Organic electrode materials are very attractive for electrochemical energy storage devices because they can be flexible, lightweight, low cost, benign to the environment, and used in a variety of device architectures. They are not mere alternatives to more traditional energy storage materials, rather, they have the potential to lead to disruptive technologies. Although organic electrode materials for energy storage have progressed in recent years, there are still significant challenges to overcome before reaching large-scale commercialization. This review provides an overview of energy storage systems as a whole, the metrics that are used to quantify the performance of electrodes, recent strategies that have been investigated to overcome the challenges associated with organic electrode materials, and the use of computational chemistry to design and study new materials and their properties. Design strategies are examined to overcome issues with capacity/capacitance, device voltage, rate capability, and cycling stability in order to guide future work in the area. The use of low cost materials is highlighted as a direction towards commercial realization.

Inorganic perovskite ferroelectrics are widely used in nonvolatile memory elements, capacitors, and sensors because of their excellent ferroelectric and other properties. Organic ferroelectrics are desirable for their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. Although almost a century has passed since the first ferroelectric, Rochelle salt, was discovered, examples of highly desirable organic perovskite ferroelectrics are lacking. We found a family of metal-free organic perovskite ferroelectrics with the characteristic three-dimensional structure, among which MDABCO (-methyl--diazabicyclo[2.2.2]octonium)-ammonium triiodide has a spontaneous polarization of 22 microcoulombs per square centimeter [close to that of barium titanate (BTO)], a high phase transition temperature of 448 kelvins (above that of BTO), and eight possible polarization directions. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

Molecular piezoelectrics are highly desirable for their easy and environment-friendly processing, light weight, low processing temperature, and mechanical flexibility. However, although 136 years have passed since the discovery in 1880 of the piezoelectric effect, molecular piezoelectrics with a piezoelectric coefficient comparable with piezoceramics such as barium titanate (BTO; ~190 picocoulombs per newton) have not been found. We show that trimethylchloromethyl ammonium trichloromanganese(II), an organic-inorganic perovskite ferroelectric crystal processed from aqueous solution, has a large of 185 picocoulombs per newton and a high phase-transition temperature of 406 kelvin (K) (16 K above that of BTO). This makes it a competitive candidate for medical, micromechanical, and biomechanical applications.

To predict or identify ferroelectricity is essential for extending the family of molecular ferroelectrics and thereby promoting their practical applications in nonvolatile memories, capacitors, piezoelectric sensors and nonlinear optical devices. In this respect, symmetry breaking is of particular importance, since the paraelectric phase adopting any of the 32 crystallographic point groups is always broken into one of the 10 ferroelectric point groups, i.e. C1, C2, C1h, C2v, C4, C4v, C3, C3v, C6 and C6v.1 It is the Curie symmetry principle that determines the group-subgroup relationship between paraelectric and ferroelectric phases, and thus 88 species of potential ferroelectric phase transitions are deduced. However, in some cases such as croconic acid and triglycine sulfate (TGS), the existence of pseudo center of symmetry makes it difficult to accurately recognize the ferroelectric phase. Then inspired by the Neumann's principle, which states that the symmetry of any physical property of a crystal must include the symmetry elements of the point group of the crystal, the temperature-dependent SHG effect and dielectric property become useful for detecting symmetry breaking and ferroelectricity. Consequently, in the light of the Curie symmetry principle and Neumann's principle, ferroelectrics can be effectively distinguished from innumerable compounds with various crystal structures collected in the Cambridge Structural Database. Taking advantage of such strategy and combining with the measurements of ferroelectric hysteresis loops and ferroelectric domains, we have successfully discovered a series of low-temperature and high-temperature molecular ferroelectrics with high performance.2-6 This study does help to avoid blindly searching for molecular ferroelectrics.