RS Fellow - EPSRC grant (2014): Lattice-matched electrode-electrolyte interfaces for high-performance Li-batteries

锂离子电池就在我们身边。它们被用于便携式电子产品、混合动力汽车，甚至是Currency火星探测器的能量存储。随着我们减少对化石燃料的依赖，锂离子电池的使用只会增加。然而，今天的商用锂离子电池面临着寿命短的问题--笔记本电脑或手机电池的充电时间缩短--以及安全问题--最近报道的飞机和电动汽车起火事件就说明了这一点。如果锂离子电池要得到更广泛的应用，它们必须有更长的寿命和更好的稳定性。可以潜在解决的一种技术是“全固态”锂离子电池。常规电池具有液体电解质，当电池充电或放电时，液体电解质在阴极和阳极之间输送锂离子。这些电解质在化学上不稳定，并且随着时间的推移而降解-导致容量降低-或者在短路或强烈加热的情况下可能发生爆炸性反应。在全固态电池中，这些被化学惰性和坚固的陶瓷固体所取代，但仍然允许锂离子在电极之间移动。目前的挑战是开发出既能满足化学稳定性和良好的锂离子导电性要求，又能实现电子绝缘的固体电解质。电池性能的限制因素之一是锂在设备中移动的速度：这决定了锂的充电速度和电池的总功率密度。全固态电池的一个具体问题是，即使在允许快速锂传输的固体电解质的情况下，锂在电解质和电极之间的传输（或反之亦然）也可能很慢，最终限制了总的锂传输速率。电极和电解质之间的边界阻碍锂离子传导的能力是“界面电阻”，对于高功率应用来说，界面电阻应该很低。1985年，一项专利申请提出了一种设计具有低界面电阻的固态电池的策略：如果电极和电解质都具有基于相同的下层晶格的结构，那么应该可以找到“晶格匹配”的材料的组合，其具有用于电极和电解质的类似晶格尺寸。有人提出，这将允许锂传导路径在电极-电解质界面上排列，形成具有低界面电阻的连续通道，并且使得能够通过电池快速离子传输。直到最近，这种想法的应用一直受到缺乏有前途的固体电解质的限制，所述固体电解质具有允许与已知固体电极晶格匹配的晶体结构。然而，在2013年，一种新的固体电解质家族被报道具有与许多广泛研究的固体电极相同的潜在“尖晶石结构”，为这一概念注入了新的活力。该研究项目试图使用计算机模拟来了解尖晶石结构锂电解质的化学组成如何影响其对晶格匹配固态电池的适用性。我们将构建新的模型，能够用数字描述构成这些材料的原子的运动和相互作用。然后，这些模型将用于大规模计算机模拟，以研究化学差异如何控制这些材料中晶体结构和锂离子传导速率的细节。通过扩展这些模型来描述电极-电解质界面，我们将直接计算界面电阻，并探索“晶格匹配”作为高性能固态锂离子电池设计策略的潜力。

Lithium-ion batteries are all around us. They are used for energy storage in portable electronics, hybrid-electric vehicles, even the Curiosity Mars rover. As we reduce our dependence on fossil fuels, the use of lithium-ion batteries will only increase. Today's commercial lithium-ion batteries, however, face problems of short lifespans - the degrading charge-time of a laptop or phone battery - and safety issues - illustrated recently by the fires reported on aircraft and in electric cars. If lithium-ion batteries are to be used more widely they must have both longer lifetimes and improved s!afety characteristics.One technology that may potentially address is an "all-solid-state" lithium-ion battery. Conventional batteries have liquid electrolytes that transport lithium ions between the cathode and anode as the battery is charged or discharged. These electrolytes are chemically unstable, and degrade with time - leading to reduced capacity - or can react explosively in the case of a short-circuit or intense heating. In an all-solid-state battery, these are replaced by ceramic solids that are chemically inert and robust, yet still allow lithium ions to move between the electrodes. The challenge is to develop solid electrolytes that meet these specifications of chemical stability and good lithium-ion conductivity, while also being electronically insulating.One of the limiting factors for battery performance is the rate at which lithium can move through the device: this dictates the speed with which it charges and the total power density of the battery. a specific issue for all-solid-state batteries is that even in the case of solid electrolytes that allow rapid lithium transport, transferring lithium between the electrolyte and an electrode (or vice-versa) can be slow, ultimately limiting the overall lithium transport rate. This ability of the boundary between electrode and electrolyte to impede the conduction of lithium ions is the "interfacial resistance", which should be low for high-power applications.In 1985 a patent was filed that proposed a strategy for designing solid-state batteries with low interfacial resistance: if the electrodes and electrolyte all have structures based on the same underlying crystal lattice, then it should be possible to find combinations of materials that are "lattice-matched", with similar crystal lattice dimensions for the electrodes and electrolyte. It was proposed that this would allow the lithium- conduction pathways to line up across the electrode-electrolyte interfaces, forming continuous channels with low interfacial resistance, and enabling fast ionic transport through the battery.The application of this idea has until recently been limited by an absence of promising solid electrolytes with crystal structures that permit lattice-matching with known solid electrodes. In 2013, however, a new family of solid-electrolytes was reported with the same underlying "spinel-structure" as a number of widely studied solid electrodes, breathing new life into this concept.This research project seeks to use computer simulations to understand how the chemical composition of spinel-structured lithium-electrolytes affects their suitability for lattice-matched solid-state batteries. We will construct new models that are able to describe numerically the motions and interactions of atoms that make up these materials. These models will then be used in large-scale computer simulations to study how differences in chemistry control the details of crystal structure and rates of lithium ion conduction in these materials. By extending these models to also describe electrode-electrolyte interfaces, we will directly calculate interfacial resistances, and explore the potential for "lattice-matching" as a strategy for the design of high-performance solid-state lithium-ion batteries.