Exploring The Mechanical And Electrochemical Properties Of Amorphous Silicon Dioxide In Lithium-ion Batteries

Lithium-ion batteries(LIB)are a popular choice for energy storage due to their longer charge and discharge lifespan,lower self-discharge rates,and greater environmental friendliness than traditional batteries.While graphite is commonly used as the anode material in traditional LIB,its lower energy density is insufficient to meet market demands.Silicon oxide(SiO2)is a highly promising anode material for next-generation LIB due to its abundant reserves,relatively low working potential,and high specific capacity.However,during electrochemical cycling,there is a considerable degree of volume change in SiO2 anode,which can lead to internal stress and result in decreased battery performance.This paper utilizes a molecular dynamics(MD)simulation approach based on Reaction force field(ReaxFF)to conduct a series of simulation studies on SiO2 as a LIB anode.The simulations investigate the underlying atomic mechanisms of SiO2 lithiation at the nanoscale to identify ways to improve current defects in the SiO2 anode.Additionally,the paper simulates the entire battery cycle by conducting lithiation/delithiation simulations to calculate the ChemoMechanical and electrochemical properties of SiO2 anode,with particular focus on the role played by stress.(1)We investigated the lithiation behavior of SiO2.Unlike the twophase lithiation of Si-based anodes,single-phase lithiation is observed in SiO2.By analyzing the radial distribution function(RDFs)and coordination number during lithiation,we analyze the bond breaking and formation as well as the structural evolution of SiO2.Comparing the crystalline phase(c_SiO2)with amorphous phase(a_SiO2),the latter exhibits a faster phase boundary movement rate and a lower internal stress generated during lithiation,attributed to the weaker chemical bonds in the a_SiO2,which provides a greater strain range for the structure to relax through atomic displacement during lithiation.We applied external biaxial stress on both sides of the simulation box and found that compressive stress delays lithiation while tensile stress accelerates it.(2)We studied the stress-dependent lithiation behavior of SiO2 anode by simulating the lithiation process of a_SiO2 anode under various external stresses.This work was carried out with the aim of shedding light on the fundamental mechanisms governing the behavior of LIB.Our simulations revealed that the mechanical properties of LixSiO2 transform as lithiation proceeds,leading to a gradual brittle-to-ductile transition.We calculated the variation of generation enthalpy with external stresses and concluded that external tensile/compressive stresses can increase/decrease the thermodynamic stability accordingly.The voltage profiles under different stresses are calculated and our results are more accurate compared to those of the conventional continuum model.It is found that the tensile stress helps to increase the generation enthalpy and potential of a_LixSiO2,while on the contrary the compressive stress triggers a significant decrease in the negative open circuit voltage of a_SiO2 and the consequent capacity decay.(3)During the delithiation process of LIB negative electrodes,structural defects are inevitably formed,which cannot be completely eliminated during rapid delithiation,resulting in accumulation and significant impacts on battery performance.Therefore,we have developed a rational model to explain the structural changes and performance degradation of a_SiO2 anode during battery cycling.We quantitatively describe the degree of structural disorder by calculating some material state variables,such as excess energy(Eex),free volume(fV),and pore volume(Vpore)during the delithiation process.We investigate the potential link between the discharge rate of lithiated a_SiO2 anode and structural disorder,where a higher discharge rate leads to more structural disorder accumulation.We also discuss the changes in mechanical properties resulting from structural disorder.We found that at the same lithium concentration x,a higher starting point of rapid delithiation,indicating more accumulated structural disorder,causes brittle-to-ductile transition in a_LixSiO2.Finally,we calculated the generation enthalpy and voltage,and found that the accumulation of structural disorder reduces the thermodynamic stability of a_LixSiO2,causing a loss in battery capacity.Our study not only provides an atomic-level understanding of the lithiation of SiO2 anode,but also reveals the microscopic structure and the relationship between stress and performance during the battery cycling process.These findings provide theoretical guidance for the development of a chemical-mechanical model for alloy anodes.