Review on reliability of supercapacitors in energy storage applications
Introduction
Energy storage (ES) technology is highly demanded along with the trends of electrification and renewable energy generation [1], [2], [3]. Among various kinds of ES elements, supercapacitors (SCs, also called ultracapacitors) are emerging with promising potentials to applications, such as wind power generation [4], [5], photovoltaic (PV) generation [6], [7], railway [8], [9], [10], electric vehicles (EVs) [11], [12], [13], more electric aircraft [14], [15], electric grid [2], [16], [17], etc., due to its performance in power density, response time, and lifetime expectancy. On the one hand, the SC technology is evolving with new materials applied and new concepts developed, such as hybrid SCs which have a compromised power density and energy density, creating new application possibilities. On the other hand, the applications of SCs need to deal with challenges in fulfilling functionality, reliability, maintenance requirements, and cost constraints. The lifecycle performance and lifecycle cost are of great interest, and comprehensive studies on reliability are needed.
Reliability is the probability that a product can perform a required function under given conditions for a given time interval [18]. Traditional reliability analysis mainly focuses on statistics based on operation data [19], [20]. In recent years, physics-of-failure (PoF) based reliability analysis is widely used in power electronics [21], [22], from component-level ([23], [24], [25], inductor [26], semi-conductor [27]) and module-level (e.g., capacitor bank in wind power converters [28]) to system-level (e.g., DC/DC converter in a fuel cell system [29], back-to-back power converter in a wind turbine [30], PV inverter [31]). This approach can help users understand the failure mechanism of the product, and optimize it from the design phase, in order to meet requirements for lifecycle performance and lifecycle cost.
Concerning the energy storage system (ESS), reliability plays an important role as well. B. Zakeri et al. [32] analyzed the life cycle cost of electrical ESS, considering uncertainties in cost data and technical parameters. O. Schmidt et al. [33] discussed the levelized cost of storage (LCOS) for 9 technologies in 12 power system applications from 2015 to 2050. J. Teh et al. [34] modeled the reliability impacts of ESSs on wind-integrated power networks to improve the penetration of renewable energy. From the owner’s perspective, lifecycle cost, which relates a lot to the useful lifetime, is a more important indicator to evaluate and compare different ESSs. Thus, more and more researchers focus on reliability. W. Jing et al. [35] established a hybrid ESS (HESS) with SCs and Lead-acid battery in a standalone PV power system, using SCs to mitigate the charge/discharge stress on battery due to fluctuating power exchange and improve its service life. Z. Song et al. [12] optimized the design of a battery/SC HESS for the electric city bus and investigated the energy management strategy, to extend the lifetime of battery by reducing its current stress.
Particularly for SCs, different reliability-related issues have been investigated over the past years. L. J. Hardwick et al. [36] analyzed the failure mechanism of electrode based on in situ Raman microscopy test and proposed the electrolyte ion intercalation may cause electrode damage. P. Azais et al. [37] proposed electrolyte was decomposed on the electrode materials, leading to pores blockage and gas evolution. A. M. Bittner et al. [38] also investigated the failure mechanism of the electrode with post mortem analysis, and they found the electrolyte decomposition and electrode reduction and oxidation led to the structural modification of the electrode. O. Bohlen et al. [39] and R. Kotz et al. [40], [41] investigated the stressor impact on lifetime and concluded that the acceleration factor for the SC degradation is about 2 for a temperature increase of 10 C, and also about 2 for a voltage increase of 0.1 V. According to Eyring Law, P. Kreczanik et al. [42], [43] introduced root-mean-square (RMS) current in the lifetime model. R. German et al. [44] described the failure process and presented physical interpretations for the aging increase with voltage resets. M. Hahn et al. [45], [46] analyzed the effect of voltage and concluded even the nominal voltage would cause electrolyte or electrode decomposition, and the over-voltage would cause significant expansion of the electrode. Although so many works on failure mechanism and the lifetime of SCs, it is still lack of a systematic review for reliability, especially compared with other aspects of SCs which have been reviewed comprehensively, e.g., electrode and electrolyte materials [47], [48], [49], [50], [51], [52], [53], [54], [55], energy storage mechanism [48], [49], [51], [52], [53], [56], commercial applications [48], [50], [54], [56], [57], electrical and thermal modeling [58], [56], [59], [57], voltage balancing [60], [61], [62], energy management system [63], etc.
This paper aims to give an overview of the reliability research on SCs, from a PoF perspective and involves both mechanism and application. It covers three major categories: (i) Failure analysis for different types of SCs. We intend to clear the failure mechanisms of SCs, as the fundamental of reliability research. (ii) Stressor impacts on lifetime models, model parameterizations, and mission profile based lifetime prediction. We intend to obtain a more accurate lifetime prediction by considering the stressor impact and mission profile. (iii) Reliability-oriented design for SC-based ESS. We intend to optimize the lifecycle cost of ESS by utilizing the concept of reliability during the design phase.
This paper is organized as follows: Section 2 introduces the classification and operating principle, as the fundamental of reliability research of SCs; Section 3 discusses the failure modes, failure mechanisms, root causes, and critical stressors of different types of SCs; Section 4 analyzes impacts of different stressors on the lifetime, discusses the model parameterization, and proposes a mission profile based lifetime prediction approach; Section 5 presents a reliability-oriented system design procedure and discusses the key steps; Section 6 discusses the research gap at present and future opportunities in reliability research on SCs; Section 7 gives the conclusion.
Section snippets
Classification and operating principles of supercapacitors
As shown in Fig. 1, SCs can be divided into three main categories, based on the charge storage principles: electric double-layer capacitor (EDLC), pseudo-capacitor (PC), and hybrid supercapacitor (HSC) [64], [47], [48]. PC and HSC can be further divided into several sub-categories [47], [49]. Besides, X. Li et al. [50] considered EDLC and PC as symmetric SCs, and highlighted the asymmetric feature of HSC, from the perspective of material and construction of electrodes. Y. Shao et al. [49]
Failure analysis for supercapacitors
This section discusses failure modes, failure mechanisms, and critical stressors of different SCs. Failure mechanisms are especially highlighted as they are inherent explanations of degradation and failure behaviors, and the PoF fundamentals of lifetime modeling and reliability assessment.
Lifetime prediction of supercapacitors
Manufacturer’s datasheets usually provide the projected life of SCs under specific conditions, e.g., at rated voltage and constant ambient temperature [68], [69], [70], [71], [72], [73], [74]. This value can be used as a reference for the preliminary SC sizing, but cannot reflect the practical lifetime, especially under complex operation conditions, as discussed below.
Reliability-oriented design for supercapacitors system
In practical applications, there are different system solutions for SC-based ESS. For example, HESS with more than one kind of energy storage element is popular aiming to optimally exploit the benefits of different ES elements [35], [124], [125]. It should be noted that SCs are usually used as a module (series and parallel connected inside) in applications, as the rated voltage of a cell is typically low (e.g., 2.7–5 V).
With requirements for better performance of ESS, the design focus is not
Outlook of reliability research for supercapacitors
Reliability related works have been investigated more and more in recent years, but research gaps still exist in both mechanism and application. The important challenges need to be considered are:
- (1)
New failure mechanisms – as the SC technology is evolving, the existing understandings of the SC failure mechanisms are still limited, especially for PCs and HSCs. For example, the fundamental understanding of the electrolyte ion dynamics inside confined pores [147], [148] and intercalation
Conclusions
This paper gives an overview of the reliability of SCs in energy storage applications. To understand the reliability in-depth, the classification, operating principles, and performance comparison of SCs are introduced in advance. Failure effects, failure modes, failure mechanisms, critical stressors, root causes, and the interactions between them are analyzed for EDLCs and HSCs. The available lifetime models and the corresponding range of the model parameters are compared and discussed, then a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The presented study was supported by the National Natural Science Foundation of China (No. E51777141); the Fundamental Research Funds for the Central Universities (inter-disciplinary program) under contract No. KX00800201 73427 and the China Scholarship Council (the International Clean Energy Talent Program, 2017).
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