Short communication
All solid state lithium batteries bad on lamellar garnet-type ceramic electrolytes
Fuming Du a ,1,Ning Zhao a ,1,Yiqiu Li a ,Cheng Chen a ,Ziwei Liu b ,Xiangxin Guo a ,*
a
State Key Laboratory of High Performance Ceramics and Super fine Microstructure,Shanghai Institute of Ceramics,Chine Academy of Sciences,Shanghai 200050,China b
Analysis and Testing Center for Inorganic Materials,Shanghai Institute of Ceramics,Chine Academy of Sciences,Shanghai 200050,China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
All solid state batteries were con-structed with a mild and convenient approach.
Den and highly conducting garnet electrolytes were ud as supports of the cells.
Composite cathodes with highly conducting ionic and electronic net-works were built.
The solid state batteries showed good performance at 60 C.
Further elevated temperature like 100 C led to greatly improved cell
performance.
a r t i c l e i n f o
Article history:
Received 26May 2015Received in revid form 28July 2015
Accepted 15September 2015
Available online 22September 2015Keywords:
All solid state batteries Lithium garnets
Ceramic electrolytes
Ionic and electronic conducting networks
a b s t r a c t
All solid-state lithium batteries are constructed by using highly conducting Ta-doped Li 7La 3Zr 2O 1
2(LLZTO)as the solid electrolytes as well as the supports,coated with composite cathodes consisting of poly(vinylidene fluoride)(PVdF):LiTFSI,Ketjen Black,and carbon-coated LiFePO 4on one side and attached with Li anode on the other side.At 60 C,the batteries show the first discharge capacity of 150mAh g À1at 0.05C and 93%capacity retention after 100cycles.As the current density increas from 0.05C to 1C,the speci fic capacity decreas from 150mAh g À1to 100mAh g À1.Further elevated tem-perature up to 100 C leads to further improved 126mAh g À1at 1C and 99%capacity retention after 100cycles.This good performance can be attributed to the highly conducting ceramic electrolytes,the optimum electronic and ionic conducting networks in the composite cathodes,and cloly contacted cathode/LLZTO interface.The results indicate that the prent strategy is promising for development of high-performance solid-state Li-ion batteries operated at medium temperature.
©2015Elvier B.V.All rights rerved.
1.Introduction
Development of large-scale storage grids and electric vehicles calls for the lithium batteries to be safer as well as with greater energy density than the currently commercialized lithium-ion batteries [1]
.Under such circumstance,the rechargeable solid state lithium batteries have attracted much attention [2e 5].Using
*Corresponding author.
E-mail address:XXGuo@mail.sic.ac (X.Guo).1
The authors contributed equally to this
work.Contents lists available at ScienceDirect
Journal of Power Sources
journal ho mep age:/locate/jpo
wsour
dx.doi/10.1016/j.jpowsour.2015.09.0610378-7753/©2015Elvier B.V.All rights rerved.
临床医学概要Journal of Power Sources 300(2015)24e 28
solid state electrolytes instead of combustible liquid counterparts not only can relieve the safe hazard,but can allow incorporation of lithium metal anodes and high-voltage cathodes into the batteries as well,which may greatly enhance the energy density of batteries [6e9].However,to fulfil potential of the solid state batteries,two key problems have to be solved,that is,the availability of a stable and highly ionic-conducting solid electrolyte and reduction of the large resistance resulting from solid e solid interfaces between electrolytes and cathodes.
Recently,the garnet-type Li7La3Zr2O12(LLZO)ceramics which werefirst reported by Murugan et al.showed promising properties for using as the solid state electrolytes[10].With optimization of element substitution,their ionic conductivities can be as high as 10À3S cmÀ1at room temperature[11e14].Moreover,they are chemically stable against the lithium metal and have electro-chemical window above5V[12,15].In conjunction with the highly conducting solid electrolytes,the low resistive electrolyte/cathode interfaces are crucial to obtain high-performance batteries.Thin-film growth techniques such as sputtering and plud lar depo-sition were often ud to grow cathode layers on ceramic electro-lyte supports[16,17].Through careful control of the composition at the interfaces,the battery performance could be greatly improved. However,such configuration limits achi
evement of the high ca-pacity owing to the small amount of electrode materials available by the growth[18].In contrast,bulk synthesis techniques are considered as powerful tools to prepare high-energy batteries[19]. In this ca,composite cathodes consisting of active LiCoO2,Li4Ti5O12)combined with Liþor eÀconducting particles (e.g.LLZTO,Li3BO3,Ag)were sintered together with the LLZTO electrolytes at700e900 C,aiming to build good ionic and elec-tronic conducting networks in the cathodes as well as cloly connected electrolyte/cathode interfaces[20e25].However,the ud temperature below900 C)on one hand is difficult to fabricate den and thus highly conducting electrolyte mem-branes,on the other hand is apt to cau pha transition of crys-talline cathodes and/or reactions between cathodes and electrolytes.Therefore,it is suggested that combination of suffi-cient high-temperature prepared ceramic electrolytes with low-temperature procesd electrolyte/cathode interfaces might be necessary to obtain high-performance solid-state batteries[26].
In this work,we ud the Li6.4La3Zr1.4Ta0.6O12(LLZTO)ceramics in thickness of0.1cm as the solid electrolytes.They were sintered by the hot-pressing technique with the relative density of99.6% and ionic conductivity of1.6Â10À3S cmÀ1at room temperature(as prented in Fig.S1).The composite cathodes compod of carbon-coated LiFePO4(LFP),Ketjen black(KB),LiTFSI in PVdF were coated
on one side of the LLZTO ceramic,and the Li anodes were attached on the other side by high pressure.It is worth noting that LiTFSI in PVdF may also be a good kind of polymer electrolyte[27e30].In terms of building electrolyte/cathode interface,such configuration not only allows the high-temperature sintering which is necessary for obtaining the highly conducting electrolyte part but also allows construction of cloly contacted electrolyte/cathode interfaces at low temperatures with the help of PVdF.Moreover,the composite cathodes have good conduction networks for both ions and elec-trons,the LLZTO/PVdF:LiTFSI interfaces can endure the volume changes during charge and discharge cycles.In the following dis-cussion,we denote the solid state batteries as LFP:KB:PVdF:LiTFSI/ LLZTO/Li.The performance of solid-state lithium ion batteries bad on garnet electrolytes and the relevant mechanism are discusd.
2.Experimental
Detailed decription of LLZTO powders and den ceramics can be found in Ref.[31]and Fig.S1.Composite cathodes consisted of poly(vinylidenefluoride)(PVdF)(Alfa Aesar),Li(CF3SO2)2N(LiTFSI) (99.95%,Sigma e Aldrich),carbon-coated LiFePO4(LFP)and Ketjen black(KB).The PVdF polymers were dissolved in N-methyl-2pyr-rolidon(NMP)and stirred for24h,which were followed by addi-tion of LiTFSI,LFP and KB in an agate mortar and ground for1h.
The LiTFSI was prebaked at80 C in vacuum for48h.The obtained slurries were coated on one side of the LLZTO ceramic membrane by blades,then dried in an oven at80 C for2h to remove NMP. After being presd with stainless-steel plate,the cathodes were dried in a vacuum oven at80 C again for12h to remove the trace amount of NMP and moisture.The typical mass of each cathode was approximately2mg.The weight ratio between LFP,KB,PVdF is fixed at7.5:1.5:1.0according to the conventional liquid-electrolyte-bad lithium ion batteries and the mass ratio of LiTFSI to LFP is varied from0%,25%,50%,75%and100%for comparison.Li anodes were attached on the other side by high pressure in an Ar-filled glove box with oxygen and moisture levels below0.1ppm. Finally,each laminated all-solid-state battery was asmbled in a Swagelok-type or coin cell.The stainless steel(SS)foils instead of Al foils were cho as current collectors in order to avoid the reaction between the Al and the PVdF:LiTFSI.The current density is normalized to the mass of LFP as well as the area of 1C being170mA gÀ1or200m A cmÀ2.
Surface and cross-ction morphologies of the LLZTO ceramics and the cathodes were tested by scanning electron microscopy (SEM,FEI Magellan400).Galvanostatic charge and discharge be-haviors of the batteries were investigated using an Arbin battery cycler with the potential ranging from4.0to2.76V at25,60and 100 C.Before the tests,the cells werefirstly rested in a thermo-static oven
for6h to reach the t temperature.Electrochemical impedance spectroscopy(EIS)measurements were performed in a frequency range of1MHz to0.1Hz with an amplitude of10mV using an Autolab instrument.
3.Results and discussion
Scanning electron microscope(SEM)image of a typical interface between the composite cathodes and the LLZTO electrolyte is shown in Fig.1a.It can be clearly en that the cathode in thickness of approximately17m m is tightly coated on the LLZTO ceramic
and
Fig.1.(a)A typical scanning electron microscope(SEM)of the interface between the composite cathodes and the LLZTO electrolyte;(b)SEM image for the surface of LLZTO ceramic electrolyte;(c)and(d)display the SEM images of the composite cathodes which were measured in the cond-electron and the back-scatter-electron mode, respectively.The oval-shaped LFP particles can be identified.
F.Du et al./Journal of Power Sources300(2015)24e2825
there are no obrvable voids at the interface.Fig.1b prents the SEM image for the surface of LLZTO ceramic electrolyte,which is compod of compacted grains in size of2e5m m with rather thin grain boundaries.Fig.1c and d displays the SEM images of the composite cathodes which were measured in the cond-electron and the back-scatter-electron mode,respectively.It is known that the former mode gives information on surface morphology of materials consisting of both light and heavy elements,while the latter does that for the materials with solely heavy elements. Therefore,comparison between the two images indicates that the oval-shaped particles,which are bright(Fe and P elements)in Fig.1c and d,can be attributed to the LFP.The cathode is compod of L
FP,KB,PVdF and LiTFSI,which homogeneously distribute in the matrix and form continuous networks for ionic and electronic carriers.
Galvanostatic discharge and charge curves for the solid-state batteries measured with0.05C at60 C are displayed in Fig.2a. During charge,the voltage plateau isflat at3.45V,which is only 0.05V larger than that of the discharge plateau,indicating a very
3英语怎么写small polarization.For thefirst discharge,a specific capacity of approximately150mAh gÀ1is delivered,which is88%of the theoretical capacity of LiFePO4.The behaviors clearly indicate that the composite cathodes have the well conduction networks for both electrons and ions.The battery can run100cycles with ca-pacity retention of93%,with thefirst Coulombic efficiency of80% and approximately95%in the following cycles,as can be en in Fig.2b.The low value of thefirst Coulombic efficiency could be related to the side decomposition of LiOH or Li2CO3 existing in the surface of LLZTO ceramic at high voltage)occurred during thefirst charge[32].During the following cycles,LiOH or Li2CO3disappears,which leads to the incread Coulombic effi-ciency.Whereas,the Coulombic efficiencies still below100%in the following cycles are most probably related to the fact that the lithium ion transference number of PVdF:LiTFSI is below1,which is conventionally obrved in the polymer-bad electrolytes[33]. This problem can be overcome by th
e elevated temperature as discusd in the following.As the current density increas respectively from0.05C,0.1C,0.2C,0.5C and1C,the specific capacity decreas from150mAh gÀ1,145mAh gÀ1,140mAh gÀ1, 130mAh gÀ1and100mAh gÀ1.As the current density turns back 0.05C,the specific capacity goes back150mAh gÀ1(as displayed in Fig.2c).The results reveal that the interfaces including internal ones inside the composite cathodes and the electrolyte/cathode interface can sustain a large number of repeated as well as high-rate cycles.Good inherent mechanical stability of micro-sized LFP particles due to the strong cohesion of P e O bond during the Liþion intercalation/deintercalation process,good interfacial contact and charge transport between the inorganic LFP particles and the flexible organic PVdF:LiTFSI are most probably responsible for such sustainability.All of the factors are beneficial to relieve the strain and sustain the volume change during cycles.It is worth noting that incorporation of LiTFSI with an optimized concentration is crucial to obtain the good battery performance as prented here.As depicted in Fig.2d,the battery shows the maximum charge and discharge capacities when the mass ratio of LiTFSI to LFP is75%. Electrochemical Impedance spectroscopy(EIS)measurement of
the
Fig.2.(a)Galvanostatic discharge and charge curves of the LFP:KB:PVdF:LiTFSI/LLZTO/Li batteries with75%mass ratio of LiTFSI to LFP,measured at0.05C and60 C;(b)Coulombic efficiency and the specific discharge capacity as a function of cycle number;(c)Rate performance;(d)The1st charge and discharge capacities of the batteries with various mass ratio of LiTFSI to LFP.The specific capacity is normalized to the weight of LFP in the
cathode.
Fig.3.Impedance spectroscopy measurement of the batteries with different mass
ratio of LiTFSI to LFP at60 C with frequency ranging from1MHz to0.1Hz.
F.Du et al./Journal of Power Sources300(2015)24e28
26
batteries with different mass ratio of LiTFSI to LFP consistently re-veals the smallest internal battery resistance at 75%(as shown in Fig.3).This means that the mass ratio of 75%corresponds to the optimum ionic-conducting network,promoting Li þion transport during charge and discharge.In this situation,the volume ratio of LFP,KB,PVdF and LiTFSI can be estimated to be 40:15:10:35taking into account the density and the weight ratio of each component.Furthermore,the conductivities of LFP:LiTFSI:PVdF with stainless steel electrodes are studied (mostly attributed to ionic contribu-tion),which are 4.3Â10À5S cm À1at 60 C and a factor of 2smaller than that of the composite LFP:LiTFSI:PVdF:KB (Details can be found in Fig.S2and Table S1).
It is known that elevation of temperature is a powerful tool to kinetic improvement.In fact,this is indee
d obrved in the batte-ries investigated here.As shown in Fig.4a,the batteries measured at 100 C exhibit the speci fic discharge capacity of 126mAh g À1at 1C,which is 74%of the theoretical capacity of LiFePO 4.Moreover,the batteries can run 100cycles without obvious capacity loss (Fig.4b),and the Coulombic ef ficiencies after the first cycle are improved to be above 99%.In Fig.4c,at 1C,2C,5C and 8C,the batteries deliver speci fic discharge capacities of 126mAh g À1,123mAh g À1,108mAh g À1,and 78mAh g À1,respectively,which are much greater than the corresponding values at 60 C.Impedance spectroscopy measurement indicates that the internal resistance of battery at 100 C is one order of magnitude smaller than that at 60 C,as shown in Fig.4d.It can be expected that the high-temperature treatment in fluences the following respects:i)The ionic conductivities of ceramic membranes are greatly enhanced.As can be found in Fig.S1,the conductivity of ceramics becomes nearly 1.0Â10À2S cm À1at 100 C,which is comparable to the value of liquid electrolyte at room temperature;ii)The conduction of polymer bad components (i.e.PVdF:LiTFSI)should be enhanced,which is also bene ficial to improvement of ion transport in the conduction network between the LFP particles;iii)The interface between the ceramic plate and the composite cathode becomes more connected and thus the charge transfer therein is improved.
Concerning that tolerance of elevated temperature is one of distinguish features of solid state batteri
es,the results prented here demonstrate a promising battery con figuration applied in the medium temperatures.
Apart from,we also measured the galvanostatic discharge and charge curves at room temperature (25 C),shown in Fig.S3.Since the internal resistance of RT is large (20k U ),the strong polarization results in low capacity at even 0.05C-rate.However,we find that the problem can be tackled by activating at 100 C and the work is still going on.4.Conclusions
In conclusion,the solid-state lithium ion batteries have been constructed by combining the highly conducting garnet-type ceramic electrolytes with composite cathodes.This con figuration allows low-temperature coating of hybrid LFP/KB/PVdF:LiTFSI composite cathodes on the high-temperature-sintered LLZTO ce-ramics.At 60 C,the batteries show the first discharge capacity of 150mAh g À1at 0.05C and 93%capacity retention after 100cycles.Further elevated temperature up to 100 C leads to further improved performance (e.g.126mAh g À1at 1C and 99%capacity retention after 100cycles).Such good performance is attributed to the highly conducting ceramic electrolytes,the optimum electronic and ionic conducting networks in the composite cathodes,and cloly contacted cathode/LLZTO interface.The results here demonstrate that lamellar garnet-type ceramic electrolytes are promising for development of high-performance solid-state Li-ion batteries operated
at medium temperature.The solid-state-batteries with higher energy density through remarkable thick-ness reduction of ceramic plate are studied underway in our lab.Acknowledgements
Financial supports from National Key Basic Rearch Program of China (Grant No.2014CB921004),Key Rearch Program of取义成仁
Chine
Fig.4.(a)Galvanostatic discharge and charge curves of the LFP:KB:PVdF:LiTFSI/LLZTO/Li batteries with 75%mass ratio of LiTFSI to LFP,measured at 1C and 100 C;(b)Coulombic ef ficiency and the speci fic discharge capacity as a function of cycle number;(c)Rate performance;(d)Impedance spectroscopy of the batteries measured at 100 C and 60 C,the former enlarged in the int for clear.The speci fic capacity is normalized to the weight of LFP in the cathode.
卡累利阿地峡F.Du et al./Journal of Power Sources 300(2015)24e 2827
Academy of Sciences(Grant No.KGZD-EW-T06)and National Nat-ural Science Foundation of China(Grant No.U1232111)are acknowledged.
Appendix A.Supplementary data
Supplementary data related to this article can be found at dx.doi/10.1016/j.jpowsour.2015.09.061.
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