Lithium metal is an ideal anode material for Li batteries due to the following properties[1]:
- Low density: 0.534 g cm-3. Low density helps to reduce overall cell mass and helps to improve both gravimetric capacities of Li battery.
- Low reduction potential: -3.04 V vs SHE. The low reduction potential of Li enables the cell to operate at relatively high cell voltage that also increases the energy density of the Li battery
- High theoretical specific capacity: 3861 mAh g-1 and 2061 mAh cm-3
The theoretical gravimetric and volumetric capacity of Li metal batteries can be calculated by the following equation:
| Qg | = | n·F/Mw = 3861.328 mAh g-1 |
| Qv | = | Qg·ρ = 2061.949 mAh cm-3 |
Where
| Qg | Gravimetric capacity of Li [mAh g-1] |
| n | Number of electrons transferred = 1 |
| F | Faraday’s constant = 26.8014814 [Ah mol-1] |
| Mw | Molecular weight of Li = 6.941 [g mol-1] |
| Qv | Volumetric capacity of Li [mAh cm-3] |
| ρ | Density of Li = 0.534 [g cm-3] |
| e charge | 1.60217663 × 10-19 coulombs |
| 1 coulomb | 1 C = 1 A ⋅ 1 s |
| Avogadro’s Constant | 6.02214076 × 1023 mol-1 |
| Faraday’s constant | Avogadro’s constant X electron charge/3600 |
N/P ratio
The N/P ratio describes the capacity ratio between the electrodes in the battery cell. There are two major types of mechanisms responsible for electrochemical reactions in batteries:
- Intercalation
- Plating/striping
For intercalation type battery, e.g. typical lithium ion battery (LIB), N/P ratio is between 1.03 to 1.2.
For anodeless Lithium metal batteries (LMB), the N/P ratio is zero. While for LMBs with Lithium foil anode, the 0<N/P<1. The lithium capacity is proportional to the thickness of the lithium layer plated as the following figure1.
[1] http://large.stanford.edu/courses/2020/ph240/kim1/