In-depth reports on lithium metal battery
- Research status of lithium metal battery
- Negative lithium metal battery layer
- Challenges facing lithium metal battery production
Lithium metal battery is also known as metal lithium batteries. Lithium metal is a battery technology route that uses lithium metal as the negative electrode. The capacity of lithium metal is about 10 times that of graphite, and the energy density of lithium metal battery can easily reach more than 400wh/kg.
Experts believe that lithium metal battery can effectively solve the problems of difficulty in improving energy density and limited cruising range, and are also considered to be the ultimate form of high specific energy batteries. Therefore, many capital and battery companies have competed to develop lithium metal battery.
The specific capacity of commercial cathode materials of lithium metal battery is lower (~150-200mAh/g), so the increase in the capacity of ternary cathode material has a significant effect on the improvement of battery (single) energy density. On the other hand, the increase in the capacity of the negative electrode still plays a considerable role in improving the energy density of the battery. The commercial graphite anode has a capacity of about 360mAh/g, which is very close to its theoretical capacity of 372mAh/g.
Research status of lithium metal battery
As the lithium ion pathway inside the battery, the comprehensive performance requirements of the electrolyte (without distinguishing between phases) include high lithium ion conductance and migration number, low electronic conductance, wide electrochemical window, low interfacial impedance, thermal stability, low volatility, and cheap and easy to scale.
Additional requirements for the electrolyte include polarity, low viscosity and easy electrode wetting. When paired with a graphite anode, the electrolyte system has high ionic conductance, good wettability, and the resulting SEI film has high stability under normal conditions.
However, when the lithium metal anode is paired with a conventional electrolyte, it is quite difficult to form a uniform, stable and highly protective SEI film, which makes the formation and evolution of lithium dendrites, the advancement of side reactions at the lithium anode-electrolyte interface, and the volume change of the battery caused by the reaction products. The negative effects are obvious, amplifying the shortcomings of the battery.
Negative lithium metal battery layer
During battery cycling, the side reactions of lithium metal and electrolyte lead to a linear battery capacity decay; while the sudden “dive” of capacity is caused by polarization caused by the growth of a poorly performing SEI layer during cycling. Excessive lithium metal in the negative electrode causes the electrolyte to be consumed in large quantities, and the formed “dry SEI” is the SEI layer with poor performance.
A negative electrode that is completely devoid of lithium (“anode-free” battery) will entrap dead lithium in the SEI that does not contribute to capacity, also degrading battery performance. On the other hand, a “wet SEI” with suitable thickness and good morphology can be grown on the surface of the 20-micron-thick lithium foil, which significantly improves the cycle performance of the battery.
The scientists found that when the lithium metal anode is working, a natural solid-state electrolyte interface film (SEI film) is formed, which can prevent the lithium metal formed during the deposition process from being corroded by the electrolyte. However, as more and more Li metal is deposited, accompanied by the formation of non-uniform Li metal surface topography (Li dendrite growth), the SEI film will eventually burst.
When this happens, a portion of the lithium metal is exposed to the electrolyte-filled environment, which can cause direct damage to the battery, resulting in direct and intimate contact between the positive and negative electrodes, resulting in a short circuit inside the battery.
There are three kinds of lithium ion battery anode materials that have been competing with each other: one is the most mature graphite anode material, based on which the theoretical energy density limit of lithium-ion batteries is about 300 Wh/kg. The second is the currently very competitive silicon carbon/silicon oxygen anode materials, the theoretical energy density limit of lithium-ion batteries based on this is about 400 Wh/kg, but the current application is limited.
The third is lithium metal anode materials, the theoretical energy density limit of lithium metal battery exceeds 500 Wh/kg. The cruising range of the corresponding electric vehicle can exceed 1200 kilometers.
Challenges facing lithium metal battery production
At present, from the perspective of the world, lithium metal battery are still in their early stages. There are three main challenges in its mass production.
The first is that the industrial chain at the production level is limited. Although lithium metal battery and lithium ion batteries have more than 60% overlap, in terms of anode material research and development, very few companies in the world can mass produce ultra-thin and ultra-wide lithium metal materials.
The second is the poor processing technology. Advanced processing methods have created opportunities to develop and improve new materials, but many challenges related to materials and interfaces remain unsolved. According to a research, a gap in the treatment science of solid state lithium ion battery is to determine whether there is a mechanism to enhance the thin solid electrolyte and thick cathode without hindering transport.
This material property requires new processes in the industry to efficiently handle thin amorphous materials. Well-known methods such as precipitation hardening, transformation toughening and tempering can strengthen structural ceramics and violent materials, but for solid electrolyte, no similar mechanism has been reported yet, and the industry has not yet found a successful method.
The hybrid lithium-metal battery selected by SES combines the advantages of traditional lithium-ion and solid-state lithium-metal batteries—good manufacturability and high energy density. Compared with the current lithium-ion battery’s energy density of around 260 Wh/kg, the SES hybrid lithium metal battery can increase the energy density by 43% at a lower weight. Hybrid lithium metal battery show excellent performance from both life cycle and fast charging perspectives.
Hybrid lithium metal battery still have safety problems in the future. Experts frankly said that the higher the energy density, the more difficult it is to guarantee security. SES plans to use both hardware and software to control safety risks, including using AI-based battery safety algorithms to predict hidden dangers.