Evaluation of redox flow batteries goes beyond round-trip efficiency: A technical review

Abstract The flow battery is a promising technology for large-scale storage of renewable energy owing to its unique advantages such as independence of power and energy capacity, scalability and versatility. The evaluation method is extremely important for the developments of both researches and applications of flow batteries. However, there is a lack of clear and uniform evaluation criteria in the open literature. The round-trip energy efficiency is commonly used to evaluate cell performance, whereas other different evaluating criteria may be suitable for different situations, with respective emphases. This paper reviews the development of performance evaluation criteria for redox flow batteries and clarifies the selection principle of evaluation criteria, stating that the system energy efficiency is the primary criterion, and power density or/and energy density are also vital evaluation criteria on the premise of maintaining high system energy efficiency for diverse types of redox flow batteries. The recent applications of these evaluation criteria on flow batteries are demonstrated afterwards. Finally, some exceptional conditions under what the system energy efficiency criterion is unsuitable are discussed, and emphasis is addressed on the new types of flow batteries. Applying a proper evaluation criterion helps to circumvent the remaining challenges of redox flow batteries, therefore, this review paper will be a useful guideline for the technology development and practical deployment of flow batteries.