Capacity loss
Capacity loss, also known as capacity fading, refers to the irreversible reduction in a rechargeable battery's ability to store and deliver its nominal amount of charge over time, resulting from chemical, physical, and mechanical degradation processes that increase unusable inactive material within the cell from the moment of manufacture.[1] This phenomenon is observed across various battery chemistries, but it is most prominently studied in lithium-ion batteries, where it manifests as a linear decline primarily driven by cycle count, calendar aging, and operational stresses like deep discharges.[1][2] In lithium-ion batteries, the dominant mechanisms of capacity loss include the loss of lithium inventory (LLI), caused by the growth of the solid electrolyte interphase (SEI) layer and lithium plating on the anode, as well as the loss of active material (LAM) due to electrode cracking, particle pulverization, and electrolyte decomposition.[2] These processes not only diminish the battery's state of health (SOH)—defined as the ratio of actual to nominal capacity, typically expressed as a percentage—but also correlate strongly with internal resistance increases, with a negative Pearson's correlation coefficient (r < -0.8) observed in over 97% of tested cells across nickel-manganese-cobalt (NMC), nickel-cobalt-aluminum (NCA), and lithium-iron-phosphate (LFP) chemistries.[2] For instance, high-energy-density lithium-ion batteries such as NMC and NCA typically retain about 80% of their initial capacity after 300–500 full charge-discharge cycles, whereas LFP batteries can retain 80% after thousands of cycles, marking a practical end-of-life (EOL) threshold often set between 70% and 80% SOH for applications like electric vehicles.[1][2] Capacity loss impacts battery performance by reducing energy density, extending charge times due to diminished charge transfer efficiency, and lowering overall system reliability, particularly in high-demand sectors such as electric vehicles (EVs), renewable energy storage, and consumer electronics.[1] In EVs, for example, degradation can shorten driving range and necessitate earlier replacements, influencing lifecycle costs and sustainability.[2] Mitigation strategies include avoiding full discharges, implementing partial charging regimens (e.g., maintaining 20–80% state-of-charge), and using advanced battery management systems to monitor and balance cells, which can extend usable life by minimizing stress factors like high temperatures and overcharging.[1] While some capacity fade is inevitable and linear with age, ongoing research focuses on material innovations, such as improved electrolytes and electrode designs, to slow degradation rates and enhance long-term performance.[2]Fundamentals
Definition
Capacity loss, also known as capacity fading, refers to the progressive and often irreversible reduction in the amount of charge a rechargeable battery can store and deliver at its rated voltage over time or through repeated charge-discharge cycles.[3] This decline arises from various degradation processes that diminish the battery's active material availability or lithium-ion transport efficiency, leading to a shorter runtime for a given load.[4] The nominal capacity of a battery, denoted as Q, is fundamentally defined by the relation Q = I \times t, where I is the discharge current and t is the discharge time; capacity loss manifests as a consistent decrease in this Q value with aging or cycling.[5] Capacity loss is distinct from related degradation phenomena such as efficiency loss or power fade. Efficiency loss typically involves a drop in coulombic efficiency, reflecting the mismatch between charge input and output due to side reactions, whereas power fade stems from increased internal resistance that limits the battery's ability to deliver high currents or rates.[6] In contrast, capacity loss focuses exclusively on the erosion of the battery's total deliverable charge storage, independent of rate or round-trip energy recovery.[7] The phenomenon was first prominently observed in nickel-cadmium batteries during the 1970s, where capacity reductions were frequently misattributed to a "memory effect" from incomplete discharges, though true fading involved electrode degradation and electrolyte issues.[8] Since the commercialization of lithium-ion batteries in the early 1990s, capacity loss has become a central focus of research, driven by the need to extend cycle life for applications like electric vehicles and portable electronics, with studies emphasizing solid-electrolyte interphase growth as a primary culprit.[9]Measurement and Quantification
Capacity loss in batteries is primarily assessed through standardized cycling protocols that involve repeated full charge and discharge operations. The most common technique is the constant current (CC) charge-discharge cycling test, where the battery is charged to its full capacity and then discharged at a fixed current rate, such as 1C (equivalent to the current that fully discharges the battery in one hour), while monitoring the delivered discharge capacity over successive cycles. This method allows for direct tracking of capacity fade by comparing the discharge capacity at each cycle against the initial value, providing a reliable baseline for evaluating degradation in lithium-ion and other rechargeable systems.[10][11] Key metrics derived from these tests quantify the extent of capacity loss in a standardized manner. The capacity retention ratio (CR) is calculated as\text{CR} = \frac{Q_n}{Q_0} \times 100\%
where Q_n is the discharge capacity at cycle n and Q_0 is the initial discharge capacity; this ratio expresses the percentage of original capacity remaining after cycling. Cycle life is typically defined as the number of full cycles until CR drops to 80%, marking a common threshold for end-of-life in practical applications. For lithium-ion batteries, empirical data from early comprehensive reviews indicate typical loss rates of 0.025–0.048% per cycle after 500 cycles, corresponding to overall losses of 12.4–24.1% at that point, with end-of-life often reached at approximately 20% total capacity loss (80% retention).[12][13][3] For more nuanced evaluation, advanced techniques like electrochemical impedance spectroscopy (EIS) enable differentiation between reversible and irreversible capacity loss components. EIS applies a small alternating current perturbation across a range of frequencies to measure the battery's impedance spectrum, revealing contributions from processes such as solid electrolyte interphase (SEI) growth (associated with irreversible lithium loss) versus reversible charge transfer or diffusion limitations. This separation aids in isolating permanent degradation from recoverable effects, enhancing predictive models for battery health without relying solely on cycling data.[14][15]