Aging Effects on Lithium-ion Batteries
A series of test results that demonstrate the impact and implications of aging on the safety performance of small-form lithium-ion batteries.
WHY STUDYING AGING EFFECTS ON LITHIUM-ION BATTERY SAFETY MATTERS
Today, lithium-ion batteries are increasingly being used for longer periods. Many lithium-ion cells are recycled and reused, while others are used in applications — such as electric vehicles and stationary energy storage — with an expected battery life ranging from five to 20 years. These longer-term uses are important because field failures of lithium-ion batteries, though rare, are highly publicized and suggest that some failure mechanisms may be dependent on how the state of the lithium-ion cell changes over time. Equally important is the fact that current safety standards do not address the potential impact of battery aging.
Some of the most common and well-known uses of lithium-ion batteries are in consumer electronics, where the expected battery life — whether for a single-cell pack in a cellphone or a six- to 12-cell pack in a laptop computer — is one to three years.1 In contrast, batteries used in electric vehicles (EVs) and hybrid electric vehicles (HEVs) are expected to have a five- to 15-year battery life, while those used in stationary energy storage applications are expected to have a 10- to 20-year life.2 In addition, while lithium-ion battery recycling is still in its infancy, the market is projected to grow to $2 billion by 2022.3 This translates to approximately 10 percent of the 2022 lithium-ion battery market. And now emerging are “second life” uses of lithium-ion batteries, typically from EVs or HEVs. The automobile industry generally defines “end of life” as the point in time when a lithium-ion battery has lost 20 percent of its original energy storage capacity or 25 percent of its peak power capacity — a milestone that is typically reached at 200,000 miles or 2,000 charging cycles.4 Second-life applications include resale of the highest-quality used lithium-ion batteries for EVs and HEVs, particularly those used in urban areas, and for stationary energy storage in grid applications, either repacked into larger installations (at the megawatt level) or simply used as they are.5 Whether in first or second life in EVs or stationary energy storage, lithium-ion batteries are being used for longer periods and over more cycles than ever before.
A common belief about lithium-ion batteries is that they become more safe over time, primarily because aging tends to degrade performance with batteries losing some of their energy storage capacity as well as some of their efficiency in discharging energy. It would seem to make sense that a battery with less energy stored and a more limited ability to discharge that energy would be a lower risk and that over time the potential severity of failures would decrease. A contrary hypothesis about lithium-ion batteries is that the degradation of lithium-ion battery materials through aging would give the batteries a higher risk of failure.
The safety of lithium-ion batteries encompasses both the frequency and severity of failure. Given the trend toward longer lithium-ion battery usage and reuse cycles, UL believed that the effects of aging on lithium-ion battery safety should be studied to understand how aging mechanisms affect battery failure.
WHAT DID UL DO?
UL developed a first-stage research study to understand what was suggested in the field failures. The initial research focused on one commonly used lithium-ion cell type: the 18650-type lithium-ion battery with a lithium cobalt oxide (LiCoOx) chemistry and 2,800 milliamp hours (mAh) energy storage capacity.6 The plan was to conduct tests on the batteries at 25°C and 45°C over 50, 100, 200, 300, 350 and 400 charging cycles. The research included nondestructive analysis, abuse tests and material analysis to investigate the potential correlation between the mechanism of aging on materials and a cell’s tolerance of abuse conditions.7
UL aging effects research8
Our innovative research into the effects of aging on lithium-ion batteries identified two critical safety concerns.
The first safety concern is the polarization effect on aged batteries, which can be detected from temperature and cell voltage profiles during overcharging. When polarization occurs in a battery, a higher voltage plateau can usually be observed under charging, while a lower voltage plateau can be observed under discharging. Further, an increased thermal effect, which is also a potential safety concern, results from the increased cell impedance and the decay of charging and discharging efficiency.
An increased thermal effect can also lead to a greater risk of side chemical reactions that are unfavorable to the safety performance of a lithium-ion battery. For example, the solid electrolyte interface (SEI) usually works as the protective layer to prevent the electrolyte material from further interaction with the electrode in a lithium-ion cell.9 However, SEI is thermally unstable and can decompose at 60°C in some specific situations.10 And the failure of SEI may become the root cause that eventually leads to a catastrophic thermal runaway.11
Another critical safety concern that our research identified is the thermal stability of active materials in aged batteries. Based on the result of the “hot box” test, thermal runaway was triggered earlier in aged samples. In aged cells, separator melting and venting were delayed when compared with that of a fresh cell during the test. Data from a differential scanning calorimeter suggests that heat-generating reactions with the cells occur earlier for an aged cell.12
Hot Box Test on fresh and aged 18650-type lithium-ion cells 13
The results of electrochemical impedance spectroscopy and material analysis provide both indirect and direct evidence that the bulk composition of active materials does not change in aged cells. Instead, the composition and crystalline structure in the interfaces of active materials show significant changes in aged cells versus fresh cells. The implication here is that the aging effect primarily occurs near the surface region of active materials in the tested cells, which is also the region where the process of ion exchange occurs.
UL’s research to assess the effects of aging on lithium-ion battery safety is still in its early stages. However, based on the results to date, we are expanding our research program. In order to establish more general results, the research will move beyond the single chemistry studied so far into other common cell chemistries, such as NMC (lithium nickel manganese cobalt oxide) and LFP (lithium iron phosphate).14 The research will also be extended over more cycles and conducted on large-format lithium-ion battery systems such as those used in electric vehicles and stationary energy storage applications. Once the full impact of aging on lithium-ion battery safety is determined, UL will update the relevant safety standards to reflect the findings and to help ensure the safe use of lithium-ion batteries over time and across applications.