N-heterocycle-based Organic Cathode Materials For Rechargeable Lithium Batteries

Organic electrode materials（OEMs）,which consist of abundant and light elements like C,H,O,N,and S,with the advantages of sustainable resources and high theoretical specific capacity,are considered to have great potential for constructing future green and sustainable energy storage devices including but not limited to rechargeable lithium and sodium batteries.In the past decades,although various types of OEMs have been reported,those with reasonably designed structure,facile synthesis,high capacity,and good cycling stability are still very lacking.Pyrazine is a highly efficient electroactive unit with similar redox mechanism to carbonyl（C=N/C–N^–vs.C=O/C–O^–）,which has attracted more attention in recent years.For example,phenazine（PZ）,2,3-diaminophenazine（DAPZ）,and hexaazatrinaphthalene（HATN）,which contain pyrazine electroactive unit,have been reported as cathode materials for lithium secondary batteries.However,similar to carbonyl compounds,these materials also suffer from high solubility in electrolytes and poor electrical conductivity,and their electrochemical performance is still far from the requirements of practical application.To solve the above problems,we constructed three types of nitrogen heterocyclic compounds from the perspective of molecular structure design and investigated their electrochemical properties.The results are details as follows:1.Synthesis and electrochemical properties of imidazophenazine dimers（BIPB）and trimers（TIPB）.By increasing molecular weight and enlargingπ-conjugated system,the dissolution of organic small molecules in the electrolyte can be effectively suppressed and the cycling performance can be improved.Based on this,in order to avoid dissolution and electropolymerization side of pyrazine-based small molecules such as PZ and DAPZ,we designed and synthesized imidazophenazine dimers（BIPB）and trimers（TIPB）with largerπ-conjugated system by encapsulating the amino groups of DAPZ,and we also explored their electrochemical properties and behaviors.Compared with BIPB,TIPB with largerπ-conjugated system and molecular weight showed lower solubility,thus exhibiting higher specific capacity and better cycling stability.2.Synthesis and electrochemical properties of poly（o-phenylenediamine）（Po PDA）.Compared with oligomers,synthesizing polymers with larger molecular weight can completely solved the dissolution of organic materials.Though pyrazine-based polymers currently reported such as PHATN has good electrochemical performance,it is expensive to synthesize and of limited practical value.Based on this,Po PDA with the advantage of facile synthesis,low cost and electrochemical activity,was selected as the research object and used as the cathode material of rechargeable lithium battery for the first time in this chapter.Although Po PDA has been widely reported,after reviewing the literature systematically,we found that there remains a lot of confusions about the synthesis,structure,and electrochemical application,so it is necessary to conduct a systematic study to clarify them.Using o-phenylenediamine（o PDA）as raw material and ammonium persulfate（APS）as oxidant,we prepared two typical Po PDA samples,namely Po PDA-R and Po PDA-H,at room temperature and high temperature by chemical oxidation method,respectively.Subsequently,by a series of structural characterizations,we verified that Po PDA-R is actually a dimer of o PDA,namely DAPZ,while Po PDA-H is a polymer with DAPZ as a repeating structural unit.Therefore,Po PDA-H showed significantly superior insolubility and structural stability than PZ and Po PDA-R（DAPZ）.As a cathode material for lithium secondary batteries,Po PDA-H exhibited high energy density(2.12×231 m Ah g^–1=490 Wh kg^–1)and excellent cycling stability（79%capacity retention after 1000 cycles）,and its electrochemical performance is comparable to that of PHATN and typical carbonyl polymers.And at the same time,it has significant cost advantage and large-scale production potential.This work clarifies many misunderstandings about the synthesis and structure of Po PDA,which is beneficial to facilitate its electrochemical applications.3.Synthesis and electrochemical properties of pyrolytic polyacrylonitrile（PPAN）.In addition to pyrazine,many other nitrogen heterocyclic structures such as pyridine and pyrrole also have electrochemical activity.In order to solve the poor conductivity of organic polymer materials,we chose PPAN as the research object,aiming to achieve high specific capacity through its rich nitrogen heterocyclic structure and high electronic conductivity through its graphite-like structure.A series of PPAN were prepared using inexpensive polyacrylonitrile（PAN）as raw material and Zn Cl₂ as catalyst and porogen at different pyrolysis temperatures（400°C,500°C,600°C,700°C and 800°C）by a simple one-step solid-phase method,and the relationship between their structure and electrochemical properties was investigated.By various characterization and testing methods,we found that the specific surface area and electronic conductivity of PPAN increased with the rising of pyrolysis temperature,while the nitrogen content of PPAN decreased and the nitrogen heterocyclic structure was mainly n-type pyridine nitrogen and p-type pyrrole nitrogen and graphite nitrogen.Comparing the electrochemical properties of PPAN prepared at different pyrolysis temperatures,we found that the PPAN-600 sample synthesized at 600°C exhibited the best comprehensive electrochemical performance due to its balanced specific surface area,electronic conductivity and nitrogen content.Even without the addition of conductive carbon,the PPAN-600 electrode can achieve the comparable reversible specific capacity and rate performance of PPAN-600 electrode with the addition of conductive carbon.This is a huge improvement compared to the previous organic electrode materials that need to add a large amount of conductive carbon（usually 30wt%）to fully exert electrochemical activity,which is beneficial to improve the practical energy density of electrodes and batteries.Combining ex-situ characterizations,we reveal the redox mechanism of PPAN,proving that PPAN is a bipolar pseudocapacitive material with both n-type and p-type reaction capacities.