First author: researcher Zhao Xiangyu; corresponding author: researcher Zhao Xiangyu
Communication unit: Nanjing University of technology paper doi: 10.1002/anie.201902842
Background introduction
The urgent demand for "zero emission" of energy consumption in human society and the rapid development of electronic technology put forward higher requirements for the high energy density, low cost and sustainability of battery energy storage technology, thus promoting the extensive research of various battery systems such as lithium-ion battery, lithium sulfur battery, lithium air battery, sodium ion battery, halogen ion battery and multivalent ion battery. However, the development of related battery technology faces many problems and challenges, such as the use of metal negative electrode, the development of high-voltage positive electrode and high-stability electrolyte, the design of new high-energy electrode system and so on.
The strategy based on the chemistry of halide materials has greatly promoted the theoretical development and technological progress of secondary battery research, including the use of halide electrode materials, halogen doping in the electrode phase or surface, electrolyte design or additives to achieve rapid ion transfer and interface stability between electrodes and electrolytes, as well as the development of new battery chemistry. It covers a variety of battery systems based on univalent cation, multivalent cation, anion or double ion transport.
Highlights of this article
Zhao Xiangyu, researcher of Nanjing University of technology, Shen Xiaodong, Professor, and Dr. Zhirong Zhao Karger, Prof. Dr. Maximilian of the Helmholtz Ulm Institute in Germany Fichtner mainly summarized the research progress of halide material chemistry in lithium-ion batteries, magnesium batteries, aluminum batteries, halogen-ion batteries, liquid metal batteries, metal halide batteries and other secondary batteries, put forward the opportunities and challenges of related research, and looked forward to the development direction of new high-performance electrochemical energy storage materials and secondary battery system, in order to explore new electrochemical energy storage Materials and systems provide a new perspective.
The main contents of this paper include: ■ analysis of halide based electrode materials in lithium-ion batteries, halogen substitution and surface modification in electrode materials, design of electrolyte, additives and other halide materials; ■ This paper summarizes the functions of halide containing electrolyte and halide in magnesium battery and halide based electrolyte in aluminum battery, and emphatically analyzes the interaction between halide containing electrolyte and electrode / electrolyte in two battery systems; ■ In this paper, the latest development of halogen ion batteries (fluorine ion batteries, chloride ion batteries) is introduced, and the halide materials in liquid metal batteries and metal halide batteries are briefly summarized.
Article analysis
3.1 the research scope of halide material chemistry in lithium-ion battery (LIB) is very wide, including the development of new electrode materials, the change of electrode material structure, the modification of electrode material surface, the modification of electrode / electrolyte interface and the enhancement of diaphragm performance, and the multi angle and all-round use of halide material chemistry to improve battery performance.
There are several kinds of halide electrode materials in lithium battery: binary metal fluoride (CUF, bif3, mnf3, fef3, etc.), ternary lithium metal fluoride (li3vf3, li2mnf5, li3crf6), metal fluoride oxide (li2vo2f) and LiBr. Compared with metal oxides and metal sulfides, the transition metal fluoride can provide a higher voltage when it reacts with lithium, which is a good electrode material for conversion reaction. For example, fef3 (Figure 1) has a high electromotive force of 2.74 V and a large theoretical capacity of 712 MAH g-1.
However, in the two-step reaction of intercalation and conversion, the electrochemical activity of the material is limited and the lithium ion diffusion is slow. Therefore, many researchers have designed various nanostructured fef3 composites to improve their structural stability, reaction rate, cycle performance, reversible capacity and so on. Among the intercalation materials, the typical one is metal fluoride oxide (li2vo2f) (Figure 2), which overcomes the capacity limitation of LiCoO2, LiMn2O4, LiFePO4, etc., and can realize the multi electron reaction and high electrochemical performance in a wide temperature range.
In addition to the halide electrode, the structure of other intercalation materials can also be improved by the replacement of halogen. For example, fluorine doped lixmno2-yfy, bromine doped Li4Ti5O12, halogenated graphene nano sheet, fluorine doped SnO2 @ graphene porous composite, etc.
Of course, in order to have a good compatibility between the electrode and the electrolyte, it is necessary to modify the surface of the electrode material. For example, fluorinated surface is used to improve reversible capacity and thermal stability, halide coating is used to limit the formation of SEI film, and metal chlorides such as MgCl2 are used to fix polysulfides. The electrode / electrolyte interface modification can also be realized by using fluorine-containing solvent or halogen containing electrolyte, such as fluorine doped solid-state anti perovskite structure electrolyte, fluorine-containing carbonate organic electrolyte, etc. In addition, due to the high chemical and electrochemical stability of fluoropolymers, some researchers have used fluorine to treat our commonly used membranes and improve their functionality.
3.2 the halogen-containing electrolyte rechargeable magnesium battery (RMB) is considered to be one of the most promising battery energy storage technologies in the next generation because of its higher volume energy density and earth abundance, as a negative electrode without dendrite deposition and as a direct negative electrode material. However, the previous development strategy of "lib" can not be used. Magnesium battery can only show its advantages when using the appropriate electrolyte and positive material. Among them, the halogen-containing electrolyte makes RMB cross domain impossible, which is the key to promote the development of RMB.
The magnesium electrolyte based on Lewis acid has the first generation of format reagent DCC, with better stability of APC and hmdsmgcl; the introduction of other magnesium salts, such as romgcl (r = alkyl or aryl group), (HMDS) 2mg, MgCl2 and other electrolytes, improves the shortcomings of the strict synthesis conditions of format reagent and poor stability in the air (Fig. 3); and then there is the inorganic magnesium salt electrolyte represented by MAcc. In addition, there are binary magnesium based electrolyte similar to MgCl2 and Mg (TFSI) 2. The formation of mg2cl3 + / MgCl + by using the chloride ion can stabilize the magnesium ion and prevent the passivation of the negative electrode surface.
However, it is not perfect. The corrosion phenomenon caused by chloride ion reduces the application scope of electrolyte. As a result, the balance of increased performance and reduced applicability is swinging. Some researchers added the inhibitor py14cl to MAcc electrolyte, but the inhibitor may also inhibit the electrochemical reaction while protecting the collecting fluid. In addition, MgCl + in the intercalation material will preferentially enter the positive material during the discharge, which will affect the capacity and reversibility of RMB. The key to make the balance no longer swing is to develop corrosion-free magnesium salt electrolyte. For example, Mg (TFSI) 2 / DME electrolyte with good stability and ionic conductivity is limited by its poor electrochemical performance.
It is found that adding a small amount of non corrosive halogen element "iodine" can form a conductive layer of magnesium ion to improve its kinetics, which provides a new idea for the development of electrolyte with simple magnesium salt and polar solvent as the combination. In recent years, there are also chloride free and corrosion-free electrolytes based on Mg (BH4) 2, Mg (cb11h12) 2 (MMC), Mg [al (HFIP) 4] 2, and borate based magnesium salts (Fig. 4), etc.
3.3 AlCl3 based electrolyte in aluminum battery benefits from the high theoretical capacity and rich raw materials of aluminum metal anode, and aluminum battery (RAB) is also a multivalent battery system worth studying (Figure 6). There have been many reviews on the research progress of aluminum battery, so this paper focuses on the chemical reactions of aluminum AlCl3 based electrolyte and halides of electrode materials on both sides.
AlCl3 based nonaqueous electrolyte has the advantages of high ionic conductivity, high coulomb efficiency, fast electrochemical kinetics and wide electrochemical window. It can be divided into three types: AlCl3 / chloride ionic liquid system, AlCl3 / organic solvent system and AlCl3 / inorganic alkali metal chloride system. The representative of the first system is the acid mixture of AlCl3 and [EMIM] Cl (Fig. 5). It has strong corrosiveness and high reaction activity, but it is not compatible with a variety of collecting fluids and commonly used adhesives. Therefore, it has been studied to reduce its corrosiveness by substituting partial chlorine with bromine or iodine. Compared with the first one, the second one has lower cost and less corrosiveness, but the strong coordination effect makes the system dynamics slow. The third is the binary AlCl3 NaCl or ternary AlCl3 NaCl KCl Lewis acid electrolyte prepared above the melting point of the mixture. When it is used in Al graphite system, it has good cycle stability and high power performance, which is a hope of high performance and low cost Rabs.
There are two types of ion transfer between the positive and negative electrode materials in AlCl3 based electrolyte system. The first is aluminum ion transfer. Both positive and negative electrodes participate in the reaction, such as aluminum sulfur battery. The second type is based on the transfer of chloride aluminate anion (mainly alcl4 -) on the positive side, such as graphite as the intercalation anode. However, the source of alcl4 - is doomed to limit the capacity, so high-capacity batteries still rely on Al3 + transfer. Based on the shortcomings of AlCl3 / CL ionic liquid system, the development of corrosion-free aluminum salt electrolyte which can transport Al3 + is the best solution for Rab to further develop.
3.4 in addition to the batteries based on cation transport, another way to get high energy density is to develop halogen ion batteries (HIBS) based on F -, Cl - anion transport. The halide battery has a theoretical energy density of up to 5000 wh L-1 and abundant raw materials. The electronegativity of halogen element is high and it can react with many kinds of metal to form salt. For example, the combination of F, CL element and metal can produce large Gibbs free energy, so it has large electromotive force. So far, the cathode materials used for Hib are metal halides (for example, bif3, SNF2, cuf3, bicl3, VCl3 and CoCl2), metal halide oxides (for example, BiOCl, FeOCl, vocl and vocl2), and chloride doped conductive polymers. There are many kinds of negative electrode materials, such as alkali metal, alkaline earth metal (such as Li, Na, Mg, etc.) and rare earth metal (such as La, CE), although the theoretical volume energy density of fibs can reach 5000 wh L-1 is much higher than lithium sulfur and magnesium sulfur, but the current research on FIB electrode mainly focuses on the construction of feasible electrode system and its reaction mechanism, which is still in the initial stage.
Many researchers are improving the performance of fluoride ion electrolytes. It has been reported that the solid-state inorganic electrolyte with fluorite or Cerite structure (Figure 7), room temperature organic electrolyte, and fibs electrolyte composed of inorganic fluoride salt (CSF) and organic solvent (tetraethoxy). The key to the development of CIB is also to develop appropriate conductive electrolyte. Different from inorganic metal fluoride electrolyte, CIB electrolyte materials have many limitations. The Lewis acid-base reaction between anode and electrolyte will lead to the dissolution of anode and large volume change, so the development of solid electrolyte is the trend of CIB development.
Of course, we also need suitable cathode materials with high matching degree. Metal chlorine oxides with better chemical stability, such as FeOCl (Figure 8), BiOCl and vocl, are good choices. In addition, there are some stable organic materials, such as polypyrrole doped with chloride and polyaniline conducting polymer. Some scholars have also studied CIBS in water system, using NaCl as electrolyte. From the perspective of environmental protection, chloride ion energy storage electrode can be further developed into an efficient electrochemical desalination system.
3.5 halogen in other secondary batteries due to the high potential in the process of halogen oxidation-reduction, such as I2 / I - and Br2 / Br -,