**Author details**

**5. Conclusions and perspectives**

130 Supercapacitors - Theoretical and Practical Solutions

neutral salts (Li<sup>2</sup>

**Acknowledgements**

**Conflict of interest**

SO<sup>4</sup>

Battery Research Team in the University.

**Appendices and Nomenclature**

SCs supercapacitors

SSCs symmetric supercapacitors

All authors declare that they have no conflict of interests.

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

, 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.

This work was financially supported by the National Natural Science Foundation of China (21701101), the Shandong Scientific Research Awards Foundation for Outstanding Young Scientists (ZR201702180243), and the Program for Tsingtao Al-ion Power and Energy-storage

Yaqun Wang and Guoxin Zhang\*

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

College of Electrical Engineering and Automation, Shandong University of Science and Technology, Tsingtao, China
