Deep discharge' in Lithium-ion batteries

'Deep discharge' in lithium-ion batteries refers to the condition where the cell is discharged to a voltage considerably lower than its recommended cut-off voltage. A typical lithium-ion cell chemistry operates between 4.1V and around 2.5–3.0V (fully discharged). Deep discharge occurs when the voltage falls below this lower threshold, usually into the range of 1.5V–2.0V, or even lower, which is not recommended during the lifetime of these batteries.

At the core of the problem are the electrochemical changes that happen within the battery during deep discharge. When you discharge a battery, lithium ions migrate from the anode to the cathode through the electrolyte, releasing energy in the process.

Issues with the deep discharge:

1. Lithium Depletion: An almost complete migration of lithium ions occurs, depleting the anode.

2. Structural Changes: The cathode undergoes structural alterations, which can permanently affect its ability to intercalate/deintercalate lithium ions.

3. Electrolyte Decomposition: The lower voltage can induce electrolyte decomposition, producing undesirable byproducts that form on the electrodes.

4. Electrical Imbalance: In the context of battery packs, deep discharge can result in different voltages across the cells, making it difficult for them to be equally charged or discharged. This exacerbates degradation and reduces the overall life of the battery pack.

5. Capacity Fade: One of the most obvious repercussions is capacity fade. A battery subjected to repeated deep discharge cycles loses its ability to hold charge over time, leading to diminished energy storage capacity.

6) Safety Risks: Deep discharge can compromise the safety mechanisms built into lithium-ion batteries. The formation of metallic lithium on the anode and the structural degradation of the cathode material can create internal short circuits, elevating the risk of thermal runaway and, potentially, fires.

Avoiding Deep Discharge:

1) Battery Management Systems (BMS)

Modern lithium-ion batteries usually come with an integrated BMS that actively monitors voltage, current, and temperature. The BMS prevents the battery from discharging below the recommended voltage threshold.

2) User Education

Educating end-users about proper charging habits can also mitigate the effects of deep discharge. For instance, keeping/ ensuring an electric vehicle partially charged when not in use can considerably prolong battery life.

3) Energy Buffer Zones

Implementing energy buffer zones within the battery pack, which keep a reserve of energy that is not to be used in normal operation, can act as an emergency backup to prevent deep discharging.