Depth profiling Li in electrode materials of lithium ion battery by 7Li(p,γ)8Be and 7Li(p,α)4He nuclear reactions

Abstract

A proton induced γ-ray emission method based on 7Li(p,γ)8Be proton capture reaction and a nuclear reaction analysis method involving 7Li(p,α)4He reaction are described for depth profiling Li in the electrode materials, graphite and lithium cobalt oxide for example, of a Li-ion battery. Depth profiling by 7Li(p,γ)8Be reaction is accomplished by the resonance at 441keV and involves the measurement of 14.6 and 17.6MeV γ-rays, characteristic of the reaction, by a NaI(Tl) detector. The method has a detection sensitivity of ∼0.2at% and enables profiling up to a depth ≥20µm with a resolution of ≥150nm. The profiling to a fairly large depth is facilitated by the absence of any other resonance up to 1800keV proton energy. The reaction has substantial off-resonance cross-sections. A procedure is outlined for evaluating the off-resonance yields. Interferences from fluorine and aluminium are major limitation of this depth profiling methodology. The depth profile measurement by 7Li(p,α)4He reaction, on the other hand, utilises 2–3MeV protons and entails the detection of α-particles at 90° or 150° angles. The reaction exhibits inverse kinematics at 150°. This method, too, suffers interference from fluorine due to the simultaneous occurrence of 19F(p,α)16O reaction. Kinematical considerations show that the interference is minimal at 90° and thus is the recommended angle of detection. The method is endowed with a detection sensitivity of ∼0.1at%, a depth resolution of ∼100nm and a probing depth of about 30µm in the absence and 5–8µm in the presence of fluorine in the material. Both methods yielded comparable depth profiles of Li in the cathode (lithium cobalt oxide) and the anode (graphite) of a Li-ion battery.