In-situ Optical Visualization And Interfacial Modulation Of Li Plating/stripping Behavior

Lithium(Li)has been regarded as the most ideal anode for next-generation high-energy batteries due to its desirable specific capacity(3860 m Ah g^-1),density(0.59 g cm^-3)and reduction potential(-3.04 V).Corresponding Li-S and Li-O₂ batteries possess unprecedented energy densities of 2500 Wh kg^-1 and 3600 Wh kg^-1 respectively,promising a practical implementation in future high-end 3C and power markets that demand high storage density or long driving range.However,lithium anode in use still faces some severe problems including the low coulombic efficiency(CE),unsatisfying cycling lifespan and tough safety issues,which are laterly proved to be closely related to the unique Li plating/stripping behavior.In contrast to the insertion/extraction mode of Li ions in commercialized lithium ion batteries(LIBs),sharp dendrites and inert dead Li particles plus huge volume changes are readily induced over Li surface upon cycling in lithium metal batteries(LMBs).With the development of myriad in-situ characterization techniques and simulation/calculation methods,the onset of dendrite growth and dead Li formation are highly elucidated.Albeit powerful,the dependency relationships between Li nucleation/growth,morphology evolutions and electrochemical performances have not been fully understood.Besides,though amounts of tactics from perspective of anode/interface/electrolyte optimization have been put forward based on the failure mechanisms,it is still challenging to mightily manipulate the Li behavior.Therefore,in this thesis,we started from in-situ optical visualization of Li deposition/dissolution process,then developed several interfacial strategies in terms of tailoring the Li deposition path,ion and electric field distribution,as follows:(1)In-situ optical visualization of Li plating/stripping and behavior investigations:Though the time-dependent Li depositions under varied battery systems have been vastly reported,the evolutions of nuclei density,size and distribution are inadequately studied.Regarding this,an in-situ optical platform was built to visualize the Li anode.Several battery configurations in view of different purposes were designed to investigate the Li plating/stripping behaviors.Firstly,the key nucleation parameters and their progressions under different currents,capacities,additives and upon cycling were monitored to establish the relationships between morphologies and coulombic efficiency(CE).Afterwards,evolutions of nucleation/growth states particularly under dynamic current or potential upon continuous plating were captured.And it was firstly observed that the Li growth underwent a transformation from isotropic mode to a face to face mode between Li clusters upon increasing the current.Besides,it was revealed that the nuclei density and location were solely determined by the initial nucleation condition,and changes in current and potential made little difference.Right based upon these Li plating/stripping behavior,a tactic of using high current for nucleation and low current for growth was proposed in order to dynamically control the Li deposition process and achieve a potential high reversibility.Results showed that the tactic could truly elevate the CE of Li-Cu half cells from 83.06%to 93.53%together with that of anode-free Cu-LFP full cells from 30.16%to62.89%.(2)The introduction of ferroelectric materials and the Li nucleation/growth manipulation:Previous optical monitoring implied that the distribution of Li ions and electric field over the whole substrate greatly determined the Li nucleation/growth process.And affected by these deposition kinetics,Li deposits gradually evolve into whiskers and dendrites and barely present an isotropic growth in their thermodynamic-stable morphology.Regarding this,the ferroelectric Ba Ti O₃(BTO)was introduced which rendered a deposition full of regular Li hexagon/tetragon for the first time.Several characterization and electrochemical techniques were employed to validate the polarization behavior of BTO and the spontaneously generated internal electric field and polar ends.The effects of polarized BTO on the interfacial ion state,local electric field distribution and the electrolyte decomposition process were also thoroughly studied,which were then correlated to the process of Li nucleation/growth.(3)The introduction of electrochemistry-active additive and optimization of Li deposition morphology:In addition to manipulate the Li nucleation and growth,optimizing the deposition morphologies is another key strategy to promote Li performances.And in this chapter,3D continuous sponge-like Li deposits without any dendrites were achieved merely via adding a low amount of electrochemistry-active PMMA into the commercial ester-based electrolyte.Such morphology thus contributed to a greatly enhanced rate performances.Then,the function mechanism of PMMA was investigated.In contrast to prevailing electrolyte additive which are irreversibly decomposed to ameliorate the Li surface,PMMA here was proved able to reversibly participate in the Li plating and stripping steps.More importantly,the Li ions which were inserted into PMMA molecules could be further reduced to metallic Li at the very first start of Li plating and then served as nuclei to guide Li deposition.Actually,this new function mechanism not only enriched the functionality of additives,but also shed a new light for future additive design.(4)The Construction of auto-transferable interfacial layer and promotion of Li cycling performances:Aforementioned tailored morphology without dendrite formation greatly enhanced the rate capability of Li anode,while the introduction of interfacial layer is highly promising to promote Li cycling lifespan.First of all,considering harsh agent and ambience selectivity upon building interfacial layers via existing methods,a brand-new method based on auto-transfer with little selectivity was proposed,which vastly expand the kinds of interfacial materials.Afterwards,g-C₃N₄ is deliberately chosen especially for its ultrahigh content of N-related species.Its interaction with Li ions was elucidated and the as-formed Li-N bond could help to uniform the interfacial ion distribution stably and sustainably,thus leading to a hugely enhanced CE and long cycling performances.