Suitability of late-life lithium-ion cells for battery energy storage systems

The globally installed capacity of battery energy storage systems (BESSs) has increased steadily in recent years. Lithium-ion cells have become the predominant technology for BESSs due to their decreasing cost, increasing cycle life, and high efficiency. However, the cells are subject to degradation due to a multitude of cell internal aging mechanisms, which result in reduced capacity, efficiency, and usable nominal power range over a BESS life cycle. Towards their end-of-life, the cells often show a steep increase in their degradation rate, called nonlinear aging. This work investigates how these "late-life" lithium-ion cells perform in typical BESS applications. We show how decreased capacity, efficiency, and nominal power range impact the profitability of a home storage system for self-consumption increase (SCI) and a large-scale storage system for energy arbitrage (EA). A physicochemical aging model, which accounts for lithium-plating-induced nonlinear aging, is developed and parameterized based on an experimental cell aging study. The aging study and model show that cells that have entered the nonlinear aging phase can transition back to a significantly reduced degradation rate through adapted operating conditions, namely a reduced charging rate and operating voltage window. The case study highlights that the battery cells enter the nonlinear aging phase significantly earlier when used for the EA application compared to the SCI application. However, by adapting the operating conditions below 80% state of health, the obtainable net present value over the investigated ten-year timeframe in the EA application increases by 39.8%, and the lifetime is significantly prolonged.