In 𝛿-MnO 2 , ion insertion voltages are closely spaced at low capacities (i.e. high Mn oxidation states) (Fig. ), as the distance between Mn layers can expand or contract to accommodate inserted ions in their preferred coordination environment. This finding helps explain the pseudocapacitive behavior of layered MnO 2 in aqueous electrolytes. In λ-MnO 2 (Fig. ), Zn 2+ and Li + are favored to insert at low capacities into the 8a tetrahedral sites. At higher capacities, once the tetrahedral sites are full, continued insertion forces these ions to occupy octahedral sites (above Mn 3.5+ for Li + and above Mn 3+ for Zn 2+ ), leading to a corresponding drop in the insertion voltages. The relatively high stability of Zn 2+ in tetrahedral sites explains why hetaerolite (spinel ZnMn 2 O 4 ) is a common degradation product in primary alkaline batteries with Zn anodes and in secondary Zn 2+ shuttle batteries with MnO 2 cathodes. Spinel LiMn 2 O 4 is also a well-known Li-ion battery cathode and was the first demonstrated positive electrode in aqueous Li-ion batteries, although its lower voltage compared to Ni and Co-based cathodes limits its desirability for volumeconstrained applications.