Lattice Mismatch The simulations above do not include long-range elastic energy from lattice mismatch. It has been suggested that the surface phase suppresses the lattice collapse, thereby preventing H3 phase formation. Since PBE+U cannot capture the lattice collapse of the H3 phase due to the lack of van der Waals interactions, SCAN+rVV10 was used to quantify the lattice pinning effect caused by the surface phase. The calculated equilibrium c lattice parameters are 13.39 Å for the H3 phase and 14.24 Å for the H2 phase, while the experimental values are 13.338 Å and 14.399 Å, respectively. The Li layer spacing of the H3 phase was then fixed to that of the H2 phase to eliminate the change in lattice mismatch between the bulk and surface phases after the H2-H3 phase transition. The energy increase due to this constraint reflects the energy penalty from the lattice pinning. The resulting charge voltage increases is 0.040 V (0.058 V using experimental c), which accounts for only a small fraction of the overpotential observed. In reality, strain decays to zero far from the interface, so applying this constraint to the entire bulk provides an upper bound for the energy penalty.