Investigation On Ionic Liquids In The Secondary Battery

The accelerating energy consumption,as well as a series of environmental issues caused by the abuse of traditional fossil energy,have led to an unprecedented boom of clean and renewable energy.As a predominant role in clean and renewable energy,secondary batteries have been widely studied and its application expands from portable electronic devices to electric vehicles and stationary energy storage.However,in the future market,whether secondary batteries can continue to expand their influence and stay ahead depends on the following aspects:high charging-rate,high energy density,sustainable and cost-efficient,and high environmental adaptability.Among the four key factors,high charging-rate means low time cost during the battery use and it mainly involves the fast charging technology.As a typical commercial battery,graphite/LCO battery has always been bothered by its power capability and slow Li⁺insertion,and the risk of Li dendrites growth greatly makes any breakthrough in power density extremely difficult.Aiming to these points,researchers tried to improve the ion transfer on graphite surface through electrolyte screening and optimization,but the efforts always went in vain because all the strategies had been confined within carbonate electrolytes.On the other side,as the leader in new generation battery,Li/NMC has achieved some breakthrough in energy density,however,it similarly suffered from the dendrite growth,additionally,the mismatch between the high-voltage cathode and the electrolyte incurs new issues,and thus its further progress towards practical applications are beset with difficulties.All these problems point to the same root,electrolyte.Therefore,finding an electrolyte that is highly adaptable with the high voltage cathode and can effectively adjust the lithium deposition behavior without any concomitant side reaction becomes the most likely and even only solution.Besides,green and low price is always the goal for battery industry and it now becomes more important against the current backdrop because sustainable development has been occasionally highlighted.However,the unbalanced distribution of mineral resources and industrial in-coordination continuously pushed up the prices of traditional electrode materials.Meanwhile,the inherent defects of the inorganic salt/organic solvent-electrolytes,such as flammability,explosion and instability,casts a shadow over the battery safety.In comparison,organic materials,both electrodes and electrolytes,provides an alternate solution due to the unique characteristics,such as,natural renewability,wide sources,and structural tunability.Environmental adaptability is the new requirement for battery,which has been proposed on account of the increasing all-weather application of batteries.It requires a stable energy/power output with temperature variation,especially the temperature range below ice point.However,low-temperature property is always the biggest challenge for batteries,and down to–20^oC can cripple most existing batteries and lead significant shrinkage in battery energy.The main reasons include the sluggish electrode kinetics and slow mass transfer within electrode,electrode surface as well as electrolyte.To address all these issues,innovation in electrode,electrolyte and even underlying reaction mechanism is necessary.In general,electrolyte is greatly involved in battery performance and battery development.From the energy/power density,to the（thermal）safety or the environmental adaptability or friendliness,the progress cannot be achieved without electrolyte improvement.As a room temperature molten salt,ionic liquid has many unique properties,such as super thermal stability,wide electrochemical window,flame retardancy,high ionic conductivity,strong dissolving ability for salt,and high miscibility with other solvent.Now it has been widely investigated as an electrolyte solvent or additive.Hereon,we will start at developing safe electrolyte,designed a series of ionic liquid electrolyte,and tried to address the four key issues of battery with ionic liquid-based electrolyte.The main results are detailed in the following:1.Fast-charging of graphite electrode.An interesting phenomenon was observed that mixing two solid compounds of tetraethylammonium bis（fluorosulfonyl）imide（Et₄NFSI）and Li FSI can form a liquid characteristic of ionic-liquid.The liquid exhibits good ionic conductivity,thermal stability,and relatively wider electrochemical window,and excellent flame retardancy,and it can be further used directly as electrolyte.We thus used it on graphite anode.The results showed that it had good compatibility with graphite during lithium intercalation process.The charge/discharge profiles,XRD and AFM observation all indicted that no co-intercalation into graphite could happen other than Li⁺.In addition,the rate performance of graphite in this novel electrolyte under ambient or high temperature both got greatly improved comparing with traditional ester electrolytes.2.High energy density Lithium metal batteryThe study of ionic-liquid electrolyte（Li FSI/Et₄NFSI）inspired us to use it on Li anode.The drawback of high viscosity and poor wettability was improved by a new electrolyte design in which the concept of localized high concentration salt was introduced and realized by the diluent of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether（TTE）.Thus-obtained electrolyte showed verry low viscosity,good wettability as well as ultra-wide electrochemical window.High coulombic efficiency was hence obtained on Li/Cu cell and lithium dendrite growth was greatly inhibited.On the other hand,it also has good compatibility with the high-voltage NMC622 cathode.Even when being charged to as high as4.8V,98%efficiency could be maintained on NMC622 cathode with 227m Ah g^–1 capacity release.None-Li Cu/NMC622 full cell showed a specific capacity of 190m Ah g^–1（4.5V cut-off voltage）,and the coulombic efficiency of 99%was obtained.3.Metal-free all-organic battery.Metal-free all-organic battery was constructed by N-type organic anode（polyimide,PI5）,P-type organic cathode（polytriphenylamine,PTPAn）and pure ionic liquid electrolyte（1-ethyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide,EMITFSI）.The unique redox mechanism of association/de-association of organic cations with N-type organic materials was revealed in the study.Owing to the pseudo-capacitance characteristic,both the cathode and anode exhibited ultra-fast reaction kinetics.In addition,the full cell presented very stable long-cycling behavior and potential application in wide-temperature conditions.The pure ionic liquid electrolyte not only essentially guaranteed the battery safety,but also well addressed many issues incurred with conventional electrolyte,such as dendrite growth,SEI resistance and de-solvation barrier,meanwhile,it greatly simplifies the electrolyte recipe and provides an alternate path for improving battery power output.4.Low-temperature batteries.The amazing low-temperature performance of the metal-free battery inspired its further application in all-weather battery.Next we used ionic-liquid based electrolyte of 1 M EMITFSI methyl acetate（MA）/acetonitrile（AN）（1/2,v/v）electrolyte,and PI5 anode,PTPAn cathode,fabricated the battery that could work down to–80⁰C.Viscosity,freezing point,conductivity as well as battery performance was all assessed during electrolyte optimization and the function/role of each electrolyte component was well investigated.Working temperature breakthrough was achieved owing to the newly designed electrolyte and the unique reaction mechanism of the electrodes.The battery exhibited as high as 79%capacity retention at–80℃under 1 C current density and extremely good rate capability at–60℃,namely,more than 50%capacity release under 200 C.