1. All-solid-state lithium ion batteries
The current commercial lithium ion battery electrolyte is liquid, so it is also called liquid lithium ion battery. To put it simply, all-solid lithium ion battery means that all parts of the battery structure are in solid form, replacing the liquid electrolyte and diaphragm of traditional lithium ion battery with solid electrolyte.
Compared with liquid lithium ion battery, all-solid electrolyte has the following advantages: high safety/excellent thermal stability, can work normally at 60-120℃ for a long time; Wide electrochemical window, can reach more than 5V, can match high voltage materials; It only conducts lithium ions but not electrons; Simple cooling system, high energy density; It can be applied in the field of ultra-thin flexible battery. But the disadvantages are also obvious: low ionic conductivity per unit area, poor specific power at room temperature; Extremely expensive; Industrial production of large - capacity batteries is difficult.
The performance of electrolyte material largely determines the power density, cycle stability, safety performance, high and low temperature performance and service life of all-solid lithium ion battery. Solid electrolytes can be divided into polymer electrolytes (generally a mixture of PEO and lithium salt LiTFSI as the electrolyte base material) and inorganic electrolytes (such as oxides and sulfides). All-solid-state battery technology is recognized as the next generation of innovative battery technology focused on development. It is believed that in the near future, the technology is more and more mature, and these problems can be easily solved.
2. High energy density ternary material battery
With the pursuit of battery energy density, ternary cathode materials have attracted more and more attention. Ternary cathode material has the advantages of high specific capacity, good cycling performance and low cost, generally refers to the layered structure of nickel-cobalt-manganese lithium material. By increasing the battery voltage and nickel content, the energy density of the ternary cathode material can be effectively increased.
Theoretically, the ternary material itself has the advantage of high voltage: the standard half-cell test voltage of the ternary cathode material is 4.35V, under which the common ternary material can show very good cycling performance; By increasing the charging voltage to 4.5V, the capacity of symmetrical materials (333 and 442) can reach 190, and the cycling is not bad, while the cycling of 532 is not good. When charging to 4.6V, the cyclicity of ternary materials begins to decline, and the phenomenon of gas swelling becomes more serious. At present, it is difficult to find the matching high voltage electrolyte that restricts the application of high voltage ternary cathode materials.
Another method to improve the energy density of ternary materials is to increase the content of nickel element in the material. Generally speaking, the ternary cathode materials with high nickel content refer to the mole fraction of nickel in the material greater than 0.6. Such ternary materials have the characteristics of high specific capacity and low cost, but their capacity retention rate is low and thermal stability is poor. The properties of the material can be improved effectively by improving the preparation process. Micro/nano size and morphology have a great influence on the properties of high nickel ternary cathode materials, so most of the current preparation methods focus on uniform dispersion to obtain spherical particles with small size and large specific surface area.
Among many preparation methods, coprecipitation method combined with high temperature solid phase method is the mainstream method. Firstly, coprecipitation method is used to get the precursors with uniform raw material mixing and uniform particle size. Then, ternary materials with regular surface morphology and easy process control are obtained by high temperature calcination. This is also the main method used in industrial production at present. Compared with coprecipitate method, spray drying method has the advantages of simple process and fast preparation speed. The defects of high nickel ternary cathode materials, such as cation mixing and phase transformation during charge and discharge, can be effectively improved by doping modification and coating modification. At the same time of restraining side reaction and stabilizing structure, improving electric conductivity, cycling performance, rate performance, storage performance and high temperature and high pressure performance will still be the focus of research.
3, high capacity silicon carbon anode
As an important part of lithium ion battery, the cathode material directly affects the energy density, cycle life and safety performance of the battery and other key indicators. Silicon is given a known (4200 mah/g), the highest lithium ion battery cathode material, but because of its more than 300% of the volume effect, silicon electrode materials in the process of charging and discharging will pulverization and peeling from the collection of fluid, make active substances and active substances, loss of electrical contact between the active materials, SEI constantly forming new solid electrolyte layer at the same time, Ultimately resulting in deterioration of electrochemical performance. In order to solve this problem, researchers have carried out a lot of exploration and attempts, among which silicon carbon composite materials are very promising materials.
As the anode material of lithium ion battery, carbon material has little volume change in the charging and discharging process, has good cycling stability and excellent electrical conductivity, so it is often used to compound with silicon. In the carbon silicon composite anode materials, according to the types of carbon materials can be divided into two categories: silicon and traditional carbon materials and silicon and new carbon materials, the traditional carbon materials mainly include graphite, mesomorphic microspheres, carbon black and amorphous carbon; New carbon materials mainly include carbon nanotubes, carbon nanowires, carbon gels and graphene. Using silicon carbon composite, the porous effect of carbon material is used to restrain and buffer the volume expansion of silicon active center, prevent the agglomeration of particles, prevent the infiltration of electrolyte to the center, and maintain the stability of interface and SEI film.
Many enterprises around the world have begun to work on this new anode material, for example, Shenzhen Beitrui and Jiangxi Zichen have taken the lead in launching a number of silicon carbon anode material products, Shanghai Shanshan is in the process of silicon carbon anode material industrialization, Star City graphite has taken silicon carbon new anode material as the direction of future product development.
4, high voltage and high capacity rich lithium materials
Li-rich manganese (xLi[Li1/3-Mn2/3]O2); (1 -- x)LiMO2, M is transition metal 0≤x≤1, structure similar to LiCoO2) has a very high specific discharge capacity, which is about 2 times of the actual capacity of the cathode material currently used, so it has been widely studied for lithium battery materials. In addition, because the material contains a large amount of Mn, compared with LiCoO2 and Li[Ni1/3MN1/3Co1/3]O2, it is more environmentally friendly, safe and cheap. Therefore, xLi Li1/3 - Mn2/3 O2. (1 -- X)LiMO2 is regarded as the ideal cathode material for the next generation of lithium ion batteries by many scholars.
At present, coprecipitation method is mainly used to prepare li-Mn-rich materials, and some researchers also use sol-gel method, solid phase method, combustion method and hydrothermal method to prepare materials, but the obtained material properties are not as stable as coprecipitation method. Although this material has a high specific capacity, its practical application still has several problems: the first cycle irreversible capacity is up to 40 ~ 100mAh/g; Poor rate performance, 1C capacity below 200mAh/g; High charging voltage causes electrolyte decomposition, which makes the cycling performance not ideal, as well as the use of safety problems. The above problems can be solved by using metal oxide coating, composite with other cathode materials, surface treatment, special structure, low upper limit voltage precharge and discharge treatment.
In 2013, Ningbo Institute of Materials Developed a novel gas-solid interface modification technology to form uniform oxygen vacancies on the surface of lithium manganese-rich cathode material particles, thus greatly improving the first charge and discharge efficiency, discharge specific capacity and cycle stability of the material, and effectively promoting the process of practical application of lithium manganese-rich cathode material.
5, high voltage tolerance electrolyte
Although high voltage lithium battery materials have been paid more and more attention, these high voltage anode materials still cannot achieve good results in practical production and application. The biggest limiting factor is that the electrochemical stability window of carbonate based electrolyte is low. When the battery voltage reaches about 4.5(vs.Li/Li+), the electrolyte begins to undergo violent oxidative decomposition, leading to the normal lithium imlocation reaction of the battery. The development of electrolytic liquid system with high voltage tolerance is an important step to promote the practical application of this new material.
Improving the stability of electrode/electrolyte interface through the development and application of new high voltage electrolytic liquid system or high voltage film forming additives is an effective way to develop high voltage electrolytes. From the economic point of view, the latter is often preferred. Such additives to improve the voltage tolerance of the electrolyte generally include boron, organophosphorus, carbonates, sulfur, ionic liquids and other types of additives. Containing boron additives are three (trimethyl alkyl) borate enzyme, lithium bioxalate borate, lithium bifluoroxalate borate, tetramethyl borate ester, trimethyl borate and trimethyl ring triboroxane. Organophosphorus additives include phosphite ester, phosphate ester. Carbonate additives include fluorinated anhydride compounds. Sulfur-containing additives include 1, 3-propanesulfonate lactone, dimethyl sulfonyl methane, trifluoromethylbenzene sulfide, etc. Ionic liquid additives include imidazole and quaternary phosphorous salts.
From the domestic and foreign studies that have been publicly reported, the introduction of high-voltage additives can make the electrolyte withstand 4.4-4.5V voltage, but when the charging voltage reaches 4.8V or even more than 5V, it is necessary to develop the electrolyte that can withstand higher voltage.
6, high temperature resistant diaphragm
The membrane of lithium battery mainly plays the role of conducting lithium ions and isolating the electronic contact between positive and negative electrodes in lithium ion battery, which is an important component to support the battery to complete the electrochemical process of charge and discharge. In the process of using lithium battery, when the battery is overcharged or the temperature rises, the diaphragm needs to have enough thermal stability (thermal deformation temperature >200℃), in order to effectively isolate the contact between the positive and negative electrodes of the battery, prevent the occurrence of short circuit, thermal control and even explosion accidents. Currently, the widely used polyolefin separator has a low melting point and softening temperature (<165℃), which is difficult to effectively ensure the safety of the battery. However, its low porosity and low surface energy limit the performance of the battery. Therefore, it is very important to develop high safety high temperature resistant diaphragm.
A new type of high-temperature resistant porous diaphragm has been developed by the lithium Battery Engineering Laboratory of Ningbo Institute of Materials and the Energy storage Technology Research Department of Dalian Institute of Chemical Physics using the wet process one-time molding technology. The porous diaphragm has low production cost and is easy to quantify production. The preliminary results show that the thermal deformation temperature of the diaphragm is much higher than 200℃, which is similar to the thermal stability of the commercial non-woven diaphragm, and can effectively guarantee the safety of the battery. At the same time, the porous membrane has high porosity and high curvature pore structure, which can ensure the capacity of the battery and effectively avoid the micro short circuit and self-discharge phenomenon of the battery. In addition, NINGBO Institute of Materials has also developed a heat-resistant composite diaphragm with an ultrathin ion-exchangeable functional layer, a gel composite diaphragm based on a three-dimensional heat-resistant framework and a ceramic diaphragm.
In addition to Ningbo Institute of Materials, in 2015, Mitsubishi resin coated on the diaphragm with high heat resistance inorganic filling, so that the diaphragm can still maintain appropriate resistance value at 220℃, blocking the current through.
7. Lithium sulfur batteries
Lithium sulphur battery is a kind of lithium battery with sulphur element as the positive electrode and lithium metal as the negative electrode. The biggest difference from the general lithium ion battery is that the reaction mechanism of lithium sulfur battery is electrochemical reaction, rather than lithium ion deembedding. The working principle of lithium-sulfur batteries is based on complex electrochemical reactions. So far, there has been no breakthrough characterization of the intermediates formed during the charging and discharging process of sulfur electrodes. It is generally believed that: when discharging, the negative reaction is lithium loses electrons to lithium ions, and the positive reaction is sulfur reacts with lithium ions and electrons to generate sulfide. The potential difference between the positive and negative reactions is the discharge voltage provided by the lithium sulfur battery. Under the action of applied voltage, the positive and negative reactions of lithium sulfur battery reverse, namely charging process.
The biggest advantage of lithium-sulfur battery lies in its higher theoretical specific capacity (1672mAh/g) and specific energy (2600Wh/kg), which is far higher than other types of lithium-ion battery widely used in the market at present. Moreover, due to the rich storage of elemental sulfur, this kind of battery is cheap and environmentally friendly. However, lithium-sulfur batteries also have some disadvantages: the electronic and ionic conductivity of elemental sulfur is poor; The intermediate discharge products of lithium-sulfur batteries will dissolve into organic electrolyte, and polysulfide ions can migrate between positive and negative electrodes, resulting in the loss of active substances. The volume of lithium anode will change during charging and discharging and dendrite will be formed easily. The positive sulfur electrode has up to 79% volume expansion/contraction during charging and discharging.
The main method to solve the above problems is generally from the electrolyte and cathode materials two aspects: electrolyte, mainly use ether electrolyte as the battery electrolyte, electrolyte with some additives, can be very effective to alleviate the solution of lithium polysulfide compounds; Cathode materials, mainly sulfur and carbon material composite, or sulfur and organic compound, can solve the problem of sulfur non-conductivity and volume expansion.
Lithium-sulfur batteries are still in the laboratory development stage, with the Chinese Academy of Sciences, Nanyang Technological Institute, Stanford, Nippon Institute of Industrial Technology and The University of Tsukuba leading the research. SionPower has already carried out significant trials in notebook and uav applications.
8, lithium empty battery
Lithium-air battery is a new type of large-capacity lithium-ion battery, which is jointly developed by Japan Research Institute of Industrial Technology and Japan Association for Academic Promotion (JSPS). The battery uses lithium metal as the negative electrode and oxygen in the air as the positive electrode, separated by a solid electrolyte. The negative electrode uses an organic electrolyte and the positive electrode uses a water-based electrolyte.
During discharge, the negative electrode dissolves in organic electrolyte in the form of lithium ions, and then migrates through solid electrolyte to water-based electrolyte of the positive electrode. The electrons travel through a wire to the positive electrode, where oxygen and water in the air react on the surface of the micro-carbon to form hydroxide, which combines with lithium ions in a water-based electrolyte in the positive electrode to form water-soluble lithium hydroxide. When charging, electrons are transmitted to the negative electrode through a wire, and lithium ions arrive at the surface of the negative electrode through the solid electrolyte through the water-based electrolyte of the positive electrode, and react on the surface of the negative electrode to form lithium metal. Oh in the positive end loses electrons to form oxygen.
Lithium empty battery can be replaced by positive electrolyte and negative lithium without charging, discharge capacity up to 5000mah /g, high energy density, theoretically 30kg lithium metal and 40L gasoline release the same energy; The product lithium hydroxide is easy to recycle and environmentally friendly. However, cycle stability, conversion efficiency and multiplier performance are its shortcomings.
In 2015, Gray of The University of Cambridge developed lithium air with high energy density, which can be charged "more than 2,000 times" and has a theoretical energy efficiency of more than 90%, making the practical application of lithium air batteries a step forward. Back in 2009, IBM launched a sustainable transportation project to develop a lithium-air battery suitable for home electric vehicles that could travel about 500 miles on a single charge, and recently asahi Kasei and Chuo Glass have joined the project. Research institutions and well-known companies in the field of lithium-air battery development will greatly promote the application of this battery technology.
