Existing problems

Dendrite problem

Figure1. Dendrite [1]

Why dendrite appears in Li-metal battery?

When the battery is charged, the Li+ drops from anode. In normal condition, when the cathode’s electric potential is larger than 0 verse anode, the Li+ will embed into the anode. However, as the charging process continuing, if when the cathode’s electric potential is smaller than 0 verse anode, the Li+ cannot stay at the cathode, so it will be stored on the surface of anode and get electrons, so the solid sediment dendrite appears. [2]

Figure2. Mechanism of Li-ion battery and Li-metal battery [1]

The hazards that dendrite brings to the battery:

(1) the lithium dendrites on the surface of the negative electrode continue to grow, piercing the diaphragm, causing internal short circuit.

(2) The metal lithium deposited on the surface of the negative electrode and the "dead lithium" inside the battery react with the electrolyte to release a large amount of heat.

(3) When overcharging, the large current density further aggravates the side reaction of lithium deposition and increases the temperature of the battery.

(4) When the temperature continues to rise, the gas generated by the decomposition of the electrolyte makes the internal pressure of the battery continue to rise, eventually causing the battery to deflate and the metal lithium to melt. In this process, water and oxygen in the air react violently with lithium metal, resulting in combustion or even explosion.[2]

SEI problem

As the most active alkali metal element in the IA group, lithium has the lowest redox potential among all electrode materials, and the chemical and electrochemical properties of lithium metal are relatively active. Therefore, lithium metal electrodes are very susceptible to a series of redox reactions of polar solvents and anions in the electrolyte to form SEI films. SEI films do not conduct electrons, but are ionically conductive and have a thickness of about 20 nm. Typically, this in situ formed SEI film is not chemically and mechanically stable enough to stop the growth of lithium dendrites, which can pierce the SEI membrane, causing the active lithium to be exposed to the electrolyte and undergo further side reactions with the electrolyte. Continuous side reactions can lead to excessive consumption of active lithium and electrolyte, resulting in reduced battery cycle life.[4]

How does SEI formed？

SEI is formed the moment the electrode (metal) comes in contact with the electrolyte (solution). The thickness of SEI depends on the tunneling of electrons. The constituents of the SEI are the insoluble products formed due to the reaction between the electrode and the electrolyte. SEI has the properties of a solid electrolyte and as the name indicates, it acts as an interphase between the electrode and the electrolyte, and electrons are not allowed to pass through this barrier SEI seems to grow over repeated cycles of charge and discharge.[5]

Literature review (Long) v.2 This page is part of the collection 'Li-Metal Battery v.2'.

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