Development of Electrolytes towards Achieving Safe and High-Performance Energy-Storage Devices

DOI:10.1002/celc.201402277

Development of Electrolytes towards Achieving Safe and High-Performance Energy-Storage Devices:A Review

Yu Wang and Wei-Hong Zhong*[a]

CHEM ELECTRO CHEM

REVIEWS

1.Introduction

1.1.Electrolytes in Lithium-Ion Batteries

From a materials point of view,energy-storage devices (ESDs)are composites with special distribution of constituents,includ-ing a cathode and an anode,a salt solution as the electrolyte,highly conductive substrates (metals)as current collectors,and other functional additives.These components work together in response to charging and discharging.The structures and the distribution of the constituents inside an ESD [a lithium-ion battery (LIB),for example]are illustrated in Figure 1a.Specifi-

cally,the cathode and anode are electronically insulated,but connected by the electrolyte/separator.On both sides,the electrodes are connected to the current collectors.To achieve a high volumetric capacity,the electrodes are dominated by active electrode particles (the weight percentage in the elec-trodes is usually above 80%).As a result,the particles stack with other additives (usually,polymer binder or conductive fill-ers)and form porous electrodes.With so many components inside,LIBs are rich in interfaces,as demonstrated in Figure 1b.These interfaces fundamentally contribute to the overall per-formances of LIBs,including overall capacity,energy/power density,cycle performance,and so on.Among these interfaces,

the interface between the electrolyte and the electrodes is most important,as it controls the dynamics of lithium ions inside the battery.Moreover,from the structures illustrated in Figure 1a,we can see that the electrolyte is the only compo-nent that contributes to all interfaces,owing to its continuous distribution inside the battery.1.2.Electrolyte Properties

The desired electrolyte properties are determined by the work-ing mechanism as well as the structure and type of the ESD.An electrolyte,in a narrow sense,refers to a liquid or solid so-lution with the ability to conduct ions,but not electrons.In a broad sense,it can be a material with ion-conducting com-ponents composited of other functional or structural compo-nents,such as additives,[1]structural enhancements,[2,3]and so on.Based on the states of matter at room temperature,there are basically three types of electrolytes used for LIBs,that is,liquid,solid and gel electrolytes.Irrespective of the state,the primary function of an electrolyte in a LIB is to efficiently trans-port Li +between two electrodes while being as thin as possi-ble.[4]However,it is noted that,with the development of ESDs,the number of desired properties and functions of electrolytes are increasing.In general,the significant electrolyte properties for various LIBs can be summarized and are compared for dif-ferent types of electrolytes in Table 1.

There are several points to be noted from Table 1.Firstly,the evaluation of the properties listed in the table is not compre-hensive and is only based on the authors’knowledge.It mainly reveals the effects of the states of matter on electrolyte prop-erties.Some special electrolytes are not included in this table.For example,ILEs are not included in the column of liquid elec-trolytes and will be discussed individually.A detailed discussion of the electrolyte properties can be found in the specific sec-tion for each type of electrolytes.Secondly,all of the properties are qualitatively compared,owing to the fact that some elec-trolyte properties cannot be quantitatively described.The ionic conductivity and contact/interfacial properties are most impor-tant for practical applications.It is noted that the contact/inter-facial properties of electrolytes are strongly affected by the states of matter,and the study of interfacial properties is seldom reported.[5]Moreover,one can find that Table 1only summarizes the most significant properties for electrolytes.The fundamental properties are summarized for any kind

of

Figure 1.a)Illustration of the structures of a planar battery (e.g.a LIB)and b)interfaces inside the battery

[a]Y.Wang,Prof.W.-H.Zhong

School of Mechanical and Materials Engineering

Washington State University,Pullman,WA 99164(USA)E-mail:katie_zhong@