Earth-abundant Cathodes for Na-ion Batteries

钠离子电池被认为是锂离子的主要竞争者，因为它们更便宜，而且由更丰富、更可持续的材料制成。然而，钠离子电池目前无法提供远程电动汽车所需的高体积和高重量能量密度。一个关键的制约因素是阴极。由于Na+离子的离子半径比Li+离子半径大，Na-离子阴极表现出更严重的结构转变。这限制了它们可以可逆循环的成分范围，从而导致较低的能量密度。尺寸差异的另一个后果是，Na+离子通常太大，无法被替换到3D基层状氧化物阴极的过渡金属位置，这意味着像Na2MnO3和Na1.2Ni0.2Mn0.6O2这样的富钠阴极在合成上是不可行的。已知的用于阴极的富Na材料主要有Na2RuO_3和Na_2IrO_3，它们所用的稀土金属价格昂贵，不适合大规模市场应用。本项目旨在探索制备富Na过渡金属氧化物的新的合成路线，以获得更高容量的Na离子阴极。在一种策略中，将选择在尺寸上更接近Na+(1.02A)而不是Mn4+(0.53A)的高价过渡金属离子，以有利于过渡金属层中存在Na+离子。它们将与低价的过渡金属离子结合，作为电荷补偿氧化还原中心。另一种策略将涉及利用无序获取富含钠的成分。具有d0电子组态和零晶场稳定能的过渡金属离子如Ti4+和Nb5+促进了富锂岩盐阴极中的阳离子无序。同样的原理将在钠离子体系中进行探索，以检验相同的效应是否转化为钠离子阴极以产生富钠材料。为了制备这些相，将探索在不同气氛和温度条件下的传统固相合成。所得到的材料将用一系列表征工具进行研究，以确认结构、形态和组成，并验证每种策略的成功程度。它们将作为正极材料在钠离子电池中进行检测，并将用衍射和光谱分析研究充放电反应过程中的结构和化学变化。将特别关注氧阴离子氧化还原的作用，预计在这些富钠材料中，氧阴离子氧化还原的程度很高。作为这一博士项目的结果建立的材料和理解将对推动更高能量密度的钠离子电池的发展具有重要意义。这项技术可以在廉价、大众市场的电动汽车中提供一种可行的锂离子替代品，并有助于绕过锂离子电池面临的资源稀缺和价格波动的问题。该项目属于EPSRC能量存储、电化学科学和能源应用材料研究领域。它是由EPSRC全额资助的。

Sodium-ion batteries are considered to be a main contender to lithium-ion as they are cheaper and made from more abundant, sustainable materials. However, Na-ion batteries cannot currently deliver the high volumetric and gravimetric energy densities that are required for long-range electric vehicles. A key constraint is the cathode. Due to the larger ionic radius of Na+ compared with Li+, Na-ion cathodes exhibit more severe structural transitions. This limits the compositional range over which they can be cycled reversibly leading to lower energy density. Another consequence of the difference in size is that Na+ ions are typically too large to be substituted into the transition metal sites of 3d-based layered oxide cathodes, meaning Na-rich cathodes such as Na2MnO3 and Na1.2Ni0.2Mn0.6O2 are not synthetically viable. The primary known examples of Na-rich materials which have been used as cathodes, Na2RuO3 and Na2IrO3, involve expensive rare earth metals which are not suitable for mass market application.This project aims to explore new synthetic routes to make Na-rich transition metal oxides in order to achieve Na-ion cathodes with higher capacities. In one strategy, high valence transition metal ions which are closer in size to Na+ (1.02 A) than Mn4+ (0.53 A) will be selected to favour the presence of Na+ ions in the transition metal layer. These will be combined with low valence, transition metal ions to act as charge compensating redox centres. Another strategy will involve using disorder to access Na-rich compositions. Transition metal ions such as Ti4+ and Nb5+ with d0 electron configurations and zero crystal field stabilisation energy are found to promote cation disorder in Li-rich rocksalt cathodes. This same principle will be explored in Na-ion systems to examine whether the same effect translates to Na-ion cathodes to yield Na-rich materials.To make these phases, conventional solid state synthesis under different atmospheres and temperature regimes will be explored. The resulting materials will be investigated with a range of characterisation tools to confirm structure, morphology and composition and verify the extent of success of each strategy. They will be examined as cathode materials in Na-ion cells and the structural and chemical changes during the charge and discharge reactions will be studied with diffraction and spectroscopy. A particular focus will be paid to the role of oxide anion redox which is anticipated at high degrees of desodiation in these Na-rich materials.The materials and understanding established as a result of this PhD project will be important in advancing the development of higher energy density Na-ion batteries. This technology could provide a viable alternative to Li-ion in cheap, mass market electric vehicles and help to side-step the issues of resource scarcity and price volatility that Li-ion batteries face.This project falls within the EPSRC Energy Storage, Electrochemical Sciences and Materials for Energy applications research areas. It is fully funded by the EPSRC.