Review on reliability of supercapacitors in energy storage applications

Highlights

• Failure mechanisms and root causes are analyzed for different types of supercapacitors.

• The existing supercapacitor lifetime models are reviewed systematically.

• A reliability-oriented design approach is proposed for the supercapacitors system.

Abstract

With the increasing use of supercapacitors (SCs) in the transportation and energy sectors, reliability which relates to the lifecycle performance and cost, becomes an important aspect to consider. While existing overviews of SCs mainly focus on materials, electrical and thermal modeling, voltage balancing, etc., this paper reviews the failure mechanisms, lifetime modeling, and reliability-oriented design of three types of SCs in energy storage applications. Systematic discussions on electric double-layer capacitors, pseudo-capacitors, and hybrid SCs are given. Scientific challenges and opportunities are also identified from an application perspective.

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.