Structural evolution of the MoO3(010) surface during lithium intercalation

Atomic force microscopy (AFM) has been used to characterize the structural evolution of the MoO3(010) surface during the initial stage of Li+ intercalation. Lithiation was observed in situ using model cells comprised of a single crystal MoO3 cathode, a dilute propylene carbonate (PC)-based electrolyte, and an Li metal anode. The insertion of Li+ into MoO3 results in the topotactic nucleation and growth of acicular LixMoO3 (x∼0.25) precipitates at the (010) surface. Because the interlayer spacing of LixMoO3 (d=7.88 Å) is greater than that of MoO3 (d=b/2=6.93 Å), the LixMoO3 precipitates expand out of the (010) surface as they grow into the MoO3 crystal along 〈010〉. The local strain associated with this expansion causes the LixMoO3 precipitates to crack parallel to the MoO3〈001〉 and 〈010〉 axes once their height exceeds approximately 20 nm. With continued Li+ intercalation, cracking becomes more prominent, and the (010) surface begins to fragment and disintegrate. Anisotropies in Li+ ion uptake and the influence of surface morphology on LixMoO3 precipitation and crack propagation are described.