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Advanced materials: freeze electron microscopic analysis of energy materials

wallpapers News 2020-09-14

high performance rechargeable battery plays an indispensable role in the non fossil energy society. However it is not easy to develop safe high energy density next generation secondary batteries. The innovation of new battery requires researchers to have a deeper understing of the failure mechanism working principle of battery components such as electrode electrolyte solid-liquid interface so as to achieve a more reasonable design development of the next generation of high-performance battery. However due to the high air electron beam sensitivity of the battery components especially the hot most potential lithium metal anode in recent years impurities such as Li2O Li3N Li2CO3 will be produced rapidly when exposed to air at room temperature the lithium metal some of its interface components will melt volatilize rapidly when exposed to high energy electron beam at room temperature which is the reason These problems seriously hinder researchers to accurately characterize the chemical structural information of electrode materials even lead to misleading results. In recent years the development application of cryo em in the field of battery play a key role in solving the above difficulties. Freeze electron microscopy (SEM) can characterize electron beam sensitive energy materials with micro nano even atomic scale resolution which provides a strong support for exploring the chemical structural information of various parts of the secondary battery.

recently the research group of Professor Zhang Qiang of the Department of chemical engineering of Tsinghua University published a review article entitled "analyzing energy materials by cryogenic electron microscope: a review" in the international famous journal advanced materials( DOI:10.1002/adma.201908293 )。 This review starts with the development of freeze electron microscopy its advantages in characterizing energy materials summarizes the preparation process of samples required for characterizing energy materials by freeze electron microscopy sorts out the new research findings in the field of energy materials according to the functions of freeze electron microscopy especially the latest research results of lithium metal its surface interface. Finally the application development direction of freeze electron microscopy in the field of energy materials are prospected. This review provides an important reference for accurately characterizing the structure chemical information of electron air sensitive electrode materials evaluating the working failure mechanism of batteries in the future.

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1. The development history of cryo electron microscopy in the field of energy materials

Fig. 1. Cryo transmission electron microscopy (cryo TEM) has been used in the structural analysis of biological macromolecules since it was manufactured in 1974. It was not until 2017 that cryo-TEM Professor Cui Yi of Stanford University first used to study lithium metal negative electrode its surface solid-liquid interfacial film (SEI). In 2018 cryo-TEM was used to study the solid-liquid interfacial film (CEI) of oxide cathode for lithium ion batteries the lithium dendrite structure composed of LiH was first discovered in the same year. In 2019 the structure chemical composition of sulfur cathode CEI were observed by cryo tem for the first time. Cryo scanning electron microscope (cryo SEM) has been used in the research of battery materials since 2015. In recent years frozen ion beam scanning electron microscopy (cryofib) has been better applied to the cutting surface observation of lithium metal anode the preparation of cryoTEM samples.

2. The preparation transfer process of frozen electron microscope sample

2 In order to obtain more real accurate information of surface interface chemistry structure of energy materials especially the characterization of air electron beam sensitive electrode materials such as lithium anode SEI CEI there are three main sample preparation processes for freeze electron microscopy: (a) large lithium anode materials can be cut by cryofib high-energy GA ion beam to expose fresh cross-section then the samples can be processed The micro resolution was characterized. Cutting lithium metal at liquid nitrogen temperature can keep stable for several hours avoiding the damage of material structure caused by the heat generated by the convergence of high energy ion beam. (b) For the nano resolution characterization of the bulk materials it is necessary to cut the bulk materials with the help of cryo FIB to a thickness less than 100 nm fix them on the copper mesh then transfer them to cryo tem for observation. It should be noted that during the opening sampling of cryofib the samples are easy to directly contact with the air at room temperature which will cause the samples to be polluted damaged to a certain extent. (c) For the observation characterization of nano resolution the materials can also be grown on the Cryo TEM copper mesh with nano size. For example lithium metal can be plated on the copper mesh by in-situ deposition of the battery then directly transferred to the Cryo tem for observation. The sample transfer process of cryo tem is particularly important. At present there are two kinds of sample rods developed. The first is cryo transfer holder which needs to install the sample in liquid nitrogen. It is difficult to operate easy to introduce crystal ice. It may also lead to loose deposition of lithium unstable surface SEI dissolved in liquid nitrogen resulting in structural damage. The other is cooling holder. Compared with the previous sample rod the operation of freezing sample rod is simpler the liquid nitrogen cooling process can be carried out after the sample rod is inserted into the freezing electron microscope chamber. However in the process of normal temperature transfer the sample should be protected by inert gas to prevent pollution damage caused by direct contact with the air at normal temperature. High resolution electrode materials based on cryo tem are characterized by

. Fig. 3. (a) it can be seen from the photos of lithium metal dendrites taken by ordinary TEM cryo tem that the lithium dendrites melt transform into polycrystalline structure after exposure to ordinary tem for 1 second while the lithium dendrites can be stable in single crystal form for more than 10 minutes under cryo TEM electron beam exposure. (b) The atomic resolution photos of lithium dendrites were taken by cryo TEM. (c) The photos collected by cryo TEM show that the lithium dendrites have oriented growth characteristics. (a) the SEI structure on the surface of lithium metal was photographed by cryo TEM. It was found that the addition of FEC to EC / DEC electrolyte could change the SEI structure from mosaic structure to ordered bilayer structure. (b) LIF was found in SEI on the surface of lithium deposited in EC / DEC lifp6 electrolyte. (a) using cryo TEM it was found that the electrolyte of EC / DEC was filled withWith the addition of LiNO3 additive the surface SEI of the deposited lithium metal also presents a double-layer structure. (b) Using high salt lifsi electrolyte dimethyl carbonate (DMC) as solvent without any other additives the metal lithium was deposited on the graphene cage. The SEI formed on the surface of graphene cage also showed an ordered double-layer structure which was similar to that in ester EC / DEC electrolyte with 10% FEC 1% FEC The structure of SEI formed by vinyl carbonate (VC) additive is similar but the SEI produced by VC additive is thinner. (a) using cryo TEM it was observed that different SEI structures with or without 10% FEC in EC / DEC electrolyte had a significant effect on the degree of lithium removal. The ordered double-layer SEI structure had more uniform ionic conductivity which could promote the uniform lithium removal reduce the production of dead lithium. (b) In different electrolytes the growth morphology of lithium dendrite affects the production of dead lithium the "short fat" morphology is more conducive to the uniform removal of lithium which can reduce the production of dead lithium. (a) cryo TEM observation shows that there are two forms of SEI formed on the surface of carbon black anode after repeated deintercalation of lithium in EC / DEC electrolyte without additives: one is a thicker SEI layer mainly composed of amorphous components; the other is a sei layer mainly composed of inorganic components with a compact thin structure. (b) When lithium was deposited on CuO substrate in EC / DEC with 10% FEC as electrolyte different SEI films were formed on the surface of CuO at different potentials. The amorphous state at high potential is transformed into a bilayer structure at low potential. (a) the artificial SEI layer wrapped on the surface of lithium metal can be clearly observed by cryo TEM. (b) Cryo TEM can identify the size uniformity of CEI particles on the surface of LiNi0.5Mn1.5O4. (c) As the cathode material of lithium sulfur battery the CEI formed on the surface of vulcanized polyacrylonitrile can be captured by cryo TEM its main composition is the mixed crystal particles of LIF lino2.


were characterized by cryo stem XEDs eels. Fig. 9. Cryo stem combined with electron energy loss spectroscopy (EELS) or X-ray energy scattering spectroscopy (XEDs) can be used to characterize the chemical information of each component in the sample micro region with high resolution. (a) It is found that there are lithium dendrites composed of LiH in the deposited lithium metal. (b) In the sulfur cathode composite material of lithium sulfur battery the uniform distribution of sulfur in the carrier material directly affects the performance of lithium sulfur battery. However the evaluation accuracy of battery material is affected because sulfur is easy to sublimate redistribute under electron beam irradiation. The distribution of sulfur in the carrier is more reliable.

5. Characterization of

based on the micron resolution of cryofib bulk materials Fig. 10. (a) the cross-section morphology of the deposited lithium bulk materials is obviously different after using cryofib ordinary fib. The cross section of lithium block cut by cryofib is smooth complete. After cryo FIB cutting the deposition of lithium dendrites is obvious. However when the dendrite is cut by conventional FIB the lithium sublimates under high energy ion current leaving only mossy SEI shell. (b) For the characterization of artificial SEI film on lithium surface the cross section of the sample can be obtained by cryofib cutting so as to evaluate the thickness of artificial SEI film with micron resolution obtain the information of element distribution at the cross section with EDs.


Fig. 11. The future development direction of cryoelectron microscopy in the field of secondary batteries. Summary Prospect of

6. The fine sample preparation process low temperature test conditions of

cryoelectron microscopy are summarized prospected. The original information without pollution damage can be obtained successfully. Cryoelectron microscopy has gradually become an important tool for researchers to evaluate the working failure mechanism of batteries analyze the chemical structural information of electrodes surface interfaces even electrolytes. Therefore it is necessary to conduct in-depth research excavation on the field of freeze electron microscopy in battery in order to provide a more comprehensive understing of energy materials.

(1) sample preparation transfer. At present it is a simple common method to study the surface interface of electrode materials by depositing Li or Si in situ on the TEM carrier (such as copper micro grid or lace carbon film) in nano size transferring them to cryo TEM. However the structural differences (such as surface uniformity etc.) between TEM carrier conventional battery copper foil collector will affect the local current density electric field ion concentration distribution interfere with the deposition of lithium on the carrier the formation of SEI. Therefore the consistency of lithium deposition SEI formation on TEM carrier copper foil collector is the premise of accurate characterization of cryo TEM. Because the observation horizon of cryo tem is limited the selection of sample characterization area should be representative repeatable to avoid the inaccuracy of observation results due to individual differences. Furthermore the effect of liquid nitrogen on the composition structure of lithium SEI should be considered when using low temperature transfer sample rod. In addition a more simplified sample preparation transfer process should be considered.

(2) in situ equipment. The development of cryo TEM based in-situ frozen sample rods may realize the dynamic capture of Li nucleation growth SEI formation structural changes of electrode materials at high resolution. In situ high-resolution detection of commercial graphite anode such as the formation of SEI lithium evolution on graphite surface will also become a powerful means to solve the safety of commercial lithium-ion batteries. The development of cryo TEM combined with spectrometer can realize the real-time structural evolution of materials the synchronous analysis of regional components in the experimental process. The mechanical properties of lithium surface SEI can be characterized by cryo TEM combined with AFM cantilever or other in-situ mechanical testing equipment. Of course the extremely low temperature is also a great challenge for the combination of ordinary instruments.

(3) table interface. The surface interface is the main place for electrochemical reaction of battery materials. The properties of SEI formed on the surface of anode materials are subject to the working conditions of the battery itself. The structure composition of SEI are related to solvents lithium salts additives in electrolyte
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