The solid conduction phenomenon caused by ion movement was first discovered and applied by the broadband light source made of doped zirconia at the end of last century, which is commonly called Nernst light source.The important progress made around 1935 was due to Strock's discovery that the conductivity of AgI increased by more than three orders of magnitude to 1.3S/cm when it changed from low temperature β phase to high temperature α phase at 146 ℃.Later, it was found that Ag+The composite silver iodide with ions as carriers has particularly high ionic conductivity at low temperatures, and in 1961, the first room temperature fast ionic conductor Ag with α - AgI was synthesizedthreeSI. In 1967, Bradley and Greene, Owens and Argue discovered RbAg respectivelyfourIfiveThe room temperature conductivity reaches 0.27S/cm.At the same time, Weber and Kummer of Ford found that Beta Al with sodium ion as carriertwoOthreeEspecially high ionic conductivity (up to 10-1S/m），It is possible to be used as diaphragm material of high-energy sodium sulfur battery.Since then, the internationalFast ionic conductorA wide range of research has been carried out: on the one hand, we have carried out in-depth work on the discovered fast ionic conductors and further explored new ionic conductors;On the other hand, extensive research has been carried out from the perspective of crystal structure, ion conduction mechanism and conduction dynamics, in order to obtain the structural conditions of high ion conductivity and a unified understanding of fast ion conduction theory.
In September 1972, "Fast Ion Transport in Solids" first appeared in Italy as the name of an international conference.In 1976, an international conference with the same name was held in the Research and Development Center of GE Corporation in New York, the United States. At the conference, it was discussed that ionic conductive materials might be used in new batteries and sensors.In 1979, the international conference held in Lake City, Geneva, the United States, tried to combine basic research with practical application.Since the international conference held in Tokyo, Japan in 1981, the name of the conference has been changed to“Solid state ionics”(Solid State Iones).At the same time, in 1980, it launched the international monthly magazine "Solid State Ionics".In 1986, the first Asian Solid Ionics Conference was held in Singapore. Like the aforementioned international conferences, it is held every two years.
In China, stabilized zirconia was used as diaphragm material at the end of 1960sHigh-temperature fuel cellResearch.In the early 1970s, sodium Beta AltwoOthreeStudy on sodium sulfur battery with membrane material.Since then, other ionic conductors have also been studied, which is reflected in the academic papers on various ionic conductors such as silver, lithium, sodium, oxygen and glassy state at the first National Symposium on Fast Ionic Conductors held in Huangshan in 1980.In the subsequent national conferences held every two years, the number, quality and number of participants of articles have increased year by year, the scope of research has gradually expanded, and has been applied in some aspects.
As mentioned earlier, in the late 1960s, the phenomenon of fast ion conduction has attracted people's attention.In the middle and late 1970s, it gradually formed categories.1983Shanghai Silicate Research Institute, Chinese Academy of SciencesLin Zuhuan and others wrote a book on fast ionic conductors (solid electrolytes), which also describes various types of ionic conductors.[1]
Ionic conduction theory
Announce
edit
Defects in ionic crystals
Ion conduction mainly occurs in ionic solids, as shown in the figure Defects in Ionic Crystals.There are four main types of thermal defects in ionic crystals: positive interstitial ions, positive blank points, negative interstitial ions and negative blank points. Take univalent ionic crystals as an example, positive interstitial ions are charged+e, positive blank points are charged - e, and negative interstitial ions are charged - e.The negative space point is charged+e.Let the direction of electric field E be positive.The direction of the electric field is from left to right, and the opposite direction is negative.Then the direction of the electric field acting on various defects, the movement direction of various defects, and the current direction generated by them are listed in the table.It can be seen from the table that the current direction generated by the four defects in the ionic crystal is positive, that is, the direction of the electric field.In the absence of electric field, ions move in the crystal in the form of diffusion instead of lattice vacancy. This movement is disordered, so no net charge flow is given.The diffusion of various defects is also disordered, so no current is generated.Under the action of external electric field, the probability of ion replacing vacancy moving along the electric field direction is greatly increased, as shown in the table, thus generating ion current along the electric field direction.According to this mechanism, the formula of its conductivity σ can be derived:
Where: n is the number of ions per unit volume;
The valence number of z-ion;
E - electronic charge;
K - Boltzmann constant;
T - absolute temperature;
D - diffusion coefficient.[2]
Ion movement direction table
Conductivity and migration number
Ionic conduction is related to the transition of ions in crystals.The relationship between conductivity and diffusion coefficient can be determined by Nernst -Einstein equation(Nernst Einstein) Export:
Where σ is conductivity, DiIs the diffusion coefficient of interstitial atoms, CiIs the number of particles per unit volume, QiIs the charge of ion i, K is the Boltzmann constant, and T is the absolute temperature.
The important equation of ionic conductivity can be deduced from the above equation -Arrhenius equation：
Where σzeroIs the pre exponential factor, EaIs the activation energy, R is the ideal gas constant, and T is the absolute temperature.
When several carriers exist in the conductive material at the same time, the contribution of i ion to the total conductivity can be usedIon migration numbertiexpress:
Characteristics of fast ionic conductors
Materials with much higher conductivity than classical ionic conductors are called "fast ionic conductors"“solid electrolyte”Or "super ionic conductor".In order to distinguish from superconducting materials, the name "super ionic conductor" is generally not used, but only the first two.They have unique structural and electrical characteristics.
1、 Structure
The crystal structure of fast ion conductor generally consists of two sets of lattices, one is a rigid lattice composed of skeleton ions, and the other is a sublattice composed of migrating ions.In the migration ion sublattice, the defect concentration can be as high as 10twenty-two/cmtwoSo that the number of migrating ion positions greatly exceeds the number of migrating ions themselves.This state where a few ions are statistically distributed in a large number of positions results in a high degree of disorder, which is called sublattice disorder.This disorder enables all ions to migrate, increasing the carrier concentration.At the same time, it can also produce coordinated movement of ions, reduce the activation energy of conductivity, and greatly increase the conductivity.
2、 Factors affecting ion migration.
There are many factors affecting ion migration:
one
The size of the ion migration channel.Generally, the bottleneck size of interconnected channels should be more than twice the sum of the radius of conducting ions and skeleton ions, but too large is not good.
two
The concentration of migrating ions should be high and the activation energy should be low.
three
Generally speaking, the positions of the migrating ions that are not equal in crystallography should be similar in energy, so that the potential barrier crossed by the ions when they move from one position to another is low, thus reducing the activation energy.
four
When ions move from one position to another, they must pass through one or more intermediate states, that is, a series of coordination polyhedra.The coordination number directly affects the difficulty of ion migration.Generally, the smaller the coordination number, the easier the ion migration.
five
Both skeleton ions and migration ions hope to have larger polarizability, because the polarizability indicates the deformability of ions, and high polarizability is conducive to ion migration.
six
From the perspective of the stability of the compound, it is hoped that there is a strong covalent bond in the rigid skeleton, and a weak ionic bond is expected between the skeleton ion and the conduction ion, so that the conduction ion is easy to migrate.
The above factors affecting ion migration are not absolute.The reality often depends on the comprehensive effect.Therefore, experimental verification is needed.[1]
Hydrogen ion conductor has the characteristics of energy storage. It can be used for the storage of nuclear energy and solar energy, as well as chemical energy storage.So it is very eye-catching.For example, hydrogen ion conductor or hydrogen ion conductor diaphragm material can be used for fuel cells.imageOxyhydrogen fuel cellIt uses hydrogen obtained by hydrolysis as fuel to realize energy storage.Fuel cells also have a unique feature different from other batteries, that is, the lower the temperature, the higher the output power.
The conduction mechanism of hydrogen ion conductor is hydrogen ion H+(Proton) has no outer electrons and has a small radius, so it is easier to approach other ions.Such as proton and water reaction
When Hthree+O and HtwoProton transitions occur when O approaches, resulting in proton conduction.
Oxygen ion conductors include fluorspar type and perovskite type oxygen ion conductors.With ZrOtwoThe based solid solution is a fluorite type oxygen ion conductor, which was first discovered in 1900.ZrOtwoThe conductivity of the based solid solution is mainly Otwo-Ions.Although their conductive activation energy is as high as 0.65~1.10 ev, the conductive activation energy of ionic conductive materials is less than 0.5 According to this index, they cannot be called ionic conductive materials, but they have relatively high Otwo-Ionic conductivity has been applied in scientific research and industrial production.
Fluorine ionic conductor
F-Ions are the smallest anions with only one charge, which is conducive to migration. Some fluorides with larger cation size have higher F-Ionic mobility。For example, fluorspar structurealkaline-earth metalFluoride, rare earth metal fluoride with fluocerite structure, etc.F-The migration of ions is similar to that of O2 - ionic conductor, but also by means of F-The movement of ionic vacancies.Therefore, the method of doping heterovalent fluoride can also be used for the conductivity of high fluoride ion conductors.
Polymer ionic conductor
Since the 1980s, polymer ionic conductors have made a lot of progress.Polymer ionic conductor was first discoveredPolyoxyethyleneComplexes with alkali metal salts.There are a variety of polymer metal salts, which can be divided into 5 categories according to polymer:
① Main chain polyether, based on polyethylene oxide (PEO) andPropylene oxide(PPO) - salt, for example, its room temperature conductivity is 10-6~10-7S/cm；
② Polyester type, mainly polyDiethyl succinate(PESC) and poly - β - propionolactone (PLL) - salt have lower room temperature conductivity than PEO;
③ Polyimide type, with lower conductivity than PEO;
④ Polyether network type, such as PEO based crosslinker and LiClOfourThe conductivity of the formed complex reaches 10 at 30 ℃-5S/cm；
⑤ Side chain polyether, introducing oligomeric ethylene oxide salt with lower degree of polymerization into the main chain of the polymer, whose room temperature conductivity can reach 10-5～10-4S/cm（25℃）。
The conduction mechanism of polymer ionic conductor is also different from that of ceramic fast ionic conductor.Its ions do not diffuse and move in the crystal vacancy, but move in the polymer chain segment.Although the conductivity of polymer ionic conductor is not as high as that of inorganic fast ionic conductor, and there is a contradiction between conductivity and mechanical properties, it is easy to process into large-area films, excellent viscoelasticity is conducive to ion contact with solid electrode materials, and has goodchemical stabilityIt has considerable practical value.[2]
Ionic conductor application
Announce
edit
1. Solid ion selective electrode
oxygenion selective electrode(Oxygen sensor) can be used to measure the oxygen content in the metal melt, the oxygen content in the gas, and the humidity and vacuum of other oxygen related substances.The sodium ion selective electrode can determine the sodium content in the alloy.Silver ion selective electrode can determine AgNOthreeSilver ion concentration in.Halogen ion selective electrode can measure the concentration of chlorine, bromine and iodine.
2. Solid electrochemical devices
It can be used as coulometer to measure electric quantity, as well as integral element, timer, electric switch, etc. of microcircuit.Can be doneVariable resistor, electrochemical switch, electrical integratorElectric double layer capacitorEtc.
In addition, using Na+、Li+The oxidation-reduction coloring effect of some ions in the ionic conductor can make an electrochromic display with large contrast, large area display and memory.It can also be usedBattery diaphragmMaterial Science.
3. Electrolyte of all solid battery and electrochromic display
It can be used as electrolyte of high specific energy solid state battery,Photoelectrochemical cellElectrolyte and electrolyte of all solid electrochromic display.
Development trend
Announce
edit
1. Research on fast ionic conductors with higher ionic conductivity, especially fast ionic conductors with high room temperature conductivity. Research on nano fast ionic conductors is a new way, with the goal of achieving room temperature conductivity of 10two~10threeS/cm。
2. Research new polymer ionic conductor, and the conductivity of polymer salt is difficult to exceed 10-4S/cm (25 ℃), the polymer with tunnel structure, layered structure or high crystallinity shall be designed, and the charge dispersed anion (such as with aromatic ring) shall be selected to obtain a wide and shallow potential well, so that its conductive mechanism is similar to that of inorganic ionic conductor.
3. Study the polymer single ion conductor, which refers to the polymer ionic conductor with only a single positive (or negative) ion conducting rapidly and no pair of ions migrating.The conductivity of polymer lithium ion conductor developed in China has reached 10-6S/cm，This is an important material for lithium batteries without anion migration.[2]
