**5. Conclusions and perspectives**

There have been many strategies proposed to pursue high-energy symmetric supercapacitors (SSCs), including two main parts: (1) the electrode materials' selection and design and (2) the electrolyte optimization. As described in Section 3, commonly, desirable electrode materials (mainly carbon-based materials) for SSCs can be obtained by meeting the following criterions: large specific surface area, hierarchical porosity, optimized heteroatom doping, and compositing with pseudo-capacitive materials such as electronically conductive polymers (ECPs) or stable metal oxides. Section 4 summarized the commonly used electrolytes and illustrated a few key principles of how to select a proper electrolyte for SSCs for different purposes. Three main types of liquid electrolytes: aqueous electrolytes, organic electrolytes, and ionic liquids (ILs) electrolytes hold different advantages, for instance, aqueous ones are low cost and highly ionic conductive while organic and IL ones can be operated at high voltages. However, there are also limits for each type of electrolytes, for example, the operating voltages of aqueous electrolytes are restricted in between 1.0 and 1.8 V and organic and ILs electrolytes are expensive and risky to environments. Based on the abovementioned advances in supercapacitors, we provided two feasible ways to further improve the capacitive performance of SSCs about the interface-related subjects, including (1) optimizing the types and contents of heteroatom dopants to involve pseudo-capacitance and expand the operating voltage by suppressing the activity of splitting electrolytes and (2) designing new types of electrolytes such as aqueous electrolytes containing neutral salts (Li<sup>2</sup> SO<sup>4</sup> , Li-TFSI, etc.). In conclusion, on the basis of proper electrode/electrolyte's selection, we can achieve further development in SSCs through engineering the abovementioned two important aspects that are used for the construction of robust electrode/electrolyte interface.

EDLCs electrochemical double-layer capacitors

Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering

http://dx.doi.org/10.5772/intechopen.73131

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EASA electrochemically accessible surface area

PNTE polynaphthalene four formyl ethylenediamine

TEMA-BF<sup>4</sup> triethylmethylammonium tetrafluoroborate

SBP-BF<sup>4</sup> spiro-bipyrrolidinium tetrafluoroborate

\*Address all correspondence to: zhanggx@sdust.edu.cn

College of Electrical Engineering and Automation, Shandong University of Science and

[1] Wu S, Zhu Y. Highly densified carbon electrode materials towards practical supercapacitor devices. Science China Materials. 2017;**60**:25-38. DOI: 10.1007/s40843-016-5109-4

ECPs electronically conducting polymers

NDPC non-doped porous carbon materials

ASSCs asymmetric supercapacitors

AC activated carbon

PANI polyaniline PPy polypyrrole PTh polythiophene ILs ionic liquids

AN acetonitrile

PC propylene carbonate

DMF N,N-dimethylformamide

NMP N-methylpyrrolidone DMA N,N-dimethylacetamide

EMIM-BF<sup>4</sup> tetrafluoroborate

Yaqun Wang and Guoxin Zhang\*

Technology, Tsingtao, China

**Author details**

**References**

GBL γ-butyrolactone
