Exploration of vanadium sulfates as positive electrodes for battery applications

Abstract

The exploration and development of safer and less expensive positive electrode materials, for high energy density lithium ion batteries (LIBs), are essential for large scale applications, such as electric vehicles and grid energy storage. Of all the candidates, vanadium based compounds have attracted considerable interest. This is due to vanadium’s ability to adopt a range of oxidation states, with oxidation states of III, IV and V being particularly common in battery materials. This enables vanadium to undergo multi electron transfer processes, which allow reversible insertion/extraction of more than one Li⁺ ion per transition metal.

This thesis explores a range of vanadium based materials as candidate positive electrode materials for LIBs. The materials were separated into three groups (i) phases containing the mononuclear [VO] ³⁺ species, AVO(SO₄)₂ (A = NH₄⁺, K⁺), (ii) phases containing the mononuclear [VO₂]⁺ species, AVO₂SO₄ (A = NH₄⁺, K⁺), and (iii) V₂O₃(SO₄)₂ which contains the [V₂O₃]⁴⁺ species.

NH₄V⁽ⱽ⁾O(SO₄)₂ and KV⁽ⱽ⁾O(SO₄)₂ were studied initially. While these materials have previously been synthesised, their electrochemical properties have not been evaluated before. NH₄VO(SO₄)₂ and KVO(SO₄)₂ were prepared via solution based methods using low temperatures (≤200 °C) and were characterised using X-ray diffraction, infrared spectroscopy, scanning electron microscopy and optical microscopy. These compounds are isostructural and are made up of distorted VO₆ octahedra corner sharing with five SO₄ tetrahedra, forming layered structures. The thermal stability of these materials was examined by means of thermogravimetric analysis. Additionally, the intermediate phases identified from thermogravimetric analysis were characterised and this work led to the structural study of the layered material, 2VOSO₄·H₂SO₄, in order to determine the positions of the hydrogen atoms and evaluate the disorder associated with the H₂SO₄ layer. The electrochemical performance of NH₄VO(SO₄)₂ and KVO(SO₄)₂ as positive electrode materials for LIBs were tested for the first time. Both materials were found to operate at a high voltage of 4.05 V versus Li⁺/Li⁰ and presented similar discharge profiles and specific discharge capacities of <20 mA h g⁻¹.

Thereafter, the dioxovanadium phases, KV⁽ⱽ⁾O₂SO₄ and NH₄V⁽ⱽ⁾O₂SO₄ were explored. The synthesis of the KVO₂SO₄ phase has previously been reported in literature but its electrochemical properties were tested here for the first time. KVO₂SO₄ was synthesised by a solid state route at 300 – 400 °C, forming a three dimensional framework structure consisting of corner sharing VO₆ octahedra interconnected by corner sharing and edge sharing SO₄ tetrahedra. This material delivered a discharge capacity of 22 mA h g⁻¹ and its discharge profile showed multiple reduction processes at 3.05, 2.32 and 1.97 V versus Li⁺/Li⁰. Additionally, a new material, NH₄VO₂SO₄, was synthesised for the first time via a low temperature solution based route. The composition, structure, morphology, thermal stability and electrochemical properties of this new material were analysed using a range of techniques. Single crystal X-ray diffraction showed NH₄VO₂SO₄ crystallises in a large monoclinic unit cell, consisting of two chain types; (i) edge sharing VO₆ octahedra with bridging SO₄ tetrahedra and (ii) isolated VO₆ octahedra interconnected by bridging SO₄ tetrahedra. Despite the structural differences between NH₄VO₂SO₄ and NH₄VO(SO₄)₂, these materials exhibited similar thermal stabilities as well as comparable open circuit voltages (4.10 V versus Li⁺/Li⁰), discharge profiles and specific discharge capacities of <20 mA h g⁻¹.

Finally, a systematic investigation of the electrochemical and chemical insertion of Li⁺ into V⁽ⱽ⁾₂O₃(SO₄)₂ was performed. Although the synthesis of V₂O₃(SO₄)₂ has previously been reported, here it was prepared at a lower temperature of 140 °C and its electrochemical performance tested for the first time. This material contains the [V₂O₃]⁴⁺ species which consists of pairs of VO₆ octahedra linked together by an oxygen bridge. The corner sharing VO₆ octahedra are interconnected by bridging SO₄ tetrahedra, creating a three dimensional framework with open channels running down the c axis. A series of electrochemical tests were carried out to evaluate the lithiation and delithiation process for this material. V₂O₃(SO₄)₂ displayed an interesting electrochemical profile, with multiple reversible processes occurring between 1.95 and 4.20 V versus Li⁺/Li⁰. A discharge capacity of 160 mA h g⁻¹ was obtained, corresponding to the insertion of 2.0 mol of Li per mol of V₂O₃(SO₄)₂. Consequently, ex situ studies were carried out to determine the phase evolution and structural changes which occurred during of the lithiation and delithiation process. To further evaluate the Li⁺ insertion into this phase, V₂O₃(SO₄)₂ was chemically lithiated using n butyllithium. Inductively coupled plasma optical emission spectrometry, X-ray absorption spectroscopy, and infrared spectroscopy measurements showed that a composition of Li₄V₂O₃(SO₄)₂ can be attained by chemical lithiation. Interestingly, the structural studies evidenced by X-ray and neutron powder diffraction on the lithiated samples showed that a significant amount of Li⁺ can be inserted into V₂O₃(SO₄)₂, while it maintains its three dimensional framework structure.