Study On The Dual-functional Electrolyte And Its Compatibility With LiFePO₄-AC/AG-AC Capacitor Enhanced Battery Electrode Materials

Capacitor enhanced battery is a new kind of energy storage device possessing high energy density of lithium ion battery as well as high power density of super-capacitor. It has great application aspects in fields of transportation, mobile communication and information technology, etc. As one of the core material of capacitor enhanced battery, electrolyte has an extremely impact on capacity, resistance, cycling life and temperature properties, etc. In this paper, tetraethylammonium tetrafluoroborate (TEABF4) which is the most common electrolyte salt of super-capacitor was firstly blend into LiPF6 based electrolytes. The compatibility of LiFePO4 cathode, artificial carbon (AG) anode and LiPF6-TEABF4 blend salts electrolyte was studied; and then expanding solvent system and change concentration of TEABF4, the compatibility of LiFePO4/AG cells, AC/AC capacitor and electrolytes was studied. The "dual-functional" electrolyte which shows good performance in this two energy storage devices was optimized; finally, the electrochemical properties of LiFePO4-AC/AG-AC-type capacitor enhanced battery were investigated in dual-functional electrolyte. The constant current charge-discharge, cyclic voltammetry, AC impedance and infrared spectral analysis were applied in this paper, and obtain the following conclusions:(1) The initial charge-discharge effeciency of LiFePO4/Li half-cells is over 92% in LiPF6-TEABF4 blend salts electrolytes. From the results of capacity tests, LiFePO4/Li half-cells show higher discharge capacity (over 142 mAh) in A system electrolytes than D system. Regardless of whether containing PC in electrolyte, adding a certain amount of TEABF4 can improve the charge-discharge capacity. That is to say LiFePO4 cathode has good compatibility with LiPF6-TEABF4 blend salts electrolytes.(2) AG/Li half-cells show high charge-discharge capacity and initial charge-discharge effeciency in LiPF6-TEABF4 blend salts electrolytes. The interface impedance of AG and electrolyte A is reduced by adding 0.1M TEABF4. It is can be seen from the results of EIS during initial discharge and FTIR analysis that, SEI film forms at the potential range of 1.0 V-0.5 V, and repair below 0.5 V. The main components of SEI film are ROCO2Li, Li2CO3, ROLi and so on. Similar to A system, it can be seen apparent formation and repairment of SEI film on AG interface during intial dischrge in electrolyte D3. And the impedance of SEI is lower than electrolyte D, TEABF4 improves the compatibility between AG and PC-containing electrolyte.(3) The charge-discharge capacity, rate and cycly performances of LiFePO4/AG batteries as well as the capacitance of AC/AC super-capacitors are improved by adding some TEABF4 to LiPF6-based electrolytes. 1.0M LiPF6+0.1M TEABF4/EC-PC-DMC-EMC electrolyte plays the optimal compatibility with electrodes. With this electrolyte, the discharge capacity of LiFePO4/AG cells is 423 mAh, capacity retention after 100 times are 98.4% and 89.4%, respectively. With this electrolyte, the charge-discharge curves of AC/AC capacitor show good linear relationship, and the capacitance is 2.4 F after 1500 cycles. The dual-functional electrolyte which shows good compatibility with LiFePO4/AG cell as well as AC/AC capacitor was employed into LiFePO4-AC/AG-AC type capacitor enhanced battery. Capacitor enhanced battery with dual-functional electrolyte show higher capacity (more than 8.5%) than with common lithium ion battery electrolyte (LiPF6/EC-PC-DMC-EMC).To sum up, we point out that the most applicable electrolyte to LiFePO4-AC/AG-AC capacitor enhanced battery is 1.0M LiPF6+0.1M TEABF4/EC-PC-DMC-EMC.