With the continuous improvement of the energy density of lithium batteries, traditional graphite materials (theoretical specific capacity of 372 mAh/g) are far from meeting the design requirements of the new generation of high specific energy batteries. Although Si materials have high specific capacity, they have large volume expansion during lithium intercalation, resulting in poor cycle life. In contrast, metallic Li materials have a theoretical specific capacity of 3860 mAh/g and excellent conductivity, making them an ideal anode material.
However, lithium metal materials develop Li dendrites during charge and discharge, which not only reduces coulombic efficiency but can also puncture the separator in severe cases, causing a short circuit between the positive and negative electrodes and making lithium metal anodes difficult to apply. Recently, Peng Shi (first author) and Qiang Zhang (corresponding author) from Tsinghua University verified the failure mechanism of lithium metal anodes under different cycling conditions in pouch lithium batteries.
Traditional research on lithium metal anodes has primarily been conducted in coin cells, where lithium metal is subject to stress. However, in pouch lithium batteries, the Li metal sheet is completely unconstrained, meaning data obtained from coin cells cannot be directly applied to pouch lithium batteries. Therefore, in this experiment, the authors fabricated a symmetrical Li/Li pouch lithium battery with an electrode area of 28 cm², a Li sheet thickness of 50 μm, and an areal capacity density of 10 mAh/cm², to simulate the performance of real-world Li metal batteries as closely as possible.
Cycling tests were conducted at 3.0 mA/cm² and 3.0 mAh/cm², 7.0 mA/cm² and 7.0 mAh/cm², and 10.0 mA/cm² and 10.0 mAh/cm², respectively. The battery test results at 3.0 mA/cm² and 3.0 mAh/cm² are shown in Figure a below. We can see that the polarization voltage of the battery gradually increases during cycling, indicating that the diffusion resistance of Li+ is increasing. This process is mainly related to the continuous pulverization of the metallic Li electrode and the appearance of dead lithium.
If we increase the current density (7.0 mA/cm² and 7.0 mAh/cm²), we can see that the cycle life of the battery is significantly shortened. If we further increase the current density (10.0 mA/cm² and 10.0 mAh/cm²), the battery will transform into a regular square wave voltage pattern after a few cycles. This is mainly because at such a high current density, the Li dendrites generated on the surface of the metallic Li electrode break through the separator, causing a short circuit between the positive and negative electrodes, thus turning the battery into a pure resistor. Therefore, we can see that the polarization of the battery at this time is only about 20 mV.
The morphological changes of the Li electrode under different current densities can better reflect the failure mechanism of the metallic Li electrode under different current densities. Regarding the current density of 3 mA/cm2, there was no significant change on the surface of the metallic Li anode after one cycle. After five cycles, the presence of dead Li was clearly observed. After 25 cycles, the surface of the Li electrode was covered with a dead Li layer with a thickness of 55 μm. When dissecting the battery, some dead Li could also be observed adhering to the separator. After 50 cycles, the electrode was completely pulverized, and the thickness of the dead Li layer reached 95 μm.
If we increase the current density to 7 mA/cm2, we will find that the pulverization of the Li electrode will be more severe. After only 5 cycles, the morphology of the Li electrode is equivalent to that of the electrode after 25 cycles at 3 mA/cm2. Furthermore, scanning electron microscopy also shows that more pores appear inside the electrode after cycling at high current density. After 10 cycles, the thickness of the dead Li layer on the surface of the Li electrode reaches 70 μm.
If we further increase the current density to 10 mA/cm2, the Li electrode will exhibit a completely different morphology. Most of the Li electrode is retained, but very coarse Li dendrites can be observed. There is no Li stuck on the separator, but we can observe pores with a diameter of 10 μm on the separator. This indicates that the abnormal characteristics of the battery under high current cycling are caused by short circuits caused by Li dendrites piercing the separator.
Based on the behavior of the Li electrode under different current densities, the authors divided the failure mechanism of the Li electrode into three regions: 1) polarization; 2) transition; and 3) short circuit. In the polarization region, the current density is very small (<4 mA/cm², 4 mAh/cm²), and Li is gradually consumed, forming a porous dead Li layer. Therefore, within the polarization range, the failure of the Li electrode is mainly due to the pulverization of the Li electrode and the formation of dead Li.
Within the short-circuit region, Li dendrite piercing of the separator due to high current is an important failure mechanism. This is mainly related to the areal capacity density. For example, at an areal density of 10 mAh/cm2, once the current exceeds 3 mA/cm2, short circuit will become an important failure mechanism.
There is a transition region between the polarization and short-circuit regions. In this region, dead Li is not the only mechanism causing Li electrode failure. In this region, the Li dendrites caused by high current density and high capacity density also cause short circuits between the positive and negative electrodes, which is also one of the reasons for Li electrode failure.
The difference in the decay mechanism of Li electrode due to different current densities is important because a SEI film is formed at the interface between Li metal and electrolyte, which increases the diffusion resistance of Li+. At low current densities, the driving force is small, so the newly deposited metallic Li is preferentially decomposed, and the morphology of the bulk phase of the Li electrode remains flat. However, at high current densities, a greater driving force appears, so the difference in driving force between the bulk Li and the newly deposited Li is small, which leads to the consumption of more bulk Li and thus less deposited Li, resulting in the appearance of dead Li.
Besides the influence of current density, areal capacity density also significantly affects the cycling performance of Li metal electrodes. The diameter of deposited Li is 5 μm at a density of 3 mAh/cm², increasing to 10 μm at a density of 9 mAh/cm². The thickness of deposited Li is 75 μm at a density of 3 mAh/cm², increasing to 120 μm at an areal density of 7 mAh/cm². The three decay mechanisms of Li metal electrodes under different current densities and areal capacities are shown in the figure below.
From the current and capacity boundary conditions in the three regions, it can be seen that the capacity surface density of the Li electrode is crucial in the short-circuit region. A large capacity surface density will more easily lead to the growth of Li dendrites, which will pierce the separator and cause a short circuit between the positive and negative electrodes. In the polarization curve and transition region, the current has a greater impact. A large current will more easily lead to dead Li and cause electrode pulverization.
Peng Shi's work shows that the behavior of metallic Li electrodes in pouch lithium batteries differs from that in constrained coin cells. The failure of Li metal anodes in pouch lithium batteries mainly exists in three regions: 1) polarization region; 2) transition region; 3) short circuit region. Current density and areal capacity density are important boundary conditions for these three regions. By selecting appropriate boundary conditions, the Li electrode can be controlled to obtain better cycle performance and reduce the risk of short circuit.
