As the most mature and reliable high-density energy storage method, it has placed all the hopes for the development of electric vehicles. In order to promote the development of electric vehicles, the relevant national departments have proposed that the 2020 power battery needs more than 300Wh/kg, so high. The specific energy needs to be applied to higher-capacity high-nickel ternary materials and silicon-carbon composite anode materials. In the current slow increase in cathode material capacity, the application of silicon-carbon anodes has become the most effective way to improve the energy density of lithium-ion batteries. However, the silicon carbon material will have a huge volume expansion during the lithium intercalation process, causing particle pulverization and electrode structure damage. Therefore, the current mainstream silicon carbon negative electrode is added at about 10%. To continue to increase the negative electrode capacity, it is necessary to adopt a new one. Material system.
B element is one of the lightest elements and can form an alloy with Li. Its theoretical capacity is up to 12395 mAh/g (forming Li5B alloy). However, element B can only alloy with Li in atomic state. Conventional elemental substance Both B and B oxides are difficult to react with Li, so B element has received less attention. To solve this problem, WujieDong (first author) of Shanghai Institute of Ceramics, FuqiangHuang (corresponding author) tried to B element Dispersed in the Fe conductive network to form Fe/B alloy (actually B is also a common steel alloy element, which plays a role in refining grains and enhancing steel toughness). According to the amount of B added, the anode material is different. The maximum capacity can reach 10700mAh/g (only considering the weight of B element). In order to improve the practicality, WujieDong designed the B2O3/FeOx composite electrode. The initial capacity of the electrode can reach 800mAh/g (0.1A/g), after 250 cycles. Increased to 1500mAh / g, and showed good rate performance, stable capacity up to 1250mAh / g at 0.5A / g current density, stable capacity up to 1200mAh / g at 1A / g current density, at 2A / The stable capacity of g at a current density of up to 800 mAh/g, More critically, the material has a tap density of 2.12 g/cm2, which is almost twice that of graphite. It is an ideal lithium ion battery anode material.
WujieDong prepares a certain proportion (1-11%) of B powder into the Fe powder, and then heats it by solid phase reaction to prepare a negative electrode material containing Fe and Fe2B alloy, and then the alloy negative electrode material is ground by high energy spheroidal ink. In order to reduce the particle size of the particles. The following figure shows the cycle curve of pure B powder and 1% B Fe alloy powder. It can be seen from the figure that the initial discharge capacity of pure B powder is only 92 mAh/g, and it drops to 6 mAh after 200 cycles. /g (0.1A/g, 3V-0.01V), indicating that the pure B powder activity is very low and cannot be used as a negative electrode material. The initial capacity of the 1% B Fe alloy powder material is only 30 mAh/g, which is close to the pure B material. However, the capacity of the material continued to increase during the cycle, and its reversible capacity increased to 107 mAh/g after 1400 cycles. If only the content of B element is considered, the reversible capacity can reach 10700 mAh/g, which is close to B. The theoretical capacity of the element.
The cyclic voltammetry curve of Fe/B alloy anode materials with B content of 1%, 7% and 11% respectively is shown in Figure DF. Figure C below shows the cyclic voltammetry curve of element B. The material can be seen from the figure. The reduction peak appears near 0V, and the corresponding reaction is the formation of LiXB. The current peak appearing at 0.5-0.75V during the first lithium insertion process is mainly the reduction of electrolyte to form SEI film, which disappears in the subsequent cycle.
The figure below shows the charge-discharge curves of B/Fe alloy anodes with B powder and 1-7%B content in different cycle periods. It can be seen from the figure that the capacity of B/Fe alloy anodes is significantly improved compared to pure B anodes. This is mainly because the B element is dispersed in the Fe phase, which greatly shortens the diffusion distance of Li+, while the Fe phase provides a good conductive network, improves the kinetic conditions of lithium intercalation, and continuously circulates the B element because Lithium intercalation and volume expansion will further promote the dispersion of B element in the Fe phase. Therefore, the capacity of the B/Fe alloy negative electrode will continue to increase with the cycle. It should be noted that although the B element is used as the reference B/ The specific capacity of the Fe alloy anode is very high, but when we take the quality of the Fe element into account, the overall capacity of the material is very low (100 mAh/g, and it takes a long time to activate), so there is no practical value. In order to solve this problem, the author turned his attention to B2O3 materials.
The theoretical maximum capacity of element B can reach 12395 mAh/g, but it is accompanied by a huge volume expansion. Generally, we believe that metal oxide can effectively inhibit the volume expansion during lithium insertion. For example, SnO2 material can inhibit lithium insertion well. The volume expansion in the process, and the Gibbs free energy of Li and B2O3 reaction is -489.3kJ/mol, which is theoretically a spontaneous reaction, but the conductivity of B2O3 material is extremely poor (<10-13S/cm) , 导致B2O3无法正常嵌锂, 为了解决这一问题WujieDong设计了B2O3/ FeOx复合电极, 复合电极经过烧结后电导率提高到了1.6S/cm.
It can be seen from the figure below that the B2O3 material has a reversible capacity of only 20 mAh/g due to the extremely poor conductivity. The reversible capacity of the Fe2O3 material is as high as 1000 mAh/g, but it decays rapidly in the cycle, and the sintered B2O3/FeOx The initial capacity of the composite electrode is about 800 mAh/g. As the cycle increases, it reaches 1500 mAh/g after 200 cycles. At the same time, since the tap density of the composite electrode is as high as 2.12 g/cm3, it has no volume energy density. The advantage of Lenby.
At the same time, the B2O3/FeOx composite electrode also exhibits excellent rate performance at current densities of 0.2, 0.5, 1.0, 2.0, 5.0 and 10 A/g, with reversible capacities of 850, 810, 750, 680, 550 and 430 mAh/g, respectively. After recovering to a current density of 0.1A/g, the capacity of the composite electrode continues to rise, and finally reaches 1500mAh/g.
The theoretical specific capacity of element B can reach 12395mAh/g, which is an ideal anode material for lithium ion batteries. However, due to its poor dynamic conditions, it is difficult to use as a negative electrode material. B/Fe alloy negative electrode only considers B element. Its weight, its specific capacity can reach 10700mAh / g, but considering the weight of Fe element, its reversible capacity is only about 100mAh / g, so the author turned his attention to the B2O3 / FeOx composite electrode, the initial reversible capacity of 800mAh /g, stable and reversible capacity up to 1500mAh/g, and high tap density (2.12g/cm3), so it has more advantages in volumetric energy density, even better than Si negative electrode material known for high capacity, it has a very broad Application prospects.
