High Capacity and Stability Lithium Metal Anode for Ultra-High Energy Density and High Safety Lithium Metal Batteries
Authors: Wu Wei-Hsin , Chang Chih-Ching , Chiou Shian-Hau , Chao Chung-Hsiang , Wu Sheng-Hui , Fang Jason 
Published: February 2020

The advantages of lithium-metal batteries (i.e., a higher energy density and a smaller size) have existed for decades. However, these batteries have been so far non-rechargeable and have been known to burst into flame. These two characteristics stem from the reaction which takes place between the lithium metal and the battery’s electrolyte. This reaction not only produces compounds which increase the resistance in the battery and reduce the cycle life, but also forms mossy lithium-metal bumps on the anode which leads to short circuits. A short circuit generates high heat and ignites the flammable electrolyte.

The current approach to prevent from lithium dendrite formation on anode is simply divided into four aspects: 1). Electrolyte with additives; 2). Functional artificial SEI modification (FASEI); 3). Lithium physical morphology and 4). Current collector. All these the state-of-the-art approaches must well coordinate with each other so that at least having a chance to improve both cell efficiency and suppressing dendrite after long cycling. In other words, neither side can completely solve the problems of lithium metal battery. The strategies must be comprehensive and well designed. (ie. an integrated safety materials and compact cell design should take into consideration as well).

First is from electrolyte with additives controlled, second is ex-situ SEI formation by polymer film or inorganic species, third is from the lithium bulk surface morphology or lithium physical shape. However, these technology or scientific innovations had been restricted anyway due to the following reasons:

1. Solvent reduction species are often prominent for reactive solvents (carbonates or ethers).

2. Anion reactions can be more influential when more stable solvents (ether type) are present.

3. The electrolyte salt concentration can have a major impact on the CE (coulomb efficiency) for highly concentrated electrolytes, but this is strongly influenced by the solvent and Li salt employed.

4. The replacement of aprotic solvents with ILs reduces, but does not fully prevent, formation of Li dendrite.

5. Block copolymer electrolytes do hinder dendrite infiltration. They do not ultimately prevent SC. of cells due to dendrites. This is also the case for solid inorganic (crystalline or glassy) separators.

6. The substrate on which the Li is plated is a significant factor governing the deposition.

7. Substrate surface roughness (pits, ridge lines, etc.) also dramatically affects where and how Li deposits.

8. Alloy formation further complicates the interpretation of the electrochemical redox reactions occurring.

9. Pressure generally results in less dead Li due to better electrical contact within the deposited lithium as well as results in superior plating/stripping behavior due to both lower side reaction rates.

Last but not least, there is no way that either of above mention can act as one antidote for lithium dendrite, but being a trade-offs instead. Eventually, a well distribution electro field with homogeneous lithium surface and stable SEI having good ion conversion efficiency are both the keystone to the lithium anode preventing from dendrite formation and poor coulomb efficiency.

Figure 1