Design And Properties Of High Capacity Carbonyl Cathode Materials Based On Proton Transfer

The growing problems of global ecological environment and climate are urgently requiring the deep change of energy structure.The replacement of fossil energy by wind,water,solar and other clean energy has become the trend of the times.The development of energy storage technology is crucial to realize the efficiently use of these energy.Electrochemical energy storage is one of the most universal energy storage technologies.There are many problems to be solved in chemical energy storage,among which the cathode material needs to meet the requirements of high energy density,low cost and sustainability.Due to its low theoretical capacity and limited raw material reserves,the present cathode materials mainly of transition metal oxides cannot meet the rapidly growing power battery and large-scale energy storage market.Because of the inherent characteristics of high capacity,low cost,sustainable,low environmental footprint and structural designability,organic electrode materials are considered as the most promising candidates for the next-generation ones.Among them,organic carbonyl materials have the most balanced performance in energy density,power density and cyclic stability.And following the free electron conjugate addition mechanism,each carbonyl group can accept one electron,forming free radical,which is going to participate in the new conjugation.Therefore,the upper designing limitation of its theoretical specific capacity is 957 mAh g-1,which has a wide application prospect.To break through the shackles of conjugate mechanism and its upper limitation of theoretical capacity,this paper intends to propose a new non-conjugated addition mechanism of carbonyl based on H transfer,forcing each free radical,produced by carbonyl after receiving an electron,cannot participate in conjugate further,and must to caption second electron to form a relatively stable negative ions,negative ions grab active H,realizing the proton transfer.In this process,each carbonyl group gains two electrons,and its effective utilization rate is significantly improved,which improves the theoretical specific capacity and provides a new ideology for design and development of organic electrode materials.In this paper,three kinds of organic electrode materials for proton transfer are designed,one is N-H proton transfer,another is C-H proton transfer,and N-H and C-H proton transfer together.First of all,cyanuric acid(CA)and trithiocyanuric acid(TTCA)has both carbonyl unit that can accepts double electrons to form negative ions,and active hydrogen that can realize proton transfer.We used CA and TTCA as cathode materials in secondary lithium batteries(LiBs)to realize proton transfer in electrochemical process and improve specific capacity.NMR test and theoretical calculation show that N-H proton transfer occurs in the discharge process,thus achieving the 1,3-addition of lithium atom to imide unit.According to this mechanism,the theoretical specific capacity of CA is 1246 mAh g-1,which is 31%higher than the theoretical upper limit of conjugated carbonyl compounds.In practical tests,at current density of 500 mA g-1,and in a discharge voltage range of 1 to 4 V,ultra-high specific capacity of 1257 mAh g-1 can be provided at 60℃,and 480 mAh g-1 at room temperature.The reactivity of the thio-substituted compound TTCA is greatly improved,under the same current density,820 mAh g-1 was provided at room temperature,close to its theoretical capacity of 908 mAh g-1,and it shows an obvious discharge voltage platform between 2.5V and 2.2V.Secondly,a novel electrode reaction mechanism involving C-H proton transfer was developed by using barbituric acid(BA)and 1,3-dimethyl barbituric acid(1,3-DMBA)as active cathode materials for LiBs,which provides an evidence for the feasibility of using 1,3-dicarbonyl compounds as cathode materials for secondary batteries.When the discharge voltage is stop to 1 V,BA can provide a specific capacity of up to 739.1 mAh g-1,1,3-DMBA provides 473.4 mAh g-1,they should undergo four electron transfer reaction.NMR tests confirmed that the protons of the active methylene were transferred during the discharge.It means that the discharge capacity of BA is mainly contributed by the 1,3-dicarbonyl structural unit.The discharge capacity and theoretical calculations of N-substituted 1,3-DMBA also support this conclusion.Finally,active cathode materials of 2-TBA and TTBA were obtained by selective thio-substitution of carbonyl group in BA molecule.In their electrochemical reaction,both the above two type of proton transfer mechanisms work together.The significant improvement in the electrochemical activity of carbonyl group by thioylation was verified again.At the current density of 500 mA g-1 and the discharge voltage up to 1 V,the 2-TBA material can provide a specific capacity of 774 mAh g-1,which is 69.3%of the theoretical data estimated by the 6-electron reaction model.Under the same conditions,the specific discharge capacity of TTBA is as high as 911.3 mAh g-1,which is 99.8%of the theoretical value of the 6-electron reaction model.