| Our society is facing a serious energy and environmental crisis at this moment.We need to stop using the fossil energy that we heavily rely on,to use cleaner energy urgently.But new energy power generation technology has many uncertainties,and it is unable to continuously deliver electricity to the grid,which will affect the stability of the grid.Therefore,its development relies heavily on the advancement of high-efficiency energy storage technology.Consumer electronics and new energy electric vehicles also place high demands on the energy density and power density of electrical energy storage devices.Therefore,vigorously developing electric energy storage equipment is the only way to solve the energy and environmental crisis we are facing.Lithium-ion battery has the highest energy density and power density among many electrical energy storage devices,so it has become an irreplaceable energy carrier in our lives,and its performance performance still needs to be improved.Researchers need to further explore to meet people's increasing demand on high performance lithium-ion battery.However,the development of lithium-ion batteries has encountered bottlenecks currently.We must put an eye on the electrode materials of lithium-ion batteries and replace them to make the performance of lithium-ion batteries a breakthrough.At present,most commercial lithium-ion batteries still use carbon based materials as anode materials,and carbon based materials have better cycle stability,but lower specific capacity.It can be found that the anode material of the lithium ion battery has not undergone a breakthrough upgrade since the beginning of the commercial application of the lithium battery.Therefore,it is very necessary for us to develop new anode materials with high specific capacity,high cycle stability,high rate performance and high safety.We have developed a series of new anode materials with high energy density and high power density by using ingenious material preparation technology,advanced material characterization technology and mature electrochemical testing methods.The details are shown as follows:(1)Among many anode materials,silicon material has an ultra-high theoretical specific capacity of 4200 mAh g-1.In addition,the working potential of the silicon anode material is suitable,which is not easy to lead to the formation of lithium dendrites to cause safety problems,and can be matched with the positive electrode to output a high voltage.These advantages have caused widespread concern among researchers.However,precisely because of its huge lithium storage capacity,the silicon anode material undergoes a large volume change of nearly 300%caused by a large amount of lithium insertion/extraction.These large strains will destroy the conductive network between the active materials and cause the electrode material to break or the shape of the electrode to change.Eventually,these electrode materials will fall off the electrode and lose electrical contact,resulting in rapid decay of the reversible capacity of the electrode.In order to make silicon materials commercially available as soon as possible,researchers have proposed various solutions to overcome these problems.However,these solutions have some fatal shortcomings:some of the solutions are very expensive,resulting in high cost;some solutions use explosive or highly toxic raw materials,which will bring huge safety hazards when the experiment is amplified to increase production capacity;some solutions can not achieves a high mass loading to meet the require of battery manufacturing;some solutions use some very complicated processes or equipment,and the industry cannot achieve amplification production at all.Here,we have successfully prepared a mesoporous amorphous silicon material by a simple self-template solvothermal method,and the preparation process of the material avoids many of the disadvantages mentioned above.More notably,this material has excellent cycle stability and can retain a specific capacity of 1025 mAh g-1 after 700 cycles at a high current density of 3 A g-1.The retention rate has also reached 88%.This material exhibits such outstanding performance because its porous structure provides sufficient space for the volume expansion of the silicon material to maintain the stability of the entire electrode.In addition,these holes allow the penetration of electrolyte,shorten the diffusion distance of Li-ions in the solid phase of Si material and increases the rate performance of the battery.(2)Although the silicon material has an ultra-high theoretical specific capacity,its energy density is still not high enough,compared to the direct deposition of metallic lithium onto the current collector.Because we use any other additional material to store lithium will reduce the energy density of the entire electrode.Therefore,the lithium metal anode is the ultimate status of the lithium battery anode,which has the highest theoretical capacity and the most negative operating potential.However,the deposition of metallic lithium spontaneously tends to be dendrites like,which has a great risk of penetrating the separator,causing internal short circuit of the battery,causing safety problems.It is also not friendly to the formation of a stable SEI film on the surface of the lithium anode and aggravate the generation of "dead Li",resulting in a lower coulombic efficiency and a higher voltage polarization.Therefore,the lithium metal anode was abandoned when the lithium battery just starts to develop.However,as lithium-ion battery anode materials continue to approach their capacity limits,the call for resurrection of lithium metal anode has become higher and higher.Researchers have also proposed various solutions to solve the problem of lithium dendrite growth.But most of these solutions are too dependent on the protection of the lithium metal anode by using the ether based electrolyte,and the ether based electrolyte itself has significant defects that have been excluded from most commercial batteries.In addition,although the coulombic efficiency of many lithium metal anode has been greatly improved,it is still far from commercialization.Even if the Coulomb efficiency reaches 99%,this means that 1%of the lithium resources are irreversibly consumed in each cycle,and the capacity retention of battery will be less than 80%only after 20 cycles.Of course,there are still some solutions get excellent performance,but the high cost in the raw materials,processes and equipments basically hamper them from practical use.In this context,we continue to insist on the use of commercial carbonate based electrolyte,and developed a simple method to protect the surface of the lithium metal anode with a Li3P/LiCI layer.The protective layer has good lithium ion diffusion performance and is electronically insulated,so that the formation of lithium dendrites can be well suppressed,and the metallic lithium is uniformly deposited under the protective layer,thereby realizing the stable cycling of lithium metal anode in the carbonate based electrolyte.(3)On the basis of the second work,we found that the protective effect of Li3P/LiCl layer on the Li surface can be strengthened even under much higher current density and areal specific capacity,by simply applying pressure onto the battery.Cycle stability and rate performance of battery have been greatly improved when applying pressure.Finally,the lithium metal battery assembled by using Li3P/LiCl protected lithium foil as anode and LiFePO4 as cathode can achive a specific capacity of 129.6 mAh g-1(areal specific capacity of 1.69 mAh cm-2)after 1000 cycles at a high current density of 3.9 mA cm-2 by applying pressure onto the battery.The specific capacity at a high current density of 6.5 mA cm-2 can still be maintained at 115 mAh g-1,which is 76%of the specific capacity at 1.3 mA cm-2.This program has shown a huge improvement in the performance of lithium metal battery by directly applying pressure onto it,and has a very positive significance for the future development of lithium metal batteries. |
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