  Lithium ion batteries (LIBs) work under sophisticated external force field
and its electrochemical properties could be modulated by strain. Owing to the
electro-mechanical coupling, the change of micro-local-structures can greatly
affect lithium (Li) diffusion rate in solid state electrolytes and electrode
materials of LIBs. In this study, we find that strain gradient in bilayer
graphene (BLG) significantly affects Li diffusion barrier, which is termed as
the flexo-diffusion effect, through first-principles calculations. The Li
diffusion barrier substantially decreases/increases under the positive/negative
strain gradient, leading to the change of Li diffusion coefficient in several
orders of magnitude at 300 K. Interestingly, the regulation effect of strain
gradient is much more significant than that of uniform strain field, which can
have a remarkable effect on the rate performance of batteries, with a
considerable increase in the ionic conductivity and a slight change of the
original material structure. Moreover, our ab initio molecular dynamics
simulations (AIMD) show that the asymmetric distorted lattice structure
provides a driving force for Li diffusion, resulting in oriented diffusion
along the positive strain gradient direction. These findings could extend
present LIBs technologies by introducing the novel strain gradient engineering.
