Understanding structural changes in NMC Li-ion cells by in situ neutron diffraction

• NMC cell was studied in-situ using neutron diffraction and electrochemistry.
• Transformation of the anode into LiC6 and LiC12 was found in higher charge states.
• A change of the Li occupation and lattice parameters in the cathode were observed.
• The cation mixing in the cathode vanishes if the cell capacity is above ∼0.8Qmax.
• The cation mixing can be one of the reasons of nonlinear lattice parameter changes.

Commercial NMC cells of 18650-type based on a Lix(Ni0.5Mn0.3Co0.2)O2 cathode and a graphitic anode were studied in situ using a combination of high-resolution monochromatic neutron powder diffraction and electrochemical analysis. The structural changes of the electrode materials during cell charge/discharge have been determined using Rietveld refinement and single profile decomposition techniques. A transformation of the graphitic anode to LiC12 and LiC6 through the formation of higher ordered lithium intercalated carbons was observed. A different behavior of electrochemically-driven lattice distortion was observed for NMC material in comparison to LixCoO2 and its influence on the overall cell performance has been discussed in brief. Detailed analysis of the structural changes in the Lix(Ni0.5Mn0.3Co0.2)O2 cathode material revealed reversible Li/Ni cation mixing (5.6(8)%), which is state-of-charge independent below 1600 mAh and vanishing above 1800 mAh (∼0.8Qmax).