Preparation And Performance Investigation Of P-doped Carbon Materials For Lithium Ion Capacitors

As new electrochemical devices,Lithium ion capacitors（LICs）consist of a capacitive cathode and a battery-type anode,fully attaining the excellent power density of supercapacitors and the outstanding energy characteristic of lithium ion batteries.However,the kinetic mismatch and capacity mismatch between anode and cathode still restrict the development of LICs.Thus,development of anode with superior rate capability and cathode with high capacity is of great significance for boosting the performance of LICs.Among the carbon materials,hard carbon（HC）is a good choice for the anode of high-power LICs,but its capacity and rate capability are still deficient and need to be further enhanced.In addition,as the widely used cathode material for LICs,the low capacity of activated carbon（AC）restricts the energy-power characteristics of LICs.Phosphorus（P）is a significant dopant owing to its relatively low electronegativity,high theoretical capacity and high electron-donating ability,which has drawn increasing attention in recent years.P-doping is a hopeful way to elevate the electrochemical performance of carbon materials.In this paper,P-doped hard carbon anode materials and P-doped activated carbon cathode materials are prepared by the esterification reaction between starch and phytic acid（PA）.The microstructure of materials is controlled by PA.The influence of material structure on electrochemical performance and the application of these materials in LICs is studied.The main contents are summarized as follows:（1）In aspect of anode materials for LICs,the P-doped hard carbon（PHCs）with high P content are prepared via carbonizing cross-linked starch/PA precursors.PA acting as crosslinking agent,activation agent and P dopant,could inhibit orderly massing of graphene layers,enlarge specific surface area and enhance the P content,which not only offers more sites in favor of fast electrochemical reactions but also promotes the insertion/extraction of Li ions,bringing about a superior electrochemical performance.The PHC-4 sample prepared by adding 4 m L of PA shows a high reversible capacity(1040.2 m Ah g^-1 at 0.05 A g^-1)and excellent rate capability(231.7 m A h g^-1 at 6.4 A g^-1).Furthermore,the kinetic analysis demonstrates that Li ion storage is dominated by the surface-induced capacitive process.In addition,PHC-4//AC LIC assembled with pre-lithiated PHC-4 anode and AC cathode has a maximum energy density of 104 Wh kg^-1 and a high power density of 11.9 k W kg^-1.The PHC-4 anode with high rate capability effectively mollifies the dynamics gap with the cathode,increasing the electrochemical performance of LICs.（2）In aspect of cathode materials for LICs,the P-doped activated carbon（PACs）are also synthesized by carbonizing cross-linked precursor acquired from starch and PA.By setting the ratio of PA to starch,the specific surface area and the ratio of micropores/mesopores could be adjusted.Among them,the 1.25～3 nm pore size gives a balance of capacity and rate capability,which not only supplies abundant sites for electrolyte ions adsorption/desorption,but also facilitates rapid transport of ions.Moreover,P-doping not only improves electronic conductivity,but also provides active sites for redox reactions,further enhancing pseudocapacitance.The PAC-6 sample prepared by adding 6 m L of PA shows a high specific capacitance of 147.1 F g^-1 with outstanding rate capability(76 F g^-1 at 6.4 A g^-1).Besides,PHC-4//PAC-6 LIC assembled with PAC-6 cathode and pre-lithiated PHC-4 anode delivers superior energy-power characteristics(maximum 122 Wh kg^-1,9775 W kg^-1).In general,the PAC-6 with enhanced capacitive storage could effectively bridge the gap with the anode,improving the electrochemical performance of LICs.