Beyond Li: Challenges in Moving Towards Earth-Abundant Battery Materials

Batteries are a necessary component towards the advancement and proliferation of modern day technology, and are also an essential piece of the transition towards renewable energy. The lithium-ion battery (LIB) is the most common type of rechargeable battery, and the archetype relies on a traditional layered transition metal oxide cathode, organic electrolyte with a lithium salt, and a graphite anode. The design of these cells has been optimized to the point that the energy densities in these batteries are approaching their theoretical capacities. Combined with the supply chain challenges associated with many typical cathode elements and increasing energy demand, this highlights the need for new earth-abundant, high energy density battery technology. This thesis addresses challenges in two such systems: Mg-S and sodium-ion batteries (SIBs). Mg-S batteries suffer from capacity fade related to the polysulfide shuttle effect, which results in loss of active material and passivation of the anode. Here, we demonstrate that the rate of passivation is inversely proportional to the chain length of the polysulfides present in solution, and that passivation can be slowed or even reversed through addition of S₈ and the consequent perturbation of existing polysulfide speciation equilibria. SIBs are frequently touted as a "drop-in" technology for LIBs due to both systems relying on mobile alkali ions, but SIBs have inherently lower energy densities due to larger Na⁺ ion. In Chapters 3 and 4 we explore anion redox as a method of increasing energy densities in SIBs--Chapter 3 shows that in LiNaFeS₂, the charge compensation mechanisms from Li and Na cycling are identical. However, Na⁺ cycling is worsened compared to Li⁺ by structural degradation from the removal and insertion of the bulky Na⁺ ion, emphasizing the differences that exist between optimizing SIB cathode performance compared with that of LIBs. In Chapter 4, we aim to develop structure-property relationships that enable a stronger understanding of anion redox that can be leveraged to design high energy density, multielectron redox cathodes. Through the examination of the electrochemically inactive NaCu1.5Fe0.5S₂ and its vacancy-containing derivative NaCu1.125Fe0.625S₂, we show that vacancies in the transition metal layer enable redox although the redox is observed occurs on the transition metals. The study also demonstrates potential limitations of ideal model systems and bulk spectroscopic analysis techniques in materials with low degrees of redox.