At present, most of the negative electrode materials of lithium ion batteries use various lithium intercalated carbon materials. However, the potential of the carbon electrode is very close to the potential of the metal lithium. When the battery is overcharged, the surface of the carbon electrode is liable to precipitate metallic lithium, which may form dendrites and cause short circuit; when the temperature is too high, heat is easily lost. At the same time, during the repeated insertion and deintercalation of lithium ions, the structure of the carbon material is destroyed, resulting in a decrease in capacity. Titanium-oxygen compounds are also a class of negative electrode materials which are currently studied more, including TiO2 , LiTi 2 O 4 , Li 4 Ti 5 O 12 , Li 2 Ti 3 O7 and their doped modifying materials. A battery in which spinel Li 4 Ti 5 O 12 is used as a negative electrode material has been used in watches. The authors reviewed the structure, synthesis and physicochemical properties of spinel Li 4 Ti 5 O 12 anode materials in recent years . 1 Â Â Â Â Â Â Â Â
Structure and electrochemical properties of Li 4 Ti 5 O 12 Spinel Li 4 Ti 5 O 12 is a "zero strain" insert material, which has become a negative electrode material for lithium ion batteries with excellent cycle performance and extremely stable structure. Although the theoretical capacity of Li 4 Ti 5 O 12 is only 175 mAh/g (discharge to 1 V ), its reversible lithium ion deintercalation ratio is close to 100% , so its actual capacity is generally maintained at 150-160 mAh/g ( Discharge to 1 V ). Li 4 Ti 5 O 12 belongs to the spinel type and is a face-centered cubic structure ( space group Fd3m) . Among them, O 2- ions form a lattice of FCC , which is located at 32e , and some lithium ions are located at the position of tetrahedron 8a . Lithium ions and titanium ions (Li : Ti = 1 : 5) are located at the 16d position of the octahedron , as shown in Figure 1 . Thus, Li 4 Ti 5 O 12 can also be expressed as [Li] 8a [Li 1/3 Ti 5/3 ] 16d [O 4 ] 32e , and the lattice constant a = 0.8364 nm . In the process of lithium insertion , the principle of structural change is as follows : [Li] 8a [Li 1/3 Ti 5/3 ] 16d [O 4 ] 32e +e - +Li + → [Li2] 8a [Li 1/3 Ti 5/3 ] 16d [O 4 ] 32e Most spinel-type materials are compounds in which single-phase ions are randomly inserted, while Li 4 Ti 5 O 12 has a very flat charge and discharge platform when foreign Li + is embedded in the crystal lattice of Li 4 Ti 5 O 12 , Li + began to occupy the position 16 c, the lattice of Li 4 Ti 5 O 12 is positioned 8 c of the original Li + began to migrate to position 16 c, 16 c and finally all of the positions are occupied by Li +, so the The capacity is also primarily limited by the number of octahedral voids that can accommodate Li + . The reaction product Li 4 Ti 5 O 12 is light blue, and its electron conductivity is good due to the occurrence of Ti 4+ and Ti 3+ valence, and the electrical conductivity is about 10-2 S/cm . Ohzuku et al. showed that Li 4 Ti 5 O 12 is used as a negative electrode material for lithium ion batteries. During charge and discharge, lithium ion insertion and deintercalation have little effect on the material structure, and the unit cell parameter a changes little, only increasing from 0.836 nm to 0.837 nm , and after 100 charge and discharge cycles, the capacity loss is very small. This is of great significance, and can avoid the destruction of the structure due to the back and forth expansion and contraction of the electrode material in the charge and discharge cycle , thereby improving the cycle performance and the service life of the electrode, and reducing the large-scale attenuation of the specific capacity as the number of cycles increases. 12 so that more excellent than carbon cycle with Li 4 Ti 5 O. At 25 °C, the chemical diffusion coefficient of Li 4 Ti 5 O 12 is 2 × 10-8 cm2/s , which is an order of magnitude larger than the diffusion coefficient in carbon anode materials. The high diffusion coefficient makes the anode material fast and multiple times. Cycle charging. Therefore, Li 4 Ti 5 O 12 has been extensively studied as a negative electrode material for lithium ion batteries. The electrode potential of Li 4 Ti 5 O 12 relative to lithium metal is 1.55 V. The reaction has a very flat charge and discharge platform, which exceeds 90% of the whole process of the reaction . This indicates that the two-phase reaction runs through the whole process, and the voltage of charge and discharge is close. This can be combined with high-potential (about 4 V ) positive active materials such as LiNiO 2 , LiCoO 2 , and LiMn 2 O 4 to form an open circuit voltage of 2.4 to 2.5 V ( about twice that of Cd-Ni or MH-Ni batteries ) . battery. And as the chip core voltage drops, this voltage battery will be available in current mainstream electronics. As a battery negative electrode material, Li 4 Ti 5 O 12 has advantages such as good safety, high reliability, and long life with respect to carbon materials such as graphite, and thus may be applied to electric vehicles and energy storage batteries. 2        Â
Synthesis method of Li 4 Ti 5 O 12 The solid phase method is simple in operation, low in equipment requirements, and suitable for mass production. Therefore, in many studies, Li 4 Ti 5 O 12 is synthesized by a solid phase reaction. Usually a certain ratio according to the material (typically Li: TiO 2 = 4: 5 ) to LiOH · H 2 O and TiO 2 dispersed in an organic solvent or water, the solvent was removed at a high temperature, and then sintered in an air atmosphere in 800 ~ 1000 ℃ 3~24 h , after cooling with the furnace temperature , ball milling , to obtain the ideal spinel structure of Li 4 Ti 5 O 12 . K.Nakahara et al. synthesized Li 4 Ti 5 O 12 according to this method . The average particle size of the sample was 0.7 μm , and showed good high-rate charging performance and cycle life. At 25 ° C, 1 C was charged. The capacity remained 99% after 100 cycles of discharge . The main disadvantages of solid phase synthesis are that the product particles are not uniform, the crystal shape is irregular, the particle size distribution is wide, the synthesis cycle is long, and the stoichiometry is difficult to control. The sol - gel method has the following advantages: (1) The sol - gel method precursor solution has good chemical uniformity and low heat treatment temperature; (2) can effectively improve the purity of the synthesized product and the crystal grain size, and the reaction process is easy to control; Nano-powder and film can be prepared; however, organic matter generates a large amount of CO 2 gas during sintering, has large drying shrinkage, long synthesis cycle, and is difficult to industrialize. S. Bach et al. used isopropanol titanium and lithium acetate as raw materials, dissolved lithium acetate containing crystal water in ethanol, and then added titanium isopropoxide. The yellow solution became white gel after 1 h . After the gel is placed in the air at 60 ° C for 1 day, it is dried and calcined to obtain the product Li 4 Ti 5 O 12 . Li + diffusion coefficient of the product was 3 × 10-12 cm 2 / s, at a C / 60 rate, initial discharge capacity of 150mAh / g, the charge-discharge platform 1.55 V. 3        Â
Doping modification of Li 4 Ti 5 O 12 The doping modification of Li 4 Ti 5 O 12 can not only improve the conductivity of the material, but also reduce the resistance and polarization, and also lower the electrode potential and increase the energy density of the battery. The doping modification can, on the one hand, carry out a bulk doping of the material and, on the other hand, can introduce a highly conductive phase directly. In order to improve the electronic conductivity of the material, free electrons or electron holes can be introduced into the material. The doping modification of Li 4 Ti 5 O 12 can be carried out by substituting Li + , Ti 4+ or O 2- . Chen et al . added Mg 2+ in the preparation of Li 4 Ti 5 O 12 to replace the position of Li + , after ball milling in methanol medium, heat treatment at 1000 °C in a mixed gas stream of 3% H2 and 97% He . 5 h , Li 4-x Mg x Ti 5 O 12 (0.1<1.0) P. Kubiak et al. used V , Mn and Fe as doping elements to synthesize Li 4 Ti 5 O 12 products by sol - gel method . The results show that the specific specific capacity of Li 4 Ti 5 O 12 sample is 154 mAh/g , and the cycle specific capacity of Li 4.25 Ti 4.75 Fe 0.25 O 12 sample is 106 mAh/g , and the cycle of Li 4.25 Ti 4.75 V 0.25 O 12 sample The specific capacity is 74 mAh/g . Doping changes the structure of the sample and reduces the specific capacity of the sample. AD Robertson et al. studied the electrochemical properties of Li 1.3 M 0.1 Ti 1.7 O 4 (M = Fe , Ni , Cr) with a substitution mechanism of: 3M 3+ â†â†’ 2Ti 4+ +Li + . Fe is rich in resources and low in toxicity, and is superior to other transition metal elements. Ni 2+ and Cr 3+ are selected as doping elements, mainly because they are similar to the ionic radius of Ti 4+ and can preferentially occupy the octahedral position. The Li 2 CO 3, Fe 2 O 3 ( or NiO, Cr 2 O 3), and the TiO 2 were weighed sufficiently dried, after adding ethanol, a ball mill at room temperature 30 min, for homogenization, after drying, the powder Heat treatment at 600~700 °C for several hours, re-grind, heat treatment at 900~1000 °C for 1~2 h , and finally mill the sample for 30 min to obtain the product. Doping reduces the discharge platform voltage of the sample. Nickel and chromium doping increase the theoretical specific capacity of the sample, but reduce the cycle performance; after doping with iron, the cyclic specific capacity is significantly reduced. In the study [11], Tsutoum et al. [11] obtained a spinel structure of Li[CrTi]O 4 with an open circuit voltage of 1.5 V and a recyclable capacity of 150 mAh/g . To Li 2 CO 3, TiO 2 and AgNO 3 as a raw material, air temperature solid state reaction Ag doped Li 4 Ti 5 O 12. The conductivity of the sample after doping with Ag increases. XRD analysis shows that Ag is present in the doped sample as a separate phase in the Li 4 Ti 5 O 12 matrix, that is, the doped sample is actually a composite of Ag and Li 4 Ti 5 O 12 , so the increase in conductivity is mainly The increase in the electronic conductance of the sample. The constant current charge and discharge test was carried out at 0.2~4 C , and it was found that the capacity decreased as the charge and discharge current density increased. Except at 0.2 C rate, the doping sample has much higher capacity than undoped, and Ag doping significantly improves the high rate performance of Li 4 Ti 5 O 12 . Westland synthetic spinel structure doped Ni, W, Sn of Li 4 Ti 5 O 12 material. The TiO 2, Li 2 CO 3 and the doping element according to a certain proportion polishing uniformity charged crucible, placed in a high temperature furnace and heated for 24 h at 1000 ℃, i.e. to obtain a doped Li 4 Ti 5 O 12. The experimental battery voltage platform of the doped Li 4 Ti 5 O 12 assembly is lower than that of the undoped composite oxide experimental battery, and the first irreversible capacity of the battery is also small. Belharouak other by solid-phase synthesis method Li 4 Ti 5 O 12, the Li 2 CO 3, SrCO 3 or BaCO 3, TiO 2 mixture was rapidly heated to 600 ℃, decomposition of carbonate, after grinding, then at 1000 ℃ After incubation for 24 h , Li 2 MTi 6 O 14 (M=Sr, Ba) was synthesized . The obtained product was a three-dimensional network structure. It was found by ASI test that Li 2 MTi 6 O 14 (M=Sr, Ba) has very Good ionic or electronic properties, which may be due to a mixed valence state in the 3- step synthesis process, which improves the conductivity of the material. The reversible capacity of Li 2 MTi 6 O 14 after 40 cycles is stable at 140. mAh / g or so, with good cycle performance. Al is highly stable and light in octahedron and is an ideal doping element. At the same time, doping the anion F- can also increase the electronic conductance. It is found that Al doping can significantly improve the reversible capacity and cycle stability of Li 4 Ti 5 O 12 , while F doping reduces it. Al and F co-doped samples exhibited better electrochemical performance than F- doped samples, but were inferior to Al- doped samples. In addition, the electron conductance of the electrode can be improved by adding C to improve the performance of the Li 4 Ti 5 O 12 material. C has three main functions: as a reducing agent, promoting the diffusion of lithium to enable complete reaction; Particle size; increase the interparticle bonding force and inhibit the growth of interfering ions. Gao et al. studied the carbon-coated Li 4 Ti 5 O 12 electrode material. The results show that the high-rate discharge performance of the material is greatly improved after coating C. When discharging at 3.2 mA/cm 2 , the initial discharge capacity is 132.4. mAh/g , after 50 cycles, the capacity retention rate reached 86.7% . However, this work did not directly and quantitatively measure and discuss the improvement of the electrical resistance and electrical conductivity of the material. Liu et al. reported that LiMn 2 O 4 coated with Li 4 Ti 5 O 12 has quite good high-temperature discharge performance and cycle performance. Yi Tingfeng and other research groups synthesized LiCr 0.2 Ni 0.4 Mn 1.4 O 4 cathode material by high temperature solid phase method , but its electrochemical performance was poorer than that of sol - gel method. After coating with Li 4 Ti 5 O 12 , The electrochemical performance, especially the cycle performance, is quite good. Ohta et al reported that, after spray coating method in a thickness of 5 nm 2 after the surface of LiCoO Li 4 Ti 5 O 12, which resistance decreases as the raw material of 1/20 to when 5 mA / cm 2 current discharge, coated using The sample discharge capacity is 16 times that of the uncoated , and its capacity is still as high as 44 mAh/g when discharged at 10 mA/cm 2 (0.88 A/g) . Wang et al reported that the 30 -cycle ( current density: 0.2 mA/cm 2 ) of the 3 V battery consisting of LiNi 0.5 Mn 1.5 O 4 / Li 4 Ti 5 O 12 has a capacity decay of only 0.28% of the initial capacity , which is quite good. Cyclic performance; however, this work has not been further reported on its high current discharge performance and cycle performance. 4        Â
Calculation of the principle of Li 4 Ti 5 O 12 and its doping compounds Currently, less about 4 Ti 5 O 12 ** principle calculation Li and dopant compound reported. Liu et al. used the principle of ** to calculate the effect of cation doping on the electronic conductivity of Li 4 Ti 5 O 12 . The results show that Fe and Ni doping can not improve the electronic conductivity of Li 4 Ti 5 O 12 , Cr and Mg. The doping can increase the electronic conductivity of Li 4 Ti 5 O 12 . Ouyang et al. calculated the structure and electronic properties of Li 4 Ti 5 O 12 by density functional plane wave pseudopotential method . The results show that the volume and Gibbs free energy of Li 4 Ti 5 O 12 change during lithium ion intercalation. It is smaller than LiMeO 2 and LiMn 2 O 4 , and after the lithium ion is embedded, the d orbital of Ti is partially filled, and its electronic structure exhibits metallic properties. Zhong and other studies suggest that, Li 4 Ti 5 O 12 can not only lithium into Li 7 Ti 5 O 12, can also be into a lithium Li 8.5 Ti 5 O 12, which is the theoretical capacity of 1.5 times of the former. 5 Â Â Â Â Â Â Â Â Conclusion Spinel Li 4 Ti 5 O 12 is a "zero strain" insert material that has received much attention due to its excellent cycle performance and extremely stable structure. Also. The main technical bottleneck in the development of hybrid vehicle- powered lithium-ion batteries is rate performance and safety. Toshiba Corporation of Japan reported the safety hazard caused by internal short circuit to lithium-ion batteries, and proposed the use of Li 4 Ti 5 O 12 anode to reduce the safety hazard of internal short circuit. The hybrid lithium-ion battery designed with Li 4 Ti 5 O 12 can be smaller than the battery designed with carbon negative electrode, which reduces the cost of the battery. Compared with carbon anode materials, Li 4 Ti 5 O 12 has good electrochemical stability and safety, so it has become a popular target for designing hybrid vehicle power batteries. However, at present, only a few companies such as the United States and Japan can mass produce Li 4 Ti 5 O 12 electrode materials, and the annual domestic supply and usage are obviously insufficient. In the global field of power batteries, the high-rate operating characteristics of lithium-ion batteries are one of the key factors determining whether they can be commercialized. The low high-rate performance is the bottleneck affecting the development of Li 4 Ti 5 O 12 as a negative electrode material. Therefore, how to improve the high-rate performance of Li 4 Ti 5 O 12 has become one of the hotspots of current concern. Space Truss Structure Building Space Truss Structure Building,Steel Construction,Space Truss Metal Roof Building Steel Structure Workshop Co., Ltd. , http://www.nssteelstructure.com
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2.1 solid phase method
2.2 Sol - gel method
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The Sn- doped Li 4 Ti 5 O 12 electrode material has stable cycle performance and large charge and discharge capacity.
Wang et al. synthesized a Li 4 Ti 5 Cu 0.15 O 12.15 anode material by high temperature solid phase method . The capacity of the 5 C discharge was 107 mAh/g , which was much higher than that of the pure phase Li 4 Ti 5 O 12 .
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