Magnetometry offers a powerful tool for studying Li-ion battery electrode materials since their magnetic properties are highly sensitive to phase composition, structural disorder, defects and the oxidation state of the transition metals ions. A novel pathway in this field has recently opened up by developing and applying in-situ techniques for studying magnetic behaviour during electrochemical charging. Based on our recent in-situ experiments in a SQUID , we have designed an electrochemical cell allowing operando measurements of the magnetic susceptibility of battery electrodes during charging and discharging.
As case study, charging and discharging of a commercial used paramagnetic LiCo-Oxide cathode were monitored during consecutive cycles . The magnetic susceptibility of this cathode material changes by more than a factor of 1.5 upon charging/discharging and, therefore, serves as highly sensitive fingerprint for the charge state. The continuous monitoring of the suseceptibility with varying Li content yields evidence for a nonmetal–metal transition of Anderson-type and of charge compensation where in addition to Co oxidation a joint charge transfer with O is involved.
Operando SQUID measurements on the technological highly relevant cathode material LiNiCoMn-Oxide reveal - among others - a kink-like variation of the susceptibility with Li content x indicating the subsequent oxidation of Ni from 2+ to 3+ (1≥ x ≥0.66) and from 3+ to 4+ (0.66 ≥ x ≥0.33) with decreasing x. The present studies therefore demonstrate that continuous monitoring of the susceptibility allows a clear distinction between different magnetic behavior for various lithium contents which yields novel insights into the electronic and chemical processes in the electrode during charging and discharging.
Financial support by the Graz inter-university cooperation on natural sciences (NAWI Graz) is appreciated.
 Topolovec, S.; Krenn, H.; Würschum, R.: Electrochemical cell for in situ electrodeposition of magnetic thin films in a superconducting quantum interference device magnetometer; Review of Scientific Instruments 86 (2015) 063903. DOI: 10.1063/1.4922462
 Topolovec, S.; Kren, H.; Klinser, G.; Koller, S.; Krenn, H.; Würschum, R.: Operando magnetometry on LixCoO2 during charging/discharging; Journal of Solid State Electrochemistry, accepted. DOI: 10.1007/s10008-015-3110-6