Mechanosynthesis of Energy Materials

本文重点介绍了机械化学合成作为一种相对较新的方法来制备用于能源应用的氧化物和混合阴离子材料。球磨广泛用于在随后的高温反应前减小试剂的粒度，增加其表面积，提高其反应活性。通过使用高能行星球磨机，反应可以在环境温度下完成，而不需要后续的高温反应。一个主要的优点是可以制备在高温下不稳定的新材料，并且不能通过传统方法制备。该方法最近已成功用于合成锂电池的新型正极材料。本提案旨在为机械化学合成作为一种多功能合成程序的成功使用创造条件，并通过对具有无序岩盐晶体结构的材料进行有针对性的计划来实现这一目标。通过掺杂和成分控制，可以获得具有长循环寿命的可重复、高充放电容量的材料。可充电锂电池是世界范围内的主要能源来源，用于为各种便携式电气设备、混合动力和全电动汽车提供动力，以及与风能和太阳能可再生能源的日益使用相关的大型发电站负荷平衡装置。这些电池的范围从可以植入人体的微型设备到作为电网一部分的兆瓦级储能设备。虽然目前的锂电池技术已经很成熟，但仍然需要改进系统，以实现更高的能量存储容量，增加循环寿命，并通过开发无毒和低成本的替代品来改善环境兼容性，而不是使用钴作为阴极的关键氧化还原活性成分。新型锂阴极的开发需要一种合成方法，这种方法可以扩大规模，使材料在电池多次充放电过程中表现出可重复性，而不会损失性能。作为LiCoO2的潜在替代品，有几组材料已经进行了深入的研究和开发，其中包括三组具有岩盐相关晶体结构的材料。它们是：类似于LiCoO2的层状结构，但Co被其他过渡金属组合取代；最近，具有较高锂含量的类似层状结构因其高容量和氧氧化还原对容量的贡献而受到特别关注；最近，立方体无序岩盐结构具有看似简单的晶体结构，其阳离子随机分布在立方体紧密排列的阴离子亚晶格中的八面体位点上。这些后一种材料是本提案的主要焦点，既可以利用机械合成优化操作变量，也可以在研究较少的材料家族中瞄准新成分，这些材料具有相当大的潜力作为锂电池阴极。对这些无序岩盐结构材料的了解尚不完整，因为它们的纳米级尺寸、成分的不确定性，特别是它们的氧化学计量、它们的局部晶体结构可能具有有序域或缺陷簇，以及它们的表面结构在许多情况下对大气敏感。许多具有高电荷存储容量，但电化学性能，材料合成过程和结构/组成特性之间的相关性尚未很好地建立。在该项目中，将制备许多新材料，更好地了解无序岩盐结构的电化学性质，并为机械合成技术的广泛应用建立指导方针。

This proposal focuses on mechanochemical synthesis as a relatively new method to prepare oxide and mixed anion materials for energy applications. Ball milling is widely used to reduce particle size of reagents, increase their surface areas and increase their reactivity before subsequent high temperature reaction. By using high energy, planetary ball mills, reaction can be carried out to completion at ambient temperatures without the need for follow-on high temperature reaction. A major advantage is the possible preparation of new materials that are unstable at high temperatures and cannot be prepared by traditional routes. The method has been used with some recent success to synthesise new cathode materials for lithium batteries. This proposal aims to establish the conditions for successful use of mechanochemical synthesis as a versatile synthesis procedure and to achieve this by a targetted programme on materials with disordered rock salt crystal structures. There is much scope, by doping and compositional control to achieve reproducible, high charge-discharge capacity materials that have long cycle lives.Rechargeable lithium batteries provide a major source of energy worldwide that are used to power a wide range of portable electrical devices, hybrid and all-electric vehicles, together with large-scale power station load levelling installations linked to the increasing use of wind and solar renewable sources of energy. The batteries range from miniature-scale devices that can be implanted into the human body to megawatt-scale energy storage as part of electricity grid networks. Although current lithium battery technologies are well-established, there is still a great need for improved systems with the objectives of higher energy storage capacity, increased cycle lifetimes and improved environmental compatibility through development of non-toxic and lower cost alternatives to the use of cobalt as a key redox active component of cathodes.The development of new lithium cathodes requires synthesis methods which can be scaled up to give materials that behave reproducibly during battery charge - discharge over many cycles without loss of performance. Several groups of materials have undergone intensive research and development as potential substitutes for LiCoO2, including three groups with rock salt-related crystal structures. These are: layered structures similar to LiCoO2, but with Co replaced by other transition metal combinations; more recently, similar layered structures with a higher Li content that are of particular interest for their high capacities and oxygen redox contributions to capacity; most recently, cubic disordered rock salt structures which have a deceptively simple crystal structure that has cations distributed at random over octahedral sites in a cubic close packed anion sublattice. These latter materials are the main focus of this proposal, both to optimise operational variables using mechanosynthesis and to target new compositions in a poorly-studied family of materials that have considerable potential as lithium battery cathodes.Understanding of these disordered rock salt structure materials is incomplete due to a combination of their nanoscale size, compositional uncertainty, especially concerning their oxygen stoichiometry, their local crystal structures with possible ordered domains or defect clusters and their surface structures which in many cases are atmosphere sensitive. Many have high charge storage capacities but correlations between electrochemical performance, materials synthesis procedures and structural / compositional characteristics are not well established. Many new materials will be prepared during this project, better understanding of the electrochemical properties of disordered rock salt structures gained and guidelines established for wider use of the mechanosynthesis technique.