| The ever-increasing needs of energy has stimulated the research on second metal batteries(i.e.,Li,Na,Zn,etc.),which hold high energy density and show widespread uses in emerging applications represented by electrical vehicles,portable electronics,aerospace application,and etc.Lithium metal battery(LMB)stirs intense research due to its highest theoretical capacity(3860 mAh g-1)and lowest electrochemical potential(-3.04V versus standard hydrogen electrode).However,the uncontrolled dendrite formation during charging process leads to safety concern and shortened life for lithium battery.In detail,the formidable lithium dendrites during repeated cycling tend to connect the anode and cathode,resulting in internal short circuits and safety issues.Also,after piercing solid electrolyte interface(SEI layer)by dendrites,the undesired reaction between deposited lithium and electrolytes as well as the dead lithium during discharge process deteriorate the lifespan of batteries.Thus,inhibiting lithium dendrites is the primary prerequisite for developing batteries with high power density and long lifespan.During the past decades,numerous investigations have been focused on suppressing dendritic lithium via electrolyte modification and additive,protecting SEI layer or in-situ formation robust SEI layer,all solid-state electrolyte,3D and structure anodes,charging style,separator modification,and etc.Although significant progress has been achieved,this problem is still unsolved.However,there are few efforts to inhibit lithium dendrites based on the formation mechanism of dendritic structures.A good understanding on the growth mechanism of dendrites may lead to a breakthrough solution to mitigate or eliminate these structures in recharging batteries.In our previous studies,the dendritic structures are prone to form at diffusion limitation of chemicals.However,the dendritic structures can be inhibited by enhancing the transport of metal ions.Thus,in this dissertation,diffusion enhancements by external fields(i.e.,electric fields,magnetic fields)are employed to suppress the formation of dendrites in lithium battery at a quick charging process.Firstly,the factors(i.e.,reaction rate(current density),reaction time,and cycles)were examined to probe their effects on dendrite proliferation.Also,the battery is also tested to explore the lifespan of battery without external fields.The conclusions in this part are presented as follows:(1)higher reaction rate gives rise to the slender and elongated dendrites.With the increasing current density from 0.2 to 10 mA cm-2,the dendritic morphologies are evoluted from needle-like dendrites with length less than 5μm to wire-like dendritic structure with a length of approximate 50 μm.When the reaction rate is faster than the diffusion rate of lithium ions,a concentration gradient is formed at the growth front of crystals.Once the gradient is stable,lithium ions tend to accumulate at the tip of dendrites to deposit,lengthening the dendrites.(2)With the repeated charging process at a high current density,three dendritic morphologies appear on the anode surface at the sequence of wire-like dendrites,moss-like dendrites,and shrub-like dendrites.At the initial stage,the transport of lithium ions near the anode surface is slower than the consumption rate,which results in the established of stable gradients at the growth front of crystals.Lithium ions tend to accumulate,nucleate,and grow at the tip of dendrites,lengthening the dendritic structures.During the middle stage,lithium ions are further consumed and the concentration gradients have changed,inducing the unstable growth direction and forming moss-like structures.At the last stage,the lithium ions are extremely consumed,which generates defects in the craystal and shapes twisted shrub-like dendrites.(3)The lithium dendrites are elongated with the repeated cycling.These structures tend to break into electrolyte,forming dead lithium and lowering battery capacity.In addition,more and more inactive dendrites still remain on the anode surface at the end of discharge in each cycle,deteriorating battery life.(4)The COMSOL simulation shows that lithium ions fluxs are uneven distribution at the anode surface.Also,there appears steep concentration gradient at the tip of dendrites,confirming the interface concentration gradient is the driving force for dendrite growth.Secondly,external electric field was employed to enhance the diffusion of lithium ions,lower the concentration gradient at the deposition interface,and suppress lithium dendrites even at a high charge current density:(1)An external alternating current field(ACF)is delicately built up perpendicular to the anode to perturb lithium ion distribution around the anode.The ACF with 30 Hz shows excellent performance in lengthening battery life and suppressing lithium dendrites due to its superior uniform distribution of Li+ along the anode surface.In detail,the battery life with ACF(5 V cm’1 and 30 Hz)are three times of the control case.The deposited morphologies at high current density are small particles,confirming the efficacy of dendrite inhibition.(2)An external direct current field(DCF)was applied to the battery to speed up the diffusion of Li+ in electrolytes.The DCF was parallel to the internal electric field,which was expected to accelerate the transport of lithium ions in electrolytes.The results show both the transfer number and diffusion coefficient of lithium ions increase with the intensity of DCF.Also,the concentration gradient decreases with the intensity of DCF,indicating that the DCF speeds up the lithium ions transport from cathode to anode.The COMSOL simulation shows that the deposited morphologies are uniform deposition layer after enhancing the transport of lithium ions.The life of the battery increases two times after the introduction of the DCF(5 V cm-1)even at high current density of 2 mA cm-2.There are no dendritic structures after cycling at the anode,confirming the effectiveness of DCF in suppressing lithium dendrites.(3)A simultaneous adoption of ACF(5 V cm-1 and 30 Hz)and DCF(5 V cm-1)was employed to investigate their effects on the electrochemical performance of the lithium battery.The lifespan of the lithium battery is almost five times of the control case at the high current density of 2 mA cm-2,showing excellent performance of DCF and ACF in protecting the anode surface,prolonging battery life,and suppressing lithium dendrites.Finally,the external magnetic field(EMF)was employed to examine its effect on battery electrochemical performance and dendrite inhibition.The EMF was parallel to the anode surface,namely vertical to the lithium ions movement.The magnetic field force(Lorentz force)could be exerted on the motional lithium ions,inducing micro fluids at the anode surface,which avoids the accumulation of lithium ions at the tip,resulting in uniform distribution of lithium ions and dendrite-free deposited morphology.The conclusions in this part are as follows:(1)Li+ diffusion coefficient increases with the intensity of EMF Also,the increased intensity of EMF leads to the decrease of concentration gradients at the anode surface.This validates the effectiveness of EMF in the enhancement of mass transport.(2)A moderate EFM with 0.8T during battery tests is evaluated for well inhibiting lithium dendrites and prolonging battery life.The life of battery with EMF of 0.8 T is 5.5 times of the control case at the high current density of 2 mA cm-2.With the increased strength of magnetic field,the deposited morphologies are needle-like dendrites,wire-like dendrites,large particles,and tiny particles.This confirms that the enhancement of mass transport via EMF could effectively shape the deposited lithium morphologies.(3)The COMSOL simulations were conducted to track the morphologies evolution.It shows that the sharp concentration gradient exists on the tip of dendrites and drives the proliferation of dendrites.However,the enhancement of transport via EMF lowers the concentration gradient,forming dendrite-free deposited morphologies. |
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