and relatively light weight. The current commercial lithium-ion battery is based on a LiCoO 2 cathode and a graphitized anode. LiCoO 2 is an excellent cathode material, with good capacity, reversibility and charge/ discharge rate capability. However, due to the high cost of Co there has been a considerable interest in developing cathode materials based on Mn, V, or Ni. The key attributes required for a successful cathode material are: (1) a high intercalation potential (the voltage at which the material exchanges lithium), (2) a high lithium ion capacity (at least one lithium reacting per transition metal atom), (3) fast lithium ion diffusion kinetics (to allow high-rate cycling) and (4) complete chemical reversibility during repeated cycling (most easily attained if the crystal structure does not change drastically during lithiation/delithiation). Attributes (1) and (2) determine the energy density, (3) determines the power density and (4) determines the rechargeability and cycle life of the battery. In order to help design cathode materials with these attributes, it is of fundamental importance to understand the changes in the structural and electronic properties of the cathode materials during electrochemical cycling. In situ x-ray diffraction plays a vital role in elucidating the long-range structural changes that accompany lithiation/delithiation. In this report we highlight some of our recent results and findings, using in situ x-ray absorption spectroscopy (XAS), on the electronic and atomic structure of nickel/manganese oxide-based cathode materials. The element specific nature of XAS, its dependence only on the short-range order and its sensitivity to dilute elements make it an ideal tool to study this class of materials. The XAS experiments were performed at beamline X11A. All experiments were performed in situ using a specially designed spectro-electrochemical cell. The cell pack consists of the cathode, a Li foil anode, a Celgard separator and electrolyte (1 M LiPF 6 in an ethylene carbonate-dimethyl carbonate solvent). The cell was housed between two blocks of aluminum, machined to provide windows for the passage of x-rays and holes for bolts. The windows were sheets of 250μm thick mylar. A rubber gasket was used to make a hermetic seal. Provisions were made for current collection by using thin copper and aluminum strips. The cells were assembled in an argon-filled glove box.