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

Abstract

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 (4.03V) and then decreases during the first charge. The decreasing trend of the c lattice parameter with time during the hold at 4.7V and 4.9 V agrees very well with the change of current. Overall the structural changes appear highly reversible when 4.7V is used as an upper cutoff voltage, even following a 10h hold at 4.7V. However, the electrode/electrolyte changes dramatically when charged and held at 4.9V. There is a significant drop in background attributed to electrolyte decomposition and an unexpected increase in the a lattice parameter is noted after the 4.9V hold. Therefore, the electrolyte system used is both beneficial for in-situ neutron diffraction studies and battery performance until 4.7V, but appears to degrade in combination with the electrode at 4.9V. 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 4.0V.