Research On High-performance Hvbrid Lithium-ion Capacitors

Combining the advantages from lithium-ion batteries（LIB）and supercapacitors（SCs）,hybrid lithium-ion hybrid capacitors（LIC）possess enhanced energy density,power density,and cycle life,thereby standing for an important research direction in the area of energy storage technology.Nevertheless,due to the narrow electrochemical stable potential window（ESPW）of conventional organic carbonate electrolytes and the low charge/discharge potential of commonly used cathode materials（4 V vs.Li/Li⁺）in LIC,the operating voltage of previously reported LIC is still low（≤3.3 V）,limiting the improvement of energy density for LIC.Furthermore,the inferior electrical conduction network of conventional electrodes and the improper capacity ratio between the cathode and anode hinder the enhancement of power density and cycle life of LIC.All of these indicate a strong need to further enhance the energy density,power density,and cycle life for currently available LIC to meet the practical application demands.In this thesis,therefore,we developed high-voltage LIC（composed of a high-potential lithium nickel manganese oxide（LNMO）cathode and an activated carbon（AC）anode,i.e.,LNMO//AC）by studying a functional high-voltage electrolyte,compositing electrodes with multidimensional carbon nanomaterials,and modulating the capacity ratio between the cathode and anode.The comprehensive research performed on these three aspects is summarized as follows:First,by introducing tris（trimethylsilyl）phosphite（TMSP）as a functional additive into a conventional carbonate electrolyte,the resultant electrolyte possessed a wide ESPW and formed a stable cathode/electrolyte interface（CEI）at cathode,leading to a high operating voltage and hence enhanced energy density for LIC with improved cycling stability.Specifically,our high-voltage electrolyte was obtained by adding 1 wt.%TMSP into the regular carbonate electrolyte（Reg）(1 mol L^-1 Li PF₆ in EC/DMC/EMC,1:1:1 by vol.),resulting in 1 wt.%TMSP+Reg electrolyte,which,upon electrochemical characterization was tested by three-electrode system,has showed a widened ESPW than its Reg counterpart.Furthermore,the investigation through the LNMO//Li and AC//Li half cells indicated that TMSP can decompose to form CEI at the LNMO cathode surfaces,respectively.The as-formed CEI possessed a high ionic conductivity and charge transfer kinetics,TMSP can remove decomposition products in the electrolyte,effectively enhancing the rate capability and cycling stability for both the LNMO cathode and AC anode.For the comparison between the Reg electrolyte and our 1 wt.%TMSP+Reg electrolyte,the capacity retention of a LNMO//Li half cell upon cycling at 1 C for 200cycles increased from 57.88%to 94.51%,while that for an AC//Li half cell upon cycling at 0.6 A g^-1 for 1000 cycles from 79.41%to 89.68%.Second,compositing LNMO with multidimensional carbon nanomaterials（including one-dimensional carbon nanotube（CNT）and two-dimensional graphene（GN））and conventional conductive additives（including zero-dimensional carbon black（SP）and conductive graphite（KS-6））,the resultant composite electrode had a well-defined“point-to-line-to-plane”electrical conduction network to facilitate its charge transfer and ion diffusion and thus lowering its resistance and improving its rate capability.Specifically,during the compositing of LNMO with CNT or GN,the contents of CNT and GN in the composite were optimized,respectively.As a result,with respect to the discharge capacity of a conventional electrode（i.e.,LNMO/SP/KS-6）at 20 C,those of the optimized composite electrodes of LNMO/SP/KS-6/CNT（CNT:0.2 wt.%）and LNMO/SP/KS-6/GN（GN:0.025 wt.%）were increased to 40.91 m Ah g^-1 and 38.06 m Ah g^-1,respectively.Moreover,upon the compositing of LNMO with CNT and GN synergistically,the resultant multi-nanocarbon composite electrode（i.e.,LNMO/SP/KS-6/CNT/GN with 0.2wt.%CNT+0.15 wt.%GN）was characteristic of a unique“point-to-line-to-plane”electrical conduction network,thereby showing a further enhanced discharge capacity at20 C up to 64.11 m Ah g^-1.Finally,with the high-voltage electrolyte and multi-nanocarbon-composited LNMO electrode optimized above,we developed our high-voltage LIC by coupling the LNMO composite electrode as cathode with a high-rate AC electrode as anode and modulating the capacity ratio between the cathode and anode for the resultant LNMO//AC LIC.It should be pointed out that the research about the cathode/anode capacity ratio of LIC has been rarely reported in the literature,which indeed represents a shortcoming in this area.In our work,therefore,we conducted this kind of research by fabricating LIC at a range of cathode/anode capacity ratios of 1:1,2:1,3.5:1,4:1,and 5:1.By evaluating the rate capability and cycling stability of these LIC,an optimal ratio has been determined,which allowed the low-polarization-operation of both the cathode and anode within their safe voltage limits to ensure a stable charge/discharge cycling for the optimized LIC at a high operating voltage.In particular,our experiments determined the cathode/anode capacity ratio of the optimal LIC to be 3.5:1.With a high operating voltage of 3.45 V,this optimal capacitor showed a high energy density of 63.9 Wh kg^-1,high power density of 2892.6 W kg^-1,and long cycle life with a capacity retention of 78.8%after cycling at 0.6 A g^-1 for7000 cycles,significantly outperforming its pristine counterpart with the cathode/anode capacity ratio of 1:1(energy density:54.8 Wh kg^-1,power density:1211.6 W kg^-1,capacity retention after cycling at 0.6 A g^-1 for 1000 cycles:15.1%).In summary,we systemically studied a functional high-voltage electrolyte and the compositing of LNMO with multidimensional carbon nanomaterials.With the high-voltage electrolyte and multi-nanocarbon-composited LNMO electrode optimized,we have developed our high-voltage LIC by coupling the high-rate LNMO composite electrode as cathode with a high-rate AC electrode as anode.By modulating the capacity ratio between the cathode and anode,we have successfully achieved our high-voltage LIC that can be safely operated at a high voltage of 3.45 V with enhanced performances.Moreover,the commercial availability of all materials we used indicates the great feasibility of our results towards practically useful LIC,while the overall strategy developed in our work would benefit the research of other similar hybrid electrochemical energy storage devices.