面向可充电电池的锂金属负极的枝晶生长:理论基础、影响因素和抑制方法

在全球能源与环境问题日趋紧迫的大背景下,可再生能源的获取与利用途径及高效安全的储能技术的研发一直是工业界和科学界关注的热点之一。锂离子二次电池作为能量存储器件,拥有高比能量、长循环寿命等优点,近十几年来其研究取得了长足进展,并在各类便携式电子设备和电动交通工具中获得了广泛应用。然而,随着各种高性能设备的不断涌现,商业化的锂离子电池越来越难以满足其在能量密度、循环稳定性和安全性等方面的要求。为了进一步提高锂离子电池的能量密度,需要开发出高比容量的负极材料(硅、锡和锂等)以取代传统石墨负极。硅、锡等新式负极材料通过与锂离子反应形成含锂化合物的原理来存储与释放锂离子,完成电池的一个充放电过程。这个过程往往伴随着负极材料体积的剧烈变化,经历较长时间循环使用后会导致负极材料的粉化甚至从集流体上剥离,引起电池容量迅速衰减甚至失效。而锂负极通过锂在负极上的溶解和沉积来完成电池的充放电过程,该过程不存在反应相变所导致的体积变化。另外,锂金属负极材料具有极高的质量比容量(3 860mAh/g)、低密度(0.59g/cm3)和低的还原电位(-3.04V,相比于氢标准电极),被认为是一种理想的可充电电池负极材料。然而,锂的枝晶生长、锂金属电池低的库伦效率和锂的无主体沉积引起的体积膨胀等一些关键问题长期以来制约着锂负极的商业应用。锂的每次沉积都会产生枝晶,在充放电循环中,锂枝晶会导致电池内部短路甚至发生爆炸,带来严重的安全问题。除此之外,锂枝晶还会增加负极表面积,新暴露的锂金属会与电解液反应生成固态电解质膜(Solid electrolyte interface,SEI),这会损耗活性材料以及降低电池的库伦效率。为了解决以上问题,研究者们对锂金属电极进行了许多探索,尤其是在锂枝晶生长的机理及其抑制方法方面。一些理论模型如扩散模型、SEI保护模型、电荷诱导生长模型和薄膜生长模型等,以及与这些模型相对应的一些抑制方法如均匀锂离子流法、SEI膜保护法、稳定沉积主体法和静电屏蔽保护法等被提出。这些抑制方法能够在一定程度上缓解锂枝晶的生长问题,但都未能达到商业化应用的要求。本文总结了近几年研究人员针对锂离子电池锂金属负极的一些重要研究,系统地介绍了业内较为认同的枝晶生长模型和影响因素,并着重叙述了抑制枝晶生长的方法及成效,最后就锂金属负极将来的研究方向给出一些建议。

Dendrite Growth of Lithium Metal Anode for Rechargeable Batteries: Theoretical Basis, Influencing Factors and Inhibition Methods

The worldwide increasingly tough circumstance of energy and environment urges the continuous and intensive inte-rest both in industry and in science upon the utilization of renewable energy sources and the development of efficient and safe energy storage techniques.Lithium-ion secondary batteries are a kind of high-specific-capacity and long-cycle-life energy storage devices which over the past decade have acquired considerable and fruitful research attentions and found wide applications in a rich variety of portable electronic devices and electric vehicles.On the other hand,the fast emergence of high-performance electronics has exaggera-ted the requirements of energy density,cycle stability and safety to a remotely high level for the commercialized lithium ion batteries. For the sake of increasing Li-ion batteries’energy density,anode materials with high capacity,e.g.silicon,tin,lithium,have been developed to substitute for the conventional graphite anode.Either silicon or tin anode stores and releases Li ions by reacting with them and forming lithium-containing compounds within a charge-discharge process,which generally accompanies huge volume change of the anode.This will cause these anode materials to be pulverized or even stripped from the current collector after expe-rienced relatively long time usage,and in consequence,the rapid fading of battery capacity and even failure.By contrast,Li metal anode’s charge-discharge process is based on dissolution/deposition of lithium on the current collector,and this process involves no reaction-phase-transition-induced volume change.Furthermore,lithium metal is regarded as an ideal candidate material for rechar- geable batteries due to its high theoretical specific capacity (3 860 mAh/g),low density (0.59 g/cm3)and the lowest electrochemical potential (-3.04 V vs RHE)within all anodes.Unfortunately,some key issues,such as Li dendrite growth,low columbic efficiency and uncontrolled-deposition-induced volume expansion,have restrained the commercialization of Li metal anodes. Each time of lithium deposition will cause the growth of dendrites,and thus the Li dendrites will lead to internal short circuits and even battery explosion,which is considered as a serious safety problem.In addition,lithium dendrites will also increase the anode surface area,enabling the reaction between newly exposed lithium and electrolyte to form solid electrolyte interphase (SEI)that cau-ses low columbic efficiency.In order to solve these problems,researchers have done a lot of studies on the lithium metal anodes,es-pecially on the mechanism and inhibition methodology of dendrite growth.There have been proposed several theoretical models which describe the formation and growth behavior of Li dendrites,including diffusion model,SEI protection model,charge-induced growth model and film growth model,and furthermore,the corresponding inhibition solutions,such as uniform lithium ion flux method, SEI protection method,host-assisted stable deposition and electrostatic shielding method.By adopting these methods,the growth of lithium dendrites can be alleviated to a certain extent,but still the Li metal anodes are unsatisfactory for commercialization. Herein,we summarize the global research works of lithium metal anode in recent years,introduce systematically several gene-rally recognized Li dendritic growth models and the corresponding influencing factors,and emphatically clarify the inhibition methods of dendrite growth and their effectiveness.Finally,we also put forward some suggestions for the future research direction of lithium metal anodes.