Understanding the Factors Behind the Fading Lithium-Ion Battery Capacity

Lithium-ion batteries have brought about a shift in the world of electronics and electric vehicles owing to their high energy density, lightweight design and rechargeable nature. However like any battery they are prone to capacity fading over time and with repeated usage. Capacity fading refers to the decrease in a battery's ability to hold charge. In this article we will explore the reasons behind this phenomenon of capacity fading in lithium-ion batteries and shed light on the mechanisms involved.

Anode and Cathode Materials

The materials employed for both anode and cathode in lithium-ion batteries play a role in determining their performance and longevity. These components consist of materials that facilitate the movement of lithium ions during charge discharge cycles.

Anode Material;

Traditionally graphite has been favored as the material for constructing lithium ion battery anodes due to its ability to intercalate lithium ions. Nevertheless with this advantage graphite anodes are not completely immune, to capacity fading. Several factors contribute to the fading of capacity related to the anode;

Growth of Solid Electrolyte Interphase (SEI); Over time when lithium ions are repeatedly inserted and extracted a layer called Solid electrolyte interphase (SEI) forms, on the anode’s surface. While this layer is crucial for battery stability, its continuous growth can hinder the flow of lithium ions, resulting in reduced capacity.

Loss of Active Material; Another contributor to capacity fading in the anode is the loss of active material. As the battery undergoes charge discharge cycles tiny particles from the graphite anode can break away causing a reduction in its capacity.

Cathode Materials;

Lithium ion battery cathodes typically consist of materials like lithium cobalt oxide (LiCoO2) lithium manganese oxide (LiMn2O4) lithium iron phosphate (LiFePO4) and others. Each cathode material exhibits properties that can contribute to capacity fading through mechanisms;

Transition Metal Dissolution; Cathode materials often contain transition metals, like cobalt, manganese and iron. During charge and discharge cycles these metals can dissolve into the batterys electrolyte. The presence of dissolved metal ions can then interfere with battery performance resulting in capacity fading.

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Structural Changes; Some materials used in cathodes can undergo changes, in their structure as the battery goes through charging and discharging cycles. This can cause a decrease in the battery's capacity over time. One example of this is seen in lithium cobalt oxide cathodes.

Current Collector; 

The current collector plays a role in lithium ion batteries by connecting the materials in the anode and cathode to the external circuit. While current collectors are typically made of materials like copper or aluminum they can still contribute to capacity loss.

Corrosion of Current Collector; Over time the current collector may corrode due to exposure to the electrolyte and the electrochemical processes that happen during charge and discharge cycles. This corrosion weakens the connection between the materials and the external circuit leading to increased resistance and loss of capacity.

Delamination; Delamination refers to when there is a separation between the collector and the active material within an electrode. This separation can occur due to stresses experienced during battery operation. When delamination happens it disrupts electron flow and movement of lithium ions resulting in reduced battery capacity and overall performance.

Charge and Discharge Factors

The charge and discharge processes play a role in the functioning of a lithium-ion battery. Several factors associated with these processes can contribute to the decline in capacity;

Overdischarging; Exposing lithium ion batteries to charging or discharging can speed up capacity deterioration. Overcharging can cause the formation of lithium metal on the anode leading to the growth of dendrites and short circuits that compromise both capacity and safety. On the other hand overdischarging can result in breakdown of the cathode material.

High Operating Temperatures; Elevated temperatures during operation can significantly impact a lithium ion battery's capacity. Higher temperatures promote the growth of a layer called Solid electrolyte interphase (SEI) that hinders the flow of lithium ions. Moreover it accelerates dissolution of transition metals in the cathode.

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Cycling Depth; The extent to which a battery is cycled, i.e. how much charge is drawn and discharged during each cycle influences capacity degradation. Batteries subjected to cycling tend to experience rapid loss in capacity compared to those cycled within a narrower range of depth of discharge.

Charge Rate; The speed at which a lithium ion battery is charged can affect its long term capacity. Fast charging generates heat and stress within the battery, thereby accelerating capacity degradation. Slower and controlled charging rates generally have a impact, on the long term capacity of a battery.

Discharge Rate; The rate at which a battery is discharged can affect its capacity. High discharge rates like those needed for acceleration in vehicles can contribute to an increase in capacity fade. Managing discharge rates and employing buffering systems can help mitigate this effect.

Rest Duration; Taking rest periods between charge and discharge cycles for lithium ion batteries can aid in reducing capacity fading. During these rest periods the chemical reactions within the battery stabilize decreasing the accumulation of SEI (Solid Electrolyte Interphase) and extending its capacity.

Depth-of-Discharge (DoD) Cycling; Maintaining a depth of discharge range for lithium-ion batteries during their cycles can minimize capacity fading. Batteries that frequently undergo discharges, such as those employed in applications requiring energy output are more prone to experiencing capacity loss.

Cycle Frequency; The frequency at which charge and discharge cycles occur also plays a role in capacity fading. Batteries that undergo frequent cycling tend to experience rapid capacity fade compared to those used intermittently.

Considering these factors related to charging and discharging, battery manufacturers and researchers can develop strategies to extend the lifespan and maintain the capacity of lithium-ion batteries. This in turn will enhance the performance of devices and systems that rely on these energy storage solutions.

Conclusion 

To improve battery technology and increase the life of lithium-ion batteries it is crucial to understand the causes of capacity fading. While all batteries experience this phenomenon to some extent researchers and engineers are continuously working on strategies to minimize its impact.

Efforts are being made to develop materials for both the anode and cathode that offer improved stability while reducing growth on the solid electrolyte interface (SEI). Additionally, innovations in collector design and materials aim at minimizing issues like corrosion and delamination. Moreover, advancements in battery management systems are helping prevent problems such as overcharging, overdischarging and operating at high temperatures.