【Background Introduction】

In today‘s world, mankind is facing the growing depletion of fossil energy and the serious problem of increasing demand for sustainable energy, thus promoting the research of low-cost, environmentally friendly and high-performance energy conversion and storage systems. At the same time, with the rapid increase in demand for electric vehicles and smart grid applications, the market demand for batteries that can provide high energy density, stable recyclability, and excellent cost-effectiveness is increasing. Among them, lithium ion batteries have become one of the most promising energy storage devices due to their advantages of high energy density, excellent cycle stability, and light weight. However, the current state-of-the-art lithium-ion batteries still cannot meet the increasing demand for high energy density, because when lithium metal is used as the anode, there is mainly dendritic growth, which may cause short circuits (leading to thermal runaway) and low coulombic efficiency, cycle life Poor problem.

When developing lithium metal anodes and other high-capacity cathode chemicals (such as sulfur and oxygen), researchers found that using solid electrolytes (SSE) to replace traditional electrolytes has good safety, so they developed lithium metal batteries based on solid electrolyte Perhaps the problem of security can be fundamentally solved. At the same time, the results of the study indicate that the ionic conductivity at room temperature is higher than that of the 10 ^-3 S cm ^-1 superionic conductor. However, the problem of high interfacial impedance caused by poor compatibility between SSE and electrodes limits their practical application. At present, researchers have proposed new methods such as advanced separators, electrolyte additives and positive temperature coefficient (PTC) improved current collectors to improve the safety of lithium metal batteries. Therefore, there is an urgent need to develop new chemicals or technologies for lithium metal batteries with higher energy density, longer cycle life and higher safety.

【Achievement Introduction】

Recently, Chem online published Professor Cui Yi of Stanford University in the United States and Professor Liu Wei of Shanghai University of Science and Technology (co-corresponding author), Bo Xia Shuixin of Shanghai University of Science and Technology (first author) and Wu Xinsheng, a 15-level undergraduate of Shanghai University of Science and Technology (A work) and others summarized the development status and future prospects of all-solid-state lithium metal batteries. The title is "Practical Challenges and Future Perspectives of All-Solid-State Lithium-Metal Batteries" . In this review, we first summarize the main challenges and latest developments of high-conductivity solid electrolyte (SSE), including polymers, inorganic and composite materials, and lithium batteries for the next generation of high energy density, from basic understanding to technological innovation . Secondly, the strategy about the problem of SSE and electrode interface is summarized. Next, the current progress and practical challenges of ASSLMBs combining lithium metal anodes with lithium intercalation compounds, sulfur and oxygen cathodes are introduced. Finally, the future prospects of ASSLMBs based on lithium metal anodes are also forecasted.

【Graphic Interpretation】

1. Brief introduction to the development history of lithium-ion batteries

The development of ion batteries (LIBs) has gone through a renaissance process from Li-metal batteries (LMBs) based on lithium (Li) -metal anodes to LIBs using lithium-intercalated compound electrodes, and then to LMBs.

Figure 1: A brief chronology of typical solid electrolytes (SSE) and solid-state batteries (ASSLBs)

to ASSLMBs Schematic diagram of the development trend

2. Solid electrolyte of lithium battery

2.1 The following problems exist in the practical application of solid electrolyte (SSE):

(1) Low ionic conductivity of SSE, especially at low temperature;

(2) The interface resistance at the solid-solid interface of the electrode-electrolyte is large;

(3) Poor electrochemical compatibility with electrodes, such as lithium metal anodes and high voltage cathode materials;

(4) The physical stability of the electrode decreases leading to large changes in interface stress.

2.2. Basic understanding of solid electrolyte

Lithium ion transmission in SSE is mainly divided into two categories: ion transmission in polymers and inorganic materials. The temperature dependence of ionic conductivity in SSE is usually modeled by Arrhenius (for crystalline materials) or Vogel Tammann-Fulcher (VTF) equations (for amorphous materials).

Figure 3. Schematic diagram of two lithium ion conduction mechanisms for SPEs

(A) Li ion conduction in the amorphous phase of SPE;

(B) Li ion conduction in the SPE crystal phase.

2.3. Lithium ion transmission at the electrolyte-electrode interface

The high interface resistance between the electrolyte and the electrode has a significant impact on the overall performance of the battery, which hinders the development of ASSLBs. The electrochemical reaction of ASSLBs is different from a lithium battery using a liquid electrolyte with a solid-liquid interface, which proceeds through the solid-solid electrolyte-electrode interface. Lithium ions diffuse from the electrolyte to the electrode through their interconnected regions, and redox reactions with active materials and electrons occur at the contact electrolyte-electrode interface.

Figure 4. Schematic diagram of SSEs electrode-electrolyte interface and electrochemical stability area

(A) Li defect layer formed at the positive electrode-electrolyte interface;

(B) The reduced decomposition level of Li is to the Li-metal anode;

(C) The electrochemical stability region of various SSE materials.

Figure 5. Strategies for reducing high solid-solid interface impedance

2.4. Observation of electrolyte-electrode interface

Understanding and improving the behavior of the electrolyte-electrode interface through nanoengineering and material design techniques is absolutely necessary to construct a safe lithium battery with improved electrochemical performance. At the same time, observing the evolution of the interface in the battery in real time on the atomic scale will be more helpful for researchers to understand and master the changes in the battery.

Figure 6. Various advanced technologies to observe the microstructure and morphology of the electrolyte-electrode interface

(A) In-situ STEM settings;

(B) EH operated by TEM, the distribution of lithium ions and electrons at the electrode-electrolyte interface under the charged state (top) and the measured potential distribution (bottom);

(C) Operating liquid imaging platform for X-ray microscope;

(D) Procedure for characterization by freezing TEM.

3. Solid electrolyte

3.1, solid polymer electrolyte

Dry polymer SPEs can dissolve lithium salts, and have the advantages of good flexibility, light weight, good processability and low cost, and are obviously superior to inorganic solid electrolytes.

Figure 7. Single ion polymer electrolyte and its corresponding conductivity performance

(A) The chemical structure of single ion conductive polymer electrolyte;

(B) Conductive properties of polymer electrolytes with different proportions of P (STFSILi);

(C) Synthesis route of single lithium ion conductor LiPSsTFSI polymer;

(D) Conductivity of different electrolytes.

3.2, inorganic solid electrolyte

Inorganic solid lithium ion conductors mainly include garnet type, perovskite type, sodium super ion conductor (NASICON) type and lithium super ion conductor (LISICON) type materials, and sulfide glass. They can be roughly divided into two categories: oxides and sulfides.

3.2.1, oxide

The general formula of garnet-type materials is A 3 B 2 (XO 4 ) 3 (A = Ca, Mg, Y, La or rare earth; B = Al, Fe, Ga, Ge, Mn, Ni, V; X = Si, Ge, Al), where A and B cations have 8 and 6 coordination, respectively. The ideal general formula of the perovskite structure is ABO 3 (A = Li, La; B = Ti), where the A site is 12 coordinated and the B site is 6 times coordinated. NASICON structured crystalline phosphates such as Li 1 + x Al x Ti 2-x (PO 4 ) 3 (LATP) and Li 1 + x Al x Ge 2-x (PO 4 ) 3 (LAGP) have high ionic conductivity The excellent lithium ion conductor has a conductivity of ~ 7 × 10 ^-4 S cm ^-1 at room temperature, and has good stability in a humid environment.

Figure 8. Crystal structure of inorganic materials

(A) Crystal structure of cubic Li 7 La 3 Zr 2 O 12 ;

(B) Three-dimensional conductive network of Li atoms arranged in cubic Li 7 La 3 Zr 2 O 12 ;

(C) Crystal structure of Li 3x La (2/3)-× ☐ 1 / 3-2x TiO 3 ;

(D) The crystal structure of Li 10 GeP 2 S 12 .

3.2.2, sulfide

The earliest studied sulfide solid electrolyte was Li 2 S-SiS 2 system. Commonly used sulfide glass electrolytes include Li 2 S-P 2S5 , Li 2 S-GeS 2 , Li 2 S-B 2 S 3 and Li 2 S-SiS 2 , with a conductivity of ~ 10 ^-4 S cm ^-1 . At room temperature, the sulfur element in sulfur-LISICON type Li 3 + x (P 1- x Si x ) S 4 obtained by substitution with oxygen element shows an improved ionic conductivity of 2 times, making its ionic conductivity The rate is as high as 6 × 10 ^-4 S cm ^-1 .

3.3. Organic-inorganic composite electrolyte

CSEs are divided into two categories based on the main components: polymer matrices (CPEs) and inorganic material matrices. CPEs with SPEs with inorganic fillers usually show higher ionic conductivity, better mechanical properties and compatibility with electrodes. Constructing CPEs that combine the advantages of organic and inorganic electrolytes is considered to be a very promising method for manufacturing high-performance flexible batteries, while enhancing mechanical properties helps prevent lithium dendrite growth to improve safety.

Figure 9. Comparison of recently reported ion conductivity of CSEs

3.3.1, nanoparticle filler, nanowire filler, nanosheet filler

Nanoparticle-filled CPEs have been extensively studied. Unlike nanoparticles with particle-particle junctions, nanowires can construct a 3D network for rapid lithium ion transmission due to the continuous and extended jumping path on the surface of the nanowire. At the same time, two-dimensional (2D) mesoporous nanosheets such as montmorillonite, clay and mica are also fillers for high-performance CPEs.

Figure 10. Schematic diagram of CPEs with different forms of fillers

(A) Schematic diagram of PEO / MUSiO 2 NPs;

(B) Schematic diagram of 3D NWs network in CPEs;

(C) Schematic diagram of SiO 2 aerogel enhanced CPEs;

(D) Comparison of possible lithium ion conduction pathways between CPEs and NPs, random NWs, and aligned NWs.

4. All-solid battery based on lithium metal anode

In recent years, the research of using lithium metal as a negative electrode has gradually become a hot spot. Lithium metal has high theoretical specific capacity (3860 mAh / g), lowest negative electrochemical potential (-3.040 V relative to standard hydrogen electrode) and low density (0.59 g / cm ³ ), making it ideal for the next generation of high energy density batteries Negative candidate. However, lithium metal tends to deposit unevenly in the form of lithium dendrites, making it possible to penetrate the separator and cause thermal runaway and battery failure. At the same time, the SEI layer formed spontaneously on the lithium metal surface by consuming electrolyte contributes to uneven nucleation and leads to low CE. In addition, when the deposition and dissolution cycles are repeated, the large volume change of the lithium metal anode may cause cracks in the SEI layer, thereby promoting the continuous reaction of lithium and lithium metal below. At present, researchers have proposed methods such as electrolyte additives, artificial SEI layers and engineering interface layers, and lithium bodies to solve the problem of lithium metal anodes. Among them, SSEs are considered to be the most promising solutions to these problems.

4.1. All-solid lithium metal battery based on intercalated lithium anode

The main problems of SPEs in battery applications are their relatively limited ionic conductivity at room temperature, low voltage region, and narrow operating temperature. However, cross-linking is an effective way to improve the ionic conductivity and mechanical strength of PEO-based electrolytes. A solid sp ³ boron-based single ion conductive PEO-based polymer electrolyte membrane (S-BSM) for ASSLMBs construction has also been reported . The weak correlation between lithium ions and sp ³ boron atoms helps to improve the mobility of lithium ions, and the measured LTN number of S-BSMs is close to 1. Their development is an important step in building ASSLMBs that will operate at ambient temperature in the future.

Figure 11. Morphology and electrochemical properties of CPEs containing Mg 2 B 2 O 5

(A) TEM image of Mg 2 B 2 O 5 NWs;

(B) Schematic diagram of lithium ion conduction in CPEs with Mg 2 B 2 O 5 NWs;

(C) Rate performance of SSEs with and without Mg 2 B 2 O 5 NW to Li / LFP cells at 50 ℃ .

Figure 12. Electrochemical performance of electrolytes and batteries

(A) Cycling performance of LTO / Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 / LCO ASSLBs at 18 C and 100 ° C ;

(B) Electrochemical stability of LGPS series;

(C) Lagni diagram of the battery.

4.2 All-solid lithium-sulfur battery

Because lithium-sulfur batteries have the advantages of high theoretical energy capacity (1672 mAh / g), cost-effectiveness, non-toxicity and natural abundance, sulfur is considered to be the most promising cathode candidate for the next generation of high-energy systems. However, lithium-sulfur batteries also have the disadvantages of low electron and ion conductivity of sulfur, sulfur-containing organic compounds, and the shuttle effect of polysulfides, resulting in insufficient utilization of sulfur as a positive electrode, hindering its commercialization. The shuttle effect actually originates from the dissolution and diffusion of polysulfides in organic liquid electrolytes. The function of the shuttle effect is that the polysulfides formed at the positive electrode can be transferred to the lithium negative electrode, after which they are reduced to lower polysulfides, and the resulting lower polysulfides can be transported back to the positive electrode Return to the negative electrode after oxidation. The shuttle effect leads to low active substance utilization and low CE, and therefore short cycle life.

Figure XIII. CPEs schematic diagram and CPEs-based batteries and their corresponding battery cycle performance

(AC) The schematic diagram shows the preparation method and HNT addition mechanism of HNT modified flexible PEO CPEs, which is used to improve the ionic conductivity (A) and cycle performance of the battery at 25 ℃ (B) and 100 ℃ (C);

(DE) Schematic diagram of ASSLSBs based on PEO / LLZO CPEs (D) and cycle performance and coulombic efficiency (E) of S @ LLZO @ C cathode with a current density of 0.05 mA / cm ² at 37 ° C.

4.3 All-solid lithium-air battery

Lithium-air batteries (LABs) are a potential device considered to have large-scale energy storage technologies because they have the highest energy density (11140 Wh / kg) among various types of batteries. However, the problems of extreme polarization, capacity reduction, and safety caused by the decomposition of the organic liquid electrolyte hinder the practical application of LABs. The SSEs developed based on LABs can fundamentally eliminate the security problems. Although ASSLABs have not been widely studied due to extremely challenging problems such as extremely large polarization resistance and rapid capacity decay, the problem of forming dendrites with metallic lithium has achieved a certain degree of success.

Figure 14. Schematic diagram of ASSLABs and their corresponding electrochemical cycling performance

(A) A schematic diagram of ASSLABs based on PEO;

(B) Cycling performance of ASSLABs at 0.2 mA / cm ² and 80 ℃;

(C) Schematic diagram of ASSLABs with LAGP SSEs;

(D) Cycle performance of ASSLABs at 400 mA / g.

5. Summary and Outlook

With the development of high-capacity chemicals (such as lithium metal anodes, sulfur and oxygen cathode materials), SSEs are playing an increasingly important role in the application of the advantages of "beyond lithium" batteries with high energy density and suitable for large-scale energy storage. The role. The use of SSEs can fundamentally solve the problems of flammable organic liquid electrolytes, low CE and lithium dendrite formation of lithium metal anodes, the shuttle effect of sulfur positive electrode soluble polysulfides, and the instability of lithium air battery air components caused by opening. Although many progress has been made in SSEs, there are still some problems to be solved in large-scale promotion. For example: ion conductivity, interface impedance, mechanical strength and compatibility with electrodes, cost-effectiveness. At the same time, we need to note that in addition to the high energy density required, different advantages should be effectively utilized according to different applications, such as high power density of portable electronic devices and electric vehicles and low maintenance costs of smart grid storage. In addition, transforming existing battery manufacturing processes or new manufacturing technologies of ASSLMBs is also very important for short-term practical applications. It is believed that with the development of science and technology, ASSLMBs that provide high energy density, high power density, long cycle life and high security will gradually enter the market in the future.

Literature link: Practical Challenges and Future Perspectives of All-Solid-State Lithium-Metal Batteries (Chem, 2018 , DOI: https://doi.org/10.1016/j.chempr.2018.11.013)

Corresponding author and team profile

Brief introduction of Liu Wei‘s research group: Liu Wei‘s research group of Shanghai University of Science and Technology covers the fields of high energy density all-solid lithium batteries, nano-solid ionics, and flexible electronic devices. Liu Wei, Assistant Professor / Researcher / PhD Supervisor of School of Material Science and Technology, Shanghai University of Science and Technology, the 14th batch of thousands of young people. He obtained a bachelor of science degree from Beijing Normal University in 2008, and a doctorate degree in engineering from the School of Materials Science and Technology of Tsinghua University in 2013. During his doctorate, he exchanged visits at the University of Tokyo in Japan. He was engaged in postdoctoral research at Stanford University from 2013 to 2017. He has published more than 50 academic papers and published 22 articles including Chem, Nature Energy, Nature Communications, Science Advances, Advanced Materials, Journal of the American Chemical Society, Nano Letters, ACS Nano, Advanced Energy Materials, etc. 1 Chinese patent and 1 US patent are disclosed . He has served as a reviewer for more than ten journals including Joule, Nano Letters and Nano Energy.

Teacher Liu Wei‘s research group has long been engaged in post-doctoral research and assistant research in the fields of lithium batteries and bioelectrochemistry.

Yi Cui TF Profile: Yi Cui Stanford University nanotechnology research group widely used in lithium ion batteries, electrochemical catalysis art tip, environmental protection, solar energy conversion, smart clothes, the biological signal detection, etc., to obtain a series of original breakthrough , in Science and other international top magazines published 370 articles and converted two technologies into mass production, creating Amprius Battery Company and 4C Air Air Filter Company.

Related literature:

1. Wei Liu , Seok Woo Lee, Dingchang Lin, Feifei Shi, Shuang Wang, Austin D. Sendek, Yi Cui. Enhancing ionic conductivity in composite polymer electrolytes with well-aligned ceramic nanowires. Nature Energy , 2017, 3, 17035.

2. Wei Liu , Weiyang Li, Denys Zhuo, Zhenda Lu, Kai Liu, Yi Cui. Core-Shell Nanoparticle Coating as an Interfacial Layer for Dendrite-Free Lithium Metal Anodes. ACS Central Sci. 2017, 3, 135-140.

3. Wei Liu , Dingchang Lin, Jie Sun, Guangmin Zhou, Yi Cui. Improved lithium ionic conductivity in composite polymer electrolytes with oxide-ion conducting nanowires. ACS Nano, 2016, 10, 11407-11413.

4. Wei Liu ^# , Dingchang Lin ^# , Allen Pei ^# , Yi Cui. Stabilizing lithium metal anodes by uniform Li-ion flux distribution in nanochannel confinement. J. Am. Chem. Soc. 2016, 138, 15443-15450.

5. Wei Liu , Nian Liu, Jie Sun, Po-Chun Hsu, Yuzhang Li, Hyun-Wook Lee, Yi Cui. Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers. Nano Lett. 2015, 15, 2740-2745.

Information source: material cattle