Solid-state lithium-ion batteries using solid electrolyte instead of traditional organic liquid electrolyte, is expected to fundamentally solve the problem of battery safety, electric vehicles and large-scale chemical storage ideal energy source.
The key points include the preparation of solid electrolyte with high room-temperature conductivity and electrochemical stability, and high-energy electrode materials suitable for all-solid-state lithium-ion batteries to improve the electrode / solid electrolyte interface compatibility.
Solid-state lithium-ion battery structure, including the positive, electrolyte, negative, all made of solid materials, compared with the traditional electrolyte lithium-ion batteries have the following advantages:
① completely eliminate the potential safety problems of electrolyte corrosion and leakage, thermal stability higher;
② do not need to package the liquid, support serial arrangement and bipolar structure, improve production efficiency;
③ Due to the solid electrolyte solid state characteristics, you can superimpose multiple electrodes;
Electrochemical stable window wide (up to 5V), can match the high voltage electrode materials;
⑤ solid electrolyte is generally a single ion conductor, almost no side effects, longer service life.
Solid electrolyte
Polymer solid electrolyte
Polymer solid electrolyte (SPE) is composed of polymer matrixes (such as polyester, polylactide and polyamine) and lithium salts (such as LiClO4, LiAsF4, LiPF6 and LiBF4). Because of its light weight and good viscoelasticity, Mechanical processing and other characteristics of excellent and received widespread attention.
So far, the common SPE includes polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polymethylmethacrylate (PMMA), polypropylene oxide (PPO), poly Vinylidene chloride (PVDC), and other systems such as single ion polymer electrolytes.
At present, the mainstream of SPE matrix is still the earliest proposed PEO and its derivatives, mainly due to the stability of PEO lithium metal and lithium ions can be better resolved.
However, since the ion transport in the solid polymer electrolyte mainly occurs in the amorphous region, the unmodified PEO at room temperature has a high degree of crystallinity, resulting in low ionic conductivity and seriously affecting the charge-discharge capacity at high current.
Researchers through the method of reducing the crystallinity of PEO segment to improve the movement capacity, thereby increasing the conductivity of the system, the simplest and most effective method is to polymer matrix hybridization of inorganic particles.
At present, more inorganic fillers include metal oxide nanoparticles such as MgO, Al2O3 and SiO2, as well as zeolites and montmorillonites. The addition of these inorganic particles disrupts the orderliness of the polymer segments in the matrix and lowers the crystallinity , The interaction between polymer, lithium salt, and inorganic particles increases the lithium ion transport channels and increases the conductivity and ion mobility. Inorganic fillers also can adsorb trace impurities (such as moisture) in the composite electrolyte and increase The role of mechanical properties.
In order to further improve the performance, researchers have developed some new types of fillers, in which the self-assembly of transition metal ions and organic linking chains (generally rigid) of unsaturated coordination sites forms a metal-organic framework (MOF) due to its porosity And high stability and attention.
Oxide solid electrolyte
In accordance with the substance structure, the oxide solid electrolyte can be divided into two categories: crystalline state and glassy state (amorphous state), wherein the crystalline state electrolytes include perovskite type, NASICON type, LISICON type and garnet type, etc., glassy oxide electrolytes Research hot spot is used in thin-film batteries LiPON electrolyte.
Oxide crystalline solid electrolyte
The oxide crystalline solid electrolyte has high chemical stability and can stably exist in the atmosphere. It is in favor of large-scale production of all-solid-state batteries. The current research focus is to improve the room temperature ionic conductivity and its compatibility with the electrode. At present, the methods for improving the conductivity are mainly element substitution and elemental doping. In addition, the compatibility with the electrode is also an important issue that restricts its application.
LiPON-type electrolyte
In 1992, the United States Oak Ridge National Laboratory (ORNL) in a high purity nitrogen atmosphere using RF magnetron sputtering device sputtering LiPP04 targets prepared LiPON electrolyte film.
The material has excellent comprehensive properties, room temperature ionic conductivity of 2.3x10-6S / cm, the electrochemical window of 5.5V (vs.Li/Li +), good thermal stability, and LiCoO2, LiMn2O4 and other positive electrodes and lithium metal , Lithium alloy cathode compatibility good.LiON thin film ion conductivity depends on the size of the amorphous material in the film structure and the content of N, N content can increase the ionic conductivity of the conductivity.
It is generally accepted that LiPON is the standard electrolyte material for all-solid-state thin-film batteries and has been commercially used.
However, at the same time, it is difficult to control the thin film composition and the deposition rate is low. Therefore, the researchers try to use other methods to prepare LiPON thin films, such as pulsed laser deposition , Electron beam evaporation and ion beam assisted vacuum thermal evaporation and the like.
In addition to the changes in the preparation methods, elemental substitution and partial substitution methods have also been used by researchers to prepare a variety of LiPON-type amorphous electrolytes with superior properties.
Sulfide crystalline solid electrolyte
The most typical sulfide crystalline solid electrolyte is thio-LISICON, KANNO professor from Tokyo Institute of Technology first found in Li2S-GeS2-P2S, the chemical composition of Li4-xGe1-xPxS4, room temperature ionic conductivity up to 2.2 x10-3S / cm (where x = 0.75), and the electronic conductivity is negligible thio-LISICON has the general chemical formula Li4-xGe1-xPxS4 (A = Ge, Si and the like, B = P, A1, Zn, etc.).
Sulfide glass and glass ceramic solid electrolyte
Glassy electrolytes usually consist of networks such as P2S5, SiS2 and B2S3 and network modified Li2S. The system mainly consists of Li2S-P2S5, Li2S-SiS2 and Li2S-B2S3. The composition of the glassy electrolyte is wide ranging in composition and ion conductivity is high at room temperature, High thermal stability, good safety performance and wide electrochemical stable window (up to 5V), it has outstanding advantages in high power and high and low temperature solid-state batteries, and is a promising solid-state battery electrolyte material.
TATSUMISAGO, Osaka Prefecture University, Japan The research on Li2S-P2S5 electrolytes is at the forefront of the world. They first found that the Li2S-P2S5 glass is partially crystallized to form glass ceramics by pyrolyzing the Li2S-P2S5 glass. The crystal phase deposited in the glass matrix makes the electrolyte The conductivity has been greatly improved.
All solid state battery electrode material
Although there is almost no side reaction of the solid electrolyte decomposition at the interface between the solid electrolyte and the electrode material, the solid property makes the electrode / electrolyte interface poor in compatibility and the high interface impedance severely affects the ion transport and eventually leads to low cycle life of the solid state battery , The rate of performance is poor.In addition, the energy density can not meet the requirements of large batteries.For the electrode material research mainly in two aspects: First, the electrode material and its interface modified to improve the electrode / electrolyte interface compatibility; The second is the development of new electrode materials, thereby further enhancing the electrochemical performance of solid-state batteries.
Cathode material
All-solid-state batteries generally use composite electrodes, in addition to the electrode active material also includes solid electrolyte and conductive agent, in the electrode play a role in the transfer of ions and electrons. LiCoO2, LiFePO4, LiMn2O4 and other oxide positive electrode in the whole solid battery more universal.
When the electrolyte is sulfide, due to large difference in chemical potential, the attraction of Li + by the positive electrode of the oxide is much stronger than that of the sulfide electrolyte, resulting in a large amount of Li + moving towards the positive electrode and poor lithium at the interface electrolyte.
If the positive electrode of the oxide is an ion conductor, the space charge layer will also be formed at the positive electrode. However, if the positive electrode is a mixed conductor (such as LiCoO2, which is both an ion conductor and an electron conductor), the Li + concentration at the oxide is diluted by the electron conductivity. The charge layer disappears when Li + at the sulfide electrolyte moves toward the positive electrode again and the space charge layer at the electrolyte further increases, thereby creating a very large interface impedance that affects cell performance.
The addition of only the ion conductive oxide layer between the positive electrode and the electrolyte can effectively suppress the generation of the space charge layer and reduce the interface impedance, and in addition, the ionic conductivity of the positive electrode material can be improved to optimize the battery performance and improve the energy density.
In order to further improve the energy density and electrochemical performance of all-solid-state batteries, people are also actively researching and developing new high-energy positive electrodes, including high-capacity ternary positive materials and 5V high-voltage materials.
Typical representatives of ternary materials are LiNi1-x-yCoxMnyO2 (NCM) and LiNi1-x-yCoxA1yO2 (NCA), both with a layered structure, and the theoretical specific capacity is high.
Compared with spinel LiMn2O4, 5V spinel LiNi0.5Mn1.5O4 has higher discharge platform voltage (4.7V) and rate performance, so it becomes a powerful candidate material for all-solid-state battery positive pole.
In addition to the positive electrode of oxide, the positive electrode of sulfide is also an important part of all-solid-state battery cathode material. Such materials generally have high theoretical specific capacity, several times or even an order of magnitude higher than the positive electrode of oxide, Electrolyte matching, due to similar chemical potential, will not cause serious space charge layer effect, the resulting all-solid-state battery is expected to achieve high capacity and long life of the real weeks requirements.
However, the solid positive interface between the sulfide positive electrode and the electrolyte still has problems of poor contact, high impedance, and failure to charge and discharge.
Anode material
Metal Li anode material
Because of its high capacity and low potential advantages of all-solid-state batteries become the most important anode material, however, the metal Li in the cycle there will be lithium dendrites will not only make available for embedded / off the amount of lithium reduction, more Seriously, it will cause safety problems such as short circuit.
In addition, the metal Li is very lively and easy to react with the oxygen and moisture in the air, and the metal Li can not withstand high temperature, which makes the assembly and application of the battery difficult.Adding other metals and lithium alloy is the main method to solve the above problems One of these alloy materials generally have a high theoretical capacity, and lithium metal activity due to the addition of other metals to reduce, can effectively control the formation of lithium dendrites and electrochemical side reactions, thereby promoting the interface stability. The general formula for lithium alloys is LixM, where M may be In, B, Al, Ga, Sn, Si, Ge, Pb, As, Bi, Sb, Cu, Ag, Zn and the like.
However, there are some obvious defects in the lithium alloy negative electrode, mainly due to the large volume change of the electrode during the cycling, serious failure of the electrode powdering and substantial decrease of the cycle performance. Meanwhile, since lithium is still the electrode active material, Security risks still exist.
At present, the ways to improve these problems include the synthesis of new alloy materials, the preparation of ultra-fine nano-alloy and composite alloy system (such as activity / non-activity, activity / cleanliness, carbon-based composite and porous structure) and so on.
Carbon family anode material
The carbon-based, silicon-based and tin-based materials of carbon group are another important negative electrode material for all-solid-state batteries.Carbon-based materials are typically represented by graphitic materials, which have a layered structure suitable for lithium ion intercalation and delamination, Good voltage platform, charge-discharge efficiency of 90% or more, but the theoretical capacity is low (only 372mAh / g) is the biggest shortcoming of such materials, and the practical application has basically reached the theoretical limit, unable to meet the high energy density demand.
Recently, nanocarbons such as graphene and carbon nanotubes appeared on the market as new types of carbon materials, enabling the battery capacity to be expanded by 2-3 times.
Oxide anode material
Mainly including metal oxides, metal-based composite oxides and other oxides typical fireworks non-negative materials are: TiO2, MoO2, In2O3, Al2O3, Cu2O, VO2, SnOx, SiOx, Ga2O3, Sb2O5, BiO5, these oxides All have a high theoretical specific capacity. However, during the replacement of metal elements from oxides, a large amount of Li is consumed, resulting in huge capacity loss, accompanied by a huge volume change during cycling, resulting in battery failure, This problem can be ameliorated by compounding with carbon-based materials.
in conclusion
The most current solid electrolyte materials that are most likely to be used in all-solid-state lithium-ion batteries include PEO-based polymer electrolytes, NASICON type and garnet oxide electrolytes, and sulfide electrolytes.
On the electrode side, in addition to the traditional transition metal oxide positive electrode, lithium metal, graphite anode, a series of high-performance positive and negative materials are also being developed, including high voltage oxide positive electrode, high capacity sulfide positive electrode, good stability The composite negative and so on.
But there are still problems to be solved:
1) The conductivity of PEO-based polymer electrolytes is still low, resulting in poor battery magnification and low temperature performance, poor compatibility with high voltage positive electrodes, new polymer electrolytes with high conductivity and high pressure resistance to be developed;
2) In order to realize high energy storage long life of all-solid-state battery, development of new high energy and high stability positive and negative materials is imperative. The best combination and safety of high-energy electrode material and solid electrolyte needs to be confirmed.
3) The electrode / electrolyte solid-solid interface in all-solid-state battery has always been a serious problem, including the interface impedance is large, the interface stability is poor, the interface stress changes, which directly affect the performance of the battery.
Although there are many problems, on the whole, all-solid-state battery development prospects are very bright, in the future to replace the existing lithium-ion battery to become the mainstream energy storage power is the trend.
