Design And Electrochemical Performance Of High Specific Energy Lithium Ion Capacitor

The construction of new energy system and the rapid development of electronicequipment put forward higher requirements for energy storage devices, their energy density, power density and cycle stability of lithium ion battery and capacitor need to be improved. Lithium-ion capacitor (LIC) is a novel storage device which based on the dual energy storage mechanism of lithium-ion battery (LIB) and electrochemical double-layer capacitor (EDLC). LIC can deliver higher energy density than EDLC and higher power density than LIB, it’s one of the optimum choices for hybrid electric vehicles and electric vehicles. In terms of future industrialization of lithium ion capacitor, carbonaceous materials seem to be the best alternative because of its cheapness and easy availability. In this paper, we construct four different systems of LIC and undertake a more detailed evaluation of the electrochemical performance of LIC’s systems.In the activated carbon/graphite LIC system, the pre-lithiaiton degree of graphite anode, lithium ion intercalation plateaus utilization of anode, and the capacity design of activated carbon cathode can realize the control of the specific capacity, specific energy, specific power and cycle performance of the LIC. When the capacity design of activated carbon cathode was set at 50 mAh g-1, the second lithium ion intercalation plateau of graphite anode was used and the pre-lithiuon capacity of anode was set at 300 mAh g-1, the activated carbon/graphite LIC system exhibited the optimal electrochemical performance. The energy density of LIC was up to 92.3 Wh kg-1, the power density as high as 5.5 kW kg-1 and the capacity retention was 97.0% after 1000 cycles at the working voltage of 2.0-4.0 V. The power density and cycle performance of LIC could be further improved by reducing the AC positive electrode designed capacity or using the first lithium ion intercalation plateau of graphite anode.In the activated carbon/soft carbon LIC system, the mesocarbon microbead (MCMB) was used as the precursor to prepare the soft carbon material under different carbonization temperatures, and the soft carbon material could keep good spherical structure. With the increase of carbonization temperature, the layer spacing of soft carbon graphite crystallite first increased then decreased. The layer spacing of soft carbon material prepared with the carbonization temperature of 1500℃ was about 0.3512 nm, which was greater than the layer spacing of graphite materials. The layer spacing variation of soft carbon would affect the rate performance as the capacitor anode, as well as the charge and discharge process and electrochemical potential changes of the positive and negative electrode, and affect the electrochemical performance of LIC. Within the working voltage range of 2.0-4.0V, LIC constructed with activated carbon cathode and soft carbon anode which prepared with the carbonization temperature of 1500℃ showed the optimal electrochemical performance. The energy density of LIC1500 was up to 85.1 Wh kg-1, the power density as high as 6.2 kW kg-1 and the capacity retention was 93.7% after 1000 cycles.In the activated carbon/hard carbon LIC system, the irregular hard carbon material had a distinct lithium ion intercalation plateau near about 0V in the charge-discharge process in comparision with the spherical hard carbon. The lithium ion intercalation plateau could make the HC negative electrode at the lower potential range, which was conductive to the sufficient utilization of AC positive electrode and the improvement of LIC’s energy density. The irregular HC material had disordered structure and the space gap of irregular HC was larger than the interlayer spacing of the spherical HC, which resulted in enabling a faster lithium ion intercalation/de-intercalation process, the structural characteristics of irregular HC decided the higher power density of LIC. LIC exhibited the optimal electrochemical performance at the working voltage of 2.0-4.0 V. The energy density of LIC was up to 85.7 Wh kg-1, the power density as high as 7.6 kW kg-1 and the capacity retention was 96.0% after 5000 cycles at 2C rate. In addition, LIC-IH showed excellent electrochemical performance at low temperature. LIC-IH still could maintain the energy density of 76.6 Wh kg-1, the power density of 5.8 kW kg-1 and 80.1% capacity retention after 5000 cycles.In the graphite-based lithium ion capacitor system, the mesoporous carbon (MC) material was prepared through an one-step facile template approach wherein MgO was used as the template. The MC material showed appropriate mesopore structure (about 6.5 nm), high specific surface area (1432 m2 g-1) and large pore volume (2.894 cm3 g-1). The appropriate mesopore size distribution and high surface area of MC were beneficial to the charge accumulation and the transmission. The specific capacity and cycle performance of mesoporous carbon mateials were superior to commercial activated carbon materials in the lithium salt electrolyte. In LIC, the energy characteristics of LIC depended on the charge adsorption-desorption behavior of the cathode. Therefore, the increased anion adsorption-desorption amount of MC cathode would result in the improvement of graphite-based LIC energy density. Compared with the graphite electrode, the MH composite electrode was prepared by adding the HC material into graphite electrode, and the MH composite electrode with different HC content exhibited improved rate performance. The power capability of LIC would be limited by the rate characteristics of the negative electrode, the improved rate performance of MH composite electrode was beneficial for the improvement of power density and cycle stability of graphite-based LIC.