Characterization of the interface between LiCoO 2 and Li 7La 3Zr 2O 12 in an all-solid-state rechargeable lithium battery
The interfacial layer formed between a lithium-ion conducting solid electrolyte, Li 7La 3Zr 2O 12 (LLZ), and LiCoO 2 during thin film deposition was characterized using a combination of microscopy and electrochemical measurement techniques. Cyclic voltammetry confirmed that lithium extraction occurs across the interface on the first cycle, although the nonsymmetrical redox peaks indicate poor electrochemical performance. Using analytical transmission electron microscopy, the reaction layer (∼50 nm) was analyzed. Energy dispersive X-ray spectroscopy revealed that the concentrations of some of the elements (Co, La, and Zr) varied gradually across the layer. Nano-beam electron diffraction of this layer revealed that the layer contained neither LiCoO 2 nor LLZ, but some spots corresponded to the crystal structure of La 2CoO 4. It was also demonstrated that reaction phases due to mutual diffusion are easily formed between LLZ and LiCoO 2 at the interface. The reaction layer formed during high temperature processing is likely one of the major reasons for the poor lithium insertion/extraction at LLZ/LiCoO 2 interfaces.