LR6 battery

Review of the research on lithium titanate, the negative electrode material of LR6 battery

At present, the negative electrode materials of LR6 battery are mostly various lithium-intercalated carbon materials. However, the potential of the carbon electrode is very close to that of metallic lithium. When the battery is overcharged, metallic lithium is easily precipitated on the surface of the carbon electrode, forming dendrites and causing short circuits; when the temperature is too high, it is easy to cause thermal runaway. At the same time, the structure of the carbon material will be destroyed during the repeated insertion and deinsertion of lithium ions, resulting in capacity decay. Titanium-based compounds are also a type of negative electrode material that is currently being studied more, including TiO2, LiTi2O4, Li4Ti5O12, Li2Ti3O7 and their doped and modified materials. Among them, batteries using spinel Li4Ti5O12 as negative electrode materials have been used in watches. The author of this article reviews the research on the structure, synthesis and physical and chemical properties of spinel-type Li4Ti5O12 negative electrode materials at home and abroad in recent years.

1Structure and electrochemical properties of Li4Ti5O12

Spinel Li4Ti5O12 is a "zero strain" insertion material. It has become a widely-regarded negative electrode material for LR6 battery due to its excellent cycle performance and extremely stable structure. Although the theoretical capacity of Li4Ti5O12 is only 175mAh/g (discharged to 1V), its actual capacity is generally maintained at 150~160mAh/g (discharged to 1V) because its reversible lithium ion extraction/insertion ratio is close to 100%. Li4Ti5O12 belongs to the spinel type and is a face-centered cubic structure (space group Fd3m), in which O2- ions constitute the FCC lattice and are located at the 32e position, some lithium ions are located at the tetrahedral 8a position, and the remaining lithium ions and titanium ions (Li∶Ti＝1∶5) are located at the octahedral 16d position, as shown in Figure 1. Therefore, Li4Ti5O12 can also be expressed as [Li]8a[Li1/3Ti5/3]16d[O4]32e, with lattice constant a=0.8364nm. During the lithium insertion process, the structural change principle is as follows:

[Li]8a[Li1/3Ti5/3]16d[O4]32e+e-+Li+→[Li2]8a[Li1/3Ti5/3]16d[O4]32e

Lithium titanate crystal structure

Most spinel-type materials are compounds with random insertion of single-phase ions, while Li4Ti5O12 has a very flat charge and discharge platform. When the external Li+ is embedded in the lattice of Li4Ti5O12, these Li+ begin to occupy the 16c position, and the Li+ originally located at 8c in the lattice of Li4Ti5O12 also begins to migrate to the 16c position. Finally, all 16c positions are occupied by Li+, so its capacity is also mainly limited by the number of octahedral voids that can accommodate Li+. The reaction product Li4Ti5O12 is light blue. Due to the valence change of Ti4+ and Ti3+, its electronic conductivity is good, and the conductivity is about 10-2S/cm. Ohzuku et al.'s research shows that when Li4Ti5O12 is used as a negative electrode material for LR6 battery, the insertion and extraction of lithium ions has almost no effect on the material structure during charge and discharge. The unit cell parameter a changes very little, only increasing from 0.836nm to 0.837nm, and after 100 charge and discharge cycles, the capacity loss is very small. This is of great significance. It can avoid the structural damage caused by the back-and-forth expansion and contraction of the electrode material during the charge and discharge cycle, thereby improving the cycle performance and service life of the electrode, reducing the significant attenuation of the specific capacity with the increase in the number of cycles, and making Li4Ti5O12 have better cycle performance than carbon. At 25°C, the chemical diffusion coefficient of Li4Ti5O12 is 2×10-8cm2/s, which is an order of magnitude larger than the diffusion coefficient in carbon negative electrode materials. The high diffusion coefficient allows the negative electrode material to be quickly and repeatedly charged. Therefore, Li4Ti5O12 has been widely studied as a negative electrode material for LR6 battery. The electrode potential of Li4Ti5O12 relative to metallic lithium is 1.55V, and the reaction has a very flat charge and discharge platform, which exceeds 90% of the entire reaction process, indicating that the two-phase reaction runs through the entire process and the charge and discharge voltages are close. This can form a battery with an open circuit voltage of 2.4~2.5V (about twice that of Cd-Ni or MH-Ni batteries) with high potential (about 4V) positive electrode active materials such as LiNiO2, LiCoO2, and LiMn2O4. And as the core voltage of the chip decreases, batteries with this voltage will be used in current mainstream electronic products. As a battery negative electrode material, Li4Ti5O12 has the advantages of good safety, high reliability and long life compared to carbon materials such as graphite, so it may be used in electric vehicles, energy storage batteries, etc.

2 Synthesis method of Li4Ti5O12

2.1 Solid phase method

The solid phase method is simple to operate, has low equipment requirements, and is suitable for large-scale production. Therefore, in many studies, Li4Ti5O12 is synthesized by solid phase reaction. Generally, LiOH·H2O and TiO2 are dispersed in an organic solvent or water according to a certain molar ratio (generally Li∶TiO2=4∶5), the solvent is removed at high temperature, and then sintered at 800~1000℃ in an air atmosphere for 3~24h. After cooling with the furnace temperature, ball milling is performed to obtain the ideal spinel structure Li4Ti5O12. K.Nakahara et al. synthesized Li4Ti5O12 according to this method. After testing, the average particle size of the sample was 0.7μm, and it showed good high-rate charging performance and cycle life. At 25℃, the capacity remained 99% after 100 cycles of 1C charge and discharge. The main disadvantages of solid-phase synthesis are uneven product particles, irregular crystal shape, wide particle size distribution range, long synthesis cycle, and difficult stoichiometry control.

2.2 Sol-Gel Method

The sol-gel method has the following advantages: (1) The chemical uniformity of the precursor solution of the sol-gel method is good, and the heat treatment temperature is low; (2) It can effectively improve the purity and crystal size of the synthesized product, and the reaction process is easy to control; (3) Nanopowders and thin films can be prepared; but organic matter produces a large amount of CO2 gas during the sintering process, has large drying shrinkage, a long synthesis cycle, and is difficult to industrialize. S. Bach et al. used titanium isopropoxide and lithium acetate as raw materials, dissolved lithium acetate containing crystal water in ethanol, and then added titanium isopropoxide. After 1 hour, the yellow solution turned into a white gel. After the gel was placed in the air at 60°C for 1 day, it was dried and calcined to obtain the product Li4Ti5O12. The Li+ diffusion coefficient of the product is 3×10-12cm2/s. At the C/60 rate, the first discharge specific capacity is 150mAh/g, and the charge and discharge platform is 1.55V.

3Doping modification of Li4Ti5O12

Doping modification of Li4Ti5O12 can not only improve the conductivity of the material, reduce resistance and polarization, but also reduce its electrode potential and increase the energy density of the battery. Doping modification can be used to dope the material in bulk on the one hand, and directly introduce a highly conductive phase on the other hand. 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 Li4Ti5O12 can be carried out from three aspects: replacing Li+, Ti4+ or O2-.

Chen et al. added Mg2+ to replace the position of Li+ when preparing Li4Ti5O12. After ball milling and dispersion in methanol medium, the Li4-xMgxTi5O12 (0.1<1.0) sample was obtained by heat treatment at 1000℃ for 5h in a mixed gas flow composed of 3% H2 and 97% He. Due to the different valences of Mg and Li, the valence of titanium changes from +4 to +3, which greatly improves the conductivity of electrons. At the same time, doping will lead to a decrease in capacity, which may be because Mg2+ occupies part of the 8a position of the tetrahedron in the spinel structure, but only within the range of 10%, so a small amount of doping can make the battery have good stability.

P. Kubiak et al. used V, Mn and Fe as doping elements and used the sol-gel method to synthesize doped Li4Ti5O12 products. The results showed that the cycle capacity of the Li4Ti5O12 sample was 154mAh/g, the cycle capacity of the Li4.25Ti4.75Fe0.25O12 sample was 106mAh/g, and the cycle capacity of the Li4.25Ti4.75V0.25O12 sample was 74mAh/g. Doping changed the structure of the sample and reduced the specific capacity of the sample.

A.D. Robertson et al. studied the electrochemical properties of Li1.3M0.1Ti1.7O4 (M=Fe, Ni, Cr), and its substitution mechanism was: 3M3+←→2Ti4++Li+. Fe is abundant in resources and has low toxicity, which is superior to other transition metal elements. Ni2+ and Cr3+ were selected as doping elements mainly because they have similar ionic radius to Ti4+ and can preferentially occupy octahedral positions. Li2CO3, Fe2O3 (or NiO, Cr2O3) and TiO2 were fully dried and weighed, and after adding ethanol, they were ball-milled at room temperature for 30 minutes to make them evenly mixed. After drying, the powder was heat-treated at 600~700℃ for several hours, then re-ground, and heat-treated at 900~1000℃ for 1~2h. Finally, the sample was ball-milled for 30 minutes to obtain the product. Doping reduces the discharge platform voltage of the sample. Doping with nickel and chromium increases the theoretical specific capacity of the sample, but reduces the cycle performance; after doping with iron, the cycle specific capacity is significantly reduced. Tsutoum et al. [11] obtained spinel structure Li[CrTi]O4, whose open circuit voltage relative to metallic lithium is 1.5V and the cyclable capacity is 150mAh/g.

Ag-doped Li4Ti5O12 was synthesized by high-temperature solid-phase reaction in air using Li2CO3, TiO2 and AgNO3 as raw materials. The conductivity of the sample increased after Ag doping. XRD analysis showed that Ag in the doped sample existed as a separate phase in the Li4Ti5O12 matrix, that is, the doped sample was actually a composite of Ag and Li4Ti5O12, so the improvement in conductivity was mainly due to the increase in the electronic conductivity of the sample. Constant current charge and discharge tests were performed at 0.2~4C, and it was found that the capacity decreased with the increase in charge and discharge current density. Except at a rate of 0.2C, the capacity of the doped sample was much higher than that of the undoped sample, and Ag doping significantly improved the high-rate performance of Li4Ti5O12.

Hua Lan et al. synthesized spinel-structured Li4Ti5O12 materials doped with Ni, W, and Sn. TiO2, Li2CO3 and doping elements are ground and mixed evenly in a certain proportion, put into a crucible, placed in a high-temperature furnace, and heated at 1000℃ for 24h to obtain doped Li4Ti5O12. The voltage platform of the experimental battery assembled with doped Li4Ti5O12 is lower than that of the undoped composite oxide experimental battery, and the first irreversible capacity of the battery is also smaller.

The Sn-doped Li4Ti5O12 electrode material has stable cycle performance and large charge and discharge capacity.

Belharouak et al. synthesized Li4Ti5O12 by solid phase synthesis. The mixture of Li2CO3, SrCO3 or BaCO3, and TiO2 was quickly heated to 600℃ to decompose the carbonate. After grinding, it was kept at 1000℃ for 24h to synthesize Li2MTi6O14 (M=Sr,Ba). The obtained product was a three-dimensional spatial network structure. Through ASI test analysis, it was found that Li2MTi6O14 (M=Sr,Ba) had good ionic or electronic properties. This may be due to the generation of a mixed valence state in the three-step synthesis process, which improved the conductivity of the material. After 40 cycles, the reversible capacity of Li2MTi6O14 can be stabilized at about 140mAh/g, with good cycle performance.

Wang et al. synthesized Li4Ti5Cu0.15O12.15 negative electrode material by high temperature solid phase method. The capacity reached 107mAh/g at 5C discharge, which is much higher than pure phase Li4Ti5O12.

Al is highly stable and light in the octahedron, making it an ideal doping element. At the same time, doping with anion F- can also improve electronic conductivity. Studies have found that Al doping can significantly improve the reversible capacity and cycle stability of Li4Ti5O12, while F doping reduces it. The electrochemical performance of Al and F co-doped samples is better than that of F-doped samples, but worse than that of Al-doped samples.

In addition, the performance of Li4Ti5O12 materials can also be improved by adding C to increase the electronic conductivity of the electrode. C has three main functions: as a reducing agent, it promotes the diffusion of lithium so that it can react completely; reduces the particle size of the product particles; increases the interparticle bonding force and inhibits the growth of interfering ions. Gao et al. studied carbon-coated Li4Ti5O12 electrode materials. The results showed that after coating with C, the high-rate discharge performance of the material was greatly improved. When discharged at a current of 3.2mA/cm2, the first discharge capacity was 132.4mAh/g, and 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 material's resistance and conductivity. Liu et al. reported that LiMn2O4 coated with Li4Ti5O12 has quite good high-temperature discharge performance and cycle performance. Yi Tingfeng et al.'s research group synthesized LiCr0.2Ni0.4Mn1.4O4 positive electrode material using a high-temperature solid-phase method, but its electrochemical performance was worse than that synthesized by the sol-gel method. After coating with Li4Ti5O12, the electrochemical performance, especially the cycle performance, was quite good. Ohta et al. reported that after coating LiCoO2 with 5nm thick Li4Ti5O12 on the surface by spraying, its resistance was reduced to 1/20 of the raw material. When discharged at a current of 5mA/cm2, the discharge capacity of the coated sample was 16 times that of the uncoated sample. When discharged at 10mA/cm2 (0.88A/g), its capacity was still as high as 44mAh/g.

Wang et al. reported that the capacity decay of a 3V battery composed of LiNi0.5Mn1.5O4/Li4Ti5O12 in the first 30 cycles (current density: 0.2mA/cm2) was only 0.28% of the initial capacity, which has a fairly good cycle performance; but this work did not further report its high current discharge performance and cycle performance.

4 First-principles calculation of Li4Ti5O12 and its doped compounds

Currently, there are few reports on first-principles calculations of Li4Ti5O12 and its doped compounds. Liu et al. used first-principles calculations to study the effect of cation doping on the electronic conductivity of Li4Ti5O12. The study showed that Fe and Ni doping could not improve the electronic conductivity of Li4Ti5O12, while Cr and Mg doping could improve the electronic conductivity of Li4Ti5O12. Ouyang et al. used the density functional plane wave pseudopotential method to calculate the structure and electronic properties of Li4Ti5O12. The results show that during the lithium ion insertion process, the volume and Gibbs free energy changes of Li4Ti5O12 are smaller than those of LiMeO2 and LiMn2O4.After insertion, the d orbital of Ti is partially filled, and its electronic structure presents metallic properties. Zhong et al. believe that Li4Ti5O12 can not only be lithiated into Li7Ti5O12, but also into Li8.5Ti5O12, the latter of which is 1.5 times the theoretical capacity of the former.

5 Conclusion

Spinel Li4Ti5O12 is a "zero strain" insertion material, which has attracted wide attention for its excellent cycle performance and extremely stable structure. In addition. The main technical bottlenecks in the development of hybrid electric vehicle power LR6 battery are rate performance and safety. Toshiba Corporation of Japan reported the safety hazards caused by internal short circuits to LR6 battery, and proposed that the use of Li4Ti5O12 negative electrodes can reduce the safety hazards of internal short circuits. The hybrid electric vehicle power lithium-ion battery designed with Li4Ti5O12 can be smaller in size than the battery designed with carbon negative electrodes, reducing the cost of the battery. Compared with carbon negative electrode materials, Li4Ti5O12 has good electrochemical stability and safety, so it has become a popular object for designing hybrid electric vehicle power batteries. However, only a few companies in the United States and Japan can mass-produce Li4Ti5O12 electrode materials, and the domestic annual supply and usage are obviously insufficient. In the field of power batteries, which has attracted global attention, the high-rate working characteristics of LR6 battery are one of the key factors that determine whether they can be commercially applied. The low high-rate performance is the bottleneck that affects the development of Li4Ti5O12 as a negative electrode material. Therefore, how to improve the high-rate performance of Li4Ti5O12 has become one of the hot topics that people are currently paying attention to.

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