Crystallographic insight into the size-reduction effect of MnNb2O6 on the enhanced Li+-storage capability of its Nb species

Despite the multiple advantages of MnNb 2 O 6 -based anodes for Li + storage, the relatively low Li + -storage activity of their Nb species limits their Li + -storage performances. Herein, two significantly different-sized MnNb 2 O 6 /N-doped carbon (NC) composites with primary MnNb 2 O 6 nanoparticles uniformly confined in NC substrate are synthesized, denoted as P-MNO-NC (as tiny as approximately 5 nm for primary MnNb 2 O 6 nanoparticles) and H-MNO-NC (20–50 nm for primary MnNb 2 O 6 nanoparticles), respectively. Compared with H-MNO-NC, the significantly downsized primary MnNb 2 O 6 nanoparticles of P-MNO-NC enhance surface atom density and MnNb 2 O 6 /NC interface density, thus raising surface energy and residual stress to promote lattice relaxation. Consistent with this, systematic characterizations and theoretical calculations confirm that P-MNO-NC features widened intra-lattice Nb-layer Li + channels and reduced Nb-layer Li + -diffusion energy barrier to facilitate Li + migration through Nb layers, which makes the Nb 5+ ions of P-MNO-NC more accessible to the intercalated Li + ions and thereby substantially enhances the Li + -storage capability of its Nb species. Correspondingly, P-MNO-NC exhibits significantly higher Nb 3+ content after lithiation than H-MNO-NC. The intercalation-type Li + -storage mechanism is also verified for both composites. On this basis, P-MNO-NC significantly outperforms H-MNO-NC in Li + -storage performances as anode for both lithium-ion batteries (LIBs) and hybrid lithium-ion capacitors (HLICs). This work provides a fresh crystallographic insight into the size-reduction effect of P-MNO-NC on its significantly enhanced Li + -storage capability, which can be adopted to optimize the intrinsic Li + -storage performances of other Nb-based anodes.