The octahedral void exposes SS in FeS2FeS2 on the surface and made it vulnerable to a free Li2Li2. One Fe−SFe−S ionic bond is broken with the formation of one Li2−SLi2−S bond. This one broken bond out of six with a big LiLi jammed in the lattice causes the lattice structure to crack, and data from X-ray absorption suggests that Li2FeS2Li2FeS2 is amorphous. At room temperature, on recharge the lattice does not reform and a different sets of redox reactions applies which results in the precipitation of FeFe and SS. Both are undesirable in the operation of the battery.
It's the hole's fault, we need a bigger hole.
A mix of smaller "late" transitional metal might allow FeFe to open up. Cobalt CoCo is a suitable candidate.
And CoS2CoS2 (beta form) can serve as a backing lattice to keep FeS2FeS2 structural integrity. But CoCo is further down the reactivity series, it is likely that CoCo is reduced by LiLi before FeFe in which case we would use FeS2FeS2 as the backing lattice and use CoS2CoS2 to receive Li2Li2. CoS2CoS2 will insulate FeS2FeS2 from Li2Li2 while FeS2FeS2 provides structural integrity.
Another big "late" transitional metal is MnMn with crystal radius of 0.81×10−100.81×10−10m. MnMn is more reactive than FeFe. In place of FeFe as the backing lattice and dopant for a reactive CoS2CoS2 layer might open the lattice (CoS2CoS2 doped with MnMn lattice) further and make SS more accessible to Li2Li2.
