Study On Porous Silicon Prepared From Pelagic Clay By Magnesium Thermal Reduction Applied To The Lithium Ion Batteries

In recent years,the popularity of mobile electronics and electric vehicle applications has led to a large demand for high-energy density energy conversion devices,especially lithium-ion batteries（LIBs）.In the process of pursuing high energy density LIB,silicon（Si）stands out among many anode materials.Because of its advantages of high specific capacity,low electrochemical potential,low cost and environmental friendliness,silicon is regarded as the most promising anode material to replace graphite.With the decrease of high-quality clay resources on land,researchers in many countries pay more and more attention to the study of ocean clay.For reasonable development and abundant Marine clay resources,on the basis of its larger specific surface area,compared with land clay porous,crystalline and the characteristics of high activity,we chose the high silicon content of oceanic clay samples,using magnesium thermal reduction process,under the suitable reaction conditions to ocean Si⁴⁺reduction of clay minerals,Then,porous silicon with good crystallization and loose lamellar aggregate was obtained,and the obtained porous silicon was applied to the anode of lithium ion battery,and the mechanism of the process was analyzed.First,ocean clay is converted to silica（PC-SiO₂）by a one-step pickling process,which is used as a precursor for the magnesium thermal reduction reaction,and then the obtained silica is converted to porous silicon（PC-Si）by the magnesium thermal reduction reaction.The BET specific surface area,pore volume and average pore size of the obtained porous silicon are 81.473 m~2·g^-1,0.290 cm~3·g^-1 and 10.193 nm,respectively,and the structure of the obtained porous silicon is well inherited from the ocean clay.The conversion of ocean clay into porous silicon through simple acid solution treatment and magnesium thermal reduction reaction has the following advantages:（1）the loose structure of ocean clay makes it easier to be converted into silica in the process of acid solution.Under the optimized temperature conditions,the obtained silica retains the structure of ocean clay completely;（2）In the process of magnesium thermal reduction,an appropriate amount of sodium chloride is added as a heat removal agent to absorb the excess heat released during the reaction process to avoid the structure of silicon generated by high temperature destruction.Then,the PC-Si was coated with carbon to obtain the porous carbon silicon complex（PC-C@Si）,which was applied to the anode of lithium ion battery.The test results show that the PC-C@Si has good cycling performance and rate performance.At a current density of 0.5 A·g^-1,after 120 cycles,PC-C@Si still provides A reversible specific capacity of 542.3 and 531.5 m Ah·g-1,which is much higher than that of graphite.In order to further improve the electrochemical performance of the prepared silicon negative electrode,the porous silicon prepared was broken by ultrasonic cell crusher.After 0.5 h ultrasonic treatment,the nano-porous silicon（NM-PC-Si-0.5）with a particle size of about 500 nm was obtained,and its BET specific surface area was140.692 m~2·g^-1.The pore volume is 0.225 cm~3·g^-1.The nanoporous silicon nanocomposites（NM-PC-C@Si-0.5）obtained by carbon coating showed excellent cycling and rate performance.At a current density of 0.5 A·g^-1,after 200 cycles,NM-PC-C@Si-0.5 still provides reversible specific capacities of 579.9 and 574.3 m Ah·g^-1.The prepared silicon anode has good electrochemical performance due to the following points:（1）the high specific surface area of porous silicon ensures the reactivity of the silicon anode,so it can show a high specific capacity in the charging and discharging process.（2）The carbon coating on the porous silicon surface increases the electrical conductivity of the silicon anode,and restricts the expansion of silicon particles externally,slowing down the rate of specific capacity decay.（3）The pore structure of porous silicon itself is conducive to ion transport on the one hand,and on the other hand,it can alleviate internal damage caused by the expansion of silicon particles.