Fibre Optic Sensor for Characterisation of Lithium‐Ion Batteries

The interaction between a fibre optic evanescent wave sensor and the positive electrode material, lithium iron phosphate, in a battery cell is presented. The optical–electrochemical combination was investigated in a reflection‐based and a transmission‐based configuration, both leading to comparable results. Both constant current cycling and cyclic voltammetry were employed to link the optical response to the charge and discharge of the battery cells, and the results demonstrated that the optical signal changed consistently with lithium ion insertion and extraction. More precisely, cyclic voltammetry showed that the intensity increased when iron was oxidised during charge and then decreased as iron was reduced during discharge. Cyclic voltammetry also revealed that the optical signal remained unchanged when essentially no oxidation or reduction of the electrode material took place. This shows that optical fibre sensors may be used as a way of monitoring state of charge and electrode properties under dynamic conditions.