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| Fig. 1: Li metal anode areal capacity QA vs Li thickness TLi computed using Eq. (1). (Source: M. S. Kim) |
Lithium metal is an ideal anode material for Li batteries due to the following properties. [1]
Low density: 0.534 g cm-3
Low reduction potential: -3.04 V vs SHE
High theoretical specific capacity: 3861 mAh g-1 and 2061 mAh cm-3
The low density of Li helps to reduce overall cell mass and volume, which helps to improve both gravimetric and volumetric capacities and energy densities of Li battery. Also, 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.
The theoretical gravimetric and volumetric capacities of Li metal anode are
| 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] |
The capacity values calculated above are the upper limit of the all the possible anode materials that could be utilized as the anode for Li batteries, meaning that Li is intrinsically the highest energy-dense anode material for Li batteries. [1-3] However, these theoretical values are difficult to achieve in reality as the high chemical and electrochemical reactivities of Li induce side reactions during the battery cycling. [4]
In general, there are two representative energy density metrics for batteries: 1) gravimetric energy density (energy stored per unit weight of a battery) and 2) volumetric energy density (energy stored per unit volume of a battery). The low density of Li helps to improve the gravimetric and volumetric energy densities by reducing the anode weight and volume in batteries. Its low reduction potential is necessary to increase an operating cell voltage (Vcell).
A graphite anode is widely used in commercial Li-ion batteries (LiB). The graphite anode exhibits a theoretical specific capacity of 372 mAh g-1. Comparing the calculated theoretical capacity of Li (3861 mAh g-1), Li metal anode holds about 10 folds higher specific capacity than that of the graphite. However, the major capacity that dictates the energy density of the battery is the discharge capacity that depends on the cathode. This is because the electrical energy is obtained from the battery during the discharge process. On the other hand, the anode capacity dictates the total storage amount of Li ions during the charging process. In general, an unequal capacity ratio between the anode and cathode is used when constructing Li batteries. The capacity ratio between the anode (the negative electrode) and cathode (the positive electrode), known as N/P ratio, is an important cell designing parameter to determine a practical battery performance and energy density. [2] The below equations illustrate how the energy densities of the battery are calculated.
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| Fig. 2: The left image represents a conventional Li-ion cell structure with projected gravimetric and volumetric energy densities. The right image represents Li-Metal cell structure at the charged state with N/P=0.2 along with projected gravimetric and volumetric energy densities. The projected values are taken from Albertus et al. [4] (Source: M. S. Kim) |
| Vcell | = | Vcathode - Vanode |
| Eg | = | Qdis·Vcell·Mcell-1 |
| Ev | = | Qdis·Vcell·Ncell-1 |
where
| Vcell | ≡ | Operating voltage of the cell [V vs Li/Li+] |
| Vcathode | ≡ | Operating voltage of the cathode [V vs Li/Li+] |
| Vanode | ≡ | Operating voltage of the anode [V vs Li/Li+] |
| Qdis | ≡ | Cell discharge capacity [Ah] |
| Eg | ≡ | Gravimetric energy density [Wh kg-1] |
| Mcell | ≡ | Total weight of the cell [kg] |
| Ev | ≡ | Volumetric energy density [Wh L-1] |
| Ncell | ≡ | Total volume of the cell [L] |
The energy densities of the battery are a function of capacity, operating cell voltage, cell weight, and cell volume. The discharge capacity is used to calculate the battery energy density. For the operating cell voltage, the voltage reference is always with respect to Li/Li+ for Li batteries, and this shows another benefit of using Li metal anode instead of the graphite anode. Since the reference is Li/Li+, the operating voltage for Li metal anode is 0 V, which the cell voltage is the only function of the cathode potential. However, the average operating voltage of the graphite anode is approximately 0.1 V, in which there is about 0.1 V reduction in the cell operating voltage. This operating cell voltage reduction translates to about 3% decrease in energy density assuming the discharge capacity is the same, and state-of-the-art cathode materials exhibit average operating voltage of 3.9V vs Li/Li+. [2]
The N/P ratio describes the capacity ratio between the electrodes in the battery cell. The interpretation of N/P ratio is slightly different based on the lithiated states of cathode materials. Also, there are two major types of mechanisms responsible for electrochemical reactions in batteries: 1) Intercalation and 2) Conversion type mechanisms. [5,6] The intercalation mechanism is inserting Li ions into layered structures (or any other thermodynamically favorable crystal structures) of electrode materials (examples of intercalation based electrodes are graphite anode and LiNixMnyCozO2 (NMC) cathode). Therefore, the intercalation based electrode materials are considered as the host for storing Li ions. On the other hand, the conversion mechanism involves forming a new product after redox reactions with Li ions. Few examples of conversion type electrode materials are Li metal anode, elemental sulfur (S8) cathode, and oxygen (O2) cathode. Hence, the electrode materials that adopt the conversion mechanism are not considered as the host for Li ions. The difference in the term between Li ion and Li metal batteries simply arises from the electrode electrochemical reaction types. In general, Li metal battery refers to any type of battery that utilizes Li metal as the anode. Table 1 explains the general terms introduced in the battery fields:
Most of the intercalation based cathode materials are at lithiated state when the cells are firstly constructed whereas the conversion type cathode materials are not. This is due to thermodynamics associated with electrode materials. Thus, the interpretation of N/P ratio is slightly different for each of the battery types. Table 2 is the summarized interpretations of N/P ratio for each of the exact battery terms followed by a practical range of N/P ratios.
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| Table 1: This table explains the battery term based on the electrode electrochemical reaction mechanisms. |
The interpretation of N/P ratio is determined by the electrode electrochemical reaction mechanism, in which the intercalation mechanism involves Li ion host whereas the conversion type involves a bulk electrochemical reaction between Li and cathode species. However, Li metal ion battery is an outlier in this case, as it involves both conversion and intercalation mechanisms in the anode and cathode, respectively. Since there is already Li ion stored in the cathode material (i.e. any lithiated cathodes), no extra Li ion is ideally needed at the anode to operate the battery. Therefore, a new battery term is introduced for a zero N/P ratio for Li metal ion battery, which is Anode-less Li metal battery. [7] Also, the commonly used Li metal battery term represents N/P ratio greater than zero for Li metal ion battery. It is important to note that "Li metal battery" term refers to any type of batteries that use Li metal as anode; however, Li metal battery in the field is often referring to Li metal ion battery. Moreover, there are two configurations that describe Li metal ion batteries, which are Anode-less Li metal configuration (N/P = 0) and the Li metal configuration (N/P > 0).
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| Table 2: Interpretation of N/P ratio based on battery terms. Practical N/P ratio range based on the battery terms is provided. |
For the ease of calculating N/P ratio for Li metal batteries, often areal capacities in unit of mAh cm-2 for Li metal anode and cathode material are used. It is worth noting that the often theoretical capacity of Li and the practical capacity of the cathode are used for calculating N/P ratio. It is because Li metal is often industrially processed as a thin film (ranging from 500 μm to 20 μm in the thickness), and the initial capacity of Li is not dramatically affected by cycling conditions. However, the major reason for using practical capacity, equivalently measured capacity, for the cathode is that the actual cathode capacity is sensitive to operating conditions such as operating voltage window, C rate, temperature, and etc. Therefore, it is common in the field to consider measured cathode capacity from the defined operating conditions to calculate N/P ratio. Since the battery electrodes are relatively thin, the capacity values are normalized by the electrode size to derive the areal capacity of the anode and cathode. Gravimetric capacities are reported as well, but it became common in the field to use areal capacities to find N/P ratio. Since Li metal anode comes as a thin film, a useful equation to calculate N/P ratio with Li metal foil is relating Li thickness to areal capacity. The below equation relates to how the thickness of Li metal foil is related to its theoretical areal capacity.
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| Table 3: Areal capacity of Li with respect to its thickness. Note that these are theoretical values. The thickness is calculated by assuming perfectly flat and smooth film of Li. |
| TLi | = | 10000 QA Mw n F ρ |
(1) |
where
| TLi | ≡ | Li thickness [μm] |
| QA | ≡ | areal capacity of Li [mAh cm-2] |
| Mw | ≡ | molecular weight of Li = 6.941 [g mol-1] |
| n | ≡ | number of electrons transferred = 1 |
| F | ≡ | Faraday's constant = 26801.4814 [mAh mol-1] |
| ρ | ≡ | density of Li = 0.534 [g cm-3] |
Using the equation above, the thickness of Li can be calculated as a function of the areal capacity of Li or vice versa. Table 3 shows three corresponding Li thickness based on the three representative areal capacities commonly used in the field. Fig. 1 shows a more complete spectrum of the corresponding thickness of Li based on the areal capacity. Also, one may use this chart to quickly estimate the areal capacity of Li based on the thickness of Li foil used.
Fig. 2 illustrates cell structure comparisons between Li-ion cell and Li-metal cell (N/P > 0) with relevant battery components in the cell such as current collectors, separators, and electrodes. Based on the cell structures, Li-metal cell configuration inceease about 1.67 folds higher gravimetric energy density and 3 folds higher volumetric energy density based on that of Li-ion cell configurations. Please note that these are projected values where the exact values could change based on different cell specifications and dimensions; however, the relative increase of the energy densities are reliable values. Therefore, it is evident that that Li-metal cell configuration has a bigger advantage of improving the volumetric energy density than the gravimetric energy density of the Li battery. Electric vehicles require high volumetric energy density because there is a limited space afforded by a vehicle form factor, and the powertrain is not so much affected by the weight added to the vehicle. This became the main driver to develop Li-metal batteries, and there is still ongoing research to further improve the energy density of Li batteries.
It has been reported that the gravimetric energy density of Li metal batteries (N/P > 0) can be enhanced by systematically optimizing battery parameters. [2-4] There are numerous parameters that affect the battery energy density such as N/P ratio, electrolyte loading, cathode types and capacities, the tap density of cathode, separator thickness, current collector thickness, operating voltage window, inactive battery components, and so on. By optimizing the aforementioned parameters, Li metal battery is able to surpass 500 Wh kg-1. [2] With the improved energy density, the mileage for electric vehicles could be enhanced from roughly 300 miles to 600 miles.
The anode-less Li metal cell (N/P=0) is the ultimate cell configuration as no excess Li is present in the cell. Thus, the anode-less Li metal battery is considered as a "holy grail" for Li battery. With the anode-less Li metal cell configuration, the practical volumetric energy density of 1,200 Wh L-1 is achieved at the stack level. [3] This is a promising outcome for developing more reliable electric cars. Although improving batteries require intensive research and developments, high-performance Li batteries can soon to be realized, and this improved battery technology will indeed further facilitate the 4th industrial revolution.
© Mun Sek Kim. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Lin, Y. Liu and Y. Cui, "Reviving the Lithium Metal Anode For High-Energy Batteries," Nat. Nanotechnol. 12, 194 (2017).
[2] J. Liu et al., "Pathways For Practical High-Energy Long-Cycling Lithium Metal Batteries," Nat. Energy 4, 180 (2019).
[3] A. J. Louli et al., "Diagnosing and Correcting Anode-Free Cell Failure Via Electrolyte and Morphological Analysis," Nat. Energy 5, 693 (2020).
[4] P. Albertus et al., "Status and Challenges in Enabling the Lithium Metal Electrode For High-Energy and Low-Cost Rechargeable Batteries," Nat. Energy 3, 16 (2018).
[5] J. Zheng et al., "Regulating Electrodeposition Morphology of Lithium: Towards Commercially Relevant Secondary Li Metal Batteries," Chem. Soc. Rev. 49, 2701 (2020).
[6] X.-B. Cheng et al., "Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review," Chem. Rev. 117, 10403 (2017).
[7] J.-G. Zhang, "Anode-less," Nat. Energy. 4, 637 (2019).