First-principles Studies On Carbon Materials For Energy Storage

The carbonaceous materials have been extensively applied in the lithium-ion batteries.These materials have complicated structures,and their energy storage mechanisms are highly dependent on their micro-structures.Herein,we explore the energy storage mechanisms in some carbon materials by using the first principles calculations.The topics of our study cover the stacking structure of the lithium-graphite intercalation compounds?Li-GICs?,the distribution of the lithium ion in Li-GICs,the edge effect of the hard carbon materials,and the influence of hydrogen in porous hard carbons.Graphite is one of the most popular anode materials for the commercial lithium-ion batteries and is considered a promising material for the storage of other alkaline metal ions.The reaction between graphite and lithium?Li?is an intercalation reaction,eventually forming lithium-graphite intercalation compounds?Li-GICs?with the chemical formula of LiC₆.As the number of the intercalated Liions increases,graphite undergoes a series of phase transitions.To better understand the changes in the structure of Li-GICs during the Liintercalation,the first-principles calculations were applied to explore the changes in the layer spacing of Li-GICs,the stacking transformation of the graphite layers,and the distribution and migration of the Liions in Li-GICs.Our studies show that the average layer spacing of Li-GICs gradually increases from 3.34?to 3.65?with the intercalation of lithium ions.The graphite layers slide continuously with the increase of the number of the intercalated Liions.The stacking of the graphite layers changes from AB to AA as the Li/C ratio increases and reaches 1/24.We further studied the distribution of Liions in LiC₁₂ of Li-GICs.Calculations show that the“crossing”distribution of Liions can reduce the layer spacing,thereby greatly reducing the free energy of LiC₁₂.Meanwhile,the“un-crossing”distribution of Liions in LiC₁₂ tends to migrate toward the formation of“crossing”distribution.The“crossing”distribution of Liions is similar to the“stage 2”of the Daumas–Hérold model.Thus,we believe that the Daumas–Hérold model is more reasonable than other models in Li-GICs.Amorphous carbon materials usually have a much higher specific capacity for Lior sodium?Na?storage than graphite.Among them,hard carbons are known for their high specific capacities and good cycle performance in Liand Na storage.Due to the lack of layered graphitic structure but an abundance of pores and high specific surface areas,the hard carbons have a complex energy storage environment.Meanwhile,the presence of the impurity elements such as nitrogen,hydrogen and oxygen further changes the storage environment of Li?Na?.To have a deeper understanding of the energy storage mechanism in hard carbon materials,graphene nanoribbons which have many similarities with hard carbons were studied.The results show that,with an increasing number of the adsorbed atoms,the adsorption process can be divided into three areas on the voltage curve:voltage decay,nucleation,and metal formation.The edges can contribute a specific capacity of approximately 460 m Ah g^-1 for Listorage and approximately 185 m Ah g^-1 for Na storage in the voltage decay region.Studies of the influence of the presence of nitrogen and hydrogen on the energy storage of hard carbon show that there is a strong interaction between Liand the H-free bare zigzag edges?GC?or pyridinic nitrogen edges?GN?.However,the presence of H on the edges lowers the average potential,resulting in a significantly reduced interaction.The Liatoms can replace the H atoms and form Li-N bonds on the H-terminated GN edges,but this hardly takes place on the H-terminated GC.