Understanding the performance of lithium-ion capacitors under slow charge conditions: An interplay between power, energy and cycle life

Lithium-ion capacitors combine the characteristics of lithium-ion batteries and supercapacitors to achieve high energy and power densities. We analyze the effects of slow charging and electrode mass balancing on the power, energy, and cyclability of lithium-ion capacitors. Results show that graphite-activated carbon lithium-ion capacitors can simultaneously achieve high energy and power densities under optimized slow-charging protocols. Specifically, energy densities ranging from 60 to 104 Wh kg −1 and power densities of 20–113 kW kg −1 (maximum measured) or 1.7–14.5 kW kg −1 (practical) can be achieved depending on electrode mass ratio and operating protocol. Energy density can be increased through basic cell engineering without compromising power density. However, adjusting garphite-activated carbon electrode ratio to 1:3 reduces cell power and is not beneficial. Cyclability under slow charge conditions depends on the discharge current. If the discharge current is insufficiently elevated, lithium depletion in the negative electrode occurs, leading to poor cyclability. The combination of slow charging and fast discharging extends cyclability to 150,000 cycles for a 1:1 electrode ratio. The study reveals distinct performance regimes in lithium-ion capacitors under slow charge conditions: an ultra-high-power regime achieved by limiting charging current, and a high-energy and ultra-high-power regime achieved by increasing the positive electrode content and limiting charging current. Cyclability improves by fine-tuning lithium intercalation and deintercalation through appropriate balancing of charge and discharge currents. This study bridges a significant research gap and improves understanding of lithium-ion capacitors. The findings offer actionable strategies for customizing electrode ratios and charge-discharge cycles to achieve targeted application goals. • Operation of lithium-ion capacitors under slow charge conditions is investigated • Exceptionally high practical power densities are measured (14.5 kW kg −1 ) • Simultaneously enhanced energy and power densities are possible • Two performance regimes for ultra-high power are identified • Good cyclability requires avoiding lithium depletion in the graphite electrode