In-situ Neutron Diffraction Study of a High Voltage Li(Ni0.42Mn0.42Co0.16)O2/Graphite Pouch Cell

The application of detailed in-situ neutron diffraction studies on lithium ion batteries has been limited in part due to the requirement of expensive deuterated carbonate-based electrolyte. This work presents an in-situ neutron diffraction study of the structural evolution of the Lix (Ni0.4Mn0.4Co0.2)O2 (NMC442) positive electrode material using a recently-developed low-cost deuterated ethyl acetate-based electrolyte. Rietveld analysis show that the NMC442 c lattice parameter gradually increases until x = 0.47 (4.03 V) and then decreases during the first charge. The decreasing trend of the c lattice parameter with time during the hold at 4.7 V and 4.9 V agrees very well with the change of current. Overall the structural changes appear highly reversible when 4.7 V is used as an upper cutoff voltage, even following a 10 h hold at 4.7 V. However, the electrode/electrolyte changes dramatically when charged and held at 4.9 V. There is a significant drop in background attributed to electrolyte decomposition and an unexpected increase in the a lattice parameter is noted after the 4.9 V hold. Therefore, the electrolyte system used is both beneficial for in-situ neutron diffraction studies and battery performance until 4.7 V, but appears to degrade in combination with the electrode at 4.9 V. By comparison to Li(Ni0.8Mn0.1Co0.1)O2 (NMC811), the contraction of the c lattice with increasing voltage and decreasing lithium content of the NMC442 is less rapid. The transition metal composition significantly affects the c lattice contraction above 4.0 V.