| In the field of electric vehicles, good energy recovery performance means that the battery possesses good rate performance, which was mainly determined by the anode materials.Anode materials used in lithium ion battery mainly include graphite, soft carbon and hard carbon. Graphite is the most popular material, higher capacity and efficiency, of which d002 ranges from 0.3330nm to 0.3370nm, however graphite shows bad rate performance, and it is not safe enough at high rate charge/discharge. Hard carbon has a wider d002 from 0.3700 to 0.400nm, which could offer wider paths for lithium ions.The rate performance and low temperature properties of hard carbon are perfect, but its cost is high, and the process requirements was rigorous, such as an hydrous oxygen-free environment. Soft carbon could be easily graphitized at a high enough temperature, the d002 is 0.342～0.347 nm. Soft carbon was with a wide layer spacing, good rate performance, low temperature performance between graphite and hard carbon. Furthermore, the cost is low, the solvent can be water, which is out of pollution. Soft carbon is a developing anode material for power battery. However, the application of soft carbon as the anode still needed to settle the following issues:First, the manufacture of materials with good low temperature performance; second, the specific capacity is low;third,the large voltage drop of soft carbonduring discharge, DC internal resistance, resulting in excessive polarization of heat and energy waste. Application of soft carbon in lithium-ion battery is still a changelling tasks. This dissertation studied the rate performanceof soft carbon with different sizes, and tried to improve the low temperature properties of the materials; To improve the compaction density and energy density by mixing conductive graphite, reducing the DC resistance of the battery. Test various aspects performance of the battery, and achieved good results. The main contents are as follows:1. A comparison of Rs and Rct with different size materials D50 = 15μm,12μm, 10μm was conducted, and found the rate capability increased with decreasing the particle size, the D50 of particle reduced from 15μm to 10μm, the Rct decreased. Reducing the size material particle, the lithium-ion transmission path can be shortened in the solid phase, reducing Rct of cell;2. Discharge capacity of soft carbon at low temperature was improved by coated asphalt.-20℃ 3C discharge rate improved from 55% to 62% after coated, and the discharge platform was also greatly improved.3. Soft carbon has low first coulomb efficiency, part of lithium of cathode lost during the first charging, which led to low energy. To solve this question, we found an effectively way. Sprinkling quantificational lithium powder on the anode electrode surface, the added lithium could go to cathode when first discharge, compensate the lithium loss of SEI film formation, which improved the coulomb efficiency. The specific capacity of active material could be greatly improved, the cathode was up to 150mAh/g from 120mAh/g, the soft carbon increased from 207mAh/g to 257mAh/g. The energy density increased nearly 25%. We found a good way to improve the first efficiency of soft carbon.4. We improved the adhesive strength by optimizing the soft adhesive of soft carbon between the pole piece and the collector, improved the performance of battery; improved the conductivity of the pole pieces and compaction density by optimizing a conductive agent, the volume energy density of the battery increased. Then, the battery has a good optimization rate charge-discharge performance, high rate cycle characteristics, high temperature cycle performance and good safety characteristics. |
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