Tailoring Stepped Layered Titanates for Sodium-Ion Battery Applications

ConspectusConcerns about sustainability and supply chain issues associated with lithium-ion batteries (LIBs) have led researchers and companies around the world to investigate alternative technologies. Of all the so-called "beyond LIBs", sodium-ion batteries (NIBs) are in the most advanced stage of development, and are being considered for grid storage applications as well as moderate-range electric vehicles. While graphite is the most commonly used anode material for LIBs, hard carbons are used in NIBs because sodium insertion into graphite does not occur to a useful extent. Other possibilities, based on cost and availability arguments, are titanates, which are generally denser than disordered carbons, meaning more material can be packed into a given volume, leading potentially to greater energy density. We have researched stepped layered titanates for use as anode materials, focusing on two types of structures. The first is "sodium nonatitanate" or NNT, with the composition NaTi3O6(OH)·2H2O having six Ti4+O6 octahedra joined together in steps to form layers with sodium ions and water in-between. The lepidocrocite-type titanate structure, contains zigzag layers (or steps one Ti4+O6 unit across). These exist in a wide range of compositions, and contain large exchangeable cations between the layers. An unusual feature of both NNT and the lepidocrocite titanates is the very low potentials (0.3–0.5 V vs Na+/Na) at which they insert sodium. This makes them particularly attractive for anode applications. Another interesting feature is the ability to tailor the electrochemical properties by various modifications, such as heat-treatment to remove water and change the structure, introduction of vacancies, ion-exchange, surface modifications, and carbon coating or graphene wrapping, all of which alter the electrochemical properties. Finally, heterostructuring (interleaving titanate layers with carbon) results in new materials with different redox properties. For all the titanates, the first cycle Coulombic efficiency (C.E.) is very sensitive to the binder used in the electrode fabrication and the electrolyte used. Because sodium insertion occurs at such a low potential, some electrolyte and binder are irreversibly reduced during the first cycle to form a protective solid electrolyte interphase (SEI). In a full cell, it is important to maximize the C.E. because all the cyclable sodium must come from the cathode, so cells must be overbuilt to compensate for these losses. Proper selection of binder and electrolyte results in improved cycling performance and minimal first cycle losses. Finally, examples of full cells containing some of the materials under discussion are provided.