The effect of chemically preintercalated alkali ion on structure of layered titanates and their electrochemistry in aqueous energy storage systems

Chemical Preintercalation Synthesis of Versatile Electrode Materials for Electrochemical Energy Storage

ConspectusThe widespread use of electrical plants and grids to generate, transmit, and deliver power to consumers makes electricity the most convenient form of energy to transport, control, and use. Balancing electricity demand with electricity supply requires a mechanism for energy storage, which is enabled by electrical energy storage devices such as batteries and supercapacitors. In addition to the grid-level energy storage, we have all witnessed the quick growth of a number of applications that require autonomous power, illustrated by the Internet of Things, and electrification of transport. Batteries, when developed for targeted applications with specific requirements, require new materials with improved performance enabled by rational design on the atomic level. The material tunability knobs include chemical composition, structure, morphology, and heterointerfaces, among others. Synthesis methods that could enable control of these parameters while offering versatility and being facile are highly desired.In this Account, we describe a synthesis strategy for the creation of new intercalation host oxides, hybrid materials, and compounds with oxide/carbon heterointerfaces for use as electrodes in intercalation batteries. We begin by introducing a strategy called the chemical preintercalation synthesis approach and describing processing steps that can be used to tune the material's chemical composition, structure, and morphology. We then show how the chemical preintercalation of inorganic ions can be used to improve the ion diffusion and stability of the synthesized materials. We reveal how confined interlayer water can be controlled and how the degree of hydration affects the electrochemical performance. This is followed by a demonstration of the chemical preintercalation of organic molecules leading to unprecedented expansion of the interlayer region up to ∼30 Å and initial electrochemical characterization of the obtained hybrid materials. We then present evidence that the carbonization of the interlayer organic molecules is an efficient synthetic pathway for creating oxide/carbon heterointerfaces and improving the electronic conductivity of oxides, which leads to improved stability and rate capability during electrochemical cycling. The examples discussed in this Account show that the chemical preintercalation synthesis approach opens pathways for the preparation of materials that have not been synthesized previously, such as new phases, hybrid materials, and 2D heterostructures with advanced functionalities. We demonstrate that chemical preintercalation can be used to effectively tune the chemistry of the confined interlayer region in layered phases and form tight oxide/carbon heterointerfaces enabling control of the material properties at the atomic level.

Intercalation-exfoliation processes during ionic exchange reactions from sodium lepidocrocite-type titanate toward a proton-based trititanate structure

Topochemical reactions involving ionic exchange have been used to assess a large number of metastable compositions, particularly in layered metal oxides. This method encompasses complex reactions that are poorly explored, yet are of prime importance to understand and control the materials' properties. In this work, we embark on investigating the reactions involved during the ionic exchange between a layered Na-titanate (lepidocrocite-type structure) and an acidic solution (HCl), leading to a protonic (H3O+) titanate (trititanate structure). The reactions involve an ionic exchange provoking a structural change from the lepidocrocite-type to the trititanate structure as shown by real-space refinements of ex situ pair distribution function data. Mobile Na+ ions are exchanged by hydronium ions inducing high proton mobility in the final structure. Moreover, the reaction was followed by ex situ23Na and 1H solid-state MAS NMR which allowed, among other things, confirming that the Na+ ions are in the interlayer space and specifying their local environment. Strikingly, the ionic exchange reaction induces progressive exfoliation of the Na-titanate particles leading to 2-5 nm thin elongated crystallites. To further understand the different steps associated with the ionic exchange, the evolution of the electrolytic conductivity, using conductimetric titration, has been monitored upon HCl addition, enabling characterization of the intercalation(H+)/de-intercalation(Na+) reactions and assessing kinetic parameters. Accordingly, it is hypothesized that the exfoliation of the particles is due to the accumulation of charges at the particle level in relation to the rapid intercalation of protons. This work provides novel insights into ionic exchange reactions involved in layered oxide compounds.