# Feasible Energy Density Pushes of Li-Metal vs. Li-Ion Cells
## 1 Introduction  
Existing conventional lithium-ion battery (LIB) technology is reaching its performance limits, as electric vehicle (EV) manufacturers aim to offer new EVs with extended driving range. To achieve this goal, the first approach was to increase nickel content in the cathode. However, the energy density of a battery does not only depend on the cathode but also the anode chemistry. Silicon based anodes are considered to be one of the most recent advances in Li-ion technology but they are also not expected to exceed energy densities of 800 Wh/L or 300 Wh/kg on a cell scale [1, 2]. Therefore, the utilization of metallic lithium shows huge potential to meet the energy density requirements of future applications (up to 900 Wh/L) [3]. Li-metal has a specific capacity (3.860 mAh/g,) about ten times higher than that of graphite [4]. Although many studies used lithium metal (Li-metal) foils with a thickness of hundreds of microns in order to have a lithium reservoir against capacity loss, this approach is not practical for long-term developments [5, 6]. The direct use of thick Li-metal foil will not only cause more rapid depletion of global lithium reserves and increase manufacturing costs, but will also bring some specific challenges associated with dendrite nucleation, limited coulombic efficiency (CE) and air and moisture sensitivity of the cell. Additionally, increasing the thickness of Li-metal foil (with excess lithium) decreases the volumetric capacity of lithium. For example, by a simple calculation, [PERSON] et al., have shown that if there is a 10 times excess of lithium (10\(\times\)), the volumetric capacity decreases from 2060 mAh/L to 187 mAh/L, which is almost 4 times less than that of a graphite anode (719 mAh/L) [7]. Therefore, we should first of all define the term \"excess\" lithium clearly. According to a study by [PERSON] et al. [8], it is suggested that maximum extraction of Li\({}^{+}\) ions from an NMC 811 (LiNi\({}_{0.8}\)Mn\({}_{0.1}\)Co\({}_{0.1}\)O\({}_{2}\)) cathode was 0.5 and after this point a significant capacity fade was observed. For this reason, in this paper the extraction of Li\({}^{+}\) ions will be assumed as 0.5. In this case, the delithiation reaction of NMC 811 can be given as following:  
\[\text{LiNi}_{0.8}\text{Co}_{0.1}\text{Mn}_{0.1}\text{O}_{2}\to 0.5\text{ Li}+\text{Li}_{0.5}\text{Ni}_{0.8}\text{Co}_{0.1}\text{Mn}_{0.1}\text{O}_{2}\]  
According to the reaction above, the amount of Li of 0.5 will be released during charging and plate the current collector. When there is no additional Li-metal on the current collector, this layer will act as an anode (zero excess lithium). Recently, anode-free (or zero-excess configuration) concepts have been proposed in many studies [3; 7; 9; 10; 11; 12; 13]. In this configuration, only a thin film current collector or surface modified current collectors are used as an anode (current collector), and the cathode material is the only lithium resource. During the first charge, lithium ions migrate from the cathode side to the negative current collector, are electrodeposited in situ on the current collector, and build a temporary Li-metal anode. Conversely, during discharge lithium ions move back into the cathode. The CE of such cells determines the reversibility of the cathode intercalation reaction and the cycle life since there is no extra Li source in the system. Moreover, from a safety point of view, since there isn't any Li-metal foil for the cell preparation, a protected environment is not required during cell manufacturing. However, it is still challenging to establish a highly reversible in situ lithium electrodeposition & intercalation process. [PERSON] et al. showed that an anode-free cell with 99.9% of CE presented a cycle life less than 400 cycles [13]. As an increase of CE is the major key to success, it is also important to conduct research with thin film lithium metal anodes. Therefore, it has been recommended to use lithium foils thinner than 50 \(\upmu\)m and most ideally less than 30 \(\upmu\)m [6; 7; 14].