The key to improving the specific energy of lithium-ion batteries lies in the positive and negative active materials. At present, the mainstream high-nickel ternary NCM and NCA materials, matching negative electrode silicon carbon materials can basically meet the requirements of 300Wh/kg, and even 350Wh/kg high specific energy batteries. Demand, but to further increase the specific energy of lithium-ion batteries to 400Wh/kg, or even more than 500Wh/kg, the existing system is powerless. From the current technical level, the negative metal lithium seems to be a very good option, it The theoretical specific capacity is 3860mAh/g, the voltage platform is -3.04V (vs standard hydrogen electrode), and it has excellent conductivity. It is very suitable as a negative electrode of lithium ion battery, and people have not tried it without lithium ion. Before the birth of the battery, there was once a wave of lithium metal secondary batteries on the global chemical battery market. However, this attempt ended in failure. The reason is that the lithium dendrites generated during the cycling of metal lithium negative electrodes may cause lithium. Ionic battery short circuit, leading to serious safety problems.
In order to solve the problem of lithium dendrites, people have also done a lot of work and obtained a lot of research results from electrolytes, artificial SEI films, and the production and growth mechanism of Li dendrites. This was also in our previous article. A large number of reports have been made. Recently, Zhengyuan Tu et al. of Cornell University in the United States have made a metal negative electrode with a composite structure by depositing a layer of Sn on the surface of an alkali negative electrode (Li, Na, etc.). This structure enables Li+ to be deposited on the electrode. The rapid diffusion of the surface, which effectively inhibited the growth of Li dendrites, greatly improved the cycle life of batteries using metal anodes.
The compound electrode used by Zhengyuan Tu has a very simple preparation process. By adding the salt of the target metal element Sn to a common carbonate electrolyte, the target metal element can be deposited at room temperature by ion exchange reaction on the surface of the metal Li anode. (As shown above), when talking about why Sn is used as a target element, Zhengyuan Tu said, 'The reason why Sn was selected as the target metal is mainly because Li diffuses very fast in Sn and Li in Sn. The potential difference between the embedding process and the embedding process is less than 500mV, which is conducive to the rapid diffusion of Li into the Li metal anode through the Sn layer.
Zhengyuan Tu used the AC impedance tool to analyze the Li negative electrode after depositing Sn (the result is shown in the following figure c). From figure c below, we can see that the interface resistance of Li negative electrode after deposition of Sn has apparently dropped. The interface resistance of the negative electrode is about 80W/cm2, and the interface resistance after deposition of 2um Sn drops to about 25W/cm2, and the decrease is more than three times. In addition, we noticed that after depositing a layer of Sn on the surface of Li, it was not in the EIS. An additional semicircle is added to the spectrum, which means that the deposition of Sn on the surface of Li negative electrode does not increase the additional interface resistance. The deposition of Sn on the surface of metallic Li negative electrode from the EIS data not only does not increase the impedance of Li+ at the electrode interface, but is due to the Sn layer. The presence of Li+ promotes the diffusion of Li+ at the interface. This is mainly because metal Li is a very active metal. Even if it is stored in argon, its surface will slowly grow an inert oxide layer, impeding Li+. The charge exchange at the interface, and the deposition of Sn on the surface of the Li layer suppresses the oxidation of the lithium negative surface, which reduces the diffusion resistance of Li+ at the interface.
The following figure d shows the relationship between the ionic conductivity and the temperature of the electrolyte in contact with different thicknesses of Sn-layered Li anodes. It can be seen from the figure that the Li-anode with the 500-nm-thick Sn layer has the highest conductivity and the conductivity increases with temperature. Also showed a significant increase.
The following figure shows the cyclic voltammogram of Sn-coated Li cathode and common Li cathode. Using Tafel formula to calculate the exchange current at the electrode interface, the exchange current of Sn-Li composite electrode reaches 7.5mA/cm2, which is significantly higher. In ordinary metal Li electrodes, this is consistent with our previous results obtained from the EIS test. The presence of the Sn layer reduces the electrode interface resistance and accelerates the diffusion of Li+ at the electrode interface.
The following figure shows the Li deposition of Li-Sn negative electrode and common metal Li negative electrode at a current density of 4 mA/cm2. In the same figure, we can use the Li-Sn negative electrode (upper half of the figure below) during electrodeposition of Li. It is very smooth, without the formation of Li dendrites. In contrast, the surface of ordinary lithium negative electrodes becomes very rough during Li deposition, and Li dendrites begin to appear during continuous deposition. Li-Sn composite electrodes are inhibited The role of lithium dendrite growth can also be verified by the button cell test results. Zhengyuan Tu made two identical Li sheets into a button cell and repeated charge and discharge to verify the growth characteristics of Li dendrites for the two types of negative electrodes. Fig. c shows the Li-Sn electrode, the following figure d shows the normal Li electrode, the current density is 3mA/cm2, and the charge-discharge capacity is 3mAh/cm2). From the figure, we can see that the common Li anode is due to lithium dendrites after cycling for 50h. Piercing the diaphragm caused a short circuit in the battery to cause a sudden drop in the battery voltage, while the Li-Sn battery was stably cycled for more than 500 hours without Li dendrite piercing the diaphragm.
Zhengyuan Tu used the above Li-Sn electrode and NCA electrode together to prepare a full battery, and the full battery also showed very excellent cycle performance. After 300 cycles, the capacity retention rate exceeded 80%, while the battery using ordinary metal Li anode was After several dozen cycles, it failed due to internal short circuit. In addition, ZhengyuanTu's research also showed that deposition of Sn on the surface of Na negative electrode can also play an important role in inhibiting dendrite growth, and significantly improve the cycle life of Na negative battery.
Sn element has the ability of rapid Li diffusion, but because the volume expansion during charging and discharging is too large, it can not be applied due to the volume expansion, and Zhengyuan Tu has another way to deposit Sn on the surface of Li or Na negative electrode, not only fully utilizing Sn fast Li. The ability to diffuse inhibits the growth of Li dendrites. At the same time, because Sn is in direct contact with the Li metal, it is always in a Li-rich state without violent volume expansion, thereby stabilizing the Sn-electrolyte interface and reducing the SEI. The destruction and reconstitution of the membrane greatly improved the cycle stability of Li, Na negative electrodes for alkali metals, which opened up a new way for the application of metallic Li batteries.
